Multi-scale electromagnetic exploration method based on single-electrode potential detection

Abstract

The invention discloses a multi-scale electromagnetic exploration method based on single-electrode potential detection, and relates to the technical field of exploration geophysics, and the method comprises the steps: determining a measurement region, determining an exploration target, and determining the area of the measurement region and the density of measurement points according to the depth of the exploration target; arranging a plurality of emission sources and receiving arrays in a measurement area, collecting surface potential data of the measurement area in real time, and processing the data frequency point by frequency point according to a preset frequency sequence; on the basis of the single-electrode potential data, generating a surface potential distribution surface, constraining and suppressing a field source effect, obtaining electric field distribution information of a measurement surface, and calculating apparent resistivity parameters in combination with magnetic field measurement data; a multi-azimuth excitation mode and a multi-source emission scheme are adopted, a field source effect is inhibited, a three-dimensional resistivity profile is constructed, and a low-resistance body or a high-resistance body is identified. According to the invention, the static effect and the volume effect are solved, and the limitation of the existing electromagnetic instrument on the detection accuracy and depth is broken through.

Description

Technical Field

[0001] The present invention relates to the technical field of exploration geophysics, and particularly to a multi-scale electromagnetic exploration method based on single-electrode potential detection. Background Art

[0002] Electromagnetic exploration extracts detection parameter information including apparent resistivity, dispersion rate, polarization rate, magnetic susceptibility, and inclination angle, etc. through the amplitude / phase characteristics (frequency domain) and attenuation characteristics (time domain) of ground electromagnetic signals, intuitively obtains geoelectric characteristics, and combines forward and inverse modeling techniques to complete the detection of formation structures. Therefore, observing the amplitude / phase and attenuation characteristics of electromagnetic signals is a key task of electromagnetic exploration instruments.

[0003] The current surface electric field detection method uses the mode of measuring the potential difference between two electrodes MN, and the electric field observation scale is fixed. This surface electric field detection method not only has a large construction volume, but also cannot overcome the influence of rock masses near the field source, low resistivity on the surface of the measurement area, and high resistivity in the shallow layer.

[0004] The detection methods in foreign countries are difficult to meet the requirements of China's complex geological conditions. When there is a certain scale of geological body near the emission field source or between the transmitter and receiver, the secondary field excited by this geological body acts on the measurement area as part of the total excitation field, forming a field source effect; when there is an uneven area (such as rivers, lakes, etc.) on the surface of the measurement area, the induced charges around this uneven area form an induced electric field as part of the total measurement field, and the measured electric field shifts, forming a static effect; when there is a large-scale high-resistivity body (such as granite, etc.) in the shallow part of the measurement area, this high-resistivity body hinders current conduction and forms an electromagnetic energy reflection area, and deep geological bodies cannot be identified, forming a volume effect.

[0005] China's geological conditions are extremely complex. Conducting electromagnetic exploration in China will almost inevitably face these problems, and the existing electric field detection method using double-electrode potential difference cannot solve these problems. There is an urgent need to study a new detection method and develop corresponding new detection equipment to break through the limitations of existing electromagnetic exploration instruments on detection accuracy and depth. Summary of the Invention

[0006] The present invention proposes a multi-scale electromagnetic exploration method based on single-electrode potential detection, which overcomes problems such as field source effect, static effect, and volume effect that existing detection instruments cannot overcome, and realizes fine scanning of the formation from shallow to deep.

[0007] To achieve the above object, the present invention provides a multi-scale electromagnetic exploration method based on single-electrode potential detection, including:

[0008] Determine the measurement area, clarify the exploration target, and determine the area of the measurement area and the density of measurement points according to the depth of the exploration target;

[0009] Deploy a number of emission sources and receiving arrays in the measurement area, collect the surface potential data of the measurement area in real time, and process the data frequency by frequency according to a preset frequency sequence;

[0010] Based on the single-electrode potential data, generate a surface potential distribution plane, constrain and suppress the field source effect, obtain the electric field distribution information of the measurement plane, and calculate the apparent resistivity parameter in combination with the magnetic field measurement data;

[0011] Adopt a multi-directional excitation mode and a multi-source emission scheme to suppress the field source effect, and construct a three-dimensional resistivity profile to identify low-resistivity bodies or high-resistivity bodies.

