Electromagnetic survey method and system, electronic equipment and computer readable storage medium

Abstract

The invention provides an electromagnetic survey method and system and electronic equipment, and belongs to the technical field of exploration geophysics. The method comprises the steps of collecting an initial signal; the initial signal comprises a natural electric field and / or magnetic field entering underground and meeting with the natural electric field and / or magnetic field entering underground at the moment at the sensor at the moment when the natural electric field and / or magnetic field enters underground and returns to the sensor through reflection and refraction of an underground electrical impedance interface, and the two signals are superposed to obtain an initial time sequence of a synthetic electric field and / or magnetic field intensity; obtaining an initial frequency spectrum of the initial time sequence based on Fourier transform of the initial time sequence; travel time inversion: based on an electromagnetic field superposition principle, performing superposition operation on the initial frequency spectrum through a harmonic synthesis algorithm to obtain a synthesized travel time sequence of the electric field and / or magnetic field intensity; the synthesized travel time sequence is used for studying and judging geological information. According to the method provided by the embodiment of the invention, high-resolution geological exploration and engineering exploration based on the natural electromagnetic field source are realized.

Description

[0001] This application is a divisional application of the patent application with the application date of March 8, 2023, the application number of "202310220533.X", and the invention name of "Electromagnetic Survey Method, System, Electronic Device and Computer Readable Storage Medium". Technical Field

[0002] This application relates to the technical field of exploration geophysics. Specifically, it relates to an electromagnetic survey method, system, electronic device and computer readable storage medium. Background Art

[0003] The electromagnetic method survey method is a large class of geophysical exploration means for exploring geological structures, tectonic and other geological problems based on the principle of electromagnetic induction for engineering surveys. Existing electromagnetic method survey technologies mostly use the skin effect phenomenon existing in the electromagnetic induction process as the specific physical basis, which is a volume exploration technology. The electromagnetic method survey method obtains the electrical property distribution characteristics of underground rocks and ores through the measurement and analysis of electromagnetic field data on the ground, and combines the electromagnetic (resistivity, polarizability, magnetic susceptibility, etc.) property data of rocks and ores, geological and other geophysical exploration data. Through the comprehensive interpretation of various data, it solves problems related to geological exploration and engineering surveys.

[0004] The use of natural electromagnetic fields as excitation field sources for geological exploration and engineering surveys can be traced back to the magnetotelluric method (MT, Magnetotelluric) around 1950. Subsequent electromagnetic method survey methods are all the results of optimization and improvement based on the magnetotelluric method principle, and their survey accuracy has not been substantially improved.

[0005] Currently, when using natural electromagnetic fields as excitation field sources to carry out geological exploration and engineering survey activities, most are dedicated to the study of underground electrical properties (resistivity, polarizability) distribution (structure), with the main research objectives of solving the distribution of strata and geological structures. However, due to the characteristics of the skin effect and limited volume exploration resolution ability, only rough information on the underground electrical structure can be obtained, and the underground electrical stratification cannot be detailedly characterized.

[0006] Therefore, there is an urgent need for a new electromagnetic method survey method for detailedly characterizing underground electrical stratification. Summary of the Invention

[0007] To solve the existing technical problems, the embodiments of this application provide an electromagnetic survey method and system based on natural field sources.

[0008] In a first aspect, the embodiments of this application provide an electromagnetic survey method, and the method includes:

[0009] Collect the initial signal; the initial signal includes the natural electric field and / or magnetic field incident on the ground. When it is reflected and refracted by the underground impedance interface and returns to the sensor position, it meets the natural electric field and / or magnetic field incident on the ground at the same time at the sensor, and the two are superimposed to obtain the initial time series of the synthetic electric field and / or magnetic field intensity.

[0010] Perform Fourier transform on the initial time series to obtain the initial frequency spectrum of the initial time series.

[0011] Travel-time inversion. The travel-time inversion is based on the principle of electromagnetic wave superposition, and the initial frequency spectrum is superimposed through the harmonic synthesis algorithm to obtain the synthetic travel-time time series of the natural electric field and / or magnetic field; the synthetic travel-time time series is used to analyze geological information.

[0012] Optionally, the initial signal at least satisfies:

[0013]

[0014] Among them, is the phase difference between the natural electric field and / or magnetic field of any frequency after being reflected and refracted by the underground impedance interface and returning to the sensor and the natural electric field and / or magnetic field of the same frequency continuously incident at the same time; f 0 is the frequency of the electric field and / or magnetic field of any frequency; ρ 1 is the resistivity of any layer of impedance interface; h 1 is the buried depth of any layer of impedance interface.

[0015] Optionally, the method further includes:

[0016] Screen the dominant frequency components in the initial frequency spectrum; the dominant frequency components are related to any underground impedance interface.

[0017] Optionally, the method further includes:

[0018] Screen the dominant frequency components in the initial frequency spectrum; any frequency component in the dominant frequency components is inversely proportional to the buried depth of the impedance interface corresponding to the frequency component.

[0019] Optionally, the dominant frequency components at least satisfy:

[0020]

[0021] Among them, n is an integer greater than or equal to 0; It is the phase difference between the natural electric field and / or magnetic field of any frequency component among the dominant frequency components when it returns to the sensor after being reflected and refracted by the underground electrical impedance interface, and the natural electric field and / or magnetic field of the same frequency component that continues to be incident at the said moment.

[0022] Optionally, the dominant frequency component also satisfies:

[0023] The phase difference of any frequency in the dominant frequency component when superimposed at the sensor is located in the frequency band Within.

[0024] Optionally, the harmonic synthesis algorithm includes:

[0025] and / or

[0026]

[0027] Where f(i) is the frequency of the i-th harmonic, H(t) is the time series of the magnetic field intensity inverted by the travel time in the time domain, and H i (t) is the amplitude of the ith magnetic field harmonic; E(t) is the time series of electric field intensity inverted by travel time in the time domain, E i (t) is the amplitude of the ith electric field harmonic, is the phase of the ith harmonic, k is not equal to 0; t is the travel time; N is the number of harmonics.

[0028] Optionally, the harmonic synthesis algorithm includes:

[0029] and / or

[0030]

[0031] Where f(i) is the frequency of the i-th harmonic, H(t) is the time series of the magnetic field intensity inverted by the travel time in the time domain, and H i (t) is the amplitude of the ith magnetic field harmonic; E(t) is the time series of electric field intensity inverted by travel time in the time domain, E i (t) is the amplitude of the ith electric field harmonic, is the phase of the ith harmonic, k is a natural number greater than 0, t is the travel time, and N is the number of harmonics.

[0032] Optionally, the harmonic synthesis (superposition) algorithm includes:

[0033] and / or

[0034]

[0035] Among them, f(i) is the frequency of the i-th harmonic, H(t) is the travel-time inversion magnetic field strength time series in the time domain, and H i (t) is the amplitude of the i-th magnetic field harmonic; E(t) is the travel-time inversion electric field strength time series in the time domain, and E i (t) is the amplitude of the i-th electric field harmonic, is the phase of the i-th harmonic; t is the travel time; N is the number of harmonics; the value of k is selected from any one of 1, 1.2, 1.4, 2, 2.5, 3, 3.8, and 4.

[0036] Optionally, in the collected initial signal, the number of channels of the acquisition instrument is greater than or equal to 1.

[0037] Optionally, the method further includes:

[0038] Setting acquisition parameters before collecting the initial signal;

[0039] The acquisition parameters include at least one or more of the sampling rate, the lowest frequency to be acquired, the highest frequency to be acquired, and the acquisition duration.

[0040] Optionally, the sampling rate is negatively correlated with the depth to be surveyed.

[0041] Optionally, the sampling rate is greater than or equal to 1000 times per second and less than or equal to 100000 times per second.

[0042] Optionally, the lowest frequency to be acquired is negatively correlated with the maximum depth to be surveyed.

[0043] Optionally, the collecting of the initial signal includes:

[0044] When jointly collecting the electric field strength and the magnetic field strength, the lowest frequency to be acquired is greater than or equal to the lower limit of the frequency that the magnetic sensor can receive;

[0045] When separately collecting the electric field strength or the magnetic field strength, the lowest frequency to be acquired is greater than or equal to 0.01 Hz and less than or equal to 10 Hz.

[0046] Optionally, the acquisition duration is positively correlated with the maximum depth to be surveyed.

[0047] Optionally, the method further includes:

[0048] Performing a first notch processing and / or a low-pass filtering before collecting the initial signal according to the highest frequency to be acquired;

[0049] The first notch processing is to suppress the power frequency and its harmonics;

[0050] The low-pass filtering is to filter out the frequency components higher than the highest frequency to be acquired.

[0051] Optionally, the first notch processing includes first comb filtering processing;

[0052] The first comb filtering processing is a hardware-based filtering processing; the hardware includes a filter.

[0053] Optionally, the frequency of the initial signal is greater than or equal to 0.01 Hz and less than or equal to 50000 Hz.

[0054] Optionally, the obtaining the initial frequency spectrum of the initial time series by performing a Fourier transform on the initial time series includes:

[0055] Converting the original time series data of the time-domain electric field and / or magnetic field intensity components of the initial signal collected by each acquisition channel into component frequency spectra of the frequency-domain electric field and / or magnetic field intensity components;

[0056] Using the vector synthesis rule to synthesize the electric field and / or magnetic field intensity component frequency spectra corresponding to all acquisition channels into the initial frequency spectrum of the electric field and / or magnetic field intensity.

[0057] Optionally, the initial frequency spectrum includes the frequency, real part, imaginary part, amplitude, and phase of the electric field and / or magnetic field.

[0058] Optionally, screening for dominant frequency components in the initial frequency spectrum includes:

[0059] Determining the lowest dominant frequency component based on the survey object and requirements; and

[0060] Suppressing interfering frequency components;

[0061] Wherein the interfering frequency components include background noise, power frequency, and multiples of the power frequency; the background noise includes all frequency components higher than and / or lower than the dominant frequency components.

[0062] Optionally, the suppressing of the interfering frequency components includes: second comb filtering processing and band-pass filtering processing;

[0063] Both the second comb filtering processing and the band-pass filtering processing are processing methods based on the digital signal processing principle.

[0064] Optionally, the method further includes:

[0065] Based on the comparison between the geological data of the survey area and the synthetic travel time series, establishing a correspondence between the geological horizons of the exploration area and the underground impedance interface determined by the synthetic travel time series;

[0066] Determining a marker layer based on the correspondence; the marker layer is the layer in the geological horizon with the highest matching degree with the synthetic travel time series;

[0067] Based on the depth of the said marker layer and the time corresponding to the marker layer in the synthetic travel time series, obtain the time-depth conversion parameters.

[0068] Optionally, the travel time inversion further includes dispersion effect correction;

[0069] The dispersion effect correction includes performing velocity compensation on electromagnetic waves of different frequencies in the harmonic synthesis stage, so that the electromagnetic waves of different frequencies have the same propagation velocity.

[0070] Optionally, the method further includes:

[0071] Before collecting the initial signal, select the initial value of the propagation velocity of the electromagnetic wave corresponding to the lowest frequency in the initial spectrum according to the depth to be surveyed and the geological characteristics of the surveyed area; the initial value is used for time-depth conversion of the initial time series, so as to obtain the initial time series in the depth domain.

[0072] Optionally, the initial spectrum satisfies:

[0073]

[0074] where f min is the lowest frequency in the initial spectrum; v min is the initial value of the propagation velocity of the electromagnetic wave corresponding to the lowest frequency; σ is the conductivity of the underground electrical medium; μ is the magnetic permeability of the underground electrical medium.

[0075] Optionally, the sensor is located on the ground, in the air or on the water surface.

[0076] In a second aspect, an electromagnetic survey system is further provided in an embodiment of the present application, which is characterized in that it is applicable to the electromagnetic survey method provided in any of the above embodiments, and the electromagnetic survey system includes:

[0077] A collection module, which is configured to collect the initial signal; the initial signal includes the moment when the natural electric field and / or magnetic field enter the ground, are reflected and refracted by the underground impedance interface, and return to the ground, and meet the natural electric field and / or magnetic field that enter the ground at the moment at the sensor, and the two are superimposed to obtain the initial time series of the synthetic electric field and / or magnetic field intensity;

[0078] An initial signal processing module, which is configured to perform Fourier transform on the initial time series to obtain the initial spectrum of the initial time series;

[0079] A travel-time inversion module, configured to superimpose the initial spectrum through a harmonic synthesis algorithm based on the principle of electromagnetic field superposition, so as to obtain a synthetic travel-time time series of the natural electric field and / or magnetic field; the synthetic travel-time time series is used to judge geological information.

[0080] Optionally, the electromagnetic exploration system further includes a sensor;

[0081] The sensor is located on the ground, in the air or on the water surface.

[0082] In a third aspect, an embodiment of the present application further provides an electronic device, including a bus, a transceiver, a memory, a processor, and a computer program stored on the memory and executable on the processor. The transceiver, the memory, and the processor are respectively connected through the bus. When the computer program is executed by the processor, it implements the steps in the electromagnetic exploration method provided in any of the above embodiments.

[0083] In a fourth aspect, an embodiment of the present application further provides a computer-readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, it implements the steps in the electromagnetic exploration method provided in any of the above embodiments.

