“The strong attraction of gravity at Geoscience Australia” (Richard Lane, Geoscience Australia,...

22nd ASEG International Conference & Exhibition 2012© Commonwealth of Australia (Geoscience Australia) 2012

The strong attraction of gravity at Geoscience Australia

Richard Lane

( [email protected] )

Geoscience Australia

© Commonwealth of Australia (Geoscience Australia) 2012

Gravity at GA ...More than just a ground gravity database

• Significant scope to improve The quality and extent of the coverage By providing better support to help interpret and integrate gravity with

other geoscientific information

• Monitoring developments in AG and AGG

• Expanding and improving the AFGN and the ANGD

• Developing the Kauring test site Analysing test results Working towards a Deed-Of-Standing-Offer for service providers

• Assisting State and Territory geoscience agencies, and other government groups with proposals calling for AG and AGG

• Enhancing the rock properties database and web delivery application

• Improving 3D modelling capabilities


The Long Term (Ideal) Objective

Global Satellite or Satellite+Shipborne+Ground data

Airborne Gravity (AG)

Airborne Gravity Gradiometry(AGG)

Onshore coverage:Airborne gravity gradiometer data at 1 km line spacing

Coastal zone, offshore (EEC) and Antarctic coverage:Airborne gravity data at 2 km spacing

Base coverage:Global Satellite or Satellite+Shipborne+Ground gravity models


Airborne Gravity Gradiometry (AGG)Ideal requirement: 1 Eo/√(Hz) with 100 m resolutionStatus: Typically 5 Eo/√(Hz) with 400 m resolutionOutlook: Several systems with desired specifications set for flight

trialsRecommendation: Hold off on systematic surveying

Consider special instances on a case-by-case basis

Airborne Gravity (AG)Ideal requirement: 0.1 mGal with 1 km resolutionStatus: Typically 1 mGal with 3 km resolutionOutlook: No breakthroughs in sightRecommendation: Pursue opportunities in coastal and marine settings, and

in Australian Antarctic TerritoryConsider onshore usage on a case-by-case basis

Satellite GravityRecommendation: Pursue opportunities with GRACE and GOCE projects


Satellite Gravity


GRACE - Gravity Recovery and Climate Experiment

(Courtesy of NASA and DLG)


GOCE - Gravity Field and Steady-State Ocean Circulation Explorer

(Courtesy of ESA)


http://icgem.gfz-potsdam.de/ICGEM/

Global model GGM03C

Combined GRACE, ground and satellite

altimetric gravity model

(Courtesy of ICGEM - IAG)


Airborne Gravity(AG)


Gippsland Nearshore Airborne Gravity SurveyVictorian Department of Primary Industries (DPI)

• Consistent mapping of

geological structure

between onshore and

offshore• Sander AirGRAV

120 x 70 km area 10,500 km 1 km spacing RMS of 1.6 µms-2 for 3

km half wavelength

• Kauring Test Site in WA also flown

( http://www.dpi.vic.gov.au/energy/sustainable-energy/carbon-capture-and-storage/the-carbonnet-project/airborne-gravity-survey )

Survey outline


Airborne GravityGradiometry

(AGG)


Status update on existing AGG systems

• ARKeX – FTGAdding a GMA Gz sensor to an existing FTG system

• Bell Geospace – Air-FTGTesting a combination of Air-FTG and ZTEM AEM (an

ambient EM system – Geotech)

• Fugro – FALCONLatest system (#5, “Cavendish”) delivered

HeliFALCON in commercial use since early 2011

Successful test flight of FALCON and TEMPEST AEM systems


Status update on new AGG systems

• Rio Tinto – VK1 – 1 Eo per root Hz “Active period of airborne testing at the Kauring Test Site”

• Lockheed Martin - Enhanced FTG - 2 Eo per root Hz “The system build and initial test will be completed in July

2012”

• ARKeX - EGG Exploration Gravity Gradiometer – 1 Eo per root Hz “2 instruments built, ongoing flight trials”

• Gedex - High Definition Airborne Gravity Gradiometer HD-AGG™ - 1 Eo per root Hz “Upgrades are being implemented (following initial flight

trials), and the flight testing will resume in April 2012”


“Certainly the benchmark for airborne gravity.”(GW – University of Utah / TechnoImaging)

“… the most comprehensive resource for this rapidly evolving technology. I believe that this is an important contribution by GA to the exploration and geological mapping community.”(CF – CSIRO)

“… the proceedings was a very professionally put together document and will be an important resource for years to come.”(DH – Gedex Inc.)

