Nonlinear frequency dependence of sound attenuation in sea bottoms at low frequencies

Abstract

In the 1980s, several papers were published that stressed the importance of non-linear frequency dependence (NLFD) of sound attenuation in marine sediments al low frequencies, and showed experimental evidence that the NLFD can have a significant effect on long-range sound propagation [Zhou et al., J. Acoust. Soc. Am., 78, 1003–1009 (1985); 79, Suppl. 1, S68, (1986); 82, 287–292(1987); 82, 2068–2074 (1987)]. According to Kibblewhite [JASA, 86, 718–738 (1989)], “the only program directly related to marine sediment attenuation and not identified by Hamilton is that of Zhou et al.” This analysis of NLFD was extended later to cover 20 locations in different coastal zones around the world, resulting in an effective geoacoustic model for sandy to sand-silt-clay bottoms. [Zhou, Zhang and Knobles, JASA, 125, 2847–2866 (2009)]. However, the attenuation-frequency relationship is still an open issue. There is a pressing need to have quality sound-speed and attenuation data in the low- to high-frequency transition band from one location by using both inverse and direct techniques. Thus, a sea-going experiment in an area with shallower water depth is desirable. This paper will discuss some physical and technical issues related to broadband geoacoustic inversions and examine some data from muddy bottoms.