Methods and Systems for Constructing Multi-Level Geological Structure Models Based on Electromagnetic Detection

Abstract

This invention discloses a method and system for constructing a multi-level geological structural model based on electromagnetic detection. The method includes: emitting electromagnetic signals of different frequencies at various measuring points using an electromagnetic detector; obtaining the geoelectric structure through resistivity inversion using apparent resistance and impedance phase; classifying heterogeneous and homogeneous geotechnical structures based on electromagnetic static displacement; analyzing orthogonal magnetic field components to obtain electromagnetic response parameters and polarization ellipse parameters; determining the electric field diffusion curve; obtaining the electric field distribution model through finite difference inversion; inferring the underground geological spatial morphology and distribution patterns; dividing fault planes accordingly; calculating a two-dimensional mapping plane to generate a triangulated network model; finally, calculating the electromagnetic response surface to obtain a two-dimensional contour curve; and performing three-dimensional reconstruction to obtain a multi-level geological structural model. This method not only improves the accuracy and reliability of the structural model but also has good interpretability and can be directly applied to the construction system of a multi-level geological structural model based on electromagnetic detection.

Description

Technical Field

[0001] This invention relates to the field of geology, and in particular to a method and system for constructing a multi-level geological structure model based on electromagnetic detection. Background Technology

[0002] Geological structures are the composition and structural forms of the Earth's lithosphere, and they have a crucial impact on the distribution of mineral resources, groundwater migration, the occurrence of geological disasters such as earthquakes, and the stability of engineering projects. An accurate understanding of underground geological structures is of irreplaceable significance for numerous fields, including resource exploration and development, environmental protection, geological disaster prevention, and infrastructure construction.

[0003] However, traditional research methods have many limitations: geological drilling is costly and time-consuming, seismic exploration requires specific terrain conditions, and gravity exploration has low resolution.

[0004] Electromagnetic detection technology is non-invasive, enabling detection without damaging geological bodies. It also responds well to differences in the electromagnetic properties of different geological formations, effectively identifying geological structures based on differences in conductivity, magnetism, and other characteristics. These significant advantages have made electromagnetic detection technology a focus of attention in the field of geological structure research, providing a powerful new tool for constructing more accurate and detailed multi-level geological structure models, and contributing to the advancement of geological scientific research and related engineering applications. Summary of the Invention

[0005] The purpose of this invention is to provide a method and system for constructing a multi-level geological structure model based on electromagnetic detection.

[0006] To achieve the above objectives, the present invention is implemented according to the following technical solution: The first aspect of this invention provides a method for constructing a multi-level geological structure model based on electromagnetic detection, comprising the following steps: The S100 uses an electromagnetic detector to emit electromagnetic signals of different frequencies at each measuring point. Based on the apparent resistivity and impedance phase at different frequencies, resistivity inversion is performed to obtain the geoelectric structure. S200 acquires the static displacement of the electromagnetic field, and divides the geoelectric structure into non-uniform and uniform geoelectric regions based on the static displacement. It then performs a difference analysis on the orthogonal components of the magnetic fields of the non-uniform and uniform geoelectric regions to obtain electromagnetic response parameters and polarization ellipse parameters. S300 determines the diffusion curve of the electric field based on the electromagnetic response parameters and the polarization ellipse parameters, and solves the diffusion curve by the finite difference method to invert the structure of the underground geology and obtain the distribution model of the electric field. S400 infers the spatial morphology and distribution pattern of underground geology based on the electric field distribution model; S500 divides geological layers based on the spatial morphology and distribution patterns of the underground geology, calculates the two-dimensional mapping plane of each geological layer, and generates a multi-level geological triangulation model. The S600 calculates the electromagnetic response surface to obtain an irregular closed two-dimensional contour curve. Based on the two-dimensional contour curve and the multi-level geological triangulation model, a three-dimensional reconstruction is performed to obtain a multi-level geological structure model.

[0007] As a further method, the method for resistivity inversion based on apparent resistivity and impedance phase at different frequencies includes: Apparent resistivity and impedance phase at different frequencies are calculated using the integral equation method. Inversion is then performed based on apparent resistivity and impedance phase, with the following expression: in, Let be the resistivity distribution vector. For inversion function, and These are the apparent resistivity and impedance phase values ​​at the observation point, respectively; The objective function in the inversion process is expressed as follows: in, Let be the resistivity distribution vector. The number of observation points, For the first Voltage at each observation point For the first The resistance at each observation point For regularization parameters, The number of resistivity values. For the first All neighboring nodes of a resistivity value and The first The resistivity value and the first One resistivity value; The resistivity distribution is iteratively updated using the least squares method until the objective function no longer decreases, as expressed in the following expression: in, and The first Second and third The resistivity distribution vector of the next iteration. For Jacobian matrices, This is represented as a transpose operation. For regularization parameters, For regularization matrix, To observe the voltage vector, The voltage vector predicted based on the current resistivity distribution vector; Based on the resistivity distribution, different geological strata and structures are divided, the boundaries and contact relationships of the geological strata are determined, and the geological electrical structure is obtained.

[0008] As a further method, the method for obtaining the static displacement of the electromagnetic field and dividing the geoelectric structure into non-uniform and uniform geoelectric regions based on the static displacement includes: A theoretical electromagnetic propagation model is established based on prior knowledge of the geological region. The established model is used to simulate the normal electromagnetic response under electromagnetic signals of different frequencies, including the theoretical value of apparent resistivity. The static displacement is calculated based on the measured apparent resistivity. Calculate the mean and standard deviation of the static displacement. Add twice the standard deviation to the mean as a threshold. Points with measured apparent resistivity greater than the threshold are classified as non-uniform electrical geology; otherwise, they are classified as uniform electrical geology. After completing the point-by-point judgment, adjacent measuring points of the same type are integrated to form uniform and non-uniform electrical geological regions.

