2D Resistivity (Geoelectrical Resistivity Tomography)

Method

For 2D Resistivity imaging numerous 4-point-measurements (see 1D Resistivity) with different electrode combinations are taken along a measuring line. The 2D model is thus filled systematically with measured values (Figure 1). There are different electrode arrays that can be used to collect the data (Figure 2).

Figure 1: 2D Resistivity model of datum points (Wenner array) Figure 2: 2D Resistivity arrays

A forward modeling subroutine is used to calculate the resistivity values. The software is based on the smoothness-constrained least-squares method. It amortizes the bulk data into a series of horizontal and vertical rectangular blocks, with each box containing a number of records. Resistivities of each block are then calculated to produce a resistivity pseudosection. The pseudosection is compared to the actual measurements for consistency. Topographic corrections can be applied to the profiles (Figure 3).

Figure 3: Model Resistivity 2D, cross valley profile, Arctic Geophysics Inc., Yukon 2007 Figure 4: 2D Resistivity imaging system with active electrodes
Instrumentation

The data acquisition is carried out by the automatic activation of 4-point-electrodes, several thousand measurements are taken, one every 1-2 seconds. The AC transmitter current of 0.26 to 30 Hz is amplified by control modules, connected with the electrodes, up to a maximum of 100mA and 400V. The voltage measured at the receiver electrodes (M, N) is also amplified. Our system uses little (solar-) electricity for high-performance 2D measurements up to a depth of 450 ft..

Application

2D Resistivity is an excellent technology for placer prospecting: Our profiles accurately detect the stratification of muck-gravel-bedrock: targets for placers such as paleochannels, reefs, crevices, and terraces in bedrock, permafrost and groundwater table.

2D Resistivity is a suitable technique for mineral exploration: It locates veins/dikes, ore bodies, and kimberlite. In combination with Induced Polarization rocks with similar mineral content can be differentiated.