HR: 1340h
AN: H13C-1343    [Abstracts]
TI: 2D Elastic Full Waveform Inversion in a space frequency domain formulation: application to near surface areas characterization
AU: * Gelis, C
EM: gelis@geoazur.unice.fr
AF: Geociences Azur, 250 Rue A. Einstein Sophia Antipolis, Valbonne, 06560 France
AU: Virieux, J
EM: viri@geoazur.unice.fr
AF: Geociences Azur, 250 Rue A. Einstein Sophia Antipolis, Valbonne, 06560 France
AU: Grandjean, G
EM: g.grandjean@brgm.fr
AF: BRGM Dept ARN, 4 Rue Claude Guillemin BP 6009, Orleans, 45060 France
AU: Operto, S
EM: operto@geoazur.obs-vlfr.fr
AF: Geosciences Azur, Observatoire Océanologique, La Darse, B.P. 48, Villefranche sur mer, 06235 France
AB: The superficial weathered zone, few hundreds meters thick, presents highly variable and complex near-surface structures. Energetic seismic surface waves, often called ground roll, may hide information coming from deeper areas. Moreover near-surface anomalies (cavities, overthrusts) are quite important and, therefore, detecting heterogeneities in near-surface areas and quantifying their physical properties is still a challenge for seismic imaging.
Since heterogeneities are located in near-surface areas, both surface and body waves induce complex footprints in seismic data. The propagation of 2D P-SV is performed in a frequency domain modeling. This frequency formulation takes into account attenuating behavior and efficiently takes benefit of multisource and multireceiver configurations. A new finite-difference stencil of second order using rotated derivatives axes (Saenger et al., 2000) simulates surface waves precisely and remains stable nearby the free surface and/or rapidly-varying zones. It will be the forward problem kernel of our approach.
We follow the matrix formalism of Pratt et al. (1998) and perform a linearized inversion in the least-square sense, since heterogeneities of reasonable amplitudes towards the surrounding medium are considered. We use the gradient method to perform the full waveform inversion for elastic waves. In this formulation we take only the Hessian diagonal part and use a parabolic approximation to find the stepping in the gradient direction. To compute Fréchet derivatives, we resort to the Born and the Rytov approximations and evaluate their performances in transmission and reflection acquisitions geometries. We test as well the influence of the inverted parameters choice. Moreover we highlight the influence of data preconditioning that must be particularly efficient to deal with both body and surface waves.
We apply this elastic full waveform inversion to near-surface data, containing strong surface waves and acquired just above a cavity that has to be characterized. We highlight the potentiality and limits of this method in this real example to reconstruct near surface areas and determine the cavity parameters and geometrical properties.
Saenger E. H., Gold N. & Shapiro S. A., 2000. Modeling the propagation of elastic waves using a modified finite-difference grid. Wave Motion, 31, 77-92. Pratt G., Shin C. & Hicks G.J., 1998. Gauss-Newton and full Newton methods in frequence space seismic waveform inversion. Geophys. J. Int., 133, 341-362.
DE: 0902 Computational methods: seismic
DE: 0935 Seismic methods (3025, 7294)
DE: 3260 Inverse theory
DE: 3285 Wave propagation (0689, 2487, 4275, 4455, 6934)
DE: 7255 Surface waves and free oscillations
SC: Hydrology [H]
MN: Fall Meeting 2005