Imaging method based on ground-air transient electromagnetic attenuation field under complex terrain

Abstract

The invention belongs to the technical field of geophysical exploration, and discloses a ground-air transient electromagnetic attenuation field imaging method based on a complex terrain, which simulates a ground-air transient electromagnetic scene, constructs a model with terrain resistivity for an exploration target area, and performs electromagnetic field finite volume numerical simulation on the model to obtain a ground-air transient electromagnetic attenuation field. Calculating diffusion field induced electromotive force distribution under different half-space resistivity conditions; and then, constructing a loss function of two norms of the induced electromotive force and the observed induced electromotive force under the condition of uniform background resistivity, and searching the optimal background resistivity to enable observation data to reach the minimum value after deducting diffusion field information, thereby extracting an attenuation field for imaging. According to the method, the influence of diffusion information and terrain in a secondary field in the transient electromagnetic method can be effectively removed, the spatial distribution difference of an attenuation field is highlighted, rapid imaging of three-dimensional transient electromagnetic data can be realized, the influence of complex terrain on transient electromagnetic inversion imaging is avoided, and the detection efficiency and reliability of the transient electromagnetic method are improved.

Description

Technical Field

[0001] The invention belongs to the field of geophysical detection technology, and in particular relates to an imaging method based on ground-to-air transient electromagnetic attenuation field under complex terrain. Background Art

[0002] The Grounded Source Transient Electromagnetic Method (SATEM) is a transient electromagnetic method that uses a long ground wire source for transmission and a receiving coil on a drone for aerial observation. This transmission-receiving configuration has been proven to be more efficient than traditional ground-based electromagnetic methods and can achieve a deeper detection depth than traditional aerial electromagnetic methods. Because SATEM has a wide spatial coverage, intensive sampling, and synchronous observations during flight, it has high experimental efficiency and strong adaptability to complex terrain. Therefore, it has unique advantages in surveying in mountainous areas, deserts, hills, lakes, and areas covered with vegetation, which makes it have broad application prospects in various fields (mineral, groundwater, geothermal and other resource exploration fields). In terms of resource exploration, SATEM can provide high-resolution underground structure information, help accurately explore mineral resources and oil and gas reservoirs, and promote the rational development and utilization of resources; in basic earth science research, SATEM technology can be used to conduct in-depth research on underground rock and soil structure, tectonic movement, crustal evolution, etc., and promote the development of earth science; in terms of environmental protection and disaster prevention, SATEM can be used to monitor the migration and diffusion of groundwater bodies and underground pollutants, provide assessments and early warnings of groundwater resources and environmental pollution, and help protect the environment and reduce disaster risks.

[0003] In recent years, the existing technology has made significant progress in the fields of SATEM data processing and imaging methods, but there are still some problems, such as: data interpretation is difficult, mainly because after the transmitting power is turned off, the original static magnetic field in the space disappears instantly, generating an induced electromotive force and exciting the underground medium to generate an induced secondary field. The area closer to the field source produces a stronger secondary field and diffuses to the area with weaker secondary field; at the same time, the secondary field of rocks with lower conductivity decays more slowly, which also affects the diffusion speed of the secondary field. Therefore, in the transient electromagnetic system, the secondary field information collected by the sensor includes two parts: ① The diffusion information of the secondary field (abbreviated as the diffusion field E d ), which accounts for the main component and is mainly determined by the spatial position relationship from the transmitting source to the receiving point and the resistivity distribution on the diffusion path; ② The attenuation difference information of the secondary field (abbreviated as attenuation field E a), which is a minor component, generally one order of magnitude different from ①, and is mainly affected by the local resistivity structure, especially the good conductor. The smaller the resistivity, the slower the attenuation. The transient electromagnetic attenuation field contains key information about the underground resistivity structure. Imaging the attenuation field can more intuitively display the spatial distribution of the underground resistivity structure, which is of great significance for the interpretation of transient electromagnetic data. There is no obvious boundary between the diffusion field and the attenuation field. During the propagation of the transient electromagnetic field, the two always exist at the same time. Current technologies all use the total field for inversion imaging. Current research is still blank in attenuation field imaging. The core difficulty of attenuation field imaging is how to remove the diffusion information in the secondary field and the influence of terrain. In addition, complex terrain also affects the size of the secondary field response, and its abnormal amplitude even exceeds the attenuation anomaly, which brings huge challenges to SATEM imaging under complex terrain conditions. Summary of the invention

[0004] To solve the above technical problems, the present invention provides an imaging method based on the Semi-airborne Transient Electromagnetic Method (SATEM) attenuation field under complex terrain. By designing a diffusion field and terrain influence subtraction algorithm, the attenuation field information that directly responds to the resistivity structure information is extracted to achieve rapid attenuation field imaging, thus solving the problem of the influence of complex terrain on transient electromagnetic inversion imaging.

