Method for classifying field zones for electric dipole source csamt
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2025-07-31
- Publication Date
- 2026-08-13
Smart Images
Figure CN2025111662_13082026_PF_FP_ABST
Abstract
Description
A method for dividing the field region of an electric dipole source CSAMT Technical Field
[0001] This invention belongs to the field of electromagnetic exploration, specifically relating to a method for delineating the field area of an electric dipole source (CSAMT). Background Technology
[0002] In CSAMT (controlled source audio-frequency magneto-tellurics) exploration, the delineation of the field zone is an important issue. Zonge and Hughes (Zonge and Hughes, 1991: pp. 723-725, 740), based on simulation results of a uniform electric dipole source field with a resistivity of 100 Ω·m, defined the region where the difference between the Cagnird resistivity and the true resistivity does not exceed 5% as the far zone: for observed E... x and H y The equatorial device, when B = (r min / δ FD )>4 is the far-field region; for observation E x and H y The axial device, when B = (r min / δ FD )>5 is the far zone; for observation E y and H x The device, when B = (r min / δ FD A value greater than 3 indicates a far-field region. This method of dividing the far-field region by a defined inductance number B has been widely used for a long time, such as in the minimum offset distance r specified in technical specifications. min The selection criteria (such as those of the Ministry of Land and Resources of the People's Republic of China, 2015: p3, 27) and the recently published patent CN 117031561A (Cao Qinghua and Yan Shu, 2023) all use the above-mentioned B value as the standard for entering the far zone.
[0003] Since the inductance values used to delineate the field area are obtained under specific geoelectric conditions, experience is often still required to delineate the field area. To extend the method of determining the inductance values for field area delineation from uniform ground with a specific resistivity to uniform ground with arbitrary resistivity, layered ground with arbitrary layer parameters, and two-dimensional and three-dimensional geoelectric structures with arbitrary geoelectric parameters, this invention proposes a new method for delineating the field area of an electric dipole source (CSAMT). Summary of the Invention
[0004] The method for dividing the field region of an electric dipole source (CSAMT) proposed in this invention lies in determining the inductance number B = cri, which serves as the standard for entering the far region, based on the defined far-region separation point between the full-region apparent resistivity curve and the far-region apparent resistivity curve. far-tranBased on the defined near-field separation point between the full-area apparent resistivity curve and the near-field apparent resistivity curve, the inductance number B = cri, which serves as the near-field standard, was determined. tran-near The site delineation method proposed in this invention is no longer limited to specific geoelectric conditions and can adapt to different geoelectric structures. This method not only expands the applicability of site delineation methods used in exploration engineering and provides a more universal standard for selecting the minimum offset in construction design, but also provides new means and tools for exploration theory research and data interpretation.
[0005] The technical solution adopted in this invention is as follows:
[0006] A method for dividing the field region of an electric dipole source (CSAMT) involves dividing the electric field E from the high-frequency end to the low-frequency end. x The last point where the apparent resistivity curves of the whole area and the far area of the component coincide is defined as the far-field separation point, and the inductance value corresponding to the far-field separation point is denoted as B = cri. far-tran From the low-frequency end to the high-frequency end, the electric field E is... x The last point where the apparent resistivity curve of the whole region and the apparent resistivity curve of the near region coincide is defined as the near-region separation point, and the inductance number corresponding to the near-region separation point is denoted as B = cri. tran-near ;
[0007] Based on the far-field separation point and near-field separation point defined above, and the corresponding inductance values at the separation points, the field area is divided into:
[0008] B≥cri far-tran The area is the far zone, cri tran-near <B<cri far-tran The region is a transition zone, where B ≤ cri tran-near The area is the near zone.
[0009] Furthermore, the electric field E at each frequency point x The full-area apparent resistivity of the component Obtained by the following simple iterative formula
[0010] electric field E x Far-field apparent resistivity of components Obtained from the following direct calculation formula
[0011] electric field E x Near-zone apparent resistivity of components Obtained from the following direct calculation formula
[0012] The number of inductors B is defined by the following formula.
[0013] The detection depth δ FD It can be calculated using the skin depth formula.
[0014] Where I is the emission current intensity in A; l is the electric dipole length in m; r is the offset distance from the source point to the observation point in m; f is the operating frequency in Hz; j is the imaginary unit; and μ0 is the vacuum permeability of the nonmagnetic earth. The angle between the observation point and the source point.
