Seismoelectrical Method - Operation

Operation

When a seismic wave encounters an interface, it creates a charge separation at the interface forming an electric dipole. This dipole radiates an electromagnetic wave that can be detected by antennae on the ground surface.

As the seismic (P or compression) waves stress earth materials, four geophysical phenomenon occur:

1. The resistivity of the earth materials is modulated by the seismic wave;
2. Electrokinetic effects analogous to streaming potentials are created by the seismic wave;
3. Piezoelectric effects are created by the seismic wave; and
4. High-frequency, audio- and high-frequency radio frequency impulsive responses are generated in sulfide minerals (sometimes referred to as RPE).

The dominant application of the electroseismic method is to measure the electrokinetic effect or streaming potential (item 2, above). Electrokinetic effects are initiated by sound waves (typically P-waves) passing through a porous rock inducing relative motion of the rock matrix and fluid. Motion of the ionic fluid through the capillaries in the rock occurs with cations (or less commonly, anions) preferentially adhering to the capillary walls, so that applied pressure and resulting fluid flow relative to the rock matrix produces an electric dipole. In a non-homogeneous formation, the seismic wave generates an oscillating flow of fluid and a corresponding oscillating electrical and EM field. The resulting EM wave can be detected by electrode pairs placed on the ground surface.

Surface seismic sources and measurement electrode pairs are generally used to measure the electrokinetic effect. Borehole systems also have been recently developed. A hammer blow or small explosive (black powder) are typically used for the seismic source. Two, short (few meters) electrode pairs are typically located collinear and symmetric to the seismic source. Stacking of repeat seismic "shots" is often used to improve signal to noise.