Geology Reference
In-Depth Information
The Wenner and two-electrode arrays score more highly in this respect than
most others.
The voltages measured using a Schlumberger array are always less than
those for a Wenner array of the same overall length, because the separation
between the voltage electrodes is always smaller. For the dipole-dipole array,
the comparison depends upon the '
n
' parameter, but even for
n
=
1 (i.e. for
an array very similar to the Wenner in appearance), the signal strength is
smaller by a factor of three.
Differences are even greater when the gradient and two-electrode recon-
naissance arrays are compared. If the distances to the fixed electrodes are
30 times the dipole separation, the two-electrode voltage signal is more than
150 times the gradient array signal for the same current. However, the shorter
gradient-array voltage cable is easier to handle and less vulnerable to in-
ductive noise, and much larger currents can be used with safety because the
current electrodes are not moved.
6.3 Resistivity Profiling
Resistivity traverses are used to detect lateral changes. Array parameters
are kept constant and the penetration therefore varies only with changes in
subsurface layering. Depth information can be obtained from a profile if
only two layers, of known and constant resistivity, are involved since each
value of apparent resistivity can then be converted into a depth using a two-
layer type-curve (Figure 6.7). Such estimates should, however, be checked
at regular intervals against the results from expanding-array soundings of
the type discussed in Section 6.4.
6.3.1 Targets
The ideal traverse target is a steeply dipping contact between two rock types
of very different resistivity, concealed under thin and relatively uniform
overburden. Such targets do exist, especially in man-modified environments,
but changes in apparent resistivity due to geological changes of interest are
often small and must be distinguished from a background due to other
geological sources. Gravel lenses in clays, ice lenses in Arctic tundra, and
caves in limestone are all much more resistive than their surroundings but
tend to be small and rather difficult to detect. Very good conductors, whether
sulphide ore bodies or metal pipes, are usually more easily found using the
electromagnetic methods described in Chapter 8.
6.3.2 Choice of array
The preferred arrays for resistivity traversing are those that can be most easily
moved. The gradient array, which has only two mobile electrodes separated
by a small distance and linked by the only moving cable, has much to
