DOI: 10.5285/af14076f-f0c4-479f-b38f-6aa0b7c7d314
title: Processed line aerogravity data over the Thwaites Glacier region (2019/20 season)

description: As part of the International Thwaites Glacier Collaboration (ITGC) ~9540 km of new airborne gravity data was acquired by the British Antarctic Survey, including ~6200 km over the Thwaites Glacier catchment. Data was collected using an iCORUS strap-down airborne gravimeter system mounted on the BAS aerogeophysical equipped survey aircraft VP-FBL. The survey operated from Lower Thwaites Glacier camp, and focused on collecting data between 70 and 180 km from the grounding line. Additional profiles from the coast to the Western Antarctic Ice Sheet (WAIS) divide and over the eastern shear margin were also flown. Navigation, aircraft attitude, sensor temperature, initial and levelled free air gravity anomalies are provided as an ASCI table. The Thwaites 2019/20 aerogeophysical survey was carried out as part of the BAS National Capability contribution to the NERC/NSF International Thwaites Glacier Collaboration (ITGC) program. Data processing was supported by the BAS Geology and Geophysics team.
description: The ITGC aerogeophysical survey relied on an iCORUS strap-down airborne gravimeter system. This high specification IMU was mounted in a thermally stabilised enclosure close to the centre of mass of the survey aircraft. GNSS and IMU data were recorded to a windows tablet PC for post processing. Post processing used base-station free Precise Point Positioning (PPP) implemented in the Terrapos GNSS processing software. This software uses a Kalman filtering approach to simultaneously solve for position, attitude and the Earth's gravity field (output as a free air anomaly). As no GNSS base station is used ionosphere corrections are derived from published global solutions available ~1 month after the survey. Due to operational procedures system temperature varied by up to 6 degrees, within the main survey area. However, temperature stabilisation meant that temperature generally varied by less than 1 degree for much of the survey. Comparison of errors and sensor temperature did not find a strong correlation, so a temperature correction was not applied. After initial processing the position and free air anomaly data was imported into a Geosoft database for further processing. The free air gravity for each flight were initially shifted by a DC value so that the value at the Lower Thwaites Glacier camp (LTG) was the same, and so there was a zero mean offset with integrated OIB/ITGC gravity data from the previous season. After this simple DC levelling the internal crossovers for the data-set had a standard deviation of 3.44 mGal. To further mitigate trends in the data within flights previous surveys were considered. These included integrated OIB/ ITGC gravity data, and data from the older AGASEA/BBAS survey. Data from these earlier surveys was sampled from grids and compared to the new data. The difference to the OIB data had a standard deviation of 3.37, in-line with the errors seen in the crossover analysis. The AGASEA/BBAS dataset showed a higher standard deviation (&gt;6 mGal), however, several areas appeared systematically different both to OIB and the new dataset. The difference to OIB was therefore taken as the primary reference field, and where the difference to AGASEA/BBAS was &lt;5 mGal this was used as a secondary reference. This integrated difference was fit with a B-spline, with a tension of 0.7 and smoothness of 0.7. The resulting best fit correction was added to the simple levelled free air gravity anomaly, to give the final spline-levelled free air gravity anomaly. The internal crossover error of this levelled dataset is 2.36 mGal, which equates to an error of 1.67 mGal. In three instances the gravity system failed, and in these places positional data from an alternative 1Hz GPS only positional solution is provided. In these cases values for Roll, Pitch and Heading are also set to NaN. DATA FORMAT: Data are provided in ASCII file format (.csv). The data set includes the following channels: FlightID: Flight name e.g. T05 Date: Date GPS satellite time (YYY/MM/DD) Time: Time GPS satellite time (HH:MM:SS.SSS) Lat: Latitude DD.DDDDDDDDD Lon: Longitude DD.DDDDDDDDD X: X Polar Stereographic m (true scale -71) Y: Y Polar Stereographic m Elev: WGS1984 ellipsoidal elevation of aircraft IMU (m) Roll: Aircraft roll (degrees) Pitch: Aircraft pitch (degrees) Heading: Aircraft heading (degrees true north) Internal_temperature: IMU accelerometer temperature (Celcius degrees). FAA: Initial calculated free air gravity anomaly (mGal) FAA_Level_I: Free air gravity anomaly after levelling (mGal). Note dummy values are reported as NaN. This includes flights outside the main survey area where no levelling was carried out.
description: Data was collected using the BAS aerogeophysically equipped twin otter VP-FBL. The gravity system was an iCORUS Airborne Gravimeter System, from iMAR Navigation.
description: Analysis of 76 crossover points within the main survey area indicates a standard deviation for the bed elevation of 2.36 mGal, which equates to a standard error of 1.67 mGal. Line spacing is on the order of 15 km (designed to inter-leave with previous surveys). Airborne gravity processing imposes a correlation constant of 5km, hence the resolved wavelengths are assumed to be &gt;5 km.