DOI: 10.5285/b9b28a35-8620-4182-bf9c-638800b6679b
title: Processed line aerogravity data over the Thwaites Glacier region (2018/19 season)

description: Aerogravity data has an important role to play in constraining sub-surface geology under grounded ice and bathymetry beneath floating ice shelves. This dataset contains aerogravity collected by the British Antarctic Survey as part of the International Thwaites Glacier Collaboration (ITGC). Data were collected using both a traditional stabilised platform approach, and a more modern strapdown gravity system. Flights were flown at a constant altitude ~450 m above the ice surface where surface topography was flat. Gravity data is also recovered along draped sections by the strapdown system. In total 9872 km of data is presented, of this 6033 km was collected in the main survey area, while other data was collected on input and output transit flights. The aircraft used was the BAS twin otter VP-FBL equipped for aerogeophysical surveys. Data are available in ASCII file format (.xyz). Three databases are provided with aerogravity data: one with the Strapdown processing flow, a second with the LaCoste & Romberg processing flow, and a final simplified database with the optimal free air gravity anomalies from the strapdown system.
description: PROCESSING: Gravity data during the ITGC survey were collected using both a strapdown (Jordan and Becker 2018) and traditional stabilised platform approach (Valliant 1992). The initial strapdown gravity signal was recovered from raw INS and GPS data using a Kalman filtering technique implemented in the TerraPos GPS processing software. Raw data were processed in a tightly coupled manner solving for gravity, position, and sensor orientation simultaneously. Ionospheric and clock errors were solved using Precise Point Positioning (PPP) technique with accurate satellite clock and orbit corrections used in the 'Final' ephemeris data downloaded more than a month after the survey was completed. The applied Kalman filter included a gravity smoothing filter assuming a 5km gravity correlation constant and 10 mGal permitted standard deviation. In order to minimise thermal effects on the QA2000 accelerometers, the IMU was warmed up from ambient temperatures inside an insulating enclosure for at least three hours before each flight. However, very significant thermal drift of &gt;30 mGal across a flight remained apparent when comparing the data to an independent Operation IceBridge (OIB) dataset (Cochran and Bell 2010, updated 2018). A thermal correction (C) based on the sensor internal temperature (T) was calibrated by comparison with the existing regional OIB data. The thermal correction took the form of a polynomial: C= -0.0174T^2 + 2.2619T + 5.8554 After thermal correction long wavelength residual errors (&gt;~30 minutes/100 km) attributed to hysteresis in the thermal response of the gravity system were still apparent. These were corrected for by a combination of statistical levelling, and fitting the long wavelength residual errors to the gravity field revealed by the OIB dataset. The stabilised platform gravity data was collected using a LaCoste & Romberg sensor (S83) mounted in a ZLS stabilised platform. Raw gravity data was collected at 1Hz. The GPS positional data processed using TerraPos, described above, was used to calculate standard gravimetric corrections for vertical acceleration, Eotvos effect, Latitude and elevation (free air). Sensor drift was low and stable, and was calculated based on a series of still reading taken before and after every survey flight. The final free air anomaly was filtered with two passes of a B-Spline filter with smoothness 1 and tension of 0.05. Before each pass of the B-Spline filter the data was manually edited to remove spikes and other data errors at the start of line segments. All the processing has been carried using Geosoft Oasis Montaj software. RESOLUTION: The line spacing for the survey is variable between 5 km and 10 km. Line data is generally designed to interleave with existing OIB data. Along-track resolution of the strapdown data is ~5 km. DATA FORMAT: Data are provided