the use of physics for the detection of remote objects, and both radar and SONAR
methods were well advanced (Jones 1999).
Modern applications of acoustic remote sensing techniques persist in part
within the military realm, but civilian application has also become increasingly
important. As agencies entrusted with the management of marine resources strive
to better understand the systems put into their charge, habitat mapping has become
of prime importance. Fisheries management, conservation management, and
marine spatial planning rely increasingly on large-scale habitat maps that can, in
depths beyond the resolution of optical sensors, only be produced by acoustic
means. Thus, acoustic applications directly related to the needs of habitat mapping,
as well as biological and fisheries assessment, are proliferating.
8.2 Physical and Technical Principles
Sound waves exhibit different physical characteristics from electromagnetic waves
(e.g., visible and infrared wavelengths) that make them well suited for use in
underwater investigations. Radio, radar and other electromagnetic waves propagate at the speed of light for long distances in the atmosphere, but as soon they
enter the water, these properties become rapidly limited due to attenuation. Water,
especially salt water, has high conductivity and is highly dissipative. Assessment
of objects and surfaces at greater depths thus requires another means of transmitting energy, such as sound, that utilizes different physical properties. The
mechanical propagation of a disturbance underwater can travel great distances
(thousands of kilometers in the case of a whale song). Sound waves are described
by the wave equation and compensate for lower propagation speeds with lower
transmission loses (e.g., lower speeds and losses than light).
8.2.1 The Sound Wave
Sound is a mechanical disturbance that travels through a medium, in our case
seawater (Fig. 8.1). The propagating disturbance is identified as an incremental
acoustic pressure, the magnitude of which is smaller than ambient pressure (Medwin and Clay 1998). Sound is transmitted through gases and liquids as a longitudinal, or compressional, wave characterized by local regions of compression and
rarefaction (Fig. 8.1). The term ‘‘longitudinal’’ refers to the displacements in the
medium occurring along the direction of wave propagation. In contrast, sound
transmission in solids can also be expressed as a ‘‘transverse’’ wave characterized
by waves of alternating shear stress (i.e., particle displacement) at right angles to
the direction of propagation. These types of waves have about half the velocity of
longitudinal waves and cannot pass through liquids, since they cannot sustain the
shear forces.
8 Acoustic Methods Overview
197
methods were well advanced (Jones 1999).
Modern applications of acoustic remote sensing techniques persist in part
within the military realm, but civilian application has also become increasingly
important. As agencies entrusted with the management of marine resources strive
to better understand the systems put into their charge, habitat mapping has become
of prime importance. Fisheries management, conservation management, and
marine spatial planning rely increasingly on large-scale habitat maps that can, in
depths beyond the resolution of optical sensors, only be produced by acoustic
means. Thus, acoustic applications directly related to the needs of habitat mapping,
as well as biological and fisheries assessment, are proliferating.
8.2 Physical and Technical Principles
Sound waves exhibit different physical characteristics from electromagnetic waves
(e.g., visible and infrared wavelengths) that make them well suited for use in
underwater investigations. Radio, radar and other electromagnetic waves propagate at the speed of light for long distances in the atmosphere, but as soon they
enter the water, these properties become rapidly limited due to attenuation. Water,
especially salt water, has high conductivity and is highly dissipative. Assessment
of objects and surfaces at greater depths thus requires another means of transmitting energy, such as sound, that utilizes different physical properties. The
mechanical propagation of a disturbance underwater can travel great distances
(thousands of kilometers in the case of a whale song). Sound waves are described
by the wave equation and compensate for lower propagation speeds with lower
transmission loses (e.g., lower speeds and losses than light).
8.2.1 The Sound Wave
Sound is a mechanical disturbance that travels through a medium, in our case
seawater (Fig. 8.1). The propagating disturbance is identified as an incremental
acoustic pressure, the magnitude of which is smaller than ambient pressure (Medwin and Clay 1998). Sound is transmitted through gases and liquids as a longitudinal, or compressional, wave characterized by local regions of compression and
rarefaction (Fig. 8.1). The term ‘‘longitudinal’’ refers to the displacements in the
medium occurring along the direction of wave propagation. In contrast, sound
transmission in solids can also be expressed as a ‘‘transverse’’ wave characterized
by waves of alternating shear stress (i.e., particle displacement) at right angles to
the direction of propagation. These types of waves have about half the velocity of
longitudinal waves and cannot pass through liquids, since they cannot sustain the
shear forces.
8 Acoustic Methods Overview
197