[0009] As a further method, the method for difference analysis of the orthogonal components of the magnetic fields of non-uniform and uniformly electrically charged geological regions includes: At each measuring point and frequency, the ratio of the amplitudes and the difference in phases of the orthogonal components of the magnetic field in the non-uniform and uniformly electrically charged geological regions at the same frequency are calculated to obtain the amplitude response and phase response, which are then used as electromagnetic response parameters. Based on the data of the orthogonal components of the magnetic field, the equation for the polarization ellipse is constructed, and its expression is: in, and These represent the horizontal component of the magnetic field and its amplitude, respectively. and These represent the perpendicular component of the magnetic field and its amplitude, respectively. This represents the phase difference between the horizontal and vertical components of the magnetic field. For both homogeneous and non-homogeneous geological regions, solve their respective polarization ellipse equations to obtain the inclination angle, major axis, and minor axis of the polarization ellipse. Calculate the difference between the homogeneous and non-homogeneous geological regions as the polarization ellipse parameters.

[0010] As a further method, the method for determining the diffusion curve of the electric field based on the electromagnetic response parameters and the polarization ellipse parameters includes: A theoretical model is established based on Maxwell's equations, relating the electric field to electromagnetic response parameters and polarization ellipse parameters. The expression is as follows: in, It is the amplitude of the magnetic field. For frequency, The imaginary unit, Angular frequency, Permeability, For electric field, For electrical conductivity, For volume, The inclination angle of the polarization ellipse. and These are the components of the electric field in the horizontal and vertical directions, respectively. for The complex conjugate, for and The real part of the product, and These are the major and minor axes of the polarization ellipse, respectively. The formulas for selecting the major and minor axes are: plus sign for calculating the major axis and minus sign for calculating the minor axis. The underground geological region is divided into multiple grid points. For each grid point, an electric field diffusion equation is established. The theoretical model is solved using the finite difference method to obtain the distribution of the electric field throughout the region, resulting in the electric field diffusion curve, expressed as: in, For in position and time The total electric field at that location, For the observation point, For time, The amplitude of the incident electric field. The attenuation coefficient is a location-dependent factor. For vector field operators, The phase factor of the sine wave. For wave vectors, As the source point, The source term of the electric field is the electric field generated by an external source.

[0011] As a further method, the method of solving diffusion curves using the finite difference method to invert the structure of subsurface geology includes: The partial differential equation of the diffusion curve is converted into a difference equation, specifically: for the spatial derivative, the central difference scheme is used, and for the time derivative, the backward difference scheme is used. Boundary conditions are determined based on the actual geological environment and electromagnetic characteristics, and initial conditions are set based on the initial state of electromagnetic detection. Each grid is assigned an initial electric field strength value based on the initial conditions. Starting from the initial time, time iteration is performed within the boundary conditions according to the preset time step. Specifically, within each time step, the electric field value of the next time step is calculated using the difference equation based on the electric field value at the current time, until all electric field values ​​of each grid within the time range are obtained.

[0012] As a further method, the method of inferring the spatial morphology and distribution law of underground geology based on the electric field distribution model includes: determining the type of underground geological body based on the electric field value in the electric field distribution model, outlining the shape of the geological body in combination with the electric field gradient change, analyzing the electric field characteristics of different regions to obtain the distribution law of geological structure, detecting abnormal points in the electric field distribution model and determining the abnormality type.

[0013] As a further method, the method for calculating the two-dimensional mapping plane of each geological layer to generate a multi-level geological triangulation model includes: The depth of each geological layer is calculated based on the changing trend of the secondary induced electromotive force in the receiving coil. The geological layer is vertically projected onto the horizontal plane through orthophoto projection to determine the coordinate system of the projection plane. For each geological layer, the coordinates on the projection plane are calculated using trigonometric functions based on the strike, dip angle and displacement. Using the coordinates of the geological bedding plane on the projection plane as input, a triangulated mesh is generated using the triangulation method. The expression for triangulating the point set on the geological bedding plane is as follows: in, It indicates that there are three points , and The triangle formed For point set, For any point in the point set, From point arrive The vector, From point arrive The vector, From point Time The vector, From point Time The vector.

[0014] As a further method, the method for calculating the electromagnetic response surface to obtain an irregularly closed two-dimensional profile curve includes: The partial differential equation of the electromagnetic response surface is expressed as follows: in, For the Laplace operator, For electric field strength, Permeability, Where is the dielectric constant. For current density, For time; Using the finite difference method, the partial differential equation is transformed into a difference equation, which is then discretized in space and time to solve for the electromagnetic response. The expression for the difference equation is as follows: in, , and They are respectively at time step of , and When the coordinates in two-dimensional space are The electric field strength at that location, For time step, , , and They are respectively at time step of Time, and coordinates The electric field strength values ​​at four adjacent grid points, and They are respectively in In direction and in Spatial step size in direction, Permeability, Where is the dielectric constant. For time step Time, coordinates The current density vector at that location, For time, The rate of change of current density with time; The calculated electromagnetic response values ​​of each grid point are arranged according to geographical coordinates to form a two-dimensional data matrix, and the data matrix is ​​then converted into an electromagnetic response surface. The range threshold of electromagnetic response for various geological strata is obtained, and the contour is extracted from the electromagnetic response surface to obtain the irregular closed two-dimensional contour curve of each stratum.

