A method for electric dipole source frequency bathymetry based on horizontal magnetic field primary field cancellation strategy

Abstract

This invention discloses a frequency-based electromagnetic sounding method using an electric dipole source based on primary field cancellation measurement of a horizontal magnetic field. The method includes: deploying a horizontal electric dipole source; establishing observation points in the near-field, transition, and far-field regions of the source; and recording the transmitted and received data at each observation point. Based on the transmitted and received data, a synthetic magnetic field is constructed, and the apparent resistivity parameters of the subsurface medium are calculated. This method, based on a primary field cancellation strategy of the horizontal magnetic field, extracts the secondary field through a rationally designed synthetic magnetic field, thereby obtaining the electrical parameters in the near-field, transition, and far-field regions. This allows for frequency-based electromagnetic sounding under different transmission and reception distances, effectively improving the detection depth of frequency-based electromagnetic sounding and expanding the spatial measurement range.

Description

Technical Field

[0001] This invention relates to an electromagnetic sounding method in the field of exploration geophysics, and more particularly to a frequency sounding method based on a horizontal magnetic field primary field cancellation strategy using an electric dipole source. Background Technology

[0002] In the field of exploration geophysical electromagnetic methods, the Audio-frequency Magnetotelluric (AMT) method offers deep probe depths and lightweight acquisition equipment, but suffers from low signal-to-noise ratios and weak noise immunity. The Controlled Source Audio-frequency Magnetotelluric (CSAMT) method uses artificial electromagnetic sources as excitation, observing the artificial electromagnetic field within a defined observation area, thus improving the data signal-to-noise ratio. However, effective impedance data requires acquisition in the "far zone," and in the "near zone" and "transition zone" of the artificial source, impedance data is distorted, leading to incorrect interpretations. Although researchers have developed a series of processing methods, such as various data correction strategies and definitions and calculation schemes for apparent resistivity across the entire area, partially extracting geoelectric information within the "transition zone," observation data from the "near zone" and "transition zone" remains difficult to utilize. The insufficient weight of the secondary field in the total field is a key reason limiting sounding at low induction frequencies in practical situations. The sensitivity of an electromagnetic observation system largely depends on its ability to accurately resolve small secondary fields. However, the presence of the primary field makes secondary field observation difficult under low induction number conditions. Currently, there is limited research on the cancellation of primary fields by electric dipole sources in the frequency domain, making it difficult to fully utilize the theoretically small-induction frequency sounding capability of electric dipole sources. Summary of the Invention

[0003] To address the problems in the background art, this invention provides a method for frequency sounding of an electric dipole source based on the cancellation of the primary field of a horizontal magnetic field. By reasonably observing and utilizing the components of the horizontal magnetic field to cancel the primary field and retain the secondary field, the apparent resistivity information is calculated using the secondary field, thereby enabling the acquisition of the electrical structure of the underground medium in the near-field, transition, and far-field regions of the field source.

[0004] A method for frequency sounding of a dipole source based on primary field cancellation measurement of a horizontal magnetic field includes:

[0005] Deploy a horizontal electric dipole field source, set up observation points in the near-field, transition, and far-field regions of the field source, and record the transmitted and received data of the field source at each observation point;

[0006] A synthetic magnetic field is constructed based on the transmitted and received data, and the apparent resistivity parameters of the underground medium are solved.

[0007] The method described above utilizes orthogonal horizontal magnetic field components to cancel the primary field and retain the secondary field through a well-designed observation system, thereby acquiring subsurface electrical information in the near-field, transition, and far-field regions of the field source. Compared to the electromagnetic method using natural field sources, the artificial field source achieves a better signal-to-noise ratio. Compared to the controlled-source audio-frequency magnetotelluric method, frequency sounding is conducted in the near-field, transition, and far-field regions of the artificial field source to extract geoelectric information that varies with frequency, thus expanding the observation range and increasing the detection depth. By receiving electromagnetic signals of different frequencies, the distribution of electrical media at different depths underground can be obtained. By observing the underground electrical distribution, the distribution of subsurface geoelectric characteristics, geological structures, and mineral resources can be identified, or other engineering, hydrological, and environmental geological problems can be solved.

[0008] Furthermore, the data transmitted and received by the field source includes: the transmitting current intensity of the field source, the coordinate position of the transmitting electrode, the coordinate position of the receiving electrode, the azimuth of the measuring point, and the orthogonal horizontal magnetic field components.

