|
|
|
ELECTRICAL RESISTANCE TOMOGRAPHY
|
Resistivity The resistivity method measures the electrical resistivity of the subsurface that includes soil, rock, and groundwater. Resistivity provides information on layering and depths of subsurface strata including lateral changes in the subsurface. Resistivity can locate water-saturated and unsaturated voids, leaking dams, and map stratigraphic and structural features. Resistivity surveys can be conducted across a total survey area, as a profile, or as soundings at discrete locations. A current is inserted into the ground by a pair of surface electrodes. The subsurface resistivity is calculated from the electrode separation, applied current, and measured voltage. Most soil and rock minerals are electrical insulators (high resistivity), and current flow is conducted primarily through the moisture-filled pore spaces within the medium. The resistivity of porous media is largely controlled by the amount of pore water, porosity and permeability of the porous media, and the dissolved solids concentration of the pore water. Apparent resistivity sounding can be entered in a model such as RES2DINV, which produces a log-log plot of depth versus resistivity. The resistivity can be converted into geologic lithologies from published data. The direct application to published data can be erroneous or misleading because published data are usually from surveys from different regions and for different purposes. Local geologic data and all available data at a particular site are very important in interpreting resistivity data.
|