A data processing method for magnetic source multi-waveform combined fine prospecting

Abstract

The present application relates to a kind of magnetic source multi-waveform combination fine geophysical prospecting data processing method, it is in order to the electromagnetic response of triangular wave, trapezoidal wave, half-sine wave and its combination emission current, carry out high-precision record and data joint interpretation realizes transient electromagnetic fine detection. Large loop multi-waveform combination electromagnetic response is recorded using low-noise receiver, the response of different emission current is converted into step wave response by the primary field removal method based on measured current waveform and the deconvolution method based on SVD;Small loop dual-waveform combination electromagnetic response is stored using double sampling rate receiver, and the attenuation curve of triangular wave and trapezoidal wave is spliced together by residual primary field removal method and response splicing method. According to data interpretation method, the converted step wave response and spliced response are resistivity imaging, realize the fine detection of shallow and deep target body, solve the problem of low precision of multi-waveform combination emission data interpretation of transient electromagnetic method.

Description

Technical Field

[0001] This invention relates to a data processing method for refined detection of magnetic sources using multi-waveform combinations, applicable to electromagnetic geophysical exploration or refined detection of urban underground space, and particularly for data processing and interpretation of targeted measurements using multi-waveform combinations of magnetic sources. Background Technology

[0002] Magnetic source transient electromagnetic methods offer advantages such as a wide detection depth range and cost-effectiveness, and have been widely applied in mineral exploration, hydrogeological surveys, archaeological research, urban underground space exploration, and disaster prediction. In recent years, with the continuous advancement of urbanization, the requirements for refined exploration of urban underground spaces have become increasingly stringent, mainly involving subway rail transit planning, underground pipeline laying, and the detection of underground cavities. Furthermore, with my country's rapid economic development, the demand for minerals such as metals has significantly increased, while shallow mineral resources are almost exhausted, necessitating the use of new technologies and methods for refined exploration of deep mineral deposits.

[0003] The magnetic source transient electromagnetic detection system mainly includes a transmitter, transmitting coil, receiver, and receiving coil. The transmitter outputs bipolar triangular waves, trapezoidal waves, half-sine waves, and their combined current waveforms. The transmitting coil is flexibly adjusted according to the depth of the target body. When detecting deep targets, a large-loop transmitting coil is used, and when detecting shallow surface targets, a small-loop transmitting coil is used. During the current transmission process, the receiving coil converts the time-varying derivatives of the primary and secondary fields into voltage. The receiver samples and records the induced electromotive force information of the transmitted current by sampling the voltage. Data interpretation methods are used to process and interpret the data recorded by the receiver to obtain the resistivity distribution information of the underground medium in order to identify underground anomalies in the target area.

[0004] Traditional magnetic source time-domain electromagnetic methods are limited by the performance of the transmitting system, and the transmitted waveform is mainly trapezoidal wave. With the development of electronic technology, the transmitted waveform can now include triangular wave, trapezoidal wave, half sine wave and their combination waveforms. However, the receiving device has problems such as low sampling rate, inability to continuously acquire data and high noise level, which cannot meet the needs of multi-waveform combined transmission. This results in the loss of early electromagnetic response signals and low signal-to-noise ratio of late data, which is not conducive to the detection of shallow surface targets and deep targets. The poor performance of the receiving device seriously limits the further development and application of transient electromagnetic methods for multi-waveform combined transmission.

[0005] To meet the needs of refined detection using multi-waveform combinations, traditional receiving devices are no longer suitable. Low-sampling-rate continuous acquisition receivers have lower noise levels and can record the electromagnetic response of the combined current waveform of the large-loop transmitting coil, improving the resolution of the late attenuation curve and making them more suitable for deep-level detection. High-low dual-sampling-rate continuous acquisition receivers can change the sampling rate and store the electromagnetic response of the triangular-trapezoidal wave combined current waveform of the small-loop transmitting coil. The early response is acquired using a high sampling rate to increase the time to the first valid signal, while the late response is acquired using a low sampling rate to improve the signal-to-noise ratio of the electromagnetic response, making them more suitable for near-surface detection.

[0006] Primary field removal is a crucial step in data interpretation. Since the acquired electromagnetic response is the sum of the primary and secondary fields, the primary field needs to be subtracted from the measured data to obtain the pure secondary field response for data interpretation. The primary field is calculated based on the transmit current waveform and the parameters of the transmitting and receiving coils, and is often calculated using an ideal transmit current waveform. However, the actual transmit current waveform is not equivalent to the ideal transmit current waveform, resulting in significant differences in the obtained primary field. To obtain a high-quality pure secondary field response, current information needs to be acquired using a current acquisition device, and primary field removal needs to be performed based on the measured current waveform.