[0084] The electromagnetic exploration method, system, electronic device, and computer-readable storage medium provided by the embodiments of the present application collect the natural electric field and / or magnetic field incident on the ground. After being reflected and refracted by the underground impedance interface and then returning to the ground, the time when they meet the natural electric field and / or magnetic field incident on the ground at this time. The two are superimposed to obtain the initial time series of the synthetic electric field and / or magnetic field. After screening out the dominant frequency components in the total spectrum, through travel-time inversion, the harmonic synthesis algorithm is used to superimpose the initial spectrum components to form the synthetic travel-time series of the natural electric field and / or magnetic field. Thus, the electromagnetic exploration method and system provided by the embodiments of the present application collect the time when the natural electromagnetic field returns to the ground and superimpose it with the natural electromagnetic field incident at this time. The superimposition process has the general characteristics of frequency selectivity, making the dominant frequency components corresponding to the underground impedance interface have the basic characteristics of high signal-to-noise ratio, thereby achieving a greater exploration depth, higher resolution, and higher working efficiency than the traditional method based on natural field sources. Description of the Drawings

[0085] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the background art, the following will describe the drawings required to be used in the embodiments of the present application or the background art.

[0086] Figure 1 Shows a flowchart of an electromagnetic exploration method based on natural field sources provided by an embodiment of the present application;

[0087] Figure 2 A schematic diagram showing a time series of initial signal magnetic field strength collected by the electromagnetic survey method provided in an embodiment of the present application;

[0088] Figure 3 An optional time series of magnetic field intensity in an initial signal collected in an embodiment of the present application is shown, and an initial spectrum diagram (partial) of the initial time series obtained by Fourier transform;

[0089] Figure 4 The figure shows a synthetic time series curve obtained by travel time inversion of a dominant frequency component of the magnetic field intensity selected in an optional initial spectrum provided by an embodiment of the present application;

[0090] Figure 5 A synthetic travel time time series curve graph obtained by inverting the travel time of magnetic field intensity spectrum data of another optional screened dominant frequency component provided in an embodiment of the present application is shown;

[0091] Figure 6 A synthetic travel time time series curve graph obtained by inverting the travel time of the magnetic field intensity / electric field intensity spectrum data of the dominant frequency component selected from an optional initial spectrum provided in an embodiment of the present application is shown;

[0092] Figure 7 A comparison diagram of an optional synthetic travel time time series curve provided in an embodiment of the present application and the skin effect of electromagnetic waves propagating underground and a synthetic signal of a ground penetrating radar is shown;

[0093] Figure 8 The comparison diagram of the travel time section and geological and seismic sections corresponding to a certain structural area in the second place is shown;

[0094] Figure 9 Another time series curve of the inversion synthesis of the travel time of the magnetic field intensity spectrum of the collected initial signal provided in an embodiment of the present application is shown;

[0095] Figure 10 An optional structural schematic diagram of an electromagnetic survey system based on a natural field source provided in an embodiment of the present application is shown;

[0096] Figure 11 Another optional structural schematic diagram of an electromagnetic survey system based on a natural field source provided in an embodiment of the present application is shown;

[0097] Figure 12 Another optional structural schematic diagram of an electromagnetic survey system based on a natural field source provided in an embodiment of the present application is shown;

[0098] Figure 13 An optional structural diagram of an electronic device provided in an embodiment of the present application is implemented.

[0099] The reference numerals in the figure respectively represent:

[0100] 801 - Acquisition module; 802 - Initial signal processing module; 803 - Screening module; 804 - Travel - time inversion module;

[0101] 805 - First notch device; 806 - Low - pass filter; 807 - Suppression device; 808 - Compensation device;

[0102] 1110 - Bus; 1120 - Processor; 1130 - Transceiver; 1140 - Bus interface; 1150 - Memory; 1160 - User interface. Detailed implementation manners

[0103] In the description of the embodiments of the present application, those skilled in the art should know that the embodiments of the present application can be implemented as a method, a device, an electronic device, and a computer - readable storage medium. Therefore, the embodiments of the present application can be specifically implemented in the following forms: complete hardware, complete software (including firmware, resident software, microcode, etc.), and a combination of hardware and software. In addition, in some embodiments, the embodiments of the present application can also be implemented in the form of a computer program product in one or more computer - readable storage media, and the computer - readable storage medium contains computer program code.

[0104] The above - mentioned computer - readable storage medium can adopt any combination of one or more computer - readable storage media. The computer - readable storage medium includes: electrical, magnetic, optical, electromagnetic, infrared, or semiconductor systems, devices, or components, or any combination of the above. More specific examples of the computer - readable storage medium include: portable computer disks, hard disks, random access memory (RAM), read - only memory (ROM), erasable programmable read - only memory (EPROM), flash memory, optical fibers, CD - ROMs, optical storage devices, magnetic storage devices, or any combination of the above. In the embodiments of the present application, the computer - readable storage medium can be any tangible medium that contains or stores a program, and the program can be used by or in combination with an instruction - execution system, device, or component.

[0105] The computer program code contained in the above - mentioned computer - readable storage medium can be transmitted by any suitable medium, including: wireless, wire, optical cable, radio frequency (RF), or any suitable combination of the above.

[0106] The computer program code for performing the operations of the embodiments of the present application can be written in assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-related instructions, microcode, firmware instructions, status setting data, integrated circuit configuration data, or in one or more programming languages or combinations thereof. The programming languages include object-oriented programming languages such as Matlab, LabVIEW, Java, Smalltalk, C++, and also include conventional procedural programming languages such as the C language or similar programming languages. The computer program code can be executed entirely on the user's computer, partially on the user's computer, executed as an independent software package, partially on the user's computer and partially on a remote computer, and entirely on a remote computer or server. In the case of a remote computer, the remote computer can be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), and can also be connected to an external computer.

[0107] The methods, apparatuses, and electronic devices provided in the embodiments of the present application are described by flowcharts and / or block diagrams.

[0108] It should be understood that each block of the flowchart and / or block diagram, and the combinations of blocks in the flowchart and / or block diagram, can be implemented by computer-readable program instructions. These computer-readable program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, or other programmable data processing device, thereby producing a machine. These computer-readable program instructions, when executed by a computer or other programmable data processing device, produce an apparatus that implements the functions / operations specified in the blocks of the flowchart and / or block diagram.

[0109] These computer-readable program instructions can also be stored in a computer-readable storage medium that can cause a computer or other programmable data processing device to work in a specific manner. In this way, the instructions stored in the computer-readable storage medium produce an instruction device product that includes the instructions for implementing the functions / operations specified in the blocks of the flowchart and / or block diagram.

[0110] The computer-readable program instructions can also be loaded onto a computer, other programmable data processing device, or other device, such that a series of operation steps are executed on the computer, other programmable data processing device, or other device, to produce a computer-implemented process. Thus, the instructions executed on the computer or other programmable data processing device can provide a process for implementing the functions / operations specified in the blocks of the flowchart and / or block diagram.

[0111] In the description of the text of this application, the terms "electromagnetic field" and "electromagnetic wave" are used interchangeably. This is because the electromagnetic field propagates in the underground medium in the form of waves, and the difference in terms is purely related to the "scenario" of the words used, without indicating a difference in their meanings.

[0112] The embodiments of this application will be described below with reference to the accompanying drawings in the embodiments of this application.

[0113] The problem of the propagation of the natural electromagnetic field in the underground medium belongs to the category of macroscopic electrodynamics, and the classical Maxwell's equations are the basic physical laws it follows. Both theory and practice have shown that for the propagation of the natural electromagnetic field (wave) in the underground medium in the audio frequency band and below, the wave equation is the basic equation it satisfies.

[0114] Among many electromagnetic exploration technologies, the magnetotelluric MT technology, such as the continuous electromagnetic profiling method (CEMP), is the most widely used electromagnetic exploration technology, which was proposed by the scholar Tikhonov in 1950. In the 1950s, the French scientist Garnier defined the basic relationship of the magnetotelluric method (also known as Garnier's formula) on the basis of the concept of the intrinsic impedance of electromagnetics, as follows:

[0115] ρ(f) = 0.2 * T * |(E(f)) / (H(f))|; (1)

[0116] In formula (1), f is the frequency of the electromagnetic field, T = 1 / f is the period of the electromagnetic field, ρ(f) is the resistivity, |E(f)| is the electric field amplitude, and |H(f)| is the magnetic field amplitude. As the theoretical basis of electromagnetic exploration technology, the Cagniard relation is concise. At the same time, because the operation process uses the operation of the electric field compared with the magnetic field, the "dynamic" form of the wave term (i.e., the primary field effect of the incident electromagnetic field) jointly possessed by the electric field and the magnetic field is implicitly canceled. Therefore, the electromagnetic exploration technology based on the magnetotelluric method and the Cagniard formula both implicitly cancel the primary field effect in the incident electromagnetic field, and only retain the basic relationship between the attenuation of the electromagnetic field and the frequency characterizing the skin effect (i.e., the secondary field effect). On this basis, combined with the existing signal acquisition and processing technology, only the impedance values at a small number of frequency points can be obtained. For example, in the study of a sedimentary basin of clastic rocks over a range of several kilometers, there are generally only 20 to 30 frequency points. However, the information content that can be characterized by 20 to 30 frequency points is far from sufficient for the requirements of geological exploration and engineering survey in reality. This also results in the fact that the resolution of the electromagnetic exploration method based on natural field sources in the existing technology can only reach the order of hundreds of meters. The low resolution leads to the fact that the electromagnetic exploration method based on natural field sources can only be used for rough exploration in the early stage of geological exploration. To date, traditional magnetotelluric technologies, including the later developed CEMP technology and the controlled-source audio-frequency magnetotelluric technology (CSMAT), still remain at the stage where they can only reflect the distribution of several large electrical layers underground and cannot depict the detailed structure of the electrical distribution.

[0117] As is well known in the art, since the underground medium is not always uniform, the propagation of electromagnetic waves in a non-uniform medium is equivalent to a process of repeated refraction and reflection between different media. Therefore, the propagation path of electromagnetic waves in the underground medium is more complex than that in a uniform medium, and the final outgoing direction is also very complex. This complex and long propagation path also brings more obvious attenuation or loss. Intuitively, under the actual underground electrical medium conditions, as the electromagnetic wave penetrates deeper into the ground, the amplitude of the electromagnetic wave amplitude attenuates strongly with depth. Based on the above characteristics, those skilled in the art generally believe that the propagation of electromagnetic waves in the underground medium has the basic characteristics of "two fast and one low", that is, fast attenuation, fast speed, and low frequency (i.e., long wavelength). The fast attenuation severely limits the exploration depth of electromagnetic exploration, while the low frequency limits the exploration accuracy of electromagnetic exploration (that is, the resolution is low).

[0118] In view of the above characteristics of the electromagnetic field propagating underground, in the past 70 years, those skilled in the art have advanced in multiple directions and committed to solving the "two fast and one low" problems of the electromagnetic field. For example, in view of the problem of rapid attenuation of the electromagnetic field, those skilled in the art mostly adopt electromagnetic exploration techniques based on artificial field sources. The electromagnetic exploration techniques based on artificial field sources require numerous devices and complex device layout methods. They not only require long-time detection but also the arrangement of a large number of points. Therefore, it is inevitable that the cost is high, the efficiency is low, and it is extremely unsuitable for use in complex terrains such as mountainous areas.

[0119] In view of the problem of low frequency of electromagnetic waves propagating underground, those skilled in the art usually use high-frequency signals for detection. For example, the ground penetrating radar technology is a means of detecting underground using high-frequency electromagnetic waves. However, the higher the frequency of the electromagnetic wave, the faster its attenuation underground. That is to say, based on the detection method of high-frequency electromagnetic waves, its detection depth is very limited. Therefore, the exploration methods based on electromagnetic methods cannot have both high resolution and large exploration depth and cannot occupy a place in the field of high-resolution exploration.

[0120] The inventor found that the wave terms canceled out in the Cagniard formula on which the traditional electromagnetic method is based have the potential to improve the resolution of electromagnetic exploration techniques. However, there are three major obstacles to the development of natural electromagnetism based on wave theory:

[0121] First, compared with seismic elastic waves, natural electromagnetic waves propagate faster and have longer wavelengths in underground media. Developing an electromagnetic exploration method based on natural field sources based on wave theory with the technical idea of seismic exploration has relatively limited resolution.

[0122] Second, the research object of the technical route of wave theory research is a single underground reflected electromagnetic signal. Since the natural electromagnetic field is used as the excitation field source, due to its continuous incidence characteristics, the underground reflected electromagnetic long signal is always superimposed and coexists with the continuously incident external electromagnetic field, "being interdependent". Existing acquisition and processing means cannot effectively separate the two to obtain a pure underground reflected electromagnetic field signal.

[0123] Third, since the natural electromagnetic field continuously enters the ground, existing acquisition and processing means cannot obtain the initial incidence time of the electromagnetic signal excited by the natural field source and the initial zero time data of the underground reflected electromagnetic signal. That is, the existing technology cannot measure the actual travel time sequence of the natural electromagnetic field.