“I think it is terrific that Geoscience Australia supports such initiatives, and produces the record of proceedings.”(GW – BHP Billiton)

“… the GA publication of the workshop proceedings is of the highest quality.”(DDF – Lockheed Martin)

( https://www.ga.gov.au/products/servlet/controller?event=GEOCAT_DETAILS&catno=70673 )


AFGN and ANGD


Land relative gravimeters

Scintrex CG-5 AUTOGRAVLaCoste Romberg Model G


Land absolute gravimeter

Micro-g LaCoste A10

(Photographs courtesy of Ray Tracey - GA)


AFGN absolute gravity observations

(Courtesy of Ray Tracey, GA)


Absolute and relative gravity measurements in Antarctica

• Absolute gravity reference points

• Relative gravity observations to tie existing survey data to the absolute reference points

(http://www.ga.gov.au/ausgeonews/ausgeonews201006/inbrief.jsp )(Photograph of Ray Tracey - GA)


AAGD07 and revised processing equations and procedures• Absolute gravity observations for ~ 60 stations in the Australian

Fundamental Gravity Network (AFGN) define AAGD07 New datum replaces the Isogal84 datum

Linked by relative observations to the IGSN71 global datum

(Approximately) Supported by sparse, older absolute gravity observations

• g(AAGD07) = g(ISOGAL84) – 0.78 μms-2

• Applied to >1.4 M observations in the Australian National Gravity Database (ANGD)

• Also implemented revised processing equations and procedures More accurate equations Ellipsoid height datum and GDA94 horizontal datum


Observation uncertainty

• Australian National Gravity Database (onshore)• Uncertainty estimates

Standard deviation assuming a Gaussian error distribution Horizontal location Vertical location Observed vertical gravity (Terrain corrections)

• Combined uncertainty estimate for simple Bouguer vertical gravity values

• 0.2 μms-2 to more than 20 μms-2


Sources of uncertainty values

1. Operations/Acquisition Report2. Processing Report3. Typical performance of reported

method/equipment used4. Typical performance of assumed

method/equipment5. Estimated from the date of the survey by

reference to other surveys of a similar vintage

6. Analysis of the external network adjustment errors

7. Unknown source


Observation uncertainty – example of estimation from supplied uncertaintiesh uncertainty in horizontal position in

metres

v uncertainty in vertical position in metres

εobs vertical gravity observation error in μms-2

φ latitude in degrees north

εht vertical gravity error in μms-2 associated

with uncertainty in horizontal positioning

εvfa error in μms-2 for free-air corrected

vertical gravity due to uncertainty in vertical positioning

εv error in μms-2 for simple Bouguer

anomaly vertical gravity due to uncertainty in vertical positioning, given a correction density of 2670 kg/m3

εtc uncertainty in the accuracy of terrain

corrections in μms-2

2sin01.0 hht

vvfa 3

vv 2

222obsvfahtFA

2222tcobsvhtCBA


Kauring Test Site


Kauring Test Site, Western Australia

~ 110 km

(Courtesy of David Howard)


Kauring referencegravity data

• Continental data Variable spacing

• Regional data 150 x 150 km @ 2 km spacing

• Airborne Gravity site 20 x 20 km @ 500 m spacing

• Airborne Gravity Gradiometer site

5 x 6 km area at variable spacing

50 x 50 m; 100 x 50 m; 250 x 100 m

Plus LiDAR DEM with 1 m grid cells

*Perth


FALCON AGG data

• Flown July 2011

• Height ~70 m

• Line spacing 50 m

• Noise 1.9 Eo RMS

• Bandwidth 0.18 Hz

GDD from ground gravity

GDD from FALCON(Courtesy of Mark Dransfield, FAS)


Ground reference gravity Equivalent source model

Recovered inversion model

Simulate data for various systems

CSM CGEM synthetic studies

(Courtesy of Cericia Martinez and Yaoguo Li, CSM)