[0007] Deconvolution methods based on Singular Value Decomposition (SVD) offer advantages such as robustness, high signal-to-noise ratio, zero phase shift, and low waveform distortion, making them widely used in electromagnetic signal reconstruction. Because the electromagnetic response patterns of triangular waves, trapezoidal waves, and half-sine waves differ significantly, there is no unified standard for interpreting the electromagnetic response of a large-loop transmitting coil. Therefore, deconvolution methods are needed to reconstruct the electromagnetic responses of different transmitting current waveforms into step wave responses, normalizing the responses before interpreting the data for different transmitting current waveforms.

[0008] Data stitching technology combines multiple, multi-dimensional, or different-scale data sets to integrate their strengths for more accurate interpretation. In recent years, data stitching technology has been widely applied in fields such as lidar imaging, power grid big data fusion, image processing, and electromagnetic methods. To perform joint data interpretation of the triangular-trapezoidal wave electromagnetic response of a small-loop transmitting coil, it is necessary to stitch the triangular wave attenuation curve and the trapezoidal wave attenuation curve together. The stitched attenuation curve not only includes early signals but also has a high signal-to-noise ratio, making it more beneficial for interpreting shallow subsurface anomalies.

[0009] To address the problem that strong electromagnetic interference can easily mask weak useful magnetotelluric signals, Li Jin et al. employed the Adaptive Multi-Resolution Singular Value Decomposition (AMRSVD) method to process magnetotelluric data. The results showed that this method can effectively separate highly correlated noise, and the curve shape is significantly improved. Esben Auken et al., addressing the data processing problem of ground-towed TEM systems, used a data stitching method to stitch together trapezoidal wave responses of two different amplitudes, achieving resistivity imaging within a 70m subsurface depth.

[0010] Chinese patent CN113866835B discloses an electromagnetic launch system and control method with time-domain three-waveform combination. By adopting a launch bridge circuit, a passive clamping circuit and an RLC series resonant circuit, it outputs triangular waves, trapezoidal waves, half-sine waves and their combination current waveforms with controllable turn-off time, which significantly improves the excitation capability of the magnetic source time-domain electromagnetic launch system.

[0011] Chinese patent CN115128680B discloses a transient electromagnetic targeted measurement method based on a combination of multiple waveforms of a magnetic source. The method analyzes the detection effect of triangular waves, trapezoidal waves, half-sine waves and their combinations of current waveforms through numerical simulation, establishes a set of targeted excitation parameters, and performs targeted excitation based on the characteristics of the target body to achieve refined transient electromagnetic detection.

[0012] The methods described above disclose the application of singular value decomposition (SVD) and data stitching techniques, as well as a method for measuring transient electromagnetic targets using multi-waveform combinations of magnetic sources. However, existing data processing methods are almost entirely inadequate for the data processing and interpretation of refined detection using multi-waveform combined emission currents, failing to provide high-precision data interpretation based on these currents. Most time-domain electromagnetic data processing methods are based on trapezoidal waves, which cannot provide high-precision data processing and interpretation when dealing with complex geological structure identification and refined multi-waveform combined detection. How to achieve high-precision data recording and multi-waveform joint data processing for targets at different depths and with different combined emission current waveforms in time-domain electromagnetic multi-waveform combined targeted detection, thus enabling refined detection of complex targets, is a pressing technical problem that those skilled in the art must solve. Summary of the Invention

[0013] The technical problem this invention aims to solve is to provide a data processing method for refined detection of multi-waveform combinations of magnetic sources. The purpose is to achieve high-precision recording and joint interpretation of the electromagnetic responses of triangular waves, trapezoidal waves, half-sine waves, and their combined transmitted currents, thus realizing refined detection of transient electromagnetic sources. A low-noise receiver is used to record the combined electromagnetic responses of large-loop multi-waveform combinations. The responses of different transmitted currents are converted into step wave responses using a first-order field removal method based on measured current waveforms and a deconvolution method based on SVD. A dual-sampling-rate receiver is used to store the combined electromagnetic responses of small-loop dual-waveform combinations. The attenuation curves of triangular waves and trapezoidal waves are stitched together using a residual first-order field removal method and a response stitching method. Resistivity imaging is then performed on the converted step wave response and the stitched attenuation curves according to the data interpretation method, enabling refined detection of shallow and deep targets and solving the problem of low data interpretation accuracy in transient electromagnetic multi-waveform combination transmission methods.

