Magnetotelluric sounding measurement method for multidirectional aggregation observation

Abstract

The invention discloses a magnetotelluric sounding measurement method for multidirectional aggregation observation, and the method comprises the steps: building a novel device layout: designing a reasonable measurement point layout scheme according to geological features and electromagnetic response distribution; processing data and establishing an unstructured grid: acquiring an electromagnetic field component, completing data acquisition, and inputting the processed electromagnetic field data into an unstructured grid model; impedance tensor calculation: on the basis of the unstructured grid, performing impedance calculation by utilizing multi-component electromagnetic field data and combining an interpolation idea, and constructing an extended impedance tensor of an electromagnetic field component to obtain an impedance value component; and three-dimensional inversion and visualization: performing three-dimensional inversion through a vector finite element method to obtain an inversion result. According to the method, richer observation data are provided for deep geologic structure inversion.

Description

Technical Field

[0001] This invention relates to the field of geophysical exploration technology, and in particular to a magnetotelluric sounding method based on multi-directional convergent observation. Background Technology

[0002] Currently, my country has made certain achievements in the research and application of magnetotelluric sounding devices, and some of the developed equipment has been widely used in various fields such as geological exploration, oil and gas and mineral resource exploration, geothermal surveys, and geological disaster monitoring. Traditional electromagnetic exploration devices typically use a cross-shaped layout for electromagnetic component acquisition, but under certain specific geological conditions, they cannot effectively improve the resolution of deep structures. Existing devices still have technical shortcomings in terms of anti-interference capabilities, limitations of single observation directions, and the level of automation in data acquisition. Especially in complex or heterogeneous media, the response sensitivity in non-orthogonal directions is low, and the ability to measure the Hz vertical component is weak. Traditional devices are also affected by high and low frequencies and the underground medium, which obviously lags behind international advanced levels and needs improvement and innovation. At present, although the mainstream devices in China have achieved conventional observation with a cross-shaped layout, there are still shortcomings in sensor layout optimization and multi-dimensional observation capabilities (especially the coordinated acquisition of oblique and vertical components), and a high-precision observation technology system that can adapt to complex geological conditions has not yet been formed.

[0003] To date, the following patents have been publicly disclosed in China: ① Chen Xiaobin, Wang Peijie, et al., a magnetotelluric observation method, device, equipment and medium for arbitrary observation directions; ② Jiang Xikun, Ma Ruihua, et al., a controllable source audio magnetotelluric device.

[0004] The two patents mentioned above, patent ① CN120233452A, entitled "Method, Device, Equipment and Medium for Magnetotelluric Observation in Arbitrary Observation Directions," proposes a new magnetotelluric sounding method that breaks through the traditional limitation of "orthogonal deployment." Its core idea is to allow electric and magnetic field observation channels to be deployed in arbitrary directions (non-orthogonal). By constructing an electromagnetic field correction matrix, the observation data is vector-transformed, thereby accurately converting the non-orthogonal data into electromagnetic spectrum data in an orthogonal coordinate system, and calculating the impedance tensor accordingly. This method greatly improves the flexibility of measurement point deployment, especially suitable for areas with complex terrain and limited observation, such as mountain valleys and urban underground areas. The technical route covers data acquisition, frequency domain conversion, angle correction, and tensor calculation, ensuring measurement accuracy while significantly expanding the applicability of the magnetotelluric method. The innovation lies mainly in the integration of "non-orthogonal deployment + mathematical vector correction," making the traditional method, which can only be deployed in "flat and open areas," reliable and effective even under harsh conditions. However, this invention does not take into account factors such as measurement errors of the angle between the electromagnetic field observation channel and the reference axis, and external interference conditions, which may affect the accuracy of subsequent vector transformation and impedance tensor calculations. It also does not consider the applicability of three-dimensional effects and anisotropy. When the polarization direction of the field source changes, this invention still has inherent limitations in noise suppression and distortion identification compared to methods using multi-polarized field sources.

[0005] Another patent, CN116466403A, proposes an array-type magnetotelluric data acquisition device. The core method is to improve exploration efficiency by connecting electrodes in series via a single cable and sharing magnetic field sensors at intervals. The idea is to connect multiple electrodes (near electrodes) in series on a single cable, with each electrode equipped with an electric field acquisition device; a magnetic field sensor is placed every odd number (M) of electric field acquisition devices, allowing (M-1) / 2 electric field points before and after to share the magnetic field data. This solves the problems of traditional magnetotelluric methods, which require independent placement of cross electrodes and magnetic field sensors at each measuring point (e.g., 41 magnetic field sensors are needed for a 1-kilometer survey line), resulting in cumbersome deployment, low efficiency, and high cost. Unlike traditional methods, this invention employs ① electrode series connection + magnetic field sharing, significantly reducing the number of sensors (90% reduction in magnetic field sensors for scalar measurements and approximately 90% reduction for vector measurements); ② a bidirectional expansion design, achieving vector measurements by adding vertically positioned far electrodes and magnetic field sensors, requiring only a small amount of additional equipment; and ③ a synchronous triggering circuit to ensure synchronized acquisition of electric and magnetic field data. This invention has limitations and constraints, relying on the assumption of spatial homogeneity of the magnetic field, which may limit the accuracy of shared magnetic field data under complex geological conditions; it also does not provide a detailed analysis of the impact of shared sensors on the resolution of high-frequency signals. Overall, this invention significantly reduces equipment deployment costs and fieldwork, making it suitable for large-scale exploration, but further verification of its adaptability to complex environments is needed. Summary of the Invention

