Tunnel hole frequency domain electromagnetic advanced detection method and system based on decoupling coil

Abstract

The present application relates to the technical field of tunnel frequency domain electromagnetic advanced detection, and provides a tunnel hole frequency domain electromagnetic advanced detection method and system based on decoupling coils, which comprises the following steps: acquiring vertical magnetic field measurement data of each receiving coil in the decoupling coil device; combining the vertical magnetic field measurement data of the receiving coils two by two, and obtaining multi-directional gradient observation data through differential observation; for each receiving coil, calculating a first apparent resistivity based on the vertical magnetic field measurement data; for each direction, calculating a second apparent resistivity based on the gradient observation data; and outputting the apparent resistivity distribution after error verification based on the first apparent resistivity and the second apparent resistivity. The apparent resistivity imaging distribution in front of the tunnel face can be quickly, comprehensively and accurately obtained.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of tunnel frequency domain electromagnetic advanced detection, and particularly relates to a tunnel hole frequency domain electromagnetic advanced detection method and system based on a decoupling coil. BACKGROUND

[0002] The statements in this section merely provide background information related to the present application and do not necessarily constitute prior art.

[0003] Affected by strong plate activity, the construction of dangerous mountainous tunnel (hole) projects faces extremely complex geological conditions such as super-high water pressure and high ground stress, the geological genesis and occurrence characteristics of disaster-causing water bodies and disaster sources such as strong karst are increasingly complex and variable, and the disaster-causing effect is more sudden and destructive, which puts forward higher requirements for the long-distance detection of disaster-causing water bodies.

[0004] At present, the detection distance of the mainstream tunnel electrical method and electromagnetic water detection method is only tens of meters, for example, the detection distance of the transient electromagnetic method is generally about 60 meters (there is a detection blind area), the advanced detection distance of the tunnel induced polarization method is 30-40 meters, and the advanced detection distance of the geological radar method is only 20-30 meters.

[0005] In comparison, the controlled source frequency domain electromagnetic method has the advantages of strong anti-interference ability and large detection depth, and has been widely applied in the fields of mineral resource exploration and groundwater exploration. Based on this, a tunnel frequency domain electromagnetic advanced detection method is proposed, which uses the advantage of "frequency sounding" to realize the detection of disaster-causing water bodies at a farther distance. However, the main problem is that the traditional artificial source frequency domain electromagnetic method (such as the controlled source audio-frequency magnetotelluric method CSAMT) usually requires observation in the far zone, because in the far zone the primary field interference has decayed completely, the wave field form is relatively simple, and it is easy to solve, while the internal space of the tunnel is narrow, the observation distance is limited, resulting in a small transmitting-receiving distance and a high primary field amplitude, which completely covers the secondary field response signal containing geological information, making it difficult to obtain effective geological information. SUMMARY

[0006] In order to solve the technical problems existing in the background art, the present application provides a tunnel hole frequency domain electromagnetic advanced detection method and system based on a decoupling coil, which simultaneously carries out single-point measurement and differential measurement, and through multi-point parallel acquisition and differential analysis, rich multi-gradient observation data can be obtained, and the apparent resistivity obtained based on single-point measurement and differential measurement can be mutually checked, so that the apparent resistivity imaging distribution in front of the tunnel face can be quickly, comprehensively and accurately obtained.

[0007] In order to achieve the above purpose, the present application adopts the following technical solutions:

[0008] The first aspect of the present application provides a tunnel hole frequency domain electromagnetic advanced detection method based on a decoupling coil, which comprises:

[0009] Obtaining vertical magnetic field measurement data of each receiving coil in the decoupling coil device;

[0010] Combining the vertical magnetic field measurement data of the receiving coils two by two, and obtaining multi-directional gradient observation data through differential observation;

[0011] For each receiving coil, calculating a first apparent resistivity based on the vertical magnetic field measurement data;

[0012] For each direction, calculating a second apparent resistivity based on the gradient observation data;

[0013] After error verification based on the first apparent resistivity and the second apparent resistivity, outputting the apparent resistivity distribution.

[0014] Further, the decoupling coil device comprises a transmitting coil, a compensating coil and a plurality of receiving coils; the transmitting coil and the compensating coil are arranged concentrically and coplanarly, forming a weak magnetic flux region in the middle region of the transmitting coil and the compensating coil, and the receiving coils are coplanar with the transmitting coil and arranged in the weak magnetic flux region.

