Optical Variability and Bottom Classification in Turbid Waters: HyMOM Predictions of the Light Field in Ports and Beneath Ship Hulls

Abstract

LONG-TERM GOALSThe long-term goal of this research effort is to develop a method to predict the visible and ultraviolet radiance distribution in complex three-dimensional marine environments as found in ports, anchorages, and coastal waters. Effective deployment of AUV-or ROV-mounted sensors to inspect ship hulls and port facilities will depend on accurate, real-time prediction of the sub-surface optical environment at the time and place of inspection. Furthermore, since the active or passive camouflaging of divers, AUVs, and bottom objects is quite dependent upon masking the effects of three-dimensionality, 3-D optical models to evaluate such problems are being developed and implemented. While HyMOM will be used to calculate light fluxes to the bottom for foreign-object detection near structures such as coral heads, seawalls, and pilings, it is equally useful in ecological models dealing with the bleaching of corals and foraminifera and the photosynthesis of benthic plants. OBJECTIVESThe initial objective of this work is to extend the existing model system from two-dimensional environments into realistic three-dimensional environments. The model concept involves four distinct phases: 1) Modeling the optical response of individual three-dimensional elements using Monte Carlo techniques, analytic expressions, and ad-hoc definitions; 2) Building the three-dimensional region to be modeled by combining the elements developed in the previous phase; 3) Determining the radiance field around each element of the modeled environment by employing an iterative, relaxation technique to diffuse the source radiance throughout the environment; 4) Displaying the results of the model numerically and graphically. The final objectives are to evaluate and validate the model results with field radiance and irradiance data for a variety of settings, and to apply the model to predicting the perceptibility of underwater objects for all angles and fields of view. APPROACHConsider a point, ρ i , on the surface of an arbitrary region (Fig. 1). This point is being illuminated by a pencil of radiant power, E i , at wavelength λ i and from direction ε i . E i is that portion of the irradiance at ρ i that arrives from direction ε i , expressed in units W m -2 sr -1 . As a result of this illumination, pow may be emitted from the region at wavelength λ er o , point ρ o , and direction ε o . In the following