Introduction to Acoustics

Underwater
5. Underwater Acoustics
A
It is well established that sound waves, compared
to electromagnetic waves, propagate long
distances in the ocean. Hence, in the ocean
as opposed to air or a vacuum, one uses
sound navigation and ranging (SONAR) instead
of radar, acoustic communication instead of
radio, and acoustic imaging and tomography
instead of microwave or optical imaging or X-ray
tomography. Underwater acoustics is the science
of sound in water (most commonly in the ocean)
and encompasses not only the study of sound
propagation, but also the masking of sound signals
by interfering phenomenon and signal processing
for extracting these signals from interference. This
chapter we will present the basics physics of ocean
acoustics and then discuss applications.
5.1 Ocean Acoustic Environment ................. 151
5.1.1 Ocean Environment ................... 151
5.1.2 Basic Acoustic Propagation Paths 152
5.1.3 Geometric Spreading Loss........... 154
5.2 Physical Mechanisms............................ 155
5.2.1 Transducers .............................. 155
5.2.2 Volume Attenuation .................. 157
5.2.3 Bottom Loss.............................. 158
5.2.4 Scattering and Reverberation ..... 159
5.2.5 Ambient Noise .......................... 160
5.2.6 Bubbles and Bubbly Media......... 162
5.3 SONAR and the SONAR Equation............. 165
5.3.1 Detection Threshold and Receiver
Operating Characteristics Curves .. 165
During the two World Wars, both shallow and deepwater
acoustics studies were pursued, but during the
Cold War, emphasis shifted sharply to deep water.
The post-World War II (WWII) history of antisubmarine
warfare (ASW) actually started in 1943 with
Ewing and Worzel discovering the deep sound channel
(DSC) caused by a minimum in the temperaturedependent
sound speed. (Brekhovskikh of the Soviet
Union also discovered it independently, but later.)
5.3.2 Passive SONAR Equation ............. 166
5.3.3 Active SONAR Equation ............... 167
5.4 Sound Propagation Models................... 167
5.4.1 The Wave Equation
and Boundary Conditions........... 168
5.4.2 Ray Theory ............................... 168
5.4.3 Wavenumber Representation
or Spectral Solution ................... 169
5.4.4 Normal-Mode Model ................. 169
5.4.5 Parabolic Equation (PE) Model .... 172
5.4.6 Propagation
and Transmission Loss ............... 174
5.4.7 Fourier Synthesis of
Frequency-Domain Solutions...... 175
5.5 Quantitative Description of Propagation 177
5.6 SONAR Array Processing ........................ 179
5.6.1 Linear Plane-Wave
Beam-Forming and
Spatio-Temporal Sampling ......... 179
5.6.2 Some Beam-Former Properties ... 181
5.6.3 Adaptive Processing................... 182
5.6.4 Matched Field Processing, Phase
Conjugation and Time Reversal ... 182
5.7 Active SONAR Processing ....................... 185
5.7.1 Active SONAR Signal Processing.... 185
5.7.2 Underwater Acoustic Imaging ..... 187
5.7.3 Acoustic Telemetry..................... 191
5.7.4 Travel-Time Tomography............ 192
5.8 Acoustics and Marine Animals............... 195
5.8.1 Fisheries Acoustics..................... 195
5.8.2 Marine Mammal Acoustics .......... 198
5.A Appendix: Units .................................. 201
References .................................................. 201
This minimum has been mapped (dotted line in
Fig. 5.1), and typically varies from the cold surface
at the poles to a depth of about 1300 m
at the equator. Since sound refracts toward lower
sound speeds, the DSC produces a refraction-generated
waveguide (gray lines) contained within the ocean,
such that sound paths oscillate about the sound
speed minimum and can propagate thousands of
kilometers.
149
Part A 5