[0006] To address the problems existing in the prior art, the purpose of this invention is to provide a multi-directional convergent observation magnetotelluric sounding method. This invention can be applied to magnetotelluric sounding in fields such as mineral resource exploration, oil and gas reservoir detection, and geological structure research. This method utilizes natural electromagnetic fields to detect the electrical structure inside the Earth and has broad application prospects. This invention is an innovative improvement based on the existing cross-shaped layout device, realizing the measurement of multi-directional electromagnetic field components. It is especially suitable for high-precision detection of complex geological structures (such as two-dimensional and three-dimensional non-homogeneous media), providing richer observational data for the inversion of deep geological structures.

[0007] To achieve the above objectives, the technical solution adopted by this invention is: a magnetotelluric sounding method based on multi-directional convergent observation, comprising the following steps:

[0008] Step 1: Establishment of the layout of the new device: Design a reasonable layout scheme for measuring points based on geological characteristics and electromagnetic response distribution;

[0009] Step 2, Data Processing and Unstructured Mesh Creation: Obtain electromagnetic field components, complete data acquisition, and input the processed electromagnetic field data into the unstructured mesh model;

[0010] Step 3, Impedance Tensor Calculation: Based on the unstructured mesh, using multi-component electromagnetic field data and combining interpolation, impedance is calculated to construct an extended impedance tensor of the electromagnetic field components, thus obtaining the impedance value components.

[0011] Step 4, 3D Inversion and Visualization: The 3D inversion is performed using the vector finite element method to obtain the inversion results.

[0012] As a further improvement to the present invention, step 1 is specifically as follows:

[0013] An orthogonal, oblique, and L-shaped layout framework is adopted. In the orthogonal directions of the north-south x-axis and the east-west y-axis, oblique directions p and q are added at 45° and 135° to the x-axis, respectively. Two sets of L-shaped local coordinate systems are configured on the periphery to form a fully enclosed observation system for simultaneously acquiring multiple electromagnetic field components.

[0014] As a further improvement of the present invention, the electromagnetic field components include orthogonal electromagnetic field components, oblique electromagnetic field components, two sets of L-shaped electromagnetic field components, and vertical magnetic field components.

[0015] As a further improvement of the present invention, in step 2, the electric field components are obtained in the following way:

[0016] The electric field data is calculated using an interpolation method. For any node on the plane of the device, its electric field component value is obtained by calculating the arithmetic mean of all edges connected to it and in the same direction.

[0017] As a further improvement of the present invention, in step 2, the magnetic field components are obtained in the following way:

[0018] Extract the electric field value at the measuring point. According to Maxwell's equations, the electric field and magnetic field are related through curl. Calculate the curl using the finite difference method to obtain the magnetic field component.

[0019] As a further improvement of the present invention, in step 4, a theoretical model based on finite element method is established to describe the electromagnetic response of the underground medium; the electrical parameters of the underground medium are deduced from the measured electromagnetic data, and the extended impedance tensor is used to optimize the model parameters by constructing a three-dimensional inversion objective function. The objective function is used to minimize the difference between the measured data and the theoretical model.

[0020] As a further improvement of the present invention, the objective function is expressed as:

[0021] ;

[0022] in, It is the data objective function, which measures the difference between observed data and theoretical models; λ is the model objective function, used to constrain the smoothness and rationality of the model; λ is the regularization parameter, used to balance the weights of data fitting and model smoothness.

[0023] As a further improvement of the present invention, the data objective function It consists of orthogonal components, two sets of L-shaped layout components, and oblique components, as shown in the following formula:

[0024] = + ;

[0025] in, For the cross-shaped layout components,; This is the first group of L-shaped layout components; This is the second group of L-shaped layout components; This is a component for diagonal layout.

