influences the initiation and distribution of coral mounds (Del Mol et al. 2002; van
Weering et al. 2003; van Rooij et al. 2003).
The finding of large mound structures along the European margin (e.g., Kenyon
et al. 2003) as well as the improvement of acoustic mapping tools triggered a new
phase in the study of cold-water coral ecosystems in the last decade. Deep-towed
side-scan sonar and more recently multi-beam systems are now the most common
acoustic sensors used to map these habitats (Paul et al. 2000; Huvenne et al. 2002;
Foubert et al. 2005; Wheeler et al. 2005a; Roberts et al. 2005; Mienis et al. 2006;
Dolan et al. 2008; Guinan et al. 2009; Dorschel et al. 2009). Both sensors can
cover a reasonable subset (e.g., tens of km
2 ) of the spatial area of a typical coldwater coral field. However, the datasets that these sensors generate are often
limited by a poor underwater positioning system and/or coarse resolution.
Side-scan sonar (SSS) produces images of the seabed by transmitting acoustic
waves through side antennas that intercept the seafloor at grazing incidence
(Blondel 2009). SSS is frequently used to map cold-water coral ecosystems
because it can readily differentiate coral habitats from the surrounding seabed
(Fosså et al. 2005). Coral habitats produce high acoustic amplitudes and thus are
particularly visible in SSS images against a soft and smooth seabed, which are
characterized by relatively low amplitudes. Most SSS sensors are deep-towed near
the seafloor which allows the use of higher frequencies. As frequency increases,
the acoustic wavelength shortens and so does the time interval between two
successive measurements of acoustic energy (i.e., ping rate). Thus, towing the
sensor near the seabed makes it possible to generate maps at meter-resolution (e.g.,
Mienis et al. 2006). The resulting layback (i.e., distance) between the sensor and
the mother ship, however, creates operational challenges that can affect survey
effectiveness and data quality (Northcutt et al. 2000).
Pulling the ‘towfish’ on which the SSS is mounted requires an extensive amount
of tow cable (up to 10,000 m), which substantially increases drag on the mother
ship (Northcutt et al. 2000). This limits survey speed to *2.5 knots, restricting the
area that can be covered in a given cruise, and thus reducing the cost effectiveness
of deep-towed SSS surveys (Northcutt et al. 2000). Traditionally, the towfish
position is calculated using trigonometric relationships between cable length,
towfish depth, and ship speed. However, sinuosity of the ship’s track and drifting of
the sensor due to strong ocean bottom currents, which are often observed in coldwater coral fields, can result in significant positioning errors (e.g., ±50 m; Wheeler
et al. 2005a). One strategy for reducing these errors is to place an additional
acoustic beacon on the cable or the towfish. This beacon relays the position of the
towfish to the mother ship; however, this extra beacon can add noise to the collected
data due to acoustic interference and/or can destabilize the cable when directly
attached to it (Fosså et al. 2005). Alternatively, the towfish position can be acquired
by placing an encompassing grid of acoustic transponders on the seafloor, but this
requires significant time and finances (Blondel and Murton 1997).
Another major limitation of acoustic images produced from SSS sensors is that
they do not acquire topographic data (see Chap. 8), although the height of objects
such as mounds can be roughly estimated from trigonometry (Blondel 2009).
10 Deep Acoustic Applications
257
Weering et al. 2003; van Rooij et al. 2003).
The finding of large mound structures along the European margin (e.g., Kenyon
et al. 2003) as well as the improvement of acoustic mapping tools triggered a new
phase in the study of cold-water coral ecosystems in the last decade. Deep-towed
side-scan sonar and more recently multi-beam systems are now the most common
acoustic sensors used to map these habitats (Paul et al. 2000; Huvenne et al. 2002;
Foubert et al. 2005; Wheeler et al. 2005a; Roberts et al. 2005; Mienis et al. 2006;
Dolan et al. 2008; Guinan et al. 2009; Dorschel et al. 2009). Both sensors can
cover a reasonable subset (e.g., tens of km
2 ) of the spatial area of a typical coldwater coral field. However, the datasets that these sensors generate are often
limited by a poor underwater positioning system and/or coarse resolution.
Side-scan sonar (SSS) produces images of the seabed by transmitting acoustic
waves through side antennas that intercept the seafloor at grazing incidence
(Blondel 2009). SSS is frequently used to map cold-water coral ecosystems
because it can readily differentiate coral habitats from the surrounding seabed
(Fosså et al. 2005). Coral habitats produce high acoustic amplitudes and thus are
particularly visible in SSS images against a soft and smooth seabed, which are
characterized by relatively low amplitudes. Most SSS sensors are deep-towed near
the seafloor which allows the use of higher frequencies. As frequency increases,
the acoustic wavelength shortens and so does the time interval between two
successive measurements of acoustic energy (i.e., ping rate). Thus, towing the
sensor near the seabed makes it possible to generate maps at meter-resolution (e.g.,
Mienis et al. 2006). The resulting layback (i.e., distance) between the sensor and
the mother ship, however, creates operational challenges that can affect survey
effectiveness and data quality (Northcutt et al. 2000).
Pulling the ‘towfish’ on which the SSS is mounted requires an extensive amount
of tow cable (up to 10,000 m), which substantially increases drag on the mother
ship (Northcutt et al. 2000). This limits survey speed to *2.5 knots, restricting the
area that can be covered in a given cruise, and thus reducing the cost effectiveness
of deep-towed SSS surveys (Northcutt et al. 2000). Traditionally, the towfish
position is calculated using trigonometric relationships between cable length,
towfish depth, and ship speed. However, sinuosity of the ship’s track and drifting of
the sensor due to strong ocean bottom currents, which are often observed in coldwater coral fields, can result in significant positioning errors (e.g., ±50 m; Wheeler
et al. 2005a). One strategy for reducing these errors is to place an additional
acoustic beacon on the cable or the towfish. This beacon relays the position of the
towfish to the mother ship; however, this extra beacon can add noise to the collected
data due to acoustic interference and/or can destabilize the cable when directly
attached to it (Fosså et al. 2005). Alternatively, the towfish position can be acquired
by placing an encompassing grid of acoustic transponders on the seafloor, but this
requires significant time and finances (Blondel and Murton 1997).
Another major limitation of acoustic images produced from SSS sensors is that
they do not acquire topographic data (see Chap. 8), although the height of objects
such as mounds can be roughly estimated from trigonometry (Blondel 2009).
10 Deep Acoustic Applications
257