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 ∼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.