Proceedings Volume 0489

Ocean Optics VII

Marvin A. Blizard
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Proceedings Volume 0489

Ocean Optics VII

Marvin A. Blizard
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 27 September 1984
Contents: 1 Sessions, 45 Papers, 0 Presentations
Conference: Ocean Optics VII 1984
Volume Number: 0489

Table of Contents

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Table of Contents

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Introduction To Ocean Optics
Howard R. Gordon, Raymond C. Smith, J.Ronald V. Zaneveld
In this introductory survey of optical oceanography we first present the fundamental inherent and apparent optical properties of natural waters. Relationships between these inherent and apparent optical properties, as related through the radiative transfer equation, are then presented. Following the first three theoretical sections brief discussions describing the application of ocean optics to geophysics, biological oceanography, and ocean remote sensing are then presented. Authorship for each section is indicated by initials after the section heading.
Microplankton And Optical Variability In The Sea: Fundamental Relationships
Dale A. Kiefer
Several fundamental relationships appear to be important for the interpretation of biological and optical measurements made at sea: 1. The growth of phytoplankton and cyanobacteria supports the entire planktonic community and such growth can only occur by the absorp-tion of light downwelling through the water column. 2. Suspended marine particles are the major source of optical variability in the open ocean and many coastal waters. 3. The numerical concentration of particles of a given size varies as the inverse fourth power of the particles' diameter, thus small cells and detritus are major sources of light scattering and absorption. Since these small particles have relatively low indices of refraction, the complexity of applying Mie Lorentz theory is diminished. 4. For pathlengths of 1 m or less the attenuance of collimated light is sufficiently low so that measurements can be interpreted in terms of single scattering. 5. The efficiency with which light absorbed by a phytoplankter is converted into cellular material appears to be predictable, dependent only upon ambient light intensities. These relationships were then used to examine changes in the absorption and scattering properties of particles within the water column.
The Effect Of Suspensoids On Optical Parameters In A Typical Estuary
Jerome Williams, Fred Skove, John Foerster
As a first step in optical modeling of coastal areas the effects of various suspensoids on optical parameters were studied in the Patuxent River, a subestuary of Chesapeake Bay. Particle populations were measured using a Coulter Counter, while both inherent and apparent optical properties were being monitored. Results are presented indicating strong relationships between beam attenuance and total suspensoids, including both suspended sediments (particle diameters between 1 and 5 x10-5 m) and phytoplankton (particle diame-ters between 15 and 35 x10-6 m). Relationships of natural phenomena such as tidal currents and daylight period with sediment and plankton populations are also demonstrated. As expected, efforts at predicting inherent optical properties from apparent, and vice versa, were not too successful. From these studies it appears possible to develop models capable of predicting, within reasonable limits, optical properties of coastal waters when local conditions such as weather, input stream characteristics, and local topographic conditions are known.
A Two Component Model Of Optical Pulse Propagation In Seawater
Charles P. Frahm
Some laser bathymetry data exhibit a leading edge shoulder in the bottom return. The occurrence of such a shoulder was unexpected and its origin is not well understood. In order to develop a theoretical framework within which the origin of the shoulder might be elucidated and our understanding of optical pulse propagation in seawater improved, work has been initiated on a two component model of pulse propagation. A simplified version of the model will be described and some preliminary results presented.
Optical Controls On The Radiant Energy Dynamics Of The Air/Water Interface: The Average Cosine And The Absorption Coefficient
R. H. Stavn, F. R. Schiebe, C. L. Gallegos
It is here demonstrated that the air-water interface is a region of energy trapping or enhanced radiant flux. The Three-Parameter Model of the submarine light field predicts aspects of this phenomenon from the average cosine parameter (an apparent optical property') and permits the direct measurement of energy trapping. This model also extracts energy absorption effects from those attributable to energy redistribution in the submarine light field.
Optical Particle Sizing For Hydrodynamics Based On Near Forward Scattering
Y. C. Agrawal, J. B. Riley
In the study of boundary layer dynamics in the ocean, the stratification induced by suspended matter enters through the parameter cnwn where cn and wn are, respectively, the concentration and fall velocity of size class n. In this paper, an optical arrangement for instantaneous measurement of the particle size distribution will be described. The basic principle is that the near forward scattering by a particle is approximated quite well by the Fraunhofer diffraction through a circular aperture. Thus, when light is scattered by particles illuminated by a collimated laser beam, the observed intensity distribution in the Fourier transform plane of a coaxial lens is the incoherent sum of the Airy patterns from all particles, weighted by area square. An inversion of this observed intensity distribution produces the size distribution. The inversion, based on a Titshmarch-Bateman formula is analytic, and has been known in the literature to reduce to a Fourier transform of the angular intensity distribution, weighted by the third power of the scattering angle, i.e. o3.I(s). In this paper the information content of the measured intensity distribution is discussed, in particular, the size resolution and minimum and maximum size of particles about which information is contained in the observations are described. Furthermore, the concept of a Nyquist size and aliasing is introduced. By enclosing the scattering volume when the decay of the size distribution is differentiated in time, the fall velocity, hence mass density, at each size can be deter-mined separately. The above reduction of the observations generates the relevant parameter cnwn and also permits the direct observation of the fluctuations in concentrations, which are theoretically predicted to follow the -5/3 power law but have never before been verified. Theoretical background and analysis of synthetic data will be presented.
