Influence Analysis And Correction Of Coil Attitude For Fixed-wing Time Domain Airborne Electromagnetic Survey

Fixed-wing Time-domain airborne electromagnetic survey is a geophysical method by using aircraft as carrier platform and it measures the secondary field produced by underground media by receiving coil after the emission current is transmitted with transmitting coil.Because fixed-wing time-domain airborne electromagnetic survey is based on flight measurements,it has many advantages,such as not limited in complex terrain condition,fast speed,good flexibility,and high efficiency.At the same time,the attitude of transmitting and receiving coils are rotated by wind direction,aircraft turbulence,flight speed and etc,.and it brings deviation to the measured data and has great influence on interpretation of subsequent data.As a consequence,dynamic response and static response caused by the change of coil attitude is deduced.The influence of coil attitude change on dynamic response and static response are analyzed,and then coil attitude is corrected.The major research contents are as follows:According to basic theory of electromagnetic field,Electromagnetic response is calculated in non-stationary flight by the vectors' transformation in coordinate of transmitting coil and receiving coil,and is composed of two parts: the dynamic response generated in the movement process of coil and the static response induced in the sloping coil.Based on forward calculation,the effect of coil attitude change on dynamic response and static response are analyzed in the range of actual flight detection angle.For the dynamic response,the attitude change of the transmitting coil has no effect on the dynamic response,and it only related to angle and angle change rate of receiving coil.For the static response,The attitude change of transmitting and receiving coilhave an effect on it,and the effect of the pitch angle of the transmitting and receiving coils on the static is much larger than that of the roll angle.In view of the influence of the coil attitude change on the dynamic response and static response,a dynamic and static response correction method is proposed.The dynamic response is calculated by using the known geomagnetic field and the coil's roll,pitch angle and angle change rate.The dynamic response is subtracted from the electromagnetic response under the coil's attitude change,and the dynamic response is corrected.According to the characteristics that the static response coefficient changes slowly with the conductivity,the same static response coefficient can be used to correct the static response in a certain range of conductivity at the same coil attitude angle.The earth model with shallow and deep anomalies is simulated,then correction algorithm is used to correct coil attitude change,and is concluded that the correction algorithm can effectively correct the coil attitude change.If the conductivity of the detection area is unknown,it is necessary to obtain the conductivity by inversion or fast imaging,and then correcting the static response with the static response coefficient under this conductivity,but it may affect the accuracy of correction.In view of the problem of acquiring earth conductivity information in detection area for static response correction,a coil attitude correction method based on BP neural network is proposed.The BP neural network with four hidden layers is designed.The coil attitude angle and the static response when the coil attitude changes under different conductivity models are taken as the output of the neural network,and the static response coefficient is taken as the output of the neural network.The relatively accurate static response coefficient can be obtained when the conductivity is unknown.,and the static response can be corrected.By simulating the correction experiments of shallow and deep anomalous earth models,the effectiveness of coil attitude correction based on BP neural network is demonstrated.