[0015] A second aspect of the present invention also provides a system for constructing a multi-level geological structure model based on electromagnetic detection, comprising: The resistivity inversion module is used to transmit electromagnetic signals of different frequencies at various measuring points using an electromagnetic detector, and to perform resistivity inversion based on the apparent resistivity and impedance phase at different frequencies to obtain the geoelectric structure. The electrical zoning analysis module is used to obtain the static displacement of electromagnetic fields. Based on the static displacement, the geoelectric structure is divided into non-uniform electrical geological regions and uniform electrical geological regions. The difference analysis of the orthogonal components of the magnetic fields of the non-uniform electrical geological regions and uniform electrical geological regions is performed to obtain electromagnetic response parameters and polarization ellipse parameters. The electric field diffusion simulation module is used to determine the electric field diffusion curve based on the electromagnetic response parameters and the polarization ellipse parameters, and solve the diffusion curve by the finite difference method to invert the structure of the underground geology and obtain the electric field distribution model. The geological morphology inference module is used to infer the spatial morphology and distribution patterns of underground geology based on the electric field distribution model. The multi-level modeling module is used to divide geological layers based on the spatial morphology and distribution pattern of the underground geology, calculate the two-dimensional mapping plane of each geological layer, and generate a multi-level geological triangulation model. The three-dimensional reconstruction module is used to calculate the electromagnetic response surface, obtain an irregular closed two-dimensional contour curve, and perform three-dimensional reconstruction based on the two-dimensional contour curve and the multi-level geological triangulation model to obtain a multi-level geological structure model.

[0016] Compared with the prior art, the embodiments of the present invention have at least the following advantages or beneficial effects: This invention utilizes apparent resistance and impedance phase at different frequencies for resistivity inversion, combined with multi-parameter methods such as electromagnetic static displacement and orthogonal component magnetic field difference analysis, to comprehensively and deeply understand the electrical structure and characteristics of underground geology, thereby improving the accuracy and reliability of geological structure detection.

[0017] This invention determines the diffusion curve of the electric field based on electromagnetic response parameters and polarization ellipse parameters, and then uses the finite difference method to solve the diffusion curve to invert the underground geological structure. It takes into account the dynamic diffusion process of the electric field in the underground geological body, and can more realistically simulate the interaction between electromagnetic signals and geological structures.

[0018] This invention divides geological layers, calculates the two-dimensional mapping plane of each layer, and generates a multi-level geological triangulation model. Then, it combines this with the irregular closed two-dimensional contour curve of the electromagnetic response surface for three-dimensional reconstruction. This approach not only considers the geometry of the geological layers themselves but also incorporates electromagnetic response information, resulting in a multi-level geological structure model that more accurately reflects the actual underground geological conditions. Attached Figure Description

[0019] Figure 1This is a flowchart illustrating the steps of the method and system for constructing a multi-level geological structure model based on electromagnetic detection, as described in this invention. Detailed Implementation

[0020] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0021] Reference Figure 1 As shown, this invention provides a method for constructing a multi-level geological structure model based on electromagnetic detection, including: The S100 uses an electromagnetic detector to emit electromagnetic signals of different frequencies at each measuring point. Based on the apparent resistivity and impedance phase at different frequencies, resistivity inversion is performed to obtain the geoelectric structure. It needs to be explained that geoelectric structure refers to the electrical characteristics and structure of underground geological bodies (such as resistivity magnitude and distribution). The geoelectric structure obtained through resistivity inversion can help us identify different geological strata. For example, high resistivity areas may correspond to dense rock layers (such as granite), while low resistivity areas may be strata containing more water or conductive minerals (such as clay or metallic mineralization zones). The boundaries between different resistivity areas may represent the interfaces of strata or the boundaries of geological structures. For a mountainous area rich in mineral resources, a detailed analysis and visualization of geological stratification is required, covering an area of ​​approximately 30 square kilometers. The geological structure of this region is relatively complex, suggesting the possible interaction of strata of different lithologies and signs of potential geological tectonic activity. In the actual assessment, 150 measuring points were arranged in a quincunx pattern within the study area. The spacing between the measuring points was 80 meters in flat areas and reduced to 50 meters in complex mountainous terrain, depending on the terrain and detection accuracy requirements. A high-precision electromagnetic detector was used to transmit electromagnetic signals at frequencies of 5Hz, 15Hz, 30Hz, and 60Hz at each measuring point. The apparent resistivity and impedance phase data of each measuring point at different frequencies were recorded. At measuring point 1, when the transmission frequency was 5Hz, the observed apparent resistivity was 40Ω·m and the impedance phase was 25°; when the transmission frequency was 15Hz, the observed apparent resistivity was 38Ω·m and the impedance phase was 23°. Detailed data for all measuring points at different frequencies were recorded. The theoretical values ​​of apparent resistivity and impedance phase at different frequencies were calculated using the integral equation method. Taking measuring point 1 at a frequency of 5Hz as an example, the theoretical value of apparent resistivity was calculated to be 39Ω·m and the theoretical value of impedance phase was 24°. In practical evaluation, inversion is performed based on apparent resistivity and impedance phase. The resistivity distribution is iteratively updated using the least squares method. The resistivity distribution vector for the first iteration is... The resistivity distribution vector in the second iteration is Jacobian matrix The resistivity distribution is obtained by calculating the partial derivative of the objective function with respect to the resistivity distribution vector. The regularization matrix is ​​constructed based on the regularization parameters and grid relationships. The observed voltage vector is a vector composed of the actual measured voltage values. The voltage vector predicted based on the current resistivity distribution vector is obtained through forward modeling. The process is iterated until the objective function no longer decreases, ultimately yielding a relatively accurate resistivity distribution. Based on the resistivity distribution, different geological strata and structures are identified. The first geological stratum, with a depth of 3-8 meters, is determined to have a resistivity characteristic of 35-45 Ω·m, exhibiting a gradual contact relationship with the lower geological strata, thus obtaining the geoelectric structure.

