Electrical Methods
Surface geophysics electrical methods detect subsurface electric properties by measuring naturally occurring (i.e., passive) or artificially introduced (i.e., active) voltage potentials, electrical currents, and/or electromagnetic fields. Subsurface electrical properties can be measured galvanically via direct electrode connection to the ground or inductively via electromagnetic field induction through the subsurface. Though electrical methods can be broadly categorized to include both the galvanic and electromagnetic techniques, this website separates the categories into electrical methods and electromagnetic methods.
All earth materials (e.g., soil and rock) have intrinsic electrical properties. Electrical resistivity(R), which is the basis for the Ohm’s Law relationship (V = IR) between current density(I) and voltage (V), can itself be a function of other properties. Variations of resistivity (or its inverse, conductivity) within the subsurface can indicate variations in the composition, structure, spatial extent, and/or physical properties of the subsurface materials. Numerous electrical geophysical techniques have been designed to distinguish materials by certain electrical property contrasts.
Because homogeneous and isotropic materials are rare in nature, earth materials can have very large electrical resistivity ranges, and several materials may have overlapping values. Table 1 shows some typical ranges of resistivity values for common natural materials (Keller and Frischknecht, 1966; Telford and others, 1976).
Table 1. Typical electrical resistivities of earth materials.
|
Material |
Resistivity (Ωm) |
|---|---|
|
Clay |
1-20 |
|
Sand, wet to moist |
20-200 |
|
Shale |
1-500 |
|
Porous limestone |
100-1,000 |
|
Dense limestone |
1,000-1,000,000 |
|
Metamorphic rocks |
50-1,000,000 |
|
Igneous rocks |
100-1,000,000 |
Table 1 divides earth materials by geology. However, due to overlapping resistivity ranges, geological variations do not necessitate electrical variations, and therefore, certain subsurface materials may be indistinguishable in an electrical survey. Thus, attempts to interpret resistivities in terms of soil types or lithology requires consideration to the various factors that affect resistivity. Such factors include porosity, water content, composition (e.g., clay mineral and metal content), salinity of the pore water, and grain size distribution.
In most shallow subsurface environments, the conduction of electric current takes place almost entirely in the water occupying the pore spaces. In its pure state, water is virtually resistive (i.e., nonconductive) but forms a conductive electrolyte when it is introduced to chemical salts. Porewater conductivity is proportional to and can be indicative of dissolved solids/chemical constituents within the pores and/or interactions occurring at the pore-water interface (i.e., pore membrane).
Electrical current follows the path of least resistance, and pore fluid is the predominant current conduit due to the high resistivities of common rock-forming minerals. However, clays and a few other minerals (e.g., magnetite, hematite, carbon, pyrite, etc.) may exist in enough concentration to measurably contribute to the subsurface resistivity. Some electrical methods (e.g., induced polarization) are designed and can be selected to measure this solid component of the electrical conductivity.
A vast array of surface electrical methods that have been applied to numerous subsurface studies are excluded from further discussions. Such geophysical methods include telluric currents, magnetotellurics, equipotential, mise-à-la-masse, and various electromagnetic techniques. The surface electrical methods most commonly used in environmental studies are as follows:
References
Keller, G.V. and Frischknecht, F.C., 1966, Electrical Methods in Geophysical Prospecting: Oxford, Pergamon Press, Oxford, 517 p.
Telford, W.M., Geldart, L.P., Sheriff, R.E., and Keys, D.A., 1976, Applied Geophysics: Cambridge, Cambridge University Press.