Compressional-and shear-wave velocity logs (V p and V % , respectively) that were run to a sub-basement depth of 1013 m (1287.5 m sub-bottom) in Hole 504B suggest the presence of Layer 2A and document the presence of layers 2B and 2C on the Costa Rica Rift. Layer 2A extends from the mudline to 225 m sub-basement and is characterized by compressional-wave velocities of 4.0 km/s or less. Layer 2B extends from 225 to 900 m and may be divided into two intervals: an upper level from 225 to 600 m in which V p decreases slowly from 5.0 to 4.8 km/s and a lower level from 600 to about 900 m in which V p increases slowly to 6.0 km/s. In Layer 2C, which was logged for about 100 m to a depth of 1 km, F p and F s appear to be constant at 6.0 and 3.2 km/s, respectively. This velocity structure is consistent with, but more detailed than the structure determined by the oblique seismic experiment in the same hole. Since laboratory measurements of the compressional-and shear-wave velocity of samples from Hole 504B at P con fj n. ing = ^differential average 6.0 and 3.2 km/s respectively, and show only slight increases with depth, we conclude that the velocity structure of Layer 2 is controlled almost entirely by variations in porosity and that the crack porosity of Layer 2C approaches zero. A comparison between the compressional-wave velocities determined by logging and the formation porosities calculated from the results of the large-scale resistivity experiment using Archie's Law suggest that the velocity-porosity relation derived by Hyndman et al. (1984) for laboratory samples serves as an upper bound for V p , and the noninteractive relation derived by Toksöz et al. (1976) for cracks with an aspect ratio a = 1/32 serves as a lower bound.
On ODP Leg 102, the JOIDES Resolution returned to Hole 418A at the southern end of the Bermuda Rise and logged the hole with a comprehensive suite of tools to determine the geophysical properties of old oceanic crust from insitu measurements. An excellent set of density, porosity, natural gamma-ray, conductivity, resistivity, full wave and mul tichannel sonic (P and S), magnetic susceptibility, three-axis magnetometer, and caliper logs was obtained over varying intervals from 0 to 488 m within the basement. In addition, the sediments were logged through the pipe using the poros ity and spectral gamma-ray tools, water samples were taken and temperature measurements made at selected depths in basement, and the oblique seismic experiment was successfully run with a three-component borehole seismometer clamped 41, 81, 230, 330, and 430 m within the basement. The results demonstrate as follows: 1. Layer 2A is absent: V p increases gradually from 4.5 km/s at the sediment/basement contact to 6.9 km/s at 1.5 km within the basement and averages 4.8 km/s in the upper 0.5 km. 2. The upper crust is anisotropic: V p varies with azimuth by ±0.2 km/s to a range of 0.6 km, with V^ fast parallel to spreading in the top of the section and fast subperpendicular to spreading near the bottom of the hole. In addition, the upper crust displays vertical/horizontal anisotropy, with V^ fast by 0.2 km/s in the horizontal propagation direction. 3. The upper 0.5 km of the crust is cracked at all scales of investigation because Vp lab > V Plog > Vp OSE. 4. The average formation porosity of the section drilled is low (15%, of which 10% is grain boundary porosity and 5% is fracture porosity) but was originally higher by an amount less than or equal to the present volume of clay in the formation (9%). 5. The original formation porosity near the top of the section (Unit 5 and Subunit 8A) approached 40%; the origi nal velocity of this interval would have been about 3.5 km/s, or that of Layer 2A. 6. The disappearance of Layer 2A was caused by infilling by an ordered sequence of alteration products formed by rock-water interaction in a closed system.
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