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

Abstract

The present disclosure provides an electromagnetic survey method, system and electronic equipment, which belongs to the technical field of geophysical exploration. The method includes collecting initial signals; the initial signals include natural electric fields and / or magnetic fields incident underground, and when reflected and refracted back to the sensor position through the underground electrical impedance interface, they meet the natural electric fields and / or magnetic fields incident underground at the sensor at the time, and the two are superimposed to obtain the initial time series of synthetic electric field and / or magnetic field strength; the initial spectrum of the initial time series is obtained based on the Fourier transform of the initial time series; travel time inversion, travel time inversion is based on the electromagnetic field superposition principle, and the initial spectrum is superimposed by the harmonic synthesis algorithm to obtain a synthetic travel time series of electric field and / or magnetic field strength; the synthetic travel time series is used to judge geological information. The method provided in the embodiment of the present application realizes high-resolution geological exploration and engineering exploration based on natural electromagnetic field sources.

Description

Technical Field

[0001] The present application relates to the technical field of geophysical exploration, and in particular to an electromagnetic survey method, system, electronic equipment and computer-readable storage medium. Background Art

[0002] Electromagnetic survey method is a kind of geophysical exploration method based on the principle of electromagnetic induction to investigate geological structures, structures and other geological problems, and to conduct engineering surveys. Existing electromagnetic survey technology mostly uses the skin effect phenomenon existing in the electromagnetic induction process as its specific physical basis, and is a volume exploration technology. The electromagnetic survey method obtains the electrical distribution characteristic data of underground rocks and ores by measuring and analyzing the electromagnetic field data on the ground, combining the electromagnetic properties (resistivity, polarizability, magnetic susceptibility, etc.) of rocks and ores, geological and other geophysical data, and solving problems related to geological exploration and engineering surveys through comprehensive interpretation of various data.

[0003] The history of using natural electromagnetic fields as excitation sources for geological and engineering surveys can be traced back to the magnetotelluric method (MT) around 1950. Subsequent electromagnetic survey methods are the result of optimization and improvement on the principle of MT, and their survey accuracy has not been substantially improved.

[0004] At present, geological exploration and engineering survey activities using natural electromagnetic fields as excitation sources are mostly devoted to the study of underground electrical properties (resistivity, polarizability) distribution (structure), with the main research objectives being to solve the stratum distribution and geological structure. However, due to the skin effect and limited resolution of volume exploration, only rough information on underground electrical structures can be obtained, and detailed characterization of underground electrical stratification is not possible.

[0005] Therefore, a new electromagnetic survey method is urgently needed to characterize the underground electrical stratification in detail. Summary of the invention

[0006] In order to solve the existing technical problems, the embodiments of the present application provide an electromagnetic survey method and system based on natural field sources.

[0007] In a first aspect, an embodiment of the present application provides an electromagnetic survey method, the method comprising:

[0008] Collecting initial signals; the initial signals include natural electric and / or magnetic fields incident underground, and when they are reflected and refracted back to the sensor position through the underground electrical impedance interface, they meet the natural electric and / or magnetic fields incident underground at the sensor at that moment, and the two are superimposed to obtain the initial time series of synthetic electric and / or magnetic field strengths;

[0009] Performing Fourier transform on the initial time series to obtain an initial frequency spectrum of the initial time series;

[0010] Travel time inversion, the travel time inversion is based on the principle of electromagnetic wave superposition, and the initial spectrum is superimposed through a harmonic synthesis algorithm to obtain a synthetic travel time series of the natural electric field and / or magnetic field; the synthetic travel time series is used to analyze geological information.

[0011] Optionally, the initial signal at least satisfies:

[0012]

[0013] in, It is the phase difference between the natural electric field and / or magnetic field of any frequency at the moment when it returns to the sensor after being reflected and refracted by the underground impedance interface, and the natural electric field and / or magnetic field of the same frequency that continues to be incident at the said moment; f0 is the frequency of the electric field and / or magnetic field of any frequency; ρ1 is the resistivity of any layer of impedance interface; h1 is the burial depth of any layer of impedance interface.

[0014] Optionally, the method further comprises:

[0015] The dominant frequency component is screened in the initial spectrum; the dominant frequency component is related to any electrical impedance interface in the underground.

[0016] Optionally, the method further comprises:

[0017] The dominant frequency components are screened in the initial spectrum; any frequency component among the dominant frequency components is inversely proportional to the buried depth of the electrical impedance interface corresponding to the frequency component.

[0018] Optionally, the dominant frequency component at least satisfies:

[0019]

[0020] Wherein, 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.

[0021] Optionally, the dominant frequency component also satisfies:

[0022] The phase difference of any frequency in the dominant frequency component when superimposed at the sensor is located in the frequency band Within.

[0023] Optionally, the harmonic synthesis algorithm includes:

[0024] ω = 2πf(i); and / or

[0025] ω=2πf(i);

[0026] 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.

[0027] Optionally, the harmonic synthesis algorithm includes:

[0028] ω = 2πf(i); and / or

[0029] ω=2πf(i);

[0030] 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.

[0031] Optionally, the harmonic synthesis (superposition) algorithm includes:

[0032] ω = 2πf(i); and / or

[0033] ω=2πf(i);

[0034] 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 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.

[0035] Optionally, in the acquisition of the initial signal, the number of channels of the acquisition instrument is greater than or equal to 1.

[0036] Optionally, the method further comprises:

[0037] Setting acquisition parameters before acquiring the initial signal;

[0038] The acquisition parameters include at least one or more of a sampling rate, a minimum frequency to be acquired, a maximum frequency to be acquired, and an acquisition duration.

[0039] Optionally, the sampling rate is negatively correlated with the depth to be surveyed.

[0040] Optionally, the sampling rate is greater than or equal to 1000 times / second and less than or equal to 100,000 times / second.

[0041] Optionally, the lowest frequency to be acquired is negatively correlated with the maximum depth to be surveyed.

[0042] Optionally, collecting the initial signal includes:

[0043] 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;

[0044] When the electric field strength or the magnetic field strength is collected separately, the lowest frequency to be collected is greater than or equal to 0.01 Hz and less than or equal to 10 Hz.

[0045] Optionally, the acquisition duration is positively correlated with the maximum depth to be surveyed.

[0046] Optionally, the method further comprises:

[0047] According to the highest frequency to be collected, performing a first notch processing and / or a low-pass filtering before collecting the initial signal;

[0048] The first notch processing is to suppress the power frequency and the multiple of the power frequency;

[0049] The low-pass filtering is to filter out frequency components higher than the highest frequency to be collected.

[0050] Optionally, the first notch processing includes a first comb filtering process;

[0051] The first comb filtering process is a hardware-based filtering process; the hardware includes a filter.

[0052] Optionally, the frequency of the initial signal is greater than or equal to 0.01 Hz and less than or equal to 50,000 Hz.

[0053] Optionally, the obtaining an initial spectrum of the initial time series based on performing Fourier transform on the initial time series includes:

[0054] Converting the original time series data of the electric field and / or magnetic field intensity components in the time domain of the initial signal acquired by each acquisition channel into component spectra of the electric field and / or magnetic field intensity components in the frequency domain;

[0055] The electric field and / or magnetic field intensity component spectra corresponding to all acquisition channels are synthesized into the initial spectrum of the electric field and / or magnetic field intensity by using the vector synthesis rule.

[0056] Optionally, the initial spectrum includes the frequency, real part, imaginary part, amplitude and phase of the electric field and / or magnetic field.

[0057] Optionally, screening the dominant frequency components in the initial spectrum includes:

[0058] Determine the lowest dominant frequency components based on the survey objectives and requirements; and

[0059] Suppress interfering frequency components;

[0060] The interference 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.

[0061] Optionally, the suppressing interference frequency components comprises: a second comb filter process and a bandpass filter process;

[0062] The second comb filter processing and the bandpass filter processing are both processing methods based on the principle of digital signal processing.

[0063] Optionally, the method further comprises:

[0064] Based on the comparison between the geological data of the survey area and the synthetic travel time series, a corresponding relationship between the geological strata of the survey area and the underground electrical impedance interface determined by the synthetic travel time series is established;

[0065] Determine a marker layer based on the corresponding relationship; the marker layer is the layer in the geological layer with the highest matching degree with the synthetic travel time time series;

[0066] A time-to-depth conversion parameter is obtained based on the depth of the marker layer and the time corresponding to the marker layer in the synthetic travel time series.

[0067] Optionally, the travel time inversion further includes dispersion effect correction;

[0068] 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.

[0069] Optionally, the method further comprises:

[0070] Before collecting the initial signal, the initial value of the electromagnetic wave propagation velocity corresponding to the lowest frequency in the initial spectrum is selected according to the depth to be surveyed and the geological characteristics of the survey area; the initial value is used to perform time-to-depth conversion of the initial time series, thereby obtaining the initial time series in the depth domain.

[0071] Optionally, the initial spectrum satisfies:

[0072]

[0073] Among them, 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 electrical conductivity of the underground electrical medium; μ is the magnetic permeability of the underground electrical medium.

[0074] Optionally, the sensor is located on the ground, in the air or on the water surface.

[0075] In a second aspect, an embodiment of the present application further provides an electromagnetic survey system, characterized in that it is applicable to the electromagnetic survey method provided in any of the above embodiments, and the electromagnetic survey system includes:

[0076] An acquisition module, wherein the acquisition module is configured to acquire the initial signal; the initial signal includes a natural electric field and / or magnetic field incident underground, and when it is reflected and refracted back to the ground through the underground electrical impedance interface, it meets the natural electric field and / or magnetic field incident underground at the moment at the sensor, and the two are superimposed to obtain an initial time series of synthetic electric field and / or magnetic field strength;

[0077] An initial signal processing module, wherein the initial signal processing module is configured to perform Fourier transform on the initial time series to obtain an initial spectrum of the initial time series;

[0078] The travel time inversion module is configured to superimpose the initial spectrum based on the electromagnetic field superposition principle through a harmonic synthesis algorithm to obtain a synthetic travel time series of the natural electric field and / or magnetic field; the synthetic travel time series is used to judge geological information.

[0079] Optionally, the electromagnetic survey system further comprises a sensor;

[0080] The sensor is located on the ground, in the air or on the water surface.

