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Ground-Penetrating Radar Examines Sand Bars in Grand Canyon
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The veteran team of Diane Minasian (left) and Ingrid Corson (right) collect
GPR data across a sand bar in Grand Canyon. Shown here is the
100-MHz transmitter (in front) and receiver (in back), which
communicate with a laptop computer via fiber optic
cables.
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Members of the Western Region CMG Team recently deployed
ground-penetrating radar (GPR) in the Grand Canyon for the first
time. Walter Barnhardt, Rob Kayen, and Diane Minasian surveyed
sand bars at seven sites during the 9-day project under the
direction of David Topping (WRD-Reston) and Dave Rubin. Dan
Dierker (master boatman) and members of the sediment-sampling
crew, including Robin Dornfest and Hank Chezar (CMG), and Jim
Bennett, Ingrid Corson, and Margie Franseen (WRD-Denver),
provided extra hands. The main objectives of the GPR surveys
were to (1) test the applicability of radar in the Grand Canyon
environment, (2) help develop a sediment budget for the Colorado
River below Glen Canyon Dam, and (3) better understand the
stratigraphic evolution of sand bars under artificially regulated
flow regimes.
The GPR system includes a transmitter, a receiver, a small
electronics console, and a laptop computer-all powered by 12-volt
batteries. The transmitter sends an electromagnetic pulse into
the ground, and the signal reflects off an interface between
electrically contrasting materials. After each shot, the receiver
listens for approximately 500 nanoseconds (500 x
10-6 seconds)
and precisely times the arrival of reflected signals. Data are
recorded on the computer and displayed in real time on the screen.
Typically, we collect 32 to 64 shots at each point and stack them
to improve the quality of the signal. Post-processing procedures
are robust and similar to seismic-reflection techniques. Common
midpoint (CMP) surveys were also performed to estimate the velocity
of GPR through the deposits, and thus permit the conversion of
travel time to depth.
A 3-person team deploying the GPR system can collect data at rates
of up to 1,000 m/hr depending on terrain and shot spacing. Relief
on the sand bars exceeded 4 m in places, requiring topographic
surveys to correct the profiles and determine the true geometry
of subsurface reflections. Waterproof boxes resembling small
catamarans were also built to tow the antennas over shallow,
submerged parts of sand bars. However, the high conductivity of
the Colorado River water minimized penetration of the GPR signals.
Consequently, most efforts were focused on subaerial parts of the
sand bars, where hot sun and blowing sand posed the greatest
problems. In the future, we will use the GPR boats to obtain
high-resolution imagery of shallow reservoirs
and sag ponds along fault zones in California.
GPR data across a sand bar in Grand Canyon. Profile begins at edge of
river (on left), crosses a gently sloping beach and sandy ridge,
then terminates near the canyon wall. Clinoform reflections
underlie most of the sand bar, recording an earlier episode
of progradation. The dipping reflections are truncated by a
sharp unconformity at approximately 200 to 250 nanoseconds
and are buried by younger material, most of which was probably
deposited by a large flood in 1983. Elevation (scale on right)
is in meters relative to the river's level at the time of the
GPR survey. When the river was at a higher level, the sand layers
labeled "platform" were deposited in a large eddy. The "return
channel" was cut by water in that eddy, flowing back upstream
along the river's edge, which is marked by a small scarp
labeled "break line."
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November 1999
in this issue:
Lake Tanganyika
Honduras
Grand Canyon Sediments
Grand Canyon GPR
Medicine Lake, CA
SWASH Project
Cruise News
cover story:
Earth Science Week in St. Pete
Earth Science Week in Woods Hole
R/V Gilbert
Nat'l Mapping Leadership
MARGINS
Great Lakes Mapping
Delmarva Bays Delta
Grand Canyon Research
Bratton in the News
Woods Hole Arrivals
Woods Hole Visitors
Marshall Islands Map
November Publications List
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U. S. Department of the Interior | U.S. Geological Survey
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