Sonar and Underwater Acoustics E-Book

Jean-Paul Marage

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 719

Veröffentlichungsjahr: 2013

Beschreibung

Sonar and Underwater Acoustics brings together all the concepts necessary for designers and users of sonar systems. Unlike other books on this subject, which are often too specialized, this book is accessible to a wider audience. The first part focuses on the acoustic environment, antenna structures, and electric acoustic interface. The latter provides knowledge required to design, as well as the development and implementation of chain processes for an active sonar from the conditioning input to output processing. The reader will find a comprehensive range of all problems encountered in underwater acoustics for a sonar application, from physical phenomena governing the environment and the corresponding constraints, through to the technical definition of transducers and antennas, and the types of signal processing involved. In one section, measures in underwater acoustics are also proposed.

Leseprobe

Table of Contents

Preface

PART 1. The Marine Environment

Part 1. Introduction

Chapter 1. Problematics

1.1. History

1.2. Underwater acoustics

1.3. Applications

1.4. Comparison with radar

1.5. Submarine detection and warfare

1.6. Submarine detection

1.7. Submarine detection: a veritable challenge

1.8. Overcoming the effects of the ocean

1.9. Sonar and information processing

Chapter 2. Sound Propagation in the Marine Environment

2.1. General points

2.2. Characteristics of the marine environment

2.3. Models used

2.4. Propagation phenomena

2.5. Application examples

Chapter 3. Noises and Reverberation

3.1. Classification of ambient noises

3.2. Analysis of noise sources

3.3. Wenz' model of sea noise

3.4. Directivity of sea noise

3.5. Reverberation

Chapter 4. Radiated and Inherent Noises

4.1. Radiated noise

Chapter 5. Transmission of the Acoustic Signal: Sonar Equations

5.1. Introduction

5.2. Detection contrast and detection index

5.3. Transmission equation

5.4. Equation of passive sonar

5.5. Equation of active sonar

PART 2. Acoustic-electric Interface Antenna Structures

Part 2. Introduction

Chapter 6. Electric-acoustic and Acoustic-electric Transformations

6.1. Transducers and hydrophones

Chapter 7. Performance and Structures of Acoustic Antennas

7.1. Antennas and radiation

7.2. Structures of sources and antennas

Chapter 8. Electronic Transducer-hydrophone Adaptation

8.1. Hydrophones

8.2. Transducers

Chapter 9. Electro-mechano-acoustic Analogies

9.1. Methods of studying transducers and hydrophones

9.2. Mechanic-electric equivalence

9.3. Electric-acoustic equivalence

9.4. Finite element method (FEM)

PART 3. Processing Chain of Active Sonar

Part 3. Introduction

Chapter 10. Selection Criteria in Active Processing

10.1. Selection criteria related to propagation

10.2. Selection criteria relative to noise

10.3. Selection criteria related to reverberation

10.4. Selection criteria related to emission power

10.5. Selection criteria related to the antenna

10.6. Selection criteria for the operating frequency

10.7. Selection criteria related to operational considerations

10.8. Selection criteria related to the nature of targets

Chapter 11. Processing Chain in Active Sonar

11.1. General points

11.2. Emission

11.3. Reception

Chapter 12. Basic Theoretical Notions in Active Processing

12.1. The Doppler effect

12.2. The Doppler effect in active sonar conditions

12.3. Treatment of the signal

12.4. Choice of an emission signal under active sonar conditions

Chapter 13. Measurement in Underwater Acoustics

13.1. Introduction

13.2. Wave train method

13.3. Precautions before measuring

13.4. Acoustic measurements and calibrations of transducers

13.5. Notion of uncertainty estimation and of maximum tolerated difference

13.6. Other types of measurements in underwater acoustics

APPENDICES

Appendix 1. Logarithmic Scales

Appendix 2. Equation of Sound in Fluids

A2.1. Equation of motion

A2.2. Continuity equation

A2.3. Equation of state

A2.4. Resolution of one dimension problem

A2.5. General case approach

A2.6. Comparison with the case of elastic solids

Appendix 3. Piezoelectricity Fundamentals

Appendix 4. Vector Analysis – Fundamentals

A4.1. Locating a point within a three-dimensional space

A4.2. Vector analysis

Appendix 5. Reciprocity Theorem

A5.1. Application of the reciprocity theorem to a piezoelectric transducer

A5.2. Sensitivities at emission and at reception of a reciprocal transducer expressed as a function of the reciprocity factor J

