Geology Reference
In-Depth Information
data-points should always be shown on contour maps. Where anisotropy
is present, maps of the same area that have been produced using arrays in
different orientations can be very different.
6.4 Resistivity Depth-Sounding
Resistivity depth-soundings investigate layering, using arrays in which the
distances between some or all of the electrodes are increased systemati-
cally. Apparent resistivities are plotted against expansion, on log-log paper
(Figure 6.7). Although techniques have been developed for interpreting dip-
ping layers, conventional depth-sounding works well only over interfaces
that are roughly horizontal.
6.4.1 Choice of array
Since depth-sounding involves expansion about a centre point, the instru-
ments generally stay in one place. Instrument portability is therefore less
important than in profiling. The Wenner array is very popular, but for speed
and convenience the Schlumberger array, in which only two electrodes are
moved, is often preferred. Interpretational literature, computer programs and
type curves are widely available for both arrays. The differences between
Wenner and Schlumberger curves are usually below the level of observa-
tional noise (Figure 6.7).
Array orientation is often constrained by topography; that is, there may be
only one direction in which electrodes can be taken a sufficient distance in a
straight line. If there is a choice, an array should be expanded parallel to the
probable strike direction, to minimise the effect of non-horizontal bedding.
It is generally desirable to carry out a second, orthogonal expansion to
check for directional effects, even if only a very limited line length can be
obtained.
The dipole-dipole and two-electrode arrays are not used for ordinary
DC sounding work. Dipole-dipole
depth pseudo-sections,
muchusedinIP
surveys, are discussed in Section 7.5.2.
6.4.2 Using the Schlumberger array
Site selection, extremely important in all sounding work, is particularly
critical with the Schlumberger array, which is very sensitive to conditions
around the closely spaced inner electrodes. A location where the upper
layer is very inhomogeneous is unsuitable for an array centre, and the offset
Wenner array (see Section 6.4.3) may therefore be preferred for landfill sites.
The outer electrodes of a Schlumberger array are usually moved in steps
that are approximately or accurately logarithmic. The half-spacing sequence
1, 1.5, 2, 3, 5, 7, 10, 15
...
is convenient, but some interpretation programs
