Seismoelectric Radiating Reflection and Transmission at Porous–Porous Interfaces: Angle- and Frequency-Dependent Coefficients

Detecting fluid contacts, such as oil-water interfaces, remains a significant challenge for conventional seismic exploration when the acoustic impedance contrast is weak. However, the seismoelectric effect, which depends on electrical conductivity and electrokinetic coupling, provides a promising method for interface detection. In this study, we investigate the radiating seismoelectric electromagnetic (EM) reflection and transmission coefficients generated at an interface between two fluid-saturated porous media under fast P-wave incidence.

We analytically derive the reflection and transmission coefficients for all generated wave modes (fast P, slow P, SV, and EM waves) using Pride’s theory and Helmholtz decomposition. To verify the validity of our derivation, we calculate the energy fluxes of the wavefields using the electrokinetic Poynting vector and confirm that energy conservation is strictly satisfied.

Analyzing the frequency- and angle-dependent behavior of the reflection and transmission coefficients, we show that seismoelectric conversion is directly governed by the contrast in elastic and electric properties of the reservoir and the fluids therein, at the interface. Building on this, we compare the response of an "elastic interface" (strong mechanical contrast) with an "electric interface" (strong conductivity contrast but weak mechanical contrast, representing an oil-water contact). The analysis shows that while the seismic reflection energy at the electric interface is negligible, the reflected seismoelectric EM wave energy is comparable to the seismic signal generated at a strong lithological interface. These findings suggest that reflected seismoelectric waves are a reliable tool for identifying oil-water interfaces that are effectively invisible to conventional seismic surveys.