A method and system for measuring apparent resistivity based on magnetic field frequency gradient using a horizontal coupler source.

Abstract

This invention discloses a method and system for measuring apparent resistivity based on a horizontal grounded dipole source using magnetic field frequency gradient. The method involves supplying power to the underground by laying a horizontal grounded dipole source on the surface, and setting up measuring points at a distance *r* from the horizontal dipole source to measure the radial and tangential magnetic field strengths. Based on the analytical expression of the magnetic field frequency gradient generated by the horizontal dipole source on a uniform half-space surface, the frequency difference between the radial and tangential magnetic field strengths is calculated using a differential algorithm, suppressing the primary field and highlighting the secondary field. The actual field value frequency difference is equivalently represented by the field value frequency gradient of the uniform half-space, and the apparent resistivity is solved using a numerical iteration method. This invention, by calculating the magnetic field frequency gradient to extract apparent resistivity, enables electromagnetic sounding under conditions of low induction numbers, effectively reflecting the deep electrical structure.

Description

Technical Field

[0001] This invention belongs to the field of exploration geophysics technology, specifically relating to a method and system for measuring the apparent resistivity of magnetic field frequency gradient based on a horizontal electric couple source. Background Technology

[0002] Frequency-domain controlled-source electromagnetic sounding generally utilizes a transmitter to supply an alternating current of a specific transmission frequency to the ground via a grounded conductor (horizontal electric dipole) or an ungrounded loop (vertical magnetic dipole). Measurement points are laid at a certain transmitter-receiver distance, and a receiver receives the electric and / or magnetic fields propagating through the earth. The apparent resistivity is then calculated based on the electromagnetic field using a specific method. This apparent resistivity should reflect the objective electrical changes underground. By successively changing the frequency, the conductivity variation from shallow to deep underground layers can be obtained. Currently, the most commonly used frequency-domain controlled-source electromagnetic sounding methods are Controlled-Source Audio-Frequency Magnetotelluric (CSAMT) and Wide-Area Electromagnetic (WFEM), which have advantages such as high efficiency and strong anti-interference capabilities.

[0003] Controlled-source audio-frequency magnetotellurics (CSAMT) follows the definition of apparent resistivity in magnetotellurics. It measures mutually orthogonal horizontal electric and magnetic fields at the measurement point and calculates the Carnia apparent resistivity. At a sufficiently large transmit / receive distance (far-field), the electromagnetic waves generated by the artificial source can be approximated as plane waves, and the Carnia apparent resistivity of CSAMT can reflect changes in subsurface electrical properties. However, in the transition and near-field regions, the electromagnetic field of the artificial source cannot be approximated by plane waves. In these conditions, the CSAMT apparent resistivity curve in equally spaced logarithmic coordinates exhibits a linear change at low frequencies, unrelated to subsurface resistivity, and cannot reflect the objective electrical properties of the subsurface.

[0004] Wide-area electromagnetic sounding (WFEM) can measure only a single electromagnetic field component, fitting the observed electric or magnetic field with a uniform half-space resistivity model, and obtaining the apparent resistivity through continuous iterative search. The WFEM apparent resistivity is derived from the analytical expression of the electromagnetic field without any approximations; therefore, the apparent resistivity sounding curve will not be distorted in non-wavelength regions and can accurately reflect changes in the underground electrical structure across the entire area. However, at low induction numbers, the proportion of the sounding secondary field to the total field is too small, only a fraction of ten thousandths. This makes it impossible to obtain effective secondary field information and achieve frequency electromagnetic sounding in noisy observation environments with low induction numbers. Therefore, WFEM still requires a large transmit / receive distance or can only obtain limited detection depths. Summary of the Invention

[0005] To address the problem that current frequency-domain controllable source electromagnetic exploration methods cannot perform electromagnetic sounding at low induction numbers to calculate the apparent resistivity that objectively reflects underground electrical information, this invention provides a magnetic field frequency gradient apparent resistivity measurement method and system based on a horizontal electric couple source. By eliminating the primary field from the total field and retaining the derivative value of the secondary field, the proportion of the measurable portion is increased, enabling the frequency gradient apparent resistivity to reflect electrical changes very well at low induction numbers and distinguish different low-resistivity layers.

