The Study On The Distortion Mechanism Of Transient Electromagnetic Signal

Transient electromagnetic method launches radio waves to the conductivegeological body. Then the conductive body produces the secondary field and thereceiving coil receives the secondary field. Finally, the conductive properties ofgeological body are acquired by analyzing the secondary field. Transientelectromagnetic sensor has been widely used in mineral exploration. However, theturn-off time of the sensor’s emission current can’t be zero, which usually results inresponse distortion during the shutting-off period. So the response during this periodis usually given up which results in blind detection area in shallow detection. In orderto reduce the early signal distortion to narrow shallow blind detection area, the factorsaffecting the sensor’s response distortion are analyzed in this paper.The ground TEM system model aiming at calibration loop body is established.The responses of the calibration model are calculated and analyzed. The calculationresults show that responses change according to the index law. First of all, the lawabout how the damping characteristics of the sensor affect the response distortion hasbeen studied. The simulation on the response based on Matlab is carried out anddamping characteristic of the TEM sensor is studied. Calculated results indicate thatan under-damping state is the best way to match the TEM sensor for the rapidestresponse and the smallest distortion in early time. In order to prove the conclusion, aseries of tests are carried out. The measured results show that under-dampingmatching is the best matching state.Secondly, the law about how the bandwidth of the sensor coil and thepre-amplifier’s bandwidth affect the response distortion has been studied. The resultsshow that the pre-amplifier’s bandwidth decreases when the filter capacitor andcompensation capacitor increase. If the pre-amplifier’s bandwidth decreases, theresponse distortions will happen. The research on the receiving coil shows that the response distortions will happen when the bandwidth of the sensor coil decreases. Sowe should take into comprehensive consideration of the bandwidth of the sensor coiland the pre-amplifier’s bandwidth. Only when their bandwidth matches well canavoid response distortion.Thirdly, the law about how the amplitude of the sensor’s input signal effects theresponse distortion has been studied. The results show that the amplitude of the inputsignal will affect the bandwidth of pre-amplifier. When the amplitude of the inputsignal increases, the pre-amplifier’s bandwidth will decrease. The low bandwidth ofthe pre-amplifier will cause response distortion. So the magnitude of input signalshould be controlled in actual measurement. The magnitude mustn’t exceed theacceptable range of pre-amplifier. Otherwise the pre-amplifier bandwidth willdecrease and response distortion will happen.Finally, the law about how the emission current turn-off time affects the responsedistortion has been studied. The results show that high frequency response willincrease and the amplitude of the response will become lager when the currentturn-off time decreases. At the same time, the increased signal amplitude will decreasethe bandwidth of the pre-amplifier. This requires the receiving coil and the amplifierhave enough bandwidth in order to decrease the response distortion.Through the analysis of the distortion factors of TEM sensor, it is concluded thatshorter current turn-off time doesn’t mean better shallow layer detection ability. It isnecessary to consider shorter turn-off time may cause high frequency response whichrequires that the sensor coil has higher bandwidth. So we should take intocomprehensive consideration of the bandwidth of the sensor coil and thepre-amplifier’s bandwidth to design the current turn-off time in order to reduce theearly signal distortion and narrow shallow blind detection area of the sensor.