A semi-airborne transient electromagnetic signal adaptive gain method and system

Abstract

The application discloses a kind of semi-airborne transient electromagnetic signal adaptive gain method and system, it is related to geophysical prospecting technical field, including: the differential signal of receiving coil is obtained, and the differential signal is chopped self-stable zero amplification processing;Under the time reference set, the total time window of differential signal is divided into multiple time channels, the length of each time channel is adjusted according to signal attenuation characteristics, according to sampling rate and total time window, the time length and data points of each time channel are calculated, data points are distributed to early time channel, middle time channel and late time channel, to switch different gain multiples, to carry out adaptive gain amplification;After adaptive gain amplification, differential signal is transmitted to receiver after being collected by ADC.The gain coefficient is dynamically adjusted by time channel, the technical problems of large dynamic range of transient electromagnetic signal and low signal-to-noise ratio of weak signal are solved, and high-fidelity acquisition of full-time domain signal is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of geophysical prospecting, in particular to a semi-airborne transient electromagnetic signal adaptive gain method and system. BACKGROUND

[0002] In semi-airborne transient electromagnetic prospecting, the amplitude of the signal induced by the receiving coil rapidly decays from the millivolt level of the early time channel to the microvolt level of the late time channel, and the dynamic range exceeds 100 dB.

[0003] The traditional fixed gain amplification circuit has the following defects: early signal saturation, leading to signal distortion; low signal-to-noise ratio of late signal, leading to signal being submerged in noise; limited dynamic range, limiting the detection depth and resolution. Although the chopping self-stable zero and low noise operational amplifier in the prior art can suppress noise, it still cannot solve the adaptation problem of large dynamic range signals. SUMMARY

[0004] To solve the above problems, the present application provides a semi-airborne transient electromagnetic signal adaptive gain method and system, which dynamically adjusts the gain coefficient by time channel to solve the technical problems of large dynamic range of transient electromagnetic signals and low signal-to-noise ratio of weak signals, and realizes high-fidelity acquisition of full-time domain signals.

[0005] To achieve the above purpose, the present application adopts the following technical scheme:

[0006] In a first aspect, the present application provides a semi-airborne transient electromagnetic signal adaptive gain method, comprising:

[0007] obtaining a differential signal of a receiving coil and performing chopping self-stable zero amplification processing on the differential signal;

[0008] under a set time reference, dividing the total time window of the differential signal after chopping self-stable zero amplification processing into multiple time channels, adjusting the length of each time channel according to the signal decay characteristic, calculating the time length and data points of each time channel according to the sampling rate and the total time window, distributing the data points to the early time channel, the middle time channel and the late time channel, switching different gain multiples, and thus performing adaptive gain amplification;

[0009] The differential signal after adaptive gain amplification is transmitted to a receiver after being collected by an ADC.

[0010] As an optional implementation, the design of the receiving coil comprises:

[0011] Optimization of internal resistance: calculating the overall winding resistance of the coil: ; wherein, N is the number of turns of the inner conductor of the hollow coil, is the resistivity of the conductor, A is the cross-sectional area of the wire, D is the diameter of the coil;

[0012] Adjustment of the matching resistance: by changing the matching resistance, the damping coefficient of the second-order transmission network and the system bandwidth are changed; in particular:

[0013] If the receiving coil works in the critical damping state, the matching resistance and the system bandwidth are:

[0014] ; ;

[0015] When the receiving coil works in the over-damped state, that is, K >1, or the receiving coil works in the under-damped state, that is, 0< K <1, the matching resistance is:

[0016] ;

[0017] Wherein, K is the working state of the receiving coil; is the winding resistance; is the distributed inductance of the coil, is the distributed capacitance of the coil, is the frequency.

[0018] As an optional implementation, the design of the receiving coil further includes:

[0019] Adjustment of the natural resonant frequency: the natural resonant frequency of the differential induction type hollow coil is: ;

[0020] The distributed inductance is: ;

[0021] The distributed capacitance is: ;

[0022] Wherein, d is the wire diameter, is the space permeability, is the equivalent radius of the coil cross section; is the number of coil segments, is the total number of coil winding layers, is the relative dielectric constant of the wire, delta is the distance between each layer, is the relative dielectric constant of the skeleton, is the height of the skeleton, e is the width between the skeleton grooves, and N is the number of turns of the coil.

[0023] As an alternative embodiment, the chopping self-stable zero amplification process comprises:

[0024] The differential signal is multiplied by the modulation signal , and is modulated from low frequency to the center frequency band, and the amplifier noise is white noise at the band;

[0025] The modulated signal is amplified by a low-noise amplifier, and the amplified signal is down-converted, i.e. the amplified signal is multiplied again by the modulation signal , and the signal is demodulated back to the low frequency band after demodulation, and the noise signal is moved to the high frequency band after modulation, and finally low-pass filtered to realize low-noise amplification of the low frequency signal.

[0026] As an alternative embodiment, the length of each time channel is adjusted according to the signal attenuation characteristics, and the transient electromagnetic data attenuation conforms to the e exponential attenuation law, so that the early, middle and late time channels are divided by exponential time channel division:

[0027] Early time channel: the first 0-50 time channels, the signal attenuates fast, the time window is narrow, and the amplification factor G1=1;

[0028] Mid time channel: 51-1586 time channels, the amplification factor is G2, G2 is set to 10, 20, 30, 40 and 50-100, as a transition zone;

[0029] Late time channel: 1587-5119 time channels, the signal attenuates slowly, the time window is long, and the amplification factor increases to G3, G3 is set to 200, 300, 400 and 500.

[0030] As an alternative embodiment, the differential signal amplified by the adaptive gain is processed by low-pass filtering and then collected by ADC; the low-pass filtering process comprises:

[0031] The low-pass filter single cutoff frequency is lower than the lowest frequency band of the high-frequency noise of the unmanned aerial vehicle, i.e. 14.2 kHz is selected as the low-pass filter cutoff frequency;

[0032] A fourth-order Butterworth low-pass filter chip is selected for low-pass filtering, and the cutoff frequency is set by an external resistor, with a maximum support of 256 kHz cutoff frequency.

