Electrical resistivity measurements in the field can be done using instruments which use either contact or non contact methods. We discuss here instruments which use contact methods. These instruments make a measurement using four electrodes: two electrodes which inject current and two electrodes which measure the resulting potential. Note that as discussed in the section on instrumentation newer instrumentation can take measurements using multiple electrodes and channels and there are even systems emerging which can inject current at multiple locations simultaneously - however, the four electrode measurement is the basic unit of measurement.
The basic configuration of a four electrode measurement is shown below on the left, with the resulting data shown on the right. In this configuration we have four electrodes: two current electrodes and two potential electrodes. A voltage source is used to apply a voltage over the two current electrodes and a current runs between the two current electrodes as a result of this voltage difference. This results in a potential field which is measured by the potential electrodes. To make a measurement one typically collects and averages data over a few cycles in which (as shown on the right) the polarity of the voltage applied over the current electrodes is reversed. In general this cycle will include a period in which no current is injected. The current and resulting voltage are measured for each cycle and averaged or stacked to give a single measurement. This thus consists of a current (given in Amperes, A) and an associate measured voltage (given in Volts, V). Note that as shown in the figure on the right this measurement can also have an induced polarization part (resulting from the fact that the earth acts both as a conductor and a capacitor).
The current and the voltage will depend on both the geometry of the different electrodes and the spatially heterogeneous electrical properties of the soil, and as discussed in the next section by collecting and combining multiple measurements we can now obtain information on these properties.