[0005] The imaging method based on the ground-to-air transient electromagnetic attenuation field under complex terrain described in the present invention comprises the following steps:

[0006] Step 1: simulate the ground-to-air transient electromagnetic scene and construct a terrain resistivity model for the exploration target area;

[0007] Step 2, perform electromagnetic field finite volume numerical simulation on the belt terrain resistivity model, and calculate the diffusion field induced electromotive force distribution under different half-space resistivity conditions;

[0008] Step 3: Construct the loss function of the induced electromotive force and the second norm of the observed induced electromotive force under uniform background resistivity conditions, and search for the optimal background resistivity so that the observed data reaches the minimum value after deducting the diffusion field information, thereby extracting the attenuation field for imaging.

[0009] Furthermore, step 2 is specifically as follows:

[0010] Set the background resistivity ρ to be distributed in 10 0 ~10 5 Ω·m and a series of models that satisfy uniform logarithmic distribution, and calculate the diffusion field of these models to obtain the diffusion field induced electromotive force data set E in the three dimensions of resistivity, space and time. d(ρ,x,t), where x represents space and t represents time; spline interpolation is performed in the resistivity dimension to obtain a denser induced electromotive force data set E in this dimension d I (ρ I ,x,t),ρ I is the resistivity after interpolation.

[0011] Further, in step 3, the observation data refers to the measured data collected in the exploration target area;

[0012] Assume that when the difference between the diffusion field simulated by the uniform resistivity model and the observed data is extremely small, the corresponding resistivity is the background resistivity; the background resistivity ρ k It is expressed as:

[0013] ρ k =argmin I (‖ E d I (ρ I ,x,t)-E obs ‖ 2 ) (1)

[0014] Among them, E obs is the observed induced electromotive force; in the transient electromagnetic system, the secondary field information collected by the sensor includes diffusion field and attenuation field information, among which the diffusion field transient electromagnetic field controlled by the background resistivity always occupies the main component, and the energy of the attenuation field differs by one order of magnitude; therefore, it is assumed that when the difference between the diffusion field simulated by the uniform resistivity structure and the observed data is the minimum, the corresponding resistivity is the background resistivity; after obtaining the background resistivity, the attenuation field can be obtained by subtracting the diffusion field from the observed total field:

[0015] E a obs =E obs -E d I (ρ k ,x,t) (2)

[0016] According to E a obs A qualitative analysis of the underground resistivity structure is made, that is, the area with attenuation field anomaly also corresponds to the underground resistivity anomaly area. The time corresponding to the attenuation field anomaly is related to the depth of the resistivity anomaly. The longer the time, the greater the depth.

[0017] The beneficial effects described in the present invention are as follows: the traditional SATEM imaging method uses total field data for three-dimensional inversion, which is usually inefficient due to the huge amount of data; the method described in the present invention develops a subtraction algorithm for the diffusion field and terrain influence, which can effectively remove the diffusion information in the secondary field of the transient electromagnetic method and the influence of the terrain, extract the attenuation field information that directly responds to the resistivity structure information, so as to highlight the spatial distribution difference of the attenuation field and explore the law of transient electromagnetic attenuation field imaging; this difference is directly determined by the abnormal resistivity distribution of the underground medium, which is conducive to the qualitative analysis of SATEM detection, and can realize the rapid imaging of three-dimensional transient electromagnetic data, greatly improving the imaging efficiency; avoiding the influence of complex terrain on transient electromagnetic inversion imaging, solving the problems of large uncertainty and low detection accuracy in transient electromagnetic imaging under mountainous conditions, and improving the detection efficiency and reliability of the transient electromagnetic method. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is the distribution characteristics of transient electromagnetic field under terrain conditions;

[0019] Figure 2 It is a schematic diagram of the relationship between the SATEM induced electromotive force and resistivity under the belt terrain condition;

[0020] Figure 3 It is a search process of the attenuation field extracted by the synthetic induced electromotive force obtained by the algorithm of the present invention;

[0021] Figure 4 It is the comparison between the extracted attenuation field and the real attenuation field.

