Transient electromagnetic data polarizability extraction method based on frequency domain resistivity

Abstract

The invention discloses a transient electromagnetic data polarizability extraction method based on frequency domain resistivity, and the method comprises the following steps: designing measuring line and wire emission source positions in a test mining area, selecting observation parameters, and collecting field electromagnetic data, the field electromagnetic data comprising transient electromagnetic method electric field data and frequency domain electromagnetic method electric field data; performing inversion on actually measured frequency domain electromagnetic method electric field data by adopting an Occam inversion algorithm to obtain resistivity characteristics of different measuring points, and further obtaining resistivity distribution information; and taking the resistivity distribution information obtained by inversion as a constraint condition of transient electromagnetic method electric field data inversion, performing transient electromagnetic method polarizability inversion by analyzing a relationship between the resistivity and a time constant, and extracting the polarizability. By analyzing and processing the frequency domain electromagnetic data and the time domain electromagnetic data, the resistivity and the polarizability can be accurately extracted, parameter separation is achieved, and the problem of traditional time domain inversion coupling is solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of geophysical exploration, and particularly relates to a method for extracting the polarization rate of transient electromagnetic data based on frequency-domain resistivity. Background Art

[0002] The transient electromagnetic method (TEM) utilizes the significant electrical property differences between target bodies and surrounding rocks in mineral exploration and has been widely applied in metal mineral exploration. Traditional transient electromagnetic data interpretation methods mainly focus on resistivity inversion and often ignore the influence of polarization effects on electromagnetic responses. Polarized bodies are widespread in metal ore deposits, such as massive sulfide ore bodies, carbonaceous rocks, porphyry copper deposits, pyrite, magnetite, and graphite. Due to the induced polarization effect of underground polarized bodies, the measured transient electromagnetic data contains not only electromagnetic responses related to resistivity but also induced polarization responses related to information such as polarization rate. The polarization characteristics (polarization rate) of underground media have important indicative significance for mineral composition, pore structure, and types of saturated liquids. The induced polarization effect caused by polarized bodies will lead to the distortion of transient electromagnetic responses, thus significantly increasing the difficulty of data processing and interpretation.

[0003] With the proposal of the Cole-Cole model, researchers began to attempt to invert parameters such as resistivity and polarization rate simultaneously from electromagnetic responses. However, the computational cost of this method is large and it is only applicable to one-dimensional inversion. Currently, the more commonly used inversion algorithm is to use early data with less influence from the induced polarization effect and obtain the underground resistivity structure through three-dimensional inversion. Subsequently, three-dimensional forward modeling is carried out using the inverted resistivity information to obtain a pure electromagnetic response. By subtracting the pure electromagnetic response from the observed data, a pure induced polarization response can be obtained, and further inversion is performed on it to obtain information such as polarization rate. However, this method requires multiple three-dimensional forward and inverse inversions, with a huge computational cost, and error accumulation will occur during this process, resulting in inaccurate resistivity results. In addition, the early data measured in the field may also contain induced polarization responses, so this method is not applicable to the inversion of all underground structures.

[0004] Aiming at the above problems, there is an urgent need to propose a method for extracting the polarization rate of transient electromagnetic data based on frequency-domain resistivity. Summary of the Invention

[0005] To solve the above technical problems, the present invention proposes a method for extracting the polarization rate of transient electromagnetic data based on frequency-domain resistivity to solve the problems existing in the above prior art.

[0006] To achieve the above object, the present invention provides a method for extracting the polarization rate of transient electromagnetic data based on frequency-domain resistivity, including the following steps:

[0007] Design survey lines and conductor emission source positions in the test mining area, select observation parameters to collect field electromagnetic data, where the field electromagnetic data includes transient electromagnetic method electric field data and frequency domain electromagnetic method electric field data;

[0008] Use the Occam inversion algorithm to invert the measured frequency domain electromagnetic method electric field data, obtain the resistivity characteristics of different measurement points, and then obtain the resistivity distribution information;

[0009] Take the resistivity distribution information obtained by inversion as the constraint condition for the inversion of the transient electromagnetic method electric field data. By analyzing the relationship between resistivity and time constant, perform transient electromagnetic method polarizability inversion and extract the polarizability.

[0010] Optionally, the selected observation parameters at least include emission frequency, emission current, observation frequency, and measurement point spacing.

