Of all the shallow geophysical exploration techniques, geo-electrical profiling is on the most robust techniques currently available. Geo electrical profiling is a highly reliable and versatile method that can be under harsh field and subsurface conditions. This technique has grown worldwide popularity within the last few decades due to advancements in both hardware and software capabilities. The accuracy of measurement has also been improved by a factor of 10 by recent advancements in the geo-electrical measurement equipment. Geo-electrical profiling typically encompasses three different measurements – resistivity, induced polarization, and self potential. Full waveform measurement makes it possible for more accurate mathematical modeling of the subsurface conditions; hence, better interpretive analysis can be performed by the end user. A combinational form of measurement is beneficial and superior then conventional one measurement exploration.
The resistivity method induces a direct current or very low frequency into the ground and the corresponding equipotential voltage field is measured. The resistance of the material is calculated based on the known current injection and measurement voltage field. Resistivity Imaging is particularly useful in fine grained soils such as silts and clays where methods such as Ground Penetrating Radar (GPR) are less effective and useful to define transitional boundaries which can be difficult to detect using other geophysical methods.
The time domain induced polarization (IP) technique energizes the ground with an alternating square wave pulse. During current injection, the apparent bulk resistivity of the ground is calculated. After the pulse has been transmitted, the IP effect is measured as a time diminishing voltage at the receiver electrodes. Combined resistivity and IP imaging is a powerful tool for mapping of subsurface conditions in a very efficient process, both in the field and in post processing.
Self potential is the natural ground voltage observed between nearby non-polarizing electrodes in the field survey. Small currents are naturally produced beneath the Earth’s surface. The Self Potential or Spontaneous Potential (SP) geophysical method measures the potential difference produced by the currents between any two points on the ground surface. The SP method is a passive, non-intrusive method that does not require the application of an electrical current.
Small potentials of the order of a few millivolts (mV) are produced by two electrolytic solutions of differing ionic concentrations being in direct contact. The flow of electrolytic fluids through porous materials (streaming potential) can also be detected using this method. Larger ground potentials are produced by conductive mineralized ore bodies that are partially immersed below the water table.