Elimination of oil spill like structures from radar image using MODIS data

Liis Sipelgas Marine Systems Institute

Tallinn University of Technology Tallinn, Estonia

liis@sea.ee

Rivo Uiboupin Marine Systems Institute

Tallinn University of Technology Tallinn, Estonia

rivo.uiboupin@phys.sea.ee

Abstract—Oil-spill detection from radar imagery is complicated task in Estonian coastal sea where look-alikes caused by ice, upwelling events, cyanobacterial blooms may occur. We compared SAR image with the MODIS optical imagery and look-alikes caused by upwelling event and cyanobacterial bloom were detected from images of August 7, 2007. Analysis of MODIS imagery from years 2000-2006 showed that during the ice free period upwellings occur frequently in the Gulf of Finland and they are strongest during summertime stratification period. Cyanobacterial blooms occurred in July and August in Estonian coastal sea. Some ice cover is present in Estonian coastal sea every winter.

Keywords-Baltic Sea, SAR, MODIS, oil-spill look-alikes

I. INTRODUCTION

Oil spills on the sea surface can be observed quite often and they are in correlation with the major shipping routs. In the Baltic Sea ship traffic is quite busy and the rate of oil transportation is increasing every year. Therefore possible environmental risks should be taken into the consideration. The damages of oil spill over the Estonian coastal area are well known. Two medium size oil accidents happened in the Gulf of Finland in 2006 and in one case the oil reached to the shore. The main environmental impacts of oil spills are assumed to be to seabirds and to the coastal ecology when the spill hits the shore.

When spill occurs it is important to describe the type, position, extent and state of oil at sea; predicting the future track of oil slicks and areas of shoreline likely to become polluted. One possibility to detect the oil spill and to describe its position and extent is satellite remote sensing. The prediction of oil spill future track can be done with different oil spill models. Methodologies of satellite based oil pollution monitoring have been demonstrated in several studies [2,4,6,9,11]. Synthetic Aperture Radar (SAR) that registers roughness of sea surface that express the reflection of the microwave backscattering properties is today most used satellite sensor for monitoring oil-spills. SAR is independent of weather and sun illumination and allows to acquire SAR images day and night under cloud cover that is the main

advantage over other remote sensing sensors. Main providers of SAR images today are RADARSAT and ENVISAT.

An important part of oil spill detection problem is distinguishing of oil slicks from look-alikes. These look-alikes can be organic algae mats on the sea surface, local temperature differences in upwelling zone, internal waves, local low wind areas etc. Some of these natural phenomena can be monitored with the optical remote sensing imagery. Chlorophyll a maps obtained from optical satellite data describe the locations of algae mats on sea surface. Satellite sea surface temperature data can be used for detecting the upwelling zones.

The objective of current study is to analyze the sources and occurrence of look-alikes in Estonian coastal sea. The possibilities of elimination of oil-spill like structures from radar images using MODIS imagery and models are discussed.

II. ANALYSES IMAGERY

A. SAR Imagery ENVISAT SAR Wide Swath Product from 7.08.2006 was

provided by KSAT.

B. Optical Imagery Optical imagery

MODIS (Moderate Resolution Imaging Spectroradiometer) data from years 2000-2006 were used in current study. MODIS spectrometers are installed onboard satellites Terra and Aqua and pass from north to south across equator in the morning. MODIS sensors register the radiance signal backscattered from the atmosphere and sea surface. The following products from different data distributes were downloaded and processed:

• Standard Chlorophyll a product (OC3) provided by Ocean Colour Web

• Standard Sea Surface Temperature product provided by Ocean Colour Web

• Level 1B reflectance data provided by MODAPs Web.

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III. OCCURICANCE OF POSSIBLE LOOK ALIKES

A. Wind The detectability of oil spills in SAR image strongly

depends on wind speed. In study [5] 700 ERS image over the Baltic Sea, North Sea and Mediterranean were analyzed and only few oil spills were detected when wind speed was >10m/s. According to study [1] the maximum wind speed of slick detection depends on oil type and age of the spill. The wind speed between 3 and 6 m/s is the best for oil slick detection, the sea surface roughness is developed and oil slicks appear as dark patches on a bright background [1,11]. In case of low wind speed >3m/s the sea surface is calm and looks dark on SAR image so the detection of dark oil spill in dark background is impossible.

