2 naR

where a is the interelectrode spacing (m). The equations for other electrode configurations can be found in Dobrin (1960), Telford et al. (1990), and Burger (1992).

The volume of measurement of the Wenner array is relatively large and includes all the soil between the inner pair of electrodes from the soil surface to a depth equal to roughly the inter-electrode spacing. Figure 2.2 illustrates the volume of measurement. For a homogeneous soil, the volume of measurement is approximately na3. The depth of penetration of the electrical current and the volume of measurement increase as the interelectrode spacing, a, increases.

Apparent soil electrical conductivity for a discrete depth interval of soil, ECx, can be obtained with the Wenner array by measuring the ECa of successive layers by increasing the interelectrode spacings (ai.1 and a) and using Equation (2.9) for parallel resistors (Barnes, 1952):

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