Alaska permafrost characterization: Geophysical and related field data collected from 2019-2020

Geophysical measurements were collected by the U.S. Geological Survey (USGS) at two sites in Interior Alaska in 2019 and 2020 for the purposes of imaging permafrost structure and quantifying variations in subsurface moisture content in relation to thaw features. In September 2019, electrical resistivity tomography (ERT) and downhole nuclear magnetic resonance (NMR) data were used to quantify permafrost characteristics across the shorelines of Big Trail Lake, a thermokarst lake outside of Fairbanks, Alaska. Three 222 m ERT survey lines were collected perpendicular to the North, East, and South shorelines, and two 110 m lines were collected parallel to the southeast and northeast shorelines. Models of electrical resistivity produced from these data revealed the distribution of frozen and thawed soil to depths of 10-40 m below the surface. NMR data were collected within two 2.3 m deep boreholes adjacent to the East and North perpendicular ERT survey lines in September 2019 and logged again in March 2020. Additional one-time NMR measurements of liquid water content were collected in September 2019 within the lakebed sediments (0-25 cm depth) in approximately 2.5 m lateral increments moving away from the shorelines in the East and North, between 0 and 12 m from shore. These NMR transects roughly coincided with the perpendicular ERT lines. A separate ERT survey was conducted at the Bonanza Creek LTER (Long Term Ecological Research) in September 2019 and was a repeat of a previous ERT survey done in the same exact location three years prior. The survey line was 125 m in length and spanned the transition between burned and unburned forest. Models of electrical resistivity for this site imaged the structure of frozen and thawed soils to depths of 10-15 m. At both sites, manual permafrost-probe measurements of thaw depths were collected at set intervals along each ERT transect and used for comparison to the resistivity models.