Determination of geophysics properties of seabed sediments with multi-scanner core logger (MSCL)

Abstract

This project aims to study the plausibility of using geophysical method alone to study the physical properties of offshore sediments in Hong Kong. GEOTEK’s Multi-scanner Core Logger is an automatic device that allows the continuous measurement of physical properties in a fast, high resolution and non-destructive manner. HKUV10, a bored core that is retrieved from the offshore area South of Lantau Island, is studied in this project. Physical properties, including bulk density, fractional porosity, P-wave velocity, P-wave amplitude, impedance and magnetic susceptibility are measured along core.

The measured geophysical properties of HKUV10 are compared against findings in the literature. The Quaternary offshore geology in Hong Kong has been well studied (Davis, 1999, Fyfe et al., 1997, Fyfe et al., 1999, Yim, 1994, Yim et al., 2004). Two established geological model are used as reference for the comparison. They are Fyfe et al. (1997) and Yim (1994) respectively.

Correlation plots are made from the physical properties measured. It is found that bulk density and fractional porosity are closely correlated as expected. Some correlation is found between magnetic susceptibility and density, as well as magnetic susceptibly and fractional porosity. However, little correlation was found between bulk density and P-wave velocity. This is because of the incorrect measurement of P-wave velocity.

Secondly, the measured physical properties are plotted against the depth of core. It is found that in the first 3 to 4m, an abruptly elevated magnetic susceptibility of sediment is observed. The observation agrees with the finding from Chan et al. (1999) and Yim et al. (2004).

Thirdly, it is suggested that core depth may be 3 to 17.2m is a marine deposit and 17.2 to 25.7m may be terrestrial deposits. The suggestion is made based on past research that shows high level of magnetic susceptibility could be found in terrestrial deposits (Tovey and Yim, 2002, Yim et al., 2004).