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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