A ground-to-air three-component observation method based on very low frequency electromagnetic method

Abstract

The present application relates to a kind of ground-to-air three-component observation methods based on very low frequency electromagnetic method, based on the existing very low frequency transmitting station of ground, establish cross stratosphere very low frequency antenna transmission-reception model, deduce very low frequency electromagnetic wave calculation expression, carry out ground transmission, air receiving numerical simulation, analyze three-component magnetic field propagation characteristics;According to the frequency range of very low frequency transmitting station, determine the receiving frequency point, according to the propagation characteristics, determine the sensitivity requirement, design the very low frequency ground-to-air three-component receiving coil sensor;Based on unmanned aerial vehicle, receiving coil and receiver are used to receive, collect and store field test data;The measured data is preprocessed, and three-component signal correction is carried out according to flight attitude;Underground structure and target body are accurately imaged by using K-H filtering method and Euler deconvolution method.The purpose of the present application is to analyze the propagation characteristics of very low frequency electromagnetic wave, design the very low frequency ground-to-air three-component receiving coil sensor, and realize high-precision detection of underground shallow target body.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of geophysical exploration, and particularly to a ground-air three-component observation method based on very low frequency electromagnetic method. BACKGROUND

[0002] The very low frequency electromagnetic method is a geophysical exploration method that has developed rapidly in recent years. This method uses a ground navigation radio transmitting station as a source to image underground structures by measuring the changes in signal strength and phase at the receiving point. Ground-air receiving is a receiving method proposed to overcome the shortcomings of ground receiving and air receiving. By mounting the receiving system on a UAV, the detection efficiency can be effectively improved, and the limitations of transmitting power and the influence of complex terrain caused by the weight of the helicopter can be avoided. Since the UAV will introduce noise interference during flight, the three-component receiving method can effectively improve the signal-to-noise ratio and correct the errors of the received signal. Therefore, the study of the ground-air three-component observation method based on the very low frequency electromagnetic method can provide a new technical means for the research of frontier scientific and technological fields such as geophysical exploration, and has important scientific and technological value and practical use value.

[0003] Currently, there are many related research results in the field of ground-air observation of the very low frequency electromagnetic method. In 2007, Pedersen et al. proposed a new type of tensor very low frequency technology and applied it to the airborne very low frequency data collected by the Swedish Geological Survey. However, this method only uses the dip angle as a parameter for data correction, resulting in some errors in the corrected data. In 2024, Ren Boyu et al. proposed a numerical analysis method for the near-field electric field of a very low frequency antenna based on the charge simulation method. This method can calculate the electromagnetic field distribution around the very low frequency transmitting antenna, but does not consider the influence of underground structures. In 2021, Sattel et al. proposed a method for processing interference noise in airborne very low frequency data, which has certain reference value. Currently, related research results at home and abroad mainly focus on airborne electromagnetic detection methods, and there are few studies on ground-air data processing.

[0004] CN202311865738.X discloses a relay transmission method, device, equipment and medium based on very low frequency electromagnetic waves. This method is used for communication transmission underground and on the ground, and has certain reference value, but there are still great differences from the ground-air method.

[0005] CN202211381091.9 discloses a quadrature multichannel very low frequency omnidirectional synthetic magnetic antenna receiver and a signal processing method thereof. The receiver has high sensitivity and strong anti-interference ability, and can increase the effective communication distance. However, this method is only suitable for single-component or vertical cross antenna reception, and cannot be used for three-component reception, which still needs to be improved. SUMMARY

[0006] The present application aims at high-precision detection of underground shallow target bodies which is difficult to achieve by existing electromagnetic detection methods, establishes a cross-sphere very low frequency antenna transmission-reception model according to existing ground very low frequency transmission stations, designs a ground-air three-component receiving coil sensor, and provides a ground-air three-component observation method based on very low frequency electromagnetic method.

