Ocean Variability & Acoustic Propagation Ocean Variability & Acoustic Propagation edited by John Potter and Alex Warn-Varnas SACLANT Undersea Research Centre, La Spezia, Italy sponsored by North Atlantic Treaty Organization US Navy Office of Naval Research Centro Ricerche Energia Ambiente (ENEA) SPRINGER SCIENCE+BUSINESS MEDIA, B.V. Proceedings of the Wor1<shop on Ocean Variability & Acoustic Propagation La Spezia, ltaly June 4-8, 1990 Ubrary of Congress Cataloglng-In-Publleatlon Data Ocun varlablllty 4 aCOUSlIC propagatlen I eOlted by John POtUl" amI Alu Harn-Varnas , sponsoree by Nertt. AtlantIC Truty Organlzatlon. US Navy Offlce of Naval Res!arch. ~8ntro rlcerche energIa a.blente (ENeAI. p. e •. P~ers fro •• workshop. Incluoes lnae~es. ISBN 978-94-010-5462-1 ISBN 978-94-011-3321-8 (eBook) DOI 10.1007/978-94-011-3321-8 1. Unceruter aecu$tles. 2. Oceanogr.phy. 1. Petter, John. II. Warn-Varnas, Alu. III. Nerth AtlantiC Treaty OrganIzat Ion. IV. Unttec States. OHlei! of Naval Researeh. V. Cent ro rlcerehi! energIa ublent! Sanu Terna. VI. Tltle: Ocun varlablllgy and lIcoust le preDag't ler. . OC2<l4 , 0311 1991 534' .23--oc20 90-25672 ISBN 976-94-010-5462-1 Printed on acid-free papef AU Rights Reserved IZ> 1991 Splinger Science+Business Media Dordrecht Orlglnally publlahed byKluwer Academic Publishers In 1991 Softcover reprint of the hardcover 15t edition 1991 No pari of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photo copylng, recording or by any information storage and retrieval system. without written permission from the copyright owner. TABLE OF CONTENTS PREFACE ix SESSION 1 Experimental results relating acoustic and oceanographic variability Deterministic-stochastic oceanographic descriptions for ocean-acoustic experiments 3 M.G. Briscoe Experimental ocean acoustic field moments versus predictions 23 IE. Ewart and S.A. Reynolds Two dimensional acoustical propagation in a stratified shear flow 41 D.M. Farmer and D. DiIorio The effects of sound speed on the shape of the ocean impulse response 57 R.L. Field and M.K. Broadhead The effect of seasonal temperature fluctuations in the water column on sediment 69 compressional wave speed profiles in shallow water S.D. Rajan and G. V. Frisk SLlCE89: A single slice tomography experiment 81 B.M. Howe. J.A. Mercer, R.C. Spindel. P.F. Worcester, J.A. Hildebrand. W.S. Hodgkiss Jr, IF. Duda and S.M. Flatte Marginal Ice Zone oceanographic variability and its effects on acoustic propagation 87 R. W. Meredith and P.M. Jackson Internal wave induced fluctuations in the oceanic density and sound speed fields 103 R. Pinke/ and J. T. Sherman Gyre-scale reciprocal acoustic transmissions 119 P.F. Worcester, B. Dushaw and B.M. Howe Summary of Session 1 135 M.G. Briscoe and IE. Ewart SESSION 2 137 Wave motion and finestructure affecting acoustic propagation: small- scale variability Chaos in underwater acoustics 139 M.G. Brown. F.D. Tappert. G.J. Goni and K.B. Smith Impulse response analysis of ocean acoustic propagation 161 S.M. Flatte. J. Colosi. T.F. Duda and G. Rovner Dependence of near-surface acoustic scatter on wind speed 173 B.E. McDonald Nonlinear effects in wind-wave generation 187 R.H. Me//en v vi Multichannel acoustic reflection profiling of ocean watermass temperature/salinity 199 interfaces J.D. Phillips and D.F. Dean Acoustic variability due to intemal waves and surface waves in shallow water 215 D. Rubenstein and M.H. Brill Observations of ocean inhomogeneities 229 J. S6l1schopp The problem of creating a synthetic aperture in a non-isotropic ocean 237 S. Stergiopou/os Prediction of coastal ocean thermal variability 251 D. Wang Summary of Session 2 261 S.M. Ratte and W.H. Munk SESSION 5 263 Stochastic modelling In oceanography and acoustics Treatments of incoherent scattering for the parabolic equation and ASTRAL propagation 265 models L.B. Dozier, J.S. Hanna and C.R. Pearson Average sound intensities in randomly varying sound-speed structures 283 H.G. Schneider Stochastic ray tracing in thermoclines 293 J. Set/schopp Modeling of sound propagation in a randomly varying ocean by stochastic mode coupling 313 R. Thiele Summary of Session 5 323 J. McCoy and H.G. Schneider SESSION 3 325 Range-dependent acoustic propagation caused by fronts and eddies: mesoscale vartablllty Radar altimetry and acoustic prediction 327 C. Boissier and H. Bouxin A range-dependent analysis of acoustic transmission across a cold filament in the 343 Califomia current L.M. Jendro, R.H. Bourke and S.R. Ramp Acoustic effects of the Iceland-Faeroe front 359 F.B. Jensen, G. Dreini and M. Prior Deep hydrographic fluctUations in the north-east Atlantic Mediterranean outflow: influence 375 on acoustic propagation J.M. Darras, R. Laval and F.R. Martin-Lauzer vii Aspects of oceanographic variability observed from thermistor chains on free-drifting 391 buoys P.J. Minnett and IS. Hopkins Theoretical determination of the fractal dimension of fluid parcel trajectories in large and 407 meso-scale flows A.R. Osborne and R. Caponio How do eddies modify the stratification of the thermocline? 