15arspc submission 48

8/8/2019 15arspc Submission 48

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THE 2009 MONTARA OIL SPILL IN THE TIMOR SEA ASOBSERVED BY EARTH OBSERVATION SATELLITES

Xiaojing Li, Linlin Ge, Zhe Hu and Hsing-Chung ChangSchool of Surveying and Spatial Information Systems

The University of New South Wales, Sydney, NSW 2052, AustraliaPhone: +61 2 9385 4177, Fax: +61 2 9313 7493

[email protected]

Abstract

On 21 August 2009, the Montara offshore oil platform in the Timor Sea, 250 km

to the northeast of Australia, started leaking oil. Over a period of ten weeks,more than two million litres of oil were lost into the sea, forming a 2000 squarekilometre slick. The Australian Maritime Safety Authority (AMSA) mobilisedaircraft to try to break up the growing oil slick, spraying around 150,000 litres ofchemical dispersant at a cost of around $5.3m, in an attempt to stop the oilfrom contaminating shoreline habitats. The oil well was finally closed at thebeginning of November 2009, stopping the flow of oil into the sea.

A range of radar and optical satellites followed the growth of the slick as the oilspill occurred. For example, the MODIS sensor aboard the Terra and Aqua

satellites took images of the Timor Sea on daily basis and results based onthese observations have been circulated widely. However, cloud cover presentsa serious challenge to optical observation made by sensors such as MODIS.

The active microwave sensors that are used on radar satellites, on the otherhand, work regardless of weather conditions, cloud cover or the absence ofdaylight and some are able to provide radar data with a resolution of down toone metre. They can give a two-dimensional representation of the reflection ofradar waves from the surface of the water. Data from two of such radarsensors, namely the ALOS PALSAR and Envisat ASAR, are processed and

compared with MODIS results. While results from these optical and radarsensors are consistent with each other, radar sensor has demonstrated uniqueadvantages in monitoring oil spill.

1. Introduction

1.1 The Incident

The incident started on Friday 21 August 2009 when the Montara wellhead

platform had an uncontrolled release of oil and gaseous hydrocarbons. The



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platform located 200 km off the Kimberley coast, or approximately 280 kmnorthwest from Truscott, Western Australia. The company operating theplatform, PTTEP Australasia Proprietary Limited, estimated there was about400 barrels (approximately 64 tonnes) of crude oil being leaked per day during

this incident. The uncontrolled release of oil continued until the 3rd

November2009 (IAT 2010).

Over 130 surveillance flights were conducted survey to gather oil spillintelligence and environmental data during the oil spill event. It was observedthat the majority of oil remained within about 35km of the platform. However,the sheen and weathered oil was still reported further away and in differentdirections from the platform, depending on the conditions of wind, currents, seaand air temperatures, etc. The Australian Maritime Safety Authority (AMSA)reviewed daily the oil spill response and the disaster relief operation based on

the observations from morning surveillance flights.

The resulting environmental disaster has been recognised as one of Australia’sworst oil spills. World Wide Fund for Nature (WWF) launched a research trip tothe affected area on 24 September 2009 to study the impacts on the region’smarine life. Surface oil was detected as glassy water, waxy residues thicksheen with strong smell. Official estimates of the size of the slick from AMSAindicate that it was about 6,000 km

2but it was believed the impact has

extended between 10,000 to 25,000 km2

(AES 2009).

Early studies on detecting and monitoring of oil slicks on ocean using remotesensing have demonstrated the great potentials with the imagery acquired byairborne systems (Estes and Golomb 1970). Remote sensing techniques canbe categorised into optical and radar remote sensing with the difference ofimages comes from airplane or satellite. The former is passive and normally isrelied on the solar illumination; the latter is an active system which has its ownenergy/illumination source.

1.2 Optical remote sensing for oil spill monitoring

Airborne optical photography has been the most common form of oil spillmapping (Fingas and Brown 1997). The sheen of oil spill shows silvery andreflects light over a wide range in visible spectrum. However, due to the variousconditions, such as solar zenith angle, sensor position, and wind condition, theoil sheens often are hard to be distinguishable from the background. Due to theabsence of particular spectral signatures that distinguish oil from thebackground, the effectiveness of using the visible and near infrared (NIR)remote sensing data is constrained (Adamo et al. 2006). An experiencedoperator may also be needed to identify the presence of oil slick in the imagesmanually.



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The remote sensing techniques in the Infrared (IR) and Thermal Infrared (TIR)spectral regions focus on measuring the difference in the thermal radianceproperties of oil and ocean. It was found that thick oil appears warmer but theintermediate thickness of oil appears cooler than the surrounding (Fingas and

Brown 1997). The temperature difference between oil and water is dependenton the thickness of oil slick. TIR system, however, does not detect thin sheens(Goodman 1994). NOAA Advanced Very High Resolution Radiometer (AVHRR)IR imagery has been used to detect the hot spots caused by oil spills (Tsengand Chiu 1994). During the day, the oil spill regions may have temperatures ofabout 2-4

0C above the surrounding sea surface temperature. There is a

research of NOAA-AVHRR TIR imagery successfully detected the oil slicks(Cross 1992). This study found that the oil slicks had higher brightnesstemperatures than the surrounding water, while the slicks appeared cooler atnight.

