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Instrumentation, Electrical Resistivity

• Electrical survey. Mapping subsurface resistivity by injecting an electrical current into the ground.
• Resistivity meter. An instrument used to carry out resistiv­ity surveys that usually has a current transmitter and volt­age-measuring circuitry.
• Electrode. A conductor planted into the ground through which current is passed, or which is used to measure the voltage caused by the current.
• Apparent resistivity. The apparent resistivity is the resistiv­ity of an equivalent homogeneous earth model that will give the same potential value as the true earth model for the same current and electrodes arrangement.
• Multi-core cable. A cable with a number of independent wires.

Introduction

The resistivity survey method is more than 100 years old and is one of the most commonly used geophysical explo­ration methods (Reynolds, 1997). It has been used to image targets from the millimeter scale to structures with dimensions of kilometers (Linderholm et al., 2008; Storz et al., 2000). It is widely used in environmental and engi­neering (Dahlin, 2001; Chambers et al., 2006) and mineral exploration (White et al., 2001; Legault et al., 2008) sur­veys. There have been many recent advances in instru­mentation and data interpretation resulting in more efficient surveys and accurate earth models. In its most basic form, the resistivity meter injects a current into the ground through two metal stakes (electrodes), and mea­sures the resulting voltage difference on the ground sur­face between two other points (Figure 1). The current (I) and voltage (V) values are normally combined into a single quantity, the apparent resistivity, which is given by the following relationship:

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