Bioluminescence In The Marine Environment
J. Losee, D. Lapota, Mark Geiger, et al.
The Naval Ocean Systems Center And The Naval Oceanographic Office Have Been Making Measurements Of Bioluminescence In Conjunction With Other Oceanographic Parameters In Surface Waters (Upper 200 Meters) And To Depths Of 3650 Meters Using Submersible Vehicles. Biological Samples And Laboratory Measurements Of Individual Organisms' Flash Signatures Are Also Obtained. Pumped/ Closed (Closed In This Text Signifies Light Baffeled) Detectors Are Routinely Used For Surface And Depth Measurements Of Bioluminescence. An Open (Open Signifies Viewing Directly Out Into The Seawater) System Is Used In Conjunction With A Pumped Detector For Deep Dives. A Brief Overview Of The Instrumentation, Some Examples Of Data Obtained, And Conclusions Based On Measurements Are Presented.
Bioluminescence Of Marine Snow: Its Effect On The Optical Properties Of The Sea
J. K. Orzech, K. H. Nealson
During two 60-meter saturation dives, U.S. Navy divers hand-collected samples of marine snow (aggregates) and of surrounding water, and made video recordings of aggregates passing through a loop used to determine their sizes and numbers. Analyses of the video recordings revealed that aggregates were abundant during both dives, accounting for 748 and 759 ml per cubic meter of seawater volume respectively. Although the total volume of aggregate material was similar, it was distributed quite differently between the two dives in 220 larger (mean volume 3.40 ml) and 595 smaller (mean volume 1.28 ml) aggregates per cubic meter. Laboratory analyses revealed that many of the aggregate samples were luminous, emitting light from two to six orders-of-magnitude greater than that produced in comparable volumes of the surrounding water. We estimated the total light flux (1)(t) per cubic meter of seawater(l.l x 109 and 3.2 x 108 quanta per second), as well as the portion of that flux which was due to the aggregates. For the two dives, 63 and 20 per cent of the aggregate samples were luminous accounting for 97 and 44 per cent of the total light flux qb(t). These results indicate that marine snow is a variable source for light-scattering material and bioluminescence in the sea.
Characteristic Vertical Patterns Of Particles, Oxygen, And Chlorophyll Pigments In The North Pacific Gyre
Hasong Pak
Characteristic vertical patterns of suspended particles, dissolved oxygen, and chlorophyll pigments in the North Pacific Gyre indicate that particle and oxygen maxima are found together in the upper part of the seasonal thermocline. The chlorophyll maximum is typically found 30 to 50 m below the particle maximum. The depth of the particle and oxygen maxima respond closely to changes in the mixed layer depth, but the chlorophyll maximum does not. The chlorophyll maximum is not only separated from the oxygen and particle maxima, but it is also little affected by changes in the surface mixed layer, suggesting that the chlorophyll maximum has its own origin different from that of the particle and oxygen maxima. Profiles of dissolved oxygen show that the high oxygen concentration layer extends to approximately 400 m, well below the euphotic zone, which points out that the primary source of the oxygen in this layer is the surface waters sunk in the high latitude Pacific and spread out in the gyre. Photosynthetical production of oxygen would be added on to the already oxygen rich water, but its net contribution appears relatively small since the oxygen concentration in the euphotic zone is not significantly higher than in the water below. Such a secondary role of the photosynthetic production of oxygen is also supported by the nutrient profiles; nutrient concentrations are uniformly low in the surface water down to the nutracline at approximately 120 m allowing only a low photosynthesis rate. The most significant change in the vertical pattern of beam attenuation coefficients is expected from the seasonal progress in the mixed layer since the attenuation maximum and the mixed layer are related.
Variability Of Particulate Spectral Absorption Coefficients In The Eastern Pacific Ocean
B.Greg Mitchell, R. Iturriaga, D. A. Kiefer
As part of the Optical Dynamics Experiment (ODEX), the spectral absorption coefficient (400-700 nm.) was measured for marine particles sampled at stations in the California current and the eastern edge of the North Central Pacific Gyre. By normalizing the spectral absorption coefficients to the concentration of chlorophyll plus phaeopigments, variability in both the spectral shapes and magnitudes of the specific coefficient can be assessed. Comparisons between samples indicated large variability vertically (mixed layer versus deep euphotic zone), horizontally (near shore versus central gyre) and seasonally (fall versus spring). When compared to chlorophyll specific absorption coefficients for healthy cul-tures, the field data suggest that a large, and variable fraction of particulate absorption in the ocean is due to detrital components. Because the origin of particulate organic material (POM) for these regions can be assumed to be derived from in situ biogenic process-es, the deviation of field spectra from those observed for cultures must in large part be due to the previous history of biological dynamics within a particular water mass.
