Combined advanced detection method based on borehole transient electromagnetism and direct current resistivity

Abstract

The invention discloses a combined advanced detection method based on borehole transient electromagnetism and direct-current resistivity, and the method comprises the steps: laying an advanced borehole in a tunnel face in a tunneling direction, carrying out the collection of borehole transient electromagnetic data, and carrying out the inversion, and obtaining a borehole transient electromagnetic inversion result; according to an inversion result and a corresponding estimation method, calculating to obtain Gaussian variation models in three main axis directions, and expanding the Gaussian variation models to any direction; then, the Gaussian variation model in any direction is substituted into a covariance calculation formula, a covariance value between any two grid units in the area needing to be detected is obtained through calculation, and a covariance constraint matrix is assembled; and finally, introducing the matrix as a regularization item into an inversion objective function of direct-current resistivity advanced detection to realize covariance constrained direct-current resistivity inversion, so that the inversion stability and the inversion imaging precision of the direct-current resistivity advanced detection are remarkably improved, and the method is suitable for scenes such as tunnel face or coal mine underground tunneling advanced detection and the like.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of geological exploration, and particularly relates to a combined advanced detection method based on borehole transient electromagnetic and direct current resistivity. BACKGROUND

[0002] In roadway excavation, it is crucial to pre-evaluate the geological conditions in front. There may be fault fracture zone containing water, collapse column containing water, goaf water and karst water-rich area containing water and other types of water-containing in front of the excavation. The common means for exploring adverse geological factors are drilling and geophysical prospecting. Although drilling has high accuracy, it has high cost, long time consumption, limited exploration range, and the formed detection holes are easy to become a channel for hidden water bodies to enter the roadway and cause secondary accidents. Drilling alone cannot meet the actual production needs. At present, drilling has developed borehole transient electromagnetic method, which can detect a certain range around the borehole, but the detection range is still limited. Geophysical exploration is widely used in water exploration due to its low cost, large exploration range and rapidity. There are many types of geophysical methods, such as seismic reflection wave method, direct current resistivity method and other methods and technologies. However, due to the sensitive characteristics of various methods and the limitation of roadway excavation space, as well as the influence of metal anchor support and other external factors, although these methods have a larger detection range, their detection accuracy is relatively low, especially the direct current resistivity method has the problem of multiple solutions after inversion (i.e., non-uniqueness of inversion results).

[0003] Combined inversion has always been a new idea for geophysical inversion. Its principle is to use different types of geophysical parameters for joint constraint inversion to improve the reliability and accuracy of the inversion results. There are currently many different types of combined inversion, such as seismic and transient electromagnetic joint, transient electromagnetic and electrical joint, seismic and electrical joint, etc. The main process of these combined methods is to detect the same area by using two different geophysical methods, then superimpose the inversion results, and finally delineate the abnormal area. This method can improve the detection accuracy compared to single detection, but since it is essentially a combination of two geophysical methods, the detection accuracy is still not high, and there may still be multiple solution problems.

[0004] Therefore, how to provide a new combined advanced detection method that can effectively improve the accuracy of advanced detection while maintaining the range of geophysical exploration is the research direction of the present application. SUMMARY

[0005] In view of the problems existing in the prior art, the present application provides a combined advanced detection method based on borehole transient electromagnetic and direct current resistivity, which extracts the inversion results of borehole transient electromagnetic as structural priori for constraining the inversion results of direct current resistivity method, thereby effectively improving the accuracy of advanced detection while maintaining the range of geophysical exploration.

[0006] In order to achieve the above object, the technical scheme adopted by the present application is as follows: a combined advanced detection method based on borehole transient electromagnetic and direct current resistivity, comprising the following steps:

[0007] Step one, laying advanced borehole: establishing a coordinate system with the tunnel axis as the x-axis, parallel to the tunnel roof and floor as the y-axis, and perpendicular to the tunnel roof and floor as the z-axis, and constructing an advanced borehole along the x-axis at the center position of the tunnel face.

