This paper provides a review of the development of profiling oceanographic lidars. These can provide quantitative profiles of the optical properties of the water column to depths of 20 to 30 m in productive coastal waters and to depths of 100 m for a blue lidar in the open ocean. The properties that can be measured include beam attenuation, diffuse attenuation, absorption, volume scattering at the scattering angle of 180 deg, and total backscattering. Lidar can be used to infer the relative vertical distributions of fish, plankton, bubbles, and other scattering particles. Using scattering as a tracer, lidar can provide information on the dynamics of the upper ocean, including mixed-layer depth, internal waves, and turbulence. Information in the polarization of the lidar return has been critical to the success of many of these investigations. Future progress in the field is likely through a better understanding of the variability of the lidar ratio and the application of high-spectral-resolution lidar to the ocean. Somewhat farther into the future, capabilities are likely to include lidar profiling of temperature in the ocean and an oceanographic lidar in space.

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-53/issue-5/051405/Review-of-profiling-oceanographic-lidar/10.1117/1.OE.53.5.051405.pdf

Review of profiling oceanographic lidar

Satellite passive ocean color instruments have provided an unbroken ∼20-year record of global ocean plankton properties, but this measurement approach has inherent limitations in terms of spatial-temporal sampling and ability to resolve vertical structure within the water column These limitations can be addressed by coupling ocean color data with measurements from a spaceborne lidar Airborne lidars have been used for decades to study ocean subsurface properties, but recent breakthroughs have now demonstrated that plankton properties can be measured with a satellite lidar The satellite lidar era in oceanography has arrived Here, we present a review of the lidar technique, its applications in marine systems, a perspective on what can be accomplished in the near future with an ocean- and atmosphere-optimized satellite lidar, and a vision for a multiplatform virtual constellation of observational assets that would enable a three-dimensional reconstruction of global ocean ecosystems

/pdf/spaceborne-lidar-in-the-study-of-marine-systems-1ljna0loxn.pdf

Spaceborne Lidar in the Study of Marine Systems.

Churnside, J. H., and Donaghay, P. L. 2009. Thin scattering layers observed by airborne lidar. - ICES Journal of Marine Science, 66: 778-789.More than 2000 km of thin (<3 m) optical scattering layers were identified in 80 000 km of airborne lidar data collected from a variety of oceanic and coastal waters. The spatial characteristics of thin layers varied dramatically from (i) those that were self-contained features consistently <3-4 m thick over their 1-12 km extent to (ii) those that were clearly parts of much longer layers that had gaps and/or regions where the layer became more intense and much thicker than the 3-m criterion. The characteristics of the lidar signal suggest that plankton was the most likely source of scattering. Examples from upwelling regions, areas with large fresh-water influx, and warm-core eddies are presented. The results are quite consistent with the characteristics observed in studies of thin plankton layers in fjords and near-coastal waters. These layers exhibit great spatial variability that is difficult to observe using traditional methods, and examples of layer perturbations by both linear and non-linear internal waves are presented. The results suggest that airborne lidar can be a powerful tool not only for detecting and mapping the spatial extent of thin scattering layers and linking their occurrence to larger scale physical processes, but also for tracking their evolution over time and guiding the ship-based sampling needed to understand their composition, dynamics, and impacts. Such a capability will be crucial in future studies designed to test the hypothesis that thin plankton layers have the spatial extent and intensity to play a key role in controlling the recruitment of fish larvae, biogeochemical cycling, trophic transfer processes, plankton biodiversity, and harmful algal bloom dynamics.

/pdf/thin-scattering-layers-observed-by-airborne-lidar-457rd1uipj.pdf

Thin scattering layers observed by airborne lidar

Human activity and natural climate trends constitute a major threat to coral reefs worldwide. Models predict a significant reduction in reef spatial extension together with a decline in biodiversity in the relatively near future. In this context, monitoring programs to detect changes in reef ecosystems are essential. In recent years, coral reef mapping using remote sensing data has benefited from instruments with better resolution and computational advances in storage and processing capabilities. However, the water column represents an additional complexity when extracting information from submerged substrates by remote sensing that demands a correction of its effect. In this article, the basic concepts of bottom substrate remote sensing and water column interference are presented. A compendium of methodologies developed to reduce water column effects in coral ecosystems studied by remote sensing that include their salient features, advantages and drawbacks is provided. Finally, algorithms to retrieve the bottom reflectance are applied to simulated data and actual remote sensing imagery and their performance is compared. The available methods are not able to completely eliminate the water column effect, but they can minimize its influence. Choosing the best method depends on the marine environment, available input data and desired outcome or scientific application.