[0015] Furthermore, electric field E x The components are obtained through theoretical formulas or actual measurements. The geoelectric model is theoretically calculated during the exploration and experimental stage and constructed based on electrical logging data or other geological data of the survey area.
[0016] Furthermore, in actual exploration, the minimum offset r required to achieve far-field conditions is... min and detection depth δ FD The relationship, or the maximum depth that can be detected given an offset r. Both are determined by the inductance number B = cri at the far-field separation point. far-tran Estimate.
[0017] Furthermore, the detection depth may be calculated using an algorithm, such as an equivalent detection depth algorithm.
[0018] Furthermore, regarding the apparent resistivity of the entire region Curve and apparent resistivity in the far region The points where the curves coincide must satisfy the following equation:
[0019] apparent resistivity of the whole area Curve and near-zone apparent resistivity The points where the curves coincide must satisfy the following equation:
[0020] In the formula, α and β are the tolerance values for the far-field separation point and the near-field separation point, respectively.
[0021] Furthermore, the apparent resistivity of the entire region at each frequency point The iterative calculation steps are as follows:
[0022] Step S1: Iterate starting from the high-frequency end, setting the initial apparent resistivity of the entire region to be [value missing].
[0023] Step S2: At each frequency point, set the initial value of the number of iterations n = 0 and the maximum value of the number of iterations N;
[0024] Step S3, set the initial value of the apparent resistivity of the entire region. By substituting the apparent resistivity of the entire region into a simple iterative formula, a new apparent resistivity value can be calculated. if Then the apparent resistivity at that frequency point And terminate the iterative calculation at that frequency point, Repeat steps S2 to S3 as the initial value for the next frequency point to calculate the apparent resistivity of the entire area at the next frequency point; δ is the accuracy value.
[0025] Step S4, if Then continue the iterative calculation of the apparent resistivity of the entire region at that frequency point, with the number of iterations n = n + 1. If n ≤ N, then... Input to step S3, and execute the iterative calculation of steps S3 to S4 again;
[0026] Step S5, if n>N then The iterative calculation of the apparent resistivity of the entire region at this frequency point is terminated.
[0027] Furthermore, the initial value of the apparent resistivity of the entire region mentioned in step S1 In theoretical calculations, the resistivity value of the first layer of the geoelectric model is used; in exploration engineering, the Cagniard resistivity or E calculated from the apparent resistivity formula in the far zone is used. x The first branch curve value of the apparent resistivity in the far region.
[0028] Furthermore, the maximum value of the iteration is N = 20.
[0029] Furthermore, the accuracy value is set to δ = 10. -5 . Beneficial effects
[0030] 1) Extend the existing applicable conditions for dividing the field area by determining the inductance number from the uniform earth with a resistivity of 100 Ω·m to the uniform and layered earth with arbitrary layer parameters, as well as the two-dimensional and three-dimensional geoelectric structures.
[0031] 2) The far-field separation point and near-field separation point defined in this invention not only make the method of dividing the field area more universal and applicable, but also supplement the long-lost standard for dividing the near-field area by determining the number of inductors.
[0032] 3) This invention not only provides a universally applicable site division method for construction design, but also provides new means and standards for research work such as CSAMT exploration theory and data interpretation methods. Attached Figure Description
[0033] Figure 1 shows the angle between the observation point and the source point. A schematic diagram.
[0034] Figure 2 shows the uniform electric field E on the ground. x Apparent resistivity curves of the components.
[0035] Figure 3 shows the electric field E on the layered ground. x Apparent resistivity curves of the components.
[0036] In the diagram: 1. Source point; 2. Observation point; 3. Electric field E x The apparent resistivity curve of the entire region of the component; 4 electric field E x The far-field apparent resistivity curve of the component; 5 electric field E x Near-field apparent resistivity curves of the components; 6. Far-field separation point; 7. Near-field separation point. Detailed Implementation
[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.
[0038] The present invention proposes a method for dividing the CSAMT field region of an electric dipole source, which divides the electric field E from the high-frequency end to the low-frequency end. x The last point of overlap between the full-area apparent resistivity curve 3 and the far-area apparent resistivity curve 4 is defined as the far-area separation point 6, and the inductance value corresponding to the far-area separation point 6 is denoted as B = cri far-tran From the low-frequency end to the high-frequency end, the electric field E is... x The last point of overlap between the full-area apparent resistivity curve 3 and the near-area apparent resistivity curve 5 is defined as the near-area separation point 7, and the inductance value corresponding to the near-area separation point 7 is denoted as B = cri. tran-near .
