An imaging method based on ground-to-air transient electromagnetic attenuation field in complex terrain

Abstract

The present invention belongs to the field of geophysical exploration technology and discloses a method for imaging the ground-to-air transient electromagnetic attenuation field under complex terrain. The method simulates the ground-to-air transient electromagnetic scene, constructs a terrain resistivity model for the exploration target area, and performs electromagnetic field finite volume numerical simulation on the model to calculate the distribution of the diffuse field induced electromotive force under different half-space resistivity conditions. Then, a loss function is constructed for the induced electromotive force under uniform background resistivity conditions and the second norm of the observed induced electromotive force. By searching for the optimal background resistivity, the observed data reaches a minimum value after deducting the diffuse field information, thereby extracting the attenuation field for imaging. The present invention can effectively remove the diffuse information in the secondary field of the transient electromagnetic method and the influence of terrain, highlight the spatial distribution differences of the attenuation field, and can achieve rapid imaging of three-dimensional transient electromagnetic data. It avoids the influence of complex terrain on transient electromagnetic inversion imaging, and improves the detection efficiency and reliability of the transient electromagnetic method.

Description

Technical Field

[0001] The present invention belongs to the field of geophysical exploration 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-based wire source for transmission and an unmanned aerial vehicle (UAV) equipped with a receiving coil for aerial observation. This transmitter-receiver configuration has been proven to be more efficient than traditional ground-based electromagnetic methods and can achieve deeper detection depths than traditional aerial electromagnetic methods. Due to SATEM's wide spatial coverage, dense sampling, and simultaneous observations during flight, it has high experimental efficiency and strong adaptability to complex terrain. Therefore, it has unique advantages in surveying mountainous areas, deserts, hills, lakes, and areas covered by vegetation. This gives it broad application prospects in various fields (mineral, groundwater, geothermal, and other resource exploration). In resource exploration, SATEM can provide high-resolution underground structure information, helping to 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 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 contribute to environmental protection and disaster risk reduction.

[0003] In recent years, existing technologies have made significant progress in areas such as SATEM data processing and imaging methods, but there are still some problems. For example, data interpretation is difficult. This is 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 generates a stronger secondary field and spreads 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 diffusion field E d ), which accounts for the main component and is mainly determined by the spatial position relationship between the emission source and 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 and is generally an order of magnitude different from ①. It 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 on attenuation field imaging is still blank. 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 magnitude of the secondary field response. 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 an algorithm to subtract the diffusion field and terrain influence, 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 present invention provides an imaging method based on the ground-to-air transient electromagnetic attenuation field under complex terrain, comprising the following steps:

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

[0007] Step 2: Perform electromagnetic field finite volume numerical simulation on the terrain resistivity model to 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] Furthermore, 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 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 total observed 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 conducted, that is, the area with attenuation field anomaly corresponds to the underground resistivity anomaly area at the same time. 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 of 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 of 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 reflects 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, can realize the rapid imaging of three-dimensional transient electromagnetic data, and greatly improve the imaging efficiency; avoid the influence of complex terrain on transient electromagnetic inversion imaging, solve the problems of large uncertainty and low detection accuracy in transient electromagnetic imaging under mountainous conditions, and improve 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 fields under terrain conditions;

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

[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 includes the following steps:

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

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

[0027] First, the terrain is discretized using an octree grid. The grid is then denser near terrain with large curvature and near the emission source to improve simulation accuracy. The coordinates of the receiving point are constructed based on the spatial position of the sensors carried by the aircraft. Simulated emission source information is constructed based on the actual emission source information (electrical or magnetic). Finite volume electromagnetic field forward modeling is then 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 the resistivity, space and time dimensions, 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] This embodiment uses a bisection method to quickly search for the resistivity corresponding to the minimum value of the loss function, i.e., the optimal background resistivity. After deducting the diffusion field information, the attenuation field can be extracted for imaging. Generally speaking, locations with large attenuation field values ​​correspond to low resistance, and the attenuation moment corresponds to the depth of the 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 are 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 terrain plane cloud map and the structure 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 moment. 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 in the figure.

[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. After 8 forward simulations, the diffusion field induced electromotive force data set E in the three dimensions of resistivity, space and time was obtained. d (ρ,x,t) Figure 2 (a) and (b) show the curves of induced electromotive force versus resistivity at 50us and 1ms respectively. A total of 8 resistivities were simulated, with different colors representing different spatial positions. Then, interpolation was performed in the resistivity dimension to obtain a more dense induced electromotive force dataset E in this dimension. d I (ρ I ,x,t) Figure 2 As shown in (c) and (d), the obtained resistivity contains a total of 60.

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

[0034] Background resistivity ρ k Expressed as:

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

[0036] Among them, E obs To observe the induced electromotive force, in a transient electromagnetic system, the secondary field information collected by the sensor includes diffusion field and attenuation field information. 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 total observed 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 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.

[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 dashed line is the optimal background resistivity, and the line parallel to it is the true background resistivity. The wavy lines represent the loss functions corresponding to different background resistivities. Based on the search algorithm of formula (1), the background resistivity corresponding to the minimum value can be found to be 2099 Ω·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, where 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) The results of the extracted attenuation field at 50 μs and 1 ms, respectively. It can be seen that the real background resistivity and the attenuation field calculated by the background resistivity searched by the algorithm designed by the present invention are 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 further limit the present invention. All equivalent changes made using the contents of the present invention description and drawings are within the scope of protection of the present invention.

Claims

1. An imaging method based on ground-to-air transient electromagnetic attenuation field in complex terrain, characterized by: The following steps are involved: Step 1: Simulate the ground-to-air transient electromagnetic scenario and construct a terrain resistivity model for the exploration target area; Step 2: Perform electromagnetic field finite volume numerical simulation on the terrain resistivity model to calculate the diffusion field induced electromotive force distribution under different half-space resistivity conditions; Step 3: Construct a loss function of the induced electromotive force and the second norm of the observed induced electromotive force under uniform background resistivity conditions. By searching for the optimal background resistivity, 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, characterized in that: Step 2 is as follows: Setting background resistivity Distributed in And a series of models that meet the uniform logarithmic distribution, and calculate the diffusion field of these models to obtain the diffusion field induced electromotive force data set in the three dimensions of resistivity, space and time ,in, Represents space, Represents time; spline interpolation is performed in the resistivity dimension to obtain a denser induced electromotive force data set in this dimension , is the resistivity after interpolation.

3. The imaging method based on the ground-to-air transient electromagnetic attenuation field under complex terrain according to claim 1, characterized in that: In step 3, observation data refers to the measured data collected in the exploration target area; 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 Expressed as: ， in, This is the induced electromotive force dataset after the resistivity dimension is interpolated. Represents space, Indicates time, is the resistivity after interpolation, 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: ， according to A qualitative analysis of the underground resistivity structure is conducted, that is, the area with attenuation field anomaly corresponds to the underground resistivity anomaly area at the same time. 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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