[0009] Furthermore, the process of constructing the synthetic magnetic field is as follows:

[0010] A spatial rectangular coordinate system is established with the horizontal electric dipole field source as the origin; the x-axis is in the same direction as the electric dipole matrix, and the z-axis is vertically downward.

[0011] Using the azimuth angle of the measuring point and the orthogonal horizontal magnetic field components in the transmitted and received data, a composite magnetic field is constructed using the following formula.

[0012]

[0013] Among them, H xy To synthesize the magnetic field; H is the azimuth angle of the observation point, i.e., the angle between the radius vector from the receiving point to the center of the transmitting dipole and the x-axis; x H represents the magnetic field component along the x-axis in an orthogonal horizontal magnetic field. y y is the magnetic field component along the y-axis in an orthogonal horizontal magnetic field.

[0014] Furthermore, the process of obtaining the parameters is as follows:

[0015] By measuring and obtaining the coordinates (x1, y1) and (x2, y2) of the two transmitting electrodes and the coordinates (x3, y3) of the measuring point, the offset distance x in the x-direction is determined. r for:

[0016]

[0017] y-direction offset distance y r for:

[0018]

[0019] The modulus of the radius vector from the receiving point to the center of the transmitting dipole, i.e., the transmit-receive distance r, is:

[0020]

[0021] Azimuth And the corresponding trigonometric functions are:

[0022]

[0023]

[0024] Furthermore, the magnetic field components along the x-axis in the orthogonal horizontal magnetic field within the first field of a horizontal magnetic field under near-field conditions of a uniform half-space. H y The expression for the magnetic field component along the y-axis in an orthogonal horizontal magnetic field is:

[0025]

[0026] Where I is the transmitted current intensity of the artificial field source; dL is the length of the transmitting dipole; θ is the azimuth angle of the observation point, i.e., the angle between the radius vector from the receiving point to the center of the transmitting dipole and the x-axis; r is the transmit-receive distance.

[0027] Furthermore, in the near-field region, the horizontal components of the primary magnetic field all follow... Attenuation, and satisfying Where r is the transmit / receive distance; H is the azimuth angle of the observation point; x H represents the magnetic field component along the x-axis in an orthogonal horizontal magnetic field. y y is the magnetic field component along the y-axis in an orthogonal horizontal magnetic field.

[0028] Furthermore, the composite magnetic field component H under uniform half-space conditions xy The expression is:

[0029]

[0030] Where I0, I1, K0, and K1 are the first and second kinds of imaginary argument Bessel functions with ikr / 2 as the argument, respectively; 0 and 1 represent the order of the imaginary argument Bessel function; I is the transmitting current intensity of the artificial field source; dL is the length of the transmitting dipole; k is the wave number of the electromagnetic wave; and i is the imaginary unit. r is the magnitude of the radius vector from the receiving point to the center of the transmitting dipole; μ is the permeability in vacuum; H xy To synthesize the magnetic field; The azimuth angle of the observation point.

[0031] Furthermore, methods for calculating apparent resistivity include the bisection method or the iterative method.

[0032] Furthermore, the formula for the apparent resistivity parameter of the underground medium is:

[0033]

[0034] Among them, C H = [4I1K1+ikr(I1K0-I0K1)]ikr, where I0, I1, K0, and K1 are the first and second kind of virtual argument Bessel functions with ikr / 2 as arguments, respectively, and 0 and 1 represent the order of the virtual argument Bessel function; P E =IdL / 2π, P E dL is the electric dipole moment; I is the transmitted current intensity of the artificial field source; dL is the length of the transmitting dipole; k is the wave number of the electromagnetic wave; i is the imaginary unit. r is the magnitude of the radius vector from the receiving point to the center of the transmitting dipole; μ is the permeability in vacuum; H xy ω represents the synthesized magnetic field; ω is the angular frequency of the signal.

