Geoscience Reference
In-Depth Information
about the same location) a vertical electrical sounding may
be created, which is a representation of the vertical vari-
ation in electrical properties. This achieved by increasing
the separation of the current and potential dipoles, or
increasing the length of both of them, or most usually,
increasing the length of just the current dipole.
It follows that a traverse of measurements made
with the separations and relative positions of the elec-
trodes maintained will comprise a profile of readings
pertaining to a constant pseudo-depth. A set of such
traverses can be used to create a map. By surveying along
the same traverse with different electrode spacings the
data from individual traverses can be combined to
create a pseudosection, or for a series of traverses a
pseudovolume.
Current electrodes
+
0
200
I
a)
Metres
Dipole-field
region
Polar-field
region
Parallel-field
region
Polar-field
region
Dipole-field
region
b)
0
200
400
Depth
(m)
c)
0
200
400
5.6.4.2
Target detection
The electric
Depth
(m)
field created by the electrodes of a current
dipole comprises several distinct regions with different
and important characteristics (
Figs. 5.34f
and
g
)
. The elec-
tric
field close to each current electrode is little affected by
the
field of the other more distant electrode and the
situation resembles that due to a single current electrode.
This is known as the polar region. The polar region can
intentionally be made very large by locating one of the
electrodes a large distance from the area of interest to
reduce its in
uence to a negligible level.
That part of the
field distant from the two current
electrodes, by at least several times their spacing (the dipole
length), is known as the dipole-field region. Midway
between the two electrodes, the equipotential surfaces are
approximately vertical and the current flow approximately
uniform, planar and horizontal (see
Figs. 5.34f
and
g
). This
is known as the parallel-field region. Increasing the distance
between the current electrodes broadens this region.
The distortion of the electric
d)
0
200
400
Depth
(m)
e)
0
200
400
Depth
(m)
High
Potential
Zone of higher conductivity
0
Low
Figure 5.39
The electric
field in the axial section of a surface current
dipole. (a) The various
field regions of the dipole. (b) The electrical
field for a half-space, for a conductive vertical dyke in (c) the parallel
field and (d) the dipole region, and (e) for a conductive layer at
depth. Contour interval is variable and is the same for all models.
field by electrical property
variations in the subsurface depends on the shape of the
anomalous zone and its position relative to the current
electrodes. Referring to
Fig. 5.39
, a narrow, steeply dipping
conductor in the parallel-
eld region has little effect on the
electric
field, but when located in the polar region the
effects are far greater. Conversely a
flat-lying conductor
has more effect in the parallel-
eld region. Clearly, for
optimal detection of targets of a given geometry, the array
needs to be positioned so that the targets are within the
appropriate part of the electric
field, e.g. the polar, parallel-
field or dipole-field regions. This leads to the use of arrays
with different relative locations of the current and potential
electrodes, some con
gurations creating a large parallel-
field regions etc.
5.6.4.3
Resolution
The relative spacing of the potential electrodes largely
controls the lateral resolution of the data; put simply, the
electrical properties of the material lying between the two
electrodes are averaged so the edges of anomalous zones
whose widths are less than this spacing will not be properly
de
ned. Vertical resolution is controlled by sensitivity to
layers with different electrical properties.
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