Design Of Portable Transmitting System For Shallow Surface Electromagnetic Detection Based On Three Frequency Resonance

As a geophysical exploration method which is widely used in urban underground space exploration and underground target sensing,the electromagnetic detection of shallow surface in frequency domain is mainly aimed at the underground target which is less than 200 m.In order to obtain higher resolution of anomalous bodies,it is necessary to increase the transmission frequency.The inductive reactance of the transmitting coil increases during high frequency transmission,and the output current is severely inhibited.In the case of single-frequency transmission,the resonant method can be used to solve the problem,but the single-frequency transmission has low working efficiency,while the multi-frequency transmission cannot use the resonant strategy.In view of the above contradictions,this paper designed a circuit structure with multi-frequency resonance matching and built a complete electromagnetic launch system.Based on this,the main research contents are as follows:1.The propagation mechanism of electromagnetic wave was studied,the electromagnetic characteristics of common underground abnormal bodies was given,and the working principle of electromagnetic wave exciting abnormal body to generate secondary field and then data processing to obtain electrical structure of underground target was explained.The development history of airborne electromagnetic detection and shallow surface electromagnetic detection was discussed.The commonly used coil arrangement in the receiving and transmitting integrated structure was introduced,the main transmitting waveforms in frequency domain was listed,and the development,optimization process and application range of different waveforms was explained.It provided theoretical guidance for the design of resonance method of multi-frequency fusion waveform.2.The problems of the conventional single frequency transmitting waveform and the multi-frequency fusion waveform were pointed out.The feasibility of simultaneous resonance of the main frequency points of the multi-frequency fusion waveform was discussed from the point of view of the solution of the mathematical equation and the amplitude frequency characteristics.The solution sets of multivariate nonlinear equations were obtained by Newton iterative method.Newton iterative method was used to obtain the solution set of multivariate nonlinear equations,and the difference of "stability" of different solutions to the system error was explained.3.The hardware circuit of shallow surface electromagnetic probe and launcher based on tri-frequency resonance was completed.According to the working conditions of resonant matching circuit,the parameters of capacitor and inductor were analyzed,and the components of metallized polypropylene film capacitor and hollow inductor were determined.The peripheral circuit of the control part was designed,and the transmitting waveform programming method including GPS synchronization was given.The driving board which meets the design requirements was selected,its working principle was explained and the peripheral circuit structure was given.A complete launch system was built.4.The signal characteristics of each node from the control signal to the waveform output were tested,the working characteristics of each part and the delay error between each node were given,and the system performance index was determined.The comparative experiments of single frequency resonance or not,low frequency and high frequency,multi frequency resonance or not were carried out.The correctness of the multi-frequency resonance matching strategy and its advantages in practical work were verified.