Lidar Sensing of Ocean Trough Disturbed Surface
The Development of the Technique for Polarization Lidar Remote Sensing of the Stratified Ocean with Allowance for the Effects of the Sea Surface Roughness
Tech Area / Field
- ENV-MIN/Monitoring and Instrumentation/Environment
- OBS-NAT/Natural Resources and Earth Sciences/Other Basic Sciences
3 Approved without Funding
Institute of Oceanology, Russia, Moscow
- VNIIEF, Russia, N. Novgorod reg., Sarov\nNIIIT (Pulse Techniques), Russia, Moscow\nMIFI, Russia, Moscow
- Group d'Etudes Sous-Marines de l'Atlantique (GESMA), France, Brest
Project summaryThe objective of the Project is to develop the technique for polarization lidar remote sensing of the stratified ocean with allowance for the sea surface roughness effects, to elaborate the lidar data processing algorithm (so-called, the “rough-surface-correction” algorithm) and to devise the optimized airborne polarization lidar.
The airborne oceanographic lidars allow one to solve a great deal of topical problems, including ecological monitoring, exploration of fishing areas, sea-truth validation of satellite data. The interest in the airborne lidar use is due to the fact that they:
(1) make it possible the fast data acquisition over large areas of water;
(2) allow one to obtain "instantaneous" spatial distributions of the seawater characteristics;
(3) can operate in conditions where passive satellite remote sensing is impossible (in the dark time or in cloudy weather);
(4) allows to make measurements where shipboard research is difficult (shallow waters, severe ice conditions).
The important advantage of the lidar technique as compared to other (acoustic, radar) techniques of the ocean remote sensing is in unique opportunity to derive vertical profiles of the sea water parameters through the air-ocean interface. Field experiments carried out by specialists from USA, Russia and other countries have demonstrated the principal potentiality of the airborne pulsed lidars, i.e. lidars based on registration of the echo-signal temporal profile (or waveform), to detect the high turbidity layers in the bulk of water, to determine the spatial distribution of the bio-production, phytoplankton, dissolved organic and suspended matter concentration, to study the inner waves, oceanic fronts, vortices and streams.
The potentiality of the lidar measurements can be significantly enhanced with the use of the polarimetric technique based on registration of two echo-signal components with orthogonal (co- and cross-) polarizations. The field experiments performed by the authors of the project have shown that the polarimetric technique increases a sensitivity of the lidar measurements to water stratification in the bulk of the ocean and, as consequence, the sounding depth.
A practical implementation of the airborne pulsed lidars is retarded due to difficulties in the lidar data interpretation. The retrieval of information on the ocean water stratification from the data of lidar measurements is complicated essentially by two important factors, namely, by the rough sea-surface effects and by multiple light scattering in water. The sharpest distortions result from the wind-induced roughness of the ocean surface. A twice crossing of the rough air-sea interface by the incident and backscattered beams of light results both in large random fluctuations of the echo-signal amplitude and in a certain distortion of the averaged echo-signal waveform. These effects present a severe problem in the lidar data interpretation, pose difficulties in the echo-signal registration and lead to loss of the essential part of information from the near-surface layers of the ocean.
All algorithms used for processing of the data of the field lidar experiments are based on assumption that the air-ocean interface is flat. The interplay between the statistics of the echo-signal parameters and the characteristics of the wind-induced sea-surface roughness is still not clearly understood. The results to date are of limited range of applicability and give no quantitative description of the observed phenomena for a great deal of practically important situations. The effect of the wind-induced sea-surface roughness on the polarization of the echo-signal has not been studied yet (there are only a few examples of numerical simulations carried out with the Monte-Carlo method).
A practical interest in the use of the lidar techniques and the existence of the above-mentioned difficulties make the subject of the Project a topical problem.
The anticipated results of the Project are:
(1) The airborne lidar based on the application of the polarization lidar technique. The lidar will be optimized in construction for maximum reduction of the wind-induced sea-surface roughness effects;
(2) The algorithm for lidar data processing taking into account the sea-surface roughness effects (so-called, "rough-surface-correction" algorithm);
(3) The computer program code for the on-line lidar data processing.
Among by-products of the Project realization, of the special interest will be the following:
– the theoretical quantitative model of propagation of a spatially confined pulsed beam of polarized light in the air - rough surface - ocean system, as applied to the problem of the lidar remote sensing of the stratified ocean;
– the computer model for rapid numerical calculations of the average and correlative echo-signal parameters versus the lidar system characteristics, the overwater wind speed and the seawater optical coefficients under actual ocean conditions.
The Project realization allows one to advance from demonstrating experiments to the practical employment of the airborne pulsed lidars to deal with various scientific and applied problems. In future an industrial sample of the airborne pulsed lidar is planned to be constructed using the Project results and to be applied in the search for fishing areas and ecological monitoring of coastal water area of the Barents Sea where mining of the underwater gas resources is proceeding.