[0015] Further, for the two receiving coils combined, the gradient observation data in the direction of the line connecting the centers of the two receiving coils is , wherein is the distance between the centers of the two receiving coils, and the vertical magnetic field measurement data of the two receiving coils are d and Hzi , respectively. Hzj .

[0016] Further, the first apparent resistivity is

[0017] ;

[0018] ;

[0019] , wherein is the first apparent resistivity ，i is a complex number, is an angular frequency, is the air permeability, is the vertical magnetic field measurement data of the receiving coil, and the wave number , is the resistivity value, f is the frequency, and are the radii of the transmitting coil and the compensating coil, respectively, and are the current values input by the transmitting coil and the compensating coil, respectively, and Nt and Nc are the number of turns of the transmitter coil and the compensation coil respectively, and Nm is an intermediate parameter , is an integral variable, is the transceiver offset distance, and denote the zeroth and first order Bessel functions respectively.

[0020] Further, the second apparent resistivity is:

[0021] ;

[0022] ;

[0023] ;

[0024] wherein, is the second apparent resistivity, r i and r j are the transceiver offset distances of the receiver coil Rxi and the receiver coil Rxj respectively, is the gradient observation data, i is a complex number, is the angular frequency, is the air permeability, and k is the wave number , is the resistivity value, f is the frequency, and are the radii of the transmitter coil and the compensation coil respectively, and are the current values inputted by the transmitter coil and the compensation coil respectively, and are the number of turns of the transmitter coil and the compensation coil respectively, and Nm is an intermediate parameter , is an integral variable, and denote the zeroth and first order Bessel functions respectively.

[0025] The second aspect of the present application provides a tunnel hole inside frequency domain electromagnetic advanced detection system based on decoupling coil, which comprises:

[0026] a data acquisition module configured to acquire the vertical magnetic field measurement data of each receiver coil in the decoupling coil device;

[0027] a difference observation module configured to combine the vertical magnetic field measurement data of the receiver coils two by two, and obtain multi-directional gradient observation data through difference observation;

[0028] The first calculation module is configured to: for each receiving coil, calculate a first apparent resistivity based on vertical magnetic field measurement data;

[0029] The second calculation module is configured to calculate the second apparent resistivity for each direction based on gradient observation data.

[0030] The verification module is configured to perform error verification based on the first and second apparent resistivity, and then output the apparent resistivity distribution.

[0031] Furthermore, the decoupling coil device includes a transmitting coil, a compensating coil, and several receiving coils; the transmitting coil and the compensating coil are arranged concentrically and coplanarly, forming a weak magnetic flux region in the middle area between the transmitting coil and the compensating coil, and the receiving coil is coplanar with the transmitting coil and arranged in the weak magnetic flux region.

[0032] Furthermore, for the two combined receiving coils, the direction of the line connecting the centers of the two receiving coils is... The gradient observation data on the above are ,in, d The distance between the centers of the two receiving coils is given. The measured data of the vertical magnetic field of the two receiving coils are as follows: Hzi and Hzj .

[0033] Furthermore, the first apparent resistivity is:

[0034] ;

[0035] ;

[0036] in, First apparent resistivity ，i It is a complex number. Angular frequency, air permeability, To receive the vertical magnetic field measurement data of the receiving coil, wavenumber , This is the resistivity value. f For frequency, and These are the radii of the transmitting coil and the compensation coil, respectively. and These are the current values ​​input to the transmitting coil and the compensation coil, respectively. and These are the number of turns of the transmitting coil and the compensation coil, respectively, and intermediate parameters. , For integration variables, This is the transmit / receive offset. and These represent the zeroth-order and first-order Bessel functions, respectively.

[0037] Further, the second apparent resistivity is:

[0038] ;

[0039] ;

[0040] ;

[0041] wherein, is the second apparent resistivity, r i and r j Rxj are the transmitting coil and the compensation coil, is the gradient observation data, i is a complex number, is an angular frequency, is the air permeability, and the wave number , is the resistivity value, f is the frequency, and are the radii of the transmitting coil and the compensation coil, and are the current values input by the transmitting coil and the compensation coil, and are the number of turns of the transmitting coil and the compensation coil, and the intermediate parameter , is an integral variable, and represent the zero-order and first-order Bessel functions, respectively.

