Transient electromagnetic detection method for efficient detection

Abstract

The invention relates to a transient electromagnetic detection method with efficient detection, which is mainly used for realizing efficient acquisition of electromagnetic data inside and outside a loop source by using a multi-channel parallel sampling method in the aspect of data acquisition, and ensures the stability and the high efficiency of the data acquisition process by optimizing a sampling strategy. In the aspect of data processing, the invention provides a three-dimensional transient electromagnetic inversion algorithm based on non-structural tetrahedral mesh discretization, the algorithm adopts an advanced numerical calculation method and an L-BFGS optimization method, an inversion model is continuously optimized, the accuracy and stability of inversion are improved, and the accuracy and stability of data acquisition are improved. And high-precision inversion is carried out on the collected transient electromagnetic data, so that accurate description of three-dimensional distribution of the underground medium is realized.

Description

[0001] Methodology Area

[0002] The invention relates to the field of geophysical detection, and in particular to a transient electromagnetic detection method for high-efficiency detection. Background Art

[0003] Transient electromagnetic method (TEM) is a cutting-edge geophysical exploration method that focuses on using transient electromagnetic fields to reveal the electrical characteristics of underground geological bodies. The core of this method is to capture and analyze the electromagnetic responses caused by underground geological bodies through specific electromagnetic field transmitting and receiving devices, thereby achieving effective detection of underground structures and properties. The traditional transient electromagnetic acquisition method is to collect data in one-third or one-half of the area within the transmitting loop, where the primary field diffusion is relatively uniform, which is suitable for imaging and anomaly detection. However, deploying the transmitting loop is the most time-consuming part of transient electromagnetic field work, and the area used for traditional data acquisition in a single loop source area is small, resulting in low data acquisition efficiency.

[0004] According to the data acquisition network designed in the early stage, people need to carry devices for mobile measurement to realize the rapid acquisition of transient electromagnetic data in the survey area. The main method of transient electromagnetic data inversion is to use one-dimensional or two-dimensional methods. This method not only has strong multi-solution, but also has insufficient resolution and is not suitable for effective exploration of complex terrain and geological conditions. Summary of the invention

[0005] In view of the above, an object of the present invention is to provide a transient electromagnetic detection method with high efficiency, which successfully improves the data acquisition efficiency and the accuracy of data interpretation.

[0006] To achieve the above-mentioned invention object, the present invention provides the following method scheme: a transient electromagnetic detection method for efficient detection, comprising the following steps:

[0007] Step 1: According to the exploration target and geological conditions, select the appropriate loop source type and size. The loop source side length can be tens of meters to hundreds of meters. Determine the layout and direction of the loop source, which can at least cover the electromagnetic signal of an area that is more than twice the transmission area;

[0008] Step 2: Carefully arrange the survey lines near the loop source to ensure that the survey lines cover key areas inside and outside the loop source, and adjust the spacing and length of the survey lines according to the geological structure and potential interference factors;

[0009] Step 3: According to the characteristics of the internal and external signals of the loop source, select the appropriate transmitter and transmission parameters. For example, the suitable working current of the T-4A transmitter of Phoenix Company of Canada is 10-20A, and the frequency of the transmission waveform is 5-25Hz, which can meet the working requirements of the exploration depth within 500 meters. At the same time, considering the signal attenuation and noise interference, set specific parameters for the survey lines in different areas;

[0010] Step 4: Use a multi-channel transient electromagnetic instrument to synchronously collect electromagnetic signals inside and outside the loop source to ensure accurate synchronization between channels, which is convenient for subsequent data processing and analysis;

[0011] Step 5: During the data collection process, the quality and stability of the internal and external signals of the loop source are monitored in real time, and the acquisition parameters or measurement line layout are flexibly adjusted according to the actual situation to cope with various interferences and signal changes;

[0012] Step 6: Preprocess the collected internal and external signals of the loop source, including denoising and filtering steps, and then use specialized transient electromagnetic data processing software for in-depth analysis to accurately extract underground structure and resistivity distribution information;

[0013] Step 7: Compare the signal characteristics inside and outside the loop source, identify and eliminate outliers or interference signals, combine geological data and field survey results, and reasonably interpret and infer the internal and external signals to improve the accuracy and reliability of detection;

[0014] Step eight: Compare and verify the analysis results with known geological information, evaluate the accuracy and reliability of the detection results, and adjust and optimize the acquisition strategy based on the verification results to improve the efficiency and accuracy of subsequent detection.

[0015] As an improvement, in the implementation, in step 1, the acquisition distribution strategy in the study area is designed according to the type and size of the loop source to ensure that the survey line covers the key areas inside and outside the loop source.