Appendix 6. Concrete Example of Uncertainty Estimation Based on theReciprocity Calibration Method

A6.1. Background

A6.2. Measuring configuration in an acoustic chamber

A6.3. Determination of the influence factors inherent to these measuring techniques

A6.4. Method used or reciprocity measuring principle

A6.5. Determination of the related uncertainty components

A6.6. Validity conditions

A6.7. Statistical confirmation from real measurements

A6.8. Conclusion

First published 2010 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc. Adapted and updated from two volumes Sonars et acoustique sous-marine published 2009 in France by Hermes Science/Lavoisier © LAVOISIER 2009

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

ISTE Ltd 27-37 St George's Road London SW19 4EU UK

www.iste.co.uk

John Wiley & Sons, Inc. 111 River Street Hoboken, NJ 07030 USA

www.wiley.com

© ISTE Ltd 2010

The rights of Jean-Paul Marage and Yvon Mori to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Cataloging-in-Publication Data

Marage, Jean-Paul.   Sonar and underwater acoustics / Jean-Paul Marage, Yvon Mori.     p. cm. Includes bibliographical references and index.   ISBN 978-1-84821-189-6   1. Underwater acoustics. 2. Sonar. I. Mori, Yvon. II. Title. QC242.2.M365 2010 620.2′5--dc22

2010021223

British Library Cataloguing-in-Publication Data A CIP record for this book is available from the British Library

Preface

Implementation of the structure of sonar chains evidently entails the analysis of the functions to be carried out and therefore the analysis of operational needs. A constant of these analyses, which is without doubt over-simplistic in relation to all the possible developments, stems from the fact that all processing will be the determination of the kinematics of mobiles present and the help of objective designation.

The processes used from the available sensors should therefore ensure as far as possible these two main functions with which a certain number of secondary functions useful for the operator (tests, plotting sound fields, propagation losses, etc.) are often associated.

Historically, the processes have been associated with particular antennas: active antenna, passive antenna, interceptor antenna, etc. This distinction, justified at the time by the low calculating capacity available, is fading little by little today where a more global view of a sonar system can be considered because of the perceptible increase in the calculating capacities and the introduction of everything digital in signal processing.

This book is mainly aimed at technicians and engineers who work in the field of underwater acoustics and in particular in the field of sonar. Its purpose is to give the maximum amount of information on the diversity of techniques related to the study and development of systems in underwater acoustics. This work also forms an introduction for engineers beginning their careers and a brief outline of the problems which they will encounter and allows the more general public to attain the necessary notions to understand sonar and underwater acoustics.

The inspiration for this book is three volumes published in French which make up part of a summary of underwater acoustics written by the underwater branch of the Thales Group1.

These three volumes are:

Volume 1: The marine environment (Le milieu marin)

Volume 2: The acoustic electric interface (L'interface acoustique électrique)

Volume 3: The chain of active sonar processing (La chaîne de traitement du sonar actif)

The author, Jean-Paul Marage, a former engineer with Thales, contributed over 40 years to the definition, study, development and perfection of high technology sonar processing systems, mainly for the military sector as well as for the public sector.

The main interest of the company at the origin of these books was the capitalization, transmission and distribution of the knowledge and experience which made up the essential basis of its profession centered on underwater acoustics, through a practical approach, with concrete examples of industrial applications.

This book is a reviewed, corrected and updated republishing of the aforementioned volumes. This new edition gives all the necessary information for the training of specialists in the processing of acoustic signals, which is the reason for its republishing and greater distribution.

All the variety of problems that designers and users of sonar systems encounter are grouped into this book. We therefore find a large, relatively detailed range of all the problems encountered in underwater acoustics with sonar application in mind. From the physical phenomena governing the environment and the corresponding sonar restrictions, to the techniques of transducer and antenna definition, as well as the associated types of signal processing. It is this which sets it apart from the original works and is one of its main concerns.

