Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008geoji.172..383z&link_type=abstract
Geophysical Journal International, Volume 172, Issue 3, pp. 383-394.
Astronomy and Astrophysics
Astronomy
1
Inverse Theory, Tomography, Body Waves, Seismic Anisotropy
Scientific paper
We have developed two, new non-linear traveltime inversion schemes for 3-D seismic tomography in anisotropic media. They differ from the traditional linearized inversion approach and offer five significant improvements: (1) they are based on an alternative form of the first-order traveltime perturbation equation, derived so as to simplify the inversion formulae and overcome the quasi shear wave singularity problem; (2) robust 3-D ray tracing is employed which enables the simultaneous computation of the first-arrival traveltimes and ray paths for the three body waves (qP, qS1 and qS2) in arbitrary anisotropic media; (3) the Jacobian matrix used in the update is based on an efficient computation for a 3-D anisotropic model, so that the inversion is applicable to both weakly and strongly anisotropic situations, unlike most previous approaches which assume weak anisotropy; (4) a local-search, constrained minimization is applied to the non-linear inversion which makes anisotropic tomographic imaging an iterative procedure; (5) there is an option to invert for the elastic moduli directly or the Thomsen parameters directly in heterogenous, tilted transversely isotropic media, using any source-receiver recording geometry. We have examined the imaging capability of the non-linear solver with individual body-wave modes using a 3-D synthetic anisotropic model incorporating two targets, a `high velocity' and a `low velocity' anomaly, embedded in a titled transversely isotropic medium. The model is illuminated by means of azimuthal VSP and crosshole measurements. The experimental results show that the two non-linear inversion schemes successfully image the `targets' and yield satisfactory 3-D tomograms of the elastic moduli and the Thomsen parameters.
Greenhalgh Stewart
Zhou Bing
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