[0012] Preferably, the area of the measurement area and the measurement point density need to cover the structural trend of the exploration target.

[0013] Preferably, deploy a number of emission sources and receiving arrays in the measurement area, including:

[0014] The emission sources are deployed at the edge of the measurement area, and the low-frequency signal ratio is adjusted according to the exploration depth;

[0015] The receiving magnetic sensors are arranged in an orthogonal manner, the electric field is measured by single-point potential, and the satellite or wireless module is used to ensure the synchronization of the emission time and the receiving time.

[0016] Preferably, collect the surface potential data of the measurement area in real time, including:

[0017] Interpolate and filter the potential of the measurement points of the receiving array to obtain a potential measurement surface, obtain the curvature distribution characteristics of the potential measurement surface according to the concavity and convexity of the potential measurement surface, analyze the influence degree of the surface or shallow part based on the potential surface curvature distribution, and adjust the density of the detection electrodes according to the requirements of the detection depth to reduce the potential surface curvature distribution form and complete the detection from shallow to deep.

[0018] Preferably, use an interpolation algorithm to generate the surface potential distribution plane, and eliminate the static effect and volume effect caused by shallow low-resistivity bodies or high-resistivity bodies through filtering;

[0019] Among them, the interpolation algorithm is Kriging interpolation or polynomial fitting interpolation, and the filtering method is spatial domain low-pass filtering or adaptive threshold filtering.

[0020] Preferably, constrain and suppress the field source effect, including:

[0021] By placing sparse detection points between the emission source and the receiving array, or adopting a multi-directional excitation mode, complete the constraint and suppression of the field source effect.

[0022] Preferably, the electric field distribution information of the measurement plane obtained is:

[0023] E = σ·J;

[0024] In the formula, E is the electric field strength, σ is the conductivity, and J is the current density.

[0025] Preferably, the apparent resistivity parameter is calculated as:

[0026]

[0027] In the formula, ω represents the angular frequency, μ represents the magnetic permeability, E x represents the electric field strength in the direction of the emission source, and H y represents the magnetic field strength perpendicular to the direction of the emission source, and ρ is the apparent resistivity parameter.

[0028] Preferably, the method further includes:

[0029] Comparing the resistivity anomaly with the known geological information through the resistivity distribution interface to demarcate the lithological boundary, fault zone or hydrothermal channel, and evaluate the scale and economic value of the target body.

[0030] Compared with the prior art, the present invention has the following advantages and technical effects:

[0031] (1) The present invention adopts a multi-directional and multi-source excitation mode to solve the influence of the field source effect, and completes multi-scale surface electric field measurement based on single-electrode potential detection, solves the static effect and volume effect, and breaks through the limitations of the existing electromagnetic method instruments on detection accuracy and depth; the single-electrode potential receiving station automatically obtains the longitude and latitude of the measurement point through the satellite module without manual positioning.

[0032] (2) Through multi-directional excitation and single-electrode potential measurement, the present invention contains the information of a single field source excitation mode and a multi-field source multi-directional excitation mode, can complete the functions of geometric sounding and induction sounding, can conduct electromagnetic exploration in the time domain and frequency domain, realizes multi-scale electromagnetic field detection on the surface, and solves the drawbacks of the traditional electromagnetic method. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The drawings constituting a part of this application are used to provide a further understanding of this application. The schematic embodiments of this application and their descriptions are used to explain this application and do not constitute an improper limitation to this application. In the drawings:

[0034] Figure 1 is a schematic diagram of the traditional electromagnetic detection mode of the embodiment of the present invention;

