Acoustic properties of coral sands, Waikiki, Hawaii

Abstract

An in situ experimental study of variations of compressional wave speed and attenuation with depth in natural coral sands has been made offshore of Oahu, Hawaii. In situ data were collected at a center frequency of 7.5 kHz. Compressional wave speed averages around 1620 m/s and attenuation (expressed as Qp−1, the reciprocal of the quality factor) decreases from 0.04 at the seafloor to 0.01 at 2 m depth. Very little change in compressional wave speed is seen to 9 m below the seafloor. Coral sand sound speeds are lower than those reported elsewhere for quartz sand. Waveforms recorded over the upper 9 m below the seafloor exhibit virtually no peak broadening, suggesting that scattering contributes little to the in situ attenuation. The relationship of attenuation to frequency in the coral sands agrees with Hamilton’s observations of attenuation in other sediments, although the coral sand attenuation is slightly higher than in other sediments. The coral sand relationship between attenuation and porosity also agrees with Hamilton’s when the volume of intraparticle voids is deducted from the total porosity.