Geology Reference
In-Depth Information
noise. A considerable body of interpretational material is available. Infor-
mation from different depths is obtained by changing
n
. In principle, the
larger the value of
n
, the deeper the penetration of the current path sam-
pled. Results are usually plotted as pseudo-sections (see Section 7.5.2).
Pole-dipole array
: Produces asymmetric anomalies that are more difficult to
interpret than those produced by symmetric arrays. Peaks are displaced
from the centres of conductive or chargeable bodies and electrode posi-
tions have to be recorded with special care. Values are usually plotted at
the point midway between the moving voltage electrodes but this is not a
universally agreed standard. Results can be displayed as pseudo-sections,
with depth penetration varied by varying
n
.
Square array
: Four electrodes positioned at the corners of a square are vari-
ously combined into voltage and current pairs. Depth soundings are made
by expanding the square. The entire array is moved laterally when travers-
ing. Inconvenient, but can provide an experienced interpreter with vital
information about ground anisotropy and inhomogeneity. Few published
case histories or type curves.
Multi-electrode arrays (not shown)
Lee array:
Resembles the Wenner array but has an additional central elec-
trode. The voltage differences from the centre to the two 'normal' voltage
electrodes give a measure of ground inhomogeneity. The two values can
be summed for application of the Wenner formula.
Offset Wenner:
Similar to the Lee array but with all five electrodes the same
distance apart (see Figure 6.9). Measurements made using the four right-
hand and the four left-hand electrodes separately as standard Wenner
arrays are averaged to give apparent resistivity and differenced to provide
a measure of ground inhomogeneity.
Focused arrays:
Multi-electrode arrays have been designed to focus cur-
rent into the ground to give deep penetration without large expansion.
Arguably, this is an attempt to do the impossible, and such arrays should
be used only under the guidance of an experienced interpreter.
6.1.4 Signal-contribution sections
Current-flow patterns for one and two-layers are shown in Figure 6.4. Near-
surface inhomogeneities strongly influence the choice of array. Their effects
are graphically illustrated by contours of the
signal contributions
made by
each unit volume of ground to the measured voltage, and hence to the
apparent resistivity (Figure 6.5). For linear arrays, these contours have
the same appearance in any plane, whether vertical, horizontal or dipping,
