Ultra-wide coal face roof electrical tomography system and method

Abstract

The invention belongs to the technical field of geophysical exploration transient electromagnetic detection, particularly relates to an ultra-wide coal face roof electrical tomography system and method, and is suitable for high-precision detection of a hidden geological structure of a working face roof with the width exceeding 300 meters and the depth within 100 meters in an underground coal mine. A transient electromagnetic perspective detection technology is adopted, and full-coverage imaging of an electrical structure of a working face roof is realized through alternate arrangement of transmitting-receiving roadways and dynamic data fusion; a time-space mapping relation is established in combination with a smoke ring effect diffusion model, and omnibearing imaging of the top plate is achieved; aiming at the refined detection of the hidden geologic structure of the top plate of the ultra-wide coal face (the width is greater than or equal to 300 meters), bidirectional detection and grid subdivision are adopted, and a single-side detection blind area is eliminated through roadway transposition of a transmitting end and a receiving end, so that grid full-coverage imaging in a detection range is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of geophysical prospecting transient electromagnetic detection, and in particular relates to an electrical tomography system and method for the roof of an ultra-wide coal mining face. Background Art

[0002] Safe, environmentally friendly, and efficient production are the core goals of coal mining. With advances in mining technology, the working face width of modern coal mines has expanded from the traditional 100-200 meters to over 300 meters. However, accurately detecting hidden roof structures is crucial to ensuring safe mining, requiring high-resolution detection systems and methods that meet the requirements of ultra-wide working faces.

[0003] In existing technologies, the detection of hidden structures mainly relies on two methods: drilling and geophysical exploration. Geophysical exploration and geophysical exploration:

[0004] Drilling in the existing technology: Although it can directly verify the structure, it is costly and inefficient. The data obtained through only one borehole is difficult to reflect the spatial distribution of the structure, and the coverage is limited. Geophysical exploration in the existing technology: The slot wave seismic method is used: it can detect faults with a fault distance greater than 1 / 3 of the coal thickness and changes in coal seam thickness, but it relies on explosive sources, has strict requirements on mine ventilation, is labor-intensive, and is costly. Existing geophysical exploration can also use radio wave perspective methods: convenient construction and high resolution, but the electromagnetic waves attenuate quickly, the perspective distance is insufficient, and it is difficult to cover ultra-wide working faces of more than 300 meters. For this reason, the present invention proposes an electrical tomography system and method for the roof of an ultra-wide coal mining working face Summary of the Invention

[0005] The purpose of the present invention is to provide an electrical tomography imaging system and method for the roof of an ultra-wide coal mining face, which is suitable for high-precision detection of hidden geological structures in the roof of a coal mine with a width exceeding 300 meters and a depth of less than 100 meters; by adopting transient electromagnetic perspective detection technology, through the alternating arrangement of transmitting and receiving tunnels and dynamic data fusion, full coverage imaging of the electrical structure of the roof of the working face is achieved; combined with the smoke ring effect diffusion model, a time-space mapping relationship is established to achieve all-round imaging of the roof; for the refined detection of hidden geological structures in the roof of an ultra-wide coal mining face (width ≥ 300 meters), two-way detection and grid subdivision are adopted, and by exchanging the transmitting and receiving end tunnels, the blind spots of one-side detection are eliminated, and full coverage imaging of the grid within the detection range is achieved.

[0006] The technical solutions adopted by the present invention are as follows:

[0007] An electrical tomography system for the roof of an ultra-wide coal mining face comprises a first sensor device and a second sensor device respectively arranged in lanes on both sides of the ultra-wide coal mining face; and further comprises a data processing and display module;

[0008] The first sensing device and the second sensing device both include transmitting coils distributed in a rectangular shape in the corresponding lane and magnetic probes distributed in an array with equal spacing;

[0009] The data processing and display module includes a preliminary data acquisition unit, a data preprocessing unit, an apparent resistivity and detection range calculation unit, an overlapping area determination unit, and a data interpretation unit;

[0010] The preliminary data acquisition unit is used to obtain the original geological and hydrological data of the mining area, and the data preprocessing unit performs filtering and denoising on the forward detection data set and the reverse detection data set obtained by the first sensing device and the second sensing device; the apparent resistivity and detection range calculation is used to calculate the apparent resistivity value in each detection range grid in the magnetic line area at the corresponding moment; the overlapping area determination unit performs fusion imaging on the bidirectional data; the data interpretation unit: observes the resistivity change characteristics of the roof through the resistivity calculation value, and in order to accurately delineate the abnormal area of ​​the working face roof, analyzes the low resistivity layer through the resistivity change cross-section diagram, and then delineates the abnormal area to achieve full coverage and fine detection of the ultra-wide coal mining working face roof.

