Measurements are normally carried out using the Wenner method and the data is used to arrive at a representative soil model for the site.
Whilst the measurements would best be carried out in representative weather conditions, this is clearly not always possible, so allowance for seasonal effects may need to be made in the model.
This would normally be done by modifying the resistivity and/or depth of the surface layer. Some typical soil resistivity values are shown in Table 8.2.
Measurements are taken for a range of probe separations, each of which is a general indicator of the depth to which the value applies. Measurements in a number of directions would be taken and averaged values (excluding obvious errors) for each separation distance would be used to derive the initial soil model.
A number of computer programmes are commercially available and used to translate the data into a representative soil model. It is useful to have both the average model and the data spread, so that the error band is known, as this will influence the subsequent calculations or suggest that the derived soil model be modified to improve its accuracy.
It is possible to use formulae or graphical methods to derive a two layer model. The formula below compares the resistivity, p1 of the upper layer of depth h1 with the lower layer of resistivity, p2:
The value ps is the resistivity measured at a depth a. IEEE 80 includes a number of graphs to achieve the same result, based on the work of Sunde.
It is unusual to use formulae now, because the interactive computer programmes available can quickly provide a model which may have a number of vertical or horizontal interfaces. Often a three layer model is necessary to provide sufficient accuracy.
The soil model values are used in formula or a computer programme to calculate the earth resistance and hazard voltages.