Sonar attenuation modeling for classification of marine sediments

An attenuation‐based model for classification of marine sediments is developed for the chirpsonar operating in the frequency range of 2–10 kHz. A relaxation‐time model is proposed that combines the various dissipative energy loss mechanisms of sound in marine sediments into a single parameter. Historical data were analyzed by converting attenuation values reported in ‘‘dB/m@kHz’’ to a single relaxation time value. Analysis of these previous attenuation measurements supports the use of a relaxation‐time model. Based on this large collection of data, an empirical equation is developed that relates relaxation time to grain size (in phi units). Using this model, very little phase dispersion is observed for a correlated chirp pulse traveling through 40 m of sand, silt, or clay. Yet, this is not so for a pulse in the ultrasonic frequency range (0.2–1.0 MHz) traveling through only 10 cm of clay. Here, significant dispersion is noted. Because of the unique Gaussian‐like shape of the correlated chirp pulse power spectrum, pulse elongation due to attenuation is minimized. Using the center frequency shift in the pulse spectrum, a new ‘‘instantaneous frequency’’ method of attenuation estimation is proposed that overcomes the problems associated with interfering reflections. Based on the relaxation‐time model, the correlated chirp pulse was synthetically attenuated to establish a relation between the relaxation time and the center frequency shift. I n s i t u sediment‐type predictions from chirpsonar data using the instantaneous frequency method and analyses of core samples taken in the Narragansett Bay, Rhode Island are in good agreement.