Introduction to the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 17

The range and resolution of these instruments is set by the acoustic frequency
used and the typical densities of 'scatterers' (suspended sediments or zooplankton)
in the water column. For the deepest parts of the shelf seas, a frequency of 150 kHz
allows coverage of the full 200-metre water column but with a vertical resolution of
∼

4 metres. Higher resolution in shallower water can be achieved with instruments
of higher frequencies but with range decreasing to

20 metres at a frequency of
2MHz. These instruments provide much fuller, non-invasive coverage of the velocity
structure than could be achieved with a string of conventional current meters. When
mounted on the seabed, they are also much less vulnerable to damage by fishing
vessels than a conventional current meter mooring.
ADCPs are also used in a hull-mounted form on research vessels to monitor the
flow field below the vessel. In the shelf seas a special 'bottom track' sound pulse
returned from the seabed allows the determination of the ship's movement relative to
the seabed. By subtracting this bottom track velocity from the water velocities
relative to the ship, a vertical profile of velocity relative to the seabed can be recorded
by the ADCP while the ship is engaged in survey work. This provides a useful
supplement to measurements from bottom-mounted ADCPs and can be a valuable
way of monitoring water movements during deployment and recovery operations.
In most situations, ADCPs are now the instrument of choice for velocity measure-
ments and in the last three decades have helped to greatly advance oceanography. They
are not, however, without disadvantages. Even when they have adequate range, the
coverage of the water column by a bottom-mounted ADCP is restricted in two ways.
First, near the transducers there is a 'blank' space ignored by the instrument because
of the time taken to switch from transmit to receive modes. This can be up to several
metres for low-frequency instruments. Second, there is a zone of no data at the water
column boundary. For instance, in the case of an upward-looking ADCP no data can
be retrieved from a thin layer at the sea surface. With a 20 beam angle, this layer has
a thickness of 6% of the distance between the ADCP and the surface. A further
limitation is that, while ADCP's can be equipped with temperature, conductivity and
pressure sensors, the measurements of these parameters are restricted to the location
of the instrument. In order to provide full water column temperature and salinity
information to complement the velocity profile data, it is necessary to have a
supplementary mooring equipped with a string of CTD sensors.

∼

1.5.4 Drifters, gliders and AUVs

The alternative to measuring currents with instruments fixed relative to the seabed
(the so-called Eulerian current) is to track drifting drogued buoys to determine the
movement of water particles (the Lagrangian current). For a limited horizontal
domain, such as a small estuary, surface currents can be determined simply by
observing the movement of passive surface drifters or dye visually from an aircraft
or a high vantage point on land. For larger scales the drifters can be tracked
by relaying their GPS position by radio to a shore station or via a satellite link.
To determine subsurface currents, it is necessary to use a large drogue located at a