The Analysis Of Ocean Optical Data
Raymond C. Smith, Karen S. Baker
A new oceanographic instrument to measure underwater optical, biological and physical properties simultaneously has been built and used extensively at sea. The Bio-Optical Profiling System (BOPS) was designed for the rapid acquisition of data to accommodate shipboard "synoptic" sampling strategies, often in conjunction with concurrent aircraft and satellite sensors. The data rates and associated quantity of data from the BOPS are some orders of magnitude larger than those traditionally encountered in optical oceanography. The rapid acquisition of optical data in a wide range of environmental conditions requires new methodologies in ocean optical data analysis.
Mueller Matrix Measurements Of Ocean Water
Edward S. Fry, Kenneth J. Voss
Measurements of the complete polarized light scattering properties of ocean water have been made for scattering angles from 100 to 1600. The observed normalized Mueller matrices had six non-zero elements whose angular dependences were reminiscent of Rayleigh scat-terers. The remaining 10 elements were zero at all scattering angles. The normalized measurements showed little variability with the location or depth of the sample even though total cross-sections varied by more than an order of magnitude. An average Mueller matrix is presented which represents ocean water scattering effects with a standard deviation of typically less than 10% over the various ocean locations and depths that we observed.
Strong Backscattering And Cross Polarization From Bubbles And Glass Spheres In Water
Philip L. Marston, Dean S. Langley
Observations were made of the near backward scattering of polarized laser light from air bubbles in a highly viscous oil and in water. Both the co-polarized and the cross-polarized scattering were observed to be enhanced due to the axial focusing of glory rays. A physical-optics model for this enhancement is described which corrects for the divergence of the backscattered irradiance predicted by geometric optics. A sum of glory and axial wave amplitudes reproduces the results of Mie theory for the near backward scattering from bubbles in water. We also model scattering from glass microspheres in water and show that proper choice of the glass gives an additional enhancement due to rainbow scattering in the backward direction. Such spheres may be useful as isotropic retroreflectors in water. The scattering of circularly-polarized incident light is also considered.
Scattering Of Laser Light From Bubbles In Water At Angles From 68 To 85 Degrees
Dean S. Langley, Philip L. Marston
The scattering of light by an air bubble in water should be enhanced in the angular region where totally reflected rays contribute. For nearly spherical bubbles this occurs at angles less than the critical scattering angle (I), which is 83°. Mie theory computations and a physical-optics approximation indica& that the transition from partial to total reflection should be accompanied by coarse oscillations in the intensity as a function of the scattering angle (I). This coarse structure is predicted as a consequence of interference and diffraction effects. In the present paper the first detailed observations are presented of the scattering of laser light from single bubbles in water. The bubble radii were in the range 26.4 - 986 pm. The angular locations of observed coarse-structure maxima and minima were in agreement with the physical-optics approximation and with Mie theory. Fine-structure oscillations in intensity were also observed, along with, in some cases, modulations of the fine-structure contrast. The angular spacings of these features were able to be modeled by the interference of certain scattered rays. The models give insight into the scattering properties of bubbles (even below 68°), and provide useful sizing techniques.
Optical Discontinuities Produced By Raindrops Falling In The Ocean
John Hallett
In 1875 Reynolds published a paper "On the action of the raindrops to calm the sea" where he showed that raindrops entering a water surface turned into vortex rings which propagated some 10 cm below the surface. In so doing, surface horizontal momentum was transported to lower levels thus reducing the velocity gradient and the tendency for short wave formation. The occurrence of such vortex rings is readily demonstrated as dyed drops ' mm diameter enter a water surface from a height of a few cm. Two aspects of this phenomenon are of importance in carrying over this simple demonstration to real raindrops falling in the ocean. First, raindrops are falling at terminal velocity as they reach the surface of the ocean, which increases with diameter up to a maximum of 9.1 m 5-1 for 6 mm drops; larger drops than this break up (Figure 1, Terminal Velocity). Second, the density of sea water relative to rain is 1.025 so that raindrops may experience upward buoyant force on entry. The purpose of the study reported here was to investigate the mechanism of the splash in relation to propagation of the raindrop as a vortex ring, and to assess the nature and persistence of the resulting optical discontinuity.
Completion Of The Marine Aerosol Spectrum
Nancy J. Taylor, Jin Wu
Field measurements of large marine aerosols (radius, r >60 pm)were performed at an elevation of 6m above the mean sea surface under high wind velocities (U>9m/s). Sizes and concentrations of large droplets were measured via an optical technique. From these measurements, aerosol size spectra have been constructed and are used to complete the marine aerosol spectrum. In the meantime, sea-salt concentrations were simultaneously measured and are incorporated with spray data to ionfirm the shape of the entire aerosol spectrum. In doing so, the Gathman aerosol model , which predicts the effect of the entire aerosol spectrum on atmospheric propagation, is verified.