[0081] In a third aspect, an embodiment of the present application further provides an electronic device, comprising a bus, a transceiver, a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the transceiver, the memory, and the processor are respectively connected via a bus, and when the computer program is executed by the processor, the steps of the electromagnetic survey method provided in any of the above embodiments are implemented.

[0082] In a fourth aspect, an embodiment of the present application further provides a computer-readable storage medium having a computer program stored thereon, wherein when the computer program is executed by a processor, the steps in the electromagnetic survey method provided according to any of the above embodiments are implemented.

[0083] The electromagnetic survey method, system, electronic device and computer-readable storage medium based on natural field sources provided in the embodiments of the present application, after collecting the natural electric field and / or magnetic field incident underground, after reflection and refraction through the underground electrical impedance interface, when returning to the ground, it meets the natural electric field and / or magnetic field incident underground at that time at the sensor, and the two are superimposed to obtain the initial time series of the synthetic electric field and / or magnetic field, and the dominant frequency components in the total spectrum are screened out and then inverted through travel time, and the initial spectrum components are superimposed using a harmonic synthesis algorithm to form a synthetic travel time sequence of the natural electric field and / or magnetic field. Therefore, the electromagnetic survey method and system provided in the embodiments of the present application, collect the natural electromagnetic field at the moment of returning to the ground, and superimpose it with the natural electromagnetic field incident at that moment, and the superposition process has the general characteristics of frequency selectivity, so that the dominant frequency component corresponding to the underground electrical impedance interface has the basic characteristics of high signal-to-noise, thereby achieving a greater survey depth, higher resolution and higher work efficiency than the traditional method based on natural field sources. BRIEF DESCRIPTION OF THE DRAWINGS

[0084] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the background technology, the drawings required for use in the embodiments of the present application or the background technology will be described below.

[0085] Figure 1 A flow chart of an electromagnetic survey method based on a natural field source provided in an embodiment of the present application is shown;

[0086] 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;

[0087] 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;

[0088] Figure 4The 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;

[0089] 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;

[0090] 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;

[0091] 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;

[0092] 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;

[0093] Fig. 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;

[0094] Fig.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;

[0095] Fig.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;

[0096] Fig.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;

[0097] Fig.13 An optional structural diagram of an electronic device provided in an embodiment of the present application is implemented.

[0098] The reference numerals in the figure represent respectively:

[0099] 801-acquisition module; 802-initial signal processing module; 803-screening module; 804-travel time inversion module;

[0100] 805-first notch device; 806-low pass filter; 807-suppression device; 808-compensation device;

[0101] 1110 - bus; 1120 - processor; 1130 - transceiver; 1140 - bus interface; 1150 - memory; 1160 - user interface. DETAILED DESCRIPTION

[0102] 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 methods, devices, electronic devices and computer-readable storage media. 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.), 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.

[0103] The above-mentioned computer-readable storage medium may adopt any combination of one or more computer-readable storage media. Computer-readable storage media include: electrical, magnetic, optical, electromagnetic, infrared or semiconductor systems, devices or devices, or any combination of the above. More specific examples of computer-readable storage media include: portable computer disks, hard disks, random access memories (RAM), read-only memories (ROM), erasable programmable read-only memories (EPROM), flash memories (Flash Memory), optical fibers, compact disk read-only memories (CD-ROM), optical storage devices, magnetic storage devices or any combination of the above. In an embodiment of the present application, a computer-readable storage medium may be any tangible medium containing or storing a program, which may be used by or in combination with an instruction execution system, device, or device.

[0104] The computer program code contained in the above-mentioned computer-readable storage medium can be transmitted using any appropriate medium, including: wireless, wire, optical cable, radio frequency (RF) or any suitable combination thereof.

[0105] The computer program code for executing the operation of the embodiment of the present application can be written in assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-related instructions, microcode, firmware instructions, state setting data, integrated circuit configuration data, or in one or more programming languages ​​or a combination thereof, wherein the programming language includes an object-oriented programming language, such as matlab, labview, Java, Smalltalk, C++, and also includes a conventional procedural programming language, such as C language or similar programming language. The computer program code can be executed completely on the user's computer, partially on the user's computer, as an independent software package, partially on the user's computer, partially on the remote computer, and completely on the remote computer or server. In the case of a remote computer, the remote computer can be connected to the user's computer or to an external computer through any type of network, including a local area network (LAN) or a wide area network (WAN).

[0106] The embodiments of the present application describe the provided methods, devices, and electronic devices through flowcharts and / or block diagrams.

[0107] It should be understood that each box in the flowchart and / or block diagram and the combination of boxes 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 to produce a machine, and these computer-readable program instructions are executed by a computer or other programmable data processing device to produce a device that implements the functions / operations specified by the boxes in the flowchart and / or block diagram.

[0108] These computer-readable program instructions may also be stored in a computer-readable storage medium that enables 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 including functions / operations specified in the blocks in the flowchart and / or block diagram.

[0109] Computer-readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device, so that a series of operational steps are performed on the computer, other programmable data processing apparatus, or other device to produce a computer-implemented process, thereby enabling the instructions executed on the computer or other programmable data processing apparatus to provide a process for implementing the functions / operations specified in the blocks in the flowchart and / or block diagram.

[0110] In 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. The difference in the terms is purely related to the word "scene" and does not reflect any difference in meaning between the two.

[0111] The embodiments of the present application are described below in conjunction with the drawings in the embodiments of the present application.

[0112] The propagation of natural electromagnetic fields in underground media belongs to the field of macroscopic electrodynamics, and the classical Maxwell equations are the basic laws of physics that they follow. Both theory and practice show that for the propagation of natural electromagnetic fields (waves) in the audio frequency band and below in underground media, the wave equation is the basic equation it satisfies.

[0113] Among many electromagnetic survey technologies, magnetotelluric (MT) technology, such as the continuous electromagnetic profiling method (CEMP), is the most widely used electromagnetic survey technology. It was proposed by Tikhonov in 1950. In the 1950s, French scientist Garnier defined the basic relationship of magnetotelluric (also known as Garnier's formula) based on the concept of electromagnetic intrinsic impedance, as follows:

[0114] ρ(f)=0.2*T*|(E(f)) / (H(f))|; (1)

[0115] 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 survey technology, the Garnier relationship is concise. However, because it uses the calculation process of comparing the electric field with the magnetic field in the calculation process, it implicitly cancels out the "dynamic" fluctuation term (i.e., the primary field effect of the incident electromagnetic field) that both the electric field and the magnetic field have. Therefore, electromagnetic survey technology based on magnetotellurics and the Garnier formula implicitly cancels out the primary field effect in the incident electromagnetic field, and only retains the basic relationship between the electromagnetic field attenuation and frequency that characterizes the skin phenomenon (i.e., the secondary field effect). On this basis, combined with the existing signal acquisition and processing technology, only impedance values ​​at a small number of frequency points can be obtained. For example, within the range of several thousand meters in the study of clastic sedimentary basins, there are generally only 20 to 30 frequency points. However, the amount of information that can be represented by 20 to 30 frequency points is far from enough for the requirements of geological exploration and engineering exploration in reality. This has also resulted in the existing technology based on natural field sources. The electromagnetic survey method based on natural field sources has a low resolution and can only be used for rough exploration in the early stages of geological exploration. To this day, traditional magnetotelluric technology, including the later developed CEMP technology and controlled-source audio-frequency magnetotelluric technology (CSMAT, Controlled-Sourse Audio-frequency Magnetotelluric), still remains at a stage where it can only reflect the distribution of several large electrical layers underground and cannot depict the detailed structure of the electrical distribution.

[0116] It is well known in the art that, since the underground medium is not always uniform, the propagation of electromagnetic waves in the unevenness is equivalent to the process of refraction-reflection repeatedly between different media. Therefore, the propagation path of electromagnetic waves in the underground medium is more complicated than the propagation path in the uniform medium, and the final emission direction is also very complicated. 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 goes deeper into the ground, the amplitude of the electromagnetic wave amplitude strongly decays with depth. Based on the above characteristics, those skilled in the art generally believe that the propagation of electromagnetic waves in underground media has the basic characteristics of "two fast and one low", namely fast attenuation, fast speed and low frequency (i.e. long wavelength). Fast attenuation severely limits the exploration depth of electromagnetic surveys, while low frequency limits the exploration accuracy of electromagnetic surveys (that is, low resolution).

[0117] In view of the above-mentioned characteristics of electromagnetic field propagation underground, technicians in this field have been advancing and committed to solving the "two fasts and one low" problem of electromagnetic fields from multiple directions in the past 70 years. For example, in view of the problem of rapid attenuation of electromagnetic fields, technicians in this field mostly use electromagnetic exploration technology based on artificial field sources. The electromagnetic exploration technology based on artificial field sources requires a large number of equipment and complex equipment layout methods. It not only requires long-term detection but also requires the layout of a large number of points. Therefore, it is inevitable that the cost is high and the efficiency is low, and it is extremely unsuitable for use in complex terrains such as mountainous areas.

[0118] In order to solve the problem of low frequency of electromagnetic waves propagating underground, technicians in this field usually use high-frequency signals for detection. For example, ground penetrating radar technology is a means of detecting underground using high-frequency electromagnetic waves. However, the higher the frequency of electromagnetic waves, the faster they decay underground. In other words, the detection depth of detection methods based on high-frequency electromagnetic waves is very limited. Therefore, the survey method based on electromagnetic method cannot achieve both high resolution and large survey depth, and cannot occupy a place in the field of high-resolution survey.

[0119] The inventors found that the wave term offset in the Garnier formula based on the traditional electromagnetic method has the potential to improve the resolution of electromagnetic survey technology. However, the development of natural electromagnetics based on wave theory faces three major obstacles:

[0120] 1. Compared with seismic elastic waves, natural electromagnetic waves propagate faster and have longer wavelengths in underground media. The electromagnetic survey method based on natural field sources developed based on wave theory with the technical ideas of seismic exploration has relatively limited resolution.

[0121] 2. Technical route of wave theory research The research object is a single underground reflected electromagnetic signal. Since the natural electromagnetic field is used as the excitation field source, due to its continuous incidence, the long electromagnetic signal reflected underground always overlaps and coexists with the continuously incident external electromagnetic field, and is "interdependent". The existing collection and processing methods cannot effectively separate the two to collect a single underground reflected electromagnetic field signal.