Of course, the advances in the techniques of signal processing, the digital technology and especially the tactical naval strategies currently being designed by military staff, propose increasingly complex and efficient systems (multi-statism, multi-platform, detection and treatment of multiple data streams obtained by satellite or other means, etc.), with most being confidential and protected.

The correctional and editing work was carried out in collaboration with Y. Mori, who was also an engineer with Thales and head of a test laboratory and environmental assessments.

The developments carried out are mainly practical, the bibliography giving works of further grounding allowing the reader to reinforce their theoretical knowledge in this field. Several appendices are provided citing theoretical aspects which are judged to be useful. These appendices come from summaries of books, articles and documents to which access is not easy, and sometimes even impossible, for every reader.

We would like to thank in particular Thales who authorized the use of the French edition in order for us to bring you our new book.

Jean-Paul MARAGE Yvon MORI June 2010

1 THALES UNDERWATER SYSTEMS S.A.S.

PART 1

The Marine Environment

Part 1

Introduction

Part 1 poses the problems of underwater acoustics and gathers the notions necessary for its understanding, along with an understanding of sonar, into several chapters. This is achieved by detailing the variety of problems that designers and users of sonar systems encounter.

It forms an introduction and outline of the problems that technicians will come across and sets the essential information out in a simple manner allowing them to understand the following chapters.

For this reason, this part's mathematical complexities have been deliberately reduced. The few formulae being, for the most part, given without demonstration or justification, relying upon the large bibliography. The theoretical aspects judged useful are given in the appendices.

The links between chapters are mainly acoustic: propagation, environment noises, artificially produced noises and sonar equations.

Chapter 1

Problematics

This chapter provides a brief history of the concept of acoustics through the ages. It explains the characteristics of acoustics, particularly underwater acoustics. This introduction allows us to arrive at the modern definition of a sonar system from a military and civil application viewpoint. Several generalized underwater acoustics problems are then discussed, including detection, information processing and underwater conflicts in the marine environment.

1.1. History

Before the Greeks, man had never gone beyond the practical observation of the effects of sound. The Chinese philosopher Fohi himself struggled, around 3,000 years BC, to liken the five notes of the range to the “five components of nature”: earth, water, fire, air and wind. In 500 BC, Zeno of Elea noted our inability to understand and explain sound, saying:

“Since a bushel of millet grains make a sound when poured into a heap, each grain and each part of the grain, be it one ten-thousandth, should make its own sound.”

It was Pythagoras, in 600 BC, who was one of the first people to study the science of acoustics. The phenomenon of the echo, reported by Roman writers, is used in ancient theater.

Aristotle carried out studies on sound at around 350 BC and wrote his treatise on physics.

The word acoustics means “science relative to sound” and comes from the Greek akoustikos. It is one of the most ancient sciences, however it was not until the Renaissance that we see the appearance of a large number of researchers interested in the phenomena involved. At the end of the 15th century, Leonardo da Vinci wrote:

“If you stop your ship, then put one end of a blowpipe in the water and the other in your ear, you will hears ships far from yourself.”

This was one of the first statements in history regarding passive sonars and the Renaissance was period during which the development of acoustics started gathering pace.

Towards 1600, Mersenne wrote Universal Harmony. He was one of the first people to measure the speed of sound in air. Towards 1700, Huygens' theory was extended to acoustics.

In 1827, Chladni determined the speed of sound propagation using the vibrations of rods and sound pipes. The first measurement of the velocity of sound in water was carried out in 1827 by Swiss physicist Daniel Colladon and French mathematician Charles François Sturm in Lake Geneva. They obtained a value of 1,435 m/s.

Figure 1.1.Measurement of the speed of sound in water in 1827

Lord Rayleigh published his Theory of Sound in 1887. This theory was used as a basis for the science of acoustics and still is today.

Towards 1912, Kennely introduced the notion of motional impedance and threw out the theory of quadripole electromechanics.