Influence Of Sea Ice And Sea Ice Biota On Downwelling Irradiance And Spectral Composition Of Light In McMurdo Sound
C. W. Sullivan, A . C. Palmisano, J.Beeler SooHoo
A dense population of microalgae grows in the lower layers of annual sea ice in McMurdo Sound, Antarctica. The attenuation of light by surface snow, congelation and platelet sea ice, and ice microalgae was measured using an underwater spectroradiometer with a cosine collector. The in vivo absorption spectrum derived from in situ light measurements was com-parable to the in vivo absorption spectrum measured in the laboratory. Microalgae demonstrated an absorption peak at about 675 nm and a broad peak between 450 and 550 nm. Absorption of light by ice microalgae affects not only the total photosynthetically active radiation (PAR) but also the spectral composition of radiation available to under-ice phytoplankton. Thus biological as well as physical properties of sea ice determine the under-ice light field in polar oceans.
Spectral Dependence Of The Diffuse Attenuation Coefficient Of Light In Ocean Waters
R. W. Austin, T. J. Petzold
A study of the spectral nature of the diffuse attenuation coefficient of light, K (A), for various types of oceanic waters has been performed. These attenuation spectra were computed from downwelling spectral irradiance data, Ed (A), obtained by U.S., French and Japanese investigators, working in widely separated oceanic regions and using different measuring techniques and equipment. Attenuation properties were calculated over the spectral region from 365 to 700 nm and for depths from near-surface to in excess of 100 meters. Examining the K (A) data, we find strong, simple, and useful relationships exist between the value of K at some selected reference wavelength, A0, and the value of K at some other wavelength such that K (x) M (X) [K (A0) Kw (x 0)] + Kw (A) , where Kw is the attenuation coefficient for pure sea water. For oceanic waters (for example, Jerlov types I through III) the relationships are linear. These relationships appear to be useful throughout the entire spectral range examined and are particularly good between 420 and say 580 nm. The significance of the existence of such relationships is that they allow the inference of the spectral attenuation coefficient at all wavelengths from the attenuation value at a single wavelength, and provide analytical expressions for modeling the spectral nature of the attenuation in ocean and clear coastal water.
Effects Of Water Reflectance At 670 Nm On Coastal Zone Color Scanner (CZCS) Aerosol Radiance Estimates Off The Coast Of Central California
James L. Mueller
An algorithm is presented and evaluated for estimating water radiance at 670 nm in CZCS images of highly reflective eddies and other oceanic features in the upwelling region of the California Current System. Such an estimate is necessary for proper atmospheric correction of CZCS images over such water masses. In the present algorithm, the measured signal at 670 nm (less the Rayleigh component) is partitioned between L(670) and L,(670) by iterative adjustment until phytoplankton pigment concentrations calculated with The ratios of water radiances at 443 and 550 nm agree reasonably well with those calculated using the ratio of water radiances at 520 and 550 nm. Optical depths estimated using this algorithm with CZCS data agree well with nearly concurrent in situ measurements, and in an example where apparent aerosol radiance patterns correlate with obviously oceanic features, the algorithm performs well but appears to overestimate Lw(670) slightly .
A Method Of Selecting Optimal Angstrom Coefficients To Obtain Quantitative Ocean Color Data From Nimbus-7 CZCS
R. A. Arnone, P. E. La Violette
The application of Coastal Zone Color Scanner (CZCS) imagery to determining quantitative measurements of bio-optical properties requires accurate methods of eliminating the large signal component due to the atmosphere. Present atmosphere correction techniques based on single-scattering theory have produced encouraging results. These results, however, are limited by the proper selection of aerosol characteristics that accurately represent the entire image. An interactive method is suggested for selecting the optimum Angstrom coefficient over an entire image that utilizes the fact that the Angstrom coefficient is wavelength dependent on the aerosol optical thickness. This interactive technique permits a real-time display of the results and grossly indicates the horizontal distribution of the aerosol types. Results of the method are applied to a sequence of CZCS imagery of the Alboran Sea. Analyses indicate that the Angstrom coefficient selections were independent of the bio-optical properties calculated from the ratio of the water-leaving radiance of 443- to 550-nm channels of CZCS.
Secchi Depth Atlas Of The World Coastlines
R. A. Arnone, S. P. Tucker, F. A. Hilder
A seasonal Secchi depth atlas has been developed for the world's coastlines. Optical data have been compiled from the National Oceanographic Data Center and open literature for water depths less than 500 meters. These data have been averaged by one-degree squares and sorted by season and placed in a category of 6 classes of Secchi depth ranges. Four charts were used to cover the world at a scale of 1:12,233,000, and four seasons were selected to encompass 3-month intervals. Additionally, annual mean Secchi depths have been compiled in 4 charts. Secchi depth data were found for approximately 50% of the world's coastlines. In the areas where no optical data were available other oceanographic, meteorologic and geomorphic data sources were used to estimate the expected Secchi depth ranges. Secchi depth values show high temporal and spacial variability in certain coastal regions, even though the amount of data was highly limited. This variability suggests that improved techniques of compiling coastal optical properties, such as through use of satellites be examined both to aid in understanding historical ship data, and to obtain additional optical data.
