Surface Shift Estimation Using TerraSAR-X Data
Report from January 27nd, 2010
Haiti Earthquake Surface Shift Estimation Using TerraSAR-X Data
Report from January 27nd, 2010
M. Eineder, N. Yague-Martinez, C. Minet, T. Fritz, A. Parizzi
Remote Sensing Technology Institute, DLR
This report updates our Intermediate Report dated January, 22nd, 2010, distributed on
Monday, 25th. We additionally processed pre- and post-seismic data pairs to correlation shift
maps and interferograms. The results confirm the validity of our methods which were
performed in a very short time.
Fig. 1 shows the co-seismic (middle) interferogram (upper) and correlation shift maps (lower)
together with pre-seismic (left) and post-seismic (right) ones.
Fig. 2 summarizes our results from quantifying the surface motion of the Haiti M7 earthquake
on January 12, 2010 and shows the geographic location of our data.
Pre-seismic pair
17-SEP-2008
18-FEB-2009
Co-seismic pair
18-FEB-2009
14-JAN-2010
Post-seismic pair
14-JAN-2010
25-JAN-2010
Fig. 1: Co-seismic (middle) interferogram (upper) and correlation shift maps (lower) together
with pre-seismic (left) and post-seismic (right) ones. Images shown in radar slant range
geometry. The colors represent the absolute value of the (assumed) horizontal surface motion.
The arrows in the upper left are the 1 m unit vectors. For geographic orientation see Fig. 2.
We did not remove single outliers such as in the ocean (ships) or along the image border.
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Surface Shift Estimation Using TerraSAR-X Data
Report from January 27nd, 2010
Fig. 2: Range displacements projected to horizontal surface (72.45°W-72.28° W, 18.40°N18.65°N) measured with image correlation techniques using TerraSAR-X data.
1. Data used
TerraSAR-X stripmap radar images from 17-SEP-2008, 18-FEB-2009, 14-JAN-2010, and 25JAN-2010, all acquired at 22:54 UTC, ascending orbit, right looking, 12° to north, incidence
angle 39.1°. Best available orbits (science) with an accuracy of about 5 cm were used for
evaluation. For the 25-JAN-2010 scene only a rapid orbits are available for now. The data
were acquired by DLR’s Centre for Satellite Based Crisis Information (ZKI) to support
immediate assessment of damage caused by the earthquake and processed in DLR’s Remote
Sensing Technology Institute.
2. Methodology
After a rough geometric coregistration of the images we determined the residual shifts. This
was done by incoherent cross-correlation of the SAR images. The parameters of the
correlation procedure were as follows:
- Number of correlation chips: 400 x 400 (co-seismic) or 200 x 200 (pre- and post-seismic)
- Size of one correlation chip: 128 x 128 pixels (270 m ground range, 238 m azimuth)
- Oversampling factor 16 (shifts of 1/16th pixel can be determined)
The results were then corrected for the expected image parallax from orbital geometry using
orbit state vectors and a DEM.
Correlation for the co-seismic pair (almost 1 year!) was very low in most places (0.1-0.4). The
Pre- (5 months) and Post-seismic (11 days) correlation was much better. We filtered the
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Surface Shift Estimation Using TerraSAR-X Data
Report from January 27nd, 2010
correlation maps using a 8 x 8 median filter and sub-sampled the result to 40 x 40 for the
vector plot (Fig. 2).
Furthermore, the shifts were corrected for solid Earth tides which made up a range offset of
up to 20 cms for some of the pairs. The Post-seismic pair showed an azimuth offset of about 2
meters which we attribute to the reduced accuracy of the rapid orbits. We removed average
range and azimuth shifts in the post-seismic shift map.
We did not correct for atmospheric path delays because up until now we could not get hold of
data from the closest GPS reference station SCUB. If anybody can provide zenith delays for
the mentioned acquisition times we could further enhance the accuracies.
For the plot the slant range displacements were projected on ground assuming only horizontal
motion. Of course, this could also be an upward motion of the same order (39° incidence
angle).
The TerraSAR-X interferograms were generated with DLR’s system GENESIS. Topography
was compensated using SRTM 3” DEM, 20 x 20 looks averaged. Near range to the left, early
azimuth at the bottom.
3. Interpretation
3.1 Based on the absolute orbit and timing accuracy we expect absolute range and azimuth
accuracies of 0.2 m for each image or about 0.3 m for the correlation (assuming uncorrelated
errors). These are already pessimistic assumptions which may be appropriate for this quick
analysis.
The co-seismic results agree with the known tectonic setting of the region (left lateral strike
slip along Enriquillo fault). We conclude that the western part north of the fault moved
- about 1.3 m to the West or
- about the same order upward or
- a linear combination of both.
On the one hand Eric Fielding has found evidence for upward motion by comparing his
ALOS descending interferogram to our results. On the other hand our results are in agreement
with west-bound motion detected by CEA using SPOT5 optical data (see UNAVCO
Supersites http://supersites.unavco.org/haiti.php).
3.2 Pre- and Post-seismic correlation shift maps show no significant relative displacements.
The interferograms show patterns that we – in lack of regional geological knowledge - can
only contribute to atmosphere and residual topographic phase.
4. Further Work
We interested to compare and combine our measurements with other data sources such as
ALOS, Envisat, GPS and CosmoSkymed.
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