Compressional and shear wave velocities in uncemented sediment containing gas hydrate

The competing hypotheses for gas hydrate formation at the particle scale in sediments describe processes of pore-filling, frame-building, or cementation. New measurements of compressional (V-P) and shear wave (V-S) velocities in fine-grained sands subjected to low confinement and monitored during formation of tetrahydrofuran hydrate indicate that hydrate nucleates in the pore space ( presumably at grain boundaries) and grows with limited impact on the sediment shear stiffness, V-P, and V-S until crystals begin to interact with the granular skeleton at similar to 40% hydrate concentration. V-S increases significantly more than V-P at higher hydrate concentrations, reflecting larger changes in the specimen's shear stiffness than its bulk stiffness. The results indicate that seismic velocities and/or their ratio (V-P/V-S) have limited capability for locating hydrate or constraining hydrate concentrations.