Geology Reference
In-Depth Information
sively expanded about a fixed central point. Conse-
quently, readings are taken as the current reaches pro-
gressively greater depths. The technique is extensively
used in geotechnical surveys to determine overburden
thickness and also in hydrogeology to define horizontal
zones of porous strata.
Constant separation traversing
(CST), also known as
'electrical profiling', is used to determine lateral varia-
tions of resistivity. The current and potential electrodes
are maintained at a fixed separation and progressively
moved along a profile.This method is employed in min-
eral prospecting to locate faults or shear zones and to de-
tect localized bodies of anomalous conductivity. It is also
used in geotechnical surveys to determine variations in
bedrock depth and the presence of steep discontinuities.
Results from a series of CST traverses with a fixed elec-
trode spacing can be employed in the production of
resistivity contour maps.
Δ
V
a
a
a
Wenner
2
L
Δ
V
2
l
x
Schlumberger
Fig. 8.6
The Wenner and Schlumberger electrode
configurations.
8.2.4 Electrode spreads
Many configurations of electrodes have been designed
(Habberjam 1979) and, although several are occasional-
ly employed in specialized surveys, only two are in com-
mon use. The
Wenner configuration
is the simpler in that
current and potential electrodes are maintained at an
equal spacing
a
(Fig. 8.6). Substitution of this condition
into equation (8.9) yields
proportion of that of the outer, current electrodes (2
L
).
In CST surveys with the Schlumberger configuration
several lateral movements of the potential electrodes may
be accommodated without the necessity of moving the
current electrodes. In VES surveys the potential elec-
trodes remain fixed and the current electrodes are ex-
panded symmetrically about the centre of the spread.
With very large values of
L
it may, however, be necessary
to increase
l
also in order to maintain a measurable
potential.
For the Schlumberger configuration
D
V
I
a
= 2
a
rp
(8.10)
DuringVES the spacing
a
is gradually increased about a
fixed central point and in CST the whole spread is
moved along a profile with a fixed value of
a
. The effi-
ciency of performing vertical electrical sounding can be
greatly increased by making use of a multicore cable to
which a number of electrodes are permanently attached
at standard separations (Barker 1981). A sounding can
then be rapidly accomplished by switching between dif-
ferent sets of four electrodes. Such a system has the addi-
tional advantage that, by measuring ground resistances
at two electrode array positions, the effects of near-
surface lateral resistivity variations can be substantially
reduced.
In surveying with the Wenner configuration all four
electrodes need to be moved between successive read-
ings. This labour is partially overcome by the use of the
Schlumberger configuration
(Fig. 8.6) in which the inner,
potential electrodes have a spacing 2
l
which is a small
2
)
(
Lx
Lx
2
-
2
V
I
p
D
=
r
(8.11)
a
(
2
2
)
2
l
+
where
x
is the separation of the mid-points of the poten-
tial and current electrodes. When used symmetrically,
x
= 0, so
2
L
l
V
I
p
D
=
(8.12)
r
a
2
8.2.5 Resistivity surveying equipment
Resistivity survey instruments are designed to measure
the resistance of the ground, that is, the ratio (
D
V/I
) in
equations (8.10), (8.11) and (8.12), to a very high accu-
racy. They must be capable of reading to the very low
