Dependence of shear wave seismoelectrics on soil textures: a numerical study in the vadose zone.

ZYSERMAN, FABIO IVÁN; LEONARDO MONACHESI; LAURENCE JOUNIAUX

GEOPHYSICAL JOURNAL INTERNATIONAL

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2017 vol. 208 p. 918 - 935

0956-540X

In this work we study seismoelectric conversions generated in the vadose zone, when this region is traversed by a pure SH wave. We assume that the soil is a one-dimensional partially saturated lossy porous medium and we use the van Genuchten´s constitutive model to describe the water saturation profile. Correspondingly, we extend Pride´s formulation to deal with partially saturated media. In order to evaluate the influence of different soil textures we perform a numerical analysis considering, among other relevant properties, the electrokinetic coupling, coseismic responses and interface responses (IR). We propose new analytical transfer functions, modified from Bordes et al. (2015) and Garambois and Dietrich (2001), for the electrical and magnetic field as a function of the water saturation. Further, we introduce two substantially different saturation dependent functions into the electrokinetic (EK) coupling linking the poroelastic and the electromagnetic wave equations. The numerical results show that the electric field IR markedly depend on the soil texture and the chosen EK coupling model, and are several orders of magnitude stronger than the electric field coseismic ones. We also found that the IR of the water table for the silty and clayey soils are stronger than those for the sandy soils, assuming a non monotonous saturation dependence of the EK coupling, which takes into account the charged air-water interface. These IR have been interpreted as the result of the jump in the viscous electric current density at the water table. The amplitude of the IR is obtained using a plane SH wave, neglecting both the spherical spreading and the restriction of its origin to the first Fresnel zone, effects that could lower the predicted values. However, we made an estimation of the expected electric field IR amplitudes detectable in the field by means of the analytical transfer functions, accounting for spherical spreading of the SH seismic waves. This prediction yields a value of 15 μV/m, which is compatible with reported values.