Seismoelectric response with gradually distributed saturation: Variable time-step finite-difference modeling and experimental investigation

The seismoelectric effects induced by seismic waves have unique advantages in geophysical exploration. However, most theoretical and experimental studies are based on a single-fluid assumption, which is not consistent with the different saturations that exist in actual formations. Hence, we perform related investigations on the seismoelectric response with nonuniform saturation configurations. By applying the water content distribution to the unsaturated seismoelectric theory, we propose a non-linear saturation model and a variable time-step method for seismoelectric simulation to solve the efficiency problem in the finite-difference algorithm, which can reduce the time consumption by more than 90% compared to the fixed time-step case. We then analyze the influence of saturation discontinuities on seismic and seismoelectric waves. Based on the theoretical analysis, we conduct seismoelectric experiments using a partial saturation model to verify the theoretical simulations. In the measurements, we not only observe the seismoelectric signals outside the rock surface but also record the seismoelectric signals inside the rock using the embedded electrode, where the interface electromagnetic waves excited by the water saturation changes can be successfully detected. Subsequently, the theoretical simulations and the experimental signals are compared with the partial saturation model. These two results show a good agreement in the time-domain waveforms. These results indicate that seismoelectric signals are more sensitive to saturation changes than seismic signals for the detection of stratigraphic fluid boundaries. These findings can provide guidance for seismoelectric research and the geophysical exploration of complex reservoirs.