Vertical seismic profiles through gas-hydrate-bearing sediments

Vertical seismic profiles (VSPs) are commonly used for linking borehole information with surface-seismic surveys. Since Ocean Drilling Program (ODP) Leg 146 offshore Cascadia, VSP surveys have been conducted in all major gas-hydrate drilling campaigns, both offshore and onshore. We have compiled results from VSPs during ODP Legs 146 (1993, Cascadia margin), 164 (1995, Blake Ridge), 204 (2002, Hydrate Ridge), the onshore Mallik campaigns in 1998 and 2002, as well as the Nankai Trough exploratory wells. We also give a brief overview of other recent VSP operations conducted to investigate gas hydrates. All of these campaigns included zero-offset VSPs, including a shear-wave VSP at the Mallik sites. Zero-offset VSPs proved invaluable for tying seismic surveys to borehole data and also for interpolating data gaps in sonic logs. Compressional-to-shear conversion was observed in both constant-offset and walkaway VSPs allowing measurement of shear-wave velocity offshore. Seismic anisotropy determined from walkaway VSPs indicated a preferential alignment of clay platelets within the Blake Ridge and gas-hydrate-bearing veins on Hydrate Ridge. Finally, a unique approach, resonance scattering, emerges as a promising tool to quantify the lateral heterogeneity of gas-hydrate deposits. Some challenges still remain for VSP operations in gas-hydrate-bearing sediments, particularly offshore due to the proximity of gas hydrates to the seafloor. Conventional VSPs have now matured to be a standard tool for gas-hydrate wells. The recent rapid developments of VSP technology, both on the acquisition and evaluation side, enable us to make new and exciting insights into natural gas hydrates.