[0008] Step two, borehole transient electromagnetic detection: setting detection points at different positions in the advanced borehole, placing the transient electromagnetic detection equipment into the borehole, and performing transient electromagnetic detection at each detection point to obtain borehole transient electromagnetic observation data of all detection points in the entire advanced borehole.

[0009] Step three, transient electromagnetic inversion: for the borehole transient electromagnetic observation data of each detection point obtained in step two, performing radial inversion on the borehole transient electromagnetic observation data of each detection point to obtain a set of circular discrete points ; Set Mapping to a resistivity model profile with the depth of the detection point as the cross section and the borehole axis as the center, thereby obtaining the resistivity profile distribution of all detection points.

[0010] Step four, calculating the effective range of borehole transient electromagnetic x-axis direction and Gauss variation model: extracting the resistivity profile distribution of each detection point (which is a set of discrete points), extracting the resistivity values with the same y and z coordinates in the resistivity profile distribution of each detection point as a discrete curve along the x-axis, thereby obtaining several discrete curves along the x-axis, and superimposing all curves to obtain the average value, specifically: calculating the average value of the resistivity profile distribution of each detection point (i.e. if there are n resistivity values on the resistivity profile distribution of the detection point, then the resistivity values are added and divided by the number n to obtain the average resistivity value of the detection point), using the above calculated average values to represent the resistivity distribution of each detection point within a certain range of borehole radial direction, thereby obtaining a unique curve x(k), where k is the x-axis coordinate of each detection point; calculating the experimental semi-variation according to the curve x(k), then fitting and estimating the parameters using the Gauss variation model, and finally obtaining the effective range of borehole transient electromagnetic x-axis direction and the Gauss variation model.

[0011] Step five, advanced detection by direct current resistivity method: laying an observation system of direct current resistivity method to perform advanced detection in front of the tunnel and obtain the direct current resistivity data of the required detection area.

[0012] Step six, calculate the effective distance and Gaussian variation model of the borehole transient electromagnetic y, z axis direction: the step five of the direct current resistivity data is inverted by using the smoothing constraint, the direct current resistivity superposition inversion result under the smoothing constraint is obtained, then the effective distance of x axis, y axis and z axis under the direct current resistivity smoothing constraint is calculated respectively according to the mode of step four, and the effective distance of x axis direction of borehole transient electromagnetic is combined, thereby the effective distance and Gaussian variation model of borehole transient electromagnetic y, z axis direction are calculated.

[0013] Step seven, calculate the effective distance and Gaussian variation model of borehole transient electromagnetic in any direction: the effective distance and Gaussian variation model of x axis, y axis and z axis of borehole transient electromagnetic are obtained according to steps four and six, and the effective distance and Gaussian variation model in any direction are calculated.

[0014] Step eight, construct the covariance constraint matrix: the three-dimensional discrete of the inversion grid in the required detection area is carried out, and the covariance value of any two grid units is calculated by using the Gaussian variation model in any direction, so as to construct the covariance constraint matrix.

[0015] Step nine, covariance constraint direct current resistivity superposition inversion: the direct current resistivity detection data is inverted by using the covariance constraint matrix, and the geological conditions of the required detection area are obtained according to the inversion result and the borehole transient electromagnetic inversion result.

[0016] Further, after the drilling construction, the coordinates, elevation, axial direction and depth mark of the hole are recorded, and the basic hole inclination is checked and the hole is cleaned (water circulation), so as to ensure that the medium in the hole is stable and the channel is unobstructed.

[0017] Further, the specific formula of the radial inversion in step three is:

[0018] (1)

[0019] Wherein, is the weighted matrix of borehole transient electromagnetic observation data; is the observation data (induced voltage decay curve) of borehole transient electromagnetic at the ith detection point, i is from 1 to N; is the forward simulation operator of borehole transient electromagnetic; is the prediction resistivity model of borehole transient electromagnetic; is the model constraint term regularization factor of borehole transient electromagnetic; is the model smoothing constraint weighted matrix of borehole transient electromagnetic, which can be constructed by taking zero order, first order and second order derivative operators.

