1. Vertical electrical sounding (VES), also known as‘electrical drilling’ or ‘expanding probe’, is used mainly in the study of horizontal or near-horizontal interfaces. The current and potential electrodes are maintained at the same relative spacing and the whole spread is progressively expanded about a fixed central point. Consequently, readings are taken as the current reaches progressively greater depths.
The Wenner configuration is the simpler in that current and potential electrodes are maintained at an equal spacing a and apparent resistivity is:

2. INTERPRETATION:

3. Figure 8.11 shows an interpretation using a set of master curves for vertical electrical sounding with a Wenner spread over two horizontal layers. The master curves are prepared in dimensionless form for a number of values of the reflection coefficient k by dividing the calculated apparent resistivity values ρa by the upper layer resistivity ρ1 (the latter derived from the field curve at electrode spacings approaching zero), and by dividing the electrode spacings a by the upper layer thickness z1.
The curves are plotted on logarithmic paper, which has the effect of producing a more regular appearance as the fluctuations of resistivity then tend to be of similar wavelength over the entire length of the curves. The field curve to be interpreted is plotted on transparent logarithmic paper with the same modulus as the master curves. It is then shifted over the master curves, keeping the coordinate axes parallel, until a reasonable match is obtained with one of the master curves or with an interpolated curve. The point at which ρa/ρ1 = a/z = 1 on the master sheet gives the true values of ρ1 and z1 on the relevant axes. ρ2 is obtained from the k- value of the best-fitting curve.

4. ELECTRIC METHOD EXERCISE:
At locations A, B, C and D along the gravity profile shown in Fig. 8.38, VES (vertical electric sounding) were performed with a Wenner array with the spread laid perpendicular to the profile. It was found that the sounding curves, shown in Fig. 8.39, were similar for locations A and B and for C and D. A borehole close to A penetrated 3 m of drift, 42 m of limestone and bottomed in sandstone.
Downhole geophysical surveys provided the values of resistivity (ρR) and density (ρD) shown in the table for the lithologies encountered.

5. A seismic refraction line near to D revealed 15 m of drift, although the nature of the underlying basement could not be assessed from the seismic velocity.
TASKS:
(a) Interpret the geophysical data so as to provide a geological section along the profile.
(b) What further techniques might be used to confirm your interpretation?