Simulation313

12.1 Background 313

12.2 Hierarchical levels 314

12.2.1 Engineering 315

12.2.2 Engagement 315

12.2.3 Mission 316

12.2.4 Theater 316

12.3 Simulation infrastructure 316

12.4 High-level architecture 317

12.5 Testbeds 318

12.6 Applications 319

12.6.1 Systems engineering 320

12.6.2 Simulation-based acquisition 320

12.6.3 Operations analysis 325

12.6.4 Training 326

Appendix A Abbreviations and acronyms 328

Appendix B Glossary of terms 342

Appendix C Websites 351

References 354

Author index 403

Subject index 415

Broadly defined, modeling is a method for organizing knowledge accumulated through observation or deduced from underlying principles while simulation refers to a method for implementing a model over time. The field of underwater acoustic modeling and simulation translates our physical understanding of sound in the sea into mathematical models that can simulate the performance of complex acoustic systems operating in the undersea environment.

This book discusses the fundamental processes involved in simulating underwater-acoustic systems and emphasizes the importance of applying the proper modeling resources to simulate the behavior of sound in virtual ocean environments. Summary tables identify available propagation, noise, reverberation and sonar performance models. Guidelines for selecting and using these various models are highlighted. Specific examples of each type of model are discussed to illustrate model formulations, assumptions and algorithm efficiency. Instructive case studies in simulation are reviewed to demonstrate practical applications.

Over the past decade, rapid changes in the world situation have opened new avenues for international collaboration in modeling and simulation. Concurrent advances in electronic communications have greatly facilitated the transfer of modeling and simulation technologies among members of the international community. The Internet now provides unprecedented access to models and databases around the world. Where appropriate, references to pertinent websites are incorporated in this edition.

The level of technical detail presented in this book is appropriate for a broad spectrum of practitioners and students in sonar technology, acoustical oceanography, naval operations analysis, systems engineering and applied mathematics. The material is organized into 12 chapters. The order of presentation of the first ten chapters follows the structure suggested by a hierarchical method of sonar model construction. Chapter 1 introduces the types of underwater acoustic models, provides a framework for the consistent classification of modeling techniques and defines the terminology common to modeling and simulation. Aspects of oceanography essential to an understanding of acoustic phenomena are presented in Chapter 2.

Chapters 3 through 9 address the observations and models dealing with propagation, noise and reverberation in the sea. In Chapter 10, the information from Chapters 3-9 is integrated into sonar performance models. Chapter 11 describes the process of model evaluation. Chapter 12, which is new to this edition, discusses the application of simulation in underwater acoustics. Since simulation is a method for implementing a model over time, it is fitting that this topic is addressed only after a firm foundation of modeling and evaluation has been established. The title of this edition has been changed to Underwater Acoustic Modeling and Simulation to reflect the inclusion of material on simulation.

Rather than purging older material from the third edition, I have intentionally retained descriptions of earlier developments (including the older models) to provide a historical account of the progress that has been achieved over the cumulative period of record covered by these three editions. I trust that this new edition will continue to serve as a useful source of information for all those engaged in modeling and simulation in underwater acoustics.

Paul C. Etter Rockville, Maryland USA

The subject of underwater acoustic modeling deals with the translation of our physical understanding of sound in the sea into mathematical formulae solvable by computers. This book divides the subject of underwater acoustic modeling into three fundamental aspects: the physical principles used to formulate underwater acoustic models; the mathematical techniques used to translate these principles into computer models and modeling applications in sonar technology and oceanographic research.

The material presented here emphasizes aspects of the ocean as an acoustic medium. It shows mathematicians and physical scientists how to use this information to model the behavior of sound in a spatially complex and temporally variable ocean. This approach diminishes the need for discussions of engineering issues such as transducers, arrays and targets. Aspects of hardware design and modeling in underwater acoustics are discussed in other excellent texts.

Recent developments in underwater acoustic modeling have been influenced by changes in global geopolitics. These changes are evidenced by strategic shifts in military priorities as well as by efforts to transfer defense technologies to non-defense applications.

The strategic shift in emphasis from deep-water to shallow-water naval operations has focused attention on improving sonar performance in coastal regions. These near-shore regions, which are sometimes referred to as the littoral zone, are characterized by complicated and highly variable acoustic environments. Such difficult environments challenge the abilities of those sonar models intended for use in deep-water scenarios. This situation has prompted further development of underwater acoustic models suitable for forecasting and analyzing sonar performance in shallow-water areas.

The policy of defense conversion has encouraged the transfer of sonar modeling technology to non-defense applications. Much of this transfer has benefited the growing field of environmental acoustics, which seeks to expand exploration of the oceans through acoustic sensing. Such technology conversion is exemplified by the utilization of naval underwater acoustic models as both prognostic and diagnostic tools in sophisticated experiments employing inverse acoustic sensing of the oceans.

xviii Preface to the second edition

These rapid developments in modeling have created a need for a second edition. The intent is to update recent advances in underwater acoustic modeling and to emphasize new applications in oceanographic research. This edition also reflects a broader international interest in the development and application of underwater acoustic models. The coming years promise to be challenging in terms of defining research directions, whether for defense or industry, and this edition should provide technology planners with a useful baseline.

