Contrast transmittance (CT) theory has been used to derive a simple two-parameter visibility model that has been used by the civilian and military sectors for over seventy years. We review this classical theory with special attention to its application to problems in estimating liminal visibility range for in-water divers. Previous work by Duntley of the MIT/SIO Visibility Lab states that calculations based on this simple model give reasonable agreement with observation provided all embedded assumptions hold. Recent dat form divers conducting shallow water observations indicate that these assumptions cause the model to give poor results in this domain. We investigate an enhanced method that eliminates some of these assumptions to provide better estimate of underwater diver visibility.
Over the past few years, several extensive exercises supported by NRL Optics Programs have taken place in US coastal waters. Analysis of optical data collected has progressed from simply observing high spatial and temporal variability, to linking this variability with physical forcing factors. Data from the most recent exercises off the Oceanside, CA and Le Jeune, NC areas are presented. Optical and oceanographic casts were taken at Oceanside in October 1995 using an ac-9 and an associated CTD. Optical properties were observed to correlate very well with the temperature and density structure of the water column. In turn, the temperature/density structure varied in conjunction with the main component of the local tidal cycle. Optical casts over a 15 hour period when other physical forcing factors are believed to be minimal are presented. During the two-week Oceanside exercise, an optical mooring recorded absorption, total attenuation, irradiance, and backscattering. Cyclic changes in the optical properties of a factor between two and four were observed, with a base of 0.5 to 1.0 m<SUP>-1</SUP>. Phase shifts in the optical data indicate that tidal influences were also modulated and at times overshadowed by large scale meteorological events, local long shore currents, and other physical forces. At Le Jeune, absorption, attenuation, temperature, and density fields were observed to fluctuate with the tidal cycle at distances of up to 12 mi from New River Inlet. The mean tidal height and density are shown to correlate with a factor of three change in both the absorption ad attenuation over a tidal cycle. Warm, high density water with low optical properties advected through the area during the measurement period and overshadowed the variability attributable to tides.
Conference Committee Involvement (2)
Ocean Sensing and Monitoring V
30 April 2013 | Baltimore, Maryland, United States
Ocean Sensing and Monitoring IV
24 April 2012 | Baltimore, Maryland, United States