[0042] Compared with the prior art, the present application has the following beneficial effects:

[0043] The present application simultaneously carries out single-point measurement and differential measurement, and through multi-point parallel acquisition and differential analysis, rich multi-gradient observation data can be obtained, and the apparent resistivity imaging distribution in front of the tunnel face can be quickly, comprehensively and accurately obtained through mutual checking of the apparent resistivities obtained based on single-point measurement and differential measurement.

[0044] The present application proposes a differential gradient observation method of randomly combining two receiving coils at a time, which can not only obtain the vertical magnetic field data of a single receiving coil, but also obtain multi-directional magnetic field gradient observation data, and the gradient data is more sensitive to the resistivity change of underground medium, especially in a narrow tunnel space, the multi-directional gradient observation can reflect the distribution characteristics of the geological body in front from different angles, and effectively improves the recognition ability and resolution of small-scale, low-resistance or high-resistance abnormal bodies. BRIEF DESCRIPTION OF DRAWINGS

[0045] The accompanying drawings, which form a part of this specification, are included to provide a further understanding of the application, and are incorporated into and constitute a part of this specification. The embodiments of the application, and their

[0046] Figure 1 is a layout of an electromagnetic device according to an embodiment of the application;

[0047] Figure 2 is a schematic diagram of a decoupling coil device according to an embodiment of the application;

[0048] Figure 3 is a schematic diagram of an observation point according to an embodiment of the application;

[0049] Figure 4 is a schematic diagram of a result of apparent resistivity calculation for scenario one according to an embodiment of the application;

[0050] Figure 5 is a schematic diagram of a tunnel double- anomaly geological model according to an embodiment of the application;

[0051] Figure 6 is a schematic diagram of a result of apparent resistivity calculation for scenario two according to an embodiment of the application. DETAILED DESCRIPTION

[0052] In order to make the objects, technical solutions and advantages of the embodiments of the application clearer, the technical solutions in the embodiments of the application will be described clearly and completely below with reference to the drawings in the embodiments of the application.

[0053] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the application. Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the application pertains.

[0054] Embodiment One

[0055] The embodiment provides a tunnel hole-in frequency domain electromagnetic advanced detection method based on a decoupling coil.

[0056] The tunnel hole-in frequency domain electromagnetic advanced detection method based on a decoupling coil provided by the embodiment, as shown in Figure 1 relies on an electromagnetic device arranged in a tunnel.

[0057] In the embodiment, the electromagnetic device is a decoupling coil device. The decoupling coil device effectively offsets the primary field interference generated by a primary coil on a receiving coil by introducing a compensation coil with a reverse magnetic moment, thereby realizing direct acquisition of a pure secondary field frequency domain electromagnetic response signal.

[0058] As shown in Figure 2As shown, the decoupling coil device includes a transmitting coil, a compensation coil and a plurality of receiving coils; the transmitting coil and the compensation coil are arranged concentrically and coplanarly, and are excited with currents in the same direction and different sizes; by precisely regulating the key parameters such as the current size, the number of turns and the radius of the transmitting coil and the compensation coil, a weak magnetic flux area is formed in the middle area of the two coils, so that the total primary magnetic flux passing through the receiving coil is zero, and the primary field is effectively canceled out; the receiving coil is coplanar with the transmitting coil and the compensation coil and is arranged in the weak magnetic flux area; the primary magnetic flux of the transmitting coil and the compensation coil to the receiving coil is equal.

[0059] The decoupling coil device introduces a compensation coil with a reverse magnetic moment in the transmitting coil, so that the primary magnetic flux at the receiving coil approaches zero, thereby effectively suppressing the primary field interference under the condition of near-zero offset, directly obtaining the frequency domain electromagnetic response signal of pure secondary field, and significantly improving the data signal-to-noise ratio.

[0060] The decoupling coil device integrates the receiving coil array, and has the significant advantage of "primary transmission and multi-point reception".

[0061] The tunnel hole frequency domain electromagnetic advanced detection method based on the decoupling coil provided in the embodiment includes the following steps:

[0062] Step 1, determine the device parameters of the decoupling coil device, and supply the set current waveform into the transmitting coil and the compensation coil.

[0063] Among them, the device parameters include the radius, current size and number of turns of the transmitting coil and the compensation coil.