[0006] To achieve the above technical objectives, the present invention adopts the following technical solution:

[0007] A method for measuring apparent resistivity of magnetic field frequency gradient based on a horizontal dipole source includes:

[0008] A time-varying current signal is introduced into a horizontally laid thermocouple source on the ground. Radial and tangential magnetic fields are continuously collected at a measuring point at a distance r from the thermocouple source. The radial magnetic field strength H is obtained by time-frequency conversion. r and tangential magnetic field strength

[0009] Calculate the radial magnetic field strength H respectively r and tangential magnetic field strength Difference value as a function of frequency

[0010] Will Substituting the values ​​into the analytical expression for the relationship between the magnetic field frequency gradient and resistivity, and using a numerical iteration method to calculate the apparent resistivity corresponding to each frequency, the calculation result under a small inductance number is selected as the final measured apparent resistivity; wherein, the analytical expression for the relationship between the magnetic field frequency gradient and apparent resistivity is:

[0011]

[0012]

[0013] In the formula: IdL is the polar moment of the horizontal thermocouple source, where I represents the current intensity, dL is the length of the horizontal thermocouple source, and k is the complex wave number. i represents the imaginary unit, ω represents the angular frequency ω = 2πf, μ represents the permeability, and f is the time-varying current frequency of the horizontal dipole source; The observation angle of the measuring point is denoted by σ; I0 and I1 are the 0th and 1st order imaginary argument Bessel functions of the first kind, respectively; K0 and K1 are the 0th and 1st order imaginary argument Bessel functions of the second kind, respectively; σ is the conductivity, and ρ is the apparent resistivity calculated iteratively. a The reciprocal of;

[0014] The inductance number is defined as P = r / δ, where r is the transmit / receive distance and δ is the skin depth.

[0015] Furthermore, the apparent resistivity in the analytical expression of the function is solved using the bisection method.

[0016] Furthermore, the horizontal dipole source is a grounded long conductor, and the dipole moment is in the x-direction.

[0017] Furthermore, by moving the measuring points, the apparent resistivity of the magnetic field frequency gradient at different measuring points as a function of frequency is obtained, and then the apparent resistivity of the magnetic field frequency gradient in the entire region is obtained.

[0018] Furthermore, the current signal supplied to the horizontal thermocouple source laid on the ground is an alternating current with a determined transmission frequency.

[0019] Furthermore, by sequentially changing the frequency of the current signal at the same measuring point, the apparent resistivity of the magnetic field frequency gradient at that measuring point based on different current frequencies is obtained, thereby obtaining the frequency detection curve of that measuring point, i.e., the frequency-magnetic field frequency gradient apparent resistivity curve.

[0020] Furthermore, the distance r between the measuring point and the horizontal dipole source refers to the distance between the dipole of the horizontal dipole source and the measuring point.

[0021] Furthermore, a magnetic field sensor is used at the measuring point to collect the radial and tangential magnetic fields.

[0022] A magnetic field frequency gradient apparent resistivity measurement system based on a horizontal thermocouple source includes: a horizontal thermocouple source, a receiver, and a processor;

[0023] The horizontal thermocouple source is laid on the ground surface and generates a magnetic field after a time-varying current signal is passed through it.

