Ground-air transient electromagnetic airborne receiving coil motion noise extraction method

Abstract

The invention is suitable for the technical field of geophysical exploration, and provides a ground-air transient electromagnetic airborne receiving coil motion noise extraction method, which comprises the following steps: designing a receiving device carrying an IMU (Inertial Measurement Unit) sensor, the receiving device comprising a receiving coil, the IMU sensor and a synchronous clock; measuring a vector geomagnetic field in a transient electromagnetic detection area based on attitude angle data of the receiving device; and extracting the motion noise of the airborne transient electromagnetic coil based on the vector geomagnetic field and the attitude angle data. The invention designs a ground-air transient electromagnetic receiving device, and provides a method for extracting motion noise of an airborne receiving coil based on the ground-air transient electromagnetic receiving device, so that the problem that the motion noise is generated due to the fact that the coil cuts a geomagnetic field is effectively solved. Compared with a mode of directly separating the motion noise from the induced electromotive force signal, the method provided by the invention can directly extract the motion noise from the generation mechanism level of the motion noise, and effectively avoids the problem of incomplete noise extraction or target signal loss.

Description

Technical Field

[0001] The invention belongs to the technical field of geophysical exploration, and in particular relates to a method for extracting motion noise of a ground-to-air transient electromagnetic airborne receiving coil. Background Art

[0002] Transient electromagnetic method is a geophysical exploration method. Due to its sensitivity to low-resistance geological bodies, it is widely used in mineral exploration and water-based natural disaster exploration. According to the location of the transceiver, transient electromagnetic method can be divided into ground transient electromagnetic, airborne transient electromagnetic and ground-to-air transient electromagnetic. Among them, the ground-to-air transient electromagnetic method adopts the form of ground transmission and air reception, which has the advantage of high exploration efficiency. However, in practical applications, when the drone carries a receiving coil for exploration, the coil will swing due to factors such as wind resistance, inertia and changes in the flight state of the drone, and then periodically cut the geomagnetic field, generating additional induced electromotive force. This additional induced electromotive force will be superimposed on the target signal to become noise, which is called motion noise. The presence of motion noise will reduce the signal-to-noise ratio of the signal and have an adverse effect on subsequent data interpretation. Therefore, the extraction and elimination of motion noise is an important step in the processing of exploration data using an airborne transient electromagnetic receiving system.

[0003] At present, the methods for processing motion noise mainly rely on direct processing of the induced electromotive force signal of the receiving coil, such as using linear interpolation, wavelet transform and other algorithms. These algorithms attempt to use the component differences between motion noise and target signal to separate the two to improve the signal-to-noise ratio. However, these methods have a common limitation: they are only based on a single source of induced electromotive force data for processing, and it is often difficult to fully and accurately extract motion noise, and sometimes even part of the target signal may be mistakenly eliminated. For this reason, the present invention proposes a method for extracting motion noise from a ground-to-air transient electromagnetic airborne receiving coil. Summary of the invention

[0004] The purpose of the present invention is to provide a method for extracting motion noise of ground-to-air transient electromagnetic airborne receiving coils, aiming to solve the problems raised in the above background technology.

[0005] The purpose of the present invention is achieved through the following technical solutions: A method for extracting motion noise of a ground-to-air transient electromagnetic airborne receiving coil comprises the following steps: Step 1: Design of receiving device equipped with IMU sensor; Design a receiving device, which includes a receiving coil, an IMU sensor and a synchronous clock; the IMU sensor has a built-in extended Kalman filter algorithm; the IMU sensor includes a magnetic sensor, an accelerometer and a gyroscope, and the IMU sensor obtains nine-component data including magnetic field, acceleration and angular velocity through the magnetic sensor, the accelerometer and the gyroscope; the IMU sensor is rigidly connected to the receiving coil, the z direction is parallel to the normal direction of the receiving coil, and the x direction and the y direction are both in the same plane as the receiving coil; Step 2: Vector geomagnetic field measurement within the transient electromagnetic detection area based on the attitude angle data of the receiving device; The information obtained by the IMU sensor is combined with the extended Kalman filter algorithm to solve the real-time attitude angle of the receiving device, including roll angle, pitch angle and heading angle; the size and direction of the total magnetic field in the detection area are calculated through the attitude angle and the three-component magnetic field data obtained by the magnetic sensor; Step 3: Extraction of airborne transient electromagnetic coil motion noise based on vector geomagnetic field and attitude angle data; Using real-time geomagnetic field vector information and attitude angle data, the induced electromotive force, i.e., motion noise, generated by the swinging of the receiving coil cutting the geomagnetic field is calculated based on the relationship between the receiving coil's magnetic flux change rate, the magnetic field change rate and the induced electromotive force. The motion noise can be eliminated by subtracting the induced electromotive force received by the receiving coil from the calculated motion noise.

