A fast apparent resistivity imaging method with strong boundary recognition capability based on magnetic field gradient

Abstract

The present invention discloses a fast apparent resistivity imaging method with strong boundary recognition ability based on magnetic field gradient, belonging to the field of urban exploration, and comprising the following steps: Step 1, adopt non-contact measurement of a drag-type rectangular array under multi-source and multi-frequency excitation, and read the magnetic field intensity H in the z direction z data; Step 2, obtain the spatial gradient H z Δx by combining the magnetic field intensities of two adjacent measuring points and the distance therebetween, and obtain the frequency gradient H z Δz by combining two adjacent measuring frequencies and their corresponding magnetic field intensities at the same measuring point; Step 3, use the spatial gradient H z Δx and the distance between two adjacent measuring points to obtain the spatial apparent resistivity change Δρ a x between two adjacent measuring points, and use the frequency gradient H z Δz and two adjacent measuring frequencies to obtain the frequency apparent resistivity change Δρ a z between two adjacent frequencies; Step 4, combine the resistivity of a measuring point in the survey area, and according to the calculation formula of the magnetic field gradient apparent resistivity, obtain the apparent resistivity of any point in the survey area at any transmitting frequency; Step 5, combine the skin depth formula to draw an apparent resistivity - apparent depth pseudo-section diagram.

Description

Technical Field

[0001] The invention belongs to the field of urban detection, and relates to an electromagnetic exploration imaging method, in particular to a rapid apparent resistivity imaging method with strong boundary recognition capability based on magnetic field gradient for underground abnormal bodies in urban detection. Background Art

[0002] The electromagnetic method is based on the differences in the electrical conductivity, magnetic permeability, and dielectric properties of underground media. By observing and studying the distribution of artificial or natural alternating electromagnetic fields, the electrical and magnetic parameters of underground media are obtained, thereby achieving the purpose of exploration. The frequency domain electromagnetic method uses the skin effect of electromagnetic induction, that is, high-frequency electromagnetic fields penetrate shallowly and low-frequency electromagnetic fields penetrate deeply. The purpose of depth measurement is achieved by changing the frequency of the electromagnetic field. Compared with the time domain electromagnetic method, the frequency domain electromagnetic method has high resolution, can penetrate high-resistance layers, has little anisotropy, and has high work efficiency. At present, the controlled source frequency domain electromagnetic detection method generally adopts absolute field measurement and processing, which is insufficient in background interference suppression and boundary identification, and it is difficult to achieve fine detection of underground electrical structures; inversion can accurately obtain convenient information about anomalies, but it consumes a lot of resources and time. On-site rapid imaging methods are of great significance in the detection of urban underground space.

[0003] As urban space resources have become increasingly scarce in recent years, the rational development of urban underground space has become increasingly important. Non-invasive underground exploration methods, as an important step in underground space exploitation, can effectively avoid the harm caused by blind exploitation. Due to the complex urban detection environment and limited available space, conventional field detection methods are difficult to apply to urban space detection scenarios. The use of array source electromagnetic gradient drag-type rectangular array measurement can effectively meet the needs of urban underground space detection: non-destructive measurement to avoid damage to urban roads in the early exploration; the use of drag-type measurement, the platform is small and portable, flexible and convenient to use, and adapts to complex urban road conditions.

[0004] The invention patent with publication number CN115508900A discloses a ground towed transient electromagnetic imaging method and system, which uses a CNN-LSTM network model to obtain a target resistivity model, improving the inversion speed. However, the accuracy of identifying the boundary of anomalies is insufficient.

[0005] The invention patent with publication number CN115437015A discloses a method for delineating the boundary of a bad geological body using apparent resistivity and resistivity. First, the regional apparent conductivity data is widely measured, then the horizontal position of the boundary of the bad geological body is delineated and detailed, and finally the boundary of the bad conductor is determined by combining the apparent resistivity data and the resistivity data. However, this method requires multiple measurements of the same area and is not suitable for large-scale detection.

