In-Situ Determination of the Variability of Seafloor Acoustic Properties: An Example from the Onr Geoclutter Area

In support of the US ONR-sponsored Geoclutter program, we have developed, built, and deployed a relatively inexpensive, robust, small-ship-deployable device (ISSAP — In situ Sound Speed and Attenuation Probe) for rapidly measuring sound speed and attenuation in near-surface sediments. We have demonstrated its ability to make reliable and precise measurements (+/- 1–2 m/s for sound speed, < +/- 1 dB/m for attenuation). We have found that in the Geoclutter area the sound speed varies on the order of 200–300 m/s over spatial scales of 10’s of kms and the attenuation (at 65 kHz) varies on the order of 60 dB/m. On scales of less than one kilometer, the sound speed can vary by more than 100 m/s and attenuation by approximately 25 dB/m. On the sub-meter scale, much of the seafloor is relatively homogeneous but some areas show sound speed variation of approximately 50 m/s and attenuation variation on the order of 25 dB/m. These variations are probably related to the presence of large clasts or shells in the measured path.