in ASCII file format (.xyz). Three databases are provided with aerogravity data: one with the Strapdown processing flow, a second with the L&R processing flow, and a final simplified database with the optimal free air gravity anomalies from the strapdown system. The strapdown data set includes the following channels: Name of the file : 'Thw_tera_pos_grav_share.XYZ' Flight: Flight number Date: GPS date (YYY/MM/DD) Time: GPS time (HH:MM:SS.SSSS) Lat_deg: Latitude (DD.DDDDDDD) Lon_deg: Longitude (DD.DDDDDDD) Hght_GRS80: Elevation on the GRS80 ellipsoid (m) X: X coordinate m Polar stereographic true scale -71 Y: Y coordinate m Polar stereographic true scale -71 Heading: Heading (degrees from Geodetic N) Roll: Role (degrees from local level Geodetic) Pitch: Pitch (degrees from local level Geodetic) FAA: Initial free air gravity anomaly (mGal) ICEBRIDGE: Reference gravity values from grid of ICEBRIDGE gravity anomalies (mGal). IMU__C_: Internal temperature of IMU (degrees C). Synthetic_Dicebridge: Calibrated temperature correction (mGal). FAA_temp_corrected: Free air anomaly after temperature correction (mGal). FAA_straight: Temperature corrected free air anomaly away from turns (mGal). Faa_straight_lev: 1st pass statistical levelling (mGal) Faa_straight_lev2: 2nd pass statistical levelling (mGal) D_icebridge_Lev2: Residual error with ICEBRIDGE (mGal). Dicebridge_temp: Residual error with ICEBRIDGE with extreme values (&gt;+/-8 mGal) removed. Correction: B-spline fit to Dicebridge_temp (mGal). FAA_spl_2_ICEBRIDGE: Final free air gravity value (mGal). The L&R dataset includes the following channels: Name of the file: 'Thw_L_and_R_grav.XYZ' Date: GPS date (YYY/MM/DD) Time: GPS time (HH:MM:SS.SSSS) Flight ID: Flight number Lon_deg: Longitude (DD.DDDDDDD) Lat_deg: Latitude (DD.DDDDDDD) Hght_GRS80: Elevation on the GRS80 ellipsoid (m) X: X coordinate m Polar stereographic true scale -71 Y: Y coordinate m Polar stereographic true scale -71 ST: Spring Tension (meter Units) CC: Cross coupling value (meter units) RB: Raw beam position (mv) XACC: Cross axis acceleration (un calibrated) LACC: Long axis acceleration (un calibrated) Still: Calculated still value based on long term linear drift (meter units) Base: Rother hanger absolute gravity value (mGal) Beam_vel: Calculated beam velocity (mv/second) Rec_grav: Recalculated raw relative gravity (mGal) Abs_grav: Absolute gravity value (mGal) VaccCor: Vertical acceleration correction (mGal) EotvosCor: Eotvos correction (mGal) LatCor: Latitude correction (mGal) FaCor: Free air correction (mGal) FA_LR_Raw: Free air gravity anomaly (un-filtered) (mGal) FA_LR_Final: Free air gravity anomaly after 2 passes of editing and B-spline filter (mGal). FAA_spl_2_ICEBRIDGE: Free air anomaly from strapdown gravity system (mGal) The final gravity dataset includes the following channels: Name of the file: '''Thw_grav_final_18_19_season.XYZ' Flight: Flight number Date: GPS date (YYY/MM/DD) Time: GPS time (HH:MM:SS.SSSS) Lat_deg: Latitude (DD.DDDDDDD) Lon_deg: Longitude (DD.DDDDDDD) Hght_GRS80: Elevation on the GRS80 ellipsoid (m) X: X coordinate m Polar stereographic true scale -71 Y: Y coordinate m Polar stereographic true scale -71 FAA_final: Final free air gravity anomaly.
description: Data was collected using the BAS twin otter VP-FBL, equipped for aerogeophysical surveys. Strapdown gravity data were recorded using an iMAR RQH-4001 inertial measurement unit (IMU) owned by Lamont-Doherty Earth Observatory. This sensor package consists of three Honeywell QA2000 accelerometers (mounted in mutually perpendicular directions), and three ring laser gyroscopes. Coincident GPS data were recorded with a NovAtel receiver. The stabilised platform gravity data were collected using a LaCoste and Romberg sensor (S83) mounted in a ZLS stabilised platform.
description: The standard deviation of the strapdown gravity crossover errors after thermal correction and levelling to OIB data was 2.2 mGal, consistent with a Root Mean Square Error (RMSE) of 1.56 mGal. The Stabilised platform gravity system did not perform as well, and the data from this system is included here for completeness.