Self Potential (SP) Surveys
Self Potential (SP) geophysical surveys measure the potential difference between two points on the ground produced by the small, naturally produced currents that occur beneath the Earth's surface. It is predominantly used for DSEAR risk assessments at new petrol stations. The SP method is passive, non-intrusive and does not require the application of an electric current. Small potentials of a few millivolts produce two electrolytic solutions of differing concentrations that are in direct contact. Followed by the flow of groundwater through porous materials (streaming potential). Larger ground potentials are generated by conductive mineralised ore bodies which are partially immersed below the water table.
Passive measurement of ground potential using the SP method
DSEAR stray current testing
Finding leaks in canal embankments
Identifying seepage in dams and reservoirs
Assessing effectiveness of water-engineering remedial measures
SP Survey Operation
Standard SP surveys utilise non-polarising, porous pot electrodes. They have been adapted to minimise contact voltages. Readings are typically taken with one electrode fixed at a base station. A second, mobile 'field' electrode is moved around the survey area. Reading stations are spaced at regular intervals along linear profiles, closed loops or grids depending upon the desired application.
The self-potential method is traditionally used as a mineral exploration tool and for downhole logging in the oil industry. More recently, it has been adapted for hydro-geological and water engineering applications. This is by the use of more sensitive equipment and the careful application of data correction processes.
Stray Current Testing
SP is a useful method for detecting sporadic, man-made electrical currents in the ground, known as stray currents. These are produced by faults in high voltage electrical plant. For example, generators, industrial machinery and sub-stations. The resultant changes to the natural electrical field are dynamic and random in nature. They can result in a potential hazard in some circumstances. Stray currents cause localised, enhanced corrosion of buried steel structures. In rare circumstances they can create an ignition risk around buried fuel tanks and fuel pipelines. Our customised SP equipment can detect random changes to the background electric field vector. This makes it a valuable tool for carrying out DSEAR risk assessments at new petrol stations and other sites where volatiles are to be stored in USTs.