[0122] 3. Since the natural electromagnetic field continuously radiates into the ground, the existing collection and processing methods cannot obtain the starting time of the electromagnetic signal excited by the natural field source, and the initial zero time data of the electromagnetic signal reflected from the ground. In other words, the existing technology cannot measure the actual travel time sequence of the natural electromagnetic field.

[0123] The natural electromagnetic field is incident vertically on the ground in the form of a plane wave, part of which is reflected back into the air, and part of which is refracted into the underground medium and continues to propagate. The inventors found that when the electromagnetic field propagates in the underground medium, it is reflected and refracted again when it encounters the underground electrical impedance interface. When the electromagnetic field reflected again returns to the sensor position, it produces a superposition effect at the sensor with the external electromagnetic field that continues to be shot into the ground from the air at this moment. At this moment, the synthetic electromagnetic field after the superposition of the above-mentioned natural electromagnetic field will be shot into the underground medium again for propagation. At this moment, the intensity of each frequency component of the electromagnetic field shot into the underground again will be the synthetic electromagnetic field after superposition. Therefore, the superimposed electromagnetic field will be referred to as the synthetic electromagnetic field in the following text. It should be understood that the external electromagnetic field in this application is referenced to the ground, and the electromagnetic field shot from the air to the ground is called the external electromagnetic field. It should also be understood that the sensor position of this application can be on the ground or in the air.

[0124] When the electromagnetic field reflected and / or refracted by the underground electrical impedance interface returns to the ground, it produces a superposition effect on the ground with the natural electromagnetic field that continues to be emitted from the air. Therefore, the embodiment of the present application provides an electromagnetic survey method based on a natural field source. Figure 1 A flow chart of a method provided by an embodiment of the present application is shown. Figure 1 As shown, the method includes at least the following steps.

[0125] Collect initial signals; the initial signals include the natural electric field and / or magnetic field incident underground, and at the moment when it is reflected and refracted back to the sensor position through the underground electrical impedance interface (the electromagnetic field reflected back to the ground is the secondary field), and the natural electric field and / or magnetic field incident underground at that moment (the electromagnetic field incident underground is the primary field) meet at the sensor, and the two are superimposed to obtain the initial time series of the synthetic electric field and / or magnetic field strength.

[0126] Performing Fourier transform on the initial time series obtains an initial frequency spectrum of the initial time series.

[0127] Travel time inversion, travel time inversion is based on the principle of electromagnetic field superposition, and the initial spectrum is superimposed by a harmonic synthesis algorithm to obtain a synthetic travel time series of the natural electric field and / or magnetic field; the synthetic travel time series is used to judge geological information. Specifically, unlike traditional electromagnetic survey methods, this electromagnetic survey method based on natural field sources is aimed at the natural electric field and / or magnetic field incident underground. At the moment when it is reflected and refracted back to the sensor position through the underground electrical impedance interface, it meets the natural electric field and / or magnetic field incident underground at this moment at the sensor, and the two are superimposed to obtain a synthetic electric field and / or magnetic field intensity. In other words, the electromagnetic survey method provided in this application studies the synthetic electromagnetic field after the superposition of the primary field and the secondary field.

[0128] The inventors found that the electromagnetic field synthesized by superposition of the primary field and the secondary field has strong frequency selectivity. That is to say, the electromagnetic waves of different frequencies reflected back to the ground by the underground electrical impedance interface meet the continuously incident electromagnetic waves on the ground. When the two are superimposed, 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 the two. In this way, the underground electrical impedance interface has a "tuning" effect on electromagnetic waves.

[0129] The enhanced frequency component meets the following basic conditions: when it propagates in the underground medium and is reflected and refracted back to the ground, it is closely related to the phase difference between it and the external electromagnetic field of the same frequency that is continuously incident at this moment.

[0130] Specifically, for the case where the underground is a uniform, single-layer medium, the initial signal at least satisfies: In the formula, It is the phase difference between the natural electric field and / or magnetic field of any frequency at the time of reflection and refraction through the underground electrical impedance interface and the natural electric field and / or magnetic field of the same frequency continuously incident at the same time, f0 is the frequency of any single harmonic electric field and / or magnetic field in the initial signal, ρ1 is the resistivity of the medium, and h1 is the burial depth of any layer of electrical medium. This formula reflects the phase difference between the single harmonic electromagnetic wave of frequency f and the subsequent incident electromagnetic wave (same frequency) when they meet on the ground after being incident on the underground good conductive medium (ρ1, ε1, μ1), propagation distance h, and reflecting back to the surface at the bottom interface of the first electrical layer, when considering the dispersion factor. As for the case of multi-layer media, when the external natural electromagnetic field is incident vertically into the ground, there are many boundary interfaces in the ground at the same time, and the superposition effect on the ground will also be the superposition process of multiple interface reflection waves. In fact, the propagation of external incident electromagnetic fields in underground multi-layer media has universal practical significance. Drawing on the concept of equivalent wave impedance of multi-layer media, the reflection and refraction problems of multi-layer media are transformed into a single layer of media for research.

[0131] Theoretically, when the phase difference between two electromagnetic waves of the same frequency is When it is equal to 0, 2π, 4π, etc., that is, an even multiple of π, the two electromagnetic waves will meet and superimpose and enhance, and the amplitude of the synthetic electromagnetic field will reach the maximum enhancement effect; and when When it is equal to π, 3π, 5π..., that is, an odd multiple of π, two electromagnetic waves with the same frequency will meet, superimpose and weaken, and the amplitude of the synthetic electromagnetic field will reach the minimum.

[0132] When the phase difference meets the above conditions, the single harmonic electromagnetic wave of this frequency will achieve the maximum enhancement or weakening effect. In fact, the single harmonic electromagnetic fields of other frequencies in a frequency band on both sides of the center frequency of the above single harmonic electromagnetic field will also be enhanced or weakened to varying degrees, but the degree of enhancement or weakening is not as good as the center frequency.

[0133] According to an embodiment of the present application, the electromagnetic survey method provided by the present application further includes screening a dominant frequency component in the initial spectrum, the dominant frequency component being related to a certain electrical structure underground. Preferably, any frequency in the dominant frequency component is inversely proportional to the buried depth of the electrical impedance interface corresponding to the frequency.

[0134] Optionally, the dominant frequency component at least satisfies:

[0135]

[0136] Wherein, 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 in the dominant frequency component 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 that continues to be incident at the said moment.

[0137] Therefore, optionally, the phase difference of any frequency in the dominant frequency component of the present application when superimposed at the sensor falls within the frequency band In the above frequency bands, int means rounding. It is the phase difference between the fundamental frequency and the external electromagnetic field with the same frequency that is continuously incident at the same time when it returns to the ground after propagating, reflecting and refracting in the underground medium. Therefore, while the frequency that meets the above conditions is enhanced by superposition, the frequency component that is in a multiple relationship with the frequency is also enhanced by superposition based on a similar principle.

[0138] The location where the initial signal is collected is not limited to the ground, but can also be on the water surface or in the air. In traditional electromagnetic surveys and seismic surveys, sensors must be set on the ground or buried in shallow layers to avoid signal attenuation and interference factors that result in the inability to receive signals that meet the signal-to-noise ratio requirements.

[0139] It is worth noting that in the prior art, in the multiple coverage and superposition methods used in seismic and ground penetrating radar technology, the signal intensity of the excitation field source is fixed, and the superposition is limited to the superposition between the reflected waves (secondary fields) reflected back. The result of the superposition is that the reflected waves are always at the same numerical level. The number of superpositions in the prior art is dozens or hundreds of times. However, the present application studies the superposition between the continuously incident external primary field and the secondary field reflected back from the underground. When incident again, the intensity of the primary electromagnetic field will no longer be the same as the intensity of the primary field of the previous incident. In other words, in the embodiment of the present application, the intensity of the excitation field source of the electromagnetic field has the characteristics of dynamic and continuous enhancement. This superposition has the characteristic of exponential amplification of the intensity of the primary field incident again. Moreover, in the embodiment of the present application, the number of superpositions of electromagnetic waves is thousands or even tens of thousands of times. Therefore, compared with the traditional natural electromagnetic method, the initial signal collected by the embodiment of the present application has the characteristics of high intensity and high signal-to-noise ratio.

[0140] In addition, based on the fact that the process of natural electromagnetic fields entering the underground has a certain degree of continuity, the above-mentioned superposition process is characterized by a long time and a large number of times, so that the fundamental frequency and its multiples in the natural electromagnetic waves entering the underground can be continuously enhanced.

[0141] Stronger incident signal intensity ensures that the electromagnetic field signal can penetrate deeper into the ground. Obviously, this technology has a greater detection depth.

[0142] Natural electromagnetic waves continuously enter the underground. In addition to the numerous enhancements of the specific wave (group) corresponding to the reflected underground reflection interface, the numerous reflected single harmonics that are in a frequency doubling (higher frequency wave) relationship with the specific wave are also enhanced at the same time. The characteristics of the numerous enhancements of higher frequency electromagnetic waves and waves of the same frequency in the incident wave and waves in a frequency doubling relationship determine that the received signal will contain more high-frequency information far higher than the "base frequency". The high frequency components contained in the collected signal determine that this technology has a high resolution. The accuracy of the electromagnetic survey method based on natural field sources provided in accordance with the embodiment of the present application is improved by one to two orders of magnitude compared to traditional methods based on natural field sources such as magnetotellurics. For example, at a detection depth of 5,000 meters, the resolution of the electromagnetic survey method provided in this application is about 10 meters.

[0143] According to the implementation mode 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, may include the intensity of the magnetic field after the superposition of the primary field and the secondary field, and may also include the intensity 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 needs of the survey task. In the following text, the magnetic field after the superposition of the primary field and the secondary field is used as an example for explanation. It should be noted that when the intensity of the magnetic field and the electric field after the superposition of the primary field and the secondary field is jointly collected, the minimum frequency of the collection is the frequency lower limit that the magnetic sensor can receive as the collection lower limit, so as to calculate the Garnier resistivity. When the intensity of the magnetic field or the electric field after the superposition of the primary field and the secondary field is collected alone, the minimum frequency of the collection can optionally take any value in the 0.01Hz-10Hz frequency band for calculation. In an optional embodiment, the frequency of the initial signal is greater than 1 Hz and less than or equal to 5000 Hz.