[0024] The receiver includes a tangential magnetic field acquisition module and a radial magnetic field acquisition module for acquiring radial and tangential magnetic fields, respectively, to obtain the radial magnetic field strength H. r and tangential magnetic field strength

[0025] The processor is used to perform the following calculations and obtain the apparent resistivity:

[0026] Calculate the radial magnetic field strength H respectively r and tangential magnetic field strength Difference value as a function of frequency

[0027] Will Substituting these values ​​into the analytical expression for the relationship between the magnetic field frequency gradient and resistivity, and using a numerical iteration method to calculate the apparent resistivity corresponding to each frequency; wherein, the analytical expression for the relationship between the magnetic field frequency gradient and resistivity is:

[0028]

[0029]

[0030] In the formula: IdL is the polar moment of the horizontal thermocouple source, where I represents the current intensity, dL is the length of the horizontal thermocouple source, and k is the complex wave number. The imaginary unit is represented by ω, which represents the angular frequency ω = 2πf, μ represents the permeability, and f is the time-varying current frequency of the horizontal dipole source. ρ is the observation angle of the measuring point; I0 ​​and I1 are the 0th and 1st order imaginary argument Bessel functions of the first kind, respectively; K0 and K1 are the 0th and 1st order imaginary argument Bessel functions of the second kind, respectively; σ is the apparent resistivity ρ calculated iteratively. a The reciprocal of;

[0031] The inductance number is defined as P = r / δ, where r is the transmit / receive distance and δ is the skin depth.

[0032] Furthermore, the transmit / receive distance r and the observation angle Calculations are performed by recording the coordinates of the horizontal thermocouple source and the measuring point. Beneficial effects.

[0033] The apparent resistivity measurement method based on the magnetic field frequency gradient of the horizontal electric couple source provided by this invention calculates the apparent resistivity by measuring the tangential magnetic field and the radial magnetic field, using the derived analytical expression of the magnetic field frequency derivative, and employing a numerical iteration method. It is suitable for the measurement of apparent resistivity at any transmit / receive distance and theoretically does not have non-wavelength distortion. Therefore, this invention has the ability to detect deep underground electrical structures at small transmit / receive distances.

[0034] Furthermore, when using this invention to measure points with small transmit and receive distances, the electromagnetic field signal is greatly enhanced, the signal-to-noise ratio is high, the anti-interference capability is strong, and the power requirement of the transmitter is small, thus enabling lightweight and deep frequency depth measurement.

[0035] Furthermore, numerical simulations have demonstrated that the apparent resistivity of the magnetic field frequency gradient obtained by this invention exhibits wave zone apparent resistivity characteristics when the transmit / receive distance is large, and can also reflect deep information at medium to short transmit / receive distances, without the near-source distortion of the apparent resistivity curve found in traditional controlled-source audio-frequency magnetotelluric methods. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the apparatus used in a specific implementation of the embodiments of this application.

[0037] Figure 2 The graph shows the analytical solution of the frequency gradient of the magnetic field in a uniform half-space and the relative error of the magnetic field frequency difference. (a) is H r The frequency gradient analytical solution and the frequency difference relative error, (b) are Frequency gradient analytical solution and frequency difference relative error.

[0038] Figure 3Here is the curve showing the normalized field frequency gradient as a function of the inductance number, (a) is The curve showing the change in the number of inductances, (b) is The curve showing the change in the number of inductances illustrates... Both have binary properties.

[0039] Figure 4 For H under two-layer media conditions with different transmit and receive distances r Comparison of apparent resistivity curves with frequency gradient: (a) shows the calculated apparent resistivity when r / H = 0.1, and (b) shows the calculated apparent resistivity when r / H = 1.

[0040] Figure 5 For different transmit and receive distances under two-layer media conditions Comparison of apparent resistivity curves with frequency gradient: (a) shows the calculated apparent resistivity when r / H = 0.1, and (b) shows the calculated apparent resistivity when r / H = 1. Detailed Implementation

[0041] The embodiments of the present invention will be described in detail below. These embodiments are based on the technical solutions of the present invention and provide detailed implementation methods and specific operation processes to further explain the technical solutions of the present invention.