[0020] Further, the calculation formula of experimental semivariance in step four is:

[0021] (2)

[0022] Where, N(h) is the number of selected lag distance; in order to ensure the accuracy of the calculation, the largest lag distance needs to have more than 5 calculation samples, that is, at least 5 groups Can be calculated, otherwise the amount of data is too small, the reliability is reduced.

[0023] The specific formula of the Gaussian variation model function is:

[0024] (3)

[0025] Where, is the variance value of all discrete points of the borehole transient electromagnetic inversion result; is the effective distance in the x-axis direction of the borehole transient electromagnetic, aiming at The least square optimization is carried out on the value to find an optimal value so that the residual error between the Gaussian variation model curve and the experimental semivariogram curve calculated before is minimized; the optimal value is brought back into formula (3), that is, the Gaussian variation model in the x-axis direction of the borehole transient electromagnetic is obtained.

[0026] Further, the objective function of the inversion using the smoothing constraint in step six is:

[0027] (4)

[0028] Where, is the model smoothing constraint weighted matrix of the direct current resistivity, which can be constructed by taking zero-order, first-order and second-order derivative operators.

[0029] Let the effective distances in the x-axis, y-axis and z-axis directions under the direct current resistivity smoothing constraint be , , respectively, and the effective distance in the x-axis direction of the borehole transient electromagnetic is ; the effective distances in the y-axis and z-axis directions of the borehole transient electromagnetic are calculated by the following formula: ,

[0030] (5)

[0031] (6)

[0032] Then, the calculated , are respectively substituted into formula (3), so as to obtain the Gaussian variation model in the y-axis and z-axis directions of the borehole transient electromagnetic.

[0033] ​Further, the step seven is specifically:

[0034] Let the angle between any direction and the plane of x, y axis be , and the angle between the projection of the arbitrary direction on the plane of x, y axis and the positive direction of x axis be , then the effective distance of the arbitrary direction is The calculation formula is:

[0035] (7)

[0036] Substitute the effective distance of the arbitrary direction calculated above into formula (3), and the Gaussian variation model of the arbitrary direction is obtained.

[0037] Further, the covariance value of any two grid cells calculated in the step eight is specifically:

[0038] (8)

[0039] Wherein, C(h) is the covariance function value, which is only related to the lag distance h after the direction is determined, so in the same direction, two grid cells with the same lag distance h have the same covariance value.

[0040] Let the number of grids be , and the dimension of the covariance matrix to be constructed is This matrix is a symmetric matrix, wherein the value of any ith row and jth column is , which represents the covariance value between the ith grid cell and the jth grid cell, and the formula of the covariance constraint matrix is:

[0041] (9).

[0042] Further, the objective function for the inversion of the direct current resistivity prospecting data by using the covariance matrix in the step nine is:

[0043] (10)

[0044] Wherein, is the data weighting matrix of the direct current resistivity method; is the potential difference data collected by the direct current resistivity method; is the forward simulation operator of the direct current resistivity method; is the predicted resistivity model in the direct current resistivity method; is the regularization factor of the covariance constraint term of the direct current resistivity method; is the covariance constraint matrix. ​

