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

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…

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

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

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

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

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

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

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

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 mGal

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

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

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

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

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…

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.

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

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.

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

R. Hackney More detail

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

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

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

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 …

R. Hackney

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

R. Hackney GOCE Launch !! 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

R. Hackney Model Constraints