Bragg waves

Fig. 8. Damping ratio, i.e., reduction of the backscattered radar power (in dB) induced by a PME slick, which was spread from ethanol, as a function of the Bragg wave-number. The measurements were performed at L-, S-. C-, X-, and >band using both W- and ////-polarisations (up- [tri-angles] and downwind [squares] look directions)...

Deploying their model Gade et al. (1998c) were able to explain the continuous increase in the measured damping ratios with Bragg wave number. A comparison of some experimental and model results is shown in Figure 2. 

It is obvious that this model can well reproduce the measured damping ratios and, moreover, that the absence of the expected Marangoni damping maximum at intermediate Bragg wave numbers (approx. 100 rad m"1) can... 

Fig. 7. Measured X- and Ka-band Doppler shifts as function of the reference wind speed. The measurements were performed with a slick-free water surface. Additionally inserted into each diagram are the theoretical curves assuming bound (dashed line) and freely propagating (solid line) Bragg waves...

It is remarkable that, over each surface film deployed during our field experiments, SIR-C/X-SAR measured lower damping ratios than HELISCAT (Figure 9). These differences are independent of the deployed substance and of wind speed, but not of Bragg wave number. Possible explanations for this finding may include different resolution scales (25 m for the SAR system, the footprint dimensions of the HELISCAT systems are... 

Table 1. Radar configurations used in Fig. 8, their Bragg wave numbers and maximum damping ratios attained.

Radar band Radar wavelength fcml Incidence angle f°l Bragg wave-number Trad m1 1 Maximum damping dB]... 

The slick imaging model has not been validated with measurements. However, a provisional comparison with observations from the literature, e.g. Gade (2006), shows that the increase of damping ratio with Bragg wave number (Table 1) is indeed observed, as well as the very low sensitivity to polarisation. 

The standard SAR image processing which allows to delineate the processes on sea the surface includes, in particular, image section and NRCS contrast estimation. The wave damping ratio (the ratio of the spectral densities of Bragg waves (S) with wave number k or the normalised radar cross section (NRCS) (cr0) of spill-free and spill-covered sea surface) or contrast ... 

It is well known that oil films floating on the sea surface attenuate Bragg waves. This gives rise to considerable reduction of SAR backscatter power. The Almaz-1 SAR was operated in S-band (3.1 GHz) under incidence angles of 25-60° that correspond to Bragg wavelengths between 6 and 14 cm. For ERS-1 this value is quasi-constant and equals 7.3 cm (see Table 1). Under the influence of crude oil films or surface-active substances the spectral density of ripples essentially decreases. 

Fig. 5.20. Variation of Bragg wave vector with concentration for the dilute lamellar phase...

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