[0035] Beneficial effects

[0036] This invention proposes a frequency sounding method for electric dipole sources based on a primary field cancellation strategy using a horizontal magnetic field. This method utilizes orthogonal horizontal magnetic field components to cancel the primary field while retaining the secondary field through a well-designed observation system, thereby acquiring subsurface electrical information in the near-field, transition, and far-field regions of the source. Compared to natural source electromagnetic methods, using artificial sources yields a better signal-to-noise ratio. Compared to controlled-source audio-frequency magnetotellurics, frequency sounding is conducted in the near-field, transition, and far-field regions of the artificial source to extract frequency-varying geoelectric information, thus expanding the observation range and increasing the detection depth. By receiving electromagnetic signals of different frequencies, the distribution of electrical media at different depths underground can be obtained. By observing the underground electrical distribution, the distribution of subsurface geoelectric characteristics, geological structures, and mineral resources can be identified, or other engineering, hydrological, and environmental geological problems can be solved. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a schematic diagram of the observation device provided in an embodiment of the present invention;

[0039] Figure 2 This is a schematic diagram of an electric dipole model on a uniform horizontal layered half-space provided in an embodiment of the present invention;

[0040] Figure 3 This is a schematic diagram of the synthetic field apparent resistivity estimation results of the model observation data provided in this embodiment of the invention. Figure 1 ;

[0041] Figure 4 This is a schematic diagram of the synthetic field apparent resistivity estimation results of the model observation data provided in this embodiment of the invention. Figure 2

[0042] Figure 5 This is a schematic diagram of the synthetic field apparent resistivity estimation results of the model observation data provided in this embodiment of the invention. Figure 3

[0043] Figure 6 This is a schematic diagram of the synthetic field apparent resistivity estimation results of the model observation data provided in this embodiment of the invention. Figure 4 . Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0045] Example 1

[0046] This embodiment provides a method for frequency sounding of a dipole source based on primary field cancellation measurement of a horizontal magnetic field, including:

[0047] S1: Deploy a horizontal electric dipole field source, set up observation points in the near-field, transition, and far-field regions of the field source, and record the transmitted and received data of the field source at each observation point.

[0048] Specifically, such as Figure 1 As shown, a horizontal electric dipole field source is deployed on the ground. Observation points are set up in the near-field, transition, and far-field regions of the field source. Magnetic field sensors measure the magnetic field in different directions and connect to a receiver to store the data. The transmitted and received data of the field source includes: the transmitting current intensity of the field source, the coordinate position of the transmitting electrode, the coordinate position of the receiving electrode, the azimuth angle of the measurement point, and the orthogonal horizontal magnetic field components. In this embodiment, r is the radius vector from the receiving point to the center of the transmitting dipole. Let r be the angle between the direction of the electric dipole moment and the direction of the electric dipole moment; at each observation position, measure the coordinate positions of the transmitting and receiving electrodes, and simultaneously record the transmitting current intensity I of the artificial field source, and the orientation of the measurement point. and orthogonal horizontal magnetic field component H x H y .

[0049] S2: Construct a synthetic magnetic field based on transmitted and received data, and solve for the apparent resistivity parameters of the underground medium.

[0050] Specifically, the process of constructing the synthetic magnetic field is as follows:

[0051] like Figure 2 As shown, in an N-layered horizontal medium, a horizontal electric dipole (grounding conductor) is located on the surface of the medium. A spatial rectangular coordinate system is established with the horizontal electric dipole field source as the origin; the x-axis and the dipole matrix of the electric dipole are in the same direction, and the z-axis is vertically downward. Specifically, a cylindrical coordinate system and a rectangular coordinate system with their common origin located at the center of the dipole are selected, such that the x-axis points in the direction of the dipole moment (i.e.,...). If the z-axis is perpendicular to the downward direction, then the horizontal magnetic field distribution H on the surface of the layered medium is... x and vertical magnetic field distribution H y The expression is as follows:

[0052]

[0053]

[0054] In the formula:

[0055]

[0056] m is called the spatial frequency, which has the dimension of the reciprocal of distance; ρ n m is the resistivity of the nth layer; j Let k be the spatial frequency of the j-th layer. j h is the wavenumber associated with the j-th layer. n r is the thickness of the nth layer; r is the transmit / receive distance; J0 is the azimuth angle of the observation point, i.e., the angle between the radius vector from the receiving point to the center of the transmitting dipole and the x-axis; J1 is the zeroth-order Bessel function; I is the transmitting current intensity of the artificial field source; and dL is the dipole moment length.

[0057] S2: Using the azimuth angle of the measuring point and the orthogonal horizontal magnetic field components in the transmitted and received data, a composite magnetic field is constructed using the following formula.