Over several decades, the progress of underwater acoustics was linked with the development of detectors; whether it was hydrophones that listened (equivalent to microphones) or transducers that emit (like a loudspeaker) but can also listen (the principle of reciprocity). The main physical phenomenon involved is piezoelectricity, i.e. the transformation of a mechanical stress (acoustic wave) into electricity and vice versa. It was Paul Langevin who first underlined this phenomenon in 1917 using a quartz crystal.

Currently, the piezoelectric materials used are amorphous ceramics, molded when hot into plates, spheres or cylinders, then polarized through the application of a strong electric field, barium titanate, lead titano-zirconate or lead niobate.

The first echoes of submarines were obtained in 1918 at a distance of 1,500 meters. The first active systems were baptized ASDIC (the abbreviation for Anti-Submarine Detection Investigation Committee) by the British and simply SONAR (SOund Navigation And Ranging) by the Americans, a word universally adopted today. They worked at around 30 kHz.

We soon realized that extenuation through propagation strongly decreased with frequency: from 2 dB/km at 20 kHz it becomes 0.5 dB/kilometer at 5 kHz. This therefore led to the lowering of working frequencies in order to increase range; a practice that persists today.

Taking into account the progress of electronics, we can see that after a period of development of sensors and dedicated small electronics there was a period of progress in signal processing that we carry out today on calculators.

Today a period of information processing (mulit-antennas, multi-targets, multi-platforms, etc.) with an ever-growing emphasis on post-treatment algorithms is added to electronic beamforming, matched filtering, spectral analysis, etc.

An example of a submarine detection system is shown in Figure 1.2 and corresponds to what is called “major ship in anti-submarine warfare”.

Figure 1.2.Submarine detection systems

1.2. Underwater acoustics

As it is meant here, underwater acoustics covers sonar systems, i.e. the techniques that use the waves of mechanical vibrations in order to transmit and receive information in the marine environment.

Figure 1.3.Use of the marine channel

Of all kinds of energy, it is mechanical vibrations that propagate best in water. Electromagnetic waves abate so quickly that the ranges obtained by using them are ridiculous for most of the intended applications. Sometimes classed as “acoustics”, these waves are therefore the main means of investigation of the underwater environment.

In a general sense, underwater acoustics look to exploit the marine channel, in a broad sense of the term, as shown in Figure 1.3. By starting “use the marine channel”, we must consider it as a propagation channel and attempt to understand it.

1.2.1. Communications channel

The marine channel can be a communication channel in its classic meaning; we shall come back to this point later. Information must be transmitted between two points, both of them situated in an underwater environment: it is a problem of underwater communication.

A communications channel can also be an obligate transmission channel. This is the case in oil prospecting, where the section a company is interested in is beneath the seabed and the liquid part has to be traversed (because of the “depth” of the solid layer) more than desired.

These two types of channel are illustrated in Figure 1.4.

Figure 1.4.Transmission channel

1.2.2. Knowledge of the channel

The sonar system can be used to find the marine channel or knowledge about it. The word sonar relates to the use of acoustic waves in water to aid navigation and obtain information. It is possible to deduce two types of characteristics of the channel (or rather incidents in the channel) sought: the seabed and the “particularities”:

– “Navigation”: seabed. The knowledge of the seabed evidently takes on a crucial character for navigation. The corresponding pieces of equipment are called sounders.

– “Ranging”: particularities. By considering the notion of the channel in a general sense, the “particularities” of the channel can be: fish, which leads to fishing sonars; mine hunting sonars for mines; and surface ships and submarines, which are the targets of “large sonar systems”.

1.3. Applications

The applications of underwater acoustics are numerous, even if the economic weight of the field is not very significant. The applications can be classed by the outcomes sought. Applications are divided into two sectors: civil and military.

1.3.1. Civil applications

Four civil applications have already been cited:

– the “measurement of the seabed” with sounders;

– the detection and localization of shoals of fish with fishing sonars;

– oil prospecting with the aid of large linear antennas called “flutes”;

– the transmission of information with the help of underwater communication systems between, for example, a surface ship and an underwater robot.

Other applications can be cited:

– marine mapping for navigation;

– aiding navigation with a sounder or owing to a positioning in relation to fixed beacons;

– oceanography, which is not a true application, however;

– hydrography.