[0022] S200 acquires the static displacement of the electromagnetic field, and divides the geoelectric structure into non-uniform and uniform geoelectric regions based on the static displacement. It then performs a difference analysis on the orthogonal components of the magnetic fields of the non-uniform and uniform geoelectric regions to obtain electromagnetic response parameters and polarization ellipse parameters. It should be explained that the differential analysis of the orthogonal components of the magnetic field between heterogeneous and homogeneous electrically charged geological regions is intended to further reveal the unique characteristics of their electromagnetic field responses. The obtained electromagnetic response parameters (amplitude ratio and phase difference) can quantify the differences in the absorption, reflection, and propagation characteristics of electromagnetic signals in different regions. These differences are closely related to the electrical properties and structure of the geological body. The polarization ellipse parameters can describe the polarization state and directional characteristics of the electromagnetic field in different geological regions from a more microscopic perspective, helping us to infer the degree of anisotropy of the geological body, the dip direction of the strata, and the possible geological structural trends, thereby providing a comprehensive and in-depth understanding of the complexity and spatial distribution characteristics of underground geological structures. In the actual assessment, a theoretical electromagnetic propagation model was established based on prior knowledge of the geological region. The region's geology mainly consists of granite, sandstone, and clay. The model was constructed based on parameters such as the conductivity and magnetic permeability of the strata. This model was used to simulate the normal electromagnetic response under electromagnetic signals of different frequencies, including the theoretical value of apparent resistivity. For a 5Hz frequency, the simulated theoretical apparent resistivity at measuring point 1 was 38 Ω·m. The static displacement was calculated based on the measured apparent resistivity. At measuring point 1, the measured apparent resistivity was 40 Ω·m, therefore the static displacement = 40 - 38 = The mean and standard deviation of the static displacement at all measuring points were calculated to be 2 Ω·m. The mean was 1.2 Ω·m and the standard deviation was 0.4 Ω·m. A threshold of 2.0 Ω·m was obtained. Measuring points with a measured apparent resistivity greater than the threshold were classified as non-uniform electrical geological zones; otherwise, they were classified as uniform electrical geological zones. Measuring point 1, with a measured apparent resistivity greater than the threshold, was classified as non-uniform electrical geological zone. Measuring point 2, with a measured apparent resistivity of 36 Ω·m, less than the threshold, was classified as uniform electrical geological zone. After point-by-point judgment, adjacent measuring points of the same type were integrated to form uniform electrical geological zones and non-uniform electrical geological zones. Ultimately, four uniform electrical geological zones and three non-uniform electrical geological zones were obtained. A magnetic field orthogonal component difference analysis was performed on the non-uniform and uniform electrical geological zones to obtain electromagnetic response parameters and polarization ellipse parameters.

[0023] S300 determines the diffusion curve of the electric field based on the electromagnetic response parameters and the polarization ellipse parameters, and solves the diffusion curve by the finite difference method to invert the structure of the underground geology and obtain the distribution model of the electric field. In the actual assessment, a theoretical model was established based on Maxwell's equations to connect the electric field with electromagnetic response parameters and polarization ellipse parameters. The underground geological region was divided into multiple grid points, each with a size of 0.4 m × 0.4 m. For each grid point, an electric field diffusion equation was established, and the finite difference method was used to solve the theoretical model to obtain the distribution of the electric field throughout the region, resulting in the electric field diffusion curve. Boundary conditions were determined based on the actual geological environment and electromagnetic characteristics. The electric field was zero at the boundary of the study area. Initial conditions were set according to the initial state of electromagnetic detection, with the initial time... At that time, the electric field distribution throughout the region is as follows: ; at time step At that time, based on the current moment The electric field value and difference equation are used to calculate the electric field value at each grid point. The electric field value at time is obtained, and so on, until all electric field values ​​of each grid within the time range (time range is 0-4s) are obtained. By analyzing the changes and distribution of these electric field values, the structural characteristics of underground geology are inverted, and information on the boundaries of geological bodies and the characteristics of stratigraphic changes is obtained.

[0024] S400 infers the spatial morphology and distribution pattern of underground geology based on the electric field distribution model; In practical assessments, the type of underground geological body is determined based on the magnitude and variation characteristics of the electric field values ​​in the electric field distribution model. In areas where the electric field value is relatively high and stable between 0.25 and 0.35 V / m, it is identified as granite. Areas with lower electric field values ​​between 0.1 and 0.2 V / m and exhibiting gradient variations are identified as clay layers. By calculating the gradient of the electric field in different directions, areas with drastic electric field changes are identified; these areas represent geological body boundaries or stratigraphic interfaces. Specifically, in certain areas, the electric field... The gradient in the direction is relatively large, in A smaller gradient in the direction indicates that the geological body in this area changes significantly in the direction, and there are inclined stratigraphic interfaces or rock body boundaries. Based on the trend and magnitude of the change in the electric field gradient, the shape of the geological body is gradually delineated. In the actual assessment, the electric field characteristics of different regions were analyzed to obtain the distribution law of geological structure. In one region, the electric field value showed periodic changes with a period of 100 meters, which is related to the layered structure of the strata. It is inferred that the thickness of the strata in this region changes periodically within a certain range. The gradient change of the electric field in a certain direction has certain regularity. The gradient change is relatively gentle in the east-west direction and larger in the north-south direction. The geological structure is trending north-south. By comprehensively analyzing the electric field characteristics of multiple regions, the distribution law of underground geological structure was summarized, including the dip direction of the strata and the fold morphology. Anomalies in the electric field distribution model were detected and further analyzed to determine the anomaly type. At a certain location, the electric field value suddenly increased to 0.5V / m, which was significantly different from the electric field value of the surrounding area. Through comparison with the electric field characteristics of the surrounding area and analysis of geological data, it is inferred that the anomaly is caused by the presence of local underground metal veins.

[0025] S500 divides geological layers based on the spatial morphology and distribution patterns of the underground geology, calculates the two-dimensional mapping plane of each geological layer, and generates a multi-level geological triangulation model. In the actual assessment, the secondary induced electromotive force in the receiving coil exhibits a specific curvilinear characteristic as it changes over time. The calculated depths of the first geological stratum below the measuring point are 6 meters, the second 12 meters, and the third 31.4 meters. Through orthographic projection, the geological strata are vertically projected onto a horizontal plane. For each geological stratum, its coordinates on the projection plane are calculated using trigonometric functions based on its strike, dip angle, and displacement. For the first geological stratum, the strike is... (The angle of rotation clockwise from due north), the tilt angle is The displacement at the geological level is 10 meters. According to trigonometric relationships, the point on the projection plane... Coordinates are This allows for the calculation of the coordinates of each point on each geological layer in the projection plane. In actual assessment, the coordinates of the geological bedding plane on the projection plane are used as input. The triangulation method is used to generate a triangulated mesh. There are 800 points on the geological bedding plane. All points are judged and combined in turn to generate a triangulated mesh of the geological bedding plane, thereby constructing a multi-level geological triangulation model, which clearly shows the spatial morphology and interrelationships of the geological bedding plane.

