Abstract
Laboratory experiments have been carried out to investigate the influence of the geological processes on the fabric of the Ishikari sandstones belonging to the Ishikari coal formation of the Tertiary era. For comparison purposes. Taiheiyo and Shirahama sandstones have also been investigated.
It was clear that the velocity of P-waves transmitted parallel to the bedding planes is higher than the perpendicular ones; and that the velocity distribution within the bedding planes can be approximated by the anisotropic model in the case of Ishikari sandstones, and by the isotropic model in case of other sandstones.
Based on microscopic observations of thin sections, the following were common features in every specimen. Rock had planar structures and sutured grains existed, indicating that the rock had experienced “pressure solution” during compaction. In the case of Ishikari sandstones, open elongated pores had preferred orientation coinciding with the direction of the minimum P-wave velocity (L-direction). In other rocks, this characteristic was not recognized. Differences in P-wave velocities perpendicular and parallel to the bedding, and anisotropies within the bedding planes could be explained by these planar structures and by the preferred orientation of open microcracks, respectively.
It is interesting to note that the L-direction was perpendicular to the folding axis which characterizes the geological structure of the Ishikari formation.