Water Column Correction for Coral Reef Studies by Remote Sensing

The primary objective of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission is to study the climate impact of clouds and aerosols in the atmosphere. However, recent studies have demonstrated that CALIPSO also collects information about the ocean subsurface. The objective of this study is to estimate the ocean subsurface backscatter from CALIPSO lidar measurements. The effects of the lidar receiver's transient response on the attenuated backscatter were first removed in order to obtain the correct attenuated backscatter profile. The empirical relationship between sea surface lidar backscatter and wind speed was used to estimate the theoretical ocean surface backscatter. Then the two-way atmospheric transmittance was estimated as the ratio between the corrected ocean surface backscatter and the theoretical one. The ocean subsurface backscatter was finally derived from the subsurface attenuated backscatter divided by the two-way atmospheric transmittance. Significant relationships between integrated subsurface backscatter and chlorophyll-a concentration and between integrated subsurface backscatter and particulate organic carbon were found, which indicate a potential use of CALIPSO lidar to estimate global chlorophyll-a and particulate organic carbon concentrations.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014JC009970

Ocean subsurface studies with the CALIPSO spaceborne lidar

A polarized lidar technique based on measurements of waveforms of the two orthogonal-polarized components of the backscattered light pulse is proposed to retrieve vertical profiles of the seawater scattering coefficient The physical rationale for the polarized technique is that depolarization of backscattered light originating from a linearly polarized laser beam is caused largely by multiple small-angle scattering from particulate matter in seawater The magnitude of the small-angle scattering is determined by the scattering coefficient Therefore information on the vertical distribution of the scattering coefficient can be derived potentially from measurements of the time-depth dependence of depolarization in the backscattered laser pulse The polarized technique was verified by field measurements conducted in the Middle Atlantic Bight of the western North Atlantic Ocean that were supported by in situ measurements of the beam attenuation coefficient The airborne polarized lidar measured the time-depth dependence of the backscattered laser pulse in two orthogonal-polarized components Vertical profiles of the scattering coefficient retrieved from the time-depth depolarization of the backscattered laser pulse were compared with measured profiles of the beam attenuation coefficient The comparison showed that retrieved profiles of the scattering coefficient clearly reproduce the main features of the measured profiles of the beam attenuation coefficient Underwater scattering layers were detected at depths of 20-25 m in turbid coastal waters The improvement in dynamic range afforded by the polarized lidar technique offers a strong potential benefit for airborne lidar bathymetric applications

Airborne polarized lidar detection of scattering layers in the ocean.

Airborne lidar polarization estimation of the vertical profile of seawater light scattering coefficient

Some advantages of lidar remote sensing methods are described along with the tasks for solving which these methods are used. Examples of modern lidar systems designed for the oceanological monitoring are also given. The lidars are successfully used to solve a wide range of fishery problems: measuring the hydrooptycal characteristics of the sea surface layer, locating fish schools and estimating the corresponding biomass (1,2), finding the phytoplankton concentration (3), registering the water pollution (4), etc. The remote character of the lidar measurements allows carrying them out from aboard a moving carrier (plane, ship). No restrictions are imposed on the speed and course of the carrier. Lidar sounding allows measurements in the water without submerging any devices and therefore without affecting the processes and objects studied. Ship lidar "Gidrobiont" (SLG) was designed by JSC "Giprorybflot" in cooperation with the Institute of Oceanology RAS for installation on research and fishing ships. The main feature of the SLG is the combination of polarization and spectral channels in its design. Two methods are used for processing the echo signal formed during sounding of sea water with short unidirectional laser pulse. The first one analyses the time dependency of degree of polarization of the echo signal and therefore provides the possibility of getting vertical profiles of hydrooptical characteristics and locating and depth measurements for different inhomogeneities in sea water. The second one uses spectral analysis of the echo signal allowing locating phytoplankton clusters in the surface water layer and determining their characteristics.

B.A. Gureev

Papers

Airborne polarized lidar detection of scattering layers in the ocean.

Airborne lidar polarization estimation of the vertical profile of seawater light scattering coefficient

Oceanological Monitoring of Fishing Areas using Lidars

LED fluorimeter with spectral registration

YAG:Nd3+ Laser Systems for Marine UV Lidar