[0026] This invention proposes a method for measuring multiple electromagnetic field components by extending to multiple directions. This method aims to acquire data information from multiple components and effectively improve the information acquisition efficiency of the vertical component. It directly provides more complete input data support for high-precision 3D inversion, improving detection capabilities and work efficiency in 3D and complex structural regions. Currently, most mainstream magnetotelluric sounding devices in my country adopt a cross-shaped layout, acquiring electromagnetic field components Ex, Ey, and Hx, Hy, and Hz respectively. The current devices have relatively limited observation directions, acquiring fewer horizontal electromagnetic components. Therefore, this invention introduces an oblique extension layout to the original device, extending four new electric and magnetic field observation directions at the original observation point. The original x-axis and y-axis are rotated counterclockwise by 45° to obtain the corresponding p-axis and q-axis, allowing the acquisition of new oblique components (Ep, Eq, Hp, Hq). This oblique extension layout enhances the acquisition sensitivity of the Hz vertical component. Two additional L-shaped layouts (not interconnected) form a fully enclosed structure. Simultaneous measurement of multiple electromagnetic field components in different directions allows for the acquisition of horizontal components (Ex, Ey, Hx, Hy), vertical components (Hz), oblique components (Ep, Eq, Hp, Hq), and L-shaped layouts (Ex1, Ex2, Ey1, Ey2, Hx1, Hx2, Hy1, Hy2). This significantly improves work efficiency. Furthermore, this design enhances the ability to acquire electromagnetic components in different directions.

[0027] This invention, by constructing a multi-directional observation layout, acquires oblique electric field components (Ep, Eq) and oblique magnetic field components (Hp, Hq) based on the traditional cross-shaped layout's horizontal electric field components (Ex, Ey), horizontal magnetic field components (Hx, Hy), and vertical magnetic field components (Hz). It also acquires electric field components (Ex1, Ex2, Ey1, Ey2) and magnetic field components (Hx1, Hx2, Hy1, Hy2) from two L-shaped layouts, achieving comprehensive electromagnetic data acquisition. This breakthrough expands the data dimension from orthogonal components to aggregated observations with oblique components, and also increases the impedance tensor, laying a solid data foundation for more accurate exploration of underground structures.

[0028] Meanwhile, it possesses strong scalability and adaptability, and has broad application value in the future. In resource exploration, it can provide high-resolution 3D inversion results. Its resolution capability is significantly enhanced, achieving a balance between high-frequency shallow resolution and low-frequency deep resolution. In particular, it has a stronger ability to identify tilted, non-orthogonal, and deeply buried anomalies, and can more efficiently locate the distribution area of ​​mineral resources.

[0029] In geological hazard assessment, this technology not only demonstrates superior detection capabilities for precursors such as landslides, subsidence, and ground fissures, but also monitors geological structural changes in volcanic activity and seismic zones, capturing potential geological anomalies that could trigger disasters in advance, thus providing strong support for geological hazard prevention and response. In urban geological exploration, it can be used for structural monitoring, such as detecting underground pipeline distribution and cavities, and provides detailed geological data for the planning and development of urban underground space. Furthermore, this invention is applicable to various scientific exploration projects, such as deep Earth structure exploration and continental dynamics research. In the field of energy exploration, it plays a crucial role in detecting the reservoir distribution and permeability of unconventional energy sources such as shale gas and coalbed methane; in environmental monitoring, it can be used to define the extent of groundwater pollution and detect the distribution of heavy metal pollution in soil; and in engineering construction, it can provide accurate underground geological structure information for tunnel and bridge construction, ensuring the safe progress of projects.

[0030] This invention will advance the development of "intelligent geophysical exploration" and "multi-source fusion inversion" in my country, upgrading electromagnetic data acquisition from two-dimensional to three-dimensional and from low-dimensional to high-dimensional, potentially leading to an efficient, convenient, and scalable observation network. This will not only provide more accurate underground information support for early warning of geological disasters and green resource development, but also expand the application boundaries of magnetotelluric technology in emerging fields such as land spatial planning, safety monitoring, and carbon reservoir assessment. This invention not only improves data dimensionality and inversion accuracy, with wide applicability, but also provides technical support for multiple industries, and is expected to play a long-term strategic role in earth sciences and national infrastructure construction. Compared with existing technologies, the advantage of this invention lies in achieving an upgrade of electromagnetic data from low-dimensional to high-dimensional through the collaborative observation of multi-directional components, providing richer observational constraints for high-precision detection of complex geological structures.

[0031] The beneficial effects of this invention are:

[0032] This invention significantly enhances the ability to identify anisotropic geological bodies underground by constructing an orthogonal, oblique, and L-shaped multi-directional observation system and combining it with an extended impedance tensor model. Especially when dealing with non-orthogonal underground structures (such as fault zones at a 45° angle to orthogonal directions, oblique veins, etc.), this invention effectively captures differences in electromagnetic response through newly added oblique electric field components (Ep, Eq) and magnetic field components (Hp, Hq), as well as electromagnetic field components (Ex1, Ex2, Ey1, Ey2, Hx1, Hx2, Hy1, Hy2) in the L-shaped layout. These newly added components enhance the system's directional perception capability and can directly characterize the electromagnetic response of complex underground structures.

