Specifications

University of Pretoria etd – Combrinck, M (2006)
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0 50 100 150 200 250 300 350 400 450 500
Depth [m]
Imaged Conductivity [S/m]
50m
150m
200m
250m
300m
Figure 4-42: Imaged conductivity for a 0.2 S/m layer of 20 m thickness at various depths in a
0.02 S/m half space – linear scale.
The last factor contributing to the thin layer anomaly investigated is the host rock
conductivity. A 20 m thick plate having conductivity of 0.2 S/m is modelled at 150 m
depth in different host rock environments (Figures 4-43 – 4-45). The behaviour of all the
curves is as expected from the previous experiments. High host rock conductivities
(resulting in low contrasts) give the bell shaped conductivity function of depth while the
high contrast (low host rock conductivity) results in the depth reversal of cumulative
conductance plots, although the extent of this reversal is less than for the same contrast in
a more conductive host rock. The depth of investigation decrease with increase in host
rock conductivity, except for when a conductive layer is situated shallower than the “first
channel depth” at which point the depth of investigation is governed by the depth of the
conductor (0.01 S/m and less in this case). The imaged conductivities in these examples
were calculated after the high contrast effects dissipated, yielding the correct host rock
response.
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