[0026] The S600 calculates the electromagnetic response surface to obtain an irregular closed two-dimensional contour curve. Based on the two-dimensional contour curve and the multi-level geological triangulation model, a three-dimensional reconstruction is performed to obtain a multi-level geological structure model.

[0027] It should be explained that the irregular, closed two-dimensional contour curves depict the horizontal boundaries of different geological layers. Extracting these curves from the electromagnetic response surface allows for the accurate determination of the extent and shape of each geological layer. These curves are crucial for identifying the distribution of strata, the boundaries of geological bodies, and potential geological structures (such as folds and faults). In actual evaluation, permeability dielectric constant The partial differential equations were discretized spatially and temporally using the finite difference method. Spatially, the study area was divided into a uniform three-dimensional grid with a grid size of 100m. Boundary conditions were determined based on the actual geological environment. At the boundaries of the study area, the normal component of the electric field intensity was zero. Initial conditions were set according to the initial state of electromagnetic detection. At the initial time, the electric field distribution of the entire area was as follows: And the initial rate of change of the electric field is Starting from the initial moment, a computer program performs iterative calculations within the boundary conditions according to a preset time step. Within each time step, the electric field value for the next time step is calculated based on the difference equation and the electric field value at the current moment, thus obtaining the electric field value of each grid point at different times. In the actual assessment, based on the electromagnetic response characteristics of different geological layers and prior knowledge, the range thresholds of electromagnetic responses for each type of geological layer were determined. Through analysis of previous geological data and research experience in similar areas, it was known that the main strata in this basin include sandstone, shale, and limestone, and their corresponding electromagnetic response characteristics differ. For sandstone strata, the electromagnetic response is between [0.02 - 0.05] V / m; for shale strata, the electromagnetic response range is [0.01 - 0.03] V / m; and for limestone strata, the electromagnetic response is relatively strong, between [0.03 - 0.08] V / m. A gradient-based edge detection algorithm was used to calculate the gradient of the electromagnetic response surface in space. When the gradient value exceeds a certain threshold, the point is considered to be located on the boundary of the geological layer. By tracking these boundary points, we can obtain the irregular closed two-dimensional contour curves of each stratum. In the end, we obtained two-dimensional contour curves of sandstone strata containing 500 points, two-dimensional contour curves of shale strata containing 400 points, and two-dimensional contour curves of limestone strata containing 600 points. These two-dimensional contour curves accurately depict the distribution range of different geological layers on the horizontal plane. In practical evaluation, for each geological layer's triangulated model, its corresponding two-dimensional contour curve on the horizontal projection plane is found. Based on this contour curve, the boundaries of the triangulated model are corrected and refined. Using 3D modeling software or algorithms (such as OpenGL or Blender), 3D reconstruction is performed based on the adjusted multi-layered geological triangulation model and the two-dimensional contour curve. Triangular meshes from different geological layers are stacked and combined according to their actual underground positions and sequences to construct a complete 3D geological structure model. During 3D reconstruction, the model is optimized by removing unnecessary triangular faces and simplifying its complexity to improve rendering efficiency and visualization effects. Simultaneously, the contact relationships between geological layers are smoothed to make the model visually more realistic and natural.

[0028] In this embodiment, the method for resistivity inversion based on apparent resistivity and impedance phase at different frequencies includes: Apparent resistivity and impedance phase at different frequencies are calculated using the integral equation method. Inversion is then performed based on apparent resistivity and impedance phase, with the following expression: in, Let be the resistivity distribution vector. For inversion function, and These are the apparent resistivity and impedance phase values ​​at the observation point, respectively; The objective function in the inversion process is expressed as follows: in, Let be the resistivity distribution vector. The number of observation points, For the first Voltage at each observation point For the first The resistance at each observation point For regularization parameters, The number of resistivity values. For the first All neighboring nodes of a resistivity value and The first The resistivity value and the first One resistivity value; The resistivity distribution is iteratively updated using the least squares method until the objective function no longer decreases, as expressed in the following expression: in, and The first Second and third The resistivity distribution vector of the next iteration. For Jacobian matrices, This is represented as a transpose operation. For regularization parameters, For regularization matrix, To observe the voltage vector, The voltage vector predicted based on the current resistivity distribution vector; Based on the resistivity distribution, different geological strata and structures are divided, the boundaries and contact relationships of the geological strata are determined, and the geological electrical structure is obtained.

[0029] In this embodiment, the method for obtaining the static displacement of electromagnetic fields and dividing the geoelectric structure into non-uniform and uniformly uniform geoelectric regions based on the static displacement includes: A theoretical electromagnetic propagation model is established based on prior knowledge of the geological region. The established model is used to simulate the normal electromagnetic response under electromagnetic signals of different frequencies, including the theoretical value of apparent resistivity. The static displacement is calculated based on the measured apparent resistivity. Calculate the mean and standard deviation of the static displacement. Add twice the standard deviation to the mean as a threshold. Points with measured apparent resistivity greater than the threshold are classified as non-uniform electrical geology; otherwise, they are classified as uniform electrical geology. After completing the point-by-point judgment, adjacent measuring points of the same type are integrated to form uniform and non-uniform electrical geological regions.

