Electrical Borehole Geophysics
Electrical borehole geophysical methods exploit the variations of electrical properties that occur within subsurface materials and can be applied to hydrogeological, contamination, and other environmental investigations. Electrical borehole methods measure naturally occurring or artificially introduced electrical currents and/or voltage potentials to deduce the electrical structure of the borehole-intersected formations. Though electrical geophysical methods may also include electromagnetic induction techniques (see Electromagnetic Methods), those considered here measure electrical properties galvanically via direct electrode connection.
Earth materials have intrinsic electrical properties, and, though various tools exist to identify whatever electrical contrasts may exist, resistivity is a predominate electrical property of interest and applicability. Electrical resistivity (and its inverse, conductivity) is the basis for the relationship between current density and voltage variability, which is relevant to many electrical methods. Electrical resistivity methods investigate variations of electrical resistivity using combinations of current (i.e., injection) and/or potential (i.e., measurement) electrodes, which allows data collection at numerous scales and depths of investigation.
The resistivity of earth materials changes with variations in composition, structure, spatial extent, physical properties, or any combination of these. Thus, interpreting resistivity data involves the consideration of numerous characteristics such as porosity, permeability, saturation, pore fluid chemistry, and minerology. Electrical current is primarily conducted through the subsurface via electrolytic pore fluid (i.e. salt-containing groundwater within pore spaces). However, clay content and certain other minerals may exist in enough concentration to heavily contribute to a resistivity measurement.
The spatial variations of earth materials that differ geologically and/or hydrologically from each other may be distinguished using a single electrical method alone. However, hydrogeologically different materials may not exhibit variations of certain electrical properties. Thus, a more comprehensive characterization of the borehole-intersected formations is produced by employing at least one electrical method along with other techniques. Common electrical borehole geophysical methods used in environmental geophysics and hydrogeology include: