EVALUATION OF TERRASAR-X FOR NATURAL OIL SEEP STUDIES

Medhavy Thankappan, Graham Logan, Magnus Wettle, Shanti Reddy and Andrew Jones

Geoscience Australia, GPO Box 378, Canberra ACT 2601, AUSTRALIA Email: medhavy.thankappan@ga.gov.au

ABSTRACT C-band SAR has been used for natural oil seep detection in Australia. There is generally a low level of confidence associated with natural oil seeps identified on SAR images over Australian coastal waters due to poor characterisation of false positives that produce low-backscatter features on SAR. The availability of high resolution TerraSAR-X data provides an opportunity to establish the effectiveness of X-band SAR for natural oil seep detection in Australian waters. The focus of this study was to assess low-backscatter features observed in the TerraSAR-X images. In this paper, we report preliminary results from initial evaluation of single polarised X-band data products from TerraSAR-X over two study areas. Our evaluation of TerraSAR data is independent of C and L-band polarimetric data from other sources being evaluated over the same study area. 1. INTRODUCTION

Satellite based C-band Synthetic Aperture Radar (SAR) sensors have been used extensively for detection and identification of natural oil seepage slicks in the Australian offshore petroleum provinces. The SAR signal responds to sea surface roughness that is modulated by wind speed and direction. Suppression of sea surface capillary waves by oil from natural or anthropogenic sources reduces the surface roughness and causes low radar backscatter areas that appear dark on SAR images. Petroleum hydrocarbons are usually detected on SAR when wind speed is between 3 to 10 m/s. Characterisation of natural oil slicks on single polarised C-band SAR in order to discriminate them from other false positives remains a challenge [1, 2, 4]. The TerraSAR-X system provides X-band SAR data at 1, 3 and 16 m resolutions in the SpotLight, StripMap and ScanSAR imaging modes respectively. The high spatial resolution of TerraSAR-X and the unexplored potential of X-band SAR for natural oil seep detection prompted this evaluation. The aim was to explore the effectiveness of X-band SAR in discriminating various mechanisms that generate low-backscatter signatures on SAR to increase the confidence of natural hydrocarbon seep identification and characterisation in Australian waters. For evaluating TerraSAR-X data, we selected study areas with petroleum potential and where we have

frequently observed the occurrence of SAR features likely to be confused with natural oil seepage slicks. This work is part of a current project on slick detection methodology development and characterisation of natural oil seepage slicks using SAR. 2. METHODS

2.1 Study Areas

The Gippsland Basin (GB) in south-eastern Australia and the Australian North-West Shelf (NWS) were the two study areas selected for the evaluation of TerraSAR-X data. The GB extends both offshore and onshore, and contains a number of oil and gas fields currently in production. Australia's largest oil discovery to date, the Kingfish field is located within the GB. The likelihood of oil platforms releasing Production Formation Water (PFW) containing small amounts of oil, and pollution from oil leaks, influenced our decision to study the GB (Fig. 1). In the NWS study area, two sites, one in the north and the second in the central part were selected for the TerraSAR-X evaluation (Fig. 2). The northern NWS study area (NWS1) has a shallow, variable bathymetry and large tidal amplitudes that generate strong currents. Variable bathymetry can interact with tidal currents to produce ocean SAR features that could be misinterpreted as natural oil seeps [4] The Jabiru and Challis Floating Production Storage and Offloading (FPSO) vessels are also located in the study site. The central NWS study area (NWS2) overlies the Rowley sub-basin. Previous remote sensing studies have suggested that liquid petroleum migration and seepage is active in the central NWS [3]. Reef systems and bathymetric features in this area have the potential to produce false positives resembling seepage slicks through air-sea interactions or coral spawning [2, 4]. 2.2 Satellite and Ancillary Data

The details of 8 single-polarised TerraSAR-X datasets used for a visual and digital assessment are summarised in Table 1. Other data include: wind field from the QuikSCAT satellite, the Geoscience Australia bathymetry grid (0.01 degree) and spatial information on petroleum resources in the two study areas.

_____________________________________________________ Proc. of ‘4th Int. Workshop on Science and Applications of SAR Polarimetry and Polarimetric Interferometry – PolInSAR 2009’, 26–30 January 2009, Frascati, Italy (ESA SP-668, April 2009)

North West Shelf

Gippsland Basin

Figure 1. Location of the two study areas (red boxes)

Figure 1. Location of oil and gas fields in the Gippsland Basin (black box)

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2

Figure 2.Study sites in the North West Shelf (black boxes 1 and 2)

Table 1. Details of TerraSAR-X datasets used in the study and wind speed conditions at the time of imaging

Data Acquisition Date and Time (UTC)

SAR Product / Resolution (Study area)

Incidence angle range and Polarisation

Wind speed in m/s and Direction

27 September 2008 (8:42)

ScanSAR/ 16m (GB) 22°-32° HH 7 E

2 October 2008 (19:28)

ScanSAR/ 16m (GB) 32°-40° HH 3 NW

7 December 2008 (8:51)

StripMap/ 3m (GB) 42°-44° VV 11 SW

7 December 2008 (19:28)

StripMap/ 3m (GB) 32°-35° VV 6 W

2 December 2008 (10:22)

StripMap/ 3m (NWS2) 27°-30° VV 6 W

7 December 2008 (10:31)

SpotLight/ 1m (NWS2) 47° VV 3 SW

10 December 2008 (21:40)

StripMap/ 3m (NWS2) 34°-37° VV 9 W

20 December 2008 (9:58)

StripMap/ 3m (NWS1) 33°-36° VV 10 W

3. OBSERVATIONS

3.1 Gippsland Basin (GB)

TerraSAR-X images acquired over the Gippsland Basin are shown in Figures 3 to 6; they represent sea surface roughness conditions and highlight the observed SAR features. For each image, a graphic identifying the azimuth (A) and range (R) directions, and the standard wind barb notation denoting speed and direction at the time of imaging is provided.

Figure 3. ScanSAR acquired on 27 September 2008 (8:42 UTC) showing AGWs © DLR 2008 Figure 3 shows the HH polarised ScanSAR image acquired on 27 September 2008 at 8:42 UTC. The wind speed was 7 m/s from the east. Five distinct groups of Atmospheric Gravity Waves (AGW) that

appear as alternating dark and light bands can be seen propagating in the nominal direction of the wind velocity. The AGWs have wavelengths ranging from 1.4 km to 2.1 km. AGWs are generated by air flow over topography and can range in wavelengths from a few to several tens of kilometres. The wind velocity fluctuations at the sea surface associated with AGWs modulate the sea surface roughness enabling them to be imaged by SAR [1]. Striping in the range direction resulting from amplified sensor noise is also apparent. The moderate wind speed of 7m/s has produced radar backscatter from the ocean that is just over the sensor noise floor. The amplification of the sensor noise pattern is more pronounced in the HH polarised ScanSAR image acquired on 2 October 2008 (Fig. 4). This is likely due to low ocean backscatter resulting from the low wind speed of 3 m/s at the time of imaging. In the azimuth direction we also observe a ‘scalloping’ effect seen as two bright bands, which result from overlapping bursts of the radar energy, a common feature of imaging in the ScanSAR mode. In the bottom right of the image some dark slicks are clearly visible; they are concentrated around the Fortescue / Halibut, Mackerel and Kingfish oil fields (see Fig. 13 for detail). PFW from the oil platforms or natural biogenic films could form such slicks, but the origin of these slicks is yet to be determined.

RWINDWIND

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Figure 4. ScanSAR acquired on 2 October 2008 (19:28 UTC) showing slicks near oil platforms © DLR 2008

WIND Two VV polarised StripMap images were acquired on 7 December 2008, from the descending and ascending orbits over the same location. The image from the ascending orbit shows the mottled SAR signature of roll vortices (Fig. 5). The most common mechanism for the development of roll vortices is thermodynamic instability coupled with adequate wind shear [1]. The wind speed was 11 m/s from the south west.

Figure 5. StripMap acquired on 7 December 2008 (8:51 UTC) showing roll vortices © DLR 2008

Figure 6. StripMap acquired on 7 December 2008 (19:28 UTC) showing convection cells © DLR 2008 The image in Fig. 6 is from the descending orbit and shows the SAR signature of convective cells which

form under relatively light wind conditions over regions of the sea that have a negative air-sea temperature difference (when the water temperature is higher than the air mass over the water). Wind speed at the time of imaging was 6 m/s from the west. As the ratio of the air-sea temperature to the mean wind speed increases beyond a threshold value, rolls develop into cellular convection [1, 5]. This transformation is exemplified by the two images acquired on the same day (Fig. 5 and Fig. 6). A detailed review of rolls and convective cells is provided in [5].