[0007] The present application is implemented by a ground-air three-component observation method based on very low frequency electromagnetic method, comprising the following steps:

[0008] 1) Establishing a cross-sphere very low frequency antenna transmission-reception model, equivalent to a vertical electric dipole for a ground very low frequency station transmission antenna, and after the reflection of very low frequency electromagnetic waves through the underground, air and ionosphere layers, the electromagnetic waves reach a receiving point, the very low frequency electromagnetic waves are calculated according to the magnetic field of the receiving point, the propagation characteristics of the variation law of three-component magnetic field values are obtained by calculating the magnetic field values of electromagnetic waves at different frequencies and the magnetic field values of electromagnetic waves with the propagation distance, including the magnetic field value curve form, the magnetic field value change process and the magnetic field value size;

[0009] 2) Determining the receiving frequency point and the tuning frequency of the receiving coil according to the frequency range of the very low frequency transmission station, and determining the sensitivity of the receiving coil according to the very low frequency signal three-component magnetic field propagation characteristics obtained in step 1);

[0010] 3) Determining the design parameters of the receiving coil according to the receiving frequency point, the tuning frequency and the sensitivity of the receiving coil in step 2), the tuning frequency f a For Wherein R represents the resistance of the receiving coil, L represents the inductance of the receiving coil; the bandwidth of the receiving coil is determined by the center frequency f0, Wherein C represents the capacitance of the receiving coil; the sensitivity S a For Wherein k represents the Boltzmann constant, T represents the temperature in Kelvin, ω represents the angular frequency, N represents the number of turns of the coil, and A represents the area of the coil; the standard very low frequency receiving coil selects the resistance of 1 ohm and the inductance of 1 millihenry;

[0011] 4) Receiving, collecting and storing the measured data collected by the receiving coil by using the receiver;

[0012] 5) Preprocessing the measured data obtained in step 4), and correcting the three-component signals according to the flight attitude of the unmanned aerial vehicle;

[0013] 6) Imaging the underground structure current density by using the K-H filtering method and the Euler deconvolution method in combination for the corrected data in step 5), and delineating and identifying the underground shallow target bodies according to the results.

[0014] Further, the very low frequency antenna transmitting-receiving model established in step 1) equivalently takes the very low frequency transmitting antenna as a vertical electric dipole, and the current density expression thereof is as follows:

[0015] J z = Idlδ(x)δ(y)δ(z-d) (1)

[0016] In formula (1), J z represents the current density of the vertical electric dipole, I represents the transmitting current, dl represents the vertical antenna height, x, y and z represent the positions of the source, δ(x) and δ(y) represent the impulse functions at x and y points respectively, and δ(z-d) represents the impulse function at z=d point;

[0017] The magnetic field expression of the receiving point is as follows:

[0018]

[0019] In formula (2), B φ represents the magnetic field component in the φ direction in the cylindrical coordinate system, C 1c represents the ionosphere forward coefficient, C 1d represents the ionosphere reverse coefficient, C 0f represents the full-space correlation coefficient, ρ represents the distance from the transmitting to the receiving, and λ represents the integral variable.

[0020] Further, the three-component signal is corrected by using the direction cosine method in step 5), and it is assumed that the angles of rotation of the carrier coordinate system are roll angle α, pitch angle β and yaw angle ψ in sequence, and the rotation matrix R m and the magnetic field B in the geographic coordinate system are as follows:

[0021]

[0022]

[0023] In formula (4), B a represents the magnetic field measured in the air by the three-component coil sensor, and T represents the transpose. According to the direction cosine method, the complete coincidence of the two coordinate systems is realized by three rotations, the rotation matrix and the measured data are used to correct the magnetic field in the geographic coordinate system, and the data preprocessing is realized.