417 J. I Allen, R. I Pollard and A.L. New Three-dImensional oceanography and acoustics 433 w.A. Kuperman and J.S. Perkins Frontal boundaries and eddies on the Icel~aeroes ridge 449 J.C. Scott and N.M. Lane Upper ocean variability associated with fronts 463 R.A. Weller and R.M. Samelson Summary of Session 3 479 J.R. Potter and J.C. Scott SESSION 4 483 Coupling acoustic and oceanographic models A mixed-layer model for predicting the acoustic structure of shallow seas 485 A.J. Elliot and Z. U The use of coupled ocean-acoustic models in the design of naval forecast systems 501 A.D. Heathershaw, C.E. Mooney and S.J. Maskell The Environmental Acoustic Tactical Support System: low frequency mesoscale ocean 517 feature environmental acoustic results G.A. Kerr, D.B. King, G.P. Cloy, B.R. Gomes and P.J. Bucca Environmental focusing and source localization in the ocean 527 M.D. Collins and w.A. Kuperman Refraction of acoustic modes in very long-range transmissions 539 W.H. Munk Environmental sensitivity studies with an interfaced ocean-acoustics system 545 A.R. Robinson, S.M. Glenn, w.L. Siegmann, D. Lee and G. Botseas Simulating temperature, salinity and currents in the ocean 561 K.D. Saunders and D.B. King A numerical investigation of semi-diumal fluctuations in acoustic intensity at a shelf edge 579 IJ. Sherwin Summary of Session 4 593 A.D. Heathershawand w.A. Kuperman Ust of Participants 597 Author Index 601 Subject Index 603 c::: <::) .2 C) C) et- Cts "Y- O) Q) Cts c:: l :::J ~ co I .0 -+:: .~..... U) ::J . 0 i::: 0 0 Q) --.J ~ ~ .0 ...... :-:::: -0 .Cts i::: ~ c::: cu Q) 0 0 viii PREFACE Fifteen years ago NATO organised a conference entitled 'Ocean Acoustic Modelling'. Many of its participants were again present at this variability workshop. One such participant. in concluding his 1975 paper, quoted the following from a 1972 literature survey: ' ... history presents a sad lack of communications between acousticians and oceanographers' Have we done any better in the last 15 years? We believe so, but only moderately. There is still a massive underdeveloped potential for acousticians and oceanographers to make significant progress together. Currently, the two camps talk together insufficiently even to avoid simple misun derstandings. such as those in Table 1. Table 1 Ocsanographic and acoustic jargon (from an idea by Pol/ardi Jargon Oceanographic use Acoustic use dbordB decibar (depth in m) decibel (energy level) PE primitive equations parabolic equations convergence zone converging currents converging rays (downwelling water) (high energy density) front thermohaline front wave, ray or time front speed water current speed sound propagation speed 1 The list goes on. The time-varying aspect of the ocean makes descriptive oceanography a 4-dimenslonal problem, with spatial scales from 107 to 10-3 m, energy cascading ever-downward over 10 orders of mag nitude. In-situ senSing alone is ultimately unable to provide a sufficient description. The ocean is fundamentally opaque to electromagnetic radiation, but this limitation on information transfer is largely compensated by its ability to propagate compressional energy over thousands of kilome ters. Sound, then, is the only natural communication mechanism in the ocean, be It for geophysical surveying, whale song or remote sensing of ocean structures. Conversely, the acoustician needs to know the ocean structure if he wishes to correctly predict an acoustic field, a 7- dimensional problem. The futures of oceanography and acoustics are thus inextricably bound together. It Is surprising, then, to find that acousticians and oceanographers have not felt each other's Interests more per tinent. The confusion in terminology Is a trivial problem to resolve, the lack of mutual Interest that gives rise to it remains a serious obstacle. Consider the most basic acoustic problem, that of predicting an acoustic field. To what spatial resolution do the acousticians need to know the ocean for modelling? This is a question often asked by oceanographers of acousticians, who can rarely agree on an answer. We believe that the question Is fundamentally unanswerable by acousticians. Acoustic energy within the body of the ocean is influenced only by gradients in the local refractive Index. The acoustician should give an answer which is limited to the acoustic parameter space. He might say, for example, that he needs an ocean data point every time the refractive index changes by 0.02%. How this Is mapped onto the physical space of the oceanographer depends on whether we are mapping a thermocline, abyssal ix x water, front or whatever. This question can only be answered by the oceanographer. Table 2 gives our idea of order-of-magnitude values forthe acoustic space, oceanographic space and the resulting required oceanographic sampling resolution (in metres) in the vertical and horizontal directions. Table 2 Required OCfNUlO9raphlc measur9t7lent resolution for acoustic modelling Oceanographic variability scale mixed region eddy front Acoustic scale [0(102) km] [0(101) km] [0(10°) km] surface deep surface deep surface deep long-range hor. 104 105 103 