Airborne lidar detection of subsurface oceanic scattering layers.

The airborne lidar detection and cross-sectional mapping of submerged oceanic scattering layers are reported. The field experiment was conducted in the Atlantic Ocean southeast of Assateague Island, VA. NASA's Airborne Oceanographic Lidar was operated in the bathymetric mode to acquire on-wavelength 532-nm depth-resolved backscatter signals from shelf/slope waters. Unwanted laser pulse reflection from the airwater interface was minimized by spatial filtering and off-nadir operation. The presence of thermal stratification over the shelf was verified by the deployment of airborne expendable bathythermographs. Optical beam transmission measurements acquired from a surface truthing vessel indicated the presence of a layer of turbid water near the sea floor over the inner portion of the shelf.

The effects of particle size on contrast improvement by polarization discrimination for underwater targets.

Luminous flux was measured from an underwater target illuminated by circularly polarized lamp. Backscatter was measured with and without a circular analyzer on the telephotometer. Turbidity of the water was controlled by adding polystyrene spheres of relative refractive index m = 1.20. Contrasts were determined as a function of particle diameter and concentration for spheres ranging from 0.126 micro to 1.099 micro and for three size distributions from 6 micro to 100 micro. A ratio comparison of the contrasts showed a definite improvement for scatterers of diameters > 1 micro. Contrast degraded for CP illuminated scattering from spheres in the 1-100-micro diam range. Considering the ocean's scatterer-size distributions, circular polarization will probably most improve contrast in the vertical region from the lower euphotic zone to a few meters above bottom.

Improvement of underwater visibility by reduction of backscatter with a circular polarization technique.

This report presents a brief resume of the underwater visibility problem and explains the circular polarization approach for improving contrast. Experimental apparatus was set up at the Morris Dam facility of the U.S. Naval Ordnance Test Station, Pasadena Annex, and tests were conducted both with and without polarization. Results show that use of the polarization technique increased the visibility range in turbid water by at least a factor of two.