Print this pageShear-wave Logs and Velocity-Porosity Relationships in Gas Hydrates
GOLDBERG, DAVE, GILLES GUERIN, ALEX MELTSER, and the ODP Leg 164 Shipboard Scientific Party
In late 1995, the Ocean Drilling Program acquired downhole log data off the Carolina coast using conventional tools and a dipole sonic tool through natural gas hydrates and gas-saturated sediments. The dipole sonic tool uses a licensed transducer design that was modified to improve signal isolation and conditioning. Our evaluation of the results indicates that the velocity inversion associated with the base of the gas hydrates, which exist over a depth interval of more than 200 m, occurs at approximately 450 m below the sea floor. Vs estimates range between 430 and 700 m/s; however, poor shear wave signals were recorded in intervals associated with free gas. The relationship of Vs with porosity agrees with published models for marine sediments, but their dependence increases in the gas hydrates. The Vp/Vs ratio, which is sensitive to both porous and mechanical sediment properties, decreases from approximately 3.8 to 3.0 over the 150 m interval containing hydrates and possibly indicates a change in their concentration. Shear wave amplitudes are lower in the gas hydrate interval and increase by a factor of 2-4 below. The analysis of seismic data off the Carolina coast implies that gas hydrates are associated with high Vp, low Vs, and anomalous seismic blanking, which is consistent with these log results. These data significantly improve our understanding of the in situ physical properties of gas hydrates since they are unstable under ambient surface conditions and may help to determine their hydrocarbon reservoir potential.