1.3.2. Military applications

The different military applications of underwater acoustics are perhaps more interesting because of the complexity of the systems involved. Other than a few tasks already mentioned (communication, aiding navigation, etc.), the different missions allocated to a sonar system can be:

– the detection, localization and recognition of small objects, in general laid on the seabed;

– guiding an underwater weapon (torpedo);

– the interception of sonar emissions;

– tracking, an operation that requires a scanning function, and is therefore a detection function, accompanied in general by monitoring and classification;

– attacking, sometimes classed as an “estimation of elements-objective”, where the preceding functions are put into action and where localization takes on a particular importance.

Two sorts of sonar systems exist: active and passive.

Passive systems, see Figure 1.5, seek to detect noises radiated by the target.

Figure 1.5.Passive listening

It is passive in the sense that it does not emit any signal, it is content with “listening”. This is the technique of submarine detection, which has the principal advantage of discretion.

As for an active system, it emits a signal and bases its detection on the signal reflected from a possible target. This is illustrated in Figure 1.6. It is the same type of approach as most radar systems.

Figure 1.6.Active listening

1.4. Comparison with radar

A quick comparison with radar is informative and will allow us to introduce several important notions.

Waves

A sonar uses mechanical waves and a medium is necessary. It is therefore logical to consider that this medium is of great importance (for noise, propagation, etc.).

A radar uses electromagnetic waves that do not need a medium for support: they can propagate in a vacuum.

Velocity

Mechanical waves propagate in water with a velocity of around 1,500 m/s (330 m/s in air). Electromagnetic waves propagate at the speed of light, so 300,000,000 m/s. This ratio of around 2.105 between the two velocities has three significant consequences.

Algorithms

The sonar signal processor has more time than its radar equivalent to process the signals and apply the appropriate algorithms to its observation.

Beamforming

The low value of the velocity of acoustic waves in the underwater environment requires a certain waiting period in order to complete a general survey.

Even though the first active sonar systems possessed such mechanical beamforming (the antenna was turned by hand), this significant time interval between two successive recurrences quickly led sonar operators to develop electrical beamforming (“electronic scanning” in radar) which, with the help of multiplexing, forms all the lines of the horizon at the same time. The complexity of this operation is obviously multiplied by a factor in the order of the number of lines formed, 36 in the previous example.

Target speed

In the first case, the Doppler effect is equal to:

whereas for sonar, we get:

The Doppler effect is almost 4,000 times greater in sonar than in radar.

1.5. Submarine detection and warfare

It is common to distinguish submarine detection, which covers detection in a broad sense of the term (detection, localization, monitoring, tracking) from submarine warfare, which includes, but is not limited to, underwater weapons (mines, torpedoes). In France, the (governmental) organizations studying submarine warfare are:

– the study group concerned with research in submarine detection (GERDSM) and the management of constructions and naval weapons (DCAN) in Toulon, for everything concerning submarine detection;

– the Atlantic submarine study group (GESMA) of the DCAN in Brest, for everything involving mine-hunting sonars;

– the organization of constructions and naval weapons (ECAN) in Saint-Tropez, which studies and develops torpedoes, and therefore the acoustic part, homing device or acoustic head and the relative electronics.

1.6. Submarine detection

Among the targets that submarine detection systems are interested in, one thing is of particular importance: submarines.

Two main types of submarine exist: sub-surface ballistic nuclear (SSBN) submarines and attack submarines, whether nuclear (sub-surface nuclear, SSN) or conventional (diesel). SSBN submarines are the main deterrent of the several countries that possess them (USA, Russia, UK and France).

Due to the “inefficiency” of submarine detection, these are in fact the only discrete carriers of strategic nuclear weapons. Attack submarines are also an inconvenience since, among other things, they paralyze forces (e.g. the blockade of Argentinean warships by a UK SSN submarine during the conflict over the Falklands after the sinking of the “General Belgrano”) or convoys (e.g. German U-Boats during the Second World War).

What's more, surface ships are detectable by means other than acoustics: infrared, electromagnetic (radar) or visible means situated on different potential carriers, including satellites.

