Electrical Surveys

The electrical methods, used for measurement of subsurface resistivity, involve passing an electrical current into the ground using two electrodes, and measuring resultant potential using two potential electrodes. Resistivity sounding involves gradually increasing the spacing between the current and/or potential electrodes to obtain deeper penetration. Under profiling, the electrode spacing is kept constant and the entire arrangement is moved along profile lines, to obtain lateral variation in subsurface resistivity.

Electrical resistivity techniques are based on the principle that the resistivity varied depending on the material encountered. Resistivity can then be used to identify different geological units by their electrical properties. If a materials resistivity value drops it could mean that the rock is water saturated and one can expect to find fractured bedrock. The variation in resistivity will correspond to a geological variation along a investigated line. By calibrating the resistivity results with known geologic materials on site, valuable geotechnical information can be obtained for the dam. This type of survey becomes interesting in determining the weathered bedrock layers and the presence of sand/ moraine under the clay. It also corroborates the information from the refraction results for better geological interpretation.

2D resistivity imaging uses an array of electrodes connected by multicore cable to provide a linear depth profile of the variation in resistivity both along the survey line and with depth.

The acquisition system consists of two units connected to a laptop computer. One unit is a resistivity meter which injects the electrical current into the ground and measures the voltage between the electrodes. The other unit is a control switch box which selects both the injection electrode pair and the potential electrode pair for each datapoint .

The data are presented as pseudo-section in which the spatial distribution of the electric properties of the investigated material can be qualified. To qualify the data , a modeling routine is applied to the dataset according to the Zohdy method. This new section can be correlated to a depth section since the Zohdy method computes from the datapoint a series of continuous vertical electrical profiles.

The Electrical surveys of used in civil engineering, water resources, mining and environmental projects to:

Experts with Parsan Overseas have been regularly involved in electrical surveys for ground water prospecting and other applications. The experts have used electrical imaging for site assessment (underground storage tanks), dam structure assessment and various related fields.