[0011] A method for electrical tomography of the roof of an ultra-wide coal mining face, the method comprising the following steps:

[0012] Step 1: Data collection;

[0013] In two lanes on both sides of the top surface of the coal mining face, the transmitting coil in the first sensing device and the magnetic probe in the second sensing device are alternately used to continuously record the attenuation curve of the vertical component (Hz) of the magnetic field and the attenuation curve of the induced electromotive force to obtain a forward detection data set; then, the transmitting coil in the second sensing device and the magnetic probe in the first sensing device are used to continuously record the attenuation curve of the vertical component (Hz) of the magnetic field and the attenuation curve of the induced electromotive force to obtain a reverse detection data set;

[0014] The transmitting coil is wound with a multi-core copper cable; the transmitting coil is rectangular and long, adapting to the length and width of the lane; the magnetic probe uses a three-component magnetic sensor, and the magnetic probes are arranged in an array with equal spacing of 10m;

[0015] The two transmitting coils are both connected to a transmitter, and the transmitter outputs a square wave current to the two transmitting coils, and the peak current output by the transmitter is 10A; when outputting the square wave current to the two transmitting coils, the coil parameters are kept consistent with the transmitting current;

[0016] The two groups of magnetic probes are used as measuring points to continuously record the attenuation curve of the vertical component (Hz) of the magnetic field and the attenuation curve of the induced electromotive force to obtain a forward detection data set and a reverse detection data set.

[0017] Step 2: Data processing;

[0018] Step 201: The preliminary data acquisition unit is used to obtain the original geological, hydrological data and well logging data of the mining area to preliminarily understand the distribution of electrical characteristics of the formation;

[0019] Step 202: The data preprocessing unit performs filtering and denoising on the acquired forward detection data set and reverse detection data set, saves the data after denoising, and stores it as a time series file;

[0020] In step 202, the original forward detection data set and the reverse detection data set are specifically filtered to eliminate early transition processes and late background noise; since the format of the collected original data is not suitable for processing, the data transmitted to the central control console is format converted and interference distortion points are eliminated. After completion, the data are stored in order from the earliest to the latest measurement date or from the largest to the smallest distance between the measurement line and the working surface during measurement to facilitate the next step of processing; the detection data set of each measuring point is normalized to a unit current (10A) response to eliminate the influence of emission current fluctuations.

[0021] Step 203: The apparent resistivity and detection range calculation is used to calculate the apparent resistivity value of each detection range grid in the magnetic field line area at the corresponding moment;

[0022] In step 203, the detection area of ​​the coal mining face roof is gridded into 10m×10m grids, and then time slices are selected. The diffusion range is calculated by the diffusion of smoke rings. For time t_i, based on the smoke ring diffusion radius formula:

[0023]

[0024] Calculate the diffusion range of the smoke ring at different times t, t 1 1-2 , t 2 1-2 , t 3 1-2 , t n 1-2 , t 1 2-1 , t 2 2-1 , t 3 2-1 , t n 2-1 The detection area within the magnetic lines of force shown at the moment, that is, the detection range, is calculated using the induced electromotive force formula:

[0025]

[0026] Calculate the apparent resistivity of the detection range.

[0027] The initial conductivity is σ0; the corresponding background resistivity is ρ0, μ = 4π×10 -7 H / m, solve for apparent resistivity and get: t 1 1-2 , t 2 1-2 , t 3 1-2 , tn1-2 , t 1 2-1 , t 2 2-1 , t 3 2-1 , t n 2-1 The apparent resistivity value within each detection range grid in the magnetic field line area at that moment.

[0028] Step 204: the overlapping area determination unit performs fusion imaging on the bidirectional data;

[0029] Overlapping area determination: For each detection area, if it is covered by both the forward detection area and the reverse detection area, it is marked as an overlapping area; the resistivity arithmetic average of the overlapping area is performed:

[0030]

[0031] For non-overlapping area resistivity: only positive coverage: directly use ρ 1→2 ; Only reverse coverage: directly use ρ 2→1 .

