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

Compressional wave velocity and attenuation were measured at frequencies of 200-1500 Hz on seafloor sediments at Lough Hyne, Ireland, using a mini-boo mer source and hydrophone array. Velocity and attenuation were also measure d in the laboratory at 200-800 kHz on a 1 m long sediment core taken from t he site. The in situ results indicate an average sediment phase velocity of about 1600 m/s and sediment quality factor of 10-20. The laboratory core m easurements give an average phase velocity of 1793 +/- 26 m/s and quality f actor of 16 +/- 5. The poorly sorted, Lough Hyne sediments are highly atten uating and highly dispersive when compared to values published in the liter ature for well-sorted, marine sediments such as clean sands and marine clay s. The results are consistent with the few published data for poorly sorted sediments, and indicate that intrinsic attenuation is highest when the mas s 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 mech anism. (C) 2001 Acoustical Society of America.