Imaging the Earth Interior using Virtual Seismic Electrodes
Surface geophysics provides information about subsurface physical
properties and their distribution. Geophysical observations depend on
the interaction of a man-made perturbation with the heterogeneous
subsurface. A seismic wave propagating through a porous rock generates
electromagnetic signals that depend on the fluid saturation in rock
pores (oil, water, etc), and that can be observed remotely. This
seismoelectric effect allows us to determine fluid saturation and rock
permeability, which cannot be observed with seismic waves
alone. However, seismoelectric exploration is difficult because the
observed signals are weak, and because seismoelectric conversions are
distributed in the medium to places where seismic waves interact with
solid/fluid contacts. Therefore, we cannot associate an observed
change in electric potential with a specific seismoelectric conversion
in the subsurface. The goal is to observe this effect for a strong
seismic wave localized at a point in space, at a given moment in
time. This would allow us to scan the subsurface and make direct
measurements of the fluid content, which would have a huge impact on
oil and gas exploration, and on groundwater management.
Our solution to the localization problem is to use multiple sources of
seismic energy synchronized in space and time in order to focus the
seismic waves at a target point. Focusing not only reduces the
ambiguity of the seismoelectric conversion origin, but also insures
strong conversions since all the energy induced into the medium
concentrates at a single point, instead of being distributed
throughout the medium. The localized electric source produced by
seismic focusing acts as a virtual electrode with an arbitrary but
known position and trigger time.
All codes come from the Madagascar software package.
Our CSM group is among the top developers of this open-source package.