## A basic resistivity measurement

In the section on Subsurface Electrical Properties electrical properties are discussed. This section discusses how we measure these properties.

The simplest conceptual way to consider how to measure electrical resistivity is to consider a measurement done on a core. A representation of such a setup is shown on the left.

This core has a length L and a surface area A. If we apply a DC (Direct Current) voltage over this rod a current will run through the rod. The electrical resistance of the rod (given as R, in Ohms) is defined as the ratio between the voltage over the rod and the current which runs in the rod. This is Ohms law: R=V/I (or V=IR).

The resistance is proportional to the ratio of L/A. Or, if L gets twice as long, we would expect the resistance to double. We are interested in the **electrical resistivity** (typically represented as the greek letter rho (ρ)), which is the proportionality factor. Rho is given in Ohm meters. As discussed here the inverse of electrical resistivity is electrical conductivity. While we can measure the electrical resistivity for a core this is obviously not a feasible way to characterize the whole subsurface.