Velocity anisotropy measured on the spherical specimens: History and applications

The anisotropy of elastic properties, including seismic velocities, has already been investigated in the lab over past seven decades. Here, we present a review related to the development of a unique apparatus for the detailed measurement of seismic velocity anisotropy. Its originality lies in measuring velocities on spherical specimens, which allows for determination of the velocity anisotropy as a function of confining pressure loading with high resolution. The 132 directions, covering the sphere in a regular 15° net of meridians and parallels, have proven to be optimal with respect to common heterogeneities of investigated rocks. The device was designed and the first measurements were performed by a research team of the Institute of Geophysics in Prague (Babuška, Pros and Klíma) in 1968, shortly following many pioneer velocity anisotropy studies. Since then, almost 100 papers have been published using the velocity anisotropy measured with this unique device. The review consists of three separate but mutually interconnected parts: (i) historical development; (ii) microstructural insights from an ultrasonic velocity measurement perspective; (iii) macroscale applications to practical problems in geophysics, structural geology and rock mechanics.