In-water reflectance spectra measured on-board a jet-ski across a complex nearshore zone of case-2 waters during the ECORS experiment

International audienceIn situ reflectance spectra have been measured during the international field experiment ECORS-Truc Vert 2008 (SW France) in the nearshore zone over a complex bathymetry and in moderately turbid waters (SPM 7 m). The jet-ski provides a valuable mean to gather optical data in shallow waters and in environments hard to sample with traditional coastal ships. Main results show reflectance spectra are typical of the presence of mineral particles with light absorption at short wavelengths. The shape and magnitude of the spectra are also very representative of the type of waters and bottom depth : the maximum of reflectance is reached around 500 nm in low turbidity waters (SPM<1 mg/l) while moving to the yellow spectral band (570-580 nm) in moderately turbid shallow waters (SPM=4 mg/l) where rip currents drive sediments outside the surf zone. A very original dataset has been obtained considering the synchronization of optical data, bathymetric surveys and Formosat-2 high resolution satellite image all collected the same day (5 April 2008). In the present study, this dataset is tested to inverse optical models for bathymetry retrieval in order to compute diachronic bathymetric maps as the coastline is changing very fast and need frequent updates of the bathymetry

Remote sensing reflectance reconstruction to obtain water optical properties from MERIS multispectral satellite images

We use a multiple regression analysis and a data bank of about 400 reflectance spectra to reconstruct hyperspectral reflectances between 400 and 900 nm with a 5 nm step using only the values known at the wavelengths of the MERIS sensor level 2 data. For in situ remote sensing reflectances measured during different oceanographic missions, the reconstruction is within 2 per cent almost over the entire spectrum. The main difference (to a maximum of 4 per cent) usually occurs at the inflexion point of the reflectance curve between 580 and 600 nm. Observed in-situ remote sensing reflectances or reconstructed spectra are inverted using a Water Colour Simulator bio-optical model (WASI) to obtain the inherent optical properties (IOP) of the water. The values derived by the model are compared with the measurements when available. To validate the reconstruction, we compare the results of the model inversion using the initial spectrum or the reconstructed one as input. Preliminary results show that the derived values from the inversion of the reconstructed spectrum are very close to the values generated from the inversion of the initial spectrum, especially in case 1 waters. This reconstruction technique is used to generate hyperspectral remote sensing reflectances from reflectance data calculated by the MERIS sensor. We use the reconstructed spectra as input to be inverted in the WASI model in order to quantify the substances' concentrations; in particular, the inversion is working well for the suspended particulate matter concentrations

Inferred IOP and particulate matter from MERIS and MODIS multi-spectral satellite images

How to accurately determine and quantify particulate and dissolved matters at the water surface in coastal areas (case 2 waters) using passive ocean colour remote sensing is still a topical issue. The best actual models, like the OC5 algorithm from Gohin et al., 2002, are empirical, and usually restricted to a limited area; hence the need to gather numerous oceanographic data is strong. To improve our understanding of in-water radiation law, we tested a numerical bio-optical model with several components including bottom reflectance. The software called WASI (Water Colour Simulator) was developed by Peter Gege (Gege, 2004) and integrates forward and inverse modelling for eight common types of optical in-situ measurements in aquatic environments (in particular remote sensing reflectance, absorption or attenuation), using well-established analytical models. In forward mode, it simulates reflectance spectra using the IOP or the particulate and dissolved matters. In inverse mode, an input hyperspectral reflectance spectra measured with a spectro-radiometer is analyzed by fitting iteratively possible reflectance curves under constraints on some bio-optical parameters (simplex method) so as to provide the IOP and the particle concentrations. We used in-situ data acquired during the oceanographic mission OPTIC-CONGO (Gulf of Guinea) to evaluate the accuracy of the model results. The substance concentrations derived by the WASI model were compared with the in-situ measurements and the results are encouraging (Schmeltz et al., 2009). To apply this inversion to satellite data from MODIS (NASA) or ENVISAT/MERIS (ESA), we developed a multiple regression program adapted from Wernand (1997) to reconstruct hyperspectral reflectances from the multi-spectral satellite channels (10 bands for MODIS and 15 bands for MERIS). These reconstructed spectra are inverted in WASI to obtain the IOP and substances concentrations

Etude de l'hydrogéologie du Liban occidental (utilisation de la télédétection)