[0064] Step 2, obtain the vertical magnetic field measurement data of each receiving coil in the decoupling coil device.

[0065] Step 3, the vertical magnetic field measurement data of the plurality of receiving coils are randomly combined two by two, and gradient observation data in multiple directions are obtained through differential observation.

[0066] Specifically, two receiving coils Rxi and Rxj are randomly selected, the corresponding vertical magnetic field measurement data Hzi and Hzj are obtained, and the gradient observation data in the direction of the line connecting the centers of the two receiving coils is obtained by differential gradient, that is: the gradient observation data in the direction of the line connecting the centers of the two receiving coils is obtained. Wherein, d is the distance between the centers of the two receiving coils; the two receiving coils are randomly combined two by two, and the multi-angle gradient observation data can be obtained.

[0067] Step 4, for each receiving coil, based on the vertical magnetic field measurement data, the first apparent resistivity is calculated in combination with the device parameters.

[0068] Wherein, the first apparent resistivity calculation formula is as follows:

[0069] ;​

[0070] ;

[0071] wherein, is the first apparent resistivity, i is a complex number, is the angular frequency, is the air permeability, is the vertical magnetic field measurement data of a certain receiving coil , the wave number , is the resistivity value, f is the frequency, and are the radii of the transmitting coil and the compensation coil, respectively, and are the current values input by the transmitting coil and the compensation coil, respectively, and are the number of turns of the transmitting coil and the compensation coil, respectively, and the intermediate parameter , is the integral variable, is the transmitter-receiver offset distance, and represent the zeroth-order and first-order Bessel functions, respectively.

[0072] wherein the derivation process of the first apparent resistivity is as follows:

[0073] The analytical formula of the vertical magnetic field intensity of a single transmitting coil in a homogeneous full-space medium is: ; wherein, I is the current value input by the single transmitting coil, a is the radius of the single transmitting coil;

[0074] According to the electromagnetic field vector superposition principle, the calculation formula of the vertical magnetic field component of the decoupling coil device in a homogeneous full-space medium is: ;

[0075] Multiply the wave number on both sides of the above formula, and rearrange the above formula, then the first apparent resistivity calculation formula can be defined as follows: .

[0076] Step 5, for each direction, based on the gradient observation data, combined with the device parameters, calculate the second apparent resistivity.

[0077] wherein the second apparent resistivity calculation formula is as follows:

[0078] ;

[0079] ;

[0080] ;

[0081] wherein, is the second apparent resistivity, r i and r j respectively the transceiver offset distance of the receiving coil Rxi and the receiving coil Rxj, is the vertical magnetic field gradient observation data .

[0082] The embodiment innovatively proposes a differential gradient observation method of randomly combining two receiving coils, which can not only obtain the vertical magnetic field data of a single receiving coil, but also obtain multi-directional magnetic field gradient observation data. The gradient data is more sensitive to the resistivity change of the underground medium, especially in a narrow tunnel space, multi-directional gradient observation can reflect the distribution characteristics of the geological body in front from different angles, effectively improving the recognition ability and resolution of small-scale, low-resistance or high-resistance abnormal bodies.

[0083] Step 6, based on the first apparent resistivity and the second apparent resistivity, after error verification, output the apparent resistivity distribution, that is, the first apparent resistivity of each receiving coil and the second apparent resistivity of each direction.

[0084] Specifically, for a certain direction, the first apparent resistivity of the two receiving coils corresponding to the direction is obtained, the mean value of the first apparent resistivity of the two receiving coils is calculated, and compared with the second apparent resistivity in the direction, if the error is less than the threshold value, the first apparent resistivity of each receiving coil and the second apparent resistivity of each direction are output, otherwise, return to step 2.

[0085] The embodiment introduces a double apparent resistivity error verification mechanism, which compares the error of the average first apparent resistivity and the second apparent resistivity corresponding to the same direction to judge the data validity, and if the error exceeds the threshold value, the data is re-collected, which can effectively eliminate noise interference and abnormal data, and ensure that the final output of the apparent resistivity distribution result is more reliable.