[0014] This invention is implemented as follows: a data processing method for refined detection of magnetic sources using multi-waveform combinations:

[0015] 1) By applying a multi-waveform combined transmitter and a variable sampling rate receiver, a transient electromagnetic precision measurement system with a magnetic source multi-waveform combination is constructed. When detecting deep targets, a magnetic source large-loop transmitting coil is used, with a side length of the transmitting coil greater than or equal to 100 meters. When detecting shallow surface targets, a magnetic source small-loop transmitting coil is used, with the transmitting current being a combination of triangular wave and trapezoidal wave waveforms. The side length of the transmitting coil is less than or equal to 4 meters, and the number of turns of the transmitting coil is greater than or equal to 4. The primary field at the center point is reduced by using a compensation coil.

[0016] 2) When using a large loop transmitting coil for deep exploration, a low-noise receiving device with a sampling rate of 30kHz is used to record the transmitted current waveform information and the electromagnetic response of different transmitted currents; when using a small loop transmitting coil for shallow surface exploration, a receiving device with dual sampling rates of 256kHz and 64kHz is used to record the transmitted current waveform information and the electromagnetic response of the triangular wave-trapezoidal wave combined transmitted current.

[0017] 3) Obtain the primary field of the transmitting current based on the measured transmitting current waveform and the receiving coil parameters. Subtract the primary field from the electromagnetic response recorded by the receiving device to obtain the pure secondary field response for different transmitting current waveforms.

[0018] 4) For the multi-waveform combined pure quadratic field response obtained under the excitation of the large-loop transmitting coil, the triangular wave, trapezoidal wave and half-sine wave responses are restored to step wave responses based on the deconvolution method of singular value decomposition (SVD) and e-exponential fitting; for the triangular wave-trapezoidal wave combined pure quadratic field response obtained under the excitation of the small-loop transmitting coil, the triangular wave and trapezoidal wave responses are spliced ​​together based on the attenuation curve splicing method of electromagnetic response normalization.

[0019] 5) Based on the data interpretation method, the restored step wave response and spliced ​​response are preprocessed, filtered and extracted, and then resistivity imaging is performed to interpret the information of underground anomalies, so as to realize the fine detection of deep and shallow surface targets.

[0020] In step 1), when detecting deep targets, the side length of the transmitting coil is greater than or equal to 100 meters, and the primary field at the center point of the transmitting coil is relatively weak, so no compensation coil is needed. When detecting shallow targets, the side length of the transmitting coil is less than or equal to 4 meters, and the primary field at the center point is very strong, which can easily lead to saturation of the receiving device. Therefore, a compensation coil is needed to reduce the primary field. The currents of the transmitting coil and the compensation coil are equal but opposite in direction. To ensure that the primary field generated by the transmitting coil and the compensation coil at the center point is 0, the expression for the coil parameters of the transmitting system is as follows:

[0021] (1)

[0022] In formula (1) For the transmitting coil current, The number of turns of the transmitting coil. This is the side length of the transmitting coil; To compensate for the coil current, To compensate for the number of coil turns, To compensate for the coil side length; ρ is the magnetic permeability of air;

[0023] When probing deep targets, the transmitted current waveform includes triangular waves, trapezoidal waves, half-sine waves, and combinations thereof. Triangular waves are used to observe the top of deep targets; trapezoidal waves and half-sine waves are used to observe the distribution pattern of deep targets. When probing shallow targets, the transmitted current includes triangular waves, trapezoidal waves, and combinations thereof. Triangular waves have rapid secondary field attenuation; trapezoidal waves have strong excitation capability, high signal-to-noise ratio, and slow secondary field attenuation. The combination of triangular waves and trapezoidal waves can achieve high-quality excitation of shallow targets.

[0024] In step 2), the receiving device consists of a receiving coil, a pre-differential amplifier circuit, a signal conditioning circuit, and a 24-bit AD conversion circuit. The higher the AD sampling rate, the higher the noise level of the receiving device. When detecting deep targets, the longer the duration of the effective secondary field signal, the greater the detection depth. Therefore, a 30kHz low sampling rate, low-noise receiving device is used to collect the electromagnetic response. When detecting shallow targets, the earlier the first effective secondary field signal appears, the stronger the detection capability. Therefore, a 256kHz high sampling rate receiving device is used to collect the early electromagnetic response of the transmitted current, advancing the first effective signal appearance time to 4µs. At the same time, to improve the quality of the late electromagnetic response, a 64kHz sampling rate receiving device is used to collect the late electromagnetic response of the transmitted current. Therefore, a dual sampling rate receiving device of 256kHz and 64kHz is designed based on the same AD module, which improves the signal quality while acquiring shallow surface information. To monitor the transmitter's operating status and facilitate the removal of the primary field, a Hall sensor is connected to the input of the receiving device to convert the transmitted current into a voltage signal for storage and record the transmitted current waveform information.

[0025] In step 3), the electromagnetic response recorded by the receiving device contains primary and secondary field information. To interpret the data, the primary field must first be filtered and removed. Subtracting the primary field response from the measured electromagnetic response yields the pure secondary field response of the transmitting current. The process of obtaining the pure secondary field of the large-loop transmitting current based on the recorded transmitting current waveform information and the receiving coil parameters is as follows:

[0026] (2)

[0027] Where u is the electromagnetic response recorded by the receiver, u1 is the primary field, u2 is the pure secondary field; R1 is the matching resistance of the receiving coil, R2 is the internal resistance of the receiving coil, dI / dt is the first derivative of the pulse current, and M... F The mutual inductance between the transmitting coil and the receiving coil, t1 is the resonant frequency of the receiving coil, and t1 is the turn-off time of the trapezoidal wave or the pulse width of the triangular wave or half-sine wave.

[0028] Since the small loop detection method includes a transmitting coil and a compensation coil, the primary field at the center point is zero. However, the receiving coil is a plane, which significantly reduces the primary field but cannot completely cancel it out. Therefore, the coefficient of the remaining primary field needs to be determined using magnetic flux calculation. Subtracting the remaining primary field from the measured data obtained by the receiver yields the pure secondary field response. The process of obtaining the pure secondary field of the small loop transmitting current based on the recorded triangular and trapezoidal wave transmitting current waveforms and the receiving coil parameters is as follows:

[0029] (3)

[0030] Where FS F is the magnetic flux of the small-loop transmitting coil in the central region. B G represents the magnetic flux of the small-loop compensation coil in the central region, and G is the coefficient of the residual primary field.

[0031] In step 4), in order to interpret the electromagnetic response of the large loop, a deconvolution method based on SVD and e-exponential fitting is used to convert the pure secondary field response of different emission current waveforms into a step wave response.

[0032] Since the impulse response and step response of the system have a calculus-integral relationship, let the impulse magnetic field response and step magnetic field response of the transient electromagnetic system be H(t) and Bs(t), respectively, and I(t) be the time-domain form of the emitted current. According to the convolution theorem and frequency-time transform, we can obtain H(t) = -dBs(t) / dt. The convolution formula for the time-domain electromagnetic response under different emitted current waveform excitation is:

[0033] (4)

[0034] Since the induced electromotive force decays approximately exponentially (e), the decay curve is decomposed into a series of exponential sums (e), assuming the induced electromotive force is... The sampling time interval is The sampling time is j, and j=m. n basis functions are selected, therefore... In discrete time, it can be decomposed into:

[0035] (5)

[0036] Expanding equation (5) into matrix form yields:

[0037] (6)

[0038] in It is called the kernel matrix, and its n column vectors are called odd functions. This represents the amplitude corresponding to the i-th time constant and the j-th sampling time. These are the decomposition coefficients multiplied by the i-th basis function; the electromagnetic data is decomposed into components that establish the relationship between the sampling time t and the time constant. Simultaneously, the data reconstruction was completed through the multiplication of related matrices.

[0039] Quality and time constant of data reconstruction The selection of the exponential basis functions is relevant, therefore it is necessary to decompose and analyze the pre-selected exponential basis functions based on the SVD method, and then convert the matrix... Perform SVD decomposition and inverse it:

[0040] (7)

[0041] Where U is an m×m matrix, V is an n×n matrix, and V T It is the conjugate transpose of V. Let m×n be a positive semi-definite diagonal matrix, and let its diagonal values ​​be... It is either positive or zero; if As the value of approaches zero, then Matrices exhibit singularity, and their decomposition is unstable; therefore, to ensure efficient matrix decomposition, a limit value k is set, if... If it is less than the limit value, then Set to zero and re-perform the matrix. Decomposition:

[0042] (8)

[0043] Where P is reset The number of non-zero values ​​in the data. For the first P columns of V, For the first P columns of U, It is a P×P order nonzero diagonal matrix;

[0044] Therefore, equation (6) can be solved to obtain the decomposed coefficient vector A and the refitted electromagnetic data. :

[0045] (9)

[0046] By adjusting the limit value k and The distribution values ​​ensure that the relative error meets the accuracy requirement of less than 10%. The expression for the relative error before and after fitting is as follows:

[0047] (10)

[0048] To obtain the magnetic field response under step wave excitation, an integral method is used:

[0049] (11)

[0050] Substituting equation (11) into equation (4), the induced electromotive force of the pulse current can be obtained according to the convolution commutative law. :

[0051] (12)

[0052] in , indicating that there are n basis elements. , indicating that there are m time channels;

[0053] The second derivative of the emission current and the original basis Convolution yields a new basis. By using the e-exponential fitting method, the induced electromotive force under different emission current waveforms can be fitted according to a new basis to obtain the decomposition coefficients. The decomposed coefficients are then recombinated with the original substrate to obtain the step wave magnetic field. Then, by taking the derivative, the step wave induced electromotive force can be obtained. The deconvolution operation is completed; this process recovers the step wave excitation response from the excitation responses of different emission current waveforms, and completes the normalization of the multi-waveform excitation response, which is a key step in the interpretation of multi-waveform emission current data.

