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The ERI method, also known as electric resistivity tomography (ERT), is used to noninvasively determine the spatial variations (both laterally and with depth) of the electrical resistivity of the subsurface [see (Binley 2015) for a recent review]. Electrical resistivity describes the intrinsic resistance of the subsurface to transport an electrical charge via conduction mechanisms. The reciprocal of resistivity gives the EC of the subsurface. Electrical resistivity is a valuable geophysical property to measure as it varies over many orders of magnitude and depends on several physical and chemical properties of interest in high-resolution site characterization.

The electrical resistivity of the subsurface is a function of lithology (porosity, surface area), pore-fluid characteristics (for example, saline water, fresh water, NAPL), water content, and temperature. Ion-rich pore fluids, formations with high interconnected porosity, and formations with a high surface area (for example, fine-grained formations, especially clays), increase the EC of the subsurface, which decreases resistivity. Unsaturated soils increase resistivity (air is a perfect insulator), as does the presence of air-filled voids and or free-phase gasses. The presence of metallic features (for example, ore minerals, infrastructure) also increases EC.

The sensitivity of ERI to multiple subsurface physical and chemical properties makes ERI a highly versatile surface geophysical method. It can be used to image the hydrogeological framework controlling groundwater transport, the distribution of inorganic contaminants in groundwater, and variations of moisture content in the subsurface. This versatility also means that ERI results in nonunique interpretations of subsurface properties, as multiple unknown factors influence subsurface resistivity.

Resistivity imaging has many advantages relative to invasive methods of exploring the subsurface and other geophysical technologies. The primary advantages of the technology result from the (1) wide range of resistivity encountered in geological media, and (2) strong dependence of resistivity on multiple subsurface physical and chemical properties (including moisture content, porosity, fluid salinity, and grain-size distribution). Consequently, ERI has a wide range of potential applications.

Electrical resistivity relies on galvanic (direct) contact between the transmitting instrument and the Earth which differentiates it from the EM methods described elsewhere in this section. The basic measurement is obtained using four electrodes placed on the surface of the Earth or in boreholes for more specialized cross-hole applications. Two current electrodes are used to drive electrical current ( ) into the subsurface and a second pair of potential electrodes record the resulting electrical potential difference between the electrodes. The source of electrical current is a transmitter (typically a few hundred Watts capacity for site investigation applications) and voltage differences are recorded using a receiver that is synchronized with the transmitter. An overview of the survey is given in Figure 5‑2.

The transfer resistance is calculated for each measurement that is often converted to an apparent resistivity. The apparent resistivity represents the equivalent resistivity for a homogenous Earth that results in the measured transfer resistance. These data are representations of the raw measurements. Inverse methods are used to generate images of the variations in the actual resistivity structure (for a heterogeneous Earth) beneath the electrodes.

The electrodes are usually rods or spikes driven into the ground at regular intervals. In some cases, it is necessary to increase the size of the electrodes to reduce the contact resistance (more technically, impedance) between the metal and porous ground. High-contact resistances reduce the current flowing in the subsurface and, consequently, the resulting voltage differences at the receiving potential electrode pairs (leading to lower signal-to-noise ratio). Large electrodes can help improve the signal-to-noise ratio, but their use may violate basic assumptions when interpreting data.

Basic components of a resistivity imaging

Electrode configurations can include:

1. Nested arrays, such as the Wenner array shown in Figure 5‑2, which are popular for sensing horizontal interfaces in the Earth.

2. TheDipole-Dipole array, where the current injection and potential recording electrode pairs are separated by some integer multiple of the electrode spacing. These arrays are popular for sensing vertical contacts in the Earth and result from the different sensitivity patterns that arise based on the relative locations of the four electrodes.

The most common application of ERI is in the form of a 2-D transect, where the objective is to obtain a 2-D cross section of the resistivity structure of the Earth. Large transects are typically built up using a set number of electrodes and associated cabling that is progressively rolled along to cover extensive terrain. A major limitation of the 2-D transect is that the imaging (see below) results in a model of the resistivity structure that is constant in the plane perpendicular to the image. Such a 2-D earth assumption may be reasonable in some cases (for example, a resistivity survey perpendicular to a buried pipeline), but in most cases the subsurface is inherently 3-D. A 3-D resistivity survey requires electrodes to be placed on a grid, rather than a survey line. Commercially available software packages for processing resistivity imaging datasets now fully support 3-D surveys.