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UTIG airborne gravity operations in Antarctica from 2008 to 2016 and future directions
Jamin Greenbaum, Tom Richter, Duncan Young, Donald Blankenship
The University of Texas Institute for Geophysics
Outline
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DHC-6 platform/gravity development heritage
Scientific motivations
BT-67 (DC3-T) fixed-wing platform
AS350 rotary-wing platform
LIONESS 2016 preliminary results
DHC-6
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Platform/Gravity system heritage
Significant platform development heritage from the Twin Otter-based CASERTZ/SOAR/AGASEA programs from 1990-2001, 2004
1990-1992: CASERTZ (PI: D. Blankenship; Co-PI: Behrendt, Brozena, Hodge) - NRL collaboration with BGM-3 (with R. Bell) - BAS collaboration on first ZLS usage in Antarctica (with V. Childers)
1993-2001: SOAR (PI: Blankenship; Sci. Coordinators: Blankenship and Bell) - Mostly BGM3 surveys
2004: AGASEA (PI: Holt; Co-PI: Blankenship) - First Air-Sea II usage in Antarctica (with T. Diehl)
2008 – 2013: ICECAP (PI: Blankenship; Co-PI: Holt, Dalziel, Lawver) - Mostly BGM3; one season with the BAS ZLS
2009 – 2013: ICECAP/OIB (PI: Blankenship; Co-PI: Young, Holt) - First GT-1A, GT-2A usage in Antarctica
BT-67
(Compiled from Fretwell et al., 2013) 1000 km
Over 600,000 line km flown (330,000 km in a BT-67) 20+ years of piloted survey experience
Gravity, magnetics, ice-sounding radar, laser altimetry
BT-67 DHC-6
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Focus on areas of Antarctica with significant sea level potential ICECAP and other UTIG airborne geophysics coverage to date
Ice Bottom Elevation (m) Wilkes
Subglacial Basin
Byrd Subglacial
Basin
Aurora Subglacial
Basin
Geophysical motivations for airborne gravity (and magnetics) at UTIG
• What are the controls on the large scale architecture of the Antarctic Ice Sheet? – e.g. tectonic controls, sedimentary basins and erosion control.
(Aitken et al., 2014)
Isostatic residual gravity anomaly reconstruction into a Gondwana fit at 160 Ma
(Aitken et al., 2016)
(Young et al., 2011)
Morphological evidence of paleo-coastlines: Erosional evidence of paleo ice sheet stability:
WSB ASB
Oates Coast
Knox Coast
Sabrina Coast
Geophysical motivations for airborne gravity (and magnetics) at UTIG
• What are the boundary conditions for ocean circulation beneath and nearby ice shelves in regions of Antarctica with the potential to contribute significantly to sea level?
– Infer bathymetry under ice shelves and fast ice using airborne gravity to understand cross- shelf exchange and heat delivery to grounding lines.
Gaps in East Antarctic bathymetry along three coasts that may be vulnerable to marine ice sheet instability
(Greenbaum et al., 2015)
Totten Glacier Ice Shelf Bathymetry:
Fixed wing installation Ski-equipped DC3-T / BT-67, Range: 2100 km
Base Magnetometer
GPS on wings and tail
GPS/GLONASS over aircraft CG
• 5 x dual-frequency, carrier
phase GPS/GLONASS receivers
• 2 x GPS-aided IMUs
GNSS & INS:
Gravity Ties GT-2A Gravimeter GT-2A Power &Temperature Control
Gravimeter -Riegl Altimeter -Scanning lidar -Camera
Phase coherent, chirped 60 MHz radar sounder
Cesium-vapor magnetometer
Ice and seafloor characterization from airborne geophysics
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Marine gravity, RV Palmer
LIONESS/ASE Icebreaker-Helicopter gravity
ICECAP
UTIG BT-67 surveys, 2008 to 2016
ICECAP
• Flight profile limitations: – Gaps in ICP1-4 results due
to drape-flying motivated migration to a 3-axis stabilized gravimeter.
– Results from ICP5-7 using GT-1a and GT-2a are significantly improved under all flight conditions.
• Accessibility – All airborne gravity (Free
Air Gravity Disturbance) and magnetics data have been posted to NSIDC, including raw meter accelerations
Map of available gravity data (at NSIDC) from ICECAP seasons 1,2,4, and 5. Seasons 1-4 were acquired with BGM-3 and ZLS meters. Season 5 (in red) was with a GT-1A. Line gaps are data rejected for excessive aircraft dynamics.
