Electrical Resistivity Imaging
- Landfill investigations
- Ground water investigations including depth to water table and aquifers
- Delineation of freshwater / saltwater interface
- Mapping and monitoring of soil salinity and inorganic contaminant plumes
- Location of paleochannels, faults / fractured zones
- Locating voids and mineshafts / cave systems
- Stratigraphic mapping including gravel and clay lenses
- Soil corrosion assessment
Electrical Resistivity Tomography (ERT) is sensitive to variations in the electrical resistivity of the subsurface measured in Ohm meters (Ωm). The dominant factors affecting the bulk electric resistivity (and its inverse, conductivity) of soil or rock are:
- Porosity and permeability
- Degree of saturation - the fraction of pore space / fractures filled with fluid
- Fluid type including salt content - the composition of the fluid filling the pore spaces / fractures
- Presence of clays with moderate to high cation exchange capacity (CEC)
Resistivity measurements are made by inducing an electrical current into the earth through two current electrodes and measuring the resulting voltage difference at two potential electrodes. Knowing the current and voltage values, an apparent resistivity value can be calculated, the investigation depth of which is relative to the spacing between electrodes. Greater depths are achieved by increasing the electrode spacing. A number of different electrode configurations exist, each being suitable under various conditions.
Modern resistivity systems employ multiple electrodes connected to a central control unit via multiple core cables. Once the electrode array is deployed and the sequence program is set in the control unit, readings are automatically taken across a number of electrode positions.
Data Analysis & Presentation
After collection of a resistivity sequence a pseudo-section is generated showing the apparent resistivity measurements at the various depths along the profile. Quantitative resistivity readings can be calculated by running the pseudo-section through mathematical inversion algorithms resulting in 2D geo-electrical cross-section showing variations in the modelled electrical resistivity of the subsurface. The resistivity section can be interpreted to provide information on subsurface layering, linear and isolated features.
The imaging depth achievable with the ERI method is dependent on the total length of the electrode array, with larger electrode spacings resulting in greater imaging depth. The overall subsurface resistivity also affects the imaging depth with highly resistive ground tending to decrease the depth after inversion.