The P- and S-wave velocities in the seawater-saturated minicores were measured at confining pressures ranging from 10-200 MPa at the Rock Physics Laboratory at Purdue University using the ultra-sonic pulse transmission technique of Birch (1960). The samples are 1-in-diameter right cylinders, ~1-in long, drilled perpendicular to the core axis from the split core. The minicores were jacketed with a screen mesh overlain by copper foil. The screen mesh is used to reduce pore pressure buildup, whereas the copper foil prevents the pressure medium from invading the samples at elevated pressures. The grain and bulk densities of the minicores were determined from their masses and volumes, and the porosity for each sample was calculated from the measured densities. Results are reported in Table 1.

Seismic velocity measurements were made at confining pressures ranging to 200 MPa, but samples came from depths between 215 and 275 mbsf, where the in situ differential pressures range from ~4 to 7 MPa. Thus, the velocities measured at the lowest confining pressures (10 MPa) are most nearly representative of the seismic velocities at true in situ pressures and range from 4.7 to 5.9 km s^-1 for the P-wave data and from 2.8 to 3.4 km s^-1 for the S-wave data.

The porosities listed in Table 1 range from 2.1% to 6.9%. The bulk porosities correlate very well with the P-wave velocities made at low pressures (e.g., 10 MPa; Fig. 2A). This strong correlation has been observed in other studies of the physical properties of basalts and diabases (e.g., Wilkens et al., 1983; Christensen and Salisbury, 1985; Barton et al., 1989; Christensen et al., 1989; Carlson and Herrick, 1990; Iturrino, 1995). The correlation shown in Figure 2A suggests that the total porosity has a primary influence on the P-wave velocities in these samples and that the relative distribution of the pore shapes must be fairly uniform among these samples. A large sample-to-sample variation of the aspect ratio spectra would cause more scatter in the velocity-porosity trend, and the good correlation between P-wave velocity and porosity would not be observed.

The grain densities listed in Table 1 range from 2740 to 2910 kg m^-3. The grain densities do not correlate as well with the P-wave velocities as the bulk porosity, as shown in Figure 2B. The average grain densities on two crushed, almost entirely altered basalt samples is 2100 ± 60 kg m^-3. Because the density of alteration minerals (largely clays) is lower than the density of the primary minerals in fresh basalts (plagioclase, augite, and olivine), a lower grain density probably indicates a higher percentage of alteration minerals in the samples, so that grain density can be used as a proxy for the relative amount of alteration products.

The P- and S-wave data show a smoothly varying velocity-pressure relationship. The scatter in the data from a smooth trend indicates that the measurement error is ± 0.01 km s^-1. In contrast, the errors in the densities and calculated porosity are not as easy to determine. The estimated uncertainty in the bulk and grain densities is 20 kg m^-3 (<1%); the corresponding error in the porosity is ~1% (one porosity unit).