[0006] The invention patent with publication number CN109541695A discloses a method for rapid imaging of far-zone apparent resistivity of artificial field source frequency domain electric field gradient, which improves the response capability of the horizontal and vertical boundaries of the abnormal body by obtaining the electric field gradient. However, a large number of electrodes need to be laid on the ground, which is inconvenient for application in urban scenes.

[0007] In summary, a convenient, fast and strong boundary recognition non-destructive detection method suitable for urban environment detection is needed. Summary of the invention

[0008] The object of the present invention is to provide a rapid apparent resistivity imaging method with strong boundary recognition capability based on magnetic field gradient in view of the shortcomings of the above-mentioned existing apparent resistivity imaging methods.

[0009] The objective of the present invention is achieved through the following technical solutions:

[0010] The method for rapid apparent resistivity imaging with strong boundary recognition capability based on magnetic field gradient includes the following steps:

[0011] Step 1: Use the towed rectangular array non-contact measurement under multi-source multi-frequency excitation to read the z-direction magnetic field intensity H received by the system under the excitation of the array element. z data;

[0012] Step 2: Combine the magnetic field strength and the distance between two adjacent measuring points to calculate the spatial gradient H according to the spatial gradient calculation formula. z Δx The frequency gradient H is calculated by combining the two adjacent measurement frequencies and the corresponding magnetic field strength at the same measurement point according to the frequency gradient calculation formula. z Δz ;

[0013] Step 3: Use the spatial gradient H z Δx The distance between the adjacent measuring points is calculated according to the calculation formula of spatial apparent resistivity change to obtain the spatial apparent resistivity change Δρ between two adjacent measuring points a x , using the frequency gradient H z Δz The frequency apparent resistivity change Δρ of two adjacent frequencies is calculated according to the frequency apparent resistivity change calculation formula. a z ;

[0014] Step 4: Combined with the resistivity of a certain measuring point in the measuring area, the apparent resistivity of any point in the measuring area at any transmitting frequency can be calculated according to the calculation formula of the magnetic field gradient apparent resistivity;

[0015] Step 5: Combine the skin depth formula to draw a pseudo-section diagram of apparent resistivity-apparent depth.

[0016] Preferably, in step 1, H z is the magnetic field intensity in the z direction under the excitation of the array source, which is formed by the superposition of the responses of multiple electric dipole sources. The specific formula is as follows:

[0017]

[0018] Where P e is the emission magnetic moment, y is the vertical coordinate, ω is the angular frequency, μ is the vacuum permeability, r i is the transmitting and receiving distance of measuring point i;

[0019] Preferably, in step 2, the spatial gradient H z Δx It is the magnetic field intensity H in the z direction at the same measurement frequency at adjacent measurement points on the same measurement line. z Make the distance difference quotient, frequency gradient H z Δz The logarithm of two adjacent measurement frequencies at the same measuring line and the same measuring point is taken, and the difference is calculated to obtain the magnetic field intensity H in the z direction at adjacent frequencies. z The amplitude is used as the difference quotient, and the specific formula is as follows:

[0020] The measuring point j on the measuring line i at the frequency f k The spatial gradient at time is:

[0021]

[0022] Where, ΔL xi Indicates the distance between adjacent measuring points.

[0023] Preferably, in step 2, at the measuring point j on the measuring line i, at the frequency f k The frequency gradient at is:

[0024]

[0025] In the formula, f k 、f k-1 (k=1,2…n, n is the total number of measurement frequencies) represents two adjacent measurement frequencies;

[0026] Preferably, in step 3, when a long wire is used as the transmitting source, when the distance from the midpoint of the transmitting source to the observation point is greater than 3-5 times the length of the wire source, the electromagnetic field at the observation point can be considered as a dipole field. When the measurement area is located in a remote area, the apparent resistivity of the earth is:

[0027]

[0028] In the formula, rj is the transmit-receive distance of measuring point j.

