When the natural state of equilibrium of the electric double layer (slipping plane) is disturbed, in this case by a passing seismic P-wave, electron transfer occurs in the re-equalization of the charge equilibrium. This flow of electrons results in the generation of an electric field, that travels to the surface to be recorded by a grounded dipole antenna. The recorded field data contains information as to the electrical properties of the rock matrix and fluid properties. An electric field is only created in the presence of a fluid saturated porous media.
How it works
In a purely homogeneous media, the electric field produced by charge separation travels along with, is contained within the propagating seismic wave. This is called the co-seismic field. When the seismic wave passes by the dipole antenna, the potencial difference can be measured and contains information about the passing seismic wave. However, when a primary seismic wave intersects a geological interface, charge imbalance occurs, and the E-Field radiates outward to the surface.
Data Collection
Impulse type seismic source systems are used to inject seismic energy into the subsurface geology. Drop weight seismic sources such as the one shown here are capable of surveying to depths of 5000 meters below surface level. The environmental footprint of the survey is extremely small, in that the seismic source leave a small 30 x 30cm indent in the ground that is easily remediated after the sounding is complete.
For shallower investigations to approximately 500 meters depth, a simple sledge hammer and plate seismic source can be used. This is the simplest, most cost effective, and most versatile method of inducing an seismic impulse.
The recording setup consists of a smart phone device such as an iphone or android device, a cable that connects the phone to the electrodes and if necessary, usb-c to audio adapter if the phone does not have an audio jack. The GeoVue web application is used to record the data and collect location information.
Alternatively, the user may use any off the shelf audio recorder to collect data, as long as it is capable of recording audio in stereo 16 bit, 44100 Hz, in WAV format. The recorded wave files can be imported directly into the GeoVue application as well as KML coordinate data, for the sounding point locations.
A simple grounded dipole antenna system, consisting of two pin electrodes inserted into the ground approximately 2 meters apart, makes taking soundings, in locations that would be difficult for other techniques, simple.
