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The Fairfax County soil survey team got a preview
of soil layers in the Lorton and Franconia areas before they
even dug the first hole. The three-member team was testing ground
penetrating radar (GPR) and electromagnetic induction (EMI).
GPR
is a noninvasive geophysical tool that helps to detect large
changes in physical characteristics of the soil. It responds
to abrupt boundaries. GPR is used for determining depths to
soil layers, water table, cobble, and bedrock.
In layman’s terms, the GPR system has two
parts—an antennae and a receiver—and needs two operators.
One person drags the antennae across the surface of the ground.
Another carries the receiver.
The antennae is a bright orange box that measures
about 2’ wide by 2 ft’ long by 1’ high. It
connects with a cable to the receiver, which resembles a laptop
computer. The antennae transmits and receives the radar signal,
and then sends the information to the receiver. The receiver
shows a line on the screen which represent the depth of soil
layers. If a layer is consistently the same depth beneath the
surface, the line remains straight. If a layer dips deeper,
the lines on the screen dip as well.
Can the GPR replace soil scientists who dig holes
to determine the different layers of soil and their characteristics?
Not likely. The GPR shows only the changes between layers. It
doesn’t identify what the changes are. Furthermore, there
are many variables that limit the applicability of GPR. Generally,
the radar depth of observation is
shallow. The depth typically decreases as water content increases
and as clay or salt content increases. The use of GPR is also
interpreter dependent. The data mean little without a proper
interpretation.
Like ground penetrating radar, electromagentic
induction (EMI) has proven to be a valuable noninvasive tool,
which allows observations without disturbing the soil. EMI transmits
and receives a magnetic pulse that measures the conductivity
of the soil, i.e., the weighted average of conductivity at 6
foot and 3 foot intervals. The apparent conductivity increases
with the amount of clay, water, and salt in the soil.
For more information about either ground penetrating
radar or electromagnetic induction, contact NRCS Soil Survey
Project Leader David Harper via e-mail
or call 703-324-1428.
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