[0058]

[0059] Among them, H xy To synthesize the magnetic field; H is the azimuth angle of the observation point, i.e., the angle between the radius vector from the receiving point to the center of the transmitting dipole and the x-axis; x H represents the magnetic field component along the x-axis in an orthogonal horizontal magnetic field. y y is the magnetic field component along the y-axis in an orthogonal horizontal magnetic field.

[0060] More specifically, the process of obtaining the parameters is as follows:

[0061] By measuring and obtaining the coordinates (x1, y1) and (x2, y2) of the two transmitting electrodes and the coordinates (x3, y3) of the measuring point, the offset distance x in the x-direction is determined. r for:

[0062]

[0063] y-direction offset distance y r for:

[0064]

[0065] The modulus of the radius vector from the receiving point to the center of the transmitting dipole, i.e., the transmit-receive distance r, is:

[0066]

[0067] Azimuth And the corresponding trigonometric functions are:

[0068]

[0069]

[0070] Furthermore, under near-field conditions of a uniform half-space, the magnetic field component along the x-axis in the orthogonal horizontal magnetic field within the primary field of a horizontal magnetic field is... Magnetic field components along the y-axis The expression is

[0071]

[0072] Where I is the transmitted current intensity of the artificial field source; dL is the length of the transmitting dipole; θ is the azimuth angle of the observation point, i.e., the angle between the radius vector from the receiving point to the center of the transmitting dipole and the x-axis; r is the transmit-receive distance.

[0073] Furthermore, in the near-field region, the horizontal components of the primary magnetic field all follow... Attenuation, and satisfying

[0074]

[0075] Where r is the transmit / receive distance; H is the azimuth angle of the observation point; x H represents the magnetic field component along the x-axis in an orthogonal horizontal magnetic field. y y is the magnetic field component along the y-axis in an orthogonal horizontal magnetic field.

[0076] Therefore, the synthesized magnetic field H xy The first field component It can achieve elimination

[0077]

[0078] Furthermore, the composite magnetic field component H under uniform half-space conditions xy The expression is:

[0079]

[0080] Where I0, I1, K0, and K1 are the first and second kinds of imaginary argument Bessel functions with ikr / 2 as the argument, respectively; 0 and 1 represent the order of the imaginary argument Bessel function; I is the transmitting current intensity of the artificial field source; dL is the length of the transmitting dipole; k is the wave number of the electromagnetic wave; and i is the imaginary unit. r is the magnitude of the radius vector from the receiving point to the center of the transmitting dipole; μ is the permeability in vacuum; H xy To synthesize the magnetic field; The azimuth angle of the observation point.

[0081] Let C H = [4I1K1+ikr(I1K0-I0K1)]ikr, which gives the composite field component H under uniform half-space conditions. xy for:

[0082]

[0083] The apparent resistivity of the subsurface medium is calculated using the bisection method or iterative method, and the formula for obtaining the apparent resistivity parameter is as follows:

[0084]

[0085] Among them, C H = [4I1K1+ikr(I1K0-I0K1)]ikr, where I0, I1, K0, and K1 are the first and second kind of virtual argument Bessel functions with ikr / 2 as arguments, respectively, and 0 and 1 represent the order of the virtual argument Bessel function; P E =IdL / 2π, P E dL is the electric dipole moment; I is the transmitted current intensity of the artificial field source; dL is the length of the transmitting dipole; k is the wave number of the electromagnetic wave; i is the imaginary unit. r is the magnitude of the radius vector from the receiving point to the center of the transmitting dipole; μ is the permeability in vacuum; H xy ω represents the synthesized magnetic field; ω is the angular frequency of the signal.

[0086] By analyzing and inverting the apparent resistivity parameters, the electrical structure of the underground medium in the near-field, transition, and far-field regions of the field source can be obtained.

[0087] The method described above utilizes orthogonal horizontal magnetic field components to cancel the primary field and retain the secondary field through a well-designed observation system, thereby acquiring subsurface electrical information in the near-field, transition, and far-field regions of the field source. Compared to the electromagnetic method using natural field sources, the artificial field source achieves a better signal-to-noise ratio. Compared to the controlled-source audio-frequency magnetotelluric method, frequency sounding is conducted in the near-field, transition, and far-field regions of the artificial field source to extract geoelectric information that varies with frequency, thus expanding the observation range and increasing the detection depth. By receiving electromagnetic signals of different frequencies, the distribution of electrical media at different depths underground can be obtained. By observing the underground electrical distribution, the distribution of subsurface geoelectric characteristics, geological structures, and mineral resources can be identified, or other engineering, hydrological, and environmental geological problems can be solved.