[0011] Optionally, the process of using the Occam inversion algorithm to invert the measured frequency domain electromagnetic method electric field data includes:

[0012] Based on the Occam inversion algorithm, transform the non - linear electromagnetic inversion problem into an iterative linear solution process, and obtain the model correction amount through iterative calculation;

[0013] In each iteration, establish a dual - optimization objective function including a data fitting term and a model smoothing term, introduce an adjustable balance parameter, and dynamically adjust the weight relationship between the data fitting term and the model smoothing term;

[0014] Automatically screen the optimal balance parameter during the iterative optimization process to make the inversion result simultaneously satisfy the best match between the simulated data and the measured data and the simplest smoothness of the inversion model;

[0015] When the preset number of iterations is reached, output the final inversion result.

[0016] Optionally, the calculation formula of the Occam inversion algorithm is as follows:

[0017] F(m k +△m)≈F(m k )+J(m k )△m,

[0018] where m k is the model parameter vector of the k - th iteration, Δm is the model correction amount, and J(m k ) is the Jacobian matrix.

[0019] Optionally, the calculation formula for extracting the polarizability is as follows:

[0020]

[0021] Among them, Δρ is the resistivity change, and ρ0 is the reference resistivity.

[0022] Optionally, after extracting the polarizability, it further includes:

[0023] Collect the existing geological model in the area, compare the extracted resistivity and polarizability information with the existing geological model to ensure that the existing geological model corresponds to the extracted resistivity and polarizability data in space.

[0024] The present invention also provides a transient electromagnetic data polarizability extraction system based on frequency-domain resistivity for implementing the above method, including: a data acquisition module, a data inversion module, and a polarizability extraction module;

[0025] The data acquisition module is used to design survey lines and wire transmitter positions in the test mining area, select observation parameters to collect field electromagnetic data, and the field electromagnetic data includes transient electromagnetic method electric field data and frequency-domain electromagnetic method electric field data;

[0026] The data inversion module is used to invert the measured frequency-domain electromagnetic method electric field data by using the Occam inversion algorithm to obtain the resistivity characteristics of different measuring points, and further obtain the resistivity distribution information;

[0027] The polarizability extraction module is used to use the resistivity distribution information obtained by inversion as the constraint condition for the inversion of the transient electromagnetic method electric field data, and by analyzing the relationship between resistivity and time constant, perform the inversion of the transient electromagnetic method polarizability to extract the polarizability.

[0028] The present invention also provides a computer device, including a memory, a processor, and a computer program stored on the memory, and the processor executes the computer program to implement the steps of the above method.

[0029] The present invention also provides a computer-readable storage medium, on which a computer program is stored, and when the computer program is executed by a processor, the steps of the above method are implemented.

[0030] The present invention also provides a computer program product, including a computer program, and when the computer program is executed by a processor, the steps of the above method are implemented.

[0031] Compared with the prior art, the present invention has the following advantages and technical effects:

[0032] (1) By analyzing and processing the frequency-domain electromagnetic and time-domain electromagnetic data, the present invention can accurately extract the resistivity and polarizability, realize parameter separation, and break through the traditional time-domain inversion coupling problem;

[0033] (2) Adopt frequency-domain analysis and optimized inversion algorithms. The step-by-step inversion strategy reduces the number of parameters to be inverted simultaneously, lowers the computational complexity, and is suitable for the rapid analysis of large-scale geological data;

[0034] (3) The present invention can be applied to various types of underground structures such as metal minerals and oil and gas, overcomes the limitations of traditional methods under specific conditions, and expands the application scope. Brief Description of the Drawings

[0035] The drawings forming 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:

[0036] Figure 1 is the method flow chart of the embodiment of the present invention;

[0037] Figure 2 is the schematic diagram of the measured electric field data of the frequency-domain electromagnetic method in the embodiment of the present invention;

[0038] Figure 3 is the schematic diagram of the measured electric field data of the transient electromagnetic method in the embodiment of the present invention;

[0039] Figure 4 is the resistivity profile of the frequency electromagnetic method in the embodiment of the present invention;

[0040] Figure 5 is the polarizability profile of the transient electromagnetic method in the embodiment of the present invention. Detailed Embodiments

[0041] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The following will refer to the drawings and combine the embodiments to detail this application.

[0042] It should be noted that the steps shown in the flowchart of the drawings can 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 can be executed in a different order than here.

[0043] Embodiment 1

[0044] Regarding the extraction of the polarizability of transient electromagnetic data, the prior art has the following problems:

[0045] (1) The influence of the induced polarization effect on the transient electromagnetic response: The induced polarization effect caused by the polarized body will lead to the distortion of the transient electromagnetic response, seriously exacerbating the difficulty of transient electromagnetic inversion and interpretation;

[0046] (2) The traditional inversion method has low efficiency and poor accuracy: The traditional inversion method involves a large amount of calculation and cannot quickly process large-scale geological data. Moreover, a large number of errors will accumulate during the inversion process, making it difficult to ensure the accuracy of the inversion results.

