In June 2010, the U.S. Geological Survey conducted airborne electromagnetic and magnetic surveys of the Yukon Flats and Fort Wainwright study areas in central Alaska. These data were collected to estimate the three-dimensional distribution of permafrost at the time of the survey. These data were also collected to evaluate the effectiveness of these geophysical methods at mapping permafrost geometry and to better define the physical properties of the subsurface in discontinuous permafrost areas. This report releases digital data associated with these surveys. Inverted resistivity depth sections are also provided in this data release, and data processing and inversion methods are discussed.
Abstract : The glacial stratigraphy of Fort Richardson reflects deposition in glacial and glacial-marine environments during multiple retreat phases following the last glacial maximum. A preliminary model relied heavily on the glacial history off the region, mapping by the U.S. Geological Survey, and limited borehole logs. This report expands on that model and describes new subsurface data obtained from field observations and descriptions of stratigraphic exposures and core samples from 28 new boreholes between 1997 and 1998. Geophysical techniques were applied to seven of the new boreholes and 25 existing monitoring wells, augmenting surface techniques (ground resistivity and ground penetrating radar). Beneath the cantonment is a thick unconfined aquifer, apparently deposited as a large alluvial fan (Mountain View fan), that overlies a fine-grained confining layer composed of mud and diamicton. The diamicton is a subglacial lodgement deposit bracketed by stratified debris flow deposits, being thickest to the southeast, dipping and thinning to the north and west where deposits of the Mountain View fan truncate the confining horizon, providing potential hydraulic communication between the tipper (unconfined) and lower (confined) aquifers. A second mud-diamicton horizon forms a deeper (38-66 m depth) confining layer and also appears to extend across the cantonment. Between the upper and lower confining diamicton horizons are coarse, sandy gravels that make up a confined aquifer. Ground water in the unconfined aquifer flows generally to the northwest, presumably recharged by Ship Creek. When ground water levels are low (i.e., winter), flow is locally diverted by irregularities in the surface of the upper confining layer. When recharge is high, regional flow is unaffected by these irregularities. Ground water in the confined aquifer also flows to the northwest, following the slope of the potentiometric surface.
Discontinuous permafrost is present in interior Alaska to depths as great as 45 m. The surface active layer seasonally thaws to depths > 2m and in many locations groundwater can exist above, within, and below permafrost. The study area soils consist of organic material and loess draped over well sorted alluvial gravels. Vegetation is predominately black spruce forest with sedges while grasses predominate in inactive stream channels. In this setting we conducted a geophysical site investigation using ground penetrating radar (GPR) and two-dimensional DC resistivity. Measurements were made in late winter and late summer to delineate permafrost distribution beneath four parallel 500 m transects. 400-MHz GPR detected active layer depth and thaw zones while 100-MHz GPR detected thaw zones and the permafrost bottom. We compared results of Wenner and dipole-dipole resistivity with GPR and found that, in most cases, the permafrost interpretations from the dipole-dipole data closely match the GPR data permafrost interpretations; however, no boreholes are yet available for ground truth. Permafrost was detected on average between 0.7 and 20 m below the surface except along a north-south trending power line clearing where permafrost was mostly absent. Within some inactive fluvial channels<br>permafrost was sporadic or absent. The GPR and resistivity methods worked well in concert to delineate permafrost and stratigraphy, providing high quality data to depths of 30 m.