Robust Satellite Techniques (RST) for oil spill detection using NOAA-AVHRRTIR has been studied in recent years (Casciello et al. 2007). That approachexploited the multi-temporal NOAA-AVHRR TIR data and can increase thereliability of detecting the oil spill. As MODIS TERRA and AQUA sensors arealso capable of imaging in the TIR region, the spectral channels 31 and 32 ofMODIS are similar to the channels 4 and 5 of AVHRR. Grimaldi has publishedhis research on using MODIS data to detect the oil spill (Grimaldi et al. 2009).The authors also have successfully applied the MODIS data in analysing thegreat dust storm event in Australia in September 2009 (Li et al 2010). Thus,MODIS dada has been analysed by the authors herein to monitor oil slicks in

the Timor Sea.

The same as all other optical satellite remote sensing applications, the majordisadvantage of using optical data for oil spill monitoring is limited by solarillumination (except for TIR) and clouds, which can be complemented by radarremote sensing.

1.3 Radar remote sensing for oil spill monitoring

Radar is an active remote sensing system which can image the Earth’s surfaceday and night. It emits electromagnetic waves in the microwave spectral regionthat allows the system to operate in all weather conditions. In radar remotesensing, the backscattering coefficient of the Earth’s surface is mainlydependent on surface roughness, incidence angle and wavelength of theelectromagnetic signals. If the surface roughness is relatively small with respectto the wavelength of incident radar signal, the scattering is more specular. Thatis the backscattered signals bouncing away from the radar antenna. As a result,it appears darker in the image. If the surface is rough, then the signal will bebackscattered diffusely in all directions and some will return to the radar. Theseobjects appear brighter in the image. Therefore, synthetic aperture radar (SAR)



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data have also been used to study the ocean waves and currents (Jain et al.1982; Li et al. 2010).

When there is an incident of oil spill, oil suppresses the short ocean waves(with wavelengths in the centimetre range) that reflect the electromagneticsignal. The smooth surface of an oil slick does not reflect the incident radarwaves back to the satellite and so it appears as a dark area in the radar image,surrounded by lighter, oil-free areas. This means that in radar images, such asthose created with the Synthetic Aperture Radars (SARs) on board the ALOS,Envisat, TerraSAR-X and COSMO-SkyMed satellites, oil-covered areas appeardark.

On 17 February 1970 a four band (X, C, L and P-bands) airborne radar system

was tested to map the oil slicks in the Chedabucto Bay area of Nova Scotia(Guinard and Purves 1970). The suitable radar wavelengths and polarizationsfor monitoring oil spill have been discussed in the early studies (Girard-Ardhuinet al. 2003; Kanaa et al. 2005). The results showed radar remote sensing canbe used as an effective tool to monitor the oil spill.

Besides radar sensor, the laser fluorosensor is another active sensor which canidentify oil on most backgrounds, such as water, soil, ice, and sand etc. Thistechnique uses a laser emitting the signals in the ultraviolet region of theelectromagnetic spectrum of 300-355 nm. However, fluorosensor has the

disadvantage of high cost. As it is outside the scope of this paper, more detailsof laser fluorosensor are referred to the earlier studies (Campbell and McStay1995; Brown et al. 1996; Fingas and Brown 2000; Brown and Fingas 2003).

2. Radar Results for the Montara Oil Spill

Due to the large affected area of oil spill, multiple ALOS PALSAR Stripmapimagery acquired in the adjacent orbit paths were used to map the region. Thisstudy used 7 paths to cover the site. In the meantime, an ENVISAT ASARScanSAR image was acquired to cover the full scene. The details of the SAR

data used in this study are summarised in Table 1. The coverage of the radarimagery was illustrated in Figure 1.

In radar data processing, the intensity images are filtered at first to reduce theimpact of noises. As a result, the oil patterns stood out. Then, the filteredproducts are geo-coded to locate the exact locations of the oil. GIS analysis isthe final step to illustrate the processed radar data.



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Table 1. ALOS PALSAR and ENVISAT ASAR data used in this study.

Satellite / Sensor Path (Track) Date Imaging Mode Polarization

401 25/10/2009

402 26/09/2009

403 13/10/2009

404 30/10/2009

405 01/10/2009

406 02/09/2009

ALOS / PALSAR

407 04/08/2009

Stripmap HH

ENVISAT / ASAR 210 08/09/2009 ScanSAR VV

Figure 1 Radar coverage of ALOS PALSAR Stripmap data in black dash lines andENVISAT ASAR ScanSAR data in white dash lines.

