DC Resistivity & Electrical Resistivity Tomography
DC Resistivity and Electrical Resistivity Tomography (ERT) are surface geophysical methods in which an electrical current is injected into the ground through two electrodes and voltages on the surface are measured revealing the direction and amount of current flow in the subsurface. The data is used to image the subsurface resistivity.
Observed measurements of the current and voltages are converted into “apparent resistivity,” a weighted average of the resistance of earth materials to current flow. Variations in fluid saturation, fluid resistivity, rock type, porosity, and permeability affect resistivity values and are often revealed with electrical resistivity methods.
DC Resistivity is commonly used to…
* delineate aggregate deposits for quarries
* measure earth impedance for electrical grounding circuits
* estimate depth to bedrock
* estimate depth to water table
* detect and map geologic features
* define mining targets as part of an induced polarization survey
Electrical Resistivity Tomography (ERT)
is high-resolution DC resistivity data processed using modern tomography imaging techniques. The dense data collected provide the basis for solving sophisticated inverse computer models of the conductivity distributions of the ground.
When both resistivity and induced polarization are measured, the resulting data set relates to the complex impedance of the earth and the term “EIT” (Electrical Impedance Tomography) is sometimes used to describe the geophysics method, particularly when the transmitters and receivers are down-hole.
ERT involves the acquisition of hundreds, even thousands, of four-electrode resistivity measurements that are possible between multiple strings of electrodes. For example, given two strings of 15 electrodes (30 electrodes total), there are 632 different dipole-dipole measurements that can be made (including all reciprocal measurements) involving transmitter and receiver dipoles with a fixed length of two electrode spacings.
To measure all of the desired transmitter-receiver electrode combinations requires a computerized acquisition system that automatically switches both transmitter and receiver electrodes and has multi-channel measurement capabilities.