[0006] Furthermore, the specific process of step 2 is as follows: The information obtained by the IMU sensor is combined with the extended Kalman filter algorithm to solve the real-time attitude angle of the receiving device, including roll angle, pitch angle and heading angle; The data is corrected based on the attitude angle information, and the formula is: Formula 1: ; in, is the data in the coordinate system of the receiving device, is the data in the reference coordinate system, is the correction matrix based on attitude angle information; It is expressed as: Formula 2: ; in, Respectively represent the roll angle, pitch angle and heading angle of the receiving device; The angle between the normal to the receiving coil plane and the Earth's magnetic field is expressed as: Formula 3: ; in, It represents the angle between the normal of the receiving coil plane and the Earth's magnetic field; The normal vector of the receiving coil plane in the receiving device coordinate system is represented by the unit vector in the z direction in the reference coordinate system. Substituting into equation 1, we get; =( , , ) is the geomagnetic field data in the reference coordinate system, and the geomagnetic field data in the receiving device coordinate system obtained by the magnetic sensor is substituted into Formula 1 to obtain: , , They represent the geomagnetic field in the x direction, the geomagnetic field in the y direction and the geomagnetic field in the z direction respectively.

[0007] Furthermore, the specific process of step 3 is as follows: According to Faraday's law of electromagnetic induction, the electromagnetic response received by the receiving coil is expressed as: Formula 4: ; in, is the induced electromotive force received by the receiving coil; is the magnetic flux of the receiving coil; is the time, obtained by the synchronized clock; is the actual receiving area of ​​the receiving coil; is the magnetic field; magnetic field By the Earth's magnetic field and target signal magnetic field Composition, then formula 4 is written as: Formula 5: ; The simplified formula 5 is: Formula 6: ; in, The induced electromotive force generated by the swing of the receiving coil cutting the earth's magnetic field is motion noise; The target signal induced by the receiving coil; The actual receiving area of ​​the receiving coil is expressed as: Formula 7: ; in, is the area of ​​the receiving coil; is the angle between the normal of the receiving coil plane and the Earth’s magnetic field; Motion noise It is expressed as: Formula 8: ; The motion noise is obtained by equation 8: , and then put it into formula 6 to calculate the target signal .

[0008] Compared with the prior art, the present invention has the following beneficial effects: The present invention designs a ground-to-air transient electromagnetic receiving device, and based on this device proposes a method for extracting motion noise of an airborne receiving coil, which effectively solves the problem of motion noise generated by the coil cutting the geomagnetic field. Compared with the method of directly separating motion noise from the induced electromotive force signal, the method proposed in the present invention can directly extract motion noise from the generation mechanism level, effectively avoiding the problem of incomplete noise extraction or loss of target signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 The present invention is a flow chart of the method.

[0010] Figure 2 is a schematic diagram of a receiving device; (a) is a structural diagram of the receiving device, and (b) is Figure 2 Part (a) shows the front view, left view and top view of the receiving device.

[0011] Figure 3 This is a schematic diagram of the working mode of the receiving device.

[0012] Figure 4 Schematic diagram of the direction of the Earth's magnetic field, the normal direction of the receiving coil plane, and various angles.

[0013] Figure 5 is the attitude angle data of the receiving device; where (a) is the roll angle ; (b) is the pitch angle ; (c) is the heading angle .

[0014] Figure 6 is the geomagnetic field data of the detection area; where (a) is x Direction of the Earth's magnetic field ; (b) is the geomagnetic field in the y direction ; (c) is the geomagnetic field in the z direction .

[0015] Figure 7 is the induced electromotive force received by the receiving coil.

[0016] Figure 8 This is the motion noise extracted by the method of the present invention.

[0017] Fig. 9 This is the target signal after the motion noise is extracted by the method of the present invention. DETAILED DESCRIPTION

[0018] In order to have a clearer understanding of the technical features, purposes and beneficial effects of the present invention, the technical solution of the present invention is now described in detail below, but it should not be construed as limiting the applicable scope of the present invention.

[0019] The specific implementation of the present invention is described in detail below in conjunction with specific embodiments.

