Experimental Acoustic Inversion Methods for Exploration of the Shallow Water Environment

295 Pages·2000·15.61 MB·English

by  O. C. Rodríguez

Experimental Acoustic Inversion Methods
for Exploration of the Shallow Water Environment
Experimental Acoustic Inversion
Methods for Exploration of
the Shallow Water Environment
Edited by
A. Caiti
DII, University of Siena, Italy
J.-P. Hermand
SACLANT Undersea Research Centre,
La Spezia, ltaly
S.M. Jesus
UCEH, University ofA lgarve,
Fara, Portugal
and
M.R. Porter
Science Applications International Corp"
San Diego, CA, U.SA
SPRINGER SCIENCE+BUSINESS MEDIA, B.V.
A CLP. Catalogue record for this book is available from the Library of Congress.
ISBN 978-94-010-5800-1 ISBN 978-94-011-4112-3 (eBook)
DOI 10.1007/978-94-011-4112-3
Printed an acid-free paper
AII Rights Reserved
© 2000 Springer Science+Business Media Dordrecht
Originally published by Kluwer Academic Publishers in 2000
Softcover reprint ofthe hardcover Ist edition 2000
No part of the material protected by this copyright notice may be reproduced or
utilized in any form or by any means, electronic or mechanical.
including photocopying, recording or by any information storage and
retrieval system, without written permission from the copyright owner.
Table of Contents
Preface vii
Acknowledgements ix
Dynamics of acoustic propagation through a soliton wave packet: observations
from the INTIMATE'96 experiment
O. C. Rod'T"tguez, S. M. Jesus, Y. Stephan, X. Demoulin, M. B. Porter and
E. Coelho 1
Acoustical effects of internal tides on shallow water propagation: an overview
of the INTIMATE96 experiment
Y. Stephan, X. Demoulin, T. Folegot, S. M. Jesus, M. B. Porter and
E. F. Coelho 19
Acoustically focused oceanographic sampling and ocean forecasting
P. ElisseeJJ and H. Schmidt 39
The Haro Strait geoacoustic tomography experiment
N. R. Chapman, L. Jaschke and A. D. Kennedy 51
Quantitative performance comparison among processors in MFP
A. T. Abawi, N. O. Booth, P. Schey, P. Baxley and W. S. Hodgkiss 73
Broadband shallow water localization with a mobile array: experimental results
J. P. Ianniello and J. M. Tattersall 91
Tidal effects on source inversion
M. B. Porter, S. M. Jesus, Y. Stephan, X. Demoulin and E. Coelho 107
Inverse acoustical determination of photosynthetic oxygen productivity of
Posidonia seagrass
J-P. Hermand, P. Nascetti and F. Cinelli 125
A test at sea for measuring acoustic backscatter from marine vegetation
A. Siccardi and R. Bozzano 145
Transient bistatic scattering from buried objects
1. Karasalo and J. Hovem 161
Inversion of normal incidence backscattered data: getting seabed geoacoustic
and morphological parameters
A. Caiti and O. Bergem 177
New theoretical basis for determining the geoacoustic parameters of the seabed
M. J. Buckingham 195
vi
Geophysical parameters in a poro-elastic sediment
N. P. Chotiros 211
Seabottom acoustic parameters from inversion of Yellow Sea experimental data
P. H. Rogers, J. Zhou, X. Zhang and F. Li 219
Adaptive 3D joint inversion of direct, reflected and refracted arrivals
G. Rossi, G. Madrussani and A. L. Vesnaver 235
Application of geo-acoustic inversion techniques to subsurface imaging of
seismic nearfield scenarios
M. Westebbe, J. F. Boehme and H. Krummel 249
Geoacoustic inversion with drifting buoys: EnVerse 1997-98 experiments
(Marettimo Is.)
J-P. Hermand, P. Boni, E. Michelozzi, P. Guerrini, M. Agate, A. Borruso,
A. D'Argenio, D. Di Maio, C. Lo Iacono, M. Mancuso, M. Scannavino 263
Index 287
Contributors 289
Preface
It is remarkable that infinitesimal acoustic perturbations can ride
through the ocean's complex water system and emerge at the other
end with a predictable shape. On a global scale, sound projected
near Heard Island in the Indian Ocean in 1991 was received at the
margins of five continents hours after its transmission. Its time-of
flight provided a record of the warmth of the waters through which it
travelled. On a much smaller scale, sound can be used to measure the
fine structure of the ocean down to the bubbles produced by breaking
waves.
