Physical properties of ODP Hole 122-762C

Compressional velocity (Vp), shear velocity (Vs), compressional quality factor (Qp), electrical resistivity (p'), bulk density (pb), grain density (pg), and porosity (phi) 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 phi 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.

本研究依托岸基实验室，对取自762C钻孔的49块固结沉积物样品开展了多参数测试：压缩波速度（Compressional velocity, Vp）、剪切波速度（Shear velocity, Vs）、压缩波品质因数（Compressional quality factor, Qp）、电阻率（Electrical resistivity, ρ'）、体积密度（Bulk density, ρb）、颗粒密度（Grain density, ρg）与孔隙度（Porosity, φ）。测试完成后，将所得数据与船载测试数据进行了对比分析。
岸基测试所得的Vp值与船载Vp数据整体吻合度较高，仅在海底以下670~820米区间存在偏差——该区间船载测试曾出现标定问题。受原位上覆地层压力影响，船载声波测井得到的Vp值平均比岸基测试结果高出0.3 km/s。
岸基测试得到的ρg与φ值整体与船载测试数据相符。但船载测试所得的ρb值始终高于岸基测试结果，该偏差源于船载作业中使用的氦置换比重瓶对湿样品的体积测量结果偏低，进而导致体积密度计算出现系统性误差。通过将湿样与干样的重量差值叠加至干样体积，即可完成船载湿样体积的校正，校正后的船载ρb值与岸基测试结果吻合良好。我们建议大洋钻探计划（Ocean Drilling Program, ODP）采用该校正方法，替代原有的比重瓶湿样体积计算流程。
白垩岩层在600~720 mbsf（meters below seafloor，海底以下米数）区间呈现负速度梯度，尽管该区间的岩性未发生明显变化。在未出现超压且矿物组分无显著改变的前提下，该负速度梯度大概率由该深度区间内沉积物微观结构改变引发孔隙度升高所导致。