The direct path impulse response in bubbly shallow water

Abstract

It is well known that bubbles, even in small numbers, cause significant changes in the acoustic properties of seawater. In shallow water bubbles enter the sea, either by wind-driven breaking waves or by surf-induced rip currents. Both have been observed in acoustic transmission experiments [IEEE, JOE 25, 507–515 (2000); J. Acoust. Soc. Am. 106, 617–625 (1999)]. Wind-driven bubbles were observed, during a period of high winds, to reduce sound speed from the undisturbed sea condition of 1520 m/s to about 1494 m/s. From bubble theory, it is estimated that the average bubble void fraction was approximately 2.6×10−6. For a distribution of bubble radius size, n(a), there will be a frequency-dependent sound speed and absorption related by the Kramers-Kronig relation, from which the change in a pulsed signal can be determined. By using a bubble size distribution of n(a)=Na×10−4, for radii between values of 200 and 10 microns and the void fraction estimated above, the dispersion and absorption are calculated. The impulse response can be calculated as a function of range. [Work supported by the Office of Naval Research.]

Quantitative Side-Scan Research for Sediment Characterization and the Development of a Multibeam Subbottom Profiler

“…In separate work with Ralph Goodman, we continued some analysis of propagation data we collected during the Scripps Pier Bubble Experiment in 1997. The results of this analysis was presented at the June, 2006, meeting of the Acoustical Society [6]. This analysis addressed pulse distortion of a signal propagating through a highly attenuative bubbly medium.…”