The SEEGOCE project

1. The SEEGOCE project Michel Diament and the SEEGOCE team (C. Basuyau, S. Bonvalot, C. Cadio, S. Déroussi, H. Duquenne, G. Martelet, V. Mikhailov, G. Pajot, I. Panet, A. Peyrefitte, C. Tiberi …) Bergen 28 june - 2 july 2010

2. SEEGOCE: Solid Earth Exploration with GOCE Bergen 28 june - 2 july 2010

3. Therefore we have to image the inner structure and understand physical processes of the Earth using indirect approaches such as gravity (and other information as seismology) After Hergé

4. Before availability of Goce data? • Preparation of validation • Combination of Goce with ground/airborne data • Interpretation of gravity anomalies using dedicated techniques (CWT: continuous wavelet transform) • Cooperative gravity-seismology modelling • Take best use of gradients

5. Validation IGN in collaboration with IPGP carried out a series of absolute (with an A10) and relative (CG3/5) surveys over France. This data set is perfectly suited for validating Goce derived gravity anomaly and gradients. Red triangles : Abs. measurements Green points: relative ties.

6. Data on the Earth’s gravity Spatial resolution (km) 1000 100 10 • Supraconductinggravimeter Absolutegravimeter • global • CHAMP (2000) • GOCE (2009) • GRACE (2002) • Topex, Jason (currents !!) • regional • Airbornegravity • local • Land & seagravity Coverage

7. Our goal: to obtain the best accurate and resolved field in areas of interest by combining Grace, Goce and ground data

8. e1 e2 For that aim we use Poisson multipole wavelets Chambodut et al., 2005 • A 3D, harmonic function, Well localized both in space and frequency, • Two parameters: scale and position. Small scale wavelet  Appropriate to combine data withdifferentspatiospectralcharacteristics Large scale wavelet  Data atany altitude Multipolar sources  Any type of data • Eachwaveletis a simple linearcombination of non-central multipoles of loworders(Holschneideret al., 2003) Equivalent gravity sources  Compact representations  Earth mean sphere

9. Example: local refinement of a global model • Our method allows to increase the resolution over chosen area of interest: zoom-in. Local zoom-in GRACE SH model Data in-situ Zoom on the Marquesas islands (res. 30 km, 9500 wavelets) Regional wavelet model (res. 75 km, 9600 wavelets) Panet et al., 2006

10. Wavelets also allow analysis of the anomalies Analyzing wavelet  The waveletmodels of the gravitypotentialthusobtainedalsolead to a multi-scaleanalysis (CWT) Scale  The correlationsbetween the waveletsY and the potential T provide an integrated, regionalizedview of the densitiesr: Weightingfunction Fe of the densities

11. Application: Central Pacific a paradise… for studying mantle processes! • a fossil alignment displaying ages between 35 and 90 m.y. • no clear linear age progression 6000 4000 2000 meters 0 -2000 -4000 -6000 Bathymetrie Gebco • superswell • numerous volcanic chains • not always linear age progression: hotspot clusters

12. Wavelets analysis bring new geodynamical results as to hot-spots CWT of the staticgeoid Panet et al., 2006 Marquesas FZ Marquesas Society Tuamotu Comparison of the gravity anomalies at different scales with the bathymetry ones: Origin of the volcanism: a plume under the Society islands, lithospheric control for Marquesas.

13. Results of the continuous wavelet analysis at longer wavelengths Two larges scales geoid anomalies are well isolated: • a geoid low on French Polynesia (-5m) • a positive geoid anomaly 600 km west of the Line Islands chain (12m) waveletscale = wavelength of the geoid anomaly

14. Testing physical models such oscillatory domes Derived from studies of convection in a heterogeneous fluid (Davaille, 1999; Le Bars and Davaille, 2004) where the equilibrium between thermal and compositional effects have been investigated. • Secondary instabilities at the origin of the short hot spots tracks. • The dome loose its thermal buoyancy, become denser again and fall back. Cavity plume Diapiric plume • It breaks through the transition zone and may produce traps.

15. French Polynesia: an upwelling dome We interpret the negative geoid anomaly as the geoid signature of a less dense, therefore rising, dome extending from the CMB up to transition zone. This rising dome can explain the volcanism in this area: stopped at the transition zone, secondary instabilities created at this interface then produce South Pacific hot spots: Society, Pitcairn. surface TZ CMB The South Pacific dome now and 100 m.y. ago. This dome probably created Darwin Rise 100 m.y. ago when it was its ascending phase. This Pacific area could have thus registered a complete pulsation of the mantle. Cadio et al., in prep.

16. Another promising way: Goce data + seismology + GROUND DATA

17. ?? Cooperative modelling cures the shortsightedness of seismologists and the indesiciveness of gravimetricians. The shortsightedness The indesiciveness

18. As shown by studies realized with ground data and seismic tomography in many areas as: Mongolia (Tiberi et al., 2008) Baikal rift (Tiberi et al., 2003) Our planned targets with Goce data: Himalaya and French Polynesia

19. Finally we must not overlook the gradients. These new data call for dedicated interpretation methods. After having proposed and tested a new method for gradients denoising (Pajot et al., 2008) and for analysis (Mikhailov et al., 2007), we started working on gradients inversion with application to Africa.

20. Conclusions: As mentionned on Monday by R. Rummel: applications to geophysics using « real » Goce data can start now. Let’s do it and ultimately realize the geoscientists (and Jules Verne’s) dream!!