The View From The Shuttle Orbiter - Observing The Oceans From Manned Space Flights
John L. Kaltenbach, Michael R. Helfert, Gordon L. Wells
Point observations of the world's oceans still remain limited. The unpredictable working environment of the air-sea interface, coupled with the seemingly boundless area, makes ship-board and aircraft sensor systems expensive and restricted investigative tools. Remote sensing from space is the answer. Requirements for near real-time information of the ocean/ atmosphere physical environment continue to grow. Space oceanography promises great improvement in meaningful coverage as orbiting satellites gather and relay data. Unmanned satelites may eventually provide the data required to satisfy many of the ocean scientist's needs, but much of the knowledge and design of these systems will come from what is seen and documented from space today. Astronauts, trained in the Space Shuttle Earth Observations project to observe the Earth from orbital altitudes, are now contributing scientific observations and photography to ocean investigations.
The Optical Variability Of The Ocean Surface From CZCS Imagery
Keith Peacock
Coastal Zone Color Scanner (CZCS) results have been used to measure the optical varia-bility of the ocean over large areas. The properties of Jerlov type I waters of areas of the North Atlantic are compared with the types II and III waters of the North Pacific. Several techniques are used to quantify the variability. Radiance variations show the reflectance changes over large and small areas and demonstrate the striking difference between types I and III waters. Color index variations (in which the color index is the ratio of radiances in two spectral bands) have been computed for small areas such as warm core rings and for large ocean areas containing different water masses. Spectral band relationships, which display the radiance at one wavelength against the radiance at another wavelength or against the color index, show a great diversity which makes it difficult to generalize the data. Spectra show the spatial variability of radiance and color for a selection of north-south and east-west tracks covering a range of water types. The results indicate that the color and radiance variations have very diverse characteristics from region to region.
Ocean Remote Sensing With A Charge Injection Device (CID) Array
S. E. Stewart, R. R. Buntzen
Ocean optical data has been remotely collected using the Advanced Solidstate Array Spectroradiometer (ASAS). ASAS is a multispectral pushbroom scanner with 32 channels extending from 400 to 850 nm. It is built around a 32 by 512 element charge injection device (CID) array with enhanced sensitivity in the blue. Twelve-bit digital output with variable gain and offset in the pre-amp and low system noise give this scanner the ability to pick up low level subsurface upwelling light from the ocean. The scanner was built by General Electric and the NASA Johnson Space Center with optics from TRW under a Naval Ocean Systems Center program for ocean remote sensing. It was first flown with the detector uncooled in September of 1983 at the Naval Coastal Systems Center in Panama City, Florida. Preliminary analysis of the data indicates a signal-to-noise ratio of at least 200 to 1. Subsequent image processing and refinements in the scanner hardware promise to improve this figure significantly. Details of the scanner design, calibration, and noise reduction will be presented. The scan-ner's potential for use in shallow water bottom mapping and chlorophyll determination will be discussed. Fi-nally, projected improvements in the scanner and its performance will be described.
Shuttle Mounted Sensors For The Analysis Of Ocean Phenomena
David D. Steller
The Space Shuttle has proven to be an excellent platform for testing sensors for the analysis of ocean phenomena. The Shuttle provides a manned, stable platform that can be precisely navigated. The ocean monitoring sensors are operated, retrieved, evaluated in the laboratory and reflown. Of major importance is the ability of the astronaut/oceanographer to use his intellect and visual acuity to recognize valuable ocean phenomena and then to interact directly with the sensors. This interaction can include real-time sensor pointing, tuning, and coordination with the ground and ship stations. In the complex task of identifying ocean features from space, man can: make rapid interpretation, evaluate ocean color changes, filter out cloud affects, make geographic location decisions and assess the contrast of subtle ocean features from background. Optional sensor mounting methods have been designed to reduce flight costs and turnaround times necessary for continuing sensor demonstration schedules. The Shuttle platform permits space testing of ocean monitoring sensors without the commitment of long range, expensive, systems programs necessary for stand-alone satellite sensor testing.
Calculation Of LIDAR Beam Spread In Stratified Media
Lawrence R. Thebaud, Stephen J. Gayer
The irradiance distribution of light propagating in a multiple scattering medium undergoes spatial spreading. A numerical technique for obtaining two-way (transmitted and received combined) spread functions in configuration space is presented. This formulation is useful for modeling lidar systems in media characterized by a strongly forward scattering function such as the ocean. The theoretical basis is a random walk small angle formalism for Fourier space represen-tations of the spread functions. The assumption of all scatters being independent events allows the expression for the Fourier transform of the photon distribution (i.e. the char-acteristic function) to be written as a product of initial conditions and exponentials of integrals of scattering particle density and the Fourier representation of the local scat-tering function of the medium. The method thus applies to a stratified medium where the direction of variation is along the beam axis. We have developed a numerical method to compute configuration space spread functions utilizing two-dimensional Fast Fourier transforms. The one way results agree well with experimental measurements. When applied to the case of few scatters, our results contrast sharply with Gaussian beam models which only approach validity in a diffusion limit.
Recent Progress In The Measurement Of Temperature And Salinity By Optical Scattering
Donald J. Collins, John A. Bell, Ray Zanoni, et al.
Recent progress is described in the use of Brillouin and Raman scattering for the measurement of temperature and salinity in the ocean. The use of Brillouin scattering is described for the measurement of the sound velocity, and the use of Raman scattering is described for the independent measurement of the temperature and salinity. Coupling these techniques permits the assessment of both temperature and salinity. The experimental techniques are described together with the results of recent experiments and an assessment of the errors to be expected.
