OCEAN SEISMO-ACOUSTICS Low-Frequency Underwater Acoustics NATO CONFERENCE SERIES I Ecology II Systems Science III Human Factors IV Marine Sciences V Air-Sea Interactions VI Materials Science IV MARINE SCIENCES Recent volumes in this series Volume 8 Structure and Development of the Greenland-Scotland Ridge edited by Martin H. P. Batt, Svend Saxov, Manik Talwani, and Jarn Thiede Volume 9 Trace Metals in Sea Water edited by C. S. Wong, Edward Boyle, Kenneth W. Bruland, J. D. Burton, and Edward D. Goldberg Volume 10A Coastal Upwelling: Its Sediment Record Responses of the Sedimentary Regime to Present Coastal Upwelling edited by Erwin Suess and Jorn Thiede Volume 10B Coastal Upwelling: Its Sediment Record Sedimentary Records of Ancient Coastal Upwelling edited by Jarn Thiede and Erwin Suess Volume 11 Coastal Oceanography edited by Herman G. Gade, Anton Edwards, and Harald Svendsen Volume 12 Hydrothermal Processes at Seafloor Spreading Centers edited by Peter A. Rona, Kurt Bostrom, Lucien Laubier, and Kenneth L. Smith, Jr. . Volume 13 Flows of Energy and Materials in Marine Ecosystems: Theory and Practice edited by M. J. R. Fasham Volume 14 Mechanisms of Migration in Fishes edited by James D. McCleave, Geoffrey P. Arnold, Julian J. Dodson, and William H. Neill Volume 15 Heterotrophic Activity in the Sea edited by John E. Hobbie and Peter J. IeB. Williams Volume 16 Ocean Seismo-Acoustics: Low-Frequency Underwater Acoustics edited by Tuncay Akal and Jonathan M. Berkson OCEAN SEISMO-ACOUSTICS Low-Frequency Underwater Acoustics Edited by Tuncay Akal and Jonathan M. Berkson SACLANT ASW Research Center La Spezia, Italy Published in cooperation with NATO Scientific Affairs Division PLENUM PRESS· NEW YORK AND LONDON Library of Congress Cataloging in Publication Data Ocean seismo-acoustics. (NATO conference series. IV, Marine sciences; v. 16) Proceedings of the Symposium on Ocean Seismo-Acoustics, organized by SACLANT ASW Research Centre, held June 10-14, 1985, at La Spezia, Italy. Includes bibliographies and indexes. 1. Underwater acoustics-Congresses. 2. Ocean bottom-Congresses. I. Akal, Tuncay. II. Berkson, Jonathan M. III. Symposium on Ocean Seismo-Acoustics (1985: La Spezia, Italy) IV. SACLANT ASW Research Center. V. Series. QC242.024 1986 551.46/01 86-4940 ISBN 978-1-4612-9293-7 ISBN 978-1-4613-2201-6 (eBook) DOl 10.1007/978-1-4613-2201-6 Proceedings of a SACLANT ASW Research Center symposium, held June 10-14, 1985, at San Terenzo di Lerici, La Spezia, Italy © 1986 Plenum Press, New York Softcover reprint of the hardcover 1st edition 1986 A Division of Plenum Publishing Corporation 233 Spring Street, New York, N.Y. 10013 All rights reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from the Publisher FOREWORD Seafloor investigation has long been a feature of not only seismology but also of acoustics. Indeed it was acoustics that produced depth sounders, giving us the first capability of producing both global and local maps of the seafloor. Subsequently, better instrumentation and techniques led to a clearer, more quantitative picture of the seabed itself, which stimulated new hypotheses such as seafloor spreading through the availability of more reliable data on sediment thickness over ocean basins and other bottom features. Geologists and geophysicists have used both acoustic and seismic methods to study the seabed by considering the propagation of signals arising from both natural seismic events and man-made impulsive sources. Although significant advances have been made in instrumentation, such as long towed geophysical arrays, ai r guns and ocean bot tom seismometers, the pic ture of the seafloor is still far from complete. Underwater acoustics concerns itself today with the phenomena of propagation and noise at frequencies and ranges that require an understanding of acoustic interaction at both of its boundaries, the sea surface and seafloor, over depths ranging from tens to thousands of meters. Much of the earlier higher frequency (>1 kHz) work included the characterization of the seafloor in regimes of reflection coefficients which were empirically derived from surveys. The results of these studies met with only limited success, confined as they were to those areas where survey data existed and lacking a physical understanding of the