Acoustic observation of oil slicks at sea

Belloul, M. B.; Thorpe, S. A.

doi:10.1029/91jc01479

Cited by 5 publications

(2 citation statements)

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“…But in the biphasic Brachyhypopomus species, local biphasy occurs at the tail but not at the head (Fig. 2) where electrosensory exploratory behaviour occurs (P.K.S., unpublished data) and where electroreceptors are most dense 10 . Therefore, among the Brachyhypopomus species with biphasic waveforms, the simplest example of signal enhancement, electrolocation could not have been involved with the transition from monophasy to biphasy in the main electric organ.…”

Section: Thus a Signalmentioning

confidence: 97%

“…The clouds are detectable to ranges of ,150 m along the sea surface from the sonar, bubbles accumulating in regions of surface convergence and downwelling 17,18 . Oil reduces wave breaking and acoustic scatter from the sea surface 10 . Acoustic observations were made, however, at least 0.5 km from the oil plume, and in a variety of wind speeds from near calm to 14 m s -1 and in tidal currents reaching 1 m s -1 .…”

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Surface effects of bottom-generated turbulence in a shallow tidal sea

Smith

Thorpe

Graham

1999

Nature

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Turbulence in shelf seas strongly affects the spread of pollution (such as oil spills 1 ) as well as the distribution of sediment 2 and phytoplankton blooms 3 . Turbulence is known to be generated intermittently close to the sea bed 4 , but little is known of its evolution through the water column, or to what extent it affects the surface. Here we present observations of the surface effects of bottom-generated turbulence in a tidally in¯uenced and well mixed region of the North Sea, as derived from acoustic and visual images. Although the sea bed in the area is¯at, we ®nd that at any one time, 20±30% of the water surface is affected by boilsÐ circular regions of local upwellingÐof diameter 0.960.2 times the water depth. The signature of individual boils persists for at least 7 minutes and, in accordance with laboratory 5,6 and numerical 7 studies, shows the appearance of eddies. The boils contribute to the replacement of surface waters from depth in unstrati®ed waters, and may therefore enhance the¯uxes of gases between atmosphere and ocean.There are no reported observations of the surface signature of boils in well mixed, open sea. It is, however, clear from dynamical measurements made using current meters that tidal¯ows generate turbulence through the action of shear stress at the sea bed in much the same manner as in channels in laboratory experiments 4,8 . Fluid is intermittently ejected away from the bottom in turbulent bursts that may reach vertical speeds of 25% of the forcing current 4 . Numerical 7 and laboratory 5,6 studies show that the upwardmoving water produces a boil as it impinges on the water surface.Upward-pointing side-scan sonar has been used to observe a wide variety of processes in the upper ocean 9±13 . In experiments designed to study the processes leading to dispersion of an oil plume in the tidally well mixedÐand consequently unstrati®ed 14 Ðsouthern North Sea, a two-beam side-scan sonar system was mounted on a frame set on the sea bed at a depth of 45 m. The sonars operate at 80 kHz and 90 kHz, are set at 908 apart in the horizontal, and produce vertical fan-like beams with axes aligned upwards, 208 from the horizontal 15 . The site is 56 km from the shore and the local sea bed is¯at, with no sand banks or other notable bed forms. The principal acoustic scatterers are bubbles, of typical diameter 20± 200 mm, produced in clouds by wind waves as they break 16 . The clouds are detectable to ranges of ,150 m along the sea surface from the sonar, bubbles accumulating in regions of surface convergence and downwelling 17,18 . Oil reduces wave breaking and acoustic scatter from the sea surface 10 . Acoustic observations were made, however, at least 0.5 km from the oil plume, and in a variety of wind speeds from near calm to 14 m s -1 and in tidal currents reaching 1 m s -1 . letters to nature NATURE | VOL 400 | 15 JULY 1999 | www.nature.com 251 Figure 1 Sonar and video images of boils at the sea surface. The scale is the same in both images (and along both axes), and the tidal current is from ...

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Section: Thus a Signalmentioning

confidence: 97%

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confidence: 99%