The magneto-seismic method in geoscience

Abstract

Key concepts and potential applications associated with a phenomenon hitherto unexplored by the geoscience community, which we have named the magneto-seismic effect, are introduced. The method is based on the simple principle that when an electric charge moves in the presence of an external magnetic field, the charge carrier experiences an instantaneous force, which is equal to the vector cross product of the current it carries and the magnetic field that is present. This “Lorentz force” can create both compressional and shear sound waves in electrical conductors by passing an alternating current through them via an electromagnetic source. In laboratory settings, this magneto-seismic effect can produce readily detectable rock frame displacements. This opens up the possibility of developing new experimental methods to interrogate elastic and poroelastic response of rocks in a broad frequency range from subhertz to megahertz, potentially closing the frequency gap between traditional ultrasonic characterization and properties of interest in the seismic frequency band. In exploration settings, electric current dipole/bipole sources, or novel rotating magnetic dipole sources, can be used to generate electric currents at depth. These currents produce seismic waves at interfaces (or boundaries) where conductivity abruptly changes. The amplitude and propagation directions of these generated seismic waves depend on the relative orientation of the interfaces (or boundaries) with respect to the earth's magnetic field. These seismic waves can then be recorded by receivers at the surface and, in principle, might be processed to yield a resistivity map with seismic resolution. It is shown that processing to obtain a signal from deep targets is significantly limited by seismic background noise. However, an acceptable signal-to-noise ratio might be achieved for shallower targets. The difference between the magneto-seismic response and the previously well-studied electro-seismic response will be discussed.