WIND

3.2 North West Shelf (NWS) For the NWS study area we evaluated a total of 4 TerraSAR-X image products all acquired in VV polarisation mode. Figure 7 shows the StripMap product acquired over the NWS2 on 2 December 2008. The wind speed was 6 m/s from the west. A bright and dark feature that runs diagonally across the image corresponds to the location of a submerged ridge. Evidence from marine survey data and the frequent occurrence of these features on C-band SAR images suggest that strong tide-current interactions modulated by variable bathymetry is the predominant mechanism that generates this ocean feature [4].

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A WIND WIND

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Figure 7. StripMap acquired on 2 December 2008 (10:22 UTC) showing tide-current interaction signatures © DLR 2000

Figure 8. StripMap acquired on 10/12/2008 21:40 UTC showing rain cell signatures © DLR 2008

Figure 9. StripMap acquired on 20/12/2008 9:58 UTC showing two FPSOs (in white circles) © DLR 2008

Figure 10. Jabiru FPSO seen in the StripMap acquired on 20/12/2008 9:58 UTC © DLR 2008

Figure 11. Challis FPSO seen in the StripMap acquired on 20/12/2008 9:58 UTC © DLR 2008 A StripMap image over NWS2 site acquired on 10 December 2008 is shown in Fig. 8; the wind speed was 9 m/s from the west. SAR signatures corresponding to rain cells are seen in the image. The mechanism for rain cell detection on SAR is described in [1]. Fig. 9 shows the StripMap image acquired over the Jabiru-Challis FPSO vessels located at the NWS1 site (marked by white circles). Dark areas adjacent to the two FPSO vessels can be seen in the image. The full resolution subsets over the Jabiru and Challis FPSOs (Fig. 11 and Fig. 12) show the ocean wave field and dark areas near the two facilities. We speculate that the dark patches are low-backscatter areas associated with the wind shadow created by a 10 m/s wind blowing from the west.

A

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R WIND WIND

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WIND

WIND A

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A WIND

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Figure 12. SpotLight acquired on 7/12/2008 10:31 UTC © DLR 2008

The only high resolution SpotLight image used in the study is shown in Fig. 12. The very low wind speed of 3 m/s coupled with a high incidence angle of 47° resulted in very low ocean backscatter and low image contrast across the entire swath. No significant slicks were observed over the ocean wave field background in the image.

4. DISCUSSION

The detection of oil slicks on SAR images over the ocean depends on environmental factors such as wind, waves, currents and tides, and radar system parameters such as frequency, incidence angle and polarisation. Signal from ocean backscatter that exceeds the background noise level is required to detect the local suppression of capillary waves by slicks and appropriate sensor resolution is required to resolve slicks. The TerraSAR-X system has a suitable resolution to detect oil slicks in the order of a few metres. Based on our observations, X-band SAR images acquired in HH and VV polarisations at a range of incidence angles over the two study sites showed typical SAR signatures of air-sea and current-bathymetry interactions including slicks of unknown origin in the GB study area. Signal from ocean backscatter decreases rapidly with increasing incidence angle at all SAR frequencies [1].

Only 2 of 8 TerraSAR-X products we evaluated had average incidence angles less than 30° (see Table 1). Wind speed thresholds for oil slick detection were met for 5 of 8 TerraSAR-X products evaluated in this study; two were acquired under low wind speed and one in high wind speed conditions. VV polarised ocean backscatter returns are nearly always higher than the HH polarised returns, and there is evidence to suggest that VV polarisation may be better suited to oil slick detection because of the higher signal to noise ratio [1]. The difference in ocean backscatter between HH and VV polarisations is largest at low wind speeds, indicating the relatively higher sensitivity of HH polarisation to wind speed [1]. This fact is particularly relevant to the two ScanSAR images over the GB; both images were HH polarised and acquired under low to moderate wind speed conditions. However, confirmation of this behaviour through a comparison of simultaneously acquired VV and HH polarised TerraSAR-X data was not done in this study. In HH polarised ScanSAR products acquired over the GB, slicks were imaged at incidence angles greater than 30° and under low wind speed conditions. Under low wind speed conditions, the amplification of the sensor noise floor could interfere with the identification of low-backscatter features of interest (eg. oil slicks) as seen in the ScanSAR image of 2 October 2008 (Fig. 13).

