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Resistivity
Depth Sounding & Profiling
Electrical surveys undertaken by a dc resistivity
device involve placement of electrodes in the
ground. There are various geometry's for the layout
of the electrodes, but most have all four electrodes in
line. The Wenner, Slumberger, and Dipole Dipole
arrangements are the most popular. The two outer
electrodes are the current source and sink; current is
driven by self contained batteries. The two inner
potential electrodes sense at the surface the electrical
potential while current is flowing between the outer
electrodes. The potential measured varies with
electrode spacing in a predictable way, and also changes
as the strata and contained fluids vary laterally and
vertically. And from this data set, layers of
differing properties can be determined.
Two modes of operation are common, namely, a) depth
sounding and b) profiling. In the depth sounding
mode, all four electrodes are initially placed in the
ground with a very short spacing between adjacent
electrodes. A reading is taken, and then the array
is reset with an incremental increase in spacing.
Another reading is taken, and the array is, in turn,
progressively expanded in this manner until the maximum
depth to be investigated is reached. The current and
potential sense progressively deeper layer as the array is
expanded.
In the profiling mode, a constant electrode spacing is
selected that senses the subsurface geology to the depth
of interest, and this constant array is "leap frogged"
along a profile line to measure lateral variations along
the profile that have geologic meaning.
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Paleo Shoreline
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Paleochannel Location
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Municipal Landfill
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Landslide
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Southwest has used resistivity to:
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Map faults and landslides
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Map fracture zones for water well sighting
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Map large karst voids and sea caves
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Find depth to groundwater, in certain specific
situations
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Generally characterize subsurface geology.
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Aid environmental contamination flow projects
when used to evaluate
locations of paleo stream channels or fault
zones.
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Find buried bedrock valleys, which can be an important
source of groundwater in unconfined aquifers.
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Vertical joints or cracks, which can
indicate subsidence features.
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