[0016] As an improvement, in the implementation, in step 3, the characteristics and distribution laws of the electromagnetic signals inside and outside the loop source are analyzed by the three-dimensional forward modeling method. Starting from the differential form of the Maxwell equations in the time domain, ignoring the displacement current and eliminating the magnetic field, the electric field birotation diffusion equation satisfied by the three-dimensional transient electromagnetic can be derived:

[0017]

[0018] Where r is the displacement vector, t is the time, E(r,t) is the electric field strength at r at time t, and J s (r, t) is the source current density, σ is the conductivity of the medium, and μ is the magnetic permeability of the medium;

[0019] The three-dimensional space is discretized using unstructured tetrahedral units, and the vector interpolation basis function is introduced to approximate the electric field diffusion equation. The electric field in any tetrahedral unit can be expressed as:

[0020]

[0021] In the formula, is the tangential electric field value on the ith edge of the eth unit, is the interpolation basis function of the ith edge of unit e. The electric field diffusion equation is discretized by finite element using the Galerkin method. The following is obtained:

[0022]

[0023] Among them, A and B are stiffness matrix and mass matrix respectively, and S is the current source term.

[0024] Using the second-order backward Euler algorithm to discretize time, the above formula becomes:

[0025] (3B+2ΔtA)E j+2 (t) = B(4E j+1 (t)-E j (t))-2ΔtS j+2 ,

[0026] The formula can be solved using the MUMPS solver. The initial electric field is 0. After all the time channels of the above formula are assembled, it can be simplified as:

[0027] K t E=s t .

[0028] In the formula, K t is the coefficient matrix, E is the electric field value to be calculated on each edge of the tetrahedral unit, s t Represents the right-hand source term.

[0029] As an improvement, in the implementation, in step six, a three-dimensional inversion method is used to process the measured data. The inversion method is an objective function based on Tikhonov regularization, namely:

[0030]

[0031] Where W d , W m are the diagonal matrix of the inverse of the data noise standard deviation and the model roughness operator, d obs ,d pre are the observed data and predicted data, respectively, and m and m0 are the calculated model parameters and reference model parameter vectors, respectively;

[0032] Reverse both sides of the above formula to the model parameter mn-1 Derivatively obtain the nth solution m n , the gradient of the objective function can be written as:

[0033]

[0034] In the formula, J is the sensitivity matrix. After the model is updated by Δm, it is then updated by m n-1 =m n +Δm linear search step method to obtain a new model 。

[0035] Acquisition strategy: The present invention adopts a unique acquisition strategy, that is, to simultaneously acquire electromagnetic signals inside and outside the loop source. By arranging receiving coils inside and outside the loop source, direct responses near the source and indirect responses far away can be received simultaneously, thereby obtaining more comprehensive geological information.

[0036] Processing method: The present invention adopts a high-precision three-dimensional inversion method to process and analyze the collected electromagnetic signals. This method uses numerical simulation methods to perform high-precision inversion of the electrical properties of underground media to obtain the three-dimensional distribution and properties of underground targets.

[0037] Compared with the existing method, the present invention has the following beneficial effects:

[0038] In a transient electromagnetic detection method for efficient detection provided by the present invention, the signal acquisition strategy is optimized to ensure that the collected electromagnetic signal has a sufficient signal-to-noise ratio and resolution. By reasonably arranging the acquisition equipment, adjusting the acquisition parameters and applying advanced signal processing methods, signal interference is effectively reduced and signal quality is improved, which provides a solid and reliable foundation for subsequent data analysis and interpretation, improves the accuracy and efficiency of exploration, and adopts a three-dimensional inversion data processing method to achieve three-dimensional imaging and interpretation of underground geological structures. By preprocessing the collected electromagnetic signal data and analyzing and interpreting it at multiple angles and scales, the three-dimensional information of the underground geological structure can be more comprehensively understood, providing strong support for the positioning and property judgment of the exploration target. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the method scheme in the embodiment of the present invention or the existing method, the drawings required for use in the description of the embodiment or the existing method will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For ordinary method personnel in this field, other drawings can be obtained based on these drawings without paying creative work.

[0040] Figure 1 It is a schematic diagram of a transient electromagnetic data acquisition strategy for efficient detection provided by an embodiment of the present invention;

[0041] Figure 2 is a schematic diagram of generating an unstructured local grid of a uniform half-space model provided by an embodiment of the present invention;

[0042] Figure 3 It is a characteristic diagram of the distribution of electromagnetic signals inside and outside the loop source of the half-space model provided by an embodiment of the present invention;

[0043] Figure 4 It is a schematic diagram of forward modeling and inverse modeling of a complex terrain model provided by an embodiment of the present invention;

[0044] Figure 5 This is a three-dimensional inversion result diagram provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0045] In order to make the purpose, method scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific implementation methods described herein are only used to explain the present invention and do not limit the scope of protection of the present invention.