[0032] Step 205: The data interpretation unit observes the resistivity change characteristics of the roof through the resistivity calculation value, analyzes the low resistivity layer through the resistivity change cross-section diagram to accurately identify the abnormal area of ​​the working face roof, and then identifies the abnormal area to achieve full coverage and fine detection of the ultra-wide coal mining working face roof.

[0033] The technical effects achieved by the present invention are:

[0034] This invention addresses the challenges presented by the prior art by proposing a transient electromagnetic fluoroscopy technique. A transmitting coil is placed on one side of the tunnel to generate a pulsed magnetic field, while the induced eddy current field signals are received on the other side. This method leverages the strong penetrating properties of low-frequency transient electromagnetic fields, achieving a detection range far exceeding that of high-frequency radio waves. It also requires no explosives source and is adaptable to complex underground environments.

[0035] The present invention eliminates blind spots by alternating transmission and reception, combines dynamic grid resistivity fusion with the smoke ring diffusion model, and achieves high-precision imaging of the electrical structure of the ultra-wide working face roof, providing reliable technical support for hidden disaster prevention and control.

[0036] The present invention is suitable for high-precision detection of hidden geological structures in the roof of working faces in coal mines with a width exceeding 300 meters and a depth of less than 100 meters. It adopts transient electromagnetic perspective detection technology, and through the alternating arrangement of transmitting and receiving tunnels and dynamic data fusion, it realizes full coverage imaging of the electrical structure of the roof of the working face. In combination with the smoke ring effect diffusion model, a time-space mapping relationship is established to realize all-round imaging of the roof. For the refined detection of hidden geological structures in the roof of ultra-wide coal mining working faces (width ≥ 300 meters), it adopts two-way detection and grid subdivision, and through the replacement of the transmitting and receiving tunnels, it eliminates the blind spot of single-side detection and realizes full coverage imaging of the grid within the detection range. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a schematic diagram of an observation system in an electrical tomography method for roof of an ultra-wide coal mining face according to the present invention;

[0038] Figure 2 This is a schematic diagram of bidirectional full coverage detection of the roof in the present invention;

[0039] Figure 3 This is a flow chart of an electrical tomography method for an ultra-wide coal mining face roof according to the present invention. DETAILED DESCRIPTION

[0040] In order to make the purpose and advantages of the present invention more clearly understood, the present invention is described in detail below with reference to the following examples. It should be understood that the following text is only used to describe one or more specific embodiments of the present invention and does not strictly limit the scope of protection of the present invention.

[0041] like Figure 1-Figure 3 As shown, an electrical tomography system for the roof of an ultra-wide coal mining face comprises a first sensor device and a second sensor device respectively arranged in lanes on both sides of the ultra-wide coal mining face; and further comprises a data processing and display module;

[0042] The first sensing device and the second sensing device both include transmitting coils distributed in a rectangular shape in the corresponding lane and magnetic probes distributed in an array with equal spacing;

[0043] The data processing and display module includes a preliminary data acquisition unit, a data preprocessing unit, an apparent resistivity and detection range calculation unit, an overlapping area determination unit, and a data interpretation unit;

[0044] The preliminary data acquisition unit is used to obtain the original geological and hydrological data of the mining area. The data preprocessing unit filters and denoises the forward detection data set and the reverse detection data set obtained by the first sensing device and the second sensing device; the apparent resistivity and detection range calculation is used to calculate the apparent resistivity value in each detection range grid in the magnetic line area at the corresponding moment; the overlapping area determination unit fuses and images the bidirectional data; the data interpretation unit: observes the resistivity change characteristics of the roof through the resistivity calculation value, and in order to accurately delineate the abnormal area of ​​the working face roof, analyzes the low resistivity layer through the resistivity change cross-section diagram, and then delineates the abnormal area to achieve full coverage and fine detection of the roof of the ultra-wide coal mining working face.