[0045] Compared with the prior art, the present application arranges the advanced borehole along the tunnel face in the tunneling direction, carries out the borehole transient electromagnetic data collection and inversion, and obtains the circular resistivity profile of the borehole transient electromagnetic at each detection point. According to the inversion result of the borehole transient electromagnetic and the corresponding estimation method, the Gaussian variation model in three principal axis directions is calculated and is extended to any direction; then the Gaussian variation model in any direction is brought into the covariance calculation formula, and the covariance value between any two grid units in the required detection area is calculated and is assembled into the covariance constraint matrix. Finally, the matrix is introduced into the inversion objective function of the direct current resistivity advanced detection as a regularization term, and the direct current resistivity inversion with the covariance constraint is realized. Since the borehole transient electromagnetic has strong sensitivity to the low-resistance body within a certain range in the radial direction of the well (borehole), and has high resolution along the borehole direction, but the coverage range is also limited; the direct current resistivity method has wide coverage, but is affected by the non-uniqueness of inversion, the present application provides the structured prior (covariance) by the borehole transient electromagnetic inversion information to constrain the inversion of the direct current resistivity method advanced detection, that is, the high resolution near the well is used to guide the inversion of a large range, so that the two different methods are complementary, which can significantly improve the inversion stability and inversion imaging accuracy of the direct current resistivity advanced detection, and finally the present application can be applied to the tunnel face or coal mine underground tunneling advanced detection scene, and can accurately identify the adverse geological anomaly bodies such as water-rich fractured zone, hidden fault, karst cave and the like in the front surrounding rock. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 is the overall flowchart of the present application;

[0047] Figure 2 is the layout schematic diagram of the borehole transient electromagnetic detection in the present application;

[0048] Figure 3 is the layout schematic diagram of the direct current resistivity method detection in the present application.

[0049] Figure 4 is the schematic diagram for calculating the effective distance in any direction in the present application. DETAILED DESCRIPTION

[0050] The present application will be further described below.

[0051] As shown in Figure 1 , the present application comprises the following steps:

[0052] Step one, layout of advanced borehole: a coordinate system is established with the tunnel axis as the x axis, parallel to the tunnel top and bottom plate as the y axis, and perpendicular to the tunnel top and bottom plate as the z axis, as shown in Figure 2As shown, the advanced drilling hole is constructed along the x-axis at the center position of the tunnel face; the hole is as consistent as possible with the axis, and the deflection is as small as possible; the hole diameter needs to meet the needs of the transient electromagnetic probe and cable lowering; the required detection area preferably uses a non-metal casing to avoid metal components from entering the measurement depth range and affecting the detection effect; after drilling construction, record the hole coordinates, elevation, axial orientation, and depth markers, and perform basic hole deviation checking and hole cleaning (water circulation) to ensure that the medium in the hole is stable and the channel is unobstructed.

[0053] Step two, drilling transient electromagnetic detection: according to the depth of the advanced drilling hole, multiple detection points are set at equal intervals in the drilling hole, the transient electromagnetic detection equipment is placed in the drilling hole, and transient electromagnetic detection is performed at each detection point, thereby obtaining drilling transient electromagnetic observation data of all detection points of the advanced drilling hole.

[0054] Step three, transient electromagnetic inversion: for the drilling transient electromagnetic observation data of each detection point obtained in step two, radial inversion is performed on the drilling transient electromagnetic observation data of each detection point, and the specific formula is:

[0055] (1)

[0056] wherein, is the drilling transient electromagnetic observation data weighting matrix; is the drilling transient electromagnetic observation data at the i-th detection point (induced voltage decay curve), i from 1 to N; is the drilling transient electromagnetic forward simulation operator; is the drilling transient electromagnetic prediction resistivity model; is the drilling transient electromagnetic model constraint regularization factor; is the drilling transient electromagnetic model smoothing constraint weighting matrix, which can be constructed by zero-order, first-order, and second-order derivative operators. The set of circular discrete points is obtained through the above formula ; the set is mapped to the resistivity model profile with the depth of the detection point as the cross section and the drilling axis as the center, thereby obtaining the resistivity profile distribution of all detection points.

