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HILL, ERICA L  University of Pittsburgh.
Abbott, Mark  University of Pittsburgh.
Finney, Bruce  Univ of Alaska Fairbanks.

As part of a larger study focused on reconstructing lake-levels across the interior of Alaska and the Yukon Territory seismic surveys were done on a series of lakes using a Triton Elics-Edgetech full spectrum (4 to 24 kHz) sub-bottom profiler with a SB-424 tow-fish. This system has a maximum penetration depth of 10 to 30 m, depending on the composition of the sediments, and a resolution of 4 to 8 cm. The data was recorded, geo-referenced and reprocessed using Delphseismic 2.5 software. Results form two of the four lakes that were surveyed are presented here. Birch Lake, Alaska, was resurveyed to compare the results obtained using the new system to those obtained using a Geopulse system in 1994. Eighteen cross-lake seismic reflection profiles from Jackfish Lake and fourteen from Birch Lake were used to trace acoustic stratigraphy and identify surfaces associated with water-level changes. The seismic surveys are compared with results from cores collected from the same locations.

    The seismic profiles from Jackfish and Birch lakes were used to identify onlap sedimentary sequences and erosion surfaces associated with water-level changes and compared with data from core transects. Two problems limit the ability to interpret the results in shallow water. First, acoustic signals may be obscured by gas produced by the decomposition of organic matter. This effect is often visible in places on seismic lines that cross the depo-center of the lake basins and in patches in shallow water. A second problem occurs where sediment thickness exceeds the water depth and ringing of the sonic signal within the water column masks the seismic stratigraphy by overprinting multiples. Despite these complications the acoustic stratigraphy is remarkably correlative with the physical and geochemical stratigraphy identified from sediment core transects taken from shallow to deep water.

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