Electrical Resistivity

Electrical Resistivity - Background

The electrical resistivity imaging technique is widely used to distinguish the different resistivity values within the subsurface in landfill investigations, Hydrogeological, geological, geotechnical and downhole logging investigations. The basic principle involves injecting a direct current (I) into the ground using a voltmeter and current electrodes, as this current travels through the subsurface it is resisted and conducted by the subsurface material, as electrical potential which is received by the corresponding potential electrodes at the surface. The resistance is calculated using Ohms law:

“The current that flows in a conductive mass is directly related to the voltage passing through it”: Ohms Law: V=I/R (i.e. Resistance = Voltage / Current)

In other words, the resistance across a cross sectional area is inversely proportional to its length, measured in Ohm meters (Ω m). To obtain the true resistivity (i.e. not a singular reading), the apparent resistivity must be calculated (i.e. pa=πaR). This is defined as the resistance, which is calculated at a distance from a point electrode at a uniform half space, in homogeneous ground. Conversely, the conductivity can also be measured as this is the reciprocal of resistivity (1/p), measured in siemens (S/m).

Most metals, metallic sulphides which are found in the near surface and commonly within brownfield sites, conduct electricity through the flow of electrons, however rock forming minerals are poor conductors and have high resistances (Table 1).

Table 1 - Shows the different resistivity values for some common rock types

These resistance values are determined using Archies Law:

“The calculated resistivity within a porous rock containing water filled pore space and differing matrix grains is equal to pore fluid resistivity divided by the fractional porosity” Considering ionically pure flow, the conduction between subsurface materials will not be present within the matrix grains of the lithological formation and can only be measured by the porosity/ pore water fluids that are present.

Generally, areas which have greater resistivity values exceeding 2000 Ohms are non-porous igneous or metamorphic rocks which have subangular, intergranular grains with high cementation. In sedimentary rocks the resistivity values will be lower (e.g. around 1000 Ωm) as there is greater pore space in these units. Conversely, clay rich soils will cause low resistivity readings due to the relationship between clay particles, porosity, quartz (SIO2) content and due to the complex relationship of matrix suction and fluid pressure (i.e. the greater the porosity/ fluid content the lower the resistivity value). The resistivity value will largely depend on the clay structure for example clays with laminar structures generally do not retain fluid as well as structural or dispersed clays, those of which have a high-water content. London clay is an example of a dispersive clay which has greater expansive qualities; therefore, it is important to map out these areas of expansive clays near infrastructure/ pre infrastructural development as these areas will swell in the winter and spring months (i.e. due to heavy rainfall) and shrink in the arid summer months. This is why preliminary resistivity surveys are needed to identify these vulnerable zones as this shrink-swell relationship causes infrastructural damage and subsidence, commonly found near old works.

Areas which have a low resistivity, usually contain pore water filled pockets and are saturated areas. If you are within a tidal estuary or mapping whether there is any saline intrusion into coastal aquifers, resistivity can be used to map salinity as this considerably lowers the resistivity. This examination and calculation of pore water is calculated using Archies Law.

Archies Law: P = a (Ꝺ)-m S-nPw

To conduct an ERI or ERT survey a series of electrodes must be placed at measured intervals across a straight line over the area of investigation. The interval spacing (a) will determine the resolution and the depth of prospection, the resistivity measurement is taken every set of 4 electrodes, before being re-surveyed at depth 2 x the interval spacing (a), 3 x a, 4 x a and so on (Fig. 1).

Fig.1. Diagram which shows how you conduct the ERT survey.

However, your survey configuration will depend on the resistivity method used for example by conducting a Dipole-Dipole array, Wenner array or Schlumberger array (Table.2).

Table 2. Showing the different array configurations including IP, SP surveys.

When conducting an Electrical Resistivity Imaging/Tomographic survey (ERI/ERT), the resistivities across a uniform area can be measured as a 2D Pseudo-section or collected as a series of 2D Pseudo-sections in a gridded (x, y) area, to obtain 3D data.

To ensure good ground coupling, the electrode must be pushed into relatively moist ground. In more arid environments which have gravel type deposits within the near surface, a copper sulphate solution can be used to saturate the ground, in order to ensure that there are good contact resistances. Alternatively, each electrode should be hammered into the ground at 50mm.