[0124] The natural electromagnetic field is incident vertically on the ground in the form of a plane wave. Part of it is reflected back into the air, and part is refracted into the underground medium and continues to propagate. The inventor found that when the electromagnetic field propagates in the underground medium and encounters the underground impedance interface, reflection and refraction occur again. When the electromagnetic field reflected again returns to the sensor position, it produces a superposition effect with the external electromagnetic field that continuously enters the ground from the air at this moment at the sensor. At this moment, the synthetic electromagnetic field after the superposition of the above natural electromagnetic field will be refracted into the underground medium again and propagate. The intensity of each frequency component of the electromagnetic field refracted into the underground again at this moment will be the synthetic electromagnetic field after superposition. Therefore, in the following text, the superposed electromagnetic field is called the synthetic electromagnetic field. It should be understood that in this application, the external electromagnetic field is referenced to the ground, and the electromagnetic field that shoots from the air to the ground is called the external electromagnetic field. It should also be understood that the sensor position in this application can be on the ground or in the air.

[0125] When the electromagnetic field reflected and / or refracted by the underground impedance interface returns to the ground, it produces a superposition effect with the natural electromagnetic field that continuously enters the ground from the air at this moment on the ground. Thus, the embodiments of the present application provide an electromagnetic exploration method based on a natural field source. Figure 1 The flowchart of a method provided by the embodiments of the present application is shown. As Figure 1 shown, the method at least includes the following steps.

[0126] Collect the initial signal; the initial signal includes the moment when the natural electric field and / or magnetic field is incident underground and is reflected and refracted by the underground impedance interface and returns to the sensor position (the electromagnetic field reflected back to the ground is the secondary field), and meets the natural electric field and / or magnetic field incident underground at this moment (the electromagnetic field incident underground is the primary field) at the sensor, and the two are superposed to obtain the initial time series of the synthetic electric field and / or magnetic field intensity.

[0127] Perform Fourier transform on the initial time series to obtain the initial spectrum of the initial time series.

[0128] Travel-time inversion. Travel-time inversion is based on the electromagnetic field superposition principle, and the initial spectrum is superposed through the harmonic synthesis algorithm to obtain the synthetic travel-time time series of the natural electric field and / or magnetic field; the synthetic travel-time time series is used to judge geological information. Specifically, different from the traditional electromagnetic exploration method, this electromagnetic exploration method based on a natural field source aims at the moment when the natural electric field and / or magnetic field is incident underground and is reflected and refracted by the underground impedance interface and returns to the sensor position, and meets the natural electric field and / or magnetic field incident underground at this moment at the sensor, and the two are superposed to obtain the synthetic electric field and / or magnetic field intensity. In other words, the electromagnetic exploration method provided by the present application studies the synthetic electromagnetic field after the superposition of the primary field and the secondary field.

[0129] The inventor has discovered that the electromagnetic field synthesized by the superposition of the primary field and the secondary field has strong frequency selectivity. That is to say, when electromagnetic waves of different frequencies reflected back to the ground by the underground impedance interface meet the continuously incident electromagnetic waves on the ground and are superposed, whether the superposition result is enhanced or weakened depends on the wavelength (frequency) of the subsequent incident wave (primary field) and the reflected wave (secondary field) when they meet and the phase difference between them. Thereby, the underground impedance interface has a "tuning" effect on electromagnetic waves.

[0130] The frequency components that are enhanced satisfy the following basic conditions: They are closely related to the phase difference between the electromagnetic field that has propagated in the underground medium, been reflected and refracted back to the ground, and the external electromagnetic field of the same frequency that is continuously incident at this moment.

[0131] Specifically, for the case of a homogeneous, single-layer underground medium, the initial signal should at least satisfy: In the formula, is the phase difference between the natural electric field and / or magnetic field of any frequency after being reflected and refracted by the underground impedance interface and returning to the sensor and the natural electric field and / or magnetic field of the same frequency that is continuously incident at this moment, f 0 is the frequency of any single-harmonic electric field and / or magnetic field in the initial signal, ρ 1 is the resistivity of the medium, h 1 is the buried depth of any electrically conductive layer. This formula reflects the phase difference between the single-harmonic electromagnetic wave of frequency f and the subsequent incident electromagnetic wave (of the same frequency) when they meet on the ground when the incident wave enters the underground good conductor medium (ρ 1 , ε 1 , μ 1 ), propagates a distance h, and is reflected back to the ground surface when encountering the bottom interface of the first electrically conductive layer, considering the dispersion factor. For the case of multiple-layer media, when the external natural electromagnetic field is vertically incident underground and there are many interface boundaries underground, the superposition effect on the ground will also be a superposition process of the reflected waves of multiple interfaces. In fact, the propagation of the external incident electromagnetic field in the underground multi-layer media has general practical significance. Drawing on the concept of the equivalent wave impedance of multi-layer media, the reflection and refraction problems of multi-layer media are transformed into the study of a single-layer medium.

[0132] Theoretically, when the phase difference between two electromagnetic waves of the same frequency is equal to 0, 2π, 4π..., that is, an even multiple of π, the two electromagnetic waves meet and are superposed to be enhanced, and the amplitude of the synthesized electromagnetic field will reach the maximum enhancement effect; while when is equal to π, 3π, 5π..., that is, an odd multiple of π, the two electromagnetic waves of the same frequency meet and are superposed to be weakened, and the amplitude of the synthesized electromagnetic field will reach the minimum.

[0133] When the phase difference satisfies the above conditions, the monochromatic electromagnetic wave of this frequency will achieve the maximum enhancement or attenuation effect. In fact, the monochromatic electromagnetic fields of other frequencies within a frequency band on both sides of the central frequency of the above-mentioned monochromatic electromagnetic field will also be enhanced or attenuated to varying degrees, but the degree of enhancement or attenuation is not as great as that of the central frequency.

[0134] According to an embodiment of the present application, the electromagnetic exploration method provided by the present application further includes screening dominant frequency components in the initial spectrum, and the dominant frequency components are related to a certain electrical structure underground. Preferably, any frequency in the dominant frequency components is inversely proportional to the burial depth of the impedance interface corresponding to this frequency.

[0135] Optionally, the dominant frequency components at least satisfy:

[0136]

[0137] where n is an integer greater than or equal to 0; is the phase difference between the natural electric field and / or magnetic field of any frequency in the dominant frequency components after being reflected and refracted by the underground impedance interface and returning to the sensor, and the natural electric field and / or magnetic field of the same frequency continuously incident at this moment.

[0138] Therefore, optionally, the phase difference when any frequency in the dominant frequency components of the present application is superimposed at the sensor falls within the frequency band Inside. In the above frequency band, int represents taking the integer, is the phase difference between the moment when the fundamental frequency returns to the ground after propagating, reflecting, and refracting in the underground medium and the external electromagnetic field of the same frequency continuously incident at this moment. Therefore, while the frequency that satisfies the above conditions is enhanced due to superposition, based on a similar principle, the frequency components with a multiple-frequency relationship with this frequency are also enhanced due to superposition.

[0139] The acquisition position of the initial signal is not limited to the ground, and can also be the water surface or the air. For traditional electromagnetic exploration and seismic exploration, the sensors must be set on the ground or buried in the shallow surface to avoid being unable to receive signals with a signal-to-noise ratio meeting the requirements due to signal attenuation and interference factors.

[0140] It should be noted that in the prior art, in the multiple coverage and superposition methods used in seismic and ground penetrating radar technologies, the signal intensity of the excitation field source is fixed. The superposition is limited to the superposition between the reflected waves (secondary fields) that are reflected back, and the results of the superposition are always at the same numerical level for each reflected wave numerically. The number of superpositions in the prior art is dozens or hundreds of times. However, what this application studies is the superposition between the continuously incident external primary field and the secondary field reflected back from underground. When incident again, the intensity of the primary electromagnetic field will no longer be the same as that of the primary field in the previous incidence. That is to say, in the embodiments of this application, the intensity of the excitation field source of the electromagnetic field has the characteristics of being dynamic and continuously increasing. This kind of superposition has the characteristic of exponential amplification for the intensity of the primary field incident again. Moreover, in the embodiments of this application, the number of superpositions of electromagnetic waves is thousands or even more than tens of thousands of times. Therefore, compared with the traditional natural electromagnetic method, the initial signals collected in the embodiments of this application have the characteristics of high intensity and high signal-to-noise ratio.

[0141] In addition, based on the fact that the process of natural electromagnetic field incident underground has a certain persistence, the above-mentioned superposition process has the characteristics of long time and many times, so that the fundamental frequency and the multiple frequencies of the fundamental frequency in the natural electromagnetic waves incident underground can be continuously enhanced.

[0142] The stronger incident signal intensity ensures that the electromagnetic field signal can penetrate deeper underground, and obviously this technology has a greater detection depth.

[0143] When natural electromagnetic waves continuously incident underground, in addition to being enhanced many times with the specific wave (group) corresponding to the underground reflection interface that is reflected back, many single harmonic waves that are in a multiple frequency (higher frequency wave) relationship with this specific wave are also enhanced at the same time. The characteristic of being enhanced many times by the higher frequency electromagnetic waves with the waves of the same frequency and the waves in a multiple frequency relationship in the incident wave determines that the received signal will contain more high-frequency information far higher than the "fundamental frequency". The fact that the collected signal is rich in higher frequency components determines that this technology has a high resolution ability. According to the embodiments of this application, the accuracy of the electromagnetic exploration method based on natural field sources is improved by one to two orders of magnitude compared with traditional natural field source methods such as magnetotelluric method. For example, at a detection depth of 5000 meters, the resolution of the electromagnetic exploration method provided by this application is about 10 meters.

[0144] According to an embodiment of the present application, the initial signal may include the intensity of the electric field after the superposition of the primary field and the secondary field, or may include the magnetic field intensity after the superposition of the primary field and the secondary field, or may simultaneously include the intensities of the electric field and the magnetic field after the superposition of the primary field and the secondary field. In the specific implementation process, the type of the initial signal can be freely selected according to the requirements of the exploration task. In the following text, the magnetic field after the superposition of the primary field and the secondary field is taken as an example for illustration. It should be noted that when jointly collecting the intensities of the magnetic field and the electric field after the superposition of the primary field and the secondary field, the lowest frequency collected is the lower limit of the frequency that the magnetic sensor can receive, so as to facilitate the calculation of the Carnot resistivity. When collecting only the intensity of the magnetic field or the electric field after the superposition of the primary field and the secondary field, the lowest frequency collected can optionally take any value in the frequency band of 0.01 Hz - 10 Hz for calculation. In an optional embodiment, the frequency of the initial signal is greater than 1 hertz and less than or equal to 5000 hertz.

[0145] In an embodiment of the present application, the method further includes, before collecting the initial signal, preparing a collection plan according to the requirements of the exploration task and determining the collection parameters. Among them, the collection parameters include at least one or more of the sampling rate, the lowest frequency to be collected, the highest frequency to be collected, and the collection duration.

[0146] According to an embodiment of the present application, the sampling rate is negatively correlated with the depth to be explored. Exemplarily, for an exploration depth of 2000 meters to 5000 meters, the sampling rate is greater than or equal to 2000 hertz. Preferably, for exploration targets within 2000 meters, the sampling rate ranges from 5000 times per second to 100000 times per second.

[0147] According to an embodiment of the present application, the collection duration is positively correlated with the buried depth of the exploration target. For example, for an exploration depth of 2000 meters, the collection duration is preferably 600 seconds. For a collection depth of 5000 meters, the collection duration is preferably 1200 seconds.

[0148] Furthermore, the electromagnetic exploration method based on natural field sources provided by the embodiments of the present application further includes: performing first notch filtering and / or low-pass filtering before collecting the initial signal according to the highest frequency to be collected. Among them, the first notch filtering is to suppress the power frequency and its harmonics; the low-pass filtering is to filter out the frequency components higher than the highest frequency to be collected. The first notch filtering includes first comb filtering, and this first comb filtering is a hardware-based filtering process. Thereby, spectral aliasing can be avoided. If software-based filtering is used, spectral aliasing will inevitably occur. The aforementioned hardware includes filters, etc. The power frequency refers to the rated frequency adopted by the equipment of the power system, industrial electrical equipment, and civil electrical equipment. Exemplarily, the power frequency is 50 Hz. In some optional cases, regardless of the burial depth of the detection object and the sampling rate during data collection, it is necessary to pre-suppress the intensity of the power frequency and its harmonic components. In some optional cases, it is necessary to perform pre-low-pass filtering according to the highest frequency to be collected to suppress the frequency components higher than the highest frequency to be collected. In most cases, the initial signal can be collected at the outdoor ground surface. The collection location provided by the embodiments of the present application can also be at sea or in the air.

[0149] It can be understood that the number of channels collected in the electromagnetic exploration method based on natural field sources provided by the embodiments of the present application is greater than or equal to 1. Preferably, the number of channels collected is greater than or equal to 8.

[0150] Example 1

[0151] Figure 2 The figure shows the initial signal collected beside an oil exploration well in Location A in December 2020. This initial signal is a single-channel magnetic field intensity time series. In the figure, the abscissa is time, with the unit of second, and the ordinate is the magnetic field intensity, with the unit of nanotesla. Optionally, the ordinate can also be other physical quantities in a proportional relationship with the magnetic field intensity, such as uncalibrated scale values, voltage values, etc. Figure 2 The upper figure in [the figure] is a time series with a length of 0.4 seconds, Figure 2 The lower figure in [the figure] is a time series with a length of 0.025 seconds. The number of acquisition channels during this acquisition is 1, the sampling rate is 24,000 times per second, the total acquisition time length is 30 seconds, and the acquisition data frequency band is 0 - 12,000 Hz.

