Measurement of the Frequency Dependence of the Sound Speed and Attenuation of Seafloor Sands From 1 to 400 kHz

摘要

Measurements of sound speed and attenuation were performed over a frequency range from 1 to 400 kHz in the sandy seafloor near Ft. Walton Beach, FL, as part of the 2004 Sediment Acoustics Experiment (SAX04). The measurements at the lowest frequencies, from 1 to 20 kHz, were performed by transmitting tone burst signals from two acoustic sources positioned at depths up to 1 m below the seafloor which were recorded on a 4 m $times$ 4 m array of 35 hydrophones emplaced to depths of between 0.1 and 1 m. Measurements were obtained at frequencies from 40 to 200 kHz with the In Situ Sediment Acoustic Measurement System (ISSAMS), a fixed linear array of four piezoelectric probes. The probes were inserted to a depth of 0.3 m, and the outer two probes were used as transmitters, with the inner probes acting as receivers. Sound-speed measurements were also made at 100, 200, and 400 kHz on diver-collected cores using four separate pairs of ultrasonic transducers. The sound-speed measurements above 40 kHz demonstrate an essentially constant sound-speed ratio with frequency of between 1.165 and 1.175, where the sound-speed ratio is the ratio of the speed through the sand to the speed through the overlying seawater. The sound-speed ratio determined from the low-frequency array data decreased from 1.135 to 1.115 with decreasing frequency below 20 kHz. Uncertainties in the sound-speed ratio values on the order of ${pm 0.02}$ do not allow meaningful tests of the ability of Biot–Stoll or Buckingham propagation models to describe the observed dispersion. The measured attenuation values show a range from 1 to 120 dB/m with frequency from 1 to 400 kHz. At frequencies above 40 kHz, the attenuation follows a linear trend parallel to the estimates from the Buckingham model, as fit to the data with two free parameters, and at the highest frequencies sign-n-nificantly exceeding the Biot model prediction, as parameterized with values selected from the range of measurements of sediment and pore-water properties made from throughout the SAX04 site. At frequencies below 20 kHz, the data straddle the trends from the two models.