ETL 1110-1-171
31 Jan 96
(e) The efficiency of the laser signal can degrade if the
sapphire window is abraded, or if the optic fiber is misaligned
or pitted. Therefore, the energy of the signal is evaluated
before and after each push. Degradation of the signal is assumed
to be linear and the data is corrected by computer-based
routines.
(f) The LIF response can be affected by fluorescent dyes,
optical brighteners, sunlight in the top of the hole, and
naturally occurring fluorescent minerals. Dyes and brighteners
can be found in antifreeze and detergents.
(3)
Resistivity Measurement:
(a) The resistivity sensor uses four equally-spaced
electrodes in a Wenner array to measure soil resistivity. The
electrodes consist of stainless steel rings encircling the outer
diameter of the probe. They are separated by nonconductive
passing a DC current between the two outer electrodes and
measuring the voltage drop across the two inner electrodes. The
SCAPS computer is used to calculate the nominal resistivity in
ohm-meters by using the excitation current, the voltage drop
across the inner electrodes, and predetermined geometric factors
derived from potential field theory.
(b) The close spacing of electrodes provides for a small
test area and limited horizontal extent of investigation (50 mm),
but provides well defined differences in measured resistance.
Since electrical resistance is related to both the soil type and
the pore fluid constituents, the sensor can be used to determine
soil type and ground water table depending on soil type. The
sensor can also determine different relative levels of
contaminants in a uniform soil.
Physical Sampling Capabilities:
c.
(1)
Soil Sampling:
(a) SCAPS can use commercially available direct push
samplers to obtain soil samples. These samplers are similar in
that all have a retractable tip and a removable stainless steel
inner barrel. Sampler inner barrels can be split or whole. The
sampler is pushed to a depth above the desired sample interval
with the tip and barrel in place. The tip is then retracted and
locked into the top of the sampler and the sampler is pushed
through the sample interval. The sample can be extracted in the
field or capped and submitted for laboratory analysis.
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