[0086] As shown in Figure 1 , scene one: there is a 80m*80m*10m water-rich fault 100m in front of the tunnel face, the background resistivity is 1000Ω.m, and the resistivity of the water-rich fault is 1Ω.m. The calculation frequency of the decoupling coil device is 10Hz to 1000kHz. The measuring points are as shown in Figure 3 , the measuring points are distributed on the tunnel face along the y direction, and the distance between adjacent measuring points is 1m. The apparent resistivity distribution is as shown in Figure 4 , the horizontal axis is the frequency, the greater the frequency, the smaller the depth reflected, and the smaller the frequency, the greater the depth reflected. Figure 4The white dashed box in the middle indicates the approximate location of the water-rich fault, so the calculated apparent resistivity distribution can basically reflect the distribution of the water-rich fault.

[0087] like Figure 5 As shown in Scenario 2: There are two water-rich faults ahead of the tunnel face. The closer fault measures 20m × 20m × 5m, is 20m from the face, and has a resistivity of 5Ω·m. The farther fault measures 100m × 100m × 10m, is 120m from the face, and has a resistivity of 1Ω·m. The surrounding rock resistivity is 1000Ω·m. The calculation frequency of the decoupling coil device is 10Hz to 1000kHz. Figure 6 As shown, the apparent resistivity distribution map clearly reflects the existence of two water-rich faults. Based on the frequency, the location of the water-rich faults can be further determined.

[0088] The frequency domain electromagnetic advance detection method for tunnels based on decoupling coils provided in this embodiment simultaneously conducts single-point measurement and differential measurement. Through multi-point parallel acquisition and differential analysis, rich multi-gradient observation data can be obtained. Furthermore, the apparent resistivity obtained based on single-point measurement and differential measurement is cross-verified, which can quickly, comprehensively and accurately obtain the apparent resistivity imaging distribution in front of the tunnel face.

[0089] Example 2

[0090] The frequency domain electromagnetic advance detection system for tunnels based on decoupling coils provided in this embodiment includes:

[0091] The data acquisition module is configured to acquire vertical magnetic field measurement data of each receiving coil in the decoupling coil device;

[0092] The differential observation module is configured to combine the vertical magnetic field measurement data of the receiving coil in pairs and obtain gradient observation data in multiple directions through differential observation.

[0093] The first calculation module is configured to: for each receiving coil, calculate a first apparent resistivity based on vertical magnetic field measurement data;

[0094] The second calculation module is configured to calculate the second apparent resistivity for each direction based on gradient observation data.

[0095] The verification module is configured to perform error verification based on the first and second apparent resistivity, and then output the apparent resistivity distribution.

[0096] Further, the decoupling coil device comprises a transmitting coil, a compensating coil and a plurality of receiving coils; the transmitting coil and the compensating coil are arranged concentrically and coplanarly, a weak magnetic flux area is formed in the middle area of the transmitting coil and the compensating coil, and the receiving coils are coplanar with the transmitting coil and arranged in the weak magnetic flux area.

[0097] Further, for the combined two receiving coils, the gradient observation data in the direction of the line connecting the centers of the two receiving coils is , wherein, d is the distance between the centers of the two receiving coils, and the vertical magnetic field measurement data of the two receiving coils are Hzi and Hzj , respectively.

[0098] Further, the first apparent resistivity is:

[0099] ;

[0100] ;

[0101] , wherein, is the first apparent resistivity ，i is a complex number, is an angular frequency, is the air permeability, is the vertical magnetic field measurement data of the receiving coil, and the wave number , is the resistivity value, f is the frequency, and are the radii of the transmitting coil and the compensating coil, respectively, and are the current values inputted by the transmitting coil and the compensating coil, respectively, and are the number of turns of the transmitting coil and the compensating coil, respectively, and the intermediate parameter , is an integral variable, is the transmitting-receiving offset distance, and represent the zero-order and first-order Bessel functions, respectively.

[0102] Further, the second apparent resistivity is:

[0103] ;

[0104] ;

[0105] ;

[0106] , wherein, ​is the second apparent resistivity, r i and r j respectively the transmitting coil and the receiving coil, is the gradient observation data, i is a complex number, is the angular frequency, is the air permeability, the wave number , is the resistivity value, f is the frequency, and respectively the radius of the transmitting coil and the compensation coil, and respectively the current value inputted by the transmitting coil and the compensation coil, and respectively the number of turns of the transmitting coil and the compensation coil, the intermediate parameter , is the integral variable, and respectively represent the zeroth order and the first order Bessel functions.

