Physical Properties of Site 762: A Comparison of Shipboard and Shore-Based Laboratory Results

Abstract

Compressional velocity (Vp), shear velocity (Vs), compressional quality factor (Qp), electrical resistivity (p'), bulk density (pb), grain density (pg), and porosity (Φ were measured in our shore-based laboratory for 49 consolidated sediment samples from Hole 762C. The results are compared with shipboard data. Shore-based Vp values agree well with shipboard Vp data except in the range 670-820 meters below seafloor, where a shipboard calibration problem occurred. Shipboard sonic log data are an average of 0.3 km/s higher than shore-based Vp values because of in-situ overburden pressure. Shore-based pg and values are generally in agreement with shipboard data. However, shipboard pb values are consistently higher than shore-based data. This discrepancy is because the helium-displacement pycnometer used aboard ship gives erroneously low volumes for wet samples, which are then used in bulk density calculations. Correct shipboard wet sample volumes can be calculated by adding the difference between the wet and dry sample weights to the dry sample volume. The corrected shipboard pb values are in agreement with shore-based data. We recommend that the Ocean Drilling Program use this calculation in place of the pycnometer wet volumes. The chalks show a negative velocity gradient between 600 and 720 mbsf, though there is no apparent change in lithology. In absence of overpressuring and mineralogical changes, the negative gradient is probably caused by increasing porosity due to the change in microstructure of the sediment over this depth interval.