[0035] Figure 2 is a schematic diagram of the static effect mechanism of the embodiment of the present invention;

[0036] Figure 3 is a schematic diagram of the volume effect mechanism of the embodiment of the present invention;

[0037] Figure 4Schematic diagram of the suppression mechanism of the shallow low-resistance static effect in the embodiments of the present invention;

[0038] Figure 5 Schematic diagram of the suppression mechanism of the volume effect of the shallow high-resistance body in the embodiments of the present invention;

[0039] Figure 6 Effect diagram of the influence of the shallow geological body in the embodiments of the present invention;

[0040] Figure 7 Schematic diagram of the field construction method in the embodiments of the present invention;

[0041] Figure 8 Schematic diagram of the construction mode of the present invention in the embodiments of the present invention. Detailed implementation manners

[0042] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments may be combined with each other. The present application will be described in detail below with reference to the drawings and in combination with the embodiments.

[0043] It should be noted that the steps shown in the flowchart of the drawings may be executed in a computer system such as a set of computer-executable instructions, and although the logical order is shown in the flowchart, in some cases, the steps shown or described may be executed in a different order than here.

[0044] Traditional electromagnetic exploration methods are as Figure 1 shown. The transmitter emits current into the ground through the AB electrodes (the distance between the electrodes is about 1 km) as an excitation. A receiver array is arranged on a survey line parallel to AB 5 km away from the transmitter. The receiver measures the potential difference of the MN measurement electrodes. Since the potential difference of the MN electrodes is the integral of the electric field between MN, it is calculated using formula (1):

[0045]

[0046] In the formula, M and N respectively represent the position coordinates of the two electrodes along the survey line direction, E represents the electric field strength between MN, V MN represents the potential between MN, and dl is the line integral variable.

[0047] For the convenience of engineering, it is considered that the electric field between MN is uniform. Therefore, the detection is completed using formula (2):

[0048]

[0049] In the formula, V N and V M respectively represent the potentials of point N and point M, |MN| represents the distance between the electrode at point M and the electrode at point N, and E MN represents the electric field strength in the MN direction.

[0050] Measure the magnetic field information simultaneously, and extract different detection information according to different parameter definitions. For example, use formula (3) to define and extract the apparent resistivity parameter:

[0051]

[0052] In the formula, ω represents the angular frequency, μ represents the magnetic permeability, and E x represents the electric field strength in the direction of the emission source; H y represents the magnetic field strength perpendicular to the direction of the emission source, and ρ is the apparent resistivity parameter.

[0053] Therefore, the electromagnetic detection ability depends on the measurement accuracy of the surface electric field and magnetic field. When there are interference sources near the measurement site, the interference sources are superimposed on the surface background field, causing errors in the actually measured electromagnetic field and unable to accurately reflect the underground information. For example, when there are inhomogeneous bodies such as rivers and lakes near the measurement, induced charges are generated around them due to the influence of the background field, forming an induced electric field as part of the total measured field, causing the measured electric field to shift by a certain amplitude from low frequency to high frequency. The mechanism is as Figure 2 shown.

[0054] Another example is that there is a large-scale high-resistivity body (such as a granite body) in the shallow underground area. This high-resistivity body hinders the conduction of current and "squeezes" the current to the surface, as Figure 3 shown.

[0055] Since the magnitude of the electric field E is related to the current density J and the conductivity σ, as shown in formula (4):

[0056] E = σ·J (4);

[0057] Therefore, the high-resistivity body in the shallow underground raises the surface electric field by a certain amplitude, affecting the detection of deep geological bodies.

[0058] There is a large low-resistivity area on the surface (such as lakes, rivers, etc.). For traditional double electrodes, it is necessary to stretch long wires to cross this area for detection, and the actual construction difficulty is extremely high, or even impossible to complete. In this embodiment, the influence of shallow low-resistivity bodies on detection construction is overcome by the method of single electrode potential measurement. It only needs to place a receiving station in a certain distance area around the surface low-resistivity area, without the need to stretch electrode wires. In addition, by increasing the detection scale of the electrodes, the influence area of shallow geological bodies is bypassed to complete the detection of deep structures and overcome the influence of shallow low-resistivity static effects. As Figure 4 shown, in the low-resistivity area, the surface electric field is distorted due to the influence of induced charges and returns to normal after leaving the low-resistivity area by a certain distance. This embodiment can directly cross the area where the electric field is distorted and use the interpolation method to "smooth" the influence area within the crossing area to obtain reliable detection data.

[0059] When there are high-resistivity rock masses in the shallow subsurface, due to the obstruction of the high-resistivity bodies to the flow of electric current, the current is "squeezed" to the surface, forming a strong electric field, and returning to normal after a certain distance from the shallow high-resistivity area. In this embodiment, a large number of single-electrode receiving stations can be placed on the surface. By analyzing the data, the distribution pattern of the surface electric potential can be "outlined", such as Figure 5 shown. According to the required detection depth, gradually increase the detection electrode spacing, cross the area where the electric field is distorted, and use the interpolation method to "smooth" the affected area within the crossed area to obtain reliable detection data. By increasing the detection scale of the electrodes and bypassing the influence area of the shallow subsurface high-resistivity bodies, the fine detection of the formation structure from shallow to deep is completed.

[0060] The multi-scale electromagnetic exploration method based on single-electrode electric potential detection provided by this embodiment includes:

[0061] Determine the measurement area, clarify the exploration target, and determine the area of the measurement area and the density of measurement points according to the depth of the exploration target;

[0062] Deploy a number of emission sources and receiving arrays in the measurement area, collect the surface electric potential data of the measurement area in real time, and process the data point by point according to the preset frequency sequence;

[0063] Based on the single-electrode electric potential data, generate the surface electric potential distribution surface, constrain and suppress the field source effect, obtain the electric field distribution information of the measurement surface, and calculate the apparent resistivity parameter in combination with the magnetic field measurement data;

[0064] Adopt a multi-directional excitation mode and a multi-source emission scheme to suppress the field source effect, and construct a three-dimensional resistivity profile to identify low-resistivity or high-resistivity bodies.

[0065] The specific implementation steps are as follows:

[0066] The first step: data collection and target positioning. Collect data on the geology, geophysics, topography and electromagnetic interference sources (such as high-voltage lines, substations) of the measurement area, and clarify the exploration target (such as fault location, ore body burial depth). Determine the area of the measurement area and the density of measurement points (100 - 200 m / point) according to the target depth (such as 1 - 3 km). The area of the measurement area and the measurement points need to cover the structural strike.

[0067] During actual field construction, the electrodes can be arbitrarily deployed at appropriate positions. Except for avoiding water areas, there are no strict requirements for the electrode deployment. Each electrode is equipped with a receiving station. The receiving station records the position information of this point through the satellite module and collects the surface electric potential of this point in real time. Send the site information to the acquisition host through the wireless / wired transmission mode, such as Figure 7 shown.

[0068] Step 2: Deployment of the emission source and the receiving array. The emission source (dipole source) is deployed at the edge of the survey area, far from the interference source, and the grounding resistance needs to be lower than 20 Ω (the resistance can be reduced by parallel connection of multiple electrodes or brine treatment). The emission frequency range is usually 0.1 Hz - 10 kHz, and the current intensity is 50 - 100 A. The proportion of low-frequency signals is adjusted according to the exploration depth (lower-frequency signals penetrate deeper). The receiving magnetic sensors are arranged orthogonally (Hx, Hy, Hz), and the electric field is measured by single-point electric potential. The electrode burial depth is 20 - 30 cm to reduce grounding noise, and the magnetic probe needs to be far from metal interference. The emission and receiving times are synchronized through a satellite or a wireless module.