1.7. Submarine detection: a veritable challenge

Submarine detection is a real challenge. In simple terms, submarine detection is about finding an information carrier signal in noise. Yet the level of signal decreases, as much in passive as in active, whereas the level of noise increases.

Decrease of the active signal

Using the case of radar, the general public is aware that there are chiefly two ways of decreasing the “radar cross-section”, meaning the surface of the plane seen by the radar:

– the shape: the conception of a plane with angled forms and no surface area perpendicular to the axis of the incoming radar signal;

– the materials: the use of “absorbing” materials or paintwork.

For obvious reasons relating to restrictions in hydrodynamics and spaciousness, the cross-section of a submarine tends to remain circular and the use of bizarre shapes in the design of a submarine is not easy. On the other hand, so-called anechoic materials exist that decrease what we call the “index of the target” at certain frequencies, meaning the proportion of acoustic energy reflected is reduced compared with the incident acoustic energy.

Decrease of the passive signal

A passive sonar looks to detect noises radiated in an involuntary manner by a target, in our example a submarine. These radiated noises are mainly produced by:

– the engines on board (motors, back-ups, pumps, etc.);

– the phenomenon of cavitation (creation of bubbles that implode around the propellers);

– transitional and impulsive sources (door closing, dropped hammer, etc.);

– hydrodynamic phenomena (turbulent boundary limit).

It is evident that all developed nations have made an effort during recent years to decrease this type of noise, as illustrated in Figure 1.7 which shows the evolution of the level of noises radiated by typical examples of SSBN and SSN submarines (the levels shown here are only to give an idea of typical levels).

It is interesting to note that SSBN submarines – which are large machines of 10,000 tons for the Soviet Typhoon class SSBN, with an order of 10 MW of power installed – only radiates a few fractions of Watts into the environment.

Figure 1.7.Evolution of the noise radiated by different types of submarine

Increase of noise

Parallel to this decrease in the level of signal that we wish to detect, we must note that nuisance noise has a tendency to increase. We will take the noise of maritime traffic as an example.

One of the disruptive noises in submarine acoustics is ambient noise, meaning the noise that existed before the apparition of a sonar system (the other classes of noises are those emitted and radiated by yourself and, in active sonar, reverberation). Ambient noise is produced by natural sources (biological, precipitation, agitation of the sea, etc.) and “artificial” sources, such as those related to industrial activity (near oil rigs, near to ports, etc.) and those related to navigation by commercial or tourist ships (traffic). It is obvious that this last source of activity (traffic) is increasing and, therefore, so is the corresponding noise.

1.8. Overcoming the effects of the ocean

In order to develop an underwater detection system we need to overcome the (detrimental) effects of the marine environment. These effects we can aggregated into four groups: acoustics, propagation, noise and signal.

1.8.1. Acoustics

As we have already said, it is acoustic waves that propagate best (or least worst) in water. We must therefore develop sensors that transform:

– electrical energy at emission, that is readily available and able to be stocked on board, into mechanical energy: these are transducers (or projectors);

– an acoustic wave, at reception, into an electrical signal that we can easily process, thanks to electronics: these are hydrophones.

Additionally, as these sensors are not generally directive and we wish to obtain directional information, we cluster these sensors into an antenna in order to give priority to given observational directions.

1.8.2. Propagation

As opposed to electromagnetic waves that propagate in an almost straight line, acoustic waves propagate in a fashion that we qualify as being “curious”. In fact, their propagation depends on their velocity which, in itself, depends on the pressure, temperature and salinity of the seawater.

Figure 1.8.Propagation of sound rays in water as a function of immersion of the emmitive source

We see that the proportion of these zones not insonified by direct rays is reduced when we increase the immersion of the source. It is this which has lead to the notion of an immersed source, the antenna being located in a towed housing body (a “fish”) behind the boat. This is a variable depth sonar (VDS).

The attenuation of sound in water is added to loss by geometric divergence, as illustrated in an ideal way in Figure 1.9. This extenuation strongly increases with frequency. Sound will be carried further if we use low frequencies.

