A method for detecting transient electromagnetic gradient tensors and locating anomalies in holes

Abstract

This invention discloses a method for transient electromagnetic gradient tensor detection and anomaly localization in boreholes. The method includes deploying three-component orthogonal transmitting coils in the borehole, with the three coils simultaneously powered on and synchronously switched off to generate a vector-superimposed primary magnetic field; deploying multiple sets of three-component orthogonal receiving coil arrays to synchronously receive induced potential signals; calculating nine components of the induced potential gradient tensor based on the spatial positional relationship of the receiving arrays using spatial difference quotients; jointly analyzing the nine gradient tensor components, determining the quadrant of the anomaly based on the number of sign change points in the gradient component response curves, and then determining the angle between the anomaly and the coordinate axes based on the amplitude and degree of overlap or separation of the specific gradient component response curves, thus achieving directional localization of the anomaly in all space. This invention requires only a single borehole to achieve directional detection of anomalies within a 360° range around the borehole, and is applicable to scenarios such as advanced detection during coal mine roadway excavation, location of hidden mine water sources causing disasters, and transparent geological construction.

Description

Technical Field

[0001] This invention belongs to the field of geophysical electromagnetic exploration technology, specifically relating to a method for transient electromagnetic detection using boreholes, and more particularly to a method for locating anomalies in the entire space by three-component orthogonal transmission in the borehole, receiving multiple sets of three-component arrays, and calculating gradient tensors. Background Technology

[0002] In my country, 53% of coal resources are buried at depths below 1,000 meters, and the mining depth is increasing at an average rate of 10 to 25 meters per year. This continuous increase in mining depth has dramatically increased the risks of geological disasters such as mine water hazards, gas outbursts, and roof collapses. According to statistics from the National Mine Safety Administration, water hazards remain one of the major threats to safe coal mine production, and accurate detection of hidden hazardous geological structures (such as concealed collapse columns, water-bearing faults, and old goaf areas) is a crucial prerequisite for preventing and controlling water hazards.

[0003] Current mainstream mine geophysical exploration methods have the following prominent problems in practical applications: First, it is difficult to balance detection range and accuracy. The advanced detection range of DC electrical methods is only about 100 meters, which is difficult to meet the needs of rapid tunneling. Although transient electromagnetic methods are sensitive to low-resistivity bodies, traditional methods mainly rely on the vertical component (z component) or the total field for data interpretation, discarding effective information in the x and y directions, and the ability to distinguish the spatial orientation and geometric shape of anomalies is seriously insufficient.

[0004] Secondly, borehole utilization is low, and the information available is limited. Drilling is an essential step in tunnel excavation, but currently, the utilization of boreholes is limited to a single, narrow view; the borehole itself is only used as a passage or pumping hole, and the geological information surrounding and in front of it is not fully explored. Directional drilling is costly (tens of thousands to hundreds of thousands of yuan per borehole), but existing geophysical exploration methods are limited, and the comprehensive utilization rate of borehole resources urgently needs to be improved.

[0005] Third, the strong interference environment underground limits the detection effect. There are a large number of magnetic conductors such as metal support nets, anchor bolts, and tunneling machines underground. Traditional transient electromagnetic devices that use both co-located and separate transceivers are difficult to effectively suppress these regional background interferences. As a result, weak signals are submerged in strong interference, making it difficult to distinguish between false anomalies and true anomalies.

[0006] Therefore, there is an urgent need for an electromagnetic detection and positioning method in holes that can make full use of single-hole resources, acquire multi-dimensional information in the whole space, and have strong anti-interference capabilities. Summary of the Invention

[0007] (a) Technical issues To address the problems of insufficient detection accuracy, low utilization of borehole information, and weak anti-interference capability in existing technologies, this invention provides a method for transient electromagnetic gradient tensor detection and anomaly localization in boreholes. In a single borehole detection operation, this method employs simultaneous transmission from three-component orthogonal coils and synchronous reception from multiple arrays of three-component orthogonal coils. An induced potential gradient tensor containing nine components is calculated using spatial difference quotient, and a full-space anomaly localization criterion based on the gradient tensor response characteristics is established, enabling the determination of the orientation and angle of anomalies under single-bore conditions.

[0008] (II) Technical Content To solve the above-mentioned technical problems, the technical solution of the present invention is: a method for detecting transient electromagnetic gradient tensors in a hole and locating anomalies, comprising the following steps: Step 1: Install a three-component orthogonal transmitting coil in the borehole. The three-component orthogonal transmitting coil consists of three coils arranged orthogonally. The three coils are powered on and switched off synchronously, generating a primary magnetic field with vector superposition in space. Step 2: Deploy multiple sets of three-component orthogonal receiving coil arrays in the borehole, wherein the receiving coil arrays synchronously receive induced potential signals; Step 3: Based on the spatial positional relationship of each group of three-component orthogonal receiving coils in the receiving coil array, the induced potential gradient tensor containing 9 components is obtained by calculating the spatial difference quotient. Step 4: Perform joint analysis on the nine gradient tensor components, determine the quadrant where the anomaly is located based on the number of sign change points of the gradient component response curves, and then determine the angle between the anomaly and the coordinate axes based on the magnitude of the specific gradient component response curves and the degree of overlap or separation, thereby achieving the orientation and location of the anomaly in the entire space.

[0009] Furthermore, in the three-component orthogonal transmitting coil, the centers of the three orthogonal coils are located at the origin of the coordinate system. By changing the current direction of at least one coil, the main axis of the magnetic induction lines is made to point to any one of the eight quadrants of space.

[0010] Furthermore, the multi-group three-component orthogonal receiving coil array includes four groups of three-component orthogonal receiving coils, which are respectively arranged at the center point and at positions offset along the x, y, and z directions. By approximating the differential with the spatial difference quotient, the total spatial rate of change information of the induced potential in the x, y, and z directions is obtained.

[0011] Furthermore, the nine gradient tensor components include Vxx, Vxy, Vxz, Vyx, Vyy, Vyz, Vzx, Vzy, and Vzz.

[0012] Furthermore, the step of determining the quadrant of the anomaly based on the number of sign-changing points of the gradient component response curves specifically includes: analyzing the sign-changing characteristics of each gradient component response curve as the measurement point position changes, and determining the quadrant of the anomaly based on the correspondence between the number of sign-changing points and the spatial orientation of the anomaly.

[0013] Furthermore, the step of determining the angle between the anomaly and the coordinate axis based on the magnitude and degree of overlap or separation of the response curves of specific gradient components specifically includes: comparing the magnitude and degree of overlap or separation of the response curves of Vzy and Vzx components, and Vxy and Vyx components, to determine the precise angle of the anomaly relative to the coordinate axis.

[0014] Furthermore, the method only requires a single borehole to determine the location of anomalies within a 360° radius around the borehole.

[0015] Furthermore, when the three-component orthogonal transmitting coils are synchronously excited, the total magnetic field strength is [amount missing] times the magnetic field strength of a single coil excited individually. times.

[0016] Furthermore, the method is applied to advanced detection during coal mine roadway excavation, location of hidden mine water sources that could cause disasters, or transparent geological construction.

[0017] (III) Technical Effects The advantages of this invention compared to the prior art are: 1. It enables a leap from point to volume detection, allowing multiple sets of data to be obtained from the borehole radial 360° and the far area in front of the borehole in a single borehole detection. The amount of information obtained from a single borehole is greatly improved, breaking through the limitation of the traditional method of "seeing only from one hole".

[0018] 2. It has a natural anti-interference capability. The gradient tensor naturally suppresses regional background fields and common electromagnetic noise. Uniform primary field and far-field interference are significantly weakened in the spatial gradient, making local abnormal signals stand out and effectively solving the problem of weak signal identification in strong interference environment downhole.

[0019] 3. Achieve precise positioning in all space. Joint analysis of multi-component gradient signals can extract the directional features and location information of anomalies. Only one borehole is needed to determine the orientation of anomalies within a 360° range around the borehole (specific to quadrants and angles), breaking through the limitation of traditional methods that require joint interpretation of multiple boreholes.

[0020] 4. To increase the excitation field strength, when the three-component orthogonal coils are excited synchronously, the total magnetic field strength is [amount missing] times the magnetic field strength of a single coil excitation. It is twice as powerful (approximately 73%), and the main axis of the magnetic field streamlines is highly symmetrical along the body diagonal and in three orthogonal planes, fundamentally solving the problems of weak auxiliary component signals and low signal-to-noise ratio when transmitting in a traditional unidirectional direction. Attached Figure Description

[0021] Figure 1 A schematic diagram of the primary field propagation of a tensor source excitation device; Figure 2 Schematic diagram of a gradient tensor detection device in a hole; Figure 3 This is a technical roadmap of the present invention; Figure 4 , Figure 5 , Figure 6 These are the distribution diagrams of each magnetic field component in the xy plane, xz plane, and yz plane, respectively. Figure 7 , Figure 8 , Figure 9 The magnetic field vectors, total magnetic field strength, and magnetic field streamline diagrams are shown for the xy, xz, and yz planes, respectively. Figure 10 A schematic diagram of the layered model (a) and a diagram of the principal axis directions of the magnetic field lines in the spatial coordinate system of the primary field of the tensor source coil (b); Figure 11 Gradient tensor response curves of measurement points at different depths in the layered model; Figure 12 The multi-channel curves of the induced potential components Vx(a), Vy(b), and Vz(c) are shown. Figure 13 Multi-channel curves of the gradient tensor components Vxx(a), Vyy(b), and Vzz(c); Figure 14 This is a schematic diagram of the hole-side anomaly model and its mesh generation. Figure 15 The gradient tensor response curves are shown when the distance between the anomaly and the measuring point is 30m (a) and 60m (b). Figure 16 Multi-channel curves of the induced potential components Vx(a), Vy(b), and Vz(c) for detecting anomalies; Figure 17 Multi-channel curves of the gradient tensor components Vxx(a), Vyy(b), and Vzz(c) for anomaly detection; Figure 18 A contour map of the apparent resistivity of a single anomalous volume; Figure 19 This is a schematic diagram of a double anomaly model near the hole; Figure 20 The multi-channel curves of the induced potential components Vx(a), Vy(b), and Vz(c) of the dual-abnormal body are shown. Figure 21 The graphs show the multi-channel curves of the gradient tensor components Vxx(a), Vyy(b), and Vzz(c) of the dual anomaly body. Figure 22This is a contour map of the apparent resistivity of a dual-anomaly volume; Figure 23 Schematic diagrams of geological models when the anomaly is located at different depths; Figure 24 The graph shows the response curves of each gradient component when the anomaly is located at different depths. Figure 25 Schematic diagrams of anomalies located at different angles; Figure 26 The gradient tensor response curves are shown for the anomalies located in the same quadrant at 0° (a), 15° (b), and 30° (c). Detailed Implementation

[0022] The present invention will now be described in further detail with reference to the accompanying drawings.

[0023] Combined with appendix Figure 1 To be continued Figure 26 A method for detecting transient electromagnetic gradient tensors in a hole and locating anomalies includes the following steps: Step 1: Install a three-component orthogonal transmitting coil in the borehole. The three-component orthogonal transmitting coil consists of three coils arranged orthogonally. The centers of the three coils are located at the origin of the coordinate system. The three coils are powered on and switched off synchronously, generating a primary magnetic field with vector superposition in space. By changing the current direction of at least one coil, the main axis of the magnetic induction lines can be made to point to any one of the eight quadrants of space.

[0024] Step 2: Deploy multiple sets of three-component orthogonal receiving coil arrays in the borehole. The receiving coil array includes four sets of three-component orthogonal receiving coils, which are respectively deployed at the center point and at positions offset along the x, y, and z directions, for synchronously receiving the three-component signals of the induced potential.

[0025] Step 3: Based on the spatial positional relationship of each group of three-component orthogonal receiving coils in the receiving coil array, the spatial difference quotient is used to approximate the differential, and the complete spatial rate of change information of the induced potential in the x, y, and z directions is calculated, which is the induced potential gradient tensor containing 9 components. The 9 gradient tensor components include Vxx, Vxy, Vxz, Vyx, Vyy, Vyz, Vzx, Vzy, and Vzz.

[0026] Step 4: Perform joint analysis on the nine gradient tensor components. First, determine the quadrant where the anomaly is located based on the number of sign change points of the response curves of each gradient component as the measurement point position changes. Then, determine the angle between the anomaly and the coordinate axes based on the magnitude and degree of overlap or separation of the response curves of specific gradient components. The specific gradient components include Vzy and Vzx components, and Vxy and Vyx components. Finally, the orientation and location results of the anomaly in the entire space are formed.

[0027] As a verification of the effectiveness of the method of the present invention, the above method steps are verified by forward simulation below.

[0028] I. Tensor Source Excitation Device and Field Source Characteristics This invention uses a three-component quadrature transmitting coil as the excitation source (e.g., Figure 1 As shown, the three coils are arranged orthogonally with their centers located at the origin of the coordinate system. They are simultaneously powered on and synchronously turned off, forming a tensor source emission mode. The three directions are excited simultaneously, generating a primary magnetic field with vector superposition in space.

[0029] Taking a coil radius of 1m and a supply current of 100A as an example, based on the Biot-Savart law, the discrete integration method is used to calculate the magnetic field distribution at all points in space when three orthogonal coils are simultaneously excited (e.g., ...). Figures 4 to 9 As shown in the figure, the results indicate that when three orthogonal coils are excited synchronously, the total magnetic field strength is the vector superposition of the individual excitation magnetic fields of each coil, and is approximately [a fraction of] the excitation strength of a single coil. This technology effectively compensates for the inherent defects of weak auxiliary component signals and low signal-to-noise ratio in traditional unidirectional transmission. The main axis of the magnetic field lines follows the right-hand screw rule, pointing in a specific direction in the eight spatial quadrants. Moreover, the projections on the three orthogonal planes are all at a 45° angle to the coordinate axes, exhibiting high symmetry. When the current direction of a coil in a certain direction is changed, the main axis of the magnetic field lines can point to any one of the eight spatial quadrants, laying the foundation for the field source of subsequent joint interpretation of multi-component signals.

[0030] II. Gradient Tensor Detection Device in Hole Multiple sets of three-component orthogonal receiving coil arrays (such as...) are deployed in the borehole. Figure 2 As shown in the diagram, specifically, a spatial array of four sets of three-component orthogonal receiving coils is used, positioned at the center point and at locations offset by a certain distance along the x, y, and z directions. By approximating the differential with spatial difference quotients, nine gradient tensor components are obtained, including Vxx, Vxy, Vxz, Vyx, Vyy, Vyz, Vzx, Vzy, and Vzz. This array design allows for the acquisition of all spatial rate of change information of the induced potential at the measurement point in the x, y, and z directions in a single measurement, achieving a leap from point detection to volumetric scanning.

[0031] III. Three-dimensional full-space forward modeling simulation To verify the effectiveness of the method and the positioning criteria, a three-dimensional time-domain electromagnetic forward modeling simulation was performed using the unstructured vector finite element method (e.g., Figure 3 As shown), complex geometric structures such as boreholes and anomalies are finely depicted through tetrahedral meshing (e.g., Figure 14As shown in the figure, Nédélec vector interpolation basis functions are used to ensure the continuity of the tangential component of the electromagnetic field. The second-order back-electrode Euler method is used for time discretization to ensure numerical stability. The Pardiso direct solver is used to solve the large sparse linear equation system.

[0032] IV. Background Field Response Characteristic Analysis Through forward modeling, the response characteristics of homogeneous full-space and layered media were systematically analyzed (e.g., Figure 10 , Figure 11 (As shown).

[0033] The uniform full-space results show that the response curves of the three components of the induced potential at the center of the field source, namely Vx, Vy, and Vz, completely overlap. When shifted along the axial direction, only the effective signal segment shows a time delay while its shape remains unchanged, verifying the feasibility of lateral comparison between the components.

[0034] For H-type (low-resistivity interlayer) and K-type (high-resistivity interlayer) layered media (such as...) Figure 10 As shown in the figure, the gradient tensor signal exhibits a more pronounced response to formation interfaces compared to the traditional three-component signal. A clear minimum value is observed in the gradient amplitude at the center of low-resistivity formations, while a maximum value is displayed at the interface, confirming that the gradient tensor has a higher resolution for electrical interfaces and stratigraphic centers.

[0035] V. Response Characteristics of Anomaly Detection The system completed multi-model forward simulations of single anomalies near the borehole, double anomalies, and anomalies in front of the final borehole (e.g. Figures 14 to 24 (As shown).

[0036] like Figures 14 to 18 As shown, in the single anomaly model near the borehole, anomalies of different distances (e.g., 30m, 60m) and sizes are set. The results show that the gradient tensor response can effectively amplify the anomaly signal compared to the induced potential response, and has a good resolution effect on the boundary of the anomaly along the measurement line direction; the change in the distance between the anomaly and the measurement point will significantly affect the width and amplitude of the response time window, and the closer the distance, the earlier the anomaly response appears and the larger the amplitude.

[0037] like Figures 19 to 22 As shown, in the hole-side double anomaly model, double anomalies with different spacing and symmetrical arrangement are set. The results show that the gradient tensor has good resolution for the boundaries of adjacent anomalies and can effectively distinguish the response ranges of the double anomalies.

[0038] like Figure 23 , Figure 24 As shown, in the model of the anomaly in front of the final hole, anomalies of different depths are set up. The response curves of each gradient component change regularly with the depth of the anomaly, providing a basis for the distance assessment of the anomaly.

[0039] VI. Full-space gradient tensor localization method Based on the aforementioned large amount of forward modeling data, the system summarizes the response characteristics of nine gradient components under different orientation anomaly conditions (e.g., Figure 25 , Figure 26 As shown in the figure, a comprehensive judgment criterion based on the number of sign change points and the degree of curve overlap or separation was established for the first time.

[0040] The specific positioning steps are as follows: The first step is to determine the quadrant of the anomaly based on the number of sign-changing points, analyze the sign-changing characteristics of the response curves of each gradient component as the measurement point position changes, and determine the quadrant of the anomaly based on the correspondence between the number of sign-changing points and the spatial orientation of the anomaly.

[0041] The second step is to determine the precise angle between the anomaly and the coordinate axis based on the magnitude of the amplitude of the specific component and the degree of curve overlap. Specifically, by comparing the magnitude of the response curves of the Vzy and Vzx components and the degree of curve overlap or separation, the precise angle of the anomaly relative to the coordinate axis is determined.

[0042] Through the above two steps, a complete system of methods for locating anomalies in the entire space is formed, which improves the detection capability from judging whether there is an anomaly to accurately locating where the anomaly is.

[0043] by Figure 25 Taking the models shown as examples where the anomalous bodies are located in the same quadrant at 0°, 15°, and 30°, Figure 26 The gradient tensor response curves shown clearly demonstrate the variation of each component under different angular conditions, verifying the effectiveness of the positioning method.

[0044] VII. Engineering Applications This invention can directly serve the following engineering scenarios: (1) Rapid advance detection of coal mine roadways: Existing boreholes are used for advance detection at the tunnel face. A single measurement can obtain geological anomaly information within a range of 100 meters ahead, which is significantly more efficient than the traditional point-by-point measurement method.

[0045] (2) Accurate location of hidden disaster-causing water sources: For hidden disaster-causing bodies such as collapse columns, water-bearing faults, and old open areas, the location of the center, boundary range, and radial orientation of the anomaly can be achieved through the joint interpretation of multiple components of the gradient tensor, providing precise target points for the design of drainage boreholes.

[0046] (3) Transparent geological construction and intelligent mining provide high-resolution borehole-side detection data, which can be integrated with other geological information to construct a more refined three-dimensional geological model.

[0047] In addition to coal mine safety, this method can also be extended to deep metal mineral exploration, urban underground space exploration, oil exploration and other fields.

[0048] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the protection scope of the present invention.

Claims

1. A method for detecting transient electromagnetic gradient tensors and locating anomalies in holes, characterized in that, Includes the following steps: Step 1: Install a three-component orthogonal transmitting coil in the borehole. The three-component orthogonal transmitting coil consists of three coils arranged orthogonally. The three coils are powered on and switched off synchronously, generating a primary magnetic field with vector superposition in space. Step 2: Deploy multiple sets of three-component orthogonal receiving coil arrays in the borehole, wherein the receiving coil arrays synchronously receive induced potential signals; Step 3: Based on the spatial positional relationship of each group of three-component orthogonal receiving coils in the receiving coil array, the induced potential gradient tensor containing 9 components is obtained by calculating the spatial difference quotient. Step 4: Perform joint analysis on the nine gradient tensor components, determine the quadrant where the anomaly is located based on the number of sign change points of the gradient component response curves, and then determine the angle between the anomaly and the coordinate axes based on the magnitude of the specific gradient component response curves and the degree of overlap or separation, thereby achieving the orientation and location of the anomaly in the entire space.

2. The method for detecting transient electromagnetic gradient tensors in a hole and locating anomalies according to claim 1, characterized in that, In the three-component orthogonal transmitting coil, the centers of the three orthogonal coils are located at the origin of the coordinate system. By changing the current direction of at least one coil, the principal axis of the magnetic induction lines is made to point to any one of the eight quadrants of space.

3. The method for detecting transient electromagnetic gradient tensors in a hole and locating anomalies according to claim 1, characterized in that, The array of multiple three-component orthogonal receiving coils includes four sets of three-component orthogonal receiving coils, which are respectively arranged at the center point and at positions offset along the x, y, and z directions. By approximating the differential with the spatial difference quotient, the total spatial rate of change of the induced potential in the x, y, and z directions is obtained.

4. The method for detecting transient electromagnetic gradient tensors in a hole and locating anomalies according to claim 1, characterized in that, The nine gradient tensor components include Vxx, Vxy, Vxz, Vyx, Vyy, Vyz, Vzx, Vzy, and Vzz.

5. The method for detecting transient electromagnetic gradient tensors in a hole and locating anomalies according to claim 1, characterized in that, The step of determining the quadrant of the anomaly based on the number of sign-changing points of the gradient component response curves specifically includes: analyzing the sign-changing characteristics of each gradient component response curve as the measurement point position changes, and determining the quadrant of the anomaly based on the correspondence between the number of sign-changing points and the spatial orientation of the anomaly.

6. The method for detecting transient electromagnetic gradient tensors in a hole and locating anomalies according to claim 1, characterized in that, The method of determining the angle between the anomaly and the coordinate axis based on the magnitude and degree of overlap or separation of the response curves of specific gradient components specifically includes: comparing the magnitude and degree of overlap or separation of the response curves of Vzy and Vzx components, and Vxy and Vyx components, to determine the precise angle of the anomaly relative to the coordinate axis.

7. The method for detecting transient electromagnetic gradient tensors in a hole and locating anomalies according to claim 1, characterized in that, The method requires only a single borehole to determine the location of anomalies within a 360° radius around the borehole.

8. The method for detecting transient electromagnetic gradient tensors in a hole and locating anomalies according to claim 1, characterized in that, When the three-component orthogonal transmitting coils are synchronously excited, the total magnetic field strength is equal to the magnetic field strength of a single coil excited individually. times.

9. The method for detecting transient electromagnetic gradient tensors in a hole and locating anomalies according to claim 1, characterized in that, The method is applied to advanced detection during coal mine roadway excavation, location of hidden water sources causing disasters in mines, or construction of transparent geology.

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