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Ultrasonic compressional and shear wave velocities of homogeneous basalt samples from the Leg 83 section of Hole 504B were measured under atmospheric and in situ conditions. In situ temperature and hydrostatic pressure were estimated based on temperature profiles (Becker et al., this volume) and on the densities of the basalt samples. In going from atmospheric to in situ conditions, compressional wave velocities increased by about 10%, whereas shear wave velocities decreased by about 10%. Electrical resistivities of dry basalt samples showed a drastic decrease in going from atmospheric to in situ temperature and pressure. INTRODUCTION A study was made of the variation with pressure (P) and temperature (T) of compressional and shear wave velocities and electrical resistivities of selected, homogeneous basalt samples from the Leg 83 section of Hole 504B (836-1350 m sub-bottom). Measurements were made under conditions ranging from atmospheric to estimated in situ temperatures and pressures (Table 1). The in situ temperature at the depth from which a given sample was cored was estimated by applying the conductive temperature profiles of Becker et al. (this volume). These indicate that temperatures could be closely approximated with linear gradients within each of three sections, each with a different thermal conductivity: the sediments (0-274.5 m), basement shallower than 836 m, and basement deeper than 836 m. Pressure at a given depth was assumed to be hydrostatic and was obtained by integrating the mass in a vertical column above the depth concerned. From the agreement of logging results (Anderson et al., 1982) and measurements of densities of samples at atmospheric conditions, it was assumed that densities could be approximated by two layers of constant value (Table 1): sediments (2.5 g/cm) and basement (3.0 g/cm). Figure 1 shows diagrams used to determine pressure-temperature values for the present studies. METHODS Samples for sonic velocity measurements were cut in cylinders ~ 20 mm long and 10 mm in diameter, moistened with seawater in a pressurizing vessel (5 bars) and sealed with copper plate (100 μm thick). Samples for electrical resistivity measurements were also cut in cylindrical shape, dried in hot air (~ 100°C), sealed in a teflon insulator tube, and again sealed with the copper plate. Ultrasonic waves were generated through lithium niobate single crystals with a high-speed and high-tension electric pulse (pulse width = 1 μs and pulse height = 250 V). Velocities were measured by a transmission method (i.e., delay time method). In addition to the pressure and temperature dependence Table 1. Temperature and density parameters used to determine pressure-temperature conditions. Sub-bottom depth (m) Mudline 275 836 1350 Depth from sea level (m) 3460 3735 4296 4810 Temperature (°C) 20 60 120 165 Density (g/cm3) 1.025 2.50 3.00 3.00 Anderson, R. N., Honnorez, J., Becker, K., et al., /nit. Repts. DSDP, 83: Washington (U.S. Govt. Printing Office). 2 Address: Dept. of Earth Sciences, Faculty of Sciences, Chiba University, Yayoi-cho, Chiba 260 Japan. of the sonic and electrical characteristics of solid samples, some frequency dependence of the electrical resistivity was measured. After several runs under various P-T conditions (up to 1000 bar and 220°C), the electrical potential of 10 Hz (15 peak-to-peak volts) was selected and used in these studies. Pressure and temperature were regulated by an oil-powered hand pump with a high-precision pressure gauge and an inner heating system, driven by an ordinary electric power supply. Temperatures were measured by a thermistor attached to the surface of the specimen; calibration runs were made by comparing the temperature in the center of the specimen and on its surface prior to the experiments. Figure 2 shows a lateral view of the P-T vessels used for the present study. RESULTS Sonic Velocities Shipboard measurements of compressional sonic velocities of homogeneous basalt samples from Hole 504B were made during Legs 69, 70 (Karato et al., 1983), and 83. We used a Hamilton Frame device, with a wave packet with a 500 kHz main frequency. Figure 3 shows 100m means and standard deviations of the results of these measurements. This velocity variation correlates well with the sonic-logging velocity value (Anderson et al., 1982). If a rebound of the elastic rock samples is to be taken in account, then the in situ velocities are probably several percentage points higher than the laboratory values. In this study, the pressure and temperature dependence of compressional (Kp) and shear (Vs) sonic velocities were measured for five selected samples. Typical homogeneous basalt samples were picked from several depth

Sonic Velocity and Electrical Resistivity of Rock Samples from Hole 504B, Leg 83: Measurements Under In Situ Conditions