[0144] In an embodiment of the present application, the method further comprises preparing a collection plan and determining collection parameters according to the survey task requirements before collecting the initial signal, wherein the collection parameters include at least one or more of the sampling rate, the minimum frequency to be collected, the maximum frequency to be collected, and the collection duration.

[0145] According to an embodiment of the present application, the sampling rate is negatively correlated with the depth to be surveyed. For example, for a survey depth of 2000 to 5000 meters, the sampling rate is greater than or equal to 2000 Hz. Preferably, for a survey target within 2000 meters, the sampling rate is taken from 5000 times / second to 100000 times / second.

[0146] According to the implementation of the present application, the acquisition time is positively correlated with the burial depth of the survey target. For example, for a survey depth of 2000 meters, the acquisition time is preferably 600 seconds. For a survey depth of 5000 meters, the acquisition time is preferably 1200 seconds.

[0147] Furthermore, the electromagnetic survey method based on natural field sources provided in the embodiment of the present application also includes: performing a first notch processing and / or low-pass filtering before collecting the initial signal according to the highest frequency to be collected. Among them, the first notch processing is to suppress the power frequency and the multiples of the power frequency; the low-pass filtering is to filter the frequency components higher than the highest frequency to be collected. The first notch processing includes a first comb filter processing, which is a hardware-based filtering processing. In this way, spectrum aliasing can be avoided. If software-based filtering processing is used, spectrum aliasing will inevitably occur. The aforementioned hardware includes filters, etc. The power frequency refers to the rated frequency used by the equipment, industrial electrical equipment and civil electrical equipment of the power system. 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 multiples. In some optional cases, it is necessary to pre-low-pass filter according to the highest frequency to be collected, and suppress the frequency components higher than the highest frequency to be collected. In most cases, the initial signal may be collected on the surface of the field. The collection location provided in the embodiments of the present application may also be at sea or in the air.

[0148] It can be understood that the number of channels collected in the electromagnetic survey method based on natural field sources provided in the embodiment of the present application is greater than or equal to 1, and preferably, the number of channels collected is greater than or equal to 8.

[0149] Example 1

[0150] Figure 2 The figure shows the initial signal collected near an oil exploration well in Location A in December 2020, which is a single-channel magnetic field intensity time series. The horizontal axis in the figure is time in seconds, and the vertical axis is the magnetic field intensity in nanometers. Optionally, the vertical axis can also be other physical quantities such as uncalibrated scale values, voltage values, etc. that are proportional to the magnetic field intensity. Figure 2 The above figure is a time series of 0.4 seconds in length. Figure 2 The figure below is a 0.025 second long time series. The number of acquisition channels in this acquisition is 1, the sampling rate is 24,000 times / second, the total acquisition time is 30 seconds, and the acquisition data frequency band is 0 to 12,000 Hz.

[0151] In an optional embodiment, performing Fourier transform on the initial time series to obtain the initial spectrum 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 signal collected by each acquisition channel into the component spectrum of the frequency domain electric field and / or magnetic field intensity components; using the vector synthesis law, 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. 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: Among them, 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. The total spectrum of the electric field strength and / or magnetic field strength synthesized based on the initial signal includes: converting the original time series data of the initial signal collected by each acquisition channel into the initial spectrum of the initial signal through DFFT fast Fourier transform or modern spectrum analysis algorithm; synthesizing the initial spectra corresponding to all acquisition channels into a total spectrum using the vector synthesis rule.

[0152] Specifically, the electric field strength and / or magnetic field strength data collected by each acquisition channel is the original time series data in the time domain. The original time series data includes time, magnetic field strength, and electric field strength. The magnetic field strength includes a first magnetic field strength component and a second magnetic field strength component, and the first and second magnetic field strengths are perpendicular to each other and respectively perpendicular to the incident direction of the natural electromagnetic field. The electric field strength includes a first electric field strength component and a second electric field strength component, and the first and second electric field strengths are perpendicular to each other and respectively perpendicular to the incident direction of the natural electromagnetic field.

[0153] Exemplarily, the original time series is recorded in a custom binary file, which also stores basic parameters such as the acquisition area name, data acquisition operator, acquisition date, channel description, sampling rate, data accuracy, and data length for use in subsequent data processing.

[0154] In the embodiment of the present application, the original time series represents the relationship between the change of the electromagnetic field over time. It is necessary to convert the original time series data into the initial spectrum data of the natural electric field and / or magnetic field intensity frequency domain of each channel based on Fourier analysis to characterize the change characteristics of the synthetic electromagnetic field after the superposition of the primary field and the secondary field with frequency.

[0155] Next, after obtaining the initial spectrum of each acquisition channel, the vector synthesis rule is used to synthesize the initial spectrum into a total spectrum. Specifically, the total electric field and / or magnetic field intensity is synthesized by the real and imaginary parts of the electric field and / or magnetic field intensity components of the single-channel electric field intensity and / or magnetic field intensity, and then the amplitude and phase of the total electric field and / or magnetic field are calculated. Taking the magnetic field as an example, the specific calculation formulas are shown in formulas (2-1) to (2-4):

[0156] DH R =HX R +HY R ; (2-1)

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

[0158]

[0159] DH 相位 =arctan(DH I / DH R ); (2-4)

[0160] Wherein, DH represents the total magnetic field intensity, HX represents the first magnetic field intensity component; HY represents the second magnetic field intensity component; I represents the imaginary part, and R represents the real part.

[0161] Taking the electric field as an example, the specific calculation formulas are shown in formulas (3-1) to (3-4):

[0162] DE R =EX R +EY R ; (3-1)

[0163] DE I =EX I +EY I ; (3-2)

[0164]

[0165] DE 相位 =arctan(DE I / DE R ); (3-4)

[0166] Wherein, DE represents the total magnetic field intensity, EX represents the first magnetic field intensity component; EY represents the second magnetic field intensity component; I represents the imaginary part, and R represents the real part.

[0167] Exemplarily, the total spectrum is consistent with the initial spectrum of the first magnetic field component and the second magnetic field component, and both include frequency, real part, imaginary part, amplitude and phase.

[0168] Example 2

[0169] Figure 3 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 data acquisition and 12 times (segments) separated by 2 hours is shown, which is the initial spectrum of the single-channel magnetic field intensity data. The initial signal was collected from August 21 to 22, 2021, and the collection location was the same as in Example 1. Within 24 hours, data was collected every two hours, starting at 16:00 p.m. on the 21st, and ended at 14:00 p.m. on the 22nd. A total of 12 data were collected, each time for 30 seconds, with a sampling rate of 24,000 times / second, and an analysis band of 0-12000 Hz. In the figure, the horizontal axis is frequency, and only the 0-5000 Hz band amplitude spectrum is drawn here, in Hertz, and the ordinate is the magnetic field intensity amplitude. Figure 3 Characterizes the basic characteristics of the magnetic field intensity amplitude spectrum. Figure 3 As shown, the magnetic field components in the natural electromagnetic field are ubiquitous in the frequency band of DC-24000 Hz; in the frequency band below 500 Hz, the magnetic field strength increases monotonically with decreasing frequency; the magnetic field components at 50 Hz industrial frequency and its multiples are the main interference factors of the electromagnetic survey method provided in this application, and usually have large amplitude values.

[0170] Taking into account that when collecting the initial signal, power frequency suppression in the field does not always achieve the expected suppression effect, the collected data can be further processed by software denoising and filtering in the indoor processing stage. Therefore, it is advantageous to suppress the interfering frequency components in the total spectrum, especially the power frequency and its frequency multiple components, so as to facilitate the screening of effective signals in the total spectrum data. In the embodiment of the present application, the effective signal is also referred to as the dominant frequency component. The dominant frequency component includes the frequency component and its frequency multiple components that can be superimposed and enhanced by the primary field and the secondary field.

[0171] According to the implementation manner of the embodiment of the present application, optionally, screening the dominant frequency components in the initial spectrum includes: determining the lowest dominant frequency component based on the survey object and requirements; and, suppressing interference frequency components; wherein the interference frequency components include background noise, power frequency and multiples of the power frequency; the background noise includes all frequency components higher and / or lower than the dominant frequency component.

[0172] The power frequency and its frequency multiple components are various types of human power frequency interference, and their identification and suppression are important parts of the electromagnetic survey method provided in the embodiment 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 mode of this application, optionally, the aforementioned suppression of interference frequency components includes: a second comb filter processing and a bandpass filter processing; the second comb filter processing and the bandpass filter processing are both processing methods based on the principle of digital signal processing.

[0173] The electromagnetic survey method based on natural field sources provided in the embodiment of the present 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 / (2 / f min ). Here, v is the speed of natural electromagnetic waves propagating in underground media, and f min It is the minimum frequency (i.e., the minimum fundamental frequency) corresponding to the electromagnetic wave. The frequency components lower than the minimum fundamental frequency participate in further data processing, which will introduce non-periodic, nonlinear background frequencies (i.e., background noise) with lower frequencies (larger wavelengths) than the minimum effective frequency into the travel time data. Therefore, it is preferred to remove the background noise. Removing frequency components lower than the minimum fundamental frequency also brings additional benefits: in the initial signal acquisition stage, the intensity of the frequency components lower than the minimum fundamental frequency is often greater than the signal intensity of the minimum fundamental frequency, which is not conducive to the travel time inversion in the electromagnetic survey method provided in the present application. Therefore, it is advantageous to determine the minimum fundamental frequency, i.e., the lowest effective frequency.

[0174] The lowest effective frequency needs to be determined according to the specific geological tasks and survey requirements, especially the survey depth and speed. For example, if the mission requires a detection depth of 6 kilometers and the electromagnetic wave speed is 110,000 meters per second, the lowest dominant frequency is 18 Hz. The highest dominant frequency is determined by experiments and generally varies between 500-50,000 Hz;

[0175] Optionally, suppressing the interference frequency includes a second notch process and a bandpass filter. The second notch process suppresses the power frequency and the multiple of the power frequency. The bandpass filter includes filtering out the frequency components lower than the background noise frequency components and higher than the highest frequency to be collected.