processes of reflection and scattering. The development in recent years of systematic methods of measuring the physical and acoustical characteristics of the seabed have led to a much better understanding of propagation. However these techniques are exceedingly slow and expensive and do not fulfill the needs of present day acoustics. The relatively recent recognition of the importance of the role that shear and evanescent waves in the seabed have on water borne sound has been a major factor in the recognition of the need for acousticians and seismologists to exchange and discuss their common goals and developments. The realization of the benefits that could be achieved from such an exchange led the Centre to organize the Symposium on Ocean Seismo-Acoustics. In their entirety these papers give both the acoustician and the geophysicist a good picture of the other's interests, approaches and techniques. The authors represent a wide span of interests and institutions and, therefore, many different points of view. v That the Symposium on Ocean Seismo-Acoustics was organized and hosted by the SACLANT ASW Research Centre is fitting for two reasons. Firstly, one of the Centre's key missions is to promote the exchange of scientific and engineering information for the common benefit and understanding of the nations of the NATO community. Secondly, the Centre, throughout its twenty-six years as a research institution, has actively and significantly contributed to seafloor acoustics. I have little doubt therefore that this volume will serve experienced scientists as' well as those who are contemplating future research in the difficult environment of the sea. Ralph R. Goodman Director SACLANT ASW Research Centre vi PREFACE Because of the ease with which sound can be transmitted in seawater, acoustic techniques have provided the widest range of tools for accumu lating knowledge of the environment below the ocean surface. Consequently, the fields of underwater acoustics and marine seismology have both used sound waves for research purposes. Traditionally, the marine seismology communi ty has used earthborne propagation of elastic waves to study the sub-oceanic solid earth; the underwater acoustics community on the other hand has concentrated on studying waterborne, compressional-wave propaga tion phenomena in oceanic waters and in shallow sub-bottoms. In recent years, the research interests of these two communities have come closer together; in some areas they have overlapped. Research in underwater acoustics has been extended to the lower part of the frequency spectrum (one to a few hundred hertz), and thus overlaps the spectral domain of the seismologists. Additionally, studies of propagation of earthborne energy into the water column, and of shear-waves, interface waves, and Love waves, have increased. Coincidentally, research in marine seismology has extended to higher frequencies and to the ve10ci ty-depth structure of the uppermost part of the seafloor, overlapping the domain of underwater acousticians. These trends have been driven by the need to understand both water column and sub-bottom phenomena and by advances in technology, particularly the development of ocean bottom seismometers and multi-sensor hydrophone arrays. The overlapping interests have therefore stimulated increased activity that has benefitted both disciplines. The Symposium on Ocean Seismo-Acoustics, sponsored by SACLANT ASW Research Centre, with partial support from the Office of Naval Research, London, was held at La Spezia, Italy, June 10-14, 1985. Its purpose was to enhance cross-disciplinary dialogue and generate ideas for new research directions. This volume contains papers originating from the Symposium and includes several additional features such as subject and author indexes. The papers are organized into eight topics: Propagation: Theoretical Developments Propagation: Modelling and Experiments Propagation: Fluctuations Boundary Scattering Sediments: Seismo-Acoustic Waves and Geoacoustic Characteristics Seismo-Acoustic Waves: Propagation, Attenuation, and Inverse Techniques Seismo-Acoustic Noise Technology and New Approaches in Seismo-Acoustic Measurements Categorization of papers covering a diversity of topics is at best inexact and ofttimes subjective. Many of -the