Physical properties of rocks: Shale anisotropy and basalt seismic velocities

Joel Edward Johnston, Purdue University

Abstract

This dissertation is comprised of four investigations dealing with the physical properties of rocks. Recent studies have shown that anisotropy can have significant effects on seismic reflection data. To investigate anisotropy in sedimentary rocks, compressional and shear wave velocities of transversely isotropic shales have been measured and used to calculate the elastic constants and velocity surfaces of the samples. In highly anisotropic shales, Vp wave surfaces are found to be strongly non-elliptical while Vsv wave surfaces exhibit cusps. The velocity surfaces also indicate that shear wave splitting is negligible at "near-normal" incidence. In addition, seismic anisotropy in the shales is correlated with the degree of clay mineral alignment parallel to bedding. The remainder of this dissertation deals with basalt physical properties. Velocities of a suite of highly vesicular basalts have been measured using 1 MHz and 160 KHz velocity transducers. The importance of scale effects in velocity measurements are recognized. Scattered velocity--porosity trends and discrepancies between 1 MHz and 160 KHz velocities are explained by comparing the size of the ultrasonic wavelengths used to probe the samples to the vesicle dimensions. As wavelengths approach vesicle dimensions, the basalts appear as heterogeneous, rather than effective, media. The ultrasonic waves take fast travel paths through the basalt matrix, resulting in relatively high recorded velocities. Age and depth related changes in oceanic layer 2 have been the subject of much speculation in the literature. Therefore, microcrack porosity and alteration-related changes in physical properties of DSDP/ODP layer 2 basalts have been examined using velocity measurements, chemical analyses, and SEM imaging. Highly altered basalts, identified by elevated K$\sb2$O contents, are found to have low velocities and elevated Poisson's ratios relative to fresh samples. Natural sealed and healed microcracks are found throughout the sample suite using SEM imaging. However, an overprint of stress-relief microcracking has also been identified in each basalt examined. Lastly, microcracking in some East Pacific Rise basalts is described using velocity measurements and SEM images. Open microcracks are attributed to high differential pressures caused by trapped pore water during original core recovery.

Degree

Ph.D.

Advisors

Christensen, Purdue University.

Subject Area

Geophysics|Geology|Geotechnology

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