Geology Reference
In-Depth Information
orientation, should be given. Considerable variations in current strengths
and voltage levels are likely, and range-switch settings should be recorded
for each reading.
Generally, and particularly if a Schlumberger array is used, operators
are able to see the inner electrodes. For comments on the outer electrode
positions at large expansions, they must either rely on second-hand reports
or personally inspect the whole length of the line.
6.4.5 Presentation of sounding data
The observer often has time while distant electrodes are being moved to
calculate and plot apparent resistivities. Minor delays are in any case better
than returning with uninterpretable results, and field plotting should be
routine. All that is needed is a pocket calculator and a supply of log-log
paper. A laptop in the field is often more trouble than it is worth, since all
are expensive, most are fragile and few are waterproof.
Simple interpretation can be carried out using two-layer type curves (Fig-
ure 6.7) on transparent material. Books of three-layer curves are also avail-
able, but a full set of four-layer curves would fill a library. If an exact
two-layer fit cannot be found using the type-curves available, a rough in-
terpretation based on segment by segment matching will be the best that
can be done. The process is controlled using auxiliary curves to define the
allowable positions of the origin of the two-layer curve that is being fitted
to the later segments of the field curve (Figure 6.10).
Step-by-step matching was the main interpretation method until about
1980. Computer-based interactive modelling is now possible, even in field
camps, and gives more reliable results, but the step-by-step approach is still
often used to suggest initial computer models.
6.5 Electrical Resistivity Imaging (ERI)
Electrical resistivity imaging uses arrays of electrodes at multiple separations
to generate resistivity-depth cross-sections (pseudo-sections). Although in
principle this could be done (laboriously) using basic simple equipment, in
practice multiple electrodes are connected by multicore cable, and laptop or
palm-top PCs are used to automatically control data collection. Logarithmic
expansions are difficult to implement under these conditions, and linear
expansion, to relatively small separations, is the norm. The systems are
generally designed specifically for shallow surveys (
<
20 m depth).
6.5.1 Basic ERI
Terminology can be confusing. In Europe, Africa and Australia, 2D elec-
trical resistivity imaging is sometimes referred to as electrical resistivity
