New insights into <i>P</i> -wave attenuation characteristics and mechanisms in methane hydrate-bearing sands

SUMMARY Accurately mapping the distribution of natural methane hydrates is crucial for understanding their role in climate change and predicting the risks associated with hydrate dissociation. Attenuation shows great potential for remote hydrate detection, yet its behaviour and underlying mechanisms are still not well understood. We conducted laboratory experiments to synthesize high-saturation methane hydrate in unconsolidated sands and measure attenuation based on ultrasonic waveforms. The resulting attenuation showed an unexpected decreasing trend during hydrate formation, contradicting previous studies in sands, where attenuation generally increases with hydrate saturation. Theoretical modelling suggests that attenuation is jointly controlled by hydrate and free gas. The gas reduction in pores due to hydrate formation substantially suppresses the attenuation induced by gas-bubble oscillation, and is therefore thought to be responsible for the observed attenuation reduction. By comparison, hydrate effects are relatively weak and strongly frequency-dependent. The discrepancy between our results and previous studies arises primarily from the distinct attenuation behaviour across different ranges of gas content. Our samples fall within a relatively low gas content range, where attenuation is particularly sensitive to gas, highlighting its impact. These findings contribute new insights into the attenuation characteristics and mechanisms due to the coexistence of hydrate and gas in sediments.