A Multi-Source Short Transmit / Receive Range Frequency Domain Electromagnetic Detection Method

Abstract

This invention provides a multi-source, short-span frequency domain electromagnetic detection method, comprising the following steps: setting up a survey line in an abnormal region; deploying different transmitting sources according to the survey line; using different transmitting sources to emit electromagnetic signals of different frequencies, and observing the horizontal electric field components of different transmitting sources at observation points; and jointly inverting the horizontal electric field components of different transmitting sources observed at the same measurement point to obtain the underground electrical structure. This invention shortens the observation-to-transmit distance to 1-3 times the detection depth, which can significantly improve the strength of the observed signal and the accuracy of identifying underground targets.

Description

Technical Field

[0001] This invention belongs to the field of geophysical exploration, specifically relating to a multi-source, small-span frequency domain electromagnetic detection method. Background Technology

[0002] Traditional land-controlled-source electromagnetic methods typically employ a single transmitter to emit a signal and observe a set of orthogonal horizontal electric and magnetic field components in the far-field region. This data acquisition mode suffers from drawbacks such as weak signal strength and low resolution, resulting in low accuracy in detecting deep targets. In fact, when the observation distance exceeds one skin depth, the electromagnetic field possesses depth-sounding capabilities. However, when closer to the transmitter, electromagnetic waves cannot approximate plane waves, rendering traditional wave impedance-based data processing methods inapplicable. Therefore, developing inversion methods based on the amplitude of a single electromagnetic field is one of the main solutions for near-source electromagnetic detection. Furthermore, using different transmitters to "illuminate" anomalies can generate different electromagnetic coupling relationships, which is particularly important for grounded conductor sources with galvanic fields. By altering the geometric relationship between the transmitter, anomaly, and receiver, more accurate localization of the subsurface medium can be achieved. Summary of the Invention

[0003] The purpose of this invention is to provide a multi-source, short-distance frequency domain electromagnetic detection method. By observing the short distance, the signal strength can be significantly improved, and by utilizing multi-source transmission and joint inversion data, the target detection accuracy can be significantly improved.

[0004] The technical solution of the present invention is as follows:

[0005] A multi-source, short-range frequency domain electromagnetic detection method includes the following steps:

[0006] Set up survey lines in the abnormal area;

[0007] Different emission sources are deployed according to the survey line;

[0008] Electromagnetic signals of different frequencies are emitted using different emission sources, and the horizontal electric field components of the different emission sources are observed at the observation point.

[0009] The underground electrical structure is obtained by jointly inverting the horizontal electric field components of different emission sources observed at the same measuring point.

[0010] Furthermore, the specific details of the different emission sources deployed along the survey line are as follows:

[0011] A first transmitting source is deployed on one side of the survey line to form a side device; a second transmitting source is deployed in the extended area of ​​a certain endpoint of the survey line to form an axial observation device.

[0012] Furthermore, the frequency range of the electromagnetic signal is 0.1 to 10000 Hz.

[0013] Furthermore, the length of the survey line is greater than twice the lateral dimension of the geological target being detected.

[0014] Furthermore, the length of the emission source is greater than 1 / 2 of the length of the measuring line.

[0015] Furthermore, the distance between the emission source and the survey line is equal to 1 to 3 times the depth of the geological target being detected.

[0016] Furthermore, the formula for jointly inverting the horizontal electric field components of different emission sources observed at the same measuring point is as follows:

[0017]

[0018] In the formula, U is the objective function of the joint inversion. Let μ be the differential operator, μ be the Lagrange operator, and F be the forward operator. For the residual term, m = (h1, h2, ..., h N log 10 ρ1, log 10 ρ2, ..., log 10 ρ N ) is a model vector containing the resistivity and thickness of each layer of the earth. Let d be a data vector. B For observation data from the side-mounted device, d I For axial device observation data, M B M represents the number of data points observed by the side-mounted device. I The number of data points observed by the axial device; Here is the weight matrix for the data error, where and These represent the errors in the observation data from the side and axial devices, respectively.

[0019] Furthermore, as needed, more launch sources can be deployed in other locations, with the arrangement of these other launch sources being the same as that of the first and second launch sources.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] This invention involves sequentially deploying different transmitting sources at different locations around the observation point, with the distance between the transmitting sources and the observation point being 1 to 3 times the detection depth. The transmitting sources emit signals of different frequencies, and the horizontal electric field component is observed at the receiving point. The signals excited by different transmitting sources observed at the same measuring point are then jointly inverted to obtain the underground electrical structure. This invention shortens the transmission-receiver distance to 1 to 3 times the detection depth, significantly improving the strength of the observed signal and the accuracy of identifying underground targets. Attached Figure Description

[0022] The accompanying drawings illustrate various embodiments generally by way of example rather than limitation, and are used, together with the specification and claims, to explain embodiments of the invention. Where appropriate, the same reference numerals are used in all drawings to refer to the same or similar parts. Such embodiments are illustrative and are not intended to be exhaustive or exclusive embodiments of the apparatus or method.

[0023] Figure 1 This is a schematic diagram of the transmitting and receiving arrangement of the method of the present invention;

[0024] Figure 2 This is a graph showing the Ex response curves at different transmit / receive distances under the side-mounted device of the present invention;

[0025] Figure 3 This is a graph showing the Ex response curves at different transmit / receive distances in the axial device of the present invention.