[0176] Optionally, compensating the dominant frequency component includes compensating a frequency component having the same frequency as the interference frequency component and a phase difference equal to π to the dominant frequency component. Preferably, compensating the dominant frequency component uses a frequency component having the same frequency as the power frequency and a multiple of the power frequency and a phase difference of π. The amplitude multiplier of the compensation frequency is determined by experiments.

[0177] The initial spectrum in any of the above embodiments and the dominant frequency components screened out in the initial spectrum are used for travel time inversion. Travel time inversion is the key to the electromagnetic survey method provided in the embodiments of the present application. Travel time inversion refers to the superposition of the initial spectrum (preferably the dominant frequency components) by the Fourier harmonic synthesis algorithm based on the electromagnetic field superposition principle, thereby obtaining a synthetic travel time sequence of the natural magnetic field and / or electric field. The synthetic travel time sequence characterizes the actual travel time sequence of the electromagnetic wave and is used to judge geological information.

[0178] Specifically, travel time inversion is designed based on the principle of electromagnetic field superposition: among the many dominant frequency components selected, the underground electrical impedance interface that plays a superposition and enhancement role on the fundamental frequency electromagnetic wave component often corresponds to multiple frequency components that are multiples of the fundamental frequency. When the fundamental frequency and the multiples of the fundamental frequency are superimposed again to form a new time series, only when the travel time, frequency (or period) of the electromagnetic wave and the burial depth of the underground electrical impedance interface are completely "matched" (that is, when the superposition and enhancement conditions are met), the intensity of the electromagnetic field is superimposed and enhanced, forming a travel time sub-wave similar to the seismic wavelet, while other frequency bands between the travel time sub-waves are superimposed and canceled or chaotic.

[0179] Based on the above electromagnetic field superposition principle, the embodiment of the present application superimposes the initial spectrum through a harmonic synthesis algorithm to obtain a synthetic travel time series of the natural electric field and / or magnetic field. Considering the dispersion effect of electromagnetic waves, the superposition calculation satisfies the following formulas (4-1) and (4-2):

[0180] ω=2πf(i); (4-1) and / or

[0181] ω=2πf(i);(4-2)

[0182] 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 i-th harmonic, k is not equal to 0; t is the travel time; N is the number of harmonics. In this way, the distortion caused by dispersion during the propagation of electromagnetic waves can be effectively suppressed to achieve a large detection depth and high resolution for detailed characterization of underground electrical structures. Preferably, in formula (4-1) and formula (4-2), k is a natural number greater than or equal to 0. More preferably, in formula (4-1) and formula (4-2), the value of k is selected from any one of 1, 1.2, 1.4, 2, 2.5, 3, 3.8, 4.

[0183] It should be understood that the travel time inversion based on the above principle can be calculated using conventional inversion algorithms in the art (such as the least damped squares method, gradient method, Newton's method, simulated annealing, genetic algorithm or neural network algorithm).

[0184] Example 3

[0185] Example 3 provides an exemplary travel time inversion, the result of which is as follows: Figure 4 Example 3 takes a natural magnetic field containing a 50 Hz power frequency and its multiples as an example to perform travel time inversion. Figure 4 Based on Figure 3 After screening the dominant frequency components of the 12 sets of magnetic field intensity spectrum data, the 12 sets of travel time inversion are combined into time series (data) curves, of which the 12 curves are respectively Figure 3 The spectrum of the initial signal is collected at each moment in the , and 12 synthetic time series curves are obtained by travel time inversion after screening the dominant frequency components. The horizontal axis in the figure is time, in seconds, and the vertical axis is the magnetic field intensity. The travel time series synthesized by travel time inversion is used to judge geological information.

[0186] Figure 3 It is shown in the figure that the data collected at the same point and different times have obvious differences between the spectra of the initial signals collected at different times due to interference and changes in the natural field source itself. Table 1 shows the spectrum of the magnetic field strength of the initial signal collected in 12 time periods of 24 hours a day. Compared with the average value of the data collected 12 times in 24 hours, the amplitude correlation coefficient is the minimum 0.32 and the maximum 0.83, and the phase correlation coefficient is the minimum 0.27 and the maximum 0.31. However, after the processing of the effective signal screening technology and travel time inversion technology of this application, the correlation coefficient of the magnetic field intensity of the synthesized 12 groups of travel time series data is the minimum 0.93 and the maximum 0.99 (Table 2). Obviously, the consistency and stability of the travel time inversion results all indicate that the travel time series data obtained by the travel time inversion of this application has better suppressed the influence of various external interference factors on the inversion results. The travel time inversion result is a reflection of the underground electrical structure-only the underground electrical structure is unchanged.

[0187] Table 1 Figure 4 Statistical table of initial signal magnetic field strength initial spectrum correlation characteristics of 12 groups

[0188]

[0189]

[0190] Table 2 Figure 4 Statistics of the correlation characteristics of 12 groups of magnetic field intensity travel time series

[0191]

[0192] 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):

[0193] t=1 / (2f);(5-1)

[0194] h=v*t;(5-2)

[0195] 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.

[0196] Example 4

[0197] 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.

[0198] 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.

[0199] Figure 7The analogy diagrams of the skin effect (a), the synthetic signal of the ground penetrating radar (b) and the synthetic travel time time series (c) of the travel time inversion of the present application when the electromagnetic wave propagates in the underground medium are shown.

[0200] Figure 7 (a) shows the general law of attenuation of electromagnetic waves of different frequencies (periods) when propagating underground under given geoelectric conditions: the skin depth (skin depth: the depth at which the electromagnetic wave attenuates to 1 / e = 0.368 times before the incident medium) of a single harmonic electromagnetic wave with a period of 100 seconds 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 second is 1,000 meters.

[0201] Figure 7 (b) in the figure is the ground penetrating radar record synthesized by the six-layer geoelectric model.

[0202] Among them, the first layer h1=2.5, ε1=9ε0, ρ1=10, ν1=0.1ν; the second layer h2=1, ε2=12ε0, ρ2=500, ν2=0.09ν; the third layer h3=0.9, ε3=20ε0, ρ3=100, ν3=0.07ν; the fourth layer h4=1, ε4=12ε0, ρ4=500, ν4=0.09ν; the fifth layer h5=0.9, ε5=20ε0, ρ5=100, ν5=0.07ν; the sixth layer is a high-resistance substrate, ε6=8ε0, ρ6=5000, ν6=0.11ν.

[0203] Here h is in meters; ρ is in Ω·m; υ=300,000,000m / s, where ν i It is a multiple of the speed of electromagnetic waves in vacuum ν, i = 1, 2…6.

[0204] Figure 7 (c) is the travel time inversion synthesis time series curve in the electromagnetic survey method provided by the present application, and the data is the processing result of the magnetic field intensity time series data collected beside a certain exploration well in the C area. The sampling rate of the collected data is 24000 times / second, the sampling time is 60 seconds, and the frequency band selected by the effective signal is 5-2000 Hz, that is, the lowest frequency is 5 Hz and the highest frequency is 2000 Hz.

[0205] When the travel time inversion is used to synthesize the travel time time series, 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 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 the dispersion effect correction coefficient, and its value is not 0; t is the travel time; N is the number of harmonics.

[0206] Figure 8 The following figure shows a comparison of the travel time section corresponding to a certain structural area in the second place after the electromagnetic survey method provided by the present application was used to survey the structural area. The structural area is located in the Hetao Basin in central Inner Mongolia. The Hetao Basin covers an area of ​​about 40,000 square kilometers. It is a Mesozoic and Cenozoic depression-rift basin developed on the Paleoproterozoic-Archean metamorphic crystalline basement of the Yimeng ancient land core. From west to east, it is divided into three depressions: Linhe, Wuqian and Huhe. A certain structural area in the second place is located in the Linhe Depression.

[0207] Linhe Depression has developed multiple sets of source rocks with great thickness and wide distribution. The source rocks of Guyang Formation in Lower Cretaceous are 200-600 meters thick, buried at a depth of 3100-3200 meters, and cover an area of ​​about 1910 km. 2 The thickness of the Linhe Formation source rock of the Oligocene is 200-550m, the burial depth is 1400-1700m, and the range area is about 3680km2. The source rock in the area is a salt lake phase deposit with high organic matter abundance. The geochemistry of the Lower Cretaceous and Oligocene source rocks shows that the organic matter abundance is relatively high, with TOC (0.4-2.6)%, an average of 1.05%, which is a good source rock according to the terrestrial saltwater lake basin source rock standard. The Paleogene source rock is mainly composed of type I-II1 kerogen, the organic matter type of the Lower Cretaceous Gu 2 source rock is mainly type II kerogen, and the Gu 1 source rock is mainly type II.

[0208] The structural location of this example is the junction of the fault block buried hill and the fault depression area ( Figure 8 The profile is east-west oriented and is divided into three structural sections from west to east with points A and B: the western section of B is the buried hill part, which is a large fault block buried hill with Archean basement. The buried hill surface is buried at a depth of 300-800 meters. The buried hill is covered with 300-800 meters thick Mesozoic and Cenozoic sedimentary strata. The weathering crust and cracks on the surface of the buried hill are the main reservoirs of oil and gas; the AB section belongs to the large fault nose structural area of ​​clastic rocks developed on the eastern wing of the buried hill. The strata have a large dip angle and reservoir conditions such as developed fractures in the buried hill gneiss and near-source clastic fault nose structures. The two reservoirs are shallowly buried (400-2000m) and have good physical properties, which have the material basis for the formation of large oil and gas fields; the area east of point B belongs to the scope of sedimentary and structural system inside the fault depression, with Mesozoic sedimentary strata with complete sedimentary stratigraphic sequence and thick strata.

[0209] The study of physical data shows that the Archean basement in this exploration area is a high-resistance basement with a resistivity of up to thousands of ohms-meters, which is a typical high-resistance marker layer, while the resistivity of the Mesozoic and Cenozoic sedimentary strata is relatively low, generally less than 10 ohms-meters. Obviously, there is a huge contrast in electrical impedance interface between the Archean and Mesozoic strata, which lays a good physical prerequisite for the success of this application technology.

[0210] Figure 8The upper left picture is a cross-section of the travel time series synthesized by the inversion of the magnetic field intensity in the depth domain after time-depth conversion. It can be clearly seen from the cross-section that the entire cross-section is divided into three sections from west to east by the two faults A and B. The positions of the dividing points A and B are consistent with Figure 7 The positions of the upper three points on the geological section diagram in the lower left picture basically correspond.