[0039] Based on the aforementioned definition of far-field separation point 6 and near-field separation point 7, and the corresponding inductance values for each separation point, the field area is divided as follows:
[0040] B≥cri far-tran The area is the far zone, cri tran-near <B<cri far-tran The region is a transition zone, where B ≤ cri tran-near The area is the near zone.
[0041] In this embodiment, the electric field E x The components can be calculated using theoretical formulas or obtained through field measurements. The geoelectric model calculated theoretically during the exploration experiment phase is constructed based on electrical logging data or other geological data of the survey area.
[0042] In this embodiment, the electric field E at each frequency point x The full-area apparent resistivity of the component (Unit: Ω·m) can be obtained by the following simple iterative formula.
[0043] In this embodiment, the electric field Ex Far-field apparent resistivity of components (Unit: Ω·m) is obtained by the following direct calculation formula.
[0044] In this embodiment, the electric field E x Near-zone apparent resistivity of components (Unit: Ω·m) is obtained by the following direct calculation formula.
[0045] The number of inductors B is defined by the following formula.
[0046] The detection depth δ FD It can be calculated using the skin depth formula.
[0047] In the above formulas, I is the emitted current intensity (unit: A), l is the electric dipole length (unit: m), r is the offset distance from the source point to the observation point (unit: m), f is the operating frequency (unit: Hz), j is the imaginary unit, and μ0 = 4π × 10⁻⁶. -7 H / m is the vacuum permeability of nonmagnetic earth. The angle between the observation point and the source point is shown in Figure 1.
[0048] In this embodiment, for the apparent resistivity of the entire area Curve and apparent resistivity in the far region The curve coincidence point (i.e., far-field separation point 6) must satisfy the following formula:
[0049] In this embodiment, for the apparent resistivity of the entire area Curve and near-zone apparent resistivity The curve coincidence point (near zone separation point 7) must satisfy the following formula:
[0050] In the formula, α and β are the tolerance values for the far-field separation point and the near-field separation point, respectively, and the tolerance values can be determined according to the actual engineering situation.
[0051] In this embodiment, due to the apparent resistivity of the entire region at each frequency point It requires iterative calculation, therefore the apparent resistivity of the entire region... The calculation can be performed using the following steps:
[0052] Step S1: Iterate starting from the high-frequency end, setting the initial apparent resistivity of the entire region to be [value missing].
[0053] Step S2: At each frequency point, set the initial value of the number of iterations n = 0 and the maximum value of the number of iterations N;
[0054] Step S3, set the initial value of the apparent resistivity of the entire region. Substituting into the simple iterative equation (1), a new apparent resistivity value is calculated. If the absolute value of the difference in apparent resistivity across the entire region before and after the iteration is less than or equal to the precision value Then the apparent resistivity at that frequency point And terminate the iterative calculation at that frequency point, Repeat steps S2 to S3 as the initial value for the next frequency point to calculate the full-area apparent resistivity of the next frequency point.
[0055] Step S4: If the absolute value of the difference in apparent resistivity across the entire region before and after the iteration is greater than the accuracy value... Then continue the iterative calculation of the apparent resistivity of the entire region at that frequency point, with the number of iterations n = n + 1. If n ≤ N, then... Input to step S3, and execute the iterative calculation of steps S3 to S4 again;
[0056] Step S5, if n > N, then The iterative calculation of the apparent resistivity of the entire region at this frequency point is terminated.
[0057] In this embodiment, the precision value δ in the above steps is 10. -5 .
[0058] The following two specific examples will further illustrate this point:
[0059] Example 1: Calculating the electric field E on a uniform ground using theoretical formulas. x The components (Table 1) are iterated starting from the high-frequency end f = 8192Hz in the second column of Table 1, according to step S1. The initial value of the apparent resistivity of the entire region is taken as the resistivity of the first layer of the geoelectric model. Maximum number of iterations N = 20, precision δ = 10 -5 Then, execute steps S2 to S5 to obtain the apparent resistivity of the entire region. Together with the far-field apparent resistivity calculated using formula (2) And the near-zone apparent resistivity calculated using formula (3) The values are listed in columns 4, 5, and 6 of Table 1, and plotted in Figure 2; columns 7 and 8 of Table 1 are calculated using formulas (5) and (4) to determine the skin depth δ corresponding to each frequency point. FD And the number of inductances B. According to Table 1 and Figure 2, from high frequency to low frequency, and The last coincidence point of the curves is approximately located at the frequency f = 2048Hz. According to formula (6a), the tolerance value α at this point is approximately 0.386%, which is considered to meet the engineering requirements. Therefore, the corresponding inductance value B = cri far-tran=5.69 as the far-field separation point. From low frequency to high frequency, and The last coincidence point of the curves is approximately located at the frequency f = 2Hz. According to formula (6b), the tolerance value β ≈ 0.361% at this point, which is considered to meet the engineering requirements. Therefore, the corresponding inductance value B = cri tran-near =0.17 is used as the near-field separation point.
