Dr. Werner Alpers
Institute of Oceanography
Troplowitzstr. 7
University of Hamburg
D-22529, Hamburg
Germany
Co-Investigators:
H. Masuko, Radio Research Laboratory
P. Trivero, Istituto di Cosmogeofisica
Comparison of
SIR-C
Simulated and Measured
SIR-CSAR
Image Spectra with Ocean Wave
Spectra Derived from Buoys and Wave Production Models in the North Sea
OBJECTIVES
Carry out measurements of two-dimensional ocean wave spectra by a pitch and roll buoy
from the Forschungsplatform Nordsee (Germany Research Platform North Sea) and from
a ship of the German Hydrographic Office in the North Sea. Collaborate with North
Sea oil rig operators to obtain wave spectra from these sites.
A third generation wave prediction model (WAMODEL) will be applied to forecast and
hindcast the wave fields in the North Sea from the measured wind history
during the SIR-C
overflight. The
WAMODEL
has a resolution of 0.5 degree longitude and 0.25 degree
latitude. This model (Komen and Zambreski, 1986; Bauer et al. 1988) seems to be
accurate in predicting two-dimensional ocean wave spectra in the North Sea and will
be refined for the
SIR-C
mission.
PROGRESS
During both shuttle missions, experimenters at the University of Hamburg performed
experiments in the German Bight of the North Sea, off the island of Sylt. The aim
of these experiments was to investigate the radar signatures which are caused by
different surface films, i.e., by different monomolecular slicks consisting of artificial biogenic
substances and, on the other hand, by different mineral oils, i.e., heavy and light
(Diesel) fuel. The
SIR-C/X-SAR
data from both experiments were acquired and processed in different ways, i.e., performing different statistical analyses, during the
last months. In addition to the experiments in the German Bight, Japanese scientists
under the leadership of Dr. H. Masuko performed similar surface film experiments
off the Japanese coast. Here, a certain substance was deployed several times to get a reliable
set of
SAR
images with radar signatures for intercomparison and polarimetric studies.
X-SAR quicklook processor data tapes provided by
DLR,
as well as the quicklook data on CD ROM provided by
JPL
(L- or C-band), and I-PAF (X-band) after both shuttle
missions have been checked for features which could be of interest for oceanographic
studies. It turned out that in several parts of the world's oceans the water surface
was covered with surface films during the shuttle flights. However, only a few data
takes showing oil spills have been found (e.g., one large oil spill in the Baltic
Sea during the first shuttle mission). Therefore, a collection of interesting scenes
taken in different
SAR
modes (i.e., only like-polarization or full polarization mode) have
been ordered and are to be processed in the near future.
Another aim of the participation of the University of Hamburg in the shuttle missions
was to compare wave spectra obtained from the
SAR
images at different radar bands
and polarizations with in-situ data from buoy measurements and spectra obtained from
a wave prediction model (WAM) provided by the European Center for Medium Range Weather
Forecast (ECMWF). For this purpose,
SAR
images from the North Sea test site have
been acquired and processed. A
SAR
image taken during the first shuttle mission
over the northeast (NE) Atlantic shows a broad, dark band crossing the
SAR
track. After watching
the videotape of this data take (96.3), scientists first thought that this dark band
was due to a mineral oil spill. However, it turned out that a strong atmospheric
(rain) front was responsible for this signature. The
SAR
images of this topic have been
ordered and processed. Other data takes of the NE Atlantic (adjacent in space and
time) have been checked for this front, however, such strong signatures have not
been found. Furthermore, bright signatures caused by strong rain cells have been found on
SAR
images of the Tropical Pacific, the Mexican Gulf, and the Central Atlantic. They
will be processed and studied during the next months.