[0033] In a second aspect, the present application provides a semi-airborne transient electromagnetic signal adaptive gain system, comprising:

[0034] The front-end conditioning module is configured to obtain the differential signal of the receiving coil and perform chopping self-stable zero amplification processing on the differential signal;

[0035] The adaptive gain module is configured to divide the total time window of the chopped self-holding zero amplified differential signal into a plurality of time channels at a set time reference, adjust the length of each time channel according to signal attenuation characteristics, calculate the time length and data points of each time channel according to the sampling rate and the total time window, and distribute the data points into early time channels, middle time channels and late time channels, so as to switch different gain multiples and perform adaptive gain amplification.

[0036] The sending module is configured to transmit the differential signal amplified by the adaptive gain to the receiver after being collected by the ADC.

[0037] In a third aspect, the present application provides an electronic device, comprising a memory and a processor, and computer instructions stored in the memory and running on the processor, when the computer instructions are run by the processor, the method of the first aspect is completed.

[0038] In a fourth aspect, the present application provides a computer readable storage medium for storing computer instructions, when the computer instructions are executed by the processor, the method of the first aspect is completed.

[0039] In a fifth aspect, the present application provides a computer program product comprising a computer program, when the computer program is executed by the processor, the method of the first aspect is completed.

[0040] Compared with the prior art, the present application has the following beneficial effects:

[0041] (1) The present application effectively reduces thermal noise by optimizing internal resistance and matching resistance, thereby significantly improving signal-to-noise ratio. This improvement not only enhances the clarity of the signal, but also works cooperatively with the subsequent adaptive gain circuit, so that the system can process signals from millivolt level in early time channels to microvolt level in late time channels, meeting the acquisition requirements of full-time domain signals. In addition, by adjusting the inherent resonant frequency and distributed parameters, the coil can more accurately induce semi-airborne transient electromagnetic signals, thereby improving the detection accuracy. In terms of structural design, the present application adopts a hollow coil with a diameter of 50 cm. This design not only ensures the portability of the structure, but also makes it very suitable for unmanned aerial vehicle load, thereby ensuring the efficiency and long endurance of the detection process.

[0042] (2) The application effectively suppresses 1 / f noise and DC offset by adopting chopper self-stabilizing zero technology, while avoiding the increase of the noise floor, thereby significantly improving the signal-to-noise ratio of the signal and realizing low-noise performance. In addition, the selected AD8629 operational amplifier has low offset voltage, low temperature drift characteristics and high common mode rejection ratio, which ensures high-precision amplification of the signal. The low-power design of AD8629 makes it very suitable for devices such as unmanned aerial vehicles with strict power requirements, effectively prolonging the endurance time of the device. At the same time, the wide operating temperature range of AD8629 (-55℃ to 125℃) enables it to adapt to various harsh environmental conditions, ensuring the stability and reliability of the system.

[0043] (3) The application realizes adaptive dynamic range expansion by adjusting the gain coefficient in time channels, enabling the system to process signals from millivolt-level early time channels to microvolt-level late time channels, effectively solving the problem of low signal-to-noise ratio caused by early signal saturation and late signal amplification deficiency when traditional fixed gain amplification circuits process large dynamic range signals. The adaptive gain amplification circuit can dynamically adjust the gain according to the time characteristics of the signal, ensuring high-precision acquisition of millivolt to microvolt full-time domain signals, significantly improving the detection accuracy and data quality. In addition, the control logic circuit based on FPGA (Field Programmable Gate Array) has high flexibility and adaptability, and can adjust the gain switching strategy according to different detection requirements and signal characteristics, further improving the overall performance of the system.

[0044] (4) The application effectively removes high-frequency noise above 20kHz through the designed low-pass filter unit, significantly improving the purity and stability of the signal. By accurately setting the cutoff frequency of the filter, it ensures that the useful signal components will not be attenuated too much due to filtering, thereby optimizing the signal quality. In addition, the optimized circuit design enhances the anti-interference ability of the filter, enabling it to maintain stable operation in complex and variable environments, providing reliable protection for high-precision acquisition of semi-airborne transient electromagnetic signals.

[0045] The advantages of the additional aspects of the application will be partially given in the following description, partially will become obvious from the following description, or will be known by the practice of the application. BRIEF DESCRIPTION OF DRAWINGS

[0046] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiment or prior art description. Obviously, the drawings in the following description are only embodiments of the application, and for those skilled in the art, other drawings can be obtained without creative labor on the basis of the provided drawings.

[0047] Figure 1 The flow chart of the adaptive gain method of the semi-airborne transient electromagnetic signal provided for the embodiment 1 of the present application is shown in the figure;

[0048] Figure 2 The equivalent circuit schematic diagram of the differential induction type air core coil provided for the embodiment 1 of the present application is shown in the figure;

[0049] Figure 3 The adaptive gain amplification schematic diagram provided for the embodiment 1 of the present application is shown in the figure;

[0050] Figure 4 The primary amplification circuit diagram provided for the embodiment 1 of the present application is shown in the figure;

[0051] Figure 5 The distribution time channel schematic diagram provided for the embodiment 1 of the present application is shown in the figure;

[0052] Figure 6 The amplification circuit diagram of the variable gain stage provided for the embodiment 1 of the present application is shown in the figure;

[0053] Figure 7 The adaptive gain amplification simulation result diagram provided for the embodiment 1 of the present application is shown in the figure. DETAILED DESCRIPTION

[0054] The present application will be further described below in conjunction with the accompanying drawings and embodiments.

[0055] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present application. Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as would be understood by one of ordinary skill in the art to which the present application pertains.

[0056] It should be noted that the terms used herein are only for the purpose of describing the specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, the singular form is intended to include the plural form unless the context clearly indicates otherwise, and it should be further understood that the terms "comprise" and "include" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product, or device that includes a series of steps or units does not have to be limited to only those steps or units clearly listed, but can include other steps or units that are not clearly listed or inherent to the process, method, product, or device.

