Investigation into the feasibility of detecting salt fingers optically

Abstract

During the last decade there has been increasing interest in discovering the existance of salt fingering in the ocean. The purpose of this investigation was to determine the feasibility of optically detecting salt fingers. Theoretical calculations were conducted on the angular deviation and displacement of parallel light rays propagating through an ordered salt finger model. It is assumed that salt fingers are square cells (one centimeter) with a checkerboard pattern of cells having high and low indices of refraction. The amplitude of index of refraction, above and below the overall average, was assumed to be between 5 X 10-4 to 5 X 10-6. From the computer work it was concluded that an optical method that detects displacement or intensity would have more promise of finding salt fingers in the ocean than an optical method that detects angular deviations. From the computer results and experimental shadowgraph investigations conducted by others, it was concluded that a shadowgraph method is the most promising optical approach to use for detection of salt fingers. Depending on the strength of the salt fingers, the optimum distance between the fingers and the shadowgraph screen may be as far as one to three hundred meters to obtain a sharp image. To shorten this optical path a telescope-microscope lens system cal led a "shadowgraph shortener" was used. Salt finger detection experiments were conducted in a large tank of saltwater. An expanded laser beam (5 centimeters) was passed through the salt water and the "shadowgraph shortener" to fall on a ground glass screen which was photographed with a 16 mm movie camera. The weakest salt fingers detected had a salinity gradient of .02 parts per thousand per 6 centimeters. A survey was conducted on the salinity gradients in the Atlantic Ocean using Nansen bottle data from hydrographic stations. The larger salinity gradients found in parts per thousand per 50 meters were only one order of magnitude weaker than the experimental results. An optical salt finger detector was designed using the same basic set-up as used in the experiments.