[0152] In an alternative embodiment, obtaining the initial spectrum of the initial time series through Fourier transform of the initial time series includes: converting the original time series data of the time-domain electric field and / or magnetic field intensity components of the initial signals collected by each acquisition channel into the component spectra of the frequency-domain electric field and / or magnetic field intensity components; using the vector synthesis rule to synthesize the electric field and / or magnetic field intensity component spectra corresponding to all acquisition channels into the initial spectrum of the electric field and / or magnetic field. The aforementioned initial spectrum includes the frequency, real part, imaginary part, amplitude, and phase of the electric field and / or magnetic field. Optionally, the initial spectrum satisfies: where f min is the lowest frequency in the initial spectrum; v min is the initial value of the electromagnetic wave propagation speed corresponding to the lowest frequency; σ is the conductivity of the underground electrical medium; μ is the magnetic permeability of the underground electrical medium. Obtaining the total spectrum of the electric field intensity and / or magnetic field intensity based on the initial signals includes: converting the original time series data of the initial signals collected by each acquisition channel into the initial spectrum of the initial signals through fast Fourier transform (DFFT) or modern spectral analysis algorithms; using the vector synthesis rule to synthesize the initial spectra corresponding to all acquisition channels into the total spectrum.

[0153] Specifically, the electric field intensity and / or magnetic field intensity data collected by each acquisition channel are the original time series data in the time domain. The original time series data includes time, magnetic field intensity, and electric field intensity. The magnetic field intensity includes a first magnetic field intensity component and a second magnetic field intensity component, and the first and second magnetic field intensities are perpendicular to each other and perpendicular to the incident direction of the natural electromagnetic field respectively. The electric field intensity includes a first electric field intensity component and a second electric field intensity component, and the first and second electric field intensities are perpendicular to each other and perpendicular to the incident direction of the natural electromagnetic field respectively.

[0154] Exemplarily, the original time series is recorded in a custom binary Bin file, and basic parameters such as the name of the acquisition work area, data acquisition operator, acquisition date, channel description, sampling rate, data accuracy, and data length are stored in the bin file for subsequent data processing.

[0155] In the embodiments of the present application, the original time series characterizes the relationship between the electromagnetic field and time. Based on Fourier analysis, the original time series data needs to be converted to obtain the initial spectrum data of the natural electric field and / or magnetic field intensity in the frequency domain for each channel to characterize the variation characteristics of the synthetic electromagnetic field after the superposition of the primary field and the secondary field with frequency.

[0156] Next, after obtaining the initial spectrum of each acquisition channel, the initial spectra are synthesized into the total spectrum using the vector synthesis rule. Specifically, the real and imaginary parts of the electric and / or magnetic field strength components of the single-channel electric field strength and / or magnetic field strength are synthesized into the total electric and / or magnetic field strength, and then the amplitude and phase of the total electric and magnetic fields are calculated. Taking the magnetic field as an example, the specific calculation formulas are shown in Formulas (2-1) to (2-4):

[0157] DH R =HX R +HY R ; (2-1)

[0158] DH I =HX I +HY I ; (2-2)

[0159]

[0160] DH 相位 =arctan(DH I / DH R );(2-4)

[0161] Among them, DH represents the total magnetic field strength, HX is the first magnetic field strength component; HY is the second magnetic field strength component; I represents the imaginary part, and R represents the real part.

[0162] Taking the electric field as an example, the specific calculation formulas are shown in Formulas (3-1) to (3-4):

[0163] DE R =EX R +EY R ; (3-1)

[0164] DE I =EX I +EY I ; (3-2)

[0165]

[0166] DE 相位 =arctan(DE I / DE R ); (3-4)

[0167] Among them, DE represents the total magnetic field strength, EX is the first magnetic field strength component; EY is the second magnetic field strength component; I represents the imaginary part, and R represents the real part.

[0168] Exemplarily, the total spectrum is consistent with the initial spectra of the first magnetic field component and the second magnetic field component, and all include frequency, real part, imaginary part, amplitude, and phase.

[0169] Example 2

[0170] Figure 3 It shows the initial spectrum of the initial time series obtained by Fourier transform of the initial signal magnetic field intensity time series collected at the same point position data and collected at 12 time intervals of 2 hours each, which is the initial spectrum of the single-channel magnetic field intensity data. The initial signal was collected from August 21st to 22nd, 2021, at the same location as in Example 1. Within 24 hours, data was collected every two hours, starting from 16:00 pm on the 21st and ending at 14:00 pm on the 22nd, with a total of 12 data collections. Each collection lasted for 30 seconds, the sampling rate was 24,000 times per second, and the analysis frequency band was 0 - 12,000 Hz. The abscissa in the figure is the frequency, and only the amplitude spectrum in the frequency band of 0 - 5000 Hz is drawn here, with the unit of Hz, and the ordinate is the magnetic field intensity amplitude. Figure 3 Characterizes the basic characteristics of the magnetic field intensity amplitude spectrum. As Figure 3 shown, the magnetic field components in the natural electromagnetic field are generally present in the frequency band from DC to 24,000 Hz; in the frequency band below 500 Hz, the magnetic field intensity increases monotonically with the decrease of frequency; the magnetic field components at 50 Hz power frequency and its harmonics are the main interference factors of the electromagnetic exploration method provided by this application, and usually have relatively large amplitude values.

[0171] Considering that when collecting the initial signal, it is not always possible to achieve the expected suppression effect for power frequency suppression in the field, further software denoising and filtering processing can be performed on the collected data in the indoor processing stage. Therefore, advantageously, it is necessary to suppress the interference frequency components in the total spectrum, especially the power frequency and its harmonic components, to facilitate screening out the effective signals in the total spectrum data. In the embodiments of this application, the effective signal is also called the dominant frequency component. The dominant frequency component includes the frequency components and their harmonics that can be enhanced by the superposition of the primary field and the secondary field.

[0172] According to the implementation manner of the embodiments of this application, optionally, screening the dominant frequency components in the initial spectrum includes: determining the lowest dominant frequency component based on the exploration object and requirements; and suppressing the interference frequency components; where the interference frequency components include background noise, power frequency, and the harmonics of the power frequency; the background noise includes all frequency components higher than and / or lower than the dominant frequency component.

[0173] Power frequency and its multiple frequencies are various types of artificial power frequency interferences, and the discrimination and suppression of them are important links in the electromagnetic exploration method provided by the embodiments of this application. It should be noted that the power frequencies in different countries and regions are not exactly the same. According to the implementation manners of this application, optionally, the aforementioned interference frequency components to be suppressed include: second comb filtering processing and band-pass filtering processing; both the second comb filtering processing and the band-pass filtering processing are processing manners based on the digital signal processing principle.

[0174] The electromagnetic exploration method based on natural field sources provided by the embodiments of this application studies natural electromagnetic field data. Preferably, the frequency range is 0.01 - 50000 Hz. Theoretically, the detection depth of electromagnetic waves can be expressed as H = v / (2πf min ). Here, v is the propagation speed of natural electromagnetic waves in underground media, and f min is the minimum frequency (i.e., the minimum fundamental frequency) corresponding to the electromagnetic wave. If the frequency components below the minimum fundamental frequency participate in further data processing, non-periodic and non-linear background frequencies (i.e., background noise) with frequencies lower (longer wavelengths) than the lowest effective frequency will be introduced into the travel time data. Therefore, preferably, the background noise is removed. Removing the frequency components below the minimum fundamental frequency also brings additional benefits: in the initial signal acquisition stage, the intensity of the frequency components below the minimum fundamental frequency is often greater than that of the signals at the minimum fundamental frequency, which is not conducive to the travel time inversion in the electromagnetic exploration method provided by this application. Therefore, it is beneficial to determine the minimum fundamental frequency, that is, the lowest effective frequency.

[0175] The lowest effective frequency needs to be determined according to specific geological tasks and exploration requirements, especially the exploration depth and speed. For example, for example: if the required detection depth of the task is 6 km and the electromagnetic wave speed is taken as 110000 m / s, then the lowest dominant frequency is taken as 18 Hz. The highest dominant frequency is determined by experiments and generally varies between 500 - 50000 Hz;

[0176] Optionally, the interference frequencies to be suppressed include second notch filtering and band-pass filtering. Among them, the second notch filtering is to suppress power frequency and its multiple frequencies. The band-pass filtering includes filtering out the frequency components below the background noise frequency and above the highest frequency to be collected.

[0177] Optionally, compensating the dominant frequency components includes compensating the frequency components with the same frequency as the interference frequency components and a phase difference equal to π into the dominant frequency components. Preferably, the frequency components with the same frequency as power frequency and its multiple frequencies and a phase difference of π are used to compensate the dominant frequency components. The amplitude magnification of the compensating frequency is determined by experiments.

[0178] The initial spectrum in any of the above embodiments and the dominant frequency components selected from the initial spectrum are used for travel-time inversion. Travel-time inversion is the key to the electromagnetic exploration method provided in the embodiments of the present application. Travel-time inversion refers to, based on the principle of electromagnetic field superposition, superimposing the initial spectrum (preferably the dominant frequency components) through the Fourier harmonic synthesis algorithm to obtain the synthetic travel-time sequence of the natural magnetic field and / or electric field. This synthetic travel-time sequence characterizes the actual travel-time sequence of electromagnetic waves and is used to judge geological information.

[0179] Specifically, the travel-time inversion is designed based on the principle of electromagnetic field superposition: among the numerous dominant frequency components selected, the underground impedance interface that plays a role in superimposing and enhancing the fundamental frequency electromagnetic wave components often corresponds to multiple frequency components that are in a multiple-frequency relationship with the fundamental frequency. When the fundamental frequency and its multiple frequency are superimposed again to form a new time series, only when the travel time, frequency (or period) of the electromagnetic wave and the buried depth of the underground impedance interface are completely "paired" (that is, meet the superposition enhancement condition), the intensity of the electromagnetic field is superimposed and enhanced, forming a travel-time wavelet similar to a seismic wavelet, while in other frequency bands between the travel-time wavelets, the superposition cancels out or is chaotic.

[0180] Based on the above principle of electromagnetic field superposition, the embodiments of the present application superimpose the initial spectrum through the harmonic synthesis algorithm to obtain the synthetic travel-time time series of the natural electric field and / or magnetic field. Considering the dispersion effect of electromagnetic waves, the superimposition calculation satisfies the following formulas (4-1) and (4-2):

[0181] and / or

[0182]

[0183] where f(i) is the frequency of the i-th harmonic, H(t) is the travel-time inversion magnetic field intensity time series in the time domain, H i (t) is the amplitude of the i-th magnetic field harmonic; E(t) is the travel-time inversion electric field intensity time series in the time domain, E i (t) is the amplitude of the i-th electric field harmonic, is the phase of the i-th harmonic, k is not equal to 0; t is the travel time; N is the number of harmonics. Thereby, the distortion caused by the dispersion during the propagation of electromagnetic waves can be effectively suppressed to achieve a detailed characterization of the underground electrical structure with a large detection depth and high resolution. Preferably, in formulas (4-1) and (4-2), k is a natural number greater than or equal to 0. More preferably, in formulas (4-1) and (4-2), the value of k is selected from any one of 1, 1.2, 1.4, 2, 2.5, 3, 3.8, 4.

[0184] It should be understood that the travel-time inversion based on the above principles can be calculated using conventional inversion algorithms in the art (such as the least-damped least squares method, the gradient method, the Newton method, simulated annealing, genetic algorithms, or neural network algorithms, etc.).

[0185] Example 3

[0186] Example 3 provides an exemplary travel-time inversion, and the result of this travel-time inversion is as Figure 4 shown. Example 3 takes the natural magnetic field containing 50 Hz power frequency and its harmonics as an example for travel-time inversion. Figure 4 For the 12 groups of magnetic field strength spectrum data based on Figure 3 after screening the dominant frequency components, the 12 combined synthetic time series (data) curves obtained by travel-time inversion, and the 12 curves among them are respectively from Figure 3 the spectra of the initial signals collected at each moment in

[0187] Figure 3 After screening the dominant frequency components, the 12 combined synthetic time series curves are obtained by travel-time inversion respectively. The abscissa in the figure is time, with the unit of second, and the ordinate is the magnetic field strength. The travel-time time series synthesized by travel-time inversion is used to judge geological information.

[0188] Table 1 Figure 4 Statistical table of the correlation characteristics of the initial spectra of the magnetic field strength of 12 groups of initial signals

[0189]

[0190]

[0191] Table 2 Figure 4 Statistical table of the correlation characteristics of the travel-time time series of the magnetic field strength of 12 groups

[0192]

[0193] According to the frequency selection characteristics of electromagnetic waves after the superposition of the primary field and the secondary field, the travel time inversion at least satisfies formula (5-1) and formula (5-2):

[0194] t=1 / (2f);(5-1)

[0195] h=v*t; (5-2)

[0196] Among them, t is the travel time of any monochromatic electromagnetic wave with a frequency of f when propagating underground; v is the phase velocity of any monochromatic electromagnetic wave with a frequency of f when propagating underground; h is the burial depth of the underground electrical impedance interface.

