Fundamentals of acoustical oceanography

739 Pages·1998·26.81 MB·English

by  Herman Medwin

Fundamentals of Acoustical Oceanography
This is a volume in
APPLICATIONS OF MODERN ACOUSTICS
Series Editors:
Richard Stern Moises Levy
Applied Research Laboratory Department of Physics
Pennsylvania State University University of Wisconsin at Milwaukee
State College, Pennsylvania Milwaukee, Wisconsin
Fundamentals of
Acoustical Oceanography
Herman Medwin Clarence S. Clay
Department of Physics Department of Geology & Geophysics
Naval Postgraduate School University of Wisconsin at Madison
Monterey, California Madison, Wisconsin
W
ACADEMIC PRESS
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Copyright 1998 by Academic Press
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Library of Congress Cataloging-in-Publication Data
Medwin, Herman
Fundamentals of acoustical oceanography/ Herman Medwin, Clarence
S. Clay.
p. cm.
Includes bibliographical references and index.
ISBN 0-12-487570-X (alk. paper)
1. Underwater acoustics. 2. Seawater-Acoustic properties.
3. Oceanography. I. Clay, Clarence S. (Clarence Samuel).
11. Title.
OC242.2.M43 1997
551.46’001’53423-DC21 97-35785
CIP
Printed in the United States of America
9798990001 IP 987654321
Table of Contents
Series Preface xvii
Author Preface xviii
Acknowledgments xx
Chapter 1 The Realm of Acoustical Oceanography: Theory and
Applications of Ocean Acoustics 1
1.1 Introduction 1
1.1.1 Definition 1
1.2 Physical and Biological Characteristics of the Sea 2
1.3 Ocean Stratification, Sound Speed, and Ray Refraction 2
1.4 A Few Examples of Acoustical Oceanography 6
1.4.1 Pulse Timing of Transmissions 8
1.4.2 Remote Sensing by Scatter 10
1.4.3 Passive Acoustics 15
Chapter 2 Sound Propagation 17
2.1 Wave Propagation 17
2.1.1 Intensity of a Diverging Wave 18
2.2 Huygens’ Principle for Impulse Waves 19
2.2.1 Plane Wave Reflection at a Plane Surface: Law of Reflection 22
2.2.2 Plane Wave Refraction at a Plane Interface: Snell’s Law 22
2.2.3 Diffraction at the Edge of a Plane Interface: Scattering of a Spherical Wave 23
V
vi Contents
2.3 CW Sinusoidal Signals: Spherical Waves 24
2.3.1 Variations in Time and Space 24
2.3.2 Traveling Spherical Waves 21
2.4 CW Interference and Phase Effects 27
2.4.1 Approximations 28
2.4.2 Sum of Sounds from Two Sources 32
2.4.3 Interference Near an Interface: Plane Standing Waves 33
2.4.4 Point Source Near a Plane Reflector: Lloyd’s Mirror Interference 34
2.5 The Wave Equation 35
2.5.1 Conservation Laws and the Equation of State for Linear Acoustics 36
2.5.2 The One-Dimensional Wave Equation and Other Relations 38
2.5.3 Acoustic Intensity 40
2.6 Reflection and Transmission at Interfaces 41
2.6.1 Plane Wave Reflection and Transmission Coefficients: Critical Angle 41
2.6.2 Plane Wave Reflection at a Sedimentary Bottom 45
2.6.3 Plane Wave Reflection and Transmission at Multiple Thin Layers 46
2.6.4 Ray Reflection beyond Critical Angle 50
2.6.5 Head Waves: Spherical Waves beyond Critical Angle 51
2.6.6 Fresnel Zones: Spherical Wave Reflection at an Interface 55
2.7 Propagation in Three Dimensions 58
2.7.1 The Three-Dimensional Wave Equation 58
2.7.2 Continuous Waves in Rectangular Coordinates 62
2.7.3 Omnidirectional Waves in Spherical Coordinates 64
Chapter 3 Transmissions along Ray Paths 70
3.1 Impulse and CW Propagation in Ocean Acoustics 72
3.1.1 Impulse Sources 72
3.1.2 Pressure, Particle Velocity, and Intensity in a Pulse 74
3.1.3 Pulse Energy Transmission 75
3.1.4 Radiated Power for Continuous Wave Signals 16
3.2 Ray Paths, Ray Tubes, and Path Times 77
3.2.1 Path Amplitude Factor and Path Time 71
3.2.2 Reflections along Ray Paths 78
3.2.3 Multiple Ray Paths 80
3.2.4 Conservation of Energy in Ray Tubes: Nonabsorbing, Nonscattering Iso-Speed
Medium 82
Contents vii
3.2.5 Sound Pressures in Ray Tubes: Homogeneous, Nonabsorbing, Nonscattenng
Medium 83
3.3 Ray Paths in a Lossless, Refracting Medium 84
3.3.1 Sound Speed in the Ocean 84
