Resistivity sends an electric current into the ground and maps out the resistance to the current flow. Resistance can be converted to resistivity, a property that is used to predict the composition, structure or strength of the underlying material. It is well known that different geological materials exhibit different resistivities, and the presence of water or ores usually reduces the resistivity of a material, and contaminants can also affect the recorded values. Generally, stronger and harder sediments have higher resistivities than structurally weakened rocks
Applications for resistivity include, but are not limited to, bedrock depth detection, bedrock topography, fracture and fault identification, cavity detection and mapping, ores and mineral deposits, geology, soil type or bedrock type bedrock changes, groundwater aquifers, groundwater contamination, saltwater intrusion, waste and contaminant mapping, etc.
Resistivity is an excellent technology that can be used as a monitoring tool to record changes in ground conditions. Typical applications include groundwater monitoring around landfills to see the spread of leachate or monitoring water seepage in earth dams. Measurements can be made by returning to the same place multiple times to repeat the measurement, or the device can be left in the field as part of an automated monitoring program.
Resistivity is an effective choice for mapping the subsurface when you need a non-destructive survey technique or want to improve data coverage compared to traditional point-by-point invasive methods such as drilling or digging. The survey can be a simple sounding (measuring only the relationship between resistivity and depth), a 2D profile or even a 3D volume. Data can be collected on land, in water or in a borehole and can be a single measurement or repeated readings to meet the need to monitor measurement changes
Depending on the location of the current and potential electrodes, the data will be collected as separate points at different depths (7 coordinates) and different surface locations (X and Y coordinates). These can then be combined to create a 1D sounding or profile, a 2D profile or a 3D volume.
Vertical electrical sounding (VES) surveys collect data from different depths below a fixed point on the surface. This means that all data points will have the same surface location (X and Y coordinates) but will be measured at different depths (Z coordinates),
The electrode is placed at the midpoint of the base, and the farther away from the midpoint the electrode is placed, the deeper the measurement. For each data point, the electrode needs to be moved manually. The data is usually presented in the form of a graph with depth on one axis and apparent resistivity on the other axis, and these raw data are then used for inversion. Resistivity surveys can also be carried out at constant depth but at different surface locations, which is usually called profiling. These data are usually not inverted, and a graph of the relationship between the resistivity or apparent resistivity and the profile distance is the final output.
ERT (Electrical Resistivity Tomography) surveys, sometimes called imaging, are completed by combining measurements at different depths and lateral positions. Since ERT is a multi-dimensional method, data points can be measured in a 2D profile or in a 3D grid.
For a 2D profile, multiple electrodes are arranged in a line at a set distance. The system automatically selects which electrodes to use for current injection and voltage readings, and can measure a large number of data points in a short period of time. The data points will have different X and Z coordinates, but a fixed Y coordinate.
For large-scale 3D surveys, data from multiple (preferably parallel) 2D measurement lines can be combined in the processing software to simulate resistivity variations in the X Y and Z directions simultaneously. This process of forming a 3D data set is specifically referred to as a 2.5D survey
For smaller survey areas, the electrodes can be arranged in a grid and a 3D data set formed directly on the instrument; this is a "true" 3D survey.