[0211] The western section west of point A corresponds to the location of the Archean buried hill. On the electromagnetic method results map, the top of the buried hill is undulating and uneven, and the interior is cut by at least seven faults, dividing it into 8 isolated hilltops; the eastern section east of point B corresponds to the sedimentary and structural system inside the fault depression to the east of point B. The Mesozoic sedimentary strata are relatively thick and the strata dip is relatively small, and 3 to 4 electrical layers can be divided inside; the section east of point A and west of point B corresponds to the large-scale fault nose structural area of ​​clastic rocks developed on the eastern wing of the buried hill. The basic feature of this section is the large strata dip.

[0212] The geological model designed in this application is a horizontal or quasi-horizontal stratum model. For high-blockage structures, the secondary field of the external incident primary field and the reflected wave from the underground electrical impedance interface cannot meet and cannot be enhanced. Therefore, the appearance of a "blank zone" of data in the high-blockage structure area is inevitable, and offset technology is needed to obtain the electromagnetic data of this section. This is the next step of this technology.

[0213] Figure 8 The upper right picture is the seismic profile of the corresponding section. Comparing the electromagnetic method technology, seismic technology results and geological section map of this application, special attention should be paid to the following three points: First, the morphology of the buried hill surface is not a slowly changing unified interface, but a small hill composed of 8 isolated fault blocks complicated by 7 faults; second, exploration practice has proved that it is not the entire top of the buried hill that contains oil, but only individual sections contain oil, and the proportion of oil-bearing hills among the 8 small hills is less than 50%; third, the formation dip and the boundary lines of the three structural units are basically consistent.

[0214] Fig. 9 After Fourier transformation of the collected initial signal magnetic field intensity time series data, the spectrum of the initial signal magnetic field intensity is obtained. Furthermore, the travel time inversion of the initial signal magnetic field intensity spectrum is performed to obtain the travel time series of the magnetic field intensity. The data was collected in December 2020 near an oil exploration well in Place A. Single-channel magnetic field data was collected with a sampling rate of 24,000 times / second and a sampling time of 30 seconds. The horizontal axis in the figure is time in seconds. The 8 curves in the figure are the travel time series of travel time inversion when different maximum frequency values ​​are selected. The vertical axis is the magnetic field intensity.

[0215] In the present application technology, the signal of the selected dominant frequency component includes the fundamental frequency and the double frequency component corresponding to the impedance interface. The use of the double frequency component is the inherent basis for improving the resolution of the travel time inversion synthesis travel time series. This example shows that the inversion results have different resolution capabilities when double frequencies are selected in different frequency ranges. The higher the high frequency end of the double frequency is, the stronger the resolution capability is, and the richer the details revealed are.

[0216] In some optional implementations, the electromagnetic survey method provided in the embodiments of the present application further includes the following steps.

[0217] Determine the marker layer, that is, based on the comparison between the geological materials in the survey area and the travel time curve corresponding to the actual travel time sequence, establish the correspondence between the geological strata in the survey area and the underground electrical 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 the geological layers in the geological strata.

[0218] The time-to-depth conversion parameters are obtained, that is, the time-to-depth conversion parameters are obtained based on the depth of the marker layer and the time corresponding to the marker layer on the travel time curve.

[0219] The time-to-depth conversion parameters are used to calibrate the results of travel time inversion. The more accurate the time-to-depth conversion parameters are, the higher the match between the actual travel time sequence and the electromagnetic wave propagation situation.

[0220] Exemplarily, the time-depth conversion parameters satisfy formula (6):

[0221] ν=h / t;(6)

[0222] In formula (6), ν is the time-to-depth conversion parameter, which has the same dimension as the velocity; h is the burial depth of the marker layer; and t is the travel time of the electromagnetic wave corresponding to the depth of the marker layer.

[0223] Collect geological and geophysical data such as electrical logging and comprehensive logging in the exploration area, especially deep induction electrical logging data, and systematically compare and interpret the travel time curves of high-resolution electromagnetic survey beside the well with the electrical logging and comprehensive logging curves to ensure that the comparison between large electrical layers and large geological layers is accurate. The layer (interface) with the clearest correspondence between the logging and logging data and the travel time curves is used as the marker layer, and the corresponding depth-time value is the conversion parameter required for the final time-to-depth conversion.

[0224] Furthermore, the electromagnetic survey method provided in the embodiment of the present application also includes:

[0225] Optimizing Parameter Selection for Traveltime Inversion Based on Time-to-Depth Conversion Parameters.

[0226] Exemplarily, the selection of the parameters for travel time inversion is optimized based on the time-depth conversion parameters, and multiple time-depth conversion parameters are obtained through repeated experiments. The principles and methods for selecting the parameters for travel time inversion are improved by calibrating a large number of time-depth conversion parameters accumulated through repeated experiments. It can be understood that the optimization of the travel time inversion parameters by the time-depth conversion parameters can be achieved through deep learning.

[0227] According to an embodiment of the present application, the electromagnetic survey method may further include velocity compensation. Electromagnetic waves of different frequencies have different propagation speeds in the same medium, which is called the dispersion characteristic of electromagnetic waves. In the electromagnetic survey method provided in the embodiment of the present application, due to the dispersion characteristic of electromagnetic waves, the morphology of the reflected sub-wave on the time domain travel time curve corresponding to the underground electrical impedance interface will be distorted, and it is difficult to effectively identify the information of the electrical impedance interface.

[0228] Therefore, in travel time inversion, it is necessary to adjust and normalize the speed of the electromagnetic wave so that different frequency components have the same speed, so that the reflected "sub-wave" on the travel time curve will no longer be distorted, which is conducive to the interpretation of travel time data and improves the accuracy of the electromagnetic survey method.

[0229] In some optional embodiments, speed compensation includes:

[0230] The time-depth conversion of the travel time curve is performed according to the initial value of the electromagnetic wave propagation speed corresponding to the lowest effective frequency of the embodiment of the present application, so as to obtain the travel time curve in the depth domain. Exemplarily, the initial value satisfies:

[0231]

[0232] Among them, f min is the lowest effective frequency, that is, the minimum value of the effective frequency; v min is the initial value of the electromagnetic wave propagation speed corresponding to the lowest effective frequency; σ is the electrical conductivity of the underground electrical medium; μ is the magnetic permeability of the underground electrical medium.

[0233] The time-to-depth conversion of the travel time curve is performed using the initial value of the electromagnetic wave propagation velocity to obtain the travel time curve in the depth domain.

[0234] The system conducts the comparison and interpretation of the depth domain curve of the high-resolution electromagnetic survey beside the well with the electric logging and comprehensive logging curves to ensure the accuracy of the comparison between the electrical layer and the geological layer. The layer (interface) with the best and clearest correspondence between the logging and logging data and the proposed logging curve is used as the marker layer. min =H / t determines the electromagnetic wave propagation velocity value corresponding to the lowest frequency when processing and interpreting actual data. Where: v min The frequency is f minThe propagation velocity value corresponding to the single harmonic electromagnetic wave; H is the depth of the marker layer, in meters; t is the time corresponding to the depth of the marker layer, in seconds.

[0235] The actual propagation speed corresponding to the lowest effective frequency is used to obtain the actual propagation speed corresponding to electromagnetic waves of other frequencies. For example, the actual propagation speed corresponding to the electromagnetic wave with a frequency of f satisfies:

[0236]

[0237] Optionally, the magnetic field expression in the travel time inversion after velocity compensation is as shown in formulas (4-1) and (4-2).

[0238] Combination of the above Figures 1 to 9 , describes in detail the electromagnetic survey method based on natural field sources provided by the embodiment of the present application. The method can also be implemented by a corresponding system. Figures 10 to 13 , describes in detail the electromagnetic survey system based on natural field sources provided in an embodiment of the present application.

[0239] Fig.10 FIG. 1 shows a schematic diagram of a structure of an electromagnetic survey system based on a natural field source provided in an embodiment of the present application. Fig.10 As shown, the electromagnetic survey system includes:

[0240] The acquisition module 801 is configured to acquire initial signals; the initial signals include the natural electric field and / or magnetic field incident underground, and when reflected and refracted back to the ground through the underground electrical impedance interface, the natural electric field and / or magnetic field incident underground at the time meets at the sensor, and the two are superimposed to obtain the initial time series of the synthesized electric field and / or magnetic field strength;

[0241] The initial signal processing module 802 is configured to perform Fourier transform on the initial time series to obtain an initial spectrum of the initial time series;

[0242] Optionally, the screening module 803 screens effective frequencies in the total spectrum; wherein the effective frequencies include a base frequency that can be enhanced and a multiple of the base frequency;

[0243] The travel time inversion module 804 is configured to perform superposition operation on the initial spectrum through a harmonic synthesis algorithm based on the electromagnetic wave superposition principle, so as to obtain a synthetic travel time series of the natural electric field and / or magnetic field; the synthetic travel time series is used to judge geological information.

[0244] Therefore, the electromagnetic survey system of the embodiment of the present application collects electromagnetic waves after being superimposed by the underground electrical impedance interface through the acquisition module 801, and synthesizes the initial spectrum of electric field strength and / or magnetic field strength based on the initial signal through the initial signal processing module 802. Thereafter, the screening module 803 screens out the dominant frequency components in the total spectrum, which are converted and superimposed by the travel time inversion module 804 using Fourier transform to obtain the actual travel time sequence.

[0245] Alternatively, if Fig.11 As shown, the electromagnetic survey system also includes:

[0246] The first trap device 805 is configured to suppress the power frequency and multiples of the power frequency; and / or,

[0247] The low pass filter 806 is configured to filter out frequencies higher than the highest frequency to be collected.

[0248] Alternatively, if Fig.12 As shown, the electromagnetic survey system also includes:

[0249] A suppression device 807, configured to suppress interference frequencies; and / or,

[0250] The compensation device 808 is configured to compensate for the dominant frequency component.

[0251] In summary, the electromagnetic survey method and system based on natural field sources provided in the embodiments of the present application do not perform "division" operations on the magnetic field and the electric field, retain the fluctuation information of the natural electric field itself, and collect the natural electric field and / or magnetic field incident underground. After reflection and refraction at the underground electrical impedance interface, the natural electric field and / or magnetic field intensity is superimposed with the incident natural electric field and / or magnetic field at the time of returning to the ground sensor position. The component of the natural electric field and / or magnetic field intensity is perpendicular to the incident direction of the natural electric field and / or magnetic field, and the initial spectrum is synthesized. After the dominant frequency components in the initial spectrum are screened out, the initial spectrum (preferably the dominant frequency components) are superimposed by travel time inversion using a harmonic synthesis algorithm to form a synthetic travel time time series of the natural electromagnetic field. Therefore, the electromagnetic survey method and system provided in the embodiment of the present application utilizes the natural electric field and / or magnetic field incident underground, and after being reflected and refracted by the underground electrical impedance interface, the electromagnetic wave formed by the superposition of the incident natural electric field and / or magnetic field at the time of returning to the ground sensor position has a higher signal-to-noise ratio than the natural electromagnetic field, thereby achieving a greater survey depth, higher resolution, and higher work efficiency than the traditional electromagnetic survey method based on natural field sources. The survey depth and resolution of the electromagnetic survey method provided in the embodiment of the present application are not inferior to those of the survey method of artificial field sources, and it has the characteristics of energy saving and environmental protection. This method is different from the principle of the traditional electromagnetic survey method, and can only collect one of the electric field or magnetic field signals, and the collection efficiency is high. Moreover, since the natural electric field and / or magnetic field is incident on the underground, it is reflected and refracted by the underground electrical impedance interface and then returns to the sensor position to be superimposed with the incident natural electric field and / or magnetic field at that time, the signal strength is enhanced, so that the collection of the initial signal is not limited to the ground, but can also be in the air and on the water surface. Compared with traditional electromagnetic survey methods and seismic survey methods, the deployment difficulty and cost of the corresponding survey equipment of this application are greatly reduced, and the application scenarios are broad.

[0252] In addition, an embodiment of the present application also provides an electronic device, including a bus, a transceiver, a memory, a processor, and a computer program stored in the memory and run on the processor. The transceiver, the memory, and the processor are respectively connected via a bus. When the computer program is executed by the processor, the various processes of the above-mentioned electromagnetic survey method embodiment based on natural field sources are implemented, and the same technical effect can be achieved.

[0253] For details, see Fig.13 As shown, an embodiment of the present application further provides 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 .

[0254] In the embodiment of the present application, the electronic device further includes: a computer program stored in 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:

[0255] Collect the initial signal. The initial signal includes the component of the intensity of the natural electric field and / or magnetic field perpendicular to the incident direction of the natural electric field and / or magnetic field obtained by superimposing the natural electric field and / or magnetic field incident at the time of returning to the sensor position after the natural electric field and / or magnetic field is incident underground and reflected and refracted by the underground electrical impedance interface.

[0256] A total spectrum of the electric field strength and / or magnetic field strength is synthesized based on the initial signal.

[0257] Optionally, the dominant frequency components in the total spectrum are screened, wherein the dominant frequency components include the spectrum corresponding to the fundamental frequency and the multiple frequencies of the fundamental frequency that can be enhanced.

[0258] Travel time inversion. Travel time inversion is based on the principle of electromagnetic waves and uses a harmonic synthesis algorithm to superimpose the initial spectrum (preferably the dominant frequency component) to obtain a synthetic travel time series of the natural electric field and / or magnetic field. The synthetic travel time series is used to judge geological information.

[0259] Optionally, when the computer program is executed by the processor 1120, the following steps may also be implemented:

[0260] According to the highest frequency to be collected, a first notch processing and / or low-pass filtering is performed before collecting the initial signal;

[0261] The first notch processing is to suppress the power frequency and its multiples; the low-pass filtering is to filter out the frequency components higher than the highest frequency to be collected.

[0262] Optionally, when the computer program is executed by the processor 1120, the following steps may also be implemented:

[0263] Based on the comparison between the geological data of the survey area and the synthetic travel time series, the corresponding relationship between the geological strata in the survey area and the underground electrical impedance interface determined by the synthetic travel time series is established;

[0264] Determine the marker layer based on the corresponding relationship; the marker layer is the layer in the geological layer with the highest matching degree with the synthetic travel time time series;

[0265] Based on the depth of the marker layer and the time corresponding to the marker layer in the synthetic travel time time series, a time-to-depth conversion parameter is obtained. Optionally, when the computer program is executed by the processor 1120, the following steps may also be implemented:

[0266] Optimizing Parameter Selection for Traveltime Inversion Based on Time-to-Depth Conversion Parameters.

[0267] Optionally, when the processor 1120 executes the step of “performing Fourier transform on the initial time series to obtain an initial spectrum of the initial time series”, the computer program enables the processor to specifically implement the following steps:

[0268] The original time series data of the initial signal collected by each acquisition channel is converted into the initial spectrum of the initial signal through Fourier analysis;

[0269] The vector synthesis rule is used to synthesize the initial spectra corresponding to all acquisition channels into the total spectrum.

[0270] Optionally, when the processor 1120 executes the step of “performing Fourier transform on the initial time series to obtain an initial spectrum of the initial time series”, the computer program enables the processor to specifically implement the following steps:

[0271] Determine the lowest dominant frequency component based on survey requirements;

[0272] Suppress interfering frequencies and / or compensate for dominant frequency components;

[0273] The interference frequency includes background frequency, power frequency and multiples of power frequency; the background frequency is lower than the frequency of the lowest dominant frequency component.

[0274] The transceiver 1130 is configured to receive and send data under the control of the processor 1120 .

[0275] In an embodiment of the present application, the bus architecture (represented by bus 1110), bus 1110 may include any number of interconnected buses and bridges, and bus 1110 connects various circuits including one or more processors represented by processor 1120 and a memory represented by memory 1150.

[0276] 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 Accelerate Graphical 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.

[0277] Processor 1120 can be an integrated circuit chip with signal processing capabilities. In the implementation process, each step of the above method embodiment can be completed by an integrated logic circuit of hardware in the processor or an instruction in the form of software. The above processor includes: a general processor, a central processing unit (CPU), a network processor (NP), a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a complex programmable logic device (CPLD), a programmable logic array (PLA), a microcontroller unit (MCU) or other programmable logic devices, discrete gates, transistor logic devices, discrete hardware components. The methods, steps and logic block diagrams disclosed in the embodiments of the present application can be implemented or executed. For example, the processor can be a single-core processor or a multi-core processor, and the processor can be integrated into a single chip or located in multiple different chips.

[0278] Processor 1120 can be a microprocessor or any conventional processor. The method steps disclosed in conjunction with the embodiments of the present application can be directly performed by a hardware decoding processor, or by a combination of hardware and software modules in the decoding processor. The software module can be located in a readable storage medium known in the art such as a random access memory (Random Access Memory, RAM), a flash memory (Flash Memory), a read-only memory (Read-Only Memory, ROM), a programmable read-only memory (Programmable ROM, PROM), an erasable programmable read-only memory (Erasable PROM, EPROM), a register, etc. The readable storage medium is located in a memory, and the processor reads the information in the memory and completes the steps of the above method in conjunction with its hardware.

[0279] The bus 1110 may also connect various other circuits such as peripheral devices, voltage regulators or power management circuits, and the bus interface 1140 provides an interface between the bus 1110 and the transceiver 1130, which are well known in the art. Therefore, the embodiments of the present application will not be further described.

[0280] The transceiver 1130 may be one element or multiple elements, such as multiple receivers and transmitters, and provides a unit for communicating with various other devices on a transmission medium. For example, the transceiver 1130 receives external data from other devices, and the transceiver 1130 is used to send data processed by the processor 1120 to other devices. Depending on the nature of the computer system, a user interface 1160 may also be provided, such as a touch screen, a physical keyboard, a display, a mouse, a speaker, a microphone, a trackball, a joystick, and a stylus.

[0281] It should be understood that in the embodiment of the present application, the memory 1150 may further include a memory remotely arranged relative to the processor 1020, and these remotely arranged 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 wireless fidelity (Wi-Fi) network, and a combination of two or more of the above-mentioned networks. For example, the cellular telephone network and the wireless network can 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, a LTE Frequency Division Duplex (FDD) system, a LTE Time Division Duplex (TDD) system, an Advanced Long Term Evolution (LTE-A) system, a Universal Mobile Telecommunications (UMTS) system, an Enhanced Mobile Broadband (Enhance Mobile Broadband, eMBB) system, a massive Machine Type of Communication (mMTC) system, an Ultra-Reliable Low Latency Communications (UltraReliable Low Latency Communications, uRLLC) system, etc.

[0282] It should be understood that the memory 1150 in the embodiment 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: a read-only memory (ROM), a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), or a flash memory.

[0283] Volatile memory includes: 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 RAM (SRAM), Dynamic RAM (DRAM), Synchronous DRAM (SDRAM), Double Data Rate Synchronous Dynamic Random Access Memory (DDRSDRAM), Enhanced Synchronous Dynamic Random Access Memory (ESDRAM), Synchronous Link Dynamic Random Access Memory (SLDRAM) and Direct Rambus RAM (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.

[0284] In the embodiment of the present application, the memory 1150 stores the following elements of the operating system 1151 and the application 1152: executable modules, data structures, or subsets thereof, or extended sets thereof.

[0285] Specifically, the operating system 1151 includes various system programs, such as a framework layer, a core library layer, a driver layer, etc., which are used to implement various basic services and process hardware-based tasks. The application 1052 includes various applications, such as a media player (Media Player) and a browser (Browser), which are used to implement various application services. The program for implementing the method of the embodiment of the present application may be included in the application 1152. The application 1152 includes applets, objects, components, logic, data structures, and other computer system executable instructions that perform specific tasks or implement specific abstract data types.

[0286] In addition, an embodiment of the present application also provides a computer-readable storage medium on which a computer program is stored. When the computer program is executed by a processor, the various processes of the above-mentioned electromagnetic survey method embodiment based on natural field sources are implemented, and the same technical effect can be achieved. To avoid repetition, it will not be repeated here.