[0030] In this embodiment, the method for difference analysis of the orthogonal components of the magnetic fields of non-uniform and uniformly electrically charged geological regions includes: At each measuring point and frequency, the ratio of the amplitudes and the difference in phases of the orthogonal components of the magnetic field in the non-uniform and uniformly electrically charged geological regions at the same frequency are calculated to obtain the amplitude response and phase response, which are then used as electromagnetic response parameters. Based on the data of the orthogonal components of the magnetic field, the equation for the polarization ellipse is constructed, and its expression is: in, and These represent the horizontal component of the magnetic field and its amplitude, respectively. and These represent the perpendicular component of the magnetic field and its amplitude, respectively. This represents the phase difference between the horizontal and vertical components of the magnetic field. For both homogeneous and non-homogeneous geological regions, solve their respective polarization ellipse equations to obtain the inclination angle, major axis, and minor axis of the polarization ellipse. Calculate the difference between the homogeneous and non-homogeneous geological regions as the polarization ellipse parameters.

[0031] In this embodiment, the method for determining the diffusion curve of the electric field based on the electromagnetic response parameters and the polarization ellipse parameters includes: A theoretical model is established based on Maxwell's equations, relating the electric field to electromagnetic response parameters and polarization ellipse parameters. The expression is as follows: in, It is the amplitude of the magnetic field. For frequency, The imaginary unit, Angular frequency, Permeability, For electric field, For electrical conductivity, For volume, The inclination angle of the polarization ellipse. and These are the components of the electric field in the horizontal and vertical directions, respectively. for The complex conjugate, for and The real part of the product, and These are the major and minor axes of the polarization ellipse, respectively. The formulas for selecting the major and minor axes are: plus sign for calculating the major axis and minus sign for calculating the minor axis. The underground geological region is divided into multiple grid points. For each grid point, an electric field diffusion equation is established. The theoretical model is solved using the finite difference method to obtain the distribution of the electric field throughout the region, resulting in the electric field diffusion curve, expressed as: in, For in position and time The total electric field at that location, For the observation point, For time, The amplitude of the incident electric field. The attenuation coefficient is a location-dependent factor. For vector field operators, The phase factor of the sine wave. For wave vectors, As the source point, The source term of the electric field is the electric field generated by an external source.

[0032] In this embodiment, the method for solving diffusion curves using the finite difference method to invert the structure of underground geology includes: The partial differential equation of the diffusion curve is converted into a difference equation, specifically: for the spatial derivative, the central difference scheme is used, and for the time derivative, the backward difference scheme is used. Boundary conditions are determined based on the actual geological environment and electromagnetic characteristics, and initial conditions are set based on the initial state of electromagnetic detection. Each grid is assigned an initial electric field strength value based on the initial conditions. Starting from the initial time, time iteration is performed within the boundary conditions according to the preset time step. Specifically, within each time step, the electric field value of the next time step is calculated using the difference equation based on the electric field value at the current time, until all electric field values ​​of each grid within the time range are obtained.

[0033] In this embodiment, the method for inferring the spatial morphology and distribution pattern of underground geology based on the electric field distribution model includes: determining the type of underground geological body based on the electric field value in the electric field distribution model, outlining the shape of the geological body by combining the electric field gradient change, analyzing the electric field characteristics of different regions to obtain the distribution pattern of geological structure, detecting abnormal points in the electric field distribution model and determining the abnormality type.

[0034] In this embodiment, the method for calculating the two-dimensional mapping plane of each geological layer to generate a multi-level geological triangulation model includes: The depth of each geological layer is calculated based on the changing trend of the secondary induced electromotive force in the receiving coil. The geological layer is vertically projected onto the horizontal plane through orthophoto projection to determine the coordinate system of the projection plane. For each geological layer, the coordinates on the projection plane are calculated using trigonometric functions based on the strike, dip angle and displacement. Using the coordinates of the geological bedding plane on the projection plane as input, a triangulated mesh is generated using the triangulation method. The expression for triangulating the point set on the geological bedding plane is as follows: in, It indicates that there are three points , and The triangle formed For point set, For any point in the point set, From point arrive The vector, From point arrive The vector, From point Time The vector, From point Time The vector.

[0035] In this embodiment, the method for calculating the electromagnetic response surface to obtain an irregularly closed two-dimensional profile curve includes: The partial differential equation of the electromagnetic response surface is expressed as follows: in, For the Laplace operator, For electric field strength, Permeability, Where is the dielectric constant. For current density, For time; Using the finite difference method, the partial differential equation is transformed into a difference equation, which is then discretized in space and time to solve for the electromagnetic response. The expression for the difference equation is as follows: ; in, , and They are respectively at time step of , and When the coordinates in two-dimensional space are The electric field strength at that location, For time step, , , and They are respectively at time step of Time, and coordinates The electric field strength values ​​at four adjacent grid points, and They are respectively in In direction and in Spatial step size in direction, Permeability, Where is the dielectric constant. For time step Time, coordinates The current density vector at that location, For time, The rate of change of current density with time; The calculated electromagnetic response values ​​of each grid point are arranged according to geographical coordinates to form a two-dimensional data matrix, and the data matrix is ​​then converted into an electromagnetic response surface. The range threshold of electromagnetic response for various geological strata is obtained, and the contour is extracted from the electromagnetic response surface to obtain the irregular closed two-dimensional contour curve of each stratum.