[0024] Further, the calculation expressions of the K-H filtering method and the Euler deconvolution method in step 6) are as follows:

[0025]

[0026]

[0027] Formula (5) is the calculation expression of the K-H filtering method, wherein, Δz represents the measurement calculation region, Ia represents the current density of the calculation point, and Δx represents the distance between data points, H -3 H3 is the data value measured by the continuous 6 points, the calculated current density value is located at the center of the 6 points used in the calculation, the depth is equal to Δx, and the position and depth of the conductive body are inferred according to formula (5);

[0028] Formula (6) is a calculation expression of Euler deconvolution method, wherein, x a , y a , z a represents the position of the calculation point, f is the real component of I a calculated by formula (5), B b is the value of the regional background field, and N represents a structure index for describing the geometric shape of the anomaly source. The method can be used to infer the depth and geometric shape information of the target. By combining the two methods, the detection of the underground shallow target body can be more accurately realized.

[0029] Compared with the prior art, the present application has the beneficial effects that: in view of the difficulty of high-precision detection of underground shallow target bodies by the existing electromagnetic detection method, a cross-layer very low frequency antenna transmission-reception model is established based on the existing ground very low frequency transmission station, the very low frequency electromagnetic wave propagation characteristics are analyzed, a ground-air three-component receiving coil sensor is designed, a ground-air three-component observation method based on the very low frequency electromagnetic method is provided, and high-precision detection of underground shallow target bodies is realized. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is a flowchart of the ground-air three-component observation method based on the very low frequency electromagnetic method according to an embodiment of the present application;

[0031] Figure 2 is a very low frequency ground-air ground-air three-component transmission-reception schematic diagram provided by an embodiment of the present application;

[0032] Figure 3 is a three-component magnetic field sensor structure schematic diagram provided by an embodiment of the present application;

[0033] Figure 4 is a receiver hardware circuit structure block diagram provided by an embodiment of the present application;

[0034] Figure 5 is a posture correction flowchart of an embodiment of the present application. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0036] EMBODIMENT

[0037] A ground-to-air three-component observation method based on very low frequency electromagnetic method, see Figure 1 shown, comprising:

[0038] 1) According to the very low frequency ground-to-air ground-to-air three-component transmission-reception schematic diagram as Figure 2 shown, a cross-stratosphere very low frequency antenna transmission-reception model is established, the ground very low frequency station transmission antenna is equivalent to a vertical electric dipole, the very low frequency electromagnetic wave reaches the receiving point after reflection through the underground, air, ionosphere and other layers, the very low frequency electromagnetic wave is calculated according to the expression, and the propagation characteristics of the three-component magnetic field are analyzed;

[0039] Among them, the very low frequency transmission antenna is equivalent to a vertical electric dipole, and the current density expression is:

[0040] J z = Idlδ(x)δ(y)δ(z-d)(1)

[0041] Among them, the magnetic field expression of the receiving point is:

[0042]

[0043] In formula (1), J z represents the current density of the vertical electric dipole, I represents the transmission current, dl represents the vertical antenna height, x, y, z represent the position of the source, δ(x) and δ(y) represent the impulse functions located at x and y points respectively, and δ(z-d) represents the impulse function located at z=d point;

[0044] In formula (2), B φ represents the magnetic field component in the φ direction of the cylindrical coordinate system, C 1c represents the ionospheric forward coefficient, C 1d represents the ionospheric reverse coefficient, C 0f represents the full-space correlation coefficient, ρ represents the distance from transmission to reception, and λ represents the integral variable.

[0045] 2) Determine the design index of the receiving coil, determine the receiving frequency point and the tuning frequency of the receiving coil according to the frequency range of the very low frequency transmission station, and the frequency points of the ground very low frequency station currently include 11.9 kilohertz, 12.65 kilohertz, 17.8 kilohertz, 22.2 kilohertz, etc., and the tuning frequency is set to 5 kilohertz; At the same time, according to the three-component magnetic field propagation characteristics of the very low frequency signal calculated in step 1), the sensitivity of the receiving coil is determined;

[0046] 3) Determine the design parameters of the receiving coil according to the design index of step 2). The tuning frequency f a of the receiving coil is The bandwidth of the receiving coil is determined by the center frequency f0, Sensitivity S of the receiving coil a for A standard very low frequency (VLF) coil typically uses a resistor of 1 ohm and an inductance of 1 millihenry; the structural design of the receiving coil is as follows: Figure 3 As shown, the three component coils are arranged perpendicular to each other.

