In total, 90.2 km
2 of the seafloor in and around the VICRNM was characterized
using MBES imagery. This area (both outside and inside the Monument’s
boundaries) was dominated by rhodoliths (i.e., calcareous algal nodules). Hard and
soft corals were present in low densities (0 B 10 %), although one 0.25 km
2 area
outside the Monument was dominated by high density live coral (i.e., 50 C 90 %).
In general, the habitat map denoted the presence of slightly more live coral outside
(than inside) the current VICRNM boundaries. These quantitative results illustrate
the utility of MBES imagery for moderate-depth habitat mapping and for ecosystem-based resource management. With that in mind, expediting the rate of
MBES seafloor mapping will require the collection of datasets that concurrently
address the needs of multiple users (Costa et al. 2009b), such as the ‘‘collect once,
use many times’’ approach of the Interagency Working Group on Ocean and
Coastal Mapping (IWG-OCM). For ecosystem-based management and marine
spatial planning purposes, maps with high thematic accuracies and resolutions are
important to have because predictions of species diversity, abundances and distributions may differ depending on the thematic properties of the input maps
(Kendall and Miller 2008).
9.2.3 Phase Differencing Bathymetric Sonar
Benthic habitats in shallow (\30 m), perpetually turbid coral reef environments
are exceptionally challenging to characterize. In particular, they are challenging
because many conventional mapping technologies are unable to synoptically map
these areas (e.g., passive and active optical sensors), or are inefficient and costly
for mapping these areas (e.g., MBES systems). The use of interferometric sonars
(IS), also known as phase differencing bathymetric SONAR (PDBS), may fill in
this informational gap, where conditions are not optimal for the operation of other
sensors. Like MBES systems, PDBS can collect co-located bathymetric and
backscatter intensity information. These two pieces of information (along with
underwater video and photos) can be used to develop maps of habitats on the
seafloor. Unlike MBES systems, however, PDBS can collect these spatially
coincident datasets over wide swaths in shallow waters (\30 m), typically 10–129
the depth versus 3–59 depth for MBES systems (Gosnell 2005). PDBS are able to
collect wide swaths in shallow waters because they are not beam forming but
rather they accurately measure depths by precisely measuring the phase offsets of
acoustic returns (Gosnell 2005). These phase offsets are used to calculate the angle
from which the return was received (e.g., Denbigh 1989). This angle is combined
with measurements of range (based on two-way travel time) to calculate the
position (and depth) of the seafloor.
Given that both depth and intensity surfaces are collected, PDBS systems can
be used to develop benthic habitat maps of shallow, turbid coral reef ecosystems.
Such habitat maps, describing the geographic location, geomorphological structure
and biological cover of seafloor habitats, were developed from an interferometric
240
G. Foster et al.
2 of the seafloor in and around the VICRNM was characterized
using MBES imagery. This area (both outside and inside the Monument’s
boundaries) was dominated by rhodoliths (i.e., calcareous algal nodules). Hard and
soft corals were present in low densities (0 B 10 %), although one 0.25 km
2 area
outside the Monument was dominated by high density live coral (i.e., 50 C 90 %).
In general, the habitat map denoted the presence of slightly more live coral outside
(than inside) the current VICRNM boundaries. These quantitative results illustrate
the utility of MBES imagery for moderate-depth habitat mapping and for ecosystem-based resource management. With that in mind, expediting the rate of
MBES seafloor mapping will require the collection of datasets that concurrently
address the needs of multiple users (Costa et al. 2009b), such as the ‘‘collect once,
use many times’’ approach of the Interagency Working Group on Ocean and
Coastal Mapping (IWG-OCM). For ecosystem-based management and marine
spatial planning purposes, maps with high thematic accuracies and resolutions are
important to have because predictions of species diversity, abundances and distributions may differ depending on the thematic properties of the input maps
(Kendall and Miller 2008).
9.2.3 Phase Differencing Bathymetric Sonar
Benthic habitats in shallow (\30 m), perpetually turbid coral reef environments
are exceptionally challenging to characterize. In particular, they are challenging
because many conventional mapping technologies are unable to synoptically map
these areas (e.g., passive and active optical sensors), or are inefficient and costly
for mapping these areas (e.g., MBES systems). The use of interferometric sonars
(IS), also known as phase differencing bathymetric SONAR (PDBS), may fill in
this informational gap, where conditions are not optimal for the operation of other
sensors. Like MBES systems, PDBS can collect co-located bathymetric and
backscatter intensity information. These two pieces of information (along with
underwater video and photos) can be used to develop maps of habitats on the
seafloor. Unlike MBES systems, however, PDBS can collect these spatially
coincident datasets over wide swaths in shallow waters (\30 m), typically 10–129
the depth versus 3–59 depth for MBES systems (Gosnell 2005). PDBS are able to
collect wide swaths in shallow waters because they are not beam forming but
rather they accurately measure depths by precisely measuring the phase offsets of
acoustic returns (Gosnell 2005). These phase offsets are used to calculate the angle
from which the return was received (e.g., Denbigh 1989). This angle is combined
with measurements of range (based on two-way travel time) to calculate the
position (and depth) of the seafloor.
Given that both depth and intensity surfaces are collected, PDBS systems can
be used to develop benthic habitat maps of shallow, turbid coral reef ecosystems.
Such habitat maps, describing the geographic location, geomorphological structure
and biological cover of seafloor habitats, were developed from an interferometric
240
G. Foster et al.