Resistivity is the material property that quantifies how strongly a material opposes the flow of electric current (ohms*m). In subsurface material this corresponds to pore volume, fluid saturation and specific resistivity of minerals present. For sediments, the dominant characteristic is grain size and material assemblage. 2D Resistivity gathers numerous 4-point-measurements using a series of electrode combinations along the measuring line. The 2D model is thus filled systematically with measured values (Figure 1). Appropriate electrode arrays will vary depending on the specific structural investigation (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 apparent 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 block containing a number of records. Models of the material's true resistivity are then generated, back calculated and compared to the amortized apparent measurements to produce a single consistent resistivity model. Topographic corrections are also applied to the profiles (Figure 3).
|Figure 3: Model Resistivity 2D, cross valley profile||Figure 4: 2D Resistivity imaging system (measuring station)|
Data acquisition is carried out by the earth resistivity meter 4point light 10 W. The AC transmitter outputs current of up to 100mA. Voltage across the two transmitting electrodes (A,B) is 400V peak to peak and the measured voltage is amplified at the receiving electrodes (M,N) for accurate signal storage and transmission. Our system uses comparatively little current for high-performance 2D measurements (.1% measurement error) with up to 100m (330 ft) depth of investigation.
2D Resistivity is particularly well suited for placer prospecting: Profiles accurately represent the stratification of sediment (e.g. black muck-gravel-bedrock). Targets for placers such as paleochannels, reefs and terraces in bedrock are identifiable. Permafrost and groundwater are also identifiable when present.
We have mapped lateral and vertical extents of sand, gravel and clay deposits using 2D Resistivity. This has been utilized in planning and extending gravel/sand pits. The ability to identify clay layers can also be helpful in the search for groundwater.
2D Resistivity is a suitable technique for mineral exploration: Veins/dikes/alteration zones, ore bodies, and kimberlite pipes can be identified. In combination with Induced Polarization rocks with similar resistivity but different mineral content can be differentiated.