[0020] When a drone carries a receiving device for aerial detection, it is assumed that the receiving coil remains stable without swinging, and that the magnitude and direction of the geomagnetic field remain unchanged within a signal cycle. At this time, the magnetic flux of the receiving coil relative to the geomagnetic field remains unchanged, and the received induced electromotive force all comes from the target signal. However, in actual situations, the receiving coil will not maintain a relatively static posture, but will swing at a non-fixed frequency. During the swinging process, the magnetic flux of the receiving coil relative to the geomagnetic field will change, thereby generating additional induced electromotive force outside the target signal, which is motion noise. This swinging mechanism is complex, and the frequency components are rich and difficult to quantify. Therefore, the present invention proposes a method for extracting motion noise from ground-to-air transient electromagnetic airborne receiving coils, and its flow chart is shown as follows: Figure 1 As shown, the method comprises the following steps: Step 1: Design of receiving device equipped with IMU sensor; The receiving coil is the most commonly used receiving device in the transient electromagnetic method. It obtains the rate of change of the magnetic field in the space where it is located through electromagnetic induction and converts it into an induced electromotive force. Since it collects the rate of change of the magnetic field rather than the magnetic field itself, it can avoid interference from the direct current of the geomagnetic field when it is in a static state. However, due to the influence of the swing, the magnetic flux of the receiving coil relative to the geomagnetic field will change with the angle of the coil swing, which will introduce additional induced electromotive force, that is, motion noise. The purpose of the present invention is to eliminate this induced electromotive force. The receiving device of the present invention is as follows Figure 2 As shown, its working method is as follows Figure 3 The receiving device includes three parts: receiving coil, IMU sensor and synchronous clock; the IMU sensor has a built-in extended Kalman filter algorithm, and the IMU sensor is rigidly connected to the receiving coil, the z direction is parallel to the normal of the receiving coil plane, and the x direction and y direction are in the same plane as the receiving coil.

[0021] IMU sensor, that is, inertial navigation sensor. The IMU sensor carried by the present invention includes a three-axis magnetic sensor, a three-axis accelerometer and a three-axis gyroscope, which can obtain nine-component data including three-component magnetic field, three-component acceleration and three-component angular velocity. Based on these data, the attitude angle of the receiving device is calculated in real time and the geomagnetic field information of the detection area is obtained.

[0022] Synchronous clock, used to record the receiving time information and to synchronize the data of the receiving coil and IMU sensor.

[0023] Step 2: Vector geomagnetic field measurement within the transient electromagnetic detection area based on the attitude angle data of the receiving device; Based on the nine-component data obtained by the IMU sensor, the attitude angle (including roll angle, pitch angle and heading angle) of the receiving device is calculated using the extended Kalman filter algorithm built into the IMU sensor.

[0024] The quaternion attitude solution algorithm based on Kalman filter adopts the extended Kalman method, which takes the quaternion as the state quantity and the measurements of the accelerometer and magnetic sensor as the observation quantity; the attitude quaternion is estimated by the data measured by the gyroscope, and the quaternion estimate is corrected by the data measured by the accelerometer and magnetic sensor.

[0025] The state quantity is composed of attitude quaternion and three-axis gyroscope output : Formula 1: ; in, is a quaternion containing attitude information, are the real part, the first imaginary component, the second imaginary component and the third imaginary component of the quaternion respectively; is the output of the three-axis gyroscope, They are x , y , z Angular velocity in direction.

[0026] The conversion relationship between quaternion and attitude angle is as follows: Formula 2: ; in, They represent the roll angle, pitch angle and heading angle of the receiving device respectively.

[0027] The sampling period of the system is , using the Quaternion of time and gyroscope output Can be Quaternion of time Make predictions: Formula 3: ; in, ; in, is the matrix containing the angular velocity; The state prediction equation can be derived from formula 3: Formula 4: ; in, The predicted amount for the current moment; is the prior information of the previous moment; is the system noise, is the identity matrix. The predicted error covariance It can also be calculated from the value of the previous moment: Formula 5: ; in, is the covariance of the previous moment; is the covariance matrix of the system noise; is from Time has come The state transfer matrix at time: Formula 6: ; By predicting the angular velocity and attitude information based on equation 4, and correcting the system attitude information based on equations 5 and 6 as well as the magnetic sensor data and accelerometer data, real-time calculation of the system attitude angle data can be achieved.