[0057] Step four, calculating the effective distance and Gaussian variation model of the borehole transient electromagnetic x-axis direction: the resistivity profile distribution of each detection point is extracted (it is a collection of discrete points itself), the resistivity values with the same y and z coordinates in the resistivity profile distribution of each detection point are extracted as a discrete curve along the x-axis, thereby obtaining a plurality of discrete curves along the x-axis, and the average value is calculated by superimposing all curves, specifically: the average value of the resistivity profile distribution of each detection point is calculated (i.e. if there are n resistivity values on the resistivity profile distribution of the detection point, the resistivity values are added and then divided by the number n to obtain the average resistivity value of the detection point), the average values calculated above are used to represent the resistivity distribution of each detection point within a certain range of the borehole radial direction, thereby obtaining a unique curve x(k), wherein the value of k is the x-axis coordinate of each detection point; the number of detection points of the borehole transient electromagnetic method is N, and the x(k) curve is a collection of N discrete points; the curve represents the overall trend of the resistivity value of the surrounding rock in front of the tunnel along the x-axis direction; the experimental half variation is calculated according to the curve x(k), and the specific calculation formula is:

[0058] (2)

[0059] Wherein, N(h) is the number of selected lag distance; in order to ensure the calculation accuracy, the maximum lag distance needs to have more than 5 calculation samples, that is, at least 5 groups can be calculated, otherwise the data volume is too small, and the reliability is reduced; finally, an experimental half variation curve that monotonically rises with h is obtained.

[0060] Then the Gaussian variation model is fitted and the parameters are estimated, and the specific formula is:

[0061] (3)

[0062] Wherein, is the variance value of all discrete points of the borehole transient electromagnetic inversion result; is the effective distance of the borehole transient electromagnetic x-axis direction, the least square optimization is carried out for the value of , and an optimal value is found to make the residual error of the Gaussian variation model curve and the experimental half variation curve calculated before be the smallest; the optimal value is brought back to formula (3), that is, the Gaussian variation model of the borehole transient electromagnetic x-axis direction is obtained.

[0063] Step five, advanced detection of direct current resistivity method: the observation system of direct current resistivity method is arranged as shown in Figure 3As shown, the embodiment adopts the fixed-point source three-pole method for layout, specifically: the positive electrode A of power supply is fixed on the tunnel face, the negative electrode B of backflow is laid in a far place behind the tunnel and kept fixed; the accurate coordinates of A and B are recorded, and A and B are both fixed in subsequent re-measurement; the measurement electrodes M and N are arranged on the line connecting A and B, and different measurement electrode M and N combinations are switched to realize comprehensive coverage of the front of the tunnel face; the coordinates of the measurement electrodes M and N and the corresponding potential difference are accurately recorded each time, so as to realize advanced detection of the front of the tunnel and obtain the direct current resistivity data of the required detection area.

[0064] Step six, calculating the effective distance and Gaussian variation model of the y and z axes of the drill hole transient electromagnetic method: since the advanced drill hole is only distributed along the x axis, the effective distance and Gaussian variation model of the y and z axes of the drill hole transient electromagnetic method cannot be directly calculated by step four, so the direct current resistivity data of step five is inverted by using smoothing constraint, and the direct current resistivity advanced detection inversion result under the smoothing constraint is obtained, and the specific objective function is:

[0065] (4)

[0066] wherein, is the model smoothing constraint weight matrix of the direct current resistivity, which can be constructed by taking zero-order, first-order and second-order derivative operators.

[0067] Since the direct current resistivity advanced detection inversion result under the smoothing constraint is a complete three-dimensional rectangular space in front of the tunnel face (not just the circular resistivity profile result at each station of the drill hole transient electromagnetic method), taking the x axis as an example, all the resistivity curves of the inversion result under the direct current resistivity smoothing constraint in the x axis direction are taken out and the average value is obtained by superposition, thereby obtaining a unique resistivity discrete distribution curve along the x axis direction. Then, the experimental semi-variation, fitting Gaussian variation model and step four are calculated in the same way; therefore, the effective distance of the x axis, y axis and z axis under the direct current resistivity smoothing constraint is calculated in the way of step four, and the effective distance of the y and z axes of the drill hole transient electromagnetic method is calculated in combination with the effective distance of the x axis of the drill hole transient electromagnetic method, thereby calculating the effective distance and Gaussian variation model of the y and z axes of the drill hole transient electromagnetic method, specifically:

[0068] Let the effective distances of the x axis, y axis and z axis under the direct current resistivity smoothing constraint be , , , and the effective distance of the x axis of the drill hole transient electromagnetic method be ; the effective distances of the y and z axes of the drill hole transient electromagnetic method are calculated by the following formula , :

[0069] (5)

[0070] (6)

[0071] Then the calculated , are substituted into formula (3) respectively, so as to obtain the Gaussian variation model of the borehole transient electromagnetic y and z axis directions.