Figure 1.9.Attenuation of sound in water (in dB/km)

As the dimension of elementary sensors (mainly transducers) and antennas increases when the frequency decreases and as, moreover, a wave cannot distinguish objects whose size is smaller than its wavelength, the ranges of frequencies used will depend on the intended application. This is illustrated in Figure 1.10.

Figure 1.10.Frequency ranges in underwater acoustics

1.8.3. Unknown noise

The underwater acoustic channel introduces adverse noises that are illusory. We want to know what these noises are. This leads to the use of techniques of statistical signal processing, which we will come back to. The observation is modeled like a random variable that depends on the time and geographical location where the measurement is carried out, and eventually other parameters. This is what leads to the use of the theory of detection.

1.8.4. Unknown signal

In the use of this theory of detection, we cannot assume that only the noise is unknown. The signal is also unknown, because we do not know its moment of arrival (the position of the target is, in general, unknown). This is what leads us to use of the theory of estimation and to reason with statistical parameters such as the mean, variance and standard deviation. The techniques of signal processing therefore have a certain importance.

1.9. Sonar and information processing

A sonar system can be seen as a communication system which, in the way of Claude Shannon, is represented as follows (Figure 1.11):

Figure 1.11.Communication diagram

A signal is produced, modulated and then emitted into the environment. It propagates in this environment when a noise disturbs it. The reflected noise is then received by an antenna, is demodulated and a decision is made.

A sonar system can also be seen as a huge machine for compressing information, as illustrated in Figure 1.12.

Figure 1.12.Sonar model

The system in question is composed of 100 sensors, with a sample frequency of 10 kHz. We want the characteristics (distance, bearing and horizontal speed) for several targets, for example three. The “input” of the sonar receiver is made up of 106 observations, whereas at its “output” it contains only nine values, be it a “compression ratio” of around 105.

It is the presence of noise and an unknown modulation from the part of the channel that requires the use of probability methods, such as the statistical theories of detection and estimation.

Chapter 2

Sound Propagation in the Marine Environment

2.1. General points

Propagation in the marine environment will essentially be considered from a descriptive and practical angle here, in order to bring to light:

– the limitations and constraints that it imposes upon systems of detection and transmission;

– the theoretical models and methods that allow us to either study a given system and determine the conditions under which it should operate or analyze and evaluate sea trials.

We will only analyze the propagation of sound, leaving other phenomena that currently only have limited applications to one side, because of their very strong extenuation in seawater (radioelectric transmissions at very low frequencies, magnetic, electric or thermal detection of submarines over short distances, various firing systems, etc.).

2.2. Characteristics of the marine environment

The propagation of sound is fundamentally governed by the classic wave equation (see Appendix 2):

(2.1)

The velocity (or speed) of sound in water is therefore the most important parameter in the study of these phenomena. The sea is a far from homologous environment and the speed of sound is influenced by the temperature, pressure and salinity, as already mentioned.

All these factors vary according to the geographical locations, time and especially with immersion at a given point. The vertical variations of velocity are in fact much larger than the horizontal variations. This velocity can be known to 0.2 m/s, either through direct measurement or through the intermediary of empirical formulae linked with the temperature, pressure and salinity. Currently the most precise formulae are those of Wilson. The velocity grows almost linearly with temperature, pressure or in depth salinity. A practical formula is as follows:

(2.2)

It is important to know the general appearance of the velocity profiles in the different oceans and seas. We can distinguish two zones: one of them stable between 100 m and the seabed; the other very variable near to the surface. In the first, the velocity is linked to variations in temperature, which decrease steadily from the surface to the seabed in the Atlantic, Pacific and Indian Oceans. The influence of decreasing temperature is compensated by that of the increasing pressure and the velocity goes through a very distinct minimum at a depth of around 1,000 m in these three oceans. The value of this minimum is in the order of 1,490–1,500 m/s; the maxima at the surface and bed are in the region of 1,520–1,550 m/s.

The closed seas, such as the Mediterranean or the Black Sea, present certain particularities. In the Mediterranean, the temperature is practically constant at around 13°C below 100 m. The depth of minimum velocity is therefore around 100–150 m.