Joint inversion of 2D resistivity and seismic travel time data to image saltwater intrusion over karstic areas

The simultaneous inversion of multiple geophysical data types has been proven to be a powerful tool to both improve subsurface imaging and help in the interpretation process. The main goal of this paper was to develop joint inversion strategies to provide improved resistivity and seismic velocity images for delineating saline water zones in karstic geological formations. The cross-gradient constraint approach was adopted to jointly invert resistivity and seismic first arrival data. The basic idea of this approach is to quantitatively estimate the structural similarity between resistivity and seismic velocity models, using the cross product of their gradients and to achieve a unified geological model which satisfies both data sets. Initially, synthetic data were employed to help develop a joint inversion strategy to be used over such complex geological structures. The proposed strategy uses a weighting factor for the cross-gradient constraints and separate damping factors for the resistivity and seismic data. This strategy was applied successfully on field data from the karstic region of Stilos, Crete, Greece.