Rapid Underwater Ocean Measurements Using Brillouin Scattering
J. G. Hirschberg, J. D. Byrne
A method is proposed to measure the speed of sound in the sea as a function of depth using a laser. A pulsed laser beam will be projected into the sea and the Brillouin scattered returning radiation will be analysed. The depths will be determined by the time delay between the outgoing and return light, and the speed of sound by the wavelength spread between the components of the Brillouin scattered light. These measurements can be made with great rapidity and accuracy.
Raman LIDAR For The Remote Measurement Of Subsurface Ocean Parameters
Donald A. Leonard, Bernard Caputo
The Raman lidar technique was developed for the remote measurement of temperature and salinity profiles. A temperature accuracy of 0.5 degrees Centigrade is attainable in a practical field system for depths of up to 3 diffuse attenuation lengths, which can be 100 meters or more in the open ocean. In this paper field test results are reviewed and performance specifications for typical Raman lidar systems are presented.
The Effect Of Detector Field-Of-View On Laser Backscatter And Bottom Reflection Measurements
Duane Bright, Jerald W. Caruthers, George Kattawar
A Monte Carlo-type mathematical model is used to simulate a laser transmitter and receiver system (LIDAR) in a marine environment. Simulations include a limited receiver aperture and water properties typical of coastal waters. Results are presented for impulse responses from laser pulses directed downward, reflected from the ocean bottom, and detected by a coaxial receiver with fields-of-view from 5 to 480 milliradians.
Effects Of Propagation-Induced Pulse Stretching In Airborne Laser Hydrography
Gary C. Guenther, Robert W.L. Thomas
Monte Carlo simulation techniques have been applied to underwater light propagation to calculate the magnitudes of propagation-induced depth measurement bias errors as well as spatial beam spreading and signal attenuation for airborne laser hydrography. The bias errors are caused by the spatial and subsequent temporal dispersion of the laser beam by particulate scattering as it twice traverses the water column. Beam spreading results dictate spatial resolution at the bottom and the receiver field-of-view requirement. Sample temporal response functions are presented. The peak power attenuation relationships developed can be used to predict maxim um penetration depths, Predicted depth measurem ent biases are reported as functions of scanner nadir angle, physical and optical depths, scattering phase function, single-scattering albedo, and receiver field of view for several diverse signal processing and pulse location algorithms. Bias variations as a function of unknown in the field) water optical parameters are seen to be minimized for limited ranges of nadir angles whose values depend on the processing protocol. Bias correctors for use on field data are reported as functions of nadir angle and depth.
LIDAR Return Pulse Profile And Its Relationship To Water Optical Parameters
H. S. Lee, N. T. O'Neill
A method of deducing the water optical parameters from the temporal and/or the magnitude of the nadir incidence bathymetry lidar return signal has been developed. A data set created by a Monte Carlo simulation is used in deriving the relationship between the temporal profile of the lidar return signal and the water optical parameters. A brief discussion of these results with relation to the lidar bathymetric bias correction is given.
Scattering Meters For Light In The Sea
Willard H. Wells
To solve radiative transfer problems in seawater, we need two inherent properties, the volume scattering function (VSF) and the absorption. The traditional direct way to obtain these quantities uses a transmissometer and a scattering meter. However, there are prob-lems with the small sample size and errors in obtaining absorption by integration of the VSF. An indirect method also shows promise. One measures the radiance field and then inverts the equations of radiative transfer to obtain the inherent properties from the apparent. The only serious shortcoming is that radiance must be a function of only one position coordinate (plus two angles). (This coordinate is depth in the case of sunlight, or distance from an isotropic lamp otherwise.) We discuss two practical implementations of this indirect approach. One would measure the radiance field with a set of fisheye cameras (following R. Smith's precedent). This very thorough method produces lots of data and requires extensive calibration and number crunching. A proposed alternate radiometer would measure certain spherical moments of the radiance field, the moments being selected to facilitate recovery of the inherent properties [Appl. Opt. 22, 2313 (Aug 83)]. This scheme would produce fewer data, but it permits recovery of absorption and moments of the VSF in (nearly) real time. Similar direct and indirect approaches apply to the measurement of very small-angle scattering, from a milliradian to a few degrees, the sort of angles that blur vision. The indirect method infers small-angle scattering from the loss of contrast in images of bar charts. In this case, the indirect method is clearly superior for the same reasons that bar charts and other test patterns are widely used (instead of point spread functions) to evaluate the performance of television and various optical systems. We built a seawater MTF meter on this principle before 1970, and its features are briefly reviewed.
Beam Attenuation And Absorption Meters
J. Ronald, V. Zaneveld, Robert Bartz
The calibration of beam attenuation meters by means of a water standard measured with a two-pathlengths device is discussed. It is shown that the particulate scattering to attenuation ratio can be assumed to be constant to the same accuracy that the shape of the volume scattering function can be assumed to be constant in sea water. This observation allows one to correct beam attenuation meters without the need to measure forward scattering. An equation for this correction is derived and a table of values for the correction at a wavelength of 665 nm is given. The principles of construction as well as potential errors and their corrections are shown fora reflecting tube light absorption meter. Preliminary results for such a device are also shown.
