Geology Reference
In-Depth Information
Schlumberger L/h
k = 1.0
0.9
0.8
k =
'Cross' for Wenner or
Schlumberger field curve
plotted using total array
length (2L or 3a) as
ordinate
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
0.1
Schlumberger
'cross'
Wenner 'cross'
-0.8
k = -1.0
-0.9
Wenner a/h
Figure 6.7
Two-layer apparent resistivity type curves for the Wenner array,
plotted on log-log paper. When matched to a field curve obtained over a
two-layer Earth, the line
a
/
h
=
1
indicates the depth of the interface and the
line
ρ
α
/
ρ
1
=
1
indicates the resistivity of upper layer. The value of
k
giving
the best fit to the field curve allows the value
ρ
2
of the lower layer resistivity
to be calculated. The same curves can be used, to a good approximation,
for Schlumberger depth-sounding, with the depth to the interface given by
the line
L
/
h
=
1
.
recommend it. However, the area that can be covered with this array is small
unless current is supplied by heavy motor generators. The two-electrode
array has therefore now become the array of choice in archaeological work,
where target depths are generally small. Care must be taking in handling
the long cables to the electrodes 'at infinity', but large numbers of readings
can be made very rapidly using a rigid frame on which the two electrodes,
and often also the instrument and data logger, are mounted. Many of these
frames now incorporate multiple electrodes and provide results for a number
of different electrode combinations.
With the Wenner array, all four electrodes are moved, but mistakes are
rare since all inter-electrode distances are the same. Entire traverses of cheap
metal electrodes can be laid out in advance. Provided that 'direct' or very-
low-frequency alternating current is used, so that induction is not a problem,
