Electrical Methods

Electrical methods such as Electrical Resistivity Imaging (ERI) and soil resistivity testing in principal utilise the method of inducing an electrical current in the ground using two current electrodes and measuring the potential difference between two potential electrodes. The current flow through the Earth is highly influenced by the porosity and fluid content within the geology under investigation. Although, in the case of the induced polarisation (IP) method, an IP effect is created when the ionic current flow through the geology is changed into an electrical current flow at the surface of metallic minerals that are present and in contact with the charged ions in the fluid within the pore spaces (Blaricom, R.V., ‘Practical Geophysics II for the Exploration Geologist’, 1992). Through an inversion of the measured potential differences between recording electrodes for the ERI and IP methods, the apparent resistivity of the ground conditions can be estimated.

Draig Geoscience offers a range of electrical geophysical options ranging from ERI, IP and soil resistivity testing in order to provide resistivity soundings or cross-sections displaying the modelled distribution of subsurface electrical resistivity.

ERI is useful geophysical tool for mapping geologic boundaries, voids and fracture zones, and defining the depth to the water table. Voids and fracture zones that are either clay or air filled are likely to have different electrical properties (resistivity) than the surrounding material. ERI investigates the subsurface immediately below the surveyed line. ERI relies on a contrast in the electrical resistivity of the features being imaged. If there is no resistivity contrast, features will be not be imaged. It will only detect individual localised features (e.g. clay zones) when they have a median dimension exceeding the location of their subsurface depth. An example output of an ERI cross-section with borehole data correlation is illustrated in Figure 1.

ERI cross-section

Figure 1. Example of an ERI cross-section with borehole data correlation.

Note: ERI can be influenced by cultural EM noise and by surface infrastructure. ERI relies on a contrast in the conductivity of the features being imaged. If there is no conductivity contrast, features will be not be imaged. ERI penetration will be adversely affected by highly conductive materials at or near surface. Survey lines must be in multiples of cable lengths (e.g. 18 m for 1 m electrode spacing). Survey lines must extend approximately 3 times the depth of investigation beyond the area of interest.

The induced polarisation method (or IP method) is generally suited to exploration investigations, which in general have a greater depth of interest (>80 m depth from surface). In order to achieve larger depth of penetrations, a larger current transmission source is generally required (e.g. a high-power generator) and also larger spacing between receiving potential difference electrodes. The IP method is particularly useful in interpreting electrically conductive geological bodies and fault zones. Figure 1 provides an example cross-section of geologically interpreted IP data.

IP Cross Section

Figure 1. An example of an IP cross-section with geological interpretation (image source. Wijns, C. and Yossi, M. (2007). “The benefit of combining downhole with surface IP”, ASEG 19th Conference and Exhibition, 18-22 Nov 2007, Perth, Australia)

The proposed methodology, Soil Resistivity Testing, is routinely used to measure earth impedance or resistance for electrical grounding circuits. The Wenner 4-electrode test is the standard configuration for Soil Resistivity Testing and a-spacing of electrode separation is dependent on Client grounding specifications. Each test location has orthogonal tests (or soundings) and the measured apparent resistivity (ρ) and a-spacing (m) are plotted against one another at a log-scale to produce a 1D curve. An example output of a Soil Resistivity Testing result is illustrated in Figure 1.

Soil Resistivity Testing 1D sounding plot

Figure 1. Example of a Soil Resistivity Testing 1D sounding plot.

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Draig Geoscience provide a wide-range of geoscience consulting services, with world-wide happy clients. We utilise the latest geoscience surveying techniques for a range of applications including environmental monitoring, geotechnical studies, groundwater mapping, infrastructure assessments and mineral exploration.