6.5.2.3 Determining the soil resistivity with the Schlumberger method

The Schlumberger method is a widely used four-pole method for measuring earth resistivity. Figure 6.5.2.3.1 shows the arrangement of the measuring probes for this measuring method. The measurement is carried out from a centre point 'M' (e.g. intended insertion point for an earth rod), which is retained for all subsequent measurements. Four measuring probes (earth spikes 30 to 50 cm long) are inserted into the ground at an interval on a marked-out route. The two inner electrodes are spaced apart by a distance 'a', while the outer electrodes are spaced apart from the respective inner electrodes by a distance 'b', which is equal to or greater than the distance 'a'. The drive-in depth of the measuring probes should not exceed 1/20 of the distance 'a'.

The earth resistivity of the soil under investigation is determined from the measured resistance (Equation 6.5.2.3.1):