10.3 Cold-Water Coral Mapping Example
10.3.1 Sonar and AUV Configuration
AUVs vary in their mapping capabilities. Here we describe one possible configuration of the C-Surveyor-II AUV from C&C Technologies Inc. (Lafayette,
Louisiana, USA). A crucial component of the C-Surveyor-II AUV is its inertial
navigation system, which allows a post-processing positioning accuracy of 3 m,
given a survey conducted at 800 m water depth (Jalving et al. 2003). A Kalman
filter combines the AUV orientation, speed over ground, and depth outputs
obtained from a gyrocompass, a 300 kHz acoustic doppler current profiler
(ADCP), and a high precision pressure sensor, respectively. Positioning drift is
minimized with measurements from an ultra short baseline (USBL) acoustic
system and a differential global positioning system installed on the deployment
vessel (Chance and Northcutt 2001; George 2006). The AUV is also equipped with
obstacle-avoidance sonar that enables the vehicle to detect and respond to abrupt
changes in relief. Mission endurance is up to 55 h due to the aluminium oxygen
fuel-cell batteries carried in the AUV (George 2006). Considering the AUV’s
travelling speed of *3.8 kts, approximately 50–60 km
2 can be mapped per mission, with parallel line spacing of 200 m.
The seabed mapping sensors on the C-Surveyor-II AUV used in this study
consisted of a 200 kHz Simrad EM2000 multi-beam sonar and a 120 kHz Edgetech SSS system. The multi-beam system contains 111 beams across a 300 m
swath, with 100 m of overlap between swaths. Overlapping the swaths is done to
increase sounding accuracy at the swath edges, since the outermost beams are
usually of lower quality (Lurton 2002). To further increase multi-beam data
quality, raw echo sounding data are corrected for heave, pitch, and roll of the
vehicle using the measurements recorded by the inertial navigation sensors in the
AUV. Although the multi-beam system itself is capable of producing a DEM of
1 m bin size, positioning accuracy limits gridding to a 3 m bin size (George 2006).
The SSS system transmits acoustic waves at about three times per second, which
results in an along-track ping distance of *60 cm (AUV speed of 3.8 kts). The
associated SSS swath is about 400 m, with a 200 m overlap for each line.
The AUV also acquires sub-bottom profiles and environmental parameters
throughout each mission. A 3 kHz Edgetech Chirp system collects sub-bottom
profiles at 200 m spacing with a time penetration of up to 40 m (Correa et al. 2011,
2012). Temperature and salinity are also measured as seawater runs continuously
through a SEB FastCAT CTD sensor. In addition to acquiring the AUV ground
velocity, the ADCP data (combined with gyrocompass measurements) are also
used to determine the velocity vector of the 40 m water column between the
vehicle and the seafloor at one second intervals.
The cruising altitude (40 m) of the AUV is too high for video ground-truthing
of the seafloor; these data must be collected separately. In the surveys described in
subsequent sections of this chapter, ground-truthing was performed using the
260
T. B. S. Correa et al.
10.3.1 Sonar and AUV Configuration
AUVs vary in their mapping capabilities. Here we describe one possible configuration of the C-Surveyor-II AUV from C&C Technologies Inc. (Lafayette,
Louisiana, USA). A crucial component of the C-Surveyor-II AUV is its inertial
navigation system, which allows a post-processing positioning accuracy of 3 m,
given a survey conducted at 800 m water depth (Jalving et al. 2003). A Kalman
filter combines the AUV orientation, speed over ground, and depth outputs
obtained from a gyrocompass, a 300 kHz acoustic doppler current profiler
(ADCP), and a high precision pressure sensor, respectively. Positioning drift is
minimized with measurements from an ultra short baseline (USBL) acoustic
system and a differential global positioning system installed on the deployment
vessel (Chance and Northcutt 2001; George 2006). The AUV is also equipped with
obstacle-avoidance sonar that enables the vehicle to detect and respond to abrupt
changes in relief. Mission endurance is up to 55 h due to the aluminium oxygen
fuel-cell batteries carried in the AUV (George 2006). Considering the AUV’s
travelling speed of *3.8 kts, approximately 50–60 km
2 can be mapped per mission, with parallel line spacing of 200 m.
The seabed mapping sensors on the C-Surveyor-II AUV used in this study
consisted of a 200 kHz Simrad EM2000 multi-beam sonar and a 120 kHz Edgetech SSS system. The multi-beam system contains 111 beams across a 300 m
swath, with 100 m of overlap between swaths. Overlapping the swaths is done to
increase sounding accuracy at the swath edges, since the outermost beams are
usually of lower quality (Lurton 2002). To further increase multi-beam data
quality, raw echo sounding data are corrected for heave, pitch, and roll of the
vehicle using the measurements recorded by the inertial navigation sensors in the
AUV. Although the multi-beam system itself is capable of producing a DEM of
1 m bin size, positioning accuracy limits gridding to a 3 m bin size (George 2006).
The SSS system transmits acoustic waves at about three times per second, which
results in an along-track ping distance of *60 cm (AUV speed of 3.8 kts). The
associated SSS swath is about 400 m, with a 200 m overlap for each line.
The AUV also acquires sub-bottom profiles and environmental parameters
throughout each mission. A 3 kHz Edgetech Chirp system collects sub-bottom
profiles at 200 m spacing with a time penetration of up to 40 m (Correa et al. 2011,
2012). Temperature and salinity are also measured as seawater runs continuously
through a SEB FastCAT CTD sensor. In addition to acquiring the AUV ground
velocity, the ADCP data (combined with gyrocompass measurements) are also
used to determine the velocity vector of the 40 m water column between the
vehicle and the seafloor at one second intervals.
The cruising altitude (40 m) of the AUV is too high for video ground-truthing
of the seafloor; these data must be collected separately. In the surveys described in
subsequent sections of this chapter, ground-truthing was performed using the
260
T. B. S. Correa et al.