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Fieldwork

Ground-Penetrating Radar Examines Sand Bars in Grand Canyon


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Collecting GPR data
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.
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 profile
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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