[0197] Example 4

[0198] Example 4 provides the initial magnetic field intensity signal spectrum collected at multiple measurement points at a certain oil exploration area at a certain location 100 meters apart. After the dominant frequency component is selected, the synthetic travel time sequence obtained by travel time inversion is as follows: Figure 5 shown. Figure 5 In the figure, the ordinate is the intensity of the natural magnetic field after the superposition of the primary field and the secondary field, and the abscissa is the travel time curve of the natural magnetic field synthesized after the superposition of the primary field and the secondary field. It can be clearly seen from the figure that: first, in the early stage of the travel time curve (geologically corresponding to the shallower depth of the underground), the amplitude of the magnetic field intensity is large, the change is drastic, and the "frequency" is high, while in the late stage of the travel time (geologically corresponding to the deeper depth of the underground), the amplitude of the magnetic field intensity becomes smaller, the change is gentle, and the "frequency" is low; second, the time series curves of each travel time inversion at different points are well comparable.

[0199] Figure 6 The 24 curves are respectively synthesized from the spectrum of 12 groups of initial electric / magnetic field signals collected every 2 hours for 24 hours at a certain oil exploration well in the third place. The horizontal axis in the figure is time in seconds, and the vertical axis is the electric field intensity / magnetic field intensity synthesized by travel time inversion. The synthetic travel time time series is used to judge geological information. Figure 6 It can be clearly seen that: first, in the early stage of the travel time curve (geologically corresponding to the shallower depth underground), the amplitude of the magnetic field strength is large, the change is drastic, and the "frequency" is high, while in the late stage of the travel time (geologically corresponding to the deeper depth underground), the amplitude of the magnetic field strength becomes smaller, the change is gentle, and the "frequency" is low; second, the time series curves of the travel time inversions at different points are well comparable; third, although there are differences in "morphology" between the magnetic field intensity travel time curve and the electric field intensity travel time curve, the characteristics of "phase alignment" are consistent.

[0200] Figure 7Analogous diagram of the skin effect (a), the synthetic signal of the ground penetrating radar (b), and the travel time inversion synthetic travel time series (c) curve when electromagnetic waves propagate in underground media.

[0201] Figure 7 In (a), it shows the general law of attenuation of electromagnetic waves with different frequencies (periods) when propagating underground under given geoelectric conditions: the skin depth of a single-harmonic electromagnetic wave with a period of 100 seconds (skin depth: the depth at which the electromagnetic wave attenuates to 1 / e = 0.368 times before entering the incident medium) is 50,000 meters, the skin depth of a single-harmonic electromagnetic wave with a period of 4 seconds is 10,000 meters, and the skin depth of a single-harmonic electromagnetic wave with a period of 1 / 25 seconds is 1,000 meters.

[0202] Figure 7 In (b), it is the ground penetrating radar record synthesized by a six-layer geoelectric model.

[0203] Among them, for the first layer, h 1 = 2.5, ε 1 = 9ε 0 , ρ 1 = 10, ν 1 = 0.1ν; for the second layer, h 2 = 1, ε 2 = 12ε 0 , ρ 2 = 500, ν 2 = 0.09ν; for the third layer, h 3 = 0.9, ε 3 = 20ε 0 , ρ 3 = 100, ν 3 = 0.07ν; for the fourth layer, h 4 = 1, ε 4 = 12ε 0 , ρ 4 = 500, ν 4 = 0.09ν; for the fifth layer, h 5 = 0.9, ε 5 = 20ε 0 , ρ 5 = 100, ν 5 = 0.07ν; the sixth layer is a high-resistance basement, ε 6 = 8ε 0 , ρ 6 = 5000, ν 6 = 0.11ν.

[0204] Here, the unit of h is m; the unit of ρ is Ω·m; υ = 300,000,000 m / s, and here ν i is the multiple of the electromagnetic wave velocity ν in vacuum, i = 1, 2…6.

[0205] Figure 7 Among them, (c) is the travel-time time series curve synthesized by travel-time inversion in the electromagnetic exploration method provided by this application. The data is the processing result of the magnetic field intensity time series data collected beside a certain exploration well in place C. The sampling rate of the collected data is 24,000 times per second, the sampling duration is 60 seconds, and the frequency band selected for the effective signal is 5 - 2000 Hz, that is, the lowest frequency is 5 Hz and the highest frequency is 2000 Hz.

[0206] When synthesizing the travel-time time series by travel-time inversion, the calculation formulas are formulas (4-1) and (4-2); where f(i) is the frequency of the i-th harmonic, H(t) is the travel-time inversion magnetic field intensity time series in the time domain, and H_i(t) is the amplitude of the i-th magnetic field harmonic; E(t) is the travel-time inversion electric field intensity time series in the time domain, and E_i(t) is the amplitude of the i-th electric field harmonic, is the phase of the i-th harmonic, k is the dispersion effect correction coefficient, and its value is not 0; t is the travel time; N is the number of harmonics.

[0207] Figure 8 It shows the comparison diagram of the travel-time time section corresponding to a certain structural area in place B with the geological and seismic profiles after the electromagnetic exploration method provided by this application is used for exploration in this area. This structural area is located in the Hetao Basin in the central part of Inner Mongolia. The Hetao Basin covers an area of about 40,000 square kilometers and is a Mesozoic-Cenozoic depression-fault basin developed on the Paleoproterozoic - Archean metamorphic crystalline basement of the Yimeng ancient land core. It is divided into three depressions, namely Linhe, Wuhou, and Huhe, from west to east. A certain structural area in place B is located in the Linhe Depression.

[0208] Multiple sets of source rocks are developed in the Linhe Depression, with large thickness and wide distribution. The thickness of the source rock of the Lower Cretaceous Guyang Formation is 200 - 600 meters, the burial depth is 3100 - 3200 m, and the area is about 1910 km 2 ; the thickness of the source rock of the Oligocene Linhe Formation is 200 - 550 m, the burial depth is 1400 - 1700 m, and the area is about 3680 km2. The source rocks in the area are of saline lake facies deposition, with high organic matter abundance. The geochemical analysis of the source rocks of the Lower Cretaceous and Oligocene shows relatively high organic matter abundance, TOC (0.4 - 2.6)%, with an average of 1.05%. According to the standard of source rocks in continental saline lake basins, they are good source rocks. The source rocks of the Paleogene are mainly of type Ⅰ - Ⅱ1 kerogen, the organic matter type of the source rock of the second member of the Lower Cretaceous Guyang Formation is mainly type Ⅱ kerogen, and the source rock of the first member of the Guyang Formation is mainly of type II.

[0209] The structural position of this example is the combined part of the fault block buried hill and the fault depression area ( Figure 8(Lower left), the cross-section runs from west to east, and is divided into three structural sections at points A and B from west to east: the B-West section is the buried hill part, belonging to a large fault-block buried hill with the Archean basement. The buried hill surface is buried at a depth of 300 - 800 meters, and is covered by 300 - 800 meters thick Mesozoic and Cenozoic sedimentary strata. The weathered crust and fractures on the top surface of the buried hill are the main hydrocarbon reservoirs; the A - B section belongs to a large clastic rock fault-nose structural area developed on the eastern wing of the buried hill. The formation dip angle is large. The reservoir conditions are that the gneiss fractures in the buried hill are developed, and the clastic rock fault-nose structure is near the provenance. The two reservoirs are shallowly buried (400 - 2000m) and have good physical properties, providing the material basis for the formation of large oil and gas fields; east of point B belongs to the category of sedimentary and tectonic systems within the fault depression, and Mesozoic sedimentary strata with complete sedimentary stratigraphic sequences and large formation thicknesses are developed.

[0210] The study of physical property data shows that the Archean basement in this exploration area is a high-resistance basement with a resistivity as high as thousands of ohm-meters, which is a typical high-resistance marker bed, while the resistivity of Mesozoic and Cenozoic sedimentary strata is relatively low, generally less than 10 ohm-meters. Obviously, there is a huge contrast in the impedance interface between the Archean and Mesozoic - Cenozoic strata, laying a good physical prerequisite for the success of the technology of this application.

[0211] Figure 8 The upper left figure is the travel-time time series cross-section synthesized by the travel-time inversion of the magnetic field intensity in the depth domain after time-depth conversion. It can be clearly seen on the cross-section that the entire cross-section is cut into three sections: west - middle - east by two faults A and B from west to east, and the positions of the dividing points A and B correspond basically to the Figure 7 positions of the three divisions on the geological cross-section in the lower left figure.

[0212] The western section west of point A corresponds to the position of the Archean buried hill. On the electromagnetic method result map, the top surface of the buried hill shows a fluctuating and uneven state, and is cut by at least seven faults inside, divided into 8 mutually isolated hilltops; the eastern section east of point B corresponds to the category of sedimentary and tectonic systems within the fault depression east of point B. The Mesozoic sedimentary strata have a relatively large thickness and a relatively small formation dip angle, and 3 to 4 electrical layers can be further divided inside; the section between point A and point B corresponds to the large clastic rock fault-nose structural area developed on the eastern wing of the buried hill, and this section is characterized by a large formation dip angle.

[0213] The geological model designed in this application is a horizontal and quasi-horizontal formation model. For high-resistance structures, since the incident primary field from outside and the secondary field reflected by the underground impedance interface cannot meet and be enhanced, it is inevitable that there will be a "blank zone" in the data in the high-resistance structural area, and migration technology is needed to obtain the electromagnetic data in this section, which is the work left for the next step of this technology.

[0214] Figure 8The upper right figure is the seismic profile of the corresponding section. When comparing the electromagnetic method technology, seismic technology achievements of this application with the geological section map, pay special attention to the following three points: First, the shape of the buried hill surface is not a uniformly changing interface, but a small hill composed of 8 isolated fault blocks complicated by 7 faults; Second, exploration practice has proved that not the entire top of the buried hill contains oil, but only individual sections contain oil, and the proportion of the small hills containing oil among the 8 small hills is less than 50%; Third, the formation dip and the boundaries of the three tectonic units are basically the same.

[0215] Figure 9 After the Fourier transform of the time series data of the magnetic field intensity of the collected initial signal, the frequency spectrum of the magnetic field intensity of the initial signal is obtained. Further, by performing travel time inversion on the frequency spectrum of the magnetic field intensity of the initial signal, the travel time series of the magnetic field intensity is obtained. This data was collected beside an oil exploration well in Jiaodi in December 2020. Single-channel magnetic field data was collected with a sampling rate of 24,000 times per second and a sampling duration of 30 seconds. The abscissa in the figure is time, with the unit of second. The 8 curves in the figure are the travel time series obtained by travel time inversion when different highest frequency values are selected. The ordinate is the magnetic field intensity.

[0216] In the technology of this application, the signal of the selected dominant frequency component contains the fundamental frequency and harmonic frequency components corresponding to the impedance interface. The utilization of the harmonic frequency component is the internal basis for improving the resolution ability of the travel time inversion synthesis of the travel time series. This example shows that when different frequency ranges of harmonic frequencies are selected, the inversion results have different resolution abilities. The higher the high-frequency end of the selected harmonic frequency, the stronger the resolution ability, and the richer the details revealed.

[0217] In some optional implementation manners, the electromagnetic exploration method provided by the embodiments of this application further includes the following steps.

[0218] Determine the marker layer, that is, based on the comparison of the geological materials in the exploration area and the travel time curve corresponding to the actual travel time sequence, establish the corresponding relationship between the geological horizons in the exploration area and the underground impedance interface determined according to the actual travel time sequence, and determine the marker layer; the marker layer is the layer with the highest matching degree with the travel time curve among all geological horizons in the geological horizon.

[0219] Obtain the time-depth conversion parameter, that is, based on the depth of the marker layer and the time corresponding to the marker layer on the travel time curve, obtain the time-depth conversion parameter.

[0220] The time-depth conversion parameter is used to calibrate the result of the travel time inversion. The more accurate the selected time-depth conversion parameter is, the higher the matching degree between the actual travel time sequence and the situation of the electromagnetic wave propagation.

[0221] Exemplarily, the time-depth conversion parameter satisfies formula (6):

[0222] ν = h / t; (6)

[0223] In formula (6), ν is the time-depth conversion parameter, which has the same dimension as velocity; h is the burial depth of the marker bed; t is the travel time of the electromagnetic wave corresponding to the depth of the marker bed.

[0224] Collect geological and geophysical data such as electric logging and comprehensive mud logging in the exploration area, especially deep induction electric logging data. Systematically carry out the comparison and interpretation of the travel time curve of high-resolution electromagnetic exploration beside the well with the electric logging curve and the comprehensive mud logging curve to ensure the accurate comparison between the major electrical layers and the major geological layers. Use the layer (interface) with the clearest corresponding relationship between the logging data and the travel time curve as the marker bed, and the corresponding depth-time value is the conversion parameter required for finally obtaining the time-depth conversion.

[0225] Furthermore, the electromagnetic exploration method provided by the embodiments of the present application further includes:

[0226] Optimizing the parameter selection of travel time inversion based on the time-depth conversion parameter.

[0227] Exemplarily, optimizing the parameter selection of travel time inversion based on the time-depth conversion parameter. Through repeated experiments, multiple time-depth conversion parameters are obtained. Calibrate and improve the principles and methods of parameter selection for travel time inversion based on a large number of time-depth conversion parameters accumulated through repeated experiments. It can be understood that the optimization of travel time inversion parameters by time-depth conversion parameters can be achieved through deep learning.

[0228] According to the implementation manner of the present application, optionally, the electromagnetic exploration method further includes velocity compensation. Electromagnetic waves with different frequencies have different propagation velocities in the same medium, which is called the dispersion characteristic of electromagnetic waves. In the electromagnetic exploration method provided by the embodiments of the present application, due to the dispersion characteristic of electromagnetic waves, the shape of the reflected wavelet on the time-domain travel time curve corresponding to the underground impedance interface will be distorted, and it is difficult to effectively identify the information of the impedance interface.

