The Design Of Ground-airborne TEM Three-component Inductive Air-core Coil Sensor

Transient electromagnetic method(TEM)has been widely used in many fields,such as coal well exploration,environmental investigation and archaeological exploration,due to its high detection efficiency,sensitivity to low-resistivity bodies and strong ability to distinguish abnormal bodies.According to different exploration methods,TEM can be divided into: ground TEM,ground-airborne TEM and airborne TEM.In recent years,the emerging ground-airborne TEM adopts the detection form of ground transmitting and air receiving.Compared with the other two detection methods,it has the advantages of convenient construction,ignoring complex terrain,high detection efficiency,safety and better spatial resolution.TEM generally uses air-core coil sensor,whose advantages of no magnetic core loss and inherent linear frequency characteristics is very suitable for receiving TEM signals.However,the research time of the ground-airborne TEM detection is not long.The research mainly focuses on the detection and processing of the vertical component,and the sensor only uses the single-component coil sensor.However,when the coil sensor is hoisted by the UAV for detection,on the one hand,the UAV and the sensor are softconnected,on the other hand,there are external wind force and inertia,so sensors are prone to tilt and shake.This phenomenon has brought great influence to the accuracy of the data,based on this,this paper design a ground-airborne TEM three-component air-core coil sensor,which can collect the X,Y,Z three component datas at the same time.In the later stage,the horizontal component data are used to compensate the vertical component data to minimize the errors caused by the tilt and shake of the sensor.The main research work of this paper is as follows:1.Firstly,the working principle of the ground-airborne TEM and its coil sensor is introduced.The structure of the sensor and the factors affecting its sensitivity and signal to noise ratio(SNR)are analyzed.Based on this,the winding mode,the number of turns and coil area of the sensor coil are determined.According to the sensor design requirements,the pre-amplification electric circuit and low-noise power supply electric circuit are designed.By analyzing the amplitude-frequency and phase-frequency characteristic errors of the sensor output,the damping coefficient of the sensor matching resistance is calibrated to minimize the output errors.By analyzing the source of background noise of the sensor and determining the dominant noise in the effective frequency band,the operational amplifier of the sensor is selected to reduce the background noise of the sensor.In order to record sensor posture in air,gyroscope is used to measure and store posture data.2.To solve the problem that the three coils of the three-component coil sensor are not orthogonal to each other,this paper gives a formula for converting the measured voltage value to the magnetic induction intensity value,and according to the principle of magnetic field intensity invariance,the forward model and the inversion model are established,and the degree of non-orthogonal of the three axes can be calculated according to the inversion model,so as to correct the data with errors,and verify the feasibility of the model through the data.3.Build a data test and collection platform for the sensor.According to the detection requirements of the three-component coil sensor of ground-airborne TEM,the acquisition card is selected,and the data processing and storage are realized by using Lab VIEW,and the interface of the upper computer is designed.In order to realize the synchronization of the ground-airborne TEM detection,the GPS module is selected and its circuit is designed to realize the synchronization of the second pulse signal and the synchronous signal.STM32 is used to analyze and store the longitude,latitude and other data collected by GPS module.4.The designed sensor are tested,and the test results shows that: sensor horizontal component background noise is about (?) vertical component of the background noise is (?),X,Y,Z three axis sensitivity are 8.4nT/s?9.8nT/s?18.5nT/s.After using the inversion model to correct the non-orthogonality of the measured data,the total field error is about 8n T.The synchronization of the GPS module is tested.The synchronization signal is well aligned with the second pulse signal,and the error is within 20?s.The error of the transmitting and receiving system's second pulse signal is less than 15 ns,reaching the design expectation.Finally,the designed three-component coil sensor is used for the outdoor comparison test.Through the later data processing,it can be found that after the horizontal component is used to compensate the tilted vertical component,the accuracy of the data is improved,which meets the design expectation.