Sound speed, pulse spreading, and reverberation in muddy bubbly sediments

Abstract

The sound speed characteristic of the high-porosity mud has been found to have sonic speeds lower than expected. Since the presence of bubbles is known to be an important factor in decreasing the sound speed, these low sound speeds are attributed to methane microbubbles that result from biological decay. A theoretical treatment of “muddy sediments,” the Card House Theory (Pierce and Carey, POMA (5), 7001, 2009), estimated the slow sound speed and frequency dispersion proportional to mud porosity, Cmud ∼ (0.91–0.97)Cw. The presence of microbubbles can lower the sound speed consistent with the Mallock-Wood equation [Carey and Pierce, A 1aUW6, J. Acoust. Soc. Am. 129(4,Pt2) April 2011]. The recent Dodge Pond experiment found low sound speed estimates consistent with bubble volume fractions between 10–4 to 10–5. The experiment has also produced estimates of pulse time spreading and reverberation. This paper interprets these results in terms of a three-component mixture with the bubbles distributed in a random Poisson process. Since measurement of the bubble size distribution within the mud is difficult, limits on the distribution may be obtained by the frequency dependent nature of the sound speed, pulse spreading, and reverberation characteristics. [Work sponsored by ONR OA and NSWC PCD.]