Underwater Detection Method of Highly Conductively Targets Based on Airborne Transient Electromagnetic Method

With the rapid development of acoustic and magnetic stealth technology, the effectiveness of mainstream acoustic detection and passive magnetic detection has been greatly reduced. The classical airborne transient electromagnetic method(ATEM), with the advantages of active detection, sensitivity to highly conductive targets, and high detection speed, has great potential in the field of underwater detection of highly conductive targets. Based on the principle of the airborne transient electromagnetic method, this paper obtained the vertical magnetic field response formula in the time domain based on the form of a circular central loop device and studied the vertical magnetic field response characteristics in the one-dimensional model with different depths of the target layer and different transmitting and receiving heights by constructing the layered model of seawater-highly conductive layer–seawater–seabed rock. Based on the three-dimensional time domain finite volume method, the forward calculation of the underwater cuboid with high conductivity was carried out, and the attenuation curve of dBz/dt at the center of the receiving coil was obtained. The one-dimensional and three-dimensional simulation results all show that the vertical magnetic field response characteristics are significantly different when there is a target with high conductivity in seawater, compared with the condition with no target. In the inversion imaging, the simulation data of different signal-to-noise ratios(SNRs) were synthesized by Gaussian white noise, and the one-dimensional calculation of underwater targets was carried out in combination with OCCAM inversion algorithm, so as to compare the detection effect of underwater targets under different SNRs. Inversion results show that when the SNR is greater than or equal to 10 dB, the highly conductive layer in the water can be effectively detected; when it reaches 40 dB, the depth and thickness information of the highly conductive layer can also be well judged. Finally, at an SNR of 40 dB, the resistivity value of the high conductive layer is reduced proportionally, and it is found that this method still has a significant response to the depth information of the target layer. According to the above forward and inversion calculation processes, the feasibility of this method in the field of underwater detection of highly conductive targets is effectively verified.