the 12 kHz Simrad EM 120 system, contains 191 beams over a range of 150°,
producing a beam width of *1° in cross-track direction (Kongsberg 2005). The
bin-size resolution (i.e., the seafloor area over which sounding occurs and backscatter is measured) and the swath width of the DEM depend on the width of the
beams when they encounter the seabed (Fig. 10.2). Beam width expands as distance increases between the sensor and the seafloor. On the hull-mounted multibeam systems, the sensor is far from the cold-water coral habitats they measure.
Thus hull-mounted systems have generated maps with relatively large swath but
low resolution (Fig. 10.2).
Advantages of mounting the system on the mother vessel are that the ship’s
motion, heading, roll, pitch, heave, and position can be accurately measured by
ancillary sensors and GPS receivers (Courtney and Shaw 2000). These ship-related
measurements can be used to correct the multi-beam data and increase DEM
quality. Even with these corrections, however, hull-mounted multi-beam sonar
systems produce cold-water coral habitat maps that are approximately 10-fold
coarser in resolution than the maps acquired by deep-towed SSS platforms. In
addition, the acoustic image recorded by the hull-mounted multi-beam system is
usually of lower quality in terms of backscatter intensity contrast than the data
produced by deep-towed SSS systems. This is mainly because the incident angle in
hull-mounted multi-beam systems is more variable in multi-beam systems relative
to those from deep-towed SSS, and the measured reflectivity is averaged within
each beam across its entire swath (Lurton 2002; Fosså et al. 2005).
Taken together, deep-towed SSS and hull-mounted multi-beam sonars have
different advantages and limitations: SSS is ideal for imaging the seabed, whereas
multi-beam provides accurate topography. Together these data are critical for
assessing the distribution of cold-water coral habitats and the environmental forces
that influence them. Therefore, the most powerful survey approach to produce both
high quality and fine-scale resolution topographic and sonar maps is to combine
both sensors in a single vehicle that can: (1) travel autonomously near the seafloor,
and (2) carry internal sensors that precisely measure the motion and position of the
platform. AUVs and ROVs have no umbilical cable to the mother vessel. AUVs
are typically torpedo-shaped platforms and therefore can maneuver on a more
exact route and at higher speeds relative to ROVs and deep-towed SSS (George
et al. 2003). For example, AUVs can travel up to 4 knots, approximately twice as
fast as most deep-towed SSS surveys and ROVs (Northcut et al. 2000). The
autonomy of an AUV also increases survey efficiency relative to deep-towed
platforms. For example, in a gridded survey, a vehicle must turn 180° when it
reaches the end of each line, and begin a new line adjacent to the previous line
(i.e., line turn). The line turn duration of an AUV is about 5 min; the same
procedure can take up to six hours in a deep-towed survey (Northcut et al. 2000).
Furthermore, AUVs can employ multiple navigation and mapping tools in order to
acquire simultaneously bathymetry, backscatter data, and sub-surface profiles as
well as environmental parameters such as current data and temperature. Thus,
AUVs can produce highly informative and accurate integrated datasets.
10 Deep Acoustic Applications
259
producing a beam width of *1° in cross-track direction (Kongsberg 2005). The
bin-size resolution (i.e., the seafloor area over which sounding occurs and backscatter is measured) and the swath width of the DEM depend on the width of the
beams when they encounter the seabed (Fig. 10.2). Beam width expands as distance increases between the sensor and the seafloor. On the hull-mounted multibeam systems, the sensor is far from the cold-water coral habitats they measure.
Thus hull-mounted systems have generated maps with relatively large swath but
low resolution (Fig. 10.2).
Advantages of mounting the system on the mother vessel are that the ship’s
motion, heading, roll, pitch, heave, and position can be accurately measured by
ancillary sensors and GPS receivers (Courtney and Shaw 2000). These ship-related
measurements can be used to correct the multi-beam data and increase DEM
quality. Even with these corrections, however, hull-mounted multi-beam sonar
systems produce cold-water coral habitat maps that are approximately 10-fold
coarser in resolution than the maps acquired by deep-towed SSS platforms. In
addition, the acoustic image recorded by the hull-mounted multi-beam system is
usually of lower quality in terms of backscatter intensity contrast than the data
produced by deep-towed SSS systems. This is mainly because the incident angle in
hull-mounted multi-beam systems is more variable in multi-beam systems relative
to those from deep-towed SSS, and the measured reflectivity is averaged within
each beam across its entire swath (Lurton 2002; Fosså et al. 2005).
Taken together, deep-towed SSS and hull-mounted multi-beam sonars have
different advantages and limitations: SSS is ideal for imaging the seabed, whereas
multi-beam provides accurate topography. Together these data are critical for
assessing the distribution of cold-water coral habitats and the environmental forces
that influence them. Therefore, the most powerful survey approach to produce both
high quality and fine-scale resolution topographic and sonar maps is to combine
both sensors in a single vehicle that can: (1) travel autonomously near the seafloor,
and (2) carry internal sensors that precisely measure the motion and position of the
platform. AUVs and ROVs have no umbilical cable to the mother vessel. AUVs
are typically torpedo-shaped platforms and therefore can maneuver on a more
exact route and at higher speeds relative to ROVs and deep-towed SSS (George
et al. 2003). For example, AUVs can travel up to 4 knots, approximately twice as
fast as most deep-towed SSS surveys and ROVs (Northcut et al. 2000). The
autonomy of an AUV also increases survey efficiency relative to deep-towed
platforms. For example, in a gridded survey, a vehicle must turn 180° when it
reaches the end of each line, and begin a new line adjacent to the previous line
(i.e., line turn). The line turn duration of an AUV is about 5 min; the same
procedure can take up to six hours in a deep-towed survey (Northcut et al. 2000).
Furthermore, AUVs can employ multiple navigation and mapping tools in order to
acquire simultaneously bathymetry, backscatter data, and sub-surface profiles as
well as environmental parameters such as current data and temperature. Thus,
AUVs can produce highly informative and accurate integrated datasets.
10 Deep Acoustic Applications
259