[0022] Figure 5 is a flow chart of the method of the present invention. DETAILED DESCRIPTION

[0023] In order to make the contents of the present invention more clearly understood, the present invention is further described in detail below based on specific embodiments in conjunction with the accompanying drawings.

[0024] like Figure 5 As shown, the imaging method based on the ground-to-air transient electromagnetic attenuation field under complex terrain described in the present invention comprises the following steps:

[0025] Step 1: simulate the ground-to-air transient electromagnetic scene and construct a terrain resistivity model for the exploration target area;

[0026] Step 2, perform electromagnetic field finite volume numerical simulation on the belt terrain resistivity model, and calculate the diffusion field induced electromotive force distribution under different half-space resistivity conditions;

[0027] First, the terrain is discretized using octree grids, and the grid is encrypted near the terrain with large curvature and near the emission source to improve the simulation accuracy. At the same time, the receiving point coordinates are constructed according to the spatial position of the sensor carried by the aircraft, and the simulated emission source information is constructed according to the emission source information (electrical source or magnetic source) under actual conditions. Then, the finite volume method electromagnetic field forward calculation is performed to construct the diffusion field induced electromotive force distribution under different half-space resistivity conditions.

[0028] The simulated induced electromotive force is interpolated in the resistivity dimension to obtain the diffusion field induced electromotive force data set in three dimensions of resistivity, space and time, which is used to search for the optimal background resistivity.

[0029] Step 3: Construct the loss function of the induced electromotive force and the second norm of the observed induced electromotive force under uniform background resistivity conditions, and search for the optimal background resistivity so that the observed data reaches the minimum value after deducting the diffusion field information, thereby extracting the attenuation field for imaging.

[0030] In this embodiment, the dichotomy method is used to quickly search for the resistivity corresponding to the minimum value of the loss function, that is, the optimal background resistivity. After deducting the diffusion field information, the attenuation field can be extracted for imaging. Generally speaking, the location with a larger attenuation field value corresponds to low resistance, and the attenuation moment corresponds to the depth of abnormal resistivity. The larger the moment, the greater the depth.

[0031] like Figure 1 As shown in Figure 2, the distribution characteristics of transient electromagnetic fields under terrain conditions were constructed, where Figure 1 The horizontal and vertical coordinates represent the coordinates in the three-dimensional coordinate system. Figure 1 (a) and (d) represent the topographic plane cloud map and the structural slice at Y=0, respectively; Figure 1 (b) and (c) respectively represent the transient electromagnetic field distribution at a flight altitude of 200m under the conditions of no terrain and terrain in the present invention at the same time, Figure 1 The black line in (b) indicates the location of the long wire electrical source; Figure 1 (e) and (f) show the difference in transient electromagnetic field distribution with and without terrain at different times. Figure 1 The influence of terrain on transient electromagnetic fields can be clearly seen.

[0032] This embodiment uses the open source software SIMPEG to complete the electromagnetic field finite volume numerical simulation, simulating the induced electromotive force distribution under different terrain conditions and different background resistivities. At this time, there is no abnormal resistivity and it is assumed that there is only a diffusion field. Based on the model constructed in step 1, 8 different background resistivities are set as follows: Figure 2 As shown, its value is 10 0 ~10 5Ω·m and satisfy the uniform logarithmic distribution. Eight forward simulations were completed to obtain the diffusion field induced electromotive force data set E in three dimensions of resistivity, space and time. d (ρ,x,t) Figure 2 (a) and (b) are the curves of induced electromotive force changing with resistivity at 50us and 1ms respectively. A total of 8 resistivities were simulated, and different colors represent different spatial positions. Then interpolation is performed in the resistivity dimension to obtain a more dense induced electromotive force data set E in this dimension. d I (ρ I ,x,t) Figure 2 As shown in (c) and (d), the resistivity obtained contains 60 in total.

[0033] Next, the search for the true optimal background resistivity is conducted based on the observed data and the simulated background electromagnetic field. The observed data refers to the measured data collected in the exploration target area.

