Petrological constraints on seismic anisotropy

Abstract

The purpose of this paper is to investigate the correlations between anisotropic parameters for different orientations and mineralogies for realistic mineralogical and petrological models of the upper mantle. Such correlations make it possible to reduce the number of independent parameters required in seismic modelling of the upper mantle. The variation with depth and with degree of random orientation is also investigated. It is shown that the anisotropic combinations involved in an equivalent transversely isotropic medium (azumithally averaged velocities) with a vertical symmetry axis are strongly correlated but the combinations involved in azimuthal anisotropy are more weakly correlated. Comparison with available seismic data does not yet allow discrimination, based on anisotropy, between different petrological models. It is shown that, even if a proportion of random orientation is introduced, it should be possible to achieve petrological discrimination from seismic data for some orientation configurations, if the inverse problem is well-posed, i.e., takes account of the correlations between anisotropic parameters. These correlations are incorporated in an inversion process to show that it is possible to explain the dataset used to derive the PREM, by alternative anisotropic models for the uppermost 220 km.

Additional details