SIGNIFICANT RESULTS
The two slick experiments in the German Bight took place under different weather conditions:
During the first mission, on April 18, the weather was fine with a wind speed of
5 m/s, but during the second mission, on October 6, the weather was stormy, and the wind speed raised up to values of more than 10 m/s. Under these strong wind conditions,
surface slicks remain on the water surface only for a short time, because they are
rapidly washed down. However, most of the deployed surface films have been recognized as dark patches on the
SAR
images taken at this test site. It turned out that
the surface wave damping behavior of one substance (oleyl alcohol), which was used
for both experiments, was different for the unlike wind conditions: Under strong
winds the measured reduction of the radar backscatter was lower and was comparable to one of
the other substances, especially mineral oil (one has to admit that, for environmental
reasons, the amount of mineral oil was kept very small, so that this oil spill obviously showed a damping behavior similar to that of the monomolecular surface films).
Significant differences between the surface films resulting from various statistical
processing have therefore not been found. On the other hand, under low to moderate
wind conditions, the monomolecular slick deployed during the first experiment showed a
different damping behavior from a mineral oil spill that was found on a
SAR
image
of the Baltic Sea. A characteristic damping maximum at C-band has been found which
can be explained by the Marangoni damping theory (which is applicable to monomolecular surface
films). This result is in agreement with the results obtained from the Japanese scientists
during their experiments (Masuko and Alpers, 1995). The mineral oil spill in the Baltic Sea showed a strong inhomogeneity, because of a drifting of the upper oil
layer, due to wind and surface currents. However, on the thick (windward) edge of
the spill, in particular, a different damping behavior was found, i.e., no damping
maximum at C-band, but an increasing radar contrast with increasing radar wave number (Gade
and Alpers 1995a) (Gade and Alpers 1995b). Recent polarimetric studies of slick-free
and slick-covered water surfaces have shown that there are differences in the polarization signatures with respect to incidence angle, wind (i.e., surface roughness), and
slick coverage. In all cases, Bragg scattering seems to be the dominant scattering
mechanism, where the fraction of spatial scattering varies with incidence and slick
coverage. The processing of
SAR
image spectra, which was extensively done with the
JPL
airborne
SAR
(AIRSAR) images taken in 1991, was extended to
SIR-C/X-SAR
data taken
over the North Sea and showed distinct wave systems. The two-dimensional image spectra
processed from these
SAR
images show unimodal wave systems and reproduce very well the
spectra obtained using the
WAM
model, with respect to peak wavelength and direction.
This can be explained by the weak nonlinearity of the imaging of these ocean waves,
except for the cases of shallow water close to the coastline. Further image spectra,
which were calculated on both sides of the atmospheric front over the Atlantic Ocean,
showed strong differences, although taken at a distance of only 25 km from each other: The obtained spectra were rotated, which was most pronounced for X-band and less
for L-band. This effect can be explained by the different dependence of the ocean
wave-radar modulation transfer function (MTF) on wind speed, which results in a different
imaging of the wave fields. This could be proved by simulations of wave spectra for
both sides of the front (Alpers et al. 1995a).
FUTURE PLANS
The results on radar signatures of different ocean surface films are not yet sufficient
for any definitive statement on whether or not active radar techniques are capable
of discriminating between natural (biogenic) slicks and (manmade) mineral oil spills. Therefore, more
SAR
scenes showing surface slicks or mineral oil spills (e.g., oil
spills around oil drilling fields in the Arabian Sea west of Bombay) are to be studied
in the future. Special attention will be focused on polarimetric studies of these
SAR
scenes. New chemical insights into the morphology of monomolecular surface films
will be gained from laboratory experiments. Results will be presented at the next
International Geoscience and Remote Sensing Symposium (IGARSS) (Alpers et al. 1996)
and in a Ph.D. thesis (Gade 1996).
Theoretical studies of the classification of
SAR
image spectra, i.e., a new method
of the best fit between simulated and measured image spectra, will be published soon
in a Ph.D. thesis (Schmidt 1995).