103 200 200 [0(103) km range, vert. SO SOO 15 15 15 25 0(101) Hz freq.] medium-range hor. 103 104 400 400 100 100 [0(102) km range, vert. 5 100 2 5 2 5 0(102) Hz freq.] short-range hor. 2SO 103 200 200 SO SO [0(101) km range, vert. 25 1 2 0.5 0.5 0(103) Hz freq.] Once we decide how to sample our acoustic and oceanographic fields, how are we to connect them? Recently, some success has been achieved in using oceanographic forecasting and nowcastlng as Input to acoustic models. This is very fine, but not a true collaboration because the process is serial. First the oceanographers do their work, independently of the acousticians. Then the acousticians do theirs, with no reference to how the oceanographers were able to provide the input, only that It exists. The reverse serial process would be that in which acoustic observations are used to calculate the oceanographic physical structure (tomography). The oceanographers are then let loose on the dynamic problems to explain these structures. A more satisfactory unification would be to work acoustically and oceanographically in parallel. This may be achieved In a number of ways, perhaps through simulated annealing; slowly 'coollng' the freedom of parameter changes In the oceanographic and acoustic parameter space simultaneously. The solution provides not only the source location but also the range-dependence of the Intervening medium. If there Is one issue to pound home the need to work properly together, it is the treatment of de terministic versus stochastic descriptions. In the ocean, larger scale features tend to determine the evolution of smaller scales, by entroplc disordering of the available energy. In general, small scales do not play a strong role In governing the behaviour of larger features. This assumption is also commonly applied to relate the ocean to acoustics. The ocean environment is split into a deterministic (large-scale) component plus a stochastic (small-scale) residue. Firstly, the classifi cation of deterministic versus stochastic may have nothing to do with scale, so we are in Immediate trouble. Secondly, the line (in the spatial frequency domain) is drawn arbitrarily; usually a choice Is forced onto the acoustician by the limited resolution of the ocean data. Acoustic models are run on xi the 'deterministic' component to obtain the mean acoustic field. The 'stochastic' component is then thought of as super-imposing a variability. This is equivalent to assuming a monotonic mapping of scales from the ocean to the acoustic field, with no cross-coupling. Clearly this is technically wrong but it has often been hopefully assumed that, apart from some special cases, the approximation is not too bad, i.e. the cross-coupling is weak and local. One of the products of this workshop was that where ocean variability is strong (almost everywhere), the coupling extends from the small (ocean scale) to the large (mean acoustic field). The mapping of oceanography into acoustics is strongly non-linear. It is time to abandon the general use of the separable assumption. Further, we must come to terms with what is truly deterministic in the ocean and what must be treated statistically, or in some other (as yet) undeveloped way. The five-day workshop comprised one session per day, with the intention of rubbing a roughly equal number of oceanographers and acousticians together over a mixed bag of presentations. A prerequisite was that all participants should also be authors of a paper. This helped to engender an active atmosphere, in which everyone felt truly involved. The allocation of papers to sessions was kept rather arbitrary, to inhibit participants from favouring particular sessions over others of equal importance. Thus they were unable to exercise preconceptions about which presentations were most pertinent to their particular interest. Over half each working day was devoted to discussion, either immediately following each presentation, or during the end-of-day 'brain-storming' period, steered by the session chairmen. Their impressions are summarised in the closing articles to be found in this book at the end of each session. For this reason the sessions and papers are published in the order in which they were presented at the workshop. To help the reader find his way around the book, both author and subject indices are included at the back. We hope that the reader will find the contents of this book informative, interesting and fun. For whatever good things come of this, the participants are to be credited; not only for their written contributions but also for their enthusiasm and creativity which made it all work. The session chairmen deserve special recognition, for theirs was the hardest job. We are most grateful to the Director and staff of ENEA, who allowed us to use their laboratory and facilities for the workshop. Finally, to our workshop secretaries - Anna Bizzarri, Caroline Durville and Jeanne van den Beuken - go our respect and thanks for having taken such good care of us throughout the week, always with suave efficiency and a smile. John Potter & Alex Wam-Vamas.