Self potential or spontaneous potential (SP) methods measure the electrical potential field caused by ambient DC electrical currents in the earth.  Electrical currents occur nearly everywhere in the earth due to:

The first four above are generally due to release or capture of charged particles in underground chemical reactions.  The fifth arises where a pressure gradient causes any fluid to flow through a permeable medium. This electrofiltration creates a charge separation along the flow path which is measurable as a positive increase in SP in the direction of flow.  Thus, SP surveys can locate zones of chemical reaction, as well as detect the actual movement and direction of underground fluid flow, to delineate preferred pathways, or detect leaks or seepage through dams, levees, or earthen liners.

Flowing electrolytic solutions such as acid mine drainage or leachate make dramatic SP targets. The concentrated infiltration of water into incipient sinkholes is also a common source of detectable SP anomalies in karst areas.  Often a charge imbalance created by fluid flow will remain long after the flow ceases – making it possible to detect former flow paths, seepage zones, or infiltration points (e.g. paleosinkholes).

Where there are contaminant plumes or other subsurface phenomena which contain decomposing or reacting chemicals of all kinds, the release or capture of charged ions creates currents detectable by SP surveys. Acid mine drainage generation zones, as well as underground fires in mines or organic waste masses (e.g. buried leaf mulch or stumps) in particular produce strong SP anomalies.

In corrosion studies, SP surveys detect not only the minute electrical currents generated by galvanic (passive) corrosion, but also the stray currents that drive electrolytic corrosion. In fact, the sense of mapped current flow can determine which of these two mechanisms is active, and can help identify stray current sources.

SP data can be presented as profiles or plan-view maps (based on GPS positioning) that can be interpreted to identify a wide variety of subsurface conditions and features.

As with all geophysics, SP is best interpreted in conjunction with other data.  For example, a conductive zone on an electrical image could indicate a water-bearing fracture – an interpretation that would be strongly supported by a coincident SP electrofiltration anomaly. In karst terranes, both microgravity and SP surveys may be used to evaluate sinkhole hazard. However, an SP survey indicates where there is active or former infiltration of the type that could drive soil piping, while microgravity shows where there are bedrock cavities of the type that may not yet be receptors for soil piping, but could accept infiltration if the hydrology of the site is altered – e.g. during regrading or construction.