The original organization of material into 11 chapters has served well and therefore remains unchanged. When required, new material has been arranged into additional subsections.

Comments from users of the first edition have evidenced appeal from acousticians, as well as oceanographers, who have enthusiastically endorsed this book as both a practical tool and an instructional aid. In this latter regard, several academic institutions have utilized this book as an adjunct text for graduate-level courses in applied mathematics and ocean sciences.

This edition has benefited from a continuation of my short courses which, since 1993, have been offered through the Applied Technology Institute of Clarksville, Maryland (USA). Continued exposure to the insightful questions posed by my students has provided me with the opportunity to further refine my presentation.

Despite the appearance of several new books in the field of ocean acoustics, this book remains unique in its treatment and coverage of underwater acoustic modeling. It is a pleasure to note that the first edition has been recognized as an authoritative compendium of state-of-the-art models and is often cited as the standard reference.

Paul C. Etter Rockville, Maryland USA

The subject of underwater acoustic modeling deals with the translation of our physical understanding of sound in the sea into mathematical formulae solvable by computers. These models are useful in a variety of research and operational applications including undersea defense and marine seismology. There has been a phenomenal growth in both the number and types of models developed over the past several decades. This growth reflects the widespread use of models for the solution of practical problems as well as the considerable advances made in our computational abilities.

The primary motivation for the development of underwater acoustic models is defense related. Researchers involved in Anti-Submarine Warfare (ASW) and associated undersea defense disciplines use models to interpret and forecast acoustic conditions in the sea in support of sonar design and sonar operation. Consequently, the emphasis in this book is placed on those models that are particularly useful in solving sonar performance problems.

Users and potential users of models are commonly ill acquainted with model formulations. As a result, the capabilities and limitations of the models are poorly understood and the models are often improperly used. Moreover, the sheer number of available models complicates the process of model selection.

This book is intended for those who have a fundamental understanding of underwater acoustics but who are not familiar with the various aspects of modeling. Sufficient mathematical derivations are included to demonstrate model formulations, and guidelines are provided to assist in the selection and proper application of these models. Comprehensive summaries identify the available models and associated documentation.

The material is organized into 11 chapters. The order of presentation follows the structure suggested by a hierarchical method of sonar model construction. Chapter 1 introduces the types of underwater acoustic models, provides a framework for the consistent classification of modeling techniques and defines the terminology common to modeling work. Aspects of oceanography essential to an understanding of acoustic phenomena are presented in Chapter 2. Chapters 3 through 9 address the observations and models dealing with propagation, noise and reverberation in the sea. In Chapter 10, the information from Chapters 3-9 is integrated into sonar performance models. Finally, Chapter 11 describes the process of model evaluation.

Since 1982, I have developed and taught a series of intensive short courses for the Technology Service Corporation of Silver Spring, Maryland (USA). Earlier versions of this course were taught in collaboration with Professor Robert J. Urick of the Catholic University of America. Professor Urick would discuss underwater acoustic measurements while I would review the related modeling techniques. As the course evolved into one in which I became the sole instructor, I borrowed heavily from Professor Urick's several books (with permission) in order to preserve the continuity of the course material. The success of this course encouraged me to publish my class notes as a book.

Many notable books have been published in the field of underwater acoustics. None, however, has dealt exclusively with modern developments in modeling, although some have addressed aspects of propagation modeling. This book is unique in that it treats the entire spectrum of underwater acoustic modeling including environmental, propagation, noise, reverberation and sonar performance models.

I have intentionally preserved the notation, terminology and formalism used by those researchers whose work I have cited. I have also intentionally emphasized aspects of oceanography since my experience has indicated that many acousticians have little appreciation for the complex role played by the ocean as an acoustic medium. Conversely, oceanographers frequently fail to appreciate the great potential of underwater acoustics as a remote sensing technique.

Paul C. Etter

The students who have attended my short courses over the past 20 years have provided both a receptive and critical audience for much of the material contained in this book. Many of my colleagues have provided useful insights and suggestions. In particular, I want to recognize Dr Michael

A. Ainslie, Dr Aubrey L. Anderson, Dr Stanley A. Chin-Bing, Dr Richard

B. Evans, Dr Robert W. Farwell, Dr Richard P. Flanagan, Dr Robert L. Martin1, Dr Peter M. Ogden, Dr Frederick D. Tappert and Dr Henry Weinberg. Robert S. Winokur provided administrative guidance in the early stages of my work in underwater acoustic modeling.

Professor Robert J. Urick1 provided much encouragement and graciously allowed me to liberally borrow material from his several books. Professor John D. Cochrane1 of Texas A&M University inspired the scholarly discipline that facilitated creation of this book.

1 Deceased.

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