[0036] A second aspect of the present invention also provides a system for constructing a multi-level geological structure model based on electromagnetic detection, comprising: The resistivity inversion module is used to transmit electromagnetic signals of different frequencies at various measuring points using an electromagnetic detector, and to perform resistivity inversion based on the apparent resistivity and impedance phase at different frequencies to obtain the geoelectric structure. The electrical zoning analysis module is used to obtain the static displacement of electromagnetic fields. Based on the static displacement, the geoelectric structure is divided into non-uniform electrical geological regions and uniform electrical geological regions. The difference analysis of the orthogonal components of the magnetic fields of the non-uniform electrical geological regions and uniform electrical geological regions is performed to obtain electromagnetic response parameters and polarization ellipse parameters. The electric field diffusion simulation module is used to determine the electric field diffusion curve based on the electromagnetic response parameters and the polarization ellipse parameters, and solve the diffusion curve by the finite difference method to invert the structure of the underground geology and obtain the electric field distribution model. The geological morphology inference module is used to infer the spatial morphology and distribution patterns of underground geology based on the electric field distribution model. The multi-level modeling module is used to divide geological layers based on the spatial morphology and distribution pattern of the underground geology, calculate the two-dimensional mapping plane of each geological layer, and generate a multi-level geological triangulation model. The three-dimensional reconstruction module is used to calculate the electromagnetic response surface, obtain an irregular closed two-dimensional contour curve, and perform three-dimensional reconstruction based on the two-dimensional contour curve and the multi-level geological triangulation model to obtain a multi-level geological structure model.

[0037] The above description is merely an example and illustration of the structure of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described, or use similar methods to replace them, as long as they do not deviate from the structure of the invention or exceed the scope defined in the claims, all of which should fall within the protection scope of the present invention.

Claims

1. A method for constructing a multi-level geological structural model based on electromagnetic detection, characterized in that, Includes the following steps: Electromagnetic detectors are used to emit electromagnetic signals of different frequencies at various measuring points. Resistivity inversion is performed based on the apparent resistivity and impedance phase at different frequencies to obtain the geoelectric structure. The static displacement of the electromagnetic field is obtained, and the geoelectric structure is divided into non-uniform and uniform geoelectric regions based on the static displacement. The difference of the orthogonal components of the magnetic field in the non-uniform and uniform geoelectric regions is analyzed to obtain the electromagnetic response parameters and polarization ellipse parameters. The diffusion curve of the electric field is determined based on the electromagnetic response parameters and the polarization ellipse parameters. The diffusion curve is solved by the finite difference method to invert the structure of the underground geology and obtain the distribution model of the electric field. The spatial morphology and distribution patterns of underground geology are inferred from the electric field distribution model. Based on the spatial morphology and distribution pattern of the underground geology, geological layers are divided, and a two-dimensional mapping plane for each geological layer is calculated to generate a multi-level geological triangulation model. Calculate the electromagnetic response surface to obtain an irregular closed two-dimensional contour curve. Based on the two-dimensional contour curve and the multi-level geological triangulation model, perform three-dimensional reconstruction to obtain a multi-level geological structure model. The method for determining the diffusion curve of the electric field based on the electromagnetic response parameters and the polarization ellipse parameters includes: A theoretical model is established based on Maxwell's equations, relating the electric field to electromagnetic response parameters and polarization ellipse parameters. The expression is: ； in, The amplitude of the magnetic field. For frequency, The imaginary unit, Angular frequency, Permeability, For electric field, For electrical conductivity, For volume, The inclination angle of the polarization ellipse. and These are the components of the electric field in the horizontal and vertical directions, respectively. for The complex conjugate, for and The real part of the product, and These are the major and minor axes of the polarization ellipse, respectively. The formulas for selecting the major and minor axes are: plus sign for calculating the major axis and minus sign for calculating the minor axis. The underground geological region is divided into multiple grid points. For each grid point, an electric field diffusion equation is established. The theoretical model is solved using the finite difference method to obtain the distribution of the electric field throughout the region, resulting in the electric field diffusion curve, expressed as: ； in, For in position and time The total electric field at that location, For the observation point, For time, The amplitude of the incident electric field, The attenuation coefficient is a location-dependent factor. For vector field operators, The phase factor of the sine wave. For wave vectors, As the source point, The source term of the electric field is the electric field generated by an external source.

2. The method for constructing a multi-level geological structure model based on electromagnetic detection according to claim 1, characterized in that, The method for resistivity inversion based on apparent resistivity and impedance phase at different frequencies includes: Apparent resistivity and impedance phase at different frequencies are calculated using the integral equation method. Inversion is then performed based on apparent resistivity and impedance phase, with the following expression: ； in, Let be the resistivity distribution vector. For inversion function, and These are the apparent resistivity and impedance phase values ​​at the observation point, respectively; The objective function in the inversion process is expressed as follows: ； in, Let be the resistivity distribution vector. The number of observation points, For the first Voltage at each observation point For the first The resistance at each observation point For regularization parameters, The number of resistivity values. For the first All neighboring nodes of a resistivity value and The first The resistivity value and the first One resistivity value; The resistivity distribution is iteratively updated using the least squares method until the objective function no longer decreases, as expressed in the following expression: ； in, and The first Second and third The resistivity distribution vector of the next iteration. For Jacobian matrices, This is represented as a transpose operation. For regularization parameters, For regularization matrix, To observe the voltage vector, The voltage vector predicted based on the current resistivity distribution vector; Based on the resistivity distribution, different geological strata and structures are divided, the boundaries and contact relationships of the geological strata are determined, and the geological electrical structure is obtained.

3. The method for constructing a multi-level geological structure model based on electromagnetic detection according to claim 1, characterized in that, The method for obtaining the static displacement of electromagnetic fields and dividing the geoelectric structure into non-uniform and uniform geoelectric regions based on the static displacement includes: A theoretical electromagnetic propagation model is established based on prior knowledge of the geological region. The established model is used to simulate the normal electromagnetic response under electromagnetic signals of different frequencies, including the theoretical value of apparent resistivity. The static displacement is calculated based on the measured apparent resistivity. Calculate the mean and standard deviation of the static displacement. Add twice the standard deviation to the mean as a threshold. Points with measured apparent resistivity greater than the threshold are classified as non-uniform electrical geology; otherwise, they are classified as uniform electrical geology. After completing the point-by-point judgment, adjacent measuring points of the same type are integrated to form uniform and non-uniform electrical geological regions.

