Enhanced electroseismic coupling at charged interfaces

Abstract

Published field-scale studies of electroseismic (ES) surveying have detected harmonics of the source frequency in the recorded seismic signal. At the time the data were collected, the cause of these high frequencies was unknown. We have found that electric fields applied to lithologic interfaces create harmonics of the source frequency. At these interfaces, there are gradients in ionic concentrations in pore fluids and in situ electric fields associated with gradients in the electrochemical (EC) potential. When an electric field is applied to the junction in the direction of the internal field, the internal voltage gradient decreases and the resistivity decreases. When the applied field opposes the internal field, the internal field increases and the resistivity is high. Harmonics of the applied AC field occur when the amplitude of the applied electric field is comparable to the internal field. We use the sandstone/limestone interface as an illustrative example. An analytical model of rock interfaces predicts the wavelet response to an applied field. The theoretically predicted signal characteristics agree with the laboratory data. The model predicts that the distortion of the electric field induces strains on the rock matrix that are qualitatively similar to field measurements of the ES conversion. The estimated response from an EC junction is substantially larger than the electrokinetic conversion associated with the displacement of the pore-surface dipole layers.