James L. Mueller

METHOD TO DERIVE OCEAN ABSORPTION COEFFICIENTS FROM REMOTE-SENSING REFLECTANCE

A method to derive in-water absorption coefficients from total remote-sensing reflectance (ratio of the upwelling radiance to the downwelling irradiance above the surface) analytically is presented. For measurements made in the Gulf of Mexico and Monterey Bay, with concentrations of chlorophyll-a ranging from 0.07 to 50 mg/m(3), comparisons are made for the total absorption coefficients derived with the suggested method and those derived with diffuse attenuation coefficients. For these coastal to open-ocean waters, including regions of upwelling and the Loop Current, the results are as follows: at 440 nm the difference between the two methods is 13.0% (r(2) = 0.96) for total absorption coefficients ranging from 0.02 to 2.0 m(-1); at 488 nm the difference is 14.5% (r(2) = 0.97); and at 550 nm the difference is 13.6% (r(2) = 0.96). The results indicate that the method presented works very well for retrieval of in-water absorption coefficients exclusively from remotely measured signals, and that this method has a wide range of potential applications in oceanic remote sensing.

Remote sensing reflectance and inherent optical properties of oceanic waters derived from above-water measurements

Remote-sensing reflectance and inherent optical properties of oceanic properties of oceanic waters are important parameters for ocean optics. Due to surface reflectance, Rrs or water-leaving radiance is difficult to measure from above the surface. It usually is derived by correcting for the reflected skylight in the measured above-water upwelling radiance using a theoretical Fresnel reflectance value. As it is difficult to determine the reflected skylight, there are errors in the Q and E derived Rrs, and the errors may get bigger for high chl_a coastal waters. For better correction of the reflected skylight,w e propose the following derivation procedure: partition the skylight into Rayleigh and aerosol contributions, remove the Rayleigh contribution using the Fresnel reflectance, and correct the aerosol contribution using an optimization algorithm. During the process, Rrs and in-water inherent optical properties are derived at the same time. For measurements of 45 sites made in the Gulf of Mexico and Arabian Sea with chl_a concentrations ranging from 0.07 to 49 mg/m3, the derived Rrs and inherent optical property values were compared with those from in-water measurements. These results indicate that for the waters studied, the proposed algorithm performs quite well in deriving Rrs and in- water inherent optical properties from above-surface measurements for clear and turbid waters.

A comparison of methods for the measurement of the absorption coefficient in natural waters

In the spring of 1992 an optical closure experiment was conducted at Lake Pend Oreille, Idaho. A primary objective of the experiment was to compare techniques for the measurement of the spectral absorption coefficient and other inherent optical properties of natural waters. Daily averages of absorption coefficients measured using six methods are compared at wavelengths of 456, 488, and 532 nm. Overall agreement was within 40% at 456 nm and improved with increasing wavelength to 25% at 532 nm. These absorption measurements were distributed over the final 9 days of the experiment, when bio-optical conditions in Lake Pend Oreille (as indexed by the beam attenuation coefficient cp(660) and chlorophyll a fluorescence profiles) were representative of those observed throughout the experiment. However, profiles of stimulated chlorophyll a fluorescence and beam transmission showed that bio-optical properties in the lake varied strongly on all time and space scales. Therefore environmental variability contributed significantly to deviations between daily mean absorption coefficients measured using the different techniques.

The Marine Optical BuoY (MOBY) Radiometric Calibration and Uncertainty Budget for Ocean Color Satellite Sensor Vicarious Calibration

For the past decade, the Marine Optical Buoy (MOBY), a radiometric buoy stationed in the waters off Lanai, Hawaii, has been the primary in-water oceanic observatory for the vicarious calibration of U. S. satellite ocean color sensors, including the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectrometers (MODIS) instruments on the National Aeronautics and Space Administrations (NASAs) Terra and Aqua satellites. The MOBY vicarious calibration of these sensors supports international effort to develop a global, multi-year time series of consistently calibrated ocean color data products. A critical component of the MOBY program is establishing radiometric traceability to the International System of Units (SI) through standards provided by the U. S. National Institute of Standards and Technology (NIST). A detailed uncertainty budget is a core component of traceable metrology. We present the MOBY uncertainty budget for up-welling radiance and discuss additional considerations related to the water-leaving radiance uncertainty budget. Finally, we discuss approaches in new instrumentation to reduce the uncertainties in in situ water-leaving radiance measurements.

Monitoring Water Transparency and Diver Visibility in Ports and Harbors Using Aircraft Hyperspectral Remote Sensing

Diver visibility analyses and predictions, and water transparency in general, are of significant military and commercial interest. This is especially true in our current state, where ports and harbors are vulnerable to terrorist attacks from a variety of platforms both on and below the water (swimmers, divers, AUVs, ships, submarines, etc.). Aircraft hyperspectral imagery has been previously used successfully to classify coastal bottom types and map bathymetry and it is time to transition this observational tool to harbor and port security. Hyperspectral imagery is ideally suited for monitoring small-scale features and processes in these optically complex waters, because of its enhanced spectral (1-3 nm) and spatial (1-3 meters) resolutions. Under an existing NOAA project (CICORE), a field experiment was carried out (November 2004) in coordination with airborne hyperspectral ocean color overflights to develop methods and models for relating hyperspectral remote sensing reflectances to water transparency and diver visibility in San Pedro and San Diego Bays. These bays were focused areas because: (1) San Pedro harbor, with its ports of Los Angeles and Long Beach, is the busiest port in the U.S. and ranks 3rd in the world and (2) San Diego Harbor is one of the largest Naval ports, serving a diverse mix of commercial, recreational and military traffic, including more than 190 cruise ships annual. Maintaining harbor and port security has added complexity for these Southern California bays, because of the close proximity to the Mexican border. We will present in situ optical data and hyperspectral aircraft ocean color imagery from these two bays and compare and contrast the differences and similarities. This preliminary data will then be used to discuss how water transparency and diver visibility predictions improve harbor and port security.