[0060] In the given case of Example 1: the region where B ≥ 5.69 is the far region, the region where 0.17 < B < 5.69 is the transition region, and the region where B ≤ 0.17 is the near region.
[0061] Table 1: Skin depth δ corresponding to the far-field separation point (f = 2048 Hz) FD = 351.48m. From formula (4) It can be seen that the minimum offset r required to detect a depth of 351.48m is... min =2000m. Or, given an offset distance r = 2000m, the maximum detection depth.
[0062] Table 1. Operating frequency and electric field E of a uniformly grounded geoelectric dipole source (CSAMT) x Components, apparent resistivity, skin depth, and number of inductors
[0063] Example 2: The electric field E on the layered earth was calculated using theoretical formulas. x Components (Table 2), following step S1, begin iterating from the high-frequency end f = 8192Hz in the second column of Table 2, and take the initial value of the apparent resistivity of the entire area as the resistivity of the first layer of the geoelectric model. Maximum number of iterations N = 20, precision δ = 10 -5 Then, execute steps S2 to S5 to obtain the apparent resistivity of the entire region. Together with the far-field apparent resistivity calculated using formula (2) And the near-zone apparent resistivity calculated using formula (3) The results are listed in columns 4, 5, and 6 of Table 2, and plotted in Figure 3; columns 7 and 8 of Table 2 show the skin depth δ calculated using formulas (5) and (4). FD And the number of inductances B. According to Table 2 and Figure 3, from high frequency to low frequency, and The last coincidence point of the curves is approximately located at the frequency point f = 32Hz. According to formula (6a), the tolerance value α at this point is approximately 0.534%, which is considered to meet the engineering requirements. Therefore, the corresponding inductance value B = cri far-tran =5.20 as the far-field separation point; from low frequency to high frequency, and The last coincidence point of the curve is approximately located at the frequency point f = 0.25 Hz. According to formula (6b), the tolerance value β at this point is approximately 0.658%. It is considered that it can also meet the engineering requirements. Therefore, the corresponding induction number B = cri tran-near = 0.21 can be used as the near - zone separation point.
[0064] In the given case of Embodiment 2: The region where B≥5.20 is the far zone, the region where 0.21 < B < 5.20 is the transition zone, and the region where B≤0.21 is the near zone.
[0065] Table 2: Skin depth δ corresponding to the far - zone separation point (f = 32 Hz) FD = 384.8 m. According to formula (4) It can be known that the minimum offset distance r required to detect a depth of 384.8 m min [[ID=仃]] = 2000 m. Or rather, the maximum detection depth for a given offset distance r = 2000 m
[0066] Table 2 Operating frequency, electric field E x component, apparent resistivity, skin depth and induction number of the electric - dipole - source CSAMT on a layered earth
[0067] The detection depth calculated by the skin - depth formula (5) can also be obtained by other improved algorithms.
[0068] In summary, a method for dividing the field area of an electric - dipole - source CSAMT provided by the present invention gives the definitions and confirmation rules of the far - zone separation point and the near - zone separation point. From this, the far zone, the transition zone and the near zone are divided, providing a generally applicable and non - empirical standard for the definition of the field area.
[0069] In the above embodiments of the present invention, only two one - dimensional geoelectric structures, i.e., a homogeneous earth and a layered earth, are exemplified. However, the method proposed by the invention is not limited to the above geoelectric structures and can also be applied to two - dimensional and three - dimensional geoelectric structures.
[0070] The above embodiments are only used to illustrate the design concept and characteristics of the present invention, aiming to enable those skilled in the art to understand the content of the present invention and implement it accordingly. The protection scope of the present invention is not limited to the above embodiments. Therefore, all equivalent changes or modifications made according to the principles and design ideas disclosed by the present invention are within the protection scope of the present invention.
