A novel approach for seismoelectric measurements using multielectrode arrangements - I: theory and numerical experiments

Seismoelectric measurements consist in recording the transient electric fields generated by seismic waves propagating in fluid-filled porous or fractured media. The electric fields are conventionally measured by voltage differences between two closely spaced electrodes. Unfortunately, this measurement protocol has a direct influence on the recorded waveforms and their amplitudes. Using a filter theory approach and full waveform numerical simulations of the coupled seismic and electromagnetic wave propagation in fluid-filled porous media, we show that the coseismic electric arrivals and the small-amplitude electromagnetic interface response can be severely distorted and/or attenuated by conventional surface electrode layouts. Unlike the low-pass wavenumber filter representing the response of two geophones connected in series, the filter associated with a voltage difference is shown to be a band-pass wavenumber filter. As a result, not only horizontally and obliquely propagating waves but also vertically propagating waves undergo selective frequency attenuation in the 0-150 Hz frequency band used in field surveys. It is shown that electrode spacing cannot be optimized to enhance the electric signature of typical seismic reflections and electromagnetic interface responses, neither with horizontal dipoles nor with reasonably sized vertical dipoles. To mitigate the damaging filtering effects of electric dipoles, we consider arrangements of 3 and 5 equidistant electrodes with alternating polarity and generalize these configurations to a higher odd number of electrodes. We show that such arrangements are ideally described by low-pass wavenumber filters which preserve the quasi-plane waves corresponding to the electromagnetic interface responses in the frequency band of interest. These properties are entirely confirmed by the computation of synthetic seismoelectrograms representing the electric potential created by a seismic excitation, which allow us to determine voltage differences or voltage combinations involving two or more electrodes. In field conditions, these benefits can be challenged by the imperfect coupling between the electrodes and the ground, or by a misplacement of the electrodes, both of which require a strict control of the electrode installation.