ASSESSMENT OF SAR OCEAN FEATURES USING OPTICAL AND MARINE SURVEY
DATA
Medhavy Thankappan, Nadege Rollet, Craig J. H. Smith, Andrew Jones, Graham Logan
and John Kennard
Geoscience Australia, GPO Box 378, Canberra ACT 2601, AUSTRALIA
Email: [email protected]
ABSTRACT
Previous studies over the Australian North West Shelf
(NWS) have reported ocean features on Synthetic
Aperture Radar (SAR) images including slicks from
natural hydrocarbon seeps. The aim of this study was to
characterise SAR ocean features to enable their
differentiation from natural hydrocarbon seepage slicks.
The assessment of SAR ocean features in two NWS
study areas is reported in this paper. In the northern
NWS, repeat occurrence of ocean features observed on
SAR images acquired on three different dates were
compared with similar features on a Landsat Thematic
Mapper (TM) image acquired 28 minutes before one of
the SAR images. Analysis of the images in conjunction
with ancillary data confirmed that the ocean features
were bathymetric signatures. The same patterns were
imaged by TM primarily through specular reflection
from sun glint. Correspondence of bathymetric features
on the SAR and TM images indicates the potential for
extracting bottom topography information from optical
images with sun glint. In the central NWS, recurring
mesoscale ocean features observed on SAR images
were assessed using data collected during a marine
survey. Evaluation of the SAR features in conjunction
with the regional bathymetry and Acoustic Doppler
Current Profiler (ADCP) data showed good correlation
between the SAR features and zones with strong current
flows. Wave-current interaction resulting from
modulation of currents by variable bathymetry is the
predominant mechanism that generates these SAR
features. Recurring local SAR current shear signatures
overlying a pelagic ridge also correlated well with field
observation and in-situ current measurements. The
presence and orientation of the pelagic ridge suggests a
strong causal link to current shear signatures observed
on SAR.
1.
INTRODUCTION
Satellite based Synthetic Aperture Radar (SAR) sensors
have been used to observe various ocean related
phenomena. Internal waves, currents, eddies, fronts and
bathymetric features have been identified on SAR
images in previous studies [1-5, 7, 10-11]. The potential
_____________________________________________________
Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland
23–27 April 2007 (ESA SP-636, July 2007)
of temporally coincident SAR and optical images for
studying ocean dynamics has been demonstrated [2].
Two study areas in the north and central regions of the
Australian North West Shelf (NWS) were selected for
this study. Ocean features observed on near-coincident
SAR and optical images over the northern study area
were compared, and the potential for deriving bottom
topography information from sun glint patterns in
optical images is highlighted.
Mesoscale and local SAR ocean features observed
regularly over the central NWS study area were
assessed in conjunction with marine survey data and
found to correlate well with in-situ current
measurements and the local bathymetry. The aim of this
work was to understand the mechanisms that generate
low-backscatter signatures on SAR and increase the
confidence of natural hydrocarbon seep identification.
2.
METHODS
2.1. Study Area
The northern NWS study area is located within 10.5˚ S
129˚ E and 11.5˚ S 130˚ E. This area of the Timor Sea
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 hydrocarbon seeps.
The central NWS study area covers approximately
165,000 km2 and overlies the Rowley sub-basin situated
on the outer continental shelf. The Mermaid, Clerke and
Imperieuse reefs form the Rowley Shoals which also lie
within the study area. Previous remote sensing studies
have suggested that liquid petroleum migration and
seepage is active in the central NWS [8]. Reef systems
in this area have the potential to produce false positives
resembling seepage slicks through air-sea interactions
or coral spawning [5]. The warm Leeuwin current
which flows southward is also a key feature of the
NWS. Internal waves have also been identified on SAR
images over the area quite frequently.
Figure 1. North West Shelf showing the two study areas
2.2. Satellite and Ancillary Data
For the northern study area, three co-located ERS SAR
scenes acquired on 10 December 1995, 5 October 1997
and 26 October 1998 were studied in conjunction with a
Landsat TM scene over the same area acquired 28
minutes before the SAR image on 26 October 1998.