4. The method for constructing a multi-level geological structure model based on electromagnetic detection according to claim 1, characterized in that, The method for differential analysis of the orthogonal components of the magnetic fields of non-uniform and uniformly electrically charged geological regions includes: At each measuring point and frequency, the ratio of the amplitudes and the difference in phases of the orthogonal components of the magnetic field in the non-uniform and uniformly electrically charged geological regions at the same frequency are calculated to obtain the amplitude response and phase response, which are then used as electromagnetic response parameters. Based on the data of the orthogonal components of the magnetic field, the equation for the polarization ellipse is constructed, and its expression is: ； in, and These represent the horizontal component of the magnetic field and its amplitude, respectively. and These represent the perpendicular component of the magnetic field and its amplitude, respectively. This represents the phase difference between the horizontal and vertical components of the magnetic field. For both homogeneous and non-homogeneous geological regions, solve their respective polarization ellipse equations to obtain the inclination angle, major axis, and minor axis of the polarization ellipse. Calculate the difference between the homogeneous and non-homogeneous geological regions as the polarization ellipse parameters.

5. The method for constructing a multi-level geological structure model based on electromagnetic detection according to claim 1, characterized in that, The method for inverting the structure of underground geology by solving diffusion curves using the finite difference method includes: The partial differential equation of the diffusion curve is converted into a difference equation, specifically: for the spatial derivative, the central difference scheme is used, and for the time derivative, the backward difference scheme is used. Boundary conditions are determined based on the actual geological environment and electromagnetic characteristics, and initial conditions are set based on the initial state of electromagnetic detection. Each grid is assigned an initial electric field strength value based on the initial conditions. Starting from the initial time, time iteration is performed within the boundary conditions according to the preset time step. Specifically, within each time step, the electric field value of the next time step is calculated using the difference equation based on the electric field value at the current time, until all electric field values ​​of each grid within the time range are obtained.

6. The method for constructing a multi-level geological structure model based on electromagnetic detection according to claim 1, characterized in that, The method for inferring the spatial morphology and distribution pattern of underground geology based on the electric field distribution model includes: determining the type of underground geological body based on the electric field value in the electric field distribution model, outlining the shape of the geological body by combining the electric field gradient change, analyzing the electric field characteristics of different regions to obtain the distribution pattern of geological structure, detecting abnormal points in the electric field distribution model and determining the type of abnormality.

7. The method for constructing a multi-level geological structure model based on electromagnetic detection according to claim 1, characterized in that, The method for calculating the two-dimensional mapping plane of each geological layer to generate a multi-level geological triangulation model includes: The depth of each geological layer is calculated based on the changing trend of the secondary induced electromotive force in the receiving coil. The geological layer is vertically projected onto the horizontal plane through orthophoto projection to determine the coordinate system of the projection plane. For each geological layer, the coordinates on the projection plane are calculated using trigonometric functions based on the strike, dip angle and displacement. Using the coordinates of the geological bedding plane on the projection plane as input, a triangulated mesh is generated using the triangulation method. The expression for triangulating the point set on the geological bedding plane is as follows: ； in, It indicates that there are three points , and The triangle formed For point set, For any point in the point set, From point arrive The vector, From point arrive The vector, From point Time The vector, From point Time The vector.

8. The method for constructing a multi-level geological structure model based on electromagnetic detection according to claim 1, characterized in that, The method for calculating the electromagnetic response surface to obtain an irregular closed two-dimensional profile curve includes: The partial differential equation of the electromagnetic response surface is expressed as follows: ； in, For the Laplace operator, For electric field strength, Permeability, Where is the dielectric constant. For current density, For time; Using the finite difference method, the partial differential equation is transformed into a difference equation, which is then discretized in space and time to solve for the electromagnetic response. The expression for the difference equation is as follows: ； in, , and They are respectively at time step of , and When the coordinates in two-dimensional space are The electric field strength at that location, For time step, , , and They are respectively at time step of Time, and coordinates The electric field strength values ​​at four adjacent grid points, and They are respectively in In direction and in Spatial step size in direction, Permeability, Where is the dielectric constant. For time step Time, coordinates The current density vector at that location, For time, The rate of change of current density with time; The calculated electromagnetic response values ​​of each grid point are arranged according to geographical coordinates to form a two-dimensional data matrix, and the data matrix is ​​then converted into an electromagnetic response surface. The range threshold of electromagnetic response for various geological strata is obtained, and the contour is extracted from the electromagnetic response surface to obtain the irregular closed two-dimensional contour curve of each stratum.

9. A system for constructing a multi-level geological structure model based on electromagnetic detection, used to execute the method for constructing a multi-level geological structure model based on electromagnetic detection as described in any one of claims 1 to 8, characterized in that, The system includes: The resistivity inversion module is used to transmit electromagnetic signals of different frequencies at various measuring points using an electromagnetic detector, and to perform resistivity inversion based on the apparent resistivity and impedance phase at different frequencies to obtain the geoelectric structure. The electrical zoning analysis module is used to obtain the static displacement of electromagnetic fields. Based on the static displacement, the geoelectric structure is divided into non-uniform electrical geological regions and uniform electrical geological regions. The difference analysis of the orthogonal components of the magnetic fields of the non-uniform electrical geological regions and uniform electrical geological regions is performed to obtain electromagnetic response parameters and polarization ellipse parameters. The electric field diffusion simulation module is used to determine the electric field diffusion curve based on the electromagnetic response parameters and the polarization ellipse parameters, and solve the diffusion curve by the finite difference method to invert the structure of the underground geology and obtain the electric field distribution model. The geological morphology inference module is used to infer the spatial morphology and distribution patterns of underground geology based on the electric field distribution model. The multi-level modeling module is used to divide geological layers based on the spatial morphology and distribution pattern of the underground geology, calculate the two-dimensional mapping plane of each geological layer, and generate a multi-level geological triangulation model. The three-dimensional reconstruction module is used to calculate the electromagnetic response surface, obtain an irregular closed two-dimensional contour curve, and perform three-dimensional reconstruction based on the two-dimensional contour curve and the multi-level geological triangulation model to obtain a multi-level geological structure model.

Images