[0287] Computer readable storage media include: permanent and non-permanent, removable and non-removable media, which are tangible devices that can retain and store instructions for use by instruction execution devices. Computer readable storage media include: electronic storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, and any suitable combination of the above. Computer readable storage media include: 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 technology, compact disk read-only memory (CD-ROM), digital versatile disk (DVD) or other optical storage, magnetic cassette storage, magnetic tape disk storage or other magnetic storage devices, memory sticks, mechanical encoding devices (such as punched cards or raised structures in grooves with instructions recorded thereon) 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, computer-readable storage media do not include temporary signals themselves, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through waveguides or other transmission media (such as light pulses passing through fiber optic cables), or electrical signals transmitted through wires.

[0288] In the several embodiments provided in 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 only schematic. For example, the division of the modules or units is only a logical function division. There may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. In addition, the mutual coupling or direct coupling or communication connection shown or discussed can be an indirect coupling or communication connection through some interfaces, devices or units, or it can be an electrical, mechanical or other form of connection.

[0289] 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, and may be located in one location or distributed over multiple network units. Some or all of the units may be selected according to actual needs to solve the problem to be solved by the embodiments of the present application.

[0290] In addition, each functional unit in each embodiment of the present application may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0291] If 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 embodiment of the present application is essentially or part of the prior art that contributes to the prior art, or all or part of the technical solution can be embodied in the form of a software product, which is stored in a storage medium, including several instructions for a computer device (including: a personal computer, a server, a data center or other network equipment) to perform all or part of the steps of the method described in each embodiment of the present application. The above-mentioned storage medium includes various media that can store program codes as listed above.

[0292] The above is only a specific implementation of the embodiment of the present application, but the protection scope of the embodiment of the present application is not limited thereto. Any technician familiar with the technical field can easily think of changes or replacements within the technical scope disclosed in the embodiment of the present application, which should be included in the protection scope of the embodiment of the present application. Therefore, the protection scope of the embodiment of the present application shall be based on the protection scope of the claims.

Claims

1. An electromagnetic survey method, characterized in that: The method comprises: Collecting initial signals; the initial signals include the initial time series of the synthetic electric field and / or magnetic field strength obtained by superimposing the natural electric field and / or magnetic field incident underground and the natural electric field and / or magnetic field incident underground at the moment of reflection and refraction back to the sensor position at the underground electrical impedance interface; Performing Fourier transform on the initial time series to obtain an 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 spectrum is superimposed by a harmonic synthesis algorithm to obtain a synthetic travel time series of the natural electric field and / or magnetic field; the synthetic travel time series is used to judge geological information.

2. The electromagnetic survey method according to claim 1, characterized in that: The initial signal at least satisfies: in, It is the phase difference between the natural electric field and / or magnetic field of any frequency at the moment when it returns to the sensor after being reflected and refracted by the underground impedance interface, and the natural electric field and / or magnetic field of the same frequency that continues to be incident at the said moment; f0 is the frequency of the electric field and / or magnetic field of any frequency; ρ1 is the resistivity of any layer of impedance interface; h1 is the burial depth of any layer of impedance interface.

3. The electromagnetic survey method according to claim 2, characterized in that: The method further comprises: The dominant frequency component is screened in the initial spectrum; the dominant frequency component is related to any electrical impedance interface in the underground.

4. The electromagnetic survey method according to claim 3, characterized in that: The method further comprises: The dominant frequency component is screened in the initial spectrum; any frequency of the dominant frequency component is inversely proportional to the buried depth of the electrical impedance interface corresponding to the frequency.

5. The electromagnetic survey method according to claim 3, characterized in that: The dominant frequency component at least satisfies: Wherein, 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 in the dominant frequency component 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 that continues to be incident at the same moment.

6. The electromagnetic survey method according to claim 3, characterized in that: The dominant frequency component also satisfies: The phase difference of any frequency component in the dominant frequency components when superimposed at the sensor is located in the frequency band Inside; Among them, int means rounding. It is the phase difference between the fundamental frequency and the external electromagnetic field with the same frequency that is continuously incident at the same time when it returns to the ground after propagating, reflecting and refracting in the underground medium.

7. The electromagnetic survey method according to claim 1, characterized in that: The harmonic synthesis algorithm includes: ω = 2πf(i); and / or ω=2πf(i); 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.

8. The electromagnetic survey method according to claim 1, characterized in that: The harmonic synthesis algorithm includes: ω = 2πf(i); and / or ω=2πf(i); 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.

9. The electromagnetic survey method according to claim 7, characterized in that: The harmonic synthesis algorithm includes: ω = 2πf(i); and / or ω=2πf(i); 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 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.

10. The electromagnetic survey method according to claim 1, characterized in that: In the initial signal acquisition, the number of channels of the acquisition instrument is greater than or equal to 1.

11. The electromagnetic survey method according to claim 1, characterized in that: The method further comprises: Setting acquisition parameters before acquiring the initial signal; The acquisition parameters include at least one or more of a sampling rate, a minimum frequency to be acquired, a maximum frequency to be acquired, and an acquisition duration.

12. The electromagnetic survey method according to claim 11, characterized in that: The sampling rate is inversely related to the depth to be surveyed.

13. The electromagnetic survey method according to claim 11, characterized in that: According to the different burial depths of the survey objects, the sampling rate is greater than or equal to 1000 times / second and less than or equal to 100000 times / second.

14. The electromagnetic survey method according to claim 11, characterized in that: The minimum frequency to be acquired is inversely correlated with the maximum depth to be surveyed.

15. The electromagnetic survey method according to claim 11, characterized in that: The collecting of the initial signal comprises: 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; When the electric field strength or the magnetic field strength is collected separately, the lowest frequency to be collected is greater than or equal to 0.01 Hz and less than or equal to 10 Hz.

16. The electromagnetic survey method according to claim 11, characterized in that: The acquisition time is positively correlated with the maximum depth to be surveyed.

17. The electromagnetic survey method according to any one of claims 1 to 16, characterized in that: The method further comprises: According to the highest frequency to be collected, performing a first notch processing and / or a low-pass filtering before collecting the initial signal; The first notch processing is to suppress the power frequency and the multiple of the power frequency; The low-pass filtering is to filter out frequency components higher than the highest frequency to be collected.

18. The electromagnetic survey method according to claim 17, characterized in that: The first notch processing includes a first comb filtering process; The first comb filtering process is a hardware-based filtering process; the hardware includes a filter.

19. The electromagnetic survey method according to any one of claims 1, 2, 10 or 11, characterized in that: The frequency of the initial signal is greater than or equal to 0.01 Hz and less than or equal to 50,000 Hz.

20. The electromagnetic survey method according to claim 1, characterized in that: The obtaining the initial spectrum of the initial time series based on Fourier transform of the initial time series comprises: Converting the original time series data of the time-domain electric field and / or magnetic field intensity components of the initial signal acquired by each acquisition channel into component spectra of the frequency-domain electric field and / or magnetic field intensity components; The electric field and / or magnetic field intensity component spectra corresponding to all acquisition channels are synthesized into the initial spectrum of the electric field and / or magnetic field intensity by using the vector synthesis rule.

21. The electromagnetic survey method according to claim 1, characterized in that: The initial spectrum includes the frequency, real part, imaginary part, amplitude and phase of the electric field and / or magnetic field.

22. The electromagnetic survey method according to claim 3, characterized in that: Screening the dominant frequency components in the initial spectrum includes: Determine the lowest dominant frequency components based on the survey objectives and requirements; and Suppress interfering frequency components; The interference 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.

23. The electromagnetic survey method according to claim 22, characterized in that: The suppressing of interference frequency components includes: a second comb filter process and a bandpass filter process; The second comb filtering process and the bandpass filtering process are both processing methods based on the principle of digital signal processing.

24. The electromagnetic survey method according to any one of claims 1 to 16, characterized in that: The method further comprises: Based on the comparison between the geological data of the survey area and the synthetic travel time series, a corresponding relationship between the geological strata of the survey area and the underground electrical impedance interface determined by the synthetic travel time series is established; Determine a marker layer based on the corresponding relationship; the marker layer is the layer in the geological layer with the highest matching degree with the synthetic travel time time series; A time-to-depth conversion parameter is obtained based on the depth of the marker layer and the time corresponding to the marker layer in the synthetic travel time series.

25. The electromagnetic survey method according to claim 1, characterized in that: The travel time inversion also includes dispersion effect correction; 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.

26. The electromagnetic survey method according to claim 1, characterized in that: The method further comprises: Before collecting the initial signal, the initial value of the electromagnetic wave propagation velocity corresponding to the lowest frequency in the initial spectrum is selected according to the depth to be surveyed and the geological characteristics of the survey area; the initial value is used to perform time-to-depth conversion of the initial time series, thereby obtaining the initial time series in the depth domain.

27. The electromagnetic survey method according to claim 1, characterized in that: The initial spectrum satisfies: Among them, 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 electrical conductivity of the underground electrical medium; μ is the magnetic permeability of the underground electrical medium.

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

29. An electromagnetic survey system, characterized in that: Applicable to the electromagnetic survey method as claimed in any one of claims 1 to 28, the electromagnetic survey system comprising: An acquisition module, wherein the acquisition module is configured to acquire the initial signal; the initial signal includes the natural electric field and / or magnetic field incident underground, and when it is reflected and refracted back to the ground through the underground electrical impedance interface, it meets the natural electric field and / or magnetic field incident underground at the moment at the sensor, and the initial time series of the synthetic electric field and / or magnetic field strength is obtained by superposition; An initial signal processing module, wherein the initial signal processing module is configured to perform Fourier transform on the initial time series to obtain an initial spectrum of the initial time series; A travel time inversion module is configured to superimpose the initial spectrum based on the electromagnetic wave superposition principle through a harmonic synthesis algorithm to obtain a synthetic travel time series of the natural electric field and / or magnetic field; the synthetic travel time series is used to analyze geological information.

30. The electromagnetic survey system according to claim 29, characterized in that The electromagnetic survey system also includes a sensor; The sensor is located on the ground, in the air or on the water surface.

31. An electronic device, comprising a bus, a transceiver, a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the transceiver, the memory, and the processor are respectively connected via a bus, and when the computer program is executed by the processor, the steps of the electromagnetic survey method according to any one of claims 1 to 28 are implemented.

32. A computer-readable storage medium having a computer program stored thereon, wherein when the computer program is executed by a processor, the steps in the electromagnetic survey method according to any one of claims 1 to 28 are implemented.

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