[0033] Compared to orthogonal layouts that rely solely on the Ex, Ey, Hx, and Hy components, this invention overcomes the limitations of traditional methods in identifying oblique structures (such as fault zones or veins), avoids missed detections due to directional blind spots, and significantly improves the detection capability of underground structures. By jointly analyzing multiple components, especially oblique and L-shaped layout components, this invention enhances the detection resolution of deep structures and strengthens the spatial resolution under heterogeneous underground media conditions.

[0034] The expanded impedance tensor (including Zxy, Zyx, Zpq, Zqp, Zx1y1, Zy1x) 1、 The addition of components such as Zx²y² and Zy²x² enriches the constraints in the inversion process, improving inversion accuracy and stability. These new components provide more independent response information, enhancing the constraint of the inversion objective function, thereby accelerating convergence and reducing the number of solutions under complex media conditions. Furthermore, by combining vector basis and interpolation concepts, this invention effectively improves data quality during the data processing stage through vector interpolation, particularly in the recovery of noisy and missing data, ensuring the coherence and accuracy between electromagnetic field components.

[0035] This method not only improves the accuracy of resistivity and phase calculations but also provides more robust input data for 3D inversion through the linkage analysis of multi-component data, effectively supporting higher-precision analysis of underground structures. In summary, this invention has significant technical advantages and practical application value in electromagnetic detection of complex geological bodies and deep structures. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the layout of the improved magnetotelluric measuring device in an embodiment of the present invention;

[0037] Figure 2 This is a schematic diagram of the survey area with an oblique layout;

[0038] Figure 3 This is a schematic diagram of the measurement components of the vector finite element method in an embodiment of the present invention;

[0039] Figure 4 This is a diagram showing the calculation of the electric field value of the cross-shaped layout in an embodiment of the present invention;

[0040] Figure 5 This is a calculation diagram of the electric field value of the first group of L-shaped layouts in an embodiment of the present invention;

[0041] Figure 6 This is a calculation diagram of the electric field value of the second group of L-shaped layouts in an embodiment of the present invention;

[0042] Figure 7 This is a diagram showing the calculation of the electric field value for the oblique layout in an embodiment of the present invention;

[0043] Figure 8 This is a flowchart illustrating the implementation of use cases in this invention.

[0044] Figure 9 This is a real-world simulation scene diagram from an embodiment of the present invention;

[0045] Figure 10 This is a diagram showing the three-dimensional inversion result of the theoretical model in an embodiment of the present invention. Detailed Implementation

[0046] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0047] Example 1

[0048] A magnetotelluric sounding method based on multi-directional convergent observations includes:

[0049] 1. Multi-component observation setup:

[0050] This embodiment constructs a combined observation framework of "cross-shaped layout + oblique layout + L-shaped layout". Based on the x-axis (north-south) and y-axis (east-west), it adds p-axis and q-axis (p-axis forms a 45° angle with x-axis, q-axis forms a 135° angle with x-axis), and introduces two independent L-shaped layouts around the overall framework, forming a near-fully enclosed observation structure. This framework acquires multiple electromagnetic components: orthogonal electromagnetic field components (Ex, Ey, Hx, Hy), oblique electromagnetic field components (Ep, Eq, Hp, Hq), two sets of L-shaped layout electromagnetic field components (Ex1, Ex2, Ey1, Ey2, Hx1, Hx2, Hy1, Hy2), and vertical magnetic field components (Hz), such as... Figure 1 As shown.

[0051] By employing a 45° / 135° oblique axis system, this invention can effectively capture the response differences of complex structures (such as oblique fractures and shear zones) at a certain angle to the orthogonal direction, avoiding directional blind spots and significantly enhancing the detection sensitivity for inclined structures and deep heterogeneous bodies. Simultaneously, the introduction of the L-shaped layout makes the observation system more comprehensive in spatial direction, improving data integrity and redundancy, and helping to reduce the uncertainties caused by observations in a single direction.

[0052] At the data processing level, the directional electromagnetic components can support the construction of a 2×2 impedance tensor, incorporating the coupling relationships between orthogonal terms, oblique terms, and L-shaped components. This design overcomes the limitations of tensors in describing complex media, provides richer constraints for three-dimensional joint inversion, and significantly improves the accuracy and reliability of underground structure analysis.

