Robert N. Swift

Airborne simultaneous spectroscopic detection of laser-induced water Raman backscatter and fluorescence from chlorophyll a and other naturally occurring pigments

The airborne laser-induced spectral emission bands obtained simultaneously from water Raman backscatter and the fluorescence of chlorophyll and other naturally occurring waterborne pigments are reported here for the first time. The importance of this type data lies not only in its single-shot multispectral character but also in the application of the Raman line for correction or calibration of the spatial variation of the laser penetration depth without the need for in situ water attenuation measurements. The entire laser-induced fluorescence and Raman scatter emissions resulting from each separate 532-nm 10-nsec laser pulse are collected and spectrally dispersed in a diffraction grating spectrometer having forty photomultiplier tube detectors. Results from field experiments conducted in the North Sea and the Chesapeake Bay/Potomac River are presented. Difficulties involving the multispectral resolution of the induced emissions are addressed, and feasible solutions are suggested together with new instrument configurations and future research directions.

Water depth measurement using an airborne pulsed neon laser system

Initial base-line field test performance results of the National Aeronautics and Space Administrations airborne oceanographic lidar (AOL) in the bathymetry mode are presented. Flight tests over the Atlantic Ocean yielded water depth measurements to 10 m. Water depths to 4.6 m were measured in the more turbid Chesapeake Bay. Water-truth measurements of depth and beam attenuation coefficients by boat were taken at the same time as the aircraft overflights to aid in determining the systems operational performance. Beam attenuation coefficient a and depth d product alphad was established early in the program as the performance criterion index. A performance product of 6 was determined to be the goal. This performance goal was successfully met or exceeded in the large number of field tests executed. Included are selected data from nadir-angle tests conducted at 0 degrees , 5 degrees , 10 degrees , and 15 degrees . Field-of-view data chosen from the 2-, 5-, 10-, and 20-mrad tests are also presented. Depth measurements obtained to altitudes of 456 m are given for additional comparison. This laser bathymetry system represents a significant improvement over prior models in that (1) the complete surface-to-bottom pulse waveform is digitally recorded on magnetic tape at a rate of 400 pulse waveforms/sec, and (2) wide-swath mapping data may be routinely acquired using the 30 degrees full-angle conical scanner. Space does not allow all the 5,000,000 laser soundings to be included. Qualified interested users may obtain complete data sets for their own in-depth analysis.

Oil film thickness measurement using airborne laser-induced water Raman backscatter

The use of laser-induced water Raman backscatter for remote thin oil film detection and thickness measurement is reported here for the first time. A 337.1-nm nitrogen laser was used to excite the 3400-cm(-1) OH stretch band of natural ocean water beneath the oil slick from an altitude of 150 m. The signal strength of the 381-nm water Raman backscatter was always observed to depress when the oil was encountered and then return to its original undepressed value after complete aircraft traversal of the floating slick. After removal of background and oil fluorescence contributions, the ratio of the depressed-to-undepressed airborne water Raman signal intensities, together with laboratory measured oil extinction coefficients, is used to calculate the oil film thickness.

Airborne measurements of laser backscatter from the ocean surface

Laser backscatter data for the ocean surface near nadir have been acquired from an airborne lidar platform. The unique capability of this lidar instrument to scan both transmitted laser beam and receiver field of view up to 15° off nadir have made these data sets possible. Backscatter data were collected on eight separate missions using laser wavelengths at 337 and 532 nm and 9.5 μm. Statistics of the mean, standard deviation, and probability density function of backscatter were computed and analyzed in terms of prior analytical work that relates backscatter to wind speed and mean-square wave-slope statistics. We found the full width at half-maximum of the Gaussian-shaped mean backscatter pattern to range from 11 to 24° and the normalized standard deviation at a nadir-viewing angle to range from 0.1 to 0.6. We calibrated mean backscatter at nadir for the ocean surface in terms of an effective Lambertian reflectance by comparison of beach sand and ocean backscatter. Results were 16 and 24% reflectance on two missions where calibration was possible. Our data are compared with prior laser backscatter measurements and the general literature on optical scattering from the ocean surface.

Feasibility of airborne detection of laser-induced fluorescence emissions from green terrestrial plants

Recent experiments conducted with the NASA airborne oceanographic lidar (AOL) have shown that laser-induced fluorescence (LIF) spectral emissions from green terrestrial plants are detectable from a remote platform. A 3-MW peak power frequency-doubled Nd:YAG laser at 532 nm was used from an altitude of 150 m to induce fluorescence from trees, bushes, and grasses growing on a barrier island. A companion 422-nm XeCl excimer pumped dye laser with a controlled maximum output power of 100 kW was used separately on additional passes in order to compare its effectiveness over the same test area. Slant range measurements obtained simultaneously from each laser on-wavelength return pulse provided valuable, comparative elevational information on the heights of plants and variations in terrain along the flight lines. Samples of airborne LIF color spectra obtained with 532-nm excitation and cross sectional profiles are given together with supporting spectral measurements performed on selected plant types with a laboratory laser system. While the results to date are very encouraging, additional laboratory and field tests are required to establish the utility of the airborne LIF technique for measuring the distribution of plant pigments and biomass remotely from an airborne platform.

Airborne dual laser excitation and mapping of phytoplankton photopigments in a Gulf Stream Warm Core Ring.