[0029] Preferably, in step 3, the spatial apparent resistivity change Δρ a x and the combined apparent resistivity change Δρ a z , is derived by combining the existing calculation formula of far-zone apparent resistivity with the spatial gradient and frequency gradient formulas, where:

[0030] The spatial apparent resistivity variation between two adjacent measuring points is:

[0031]

[0032] The frequency apparent resistivity change between two adjacent frequencies is:

[0033]

[0034] Preferably, in step 3, any measuring point j in the measuring area L At the measurement frequency f k When the magnetic field gradient apparent resistivity is calculated as:

[0035]

[0036] Wherein, l = 1, 2, 3, ..., n; k = 1, 2, 3, ..., n; ρ(i, j0, f0) is the apparent resistivity value of the selected measuring point j0 on the measuring line i in the measuring area at the frequency f0.

[0037] Beneficial effects of the present invention:

[0038] The present invention improves the lateral boundary response capability to the abnormal body by obtaining the spatial gradient of the magnetic field;

[0039] The present invention improves the longitudinal boundary response capability to the abnormal body by obtaining the magnetic field frequency gradient;

[0040] This method uses array source electromagnetic gradient dragging rectangular array measurement to perform contactless magnetic field gradient measurement. It does not require the deployment of electrodes or the digging of electrode pits in urban applications. It is a non-destructive measurement method. The dragging measurement platform is flexible and portable, which is conducive to improving detection efficiency and convenience, and is more suitable for urban underground space detection.

[0041] This method has small computational complexity and fast computational speed, can achieve rapid imaging, and can be used for on-site monitoring;

[0042] The results obtained by this method can be used as a reference for accurate inversion, and focused detection can be carried out based on the detected anomalies to achieve accurate identification of underground anomalies. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 A flow chart of a fast apparent resistivity imaging method with strong boundary recognition capability based on magnetic field gradient;

[0044] Figure 2 It is the top view of the three-dimensional earth model;

[0045] Figure 3 It is the front view of the three-dimensional earth model;

[0046] Figure 4 H x Response curve graph;

[0047] Figure 5 H y Response curve graph;

[0048] Figure 6 H z Response curve graph;

[0049] Figure 7 H x Gradient response curve diagram;

[0050] Figure 8 H y Gradient response curve diagram;

[0051] Fig. 9 H z Gradient response curve graph; DETAILED DESCRIPTION

[0052] The present invention is further described in detail below in conjunction with the accompanying drawings:

[0053] The method for rapid apparent resistivity imaging with strong boundary recognition capability based on magnetic field gradient includes the following steps:

[0054] Step 1: Use the towed rectangular array non-contact measurement under multi-source multi-frequency excitation to read the z-direction magnetic field intensity H received by the system under the excitation of the array element. z data;

[0055] Step 2: Combine the magnetic field strength and the distance between two adjacent measuring points to calculate the spatial gradient H according to the spatial gradient calculation formula. z Δx The frequency gradient H is calculated by combining the two adjacent measurement frequencies and the corresponding magnetic field strength at the same measurement point according to the frequency gradient calculation formula. z Δz ;

[0056] Step 3: Use the spatial gradient H z ΔxThe distance between the adjacent measuring points is calculated according to the calculation formula of spatial apparent resistivity change to obtain the spatial apparent resistivity change Δρ between two adjacent measuring points a x , using the frequency gradient H z Δz The frequency apparent resistivity change Δρ of two adjacent frequencies is calculated according to the frequency apparent resistivity change calculation formula. a z ;

[0057] Step 4: Combined with the resistivity of a certain measuring point in the measuring area, the apparent resistivity of any point in the measuring area at any transmitting frequency can be calculated according to the calculation formula of the magnetic field gradient apparent resistivity;

[0058] Step 5: Combine the skin depth formula to draw a pseudo-section diagram of apparent resistivity-apparent depth.