[0053] 2. Impedance tensor calculation:

[0054] This embodiment proposes an electromagnetic field analysis method combining vector finite element method and interpolation, and on this basis, achieves efficient calculation of the impedance tensor. This method can calculate newly added electromagnetic field components, including oblique components (Ep, Eq, Hp, Hq) and L-shaped layout components (Ex1, Ex2, Ey1, Ey2, Hx1, Hx2, Hy1, Hy2), in complex underground media and unstructured meshes, and further derive the various components of the impedance tensor. This method not only improves calculation accuracy but also simplifies the data processing flow, providing efficient technical support for three-dimensional forward and inverse modeling.

[0055] (1) Polarization mode:

[0056] The device described in this embodiment is deployed on a specific plane (such as the ground surface). Impedance calculation is performed in two modes: SX and SY. SX is based on the electric field component (Ex) measured in the horizontal direction (x-axis). In this mode, the electric field component primarily responds to the electromagnetic properties of geological structures (e.g., veins, faults, etc.) perpendicular to the measurement electrode layout. SY mode refers to measurements performed in the y-axis direction, primarily used to capture the electric field response along the east-west component. This mode is used to detect geological structures orthogonal to the x-axis direction. The two modes capture the electric field component through different layouts and directions, allowing data to be collected from two orthogonal directions. In this invention patent, the cross-shaped layout and the two sets of L-shaped layouts are both orthogonal directions.

[0057] In oblique layouts, due to the characteristics of unstructured grids, the internal structure and relationships of the grid remain unchanged. However, in order to capture the electromagnetic response to underground structures, especially the electromagnetic response of non-orthogonal structures oblique to geological structures, the orientation of the survey area is redefined, thus introducing two new modes: SP and SQ. These two modes are based on the original SX and SY modes, respectively, rotating the measurement range (i.e., the original survey area range) by 45°. Figure 2 As shown, this can improve the detection capability of non-orthogonal structures and effectively detect relevant electromagnetic responses.

[0058] In this way, different characteristics of underground structures can be detected simultaneously, especially in complex or heterogeneous media. This multi-dimensional electric field response has higher spatial resolution than traditional single-direction observation methods.

[0059] (2) Calculation of electromagnetic field components:

[0060] Ideally, interpolation methods are used to calculate the electric field data in SX, SY, SP, and SQ modes. In this process, to obtain the multi-directional electric field components (Ex, Ey, Ep, Eq, Ex1, Ey1, Ex2, Ey2) at the device node locations, the electric field component values ​​for any node on the device plane can be obtained by calculating the arithmetic mean of all edges connected to it in the same direction. For example, in SX mode, This edge can be named after its adjacent edges. ,like Figure 3 As shown, then the arithmetic mean is calculated to obtain... The other components are calculated similarly. Secondly, the magnetic field components of both modes can be calculated from the corresponding electric field components. This algorithm avoids complex basis function calculations, significantly improving post-processing efficiency.

[0061] 3. Theoretical calculation of impedance tensor for cross-shaped layout:

[0062] (1) Ex, Ey:

[0063] like Figure 4 The figures show the electric field values ​​in modes SX and SY, respectively. The edge ① is... Components, their calculation The value is obtained through two connected edges that are in the same direction. The arithmetic mean is obtained as shown in the following formula. (Edges ②, ③, and ④) Similarly, we calculate the values ​​under both modes and then use interpolation.

[0064]

[0065]

[0066] (2) Hx, Hy:

[0067] Once the electric field values ​​of Ex and Ey are obtained, the electric field value at the measuring point is extracted. According to Maxwell's equations, the electric and magnetic fields are related through curl. The magnetic field components Hx and Hy can be calculated using the following formula:

[0068]

[0069] The curl is calculated using the finite difference method to obtain the magnetic field components:

[0070]

[0071]

[0072]

[0073] Where i represents the phase information of the signal, w represents the angular frequency, μ represents the permeability, and Ez represents the electric field component in the z-axis direction. Thus, the values ​​of Ex, Ey, Hx, and Hy are obtained, and the impedance tensor is then calculated.

[0074] (3) Zxx, Zxy, Zyx, Zyy:

[0075] The impedance is calculated based on the electromagnetic field components and the following relationship.

[0076]

[0077]

[0078]

[0079] here , These are the electric and magnetic field values ​​obtained using the SX polarization mode. , These are the electric and magnetic field values ​​obtained using the SY polarization mode.

[0080] 4. Theoretical calculation of tensile strength of the first group of L-shaped layout components:

[0081] (1) Ex1, Ey1:

[0082] like Figure 5 The figures show the electric field values ​​in modes SX and SY, respectively. The edge ⑨ is... Components, calculation The value is obtained through two connected edges that are in the same direction. The arithmetic mean is obtained as shown in the following formula. (Edge ⑩⑪⑫) , , Similarly, we calculate the values ​​under both modes and then use interpolation.