[0229] Therefore, in travel time inversion, it is necessary to adjust and normalize the velocity of electromagnetic waves so that different frequency components have the same velocity, so that the reflected "wavelet" on the travel time curve no longer distorts, which is beneficial to the interpretation of travel time data and improves the accuracy of the electromagnetic exploration method.

[0230] In some optional implementation manners, the velocity compensation includes:

[0231] Perform time-depth conversion on the travel time curve according to the initial value of the propagation velocity of the electromagnetic wave corresponding to the lowest effective frequency in the embodiments of the present application, so as to obtain the travel time curve in the depth domain. Exemplarily, the initial value satisfies:

[0232]

[0233] where f min is the lowest effective frequency, that is, the minimum value of the effective frequency; vmin is the initial value of the electromagnetic wave propagation speed corresponding to the lowest effective frequency; is the conductivity of the underground electrical medium; μ is the magnetic permeability of the underground electrical medium.

[0234] Perform time-depth conversion of the travel-time curve with the above initial value of the electromagnetic wave propagation speed to obtain the travel-time curve in the depth domain.

[0235] Systematically carry out the comparison and interpretation of the curves in the depth domain of the high-resolution electromagnetic survey beside the well with the electric logging curve and the comprehensive logging curve, ensure the accurate comparison of the major electrical layers and the major geological layers, use the layer (interface) with the best corresponding relationship between the logging data and the logging curve to be measured as the marker layer, and use v min = H / t to determine the value of the electromagnetic wave propagation speed corresponding to the lowest frequency during the actual data processing and interpretation. In the formula: v min is the propagation speed value corresponding to the single-harmonic electromagnetic wave with frequency f min ; H is the depth of the marker layer, in meters; t is the time corresponding to the depth of the marker layer, in seconds.

[0236] Use the propagation speed actually corresponding to the lowest effective frequency to obtain the propagation speed actually corresponding to the electromagnetic waves of other frequencies. Exemplarily, the propagation speed actually corresponding to the electromagnetic wave with frequency f satisfies:

[0237]

[0238] Optionally, the magnetic field expressions in the travel-time inversion after velocity compensation are shown in formulas (4-1) and (4-2).

[0239] As described above in combination with Figures 1 to 9 , the electromagnetic survey method based on natural field sources provided by the embodiments of the present application is described in detail. This method can also be implemented through a corresponding system. Next, in combination with Figures 10 to 13 , the electromagnetic survey system based on natural field sources provided by the embodiments of the present application will be described in detail.

[0240] Figure 10 shows a schematic structural diagram of an electromagnetic survey system based on natural field sources provided by an embodiment of the present application. As Figure 10 shown, the electromagnetic survey system includes:

[0241] An acquisition module 801, configured to acquire an initial signal; the initial signal includes the moment when the natural electric field and / or magnetic field enter the ground, are reflected and refracted by the underground impedance interface, and return to the ground, and meet the natural electric field and / or magnetic field that enter the ground at the moment at the sensor, and the two are superimposed to obtain the initial time series of the synthesized electric field and / or magnetic field intensity;

[0242] The initial signal processing module 802 is configured to perform a Fourier transform on the initial time series to obtain the initial spectrum of the initial time series;

[0243] Optionally, a screening module 803 screens the effective frequencies in the total spectrum; wherein, the effective frequencies include the fundamental frequency that can be enhanced and the harmonic frequencies of the fundamental frequency;

[0244] The travel time inversion module 804 is configured to perform a superposition operation on the initial spectrum through a harmonic synthesis algorithm based on the principle of electromagnetic wave superposition, so as to obtain the synthetic travel time series of the natural electric field and / or magnetic field; the synthetic travel time series is used to judge geological information.

[0245] Therefore, the electromagnetic exploration system of the embodiment of the present application collects the electromagnetic waves superimposed by the underground impedance interface through the collection module 801, and the initial spectrum of the synthetic electric field strength and / or magnetic field strength based on the initial signal through the initial signal processing module 802. Subsequently, the screening module 803 screens out the dominant frequency components in the total spectrum, and the travel time inversion module 804 uses the Fourier transform to convert and superimpose them to obtain the actual travel time series.

[0246] Optionally, as Figure 11 shown, the electromagnetic exploration system further includes:

[0247] A first notch device 805 is configured to suppress the power frequency and the harmonic frequencies of the power frequency; and / or,

[0248] A low-pass filter 806 is configured to filter the frequencies higher than the highest frequency to be collected.

[0249] Optionally, as Figure 12 shown, the electromagnetic exploration system further includes:

[0250] A suppression device 807 is configured to suppress the interference frequencies; and / or,

[0251] A compensation device 808 is configured to compensate the dominant frequency components.

[0252] In summary, the electromagnetic exploration method and system based on natural field sources provided by the embodiments of the present application do not perform the "division" operation of magnetic fields and electric fields, but retain the fluctuation information of the natural electric field itself. After collecting the natural electric field and / or magnetic field incident on the ground and then reflected and refracted by the underground impedance interface and returning to the ground sensor position, the components of the natural electric field and / or magnetic field intensity perpendicular to the incident direction of the natural electric field and / or magnetic field at this moment are superimposed with the natural electric field and / or magnetic field incident at this moment, and the initial spectrum is synthesized. After screening out the dominant frequency components in the initial spectrum, through travel-time inversion, the initial spectrum (preferably the dominant frequency components) is superimposed using the harmonic synthesis algorithm to form the synthetic travel-time time series of the natural electromagnetic field. Thus, the electromagnetic exploration method and system provided by the embodiments of the present application utilize the fact that after the natural electric field and / or magnetic field are incident on the ground and then reflected and refracted by the underground impedance interface and return to the ground sensor position, the signal-to-noise ratio of the electromagnetic wave formed by superimposing with the natural electric field and / or magnetic field incident at this moment is higher than that of the natural electromagnetic field, thereby achieving a greater exploration depth, higher resolution, and higher working efficiency compared with the traditional electromagnetic exploration method based on natural field sources. The exploration depth and resolution of the electromagnetic exploration method provided by the embodiments of the present application are not inferior to those of the exploration method using artificial field sources, and it has characteristics such as energy conservation and environmental protection. The principle of this method is different from that of the traditional electromagnetic exploration method, and it can collect only one of the electric field or magnetic field signals, with high collection efficiency. Moreover, since the natural electric field and / or magnetic field are incident on the ground and then reflected and refracted by the underground impedance interface and return to the sensor position, and are superimposed with the natural electric field and / or magnetic field incident at this moment, the signal intensity is enhanced, so that the collection of the initial signal can be not limited to the ground, but can also be in the air and on the water surface, greatly reducing the deployment difficulty and cost of the exploration equipment corresponding to the present application compared with the traditional electromagnetic exploration method and the seismic exploration method, and having a wide range of application scenarios.

[0253] In addition, the embodiments of the present application further provide an electronic device, including a bus, a transceiver, a memory, a processor, and a computer program stored on the memory and executable on the processor. The transceiver, the memory, and the processor are respectively connected through the bus. When the computer program is executed by the processor, it implements each process of the above-mentioned embodiment of the electromagnetic exploration method based on natural field sources and can achieve the same technical effects.

[0254] Specifically, as shown in Figure 13 the embodiments of the present application further provide an electronic device, which includes a bus 1110, a processor 1120, a transceiver 1130, a bus interface 1140, a memory 1150, and a user interface 1160.

[0255] In an embodiment of the present application, the electronic device further includes: a computer program stored on the memory 1150 and executable on the processor 1120, and when the computer program is executed by the processor 1120, the following steps are implemented:

[0256] Collect an initial signal. The initial signal includes the natural electric field and / or magnetic field that, after being incident on the ground and reflected and refracted by the underground impedance interface, returns to the sensor position at a certain moment and the component of the intensity of the natural electric field and / or magnetic field obtained by the dry superposition of the natural electric field and / or magnetic field incident at this moment in the direction perpendicular to the incident direction of the natural electric field and / or magnetic field.

[0257] Based on the initial signal, synthesize the total spectrum of the electric field intensity and / or magnetic field intensity.

[0258] Optionally, screen out the dominant frequency components in the total spectrum. The dominant frequency components include the fundamental frequency that can be enhanced and the spectrum corresponding to the multiple frequencies of the fundamental frequency.

[0259] Travel-time inversion. Travel-time inversion is to superimpose the initial spectrum (preferably the dominant frequency components) through a harmonic synthesis algorithm based on the electromagnetic wave principle to obtain the synthetic travel-time time series of the natural electric field and / or magnetic field. This synthetic travel-time time series is used to judge geological information.

[0260] Optionally, when the computer program is executed by the processor 1120, the following steps can also be implemented:

[0261] Before collecting the initial signal, perform a first notch processing and / or low-pass filtering according to the highest frequency to be collected;

[0262] The first notch processing is to suppress the power frequency and its multiple frequencies; the low-pass filtering is to filter out the frequency components higher than the highest frequency to be collected.

[0263] Optionally, when the computer program is executed by the processor 1120, the following steps can also be implemented:

[0264] Based on the comparison between the geological data of the survey area and the synthetic travel-time time series, establish the corresponding relationship between the geological horizons of the exploration area and the underground impedance interface determined by the synthetic travel-time time series;

[0265] Based on the corresponding relationship, determine the marker layer; the marker layer is the layer in the geological horizon with the highest matching degree with the synthetic travel-time time series;

[0266] Based on the depth of the marker layer and the time corresponding to the marker layer in the synthetic travel-time time series, obtain the time-depth conversion parameter. Optionally, when the computer program is executed by the processor 1120, the following steps can also be implemented:

[0267] Based on the time-depth conversion parameter, optimize the parameter selection of the travel-time inversion.

[0268] Optionally, when the computer program is executed by the processor 1120 to perform the step of "performing Fourier transform on the initial time series to obtain the initial spectrum of the initial time series", the processor specifically implements the following steps:

[0269] Converting the original time series data of the initial signal collected by each acquisition channel into the initial spectrum of the initial signal through Fourier analysis;

[0270] Using the vector synthesis rule, synthesizing the initial spectra corresponding to all acquisition channels into the total spectrum.

[0271] Optionally, when the computer program is executed by the processor 1120 to perform the step of "performing Fourier transform on the initial time series to obtain the initial spectrum of the initial time series", the processor specifically implements the following steps:

[0272] Determining the lowest dominant frequency component based on the survey requirements;

[0273] Suppressing the interference frequency and / or compensating the dominant frequency component;

[0274] Wherein the interference frequency includes the background frequency, the power frequency and the multiple frequency of the power frequency; the background frequency is lower than the frequency of the lowest dominant frequency component.

[0275] The transceiver 1130 is used to receive and transmit data under the control of the processor 1120.

[0276] In the embodiment of the present application, the bus architecture (represented by the bus 1110), the bus 1110 may include any number of interconnected buses and bridges, and the bus 1110 connects various circuits including one or more processors represented by the processor 1120 and the memory represented by the memory 1150 together.

[0277] Bus 1110 represents one or more of any of several types of bus structures, including a memory bus and memory controller, a peripheral bus, an Accelerated Graphics Port (AGP), a processor, or a local bus using any of a variety of bus architectures. By way of example and not limitation, such architectures include: Industry Standard Architecture (ISA) bus, Micro Channel Architecture (MCA) bus, Enhanced ISA (EISA) bus, Video Electronics Standards Association (VESA), Peripheral Component Interconnect (PCI) bus.

[0278] Processor 1120 may be an integrated circuit chip with signal processing capabilities. In implementation, the steps of the above method embodiments may be completed by the integrated logic circuit in the hardware of the processor or instructions in software form. The above processors include: general-purpose processors, central processing units (CPUs), network processors (NPs), digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), complex programmable logic devices (CPLDs), programmable logic arrays (PLAs), microcontroller units (MCUs), or other programmable logic devices, discrete gates, transistor logic devices, discrete hardware components. The various methods, steps, and logic block diagrams disclosed in the embodiments of the present application may be implemented or executed. For example, the processor may be a single-core processor or a multi-core processor, and the processor may be integrated on a single chip or located on multiple different chips.

[0279] The processor 1120 can be a microprocessor or any conventional processor. The method steps disclosed in combination with the embodiments of the present application can be directly executed by a hardware decoding processor, or executed by a combination of hardware and software modules in the decoding processor. The software module can be located in a readable storage medium well known in the art, such as a Random Access Memory (RAM), Flash Memory, Read-Only Memory (ROM), Programmable ROM (PROM), Erasable PROM (EPROM), register, etc. The readable storage medium is located in the memory, and the processor reads the information in the memory and combines its hardware to complete the steps of the above method.

[0280] The bus 1110 can also connect together various other circuits, such as peripheral devices, voltage regulators, or power management circuits. The bus interface 1140 provides an interface between the bus 1110 and the transceiver 1130, which are all well known in the art. Therefore, the embodiments of the present application will not describe them further.

[0281] The transceiver 1130 can be a single component or multiple components, such as multiple receivers and transmitters, providing a unit for communicating with various other devices on the transmission medium. For example: the transceiver 1130 receives external data from other devices, and the transceiver 1130 is used to send the data processed by the processor 1120 to other devices. Depending on the nature of the computer system, a user interface 1160 can also be provided, such as: a touch screen, a physical keyboard, a display, a mouse, a speaker, a microphone, a trackball, a joystick, a stylus.