3.3.2 Refraction at an Interface: The Ray Parameter 85
3.3.3 Rays through Constant-Speed Layers 86
3.3.4 Rays through Continuous Sound-Speed Changes 87
3.3.5 Examples of Ray Paths 89
3.3.6 Sound Intensity and Pressure in a Horizontally Stratified Ocean 92
3.3.7 Wave Fronts and Crossing Ray Paths: Phase Shifts at Caustics 94
3.3.8 Signal Transmissions in a Refracting Medium with Caustics 96
3.3.9 General Comments on Ray Path Methods 102
3.4 Attenuation 103
3.4.1 Exponential Attenuation of Plane Waves 103
3.4.2 Absorption Losses 104
3.4.3 Scattering Losses 110
3.4.4 Amplitude SONAR Equation 111
3.4.5 Sum of Multiple Arrivals: The Transmission Fraction 111
3.4.6 Logarithmic SONAR Equations: Transmission Loss (dB) 113
3.5 Tomography 116
3.5.1 Inverting for a Map of the Speed of Sound 116
3.5.2 Inverting for Ocean Motions 119
3.6 Doppler Frequency Shift 119
3.6.1 Doppler Theory 119
3.6.2 Doppler Measurements of Particle Motion 120
3.6.3 Sea-Surface Motion 122
3.6.4 Doppler Navigation 124
Chapter 4 Sources and Receivers 127
4.1 Transducer Elements 127
4.1.1 The Pulsating Sphere 121
4.1.2 Sources of Sound 128
4.1.3 The Dipole 130
4.1.4 Materials and Mechanisms 131
4.2 Arrays of Discrete Sources 133
4.3 Directivity of a Line Source 135
4.4 Circular Piston Source 138
viii Contents
4.4.1 Far-Field Directivity 138
4.4.2 Near-Field Directivity 142
4.5 Radiation from a Transducer 143
4.5.1 Total Power Radiated 143
4.5.2 Descriptors of Beam Strength 145
4.6 Equivalence of Source/Receiver Directivity 146
4.7 Free-Field Calibration of Transducers 149
4.8 Self-Reciprocity Calibration of Transducers 150
Chapter 5 High Intensities 153
5.1 Harmonic Distortion and Shock-Wave Growth 153
5.2 Cavitation 158
5.3 Parametric Sources and Receivers 162
5.3.1 CW Parametric Sources 162
5.3.2 Parametric Receivers 168
5.3.3 Intense Tone Bursts, Self-Demodulation 170
5.4 Explosive Sources 170
5.4.1 The Shock Wave 170
5.4.2 Shock Front Propagation; the Rankine-Hugoniot Equations 173
5.4.3 The Gas Globe 174
5.4.4 Interaction with the Ocean Surface 176
5.5 Acoustic Radiation Pressure 177
5.6 Acoustic Streaming 178
Chapter 6 Processing of Ocean Sounds 183
6.1 Sampling Rules 184
6. I, 1 Spatial Sampling 185
6.1.2 Temporal Sampling 185
6.2 Filter Operations 188
*6.2.1 Finite Fourier Transformations 188
*6.2.2 Fourier Integrals and Series 191
6.2.3 Filter Response Measurements 194
6.2.4 Time Domain View of Bandpass Filtering 195
* Optional background material.
Contents ix
*6.2.5 Convolution Operations 195
6.2.6 Filter Operations in the Frequency Domain 197
6.3 Gated Signals 199
6.3.1 Dependence of Spectrum on Ping Carrier Periodicity 200
6.3.2 Dependence of Spectrum on Ping Duration 200
6.4 Power Spectra of Random Signals 200
6.4.1 Signals Having Random Characteristics 203
*6.4.2 Spectral Density and Correlation Methods 203
6.4.3 Random Signal Simulations: Intensity Spectral Density 204
6.4.4 Spectral Smoothing 206
6.4.5 Traditional Measures of Sound Spectra 208
6.5 Matched Filters and Autocorrelation 209
6.6 Sounds in the Ocean 21 1
6.6.1 Natural Physical Sounds 212
6.6.2 Natural Biological Sounds 215
6.6.3 Ship Noise: Sample Calculation 219
Chapter 7 Sound Scattered by a Body 234
7.1 Scattering Measurements: Incident Plane-Wave Approximation 235
7.1.1 Scattering Length 237
7.1.2 Differential Scattering Cross Section 237
7.1.3 Total Scattering and Extinction Cross Sections 238
7.1.4 Target Strength 240
7.1.5 Single Transducer Backscattering 240
7.2 Helmholtz-Kirchhoff Methods 24 1
*7.2.1 Theorems of Gauss and Green 24 1
7.2.2 The Helmholtz-Kirchhoff Integral 243
7.2.3 Kirchhoff Approximation 244
7.2.4 Fresnel Approximation 246
7.3 Specular Backscatter by the Kirchhoff Method 249
7.3.1 Rectangular Plane Facet 249
7.3.2 Penetrable Cylinder 252
7.3.3 Finite Fluid Cylinder 255
7.3.4 Low-Contrast Finite Fluid Cylinder 256
7.4 Modal Solutions for a Fluid Cylinder 25 8
* Optional background material.