[0072] Step seven, calculating the effective range and Gaussian variation model of borehole transient electromagnetic in any direction: according to the effective range and Gaussian variation model of borehole transient electromagnetic x, y and z axis directions obtained in steps four and six, the effective range and Gaussian variation model in any direction are calculated, which is specific as follows:

[0073] Define the angle between any direction and the plane where x and y axes are located as , and the angle between the arbitrary direction and the positive direction of x axis in the plane where x and y axes are located as As shown in Figure 4 , then the effective range of any direction The calculation formula is as follows:

[0074] (7)

[0075] Substitute the calculated effective range of any direction into formula (3), so as to obtain the Gaussian variation model of any direction.

[0076] Step eight, constructing the covariance constraint matrix: the three-dimensional discrete of the inversion grid of the required detection area is carried out, and the covariance value of any two grid units is calculated by using the Gaussian variation model of any direction, and the specific formula is as follows:

[0077] (8)

[0078] Wherein, C(h) is the covariance function value, which is only related to the lag distance h after the direction is determined, so in the same direction, the two grid units with the same lag distance h have the same covariance value.

[0079] Suppose that the number of grids is , and the dimension of the covariance matrix to be constructed is , which is a symmetric matrix, wherein the value of any ith row and jth column , which represents the covariance value between the ith grid unit and the jth grid unit, so the formula of the covariance constraint matrix is as follows:

[0080] (9).

[0081] Step nine, covariance constrained direct current resistivity advanced detection inversion: covariance constraint matrix is used to carry out inversion to direct current resistivity detection data, and the specific objective function is as follows:

[0082] (10)

[0083] Wherein, It is the data weighting matrix of direct current resistivity method; It is the potential difference data collected by direct current resistivity method; It is the forward simulation operator of direct current resistivity method; It is the predicted resistivity model in direct current resistivity method; It is the covariance constraint item regularization factor of direct current resistivity method; It is the covariance constraint matrix.

[0084] According to the inversion result and the drilling transient electromagnetic inversion result, the geological condition of the required detection area is obtained.

[0085] The above only is the preferred embodiment of the present application, and it should be pointed out that: for ordinary skilled person in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, and these improvements and refinements should be considered as the protection scope of the present application.

Claims

1. A combined advanced detection method based on borehole transient electromagnetic and direct current resistivity, characterized in that, The method comprises the following steps: Step one, establish a coordinate system with the tunnel axis as the x-axis, parallel to the tunnel roof and floor as the y-axis, and perpendicular to the tunnel roof and floor as the z-axis, and drill an advanced borehole along the x-axis at the center of the tunnel working face; Step two, perform transient electromagnetic detection on the detection points at different positions in the advanced borehole to obtain the borehole transient electromagnetic data of all the detection points; Step three, after radial inversion and processing of the borehole transient electromagnetic data of each detection point, the resistivity profile distribution of all the detection points is obtained; Step four, the resistivity values with the same y and z coordinates in the resistivity profile distribution of each detection point are extracted as a discrete curve along the x-axis, and the average value of the corresponding superposition of each discrete curve is obtained to obtain a curve x(k), wherein the value of k is the x-axis coordinate of each detection point; the experimental semivariogram is calculated according to the curve x(k), then the Gaussian variogram model is fitted and the parameters are estimated, and finally the effective range of the borehole transient electromagnetic x-axis direction and the Gaussian variogram model are obtained; Step five, obtain the direct current resistivity data of the required detection area; Step six, after inversion of the direct current resistivity data in step five using the smoothing constraint to obtain the result, the effective range of the x-axis, y-axis and z-axis under the direct current resistivity smoothing constraint is calculated in the manner of step four, and the effective range of the borehole transient electromagnetic y-axis and z-axis is calculated in combination with the effective range of the borehole transient electromagnetic x-axis direction in step four, and the Gaussian variogram model is obtained; Step seven, the effective range and the Gaussian variogram model in any direction are calculated according to the effective range and the Gaussian variogram model of the borehole transient electromagnetic x-axis, y-axis and z-axis obtained in steps four and six; Step eight, the three-dimensional discrete of the inversion grid of the required detection area is performed, and the covariance value of any two grid units is calculated using the Gaussian variogram model in any direction, so as to construct a covariance constraint matrix; Step nine, the direct current resistivity detection data is inverted using the covariance constraint matrix, and the geological conditions of the required detection area are obtained according to the inversion result and the borehole transient electromagnetic inversion result.

