satellite gravity gradients for lithospheric structure r. hackney, h.-j. götze, s. schmidt institut...
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Satellite gravity gradients for lithospheric structureSatellite gravity gradients for lithospheric structure
R. Hackney, H.-J. Götze, S. SchmidtInstitut für Geowissenschaften, Abteilung Geophysik
Christian-Albrechts-Universität zu Kiel
National Geophysical Research Institute
Hyderabad, India
B. Singh, V. Tiwari, K. Arora
R. HackneyR. HackneyOutlineOutline
• Why gradients?• What data exist?• How can we use
gradient data?• What does the future
hold?
• Why GOCE is about more than just high-resolution gravity field models…
R. HackneyR. HackneyWhy gradients?Why gradients?
• Gradient tensor:
• Better resolution of geological features:– faults, lineaments,– edges of geological bodies.
zzzyzx
yzyyyx
xzxyxx
WWW
WWW
WWW
R. HackneyR. Hackney
Component combinationsComponent combinations
• Horizontal gradient magnitude:
• Differential curvature magntiude:
• Good for highlighting edges.
• gives “horizontal directive tendency”
• emphasizes effects of shallower sources.
22zyzx WW
22
2
xxyy
xy
WWW
Simplify and “focus” complex gradient components
R. HackneyR. HackneyGradient DataGradient Data
• Torsion-balance measurements:– 1930s in Germany and Hungary
• Airborne gradiometer measurements– a 21st century occurence– FALCON®, Air-FTG™.
• Calculated from gravity field:– indirect determination.
• For the future … GOCE
R. HackneyR. HackneyEarliest applicaton…Earliest applicaton…
• Torsion-balance measurements for finding salt domes.
zzzyzx
yzyyyx
xzxyxx
WWW
WWW
WWW
R. HackneyR. HackneyMapping salt domesMapping salt domes
• Horizontal gradient used to identify salt domes.
Salt dome
22zyzx WW
R. HackneyR. HackneyModelling gradientsModelling gradients
• Gradients allow finer resolution of salt structure.
NW
SE
50 10
15
20
Profile length, km
Salt 1
-2
-4
0
Horizontal gradient, Wyz, Eötvös
Horizontal gradient, Wxz, Eötvös
Bouguer anomaly, gz, 10-
5 m/s2
mes.mod.
mod.
mod.
0
-4
40
-8
-12-16
200
-20-40-6040
-20-40-60
0
20 0
0
mes.
mes.
Sedimentary basin
Sedimentary basin Salt
2
Pre -Zechstein
R. HackneyR. HackneyStructural mappingStructural mapping
• Gradients to aid structural mapping in frontal part of southern Appennines.– large oil discoveries.– Miocene–Pliocene
NE–SW shortening + thrust-top basins.
– later, E–W cross-cutting extension.
Fedi et al. (2005) Tectonophysics
R. HackneyR. HackneyAppennines structureAppennines structure
• Bouguer anomaly:– NW–SE trends related
to thrusting.– little evidence of E–W
structures,– isolated lows related to
basins.
Fedi et al. (2005) Tectonophysics50-60 mGal
R. HackneyR. HackneyAppennines structureAppennines structure
• Wzz:– vertical gravity
gradient.– better definition of
extensional basins.
Fedi et al. (2005) Tectonophysics45Eötvös-50
R. HackneyR. HackneyAppennines structureAppennines structure
• Wzy:– highlights E–W trends,– maps trend of
basement highs and basin fill.
Fedi et al. (2005) Tectonophysics33Eötvös-27
R. HackneyR. HackneyAppennines structureAppennines structure
• Wxy:– isolated highs and
lows at lineament terminations.
Fedi et al. (2005) Tectonophysics17Eötvös-7
R. HackneyR. HackneyEnd result …End result …
• “Cumulative lineament map”– overview of all
structures,– not easily obtained
from gravity alone.
Fedi et al. (2005) Tectonophysics
R. HackneyR. HackneyGOCE gradientsGOCE gradients
• Previous examples are all local- to regional-scale.
• GOCE will fly at ~250 km:– what will it see?
• Simulating gradients from existing 3D lithospheric models gives us some idea…– e.g. gradient signature of a mountain belt…
R. HackneyR. Hackney3D Andes model3D Andes model
• Constrained 3D model of Andean structure and density distribution (Tassara et al. 2006).– based on calculating gravity effect of discrete
bodies with constant properties.
R. HackneyR. HackneyModel resultsModel results
Tassara et al. (2006) JGR.
R. HackneyR. HackneyPredicted gradientsPredicted gradients
• Gradient components from Tassara et al. (2006) 3D Andean model.
• Are the results reasonable?– test against gradient
components calculated from EIGEN-GL04C.
R. HackneyR. Hackney
Predicted + SimulatedPredicted + Simulated
“simulated”EIGEN-GL04C
“calculated”(Tassara model)
R. HackneyR. HackneyMore detailMore detail
R. HackneyR. HackneyGOCE gradients?GOCE gradients?
• So far:– gradients are calculated at the surface.
• How would they look at satellite altitude?
R. HackneyR. HackneySatellite altitudeSatellite altitude
“calculated”at 250 km
“calculated”(Tassara model)
R. HackneyR. HackneySatellite altitudeSatellite altitude
• Gradient components at GOCE altitude.
• Resolvable by GOCE?
“calculated”at 250 km
R. HackneyR. HackneySummarySummary
• Use of gravity gradients is still limited:– old torsion balances, airborne gradiometers.
• Gradients help determine details of geological structure:– but we’re still working on our intuitive understanding
of gradients.
• Large-scale gradient signatures:– e.g. Andes have a ±5 Eötvös signal at GOCE altitude.
• GOCE will be about more than just higher-resolution gravity models …
R. HackneyR. Hackney
R. HackneyR. HackneyAppennines structureAppennines structure
• –Wyy:– maps truncations of
E–W lineaments against NW–SE regional structures.
Fedi et al. (2005) Tectonophysics27Eötvös-35
R. HackneyR. HackneyGOCEGOCE• Launch 15.5.2008!!• Single satellite +
gradiometer.• High-low-SST.• Polar gap ~6°.• Highest-resolution
gravity field.• Instruments:
– gradiometer,– accelerometer,– star cameras,– GPS.
GPS satellitesGPS satellites
orbit perturbations
orbit perturbations
gradiometergradiometersurfacesurface
~250 km~250 km
>0>0<0<0
R. HackneyR. HackneyModel ConstraintsModel Constraints