approximately 30 m in the clearest tropical waters and much less for coastal or
temperate waters. Areas that cannot be mapped with satellite or aerial imagery are
both extensive and ecologically important. For example, over 55 % of the Florida
Keys National Marine Sanctuary (about 1,540 square nautical miles) has not been
mapped due to water depth or clarity limitations (FMRI 1998). The Tortugas Bank,
Pulley Ridge, and Flower Garden Banks are three examples from the Gulf of
Mexico that illustrate the potential for luxuriant communities of shallow-water
(zooxanthellate) corals to exist at ‘‘mesophotic’’ depths of 30–75 m (Miller et al.
2001; Hickerson and Schmahl 2005; Jarrett et al. 2005). In addition, coral communities can exist below the photic zone, where deep-water (azooxanthellate)
corals form mounds up to several hundred meters high (see Chap. 10). Recent
ocean exploration initiatives indicate that such deep corals are much more
extensive than previously thought (Roberts et al. 2006). Deep corals provide
important habitat for fishes, and shallow coral species may potentially find refuge
from warming surface waters at mesophotic depths (Riegl and Piller 2003).
Acoustics can also be used to obtain information that is not readily accessible by
optical techniques, including but not limited to grain size distribution, bedform
patterns, abundance and canopy height of epibiota, bathymetry, and measures of
topographic complexity. Acoustics can also be merged with optical datasets to
enhance classification accuracy (Bejarano et al. 2010).
9.1.3 Acoustic Remote Sensing Platforms
Single-beam echo sounders (SBES): The simplest systems are vertical-incident
single-beam echo sounders that measure depth only. Inexpensive bottom finders,
which are categorized as SBES, are capable of producing reasonably accurate
bathymetry. Advanced SBES can run from shallow to full ocean depth at very high
accuracy. When coupled with a tide gauge and multiplexed with a GPS, SBES can
reveal seafloor zonation patterns in areas where little is known. Heyman et al.
(2007), for example, exploited an inexpensive, off the-shelf SBES system to map
bathymetry at two reef fish spawning aggregation sites in Belize.
Acoustic seabed classification systems (ASC): ASC systems are more
sophisticated scientific versions of single-beam echo sounders. ASC systems
extract information from the returned echo waveforms that, in conjunction with
ground validation, can be related to physical and biological properties of the
seabed. As with other acoustic systems, ground validation can be accomplished by
visual observations using divers, towed video, drop cameras, or by measurement of
sediment physical properties.
Most commerically available ASC systems fall into one of two approaches for
signal processing. The first approach, exemplified by the RoxAnn and ECHOplus
systems, exploits the intensity of both the first and second seabed echoes (Chivers et al.
1990). When acquired over relatively flat and monotonous seabeds, E1 (the trailing
edge of the first echo envelope) and E2 (the complete second echo envelope resulting
9 Acoustic Applications
223
temperate waters. Areas that cannot be mapped with satellite or aerial imagery are
both extensive and ecologically important. For example, over 55 % of the Florida
Keys National Marine Sanctuary (about 1,540 square nautical miles) has not been
mapped due to water depth or clarity limitations (FMRI 1998). The Tortugas Bank,
Pulley Ridge, and Flower Garden Banks are three examples from the Gulf of
Mexico that illustrate the potential for luxuriant communities of shallow-water
(zooxanthellate) corals to exist at ‘‘mesophotic’’ depths of 30–75 m (Miller et al.
2001; Hickerson and Schmahl 2005; Jarrett et al. 2005). In addition, coral communities can exist below the photic zone, where deep-water (azooxanthellate)
corals form mounds up to several hundred meters high (see Chap. 10). Recent
ocean exploration initiatives indicate that such deep corals are much more
extensive than previously thought (Roberts et al. 2006). Deep corals provide
important habitat for fishes, and shallow coral species may potentially find refuge
from warming surface waters at mesophotic depths (Riegl and Piller 2003).
Acoustics can also be used to obtain information that is not readily accessible by
optical techniques, including but not limited to grain size distribution, bedform
patterns, abundance and canopy height of epibiota, bathymetry, and measures of
topographic complexity. Acoustics can also be merged with optical datasets to
enhance classification accuracy (Bejarano et al. 2010).
9.1.3 Acoustic Remote Sensing Platforms
Single-beam echo sounders (SBES): The simplest systems are vertical-incident
single-beam echo sounders that measure depth only. Inexpensive bottom finders,
which are categorized as SBES, are capable of producing reasonably accurate
bathymetry. Advanced SBES can run from shallow to full ocean depth at very high
accuracy. When coupled with a tide gauge and multiplexed with a GPS, SBES can
reveal seafloor zonation patterns in areas where little is known. Heyman et al.
(2007), for example, exploited an inexpensive, off the-shelf SBES system to map
bathymetry at two reef fish spawning aggregation sites in Belize.
Acoustic seabed classification systems (ASC): ASC systems are more
sophisticated scientific versions of single-beam echo sounders. ASC systems
extract information from the returned echo waveforms that, in conjunction with
ground validation, can be related to physical and biological properties of the
seabed. As with other acoustic systems, ground validation can be accomplished by
visual observations using divers, towed video, drop cameras, or by measurement of
sediment physical properties.
Most commerically available ASC systems fall into one of two approaches for
signal processing. The first approach, exemplified by the RoxAnn and ECHOplus
systems, exploits the intensity of both the first and second seabed echoes (Chivers et al.
1990). When acquired over relatively flat and monotonous seabeds, E1 (the trailing
edge of the first echo envelope) and E2 (the complete second echo envelope resulting
9 Acoustic Applications
223