GPR AND THE SEARCH FOR BURIED (LAKE-SEDIMENT) TREASURE

Lake sediment coring can provide crucial information about the sedimentary and ecological development of a drainage basin; however, it is often unclear how representative such cores are of a lake basin as a whole. In particular, lake-corers would like to know if there are more sediments below the greatest depth reached by the cores, and if the sediments in the cores are laterally continuous across the lake floor. Here, we report the initial results of tests using ground-penetrating radar (GPR) to image the subsurface stratigraphy of an alpine lake near Mt. Baker Volcano, WA. The eventual goal is to use the system to image subsurface stratigraphy of other lakes in the region.

We used a GSSI SIR-3000 GPR system and a low-frequency (80 MHz) antenna to image the subsurface sediments in a small (60 m in diameter) unnamed lake at Heather Meadows, near the Mt. Baker Ski area. We accessed the lake after it had frozen to facilitate GPR data collection, and in conjunction with a coring effort and bathymetry survey. The bathymetry survey reveals that the lake is about 3 to 4 m deep in its center, and the coring demonstrates that the lake contains at least 1 m of post-glacial sediment, dominated by organic mud (gyttja) and at least two and probably three tephras. We completed two orthogonal transects of the lake (N-S and E-W), from shore to shore. The data confirmed a lake ice thickness of 30 centimeters and a maximum lake depth along the transects of 3.3 meters. A strong reflector, likely related to the transition between lake sediments and bedrock, occurs approximately 3 m below lake bottom. Although several horizontal reflectors appear within the sediments, the 80 MHz antenna we used limits our resolution to units greater than several 10s of cm (thicker than most strata in the core). To detect thinner strata in subsequent surveys, we will use a higher frequency (up to 200 MHz) antenna. Below the lake sediments is a 21-meter thick layer of reflection-free (massive) material, which we interpret to be a lava flow based on similar flows exposed in road cuts to the north and northeast of the lake.

Our GPR survey thus demonstrates that the depth of sediments in the lake extends as much as 2 meters below the deepest strata recovered during our coring efforts, and that a higher frequency antenna is needed to discern and map the lateral extent of thin strata of sediments and tephra typical of lakes near Mt. Baker volcano.