Electrical resistivity methods involve the measurement of the apparent resistivity of soil and rock as a function of depth and position. Although one of the more costly engineering geophysical applications, resistivity surveys can provide unparalleled data quality in subsurface imaging. Proper field techniques are required for reliable data collection with special attention paid to electrode coupling, nearby anthropogenic features, line geometry, underground utilities, and surface materials. Delta personnel have extensive experience conducting resistivity surveys under all field conditions, and have the ability to make the field decisions that are critical to the success of your project.
During resistivity surveys, current is injected into the earth through current electrodes and the potential difference, or “apparent resistivity” is measured between potential electrodes. The resistivity of soils is a function of porosity, permeability, ionic content of the pore fluids, and clay mineralization. Apparent resistivity is the bulk average resistivity of all soils and rock influencing the flow of current. It is calculated by dividing the measured potential difference with the input current, and multiplying by a geometric factor related to the array being used and electrode spacing. Data are generally presented in data table, as profiles, or contour maps and interpreted qualitatively.
In resistivity soundings, the distance between the current electrodes or the distance between the current and potential dipoles is expanded in a regular manner between readings, thus providing apparent resistivity values from deeper and deeper depths at a single location. A common application of this technique is for grounding design during geotechnical investigations using a Wenner 4 electrode array per ASTM G57-95a.
With resistivity profiling the electrode spacing is fixed and measurements are taken at successive intervals along a profile, providing apparent resistivity values at a constant depth along the profile. Common electrode arrangements include the dipole-dipole, pole-pole, Schlumberger, and Wenner arrays.
When information on both the lateral and vertical extent of a subsurface feature is desired, it is common to combine the sounding and profiling techniques. The Dipole-Dipole and pole-pole arrays are commonly used for this purpose because combined sounding and profiling data can be acquired very efficiently with these arrangements. Data from these arrays are presented as a resistivity pseudo-section, which is in effect a contoured cross-section like presentation of the data. Resistivity models of the data are generated using forward and inverse modeling computer software.
The ranges of resistivity values for a single material generally indicate resistivity variations between dry and water-saturated conditions. Dry sands, gravels, and massive unweathered rock typically exhibit relatively high resistivities whereas clays, clayey tills, water-saturated sediments, and weathered rock (chemically broken down to clays) tend to have lower resistivities. However this is not always the case – often wet porous sands will have lower resistivities than tight clay, and a firm understanding of the underlying geology is need to interpret resistivity cross-sections properly. Likewise well control or test borings along a profile greatly contribute to the proper interpretation of resistivity cross-sections.
Under the right conditions resistivity methods can be a great aid in determining bedrock and overburden characteristics, locating geologic contacts, and identifying subsurface faulting and fracturing, as well as discriminating between clean and contaminated aquifers. Feel free to contact Delta Geophysics to discuss you project and learn if resistivity profiling might be successfully applied given your specific site conditions.