Geology Reference
In-Depth Information
Figure 6.9
Offset Wenner sounding, with variable inter-electrode spacing
a
. (a) Voltage readings are obtained between B and C when current is passed
between A and D, and between C and D when current is passed between
B and E. (b) Expansion system allowing reuse of electrode positions and
efficient operation with multicore cables. The array in (a) is positioned for
the 8-m readings in (b).
6.4.3 Offset Wenner depth-sounding
Schlumberger interpretation is complicated by the segmentation of the
sounding curve and by the use of an array that only approximates the
conditions assumed in interpretation. With the Wenner array, on the other
hand, near-surface conditions differ at all four electrodes for each read-
ing, risking a high noise level. A much smoother sounding curve can be
produced with an
offset Wenner
array of five equi-spaced electrodes, only
four of which are used for any one reading (Figure 6.9a). Two readings are
taken at each expansion and averaged to produce a curve in which local
effects are suppressed. The differences between paired readings indicate the
magnitudes of those effects.
The use of five electrodes complicates field work, but if expansion is based
on doubling the previous spacing (Figure 6.9b), very quick and efficient
operation is possible using multicore cables designed for this purpose.
6.4.4 Depth-sounding notebooks
In field notebooks, each sounding should be identified by location, orienta-
tion and array type. The general environment should be clearly described
and any peculiarities, for example the reasons for the choice of a particular