[0059] Figure 1 It is a fast apparent resistivity imaging method with strong boundary recognition capability based on magnetic field gradient, as follows:

[0060] The z-direction magnetic field strength H collected by the reading system z data;

[0061] The magnetic field intensity H in the z direction of adjacent measuring points on the same measuring line at the same measuring frequency z The amplitude is taken as the difference quotient and divided by the distance between adjacent measuring points. The spatial gradient H is calculated according to the spatial gradient calculation formula. z Δx , take the logarithm of two adjacent measurement frequencies on the same measuring line and the same measuring point, and make the difference to obtain the magnetic field intensity H in the z direction at adjacent frequencies. z The amplitude is used as the difference quotient, and the frequency gradient H is calculated according to the frequency gradient calculation formula. z Δz ,in:

[0062] The measuring point j on the measuring line i at the frequency f k The spatial gradient at time is:

[0063]

[0064] Where, ΔL xi Indicates the distance between adjacent measuring points;

[0065] The measuring point j on the measuring line i at the frequency f k The frequency gradient at is:

[0066]

[0067] In the formula, f k 、fk-1 (k=1,2…n, n is the total number of measurement frequencies) represents two adjacent measurement frequencies.

[0068] Then, combining the transmitting magnetic moment, vertical coordinate, angular frequency, vacuum magnetic permeability, and the transmitting and receiving distance of measuring point i, the spatial apparent resistivity change Δρ between two adjacent measuring points can be calculated. a x , and the frequency apparent resistivity change Δρ of two adjacent frequencies a z ,in:

[0069] The spatial apparent resistivity variation between two adjacent measuring points is:

[0070]

[0071] The frequency apparent resistivity change between two adjacent frequencies is:

[0072]

[0073] Then, the apparent resistivity of a certain point in the measurement area at a specific frequency is calculated using the calculation formula for the far-field apparent resistivity. Then, the apparent resistivity of any point in the measurement area at any transmission frequency can be calculated using the calculation formula for the magnetic field gradient apparent resistivity, where:

[0074] The apparent resistivity of a measuring point on a measuring line within the measuring area at a specific frequency is:

[0075]

[0076] Where P e is the emission magnetic moment, y is the vertical coordinate, ω is the angular frequency, μ is the vacuum permeability, r j is the transmitting and receiving distance of measuring point j;

[0077] Any measuring point j in the measuring area L At the measurement frequency f k When the magnetic field gradient apparent resistivity is calculated as:

[0078]

[0079] Wherein, l = 1, 2, 3, ..., n; k = 1, 2, 3, ..., n; ρ(i, j0, f0) is the apparent resistivity value of the selected measuring point j0 on the measuring line i in the measuring area at the frequency f0.

[0080] Finally, combined with the skin depth formula, a pseudo-section diagram of apparent resistivity-apparent depth is drawn.

[0081] Top view of the three-dimensional earth model Figure 2As shown, the emission source is placed along the y direction and located at the center of the xy plane of the geoelectric model. The low-resistance anomaly is located on the right side of the emission source. The dotted line is the observation plane. The front view of the three-dimensional geodetic model is shown in Figure 3 As shown, the upper boundary of the low-resistance anomaly is 200m away from the xy plane, the left boundary is 8050m away from the yz plane, and the right boundary is 8950m away from the yz plane.

[0082] H x The response curve is as follows Figure 4 As shown, it can be seen that there is no obvious abnormality; H y The response curve is as follows Figure 5 As shown, it can be seen that there is an anomaly at x = 8050m, while there is still no obvious anomaly at x = 8090m; H z The response curve is as follows Figure 6 As shown, it can be seen that there are anomalies at x=8050m and x=8950m, but their boundaries are not clear. Only the range of the anomaly can be roughly determined, and the boundary of the anomaly cannot be accurately located.