Figure 2. ERS-2 SAR image acquired on 26 October
1998 (1:35 UTC) © ESA 1998
For the central study area SAR scenes from three ERS
orbits acquired on 14 September 1995, 4 November
1995 and 5 November 1996, and Advanced Synthetic
Aperture Radar (ASAR) scenes from three ENVISAT
orbits acquired on 5 June 2006, 6 June 2006 and 9 June
2006 were examined. The ASAR images were acquired
to coincide with a marine survey of the area in June
2006.
Other satellite based data used in this study include, Sea
Surface Temperature (SST) from NOAA and wind field
from Quikscat and DMSP satellites. Ancillary data
include tide tables, tidal currents modelled for
Geoscience Australia’s sediment transport studies [9],
oceanographic charts from the Australian Hydrographic
Service, the Geoscience Australia bathymetry grid (0.01
degree), Acoustic Doppler Current Profiler (ADCP) and
multi-beam bathymetry data from the marine survey.
3.
OBSERVATIONS
3.1. Northern NWS
Similar features, oriented in NE to SW direction, are
seen on three ERS SAR images acquired on 10
December 1995, 5 October 1997 and 26 October 1998.
The ERS-2 SAR image of 26 October 1998 is shown in
Figure 2. A near-coincident Landsat-5 TM image
acquired 28 minutes before the ERS-2 SAR image also
shows similar patterns that correspond to features seen
on the three SAR images (Figure 3).
Figure 3. Landsat-5 TM image acquired on 26 October
1998 (1:07 UTC)
Hydrographic charts of the area show a series of
submarine channels and shoals. The channels are
approximately 2 to 4 km wide and up to 80 m deep. The
shoals rise up to 40 m of the sea surface. Following
analysis of SAR, TM and bathymetry data, it was
evident that features in the images corresponded with
the shoals and channels observed in the regional
bathymetry map (Figure 4).
Figure 5 shows an area of convergent channels as it
appears in the TM visible and infrared wavelength
bands. The depth of these convergent channels range
from 25 to 75 m. The visibility of these features has a
weak dependence on TM wavelength, indicating that
the features are the result of near-surface phenomena.
This raises the question of whether the features in the
Landsat TM image are caused by the same mechanism
as in the case of the SAR images. As the bathymetric
features are visible in the TM near and middle infrared
wavelength bands, the only mechanism that could
adequately explain the correlation between submarine
topography and brightness variations in the image is
specular reflection from the sea surface with variable
roughness where roughness is dependent on the
interaction between tidal currents and bottom
topography. In this case, sun glint modulated by ocean
surface roughness is the predominant mechanism
producing the bathymetric features imaged by TM.
Figure 4. Bathymetry map showing channels and shoals
(yellow = shallow, blue = deep)
Bottom topography is expressed on SAR images in the
presence of moderate winds of 3 to 5 m/s and tidal
current speeds of about 0.5 m/s [1]. Wind speeds at the
time of acquiring the SAR images were 7.5 m/s for the
1995 image compared to 3.0 m/s and 3.5 m/s for the
1997 and 1998 SAR and Landsat TM images
respectively. The SAR and optical imaging coincided
with ebb tide and the time to low tide varied from 1.1 to
3 hours. Modelled tidal current profiles in the study area
show 0.5 m/s currents directed east-west. Typical
monthly time-series plots indicate strong east-west
currents, with speeds up to 0.5 m/s during spring tides
and 0.2 to 0.3 m/s during neap tides [9].
From analysis of the wind, tide, current and bathymetry
data of the study area in the context of the SAR imaging
theory [1], features seen on the ERS SAR images can be
adequately explained by the variations in the sea surface
roughness caused by tidal flow over variable bottom
topography. The features were less distinct in the 1995
SAR image compared to the 1997 and 1998 SAR
images. This could be explained by the higher wind
speed of 7.5 m/s at the time acquiring the 1995 ERS
image, which diminished the overall image contrast.
The Landsat TM image shown in Figure 3 had sun glint.
The presence of sun glint in the image is consistent with
the illumination geometry at the time of Landsat
overpass; the sun elevation angle was 60.8 degrees and
sun azimuth angle was 94.6 degrees. Visibility of
bathymetric features as a function of TM wavelength
bands was examined in order to determine the influence
of sun glint.
