Specifications

University of Pretoria etd – Combrinck, M (2006)
0.0001
0.001
0.01
0.1
0 100 200 300 400 500 600 700 800 900 1000
Depth [m]
Imaged Conductivity [S/m]
Two Layers: Layer 1, 0.02 S/m, 200 m; Layer 2, 0.002 S/m
Two Layers: Layer 1, 0.02 S/m, 200 m; Layer 2, 0.0002 S/m
Figure 4-27: Imaged conductivity versus depth for two layers of decreasing conductivity; first
layer thickness 200m.
4.5.3.3 Two layer earth with increasing conductivity ( σ
1
< σ
2
)
Once again two models with different contrasts are presented (Figures 4-28 to 4-30). The
first layer still has the same parameters of 0.02 S/m and 200 m. The second layer
conductivity is now changed from 0.2 S/m to 2 S/m. In this case the effect of the second
layer is already visible before the 200 m mark. This method approximates a set of current
distributions with one single filament and it can be expected that the transition would not
be sharp and also that more conductive layers will influence the single filament even if its
position is not indicated as inside the conductive layer itself. For the low contrast case
(second layer conductivity = 0.2 S/m) the behaviour of the cumulative conductance and
conductivity curves follow the expected pattern of increasing smoothly with depth and
approximating the second layer conductivity ever closer after an initial overshoot.
However, the high contrast case produces a very interesting phenomenon. Instead of
yielding cumulative conductance and conductivity values at increasing depths with
increasing time, a turning point is reached at some stage (300m depth for this case) after
which time the equivalent S-layer filament is mapped with drastically increasing
conductivity but decreasing depths. This type of unexpected behaviour contradicts the
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