[0083] H x The gradient response curve is as follows Figure 7 As shown, it can be seen that there are small anomalies at x = 8050m and x = 8950m; H y The gradient response curve is as follows Figure 8 As shown, it can be seen that there are anomalies at x = 8050m and x = 8950m, but their boundaries are not clear; H z The gradient response curve is as follows Fig. 9 As shown, it can be seen that there are anomalies at x=8050m and x=8950m, and their boundaries are clear and the effect is obvious, and the position of the anomaly can be accurately located.

[0084] By comparison Figure 4 , Figure 5 , Figure 6 and Figure 7 , Figure 8 , Fig. 9 As a result, it is not difficult to find that the magnetic field gradient has a significantly better effect than the absolute field in boundary recognition, where the magnetic field intensity H in the z direction is z The gradient effect is particularly significant.

[0085] The calculation results of the above model show that the gradient field apparent resistivity has a stronger ability to identify the boundaries of anomalies and can locate the position of anomalies more accurately.

Claims

1. A fast apparent resistivity imaging method with strong boundary recognition capability based on magnetic field gradient, characterized in that: The towed matrix array non-contact measurement under multi-source multi-frequency excitation is used to calculate the apparent resistivity through magnetic field gradient, including the following steps: Step 1: Use the towed rectangular array non-contact measurement under multi-source multi-frequency excitation to read the z-direction magnetic field intensity H received by the system under the excitation of the array element. z data; Step 2: Combine the magnetic field strength and the distance between two adjacent measuring points to calculate the spatial gradient according to the spatial gradient calculation formula. The frequency gradient is calculated by combining the two adjacent measurement frequencies and the corresponding magnetic field strength at the same measurement point according to the frequency gradient calculation formula. ; Step 3: Using spatial gradient The distance between the adjacent measuring points is calculated according to the calculation formula of the spatial apparent resistivity change to obtain the spatial apparent resistivity change between two adjacent measuring points. , using the frequency gradient The frequency apparent resistivity change of two adjacent frequencies is calculated according to the frequency apparent resistivity change calculation formula. ; In step 3, the calculation formula for the change in spatial apparent resistivity is: ； Step 4: Combined with the resistivity of a certain measuring point in the measuring area, the apparent resistivity of any point in the measuring area at any transmitting frequency can be calculated according to the calculation formula of the magnetic field gradient apparent resistivity; Step 5: Combine the skin depth formula to draw a pseudo-section diagram of apparent resistivity-apparent depth.

2. The method for rapid apparent resistivity imaging with strong boundary recognition capability based on magnetic field gradient according to claim 1, characterized in that: In step 1, the magnetic field strength H z is the magnetic field intensity in the z direction under the excitation of the array source, which is the superposition of the responses of multiple electric dipole sources: , Where P e is the emission magnetic moment, y is the vertical coordinate, ω is the angular frequency, μ is the vacuum permeability, r i is the transmit-receive distance of measuring point i.

3. The method for rapid apparent resistivity imaging with strong boundary recognition capability based on magnetic field gradient according to claim 1, characterized in that: In step 2, the spatial gradient calculation formula is: The measuring point j on the measuring line i at the frequency f k The frequency gradient at is: ； Wherein, fk, fk-1 represent two adjacent measurement frequencies, k=1,2…n, and n is the total number of measurement frequencies.

4. The method for rapid apparent resistivity imaging with strong boundary recognition capability based on magnetic field gradient according to claim 1, characterized in that: In step 3, the frequency-dependent resistivity change calculation formula is: 。 5. The method for rapid apparent resistivity imaging with strong boundary recognition capability based on magnetic field gradient according to claim 1, characterized in that: In step 3, the frequency-dependent resistivity change calculation formula is: Any measuring point j in the measuring area L At the measurement frequency f k hour: ； Where l = 1, 2, 3, ..., n, k = 1, 2, 3, ..., n; It is the apparent resistivity value of the selected measuring point j0 on the measuring line i in the measuring area at the frequency f0.

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