[0107] It should be noted that each module in the embodiment corresponds to each step in the first embodiment one by one, and the specific implementation process is the same, which will not be repeated here.

[0108] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A frequency-domain electromagnetic advance detection method for tunnels based on decoupling coils, characterized in that, include: Acquire vertical magnetic field measurement data of each receiving coil in the decoupling coil device; The vertical magnetic field measurement data of the receiving coil are combined in pairs, and gradient observation data in multiple directions are obtained through differential observation. For each receiving coil, the first apparent resistivity is calculated based on the vertical magnetic field measurement data; For each direction, the second apparent resistivity is calculated based on gradient observation data; Based on the first and second apparent resistivity, after error verification, the apparent resistivity distribution is output. The decoupling coil device includes a transmitting coil, a compensating coil, and several receiving coils; the transmitting coil and the compensating coil are arranged concentrically and coplanarly, forming a weak magnetic flux region in the middle area between the transmitting coil and the compensating coil, and the receiving coils are coplanar with the transmitting coil and arranged in the weak magnetic flux region. The first apparent resistivity is: ； ； in, First apparent resistivity ，i It is a complex number. Angular frequency, air permeability, To receive the vertical magnetic field measurement data of the receiving coil, wavenumber , This is the resistivity value. f For frequency, and These are the radii of the transmitting coil and the compensation coil, respectively. and These are the current values ​​input to the transmitting coil and the compensation coil, respectively. and These are the number of turns of the transmitting coil and the compensation coil, respectively, and intermediate parameters. , For integration variables, This is the transmit / receive offset. and These represent the zeroth-order and first-order Bessel functions, respectively; The second apparent resistivity is: ； ； ； in, The second apparent resistivity, r i and r j These are the transmit / receive offsets of receiving coils Rxi and Rxj, respectively. This is gradient observation data.

2. The frequency domain electromagnetic advance detection method for tunnels based on decoupling coils as described in claim 1, characterized in that, For a combination of two receiving coils, the direction of the line connecting the centers of the two receiving coils is... The gradient observation data on the above are ,in, d The distance between the centers of the two receiving coils is given, and the measured data of the vertical magnetic field of the two receiving coils are as follows: Hzi and Hzj .

3. A frequency-domain electromagnetic advance detection system for tunnels based on decoupling coils, characterized in that, include: The data acquisition module is configured to acquire vertical magnetic field measurement data of each receiving coil in the decoupling coil device; The differential observation module is configured to combine the vertical magnetic field measurement data of the receiving coil in pairs and obtain gradient observation data in multiple directions through differential observation. The first calculation module is configured to: for each receiving coil, calculate a first apparent resistivity based on vertical magnetic field measurement data; The second calculation module is configured to calculate the second apparent resistivity for each direction based on gradient observation data. The verification module is configured to perform error verification based on the first and second apparent resistivity, and then output the apparent resistivity distribution. The decoupling coil device includes a transmitting coil, a compensating coil, and several receiving coils; the transmitting coil and the compensating coil are arranged concentrically and coplanarly, forming a weak magnetic flux region in the middle area between the transmitting coil and the compensating coil, and the receiving coils are coplanar with the transmitting coil and arranged in the weak magnetic flux region. The first apparent resistivity is: ； ； in, First apparent resistivity ，i It is a complex number. Angular frequency, air permeability, To receive the vertical magnetic field measurement data of the receiving coil, wavenumber , This is the resistivity value. f For frequency, and These are the radii of the transmitting coil and the compensation coil, respectively. and These are the current values ​​input to the transmitting coil and the compensation coil, respectively. and These are the number of turns of the transmitting coil and the compensation coil, respectively, and intermediate parameters. , For integration variables, This is the transmit / receive offset. and These represent the zeroth-order and first-order Bessel functions, respectively; The second apparent resistivity is: ； ； ； in, The second apparent resistivity, r i and r j These are the transmit / receive offsets of receiving coils Rxi and Rxj, respectively. This is gradient observation data.

4. The tunnel frequency domain electromagnetic advance detection system based on decoupling coils as described in claim 3, characterized in that, For a combination of two receiving coils, the direction of the line connecting the centers of the two receiving coils is... The gradient observation data on the above are ,in, d The distance between the centers of the two receiving coils is given, and the measured data of the vertical magnetic field of the two receiving coils are as follows: Hzi and Hzj .

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