[0054] In step 4), to interpret the electromagnetic response of the small loop, the triangular wave and trapezoidal wave electromagnetic responses need to be spliced ​​together. The early response of the spliced ​​data uses the triangular wave response, and the late response uses the trapezoidal wave response. Because the amplitude of the triangular wave emission current is smaller and decays faster, the trapezoidal wave response is larger than the triangular wave response at the splicing moment. Therefore, the trapezoidal wave response needs to be normalized to the triangular wave response. The normalization process is as follows:

[0055] (13)

[0056] in To splice the response, It is a triangular wave response. The waveform is a trapezoidal wave response, where T is the normalization factor of the trapezoidal wave response. This is the starting time for data splicing. The width of the triangular wave pulse. The trapezoidal wave turn-off time; the early signal of the spliced ​​response is rich in shallow surface information, the late effective signal has a long decay time, the signal-to-noise ratio is higher, and the quality of the secondary field response is significantly improved;

[0057] In step 5), baseline correction, superposition, filtering, and channel preprocessing are performed on the restored step wave response and spliced ​​response based on the data interpretation method. Multi-waveform joint resistivity imaging is performed on the processed induced electromotive force signal to interpret the information of underground target bodies at different depths. This completes the data processing for the fine detection of magnetic source multi-waveform combination, effectively improving detection sensitivity and accuracy.

[0058] Compared with existing technologies, the advantages of this invention are as follows: Using a low-sampling-rate, low-noise receiver and a dual-sampling-rate receiver to record the electromagnetic responses of the large-loop and small-loop transmitting coils respectively effectively improves the quality of electromagnetic data, meeting the needs of refined detection in both deep and shallow ground. By using a first-order field removal method based on measured current waveforms and a deconvolution method based on SVD, the multi-waveform electromagnetic response of the large loop is transformed into a step-wave response, improving the accuracy and effectiveness of the data. The residual first-order field removal method and response stitching technology process the triangular-trapezoidal electromagnetic response of the small loop into a stitched response, reducing the near-surface detection blind zone. Multi-waveform joint resistivity imaging is performed on the processed step-wave response and stitched response to complete the identification and interpretation of deep and near-surface targets, improving the data interpretation accuracy of refined detection using time-domain electromagnetic methods with multi-waveform combinations. Attached Figure Description

[0059] Figure 1 This is a flowchart of a data processing method for refined detection of magnetic sources using multiple waveform combinations;

[0060] Figure 2 This is a schematic diagram of the structural principle of the magnetic source triangular wave-trapezoidal wave combined small loop transmitting coil;

[0061] Figure 3 This is an overall block diagram of the time-domain electromagnetic multi-waveform response and current waveform receiving device of the present invention.

[0062] Figure 4 This is a block diagram of the acquisition circuit of the dual sampling rate receiver of the present invention;

[0063] Figure 5 This is a flowchart of the deconvolution calculation for multi-waveform excitation electromagnetic response;

[0064] Figure 6 This is a diagram showing the deconvolution result of the combined electromagnetic response of the large loop multi-waveform transmission.

[0065] Figure 7 It is a resistivity imaging diagram of the emission step response of a large loop multi-waveform combination.

[0066] Figure 8 This is a stitched image of the electromagnetic response data of the combined transmission of the small loop triangular wave and trapezoidal wave.

[0067] Figure 9 It is a resistivity imaging map of the combined emission splicing of small loop triangular wave and trapezoidal wave; Detailed Implementation

[0068] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0069] Example

[0070] See Figure 1 A data processing method for refined detection of magnetic sources using multi-waveform combinations, comprising:

[0071] 1) See Figure 1 Combination Figure 2 As shown, a transient electromagnetic transmission system using a magnetic source with multiple waveform combinations was employed for shallow probing of a small-loop transmitting coil. The experiment was conducted south of the Hydraulic Engineering Building on the Chaoyang Campus of Jilin University. The structures of the transmitting coil and the compensation coil are as follows. Figure 2 As shown; the transmitting coil has a side length of 4m and 4 turns, the compensation coil has a side length of 1m and 1 turn, and the primary field at the center point of the transmitting coil is 0; the transmitting current is a combination of triangular and trapezoidal waves, the triangular wave amplitude is 4A, the rise time is 80us, and the turn-off time is 20us; the trapezoidal wave amplitude is 25A, and the turn-off time is 120us; the measuring line length is 20m, the measuring point spacing is 2m, and there are a total of 11 measuring points.

