15
looking instrument can provide vertical profiles of current speed and direction to
depths as great as 1 km. Instrument designs typically mount three or preferably four
transducers in a cloverleaf pattern at inclination of 20°–30° from the axis of the
cylindrical instrument pressure housing. Operational deployments typically sample
at 0.5–1 h intervals for months to years without need for servicing.
Sound attenuation in the ocean is a function of frequency; as the frequency
increases, so does the attenuation. Thus, attenuation of a 10 kHz beam is on the
order of 0.5 dB.km
−1
whereas that for a 100 kHz beam is around 13 dB.km
−1
, about
26 times greater (Marsh 1969). Acoustic profiling instruments operating across path
lengths of tens to hundreds of kilometers mostly operate at lower frequencies (in the
range of 10s to 100s of kHz) in contrast to the acoustic current meters discussed
above where the path length is measured in centimeters. In operation, each transducer creates a brief coded sound pulse emitting a conical beam and then suspends
transmission to capture echoes returning off particles drifting with the current.
Repeat-sequence coding using a seven bit code allows increased time/distance resolution (Pinkel and Smith 1992). Sound frequency is down-shifted by particles receding from the instrument or up-shifted by those approaching. Knowledge of the
speed of sound through seawater (1400–1530 m.s
−1
depending on the temperature
and pressure) allows calculation of the target distance. Particles reflecting acoustic
signals at around 500 kHz are probably zoo- and macro-plankton. Vertical structure
and migration of zooplankton aggregations can be effectively characterized using
ADCP, but the scatter around the linear response of mixed populations varies to
such an extent as to make meaningful biomass estimates difficult (Fielding 2004).
In practice, Doppler data is typically aggregated into depth bins encompassing
spans between 0.25 and 20 meters. High power instruments (about 1.5 kW) operating at low frequencies (55/75 kHz dual frequency) can now provide reliable profiles
spanning 1 km depth. Hull-mounted instruments aboard ocean going vessels operating at 75 kHz routinely achieve current velocity profiles to depths down to 700 m
while underway. Mid-range instruments for coastal applications operating at 300–
400 kHz achieve depth ranges of 50–100 m, while high frequency instruments
(1–2 MHz) provide fine detail with bin sizes down to 0.1 m over ranges of 10–20 m.
A single instrument may be equipped with two sets of transducers operating at different frequencies to take advantage of the fine resolution afforded by high frequencies and the long range achieved with low frequencies. Deep profiles may be
achieved on station by lowering the ADCP through the water column and correcting
for vessel drift using GPS. For such operations, the ADCP may be mounted in the
carousel cage so that the hydrographic profile, water sampling, and the acoustic
Doppler profile may all be taken concurrently.
Doppler profilers can be mounted in several configurations including downwardlooking such as on buoys and the ship hull mounts discussed above, upward-looking
sea bottom emplacements, or sideward looking on pilings or other fixed structures.
A combination of the two latter options is optimal for busy harbor operations where
swift currents in confined spaces may constitute a hazard concern.
The first one or two data bins in the proximity of ADCP instrument are commonly blanked out during the brief transition from the transmit mode to the receive
2.2 Electronic Sensors and Instruments for Ocean Observing
looking instrument can provide vertical profiles of current speed and direction to
depths as great as 1 km. Instrument designs typically mount three or preferably four
transducers in a cloverleaf pattern at inclination of 20°–30° from the axis of the
cylindrical instrument pressure housing. Operational deployments typically sample
at 0.5–1 h intervals for months to years without need for servicing.
Sound attenuation in the ocean is a function of frequency; as the frequency
increases, so does the attenuation. Thus, attenuation of a 10 kHz beam is on the
order of 0.5 dB.km
−1
whereas that for a 100 kHz beam is around 13 dB.km
−1
, about
26 times greater (Marsh 1969). Acoustic profiling instruments operating across path
lengths of tens to hundreds of kilometers mostly operate at lower frequencies (in the
range of 10s to 100s of kHz) in contrast to the acoustic current meters discussed
above where the path length is measured in centimeters. In operation, each transducer creates a brief coded sound pulse emitting a conical beam and then suspends
transmission to capture echoes returning off particles drifting with the current.
Repeat-sequence coding using a seven bit code allows increased time/distance resolution (Pinkel and Smith 1992). Sound frequency is down-shifted by particles receding from the instrument or up-shifted by those approaching. Knowledge of the
speed of sound through seawater (1400–1530 m.s
−1
depending on the temperature
and pressure) allows calculation of the target distance. Particles reflecting acoustic
signals at around 500 kHz are probably zoo- and macro-plankton. Vertical structure
and migration of zooplankton aggregations can be effectively characterized using
ADCP, but the scatter around the linear response of mixed populations varies to
such an extent as to make meaningful biomass estimates difficult (Fielding 2004).
In practice, Doppler data is typically aggregated into depth bins encompassing
spans between 0.25 and 20 meters. High power instruments (about 1.5 kW) operating at low frequencies (55/75 kHz dual frequency) can now provide reliable profiles
spanning 1 km depth. Hull-mounted instruments aboard ocean going vessels operating at 75 kHz routinely achieve current velocity profiles to depths down to 700 m
while underway. Mid-range instruments for coastal applications operating at 300–
400 kHz achieve depth ranges of 50–100 m, while high frequency instruments
(1–2 MHz) provide fine detail with bin sizes down to 0.1 m over ranges of 10–20 m.
A single instrument may be equipped with two sets of transducers operating at different frequencies to take advantage of the fine resolution afforded by high frequencies and the long range achieved with low frequencies. Deep profiles may be
achieved on station by lowering the ADCP through the water column and correcting
for vessel drift using GPS. For such operations, the ADCP may be mounted in the
carousel cage so that the hydrographic profile, water sampling, and the acoustic
Doppler profile may all be taken concurrently.
Doppler profilers can be mounted in several configurations including downwardlooking such as on buoys and the ship hull mounts discussed above, upward-looking
sea bottom emplacements, or sideward looking on pilings or other fixed structures.
A combination of the two latter options is optimal for busy harbor operations where
swift currents in confined spaces may constitute a hazard concern.
The first one or two data bins in the proximity of ADCP instrument are commonly blanked out during the brief transition from the transmit mode to the receive
2.2 Electronic Sensors and Instruments for Ocean Observing