[0057] The embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0058] Embodiment 1

[0059] The present embodiment provides a semi-airborne transient electromagnetic signal adaptive gain method, as shown in the figure, which comprises: Figure 1 ​

[0060] The differential signal of the receiving coil is acquired and chopped self-stabilized zero amplification is performed on the differential signal;

[0061] The total time window of the differential signal after the chopped self-stabilized zero amplification is divided into multiple time channels under a set time reference, the length of each time channel is adjusted according to the signal attenuation characteristics, the time length and the number of data points of each time channel are calculated according to the sampling rate and the total time window, the data points are distributed into early time channels, middle time channels and late time channels, different gain multiples are switched to perform adaptive gain amplification;

[0062] The differential signal after the adaptive gain amplification is transmitted to the receiver after being collected by an ADC.

[0063] In the embodiment, the receiving coil adopts an inductive hollow coil, and a layered differential winding structure is adopted to match the resistance and optimize to reduce the thermal noise.

[0064] The specific design of the receiving coil is described in detail below.

[0065] (1) Structure design and material selection.

[0066] The layered differential winding structure is adopted, the layered winding reduces the distributed capacitance and inductance, and provides a physical basis for dynamic gain switching, and the adaptive gain compensates for the inherent defects of the coil in early / late signal processing, forming a closed-loop optimization mechanism. Specifically, it includes a coil inner skeleton, an outer skeleton and a circuit cabin.

[0067] The coil diameter is 50 cm, and the coil diameter of 50 cm is the result of a comprehensive trade-off between detection performance, unmanned aerial vehicle load capacity, signal processing requirements and engineering implementation cost, which can effectively meet the needs of semi-airborne transient electromagnetic detection, and improve the detection accuracy and efficiency.

[0068] The coil height is 10-15 cm, and the overall weight is light, which is suitable for unmanned aerial vehicle load.

[0069] The tail is a circuit cabin, which is internally provided with a low-noise signal conditioning circuit, a power supply circuit and an attitude positioning module circuit, and is layered wound in a multi-turn air core structure. The interlayer spacing δ = 0.1 mm, and the skeleton slot width e = 2 mm.

[0070] Among them, the low-noise signal conditioning circuit adopts the chopping self-stabilized zero technology to suppress the 1 / f noise of the differential signal from the receiving coil, and reduce the offset voltage and time / temperature drift.

[0071] The power module adopts dual 8.4V lithium ion batteries, constructs a ±8.4V bipolar voltage input architecture, generates stable ±5V power through a DC-DC power module and a symmetric LDO (low-dropout regulator) to realize high-stable power supply for the operational amplifier and reduce signal distortion. The power module includes a low-noise LDO with adjustable voltage to stabilize the input positive and negative power to ±5V, and is equipped with an LED indicator light to display the battery power state. The performance parameters of the positive and negative LDO chips are consistent, respectively providing an output current of about 200mA and 300mA, meeting the total current demand of about 50mA of the rear-stage operational amplifier and differential output circuit.

[0072] The attitude positioning module has the function of realizing the output of longitude and latitude, attitude angle and time information of the electromagnetic induction sensor. The module outputs the longitude and latitude and time parameters by integrating the GPS and Beidou positioning systems and using the NEMA0183 protocol. Specifically, the nine-axis attitude angle sensor is used as the attitude module in the embodiment, the sensor integrates a high-precision gyroscope, an accelerometer and a geomagnetic field sensor, and uses a high-performance microprocessor and an advanced dynamic solution and Kalman dynamic filtering algorithm, which can quickly and accurately solve the real-time motion attitude of the module. The theoretical positioning accuracy of the positioning module is ±2.5 meters. The attitude positioning module integrates the positioning information and the attitude information, and outputs the information including time, longitude and latitude, height, angle (including navigation angle, pitch angle and roll angle) and temperature through the attitude module.

[0073] The differential input can adopt a symmetric circuit structure, and the values of the basic parameters in the upper and lower circuit structures are equal, as shown in Figure 2 , including the equivalent resistance, the equivalent inductance and the equivalent capacitance.

[0074] Specifically, in the upper part of the circuit structure, one end of the inductor L1 is connected to one end of the resistor R1, the other end of the resistor R1 is connected to one end of the capacitor C1, the other end of the capacitor C1 is connected to the other end of the inductor L1, and the capacitor and the resistor R T1 are connected in parallel, one end of the resistor R T1 is connected to the positive electrode, and the other end is grounded.

[0075] In the lower part of the circuit structure, one end of the inductor L2 is connected to one end of the resistor R2, the other end of the resistor R2 is connected to one end of the capacitor C2, the other end of the capacitor C2 is connected to the other end of the inductor L2, and the capacitor and the resistor R T2 are connected in parallel, one end of the resistor R T2 is connected to the negative electrode, and the other end is grounded.

[0076] Wire material: The wire size is selected as enameled wire with a diameter of <0.25 mm, made of copper, gold, silver or other materials with low resistivity, which can effectively reduce the internal resistance of the coil. Thus, reducing the resistance thermal noise and improving the signal-to-noise ratio; After each layer is wound, use insulation tape to fix it to ensure the stability of the coil. During winding, pay attention to the uniformity and symmetry of the coil to reduce the unevenness of the distributed parameters; The insulation material of the enameled wire is high-quality polyester paint, which has good electrical insulation performance and mechanical strength, and can effectively prevent short circuit and electromagnetic interference between wires.

[0077] Skeleton material: Both the inner skeleton and the outer skeleton are made of lightweight and high-strength engineering plastics (such as polycarbonate or nylon) as the inner and outer skeleton materials of the coil. These materials not only meet the load requirements of the unmanned aerial vehicle, but also have good mechanical properties and electrical insulation properties.

[0078] Circuit cabin shell: The circuit cabin shell is also made of lightweight engineering plastics, which has good protective performance and can protect the internal circuit from the external environment.

[0079] Other materials: Matching resistance: Select high-precision, low-noise metal film resistance, with a resistance of kΩ level, which can be 1kΩ-10kΩ, used to optimize the matching characteristics of the coil and perform damping matching.

[0080] Connection line: Use high-quality shielded signal line to reduce electromagnetic interference and ensure the stability of signal transmission.

[0081] (2) Internal resistance optimization: Select a wire type with low resistivity to reduce noise.

[0082] The overall winding resistance of the coil is estimated by formula (1):

[0083] (1);

[0084] where, N is the number of turns of the inner wire of the hollow coil, is the resistivity of the wire, is the cross-sectional area of the wire, is the diameter of the coil.

