Ultrasonic Velocities in Laboratory-formed Gas Hydrate-bearing Sediments

Gas Hydrates are widely distributed in the near surface at high latitudes and many ocean bottoms. Our laboratory measurements help calibrate the seismic and well log data that are used to ascertain the in situ distribution of hydrates. Ultrasonic velocities are measured in unconsolidated sand specimen under thermobaric conditions comparable to those found in shallow oceanic or permafrost regions, i.e. in the gas hydrate stability zone. Compressional-wave (p-wave) velocity data in dry Ottawa Sand samples subjected to a confining pressure that from 2 to 20 MPa range from about 700 to 1500 m/s. Gas hydrates were then formed a partially saturated Ottawa sand sample. As expected, this hydrate formation resulted in a dramatic increase in p-wave velocities from 700 m/s in the partially water saturated sample to 2100 m/s in the hydrate-bearing specimen. The hydrate effectively cements the sand such that nine hours after entering the hydrate stability region, the recorded waveforms remained constant even as the confining pressure was increased by more than 10 MPa.