RESISTIVITY METHODS - Field Geophysics - page 123

the work can be speeded up by cutting the cables to the desired lengths and
binding them together, or by using purpose-designed multicore cables.
The dipole-dipole array is mainly used in IP work (see Chapter 7), where
induction effects must be avoided at all costs. Four electrodes have to be
moved and the observed voltages are usually small.
Resistivity anisotropy can be roughly assessed using the square array
(Figure 6.2g), but more detail can be obtained by rotating a linear four-
electrode array about a central point. A sweep through 180 ◦ is normally
carried out, typically in 10 ◦ steps. Once a sweep is completed, the array is
moved to another pivot point and the process is repeated. Information on frac-
ture trends or water seepage pathways can be obtained using this approach.
6.3.3 Traverse field-notes
Since the array parameters remain constant when traversing, the array type,
spacing and orientation, and very often the current settings and voltage
ranges, can be noted on page headers. In principle, only station numbers,
remarks and V / I readings need be recorded at individual stations, but any
changes in current and voltage settings should also be noted since they affect
reading reliability.
Notes should be made on changes in soil type, vegetation or topography
and on cultivated or populated areas where non-geological effects may
be encountered. These observations will usually be the responsibility of
the instrument operator but, because local conditions around any of the
electrodes might be important, handlers of remote electrodes may have to
be involved. Since any note about an individual field point will tend to
describe it in relation to the general environment, a general description
and sketch map should be included. When using frame-mounted electrodes
to obtain rapid, closely spaced readings, the results are usually recorded
directly in a data logger and the general description and sketch become all
important.
6.3.4 Displaying traverse data
The results of resistivity traversing are most effectively displayed as profiles,
which preserve all the features of the original data. Profiles of resistivity and
topography can be presented together, along with abbreviated versions of
the field notes. Data collected on a number of traverses can be shown by
plotting stacked profiles on a base map (see Section 1.5.10), but there will
usually not then be room for much annotation.
Strike directions of resistive or conductive features are more clearly shown
by contours than by stacked profiles. The locations of traverse lines and