[0042] This invention provides a method and system for measuring apparent resistivity based on a horizontal dipole source using the magnetic field frequency gradient. The method involves measuring the tangential and radial magnetic field strengths at a measuring point, substituting these values ​​into the theoretically derived analytical expression for the magnetic field frequency gradient, and then calculating the apparent resistivity using a numerical iteration method. The theoretical formulas for the tangential and radial magnetic fields based on a uniform half-space model are derived as follows:

[0043] Since the definition of apparent resistivity is obtained by fitting the field of a uniform half-space, this invention first presents the formula for the field value of a uniform half-space. Under this condition, the tangential magnetic field at the measuring point on the Earth's surface... With radial magnetic field H r The following formula indicates that, in practical situations, the displacement current is much smaller than the conduction current, so the displacement current term is neglected. Based on this formula, this invention derives the analytical value of the frequency gradient of the magnetic field:

[0044]

[0045]

[0046] Where IdL is the electric dipole moment, I represents the current intensity, and dL is the length of the horizontal dipole source. Here, i represents the imaginary unit, ω represents the angular frequency ω = 2πf, μ represents the permeability, f is the time-varying current frequency of the horizontal dipole source, and r is the distance between the measuring point and the horizontal dipole source. is the observation angle of the measuring point; I0 ​​and I1 are the 0th and 1st order first-order virtual argument Bessel functions, respectively; K0 and K1 are the 0th and 1st order second-order virtual argument Bessel functions, respectively.

[0047] H r , The in-phase and quadrature components can be expanded into a series under small inductance numbers, which can be obtained from the series expansion expression of the imaginary argument Bessel function. Only the first two terms are written here:

[0048]

[0049]

[0050]

[0051]

[0052] Among them, H r and Previously, In and Q represented the in-phase component and quadrature component, respectively. From the above equation, it can be seen that regardless of H... r still The first term of the in-phase component is independent of the electrical structure of the underground medium (referring to changes in resistivity), and is the primary field of the horizontal magnetic field; this part does not possess the properties of frequency sounding. r and The second term of the in-phase component is proportional to the conductivity σ. r and All terms in the orthogonal component series expansion are related to conductivity σ and frequency f, thus possessing frequency-based depth sounding characteristics.

[0053] The following relationship exists between the integer-order imaginary argument Bessel function and the Bessel function of the first kind:

[0054]

[0055] The expression for the first type of Bessel function is:

[0056]

[0057] Taking its derivative, we get:

[0058]

[0059] The derivative property of the first kind of Bessel function is:

[0060]

[0061] Derivatives of the Bessel functions of the first kind of imaginary argument of the 0th and 1st order:

[0062] I'0(x)=[J0(ix)]'=iJ0'(ix)=-iJ1(ix)=I1(x)

[0063]

[0064] Derivatives of the Bessel functions of the second kind for 0th and 1st order imaginary arguments:

[0065]

[0066]

[0067] Based on the conclusions derived above, we can finally obtain... The analytical expression:

[0068]

[0069]

[0070] We will continue to perform a series expansion of the magnetic field frequency gradient with small induction numbers in the same manner as before.

[0071]

[0072]

[0073]

[0074] Compared with the H shown above r , By comparing the series expansions, it is clear that after taking the frequency derivative, each expression carries the electrical information of the underground medium and has the characteristics of frequency sounding. In this way, the influence of the primary field can be eliminated from the measured data.

[0075] Will by After normalization and transformation into a relationship with the inductance number, the normalized radial magnetic field frequency gradient response is obtained:

[0076]

[0077] Among them () N This indicates a normalization operation;

[0078] Will by After normalization and transformation into a relationship with the inductance number, the normalized tangential magnetic field frequency gradient response is obtained:

[0079]

[0080] In frequency-domain electromagnetic methods, the induction number is defined as P = r / δ, where r is the transmitter-receiver distance and δ is the skin depth. Based on the relationship between the skin depth and the complex wavenumber (the complex wavenumber obtained under the condition of neglecting displacement current), ikr can be transformed into (1+i)P, thus obtaining the relationship between the normalized magnetic field gradient value and the induction number.

