DOI: 10.5285/8e5f910b-11d6-4a9d-bdf7-175c9b98cfb8
title: Antarctica's Gamburtsev Province (AGAP) Project - Airborne gravity data (2007-2009)

description: Aerogravity data collected as part of the seven nation Antarctica's Gamburtsev Province (AGAP) expedition during the International Polar Year 2007-2009, and used to acquire a detailed image of the ice sheet bed deep in the interior of East Antarctica. Airborne geophysical methods were used to understand the fundamental structure shrouded beneath Dome A. Two twin Otter aircraft - one BAS, one United States Antarctic Program (USAP)- equipped with ice-sounding radars, laser ranging systems, gravity meters and magnetomemeters, operated from camps located on either side of Dome A. Airborne gravity measurements were acquired using LaCoste and Romberg air-sea gravimeter modified by ZLS Corporation, which is well-proven for Antarctic field work. A land-gravimeter was used to tie the still readings on the aircraft with the absolute gravity value at McMurdo Station.
description: Airborne gravity data are presented in Ferraccioli et al (2011). Only BAS data are presented here but USAP flown flight lines details can be found at http://pgg.ldeo.columbia.edu/wp/dataprojects/gambit-agap-consortium/. BAS data processing steps are outlined below, as are the full definitions of all channels. Processing steps: 1/ Calculate observed gravity. True spring tension (ST_real) is calculated from the posted spring tension (ST) correcting for the fact that for this survey the true spring tension approaches the posted value at 38 mGal per second. Beam velocity (Beam_vel) is derived from raw beam position (RB) assuming a centred difference approximation. Relative gravity (rec_grav) = (Spr_tens_real+((beam_vel)*k_fac)+CC)*scale_value, k_fac=30, meter scale value =0.9966. Still readings (Still) are in mGal and were calculating assuming a 2nd order best fit to the approximately linear drift of the meter observed at the tie down points. Airborne absolute gravity values (Abs_grav) = Rec_grav- Still + Base 2/ Corrections to derive free air anomalies (disturbances). Vertical acceleration (VaccCor) is calculated as 2nd derivative of flight altitude (Height_WGS1984), with a 3 point mean filter applied after differencing to reduce short wavelength noise. Eotvos correction (EotvosCor) follows (Harlan, 1968). Latitude correction (LatCor) = 978.03185(1+0.005278895 sin2Lat- 0.000023462 sin4Lat) (IUGG 1967). Free air correction (FaCor) = 0.3086*Height_WGS1984. NOTE subsequent free air values are defined as gravity disturbances in geodesy, as they are referred to the ellipsoid (Hackney and Featherstone, 2003). Horizontal acceleration correction (HaccCor). For this survey the approximation of (Swain, 1996) was used, assuming a damping factor of 0.707, and a platform period of 4 minutes. 3/ Free air anomaly and filtering. Free air anomaly (Free_air) = Abs_grav-VaccCor+EotvosCor+FaCor-LatCor-(0.5*HaccCor) Filtered free air anomaly (FAA_filt) used 9 km 1/2 wavelength space domain kernel filter (Holt et al., 2006). Final free air data (FAA_clip) was produced by manually masking turns, start and end of lines, and other regions of noisy data. Free air anomaly data was statistically levelled to give a consistent grid. The correction applied is Level_cor and the final anomaly channel is FAA_level. Upward continued free air anomaly (FA_4600m) was produced by upward continuing free air data from the collected flight altitude to 4600 m. Channel description: Basic Channels Date UTC date (YYYY/MM/DD) Time UTC time (HH:MM:SS.SS) FlightID Sequential flight number and survey ID e.g. W12 Line_name Line Number e.g. LW200.1:12 Lon Longitude WGS 1984 Lat Latitude WGS 1984 x x projected meters * y y projected meters * Height_WGS1984 Aircraft altitude (meters) in WGS 1984, for processing see location data page Raw gravity Channels ST Spring Tension (meter units) CC Cross Coupling (meter units) RB Raw beam position (Mv) XACC Cross axis accelerometer (Mv) LACC Long axis accelerometer (Mv) Still Airborne meter still reading value (mGal) Base Absolute gravity reference, from land gravity (mGal) Calculation Channels St_real True Spring tension value (meter units) Beam_vel Gravity meter beam velocity (Mv/sec) Rec_grav Recalculated relative gravity (mGal) Abs_grav Calculated absolute gravity (mGal) VaccCor Vertical acceleration correction (mGal) EotvosCor Eotvos correction (mGal) LatCor Latitude correction (mGal) FaCor Free air correction (mGal) HaccCor Horizontal acceleration correction (mGal) Free air Channels Free_air Un-filtered free air anomaly (mGal) FAA_filt Filtered free air anomaly data (mGal) Filtereee FAA_clip Filtered, masked free air anomaly data (mGal) FA_4600m Free air anomaly data upward continued to an altitude of 4600 m. (mGal) * Projected coordinates (x and y) are in Lambert conic conformal with two standard parallels defined as follows: Latitude of false origin: -80 Longitude of false origin: 80 Latitude of 1st standard parallel -83 Latitude of 2nd standard parallel -77 False easting 2000000 False northing 2000000
description: In total, 120, 000 line km of data were aquired from a nested survey grid with line spacing of 5km and tie lines ~ 33 km apart.
description: To obtain high quality airborne gravity data requires accurate kinematic post-processing of GPS data. Estimated cross-over errors at the intersection between lines and tie lines is better than 2 mGal, for wavelengths of 9 km.