SAR Remote Sensing in Argomarine

M. Babiker, T. Hamre, Nansen Environmental and Remote Sensing Center (NERSC) Norway New Strategies and Technologies for safety of Navigations in the Mediterranean Sea 21 November 2012, Petroferraio, Italy

Outline

n  Background n  SAR remote sensing for oil spill n  SAR for Argomarine n  Summary

Oil spills seriously affect the marine ecosystem and cause political and scientific concern since they have serious effects on fragile marine and coastal ecosystem. The amount of pollutant discharges and associated effects on the marine environment are important parameters in evaluating sea water quality. Satellite images can improve the possibilities for the detection and monitoring of oil spills as they cover large areas and offer an economical and easier way of continuous coastal areas patrolling.

Background

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• Natural seeps: 46%

• Discharges from consumption of oils (operational discharges from ships and discharges from land-based sources): 37%

• Accidental spills from ships; 12%

• Extraction of oil: 3%

ESTIMATES OF GLOBAL INPUTS OF OIL TO THE MARINE ENVIRONMENT

http://oils.gpa.unep.org/facts/sources.htm

The most commonly used remotely sensed system to detect ocean pollution is Synthetic Aperture Radar (SAR) imagery.

The Prestige oil spill, Spain, caused by the sinking of an oil tanker in 2002

SAR = Synthetic Aperture Radar

An active instrument that transmit/receive EM radiation, store data in amplitude and phase (real and imaginary part)

•  Operates at microwave (or radar) frequencies

•  Works in the presence of clouds, day and night

•  Fine resolution is independent of the platform height, so images with the same geometric resolution (order of 10 m) can be obtained from satellites as from airplanes

z

y

x Swath

Illuminated area!

Antenna

Azimuth

Range

A short pulse is emitted by the antenna and then the amplitude and phase of the backscattered signal is recorded as a function of time This is repeated over again while the platform is moving Thus a 2-dimensional image is generated.

Advantages compared to optical remote sensing All weather capability (small sensitivity of clouds, light rain) Day and night operation (independence of sun illumination) No effects of atmospheric constituents (multitemporal analysis) Sensitivity to dielectric properties (water content , biomass, ice) Sensitivity to surface roughness ( ocean wind speed, currents ,oil spill ) Accurate measurements of distance (interferometry) Sensitivity to man made objects Sensitivity to target structure (use of polarimetry) Subsurface penetration

SAR image ordering and acquisition for the test areas

The following Satellite were investigated: ENVISAT ASAR (launched March 2002) ALOS PALSAR (November 2006) TerraSAR-X (June 2007) COSMO-SkyMed (June 2007) Radarsat-2 (December 2007)

SAR-based oil spill monitoring in Argomarine:

NERSC set up automatic download of images for the two test areas, using software tools developed by NERSC in other RTD projects

During the project period 208 ENVISAT ASAR images were downloaded for the Tuscan Archipelago And 280 images for The National Marine Park of Zakynthos. 3 TerraSAR-X were ordered and purchased 3 Radarsat2 images ordered and purchased

SAR analysis for oil spill detection and classification A selection of archived images from the ENVISAT ASAR (Advanced Synthetic Aperture Radar) covering both study areas were analyzed.

The Next ESA SAR Toolbox (NEST) was used to analyse the ASAR data.

An ENVISAT ASAR Wide swath image acquired on the 7 June 2008 covering the Tuscany Archipelago (B) oil spills map generated from image (a) Red is potential oil spill, light blue currents and waves black low wind speed. A confirmed oil spill is demarcated with green circle.

An ENVISAT ASAR image mode image acquired on the 14 March 2008 covering Zakynthos (B) oil spills map generated from image (a) Red is potential oil spill, light blue currents and waves black low wind speed.

Classification of oil spills and surface phenomena in dual polarisation images

Alternating polarizations ENVISAT ASAR image (VV/VH) acquired on the 01 August 2003 (A) VV polarization (B) VH polarization (C) RBG image from VV, VH and VV/VH (D) Wind speed and direction generated from the image.

All the available methods a n d m o d e l s f o r discrimination between oil spills and look-likes using polarimetric SAR are still experimental and no concrete method or model are available for that.

Alternating polarizations ENVISAT ASAR image (VV/VH) acquired on the 11 May 2007 (A) VV polarization (B) VH polarization (C) RBG image from VV, VH and VV/VH (D) Wind speed and direction generated from t h e i m a g e .

Most of the surface phenomena, oil spills and look-alikes investigated can be detected clearly in both polarizations VV and the HH, on the other hand the same features can’t be detected using cross polarizations (VH, HV).

SAR estimates of wind and currents The CMOD4 and Doppler shift algorithms have been applied on a selected ENVISAT ASAR images from the two areas to estimate wind speed and current.

An ENVISAT ASAR wide Swath (VV) of the 27 of August 2010 covering the Tuscan Archipelago.

ENVISAT ASAR wide Swath (VV) of the 15 of October 2010 covering Zakynthos Island.

Distribution and density of oil spills in the test areas Seventy two archived ENVISAT ASAR (Advanced Synthetic Aperture Radar) Wide Swath images were analyzed for oil spill distribution and density.

The oil spill detection algorithm was applied for all the images and only dark features with high potential to be oil spill were analyzed. A density map was created for each study area.

Summary

SAR is the most applicable spaceborne sensor for operational oil spill detection, mostly because of its all-weather/all-day detection capabilities and wide coverage

The largest challenge in detection of oil spills in SAR images is accurate discrimination between oil spills and look-alikes.

For operational purposes, there is a need for coordination between satellite overpasses and aerial surveillance flights.

Future SAR missions are crucial for sustainable operational oil spill detection services.

Thanks