[0045] like Figure 1-Figure 3 As shown, a method for electrical tomography of the roof of an ultra-wide coal mining face is provided, and the imaging method comprises the following steps:

[0046] Step 1: Data collection;

[0047] In two lanes on both sides of the top surface of the coal mining face, the transmitting coil in the first sensing device and the magnetic probe in the second sensing device are alternately used to continuously record the attenuation curve of the vertical component (Hz) of the magnetic field and the attenuation curve of the induced electromotive force to obtain a forward detection data set; then, the transmitting coil in the second sensing device 1 and the magnetic probe in the first sensing device 2 are used to continuously record the attenuation curve of the vertical component (Hz) of the magnetic field and the attenuation curve of the induced electromotive force to obtain a reverse detection data set;

[0048] Each transmitting coil is wound with a multi-core copper cable; the rectangular long transmitting coil is adapted to the long length of the lane, and the magnetic probe uses a three-component magnetic sensor, which is distributed in an array with equal spacing of 10m.

[0049] Both transmitting coils are connected to a transmitter, which outputs square wave current to the two transmitting coils. The peak current output by the transmitter is 10A. When outputting square wave current to the two transmitting coils, the coil parameters are kept consistent with the transmitting current.

[0050] Two sets of magnetic probes are used as measuring points to continuously record the attenuation curve of the vertical component (Hz) of the magnetic field and the attenuation curve of the induced electromotive force to obtain the forward detection data set and the reverse detection data set.

[0051] Step 2: Data processing;

[0052] Step 201: The preliminary data acquisition unit is used to obtain the original geological, hydrological data and well logging data of the mining area to obtain a preliminary understanding of the distribution of electrical characteristics of the formation;

[0053] Step 202: The data preprocessing unit performs filtering and denoising on the acquired forward detection data set and reverse detection data set, and saves the data after denoising as a time series file;

[0054] In step 202, the original forward detection data set and the reverse detection data set are specifically filtered to eliminate early transition processes and late background noise; since the format of the collected original data is not suitable for processing, the data transmitted to the central control console is format converted and interference distortion points are eliminated. After completion, the data is stored in order from the earliest to the latest measurement date or from the largest to the smallest distance between the measurement line and the working surface during measurement to facilitate the next step of processing; the Hz data of each measuring point is normalized to a unit current (10A) response to eliminate the influence of emission current fluctuations.

[0055] Step 203: Apparent resistivity and detection range calculation is used to calculate the apparent resistivity value of each detection range grid in the magnetic field line area at the corresponding time;

[0056] In step 203, the roof detection area of ​​the coal mining face is gridded, and time slices are selected at logarithmic equal intervals. After the grid is divided into 10m×10m grids, the time slices are selected and the diffusion range is calculated by the diffusion of the smoke ring. For time t_i, based on the smoke ring diffusion radius formula:

[0057]

[0058] Calculate the diffusion range of the smoke ring at different times t, that is, the detection range, and then calculate the induced electromotive force using the formula:

[0059]

[0060] Calculate the apparent resistivity of the detection range.

[0061] The initial conductivity is σ0; the corresponding background resistivity is ρ0, μ = 4π×10 -7 H / m, solve for the apparent resistivity and we get Figure 2 Chinese: t 1 1-2 , t 2 1-2 , t 3 1-2 , t n1-2 , t 1 2-1 , t 2 2-1 , t 3 2-1 , t n 2-1 The apparent resistivity value within each detection range grid in the magnetic field line area at that moment.

[0062] Step 204: The overlapping area determination unit performs fusion imaging on the bidirectional data;

[0063] Overlapping area determination: For each detection area, if it is covered by both the forward detection area and the reverse detection area, it is marked as an overlapping area; the resistivity arithmetic average of the overlapping area is performed:

[0064]

[0065] For non-overlapping area resistivity: only positive coverage: directly use ρ 1→2 ; Only reverse coverage: directly use ρ 2→1 .

[0066] In actual use, steps 203 and 204 of the present invention perform preliminary processing, calculate the equivalent current loop lateral diffusion radius, and perform refined grid subdivision. The detection area of ​​the hypothetical coal mining face roof of 300m (width) × 100m (depth) is divided into a regular grid of 10m × 10m (a total of 30 × 10 = 300 cells). The initial model is set as a uniform half-space with background resistivity ρ. Spatiotemporal mapping of smoke ring diffusion: Based on the equivalent current loop lateral diffusion model, the relationship between time slices (t1, t2, ..., tn) and detection range is established: (equivalent current loop diffusion radius) where σ_i is the conductivity at time t_i, μ is the magnetic permeability of the medium, and then The calculation formula for the induced electromotive force of the separation loop and the formula for the lateral diffusion of the smoke ring can be used to obtain the conductivity and resistivity values ​​of the area from the time (t1, t2, ..., tn) and distance. Then, the apparent resistivity of the area on the other side can be obtained by exchanging the transmitting and receiving lanes. For the grid cells in the overlapping area of ​​the two-way detection, the arithmetic average is taken:

[0067]

[0068] Single-side data is directly used in non-overlapping areas to ensure full grid coverage.