Band 1
Band 2
Band 3
Band 4
Band 5
Band 7
Figure 5. Bathymetric feature visibility on Landsat TM
as a function of wavelength
Archived Landsat images from 1986 to 2005 were
examined to determine if Parry Shoal (Figures 2 and 3),
a prominent feature below 20 to 30 m of water, could be
identified. 411 Landsat images were examined to
determine if detection of bathymetric features was
related to a specific sun-sensor-target geometry, Parry
Shoal could be identified only on 13 images.
Illumination, sensor-target geometry, wind and tide
conditions at the time of acquiring these 13 images
suggest that sun elevation angles ranging from 52 to 63
degrees and sun azimuth angles from 57 to 111 degrees
in conjunction with low to moderate wind speed of 3 to
7 m/s at ebb tide favoured the generation of bathymetric
signatures.
The prevalence of ebb tide in all instances of optical and
SAR imaging where the bathymetric features could be
identified, is a significant observation. Increased tidal
velocity at ebb tide could have enabled better surface
expression of bathymetry on the images. In addition, the
average tide height at high tide for all observations was
3.8 m compared to an average tide height of 1.2 m at the
time of imaging. A shallower water column over the
submarine features during ebb tide is also likely to
result in stronger surface expression of bathymetry
enabling imaging of the features by SAR and optical
sensors.
influence the formation of bright and dark mesoscale
features similar to those shown in Figure 6. However,
examination of SST data acquired by the NOAA
satellite close to the time of SAR image acquisition did
not reveal strong correlation between SST and the
observed SAR features. Therefore, it is difficult to say if
the warm Leeuwin current had a role in the generation
of these SAR features.
3.2. Central NWS
In the central NWS study area, several mesoscale and
local ocean features have been identified repeatedly on
SAR images. Bright and dark mesoscale ocean features
trending NE-SW are observed regularly on SAR images
acquired at different times. SAR images from three ERS
orbits of 14 September 1995, 4 November 1995 and 5
November 1996 that typify the most frequently
occurring mesoscale ocean features, overlaid with 200,
400 and 800 m isobaths, are shown in Figure 6. Wind
speeds at the time of acquisition of the SAR images
ranged from 5 to 7 m/s. The tide phase was ebb for the
November images and flood for the September image.
A hydrocarbon seepage survey of the central NWS was
undertaken in June 2006. During the survey multi-beam
swath bathymetry, echo-sounder, side-scan sonar, subbottom profile, ADCP and fluorometric data were
acquired across 18 potential hydrocarbon seepage sites.
Figure 7 shows the location of some of these marine
survey sites. Details of the marine survey are reported in
[6]. ASAR scenes from three ENVISAT orbits were
also acquired over this site during the marine survey.
High wind speeds ranging from 10-17 m/s at the time of
acquiring the ASAR images, limited their use in this
study.
Analysis of the ERS SAR images in conjunction with
bathymetry and ADCP data showed good correlation of
the mesoscale features with zones of strong currents.
The SAR ocean features are also aligned parallel to the
regional bathymetric contours. ADCP transects acquired
during ebb tide showed that the current in the upper
water mass, measured to 60 m depth, was flowing
parallel to the SAR features towards SW. The Leeuwin
current has a major influence on the circulation in this
area and previous studies have identified the Leeuwin
current on satellite derived Sea Surface Temperature
(SST) images [12]. Temperature gradients could
Figure 6. ERS SAR images over the central NWS
showing mesoscale ocean features © ESA 1995, 1996
Imaging of the mesoscale SAR ocean features could be
enabled through temperature or salinity gradients,
wave-current interactions, current flow over variable
bathymetry or the presence of biogenic films. It is most
likely that multiple mechanisms are involved in the
imaging of the mesoscale features by SAR. While it is
difficult to isolate the influence of individual
mechanisms in the formation of these features, evidence
from the marine survey data and the regular occurrence
of these SAR features suggest that wave-current
interaction resulting from modulation of currents by the
variable bathymetry is the predominant mechanism. The
scale and characteristics of these SAR features confirm
that they are not related to internal waves, which are
often observed in SAR images over this area.
During the marine survey, a sea surface slick with an
ENE-WSW trend, located close to tidal rips marked on
the oceanographic chart was observed (Figure 8). The
slick was approximately 50 m wide and more than 500
m long. ADCP data was recorded at different phases of
the tidal cycle, across the observed slick and tidal rip
zones marked on the oceanographic chart.