[0081]

[0082] Based on the numerical simulation results of the normalized field gradient, the existence of the binary phenomenon can be intuitively observed. Numerical solutions for apparent resistivity typically employ iterative methods or inverse cubic spline interpolation, both of which require the field or field gradient to exhibit monotonicity with respect to the number of inductors. Due to the existence of binaryity, it is impossible to solve the problem over the entire area as with wide-area apparent resistivity. Therefore, in this invention, the required iterative value, constrained by a small number of inductors, is selected as the apparent resistivity value.

[0083] Different frequencies are input from the field source, and magnetic field values ​​at different frequencies are collected. A differential algorithm is then used to perform frequency difference calculations, with the difference step size set to the given frequency interval. Numerical iterations are performed at each frequency point. For example, if the gradient of the measured field value at frequency f1 is grad1, a... Substituting this into the expression for a uniform half-space transforms it into a nonlinear transcendental equation concerning the apparent resistivity ρ1. Using a bisection method for iterative solution, with a maximum number of iterations and a set solution precision, the value of the apparent resistivity ρ1 at that frequency can be obtained. Calculating the apparent resistivity values ​​at different frequencies for each frequency point allows for the plotting of sounding curves.

[0084] When solving for apparent resistivity using the bisection method in this invention, the parameters of the layered model are set in the program, the magnetic field frequency gradient of the layered model at a certain frequency is calculated, and a uniform half-space model is used for equivalence. The field gradient calculated by the uniform half-space model at that frequency is equal to that of the layered model. At this time, a transcendental equation about resistivity can be obtained, which is then solved using an iterative method.

[0085] Through the above process, the present invention can calculate the apparent resistivity of any measuring point at a certain frequency. By changing the position of the measuring point and using the same method, the apparent resistivity of all measuring points in the design area at a certain frequency can be calculated. At the same time, by changing the frequency and using the same method, the apparent resistivity of the same measuring point at different frequencies can be calculated.

[0086] In practical applications, the first step is generally to design the operating parameters, determining a suitable transmit / receive distance and transmission frequency range based on the target exploration depth. A long, grounded horizontal conductor (horizontal dipole source) is laid on the ground, with the center of the dipole source as the coordinate origin. The magnitude of the dipole moment is determined, and the transmitter position is recorded using GPS. A square wave or pseudo-random multi-frequency square wave current waveform is input, and the transmitted current waveform and current intensity I are recorded. Then, measuring points are set up on the ground. When the transmitter is operating stably, magnetic field sensors are laid in the corresponding directions according to the field source location to obtain the measured magnetic field value sequence. The coordinates of the transmitting dipole and the observation point are recorded, and the transmit / receive distance r and observation angle are calculated.

[0087] Simulation calculation:

[0088] Figure 2 The diagram shows the analytical solution for the frequency gradient of the magnetic field in a uniform half-space and the relative error of the magnetic field frequency difference. The parameters used in the calculation are: electric dipole moment 1 A m, resistivity of the uniform half-space 1000 Ω m, and emission frequency of the electric dipole source 0.01 Hz. It can be seen that the calculated values ​​are consistent with the theoretical values. The numerical simulation results show that the derived analytical solution for the frequency gradient used in this invention is correct, and also demonstrate that the differential algorithm used in this invention can meet the required computational accuracy and can be used for frequency difference calculations of magnetic fields in layered media.

[0089] Figure 3 For normalization The numerical simulation results show that It exhibits obvious binary characteristics, and the calculation results under a small inductance number need to be selected when calculating the apparent resistivity.