[0072] 2) See Figure 3 and Figure 4 The transmitting current waveform and electromagnetic response are recorded using a receiving device with sampling rates of 64kHz and 256kHz. The ADC chip ADS127L01 has multiple sampling rates, and the sampling rate can be changed by outputting control signals from a microcontroller. The acquired data is stored in a memory card, and the transmitting current waveform and triangular-trapezoidal electromagnetic response can be obtained by reading it through a computer.

[0073] 3) See Figure 8 For the measured response in the upper middle section, the triangular wave response data at sampling rates of 64kHz and 256kHz were first exported. The early data used the response at 256kHz, the overlapping middle section used the average of the responses at 256kHz and 64kHz, and the late data used the response at 64kHz. At this point, the appearance time of the first valid signal in the triangular wave response was advanced to 4µs, and the signal-to-noise ratio of the late data was also relatively high. Subsequently, the trapezoidal wave response data at sampling rates of 64kHz and 256kHz were exported, and the processing method was the same as that for the triangular wave response. At this point, the valid signal of the late data in the trapezoidal wave response lasted for about 6ms. Since the turn-off time of the triangular wave is significantly shorter than that of the trapezoidal wave, the shallow information of the triangular wave response is rich, while the signal quality of the trapezoidal wave response is better, which can increase the depth of data interpretation.

[0074] 4) See Figure 8The bottommost spliced ​​response is obtained by first subtracting the residual first field of the small loop transmitting coil using residual first field elimination technology to obtain the pure second field response of the triangular wave and trapezoidal wave; the splicing time and the normalization factor of the trapezoidal wave response are determined, and then the triangular wave response and trapezoidal wave response are spliced ​​together using data splicing technology to obtain the spliced ​​response; at this time, the early information of the spliced ​​response can be traced back to 4us, the effective time of the late response reaches 6ms, and the minimum signal resolution is significantly improved.

[0075] 5) See Figure 9 The resistivity imaging of the spliced ​​response was performed using a data interpretation method, and two high-resistivity anomalies were found at a depth of 4m underground. The anomalies were distributed on the left and right sides of the survey line, and their distribution location and depth were basically consistent with those of the ground penetrating radar anomalies. The effective detection depth exceeded 50m. The small loop survey line experiment proved the effectiveness of the data processing method proposed in this invention for multi-waveform combination detection. The experimental results significantly improved the accuracy of data interpretation and realized the fine detection of transient electromagnetic multi-waveform combination.

[0076] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A data processing method for refined detection of magnetic sources using multi-waveform combinations, characterized in that, Includes the following steps: 1) By applying a multi-waveform combined transmitter and a variable sampling rate receiver, a transient electromagnetic precision measurement system with a magnetic source multi-waveform combination is constructed. When detecting deep targets, a large loop transmitting coil is used, with a side length of the transmitting coil greater than or equal to 100 meters. When detecting shallow surface targets, a small loop transmitting coil is used, with the transmitting current being a combination of triangular wave and trapezoidal wave waveforms. The side length of the transmitting coil is less than or equal to 4 meters, and the number of turns of the transmitting coil is greater than or equal to 4. The primary field at the center point is reduced by using a compensation coil. 2) When using a large loop transmitting coil for deep exploration, a low-noise receiving device with a sampling rate of 30kHz is used to record the transmitted current waveform information and the electromagnetic response of different transmitted currents; when using a small loop transmitting coil for shallow surface exploration, a receiving device with dual sampling rates of 256kHz and 64kHz is used to record the transmitted current waveform information and the electromagnetic response of the triangular wave-trapezoidal wave combined transmitted current. 3) Obtain the primary field of the transmitting current based on the measured transmitting current waveform and the receiving coil parameters. Subtract the primary field from the electromagnetic response recorded by the receiving device to obtain the pure secondary field response for different transmitting current waveforms. 4) For the multi-waveform combined pure quadratic field response obtained under the excitation of the large-loop transmitting coil, the triangular wave, trapezoidal wave and half-sine wave responses are restored to step wave responses based on the deconvolution method of singular value decomposition (SVD) and e-exponential fitting; for the triangular wave-trapezoidal wave combined pure quadratic field response obtained under the excitation of the small-loop transmitting coil, the triangular wave and trapezoidal wave responses are spliced ​​together based on the attenuation curve splicing method of electromagnetic response normalization. 5) Based on the data interpretation method, the restored step wave response and spliced ​​response are preprocessed, filtered and extracted, and then resistivity imaging is performed to interpret the information of underground anomalies, so as to realize the fine detection of deep and shallow surface targets.

2. The data processing method for refined detection of magnetic sources using multi-waveform combination according to claim 1, characterized in that: In step 1), when detecting deep targets, the side length of the transmitting coil is greater than or equal to 100 meters, and the primary field at the center point of the transmitting coil is relatively weak, so no compensation coil is needed. When detecting shallow targets, the side length of the transmitting coil is less than or equal to 4 meters, and the primary field at the center point is very strong, which can easily lead to saturation of the receiving device. Therefore, a compensation coil is needed to reduce the primary field. The currents of the transmitting coil and the compensation coil are equal but opposite in direction. To ensure that the primary field generated by the transmitting coil and the compensation coil at the center point is 0, the expression for the transmitting system coil parameters is: (1) In formula (1) For the transmitting coil current, The number of turns of the transmitting coil. This is the side length of the transmitting coil; To compensate for the coil current, To compensate for the number of coil turns, To compensate for the coil side length; ρ is the magnetic permeability of air; When probing deep targets, the transmitted current waveform includes triangular waves, trapezoidal waves, half-sine waves, and combinations thereof. Triangular waves are used to observe the top of deep targets; trapezoidal waves and half-sine waves are used to observe the distribution pattern of deep targets. When probing shallow targets, the transmitted current includes triangular waves, trapezoidal waves, and combinations thereof. Triangular waves have rapid secondary field attenuation; trapezoidal waves have strong excitation capability, high signal-to-noise ratio, and slow secondary field attenuation. The combination of triangular waves and trapezoidal waves can achieve high-quality excitation of shallow targets. In step 2), the receiving device consists of a receiving coil, a pre-differential amplifier circuit, a signal conditioning circuit, and a 24-bit AD conversion circuit. The higher the AD sampling rate, the higher the noise level of the receiving device. When detecting deep targets, the longer the duration of the effective secondary field signal, the greater the detection depth. Therefore, a 30kHz low sampling rate, low-noise receiving device is used to collect the electromagnetic response. When detecting shallow targets, the earlier the first effective secondary field signal appears, the stronger the detection capability. Therefore, a 256kHz high sampling rate receiving device is used to collect the early electromagnetic response of the transmitted current, advancing the first effective signal appearance time to 4µs. At the same time, to improve the quality of the late electromagnetic response, a 64kHz sampling rate receiving device is used to collect the late electromagnetic response of the transmitted current. Therefore, a dual sampling rate receiving device of 256kHz and 64kHz is designed based on the same AD module, which improves the signal quality while acquiring shallow surface information. To monitor the transmitter's operating status and facilitate the removal of the primary field, a Hall sensor is connected to the input of the receiving device to convert the transmitted current into a voltage signal for storage and record the transmitted current waveform information.