2. The method according to claim 1, wherein the method is characterized by, After the drilling, the coordinates, elevation, axial direction and depth mark of the hole are recorded.

3. The method for combined advanced detection based on borehole transient electromagnetic and direct current resistivity according to claim 1, characterized in that, The specific formula of the radial inversion in step three is: （1） wherein, is a weighting matrix for borehole transient electromagnetic observation data; is the observation data of borehole transient electromagnetic at the ith survey point, i from 1 to N; is a forward modeling operator for borehole transient electromagnetic; is a predicted resistivity model for borehole transient electromagnetic; is a regularization factor for the model constraint term of borehole transient electromagnetic; is a model smoothing constraint weighting matrix for borehole transient electromagnetic.

4. The method of claim 1, wherein the method is characterized by, The calculation formula of the experimental semivariogram in step four is: （2） Wherein, N(h) is the number of selected lag distance; The specific formula of the Gaussian variogram model function is: （3） in, This represents the variance of all discrete points in the borehole transient electromagnetic inversion result; For the effective travel distance in the x-axis direction of the transient electromagnetic drilling, targeting The values ​​are optimized using least squares to find an optimal value. The value minimizes the residual between the Gaussian variation model curve and the previously calculated experimental semivariogram; the optimal value is... Substituting the value back into formula (3), we obtain the Gaussian variation model of the borehole transient electromagnetic x-axis direction.

5. The method according to claim 4, wherein, In the step six, the effective ranges of x-axis, y-axis and z-axis are respectively 、 、 , the effective range of x-axis of the borehole transient electromagnetic method is ; the effective ranges of y-axis and z-axis of the borehole transient electromagnetic method are calculated by the following formula 、 : （4） （5） Then the calculated , are substituted into formula (3) respectively, and the Gaussian variation model of the y and z axis direction of the drilling transient electromagnetic method is obtained.

6. The method according to claim 5, wherein, The specific formula of step seven is: Let the angle between any direction and the plane of the x, y axes be Let the angle between the arbitrary direction and the positive direction of the x axis be Then the effective range of the arbitrary direction is The calculation formula is: （6） The effective range of the calculation of any direction Substitute equation (3), so that the Gaussian variation model of any direction is obtained.

7. The method according to claim 6, wherein, The specific formula of the covariance value of any two grid units calculated in step eight is: （7） Wherein, C(h) is the covariance function value.

8. The method according to claim 7, wherein, In step nine, the direct current resistivity detection data is inverted using the covariance matrix, and the objective function is: （8） wherein, is a data weighting matrix for the direct current resistivity method; is a potential difference data collected by the direct current resistivity method; is a forward simulation operator for the direct current resistivity method; is a predictive resistivity model in the direct current resistivity method; is a regularization factor for the covariance constraint term of the direct current resistivity method; is a covariance constraint matrix.

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