Geology Reference
In-Depth Information
Figure 6.4
Current flow patterns for (a) uniform half-space; (b) two-layer
ground with lower resistivity (
ρ
low
) in upper layer; and (c) two-layer ground
with higher resistivity (
ρ
high
) in upper layer.
through the line of electrodes (i.e. the contours are semicircles when the
array is viewed end on).
A reasonable first reaction to Figure 6.5 is that useful resistivity surveys
are impossible, as the contributions from regions close to the electrodes are
very large. Some disillusioned clients would endorse this view. However,
the variations in sign imply that a conductive near-surface layer will in
some places increase and in other places decrease the apparent resistivity.
In homogeneous ground these effects can cancel quite precisely.
When a Wenner or dipole-dipole array is expanded, all the electrodes are
moved and the contributions from near-surface bodies vary from reading
to reading. With a Schlumberger array, near-surface effects vary much less,
provided that only the outer electrodes are moved, and for this reason the