A Solid-State Spectral Transmissometer And Radiometer
K. L. Carder, R. G. Steward, P. R. Payne
An in-situ instrument designed to measure the spectral attenuation coefficient of sea-water and the ocean remote sensing reflectance from 400 to 750 nm is under development and is described. It employs a 256-channel, charge-coupled type of linear array measuring the spectral intensities diffracted by a grating. Examples of the types of data expected to be delivered by this instrument have been simulated using a breadboard laboratory instrument and an above-water, solid-state radiometer. Algorithms using data from these instruments have been developed and tested providing measures of chlorophyll a + phaeophytin a from less than .1 to 77.0 mg/m3 chlorophyll, gelbstoffe spectral absorption coefficients, and detrital spectral backscattering coefficients for waters of the west Florida shelf.
Flow Cytometric Analysis Of The Optical Characteristics Of Marine Particulates
Richard W. Spinrad
The flow cytometer, an instrument initially developed for biomedical research, has proven to be a useful and valuable tool in the study of oceanic particles. By using a finely tuned stream flow and elliptical optics (with a laser source) in conjunction with a computer-based data acquisition and display system, the flow cytometer is capable of analyzing several thousand particles (of up to approximately 35 μm diameter) per second. The instrument is designed to allow measurement of several optical parameters on an individual-particle basis. These) parameters include forward light scatter (in two zones from 1.5 to 10 and 10 to 19 ), 90 light scatter and fluorescence at a variety of emission and excitation wavelengths. In addition, the flow cytometer has the ability to sort the sample into subsamples by their optical characteristics. Light scattering measurements have been made on samples of monoclonal unialgal cultures. The capability of measuring the single cell light scatter has allowed for the analysis of variability in 90° light scatter from particle shape effects. Spectral variability in scattering among single-species samples can also be studied. Studies with growing cultures of phytoplankton have demonstrated that the matrix of forward versus 90 light scatter as measured with the flow cytometer may provide a useful method in identifying the physiological state of a sample. A major goal in the use of the flow cytometer for marine optical work is the development of an easily applied algorithm for the rapid determination of the size and refractive index distribution of oceanic particulate matter.
Instrumentation For Measuring In-Situ Sea Truth For Laser Radar Applications
C. A. Moore, R. C. Honey, D. M. Hancock, et al.
Instrumentation has been developed to independently measure the key optical parameters of seawater that are required to verify algorithms used to interpret data from a laser radar. Instrumentation designs and performance are described in terms of their applicability to the laser radar problem. Representative data obtained during field tests are presented.
Continuous Measurement Of Optical And Thermal Properties Of Seawater Using A Towed Array
J. M. Wrabetz, C. A. Moore
The design of an in-situ digital data acquisition system to measure optical sea truth is described. The system provides efficient collection of data from 74 sensor channels at a 5-Hz rate. Representative array data obtained during field tests is presented.
In-Situ Forward Scatter And Transmittance Measurement Using A Low Power Laser Diode
F. M. Caimi, R. F. Tusting, G. Kennedy
A laser diode combined transmittance and low-angle scatterance measuring instrument is described for in situ use over long periods. Two methods of optical feedback are used to improve the stability of the 800 nm wavelength output. Use of a laser diode provides several advantages not otherwise easily achieved: 1) A possibility for narrow spectral filtering to reject ambient light. 2) Beam collimation of nearly 1 milliradian. 3) Higher light thru-put to facilitate the scattering measurement using a low total power consumption of 300 mW. Improved accuracy and sensitivity are salient features. Although operation in the near infrared produces less theoretical scattering signal than shorter wavelength instruments, the loss is substantially compensated by the large spectral radiant efficiency of the laser diode. Synchronous detection is used and the instrument is easily modifiable to sense multiple forward scattering angles. Inclusion of scatterance measurement in addition to transmittance not only provides more information per instrument deployed, but also provides greater usefulness in very clear waters when compared to a conventional transmissometer alone. Results are given which support identification of particle suspension mixtures of known characteristics. Particle size information may also be obtained in certain instances.
A Micro-Powered Underwater Logger For Recording Photosynthetically Active Radiation And Illumination
Karel F. Zabloudil
An underwater data logger is designed to use a micro-powered recording circuit. The in situ monitor can record up to eight signals from sensors onto an internally-housed, 32,000-word erasable programmable read-only memory (EPROM) unit. The very low energy requirements permit the logger to record hourly measurements for a monitoring period of up to six months without recharging the internal batteries. Equipped with underwater light sensors, the instrument can record photosynthetic photon flux density either through an instantaneous reading or as an integrated value for the period between sampling. In addition to these light measurements, other sensors can be used to simultaneously monitor further parameters such as: temperature, pressure, salinity, currents, etc.. Circuits diagrams depicting unit operation and data processing are described. Computer plots of sample survey data (irradiance between two depths, extinction coefficient, and temperature) are illustrated. This paper further describes such additional features of the underwater logger as the capability of the light sensors, designed for all types of measurements and/or manufactured by different companies, to be evaluated in situ at the same environmental conditions.
