Spontaneous potential

where: Membrane Potential develops when two electrolytes of different ionic concentrations, such as mud and formation water, are separated by shale.

The clay minerals in shale are usually made up of atom Al, Si, and O. O2− ions occupy the outer layer and cause a net negative charge.

The magnitude of the deflection depends mainly on the salinity contrast between borehole and formation fluid, and the clay content of the permeable bed.

Electrodes can be placed on the ground surface to map relative changes in the SP value (in millivolts, or mV), typically with the goal of identifying the path of groundwater flow in the subsurface, or seepage from an earthen dam.

A voltmeter measures the voltage between a fixed liquid-junction electrode and a mobile one (rover), which is moved along a dam face or over an area of investigation to collect multiple readings.

Besides, SP is also affected by the following factors: Spontaneous potential can be measured by placing one probe of a voltmeter at the Earth's surface (called surface electrode) and the other probe in the borehole (called downhole electrode), where the SP is to be measured.