Utilization of a two-color airborne lidar system in the systematic study of a major oceanographic feature is reported here for the first time. An excimer pumped dye laser was optically and electronically integrated into the NASA Airborne Oceanographic Lidar for simultaneous use with a frequency doubled Nd:YAG laser. The output beams exit the laser system along parallel paths after being produced on an alternating pulse basis at a combined rate of 12.5 pps. Results are presented for missions flown over a Gulf Stream Warm Core Ring (WCR) as well as over shelf, slope, Gulf Stream, and Sargasso Sea waters. From the airborne data a high coherence is shown between the two-color chlorophyll a data and between the Nd:YAG chlorophyll a and phycoerythrin responses within each of these water masses. However, distinct differences in the response patterns of these photopigments are shown to exist between the differing water masses. At certain of the boundaries separating the water masses a sharp transition is seen to occur, while at others a wider transition zone was observed in which the correlation between the photopigments appears to degrade.

Detection of phytoplankton pigments from ocean color: improved algorithms

Passive ocean-color data at 32 wavelengths in the visible domain and laser-induced fluorescence line heights of chlorophyll and phycoerythrin, measured simultaneously from an aircraft in the New York Bight area, are used to examine the problem of developing algorithms for pigment retrieval from ocean-color data that would be capable of distinguishing between chlorophyll and phycoerythrin. Using factor analysis, it is shown that it is indeed possible to develop such algorithms. Furthermore, the wavelengths used in the algorithms can be reduced from 32 to 6 (similar to the SeaWiFS channels) without much loss in information. These multiwavelength algorithms yield significantly higher correlation coefficients for chlorophyll compared with the conventional blue-green ratio used for retrieval of this pigment. The Coastal Zone Color Scanner wavelengths appear to be inadequate for quantitative retrieval of the phycoerythrin signal.

Multiplafform sampling (ship, aircraft, and satellite) of a Gulf Stream warm core ring.

The purpose of this paper is to demonstrate the ability to meet the need to measure distributions of physical and biological properties of the ocean over large areas synoptically and over long time periods by means of remote sensing utilizing contemporaneous buoy, ship, aircraft, and satellite (i.e., multiplatform) sampling strategies. A mapping of sea surface temperature and chlorophyll fields in a Gulf Stream warm core ring using the multiplatform approach is described. Sampling capabilities of each sensing system are discussed as background for the data collected by means of these three dissimilar methods. Commensurate space/time sample sets from each sensing system are compared, and their relative accuracies in space and time are determined. The three-dimensional composite maps derived from the data set provide a synoptic perspective unobtainable from single platforms alone.

Spatial variability in near-surface chlorophyll a fluorescence measured by the Airborne Oceanographic Lidar (AOL)

The primary purpose of the aircraft remote sensing component of the North Atlantic Bloom Experiment (NABE) was to: (1) quantify spatial patterns of surface Chl a variability and co-variability with temperature (T) within the NABE study regions along the 20°W meridian near 48 and 60°N; and (2) determine if the major NABE ship and mooring locations were representative of surrounding ocean waters with respect to large-scale distributions of surface Chl a and T. The sampling platform was a NASA P-3 aircraft equipped with the Airborne Oceanographic Lidar (AOL) system, which measures laser-induced Chl a fluorescence (LICF), upwelling spectral radiance and surface temperature (T). n nResults collected during nine AOL missions conducted between 26 April and 3 June show considerable mesoscale variability in LICF and T. Spatial statistics (structure functions) showed that the dominant scales of LICF and T were significantly correlated in the range 10–290 km. Spectral analysis of the results of long flight lines showed spectral slopes averaging −2 for both LICF and T for spatial scales in the range 1.2–50 km. As for previous investigations of this type, we interpret the correlation between LICF and T as evidence that physical processes such as upwelling and mixing are dominant processes affecting spatial variations in Chl a distributions in the North Atlantic during the period of our sampling. The minimum dominant T and LICF spatial scales (ca 10 km) we determined from structure functions are similar to minimum scales predicted from models (Woods, 1988, In: Toward a theory on biological-physical interactions in the world ocean, Kluwer Academic, Boston, pp. 7–30) of upwelling induced by vortex contraction on the anticyclonic side of mesoscale jets. n nThe NABE experiment was planned with the explicit assumption that major biological and chemical gradients are in the north-south direction in the northeast Atlantic. Our results support this assumption, and we observed no large-scale (>200 km), east-to-west trends in surface Chl a in the two principal study areas. n nOur analyses show that satellite ocean color scanners with pixel resolution of 4 × 4 km will generally detect the major spatial patterns of Chl a distributions (at scales > 0.3 km), in near surface waters during the spring bloom in the North Atlantic.

High frequency sampling of the 1984 spring bloom within the mid-Atlantic Bight: Synoptic shipboard, aircraft, and in situ perspectives of the SEEP-I experiment

Abstract Moorings of current meters, thermistors, transmissometers, and fluorometers on the Mid-Atlantic shelf, south of Long Island, suggest a seaward export of perhaps 0.20 mg Chl m −3 day −1 at depths of 75–81 m, between the 80- and 120-m isobaths during February–April 1984. Using a C/Chl ratio of 45/1, such a horizontal loss of algal carbon over the lower third of the water column would be 19–67% of the March–April 1984 primary production within the overlying euphotic zone. This possible physical carbon loss is similar to daily grazing losses to zooplankton of 32–40% of the algal fixation of carbon. Metabolic demands of the benthos could be met by just the estimated fecal pellet flux, without direct consumption of the remaining algal carbon. Similarly bacterioplankton metabolism could be fueled by excretory release of dissolved organic matter during photosynthesis, rather than by consumption of particulate carbon. Sediment traps tethered 10 and 70 m off the bottom at the 120-m isobath caught as much as 0.10–0.16 g C m −2 during March–April 1984. This presumed vertical flux is about one-third to one-half of the horizontal flux of 0.30 g C m −2 day −1 estimated over the lower 33 m of the water column at the 100-m isobath. These estimates suggest that ∼50% of the carbon export at the shelf-break might be derived from the adjacent overlying water column, with the remainder from lateral injections of near-bottom particles originating on the inner shelf.