Introduction to the resistivity surveying method. The resistivity of ...

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collected at 5 hours after the pumping began shows a reduction in the resistivity (of up to over
50 percent) near the ground surface in the vicinity of the 24 metres mark. The near-surface
low resistivity zone reaches its maximum amplitude after about 10 hours when the pumping
was stopped (Figure 25b). Twelve hours after the pumping was stopped, the low resistivity
plume has spread downwards and slightly outwards due to infiltration of the water through
the unsaturated zone. There is a decrease in the maximum percentage reduction in the
resistivity values near the surface due to migration of the water from the near surface zone.
This effect of the spreading of the plume becomes increasingly more pronounced after 24
hours (Figure 25d) and 36 hours (Figure 25e) due to further migration of the water. Note that
the bottom boundary of the zone with approximately 20 percent reduction in the resistivity
values tends to flatten out at a depth of about 3 metres (Figure 25e) where the plume from the
surface meets the water table.
Figure 25. Sections showing the change in the subsurface resistivity values with time obtained
from the inversion of the data sets collected during the irrigation and recovery phases of the
study.
Copyright (1999-2001) M.H.Loke