No other energy propagates as effectively in the ocean. For radio
waves, the ocean with its dissolved salts is a conductive medium
and essentially a radio mirror. In the visible spectrum light rapidly
fades into darkness over a few hundred metres for all but the bright
est lasers. However, even at the greatest depths one can still hear
the sound of marine mammals, breaking waves, and distant ships.
Largely for this reason, sound has become the principle tool for ob
serving the ocean.
To review the state of the art, the Portuguese Foundation for Science
and Technology (Fundar;ao da Ciencia e Tecnologia, Lisboa, Portugal)
sponsored a workshop on experimental acoustic inversion methods:
leading international researchers met in March of 1999 in the pic
turesque town of Carvoeiro on the southern coast of Portugal. This
volume, based on presentations at that workshop, summarizes the
diverse and innovative applications of sound to measuring the ocean
and its boundaries.
The papers may be loosely grouped into four categories. First, there
are several papers that use sound to observe the ocean itself. In
particular, internal tides figure prominently. This is a type of in
ternal wave excited by the tidal pull of the moon and the sun. They
propagate along the interfaces of fluid layers in the ocean and may
be visualized by comparison with the more familiar surface waves
which travel along the air-sea interface. Under conditions that are not
well-characterized, these tides can also excite a solitary wave-packet.
These solitons may be compared with tidal waves (tsunamis). Their
effects on acoustic propagation are currently of great interest.
Another important category of inverse problems in this volume might
be termed geometrical. This area strives to infer the location of an
acoustic source. Beyond the obvious naval applications, this sort of
vii
viii
inversion is important for observing the migrations of whales and
their synthetic counterparts: autonomous undersea vehicles.
'nuning to the ocean boundaries, there are several papers concerned
with surface and bottom backscatter. This sort of energy can be
used to measure bottom roughness, as well as vegetation and even its
resultant oxygen production. At small distances below the bottom,
buried objects can be detected and measured.
Probing deeper within the ocean bottom, several papers describe
experimental configurations and inversion methods to infer acoustic
properties of the sediment. Interestingly, despite decades of research
there is still disagreement on the appropriate elastic and poro-elastic
models to use. Simple acoustic models are the most widely used
but even then, there is controversy over the frequency-dependence of
attenuation laws.
These areas are all inter-related. The best example is perhaps matched
field processing applied to source localization, which requires an un
derstanding of the effects (and inversion) of such features as tides,
solitons, and sediment geoacoustic properties. One of the key be
nefits of such a workshop is to bring people with such diverse but
overlapping interests together. The collected papers in this volume
provide both a good introduction to the area and a survey of the
latest research.
Andrea Caiti
Jean-Pierre Hermand
Sergio Jesus
Michael Porter
Acknowledgements
This workshop was made possible by a grant from the Fundac;ao da
Ciencia e Tecnologia (Portuguese Foundation for Science and Techno
logy). Their financial support in organizing the workshop and compil
ing these proceedings is gratefully acknowledged. In addition, each
of the organizers received significant support from their associated
institutions.
In particular, M.B.P. acknowledges the support of New Jersey Insti
tute of Technology with which he was affiliated during the organiza
tion of the conference, as well as the Ocean Acoustics program at the
U.S. Office of Naval Research, which supported much of the research
reported in this volume.
Since 1959, the SACLANT Undersea Research Centre has brought
NATO scientists together, thus providing a foundation for interna
tional collaborations. The four of us originally met while working
there and this book is in large part a consequence of that important
role of the Centre.
We would like to thank the staff at the University of the Algarve,
especially Orlando Rodriguez, who provided assistance with the local
arrangements. The beautiful conference setting on the coast of Por
tugal will be remembered by all of us for years to corne. Its location
near the historic school of Prince Henry, the Navigator provided a
suggestive setting for a workshop devoted to ocean exploration.
