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Seismic velocities have been measured at confining pressures to 600 MPa for eight samples of sheeted dike rock obtained from Hole 504B during Leg ODP 111. The compressional-and shear-wave velocities are, in general, higher than the velocities measured in overlying dike rocks obtained from the hole during DSDP Leg 83. The velocity gradients observed in Layer 2C result from decreasing porosity with depth and increasing metamorphic grade. The laboratory-measured velocities of the Leg 111 dike rocks are similar to those of dike rocks reported for the Bay of Islands, Samail, and Troodos ophiolites.

To understand better the seismic attenuation in the upper volcanic regions of the oceanic crust, compressional wave attenuations of oceanic basalts have been measured as a function of confining pressure using an ultrasonic pulse‐echo spectral ratio technique capable of measuring attenuations to pressures of 500 MPa. Seven basalts, five from Deep Sea Drilling Project cores and two from dredge samples, have wide ranges of densities, porosities, and alterations, making possible an analysis of the parameters influencing basalt attenuation. Attenuation increases with the volume of secondary minerals present and with increasing porosity. Thus vesicularity and compositional changes associated with basalt alteration will produce variations in attenuation. With the application of hydrostatic pressure, cracks close, thereby reducing attenuations. This pressure dependence should be manifested in oceanic layer 2 by decreasing attenuation with depth. An inverse relationship between velocity and attenuation is observed at high hydrostatic pressures. Water saturation increases attenuation at pressures below 200 MPa and enhances the sensitivity of attenuation to pressure, thus making the state of saturation important in the 40 to 100 MPa range generally found in layer 2. These results provide a framework for interpreting marine attenuation data.

Compressional velocity and attenuation were measured as a function of depth in layer 3 gabbros from the recovered core at Ocean Drilling Program Site 735. High-frequency (400 kHz) acoustic transmission experiments were conducted on 117 minicore samples under room conditions of temperature and pressure on board the JOIDES Resolution. Although experimental errors were large in some deformed samples, the average of our measurements of 1000/(λ, on these samples is 49 ± 32. Ultrasonic tests on a \% porosity sample at elevated effective pressures from 10 up to 500 MPa showed an increase in V_ from 6.8 to 7.0 km/s and a decrease in \000/Q p from 26 to 23. The small pressure dependence of attenuation in these low-porosity gabbros enables one to use measurements at room conditions for relative interpretation as a function of depth.These measurements of 1000/(λ, vs. depth show (1) an overall decrease in attenuation from lithologic Units II through V as a function of depth, (2) a 10-fold decrease in the average variance in the measurement in lithologic Unit II, and (3) an inverse relationship between average velocity and average attenuation. An explanation for these results is that tectonized rock fabrics have a significant effect on the measurement of attenuation and velocity. The future use of this simple shipboard operation to measure trends in attenuation as a function of depth may be effective to determine the degree of alteration and tectonism in similar, nonpressure-sensitive rocks.