A
characteristic SAR feature observed at the same location
and aligned along the 200 m isobath, has good
correspondence with the tidal rip marked on the
oceanographic chart. This SAR feature appears either
dark or bright on SAR images acquired at different
times. Figure 9 shows bright ocean features in the ERS
image acquired on 28 March 1997. The wind speed was
2.5 m/s at the time of acquisition of the image. At the
time of acquiring the March image the tide phase was
flood.
The bright SAR ocean features, the surface slick
observed during the marine survey, and the tidal rip
zone marked on the oceanographic chart overlie a
pelagic ridge aligned with the 200 m isobath (Figures 7
& 10). The orientation of the pelagic ridge, rising 40 m
from the sea bed, appears to be related to the surface
current shear observed during the marine survey and
current shear signatures observed on the SAR image.
Alteration of current flow by the pelagic ridge could
create convergent and divergent flows imaged by SAR.
ADCP data and field observation of the surface current
suggest that the pelagic ridge has a causal link with
local current shear signatures observed on SAR.
Figure 7. Marine survey in the central NWS (magenta
lines and polygons=ADCP, dashed blue line=tidal rip
marked on oceanographic chart, A and B= ADCP
transects)
The location of the observed slick (star) and ADCP
measurements along Transects A and B are shown in
Figure 10 along with local multi-beam bathymetry.
ADCP measurements during 1.5 hours on 15 June 2006
along Transect A show currents flowing SW, the tide
phase was ebb to flood. ADCP measurements during
1.5 hours on 4 June 2006 along Transect B show
currents flowing in opposite directions on either side of
a tidal rip marked on the oceanographic chart (blue line
in Figure 10), the tide phase was ebb. Two adjacent
water masses moving at different speeds relative to each
other can create shear zones along the boundary. The
bright features in the ERS image exhibit the
characteristics of current shear signatures on SAR.
Figure 9. Current shear on ERS SAR image of 28
March 1997 © ESA 1997 (blue lines=ADCP transects,
magenta line=tidal rip, yellow lines= pelagic ridge and
green dot=surface slick location)
Figure 10. ADCP Transects A and B superimposed on
multi-beam bathymetry
Figure 8. Sea surface slick observed during the marine
survey
Australia. International Journal of Remote Sensing
27, 2063–2069.
4. CONCLUSIONS
Tidal currents flowing over undulating bottom features
produce bathymetric patterns imaged by SAR in the
northern NWS study area. Bathymetric features are
imaged by both SAR and optical sensors with striking
similarities. The SAR and optical image with sun glint
represent to a first approximation the sea surface
roughness conditions in the study area. Moderate wind
speeds, strong currents and low tide prevailed during
imaging of these features. Bathymetric feature detection
through sun glint is achieved only under very specific
sun-sensor-target
geometries.
Under
optimum
conditions, sun glint in optical satellite images could
provide useful information on bottom topography which
could complement the use of SAR.
Multiple mechanisms could contribute to SAR imaging
of mesoscale ocean features in the central NWS study
area. Data collected during the marine survey suggests
that wave-current interaction resulting from the
modulation of currents by variable bathymetry is the
predominant mechanism that regularly produces the
observed mesoscale SAR features. Repeat occurrence of
SAR current shear signatures corresponded well with
in-situ data and field observation. The presence of a
pelagic ridge on the sea bed at the same location
suggests a strong causal link with the local current shear
signatures observed on SAR.
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voyagedocs/2006/index.htm
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Lyzenga, D.R. and Marmorino, G. (1998).
Measurement of surface currents using sequential
Synthetic Aperture Radar images of slick patterns
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O’Brien, G.W., Cowley, R., Lawrence, G.,
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ACKNOWLEDGEMENT
5.
Jones, A.T., Thankappan, M., Logan, G.A.,
Kennard, J.M., Smith, C.J., Williams, A.K. and
Lawrence, G.M. (2006). Coral spawn and
bathymetric slicks in Synthetic Aperture Radar
(SAR) data from the Timor Sea, northwest
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ENVISAT and ERS data for this study was acquired
through the European Space Agency (ESA) Category-1
Research Proposal C1P-3415. The authors would like to
thank ESA for making this data available to us.