Simultaneous Determination of Dynamic Rock Properties by Critical Angle Wave Propagation Techniques

Abstract

Critical angle wave propagation measurements were taken at atmospheric pressure on slate and Green River shale pressure on slate and Green River shale samples to study the effect of stratification on dynamic elastic rock properties. The rock samples were taken in 10 degrees increments, from 0 degrees to 90 degrees to the plane of cleavage or bedding. Ultrasonic pulses with a frequency of approximately six megacycles per second were used to determine the velocity of dilatational and shear waves through rock samples immersed in a mineral oil bath. Results indicate a considerable variation of elastic properties with orientation of the bedding or cleavage plane. Data for slate samples indicate general constant values of dynamic elastic properties for cleavage plane orientation properties for cleavage plane orientation angles from 20 degrees to 60 degrees. Anisotropy was indicated for orientation angles near parallel and near normal to the cleavage planes. Green River shale lean in kerogen content exhibited a sharp change in values for elastic properties as orientation of the bedding planes increased from 0 degrees to 30 degrees. The measured rock properties continued to change, but at a lesser rate, as the bedding plane orientation increased from 30 degrees to 90 degrees. Data for samples of Green River shale rich in kerogen content were so widely scattered for individual orientations that only general trends were evident. Generally, S-wave velocities and shear modulus values were higher for GRS-2 than GRS-1, while values for bulk modulus and Poisson's ratio were lower. Poisson's ratio were lower Introduction The transmission of acoustic waves through a discontinuity surface has been studied both theoretically and experimentally by many investigators. Although the law of reflection for light waves was known to Euclid and that of refraction was discovered experimentally by Snell, Knott seems to have been the first to derive general equations for reflection and refraction of acoustic waves at a plane boundary.