Electrical sounding and profiling

Vertical electrical sounding and electrical profiling are methods for investigating the subsurface by measuring the capacity of earth materials to pass electrical current. These techniques are effective in detecting boundaries between materials that have contrasting resistivity. In engineering geological investigation, they are used to determine the thickness of overburden, to determine the tectonic zone, the boundary between rocks of different lithological composition and determine the parameter of resistivity for lightning protection design.

DC resistivity measurement

      Diagram showing the basic principle of DC resistivity measurement

Resistivity exploration can be carried out to depths that range from a few meters to a hundred meters under favorable conditions. Recording of resistivity information was carried out using only four electrodes and one of the standard geometric arrays. Methods are based on generation of an artificial electric field in the earth by introduction of current through metal electrodes and voltage is measured across two other electrodes.

vertical-electrical-sounding-survey

                                  A vertical electrical sounding survey at field

Instrument readings (current and voltage) are generally reduced to “apparent resistivity” values. The apparent resistivity is the resistivity of the homogeneous half-space which would produce the observed instrument response for a given electrode spacing. Apparent resistivity is a weighted average of soil resistivities over the depth of investigation. For soundings a log-log plot of apparent resistivity versus electrode separation is obtained. The resistivity data is then used to create a hypothetical model of the earth and it’s resistivity structure. Resistivity models are generally not unique; i.e., a large number of earth models can produce the same observed data or sounding curve.

circular electrical sounding

A circular electrical sounding for determining fracture direction

Fieldwork, processing and interpretation of vertical electrical sounding and electrical profiling are carried out in accordance with Vietnamese Standard 9432: 2012 and ASTM D 6431 – 99: Standard guide for using the direct current resistivity method for subsurface investigation.