2.1 ALOS PALSAR images analysis

As mentioned earlier, 7 paths of ALOS PALSAR images have been used hereto monitor the oil spill status since the coverage of one image is much smallerthan the affected area. The problem of using this method is that the images in

different paths were not acquired at the same time. It can only present the oil



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position in certain days rather than giving an overall picture of the oil status inthe whole area affected. However, by doing so, the oil spill status in differenttime is able to be observed, and the oil migration direction can be inferred, aswell as the probable moving speed.

Figure 2 shows the monitoring results in 7 paths with observation time spanningover two and half months. The oil can be clearly observed in the images of path406 (2

ndpath from left) which were acquired on 2 September 2009. It means

that the oil spread into the ocean to the east of the Montara Platform onlyaround 12 days after the incident. Within two months, the nearby area was stillpolluted by the oil, which can be concluded from the obvious dark patternobserved in path 405 (3

rdpath from left) on 1 October 2009 and in path 404 (4

th

path from left) on 30 October 2009.

It can also be seen from the ALOS PALSAR results that the oil was migrating tothe north and east direction of the Montara Platform. On 13 October 2009, onlya little oil was observed in path 403, but 17 days later, a large amount of oil wasvisible on path 404 to its west.

Figure 2 ALOS PALSAR Stripmap intensity images. The oil slicks can be identified asthe darker features in the figure.



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2.2 ENVISAT ASAR ScanSAR image analysis

In order to overcome the limited coverage problem with ALOS PALSAR case, afar larger coverage dataset taken in the ENVISAT ASAR ScanSAR mode wasobtained. In this case, the observation covers a large area of 400 x 400 km

2,

which makes it easier to identify the oil spill over the whole affected area. FromFigure 3, it can be clearly seen that the oil impacted a large ocean area in thenorth of the wellhead only 19 days after the spill. Obviously, this incident is aserious environmental hazard.

Figure 3 Envisat ASAR ScanSAR intensity image acquired on 8 September 2009.



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3 Optical Results for the Montara Oil Spill

In this study, optical images of MODIS satellite are also used to monitor the oil

spill status. Two results based on the MODIS AQUA true colour images areprovided in Figure 4. In these images, the dark area is the oil spill region. It canbe seen that the oil has spread to the north and east direction within a monthafter the incident from 30 August 2009 to 24 September 2009. The conclusionis consistent with the ALOS PALSAR observations.

The problem of using optical image is the interference of clouds. Althoughfortunately the percentage of cloud coverage in the two images of 30 Augustand 24 September 2009 are quite small, it is evident that cloud can causedifficulty in detecting oil spill as observed from Figure 5 (b) the image acquired

on 8 September 2009.

(a)



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(b)

Figure 4 MODIS AQUA true colour images acquired on (a) 30 August 2009 and (b) 24September 2009.

4. Comparison Between Radar and Optical Results

In order to illustrate the capacity of both radar image and optical data inmonitoring the oil spill, a comparison is presented in Figure 5. Here, the radarimage and optical one were acquired on the same day and were zoomed in tothe same region.

From the radar image, the oil spill area can be clearly observed as the blackpattern. In contrast, the oil spill area in the optical image is not that obvious,because the colour of the surrounding sea water is also fairly dark. It leads tosome difficulties to distinguish the oil spill region, although it can be seen thatthe envelope of the oil spill area in the two images are the similar in somedegree. The radar image does have some advantage in detecting oil spillevents on the sea.



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(a)

(b)

Figure 5 (a) Envisat ASAR ScanSAR intensity image acquired on 8 September 2009,and (b) MODIS AQUA true colour image acquired on the same day.



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5. Concluding Remarks

Both optical and radar satellite remote sensing techniques can play an

important role in monitoring oil spill as demonstrated in this study. Because ofthe synoptic nature of satellite imagery as well as regular revisits, earthobservation satellites can be used as a cost-effective tool to complement othermeans of surveillance, such as sending aircrafts or ships to the oil spill region toevaluate the status.

Optical sensors such as MODIS feature twice daily update and large coverage.The deployment of radar satellite constellations with flexible imaging modes(Spotlight, Stripmap and ScanSAR) such as COSMO-SkyMed has gone asignificant way to match optical satellites in terms of short revisit time and large

coverage needed for monitoring large scale oil spill. Such new features coupledwith radar intrinsic advantages of all-weather, day and night imaging willguarantee much more timely, high quality intelligence from satellite radar for oilspill management, as seen in the recent accident in the Gulf of Mexico.

Acknowledgement

ALOS PALSAR data includes material copyright by METI and JAXA [2009]. ItsL1.1 products were produced by ERSDAC, Japan. MODIS Terra and Aqua dataare obtained from NASA/GSFC, MODIS Rapid Response. This research is

jointly supported by Cooperative Research Centre for Spatial Information(CRCSI), The University of New South Wales (UNSW) and NSW Land andProperty Management Authority (LPMA).

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15arspc submission 48

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