[0026] Figure 4 These are the results of single-source and dual-source data inversion according to the present invention.

[0027] Figure 5 This is a construction layout diagram for the present invention;

[0028] Figure 6 This is a measured Ex signal diagram from the present invention;

[0029] Figure 7 This is a diagram showing the inversion results based on S1 source data in this invention;

[0030] Figure 8 This is a diagram showing the inversion results based on S2 source data in this invention;

[0031] Figure 9 This is a diagram showing the joint inversion results based on source data S1 and S2 in this invention. Detailed Implementation

[0032] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0033] Example 1

[0034] Reference Figures 1-9This invention discloses a multi-source, short-range frequency domain electromagnetic detection method, comprising:

[0035] Numerical simulation was conducted: A five-layer geodetic model was designed, with resistivity of each layer being ρ1 = 100 Ω·m, ρ2 = 10 Ω·m, ρ3 = 100 Ω·m, ρ4 = 10 Ω·m, and ρ5 = 100 Ω·m, and thickness of each layer being h1 = 500 m, h2 = 100 m, h3 = 500 m, h4 = 100 m, and h5 = ∞. The transmitter length was 1000 m, the transmitting current was 1 A, and the transmitting frequency was 0.1–10000 Hz. The transmitter at the side was designated S1, and the transmitter at the equator was designated S2.

[0036] Figure 2 Under the above design parameters, the response curves of the horizontal electric field component (Ex) at different transmit / receive distances under the side-mounted device condition are shown. Figure 3 The figures show the Ex response curves at different transmit and receive distances under equatorial device conditions. It can be seen that, regardless of the device type, the short transmit and receive distance observation proposed in this invention can significantly improve the intensity of the electromagnetic field signal. Compared with traditional long-range observation, the signal amplitude can be increased by nearly two orders of magnitude.

[0037] Considering the observation situation with a small transmit / receive distance (offset = 2000m), Figure 4 The inversion results for single source data (S1 and S2) and the joint inversion results for two source data (S1+S2) are presented. It can be seen that inverting data from a single source only provides a good representation of shallow strata, but loses resolution for deep strata. However, joint inversion of data from two sources significantly improves the resolution for deep strata, successfully revealing a second low-resistivity layer in the deep interior.

[0038] This method was used to conduct field measurements in an iron ore area in Dezhou City, Shandong Province, China, with the aim of locating the granite mass beneath a thick, low-resistivity overburden. The construction layout diagram is shown below. Figure 5 As shown. The operating parameters are: measuring line length 1800 meters, transmitter S1 length 1600 meters, transmit / receive distance from the measuring line 3300 meters, transmitter S2 length 1200 meters, transmit / receive distance from the measuring line 3800 meters, transmit current of both transmitters is 14 amperes, transmit frequency range of both is 9600 Hz to 1 Hz, observe horizontal electric field Ex component, electrode distance MN is 50 meters.

[0039] One-dimensional inversion was performed on the data obtained from the two sources S1 and S2 operating independently, resulting in resistivity-depth profiles as shown below. Figure 7 and Figure 8As shown in the figure, the inversion results based on the S1 source only reflect shallow strata, and essentially lose the ability to resolve changes in the electrical properties of the strata below a depth of approximately 800 meters. The inversion results based on the S2 source can reflect deeper strata, but the electrical structure is more chaotic, with many false anomalies caused by abrupt changes in resistivity. The results of the joint inversion of the data from the two sources are shown in the figure. Figure 9 As shown, the joint inversion results not only reflect the deep electrical properties of the layers well, but also avoid false anomalies caused by sudden changes in resistivity, resulting in more stable inversion results overall. Based on these results, a high-resistivity granite body was discovered deep within the range of 1200–1800 meters along the survey line.

[0040] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the technical scope disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A multi-source, small-transmitter-receiver frequency-domain electromagnetic surveying method, characterized in that, The method comprises the following steps: arranging a survey line on an abnormal area; arranging different transmitting sources according to the survey line; transmitting electromagnetic signals of different frequencies by using the different transmitting sources, and observing the horizontal electric field components of the different transmitting sources at observation points; jointly inverting the horizontal electric field components of the different transmitting sources observed at the same survey point to obtain the electrical structure of the underground; the length of the transmitting source is greater than 1 / 2 of the length of the survey line; the distance between the transmitting source and the survey line is equal to 1-3 times the depth of the geological target to be detected; arranging different transmitting sources according to the survey line specifically comprises: arranging a first transmitting source on one side of the survey line to form a side device, and arranging a second transmitting source in the area of the extension line of one end point of the survey line to form an axial observation device; the frequency range of the electromagnetic signals is 0.1-10000 Hz; the length of the survey line is greater than 2 times the lateral size of the geological target to be detected; the formula for jointly inverting the horizontal electric field components of the different transmitting sources observed at the same survey point is: where U is the objective function of joint inversion, is the differential operator, μ is the Lagrange multiplier, F is the forward operator, is the residual term, m is the model vector containing the resistivity and thickness of each layer of the earth, d is the data vector, where d B is the data observed by the lateral device, d I is the data observed by the axial device, M B is the number of data observed by the lateral device, M I is the number of data observed by the axial device; W is the weight matrix of data error; the method further comprises: arranging more transmitting sources at other positions as needed, and the arrangement mode of the other transmitting sources is the same as that of the first transmitting source and the second transmitting source.

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