[0083]

[0084]

[0085] (2) Hx1, Hy1:

[0086] Once the electric field values ​​of Ex1 and Ey1 are obtained, the electric field value at the measuring point is extracted. Similarly, combining Maxwell's equations and the method for calculating Hx and Hy, Hx1 and Hy1 can be obtained. The curl is calculated using the finite difference method to obtain the magnetic field components.

[0087]

[0088]

[0089] In this way, we obtain the values ​​of Ex1, Ey1, Hx1, and Hy1, and then we calculate the impedance tensor.

[0090] (3) The first group of L-shaped layout components (Zx1x1, Zy1y1, Zx1y1, Zy1x1):

[0091]

[0092]

[0093]

[0094] here it is , These are the electric and magnetic field values ​​obtained using the SX polarization mode. , These are the electric and magnetic field values ​​obtained using the SY polarization mode.

[0095] 5. Theoretical calculation of tensile strength of the second group of L-shaped layout components:

[0096] (1) Ex2, Ey2:

[0097] like Figure 6 The figures show the electric field values ​​in modes SX and SY, respectively. The edge ⑬ is... Components, calculation The value is obtained through two connected edges that are in the same direction. The arithmetic mean is obtained as shown in the following formula. (1496th edge) , , Similarly, we calculate the values ​​under both modes and then use interpolation.

[0098]

[0099]

[0100] (2) Hx2, Hy2:

[0101] Once the electric field values ​​of Ex2 and Ey2 are obtained, the electric field value at the measuring point is extracted. Similarly, combining Maxwell's equations and the method for calculating Hx and Hy, Hx2 and Hy2 can be obtained. The curl is calculated using the finite difference method to obtain the magnetic field components.

[0102]

[0103]

[0104] In this way, we obtain the values ​​of Ex2, Ey2, Hx2, and Hy2, and then we calculate the impedance tensor.

[0105] (3) The second group of L-shaped layouts Zx2x2, Zy2y2, Zx2y2, Zy2x2:

[0106]

[0107]

[0108]

[0109] here , These are the electric and magnetic field values ​​obtained using the SX polarization mode. , These are the electric and magnetic field values ​​obtained using the SY polarization mode.

[0110] 6. Theoretical calculation of tensile strength of oblique layout components:

[0111] (1)Ep, Eq:

[0112] like Figure 7 The figures show the electric field values ​​in SP and SQ modes, respectively. The edge ⑤ is... Components, calculation The value is obtained through two connected edges that are in the same direction. The arithmetic mean is obtained as shown in the following formula. (Edges 6, 7, and 8) , Similarly, we calculate the values ​​under both modes and then use interpolation.

[0113]

[0114]

[0115] ②Hp, Hq:

[0116] Once the electric field values ​​of Ep and Eq are obtained, the electric field value at the measuring point is extracted. Similarly, combining Maxwell's equations and the method used to calculate Hx and Hy, Hp and Hq can be obtained. The curl is then calculated using the finite difference method to obtain the magnetic field components.

[0117]

[0118]

[0119] In this way, we obtain the values ​​of Ep, Eq, Hp, and Hq, and then we calculate the impedance tensor.

[0120] ③Zpp, Zpq, Zqp, Zqq:

[0121] The impedance is calculated based on the electromagnetic field components and the following relationship.

[0122]

[0123]

[0124]

[0125] Here , These are the electric and magnetic field values ​​obtained using the SP polarization mode. , These are the electric and magnetic field values ​​obtained using the SQ polarization mode.

[0126] 7. Three-dimensional forward and inverse modeling:

[0127] The method in this embodiment first describes the electromagnetic response of the subsurface medium by establishing a theoretical model based on the finite element method (FEM). In this model, an unstructured mesh is used to accommodate complex geological structures, ensuring accurate capture of the coupling effects between various electromagnetic components, including orthogonal electric field components (Ex, Ey), magnetic field components (Hx, Hy), oblique electric field components (Ep, Eq), oblique magnetic field components (Hp, Hq), and L-shaped layouts (Ex1, Ex2, Ey1, Ey2, Hx1, Hx2, Hy1, Hy2). By establishing distribution models of different electromagnetic field sources and receivers, the method covers multi-directional, multi-component electromagnetic responses, providing higher resolution, particularly in distinguishing between deep and shallow structures.

[0128] By simulating multi-component electromagnetic data collected from various measuring points, the spatial distribution of electric and magnetic fields is calculated using an unstructured grid. Using this forward model, the electromagnetic response at each frequency can be calculated and compared with measured data to evaluate the model's accuracy and reliability.

[0129] After obtaining the forward modeling results from the theoretical model, a three-dimensional inversion can be performed by fitting them with measured data. The core objective of the inversion is to deduce the electrical parameters of the subsurface medium (such as apparent resistivity and phase) from the measured electromagnetic data. In this invention, the extended impedance tensor (including components such as Zxy, Zyx, Zpq, Zqp, Zx1y1, Zy1x1, Zx2y2, and Zy2x2) is used as the input data for the inversion. These data are obtained from multi-component acquisition and can provide more comprehensive information about the subsurface medium for the inversion.

