Fundamentals of Acoustical Oceanography

Abstract

What do shipwrecks, schools of fish, singing whales, and foaming seas have in common? They can all be detected and classified with acoustical methods and they are all described under one cover in this book. The seafloor, objects lying on or buried in the seafloor, suspended sediment, marine organisms of many kinds, eddies and turbulence, bubbles, temperature, salinity, and the sea surface all affect the manner in which sound propagates through the ocean. The purpose of this book is to describe methods that exploit the various effects so that sound can be used as a tool to infer important properties of the corresponding objects or processes in the ocean.Why use sound? Light and other forms of electromagnetic radiation do not travel far in the ocean, giving the ocean a dark and mysterious appearance. As a result, we know, in many respects, more about the surface of the moon than of the interior of the ocean on our very own planet! Sound can travel very large distances in the ocean, especially at lower frequencies.Because of this ability, sound has been widely used as a means to probe the ocean's interior. For example, a patent on use of underwater sound was applied for shortly after the steamship Titanic sank due to its collision with an iceberg in 1912. The patent was for using sound for ''detecting the presence of large objects underwater.'' Since then, the applications using sound as a tool to study the ocean grew in number and diversity. Along with the various applications are a multitude of challenges. Both the applications and their corresponding challenges are addressed formally in this book, with many examples given.Fundamentals of Acoustical Oceanography is written in a style appropriate for a broad audience at many levels. Much of the text is written in a simple tutorial manner so that both nonspecialists and people who are just entering the field can understand it. Furthermore, there is enough detail and references made to the literature so that the specialist can also make use of the material. The book spans areas of marine geology, marine biology, physical oceanography, and marine engineering and would be useful in applications involving ecology, commerce, and the military.This book follows Acoustical Oceanography, published in 1977 by the same authors but with author order reversed. The first book was widely used and cited. It had eleven printings, was translated into Russian, and was cited routinely by scientists in varied disciplines and in many different journals. Since 1977, there have been significant advances in the area of acoustical oceanography. This new book incorporates many of the advances along with a new format. The first work reserved advanced topics for appendices at the end of the book while the present book integrates advanced material along with the rest of the text, but denotes the material as optional to read.Acoustical oceanography, as defined by the authors early in the book, involves the so-called ''inverse problem.'' That is, given solutions to acoustic propagation, inverse me...

Acoustic monitoring of O2 production of a seagrass meadow

“…O 2 super-saturation (relative to atmospheric equilibrium) were observed during the whole period, which is the normal situation from spring to fall at this site (Champenois and Borges, 2012), and has also been noted in other studies (Wilson et al, 2012). When super-saturation occurs, free gas bubbles affect sound propagation (Medwin and Clay, 1998).…”

“…It has been established that the variability of the acoustic channel has an acoustic signature that is frequency dependent (Medwin and Clay, 1998). Analyzing the channel frequency response dependence on photosynthetic activity provides a guideline for selection of the optimal frequency band of the probe signal.…”

“…The set of parameters N, a n , τ n characterizes the propagation environment and may be used as indicators of environmental variability. Pulse compression is a widely used procedure in underwater acoustics to identify the various echoes on the received waveform y(t) (Munk et al, 1995). Pulse compression proceeds by cross-correlating the received signal y(t) with the emitted signal s(t) which, for random signals (due to the noise n(t)), is defined as…”

“…It is well known (Medwin and Clay, 1998) that the concentration and size of the bubbles affect sound propagation to a much greater extent than dissolved O 2 . Hermand et al (2000) and Hermand (2004) showed that signatures in low frequency acoustic signals (b16 kHz) transmitted (horizontally) through Posidonia oceanica meadows were highly correlated with the O 2 produced by the plants during photosynthesis.…”

“…In general, acoustic signals propagating through seawater are sensitive to seabed proprieties (Medwin and Clay, 1998). The influence of seagrass beds in the statistical distribution of scattered amplitude of high frequency signals (N20 kHz) were investigated by Lyons and Abraham (1999) and McCarthy and Sabol (2000).…”