A Calibration Technique For Photometers
I. B.C. Matheson, J. Lee, E. F. Zalewski
The oxidation of luminol, 3-aminophthalhydrazide, gives rise to light emission in the blue part of the spectrum [1]. The quantum yield of this reaction, i.e. the number of photons emitted per luminol molecule reacted was measured some time ago and the reaction proposed as a chemical light standard [2]. The absolute quantum yield of the reaction has been measured then and since using calibrated photomultipliers and small solid angle geometry [3,4], an integrating sphere arrangement with quantum counter and photo-multiplier [5], and a 4i solid angle arrangement where the sample was surrounded by the chemical actinometer ferrioxalate [6]. The present work concerns a remeasurement of the quantum yield using a silicon, Si, photodiode detector and a geometry for which the light collection efficiency is known absolutely. The luminol reaction has been used in turn to calibrate a portable photometer of similar geometry which in conjunction with a transferable light emitting fluid may be used in the field to calibrate underwater photometers.
Techniques To Measure The Spectral Reflectance Of Ice
S. J. Bolsenga, G. M. Greene
The system described here was designed to use two commercially available scanning spectroradiometers, a desk top calculator, and software to obtain simultaneous readings of incident and reflected radiation. To use such laboratory instruments in a hostile environ-ment, it was necessary to develop support apparatus. A weatherized, portable module to house the data storage and readout apparatus; heated casings for spectroradiometers; power systems; and an over-ice instrument support boom are included. Problems with poor cosine response of the instruments were largely overcome. A process for using integrating spheres under both clear and overcast conditions is being developed. The system was specifically designed to measure the spectral reflectance of snow and ice in the Great Lakes. Similar systems can be used for any field or laboratory application where incident flux might change during the spectroradiometer scan. Data collected showed large differences in the spectral reflectances of certain types of freshwater ice. However, for some ice types under certain atmospheric conditions, such as brash ice under clear or partly cloudy skies, it is difficult to determine a unique spectral signature since spectral reflectances are not only diurnally dependent but also site specific.
Wind Speed Measurement From Below The Sea Surface By Correlation
William Brown
The angular aspect of optical radiation transmitted throuyn the air-sea interface depends upon the effective slope of the particular element of sea surface illuminated at that particular instant of time. Waves and swells strongly effect the resultant rauiance distribution below the surface, by focusing and dispersing small light ray bundles, The air-sea interface, however, is influenced most by small wavelengths and ripples created by the wind. Empirical relationships between wave slope variance and wind speea nave been determined, and the underwater radiance variance is known in terms of the wave slope variance. As a result, it is possible to measure the wind speed from under the surface by sampling the variance of the radiation field there. An alternate method is described here which takes from laser transit velocimetry. Several sensors are placed below tree surface and their signals are correlated for a technique to measure wave ana wind speed.
Optimizing Optics For Remotely Controlled Underwater Vehicles
A. B. Billet
The past decade has shown a dramatic increase in the use of unmanned tethered vehicles in worldwide marine fields. These vehicles are used for inspection, debris removal and object retrieval. With advanced robotic technology, remotely operated vehicles (ROVs) are now able to perform a variety of jobs previously accomplished only by divers. The ROVs can be used at greater depths and for riskier jobs, and safety to the diver is increased, freeing him for safer, more cost-effective tasks requiring human capabilities. Secondly, the ROV operation becomes more cost effective to use as work depth increases. At 1000 feet a diver's 10 minutes of work can cost over $100,000 including support personnel, while an ROV operational cost might be 1/20 of the diver cost per day, based on the condition that the cost for ROV operation does not change with depth, as it does for divers. In the ROV operation the television lens must be as good as the human eye, with better light gathering capability than the human eye. The RCV-150 system is an example of these advanced technology vehicles. With the requirements of manueuverability and unusual inspection, a responsive, high performance, compact vehicle was developed. The RCV-150 viewing subsystem consists of a television camera, lights, and topside monitors. The vehicle uses a low light level Newvicon television camera. The camera is equipped with a power-down iris that closes for burn protection when the power is off. The camera can pan f 50 degrees and tilt f 85 degrees on command from the surface. Four independently controlled 250 watt quartz halogen flood lamps illuminate the viewing area as required; in addition, two 250 watt spotlights are fitted. A controlled nine inch CRT monitor provides real time camera pictures for the operator. The RCV-150 vehicle component system consists of the vehicle structure, the vehicle electronics, and hydraulic system which powers the thruster assemblies and the manipulator. For this vehicle, a light weight, high response hydraulic system was developed in a very small package.
Underwater Camera Calibration Using The Finite Element Method
Riadh A.Halim Munjy, Jay M. Goldfarb
A new photogrammetric mathematical model based on the collinearity equation and the finite element method for under-water cameras developed by the authors has been investigated in detail. An experimental verification of the proposed technique has been carried out on an Olympus OM-1 camera in a water tight housing. The experimental results support the conclusion that the newly derived photogrammetric mathematical model offers a theoretical and practical alternative to existing mathematical models for the calibration of underwater cameras.