Subsurface buoy

The type of subsurface buoy illustrated in Figure 1 is designed to allow the instrument to make velocity profiles unobstructed by the mooring hardware. One of the advantages is, that the buoy is located away from surface forcing, thus it is not subject to surface excitation. The disadvantage is that if the currents in the area are too strong, drag down of the mooring and unrecoverable high tilt may invalidate the measurements. 

A deployment can be improved by adding a subsurface buoy right above the anchor to the left (figure above), to avoid rubbing, and a buoy along the line resting at the bottom, to make it possible to catch and retrieve the instrument in case of surface buoy loss or mooring line snap. The subsurface buoy may rotate under the influence of tides, waves and currents. If wave data is of interest, note that using a subsurface buoy for deploying requires that the instrument is capable of collecting data that can be used with SUV processing method. This is because of the movement of the instrument. For more information about the processing methods please refer to the Principles of Operation - Waves. Most wave processing methods are mathematically incapable of solving wave parameters if there is instrument movement and thus require that the instrument does not move or rotate during data collection. This problem is solved by utilizing the SUV processing method. This method requires AST and is therefore exclusive to the AWAC and Signature system.

As an alternative to the surface buoy, the instrumentation may be recovered by disconnecting the mooring line from the anchor by use of a pop-up buoy with an acoustic release mechanism. With a pop-up buoy, the surface buoy is no longer needed.

Using a subsurface buoy with a short mooring line to the bottom (corresponding to a bottom mount) is convenient in coastal areas with moving sand, waves or very soft bottom types, as this avoids burial. In regions with irregular / sloping bottoms, use of subsurface buoy facilitates vertical orientation of the instrument, as opposed to using bottom frames. Subsurface buoy moorings are also favorable at latitudes with seasonal ice, because the buoy allows continuous collection of data undisturbed of the sea ice.

When using a subsurface buoy it is important to understand how the natural frequency of the buoy may contaminate the data collection.