Two-dimensional joint inversion of seismic velocity and electrical resistivity using seismic travel times and full channel electrical measurements based on alternating cross-gradient structural constraint

When imaging complex near surface structures using different geophysical exploration methods, because of the incompleteness of the observation system and insensitivity of geophysical data to some geophysical properties of media, there exist relatively large uncertainties and significant inconsistency in imaging results from separate inversions. For seismic travel time tomography and electrical resistivity tomography, both methods are subject to the issue of imaging shadow zones. For seismic travel time tomography, seismic rays tend to travel through high velocity zones, causing poor ray path coverage and thus low resolution of low velocity zones. For electrical resistivity tomography, the distribution of electric field lines is relatively sparse in high resistivity zones, leading to lower resolution. In order to reliably image underground structure, Gallardo and Meju (2003) proposed a joint geophysical inversion method based on cross-gradient structure constraint, which requires separate models fitting the data and at the same time structurally consistent by having approximately zero cross gradients. To more effectively realize the cross-gradient based structure constraint, we propose a new joint inversion scheme that alternatively inverts one type of data but structurally constrained by another model. With this new joint inversion scheme, we test two-dimensional cross-hole seismic travel time tomography and full channel resistivity tomography. Synthetic results show that the new scheme can efficiently realize the cross-gradient based joint inversion. Compared to separate inversions, the joint inversion can more reliably determine seismic velocity and electrical resistivity anomalies.