[0047] 4) A receiver is used to receive, acquire, and store the data collected through the receiving coil. The hardware circuit block diagram of the receiver is shown below. Figure 4 As shown; it includes, in sequence, a front-end conditioning circuit, a lock-in amplifier circuit, a chopper modulation circuit, an AD acquisition circuit, and a magnetic coupling isolation circuit, wherein the magnetic coupling isolation circuit is connected via I... 2 The S-interface is connected to the FPGA control circuit, the FPGA control circuit is connected to the ARM storage circuit, the ARM storage circuit is connected to the SD card and keyboard, and the ARM storage circuit includes the ARM storage control circuit, the ARM storage circuit, the magnetic coupling isolation circuit and the lock-in amplifier circuit are synchronized by a temperature-controlled crystal oscillator and GPS.

[0048] 5) Preprocess the measured data obtained in step 4), and perform three-component signal correction based on the UAV's flight attitude. The flowchart for attitude correction is as follows: Figure 5 As shown, it includes: extracting magnetic field information, extracting three-axis attitude, calculating the full rotation matrix based on the magnetic field information and three-axis attitude, storing the corrected magnetic field information, and completing the three-component signal correction process by iterating through all frequency points.

[0049] The three-component signal is corrected using the direction cosine method. Assuming the rotation angles of the carrier coordinate system are, in order, roll angle α, pitch angle β, and yaw angle ψ, then its rotation matrix R... m The magnetic field B in the geographic coordinate system is:

[0050]

[0051]

[0052] According to the direction cosine method, the two coordinate systems can be completely overlapped by three rotations. The magnetic field under the geographic coordinate system can be corrected by using the rotation matrix and the measured data, thus realizing data preprocessing.

[0053] 6) The corrected data from step 5) is used to perform underground structural current density imaging by combining the KH filtering method and the Euler deconvolution method, and the shallow underground target bodies are delineated and identified based on the results.

[0054] The calculation expressions using the KH method and the Euler deconvolution method are as follows:

[0055]

[0056]

[0057] Formula (5) is a K-H method calculation expression, wherein Δz represents a measurement calculation area, I a represents a current density of a calculation point, Δx represents a distance between data points, H -3 to H3 are data values measured by 6 continuous points, a calculated current density value is located at the center of the 6 points used in calculation, the depth is equal to Δx, and the position and depth of the conductive body are inferred according to formula (5); the method can be used to infer the position and depth of the conductive body. Formula (6) is an Euler deconvolution method calculation expression, wherein x a , y a , z a represent the position of a calculation point, f is a real component of I a calculated by formula (5), B b is a value of a regional background field, and N represents a structure index used to describe the geometric shape of an anomaly source. The method can be used to infer the information of the depth and geometric shape of a target. Two methods are combined, and the detection of a shallow underground target body can be more accurately realized.