[0085] (3) Natural resonant frequency adjustment: determined according to the physical structure of the coil (such as winding method, wire diameter, insulation material, etc.). The natural resonant frequency of the differential inductive hollow coil Mathematical expression:

[0086] (2);

[0087] where, is the distributed inductance of the coil, The distributed capacitance of the coil.

[0088] (4) Distributed inductance calculation: Distributed inductance can be calculated by simplifying the coil structure.

[0089] The total inductance expression of the air-core coil can be simplified as:

[0090] (3);

[0091] wherein, d is the wire diameter, is the space permeability, and is 4π× H / m, is the number of wire layers inside the air-core coil, is the equivalent radius of the coil cross section.

[0092] (5) Distributed capacitance reduction: Distributed capacitance can be divided into interlayer distributed capacitance and intersegment distributed capacitance. It is estimated by formula (4):

[0093] (4);

[0094] wherein, is the number of coil segments, is the total number of coil wire layers, represents the relative dielectric constant of the wire, delta represents the distance between each layer, represents the relative dielectric constant of the skeleton, represents the height of the skeleton, e represents the width of the skeleton slot, and N is the number of turns of the coil.

[0095] (6) Matching resistance adjustment: The matching resistance is in the range of several thousand ohms to several ten thousand ohms, and is connected in parallel at the output end to increase the system stability and adjust the system damping ratio. The damping coefficient and system bandwidth of the second-order transmission network can be changed by modifying the matching resistance.

[0096] The expression of the matching resistance and the system cutoff frequency is:

[0097] (5);

[0098] (6).

[0099] The receiving coil of the semi-airborne transient electromagnetic method works in different states by changing the matching resistance. When working in the over-damped state, i.e. K>1, the amplitude-frequency response is too flat, and the high-frequency signal cannot be highlighted, only the low-frequency signal can be reflected, the sensitivity of the coil is low, and the local anomaly of the geological body cannot be highlighted, only the general structure can be reflected. When working in the under-damped state, i.e. 0<K<1, the response signal amplitude is large, the signal attenuation is small, the sensitivity of the coil is high, and the local anomaly of the geological body can be highlighted, and the signal will have oscillation phenomenon. When working in the critical damping state, the response signal amplitude is moderate, the signal attenuation is also small, the sensitivity of the coil is moderate, and the overall index is between the other two states.

[0100] If the receiving coil works in the critical damping state, i.e. K ≈1, the matching resistance and the system bandwidth The expression is:

[0101] (7);

[0102] (8);

[0103] Wherein, is the frequency.

[0104] When K >1 or 0< K <1,

[0105] (9);

[0106] That is, by changing the matching resistance, the damping ratio can be changed while keeping the basic parameters of the coil unchanged.

[0107] As shown in Fig. Figure 3 is the adaptive gain amplification principle diagram, which specifically comprises:

[0108] (1) The differential signal of the receiving coil is first processed by chopper self-stabilizing zero amplification, wherein the AD8629 operational amplifier is selected to realize the chopper self-stabilizing zero amplification function.

[0109] (2) The differential signal processed by the chopper self-stabilizing zero amplification is input into the primary amplification circuit for processing, wherein the AD8429 low-noise primary operational amplifier is selected to realize 10 times fixed gain amplification.

[0110] (3) At the FPGA control end, the time channel of the signal output by the primary amplifier circuit is identified. The TTC timer is used, and 1PPS (Pulses Per Second) pulses are used as the time reference to divide the signal into multiple time channels (early (0-t1), middle (t1-t2), late (t2-5120), and real-time signal (on-time)) to determine the time channel of the signal. Then, in the gain state machine, the GPIO pin of the FPGA control end is used to control the analog switch ADG1419 to control the switching of the gain resistor network in real time.

[0111] (4) In the variable gain stage composed of OPA2188 operational amplifier and ADG1419 analog switch, the gain control of 1 / G1 / G2 / G3 times is achieved by switching the gain resistor network, thereby switching different gain multiples to complete adaptive gain amplification and output the amplified signal; thus, the differential signal after adaptive gain amplification is acquired by ADC (Analog to Digital Converter) and transmitted to the receiver.

[0112] The following is a detailed introduction.

[0113] In this embodiment, the preamplifier circuit design based on chopper self-stabilization technology specifically includes the following contents.

[0114] In semi-airborne transient electromagnetic (SATEM) signal acquisition systems, the performance of the amplifier circuit plays a crucial role in signal quality and detection accuracy. Traditional amplifier circuits generally suffer from 1 / f Noise, DC offset voltage, time drift, and temperature drift are all effects that can severely affect the accuracy of the operational amplifier's transmitted signal. Chopper self-stabilization technology suppresses 1 / f While reducing noise and DC offset, it does not increase the noise floor and can also be used for continuous signal amplification in the time domain.

[0115] A chopper self-stabilization technique is used in the front-end conditioning circuit to perform 1 / 2-wave stabilization on the differential signal from the receiving coil. / f Noise suppression reduces offset voltage and time / temperature drift.

[0116] (1) The core principle of chopper self-stabilization zero technology: By modulating the input differential signal from low frequency to high frequency and then demodulating the high frequency signal back to low frequency, 1 / f noise and DC offset are effectively suppressed, while avoiding increasing the background noise.

[0117] (1-1) Modulation process: Differential signal of the receiving coil With modulated signal Multiply, From low-frequency modulation to the center frequency at the frequency band, the noise floor of the amplifier is white noise. at the frequency band, the noise floor of the amplifier is white noise.

[0118] The core of the chopping technique is to move the low frequency signal (and the 1 / f noise in the same frequency band) to a higher frequency band at the input end, that is nearby, the amplifier faces white noise in this frequency band, instead of 1 / f noise, thereby reducing the overall low frequency noise of the system. The 1 / f noise of the amplifier itself will rise rapidly below a certain corner frequency The selected frequency should be lower than the corner frequency of the amplifier. This can ensure that the amplifier works in the white noise area. Do not choose too high, so as not to exceed the bandwidth of the amplifier or the filter, otherwise the modulated signal cannot be amplified or recovered in its entirety. The embodiment takes as an example.