[0130] To achieve this inversion process, this embodiment optimizes the model parameters by constructing a three-dimensional inversion objective function. The core idea of ​​this objective function is to minimize the difference between the measured data and the theoretical model. Specifically, the objective function can be expressed as:

[0131]

[0132] in, It is the data objective function, which measures the difference between observed data and theoretical models; λ is the model objective function, used to constrain the smoothness and rationality of the model; λ is the regularization parameter, used to balance the weights of data fitting and model smoothness.

[0133] The data file in this embodiment consists of orthogonal components, two sets of L-shaped layout components, and oblique components, as detailed in the following formula.

[0134] = +

[0135] in, The cross-shaped layout components include impedance components (Zxx, Zyy, Zxy, Zyx). The first group of L-shaped layout components includes impedance components (Zx1x1, Zy1y1, Zx1y1, Zy1x1). The second group of L-shaped layout components includes impedance components (Zx2x2, Zy2y2, Zx2y2, Zy2x2). The components are arranged in an oblique direction and include impedance components (Zpp, Zqq, Zpq, Zqp).

[0136] In summary, this method, based on the finite element method, enables three-dimensional inversion to simulate the electromagnetic field response under complex geological bodies, overcoming the limitations of traditional forward models in handling heterogeneous media. By incorporating an extended impedance tensor (including orthogonal and oblique components, and two sets of L-shaped layout components), the inversion model can provide more accurate images of subsurface structures, with significant advantages, especially in the identification of deep and shallow anomalies.

[0137] Example 2

[0138] This embodiment uses a tetrahedral resistivity model as the theoretical target to illustrate the method for multi-directional electromagnetic measurement of electromagnetic field components described in this invention, as well as a series of processes including data acquisition, forward modeling, and inversion. The application flow of the unstructured finite element inversion method is also explained. The technical flow is as follows: Figure 8 As shown, it includes:

[0139] Step 1, Establishment of the new device layout:

[0140] Within the study area, a reasonable layout scheme for measuring points was first designed based on geological characteristics and electromagnetic response distribution. An orthogonal + oblique + L-shaped layout framework was adopted, adding oblique directions p and q (at 45° and 135° to the x-axis, respectively) to the traditional north-south (x-axis) and east-west (y-axis) orthogonal directions. Two sets of L-shaped local coordinate systems were configured on the periphery to form a fully enclosed observation system, such as... Figure 9 As shown, this layout can simultaneously acquire multiple electromagnetic field components, including orthogonal electromagnetic field components (Ex, Ey, Hx, Hy), oblique electromagnetic field components (Ep, Eq, Hp, Hq), L-shaped layout electromagnetic field components (Ex1, Ex2, Ey1, Ey2, Hx1, Hx2, Hy1, Hy2), and vertical magnetic field components (Hz). This layout enhances the detection capabilities of underground three-dimensional structures and anisotropic anomalies, particularly showing better capture of the electromagnetic responses of oblique structures (such as fault zones and veins).

[0141] Step 2, Data Processing and Unstructured Mesh Creation:

[0142] After acquiring the electromagnetic field components (Ex, Ey, Hx, Hy), oblique electromagnetic field components (Ep, Eq, Hp, Hq), and two sets of L-shaped electromagnetic field components (Ex1, Ex2, Ey1, Ey2, Hx1, Hy1, Hx2, Hy2) for a cross-shaped layout, the processed electromagnetic field data was input into an unstructured mesh model. Unstructured meshes can flexibly adapt to complex underground geological structures and are not limited by traditional structured meshes when accurately simulating the spatial distribution of electromagnetic fields.

[0143] Step 3, Impedance Tensor Calculation:

[0144] Based on unstructured meshes, impedance calculations are performed using multi-component electromagnetic field data and interpolation techniques. An extended impedance tensor containing Ex, Ey, Ex1, Ex2, Ey1, Ey2 and Hx, Hy, Hx1, Hx2, Hy1, Hy2, Hp, Hq, Ep, Eq, and Hz is constructed, yielding impedance values ​​such as Zxy, Zyx, Zpq, Zqp, Zx1y1, and Zx2y2.

[0145] This theoretical model not only includes traditional Zxy and Zyx elements but also extends to non-orthogonal cross terms such as Zpq, Zqp, Zx1y1, and Zx2y2, thus providing more information on electromagnetic coupling in various directions. Impedance, apparent resistivity, and phase are further derived from the extended tensor. After obtaining the data, the quality of the apparent resistivity and other data needs to be assessed. This can be done through visualization analysis, such as drawing pseudo-section diagrams, for comparative analysis of multi-scale and multi-directional characteristics. If inversion is not performed, qualitative or semi-quantitative interpretation is required.

