Quantitative comparison of seismoelectric laboratory data with numerical modelling based on electrokinetic theory

When a seismic wave propagates in a wet porous medium, the transient movement of ions inside the pores will give rise to an electromagnetic (EM) wavefield that accompanies the seismic field. This coseismic field, due to electrokinetic phenomena, carries valuable information about the fluid content and the petrophysical properties. Additionally, when the seismic disturbance reaches an interface, where either the petrophysical, the fluid, or both properties change, another EM wave will be generated, due to the electric charge imbalance across the interface. This second wave propagates independently and carries information about the discontinuity where it was generated. While the first contains mainly information about the physical properties around the receivers, the second is a noteworthy alternative to near surface exploration since it can be detected away from the interface generating it. Moreover, it can detect layers thinner than what the seismic resolution allows, being specially sensitive to fluid changes. Seeking to extend the understanding of seismoelectric phenomena we developed a experimental setup able to detect both seismo-EM effects. To record seismic displacement we use a laser vibrometer and for the EM signals we measure (approximate) absolute potentials using stainless steel electrodes. We study two cases, the first is a saturated homogeneous sand, and the second includes a thin sandstone layer buried inside the sand. The experimental dataset confirms that measuring absolute potentials allows the interface-generated EM wave to be detected by receivers ten wavelengths away from its origin, whereas it is hardly detected when using dipolar arrays (which are common practice) located near the layer. Using a benchmarked numerical code we quantitatively compare theoretical predictions and experimental data, finding that seismo-EM amplitudes agree within a factor of 2. While this result validates the seismoelectric theory that the code is based on, it also opens the path for future upscaling of the experimental workflow used in the comparison and show that absolute potentials should be systematically measured. Finally, our study supports that thin layers can be detected by this method.