Remote sensing reflectance: preliminary comparisons between in-water and above-water measurements and estimates modeled from measured inherent optical properties

Remote sensing reflectances measured underwater and above- water in the Gulf of California are compared to evaluate the equivalence between methods. Each form of reflectance is also compared to concurrently measured ratios of scattering to absorption, and the mean backscattering fraction is estimated. Above-water and in-water remote sensing reflectance estimates differ by more than 20 percent, with absolute RMS difference ranging from 0.001 to 0.005. Standard deviations of estimated backscattering fractions are between 15 percent nd 20 percent of the mean at each wavelength.

An autonomous vehicle approach for quantifying bioluminescence in ports and harbors

Bioluminescence emitted from marine organisms upon mechanical stimulation is an obvious military interest, as it provides a low-tech method of identifying surface and subsurface vehicles and swimmer tracks. Clearly, the development of a passive method of identifying hostile ships, submarines, and swimmers, as well as the development of strategies to reduce the risk of detection by hostile forces is relevant to Naval operations and homeland security. The measurement of bioluminescence in coastal waters has only recently received attention as the platforms and sensors were not scaled for the inherent small-scale nature of nearshore environments. In addition to marine forcing, many ports and harbors are influenced by freshwater inputs, differential density layering and higher turbidity. The spatial and temporal fluctuations of these optical water types overlaid on changes in the bioluminescence potential make these areas uniquely complex. The development of an autonomous underwater vehicle with a bioluminescence capability allows measurements on sub-centimeter horizontal and vertical scales in shallow waters and provides the means to map the potential for detection of moving surface or subsurface objects. A deployment in San Diego Bay shows the influence of tides on the distribution of optical water types and the distribution of bioluminescent organisms. Here, these data are combined to comment on the potential for threat reduction in ports and harbors.

Diver visibility measured with a compact scattering-attenuation meter (SAM) compatible with AUVs and other small deployment platforms

An appropriate determination of water clarity is required by defense and security operations assessing subsurface threats compromising harbor and coastal security. For search and inspection operations involving divers, underwater imaging, and electro-optical identification (EOID) systems such as laser line-scanners, the key environmental parameter needed is the optical attenuation coefficient (directly related to diver visibility). To address this need, a scattering-attenuation meter (SAM) measuring attenuation and diver visibility was developed for integration on new compact surveying platforms such as ROVs and the REMUS and glider AUVs. The sensor is compact (18X8X6 cm3), low power, robust, and hydrodynamic with a flat sensing face. The SAM measures attenuation using a novel dual-scattering approach that solves the paradox of making high-resolution attenuation measurements over the long pathlengths required for natural waters with a compact sensor. Attenuation and visibility data is presented from San Diego harbor in coordination with video images of bottom topography collected with a REMUS vehicle, from around New York harbor with a SAM mounted in an autonomous Slocum glider, and from Narragansett Bay. Results show that 1) visibility and/or attenuation in harbor and coastal regions can change rapidly over small scales (meters), especially near the bottom, 2) turbid bottom nepheloid layers are common, 3) typical visibility and/or attenuation levels fall in a range where knowledge of visibility and/or attenuation can be essential in the decision making process for security operations, and 4) attenuation is a significantly more accurate proxy for diver visibility than backscattering.

Evaluation of coastal zone color scanner diffuse attenuation coefficient algorithms for application to coastal waters

The Coastal Zone Color Scannez (ZCS) and associated atmospheric and in-water algorithms have allowed synoptic analyses of regional and large scale variability of bio-optical properties [phytoplankton pigments and diffuse auenuation coefficient K(490)}. Austin and Petzold (1981) developed a robust in-water K(490) algorithm which related the diffuse attenuation coefficient at one optical depth [1/K(490)] to the ratio of the water-leaving radiances at 443 and 550 nm. Their regression analysis included diffuse attenuation coefficients K(490) up to 0.40 nm, but excluded data from estuarine areas, and other Case II waters, where the optical properties are not predominantly determined by phytoplankton. In these areas, errors are induced in the retrieval of remote sensing K(490) by extremely low water-leaving radiance at 443 nm [Lw(443) as viewed at the sensor may only be 1 or 2 digital counts], and improved cury can be realized using algorithms based on wavelengths where Lw(λ) is larger. Using ocean optical profiles quired by the Visibility Laboratory, algorithms are developed to predict K(490) from ratios of water leaving radiances at 520 and 670, as well as 443 and 550 nm.

Prediction of euphotic depths and diffuse attenuation coefficients from absorption profiles: a model based on comparisons between vertical profiles of spectral absorption, spectral irradiance, and P

A model is presented which predicts the diffuse attenuation coefficient of downwelling irradiance as a function of depth and the depth of the euphotic zone as based on the one percent level of photosynthetically active radiation from vertical profiles of spectral absorption and attenuation. The model is tested using data obtained in the Gulf of California. The modeled diffuse attenuation coefficients and PAR levels ar shown to have average errors of less than five percent when compared to the measured values.