[0090] Figure 4 For two-layer dielectric conditions, different transmit / receive distances and different resistivities of the bottom layer are tested. Comparison of apparent resistivity curves for magnetic field frequency gradient. Model parameters are: transmit / receive distance r = 120m, 1200m; resistivity of the first layer medium: 100Ω·m; resistivity of the second layer medium: 2Ω·m, 10Ω·m, 20Ω·m, 500Ω·m, 1000Ω·m, 5000Ω·m; first layer thickness: 1200m; (a) is when r / H = 0.1. Comparison of calculated apparent resistivity results, (b) is when r / H = 1. A comparison chart of apparent resistivity calculation results. The horizontal axis represents the ratio of skin depth to the thickness of the first layer, and the vertical axis represents the ratio of apparent resistivity to the resistivity of the first layer of the model. The curves represent the apparent resistivity at each depth for each frequency. Different colored curves represent the calculation results under different bottom resistivity models, and the correspondence between the different colored curves and the bottom resistivity is marked on the chart. Figure 4It can be seen that the apparent resistivity described in this invention can reflect changes in subsurface resistivity at different transmit / receive distances, without exhibiting the linear increase in apparent resistivity seen at low frequencies. The sounding curve is sensitive to low-resistivity layers and can effectively distinguish different resistivity models, but its response to high-resistivity layers is very weak. Figure 4 (a) and Figure 4 (b) The comparison shows that the magnetic field frequency gradient apparent resistivity can achieve electrical depth measurement with a very small transmit / receive distance. Increasing the transmit / receive distance helps to make the tail of the curve closer to the true resistivity.

[0091] Figure 5 For two-layer dielectric conditions, different transmit / receive distances and different resistivities of the bottom layer are tested. Comparison of apparent resistivity curves with magnetic field frequency gradient. All parameters are consistent with... The calculation parameters for apparent resistivity are the same. Figure 5 (a) When r / H = 0.1 Comparison chart of calculated apparent resistivity. Figure 5 (b) When r / H = 1 A comparison chart of apparent resistivity calculation results. The horizontal axis represents the ratio of skin depth to the thickness of the first layer, and the vertical axis represents the ratio of apparent resistivity to the resistivity of the first layer of the model. The curves represent the apparent resistivity at each depth for each frequency. Different colored curves represent the calculation results under different bottom resistivity models, and the correspondence between the different colored curves and the bottom resistivity is marked on the chart. Figure 5 It can be seen that the apparent resistivity described in this invention can reflect changes in subsurface resistivity at different transmit / receive distances, without exhibiting the linear increase in apparent resistivity seen at low frequencies. The sounding curve is sensitive to low-resistivity layers and can effectively distinguish different resistivity models, but its response to high-resistivity layers is very weak. Figure 5 (a) and Figure 5 (b) The comparison shows that the magnetic field frequency gradient apparent resistivity can achieve electrical depth measurement with a very small transmit / receive distance. Increasing the transmit / receive distance helps to make the tail of the curve closer to the true resistivity.

[0092] In summary, this invention provides an apparent resistivity calculation scheme applicable to small inductance values. This apparent resistivity can operate at short to medium transmit / receive distances and is used for deep electrical structure detection when the transmit / receive distance is short.

[0093] The above embodiments are preferred embodiments of this application. Those skilled in the art can make various changes or improvements based on them. Without departing from the overall concept of this application, these changes or improvements should fall within the scope of protection claimed in this application.

Claims

1. A method for measuring apparent resistivity of magnetic field frequency gradient based on a horizontal dipole source, characterized in that, include: A time-varying current signal is passed through a horizontal thermocouple source laid on the ground. The distance between the horizontal thermocouple source and the signal source is... The radial and tangential magnetic fields were continuously collected at the measuring point, and the radial magnetic field strength was obtained by time-frequency conversion. and tangential magnetic field strength ; Calculate the radial magnetic field strength respectively and tangential magnetic field strength Difference value as a function of frequency , ; Will , Substituting the values ​​into the analytical expression for the relationship between the magnetic field frequency gradient and resistivity, and using a numerical iteration method to calculate the apparent resistivity corresponding to each frequency, the calculation result under a small inductance number is selected as the final measured apparent resistivity; wherein, the analytical expression for the relationship between the magnetic field frequency gradient and apparent resistivity is: ； ； In the formula: dL is the polar moment of the horizontal galvanic couple source, where I represents the current intensity and dL is the length of the horizontal galvanic couple source. For complex wave number, , Represents the imaginary unit. Represents angular frequency , Indicates magnetic permeability, The time-varying current frequency of the horizontal thermocouple source; The observation angle of the measuring point; and These are Bessel functions of the first kind and second order, respectively, which are imaginary argument functions of the 0th and 1st order. and These are the 0th and 1st order Bessel functions of the second kind of imaginary argument, respectively. Here, is the conductivity, and is the apparent resistivity calculated iteratively. The reciprocal of; The definition of the number of inductors ,in For transmit / receive distance, For skin depth.

