Electroseismic Waves From Point Sources In Layered Media

Abstract

The macroscopic governing equations controlling the coupled electromagnetics and acoustics of porous media are numerically solved for the case of a layered poro-elastic medium.It is shown that these coupled equations decouple into two equation sets describing two uncoupled wavefield pictures. That is, the PSVTM picture where the compressional and vertical polarized mechanical waves drive currents in the PSV particle motion plane that couples to the electromagnetic wavefield components of the TM mode. And the SHTE picture where the horizontal polarized rotational mechanical waves drive currents in the SH particle motion plane that couples to the electromagnetic wavefield components of the T E mode. The global matrix method is employed in computing electroseismograms in layered poro-elastic media in the PSVTM picture. The principal features of the converted electromagnetic signals are the following: (1) contacts all antennas at approximately the same time; (2) arrives at the antennas at half of the seismic traveltime at normal incidence reflected P waves; and (3) changes sign on opposite sides of the shot. The seismic pulse is shown to induce electric fields that travel with the compressional wavespeed and magnetic fields that travel with the rotational wavefield. The frequency content of the converted electromagnetic field has the same frequency content of the driving incident seismic pulse, as long as the propagation distances are much less than the electromagnetic skin depth. Snapshots in time and converted electromagnetic amplitudes versus seismic point source-antenna offset-are calculated for contrasts in mechanical and/or electrical medium property. Conversion happens there where the seismic wavefront passes a contrast in medium properties due to generated imbalances in current across the contrast. The TM component amplitude radiation pattern away from the interface shows similarities with an effective electric dipole radiation pattern, or its dual, an effective magnetic current loop radiation pattern centered right beneath the source at the contrast's depth. The TM mode amplitudes decay rapidly with traveled distance and suggest the importance of a Vertical Electroseismic Profiling geometry to enhance recording of the converted electromagnetic signal by positioning the antennas closer to the target (contrast) of interest.United States. Dept. of Energy. Office of Energy Research (Grant DE-FG02-93ERI4322