In this study, velocities are reported for hydrostatic pressures up to 600 MPa at ambient temperature for 14 samples obtained from Leg 195 drilling. A core was taken from each sample using a 2.54-cm diameter diamond coring bit. Each core averages ~2 cm in length. The core ends were trimmed and ground flat and parallel to within 0.07 cm on a diamond grinding disk. The volume of each core was calculated from the length and diameter. The cores were weighed, and densities were calculated from their masses and dimensions. Each core was then fitted with a soldered copper jacket to prevent penetration of high-pressure oil into the rock samples. To make velocity measurements, 1-MHz transducers were affixed to both core ends. Gum rubber tubing was placed over the sample assembly as a further prevention of oil leakage.
Velocities were measured at room temperature and hydrostatic pressures up to 600 MPa using the pulse transmission method described by Christensen (1985). A pulse generator produced a 50-V square wave, simultaneously triggering a dual-trace oscilloscope. Readings were taken at 20, 40, 60, 80, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, and 600 MPa and repeated for downgoing pressures.
Interfacing the pressure system with a computer for data acquisition and storage permits automatic calculations of velocities as successive readings are taken. First break picks for acquired waveforms are automatically selected by the computer and can be fine-tuned by the operator. Using a least-squares routine, the computer fits a curve to the data points and calculates velocities for each pressure. A velocity vs. pressure curve is plotted along with recorded data points (Fig. F1). The sample length, density, measured pressure-velocity pairs, traces of the waveforms (Fig. F1) at selected pressures, the curve fit equations, and calculated pressure-velocity pairs are recorded and stored digitally.
Shipboard petrography of the rocks recovered from Site 1200 indicates that they are heavily serpentinized harzburgite and dunite clasts. (Shipboard Scientific Party, 2002). Minerals present include lizardite and chrysotile serpentine, olivine, orthopyroxene, Cr spinel, and trace amounts of clinopyroxene. In some cases the serpentinization obscures the original mineralogy. Some fracturing or cracking is also seen in the rocks.