OIL SPILL DETECTION AND MAPPING IN THE NORTHWEST PACIFIC OCEAN BY L-BAND ALOS PALSAR

Remote sensing technique is the most appropriate for oil spill detection and mapping. When tanker accidents happen, we need to know precisely where polluted areas are and to evaluate slick drift in order to protect coasts. Satellite detection is well adapted since it produces regularly images. Synthetic Aperture Radar (SAR) seems to be one of the most attractive and suitable instrument for the open ocean and coastal zone monitoring and earlier detection of illegal oil spills and wastewater discharges since it does not depend on cloudiness and sun illumination. It allows showing illegal discharges that most frequently appear during night and can also survey storms areas, where accident risks are increased. Analysis of satellite images acquired by X, Ku, C, S and L-bands SARs as well as several experimental multifrequencies studies with artificial slicks have shown that damping of the sea surface roughness with wave numbers < 80 m -1 (L and S-bands) are weakly affected by slicks [14]. Wave damping, function of slick nature, is more important for oil slicks than for natural monomolecular films. The higher the elasticity, the higher the image contrast is, but there is no linear relation. Therefore, it is not easy to distinguish slick nature with contrast [3, 4]. The potential of ALOS PALSAR operating at wavelength of 23.5 cm for oil spill detection is a disadvantage in relation to X-band COSMO-SkyMed and TERRASAR SARs and C-band Envisat ASAR and RADARSAT-1/2 SARs. However the possibility to detect new oil spills by PALSAR and to use PALSAR images to trace spills detected earlier by other SARs can improve monitoring of the sea surface. To realize this possibility it is necessary to emphasized two factors important also for other oceanic applications of PALSAR images. At first, observation of the ocean should be increased. Now only the coastal zones are observed regularly. At the second, the quick look (QL) images of all oceanic frames should be produced with minimum delay after SAR sensing and these QL-mages should be available for users. Impacts of oil pollution are not only related to the quantity but also to location, season, bottom topography, meteorological and oceanic conditions. The Asian Marginal Seas and the Indonesian Seas and especially their coastal zones are strongly impacted by pollution due to the enhanced shipping, tanker operation, river outflow, and other reasons. Oil spills here were studied using SAR images taken from various satellites [5-7]. There are many examples of SAR signatures interpretation caused by oil spills and other oceanic and atmospheric factors in papers [5, 6] and in web site [7]. Ancillary satellite and in situ data, visual and semiautomatic methods are used to increase probability of correct oil spill detection and decrease probability of false alarm as well as to decrease probability of missed spill detection and increase probability of correct rejection. The revealed oil spills can be devided on two classes. The first class includes oil spills from ship operation in the open sea resulting due to release of waste waters or small leakages. These spills look as patches or continous or intermittent narrow bands having the negative radar (brightness) contrast against the background. Sometimes the width of a dark band increases as the distance from a ship increses that evidences about larger volume of released oil. The second class covers ship accidents accompanying by the huge oil spills. Just these cases are discussed in the news. The whole arsenal of technical means is used to mitigate degradation of marine life especially in the coastal zone after ship accidents. ALOS PALSAR images obtained over two catastrophic oil spills will be considered. The first one was on 7 December 2007 when the oil tanker Hebei Spirit collided with a barge, releasing 10,500 metric tons of crude oil into the Yellow Sea. The oil slick is affecting approximately 160 km of coastline near an important habitat for birds. The second accident was happen on 21 August 2009 at the Montara offshore drilling rig in the Timor Sea. Estimates of oil released in the sea till 4 November when hole was finally closed change from approximately 4,000 to 30,000 metric tons (http://en.wikipedia.org/wiki/Montara_oil_spill).