Low-frequency geoacoustic model for the effective properties of sandy seabottoms

Abstract

The debate on the sound speed dispersion and the frequency dependence of sound attenuation in seabottoms has persisted for decades, mainly due to the lack of sufficient experimental data in the low-frequency (LF) to high-frequency speed/attenuation transition band. This paper analyzes and summarizes a set of LF measurements in shallow water that have resulted in the identification of nonlinear frequency dependence of sound attenuation in the effective media of sandy seabottoms. The long-range acoustic measurements were conducted at 20 locations in different coastal zones around the world. The seabed attenuations, inverted from different acoustic field measurements and characteristics, exhibit similar magnitude and nonlinear frequency dependence below 1000 Hz. The resulting effective sound attenuation can be expressed by alpha(dB/m)=(0.37+/-0.01)(f/1000)((1.80+/-0.02)) for 50-1000 Hz. The corresponding average sound speed ratio at the bottom-water interface in the 50-600 Hz range is 1.061+/-0.009. Both the LF-field-derived sound speed and attenuation can be well described by the Biot-Stoll model with parameters that are consistent with either theoretical considerations or experimental measurements. A combination of the LF-field-inverted data with the SAX99, SAX04, and other high-frequency measurements offers a reference broadband data set in the 50-400 000 Hz range for sonar prediction and sediment acoustics modeling.