Broadband acoustical superresolution imaging of breaking ocean waves
Date
2018-07-19
Authors
Andrew, Rex Kelley
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Abstract
An acoustic array was deployed in the nearsurface layer in Saanich Inlet, B.C. to
image breaking waves using only the naturally occurring acoustical radiation from the
breaking region over the band [160 Hz, 2000 Hz]. The 1.5-element array was configured
as a horizontal cross with an 8 m aperture, bottom-moored, and positioned nominally
3 m beneath the surface.
Due to sensor sparseness, the array PSF at any particular frequency was badly contaminated by grating lobes. A novel broadband scheme was devised to combine information at multiple
independent frequencies to yield unambiguous images with resolution of about 0.2 m at the sea surface.
The broadband scheme assumed space-time separability in the source mutual spectral density. This is only considered valid for breaking waves above about 400 Hz.
Nonstationarity and time-bandwidth constraints yielded at most six independent frequency
bands within the system passband.
A parametric image analysis showed that the images align closely with the wind
and can be observed moving downwind with a speed about two-thirds the phase speed
of the dominant component of the wind waves.
Absolute power levels were found to be consistent with previously published results.
The absolute power levels were parameterized by where [special characters omitted] and
λ (f) is well-described by a simple first order relation [special characters omitted], where [special characters omitted] varied
depending on the size of the wave but b1 appeared to be a more universal constant
estimated at -4.55 ± 0.47.
The source mechanism for frequencies below about, 400 Hz was modeled two ways:
(1) as a point source (which would follow if an acoustically compact “collective oscillation"
region had formed), and (2) as due to off-peak spectral contributions from
bubbles resonant at 400 Hz. Neither model achieved a satisfactory fit to the observed
data. This seems to imply that the mechanism below about 400 Hz was acoustically
extended and radiating as energetically as any resonant bubbles.
Description
Keywords
Image processing, Computer vision, Ocean waves