Offshore stereo measurements of gravity waves

Abstract

Stereo video techniques are effective for estimating the space-time wave dynamics over an area of the ocean. Indeed, a stereo camera view allows retrieval of both spatial and temporal data whose statistical content is richer than that of time series data retrieved from point wave probes. To prove this, we consider an application of the Wave Acquisition Stereo System (WASS) for the analysis of offshore video measurements of gravity waves in the Northern Adriatic Sea. In particular, we deployed WASS at the oceanographic platform Acqua Alta, off the Venice coast, Italy. Three experimental studies were performed, and the overlapping field of view of the acquired stereo images covered an area of approximately 1100 m 2 . Analysis of the WASS measurements show that the sea surface can be accurately estimated in space and time together, yielding associated directional spectra and wave statistics that agree well with theoretical models. From the observed wavenumber-frequency spectrum one can also predict the vertical profile of the current flow underneath the wave surface. Finally, future improvements of WASS and applications are discussed.

Space–time measurements of oceanic sea states

“…Video measurements were acquired in three experiments carried out during the period 2009-2010 to investigate both space-time and spectral properties of oceanic waves (Fedele et al, 2011a,b;Benetazzo et al, 2012). To maximize the common field of view of the two cameras, i.e.…”

“…21000 and 9000 snapshots, respectively. Both experiments were conducted during NorthEastern wind (namely Bora) conditions with mean wind speed and significant wave height equal to (9.6 m/s, 1.09 m) and (17.9 m/s, 2.16 m) for Experiments 2 and 3, respectively (see Benetazzo et al 2012). …”

“…As regard to vertical resolution, the WASS system is intrinsically a high-resolution system, because the Z-coordinate is a continuous function of both left and right image coordinates as well as internal/external parameters. On the other hand, the accuracy in measuring the wave surface elevation is defined for a given WASS configuration (see Benetazzo, 2006;Benetazzo et al, 2012), and it was estimated equal to 0.01 m and 0.04 m for Acqua Alta and Crimean experiments respectively. At the present stage of WASS development, specular reflections restricted the application of stereo methods to conditions with limited sun glitter.…”

“…At smaller scales, video techniques are effective to correctly estimate the space-time dynamics and associated spectral properties (see, for example, Benetazzo, 2006). In particular, stereo cameras can be exploited to retrieve both spatial and temporal data whose statistical content is richer than that of a time series retrieved from wave gauges (Fedele et al, 2011b;Gallego et al, 2011;Benetazzo et al, 2012). In practice, stereopairs are acquired simultaneously and the geometry of the stereo system is defined so to minimize errors due to sea surface specularities (Jahne, 1993).…”

“…The pixel footprint of the imaged area gives an estimate of WASS resolution on the horizontal plane, which is approximately 40 mm and 20 mm for Acqua Alta and Crimean experiments, respectively. In the epipolar method, the use of a sub-pixel detection algorithm did improve the resolution by a factor 5-10 (see Benetazzo et al 2012, Nobach, H., Honkanen, M., 2005. As regard to vertical resolution, the WASS system is intrinsically a high-resolution system, because the Z-coordinate is a continuous function of both left and right image coordinates as well as internal/external parameters.…”