SP measures a naturally occurring electric fields generated by one of two phenomenon: corrosion or weathering of ferrous minerals or the flow of groundwater. The electric potential field is very small, typically millivolts in scale, but can be measured over small or large areas / sites. The electrical potential (field) is generated by an ionic exchange between the water and grain-boundaries or minerals in the subsurface. For engineering, environmental or groundwater geophysics the SP method is used almost exclusively to map the flow of groundwater through porous materials; these can be unconsolidated porous soil deposits or fractures in weathered bedrock. SP is routinely used to map preferential flow paths for seepage through embankment dams or levees. Additionally it can be used to map the path of flow for seeps observed at the ground surface downstream of cut-off walls, abandoned mines, or naturally occurring springs. The SP method is extremely effective if measurements can be obtained during, for example in reservoirs, high- and low- pool conditions when the only change in the subsurface is the head or gradient on the flow condition. This method, called differencing, basically eliminates the impact to SP data caused by geology or appurtenant structures within the embankment. Surveys are often conducted in the reservoir pool itself to find the source of the seepage, on the crest and multiple locations along the downstream face and toe of the structure to determine if the seepage is pervasive and laterally extensive (i.e., blanket seepage) or if it is concentrated flow which could lead to higher pressures and the potential for internal erosion. SP can also be called the ‘Streaming Potential’ and in NDT it is commonly known as the ‘Corrosion Potential’ (of rebar in concrete). SP results can be quite valuable when used in conjunction with ERT.
Applicable on: Soil and Rock.
Test for: Mapping preferential flow paths through man-made structures (i.e., dams or levees) or around natural seeps and springs, and mineral exploration.