Afin d'évaluer les ressources en eau dans une vaste région tel que le Liban Occidental (environ 5000 km2), les méthodes conventionnelles sont insuffisantes. C'est pourquoi il est fait appel à la télédétection et aux méthodes appropriées pour acquérir des données complémentaires. L'interprétation des images satellites et des photographies aériennes, avec différentes caractéristiques optiques, est utilisée en relation avec de nombreuses mesures de terrain. Plusieurs indicateurs superficiels et souterrains ayant trait au régime hydrique ont pu être identifiés. De nombreuses cartes thématiques montrant les principaux éléments du régime hydrique ont été produites sous SIG, tel que la carte de la capacité d'infiltration, la carte des zones phréatiques potentielles, et la carte des pertes en eaux vers la mer. Le bilan hydrologique a pu être quantifié. Les informations rassemblées dans cette étude sont significatives pour la gestion des ressources en eau du Liban.The occidental part of Lebanon is an interesting region for studying water resources because it includes all elements of the water cycle. In order to make a compregensive assessment of water resources in such a large area (around 5000 km2), conventional methods alone woukd not be enough. However, the use of remote sensing together w ith those methods would be appreciated. this study treats water resources through delineating the whole water cycle. In this concer, remote sensing technique is used as a supplementary tool for data generation and extraction whenever applicable. In addition to the known methods of study including field survey, map interpretation, and water analysis and measurements, a variety of satellite images, with different resolutions, optical characteristics and of different time intervals are used; Thn, the extracted data from remote sensing and the phydical data are manipulated together int he Geographic Information System (GIS). Thus, a number of thematic maps, with detailed description and explanation is produced.BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceF

Qualité des eaux côtières du Sud du Golfe de Gascogne par télédétection spatiale (méthodologie de détermination et de quantification de substances particulaires et dissoutes)

Les eaux côtières du Sud du Golfe de Gascogne sont soumises à l expulsion d un panache fluvial au niveau de l estuaire de l Adour. Ces eaux douces peuvent entrainer de fortes variations des concentrations en matières minérales en suspension (MSM, en mg,l-1), en chlorophylle- a (Chla, en mg,m-3) et en matière organiques dissoutes colorées (MODC, en mg,l-1) contenues dans les eaux côtières. En modifiant la transparence de l eau et en charriant des nutriments et des bactéries, elles peuvent affecter la productivité biologique et avoir un impact significatif sur la santé humaine. Notre étude a pour but de développer des algorithmes empiriques et de tester l efficacité des images MODIS-Aqua pour la cartographie des substances particulaires et dissoutes contenues dans les eaux côtières soumises à l'influence du panache de l'Adour. Dans cet objectif, deux missions océanographiques ont été réalisées afin de mesurer simultanément les MSM, la turbidité (en NTU), la Chla et les MODC et la réflectance de télédétection de l eau in-situ. Deux algorithmes empiriques opérationnels pour la quantification des matières minérales en suspension et de la turbidité ont été développés. Une automatisation du traitement des images MODIS-Aqua déjà corrigées des effets atmosphériques et à la résolution de 250 mètres (produit MYD09GQ) a été réalisée. Ce système opérationnel a permis de cartographier les concentrations en sédiments en suspension contenus dans le panache sur 246 jours, entre 2006 et 2009. La première étude à grande échelle spatiale et temporelle de la dynamique hydro-sédimentaire du panache de l'Adour a ainsi pu être réalisée. Finalement, plusieurs algorithmes ont été proposés pour la quantification des concentrations en chlorophylle-a dans le sud du Golfe de Gascogne. Ces relations empiriques devront être testées sur de nouvelles mesures in-situ afin de préciser leurs limites de validité. Notre étude montre l apport de l imagerie satellitaire MODIS pour la surveillance du panache de l'Adour. A plus long terme, un système efficace de surveillance de la qualité des eaux côtières de la côte Aquitaine pourrait être envisagé.The southern Bay of Biscay coastal waters are under the direct influence of the freshwater plume from the Adour estuary. The river outflow leads to a strong variability in the concentration of mineral suspended matters (MSM in mg l-1), chlorophyll-a (Chla, mg, m-3) and colored dissolved organic matters (CDOM in mg l-1) contained in coastal waters. By changing the transparency of the water and carrying nutrients and bacteria, they may affect biological productivity and have a significant impact on human health. Our study aims to develop empirical algorithms and test the efficiency of MODIS-Aqua imagery to map particulate and dissolved substances contained in coastal waters under the influence of the Adour plume. In this aim, two oceanographic missions were carried out to measure simultaneously the MSM, turbidity, Chla and CDOM and the remote sensing reflectance of water. Two empirical operational algorithms for quantification of suspended mineral matters and turbidity have been developed. An automated processing of atmospherically-corrected MODIS-Aqua-250m product (MYD09GQ product) was performed. This operating system has been used to map the concentrations of suspended sediment content in the plume at 246 days between 2006 and 2009. Thus, the first study at large spatial and temporal scale of the Adour plume dynamics has been achieved. Finally, several algorithms have been proposed in order to quantify the concentrations of chlorophyll-a in the southern Bay of Biscay. These empirical relations must be tested on new in-situ measurements to clarify their validity limits. Our study shows the contribution of the MODIS satellite imagery to monitor the Adour plume. In future, an effective monitoring system of Aquitaine coastal water quality could be considered.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

