Geology Reference
In-Depth Information
Tunnel route survey
(a)
0
60
120
180
240
300
360
420
480 m
N
1
2
3
4
5
6
Measured apparent resistivity in
m
Iteration 5 completed with 7.4% RMS error
Ω
Electrode spacing = 25 m
Borehole
(b)
0
60
120
180
240
300
360
420
480 m
Clay
25
50
Microdiorite
75
Model resistivity in
Ω
m
16
32
64
128
Ω
m
Fig. 8.15
(a) Contoured apparent resistivity pseudosection measured along the route of a proposed tunnel. (b) Electrical image and depths
to bedrock determined in four boreholes. (After Barker 1997.)
VES data from several soundings can be presented in
the form of a pseudosection (Section 8.3.3) and it is now
possible to invert the data into a full, two-dimensional
geoelectric model (e.g. Loke & Barker 1995, 1996)
rather than a sequence of discrete, unidimensional geo-
electric sections. This technique is known as electrical
imaging or electrical tomography. An example of elec-
trical imaging illustrating how a pseudosection can
be transformed into a geoelectric structure is given in
Fig. 8.15. Cross-borehole tomography can also be
undertaken (Daily & Owen 1991).
If electrode spreads are arranged in parallel, many 2D
pseudosections can be determined that can be combined
into a 3D model.
traverse line, the array of electrodes being aligned either
in the direction of the traverse (longitudinal traverse) or
at right angles to it (transverse traverse). The former
technique is more efficient as only a single electrode has
to be moved from one end of the spread to the other,
and the electrodes reconnected, between adjacent
readings.
Figure 8.16(a) shows a transverse traverse across a sin-
gle vertical contact between two media of resistivities
r
1
and
r
2
. The apparent resistivity curve varies smoothly
from
r
1
to
r
2
across the contact.
A longitudinal traverse over a similar structure shows
the same variation from
r
1
to
r
2
at its extremities, but
the intermediate parts of the curve exhibit a number
of cusps (Fig. 8.16(b)), which correspond to locations
where successive electrodes cross the contact.There will
be four cusps on a Wenner profile but two on a Schlum-
berger profile where only the potential electrodes are
mobile.
Figure 8.17 shows the results of transverse and
8.2.8 Constant separation traversing
interpretation
Constant separation traverses are obtained by moving an
electrode spread with fixed electrode separation along a
