Electrical resistivity imaging uses direct current to assess the electrical resistivity of subsurface materials. Boundaries between layers of contrasting resistivity correlate strongly with geological boundaries. ERI can be used to map the depth and topography of contrasting layers and bedrock in certain geological settings.
Data Acquisition
ERI surveys are conducted by deploying multiple electrodes in contact with the ground surface along transects or grids. The acquisition array type is matched to the objectives of the survey and the expected geology. The inter-electrode spacing is matched to the investigation depth, with larger spacing reaching deeper but sacrificing resolution. The system records apparent resistivity values for multiple electrode combinations. Data can be acquired as a 2D electrical resistivity image along a single line, by combining several adjacent 2D survey lines (often referred to as a 2.5D survey), or as a 3D survey by installing the electrodes in a grid fashion and collect data in multiple directions.
Data Processing
Processing ERI records requires inversion of the measured apparent resistivity data to produce a model of resistivity distribution with depth and distance. The processing involves iteratively adjusting resistivity values to minimize the difference between measured and calculated apparent resistivity values. Processing includes quality control to remove noisy data points, assessment of model fit to measured data, and verification that the final resistivity model produces geologically plausible results. Multiple 2D resistivity sections can be combined to create 3D models of stratigraphy and bedrock depth across the survey area.
Data Interpretation
ERI data is interpreted by analyzing the inverted resistivity models and correlating resistivity values with geological features. The data are typically presented as 2D color contour plots showing electrical resistivity as a function of depth (or elevation) and horizontal distance along survey lines. Competent igneous or metamorphic bedrock is usually highly resistive relative to weathered bedrock and overburden. The resistivity of sedimentary rocks varies widely, typically ranging from low to moderate values. It is heavily influenced by porosity, pore fluid salinity, and clay content. In general, coarse grained rocks exhibit higher resistivity (in freshwater environments), while clay-rich rocks (shale) are conductive. The resistivity of carbonate rocks is variable but often reaches higher values unless fractured or porous.
Fractures within the bedrock can stand in resistivity contrast to surrounding material, especially when saturated. Interpretation is enhanced by integration with prior geological knowledge and borehole data to establish site-specific relationships between measured resistivity and geological units. Factors such as groundwater salinity, degree of saturation, porosity, and water content also influence resistivity and must be considered during interpretation. Interpretation can be straightforward when subsurface conditions involve simple layering with strong resistivity contrasts but becomes more complex when multiple factors affect resistivity or when three-dimensional features are imaged with two-dimensional survey lines.
Deliverables
Results of ERI surveys are typically provided as 2D color contour plots showing electrical resistivity as a function of depth and horizontal distance along survey lines, or as 3D resistivity models of the survey area. Resistivity models inform subsurface stratigraphy and may include profiles showing soil stratigraphy or bedrock topography and depth, including features such as weathered and fractured zones, and contacts between distinct geological units.
Advantages
ERI is a noninvasive and economical test method used for bedrock characterization. A large volume of subsurface data can be acquired in a short time frame for relatively low cost compared to drilling, allowing for higher resolution information regarding bedrock depth and condition. ERI provides continuous coverage along survey lines, revealing lateral variations in bedrock depth that would be missed by widely spaced boreholes or individual soundings.
Limitations
The depth of investigation depends on the electrode spacing used and the resistivity contrasts present in the subsurface. Investigation depth and resolution are inversely related—larger electrode spacings reach deeper but sacrifice spatial resolution. Surface conditions like asphalt, concrete, or very dry soils can limit electrode contact and reduce data quality. Conductive interference from fences, guardrails, buried utilities, and other metallic infrastructure can interfere with measurements. Single survey lines can be affected by off-line resistivity changes (3D effects), though this can be overcome by acquiring parallel survey lines. Edge effects resulting from reduced data density at survey line ends require surveys to extend beyond the area of interest.
Method
Electrical Resistivity Imaging