[0069] Step 3: Data acquisition. Point-by-point acquisition is carried out according to the preset frequency sequence (such as 10 frequency points / octave). In this embodiment, the stacking times of a single frequency point are ≥ 3 times to improve the signal-to-noise ratio.

[0070] Step 4: Data processing and interpretation. Bad point data are removed. Through single-electrode surface electric potential detection, the electrode density data are changed, and the electric potential outside the surface measurement points is interpolated and filtered. The terrain influence (such as the static effect caused by the surrounding of valleys / mountains / lakes / rivers) and the high-resistance layer shielding (shallow high-resistance volume effect) are corrected based on multi-scale electric field measurement. By placing sparse detection points between the emission and the reception, or adopting a multi-directional excitation mode, the constraint and suppression of the field source effect are completed. The electric field distribution information of the measurement surface is obtained through formula (4), and the apparent resistivity is calculated using formula (3), or other different detection information is extracted according to different parameter definitions.

[0071] Step 5: Invert the resistivity profile and optimize the model by combining geological constraints (such as borehole data). For complex structures, the finite element method or the integral equation method is used to construct a three-dimensional resistivity structure to identify low-resistance bodies (mineralized zones) or high-resistance bodies (bedrock).

[0072] Step 6: Comprehensive geological interpretation. Compare the resistivity anomalies with the known geological information, divide the lithological boundaries, fault zones or hydrothermal channels, and evaluate the scale and economic value of the target body.

[0073] Specifically, there is a resistivity distribution interface (line) between the resistivity anomaly and the background resistivity. This boundary line is verified with the major fault or the major tectonic interface (line) of the known geological profile, and small faults or small tectonic interface (lines) that do not exist in the geological profile are found, realizing the fine detection of the underground structure. Simply put, the major structures are used to verify the correctness of the resistivity anomaly detection, and then the finer lithological boundaries, fault zones or hydrothermal channels with smaller scales are identified.

[0074] Compared with the traditional MN two - electrode potential difference detection, since a very long electrode wire is required to complete this work, single - electrode potential detection does not require pulling a long wire. As long as the distance is appropriate, the influence degree can be intuitively seen through the equipotential surface. If the equipotential lines are too dense, it indicates that the near - surface influence is large, and the observation scale needs to be appropriately increased and adjusted to an appropriate equipotential line density. For example, Figure 6 shows a relatively large shallow - surface influence.

[0075] When there is a low - resistivity body on the surface, the surface electric field around the low - resistivity body is distorted relative to the background field, and the formed equipotential lines are relatively dense, as shown by the dotted line in Figure 6 . After leaving the low - resistivity body on the surface by a certain distance, the surface electric field returns to normal. If the traditional fixed - scale MN electrode potential difference is used to measure the electric field, due to the limited length of the electrode wire, it cannot cross the electric - field "pollution" area. Therefore, the picked - up electric - field information is seriously affected by the surface low - resistivity body. After adopting this technical solution, the surface potential is measured with a single electrode without pulling the electrode wire. Electrodes No. 10, 11, 16, 17, 22, and 23 are invalid electrodes. By crossing the electric - field "pollution" area and using the potential measured by the remaining electrodes, after interpolation filtering, the calculated potential values of electrodes No. 10, 11, 16, 17, 22, and 23 are obtained, thus overcoming the influence of the surface low - resistivity body.

[0076] The present invention adopts a multi - azimuth and multi - source excitation mode to solve the influence of the field - source effect, and completes multi - scale surface electric - field measurement based on single - electrode potential detection, solves the static effect and volume effect, and breaks through the limitations of the existing electromagnetic method instruments on detection accuracy and depth. The single - electrode potential receiving station automatically obtains the longitude and latitude of the measuring point through the satellite module without manual positioning.

[0077] Through multi - azimuth excitation and single - electrode potential measurement, it contains the information of single - field - source excitation mode and multi - field - source multi - azimuth excitation mode, can complete the functions of geometric sounding and induction sounding, can carry out electromagnetic exploration in the time domain and frequency domain, realize multi - scale electromagnetic - field detection on the surface, solve the drawbacks of the traditional electromagnetic method, and its construction mode is as shown in Figure 8 .