[0119] (1-2) Low noise amplification: the modulated signal is amplified by a low noise amplifier to ensure that the signal will not introduce additional noise during amplification in the high frequency band.

[0120] (1-3) Demodulation process: the amplified signal is down-converted, that is, the amplified signal is multiplied again with the modulation signal , the signal is demodulated back to the low frequency band, while the noise signal is moved to the high frequency band after modulation, and finally passes through a low-pass filter to realize low noise amplification of the low frequency signal and avoid the influence of 1 / f noise. f

[0121] (2) Selection and application of high-performance operational amplifier: AD8629 operational amplifier is selected to realize chopping self-stable zero amplification function. AD8629 device effectively ensures the low offset voltage and low temperature drift characteristics of the data acquisition system. The voltage noise density of AD8629 is as low as 22.1nV at 1kHz, and the current noise density is about 5fA at 10Hz in a relatively wide bandwidth range (0~10kHz). AD8629 can provide 16-bit precision, wider working temperature, ultra-low offset, drift and bias current characteristics, and its ultra-low low frequency noise is a better choice for high-resolution detection equipment such as half-airborne transient electromagnetic receiving system.

[0122] ​(3) The optimal selection of analog switch: The modulator and demodulator of the chopping self-biased zero amplifier are generally composed of analog switches. In the design of the chopping amplifier, it is crucial to select an analog switch with a low charge injection. The combination of the AD8629 operational amplifier and the ADG1419 analog switch solves the problem of residual offset in the traditional chopping self-biased zero circuit. Charge injection effects and clock feedthrough effects can cause the injection of charges, which not only form a residual offset voltage on the load, but also form a noise voltage on the source, significantly increasing the equivalent input noise of the chopping amplifier. Therefore, the selection of the analog switch is particularly important in the design process to minimize the charge injection and further reduce the noise and offset voltage.

[0123] In the embodiment, the adaptive gain amplification circuit specifically includes the following contents.

[0124] (1) Circuit architecture.

[0125] The adaptive gain amplification circuit is arranged before the ADC acquisition circuit and after the chopping self-biased zero amplifier circuit, and is built-in in the received coil signal conditioning circuit. The circuit is controlled by the control logic of the FPGA of the receiver, the gain control decision is made by the gain state machine, and the gain resistance network is controlled by the GPIO (general input / output port) port to realize adaptive amplification.

[0126] Specifically: the differential signal processed by the chopping self-biased zero amplifier is first amplified by the primary amplification circuit, then the PPS pulse synchronization is used as a time reference at the FPGA control end, or a timestamp is used as a time reference, the signal arrival time is detected, and the time channel (early, middle, late) of the signal is judged. According to the judgment result of the time channel, the gain state machine controls the switching of different gain multiples to realize adaptive gain based on time judgment. Finally, the signal amplified by the adaptive gain is output to the ADC for acquisition and transmitted to the FPGA for storage in the SD card.

[0127] (2) Primary amplification circuit: the 10-fold fixed gain amplification is performed by the AD8429 low-noise primary operational amplifier to ensure low noise and high bandwidth. The intrinsic noise density of the AD8429 is low, and the equivalent input noise can still be kept at a low level after the signal is amplified by 10 times, and the high common-mode rejection ratio can effectively prevent interference signals from damaging data acquisition, which is suitable for μV-level weak signal amplification.

[0128] As shown in FIG. 4, the adaptive gain amplification circuit specifically includes the following steps. Figure 4

[0129] ​The pin 1 and pin 3 of the coil input chip P15 are connected with the pin 4 and pin 1 of the AD8429 through the resistance R120 (1k±1%) and the resistance R117 (1k±1%) respectively, and the pin 2 of the coil input chip P15 is grounded;

[0130] The pin 2 and pin 3 of the AD8429 are connected through the resistance R118 (665R±1%);

[0131] The pin 1 of the AD8429 is connected with the parallel resistance R115 (2k±1%) and the capacitor C126 (10nF 50V), and the parallel one end of the parallel resistance R115 and the capacitor C126 is connected with the pin 1 of the AD8429 and the resistance R117, and the other end is grounded;

[0132] The pin 4 of the AD8429 is connected with the parallel resistance R122 (2k±1%) and the capacitor C132 (10nF 50V), and the parallel one end of the parallel resistance R122 and the capacitor C132 is connected with the pin 4 of the AD8429 and the resistance R120, and the other end is grounded;

[0133] The pin 5 of the AD8429 is connected with the-5V power supply, and grounded through the capacitor C130 (100nF 50V);

[0134] The pin 8 of the AD8429 is connected with the+5V power supply, and grounded through the capacitor C125 (100nF 50V);

[0135] The pin 6 of the AD8429 is grounded, and the pin 7 of the AD8429 outputs the processed signal.

[0136] (3) Time channel division and gain control.

[0137] Time channel division:

[0138] (3-1) Determine the total time window: determine the total time window according to the characteristics of the signal, usually the time period from the rising edge to the decay to the weak signal.

[0139] (3-2) Divide the time channel: divide the total time window into multiple time channels (≤3), and the length of each time channel is adjusted according to the decay characteristics of the signal. The early time channel has less data, the middle time channel is moderate, and the late time channel is longer.

[0140] The transient electromagnetic data attenuation conforms to the e exponential attenuation law, so the early, middle and late time channel division adopts exponential time channel division. For example, the early signal attenuates fast, the time window is narrow, and the first 0-50 time channels (about 97 μs) are selected, and the general electrical property source emission off time is about 50-100 us; the amplification multiple in the middle time channel design is a transition zone, and the middle time channel is 51-1586 (about 3 ms); the late signal attenuates slowly, and the window can be extended to 1587-5120 points (about 7 ms).

[0141] (3-3) Calculate the number of time channels: according to the sampling rate and the total time window, the specific time length and the number of data points of each time channel are calculated.

[0142] For example, the early signal attenuates fast, the time window is narrow, and the first 0-50 time channels (about 97 μs) are selected, and the general electrical property source emission off time is about 50-100 us; the amplification multiple in the middle time channel design is a transition zone, and the middle time channel is 51-1586 (about 3 ms); the late signal attenuates slowly, and the window can be extended to 1587-5120 points (about 7 ms).

