The effect of layer thickness and fluid conductivity contrasts on seismoelectric signals: insights from laboratory experiments and numerical simulations

Summary Seismoelectromagnetic (SE) signals are created in wet poroelastic media by electrokinetic conversions occurring at the pore scale. Two of these signals are frequently investigated: a co-seismic wavefield, bounded to the propagating seismic waves, and an electromagnetic (EM) wave created when a seismic wave passes through the interface between two porous media. Driven by the possibility offered by the SE signals in terms of vertical resolution, compared with seismic reflection methods, a restricted number of authors studied how a thin layer, or a combination of thin layers, can modify SE signals. In this context, the present work presents original experimental and numerical data as a means to further investigate the relation between the thickness of a layer and the EM interface-generated response (IR). In addition, the fact that the IR is very sensitive to contrasts in fluid conductivity, whereas seismic waves are not, is an appealing characteristic of SE exploration. Consequently, we explored the influence of the pore-fluid electric conductivity, combining experimental and numerical methodologies. We identified a strong similarity in how layer thinning affects both seismic waves and IR signals. Moreover, we observed that the maximum enhancement of an IR signal occurs when the layer thickness is approximately half the P-wave wavelength (λP) in the layer. When analysing the influence of fluid conductivity, we showed that the electrokinetic theory adopted in this study provides satisfactory predictions for the waveforms and amplitudes observed experimentally. Finally, we extrapolated our experimental results with field-scale simulations in order to understand how the effects observed experimentally translate to georesources applications.