This book is a research monograph on high-Frequency Seafloor Acoustics. It is the first book in a new series sponsored by the Office of Naval Research on the latest research in underwater acoustics. It provides a critical evaluation of the data and models pertaining to high-frequency acoustic interaction with the seafloor, which will be of interest to researchers in underwater acoustics and to developers of sonars. Models and data are presented so as to be readily usable, backed up by extensive explanation. Much of the data is new, and the discussion in on two levels: concise descriptions in the main text backed up by extensive technical appendices.
In its relentless pursuit of further knowledge, science tends to compartmentalize. Over the years the pursuit of What might be called geophysical acoustics of the sea-surface has languished. This has occured even through there are well-developed and active research programs in underwater acoustics, ocean hydrodynamics, cloud and precipitation physics, and ice mechanics - to name a few - as well as a history of engineering expertise built on these scientific fields. It remained to create a convergence, a dialogue across disciplines, of mutual benefit. The central theme of the Lerici workshop, perhaps overly simplified, was 'What are the mechanisms causing ambient noise at the upper surface of the ocean?' What could hydrodynamicists contribute to a better understanding of breaking wave dynamics, bubble production, ocean wave dynamics, or near-surface turbulence for the benefit of the underwater acoustics community? What further insights could fluid dynamicists gain by including acoustic measurements in their repertoire of instrumentation? While every attendee will have his or her percep tions of details, it was universally agreed that a valuable step had been taken to bring together two mature disciplines and that significant co-operative studies would undoubtedly follow. The scope of the workshop was enlarged beyond its original intent to also include the question of ice-noise generation. The success of this decision can be seen in high quality of the presentations. the contribution of its disciples in the other workshop discussions and the heightened awareness and interest of we other novices.
The characterization of high frequency, shallow water acoustic backscatter and reverberation is important because acoustic systems are used in many scientific, commercial, and military applications. The approach taken is to use data collected by the Toroidal Volume Search Sonar (TVSS), a 68 kHz multibeam sonar capable of 360 deg imaging in a vertical plane perpendicular to its direction of travel. With this unique capability, acoustic backscatter imagery of the seafloor, sea surface, and horizontal and vertical planes in the volume is constructed from data obtained in 200 m deep waters in the Northeastern Gulf of Mexico when the TVSS was towed 78 m below the surface, 735 m astern of a towship. The processed imagery provides a quasi-synoptic characterization of the spatial and temporal structure of boundary and volume acoustic backscatter and reverberation. Diffraction, element patterns, and high sidelobe levels are shown to be the most serious problems affecting cylindrical arrays such as the TVSS, and an amplitude shading method is presented for reducing the peak sidelobe levels of irregular-line and non-coplanar arrays. Errors in the towfish's attitude and motion sensor, and irregularities in the TVSS's transmitted beampattern produce artifacts in the TVSS-derived bathymetry and seafloor acoustic backscatter imagery. Correction strategies for these problems are described, which are unique in that they use environmental information extracted from both ocean boundaries. Sea surface and volume acoustic backscatter imagery is used to explore and characterize the structure of near-surface bubble clouds, schooling fish, and zooplankton. The simultaneous horizontal and vertical coverage provided by the TVSS is shown to be a primary advantage, motivating further use of multibeam sonars in these applications.
During the past decade there has been a renewed interest in active sonar systems at both low and medium frequencies. More recently this interest has been extended to very high frequencies in shallow water. Reverberation often limits the detection performance of these systems, and there is a need to understand the underlying mechanisms that cause the scattering. With more emphasis being given to reverberation phenomena in the Scientific Program of Work at the SACLANT Undersea Research Centre, it was considered an opportune time to host a meeting, bringing together scientists from NATO countries to foster cross-disciplinary dialogue and generate ideas for new research directions. Consequently the Ocean Reverberation Symposium was held 25-29 May 1992 in La Spezia, Italy. Over 60 presentations were made on a diverse selection of topics, of which ten papers will be published as a SACLANTCEN Conference Proceedings. The papers in this volume are grouped into 8 sections, usually in the same order as presented at the corresponding session of the Symposium: Section 1 - Scattering Mechanisms Section 2 - High Frequency Measurements and Mechanisms Section 3 - Reverberation Modelling Section 4 - ARSRP Mid-Atlantic Ridge Experiment Section 5 - Low Frequency Measurements Section 6 - Volume Scattering Section 7 - Signal Processing Issues Section 8 - Applications Taken together the papers show some emerging trends in the research.
The Acoustic Bubble describes the interaction of acoustic fields with bubbles in liquid. The book consists of five chapters. Chapter 1 provides a basic introduction to acoustics, including some of the more esoteric phenomena that can be seen when high-frequency high-intensity underwater sound is employed. Chapter 2 discusses the nucleation of cavitation and basic fluid dynamics, while Chapter 3 draws together the acoustics and bubble dynamics to discuss the free oscillation of a bubble and acoustic emissions from such activity. The acoustic probes that are often applied to study the behavior of a bubble when an externally-applied acoustic field drives it into oscillation is deliberated in Chapter 4. The last chapter outlines a variety of effects associated with acoustically-induced bubble activity. The bubble detection, sonoluminescence, sonochemistry, and pulse enhancement are also covered. This publication is a good reference for physics and engineering students and researchers intending to acquire knowledge of the acoustic interactions of acoustic fields with bubbles.
Presented in a clear and concise way as an introductory text and practical handbook, the book provides the basic physical phenomena governing underwater acoustical waves, propagation, reflection, target backscattering and noise. It covers the general features of sonar systems, transducers and arrays, signal processing and performance evaluation. It provides an overview of today's applications, presenting the working principles of the various systems. From the reviews: "Presented in a clear and concise way as an introductory text and practical handbook, the book provides the basic physical phenomena governing underwater acoustical waves, propagation, reflection, target backscattering and noise. ⦠It provides an overview of todayâs applications, presenting the working principles of the various systems." (Oceanis, Vol. 27 (3-4), 2003) "This book is a general survey of Underwater Acoustics, intended to make the subject âas easily accessible as possible, with a clear emphasis on applications.â In this the author has succeeded, with a wide variety of subjects presented with minimal derivation ⦠. There is an emphasis on technology and on intuitive physical explanation ⦠." (Darrell R. Jackson, Journal of the Acoustic Society of America, Vol. 115 (2), February, 2004) "This is an exciting new scientific publication. It is timely and welcome ⦠. Furthermore, it is up to date and readable. It is well researched, excellently published and ranks with earlier books in this discipline ⦠. Many persons in the marine science field including acousticians, hydrographers, oceanographers, fisheries scientists, engineers, educators, students ⦠and equipment manufacturers will benefit greatly by reading all or part of this text. The author is to be congratulated on his fine contribution ⦠." (Stephen B. MacPhee, International Hydrographic Review, Vol. 4 (2), 2003)