[0143] (3-4) Assign time channels: distribute the total number of data points to the early, middle and late time channels.

[0144] Taking the emission of a 25 Hz bipolar pulse signal as an example, after detecting the rising edge of the 1PPS signal, the early, middle and late time channels are judged every second based on the number of data points.

[0145] The first 10 ms is the emission rising edge stage, which contains 5120 data points (the number of sampling points = sampling rate x time, that is, 512000 x 0.01 = 5120), and the amplification multiple is 1.

[0146] The second 10 ms is the off falling edge stage, which is the detection of the secondary field induction signal of interest, and the 5120 data points are divided into three parts according to the data points corresponding to different time channels.

[0147] As shown in Figure 5 , 25 Hz indicates that one period is 40 ms, and only the secondary induction signal at the off time is concerned, and the three parts are only divided at the off time, i.e. the second and fourth 10 ms.

[0148] Since the transient electromagnetic data attenuation conforms to the e exponential attenuation law, the early, middle and late time channel division adopts exponential time channel division:

[0149] Early time channel: the first 0-50 time channels (about 97 μs), the signal attenuates fast, the time window is narrow, and the amplification multiple G1=1.

[0150] Middle time channel: 51-1586 time channels (about 3ms), the amplification multiple is G2, G2 can be set to 10, 20, 30, 40, 50-100, as a transition zone.

[0151] Late time channel: 1587-5119 time channels (about 7ms), the signal attenuation is slow, the time window is long, and the amplification multiple is increased to G3, G3 can be set to 200, 300, 400, 500.

[0152] (3-5) Gain control: using the GPIO pin of FPGA to control the analog switch ADG1419, creating AXI GPIO IP core, through the hardware time trigger timer and AXI GPIO to control the gain resistance network switching in real time, considering the early signal anti-saturation and late weak signal enhancement.

[0153] (4) Variable gain stage: through switching the gain resistance network to realize 1 / G1 / G2 / G3 times gain control. The switching speed of ADG1419 is about 140ns-190ns, and the data acquisition rate is 512ksps (about 1950ns), which is much larger than the analog switch speed, ensuring that the analog switch is in the closed state every time data sampling is performed, and the data sampling will not be caused by the switching of the analog switch. Jump point.

[0154] As shown in Figure 6 , specifically comprising:

[0155] The pin 7 of the chip U29B is connected to an input signal, and is connected to the pin 6 of the chip U29B through the resistance R113 (4.7k±1%);

[0156] The pin 5 of the chip U29B is connected to the parallel resistance R119 and the capacitor C128, one end of the parallel connection of the resistance R119 (4.7k±1%) and the capacitor C128 (10nF 50V) is connected to the pin 5 of the chip U29B and the RF radio frequency signal, and the other end is grounded;

[0157] One end of the parallel connection of the resistance R119 and the capacitor C128 is also connected to the pin 1 of the chip U29A through the resistance R116 (100±1%);

[0158] The pin 4 of the chip U29B is connected to a-5V power supply, and is grounded through the capacitor C131 (10nF 50V);

[0159] The pin 8 of the chip U29B is connected to a+5V power supply, and is grounded through the capacitor C127 (10nF 50V);

[0160] Pin 2 of chip U29B is connected to the gain interface of G1, G10 and G100 through resistors R114 (0R), R112 (1k±1%) and R111 (10k±1%) respectively, and is grounded through resistor R110 (100±1%);

[0161] Pin 3 of chip U29B outputs the processed signal and is grounded through the connection of resistors R121 (10k±1%) and capacitor C129 (10nF 50V) in parallel.

[0162] Among them, chip U29B and chip U29A are both OP A2188 operational amplifiers, and ADG1419 analog switch (not shown) is used.

[0163] (5) Simulation verification: build an analog circuit, the maximum amplitude of the signal is 100mV, and a two-stage resistance network is constructed to realize the segmented amplification of the signal: the early signal amplification factor is 1, and the late signal channel is applied with a 10-fold gain coefficient. To simplify the amplification process, the number of early and late time channels is equal here. The simulation output result shows that the amplitude of the late signal is increased to 540mV after amplification, which realizes a 10-fold gain compared with the original signal (54mV), which meets the designed gain coefficient, and the designed amplification circuit can effectively amplify the late signal. As Figure 7 shown is the simulation result of adaptive gain amplification (three-stage amplification).

[0164] In this embodiment, the low-pass filter unit design specifically includes the following contents.

[0165] (1) Filter design target.

[0166] Noise characteristics: The noise of the unmanned aerial vehicle is mainly concentrated in the high frequency band greater than 20kHz. In order to effectively remove these high frequency noises, the cutoff frequency of the low-pass filter unit should be lower than the lowest frequency band of the high frequency noise.

[0167] Cutoff frequency selection: According to the characteristics of SATEM signal and the distribution of unmanned aerial vehicle noise, 14.2kHz is selected as the cutoff frequency of the low-pass filter unit. This frequency can effectively remove high frequency noise and will not cause excessive attenuation to useful signal components.

[0168] (2) Filter chip selection.

[0169] Chip model: LTC1563-2IGN four-order Butterworth low-pass filter chip is selected. This chip has active RC rail-to-rail low-pass filter characteristics and can provide good filtering effect.

[0170] External resistance configuration: the cut-off frequency is set by external resistances R1-R6, supporting a maximum cut-off frequency of 256 kHz. By precisely calculating and configuring these external resistances, the required 14.2 kHz cut-off frequency can be achieved.

[0171] (3) Filter performance optimization.

[0172] Through simulation verification, the bandwidth of the low-pass filter unit at -3dB is 14.5 kHz, which can effectively remove high-frequency noise above 20 kHz and ensure the purity of the signal.

[0173] Embodiment 2

[0174] The embodiment provides a semi-airborne transient electromagnetic signal adaptive gain system, comprising:

[0175] The front-end conditioning module is configured to obtain a differential signal of a receiving coil and perform chopper self-stabilized zero amplification processing on the differential signal.

