Introduction to the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 16

Figure 1.8 A mooring frame containing an upward-looking 300 kHz ADCP about to be
deployed in the Celtic Sea. The orange buoys bring the frame back to the sea surface when,
at the end of the deployment, the acoustic release is signalled to let go of a weight in the
base of the frame. (Photo by J. Sharples.)

allows the depth of the measurements to be monitored and any set-down of the
mooring in strong flows to be detected.
A major advance in current meter technology was the development of acoustic
current meters based on the Doppler principle. Sound waves travelling away from a
fixed source are reflected back by suspended particles in the water with a shift in
frequency which is proportional to the water velocity in the beam direction. The idea
of detecting this return signal and determining the velocity from the Doppler shift has
long been known, but the signal strength of acoustic return is so weak ( 10 9 of the

transmitted power) that Doppler current meters for the ocean seemed to be technic-
ally unrealistic. It was only with significant advances in electronics and signal
processing in the 1980s that practical Doppler instruments became available.
Modern Doppler instruments, such as the one shown in Fig. 1.8 , typically have
three or four acoustic beams oriented at an angle of 20 or 30 to the vertical and may
be operated in upward- or downward-looking mode. The velocity at each level is
found by determining the Doppler shift of a section of the return signal from an
acoustic pulse sent out by the transducer. The three or four along beam velocities are
then combined to evaluate the Cartesian current components at each level. In this
way, a profile of the velocity in the water column is constructed. The instruments are
widely known as Acoustic Doppler Current Profilers or simply ADCPs.

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