[0046] Figure 1 FIG. 1 is a schematic diagram of a collection strategy of a transient electromagnetic detection method for efficient detection provided by an embodiment of the present invention. Figure 1 As shown, the embodiment provides a transient electromagnetic detection method for efficient detection, comprising the following steps:

[0047] Step 1: Arrange the acquisition equipment, including loop sources, receivers and measuring instruments, according to the exploration objectives and terrain conditions. Ensure that the equipment can fully cover the target area and meet the exploration requirements:

[0048] The specific operations are:

[0049] (1) Preliminary geological survey: According to the geological exploration needs, the electrical property differences such as resistivity of underground geological bodies are investigated through literature and geological exploration to determine whether they meet the conditions for transient electromagnetic exploration. For example, if there is a significant resistivity difference between bauxite and the surrounding limestone, transient electromagnetic exploration can be carried out. The exploration targets and exploration areas can be determined through preliminary investigations.

[0050] (2) Arrangement of loop sources: According to the exploration objectives and geological conditions, select the appropriate type and size of loop sources. The side length of the loop sources can be tens of meters to hundreds of meters. Determine the layout position and direction of the loop sources. At least the electromagnetic signal covering an area that is more than twice the transmitting area can be generated to generate an effective transient electromagnetic field.

[0051] Step 2: According to the positions of the receiving points inside and outside the array-type loop source, the receiver is used to collect data signals, and the coordinates of the receiving points and terrain information:

[0052] The specific operations are:

[0053] (1) Arrange the transmitting loop source and multi-channel transient electromagnetic instrument in the selected exploration area to ensure that each channel covers the key areas inside and outside the loop source. For example, the suitable working current of the T-4A transmitter of Canada Phoenix Company is 10-20A, and the frequency of the transmitting waveform is 5-25Hz, which can meet the working requirements of the exploration depth within 500 meters. The Phoenix Company's V8 electrical instrument can realize 3-channel transient electromagnetic data acquisition. At the same time, considering the signal attenuation and the noise interference of the ground electromagnetic interference source, set a reasonable point distance and line distance for the survey line in the exploration area, and collect the coordinates and terrain information of the transmitting source and each receiving point;

[0054] (2) Data acquisition: Start the transient electromagnetic transmitter and the transient electromagnetic signal receiving instrument, synchronously collect the electromagnetic signal data inside and outside the loop source, and compare it with the electromagnetic signal calculated by the uniform half-space model of the three-dimensional forward modeling, such as Figure 3 As shown, the accuracy and consistency of the data are ensured. At the same time, the quality and stability of the signal are monitored through the real-time monitoring system.

[0055] Step 3, data preprocessing: suppress noise, eliminate redundant data, and perform reasonable filtering and smoothing on the original data to reduce interference from background signals, improve the signal-to-noise ratio of useful signals, and provide a reliable data basis for subsequent analysis.

[0056] Step 4: construct an initial model for 3D inversion based on the coordinates and terrain information of the collected emission source and measuring points, use a high-precision 3D inversion method to invert the preprocessed data, and construct a high-precision 3D geological model. Figure 2 The figure is a schematic diagram of the tetrahedral mesh discretization of a complex terrain model. This model can reflect the underground structure and resistivity distribution geological information.

[0057] Step 5: By comparing and analyzing the electromagnetic signal characteristics inside and outside the loop source, preliminarily analyze the distribution of underground geological bodies, and combine the three-dimensional geological model to accurately identify the distribution and properties of underground target bodies.

[0058] Step 6: Compare and verify the analysis results with the known geological information, evaluate the accuracy and reliability of the detection results, and adjust and optimize the acquisition strategy and data processing methods based on the verification results to improve the efficiency and accuracy of subsequent detection.

[0059] In the implementation, the beneficial effects achieved in the implementation include optimizing the signal acquisition strategy, ensuring that the collected electromagnetic signals have sufficient signal-to-noise ratio and resolution, and effectively reducing signal interference, improving signal quality, and improving exploration accuracy and efficiency by reasonably arranging acquisition equipment, adjusting acquisition parameters, and applying advanced signal processing methods. The three-dimensional inversion data processing method is used to realize three-dimensional imaging and interpretation of underground geological structures. By preprocessing the collected electromagnetic signal data, analyzing and interpreting it at multiple angles and scales, a more comprehensive understanding of the three-dimensional information of the underground geological structure can be achieved.

