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Yogendra Viyogi - One of the best experts on this subject based on the ideXlab platform.

Rashmi Raniwala - One of the best experts on this subject based on the ideXlab platform.

Sudhir Raniwala - One of the best experts on this subject based on the ideXlab platform.

Václav Vavryčuk - One of the best experts on this subject based on the ideXlab platform.

  • Acoustic axes in weak triclinic Anisotropy
    Geophysical Journal International, 2005
    Co-Authors: Václav Vavryčuk
    Abstract:

    SUMMARY Acoustic axes can exist even under an infinitesimally weak Anisotropy, and occur when slowness surfaces of the S1 and S2 waves touch or intersect. The maximum number of isolated acoustic axes in weak triclinic Anisotropy is 16 as in strong triclinic Anisotropy. The directions of acoustic axes are calculated by solving two coupled polynomial equations in two variables. The order of the equations is 6 under strong Anisotropy and reduces to 5 under weak Anisotropy. The weak Anisotropy approximation is particularly useful, when calculating the acoustic axes under extremely weak Anisotropy with Anisotropy strength less than 0.1 per cent because the equations valid for strong Anisotropy might become numerically unstable and their modification, which stabilizes them, is complicated. The weak Anisotropy approximation can also find applications in inversions for Anisotropy from the directions of acoustic axes.

  • Acoustic axes in triclinic Anisotropy
    The Journal of the Acoustical Society of America, 2005
    Co-Authors: Václav Vavryčuk
    Abstract:

    Calculation of acoustic axes in triclinic elastic Anisotropy is considerably more complicated than for Anisotropy of higher symmetry. While one polynomial equation of the 6th order is solved in monoclinic Anisotropy, we have to solve two coupled polynomial equations of the 6th order in two variables in triclinic Anisotropy. Furthermore, some solutions of the equations are spurious and must be discarded. In this way we obtain 16 isolated acoustic axes, which can run in real or complex directions. The real/complex acoustic axes describe the propagation of homogeneous/inhomogeneous plane waves and are associated with a linear/elliptical polarization of waves in their vicinity. The most frequent number of real acoustic axes is 8 for strong triclinic Anisotropy and 4 to 6 for weak triclinic Anisotropy. Examples of Anisotropy with no or 16 real acoustic axes are presented.

Giorgio Pennacchioni - One of the best experts on this subject based on the ideXlab platform.

  • extrinsic elastic Anisotropy in a compositionally heterogeneous earth s mantle
    Journal of Geophysical Research, 2019
    Co-Authors: Manuele Faccenda, Ana M G Ferreira, Nicola Tisato, Carolina Lithgowbertelloni, Lars Stixrude, Giorgio Pennacchioni
    Abstract:

    Several theoretical studies indicate that a substantial fraction of the measured seismic Anisotropy could be interpreted as extrinsic Anisotropy associated with compositional layering in rocks, reducing the significance of strain-induced intrinsic Anisotropy. Here we quantify the potential contribution of grain-scale and rock-scale compositional Anisotropy to the observations by (i) combining effective medium theories with realistic estimates of mineral isotropic elastic properties and (ii) measuring velocities of synthetic seismic waves propagating through modeled strain-induced microstructures. It is shown that for typical mantle and oceanic crust subsolidus compositions, rock-scale compositional layering does not generate any substantial extrinsic Anisotropy (<1%) because of the limited contrast in isotropic elastic moduli among different rocks. Quasi-laminated structures observed in subducting slabs using P and S wave scattering are often invoked as a source of extrinsic Anisotropy, but our calculations show that they only generate minor seismic Anisotropy (<0.1-0.2% of Vp and Vs radial Anisotropy). More generally, rock-scale compositional layering, when present, cannot be detected with seismic Anisotropy studies but mainly with wave scattering. In contrast, when grain-scale layering is present, significant extrinsic Anisotropy could exist in vertically limited levels of the mantle such as in a mid-ocean ridge basalt-rich lower transition zone or in the uppermost lower mantle where foliated basalts and pyrolites display up to 2-3% Vp and 3-6% Vs radial Anisotropy. Thus, seismic Anisotropy observed around the 660-km discontinuity could be possibly related to grain-scale shape-preferred orientation. Extrinsic Anisotropy can form also in a compositionally homogeneous mantle, where velocity variations associated with major phase transitions can generate up to 1% of positive radial Anisotropy.