Self Potential Imaging as a Tool
for Subsurface Gas Characterization

 

 

 

 

 

 

 

 

 

 

In geophysics, self potentials refer to observable voltages that are measured on or near the surface of the Earth. Conceptually, these potentials arise from current sources within the Earth that behave as batteries. The passage of current, passing through resistive Earth materials, creates the self potential (SP) signal. Sources of SP include, but are not limited to: mechanical transport of ions by fluid flow, electrochemical variations in Earth media, and thermoelectric sources.

The figure above depicts the spatial distribution (xz-plane) of SP (left panel) and the partial pressure of oxygen (right panel) at a contaminant sparging facility at the Massachusetts Military Reservation. Sparging is a mechanical process whereby a region in the subsurface is pressurized with air pumped down a borehole into the Earth – frequently accompanied by a negatively pressurized borehole at some distance. The pressurized air not only increases the hydraulic gradient, allowing a contaminant to be pumped out of the ground, but it also introduces atomized oxygen bubbles into to the contaminant, creating bouyancy, which causes the contaminant to rise to surface. Both panels were derived when the sparging pump was active. The SP panel depicts an anomaly approaching nearly one volt (an extremely large SP anomaly) and corresponds almost exactly with the logarithmic plot of the partial pressure of oxygen in the region.

A relationship bewteen SP (mV) to the partial pressure of oxygen (Po2) can be created based upon the Nernst equation. The relation is as follows:

( E2 - E1 ) = 14.8 [ log ( Po22 / Po21 ) ] ( mV )

where E2 and E1 refer to the measured and standard potentials, respectively. The standard potential is typically taken to be the SP that would arise due to the atmospheric partial pressure of oxygen – E1 = 806 mV at Po21 = 0.21. Correspondingly, Po22 is the measured partial pressure of oxygen in the sparging environment.

Based upon the preceding results, one can see that SP could also be used to calculate Pco2, the partial pressure of carbon dioxide, in a pressurized subsurface environment. This would be an extremely useful way of monitoring a CO2 sequestration facility, for example.

Return to Environmental Research