Forward Modeling of Shallow Transient Electromagnetic Method under Ramp Turn-off Current
In order to reduce the shallow detection blind zone and improve the shallow detection accuracy, this paper presents a shallow transient electromagnetic direct forward modeling method based on ramp turn-off current. The main idea is to obtain the spectrum of the ramp turn-off current signal by Fourier transform at first, then using Maxwell's equation to solve the propagation process of the electromagnetic signal in the medium, thus obtaining the early signal of transient electromagnetic detection.In theory, this method is more suitable for the detection of shallow subsurface media than the ideal step signal. Simulation experiments on the ramp step signals with different turn-off times show that the complete ideal coincidence with the traditional ideal step signal when the turn-off time approaches zero, indicating the correctness of the algorithm. At the same time, compared with the current element superposition forward method, the forward curve obtained by this method is closer to the actual situation,and the calculation speed is much faster than the current element superposition method. The research results in this paper are of great value to reduce the detection blind zone, improve the detection accuracy and real-time performance of the transient electromagnetic detecting.