Invert data simulated for various systems

Compare inversion models


VK1a 60°headingLow noise next-generation FTG system

Simulated data and inversion models

VK1c 60°heading

vkc

Eo

(Courtesy of Cericia Martinez and Yaoguo Li, CSM)


1 km

AnomaliesAnomalies + 1 EöAnomalies + 10 Eö (400 m wavelength)

“Kauring Challenge”• Simulated the AGG response

for known bodies beneath the

Kauring terrain

Gzz, 80 m clearance, 1 Hz

sampling

• Added noise 1 Eö or 10 Eö

• Data available for inversion

studiesDeep Slab

Depth below ground 500 m

Volume 48 Million m3

Excess mass 10 MT @ 0.2 g/cc contrast

Peak response 2.5 Eö

Comments Broad wavelength, Low response

North East Dipping Dyke

Depth below ground At surface


Excess mass 34 MT @ 0.15 g/cc contrast

Peak response 12 Eö

Comments 75 m thick, dipping at 45°

Sphere (NiS body)



Excess mass 4.5 MT @ 0.3 g/cc contrast


Comments 150 m radius

Kimberlite


Volume 3.5 Million m3

Excess mass 1.5 MT @ -0.4 g/cc contrast


Comments Truncated cone, 100 m upper rad

Tunnel


Volume 8 000 m3

Excess mass 0.02 MT @ -2.67 g/cc contrast

Peak response 0.8 Eö

Comments 4 m square tunnel, 500 m long

1 km

(Courtesy of Theo Aravanis, Rio Tinto)


( http://www.ga.gov.au/minerals/projects/current-projects/kauring.html)

Kauring Test Site, WA

• www.ga.gov.au Search for “Kauring”

• Reference data sets Magnetic, radiometric,

elevation, gravity and digital geology

• Simulated data sets• Bibliography• Test flight protocols• Observed AG and AGG

data sets


Rock Properties


Geoscience Australia rock properties web delivery application

• Very simple database structure

• Basic search and download application on our website

• And now ...

• Populating the database Existing density and magnetic

property observations

• Hoping to expand the scope of the database Additional property types

Access to the data via a Web Feature Service (WFS)

( http://www.ga.gov.au/geophysics-rockpropertypub-web/rockproperties/search.htm )


Modelling


CBA as a response compensated for various model elements• GADDS

Primary observations Plus residual gravity - excluded defined source features

• Tides Sun and the Moon

• Theoretical gravity (including free air correction) Earth as a point mass or source with spherically symmetric

density• Bouguer cap

~ 167 km radius Simple Bouguer anomalies (SBA)

• Terrain correction ~ 167 km radius Complete Bouguer anomalies (CBA)


CBA extensions

• Distant relief correction The rest of the globe Beyond the (arbitrary) ~167 km radius used for Bouguer

anomaly values• Isostatic correction

Various models for isostatic compensation• Crustal models - v1, v2, v3, etc

e.g., CRUST2 global seismic-based model GA regional models Semi-regional models District models Prospect models etc ...

• Make available as options via GADDS


Ptolemy Magellan’s Expedition

Spherical mesh elementsApollo 17


Australia's maritime jurisdiction


CUG-CSM-GA International Project - Spherical mesh geometry gravity and magnetic modelling


Synthetic result for gravity modelling with a spherical mesh

Model resolution: 6°×6°× 10km# of cells: 60×30×10=18000

Data resolution: 6°×6°# of data points: 60×30=1800

(Courtesy of Qing Liang, CUG and CSM)


Theoretical data with

noise

Predicted data

(Courtesy of Qing Liang, CUG and CSM)


Synthetic

Inverted Inverted Inverted

Synthetic Synthetic

(Courtesy of Qing Liang – CUG and CSM)


Gravity at GA ...More than just a ground gravity database

• Significant scope to improve• Monitoring developments in AG and AGG• Expanding and improving the AFGN and the ANGD• Developing the Kauring test site

Analysing test results Working towards a Deed-Of-Standing-Offer for service providers

• Assisting State and Territory geoscience agencies, and other government groups with proposals calling for AG and AGG

• Enhancing the rock properties database and web delivery application

• Improving 3D modelling capabilities

“The strong attraction of gravity at Geoscience Australia” (Richard Lane, Geoscience Australia,...

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