IOP from reflectance measurements to obtain the Kd coefficient: application to the Gabon and Congo coastal waters

International audienc

In-Water Reflectance Spectra Measured On Board a Jet-Ski Across a Complex Nearshore Zone of Case-2 Waters during the ECORS Experiment

International audienc

Dynamics of the turbidity maximum zone in a macrotidal estuary (the Gironde, France): Observations from field and MODIS satellite data

Over a 1-year period, field and satellite measurements of surface water turbidity were combined in order to study the dynamics of the turbidity maximum zone (TM) in a macrotidal estuary (the Gironde, France). Four fixed platforms equipped with turbidity sensors calibrated to give the suspended particulate matter (SPM) concentration provided continuous information in the upper estuary. Full resolution data recorded by the moderate resolution imaging spectroradiometer (MODIS) sensors onboard the Terra and Aqua satellite platforms provided information in the central and lower estuary twice a day (depending on cloud cover). Field data were used to validate a recently developed SPM quantification algorithm applied to the MODIS `surface reflectance' product. The algorithm is based on a relationship between the SPM concentration and a reflectance ratio of MODIS bands 2 (near-infrared) and 1 (red). Based on 62 and 75 match-ups identified in 2005 with MODIS Terra and Aqua data, the relative uncertainty of the algorithm applied to these sensors was found to be 22 and 18%, respectively. Field measurements showed the tidal variations of turbidity in the upper estuary, while monthly-averaged MODIS satellite data complemented by field data allowed observing the monthly movements of the TM in the whole estuary. The trapping of fine sediments occurred in the upper estuary during the period of low river flow. This resulted in the formation of a highly concentrated TM during a 4-month period. With increasing river flow, the TM moved rapidly to the central estuary. A part of the TM detached, moved progressively in the lower estuary and was finally either massively exported to the ocean during peak floods or temporary trapped (settled) on intertidal mudflats. The massive export to the ocean was apparently the result of combined favorable environmental conditions: presence of fluid mud near the mouth, high river flow, high tides and limited wind speeds. The mean SPM concentration within surface waters of the whole estuary showed strong seasonal variations but remained almost unchanged on a 1-year-basis. These observations suggest that the masses of suspended sediments exported toward the ocean and supplied by the rivers were almost equivalent during the year investigated (2005). Results show the usefulness of information extracted from combined field and current ocean color satellite data in order to monitor the transport of suspended particles in coastal and estuarine waters. (c) 2008 Elsevier Ltd. All rights reserved

Télédétection et modélisation numérique des flux sédimentaires dans l'estuaire de la Gironde

L'objectif de ce travail est de mettre au point un dispositif combinant télédétection spatiale et modélisation numérique pour étudier la dynamique sédimentaire en milieu estuarien : la Gironde. Un algorithme, basé sur l'utilisation de rapports de réflectance, est développé pour quantifier les sédiments en suspension à partir de mesures satellites (SPOT, Landsat). Les résultats empiriques obtenus sont explicités à l'aide d'un modèle bio-optique. L'algorithme est appliqué à une série d'images satellites. Les cartes de concentrations établies, complétées par des profils verticaux mesurés in situ, permettent d'observer les déplacements saisonniers des maxima de turbidité. Mesures satellites et in situ sont utilisées pour calibrer et valider un modèle hydro-sédimentaire. Ainsi, les flux sédimentaires au centre dans l'estuaire sont estimés pendant une période de un an. Enfin, un processus d'initialisation du modèle de transport à partir des données (satellites et in situ) est développé.BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceF