Why measure earth resistivity

Earth resistivity $\rho$ (Ω-m) is the resistance of a 1 m cube of soil. It must be measured because every earthing design depends on it, and it changes widely (from about 10 Ω-m in wet clay to over 1000 Ω-m in rock) with soil type, moisture, salts and temperature.

Reasons for measuring earth resistivity

1. Design of earth electrodes and substation grids: the earth resistance is directly proportional to $\rho$ (e.g. rod $R = \frac{\rho}{2\pi L}\ln\frac{2L}{a}$). Without $\rho$ the number, size and depth of electrodes cannot be chosen.
2. Safety of personnel: step and touch potentials depend on $\rho$ of the soil and surface layer; IEEE 80 design needs $\rho$ to check safe limits.
3. Tower footing resistance: lightning performance and back-flashover rate of lines depend on footing resistance, which depends on $\rho$.
4. Choice of site and best location for earth pits, and the best depth (layered soil), since resistivity changes with depth.
5. Protection operation: a low earth resistance gives enough fault current for relays to operate quickly.
6. Corrosion assessment: low-resistivity soil is more corrosive for buried metal and pipelines; also needed for cathodic protection design.
7. Geophysical surveys: locating water, ore bodies and rock layers.

It is measured mainly by the Wenner four-electrode method ($\rho = 2\pi aR$), preferably in the dry season to get the worst-case value.