Comparison of in situ and laboratory acoustic measurements on Lough Hyne marine sediments

Compressional wave velocity and attenuation were measured at frequencies of 200\textendash1500 Hz on seafloor sediments at Lough Hyne, Ireland, using a mini-boomer source and hydrophone array. Velocity and attenuation were also measured in the laboratory at 200\textendash800 kHz on a 1 m long sediment core taken from the site. The in situ results indicate an average sediment phase velocity of about 1600 m/s and sediment quality factor of 10\textendash20. The laboratory core measurements give an average phase velocity of 1793\textpm26 m/s and quality factor of 16\textpm5. The poorly sorted, Lough Hyne sediments are highly attenuating and highly dispersive when compared to values published in the literature for well-sorted, marine sediments such as clean sands and marine clays. The results are consistent with the few published data for poorly sorted sediments, and indicate that intrinsic attenuation is highest when the mass ratio of mud (clay+silt) to sand grade particles is close to unity. It is proposed that compliance heterogeneities are most abundant when mud and sand grade particles are present in roughly equal proportions, and that the observations support local viscous fluid flow as the most likely loss mechanism.