Claims
1. A method for dividing the field region of an electric dipole source (CSAMT), characterized in that, From the high-frequency end to the low-frequency end, the electric field E x The last point where the apparent resistivity curve (3) of the whole area and the apparent resistivity curve (4) of the far area are coincident is defined as the far-field separation point (6), and the inductance number corresponding to the far-field separation point (6) is denoted as B = cri far-tran From the low-frequency end to the high-frequency end, the electric field E is... x The last point of overlap between the full-area apparent resistivity curve (3) and the near-area apparent resistivity curve (5) is defined as the near-area separation point (7), and the inductance number corresponding to the near-area separation point (7) is denoted as B = cri tran-near ; Based on the far-field separation point (6) and near-field separation point (7) defined above, and the corresponding induction number of the separation point, the field area is divided into: B≥cri far-tran The area is the far zone, cri tran-near <B<cri far-tran The region is a transition zone, where B≤cri tran-near The area is the near zone.
2. The method for dividing the field region of an electric dipole source CSAMT according to claim 1, characterized in that, Electric field E at each frequency x The full-area apparent resistivity of the component Obtained by the following simple iterative formula electric field E x Far-field apparent resistivity of components Obtained from the following direct calculation formula electric field E x Near-zone apparent resistivity of components Obtained from the following direct calculation formula The number of inductors B is defined by the following formula. The detection depth δ FD It can be calculated using the skin depth formula. Where I is the emission current intensity in A; l is the electric dipole length in m; r is the offset distance from the source point to the observation point in m; f is the operating frequency in Hz; j is the imaginary unit; and μ0 is the vacuum permeability of the nonmagnetic earth. The angle between the observation point and the source point.
3. The method for dividing the field region of an electric dipole source CSAMT according to claim 1, characterized in that, electric field E x The components are obtained through theoretical formulas or actual measurements. The geoelectric model is theoretically calculated during the exploration and experimental stage and constructed based on electrical logging data or other geological data of the survey area.
4. The method for dividing the field region of an electric dipole source CSAMT according to claim 1, characterized in that, In actual exploration, the minimum offset r required to achieve far-field conditions min and detection depth δ FD The relationship, or the maximum depth that can be detected given an offset r. Both are determined by the inductance number B = cri at the far-field separation point. far-tran Estimate.
5. The method for dividing the field region of an electric dipole source CSAMT according to claim 2, characterized in that, The detection depth may be calculated using an algorithm.
6. The method for dividing the field region of an electric dipole source CSAMT according to claim 1, characterized in that, For the apparent resistivity of the entire region Curve and apparent resistivity in the far region The points where the curves coincide must satisfy the following equation: apparent resistivity of the whole area Curve and near-zone apparent resistivity The points where the curves coincide must satisfy the following equation: In the formula, α and β are the tolerance values for the far-field separation point and the near-field separation point, respectively.
7. The method for dividing the field region of an electric dipole source CSAMT according to claim 1, characterized in that, The apparent resistivity of the entire region at each frequency point The iterative calculation steps are as follows: Step S1: Start iterating from the high-frequency end, setting the initial apparent resistivity of the entire region to be [value missing]. Step S2: At each frequency point, set the initial value of the number of iterations n = 0 and the maximum value of the number of iterations N; Step S3, set the initial value of the apparent resistivity of the entire region. By substituting the apparent resistivity of the entire region into a simple iterative formula, a new apparent resistivity value can be calculated. if Then the apparent resistivity at that frequency point And terminate the iterative calculation at that frequency point, Repeat steps S2 to S3 as the initial value for the next frequency point to calculate the apparent resistivity of the entire area at the next frequency point; δ is the accuracy value. Step S4, if Then continue the iterative calculation of the apparent resistivity of the entire region at that frequency point, with the number of iterations n = n + 1. If n ≤ N, then... Input to step S3, and execute the iterative calculation of steps S3 to S4 again; Step S5, if n>N then The iterative calculation of the apparent resistivity of the entire region at this frequency point is terminated.
8. The method for dividing the field region of an electric dipole source CSAMT according to claim 7, characterized in that, The initial value of apparent resistivity for the entire region in step S1 In theoretical calculations, the resistivity value of the first layer of the geoelectric model is used; in exploration engineering, the Cagniard resistivity or E calculated from the apparent resistivity formula in the far zone is used. x The first branch curve value of the apparent resistivity in the far region.
9. The method for dividing the field region of an electric dipole source CSAMT according to claim 7, characterized in that, The maximum value of the iteration is N = 20.
10. The method for dividing the field region of an electric dipole source CSAMT according to claim 7, characterized in that, The accuracy value is taken as δ = 10. -5 .