One possibility to obtain the necessary wind data with the wind forecast is HILRAM model. Use of the forecast enables to eliminate the images that can’t be used for oil spill detection because of the too high wind speed.

B. Ice Baltic Sea has seasonal ice cover annually the sea ice

covers a mean of 40% of the Baltic Sea. The dominating feature of the Baltic Sea annual maximum ice extent is a very large inter annual variability that ranges from 10 to 100% of the total Baltic Sea area [7].There is also a large inter annual variability in the date freezing begins, thickness, extent, and break up date [7,10].

Oil spill detection in and with ice is a complicated task, different technologies for detection are compared in study [3]. Radar technology often fails in case of ice cover.

Ice formation in Estonian coastal area begins at the Pärnu Bay, Gulf of Riga and Estonian Archipelago Sea. The evolution of ice season in the Baltic Sea during the 20th century was examined in study [8]. Their 100 year time series showed a general trend toward reduced ice conditions; the largest change being the length of ice season, which has decreased by 14-44 days during last century.

Our analysis of the ice cover extent in the Estonian coastal sea using MODIS imaginary was performed for the period 2000–2006. In winters 2002/2003, 2004/2005 and 2005/2006 practically the hole coast of Estonia was ice covered (ice extent up to 60000m2) and some ice cover was present in coastal sea from December to May. In mild winter 2000/2001 and 2001/2002 the area in Estonan coastal sea reached up to 20000 m2 and ice cover was present from February to April.

C. Algae blooms The problem of algal blooms is especially acute in the

Baltic Sea region where the cyanobacterial blooms occur almost every summer. In our study we used MODIS reflectance at band 1 (620-670 nm) data together with MODIS chlorophyll a maps to distinguish the presence of cyanobacteria in the Estonian coastal area during the years 2000-2006. During these years cyanobacterial blooms appeared in July and August. In the Gulf of Finland cyanobactria was apparent from the images every year. In 2002 cyanobacterial bloom was very

extensive and covered the entire Gulf of Finland. In the Gulf of Riga significant surface accumulations of cyanobacteria were detected only in July 2003.

D. Temperature differences Upwellings events are one source of look-alikes in the Estonian coastal waters therefore it is important to know the main locations and extent of upwelling areas. Gulf of Finland is one of major frontal areas where frequent upwellings occur. Temperature differences between upwelling water and surrounding water can be up to 15 C [12]. The temperature variability depends on the month when upwelling occurs. During summer months (July-August) when the thermal stratification is the strongest the temperature differences were between 8 and 15 C, while in September the difference was up to 7 C. Upwellings along the Estonian coast occurred mainly in July and August and the average upwelling area was 4070 km2. The average area of the upwellings that occurred off the northern coast of the Gulf of Finland was 6120 km2. Along the Finnish coast the upwellings occurred mostly in September.

Upwellings in the Gulf of Riga are more rear compared to the Gulf of Finland. The intensity and offshore extent of upwelling in the Gulf of Riga are considerably smaller as well.

IV. DETECTED PHYSICAL STRUCTERS ON SATELLITE IMAGE SAR (ENVISAT) and MODIS data from August 7, 2006

was analyzed. Essential wind data was obtained from HIRLAM model. Several physical structures were detected from the SAR image along the Estonian Archipelago coastal area (Figure 1). Comparison with the Sea Surface Temperature data from MODIS image (Figure 2) showed that these structures were caused by upwelling event. Upwelling’s can form in Estonian coast depending on meteorological situation several times during the ice free period.

Figure 1. SAR image from Estonian coastal area August 7, 2006 where look-alikes caused by upwelling are seen along the islands northern coast.

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Figure 2. Sea Surface Temperature (MODIS product) from Estonian coastal area August 7, 2006 where upwelling was detected along the islands northern coast.

On ASAR image from August 7, 2006 (Figure 3) near the Stockholm Archipelago dark oil-like stripes were observed. In July and August cyanobacterial blooms usually occur in Baltic Sea. Comparison of ASAR image with colour composite of MODIS (Figure 4) band 1(620-670nm) and band 2(841-876nm) gave good coincidence with cynobacterial bloom distribution on optical image.