[0047] (3) The existing methods have insufficient applicability: The existing methods have poor applicability to different underground structures, resulting in the inability to obtain accurate polarizability in special geological situations.

[0048] In view of the current situation that it is difficult to process transient electromagnetic data affected by the induced polarization effect with traditional methods, this embodiment proposes a method for extracting the polarizability of transient electromagnetic data based on frequency-domain resistivity. The inversion result of frequency-domain resistivity is not easily interfered by the induced polarization effect. Using it as a constraint to extract the polarizability information from transient electromagnetic data can improve the efficiency and accuracy of inversion. The specific steps are as follows:

[0049] Design survey lines and the positions of wire transmitters in the test mining area, select observation parameters to collect field electromagnetic data, and the field electromagnetic data includes transient electromagnetic method electric field data and frequency-domain electromagnetic method electric field data;

[0050] Use the Occam inversion algorithm to invert the measured frequency-domain electromagnetic method electric field data to obtain the resistivity characteristics of different measuring points, and then obtain the resistivity distribution information;

[0051] Take the inverted resistivity distribution information as the constraint condition for the inversion of the transient electromagnetic method electric field data, and through analyzing the relationship between resistivity and time constant, conduct the inversion of the transient electromagnetic method polarizability to extract the polarizability.

[0052] As a specific implementation manner, it specifically includes the following steps:

[0053] Step 1: In the test mining area, conduct a preliminary survey according to the geological conditions of the mining area and the target area, design the survey lines and the positions of wire transmitters, and select appropriate observation parameters (such as transmission frequency, transmission current, observation frequency, measuring point spacing) to collect field electromagnetic data. Arrange transmitters at the designed positions, arrange receiving electrodes at the selected measuring points, use the arranged transmitter to transmit a step current, and use the receiver to receive the transient electromagnetic method electric field data (TEM) and the frequency-domain electromagnetic method electric field data;

[0054] Step 2: After the data collection is completed, use the Occam inversion algorithm to invert the measured frequency-domain electromagnetic data to obtain the resistivity characteristics of different measuring points and extract the resistivity distribution information. The specific process includes:

[0055] (1) Linearization processing:

[0056] Occam inversion requires the best fit between the simulated data and the measured data, and also requires the model data to be the smoothest. According to Taylor's theorem and the idea of local linearization, the Occam algorithm transforms the non-linear problem into a linear problem:

[0057] F(m k +△m)≈F(m k )+J(m k )△m,

[0058] where m k is the model parameter vector at the k-th iteration, Δm is the model correction, and J(m k ) is the Jacobian matrix.

[0059] The expression for each component is:

[0060]

[0061] (2) Multi-objective optimization:

[0062] The Occam inversion algorithm pursues two objectives simultaneously:

[0063] Optimal data fitting: Minimize the residual between the simulated data and the measured data;

[0064] Model smoothest: Avoid overfitting by minimizing the model roughness

[0065] (3) Regularized solution:

[0066] Construct an objective function that includes the regularization parameter μ:

[0067]

[0068] where W d is the data weight matrix, and μ controls the balance between the fitting degree and the model smoothness.

[0069] (4) Parameter optimization:

[0070] Calculate the candidate model m k+1 (μ) through a series of μ values, and select the one that simultaneously satisfies: the smallest data fitting error, the lowest model roughness, and the optimal regularization parameter μ.

[0071] (5) Iterative convergence:

[0072] Repeat the above process until the fitting error meets the standard or reaches the upper limit of the number of iterations, and finally obtain an inversion result that takes into account both data matching and geological rationality.

[0073] ​Step 3: Use the inverted resistivity results as constraints for the inversion of transient electromagnetic electric field data (TEM). Use an appropriate algorithm (such as the Occam algorithm) to analyze the relationship between resistivity and time constant, perform transient electromagnetic polarizability inversion, and extract polarizability (α):

[0074]

[0075] Where Δρ is the resistivity change and ρ0 is the reference resistivity.

[0076] Step 4: Collect existing geological models in the region, including information such as strata, ore body distribution, and lithological characteristics. Verify the extracted resistivity and polarizability information, compare it with the existing geological model or other measurement results, evaluate the accuracy and reliability of the model, ensure that the existing model corresponds to the extracted resistivity and polarizability data in space, and interpret it in combination with the geological background. Analyze the geological significance of the extraction results based on the regional geological structure, ore body characteristics, stratum type and other information.

