Chapter 1

General Characteristics of the Underwater Environment

L. Bjørnø1,┼, and M.J. Buckingham2     1UltraTech Holding, Taastrup, Denmark     2Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, United States

Abstract

This chapter provides a framework and roadmap for the book. It starts with a brief history of underwater acoustics from the time of the Greek philosopher Aristotle (384-322 BC) up to the post-World War II era. This is followed by a discussion of the international system of units used in the book and a discussion on the use of the decibel scale. Next, the chapter deals with the features of oceanography including sound speed profiles, thermoclines, arctic regions, deep isothermal layers, expressions for the speed of sound, surface waves, internal waves, bubbles from wave breaking, ocean acidification, deep-ocean hydrothermal flows, eddies, fronts and large-scale turbulence, and diurnal and seasonal changes. This is followed by a discussion of the sonar equation that is fundamental to underwater acoustics.

Keywords

Arctic regions; Breaking waves; Bubbles; Deep isothermal layers; Deep-ocean hydrothermal flows; Detection probability; Detection threshold; Directivity index; Eddies; False alarm probability; Francois and Garrison equation; Fronts and large-scale turbulence; Internal waves; Noise level; Ocean acidification; Oceanography; Receiver-operating characteristic curves; Reverberation level; Sonar equations; Sound speed profiles; Source level; Speed of sound; Surface waves; Target strength; Thermoclines; Transmission loss; Underwater acoustics history

1.1. Introduction

Over the past about 100 years the exploitation of the seas and their resources has continuously increased. Acoustic waves have turned out to be a very useful tool for detecting resources and objects in the water column and on the seafloor. Other methods have been used with varying degrees of success depending on the objects to be detected or investigated. These methods include magnetics, magnetic anomaly detection, where minor changes in the earth's magnetic field due to presence of an object can be measured; optical methods; electric field changes; hydrodynamics such as pressure changes; thermal methods; and electromagnetic waves. While radar is very useful for detection of objects above water, electromagnetic radar waves are strongly absorbed in seawater. While electromagnetic waves in the visible frequency band from 4 to 8·1014 Hz are much less absorbed, with a minimum absorption coefficient of 3·10-3 cm-1 in the green-blue light near 455 nm wavelength (i.e., 6.59·1014 Hz), electromagnetic wave absorption in the normally used radar bands is several orders of magnitude higher than in the visible band. Seawater salt contains magnesium that makes the water conduct electricity since the 2+ cation constitutes 3.7% of seawater salt. A 1 GHz radar wave in the ultra-high frequency (UHF) band with a 0.3 m wavelength has a 1400 dB/m absorption coefficient while the same wavelength in the 5 kHz sound wave has a 3·10-4 dB/m absorption coefficient. Therefore, radar systems are not useful for detecting objects under water. Underwater sound is used in many applications, such as hydrography, off-shore activities, dredging, defense and security, marine research, and fishery. Hydrography includes harbor and river surveys, bathymetric surveys, flood damage assessment, engineering inspection, pipeline and cable route surveys, exclusive economic zone (EEZ) mapping, breakwater mapping, and so on. Off-shore activities include pipeline and cable installation and inspection, leakage detection, route and site surveys, subsea structure installation support, renewables, remotely operated vehicle (ROV) intervention guidance, decommissioning, reconnaissance surveys, search and recovery, oil and gas prospecting, and prospecting for minerals and resources on and in the seafloor. Dredging includes sonars used by rock and stone dump vessels, excavator and trailing suction hopper dredgers, cutter suction and bucket dredgers, clamshell grab cranes and underwater plow vessels, and placement support. Defense and security includes mine counter measures, submarine and torpedo detection, obstacle avoidance, search and recovery, underwater communication, vessel and fleet protection, waterside security, diver detection, and so on. Marine research includes environmental monitoring, ambient noise measurements, marine archeology, marine mammal research, and fishery research. Fishery includes fishery operations, fish school detection, catch monitoring and control, trawl position control, phytoplankton and zooplankton investigations, communication between monitoring sensors on fishing gear and the fishing vessel, seabed mapping, bottom discrimination, and so on. The counterpart to radar above water is sonar under water. SONAR is the acronym for sound navigation and ranging. It was originally used during World War II as an analog to the name "radar" and as a replacement for the name "asdics" for underwater detection systems using sound, which were used by the British Royal Navy during World War I. The two most common sonar types are passive and active. In a passive sonar system, the acoustic signal originates at a target and propagates to a receiver, where the acoustic signal is converted to an electrical signal for processing. In an active sonar system, an electrical signal is converted to an acoustical signal by a transmitter and the sound waves propagate from the transmitter to a target and back to a receiver, where conversion from acoustical to electrical signal takes place followed by electronic signal processing. Signal processing is aimed at enhancing the return signal from the target or reducing the noise in which the return signal may be embedded, as discussed in Chapter 11. The transmitter is normally called the projector and the receiver is called the hydrophone, as discussed in Chapter 10. If the return signal-the echo-from a target is detected, the position and the potential target movement are determined by the time delay of the echo from the target and the direction of the echo, respectively. The speed of a moving target can be estimated from the frequency shift-the Doppler shift-in the echo from the target, as discussed in Chapter 2. When a sound wave is produced in water it propagates from the site where it is produced. Sound sources can be natural, such as breaking waves, rain falling on the water surface, seismic activities in the seafloor, and so on, or man-made such as sonar signals, underwater explosions, ship noise, and so on, as discussed in Chapter 6. During propagation the sound signal is exposed to a number of processes which may change the sound signal and its propagation, such as sound signal amplitude attenuation due to absorption, divergence, and scattering, as discussed in Chapter 4. Scattering takes place during the sound wave's interaction with the sea surface, seafloor, and inhomogeneities in the water column, as discussed in Chapter 5. These inhomogeneities can be natural, such as plankton, fish and sea mammals, and variations in the sea temperature and salinity. Scattering and reflection of sound signals may cause sound waves to follow different paths, producing multi-path sound propagation, which can make detection of objects in the water column and on the seafloor difficult. The scattering of underwater sound may lead to reverberation which limits detection. Use of advanced signal processing on the transmitted and received signal opens up the possibility to avoid or reduce the degradation of the propagated sound signal, as discussed in Chapter 11. Ambient noise in the sea can also become a limiting factor for signal detection. The sound signal received by a hydrophone carries information about the signal source and what the signal has encountered while propagating from the source to the hydrophone. The signal received by the hydrophone is processed to extract information of value to the user. This complicated "underwater world," where sound propagation is influenced by many individual sources with effect on the sound signal's amplitude, phase, and spectral composition, is the basis for this book, "Applied Underwater Acoustics." Each chapter is introduced with a section giving the necessary definitions and describing the physical background for the subsequent sections of the chapter. The man-made sources of sound from sonar systems of various types are described in Chapter 10. This chapter also describes the different transducer types, their charge forming elements, and their geometries. Chapter 10 illuminates the sonar types available today, characteristic features, as well as their design, calculation, and calibration. Hydrophones, including array types, and their characteristics are also a part of Chapter 10. The sound wave propagation through the water and the different factors which influence the propagation path are discussed in...