Electrical Techniques
1D Resistivity Sounding
Detail
Resistivity measurements are made by passing an electrical current into the ground using a pair of electrodes and measuring the resulting potential gradient within the subsurface with a second (potential) electrode pair (normally located between the current electrodes). Resistivity soundings involve gradually increasing the spacing between the current/potential electrodes (or both) in order to increase the depth of investigation. The resistance data collected in this way are converted to apparent resistivity readings that can then be modelled to provide information on the thickness of individual resistivity layers within the subsurface.
Measurements are taken manually using a ground resistivity meter that will normally provide a direct readout of resistance. In order to convert the resistance reading to an apparent ground resistivity, a geometric factor is applied to the data, based on the type of electrode configuration being used. The most common electrode array used in sounding work is the Schlumberger array. In this configuration the two potential electrodes are located at the centre of the spread and are closely spaced compared to the two current electrodes that are also located symmetrically about the centre point.
In order to increase the depth of investigation the current electrode separation is increased whilst the potential separation remains constant. In this way only two electrodes require moving compared to all four in other configurations such as the Wenner array illustrated above.
The most common problem encountered in resistivity sounding work is high contact resistances at the current electrodes. Whilst this does not directly affect the measured value of resistance, high contact resistances (>2kOhms) will reduce the maximum current that can be applied with the output voltage available from the meter (typically 300-400V). In order to overcome high resistances electrodes can be watered with a saturated salt solution or placed in hole filled with bentonite or clay slurry.