[0028] Since the IMU sensor is rigidly connected to the receiving coil, the magnetic sensor also swings when the receiving coil swings. Therefore, the direction of the acquired three-component magnetic field data will be offset to a certain extent with the swing. The direction and magnitude of the total geomagnetic field can be obtained by combining the three-component magnetic field data with the attitude angle.

[0029] The formula for correcting the data based on the attitude angle information is: Formula 7: ; in, is the data in the coordinate system of the receiving device, is the data in the reference coordinate system, is the correction matrix based on the attitude angle information. It can be expressed as: Formula 8: ; in, They represent the roll angle, pitch angle and heading angle of the receiving device respectively.

[0030] The angle between the normal of the receiving coil plane and the Earth's magnetic field can be expressed as: Formula 9: ; in, It represents the angle between the normal of the receiving coil plane and the Earth's magnetic field; The normal vector of the receiving coil plane in the receiving device coordinate system can be obtained by the unit vector in the z direction in the reference coordinate system: Substituting into equation 7, we get: =( , , ) is the geomagnetic field data in the reference coordinate system, which can be obtained by substituting the geomagnetic field data in the receiving device coordinate system obtained by the magnetic sensor into Formula 7, where , , They represent the geomagnetic field in the x direction, the geomagnetic field in the y direction and the geomagnetic field in the z direction respectively.

[0031] The direction of the geomagnetic field, the normal direction of the receiving coil plane, and schematic diagrams of various angles can be seen Figure 4 .

[0032] Step 3: Extraction of airborne transient electromagnetic coil motion noise based on vector geomagnetic field and attitude angle data; First, the mathematical relationship between motion noise and target signal is derived. According to Faraday's law of electromagnetic induction, the electromagnetic response received by the receiving coil can be expressed as: Formula 10: ; in, is the induced electromotive force received by the receiving coil; is the magnetic flux of the receiving coil; is the time, obtained by the synchronized clock; is the actual receiving area of ​​the receiving coil; For the magnetic field.

[0033] magnetic field By the Earth's magnetic field and target signal magnetic field Therefore, formula 10 can be written as: Formula 11: ; Much greater than ,and Changes slowly. Therefore, Equation 11 can be simplified to: Formula 12: ; in, The induced electromotive force generated by the swing of the receiving coil cutting the earth's magnetic field is motion noise; It is the target signal induced by the receiving coil.

[0034] The actual receiving area of ​​the receiving coil can be expressed as: Formula 13: ; in, is the area of ​​the receiving coil; is the angle between the normal of the receiving coil plane and the Earth's magnetic field.

[0035] Furthermore, motion noise It can be expressed as: Formula 14: ; The motion noise is obtained by equation 14: , and then put it into formula 12 to get the target signal .

[0036] Embodiment 1: Attitude angle data and geomagnetic field data are obtained in actual airborne exploration. This embodiment uses the method proposed in the present invention to extract motion noise based on these two data.

[0037] In this embodiment, the area of ​​the receiving coil is set to 1m 2 , using the north-east coordinate system, x Direction is due north, y Direction is due east, z The direction is the ground direction.

[0038] like Figure 5 As shown in (a), (b) and (c), as the receiving device moves in the air, the three attitude angles of the receiving device (including the roll angle , Pitch angle and heading angle ) data are constantly changing, and the changing trends and frequencies are different. The changing trend of motion noise is the nonlinear superposition of these three attitude angles, so its performance is more complicated, and it is difficult for traditional methods to fully and accurately represent it. The present invention can obtain these attitude angle data more accurately by using IMU sensors for direct measurement, thus having significant advantages.

[0039] At the same time, if Figure 6 As shown in (a), (b) and (c), the geomagnetic field in the detection area (including the x-direction geomagnetic field , y-direction geomagnetic field and the z-direction geomagnetic field ) data also changes with time and space. Although the geomagnetic field can be considered to remain unchanged in a short period of time, the change of the geomagnetic field cannot be ignored when conducting long-term and large-scale exploration. Therefore, it is necessary to measure the geomagnetic field in the detection area in real time according to the method proposed by the present invention.

[0040] Figure 7 The induced electromotive force received by the receiving coil is shown, which contains a lot of low-frequency and large-amplitude motion noise. The amplitude of these motion noises is even greater than the target signal, which poses a great challenge to the subsequent inversion and imaging work. However, the motion noise extracted by the method of the present invention ( Figure 8 ) and the target signal after extracting motion noise ( Fig. 9 ) It can be seen that the motion noise that has the greatest impact on the signal is basically completely extracted. In the extracted target signal, only some fluctuations caused by the system background noise and environmental noise remain. This shows that the method proposed in the present invention can directly extract motion noise from the generation mechanism level, effectively avoiding the problem of incomplete noise extraction or loss of target signal.