[0069] Step 205: Data interpretation unit: Observe the resistivity change characteristics of the roof through the resistivity calculation value. In order to accurately delineate the abnormal area of ​​the working face roof, analyze the low resistivity layer through the resistivity change cross-section diagram, and then delineate the abnormal area to achieve full coverage and fine detection of the ultra-wide coal mining working face roof.

[0070] The present invention addresses the problems in the background technology and proposes a transient electromagnetic perspective technology; a transmitting coil is arranged on one side of the tunnel to excite a pulsed magnetic field, and an induced eddy current field signal is received on the other side. This method utilizes the strong penetrability of low-frequency transient electromagnetic fields, and its detection distance is far superior to that of high-frequency radio waves. It does not require an explosive vibration source and is adaptable to complex underground environments. The present invention also solves the limitations of the theoretical model of existing transient electromagnetic perspective technology: compared with the traditional uniform half-space assumption, it is difficult to characterize complex electrical structures, and the imaging resolution is more accurate; the present invention also solves the problem of single-sided detection blind spots in existing transient electromagnetic perspective technology; compared with the edge of the ultra-wide working face, the signal attenuation is serious, and a single transmission-reception is difficult to cover the entire area; the present invention achieves full area coverage through two alternating detections, and the present invention calculates the apparent resistivity and detection range to calculate the apparent resistivity value of each detection range grid in the magnetic field line area at the corresponding moment; it provides efficient and reliable data processing means to accurately locate the boundaries of geological anomalies.

[0071] Initial data processing: Since the format of the collected raw data is not suitable for processing, the data transmitted to the central control console is converted into a new format and interference distortion points are removed. After completion, the data is stored in order from the earliest to the latest measurement date or from the largest to the smallest distance between the measurement line and the working surface during measurement, so as to facilitate the next step of processing;

[0072] Resistivity calculation: First, using the time resistivity calculation formula for the split-loop device and the equivalent current loop horizontal diffusion calculation formula, the resistivity within the area transmitted by the transmitting coil of the first sensing device in the first lane and received by the magnetic probe of the second sensing device in the second lane is calculated. A sign change in the received data indicates that the smoke ring has reached the boundary. The transmitting coil of the second sensing device in the second lane is then used to transmit data, and the magnetic probe of the first sensing device in the first lane receives the data for further resistivity calculation within that area. Full coverage of the detection area is achieved, and the resistivity is averaged in areas of overlapping resistivity. Single-sided data is used in non-overlapping areas.

[0073] Imaging display: Imaging is performed based on the obtained apparent resistivity value to delineate the abnormal roof area.

[0074] The present invention adopts bidirectional detection and grid division, and eliminates the blind spot of unilateral detection by exchanging the lanes of the transmitting and receiving ends, thereby achieving full grid coverage within the detection range.

[0075] The foregoing is merely a preferred embodiment of the present invention. It should be noted that those skilled in the art may make various improvements and modifications without departing from the principles of the present invention, and such improvements and modifications are also within the scope of protection of the present invention. Structures, devices, and operating methods not specifically described or explained herein shall, unless otherwise specified or limited, be implemented in accordance with conventional means in the art.

Claims

1. An electrical tomography system for the roof of an ultra-wide coal mining face, characterized by: It includes a first sensor device and a second sensor device respectively arranged in the lanes on both sides of the ultra-wide coal mining working face; and also includes a data processing and display module; The first sensing device and the second sensing device both include transmitting coils distributed in a rectangular shape in the corresponding lane and magnetic probes distributed in an array with equal spacing; The data processing and display module includes a preliminary data acquisition unit, a data preprocessing unit, an apparent resistivity and detection range calculation unit, an overlapping area determination unit, and a data interpretation unit; The preliminary data acquisition unit is used to obtain the original geological and hydrological data of the mining area, and the data preprocessing unit filters and denoises the forward detection data set and the reverse detection data set obtained by the first sensing device and the second sensing device; the apparent resistivity and detection range calculation is used to calculate the apparent resistivity value in each detection range grid in the magnetic line area at the corresponding moment; the overlapping area determination unit performs fusion imaging on the bidirectional data; the data interpretation unit: observes the resistivity change characteristics of the roof through the resistivity calculation value, analyzes the low resistivity layer through the resistivity change cross-section diagram, and then delineates the abnormal area to achieve full coverage and fine detection of the roof of the ultra-wide coal mining working face.