[0282] It should be understood that in the embodiments of the present application, the memory 1150 may further include a memory remotely disposed relative to the processor 1020, and these remotely disposed memories may be connected to the server through a network. One or more parts of the above-mentioned network may be an ad hoc network, an intranet, an extranet, a virtual private network (VPN), a local area network (LAN), a wireless local area network (WLAN), a wide area network (WAN), a wireless wide area network (WWAN), a metropolitan area network (MAN), the Internet, a public switched telephone network (PSTN), a plain old telephone service network (POTS), a cellular telephone network, a wireless network, a Wi-Fi network, and a combination of two or more of the above networks. For example, the cellular telephone network and the wireless network may be a Global System for Mobile Communications (GSM) system, a Code Division Multiple Access (CDMA) system, a Worldwide Interoperability for Microwave Access (WiMAX) system, a General Packet Radio Service (GPRS) system, a Wideband Code Division Multiple Access (WCDMA) system, a Long Term Evolution (LTE) system, an LTE Frequency Division Duplexing (FDD) system, an LTE Time Division Duplexing (TDD) system, an Advanced Long Term Evolution (LTE-A) system, a Universal Mobile Telecommunications System (UMTS) system, an Enhance Mobile Broadband (eMBB) system, a massive Machine Type of Communication (mMTC) system, an UltraReliable Low Latency Communications (uRLLC) system, etc.

[0283] It should be understood that the memory 1150 in the embodiments of the present application may be a volatile memory or a non-volatile memory, or may include both a volatile memory and a non-volatile memory. Among them, the non-volatile memory includes: Read-Only Memory (ROM), Programmable ROM (PROM), Erasable PROM (EPROM), Electrically Erasable PROM (EEPROM), or Flash Memory.

[0284] The volatile memory includes: a random access memory (RAM), which is used as an external cache. By way of example but not limitation, many forms of RAM are available, such as: static random access memory (SRAM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), double data rate synchronous dynamic random access memory (DDR SDRAM), enhanced synchronous dynamic random access memory (ESDRAM), synchlink dynamic random access memory (SLDRAM), and direct rambus random access memory (DRRAM). The memory 1150 of the electronic device described in the embodiments of the present application includes, but is not limited to, the above and any other suitable types of memory.

[0285] In the embodiments of the present application, the memory 1150 stores the following elements of the operating system 1151 and the application program 1152: executable modules, data structures, or subsets or extended sets thereof.

[0286] Specifically, the operating system 1151 includes various system programs, such as: a framework layer, a core library layer, a driver layer, etc., for implementing various basic services and processing hardware-based tasks. The application program 1052 includes various application programs, such as: a media player and a browser, for implementing various application services. The program for implementing the method of the embodiments of the present application may be included in the application program 1152. The application program 1152 includes: applets, objects, components, logics, data structures, and other computer system executable instructions for performing specific tasks or implementing specific abstract data types.

[0287] In addition, the embodiments of the present application also provide a computer-readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, it implements each process of the above-described embodiments of the electromagnetic survey method based on natural field sources and can achieve the same technical effects. To avoid repetition, it will not be described in detail here.

[0288] A computer-readable storage medium includes permanent and non-permanent, removable and non-removable media, and is a tangible device that can retain and store instructions for use by an instruction execution device. The computer-readable storage medium includes electronic storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, and any suitable combination of the foregoing. The computer-readable storage medium includes phase change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read-only memory (ROM), non-volatile random access memory (NVRAM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, compact disc read-only memory (CD-ROM), digital versatile disc (DVD) or other optical storage, magnetic cassette tape storage, magnetic tape disk storage or other magnetic storage devices, memory sticks, mechanical encoding devices (such as punched cards or raised structures in grooves on which instructions are recorded), or any other non-transmission medium that can be used to store information that can be accessed by a computing device. As defined in the embodiments of the present application, a computer-readable storage medium does not include transient signals themselves, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagated through waveguides or other transmission media (such as light pulses passing through an optical fiber cable), or electrical signals transmitted through wires.

[0289] In several embodiments provided by the present application, it should be understood that the disclosed devices, electronic devices, and methods can be implemented in other ways. For example, the device embodiments described above are merely illustrative. For example, the division of the modules or units is only a logical function division, and there may be other division methods in actual implementation. For example, multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. In addition, the displayed or discussed couplings or direct couplings or communication connections to each other can be indirect couplings or communication connections through some interfaces, devices, or units, and can also be electrical, mechanical, or other forms of connection.

[0290] The units described as separate components may or may not be physically separated, and the components displayed as units may or may not be physical units. They can be located in one position or distributed to multiple network units. Some or all of the units can be selected according to actual needs to solve the problems to be solved by the solutions of the embodiments of the present application.

[0291] In addition, the functional units in the various embodiments of the present application can be integrated into one processing unit, or each unit can exist physically alone, or two or more units can be integrated into one unit. The above integrated units can be implemented in the form of hardware or in the form of software functional units.

[0292] When the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the embodiments of the present application, in essence, or the part that contributes to the prior art, or all or part of the technical solution can be embodied in the form of a software product. The computer software product is stored in a storage medium and includes several instructions for causing a computer device (including: a personal computer, a server, a data center, or other network devices) to execute all or part of the steps of the methods described in the embodiments of the present application. And the above storage medium includes various media that can store program codes as listed above.

[0293] The embodiments of the present application further include the technical solutions described in the following appendices:

[0294] Technical solution 1: An electromagnetic exploration method, the method includes:

[0295] Collecting an initial signal; the initial signal includes the natural electric field and / or magnetic field incident on the ground, and at the moment when it is reflected and refracted by the underground impedance interface and returns to the sensor position, it meets the natural electric field and / or magnetic field incident on the ground at the same moment, and the initial time series of the combined electric field and / or magnetic field intensity obtained by the superposition of the two;

[0296] Performing a Fourier transform on the initial time series to obtain the initial spectrum of the initial time series;

[0297] Travel time inversion, the travel time inversion is based on the principle of electromagnetic field superposition, and the initial spectrum is superimposed through a harmonic synthesis algorithm to obtain the combined travel time series of the natural electric field and / or magnetic field; the combined travel time series is used to judge geological information.

[0298] Technical solution 2: The electromagnetic exploration method according to technical solution 1, the initial signal at least satisfies:

[0299]

[0300] Wherein, is the phase difference between the natural electric field and / or magnetic field of any frequency after being reflected and refracted by the underground impedance interface and returning to the sensor and the natural electric field and / or magnetic field continuously incident at the same frequency at that moment; f 0 is the frequency of the electric field and / or magnetic field of any frequency; ρ 1 is the resistivity of any layer of impedance interface; h 1 is the buried depth of any layer of impedance interface.

[0301] Technical solution 3: The electromagnetic exploration method according to technical solution 2, the method further comprising:

[0302] Screening out dominant frequency components in the initial spectrum; the dominant frequency components are related to any impedance interface underground.

[0303] Technical solution 4: The electromagnetic exploration method according to technical solution 3, the method further comprising:

[0304] Screening out the dominant frequency components in the initial spectrum; any frequency in the dominant frequency components is inversely proportional to the burial depth of the impedance interface corresponding to the frequency.

[0305] Technical solution 5: The electromagnetic exploration method according to technical solution 3, the dominant frequency components at least satisfy:

[0306]

[0307] where n is an integer greater than or equal to 0; is the phase difference between the natural electric field and / or magnetic field of any frequency in the dominant frequency components after being reflected and refracted by the underground impedance interface and returning to the sensor, and the natural electric field and / or magnetic field of the same frequency continuously incident at this moment.

[0308] Technical solution 6: The electromagnetic exploration method according to technical solution 3, the dominant frequency components further satisfy:

[0309] The phase difference when any frequency component in the dominant frequency components is superimposed at the sensor is within the frequency band ;

[0310] where int represents taking the integer, is the phase difference between the moment when the fundamental frequency returns to the ground after propagating, reflecting, and refracting in the underground medium and the external electromagnetic field of the same frequency continuously incident at this moment.

[0311] Technical solution 7: The electromagnetic exploration method according to technical solution 1, the harmonic synthesis algorithm includes:

[0312] and / or

[0313]

[0314] where f(i) is the frequency of the i-th harmonic, H(t) is the time-domain travel-time inversion magnetic field intensity time series, H i (t) is the amplitude of the i-th magnetic field harmonic; E(t) is the time-domain travel-time inversion electric field intensity time series, E i (t) is the amplitude of the i-th electric field harmonic, is the phase of the i-th harmonic, k is not equal to 0; t is the travel time; N is the number of harmonics.

[0315] Technical solution 8. The electromagnetic exploration method according to technical solution 1, wherein the harmonic synthesis algorithm includes:

[0316] and / or

[0317]

[0318] wherein, f(i) is the frequency of the i-th harmonic, H(t) is the time-domain travel-time inversion magnetic field strength time series, H i (t) is the amplitude of the i-th magnetic field harmonic; E(t) is the time-domain travel-time inversion electric field strength time series, E i (t) is the amplitude of the i-th electric field harmonic, is the phase of the i-th harmonic, k is a natural number greater than 0; t is the travel time; N is the number of harmonics.

[0319] Technical solution 9. The electromagnetic exploration method according to technical solution 7, wherein the harmonic synthesis algorithm includes:

[0320] and / or

[0321]

[0322] wherein, f(i) is the frequency of the i-th harmonic, H(t) is the time-domain travel-time inversion magnetic field strength time series, H i (t) is the amplitude of the i-th magnetic field harmonic; E(t) is the time-domain travel-time inversion electric field strength time series, E i (t) is the amplitude of the i-th electric field harmonic, is the phase of the i-th harmonic; t is the travel time; N is the number of harmonics; the value of k is selected from any one of 1, 1.2, 1.4, 2, 2.5, 3, 3.8, 4.

[0323] Technical solution 10. The electromagnetic exploration method according to technical solution 1, wherein in the acquisition of the initial signal, the number of channels of the acquisition instrument is greater than or equal to 1.

[0324] Technical solution 11. The electromagnetic exploration method according to claim 1, further comprising:

[0325] setting acquisition parameters before acquiring the initial signal;

[0326] The acquisition parameters include at least one or more of the sampling rate, the lowest frequency to be acquired, the highest frequency to be acquired, and the acquisition duration.

[0327] Technical solution 12. The electromagnetic survey method as described in technical solution 11, wherein the sampling rate is negatively correlated with the depth to be surveyed.

[0328] Technical solution 13. The electromagnetic survey method as described in technical solution 11, wherein according to the different burial depths of the survey object, the sampling rate is greater than or equal to 1000 times per second and less than or equal to 100000 times per second.

[0329] Technical solution 14. The electromagnetic survey method as described in technical solution 11, wherein the lowest frequency to be collected is negatively correlated with the maximum depth to be surveyed.

[0330] Technical solution 15. The electromagnetic survey method as described in technical solution 11, wherein the initial signal to be collected includes:

[0331] When jointly collecting the electric field strength and the magnetic field strength, the lowest frequency to be collected is greater than or equal to the lower limit of the frequency that the magnetic sensor can receive;

[0332] When separately collecting the electric field strength or the magnetic field strength, the lowest frequency to be collected is greater than or equal to 0.01 Hz and less than or equal to 10 Hz.

[0333] Technical solution 16. The electromagnetic survey method as described in technical solution 11, wherein the acquisition duration is positively correlated with the maximum depth to be surveyed.

[0334] Technical solution 17. The electromagnetic survey method as described in any one of technical solutions 1-16, the method further includes:

[0335] Performing a first notch processing and / or a low-pass filtering before collecting the initial signal according to the highest frequency to be collected;

[0336] The first notch processing is to suppress the power frequency and the multiple frequencies of the power frequency;

[0337] The low-pass filtering is to filter the frequency components higher than the highest frequency to be collected.

[0338] Technical solution 18. The electromagnetic survey method as described in technical solution 17, wherein the first notch processing includes a first comb filter processing;

[0339] The first comb filter processing is a hardware-based filtering processing; the hardware includes a filter.

[0340] Technical solution 19. The electromagnetic survey method as described in any one of technical solutions 1, 2, 10 or 11, the frequency of the initial signal is greater than or equal to 0.01 Hz and less than or equal to 50000 Hz.

[0341] Technical solution 20. For the electromagnetic exploration method described in technical solution 1, the initial spectrum of the initial time series obtained by performing Fourier transform on the initial time series includes:

[0342] Converting the original time series data of the time-domain electric field and / or magnetic field intensity components of the initial signal collected by each acquisition channel into the component spectra of the frequency-domain electric field and / or magnetic field intensity components;

[0343] Using the vector synthesis rule, synthesizing the electric field and / or magnetic field intensity component spectra corresponding to all acquisition channels into the initial spectrum of the electric field and / or magnetic field intensity.

[0344] Technical solution 21. For the electromagnetic exploration method described in technical solution 1, the initial spectrum includes the frequency, real part, imaginary part, amplitude, and phase of the electric field and / or magnetic field.

[0345] Technical solution 22. For the electromagnetic exploration method described in technical solution 3, screening for dominant frequency components in the initial spectrum includes:

[0346] Determining the lowest dominant frequency component based on the exploration object and requirements; and

[0347] Suppressing interfering frequency components;

[0348] Wherein the interfering frequency components include background noise, power frequency, and multiples of the power frequency; the background noise includes all frequency components higher than and / or lower than the dominant frequency components.

