The seismic properties and preferred clay mineral orientation of a suite of shales are investigated using laboratory velocity measurements as a function of confining pressure, X ray diffraction techniques, and electron microprobe backscatter (BSE) imaging. The velocity measurements indicate that these shales are transversely isotropic with the main symmetry axis perpendicular to bedding. Anisotropy, at elevated pressures caused mainly by preferred orientation of clays (illite) parallel to bedding, ranges from 20% (Vp) and 19% (Vs) for a sample of New Albany Shale to 30% (Vp) and 35% (Vs) for a sample of Chattanooga Shale. The degree of clay mineral alignment in the shales is constrained by “orientation indices” produced using simple X ray diffraction techniques. A strong positive correlation is found between the degree of preferred orientation, as expressed in the orientation indices, and seismic anisotropy. BSE images of the shale fabrics confirm in a qualitative manner the results of the X ray study. To investigate wave propagation in the shales, elastic constants of each sample are calculated and used to produce phase and group (wave) velocity surfaces, which describe variation in velocity as a function of angle to the bedding normal. The calculated velocity surfaces, constrained by independent velocity measurements, display a lack of shear wave splitting at “near‐normal incidence” in even the most anisotropic shales. For the highly anisotropic Chattanooga shales, group velocity surfaces differ significantly from corresponding phase velocity surfaces.

S U M M A R YIn this paper the physical properties of oceanic layer 2 basalts are examined with the emphasis on microcrack porosity and alteration-related changes in physical properties. Over 160 core samples from some of the more significant Deep Sea Drilling Project/Ocean Drilling Program holes, including 417D, 418A, 504B and 801C are included in the present investigation. Overall physical property relationships are discussed in the first half of this study. At 200 MPa confining pressure, compressionaland shear-wave velocities of the sample suite have modal values of 6.1 kms-' and 3.5 kms-', respectively. A Poisson's ratio of 0.28 characterizes the majority of the basalts. Samples that have experienced extensive low-temperature alteration, identified through elevated K,O and decreased CaO and MgO contents, exhibit low velocities (and densities) and high Poisson's ratios.The microcrack porosity in the basalt suite is examined using velocity-pressure data, a pore-aspect-ratio inversion scheme, and scanning electron microscope (SEM) imaging. Microcracks sealed by alteration minerals such as smectite and calcite or healed by late crystallization are common throughout the sample set. Rocks from the transition zone and sheeted dyke section of Hole 504B are marked by numerous smectite-and chlorite-filled cracks. More significantly, the data indicate an increase of low-aspectratio open microcracks with increasing depth in samples from several DSDP holes, particularly Hole 504B. These microcracks, also observed using the SEM, are attributed to stress relief as the original drill cores were removed from in situ pressure conditions. Stress-relief microcracking is apparently more severe in the holocrystalline, coarsergrained samples from massive flows than in the phyric basalts with fine-grained groundmasses characteristic of pillow units. This difference in microcracking is explained through basic fracture theory

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