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despite the diminishing increase in depth penetration. EM
coupling (see
Section 5.6.7.2
)
is small for the two laterally
offset dipoles.
For the gradient array, the large current dipole requires
some time and effort to lay out; but once set up, a large
volume of the ground is energised and the large parallel-
field region below the central part of the current dipole
allows a large area to be surveyed rapidly. The uniform
horizontal subsurface current flow in the measurement
region produces simple anomaly shapes. The response of
a steeply dipping target much thinner than the potential
dipole length is generally poor. The current dipole can be
relocated along strike and further surveys conducted to
provide continuous coverage of a large area. The gradient
array is highly prone to EM coupling, a problem particu-
larly in highly conductive areas
5.6.5.3
Downhole surveying
It is necessary to distinguish between downhole electrical
surveys, where any of the electrode arrays described for
surface surveying are deployed within drillholes, and
electrical logging where the electrodes are within an
instrument, a sonde, which makes continuous readings as
it is raised through the drillhole. An essential requirement
for downhole electrical work is that the drillholes contain
water to ensure electrical connection between the elec-
trodes and the wall
rocks. Electrodes
for downhole
surveying are described in
Section 5.4.1
.
Downhole surveying investigates the region beyond the
drillhole environment. Some of the electrodes may be
located downhole (hole-to-surface array) or all of them
located beneath the surface, and either in the same drill-
hole (in-hole array) or distributed between several drill-
holes (hole-to-hole array). This places them closer to the
target zone, increasing the response of the target consider-
ably and increasing the resolution of close-spaced targets.
This is particularly useful in areas with conductive
overburden (see
Section 5.6.7.1
). It also allows greater
flexibility in focusing the survey to a particular depth and
in a particular direction. A common downhole surveying
technique is the applied potential method described in
Section 5.6.9
.
Descriptions of other forms of drillhole
Downhole logs of both resistivity and IP parameters are
often acquired in metalliferous environments (see
Section
5.6.8
). Resistivity-only measurements are routinely made in
drillholes, and the most common electrode con
gurations
are the
(see Electromagnetic
coupling in
Section 5.6.7
).
Potential measurements in the Wenner and Schlumber-
ger arrays are made in the large parallel-
eld region of the
current dipole, which allows good resolution of horizontally
layered electrical structures. Consequently, both arrays are
mainly usually used for vertical electrical soundings (see
Section 5.6.6.1
)
. The Wenner array requires more complex
logistics as all of the equispaced electrodes are moved.
In general, electrode arrays that allow rapid surveying
require less manpower and are therefore favoured for
reconnaissance work, but they provide limited depth infor-
mation. Conversely, those arrays providing more reso-
lution and depth information, where measurements are
made with multiple electrode spacing, are slower and
require more manpower, adding to survey cost. The bene-
fits of the additional information cannot be over-
emphasised as this is essential for reliable interpretations.
arrays shown in
Figs. 5.41a
point resistance. Here one current electrode is located at the
'
normal
'
and
'
lateral
'
a)
b)
c)
V
I
V
I
V
I
A
N
A
A
M
B
X
BM
X
MN
X
BM
Reading
Reading
M
B
N
Reading
B
V
I
Figure 5.41
Some common downhole logging arrays.
(a) Normal array, (b) lateral array and (c) single-point
resistance array. See
Table 5.2
for geometric factors.
Potential electrodes
Current electrodes
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