Figure 3. SAR image from Sweedish coast coastal area August 7, 2006 where look-alikes caused by cyanobacterial bloom is seen.

Figure 4. Colour composite of MODIS bands 1 and 2 from Sweedish coast coastal area August 7, 2006 where cyanobacterial bloom was detected

V. CONCLUSIONS On SAR image from second midsummer (August 7, 2006) several physical structures that cause oil spill look-alike were detected. Comparison with the optical remote sensing data we could eliminate the upwelling areas and areas covered with

cyanobacterial bloom. In current study we analyzed data about the possible look alkies from MODIS optical imagery from years 2000-2006. During the years 2000-2006 there were three winters when practically the whole coast of Estonia was covered with ice and therefore SAR imagery cannot be used for oil spill detection. Upwelling’s can form in Estonian coast depending on meteorological situation several times during the ice free period. Most upwelling events with the grater temperature difference occur in the Gulf of Finland during the summertime stratification period. Cyanobacterial blooms in Estonian coastal region occurred in July and August during 2000-2006 every year in smaller or larger extent. The MODIS optical imagery of chlorophyll a content together with the reflectance data at band 1 (620-670nm) gives relevant overview of locations of cyanobacteril surface accumulations which cause oil spill look-alikes on radar image.

ACKNOWLEDGMENT

This work was supported by the Enterprise Estonia project EU

REFERENCES [1] T.I.Bern, T.Wahl, T.An , „Oil spill detection using

[2] on by satellite

[3] as & C.Brown, „A Review of the Status of Advanced

[4] utine observation

[5] ectability of marine oil

[6]

[7] he Baltic Sea ice season in changing

[8] paranta,

[9] atic

[10] of the maximum annual extent

[11] E.Volden,

[12] from

23689.

dersson, R. Olsensatellite based SAR:Experience from a field experiment”, Proc. 1st ERS-1 Symposium, Nov. 1992, pp. 829-834 C.Brekke and A.H.S. Solberg, “Oil spill detectiremotesensing,” Remote Sens. Environ., vol. 95, no. 1, Mar. 2005, pp. 1–13. M.FingTechnologies for the Detection of Oil in and with Ice”, Spill Science & Technology Bulletin, vol.6, no.5/6, 2000, pp.295-302. M.Gade, W, Alpers „Using ERS-2 SAR images for roof marine pollution in European coastal waters”, The Science of the Total Environment, vol. 237, 1999, pp. 441-448. M. Gade, J. Scholz, C. von Viebahn, “On the detpollution in European marginal waters by means of ERS SAR imagery” Proceedings of the IEEE IGARSS’00, vol. 6, July 2000, pp. 2510–2512. F. Girard-Ardhuin, “Operational Oil-Slick Characterization by SAR. Imagery and Synergistic Data”. IEEE Journal of Oceancic Engineers. vol. 30, no 3, 2005, pp. 487-495 J. Haapala, M. Lepparanta, M, „Tclimate”, Boreal Environment Research vol.2, 1997, pp.93-108. S. Jevrejeva, V.V. Drabkin, J. Kostjukov, A.A. Lebedev, M. LepY.U. Mironov, N. Schmelzer, M. Sztobryn, „Baltic Sea ice seasons in the twentieth century”. Climate Research vol.25, 2004, pp.217-227. I. Keramitsogloua, C. Cartalisa, C.T. Kiranoudis, „Automidentification of oil spills on satellite image”, Environmental Modelling & Software vol.21, 2006, pp. 640–652. A. Seina, E. Palosuo, „The classificationof ice cover in the Baltic Sea 1720-1995”. Meri Report Series of the Finnish Institute of Marine Research vol.27, 1996, pp.79-91. A.H.S. Solberg, G.Storvik, R.Solberg, and “Automaticdetection of oil spills in ERS SAR images,” IEEE Trans. Geosci. Remote Sens. E, vol. 37, no. 4, Jul. 1999, pp. 1916–1924. R. Uiboupin. and J. Laanemets. Upwelling characteristics derivedsatellite sea surface temperature data in the Gulf of Finland, Baltic Sea. (Unpublished results)

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