papers were broad enough in vii scope to fit easily into categories other than the one chosen, and in this respect alone the subject and author indexes should prove valuable. This volume contains 75 research papers and 8 review papers by authors from many nations. The papers cover diverse areas of ocean seismo acoustics including theory, instrumentation, signal processing, experimen tal techniques, and experimental results. The number of quoted references is over 1000, an indication of the diversity of topics and a reflection of the fact that the papers might normally be scattered among many acoustical, seismic, geological, and geophysical journals, published . in several languages. Thus, this volume presents the current status of both fields, combining papers that report recent work as well as review papers that pro vide perspective. As such it consolidates the research activities of these two fields. These characteristics should make this volume not only useful to seismo-acoustic specialists but also to other researchers whose work may be influenced by the interaction of marine seismology and underwater acoustics. In the preparation of this volume, we received assistance from a large number of individuals at SACLANTCEN; regrettably all of their names cannot be listed. However, we would particularly like to thank O. Toso for secre tarial assistance and H. Ali for suggesting improvements to several papers. We are especially grateful to R. Nekritz for assistance in preparing the volume, and to Dr. R. Goodman for his guidance and support. Tuncay Akal Jonathan M. Berkson La Spezia, Italy Oc tober, 1985 viii CONTENTS PROPAGATION: THEORETICAL DEVELOPMENTS Sound Pulse Propagation in a Weakly Range Dependent Shallow Ocean 1 E. Topuz and L.B. Felsen Intrinsic Modes in a Wedge-Shaped Ocean with Stratified Elastic Bottom 11 I.T. Lu and L.B. Felsen A Green's Function Method for One-Way Wave Propagation in a Range-Dependent Ocean Environment . . . . . . . . . . . • . . 21 K.E. Gilbert and R.B. Evans Factorization and Path Integration of the Helmholtz Equation: Numerical Algorithms 29 L. Fishman and S.C. Wales Range Dependent Propagation Codes Based on Wave Field Factorization and Invariant Imbedding ............. . 39 J.J. McCoy, L. Fishman, and L.N. Frazer An Acoustic Reflectivity Method for Laterally Varying Layered Media . . . . . . 47 L.N. Frazer and J.J. McCoy A Vector Parabolic Equation Model for Elastic Propagation 57 S.C. Wales Influence of Bottom Refraction on the Propagation of Underwater Sound 67 J. Miller, A. Nagl, and H. fiberall Bottom Interaction Effects on Normal Modes: an Algebraic Approach . 75 D.H. Wood, M.D. Duston, and G.R. Verma Gaussian Beams and 3-D Bottom Interacting Acoustic Systems •......... 87 H:P. Bucker and M.B. Porter ix Development of a Parabolic Approximation for the Computation of Propagation Loss in a Range-Dependent Environment. . . . . . . . . . . . 103 B. Grandvuillemin PROPAGATION: MODELLING AND EXPERIMENTS *Linear and Nonlinear Ocean Acoustic Propagation Models . . . . . 115 W.A. Kuperman and B.E. McDonald Modeling of Pulse Response Functions of Bottom Interacting Sound Using the Parabolic Equation Method .. 129 F.D. Tappert and L. Nghiem-Phu A Study of Propagation Loss Dependence on Sediment Layer Thickness using the Fast Field Program . . . . . . . . . . . . . 139 C.H. Harrison and P.L. Cousins Shear Wave Effects on Propagation to a Receiver in the Substrate 149 P.J. Vidmar and R.A. Koch Description and Some Results of the Operational Propagation Loss Model ALMOST (= Acoustic Loss Model for Qverational ~tudies-and !asks). 159 P. Schippers Experimental Confirmation of Horizontal Refraction of Sound Propagation in a Wedge-like Ocean ........ . 169 R. Doolittle, A. Tolstoy, and M. Buckingham Acoustic Propagation over Large-Scale Linear Ocean Slopes . . . . . . 179 D.R. DelBalzo, J.E. Matthews, J.V. Soileau, and C. Feuillade Predicted Partitioning of VLF Acoustic Energy in a Range-Dependent Environment . . . . . . . . 191 T.W. Tunnel and G.J. Tango Experiments on Sound Propagation over Sloped Bottoms ....... . 199 H. Hobaek, C.T. Tindle, and T.G. Muir The Effect of Unconsolidated Sediment Rigidity on Low Frequency Acoustic Propagation . . . . . . . . . . . . 207 J.H. Beebe and C.W. Holland * Indicates Review Paper x