Abstract:

In April 1995, the Naval Research Laboratory performed high-frequency direct-path experiments in shallow water, near Panama City. During that period there were two days of strong weather which gave rise to slower sound speed, larger fluctuations over a few seconds, and dispersion. It led to the conclusion that only the presence of bubbles could cause these changes. The experimental results will be published soon. In an effort to understand these changes, analytical and numerical analyses were undertaken. Straightforward dispersion theory, based on the change of impressibility gives an integral expression that is a function of frequency and bubble distribution. It was found that there is a simple, exact integration for the case where the distribution of bubbles of radius a is given by the inverse of fourth power, a[sup -4]. Numerical studies have been performed that can be used to compare dispersion for other powers, from a[sup -3] to a[sup -5]. The analyses give insight into the role that void fraction, bubble sizes, and their distribution play in dispersion. The analyses exhibit, as well, the sensitivity to accuracy that would be required if dispersion is to be used to determine the distribution of bubble size in the sea. [Work supported by NRL.]