[0349] Technical solution 23. For the electromagnetic exploration method described in technical solution 22, suppressing the interfering frequency components includes: second comb filter processing and band-pass filter processing;

[0350] Both the second comb filter processing and the band-pass filter processing are processing methods based on digital signal processing principles.

[0351] Technical solution 24. For the electromagnetic exploration method described in any one of technical solutions 1-16, the method further includes:

[0352] Based on the comparison between the geological data of the exploration area and the synthetic travel time series, establishing a correspondence between the geological horizons of the exploration area and the underground impedance interface determined by the synthetic travel time series;

[0353] Determining a marker layer based on the correspondence; the marker layer is the layer in the geological horizon with the highest matching degree with the synthetic travel time series;

[0354] Based on the depth of the marker layer and the time corresponding to the marker layer in the synthetic travel time series, obtaining the time-depth conversion parameter.

[0355] Technical solution 25. The electromagnetic exploration method as described in technical solution 1, wherein the travel time inversion further includes dispersion effect correction;

[0356] The dispersion effect correction includes performing velocity compensation on electromagnetic waves of different frequencies during the harmonic synthesis stage, so that the electromagnetic waves of different frequencies have the same propagation velocity.

[0357] Technical solution 26. The electromagnetic exploration method as described in technical solution 1, the method further includes:

[0358] Before collecting the initial signal, select an initial value of the propagation velocity of the electromagnetic wave corresponding to the lowest frequency in the initial spectrum according to the depth to be explored and the geological characteristics of the exploration area; the initial value is used for time-depth conversion of the initial time series, so as to obtain the initial time series in the depth domain.

[0359] Technical solution 27. The electromagnetic exploration method as described in technical solution 1, the initial spectrum satisfies:

[0360]

[0361] wherein, f min is the lowest frequency in the initial spectrum; v min is the initial value of the propagation velocity of the electromagnetic wave corresponding to the lowest frequency; σ is the conductivity of the underground electrical medium; μ is the magnetic permeability of the underground electrical medium.

[0362] Technical solution 28. The electromagnetic exploration method as described in technical solution 1, the sensor is located on the ground, in the air or on the water surface.

[0363] Technical solution 29. An electromagnetic exploration system applicable to the electromagnetic exploration method as described in any one of claims 1-28, the electromagnetic exploration system includes:

[0364] A collection module, the collection module is configured to collect the initial signal; the initial signal includes the moment when the natural electric field and / or magnetic field enter the ground, are reflected and refracted by the underground impedance interface and return to the ground, and meet the natural electric field and / or magnetic field that enter the ground at the moment at the sensor, and are superimposed to obtain the initial time series of the synthetic electric field and / or magnetic field intensity;

[0365] An initial signal processing module, the initial signal processing module is configured to perform Fourier transform on the initial time series to obtain the initial spectrum of the initial time series;

[0366] A travel-time inversion module, which is configured to stack the initial spectrum based on the principle of electromagnetic wave superposition through a harmonic synthesis algorithm to obtain a synthetic travel-time time series of the natural electric field and / or magnetic field; the synthetic travel-time time series is used to analyze geological information.

[0367] Technical solution 30: The electromagnetic exploration system according to technical solution 29, further comprising a sensor;

[0368] The sensor is located on the ground, in the air or on the water surface.

[0369] Technical solution 31: An electronic device, comprising a bus, a transceiver, a memory, a processor, and a computer program stored on the memory and executable on the processor. The transceiver, the memory, and the processor are respectively connected through the bus. When the computer program is executed by the processor, it implements the steps in the electromagnetic exploration method according to any one of technical solutions 1-28.

[0370] Technical solution 32: A computer-readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, it implements the steps in the electromagnetic exploration method according to any one of technical solutions 1-28.

[0371] As mentioned above, it is only the specific implementation manner of the embodiments of the present application, but the protection scope of the embodiments of the present application is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed in the embodiments of the present application, and all should be covered by the protection scope of the embodiments of the present application. Therefore, the protection scope of the embodiments of the present application shall be subject to the protection scope of the claims.

Claims

1. An electromagnetic exploration method, characterized in that, the method includes: collecting an initial signal; the initial signal includes the natural electric field and / or magnetic field incident on the ground, at the moment when it is reflected and refracted back to the sensor position by the underground impedance interface, and meets the natural electric field and / or magnetic field incident on the ground at that moment at the sensor, and the initial time series of the combined electric field and / or magnetic field intensity obtained by the superposition of the two; performing a Fourier transform on the initial time series to obtain the initial frequency spectrum of the initial time series; travel-time inversion, the travel-time inversion is based on the principle of electromagnetic field superposition, and the initial frequency spectrum is superimposed by a harmonic synthesis algorithm to obtain the combined travel-time time series of the natural electric field and / or magnetic field; the combined travel-time time series is used to judge geological information; wherein, the travel-time inversion includes performing velocity compensation on electromagnetic waves of different frequencies in the harmonic synthesis stage, so that the electromagnetic waves of different frequencies have the same propagation speed; the harmonic synthesis algorithm satisfies: and / or where f(i) is the frequency of the i-th harmonic, H(t) is the travel-time inversion magnetic field intensity time series in the time domain, and H i (t) is the amplitude of the i-th magnetic field harmonic; E(t) is the travel-time inversion electric field intensity time series in the time domain, and E i (t) is the amplitude of the i-th electric field harmonic, is the phase of the i-th harmonic, k is the dispersion effect correction coefficient, and k is not equal to 0; t is the travel time; N is the number of harmonics.

2. The electromagnetic exploration method according to claim 1, characterized in that, the harmonic synthesis algorithm includes: and / or Among them, f(i) is the frequency of the i-th harmonic, H(t) is the travel-time inversion magnetic field intensity time series in the time domain, and H i (t) is the amplitude of the i-th magnetic field harmonic; E(t) is the travel-time inversion electric field intensity time series in the time domain, and E i (t) is the amplitude of the i-th electric field harmonic, is the phase of the i-th harmonic, k is a natural number greater than 0; t is the travel time; N is the number of harmonics.

3. The electromagnetic exploration method according to claim 2, characterized in that, the harmonic synthesis algorithm includes: and / or Among them, f(i) is the frequency of the i-th harmonic, H(t) is the travel-time inversion magnetic field strength time series in the time domain, and H i (t) is the amplitude of the i-th magnetic field harmonic; E(t) is the travel-time inversion electric field strength time series in the time domain, and E i (t) is the amplitude of the i-th electric field harmonic, is the phase of the i-th harmonic; t is the travel time; N is the number of harmonics; the value of k is selected from any one of 1, 1.2, 1.4, 2, 2.5, 3, 3.8, and 4.

4. The electromagnetic exploration method according to claim 1, characterized in that, the initial signal at least satisfies: wherein, is the phase difference between the natural electric field and / or magnetic field at any frequency after being reflected and refracted by the underground impedance interface and returning to the sensor, and the natural electric field and / or magnetic field continuously incident at the same frequency at that moment; f 0 is the frequency of the electric field and / or magnetic field at any frequency; ρ 1 is the resistivity of any layer of impedance interface; h 1 is the buried depth of any layer of impedance interface.

5. The electromagnetic exploration method according to any one of claims 1-4, characterized in that, the method further includes: screening out dominant frequency components in the initial frequency spectrum; the dominant frequency components are related to any underground impedance interface.

6. The electromagnetic exploration method according to claim 5, characterized in that, the method further includes: screening out the dominant frequency components in the initial frequency spectrum; any frequency in the dominant frequency components is inversely proportional to the buried depth of the impedance interface corresponding to the frequency.

7. The electromagnetic exploration method according to claim 5, characterized in that, the dominant frequency components at least satisfy: where n is an integer greater than or equal to 0; is the phase difference between the natural electric field and / or magnetic field of any frequency in the dominant frequency components, after being reflected and refracted by the underground impedance interface and returning to the sensor, and the natural electric field and / or magnetic field of the same frequency continuously incident at that moment.

8. The electromagnetic exploration method according to claim 5, characterized in that, the dominant frequency components further satisfy: The phase difference when any of the dominant frequency components is superimposed at the sensor lies within the frequency band ; wherein, int represents taking the integer part, is the phase difference between the moment when the fundamental frequency returns to the ground after propagating, reflecting, and refracting in the underground medium and the external electromagnetic field that is continuously incident at the same frequency at this moment.

9. The electromagnetic exploration method according to any one of claims 1-4, characterized in that, the method further includes: performing a first notch processing and / or a low-pass filtering before collecting the initial signal according to the highest frequency to be collected; the first notch processing is to suppress the power frequency and its multiples; the low-pass filtering is to filter out frequency components higher than the highest frequency to be collected.

10. The electromagnetic exploration method according to claim 9, characterized in that, the first notch processing includes a first comb filter processing; the first comb filter processing is a hardware-based filtering processing; the hardware includes a filter.

11. The electromagnetic exploration method according to claim 1, characterized in that, the obtaining the initial frequency spectrum of the initial time series based on performing a Fourier transform on the initial time series includes: converting the original time series data of the time-domain electric field and / or magnetic field intensity components of the initial signal collected by each acquisition channel into the component frequency spectra of the frequency-domain electric field and / or magnetic field intensity components; Using the vector synthesis rule, synthesize the spectra of the electric field and / or magnetic field strength components corresponding to all acquisition channels into the initial spectrum of the electric field and / or magnetic field strength.

12. The electromagnetic exploration method according to claim 5, characterized in that screening for dominant frequency components in the initial spectrum includes: determining the lowest dominant frequency component based on the exploration object and requirements; and suppressing interfering frequency components; wherein the interfering frequency components include background noise, power frequency, and multiples of the power frequency; the background noise includes all frequency components higher than and / or lower than the dominant frequency components.

13. The electromagnetic exploration method according to claim 12, characterized in that the suppressing of interfering frequency components includes: second comb filtering processing and band-pass filtering processing; both the second comb filtering processing and the band-pass filtering processing are processing methods based on the principle of digital signal processing.

14. The electromagnetic exploration method according to any one of claims 1-4, characterized in that the method further includes: establishing a correspondence between the geological horizons in the exploration area and the underground impedance interface determined by the synthetic travel time series based on the comparison between the geological data of the exploration area and the synthetic travel time series; determining a marker layer based on the correspondence; the marker layer is the layer in the geological horizon with the highest matching degree with the synthetic travel time series; obtaining a time-depth conversion parameter based on the depth of the marker layer and the time corresponding to the marker layer in the synthetic travel time series.

15. The electromagnetic exploration method according to claim 1, characterized in that the travel time inversion further includes dispersion effect correction; the dispersion effect correction includes performing velocity compensation on electromagnetic waves of different frequencies during the harmonic synthesis stage so that the electromagnetic waves of different frequencies have the same propagation velocity.

16. The electromagnetic exploration method according to claim 1, characterized in that the method further includes: before collecting the initial signal, selecting an initial value of the propagation velocity of the electromagnetic wave corresponding to the lowest frequency in the initial spectrum according to the depth to be explored and the geological characteristics of the exploration area; the initial value is used for time-depth conversion of the initial time series to obtain the initial time series in the depth domain.

17. The electromagnetic exploration method according to claim 1, characterized in that the initial spectrum satisfies: where f min is the lowest frequency in the initial spectrum; v min is the initial value of the electromagnetic wave propagation speed corresponding to the lowest frequency; σ is the conductivity of the underground electrical medium; μ is the magnetic permeability of the underground electrical medium.

18. The electromagnetic exploration method according to claim 1, characterized in that the sensor is located on the ground, in the air or on the water surface.

19. An electromagnetic exploration system, characterized in that suitable for the electromagnetic exploration method according to any one of claims 1-18, the electromagnetic exploration system includes: an acquisition module configured to acquire an initial signal; the initial signal includes natural electric and / or magnetic fields incident on the ground, and at the moment when they are reflected and refracted by the underground impedance interface and return to the ground, they meet the natural electric and / or magnetic fields incident on the ground at that moment at the sensor and are superimposed to obtain an initial time series of the synthetic electric and / or magnetic field strength; an initial signal processing module configured to perform Fourier transform on the initial time series to obtain the initial spectrum of the initial time series. A travel-time inversion module, which is configured to superimpose the initial spectrum based on the principle of electromagnetic wave superposition through a harmonic synthesis algorithm to obtain a synthetic travel-time time series of natural electric and / or magnetic fields; the synthetic travel-time time series is used to judge geological information; The travel-time inversion includes performing velocity compensation on electromagnetic waves of different frequencies during the harmonic synthesis stage so that the electromagnetic waves of different frequencies have the same propagation velocity; The harmonic synthesis algorithm satisfies: and / or Among them, f(i) is the frequency of the i-th harmonic, H(t) is the travel-time inversion magnetic field intensity time series in the time domain, and H i (t) is the amplitude of the i-th magnetic field harmonic; E(t) is the travel-time inversion electric field intensity time series in the time domain, and E i (t) is the amplitude of the i-th electric field harmonic, is the phase of the i-th harmonic, k is the dispersion effect correction coefficient, and k is not equal to 0; t is the travel time; N is the number of harmonics.

20. The electromagnetic exploration system according to claim 29, wherein, the electromagnetic exploration system further includes a sensor; the sensor is located on the ground, in the air or on the water surface.

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