from a portion of the first echo reflecting off the air–water interface) generally relate to
seabed roughness and hardness, respectively. In one analysis approach, user-defined
polygons can be drawn around clusters of points on a plot of E1 versus E2 for known
bottom types, typically distinguished by grain size. Subsequent survey data are then
classified according to which box each E1:E2 data pair falls within. A second
approach, exemplified by QTC IMPACT (Quester Tangent Corporation 2002; Preston
et al. 2004) and other non-commercial systems (e.g., van Walree et al. 2005) analyzes
features derived from the first echo only. Using this approach, a large number of
features describing the shape, duration, and for some systems the amplitude, of the first
echo are computed, reduced to a smaller number of uncorrelated variables using
principal components analysis and clustered. The clusters are assigned different
bottom types and used to classify subsequent survey data.
While the capability of both approaches to infer grain size is well-established,
the theoretical basis for using ASC to classify reef environments is more complicated than the analysis of sedimentary environments alone, due to factors such
as high topographic complexity, large slopes, and variable assemblages of epibenthic biota. ASC systems that store digitized waveforms (e.g., Biosonics,
Simrad, QTC) offer greater scope and utility compared to analog systems that only
output parameters derived from the raw waveforms. Customized methods of
waveform analysis, potentially at multiple frequencies, can be used to produce
multivariate datasets that provide greater bottom type discrimination for a larger
variety of applications.
Split-beam echo sounder systems: These are another specialization of singlebeam echo sounders. In contrast to simple single-beam systems, the returning echo
is received across four quadrants, and it is the phase difference across the four
quadrants that determine the angular position of the target in the beam. The ability
to determine accurate angular positioning in the beam allows for precise determination of target strength. Split-beam echo sounders are used primarily for scientific surveys of fishery resources, where accurate estimates of echo intensity
(e.g., fish target strength) are necessary to infer fish sizes and for scaling to
biomass and density. Split-beam echo sounders can also be used as bottom finders,
as part of an ASC system, or added to the compliment of instrumentation (e.g.,
MBES, SSS) in a benthic habitat mapping system. The benefit of the split-beam
echo sounders is the ability to detect fish and fauna in the water column simultaneous with bottom detection, and to make inferences on the distribution of
biomass and habitat use by fish on coral reefs.
Sidescan sonar (SSS): SSS are essentially two echo sounders in a single
housing that transmit beams port and starboard. Frequencies are typically in the
range of 100–500 kHz. Higher frequencies produce higher resolutions (1–10s of
cm) but at a reduced range (i.e., narrower swath width; see Chap. 8). SSS can be
operated over a broad range of depths, from sub-meter to 100s of meters, but geolocation of the towed instrument can be problematic. Unlike the other acoustic
systems, SSS does not directly produce bathymetry, but instead focuses on
backscatter intensity. A related sensor, known alternatively as interferometric
sonar (IS), or as phase differencing bathymetric sonar (PDBS), produces a
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G. Foster et al.
seabed roughness and hardness, respectively. In one analysis approach, user-defined
polygons can be drawn around clusters of points on a plot of E1 versus E2 for known
bottom types, typically distinguished by grain size. Subsequent survey data are then
classified according to which box each E1:E2 data pair falls within. A second
approach, exemplified by QTC IMPACT (Quester Tangent Corporation 2002; Preston
et al. 2004) and other non-commercial systems (e.g., van Walree et al. 2005) analyzes
features derived from the first echo only. Using this approach, a large number of
features describing the shape, duration, and for some systems the amplitude, of the first
echo are computed, reduced to a smaller number of uncorrelated variables using
principal components analysis and clustered. The clusters are assigned different
bottom types and used to classify subsequent survey data.
While the capability of both approaches to infer grain size is well-established,
the theoretical basis for using ASC to classify reef environments is more complicated than the analysis of sedimentary environments alone, due to factors such
as high topographic complexity, large slopes, and variable assemblages of epibenthic biota. ASC systems that store digitized waveforms (e.g., Biosonics,
Simrad, QTC) offer greater scope and utility compared to analog systems that only
output parameters derived from the raw waveforms. Customized methods of
waveform analysis, potentially at multiple frequencies, can be used to produce
multivariate datasets that provide greater bottom type discrimination for a larger
variety of applications.
Split-beam echo sounder systems: These are another specialization of singlebeam echo sounders. In contrast to simple single-beam systems, the returning echo
is received across four quadrants, and it is the phase difference across the four
quadrants that determine the angular position of the target in the beam. The ability
to determine accurate angular positioning in the beam allows for precise determination of target strength. Split-beam echo sounders are used primarily for scientific surveys of fishery resources, where accurate estimates of echo intensity
(e.g., fish target strength) are necessary to infer fish sizes and for scaling to
biomass and density. Split-beam echo sounders can also be used as bottom finders,
as part of an ASC system, or added to the compliment of instrumentation (e.g.,
MBES, SSS) in a benthic habitat mapping system. The benefit of the split-beam
echo sounders is the ability to detect fish and fauna in the water column simultaneous with bottom detection, and to make inferences on the distribution of
biomass and habitat use by fish on coral reefs.
Sidescan sonar (SSS): SSS are essentially two echo sounders in a single
housing that transmit beams port and starboard. Frequencies are typically in the
range of 100–500 kHz. Higher frequencies produce higher resolutions (1–10s of
cm) but at a reduced range (i.e., narrower swath width; see Chap. 8). SSS can be
operated over a broad range of depths, from sub-meter to 100s of meters, but geolocation of the towed instrument can be problematic. Unlike the other acoustic
systems, SSS does not directly produce bathymetry, but instead focuses on
backscatter intensity. A related sensor, known alternatively as interferometric
sonar (IS), or as phase differencing bathymetric sonar (PDBS), produces a
224
G. Foster et al.