[0176] The adaptive gain module is configured to divide a total time window of the differential signal after the chopper self-stabilized zero amplification processing into a plurality of time channels under a set time reference, adjust the length of each time channel according to signal attenuation characteristics, calculate the time length and data points of each time channel according to a sampling rate and the total time window, and distribute the data points into early time channels, middle time channels and late time channels, so as to switch different gain multiples and perform adaptive gain amplification.

[0177] The sending module is configured to transmit the differential signal after the adaptive gain amplification to a receiver after being collected by an ADC.

[0178] It should be noted that the above modules correspond to the steps described in Embodiment 1, and the above modules have the same examples and application scenarios as the corresponding steps, but are not limited to the content disclosed in Embodiment 1. It should be noted that the above modules as part of the system can be executed in a computer system such as a set of computer executable instructions.

[0179] In more embodiments, there are also provided:

[0180] An electronic device comprising a memory and a processor, and computer instructions stored on the memory and running on the processor, when the computer instructions are run by the processor, the method described in Embodiment 1 is completed. For brevity, it will not be repeated here.

[0181] It should be understood that in this embodiment, the processor can be a central processing unit (CPU), and the processor can also be other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field programmable gate arrays (FPGA) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor can be a microprocessor or the processor can also be any conventional processor.

[0182] The memory can include read-only memory and random access memory, and provide instructions and data to the processor, and a portion of the memory can also include non-volatile random access memory. For example, the memory can also store device type information.

[0183] A computer readable storage medium for storing computer instructions, which are executed by a processor to complete the method described in embodiment 1.

[0184] The method in embodiment 1 can be directly embodied as a hardware processor to complete, or be completed by a combination of hardware and software modules in the processor. The software module can be located in a random access memory, a flash memory, a read-only memory, a programmable read-only memory or an electrically erasable programmable memory, a register, etc. The storage medium is located in the memory, and the processor reads the information in the memory to complete the steps of the above method in combination with the hardware. To avoid repetition, it will not be described in detail here.

[0185] A computer program product comprising a computer program, which, when executed by a processor, implements the method described in embodiment 1.

[0186] The present application also provides at least one computer program product tangibly stored on a non-transitory computer readable storage medium. The computer program product includes computer executable instructions, such as instructions included in program modules, which are executed by devices on real or virtual processors of the target to perform processes / methods as described above. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, etc. that perform particular tasks or implement particular abstract data types. In various embodiments, the functions of the program modules can be combined or divided as desired. Machine executable instructions for program modules can be executed within a local or distributed device. In a distributed device, program modules can be located in local and remote storage media.

[0187] Computer program code for carrying out operations of the present application can be written in any combination of one or more programming languages. The computer program code can execute entirely on a computer, a special purpose computer, or other programmable apparatus to produce the functions / acts specified in the flow diagrams and / or block diagrams. The program code can execute entirely on a computer, a special purpose computer, or other programmable apparatus, as a stand-alone software package, partly on the computer and partly on a remote computer, or entirely on the remote computer or server.

[0188] In the context of the present application, the computer program code or related data can be carried by any suitable carrier to enable the device, apparatus or processor to perform the various processes and operations described above. Examples of carriers include signals, computer readable media, and the like. Examples of signals can include electrical, optical, radio, sound or other forms of propagated signals, such as carrier waves, infrared signals, and the like.

[0189] Those skilled in the art can realize that the units and algorithm steps of the examples described in conjunction with the present embodiments can be realized in electronic hardware or a combination of computer software and electronic hardware. Whether the functions are realized in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to realize the described functions for each specific application, but such implementation should not be considered beyond the scope of the present application.

[0190] The above describes the specific embodiments of the present application in conjunction with the accompanying drawings, but is not a limitation on the scope of protection of the present application. Those skilled in the art should understand that various modifications or variations made by those skilled in the art on the basis of the technical solutions of the present application without inventive labor are still within the scope of protection of the present application.

Claims

1. A method for adaptive gain of semi-airborne transient electromagnetic signals, characterized in that, The method comprises the following steps: acquire the differential signal of the receiving coil, and perform chopper self-biased zero amplification processing on the differential signal; the chopper self-biased zero amplification processing comprises: The differential signal is multiplied with the modulation signal , and the result is amplified from the low frequency to the center frequency of the band , and at the band , the amplifier's noise floor is white. The modulated signal is amplified by a low-noise amplifier, and the amplified signal is down-converted, that is, the amplified signal is multiplied again with the modulation signal The signal is demodulated and returned to a low frequency band, the noise signal is moved to a high frequency band after modulation, and finally low-pass filtering is performed to realize low-noise amplification of the low-frequency signal. AD8629 operational amplifier is selected to realize the chopper self-biased zero amplification function; the switching speed of ADG1419 is 140ns-190ns, and the data acquisition rate is 512ksps, so that the analog switch is in the closed state every time data sampling is performed, and the data sampling jump point caused by switching of the analog switch is avoided; in the gain state machine, the GPIO pin of the FPGA control end is used to control the analog switch ADG1419, and the switching of the gain resistor network is controlled in real time; the combination of the AD8629 operational amplifier and the ADG1419 analog switch solves the residual offset problem of the traditional chopper self-biased zero circuit; under a set time reference, the total time window of the differential signal after the chopper self-biased zero amplification processing is divided into a plurality of time channels, the length of each time channel is adjusted according to the signal attenuation characteristic, the time length and the data points of each time channel are calculated according to the sampling rate and the total time window, and the data points are distributed to early time channels, middle time channels and late time channels, so that different gain multiples are switched, and adaptive gain amplification is performed; the length of each time channel is adjusted according to the signal attenuation characteristic, and the early, middle and late time channels are divided by using exponential time channel division; the early time channels: the first 0-50 time channels, the signal attenuation is fast, the time window is narrow, and the amplification multiple G1=1; the middle time channels: the 51-1586 time channels, the amplification multiple is G2, G2 is set to 10, 20, 30, 40 and 50-100, and is used as a transition zone; the late time channels: the 1587-5119 time channels, the signal attenuation is slow, the time window is long, and the amplification multiple is increased to G3, G3 is set to 200, 300, 400 and 500; the differential signal after the adaptive gain amplification is transmitted to the receiver after being collected by the ADC; the differential signal after the adaptive gain amplification is collected by the ADC after low-pass filtering processing; the low-pass filtering processing comprises: the low-pass filter single cutoff frequency is lower than the lowest frequency band of the high-frequency noise of the unmanned aerial vehicle, that is, 14.2kHz is selected as the low-pass filter cutoff frequency; a fourth-order Butterworth low-pass filter chip is selected for low-pass filtering processing, and the cutoff frequency is set by an external resistor, and the highest support cutoff frequency is 256kHz; the design of the receiving coil comprises: the coil diameter is 50cm, the height is 10-15cm, and the multi-turn air core structure is layered and wound; the layer spacing δ=0.1mm, and the skeleton slot width e=2mm; The attitude positioning module has the functions of outputting longitude and latitude, attitude angle and time information of the electromagnetic induction sensor; the module integrates GPS and Beidou positioning systems, and outputs longitude and latitude and time parameters by using NEMA0183 protocol.