The trade-off between wide area coverage (swath width) and detection ability (resolution) is also compounded by increased backscatter at high wind speeds. This increases the likelihood of masking bright features of interest (eg. ships or oil platforms) and also causes surface slicks to disintegrate and become less detectable.

Figure 13. ScanSAR image of 2/10/2008 showing slicks (origin not confirmed) and oil platforms in the Gippsland Basin (oil fields in yellow) © DLR 2008 Four of 5 StripMap products were acquired at wind speeds within the threshold for oil slick detection. Also, 4 of 5 StripMap products were acquired at incidence angles larger than 30°. Typical SAR signatures of current-bathymetry interaction and air-sea interaction were observed on the StripMap products. The only high resolution VV polarised SpotLight image was acquired at an incidence angle of 47° under low wind speed conditions (3 m/s); no significant features of interest were observed in the small area of 10 km by 5 km covered by the image swath. To determine the influence of incidence angle and polarisation on the oil slick detection performance of X-band SAR for both the study areas, further assessments with near-coincident SAR data at different polarisations and incidence angles is required.

5. CONCLUSIONS

From the preliminary results of our evaluation of a limited set of single-polarised TerraSAR-X image products, we draw the following conclusions:

• Slicks of unknown origin around the oil and gas fields in the Gippsland Basin were identified on HH polarised TerraSAR-X ScanSAR image despite low wind speed conditions. Atmospheric Gravity Waves at kilometre wavelengths were also observed in the ScanSAR product over the Gippsland Basin.

• Amplification of the noise floor especially

under low wind speed was found to affect the identification of slicks and what appears to be better detection performance in the near-range part of the HH polarised ScanSAR image. It is expected that VV polarised ScanSAR at similar wind speeds would improve slick detection performance.

• SAR signatures of air-sea interaction were

observed on all five StripMap products evaluated; roll vortices, convection cells and rain cells were identified. Current-bathymetry interaction was identified on the StripMap image over the NWS.

• Only one SpotLight image acquired under low

wind speed at 47° incidence angle was evaluated; no significant features of interest were observed. Assessment of more SpotLight images is needed to evaluate detection performance.

The influence of polarisation and incidence angle on the oil slick detection performance by X-band SAR could not be adequately assessed in this study due to the limited number of single-polarisation data sets acquired mostly at larger incidence angles. We intend to evaluate multi-polarised SAR data sets currently being acquired over the study areas to understand the effects of polarisation and incidence angles on slick detection. 5. REFERENCES

1. Jackson, C.R. and Apel, J.R. (Eds) (2004) Synthetic Aperture Radar Marine User’s Manual pp 321-329

2. Jones, A.T., Thankappan, M., Logan, G.A.,

Kennard, J.M., Smith, C.J., Williams, A.K. and

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Lawrence, G.M. (2006). Coral spawn and bathymetric slicks in Synthetic Aperture Radar (SAR) data from the Timor Sea, northwest Australia. International Journal of Remote Sensing 27, 2063–2069.

3. O’Brien, G.W., Cowley, R., Lawrence, G.,

Williams, A.K., Webster, M., Tingate, P. and Burns, S. (2003). Migration, leakage and seepage characteristics of the offshore Canning Basin and Northern Carnarvon Basin: Implications for hydrocarbon prospectivity. APPEA Journal, 149-166

4. Thankappan, M., Rollet, N., Smith, C.J.H., Jones

A., Logan, G and Kennard J. (2006). Assessment of SAR ocean features using optical and marine survey data. Proceedings of the ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007).

5. Young, G.S., Kristovich, A.R., Hjelmfelt, M.R.

and Foster, R.C., (2002) Rolls, streets, waves, and more: Bulletin of the American Meteorological Society. 83, 997-1001

ACKNOWLEDGEMENT

Authors are thankful to DLR for providing TerraSAR-X data through the Pre-launch AO Research Proposal MTH0092. The authors also wish to thank Alan Forghani for assisting with data preparation.