Full waveform inversion for the borehole transient electromagnetic pseudo seismic data

The borehole transient electromagnetic method is arranged in the drill hole with transmitting and receiving devices, which are closer to the target body and suitable for finely detecting the water-bearing body around the coal mine borehole. The conventional borehole transient electromagnetic method approximates the direction of anomaly by multi-component response. resulting in difficulty characterizing the boundary position of anomalous bodies. Moreover, due to the diffusion characteristics of the transient electromagnetic field, the conventional inversion method has a low resolution of the boundary location of the geological anomaly. To address the problems above, this paper, based on the borehole transient electromagnetic scanning detection method attempts to convert the borehole transient electromagnetic field into the virtual wave field by using the wave field conversion formula. Then, a full waveform method can be applied to acquire a resistivity profile. Technically to address the strong energy of the virtual wave field direct wave and the interference wave, the shear wave transform is adopted to remove the direct wave of the virtual wave and then avoid the false interface, furthermore, to address the strong ill- posed problem of kernel function in the wave field conversion process, the virtual conductivity is introduced to reduce the virtual wave field velocity and thus improve the ill-posed condition: moreover, the damped least squares and adjoint state full waveform inversion method are performed respectively. In this way, the numerical simulation verifies that the inversion results obtained by the damped least squares full waveform inversion method are more accurate. which enables to achieve the fine portrayal of the layer information outside of the borehole wall, and improve the inversion and imaging results of the scanning detection of the borehole transient electromagnetic method.