ICP1: Green ICP2: Pink ICP4: Blue ICP5: Red
Improvements in data recoverability with 3-axis gravimetry
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ICP5 Totten data recovery ICP1-4 Totten data recovery
WG
S84
Hei
ght
Sample number
WG
S84
Hei
ght
Sample number
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Ice bottom elevation Gravity compilation, 2009-2012 Gravity field
(Greenbaum et al., 2015)
Magnetic field
Magnetics and gravity compilations used to identify ocean access to Totten Glacier, East Antarctica
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ICECAP/GIMBLE
Pink – Fixed-wing flights (OIB)
Blue – Fixed-wing flights (GIMBLE) Yellow – Helicopter gravity flights
White – Previous coverage Cyan – Compiled ship tracks
RMSE: 4.0 (2.8) mgal
RMSE: 3.2 (2.3) mgal
ICECAP/GIMBLE Geophysical Investigations of Marie Byrd Land lithospheric Evolution
ICECAP/GIMBLE Geophysical Investigations of Marie Byrd Land lithospheric Evolution
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ICECAP/GIMBLE
Pink – Fixed-wing flights (OIB)
Blue – Fixed-wing flights (GIMBLE) Yellow – Helicopter gravity flights
White – Previous coverage Cyan – Compiled ship tracks
RMSE: 4.0 (2.8) mgal
RMSE: 3.2 (2.3) mgal
Assumes uncorrelated random error
AS350 Helicopter-borne gravimeter for cruise ANA06B
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1. Demonstrate the technical and logistical feasibility of gravity observations from a ship at sea to obtain high resolution gravity data over an Antarctic ice shelf.
2. Infer the wide area bathymetry of the Getz sub-ice shelf cavity by inverting the gravity data constrained by seismic observations and ice front multibeam bathymetry for water column thickness.
3. Infer large-scale geological boundaries and structures of the Getz system.
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Gravity resolution: • Resolution varies linearly with
velocity and smoothing filter length (slower is better).
• Filter length depends on flight conditions and GPS-INS quality (i.e. oven-controlled accelerometers on every axis).
• Helicopters currently provide the highest resolution of any commonly-used airborne platform.
AS350 Helicopter-borne gravimeter for cruise ANA06B
UTIG airborne gravity platforms:
(Assumes 100-second filter length)
AS350 gravimeter for cruise ANA06B – GT-1A overview
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GT1a Sensor and restraint cage
GT1a Power electronics and GPS receiver
GT-1A kinematic gravimeter: • Consists of a sensitive vertical axis
accelerometer supported and aligned with local vertical by an inertial platform stabilized on all three axes.
• Heading information is determined with a real-time GPS input to assist the inertial unit in stabilizing the sensor platform.
• The GT-1A had previously been installed on a large number of fixed and rotary wing aircraft including land-based AS350B helicopters similar to those assigned to cruise ANA06B.
• UTIG has operated GT-1A, -1M, and -2A gravimeters on various platforms in Antarctica since 2012.
AS350 gravimeter for cruise ANA06B – Operations
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GT-1A installed aboard AS-350B onboard RVIB Araon.
Base station GPS receiver and antenna installed on Compass Deck of RV Araon during gravity data acquisition flights.
GPS antenna for the GT-1A while the AS350 was hangered. The rigid framed backpack strapped to the helo deck proved to be adequate as an antenna mount, sufficiently non-intrusive to deck operations.
LIONESS / Amundsen Sea Embayment Land-Ice/Ocean Network Exploration with Semiautonomous Systems
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Pink – Fixed-wing flights (OIB)
Blue – Fixed-wing flights (GIMBLE) Yellow – Helicopter gravity flights
White – Previous coverage Cyan – Compiled ship tracks
LIONESS/ASE
RMSE: 1.4 mgal
RMSE: 1.7 mgal
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• Preliminary processing indicates good internal crossovers and agreement with NASA OIB data. • Gravity follows topography: Ice rises and grounded icebergs correspond to high gravity anomalies; low
anomalies correspond to areas between ice rises. Two remarkably low anomalies lie near the grounding line. • The eastern ocean cavity entrance may be a narrow, curved trough between an ice rise and grounded icebergs.
Blue – Helicopter flights Yellow – ICECAP coverage Pink – Icebridge coverage Orange – Marine bathymetry
LIONESS / Amundsen Sea Embayment Land-Ice/Ocean Network Exploration with Semiautonomous Systems
Conclusions
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• UTIG’s complex flight operations require a 3-axis gravimeter • We evaluated the new GT-1A, -M, and -2A, for polar operations
– In-house, low cost operations – Full data recovery except on turns – Fast turn recovery – We achieved 1 mgal reflight, 2.4 mgal RMS crossovers in first attempt – Expect these numbers to improve with experience – Currently using Precise Point Positioning solutions
• Future work: – Intensive fixed-wing surveys in coastal East Antarctica (NSF EAGLE),
interior surveys to support site selection for the Rapid Ice Drill (NSF RAID) – Continued rotary-wing surveys in the Amundsen Sea, Terra Nova Bay