Transmitter electrodes (A, B) are used to inject current into the ground. The current flow between A and B is measured with the potential electrodes (M, N) (Figure 1).
|Figure 1: 4-point-measurement electric flow field||Figure 2: 1D Resistivity survey (Stefan and Dominique)|
Several 4-point-measurements are taken where the receiver electrodes M, N remain in place and the transmitter electrodes are symmetrically extended outwards. Thus the current penetrates successively deeper into the ground. When the current flow sinks to a layer with different electrical conductivity the current flow field is deformed, this can be measured at the receiver electrodes M, N. From this data a layer model based on the electrical conductivity of the different subsurface materials is calculated.
|Figure 3: 1D Resistivity Inversion, Yukon 2007|
The transmitted current is an AC with frequencies of 0.26 to 30 Hz automatically adjusted between 1 µA to 100 mA and up to 400V peak to peak. The voltage measured at the receiver electrodes is amplified by the measuring device. Measuring depth app. 250 ft..
1D Resistivity is a quick method to measure the depth to bedrock, groundwater table, and permafrost at a specific location. 1D Resistivity is however prone to adverse effects due to asymmetrical layers below ground. In these conditions we recommend using 2D Resistivity.