[0041] The above are only preferred embodiments of the present invention. It should be pointed out that, for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present invention. These should also be regarded as the protection scope of the present invention. These will not affect the effect of the implementation of the present invention and the practicality of the patent.

Claims

1. A method for extracting motion noise from a ground-to-air transient electromagnetic airborne receiving coil, characterized in that: The following steps are involved: Step 1: Design of receiving device equipped with IMU sensor; Design a receiving device, which includes a receiving coil, an IMU sensor and a synchronous clock; the IMU sensor has a built-in extended Kalman filter algorithm; the IMU sensor includes a magnetic sensor, an accelerometer and a gyroscope, and the IMU sensor obtains nine-component data including magnetic field, acceleration and angular velocity through the magnetic sensor, the accelerometer and the gyroscope; the IMU sensor is rigidly connected to the receiving coil, the z direction is parallel to the normal direction of the receiving coil, and the x direction and the y direction are both in the same plane as the receiving coil; Step 2: Vector geomagnetic field measurement within the transient electromagnetic detection area based on the attitude angle data of the receiving device; The information obtained by the IMU sensor is combined with the extended Kalman filter algorithm to solve the real-time attitude angle of the receiving device, including roll angle, pitch angle and heading angle; the size and direction of the total magnetic field in the detection area are calculated through the attitude angle and the three-component magnetic field data obtained by the magnetic sensor; Step 3: Extraction of airborne transient electromagnetic coil motion noise based on vector geomagnetic field and attitude angle data; Using real-time geomagnetic field vector information and attitude angle data, the induced electromotive force, i.e., motion noise, generated by the swinging of the receiving coil cutting the geomagnetic field is calculated based on the relationship between the receiving coil's magnetic flux change rate, the magnetic field change rate and the induced electromotive force. The motion noise can be eliminated by subtracting the induced electromotive force received by the receiving coil from the calculated motion noise.

2. The method for extracting motion noise of ground-to-air transient electromagnetic airborne receiving coil according to claim 1 is characterized in that: The specific process of step 2 is as follows: The information obtained by the IMU sensor is combined with the extended Kalman filter algorithm to solve the real-time attitude angle of the receiving device, including roll angle, pitch angle and heading angle; The data is corrected based on the attitude angle information, and the formula is: Formula 1: ; in, is the data in the coordinate system of the receiving device, is the data in the reference coordinate system, is the correction matrix based on attitude angle information; It is expressed as: Formula 2: ； in, Respectively represent the roll angle, pitch angle and heading angle of the receiving device; The angle between the normal to the receiving coil plane and the Earth's magnetic field is expressed as: Formula 3: ; in, It represents the angle between the normal of the receiving coil plane and the Earth's magnetic field; The normal vector of the receiving coil plane in the receiving device coordinate system is represented by the unit vector in the z direction in the reference coordinate system. Substituting into equation 1, we get; =( , , ) is the geomagnetic field data in the reference coordinate system, and the geomagnetic field data in the receiving device coordinate system obtained by the magnetic sensor is substituted into Formula 1 to obtain: , , They represent the geomagnetic field in the x direction, the geomagnetic field in the y direction and the geomagnetic field in the z direction respectively.

3. The method for extracting motion noise of ground-to-air transient electromagnetic airborne receiving coil according to claim 1 is characterized in that: The specific process of step 3 is as follows: According to Faraday's law of electromagnetic induction, the electromagnetic response received by the receiving coil is expressed as: Formula 4: ; in, is the induced electromotive force received by the receiving coil; is the receiving coil magnetic flux; is the time, obtained by the synchronized clock; is the actual receiving area of ​​the receiving coil; is the magnetic field; magnetic field By the Earth's magnetic field and target signal magnetic field Composition, then formula 4 is written as: Formula 5: ; The simplified formula 5 is: Formula 6: ; in, The induced electromotive force generated by the swing of the receiving coil cutting the earth's magnetic field is motion noise; The target signal induced by the receiving coil; The actual receiving area of ​​the receiving coil is expressed as: Formula 7: ; in, is the area of ​​the receiving coil; is the angle between the normal of the receiving coil plane and the Earth’s magnetic field; Motion noise It is expressed as: Formula 8: ; The motion noise is obtained by equation 8: , and then put it into formula 6 to calculate the target signal .

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