Geophysical measurements and related field data were collected by the U.S. Geological Survey (USGS) at the Alaska Peatland Experiment (APEX) site in Interior Alaska from 2018 to 2020 to characterize subsurface thermal and hydrologic conditions along a permafrost thaw gradient. The APEX site is managed by the Bonanza Creek LTER (Long Term Ecological Research). Nine instrument sites were established in April 2018 comprising a buried seismic station for continuous passive recording of the seismic wavefield, and seven of the nine sites were given a borehole for repeat nuclear magnetic resonance (NMR) logging. Between June 2018 and September 2019, measurements of active-layer thaw depth were regularly recorded at each of the nine instrument sites for comparison to the geophysical data. A nominal four measurements were collected in random azimuthal directions within 1 meter (m) distance of the buried seismometer at each instrument site. At sites containing an NMR borehole, an additional four to six measurements were taken within 1 m distance of the borehole location. During collection of electrical resistivity tomography (ERT) data in late August 2018, additional thaw depth measurements were collected at regular electrode intervals along each ERT survey line. Measurements were made with a 250 centimeter (cm) stainless-steel frost probe inserted to the depth of refusal.

Geophysical measurements and related field data were collected by the U.S. Geological Survey (USGS) at the Alaska Peatland Experiment (APEX) site in Interior Alaska from 2018 to 2020 to characterize subsurface thermal and hydrologic conditions along a permafrost thaw gradient. The APEX site is managed by the Bonanza Creek LTER (Long Term Ecological Research). In July 2018, soil temperature and moisture sensors were installed at six out of the nine instrument locations (APEX1, APEX2, APEX3, APEX4, APEX7, APEX9). Thermistors (PS103J2, US Sensor, Orange, CA, USA) were placed at depths of 5, 30, 60, 120, and 180 centimeters (cm) with three replicates. Three sites (APEX1, APEX4, APEX9) contained an additional single 240 cm thermistor. Each soil temperature record was processed to remove erroneous values and spikes. Two time-domain reflectometry (TDR) probes (CS-616, Campbell Scientific, Logan UT, USA) were installed at the same six instrument sites to record soil moisture. The first TDR was inserted at a 45-degree angle down from the ground surface (0-20 cm), and the second from 20-40 cm. We applied an organic soils correction to the raw TDR data following the procedure in Bourgeau‐Chavez et al. (2010). Soil temperature and moisture data were recorded continuously from July 2018 through October 2020 on Campbell Scientific data loggers, with some data gaps due to power loss.

Geophysical measurements and related field data were collected by the U.S. Geological Survey (USGS) at the Alaska Peatland Experiment (APEX) site in Interior Alaska from 2018 to 2020 to characterize subsurface thermal and hydrologic conditions along a permafrost thaw gradient. The APEX site is managed by the Bonanza Creek LTER (Long Term Ecological Research). In July 2018, soil temperature and moisture sensors were installed at six out of the nine instrument locations (APEX1, APEX2, APEX3, APEX4, APEX7, APEX9). Thermistors (PS103J2, US Sensor, Orange, CA, USA) were placed at depths of 5, 30, 60, 120, and 180 centimeters (cm) with three replicates. Three sites (APEX1, APEX4, APEX9) contained an additional single 240 cm thermistor. Each soil temperature record was processed to remove erroneous values and spikes. Two time-domain reflectometry (TDR) probes (CS-616, Campbell Scientific, Logan UT, USA) were installed at the same six instrument sites to record soil moisture. The first TDR was inserted at a 45-degree angle down from the ground surface (0-20 cm), and the second from 20-40 cm. We applied an organic soils correction to the raw TDR data following the procedure in Bourgeau‐Chavez et al. (2010). Soil temperature and moisture data were recorded continuously from July 2018 through October 2020 on Campbell Scientific data loggers, with some data gaps due to power loss.

Geophysical measurements were collected by the U.S. Geological Survey (USGS) at five sites in Interior Alaska in September 2021 for the purposes of imaging permafrost structure and quantifying variations in subsurface moisture content in relation to thaw features. Electrical resistivity tomography (ERT) measurements were made along transects 110-222 meters (m) in length to quantify subsurface permafrost characteristics. ERT transects were collected across a fireline boundary within the Bonanza Creek Long Term Ecological Research (LTER) site where repeat measurements have been made since 2014; across and adjacent to two thermokarst lakes, Vault Lake and Goldstream Lake; and along two profiles at the North Star golf course in Fairbanks, Alaska. 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. Borehole nuclear magnetic resonance (NMR) data were collected at two sites in order to determine liquid water content at depth in shallow boreholes. NMR data were collected in a 2.25 m-deep borehole at the North Star golf course adjacent to one of the ERT profiles, and in another two 1.625 m-deep boreholes adjacent to Big Trail Lake where previous NMR measurements were made in 2019 and 2020. Manual permafrost-probe measurements of thaw depths were collected at set intervals along each ERT transect and adjacent to NMR measurement locations used for comparison to the geophysical measurements, except at the North Star golf course where shallow permafrost was absent.

Geophysical measurements were collected by the U.S. Geological Survey (USGS) at five sites in Interior Alaska in September 2021 for the purposes of imaging permafrost structure and quantifying variations in subsurface moisture content in relation to thaw features. Electrical resistivity tomography (ERT) measurements were made along transects 110-222 meters (m) in length to quantify subsurface permafrost characteristics. ERT transects were collected across a fireline boundary within the Bonanza Creek Long Term Ecological Research (LTER) site where repeat measurements have been made since 2014; across and adjacent to two thermokarst lakes, Vault Lake and Goldstream Lake; and along two profiles at the North Star golf course in Fairbanks, Alaska. 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. Borehole nuclear magnetic resonance (NMR) data were collected at two sites in order to determine liquid water content at depth in shallow boreholes. NMR data were collected in a 2.25 m-deep borehole at the North Star golf course adjacent to one of the ERT profiles, and in another two 1.625 m-deep boreholes adjacent to Big Trail Lake where previous NMR measurements were made in 2019 and 2020. Manual permafrost-probe measurements of thaw depths were collected at set intervals along each ERT transect and adjacent to NMR measurement locations used for comparison to the geophysical measurements, except at the North Star golf course where shallow permafrost was absent.

Depth to frozen soil measurements taken by a variety of collaborators at the Alaskan Peatland EXeriment (APEX) bog/permafrost plateau site. Data is from 2018 - 2023.

Depth to frozen soil measurements taken by a variety of collaborators at the Alaskan Peatland EXeriment (APEX) bog/permafrost plateau site. Data is from 2018 - 2023.

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.

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.

Borehole nuclear magnetic resonance (NMR) data were collected by the U.S. Geological Survey (USGS) at Big Trail Lake, a thermokarst lake outside of Fairbanks, Alaska, to quantify unfrozen water content and soil properties at select sites in and around the lake edge. In September 2019, NMR data were collected within two 2.3 m deep boreholes adjacent to the East and North perpendicular electrical resistivity survey lines. Manual permafrost-probe measurements of thaw depths were also collected. These two boreholes were logged a second time in late 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 electrical resistivity lines.