2. The method of claim 1, wherein, The design of the receiving coil comprises: Optimization of the internal resistance: calculation of the winding resistance of the whole coil: ; wherein, N is the number of turns of the inner conductor of the hollow coil, is the electrical resistivity of the conductor, is the cross-sectional area of the conductor, is the diameter of the coil; adjustment of the matching resistance: the damping coefficient and the system bandwidth of the second-order transmission network are changed by changing the matching resistance; specifically: If the receiving coil is operated in a critically damped state, the matching resistance and the system bandwidth are: ； ； When the receiving coil is operating in an over-damped state, i.e. when K >1, or the receiving coil is operating in an under-damped state, i.e. when 0 K <1, the matching resistance is: ； wherein, K is the operating state of the receiving coil; is the winding resistance; is the coil distributed inductance, is the coil distributed capacitance, is the frequency.

3. The method of claim 1, wherein, The design of the receiving coil further comprises: Adjustment of the natural resonant frequency: natural resonant frequency of a differential inductive air core coil To: ; The distributed inductance is: ; The distributed capacitance is: ; wherein, d is the diameter of the wire, is the space permeability, is the equivalent radius of the cross section of the coil; is the number of coil sections, is the total number of coil layers, is the relative dielectric constant of the wire, δ is the distance between each layer, is the relative dielectric constant of the skeleton, is the height of the skeleton, e is the width between the grooves of the skeleton, and N is the number of turns of the wire in the hollow coil.

4. A semi-airborne transient electromagnetic signal adaptive gain system, characterized in that, comprises: The front-end conditioning module is configured to acquire a differential signal of a receiving coil and perform a chopping self-stabilized zero amplification process on the differential signal; The chopping self-stabilized zero amplification process comprises: Differential signal With modulated signal Multiply, From low-frequency modulation to the center frequency In the frequency band, At this frequency band, the amplifier's noise floor is white noise; The modulated signal is amplified by a low-noise amplifier, and the amplified signal is down-converted, that is, the amplified signal is multiplied again with the modulation signal The signal is demodulated and returned to a low frequency band, and the noise signal is moved to a high frequency band after modulation, and finally low-pass filtered to realize low-noise amplification of the low frequency signal. An AD8629 operational amplifier is selected to realize the chopping self-stabilized zero amplification function; the switching speed of the ADG1419 is 140ns-190ns, and the data acquisition rate is 512ksps, so that the analog switch is in a closed state every time data sampling is performed, and data sampling jumping points are not caused by switching of the analog switch; in the gain state machine, a GPIO pin of an FPGA control end is used to control the analog switch ADG1419, and switching of a gain resistor network is controlled in real time; the joint application of the AD8629 operational amplifier and the ADG1419 analog switch solves the residual offset problem of a traditional chopping self-stabilized zero circuit; The adaptive gain module is configured to divide a total time window of the differential signal after the chopping self-stabilized zero amplification process into a plurality of time channels under a set time reference, the length of each time channel is adjusted according to signal attenuation characteristics, the time length and the number of data points of each time channel are calculated according to a sampling rate and the total time window, and the data points are distributed into early, middle and late time channels, so as to switch different gain multiples and perform adaptive gain amplification. The length of each time channel is adjusted according to signal attenuation characteristics, and the early, middle and late time channels are divided in an exponential time channel division manner: The early time channels: the first 0-50 time channels, the signal attenuates fast, the time window is narrow, and the amplification multiple G1=1; The middle time channels: the 51-1586 time channels, the amplification multiple is G2, G2 is set to 10, 20, 30, 40 and 50-100, and serves as a transition zone; The late time channels: the 1587-5119 time channels, the signal attenuates slowly, the time window is long, and the amplification multiple is increased to G3, G3 is set to 200, 300, 400 and 500; The sending module is configured to transmit the differential signal after the adaptive gain amplification and the ADC acquisition to a receiver; The differential signal after the adaptive gain amplification is subjected to low-pass filtering and then collected by the ADC; the low-pass filtering comprises: The low-pass filtering single cutoff frequency is lower than the lowest frequency band of the high-frequency noise of the unmanned aerial vehicle, that is, 14.2kHz is selected as the low-pass filtering cutoff frequency; A fourth-order Butterworth low-pass filtering chip is selected to perform the low-pass filtering, and the cutoff frequency is set by an external resistor, and the highest support cutoff frequency is 256kHz; The design of the receiving coil comprises: The coil has a diameter of 50cm, a height of 10-15cm, and a multi-turn air-core structure and is wound in layers; the layer spacing δ=0.1mm and the skeleton slot width e=2mm; 5. An electronic device, comprising: The attitude positioning module has a function of outputting longitude and latitude, attitude angle and time information of the electromagnetic induction sensor; the module outputs the longitude and latitude and time parameters by integrating a GPS and a Beidou positioning system and using a NEMA0183 protocol. The computer program product comprises a memory and a processor, and computer instructions stored in the memory and run on the processor, and when the computer instructions are run by the processor, the method in any one of claims 1-3 is completed.

6. A computer-readable storage medium, characterized in that, A computer program product for storing computer instructions which, when executed by a processor, perform the method of any one of claims 1-3.

7. A computer program product, characterised in that, A computer program for implementing the method of any one of claims 1-3 when executed by a processor.

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