3. The data processing method for refined detection of magnetic sources using multi-waveform combination according to claim 1, characterized in that: In step 3), the electromagnetic response recorded by the receiving device contains primary and secondary field information. To interpret the data, the primary field must first be filtered and removed. Subtracting the primary field response from the measured electromagnetic response yields the pure secondary field response of the transmitting current. The process of obtaining the pure secondary field of the large-loop transmitting current based on the recorded transmitting current waveform information and the receiving coil parameters is as follows: (2) Where u is the electromagnetic response recorded by the receiver, u1 is the primary field, u2 is the pure secondary field; R1 is the matching resistance of the receiving coil, R2 is the internal resistance of the receiving coil, dI / dt is the first derivative of the pulse current, and M... F The mutual inductance between the transmitting coil and the receiving coil, t1 is the resonant frequency of the receiving coil, and t1 is the turn-off time of the trapezoidal wave or the pulse width of the triangular wave or half-sine wave. Since the small loop detection method includes a transmitting coil and a compensation coil, the primary field at the center point is zero. However, the receiving coil is a plane, which significantly reduces the primary field but cannot completely cancel it out. Therefore, the coefficient of the remaining primary field needs to be determined using magnetic flux calculation. Subtracting the remaining primary field from the measured data obtained by the receiver yields the pure secondary field response. The process of obtaining the pure secondary field of the small loop transmitting current based on the recorded triangular and trapezoidal wave transmitting current waveforms and the receiving coil parameters is as follows: (3) Where F S F is the magnetic flux of the small-loop transmitting coil in the central region. B G represents the magnetic flux of the small-loop compensation coil in the central region, and G is the coefficient of the residual primary field. In step 4), in order to interpret the electromagnetic response of the large loop, a deconvolution method based on SVD and e-exponential fitting is used to convert the pure secondary field response of different emission current waveforms into a step wave response. Since the impulse response and step response of the system have a calculus-integral relationship, let the impulse magnetic field response and step magnetic field response of the transient electromagnetic system be H(t) and Bs(t), respectively, and I(t) be the time-domain form of the emitted current. According to the convolution theorem and frequency-time transform, we can obtain H(t) = -dBs(t) / dt. The convolution formula for the time-domain electromagnetic response under different emitted current waveform excitation is: (4) Since the induced electromotive force decays approximately exponentially (e), the decay curve is decomposed into a series of exponential sums (e), assuming the induced electromotive force is... The sampling time interval is The sampling time is j, and j=m. n basis functions are selected, therefore... In discrete time, it can be decomposed into: (5) Expanding equation (5) into matrix form yields: (6) in This is called the kernel matrix, and its n column vectors are called odd functions. This represents the amplitude corresponding to the i-th time constant and the j-th sampling time. These are the decomposition coefficients multiplied by the i-th basis function; the electromagnetic data is decomposed into components that establish the relationship between the sampling time t and the time constant. Simultaneously, the data reconstruction was completed through the multiplication of related matrices. Quality and time constant of data reconstruction The selection of the exponential basis functions is relevant, therefore it is necessary to decompose and analyze the pre-selected exponential basis functions based on the SVD method, and then convert the matrix... Perform SVD decomposition and inverse it: (7) Where U is an m×m matrix, V is an n×n matrix, and V T It is the conjugate transpose of V. Let m×n be a positive semi-definite diagonal matrix, and let its diagonal values ​​be... It is either positive or zero; if As the value of approaches zero, then Matrices exhibit singularity, and their decomposition is unstable; therefore, to ensure efficient matrix decomposition, a limit value k is set, if... If it is less than the limit value, then Set to zero and re-perform the matrix. Decomposition: (8) Where P is reset The number of non-zero values ​​in the data. For the first P columns of V, For the first P columns of U, It is a P×P order nonzero diagonal matrix; Therefore, equation (6) can be solved to obtain the decomposed coefficient vector A and the refitted electromagnetic data. : (9) By adjusting the limit value k and The distribution values ​​ensure that the relative error meets the accuracy requirement of less than 10%. The expression for the relative error before and after fitting is as follows: (10) To obtain the magnetic field response under step wave excitation, an integral method is used: (11) Substituting equation (11) into equation (4), the induced electromotive force of the pulse current can be obtained according to the convolution commutative law. : (12) in , indicating that there are n basis units. , indicating that there are m time channels; The second derivative of the emission current and the original basis Convolution yields a new basis. By using the e-exponential fitting method, the induced electromotive force under different emission current waveforms can be fitted according to a new basis to obtain the decomposition coefficients. The decomposed coefficients are then recombinated with the original substrate to obtain the step wave magnetic field. Then, by taking the derivative, the step wave induced electromotive force can be obtained. The deconvolution operation is completed; this process recovers the step wave excitation response from the excitation responses of different emission current waveforms, and completes the normalization of the multi-waveform excitation response, which is a key step in the interpretation of multi-waveform emission current data. In step 4), to interpret the electromagnetic response of the small loop, the triangular wave and trapezoidal wave electromagnetic responses need to be spliced ​​together. The early response of the spliced ​​data uses the triangular wave response, and the late response uses the trapezoidal wave response. Because the amplitude of the triangular wave emission current is smaller and decays faster, the trapezoidal wave response is larger than the triangular wave response at the splicing moment. Therefore, the trapezoidal wave response needs to be normalized to the triangular wave response. The normalization process is as follows: (13) in To splice the response, It is a triangular wave response. The waveform is a trapezoidal wave response, where T is the normalization factor of the trapezoidal wave response. This is the starting time for data splicing. The width of the triangular wave pulse. The trapezoidal wave turn-off time; the early signal of the spliced ​​response is rich in shallow surface information, the late effective signal has a long decay time, the signal-to-noise ratio is higher, and the quality of the secondary field response is significantly improved; In step 5), baseline correction, superposition, filtering, and channel preprocessing are performed on the restored step wave response and spliced ​​response based on the data interpretation method. Multi-waveform joint resistivity imaging is performed on the processed induced electromotive force signal to interpret the information of underground target bodies at different depths. This completes the data processing for the fine detection of magnetic source multi-waveform combination, effectively improving detection sensitivity and accuracy.

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