2. The imaging method of the electrical tomography system for the roof of an ultra-wide coal mining face according to claim 1, characterized in that: The imaging method comprises the following steps: Step 1: Data collection; In two lanes on both sides of the top surface of the coal mining face, the transmitting coil in the first sensing device and the magnetic probe in the second sensing device are used alternately to continuously record the attenuation curve of the vertical component (Hz) of the magnetic field and the attenuation curve of the induced electromotive force to obtain a forward detection data set; then, the transmitting coil in the second sensing device (1) and the magnetic probe in the first sensing device (2) are used to continuously record the attenuation curve of the vertical component (Hz) of the magnetic field and the attenuation curve of the induced electromotive force again to obtain a reverse detection data set; Step 2: Data processing; Step 201: The preliminary data acquisition unit is used to obtain the original geological, hydrological data and well logging data of the mining area to preliminarily understand the distribution of electrical characteristics of the formation; Step 202: The data preprocessing unit performs filtering and denoising on the acquired forward detection data set and reverse detection data set, saves the data after denoising, and stores it as a time series file; Step 203: The apparent resistivity and detection range calculation is used to calculate the apparent resistivity value of each detection range grid in the magnetic field line area at the corresponding moment; Step 204: the overlapping area determination unit performs fusion imaging on the bidirectional data; Step 205: The data interpretation unit observes the resistivity change characteristics of the roof through the resistivity calculation value, analyzes the low resistivity layer through the resistivity change cross-section diagram, and then delineates the abnormal area to achieve full coverage and fine detection of the roof of the ultra-wide coal mining working face.

3. The method for electrical tomography of the roof of an ultra-wide coal mining face according to claim 2, characterized in that: The two transmitting coils are wound with multi-core copper cables; the rectangular long transmitting coil is adapted to the long length of the lane, and the magnetic probe is a three-component magnetic sensor, and the magnetic probes are arranged in an array with equal spacing of 10m. The two transmitting coils are both connected to a transmitter, and the transmitter outputs a square wave current to the two transmitting coils, and the peak current output by the transmitter is 10A; when outputting the square wave current to the two transmitting coils, the coil parameters are kept consistent with the transmitting current; The two groups of magnetic probes are used as measuring points to continuously record the attenuation curve of the vertical component (Hz) of the magnetic field and the attenuation curve of the induced electromotive force to obtain a forward detection data set and a reverse detection data set.

4. The method for electrical tomography of the roof of an ultra-wide coal mining face according to claim 3, characterized in that: In step 202, the original forward detection data set and the reverse detection data set are filtered to eliminate early transition processes and late background noise; the data transmitted to the central control console is format converted and interference distortion points are removed. After completion, the data are stored in order from the earliest to the latest measurement date or from the largest to the smallest distance between the measurement line and the working surface during measurement.

5. The method for electrical tomography of the roof of an ultra-wide coal mining face according to claim 4, characterized in that: In step 203, time slices are selected at equal logarithmic intervals, and the diffusion range is calculated by the smoke ring diffusion. For time t_i, based on the smoke ring diffusion radius formula: Calculate the diffusion range of the smoke ring at different times t, that is, the detection range, and then calculate the induced electromotive force using the formula: Calculate the apparent resistivity of the detection range. The initial conductivity is σ0; the corresponding background resistivity is ρ0, μ = 4π×10 -7 H / m, solve for apparent resistivity and get: t 11-2 , t 21-2 , t 31-2 , t n1-2 , t 12-1 , t 22-1 , t 32-1 , t n2-1 The apparent resistivity value within each detection range grid in the magnetic field line area at that moment.

6. The method for electrical tomography of the roof of an ultra-wide coal mining face according to claim 5, characterized in that: In step 204, overlapping area determination: for each detection area, if it is covered by both the forward detection area and the reverse detection area, it is marked as an overlapping area; Take the arithmetic mean of the resistivity of the overlapping area: For non-overlapping area resistivity: only positive coverage: directly use ρ 1→2 ; Only reverse coverage: directly use ρ 2→1 .

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