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IMAGING POTENTIAL HYPORHEIC ZONE EXTENT BENEATH ARCTIC STREAMS USING GROUND PENETRATING RADAR
GOOSEFF, MICHAEL N Utah State University.
Bradford, John H Boise State University.
McNamara, James P Boise State University.
Bowden, William B University of Vermont.
We are investigating the responses of arctic tundra stream geomorphology, hyporheic zone hydrology, and biogeochemical cycling to climate change. In particular, we expect that hyporheic exchange dynamics in tundra streams are controlled by 1) channel features (pools, riffles, etc.), and 2) depth-of-thaw beneath the stream channel. A key objective of this effort is monitoring sub-stream thaw through the thaw season using ground-penetrating radar (GPR).
In general, GPR is a well established tool for imaging terrestrial active layer thickness. However, sub-stream imaging presents a unique set of challenges. This is primarily related to strong frequency dependence and high levels of attenuation as the radar signal propagates through water. To test the effectiveness of GPR imaging of sub-stream permafrost we conducted a field investigation near the end of the thaw season when we expected the depth of thaw to be near its maximum. We investigated three sites located within the Kuparuk River and Toolik Lake basins, north of the Brooks Range, Alaska. The sites were characterized by low energy water flow, organic material lining the streambeds, and water depths ranging from 20 cm to 2 m. Water saturated peat with some pooled water was present along the stream banks. We acquired data using a pulsed radar system with high-power transmitter and 200 MHz antennas. We placed the radar antennas in the bottom of a small rubber boat, and pulled the boat across the bank and through the stream while triggering the radar at a constant rate. We verified depth to permafrost by pressing a metal probe through the active layer to the point of refusal.
Although there is significant shift toward the low end of the frequency spectrum due to frequency dependent signal attenuation, we achieved excellent results at all three sites with a clear continuous image of the permafrost boundary both lateral to, and beneath the stream. Depth migration was applied to the profiles to provide an accurate image of both the streambed and top-of-permafrost geometry. We found thaw bulb thickness to increase with stream depth, with a maximum thaw depth of 1.5 m measured beneath the 2 m deep stream. It should be noted that the maximum thaw depth occurred beneath the site with not only the greatest water depth but also the lowest flow rate.
Our results demonstrate that GPR is an excellent tool for measuring sub-stream thaw depth. We conclude that GRP can effectively identify the thawed-frozen interface in the permafrost under Arctic streams dominated by organic substrates. Modeling results suggest that we should be able to detect this interface in streams dominated by gravel and cobble. We plan to verify these results in the 2004 field season.
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