[0058] The above merely describes preferred embodiments of the present application but should not be used to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A three-component ground-to-air observation method based on very low frequency electromagnetic methods, characterized in that, Includes the following steps: 1) Establish a cross-sphere very low frequency antenna transmit-receive model. The ground-based very low frequency station transmitting antenna is equivalent to a vertical electric dipole. The very low frequency electromagnetic wave reaches the receiving point after being reflected by the underground, air and ionosphere. The very low frequency electromagnetic wave is calculated based on the magnetic field of the receiving point. By calculating the electromagnetic waves of different frequencies and the change of the magnetic field value of the electromagnetic wave with the propagation distance, the propagation characteristics of the three-component magnetic field value variation law are obtained, including the shape of the magnetic field value curve, the process of magnetic field value change, and the magnitude of the magnetic field value. 2) Determine the receiving frequency and tuning frequency of the receiving coil based on the frequency band range of the very low frequency transmitting station, and determine the sensitivity of the receiving coil based on the three-component magnetic field propagation characteristics of the very low frequency signal obtained in step 1). 3) Based on the receiving frequency, tuning frequency, and sensitivity of the receiving coil in step 2), determine the design parameters of the receiving coil, including the tuning frequency f of the receiving coil. a for Where R represents the resistance of the receiving coil, and L represents the inductance of the receiving coil; the bandwidth of the receiving coil is determined by the center frequency f0. Where C represents the capacitance of the receiving coil; the sensitivity S of the receiving coil a for Where k represents Boltzmann's constant, T represents temperature in Kelvin, ω represents angular frequency, N represents the number of coil turns, and A represents the coil area; the standard very low frequency receiving coil is selected with a resistance of 1 ohm and an inductance of 1 millihenry. 4) A receiver is used to receive, collect, and store the measured data acquired by the receiving coil; 5) Preprocess the measured data obtained in step 4) and perform three-component signal correction based on the UAV's flight attitude; 6) The corrected data from step 5) is used to perform underground structural current density imaging by combining the KH filtering method and the Euler deconvolution method, and the shallow underground target bodies are delineated and identified based on the results.

2. The ground-to-air three-component observation method based on very low frequency electromagnetic method according to claim 1, characterized in that, Step 1) establishes a very low frequency (VLF) antenna transmit-receive model, equating the VLF transmitting antenna to a vertical electric dipole, whose current density expression is: J z =Idlδ(x)δ(y)δ(zd) (1) In equation (1), J z Let represent the vertical electric dipole current density, I represent the transmitted current, dl represent the vertical antenna height, x, y, z represent the source position, δ(x) and δ(y) represent the impulse functions at points x and y, respectively, and δ(zd) represent the impulse function at point z = d. The magnetic field expression of the receiving point is: In equation (2), B φ C represents the magnetic field component in the φ direction in cylindrical coordinates. 1c C represents the positive ionospheric coefficient. 1d C represents the ionospheric reversal coefficient. 0f Let λ represent the total spatial correlation coefficient, ρ represent the distance from transmitter to receiver, and λ represent the integral variable.

3. The ground-to-air three-component observation method based on very low frequency electromagnetic method according to claim 1, characterized in that, In step 5), the three-component signals are corrected using the direction cosine method. Assuming the angles of rotation of the carrier coordinate system are, in order, roll angle α, pitch angle β, and yaw angle ψ, then its rotation matrix R m The magnetic field B in the geographic coordinate system is: In equation (4), B a This represents the magnetic field measured by the three-component coil sensor in the air. T represents transpose. According to the direction cosine method, the two coordinate systems can be completely coincident through three rotations. The magnetic field in the geographic coordinate system is corrected by the rotation matrix and the measured data to achieve data preprocessing.

4. The ground-to-air three-component observation method based on very low frequency electromagnetic method according to claim 1, characterized in that, The calculation expressions for step 6) using the KH filtering method and the Euler deconvolution method are as follows: Equation (5) is the calculation expression for the KH filtering method, where, Δz represents the measurement and calculation area, I a H represents the current density at the calculation point, Δx represents the distance between data points, and H represents the distance between the data points. -3 H3 is the data value measured at 6 consecutive points. The calculated current density value is located at the center of the 6 points used in the calculation, and the depth is equal to Δx. The position and depth of the conductor are inferred according to Equation (5). Equation (6) is the calculation expression for the Euler deconvolution method, where x a y a z a This indicates the location of the calculation point, and f is the I calculated by equation (5). a The real component, B b It is the value of the regional background field, and N represents the structure index, which is used to describe the geometry of the anomaly source.

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