Experimental and numerical study of the seismo-electromagnetic signals created at thin porous-porous interfaces

Seismo-electromagnetic (SE) signals are created in fluid-saturated porous media by electrokinetic conversions at the pore scale. Two of these signals are frequently investigated: the first is a co-seismic wavefield, bounded to the propagating seismic waves, and the second is an electromagnetic (EM) wave created when a seismic wave passes through the interface between two porous media. Led by the effectiveness of SE phenomena in detecting thin layers (i.e., layers with thicknesses smaller than the seismic wavelength), many authors studied how a thin layer, or a combination of thin layers, can alter SE signals. In this context, the present work brings new experimental and numerical data as a means to further investigate the relation between the layer thickness and the EM interface-generated response. Particularly, we have noticed that the effect of layer thinning on the EM interface-generated waves is similar to what is observed for seismic waves, but with a maximum signal enhancing, due only to thickness, occurring when a layer has a thickness that is equal to half the wavelength of the P-wave impinging on the layer. We have also explored the effect of the pore-fluid electric conductivity on SE signals, since this parameter plays an important role on the SE fields. The fact that the EM interface-generated wave is very sensitive to contrasts of fluid conductivity, whereas seismic waves (and therefore the co-seismic) are, in effect, insensitive, is an appealing characteristic of SE exploration. By comparing experiments with simulations we have accessed the effect of fluid conductivity on SE wavefields, complementing previous studies with a quantitative analysis of the dependency of SE signals on this physical property. Moreover, we have evaluated the ability of the electrokinetic theory adopted in this study to predict our experimental data, and showed that it performs well in terms of waveform and amplitude behavior for all fluid conductivities considered.