The Project participants have wide experience in:
– field researches of propagation of laser pulses and natural solar light through the atmosphere-ocean system, elaboration of the oceanographic lidars and application of them for the ocean remote sensing (SIO RAS);
– theoretical studies of light propagation through randomly inhomogeneous media, in particular, through the rough sea-surface and the ocean water (MEPhI);
– elaboration of the special instruments to detect high-rate processes by optical techniques in the natural conditions (RIPT and RFNC-SRIEPh).
The work on the Project:
(1) permits scientists and engineers that worked up weapons to re-orientate their unique experience and professional abilities on peaceful aims;
(2) to support applied researches in the environment protection that are carried out in Russia;
(3) to extend the opportunities of international collaboration for the project participants.
We plan continuous contacts with our foreign collaborator in items of:
(1) information exchange;
(2) elaboration of the program of field researches;
(3) elaboration of the computer model architecture;
(4) preparation of the data base for numerical computations;
(5) analysis of the results to be obtained;
(6) organization of the workshops;
(7) promoting of the Project results into the practice.
Implementation of the Project is planned to be started with solving the direct problem, namely with determining the average and correlative echo-signal parameters as a function of the sea-surface roughness properties, the seawater optical coefficients and their stratification, and of the lidar system characteristics. These researches will include field experiments, theoretical calculations and numerical simulations with the statistical Monte-Carlo method. The goal of these researches is to recognize the general physical effects resulting from transmission of light through the rough sea surface and to develop quantitative techniques for its calculations.
Field experiments are intended to be carried out under controlled conditions. We plan to use specially elaborated apparatus to be placed both on the stationary platform and on the shipboard. The shipboard and submerged photo receivers provide for synchronized receiving of laser pulses generated by shipboard YAG:Nd SHG laser. The linear dimensions of the sounding beam and the field of view of the photo receiver at the sea surface level will correspond to the case of airborne sensing.
Theoretical calculations of various components of the echo-signal are intended to be carried out on the basis of the statistical theory of wave propagation. The up-to-date data both on the spectrum of the wind-induced surface waves and on the scattering phase function of the ocean water will be used. In calculations of the average temporal profile and correlation functions of the echo-signal, we intend to go beyond the framework of the perturbation theory in small slopes of the waves on the surface and to take into account the effect of the enhanced backscattering due to focusing of light in refraction at the rough surface. The vector version for the statistical equations of the scalar theory will be developed to calculate the average and correlative parameters of the co- and cross-polarized echo-signals. In addition to the sea-surface roughness effects, depolarization of light due to multiple scattering in the ocean water will be taken into account. To do this, we intend to take advantage of our recent results concerning the transfer of polarized light in the media with sharply anisotropic scattering phase function. Eventually we plan to determine the dependence of the echo-signal as a function of the remote sensing geometry (the base of the lidar system, the angular spread of the laser beam, the photoreceiver field of view, mutual orientation of the laser axis and the receiver one etc.).
Numerical calculations of the statistical parameters of the echo-signal based on the actual data on the wind-induced sea-surface roughness, the seawater optical properties (including data on the scattering phase matrix) and the characteristics of the lidar simulation are planned to be carried out both by the analytical formulas and by the Monte Carlo simulation. Comparison of the results to be obtained allows us to estimate accuracy and range of validity of the analytical formulas. These formulas are planned to be used as the basis for the computer model of the echo-signal parameters. When developing the computer model, the friendly graphical user interface will be created. The software will include: data input, digital and graphical result output, and multidimensional optimization. The model will provide the feasibility to perform fast computations of the average temporal profile and correlation functions of the echo-signal both at the initial frequency for co- and cross-polarization of light and at the frequency of the Raman scattering. The input data for the model is the surface wind speed, the seawater optical coefficients and the lidar characteristics.
The most preferable geometry of sounding will be chosen on the basis of the calculations of the statistical echo-signal characteristics so that to minimize the surface roughness effect on the echo-signal. We intend:
(1) to determine the lidar apparatus characteristics wherein the enhanced backscattering effect and the echo-signal fluctuations are reduced to a minimum;
(2) to estimate proficiency of the echo-signal detection by several photoreceivers that pick up signals from nearby but non-overlapping areas of the ocean surface;
(3) to analyze the feasibility of the water Raman normalization at each instant of time to remove the wind-induced echo-signal variations.
The algorithm for lidar data processing - "rough-surface-correction" algorithm - is intended to be based on the echo-signal computation model and to be constructed as solving the system of equations for the temporal profile and correlation functions of the echo-signal averaged over given number of measurements. The data both on the co- and cross-polarized components of the echo-signal and on the signal at the Raman frequency will be used. The algorithm to be developed is expected to increase essentially an accuracy of retrieval of the water stratification.
The elaborated technique, the algorithm and the optimized lidar construction will be realized in the airborne pulsed lidar and in the program code for on-line data processing of the airborne measurements. The lidar would be designed as a set of separate blocks that are suitable for transporting, fast assembling and installing on the aircraft of various types.