[0078] The above is only a preferred specific embodiment of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed in the present application should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A multi-scale electromagnetic exploration method based on single electrode potential detection, characterized in that, Including: Determine the measurement area, clarify the exploration target, and determine the survey area and measurement point density according to the depth of the exploration target; Deploy a number of emission sources and receiving arrays in the measurement area, collect the surface potential data of the measurement area in real time, and process the data frequency by frequency according to a preset frequency sequence; Based on the single-electrode potential data, generate a surface potential distribution surface, constrain and suppress the field source effect, obtain the electric field distribution information of the measurement surface, and calculate the apparent resistivity parameter in combination with the magnetic field measurement data; Adopt a multi-directional excitation mode and a multi-source emission scheme to suppress the field source effect, and construct a three-dimensional resistivity profile to identify low-resistivity bodies or high-resistivity bodies.

2. The multi-scale electromagnetic exploration method based on single electrode potential detection according to claim 1, characterized in that The survey area and the measurement point density need to cover the structural strike of the exploration target.

3. The multi-scale electromagnetic exploration method based on single electrode potential detection according to claim 1, characterized in that, Deploy a number of emission sources and receiving arrays in the measurement area, including: The emission sources are deployed at the edge of the measurement area, and the low-frequency signal ratio is adjusted according to the exploration depth; The receiving magnetic sensors are arranged in an orthogonal manner, the electric field uses single-point potential measurement, and the satellite or wireless module is used to ensure the synchronization of the emission time and the receiving time.

4. The multi-scale electromagnetic exploration method based on single electrode potential detection according to claim 1, characterized in that, Collect the surface potential data of the measurement area in real time, including: Interpolate and filter the potential of the measurement points of the receiving array to obtain a potential measurement surface, obtain the curvature distribution characteristics of the measured potential surface according to the concavity and convexity of the potential measurement surface, analyze the influence degree of the surface or shallow part based on the potential surface curvature distribution, and adjust the density of the detection electrodes according to the requirements of the detection depth to reduce the potential surface curvature distribution form and complete the detection from shallow to deep.

5. The multi-scale electromagnetic exploration method based on single electrode potential detection according to claim 1, characterized in that Use an interpolation algorithm to generate the surface potential distribution surface, and eliminate the static effect and volume effect caused by shallow low-resistivity bodies or high-resistivity bodies through filtering; Among them, the interpolation algorithm is Kriging interpolation or polynomial fitting interpolation, and the filtering method is spatial domain low-pass filtering or adaptive threshold filtering.

6. The multi-scale electromagnetic exploration method based on single electrode potential detection according to claim 1, characterized in that Constrain and suppress the field source effect, including: Complete the constraint and suppression of the field source effect by placing sparse detection points between the emission source and the receiving array, or adopting a multi-directional excitation mode.

7. The multi-scale electromagnetic exploration method based on single electrode potential detection according to claim 6, characterized in that, Obtain the electric field distribution information of the measurement surface as: E = σ·J; In the formula, E is the electric field strength, σ is the conductivity, and J is the current density.

8. The multi-scale electromagnetic exploration method based on single electrode potential detection according to claim 1, wherein Calculate the apparent resistivity parameter as: where ω represents the angular frequency, μ represents the magnetic permeability, E x represents the electric field strength in the direction of the emission source, and H y represents the magnetic field strength perpendicular to the direction of the emission source, and ρ is the apparent resistivity parameter.

9. The multi-scale electromagnetic exploration method based on single electrode potential detection according to claim 1, wherein The method further includes: Compare the resistivity anomaly with the known geological information through the resistivity distribution interface, divide the lithological boundary, fault zone or hydrothermal channel, and evaluate the scale and economic value of the target body.

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