[0077] Embodiment 2

[0078] according to Figure 1 As shown, this embodiment provides a method for extracting polarizability of transient electromagnetic data based on frequency domain resistivity. Taking the Yemaquan skarn-type iron polymetallic mine in Qinghai as an example, the mining area is located in the East Kunlun orogenic belt, and the ore body is produced in the skarn of the outer contact zone between the intermediate-acidic intrusive rock mass and the Qimantag Group and the Di'aosu Formation carbonate rock. Skarn is mainly ferromagnesian skarn, followed by calcium-magnesian skarn and manganese skarn, which are strictly controlled by skarn and fault structures. Quaternary aeolian and flood deposits are widely distributed in the mining area. According to drilling data and outcrops, the strata from old to new include the Ordovician Qimantag Group, the Upper Devonian Yakniu Mountain Formation, the Upper Carboniferous Di'aosu Formation, the Lower-Middle Permian Dachaigou Formation and the Quaternary System. The rock types are mainly granodiorite, biotite-containing quartz monzodiorite, and monzogranite, followed by a small amount of monzodiorite.

[0079] Based on the known data and the latest physical property measurement results, a statistical table of physical properties of the mining area is obtained, as shown in Table 1.

[0080] Table 1

[0081] Lithology Resistivity / Ωm Polarizability / % Limestone 719 0.9 Carbonaceous limestone 48 39.2 Marble 1057 0.6 Granodiorite 1420 1.1 Mineralized skarn 367.6 6.7 Galena magnetite ore 179 18.3 Sphalerite magnetite ore 13.7 17.6 Chalcopyrite magnetite ore 12.7 9.3 Magnetite ore 10.3 14.9

[0082] The resistivity of monzonitic granite in the study area is the highest, showing obvious high-resistance characteristics. The resistivity of mineralized skarn is medium, and the resistivity of carbonaceous limestone and carbonaceous limestone is the lowest. The polarizability of mineralized rock mass and carbonaceous surrounding rock is relatively high, while the polarizability of non-carbonaceous surrounding rock and rock mass is very low. There are obvious differences in resistivity and polarizability among different lithologies in the study area, especially between the ore body and the surrounding rock, where the differences in resistivity and polarizability are relatively large. In the mining area, transient electromagnetic method detection and frequency-domain electromagnetic method detection are carried out, and typical survey lines are selected to extract polarizability information.

[0083] Figure 2 and Figure 3 are the measured electric field data of frequency-domain electromagnetic method and transient electromagnetic method respectively. It can be seen that the frequency-domain data is not easily affected by the induced polarization effect, while the transient electromagnetic method is greatly affected by the induced polarization effect, and the late-stage data is seriously distorted. First, the least squares method is used to invert the frequency-domain data to obtain the resistivity model. Figure 4 is the resistivity profile after inverting the frequency-domain data using the least squares method. According to the distribution characteristics of electrical properties, the profile is divided into two main regions. The resistivity in the regions of 960 - 2400m and 2400 - 3660m is significantly different, and there is a sudden interface between the red high-resistance and blue low-resistance.

[0084] Then Figure 4 the resistivity inversion result is used as a constraint condition, and the Bayesian algorithm is used to extract the polarizability of the transient electromagnetic data. Figure 5 is the polarizability profile of the transient electromagnetic method. The high-resistance and low-polarizability regions of the measuring points in the range of 1200 - 2600m correspond to the granodiorite rock mass, the shallow-layer region with low resistivity and high polarization corresponds to the carbonaceous strata, and the high-low resistivity transition region corresponds well to the known skarn zone. The entire section clearly reflects the deep spatial characteristics of each geological body, the rock mass interface is relatively consistent with the actual interface of the exploration line, and the iron polymetallic ore body is located in the high-low resistivity conversion region, indicating that this method can effectively extract the polarizability information of the transient electromagnetic method and divide the main geological bodies in the deep and the contact boundaries between the rock mass and the strata.

