DOI: 10.5285/883474c4-cb40-4949-b7af-efcc1dafc084
title: Processed line aerogravity data over the Evans Ice Stream, Southern Palmer Land region (1994/95 season)

description: The survey collected a total of 11,500 km of data along 22 lines, spaced 12 km apart and oriented perpendicular to the strike of both the Bouguer anomaly field, as derived from land data (McGibbon and Smith, 1991), and the major sub-ice topographical features (Doake et al., 1983). The speed of the aircraft was set to produce a sample spacing of about 60 m and the data were collected at heights between 1600 and 2000 m above sea level. The gravity signal was recorded using a LaCoste and Romberg air/sea gravimeter, S-83, which has been kindly loaned to BAS by the Hydrographic Office of the Royal Navy. The meter was modified by the ZLS company for use in an aircraft. The equipment was deployed in a BAS De-Havilland Twin Otter aircraft. Differential, dual frequency, carrier phase, GPS measurements of the aircraft's motion were made using Trimble and Ashtech geodetic receivers and antennas. Ice thickness data were obtained using a BAS-built, radio echo sounding system (Corr and Popple, 1994). Ice-bottom returns over most of the survey area were obtained at a sample spacing of approximately 28 m. GPS measurements were tied into base stations in International Terrain Reference Frame network (Dietrich et al., 1998) and gravity measurements to base stations in the IGSN71 net (Jones and Ferris, 1999). We present here the processed line aerogravity data collected using Lacoste and Romberg air-sea gravity meter S83. Data are provided as XYZ ASCII line data.
description: Airborne gravity data are presented in Jones et al (2002). Data are available on a flight by flight basis. 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. Upward continued free air anomaly (Faacont) was produced by upward continuing free air data from the collected flight altitude to 2800 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, for processing see readme Lat Latitude WGS 1984, for processing see readme 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 EotvosCor Eotvos correction LatCor Latitude correction FaCor Free air correction HaccCor Horizontal acceleration correction Free air Channels FaaCont Free air anomaly data upward continued to an elevation of 2000 m *Projected coordinates (x and y) are in Polar sterographic defined as follows: Latitude of natural origin: -71 Longitude of natural origin: 0 Scale factor at natural origin: 0.994 False easting 0 False northing 2082760.109 Positioning for the Evans ice stream survey uses kinematic differential GPS. Processing was carried out using GPSurvey software. Differential positioning used fixed base stations at each of the operational camps (Jones et al., 2002). Positions are calculated for the phase centre of the aircraft antenna. All positions (Lat, lon and height) are referred to the WGS1984 ellipsoid.
description: No values are given for Still, Base, St_Real, Beam_vel, Rec_grav, Abs_grav, VaccCor, EotvosCor, LatCor, FaCor, HaccCor. Intermediate information not available for this legacy data.