Taking advantage of the polarimetric capabilities of the
SIR-C
radar system, more
studies regarding the backscattering mechanism of the water surface (theoretical
as well as experimental) shall be done. For this purpose, some additional
SAR
scenes
may be ordered. For studies of radar signatures of rain cells over the world's oceans, more
SAR
scenes shall be ordered to get reliable statistical results out of the huge amount
of such data. First results coming out of these studies will be presented at the
IGARSS
'96 Symposium (Alpers et al. 1996). The results may be extended by laboratory
experiments. Several
SAR
images were taken over the Wadden Sea in the German Bight
showing distinct dark and bright structures corresponding to underwater bottom topography or dry-fallen sandbanks. This interesting data set is worth being studied, and the
results will be compared with those obtained from
ERS-1SAR
images and with theoretical
models for surface convergence (and the resulting radar backscatter).
PUBLICATIONS
Alpers, W., and B. Holt, Imaging of ocean features by SIR-C/X-SAR: An overview, Proceedings of the International Geoscience and Remote Sensing Symposium
(IGARSS '95)
, Florence, Italy, 1588-1590, 1995.
Alpers, W., C. Melsheimer, C. Brnning, and R. Schmidt, Imaging of ocean waves by
SIR-C/X-SAR
over the North Sea and North Atlantic, Proceedings of the International Geoscience and Remote Sensing Symposium (IGARSS '95)
, Florence, Italy, 1317-1319, 1995a.
Alpers, W., A. Schmidt, R. Schmidt, and C. Brnning, A comparison of ocean wave-radar
modulation transfer functions at different radar frequencies and polarizations determined
from tower and aircraft measurements, Proceedings of the International Geoscience and Remote Sensing Symposium (IGARSS '95)
, Florence, Italy, 1087-1089, 1995b.
Alpers, W. et al., several papers about different results obtained from
SIR-C/X-SAR
data, presented at IGARSS '96
, 1996.
Bao, M., C. Brnning, and W. Alpers, On the nonlinear imaging of two-dimensional ocean
surface wave fields by interferometric
SAR,
Proceedings of the International Geoscience and Remote Sensing Symposium (IGARSS '95)
, Florence, Italy, 775-777, 1995.
Gade, M., and W. Alpers, The German surface film experiments during the two
SIR-C/X-SAR
missions, EARSel Newsletters 3/95
, 1995a.
Gade, M., and W. Alpers, First results from the German surface film experiments during
the two
SIR-C/X-SAR
missions, Proceedings of the 15th
EARSeL
Symposium
, Basel, Switzerland, 1995b (in press).
Gade, M., Ph.D. thesis about the damping behavior of different surface films with
respect to the energy flux on the water surface, 1996.
Huehnerfuss, H., W. Alpers, H. Dannhauer, M. Gade, P. A. Lange, V. and V. Wismann,
Natural and man-made sea slicks in the North Sea investigated by a helicopter-borne
5-frequency radar scatterometer, Int. J. Remote Sens.
, 1995 (in press).
Mango, S., S. Chubb, F. Askari, J. Lee, G. Valenzuela, R. W. Jansen, R. A. Fusina,
B. Holt, R. M. Goldstein, W. Alpers, T. F. Donato, M. R. Grunes, H. H. Shih, J. Verdi,
J. C. Church, and L. K. Shay, Remote sensing of current-wave interactions with
SIR-C/X-SAR
during
SRL-1
and
SRL-2
at the Gulf Stream supersite, Proceedings of the International Geoscience and Remote Sensing Symposium (IGARSS '95)
, Florence, Italy, 1325-1327, 1995.
Masuko, H. and W. Alpers, Observation of artificial slicks with
SIR-C/X-SAR
around
Japan, Proceedings of the International Geoscience and Remote Sensing Symposium (IGARSS '95)
, Florence, Italy, 227-229, 1995.
Schmidt, R., Ph.D. thesis about a new method of classification of
SAR
image spectra,
1995.