GROUND PENETRATING RADAR AND SEISMIC REFRACTION TOOLS FOR CHARACTERIZING HYDROGEOLOGIC PROPERTIES IN RECENTLY ACTIVE GLACIAL MORAINE SETTINGS, TONGASS NATIONAL FOREST, ALASKA

Ground penetrating radar (GPR) and seismic refraction surveys were conducted at a critical location on a glacial moraine near Yakutat, Alaska, and at several road culvert sites prior to their excavation near Petersburg, Alaska. The subsequent exposure at the culvert locations allowed the hydrogeologic properties and aquifer geometry (including depth to water table and bedrock) to be observed and measured. Calculating the velocities of electromagnetic and seismic waves from both machines using known geometry of the subsurface provided constraints on hydrogeologic properties. Observations of the aquifer material on the moraine were made from exposures at the surface and from shallow excavation. The near-subsurface moraine material is composed of clean sand and poorly sorted gravel, and the culvert locations are filled with uniform gravel, clay and sand. The geology of the two locations varies, but the heterogeneity of the unconsolidated material comprising the substrate in both is quite similar. Seismic data acquired at the moraine show a subsurface depth to bedrock of 25 meters at the moraine. The expected results from the surveys should allow a reasonable geometric model of the subsurface to be made within a 10% estimated resolution of depth. Further applications of the data set and extrapolated properties of the subsurface should lead to reasonable models of hydrogeologic substrate and general flow regime. The subsurface model will prove valuable during the next cyclical closure of Russell Fjord by Hubbard Glacier, the body of water bordering the glacial moraine to the north.