W H P Hy d rograp h ic Operation s an d M eth od s
Ju ly 1991
Acoustic Doppler Current Pro ling Measurements and Navigation
Eric Firing University of Hawaii Honolulu, Hawaii 96822 U.S.A.
Shipboard Acoustic Doppler Current pro ler ADCP measurements are not di cult to make. A good system, properly set up, can and usually should be left to run continuously throughout acruise with no intervention beyond changing the oppy disk when they are full. The following discussion is intended primarily to help ADCP users ensure that they have a good system to with and that it is properly set up. This note is intended to supplement, not supplant, the information provided by ADCP manufacturers. In particular, the new ADCP user is advised to read rst the "Practical Primer"published by RD Instruments, for an introduction to the principles and terminology of Doppler pro ling. Before embarking on a cruise, the user should also become familiar with the relevant manufacturer's manuals and technical bulletins. The present note begins with a bit of motivation: a list of some of the potential scienti c bene ts from shipboard ADCP systems Section 3. and the relatednavigational instruments Section 4.. Recommendations for the con guration and operation of a shipboard ADCP are given in Sections 5. and 6., and in calibration methods are summarized in Section 7.. The steps required to process ADCP data are listed in SEction 8.. The note concludes with a discussion of the accuracy of shipboard ADCP measurements Section 9.. Appendices list sources of GPS informationAppendix A, describe the publicly available software developed at the University of Hawaii Appendices B and C, and give recommendations for using the obsolescent Transit navigation system Appendix D.
2. Scienti c rationale for ADCP measurements
ADCP measurements can make important contributions at moderate cost to almost any hydrographic cruise, and to the WHP cruises in particular. Withan ADCP one can map current structure along the cruise track with high vertical 10 20 m and horizontal 2 km resolution. Having both horizontal components of the velocity vector gives a more complete and realistic picture of the currents at the time of the cruise than can be obtained from the geostrophic cross-track component. This helps in mapping the circulation and in interpreting tracermeasurements. Accumulating ADCP sections from many cruises, one can statistically describe the upper ocean vertical shear eld as an aid in interpreting drifter measurements. In some regions, the ADCP velocity eld will be the best available reference for geostrophic calculations. At low latitudes, geostrophic calculations from single cruises are unreliable, and direct current measurements are essentialif one is to know the upper ocean currents. This appears to be true in strong nearshore current regimes as well. A signi cant part of the upper ocean current structure is ageostrophic; unlike geostrophic calculations, the ADCP measures the actual current eld. The measured currents include
Acoustic Doppler Current Pro ling Measurements and Navigation 1991
increase with increasing sonarfrequency, but at the expense of decreased pro ling range. Because the WHP is primarily concerned with large-scale phenomena, the two lowest-frequency models, operating at 75 kHz and 150 kHz, are much more appropriate for WHP work than are any of the higher frequency models. The VM-75 might indeed be the most suitable of all for the WHP, but few have been installed probably because thetransducer is so large and awkward and I have no experience with them. The most common model, the VM-150, can serve the WHP well. The depth range of a given pro ler is determined by the density of scatterers, the presence of bubbles under the transducer, and the noise generated by the propellers and by ow along the hull. In moderate weather winds less than about 20 knots, typical maximum pro le depths...
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