Measurements of turbulence for quantifying the impact of turbulence on underwater imaging

Abstract

It has long been acknowledged that turbulence affects propagation of light in the ocean. Physically, this is because turbulent inhomogeneities of the flow are associated with fluctuations in temperature and salinity. Variations in these passive scalars alter the water density, inducing variations in the refractive index, which result in near-forward scattering from turbulent inhomogeneities. In applications such as underwater imaging, the near-forward scattering from turbulence becomes a limiting factor over longer ranges and under conditions of stronger turbulence. The magnitude of this degrading effect depends upon the underwater environment, and can rapidly degrade the quality of underwater imaging under certain conditions. Overcoming this degradation through enhancement of imaging systems and post processing is important for such applications as diving, navigation, robotics, communication and target and mine detection and identification. To investigate the impact of turbulence upon underwater imaging and to compare with our previously developed model, quantified observation of the image degradation concurrent with characterization of the turbulent flow is necessary, spanning a variety of turbulent strengths. Therefore, we present field measurements of turbulence from the Skaneateles Optical Turbulence Exercise (SOTEX, July 2010), during which images of a target were collected over a 5 m path length at various depths in the water column, concurrent with profiles of the turbulent strength, optical properties, temperature, and conductivity. Turbulence was characterized by the turbulent kinetic energy dissipation (TKED) and thermal dissipation (TD) rates, which were obtained in close proximity using both a Rockland Scientific Vertical Microstructure Profiler (VMP) and a Nortek Vector velocimeter in combination with a PME CT sensor. While the two instrumental setups demonstrate reasonable agreement, some irregularities highlight the difficulties of accurately quantifying the desired parameters, which are likely associated with the spatial and temporal variability of the turbulence field. Supplementary measurements with the Vector/CT in a controlled laboratory convective tank will shed additional light on the quantitative relationship between image degradation and turbulence strength.