Measurement of the frequency dependence of the sound speed and attenuation of seafloor sands from 1 to 400 kHz

Abstract

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 × 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 ±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 significantly 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.