Andrea, Jean-Pierre, Sergio, Michael
ix
DYNAMICS OF ACOUSTIC PROPAGATION THROUGH A
SOLITON WAVE PACKET: OBSERVATIONS FROM THE
INTIMATE'96 EXPERIMENT
O.C. RODRIGUEZ AND S.M. JESUS
UCEH-Universidade do Algarve
Campus de Gambelas,
Faro, Portugal
Y. STEPHAN AND X. DEMOULIN
Service Hydrographique et Oceanographique
de la Marine
Brest, France
M.B. PORTER
Science Applications International Corp.
San Diego, USA
AND
E.F. COELHO
Instituto Hidrografico
Lisbon, Portugal
Abstract.
Experimental observations of acoustic propagation through a Soliton
Wave Packet (SWP) show an abnormally large attenuation over some fre
quencies, that was found to be significantly time dependent and anisotropic.
Nevertheless, by considering the problem of signal attenuation, the ap
proach used in most of the studies can be considered as "static" since no
additional effects were taken into account as a SWP evolves in range and
time. Hydrographic and acoustic data from the INTIMATE'96 experiment
clearly exhibit traces of the presence of soliton packets, but in contrast
with known observations of attenuation, its frequency response also reveals
a sudden increase of signal amplitude, which may be due to a focusing ef
fect. This signal increase coincides with a significant peak found in current
and temperature records. However, the correlation of both acoustic and
hydrographic features is difficult to support due to the different time scales
A. Caiti et al. (eds.), Experimental Acoustic Inversion Methods for
Exploration of the Shallow Water Environment, 1-18.
© 2000 Kluwer Academic Publishers. Printed in the Netherlands.
2 O.C. RODRIGUEZ ET AL.
between the rate of hydrographic data sampling and the rate of signal trans
missions. To study the possibility that a SWP could be responsible for the
observed signal increase, the INTIMATE'96 hydrographic data was used to
generate physically consistent distributions of "soliton-like" fields of tem
perature and sound velocity, which were used as input for a range-dependent
normal-mode model; it was found that for a particular soliton field, the set
of "dynamic" (i.e., range-dependent and time-dependent) acoustic simula
tions reveals an acoustic signature similar to that observed in the data.
These results contribute to a better understanding of underwater propaga
tion in shallow-water coastal environments and therefore provide a potential
basis for range-dependent temperature and sound-speed inversions.
1. Introduction
It is known that naturally generated solitons can often be observed in
coastal zones, as a result of nonlinear interaction of the surface tide with the
continental shelf; the generation mechanism remains however poorly under
stood. The significant circulation of organic surfactants caused by SWPs
induces small displacements of the ocean level and leads to a modulation
of the sea-surface roughness which can be clearly detected by satellite SAR
images. Such surface signatures provide a detailed information about the
propagation characteristics of SWPs (Small et at., 1995). Soliton packets
have been observed by satellite almost everywhere in coastal zones and
in particular near Portugal. Observations of propagating solitons include
also a considerable amount of current and temperature measurements. For
instance during the summer of 1994 current and temperature data taken
near Porto (Sherwin et at., 1996) allowed for the observation of a wave
packet, composed of three solitons, which could be tracked during their
propagation towards the shore. The waves were characterized by sudden
isotherm depressions of up to 45 m lasting 10-35 minutes, accompanied by
mls mls
current surges of up to 0.45 and shears of up to 0.7 (over 60 m).
SWPs propagated away from the shelf break towards the shore with an av
mls
erage speed of 0.56 and appeared each tidal cycle, which confirms the
important role of tides as a significant source for the generation of SWPs.
The problem of acoustic propagation through SWPs has been inten
sively studied in recent years, essentially To explain the anomalous fre
quency response of shallow-water propagating signals, which were found to
be strongly time dependent, anisotropic and sometimes exhibiting an abnor
mally large attenuation over some frequency range (Zhou et at., 1991),(Caille
et al., 1997). Most of the known reports noted the problem of signal atten
uation, without regarding to additional effects as a soliton packet evolves in
time and range. Hydrographic and acoustic data from the INTIMATE'96
experiment clearly exhibit traces of soliton presence. However, in contrast