[0146] Step 4, 3D Inversion and Visualization:

[0147] The inversion results are obtained through three-dimensional inversion using the vector finite element method; such as Figure 10 As shown, the tetrahedral shape is clearly defined with sharp boundaries, and its 45° orientation is accurately reproduced. The structure of the layered background and low-resistivity layer is also closer to the real model. Traditional devices lack oblique components, resulting in a "directional blind zone" for oblique structures and leading to inaccurate imaging. The measurement method of this invention achieves the acquisition of multi-directional electromagnetic field components, significantly improving the dimensionality of data information. At the same time, the device effectively eliminates the blind zone through multi-directional data, significantly improving the ability to characterize complex structures.

[0148] The data fitting error during inversion was significantly reduced, and the newly added electromagnetic field component provided additional constraints for the inversion, making the model not only more accurate but also more reliable. The joint constraints of the multi-component data from the new instrument enhanced the model's vertical resolution. The inversion results were analyzed by generating a three-dimensional visualization model of the subsurface structure and displaying results in the form of pseudo-section diagrams. The results not only accurately identified subsurface geological bodies but also demonstrated the electrical differences between shallow and deep layers, providing a higher-resolution geological interpretation, especially under complex tectonic and heterogeneous media conditions.

[0149] The embodiments described above are merely illustrative of specific implementations of the present invention, and while the descriptions are detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A magnetotelluric sounding method based on multi-directional convergent observation, characterized in that, Includes the following steps: Step 1: Establishment of the layout of the new device: Design a reasonable layout scheme for measuring points based on geological characteristics and electromagnetic response distribution; Step 2, Data Processing and Unstructured Mesh Creation: Obtain electromagnetic field components, complete data acquisition, and input the processed electromagnetic field data into the unstructured mesh model; Step 3, Impedance Tensor Calculation: Based on the unstructured mesh, using multi-component electromagnetic field data and combining interpolation, impedance is calculated to construct an extended impedance tensor of the electromagnetic field components, thus obtaining the impedance value components. Step 4, 3D Inversion and Visualization: The 3D inversion is performed using the vector finite element method to obtain the inversion results.

2. The magnetotelluric sounding method based on multi-directional convergent observation according to claim 1, characterized in that, Step 1 is described in detail as follows: An orthogonal, oblique, and L-shaped layout framework is adopted. In the orthogonal directions of the north-south x-axis and the east-west y-axis, oblique directions p and q are added at 45° and 135° to the x-axis, respectively. Two sets of L-shaped local coordinate systems are configured on the periphery to form a fully enclosed observation system for simultaneously acquiring multiple electromagnetic field components.

3. The magnetotelluric sounding method based on multi-directional convergent observation according to claim 2, characterized in that, The electromagnetic field components include orthogonal electromagnetic field components, oblique electromagnetic field components, two sets of L-shaped electromagnetic field components, and vertical magnetic field components.

4. The magnetotelluric sounding method based on multi-directional convergent observation according to claim 1 or 3, characterized in that, In step 2, the electric field components are obtained as follows: The electric field data is calculated using an interpolation method. For any node on the plane of the device, its electric field component value is obtained by calculating the arithmetic mean of all edges connected to it and in the same direction.

5. The magnetotelluric sounding method based on multi-directional convergent observation according to claim 4, characterized in that, In step 2, the magnetic field components are obtained as follows: Extract the electric field value at the measuring point. According to Maxwell's equations, the electric field and magnetic field are related through curl. Calculate the curl using the finite difference method to obtain the magnetic field component.

6. The magnetotelluric sounding method based on multi-directional convergent observation according to claim 3, characterized in that, In step 4, a theoretical model based on the finite element method is established to describe the electromagnetic response of the underground medium. The electrical parameters of the underground medium are inversely derived from the measured electromagnetic data. The extended impedance tensor is used to optimize the model parameters by constructing a three-dimensional inversion objective function. The objective function is used to minimize the difference between the measured data and the theoretical model.

7. The magnetotelluric sounding method based on multi-directional convergent observation according to claim 6, characterized in that, The objective function is expressed as: ； in, It is the data objective function, which measures the difference between observed data and theoretical models; λ is the model objective function, used to constrain the smoothness and rationality of the model; λ is the regularization parameter, used to balance the weights of data fitting and model smoothness.

8. The magnetotelluric sounding method based on multi-directional convergent observation according to claim 7, characterized in that, The data objective function It consists of orthogonal components, two sets of L-shaped layout components, and oblique components, as shown in the following formula: = + ； in, For the cross-shaped layout components,; This is the first group of L-shaped layout components; This is the second group of L-shaped layout components; This is a component for diagonal layout.

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