[0060] The basic principles and main features of the present invention and the advantages of the present invention are shown and described above. For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic features of the present invention; therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-restrictive, and the scope of the present invention is limited by the appended claims rather than the above description.

Claims

1. A transient electromagnetic detection method with high efficiency, characterized in that The following steps are involved: Step 1: According to the exploration target and geological conditions, select the type and size of the loop source. The side length of the loop source can be tens of meters to hundreds of meters. Determine the layout and direction of the loop source to at least cover the electromagnetic signal of an area that is more than twice the transmission area. Step 2: Carefully arrange the survey lines near the loop source to ensure that the survey lines cover key areas inside and outside the loop source, and adjust the spacing and length of the survey lines according to the geological structure and potential interference factors; Step 3: According to the characteristics of the internal and external signals of the loop source, select the transmitter and transmission parameters. For example, the suitable working current of the T-4A transmitter of Phoenix Company of Canada is 10-20A, and the frequency of the transmission waveform is 5-25Hz, which can meet the working requirements of the exploration depth within 500 meters. At the same time, considering the signal attenuation and noise interference, set specific parameters for the survey lines in different areas; Step 4: Use a multi-channel transient electromagnetic instrument to synchronously collect electromagnetic signals inside and outside the loop source to ensure accurate synchronization between channels, which is convenient for subsequent data processing and analysis; Step 5: During the data collection process, the quality and stability of the internal and external signals of the loop source are monitored in real time, and the acquisition parameters or measurement line layout are flexibly adjusted according to the actual situation to cope with various interferences and signal changes; Step 6: Preprocess the collected internal and external signals of the loop source, including denoising and filtering steps, and then use specialized transient electromagnetic data processing software for in-depth analysis to accurately extract underground structure and resistivity distribution information; Step 7: Compare the signal characteristics inside and outside the loop source, identify and eliminate outliers or interference signals, combine geological data and field survey results, and reasonably interpret and infer the internal and external signals to improve the accuracy and reliability of detection; Step eight: Compare and verify the analysis results with known geological information, evaluate the accuracy and reliability of the detection results, and adjust and optimize the acquisition strategy based on the verification results to improve the efficiency and accuracy of subsequent detection.

2. The method for efficient transient electromagnetic detection according to claim 1, characterized in that: In step 1, the acquisition distribution strategy in the study area is designed according to the type and size of the loop source to ensure that the survey line covers the key areas inside and outside the loop source.

3. The method for detecting transient electromagnetic data with high efficiency as claimed in claim 1, characterized in that: In step 3, the characteristics and distribution of electromagnetic signals inside and outside the loop source are analyzed by the three-dimensional forward modeling method. Starting from the differential form of Maxwell's equations in the time domain, ignoring the displacement current and eliminating the magnetic field, the electric field birotation diffusion equation satisfied by the three-dimensional transient electromagnetic can be derived: Where r is the displacement vector, t is the time, E(r,t) is the electric field strength at r at time t, and J s (r, t) is the source current density, σ is the conductivity of the medium, and μ is the magnetic permeability of the medium; The three-dimensional space is discretized using unstructured tetrahedral units, and vector interpolation basis functions are introduced to approximate the electric field diffusion equation. The electric field in any tetrahedral unit can be expressed as In the formula, is the tangential electric field value on the ith edge of the eth unit, is the interpolation basis function of the ith edge of unit e. The electric field diffusion equation is discretized by finite element using the Galerkin method. The following is obtained: Among them, A and B are stiffness matrix and mass matrix respectively, S is the current source term, Using the second-order backward Euler algorithm to discretize time, the above formula becomes: (3B+2ΔtA)E j+2 (t)=B(4E j+1 (t)-E j (t))-2ΔtS j+2 , The formula can be solved using the MUMPS solver. The initial electric field is 0. After all the time channels of the above formula are assembled, it can be simplified as: K t E=s t . In the formula, K t is the coefficient matrix, E is the electric field value to be calculated on each edge of the tetrahedral unit, s t Represents the right-hand source term.

4. The method for efficient transient electromagnetic detection according to claim 1, characterized in that: In step six, the measured data are processed using a three-dimensional inversion method. The inversion method is an objective function based on Tikhonov regularization, namely: Where W d , W m are the diagonal matrix of the inverse of the data noise standard deviation and the model roughness operator, d obs ,d pre are the observed data and predicted data, respectively, and m and m0 are the calculated model parameters and reference model parameter vectors, respectively; Reverse both sides of the above formula to the model parameter m n-1 Derivatively obtain the nth solution m n , the gradient of the objective function can be written as: In the formula, J is the sensitivity matrix. After the model is updated by Δm, it is then updated by m n-1 =m n +Δm linear search step method to obtain a new model.

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