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Satellite gravity gradients for lithospheric structure R. Hackney, H.-J. Götze, S. Schmidt Institut für Geowissenschaften, Abteilung Geophysik Christian-Albrechts-Universität zu Kiel National Geophysical Research Institute Hyderabad, India B. Singh, V. Tiwari, K. Arora

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R. Hackney Outline 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…

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R. Hackney Why gradients? Gradient tensor: Better resolution of geological features: –faults, lineaments, –edges of geological bodies.

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R. Hackney Component combinations Horizontal gradient magnitude: Differential curvature magntiude: Good for highlighting edges. gives “horizontal directive tendency” emphasizes effects of shallower sources. Simplify and “focus” complex gradient components

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R. Hackney Gradient Data Torsion-balance measurements: –1930s in Germany and Hungary Airborne gradiometer measurements –a 21 st century occurence –FALCON®, Air-FTG™. Calculated from gravity field: –indirect determination. For the future … GOCE

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R. Hackney Earliest applicaton… Torsion-balance measurements for finding salt domes.

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R. Hackney Mapping salt domes Horizontal gradient used to identify salt domes. Salt dome

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R. Hackney Modelling gradients Gradients allow finer resolution of salt structure.

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R. Hackney Structural 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

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R. Hackney Appennines structure Bouguer anomaly: –NW–SE trends related to thrusting. –little evidence of E–W structures, –isolated lows related to basins. Fedi et al. (2005) Tectonophysics 50-60 mGal

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R. Hackney Appennines structure Wzz: –vertical gravity gradient. –better definition of extensional basins. Fedi et al. (2005) Tectonophysics 45 Eötvös -50

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R. Hackney Appennines structure Wzy: –highlights E–W trends, –maps trend of basement highs and basin fill. Fedi et al. (2005) Tectonophysics 33 Eötvös -27

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R. Hackney Appennines structure Wxy: –isolated highs and lows at lineament terminations. Fedi et al. (2005) Tectonophysics 17 Eötvös -7

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R. Hackney End result … “Cumulative lineament map” –overview of all structures, –not easily obtained from gravity alone. Fedi et al. (2005) Tectonophysics

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R. Hackney GOCE 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…

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R. Hackney 3D 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.

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R. Hackney Model results Tassara et al. (2006) JGR.

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R. Hackney Predicted gradients Gradient components from Tassara et al. (2006) 3D Andean model. Are the results reasonable? –test against gradient components calculated from EIGEN-GL04C.

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R. Hackney Predicted + Simulated “simulated” EIGEN-GL04C “calculated” (Tassara model)

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R. Hackney More detail

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R. Hackney GOCE gradients? So far: –gradients are calculated at the surface. How would they look at satellite altitude?

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R. Hackney Satellite altitude “calculated” at 250 km “calculated” (Tassara model)

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R. Hackney Satellite altitude Gradient components at GOCE altitude. Resolvable by GOCE? “calculated” at 250 km

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R. Hackney Summary 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 …

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R. Hackney

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Appennines structure –Wyy: –maps truncations of E–W lineaments against NW–SE regional structures. Fedi et al. (2005) Tectonophysics 27 Eötvös -35

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R. Hackney GOCE Launch 15.5.2008!! Single satellite + gradiometer. High-low-SST. Polar gap ~6°. Highest-resolution gravity field. Instruments: –gradiometer, –accelerometer, –star cameras, –GPS. GPS satellites orbit perturbations gradiometer surface ~250 km >0 <0

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R. Hackney Model Constraints

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