Acoustic doppler current profilers (ACDPs) have the potential to make a significant impact on the measurement of currents in continental shelf seas Progress made in the development of one ADCP system is described, and results from five different ADCPs deployed in three different ways-mounted in a sea bed frame, on a mooring and on a ship-are presented Only by deploying the instruments and by comparing their results with other observations will faults in the instrumentation become apparent and the necessary confidence in the technique be gained.
Good quality observations of current profiles are essential if optimum benefit is to be obtained from continental shelf seas The time varying elevation and transport fields of tides and surges, which together with waves dominate shelf sea dynamics, are largely understood, through observations and modelling It is now feasible, because of both new instrumentation such as ADCPs and new more powerful computers, to develop this understanding and to tackle the real, three-dimensional world of shelf seas.
ADCPs have the potential, in one instrument, to measure the current profile over much of the water column in shelf seas This chapter will investigate this potential, look at some of the constraints, and discuss ways of realizing the potential The basic oceanographic design parameters that need specifying (Table I) are the maximum range, the cell size, the accuracy of the measurement, the sample interval, and the duration To satisfy these the instrument designer has at his disposal the acoustic frequency, f, the beamwidth, f, the beam angle to the vertical, ¿, the speed and memory of the microcomputer, the data logger, and battery capacity In brief.
(Formula available in full paper)
TABLE I Development specifications POL ADCP(available in full paper
The minimum value for dt is determined by the time delay between the end of transmission and the return of the backscattered signal from the furthest cell (in shelf seas less than 0 5 s) In practice, however, dt is determined by the time taken for the microcomputer to process the backscattered signal Given the range determined by the operating frequency, the range over which results can be obtained is constrained both near the instrument and, possibly, at the far field First, interpretation of the measurement close to the transducers (the first one or two cells) is difficult because of decaying transients Secondly, measurements near the sea surface for upward-looking sonars and near the sea bed for downward-looking sonars suffer interference between the spectacular return of a (vertical) side lobe and the reverberation return along the main slant beam Interpretation of the results within about (1 -cos ¿) water depth of the sea surface/bed is usually difficult Two further points are worth detailing First, measurements which cover the whole shelf seas water column are not relevant to small scale processes the beams can be hundreds of metres apart near the far end of their range where the cell volumes are hundreds of cubic metres. Secondly, the choice of acoustic frequency is fundamental to the to the instrument it.