[0088] based on Figure 2 The electric dipole model on the uniform horizontal layered half-space shown was simulated and verified:

[0089] like Figure 3 The diagram shows the estimated results of the apparent resistivity of the combined magnetic field. Figure 1 The calculation parameters are: N = 2, r = 1 km, ρ1 = 100 Ωm, h1 = 3 km; the resistivity ρ2 of the second layer takes multiple values, including: ρ2 / ρ1 = 100, ρ2 / ρ1 = 10, ρ2 / ρ1 = 1, ρ2 / ρ1 = 0.1, ρ2 / ρ1 = 0.01, etc., and their responses are represented by curves of different colors. In the figure, the horizontal axis is the ratio of skin depth δ1 to the thickness of the first layer h1, and the vertical axis is the apparent resistivity response data; ρ s / ρ1(Hxy) represents the ratio of the apparent resistivity obtained from the calculation formula of the apparent resistivity parameter of the underground medium to the resistivity ρ1 of the first underground layer.

[0090] like Figure 4 The diagram shows the estimated results of the apparent resistivity of the synthesized magnetic field. Figure 2 ,and Figure 3 The difference in the calculated parameters is that r = 0.5 km.

[0091] like Figure 5 The diagram shows the estimated results of the apparent resistivity of the synthesized magnetic field. Figure 3 ,and Figure 3 The difference in the calculated parameters is that r = 0.1 km.

[0092] like Figure 6 The diagram shows the estimated results of the apparent resistivity of the synthesized magnetic field. Figure 4 ,and Figure 3The difference lies in the calculation parameters: N = 2, r = 3km, ρ1 = 100Ωm, h1 = 10km; the resistivity ρ2 of the second layer takes multiple values, including: ρ2 / ρ1 = 100, ρ2 / ρ1 = 10, ρ2 / ρ1 = 1, ρ2 / ρ1 = 0.1, ρ2 / ρ1 = 0.01, etc., and its response is represented by curves of different styles.

[0093] It can be seen that, under two-layer conditions, the new apparent resistivity parameters obtained by this invention can achieve an effective geoelectric response. Analysis shows that: (1) Under uniform half-space conditions (i.e., when ρ2 / ρ1=1), the synthesized field H defined by this invention… xy Apparent resistivity reflects the electrical properties of a half-space. Similarly, when the electromagnetic field propagates only in the first layer, the resultant field H... xy Apparent resistivity can reflect the electrical properties of the first layer. (2) Under the condition of layered medium (i.e., when ρ2 / ρ1≠1), the composite field H xy The apparent resistivity can be used to obtain the change in the resistivity of the lower layer. (3) Under the condition that the accuracy of observation and calculation can be guaranteed, the composite field H xy Apparent resistivity is less affected by the transmit / receive distance, even at extremely small transmit / receive distances (such as...). Figure 5 (4) Ensure a certain level of observation accuracy, under conventional transmitter-receiver distance conditions (such as...) Figure 6 The transmit / receive distance shown is r = 3 km, which is expected to extend the detection depth to 10 km or even deeper (e.g., Figure 6 The thickness of the cover layer shown is h1 = 10 km.

[0094] Analysis shows that the frequency sounding method based on a horizontal magnetic field primary field cancellation strategy provided by this invention can obtain geoelectric information varying with frequency under low induction number conditions. By receiving electromagnetic signals of different frequencies, the distribution of electrical media at different depths underground can be obtained, achieving the purpose of electromagnetic exploration. The advantages of this invention are that, compared to conventional natural field source electromagnetic methods, this method can utilize artificial field sources to obtain a better observation signal-to-noise ratio; compared to conventional controlled-source audio-frequency magnetotelluric methods, this method can conduct frequency sounding in the near-field, transition, and far-field regions of artificial field sources, extracting geoelectric information varying with frequency, thereby expanding the observation range and increasing the detection depth.

[0095] It is understood that the same or similar parts in the above embodiments can be referred to each other, and the contents not described in detail in some embodiments can be referred to the same or similar contents in other embodiments.

