24
directional component. Such devices can be mounted on a buoy or a manned or
autonomous surface vessel. Buoys designed expressly or primarily for wave measurements are discussed in the chapter referring to observing platforms. Alternatively,
stand- alone devices can be custom mounted aboard general purpose ODAS buoys.
Buoy- mounted sensors can achieve wave height precision down to about 1 cm but,
for operational purposes, wave heights are commonly reported at 0.5 m intervals
with the smallest bin thus being 0.5 m (50 cm) amplitude.
Alternatively, buoy-mounted wave measurements can now be achieved using
GPS. Remarkably, directional GPS buoys now achieve a precision of 1–2 cm based
solely on GPS positioning data. These buoys can be operated in the moored mode
but can also be towed or be operated in free-floating Lagrangian mode.
Bottom-Mounted Wave Sensors
Bottom-mounted acoustic instruments also effectively provide wave measurements
in the coastal zone. Specialized bottom-mounted ADCPs equipped with dedicated
upward-looking wave transducers are available commercially. The upward-looking
transducer tracks the air-sea interface while the standard slanted transducers sample
the wave-induced orbital motion of near-surface suspended particles. Such instruments, operating at 400 kHz, can be effectively installed at depths down to 100 m.
Higher frequencies may be used for shallower emplacements. Resolution is on the
order of 1 cm. Manufacturers provide proprietary software to derive the common
variables of significant wave height, period, and direction from the acoustic data
(Fig. 2.9).
Bottom-mounted pressure sensors were once the preferred solution for wave
measurements (Williams 1973), but their use today is restricted to special applications such as the determination of tidal elevation or characterization of surf zone
waves where the newer technologies may de inapplicable. These instruments are
commonly fitted with high-precision piezoresistive pressure sensors which provide
greater resolution than the formerly used strain gauge transducers.
One exception to the above is the NOAA DART tsunami detection system. In
this system, an ocean bottom emplacement housing a bottom pressure recorder and
an acoustic modem is deployed at basin depths between 4 and 5 km. The modem
transmits data at 15–18 kHz to a moored surface buoy with a satellite link.
Operational DART systems are deployed along the northwestern Atlantic coast, in
the Gulf of Mexico and Caribbean Sea, and in the Pacific and Indian Oceans.
Internal Waves
Internal waves propagate along density discontinuities. The main density discontinuity in the world oceans is the thermocline separating the warm mixed layer waters
from water masses at greater depth. A pronounced density discontinuity is apparent
2 Electronic Sensors and Instruments for Coastal Ocean Observing
directional component. Such devices can be mounted on a buoy or a manned or
autonomous surface vessel. Buoys designed expressly or primarily for wave measurements are discussed in the chapter referring to observing platforms. Alternatively,
stand- alone devices can be custom mounted aboard general purpose ODAS buoys.
Buoy- mounted sensors can achieve wave height precision down to about 1 cm but,
for operational purposes, wave heights are commonly reported at 0.5 m intervals
with the smallest bin thus being 0.5 m (50 cm) amplitude.
Alternatively, buoy-mounted wave measurements can now be achieved using
GPS. Remarkably, directional GPS buoys now achieve a precision of 1–2 cm based
solely on GPS positioning data. These buoys can be operated in the moored mode
but can also be towed or be operated in free-floating Lagrangian mode.
Bottom-Mounted Wave Sensors
Bottom-mounted acoustic instruments also effectively provide wave measurements
in the coastal zone. Specialized bottom-mounted ADCPs equipped with dedicated
upward-looking wave transducers are available commercially. The upward-looking
transducer tracks the air-sea interface while the standard slanted transducers sample
the wave-induced orbital motion of near-surface suspended particles. Such instruments, operating at 400 kHz, can be effectively installed at depths down to 100 m.
Higher frequencies may be used for shallower emplacements. Resolution is on the
order of 1 cm. Manufacturers provide proprietary software to derive the common
variables of significant wave height, period, and direction from the acoustic data
(Fig. 2.9).
Bottom-mounted pressure sensors were once the preferred solution for wave
measurements (Williams 1973), but their use today is restricted to special applications such as the determination of tidal elevation or characterization of surf zone
waves where the newer technologies may de inapplicable. These instruments are
commonly fitted with high-precision piezoresistive pressure sensors which provide
greater resolution than the formerly used strain gauge transducers.
One exception to the above is the NOAA DART tsunami detection system. In
this system, an ocean bottom emplacement housing a bottom pressure recorder and
an acoustic modem is deployed at basin depths between 4 and 5 km. The modem
transmits data at 15–18 kHz to a moored surface buoy with a satellite link.
Operational DART systems are deployed along the northwestern Atlantic coast, in
the Gulf of Mexico and Caribbean Sea, and in the Pacific and Indian Oceans.
Internal Waves
Internal waves propagate along density discontinuities. The main density discontinuity in the world oceans is the thermocline separating the warm mixed layer waters
from water masses at greater depth. A pronounced density discontinuity is apparent
2 Electronic Sensors and Instruments for Coastal Ocean Observing