[0034] Background resistivity ρ k It is expressed as:

[0035] ρ k =argmin I (‖ E d I (ρ I ,x,t)-E obs ‖ 2 ) (1)

[0036] Among them, E obs For the observed induced electromotive force, in the transient electromagnetic system, the secondary field information collected by the sensor includes diffusion field and attenuation field information, among which the diffusion field transient electromagnetic field controlled by the background resistivity always occupies the main component, and the energy of the attenuation field differs by an order of magnitude. Therefore, we assume that when the difference between the diffusion field simulated by the uniform resistivity structure and the observed data is the minimum, the corresponding resistivity is the background resistivity. After obtaining the background resistivity, the attenuation field can be obtained by subtracting the diffusion field from the observed total field:

[0037] E a obs =E obs -E d I (ρ k ,x,t) (2)

[0038] According to E a obs A qualitative analysis of the underground resistivity structure shows that generally speaking, the area with attenuation field anomaly also corresponds to the underground resistivity anomaly area. The time corresponding to the attenuation field anomaly is related to the depth of the resistivity anomaly. The greater the time, the greater the depth.

[0039] Figure 3 The search process of the attenuation field extracted by the synthetic induced electromotive force obtained by the above algorithm is demonstrated. Figure 3 The coordinates represent the difference between the synthetic induced electromotive force and the true background resistivity. The red dotted line is the optimal background resistivity, the line parallel to it is the true background resistivity, and the wavy line represents the loss function corresponding to different background resistivities. Based on the search algorithm of formula (1), the background resistivity corresponding to the minimum value can be searched to be 2099Ω·m, which is very close to the true background resistivity.

[0040] Figure 4 The comparison between the extracted attenuation field and the real attenuation field is shown. Figure 4 (a) and (b) are the results of the real attenuation field at 50us and 1ms respectively. Figure 4 (c) and Figure 4 (d) are the results of the extracted attenuation field at 50us and 1ms, respectively. It can be seen that the attenuation field calculated by the real background resistivity and the background resistivity searched by the algorithm designed by the present invention is almost the same, which verifies the effectiveness of the algorithm of the present invention on synthetic data.

[0041] The above description is only a preferred embodiment of the present invention and is not intended to be a further limitation of the present invention. All equivalent changes made using the contents of the present specification and drawings are within the protection scope of the present invention.

Claims

1. An imaging method based on ground-to-air transient electromagnetic attenuation field under complex terrain, characterized in that: The following steps are involved: Step 1: simulate the ground-to-air transient electromagnetic scene and construct a terrain resistivity model for the exploration target area; Step 2, perform electromagnetic field finite volume numerical simulation on the belt terrain resistivity model, and calculate the diffusion field induced electromotive force distribution under different half-space resistivity conditions; Step 3: Construct the loss function of the induced electromotive force and the second norm of the observed induced electromotive force under uniform background resistivity conditions, and search for the optimal background resistivity so that the observed data reaches the minimum value after deducting the diffusion field information, thereby extracting the attenuation field for imaging.

2. The imaging method based on the ground-to-air transient electromagnetic attenuation field under complex terrain according to claim 1 is characterized in that: Step 2 is as follows: Set the background resistivity ρ to be distributed in 10 0 ~10 5 Ω·m and a series of models that satisfy uniform logarithmic distribution, and calculate the diffusion field of these models to obtain the diffusion field induced electromotive force data set E in the three dimensions of resistivity, space and time. d (ρ,x,t), where x represents space and t represents time; spline interpolation is performed in the resistivity dimension to obtain a denser induced electromotive force data set E in this dimension d I (ρ I ,x,t),ρ I is the interpolated resistivity.

3. The imaging method based on the ground-to-air transient electromagnetic attenuation field under complex terrain according to claim 1 is characterized in that: In step 3, the observation data refers to the measured data collected in the exploration target area; it is assumed that when the difference between the diffusion field simulated by the uniform resistivity model and the observed data is extremely small, the corresponding resistivity is the background resistivity; the background resistivity ρk is expressed as: r k =argmin I (||E d I (r I ,x,t)-E obs | 2 ) (1) Among them, E obs is the observed induced electromotive force; after obtaining the background resistivity, the attenuation field is obtained by subtracting the diffusion field from the observed total field: AND a obs =And obs -AND d I (ρ k ,x,t) (2) According to E a obs A qualitative analysis of the underground resistivity structure is made, that is, the area with attenuation field anomaly also corresponds to the underground resistivity anomaly area. The time corresponding to the attenuation field anomaly is related to the depth of the resistivity anomaly. The longer the time, the greater the depth.

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