[0096] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for frequency sounding of a dipole source based on primary field cancellation measurement of a horizontal magnetic field, characterized in that, include: Deploy a horizontal electric dipole field source, set up observation points in the near-field, transition, and far-field regions of the field source, and record the transmitted and received data of the field source at each observation point; A synthetic magnetic field is constructed based on the transmitted and received data, and the apparent resistivity parameters of the subsurface medium are solved. The process of constructing the synthetic magnetic field is as follows: A spatial rectangular coordinate system is established with the horizontal electric dipole field source as the origin; the x-axis is in the same direction as the electric dipole matrix, and the z-axis is vertically downward. Using the azimuth angle of the measuring point and the orthogonal horizontal magnetic field components in the transmitted and received data, a composite magnetic field is constructed using the following formula. ； in, To synthesize the magnetic field; The azimuth angle of the observation point is the angle between the radius vector from the receiving point to the center of the transmitting dipole and the x-axis. The x-axis magnetic field component in an orthogonal horizontal magnetic field; The y-axis magnetic field component in an orthogonal horizontal magnetic field; The specific process for obtaining the parameters is as follows: By measuring and obtaining the coordinates (x1, y1) and (x2, y2) of the two transmitting electrodes and the coordinates (x3, y3) of the measuring point, the offset distance x in the x-direction is determined. r for: ； y-direction offset distance y r for: ； The modulus of the radius vector from the receiving point to the center of the transmitting dipole, i.e., the transmit-receive distance r, is: ； Azimuth And the corresponding trigonometric functions are: ； ， ； The formula for the apparent resistivity parameter of the underground medium is: ； in, I0, I1, K0 and K1 are the first and second kinds of virtual argument Bessel functions with ikr / 2 as the argument, respectively, and 0 and 1 represent the order of the virtual argument Bessel function; , It is the electric dipole moment; dL is the transmitted current intensity of the artificial field source; dL is the length of the transmitting dipole; k is the wave number of the electromagnetic wave; i is the imaginary unit. ; r is the modulus of the radius vector from the receiving point to the center of the transmitting dipole; The value is taken as the permeability in vacuum; To synthesize the magnetic field; This is the angular frequency of the signal.

2. The method for frequency sounding of a dipole source based on primary field cancellation measurement of a horizontal magnetic field according to claim 1, characterized in that, The data transmitted and received by the field source includes: the transmitting current intensity of the field source, the coordinate position of the transmitting electrode, the coordinate position of the receiving electrode, the azimuth of the measuring point, and the orthogonal horizontal magnetic field components.

3. The method for frequency sounding of a dipole source based on primary field cancellation measurement of a horizontal magnetic field according to claim 1, characterized in that, The magnetic field component along the x-axis in a primary horizontal magnetic field within a uniform half-space near-field condition. ; The expression for the magnetic field component along the y-axis in an orthogonal horizontal magnetic field is: ， ； in, The transmitted current intensity of the artificial field source; The length of the emitted dipole; The azimuth angle of the observation point is the angle between the radius vector from the receiving point to the center of the transmitting dipole and the x-axis. For the distance between the transmitter and receiver.

4. The method for frequency sounding of a dipole source based on primary field cancellation measurement of a horizontal magnetic field according to claim 3, characterized in that, In the near region, the horizontal component of the primary magnetic field follows the... Attenuation, and satisfying ,in, For transmit and receive distance; The azimuth angle of the observation point; The x-axis magnetic field component in an orthogonal horizontal magnetic field; y is the magnetic field component along the y-axis in an orthogonal horizontal magnetic field.

5. The method for frequency sounding of a dipole source based on primary field cancellation measurement of a horizontal magnetic field according to claim 1, characterized in that, Synthetic magnetic field components under uniform half-space conditions The expression is: ； Where I0, I1, K0 and K1 are the first and second kinds of virtual argument Bessel functions with ikr / 2 as the argument, respectively, and 0 and 1 represent the order of the virtual argument Bessel function; dL is the transmitted current intensity of the artificial field source; dL is the length of the transmitting dipole; k is the wave number of the electromagnetic wave; i is the imaginary unit. ; r is the modulus of the radius vector from the receiving point to the center of the transmitting dipole; The value is taken as the permeability in vacuum; To synthesize the magnetic field; The azimuth angle of the observation point.

6. The method for frequency sounding of a dipole source based on primary field cancellation measurement of a horizontal magnetic field according to claim 1, characterized in that, Apparent resistivity can be calculated using either the bisection method or the iterative method.

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