Geology Reference
In-Depth Information
mapping equipotential lines (lines joining the electrodes
when the indicated potential difference is zero). The
method provides much more information on the extent,
dip, strike and continuity of the body than the normal
CST techniques. An example of the delineation of a
massive sulphide body by the mise-à-la-masse method is
given in Bowker (1991).
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8.2.9 Limitations of the resistivity method
Resistivity surveying is an efficient method for delineat-
ing shallow layered sequences or vertical discontinuities
involving changes of resistivity. It does, however, suffer
from a number of limitations:
1.
Interpretations are ambiguous. Consequently,
independent geophysical and geological controls are
necessary to discriminate between valid alternative
interpretations of the resistivity data.
2.
Interpretation is limited to simple structural configu-
rations. Any deviations from these simple situations may
be impossible to interpret.
3.
Topography and the effects of near-surface resistivity
variations can mask the effects of deeper variations.
4.
The depth of penetration of the method is limited by
the maximum electrical power that can be introduced
into the ground and by the physical difficulties of laying
out long lengths of cable. The practical depth limit for
most surveys is about 1 km.
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100
0
10
20
m
Fig. 8.21
CST resistivity profile across a buried mineshaft. (After
Aspinall & Walker 1975.)
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8.2.10 Applications of resistivity surveying
Resistivity surveys are usually restricted to relatively
small-scale investigations because of the labour involved
in physically planting the electrodes prior to each
measurement. For this reason resistivity methods are
not commonly used in reconnaissance exploration. It is
probable, however, that with the increasing availability
of non-contacting conductivity measuring devices (see
Section 9.7) this restriction will no longer apply.
Resistivity methods are widely used in engineering
geological investigations of sites prior to construction
(Barker 1997).VES is a very convenient, non-destructive
method of determining the depth to rockhead for foun-
dation purposes and also provides information on the
degree of saturation of subsurface materials. CST can
be used to determine the variation in rockhead depth
between soundings and can also indicate the presence
of potentially unstable ground conditions. Figure 8.21
shows a CST profile which has revealed the presence of a
buried mineshaft from the relatively high resistivity val-
26
0
10
20
m
Fig. 8.22
Resistivity profiles across a buried ditch 4 m wide.
(After Aspinall & Walker 1975.)
ues associated with its poorly-compacted infill. Similar
techniques can be used in archaeological investigations
for the location of artefacts with anomalous resistivities.
For example, Fig. 8.22 shows CST profiles across an
ancient buried ditch.
Probably the most widely-employed use of resistivity
surveys is in hydrogeological investigations, as important
information can be provided on geological structure,
lithologies and subsurface water resources without the
large cost of an extensive programme of drilling. The
results can determine the locations of the minimum