[0085] Example 3

[0086] This embodiment also provides a system for extracting the polarizability of transient electromagnetic data based on frequency-domain resistivity for implementing the above method, including: a data acquisition module, a data inversion module, and a polarizability extraction module;

[0087] The data acquisition module is used to design survey lines and the positions of wire emission sources in the test mining area, and select observation parameters to collect field electromagnetic data, where the field electromagnetic data includes transient electromagnetic method electric field data and frequency-domain electromagnetic method electric field data;

[0088] The data inversion module is used to invert the measured frequency-domain electromagnetic method electric field data by using the Occam inversion algorithm to obtain the resistivity characteristics of different measuring points, and further obtain the resistivity distribution information;

[0089] The polarizability extraction module is used to use the resistivity distribution information obtained by inversion as the constraint condition for the inversion of the transient electromagnetic method electric field data. By analyzing the relationship between resistivity and time constant, the polarizability of the transient electromagnetic method is inverted to extract the polarizability.

[0090] Embodiment 4

[0091] This embodiment also provides a computer device, including a memory, a processor, and a computer program stored on the memory. The processor executes the computer program to implement the steps of the method.

[0092] Embodiment 5

[0093] This embodiment also provides a computer-readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, the steps of the method are implemented.

[0094] Embodiment 6

[0095] This embodiment also provides a computer program product, including a computer program. When the computer program is executed by a processor, the steps of the method are implemented.

[0096] 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 by 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 method for extracting the polarization rate of transient electromagnetic data based on frequency-domain resistivity, characterized in that, It includes the following steps: Design survey lines and conductor transmitter source positions in the test mining area, select observation parameters to collect field electromagnetic data, and the field electromagnetic data includes transient electromagnetic method electric field data and frequency domain electromagnetic method electric field data; Use the Occam inversion algorithm to invert the measured frequency domain electromagnetic method electric field data, obtain the resistivity characteristics of different measurement points, and then obtain the resistivity distribution information; Take the resistivity distribution information obtained by inversion as the constraint condition for the inversion of the transient electromagnetic method electric field data, analyze the relationship between resistivity and time constant, perform transient electromagnetic method polarizability inversion, and extract the polarizability.

2. The method according to claim 1, wherein The selected observation parameters at least include emission frequency, emission current, observation frequency, and measurement point spacing.

3. The method according to claim 1, wherein The process of using the Occam inversion algorithm to invert the measured frequency domain electromagnetic method electric field data includes: Based on the Occam inversion algorithm, transform the non-linear electromagnetic inversion problem into an iterative linear solution process, and obtain the model correction amount through iterative calculation; In each iteration, establish a dual optimization objective function including a data fitting term and a model smoothing term, introduce an adjustable balance parameter, and dynamically adjust the weight relationship between the data fitting term and the model smoothing term; Automatically screen the optimal balance parameter during the iterative optimization process to make the inversion result simultaneously satisfy the best matching of the simulated data and the measured data and the simplest smoothness of the inversion model; When the preset number of iterations is reached, output the final inversion result.

4. The method according to claim 3, wherein The calculation formula of the Occam inversion algorithm is as follows: F(m k + Δm) ≈ F(m k ) + J(m k )Δm, where m k is the model parameter vector of the k-th iteration, Δm is the model correction, and J(m k ) is the Jacobian matrix.

5. The method according to claim 1, wherein The calculation formula for extracting the polarizability is as follows: where Δρ is the resistivity change and ρ0 is the reference resistivity.

6. The method according to claim 1, wherein After extracting the polarizability, it further includes: Collect the existing geological models in the area, compare the extracted resistivity and polarizability information with the existing geological models to ensure the spatial correspondence between the existing geological models and the extracted resistivity and polarizability data.

7. A transient electromagnetic data polarizability extraction system based on frequency domain resistivity, characterized in that, Used to implement the method according to any one of claims 1-6, including: a data acquisition module, a data inversion module, and a polarizability extraction module; The data acquisition module is used to design survey lines and conductor transmitter source positions in the test mining area, select observation parameters to collect field electromagnetic data, and the field electromagnetic data includes transient electromagnetic method electric field data and frequency domain electromagnetic method electric field data; The data inversion module is used to use the Occam inversion algorithm to invert the measured frequency domain electromagnetic method electric field data, obtain the resistivity characteristics of different measurement points, and then obtain the resistivity distribution information; The polarizability extraction module is used to take the resistivity distribution information obtained by inversion as the constraint condition for the inversion of the transient electromagnetic method electric field data, analyze the relationship between resistivity and time constant, perform transient electromagnetic method polarizability inversion, and extract the polarizability.

8. A computer device, comprising a memory, a processor, and a computer program stored on the memory, characterized in that, The processor executes the computer program to implement the steps of the method according to any one of claims 1-6.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by a processor, it implements the steps of the method according to any one of claims 1-6.

10. A computer program product, comprising a computer program, characterized in that, When the computer program is executed by a processor, it implements the steps of the method according to any one of claims 1-6.

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