[SSS09-P06] Seismoelectromagnetic conversions generated by a double-couple source in
partially saturated media.
Keywords:Electrokinetic conversions, Seismoelectric imaging, Induced seismicity monitoring
the theory for the coupled propagation of seismic and electromagnetic (EM) waves was
reformulated by Pride (1994). Over the last couple of decades, several modelling programs were
developed to simulate seismoelectromagnetic propagation. Most of these programs share in
common that they only allow to model point sources, well suited to simulate seismic surveys, but
inappropriate when attempting to simulate earthquakes, often described using a double-couple
In this work, we present a program allowing to model the seismoelectromagnetic waves triggered
by a double-couple source, based on the Synthetic Kennett Bouchon Program (SKBP) developed by
Garambois & Dietrich (2002); this program is modified to account for partial saturation conditions
(Warden et al., 2013) and high salinities (Vinogradov et al., 2010). We implement both M13+M31
and M12+M21 geometries. We validate our program by comparing its results with those obtained by
modelling a double-couple source through a numerical derivative and find excellent agreement
between both approaches. We then study the symmetry properties of the waves generated by this
new source type for a homogenous half-space.
We then perform a sensitivity study considering a simple tabular medium consisting of a
homogeneous layer overlying a homogeneous half-space. When considering a source of frequency
fpeak=40Hz of magnitude Mw=1, located 4km below the ground surface and 1.5km below the
boundary between both media, we model coseismic electric field amplitudes of the order of several
hundreds of μV.m−1. Analyzing these amplitudes in the light of magnetotelluric measurements
suggests that even a microearthquake could trigger a coseismic response large enough to be
measured at the ground surface.
We also study the characteristics of the interface response (IR) generated at the boundary between
the two homogenous media for a M13+M31 double-couple and show that this IR consists of a
conversion between an incident S wave and a converted EM wave; its amplitude variations can be
approximated by those of a horizontal dipole located at the boundary. We also quantify the
contributions of the SH and SV polarizations to the total converted amplitude and find that although
the SH waves are coupled to the Transverse Electric (TE) mode, they contribute to the Transverse
Magnetic (TM) mode component of the electric field through the IR.
We conclude our study by considering a more realistic model, designed to approximate the context
of a geothermal field. We also provide examples of possible coseismic signals observed in electric
and magnetic data acquired during geothermal monitoring experiments.