Rapid swing and spin of [deep] taut-wire-moored instruments |
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| Authors: | Hans van Haren |
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| Institution: | Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, The Netherlands |
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| Abstract: | Rapid ‘swing’, compass variations O(10°) in O(10 s), and ‘spin’, complete rotations around the vertical axis within a few minutes, are a concern of acoustic current meters moored in-line. Observations are used from fast sampling, at once per 1 and 30 s, instrumentation on deep-ocean moorings mainly outside surface wave and bottom boundary influences. Such instruments do not require a vane common to some historic mechanical current meters and they are often moored in a much easier to handle sub-surface buoy or mounting rack, without vanes. In their mountings they are nearly symmetric, so that they can spin freely in (turbulent; shear) flows. A comparison is made between noise levels of such free spinning instrumentation with those of instruments mounted in a fixed bottom-frame and with those of instruments equipped with a vane to one side. Typical spinning has a single rotation varying between 40 and 200 s. Spinning is shown to be highly binary: on or off. Its effects are found negligible on estimates of ocean currents, provided compass updates are adequate as in existing instrumentation. Acoustic noise is O(10) times larger than noise due to spinning. Some effects of spinning are noticed in the acoustic echo amplitude showing higher noise at frequencies >100 cpd, cycles per day. The character of this noise changes dramatically due to spinning. However, it is mainly in the ocean turbulence range and does not affect measurements of internal waves or periodic zooplankton motions. |
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