2. The method for measuring apparent resistivity of magnetic field frequency gradient according to claim 1, characterized in that, The apparent resistivity in the analytical expression of the function is solved using the bisection method.

3. The method for measuring apparent resistivity of magnetic field frequency gradient according to claim 1, characterized in that, The horizontal thermocouple source is a grounded long conductor, and the thermocouple moment is... direction.

4. The method for measuring apparent resistivity of magnetic field frequency gradient according to claim 1, characterized in that, By moving the measuring point, the apparent resistivity of the magnetic field frequency gradient at different measuring points as a function of frequency is obtained, and then the apparent resistivity of the magnetic field frequency gradient in the whole area is obtained.

5. The method for measuring apparent resistivity of magnetic field frequency gradient according to claim 1, characterized in that, The current signal supplied to the horizontal thermocouple source laid on the ground is an alternating current with a defined transmission frequency.

6. The method for measuring apparent resistivity of magnetic field frequency gradient according to claim 5, characterized in that, By sequentially changing the frequency of the current signal at the same measuring point, the apparent resistivity of the magnetic field frequency gradient at that measuring point based on different current frequencies is obtained, and thus the frequency detection curve of that measuring point is obtained, namely the frequency-magnetic field frequency gradient apparent resistivity curve.

7. The method for measuring apparent resistivity of magnetic field frequency gradient according to claim 1, characterized in that, Distance between measuring point and horizontal thermocouple source , refers to the distance between the dipole of the horizontal dipole source and the measuring point.

8. The method for measuring apparent resistivity of magnetic field frequency gradient according to claim 1, characterized in that, A magnetic field sensor is used at the measuring point to collect radial and tangential magnetic fields.

9. A magnetic field frequency gradient apparent resistivity measurement system based on a horizontal dipole source, characterized in that, include: Horizontal thermocouple source, receiver, and processor; The horizontal thermocouple source is laid on the ground surface and generates a magnetic field after a time-varying current signal is passed through it. The receiver includes a tangential magnetic field acquisition module and a radial magnetic field acquisition module for acquiring radial and tangential magnetic fields, respectively, to obtain the radial magnetic field strength. and tangential magnetic field strength ; The processor is used to perform the following calculations and obtain the apparent resistivity: Calculate the radial magnetic field strength respectively and tangential magnetic field strength Difference value as a function of frequency , ; Will , Substituting the values ​​into the analytical expression for the relationship between the magnetic field frequency gradient and resistivity, and using a numerical iteration method to calculate the apparent resistivity corresponding to each frequency, the calculation result under a small inductance number is selected as the final measured apparent resistivity; wherein, the analytical expression for the relationship between the magnetic field frequency gradient and resistivity is: ； ； In the formula: dL is the polar moment of the horizontal galvanic couple source, where I represents the current intensity and dL is the length of the horizontal galvanic couple source. For complex wave number, , Represents the imaginary unit. Represents angular frequency , Indicates magnetic permeability, The time-varying current frequency of the horizontal thermocouple source; The observation angle of the measuring point; and These are Bessel functions of the first kind and second order, respectively, which are imaginary argument functions of the 0th and 1st order. and These are the 0th and 1st order Bessel functions of the second kind of imaginary argument, respectively. Apparent resistivity for iterative calculation The reciprocal of; The definition of the number of inductors ,in For transmit / receive distance, For skin depth.

10. The magnetic field frequency gradient apparent resistivity measurement system according to claim 9, characterized in that, Transmit / receive distance r and observation angle Calculations are performed by recording the coordinates of the horizontal thermocouple source and the measuring point.

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