Detection and measurement of wave breaking - Breaking and Dissipation of Ocean Surface Waves - page 77

The subdivision into active and passive acoustic methods, on the other hand, is essential
in wave-breaking studies. There are two basic active-acoustic techniques employed in this
regard. One of the methods of active probing makes bubbles or bubble clouds resonate
when they are exposed to an external source of sound (e.g. Thorpe , 1992 ; Farmer et al. ,
1998 ; Terrill & Melville , 2000 ; Gemmrich & Farmer , 2004 ). While they resonate at the
same frequencies that they emit when being produced or collapse naturally, technically this
means the presence of sonar in the water which has to be powered and maintained. This
limits applications of the technique, particularly in extreme weather conditions, and on a
long-term or even regular basis. As a result, such active acoustic methods have been rather
extensively employed to investigate oceanic phenomena related to breaking such as bubble
clouds, bubble size distributions and void fraction (e.g. Thorpe , 1992 ; Farmer et al. , 1998 ;
Terrill & Melville , 2000 ; Gemmrich & Farmer , 2004 ), but not so much wave-breaking
physics and statistics.
Another active acoustic method, which has been gaining momentum over the past
two decades or so, is based on using the reflective properties of water heterogeneities
(such as small particles of matter, bubbles or even turbulent vortices) or of the water
surface. The former uses Doppler shift of the sound reflected by moving inhomogeneities
to determine the motion velocity, and the latter simply monitors sound reflected from
the surface to measure surface oscillations. The velocity records can be acquired as
three-dimensional time series (so-called acoustic Doppler velocimeters (ADV)) or as
a sequence of spatial slices of one-dimensional velocity fields (pulse-to-pulse coherent
Doppler profilers) (Dopbeam, Veron & Melville , 1999 ). Another type of active acoustic
device of this kind are acoustic Doppler current profilers (ADCP) which, if, for example,
positioned at a not-very-deep bottom, can measure velocity time series at a number of
points between the bottom and the surface, as well as time series of surface elevations.
The ADVs, Dopbeams and ADCPs are manufactured by industry both as research and
applied instruments.
Such acoustic velocimeters have been used to investigate velocity fields beneath break-
ing waves ( Doering &Donelan , 1997 ; Young et al. , 2005 ), to quantify differences in kinetic
energy due to wave breaking ( Young & Babanin , 2006a ) and to measure turbulence caused
by breaking, even within the crests of breaking waves ( Gemmrich & Farmer , 2004 ). Com-
pared to sonars, Doppler velocimeters have been in much broader use in wave-breaking
studies, both in the laboratory and in the field. On one hand, this is because of the avail-
ability of industrially produced battery-operated models of ADVs and ADCPs designed for
field use. On the other hand, even laboratory high-precision cable-powered versions made
their way into dedicated field experiments ( Veron & Melville , 1999 ; Gemmrich & Farmer ,
2004 ; Young et al. , 2005 ; Young & Babanin , 2006a ).
Among breaking-detection methods, passive acoustic determination of breaking and
its properties has a potential advantage. The instrumentation (hydrophones) is relatively
cheap, robust and easy to maintain. The hydrophones are deployed below the surface
and are solid-state devices, therefore escaping most of the destructive power of break-
ing waves. They have been used lately even in hurricanes, to quantify and classify them