How to evaluate the principal axes for anisotropic rocks.

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  • 異方性岩石における主軸の決定
  • イホウセイ ガンセキ ニ オケル シュジク ノ ケッテイ

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Abstract

Almost all rocks are more or less anisotropic and each rock has its own anisotropy in microstructures. The possible effect of anisotropy should be evaluated rather than neglected. To check the microdefects which characterize each rock's anisotropy, we analyzed two or three thin sections oriented parallel to and/or normal to the quarry plane for four kinds of rocks, Ogino tuff, Shimazaki andesite, Ukigane gabbro and Oshima granite. To check physical anisotropy, we used the P-wave transmission method for each rock. Thin section analyses showed that, in the case of Ogino tuff, flat pores oriented parallel to the quasi-horizontal plane and had preferred orientation, while in the case of Shimazaki andesite, plagioclase grains were oriented parallel to the quasi-horizontal plane. Healed cracks preferentially oriented within hornblende and plagioclase grains were typical in Ukigane gabbro. Open cracks and/or healed cracks oriented parallel to the three mutually perpendicular planes, one of which is quasi-horizontal, were found in Oshima granite. The ultrasonic transmission method indicates that P-wave is strongly sensitive to open crack orientation and sensitive to low aspect ratio pore orientation and mineral grain orientation, but insensitive to healed crack orientation. This study suggests the method to be available for qualitative evaluation of rock anisotropy.

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