The document summarizes two seminar presentations that used InSAR techniques to analyze land subsidence in Beijing, China and the Perth Basin in Australia. In Beijing, InSAR detected 790 mm of cumulative subsidence over 8 years, with rates comparable to GPS data. Subsidence correlated with over-exploitation of groundwater. In the Perth Basin, Sentinel-1A detected subsidence up to 15 mm/year over broad areas and 20 mm/year locally, agreeing with independent TerraSAR-X data and indicating seasonal groundwater impacts. Both studies demonstrated InSAR's ability to detect small subsidence magnitudes and would benefit from considering human impacts.

1. Summary Report of Seminar Presentations
1.[2017/03/23] “Imaging Land Subsidence Induced by Groundwater
Extraction in Beijing (China) Using Satellite Radar Interferometry”
2.[2017/05/18] “First Results from Sentinel-1A InSAR over Australia:
Application to the Perth Basin”
By
Minh Nguyen 阮銘 (105022601@cc.ncu.edu.tw)
Student ID: 105022601
At
國立中央大學 遙測科技碩士學位學程
Master of Science Program in Remote Sensing Science and Technology
National Central University
June 1, 2017

2. I. Introduction
Synthetic Aperture Radar Interferometry (InSAR) uses active sensor from remote
sensing satellite to observe our planet and provides useful observation over the time.
In previous semester, the studies focused on applicability of InSAR for ground
displacement and illustrated the powerful of this technique in land subsidence
detection and monitoring for urban area with high precision, in particularly.
In this period, the two presentations also exploited the thread of advanced
InSAR techniques and discovered varying aspects of land subsidence due to
over-exploitation groundwater. One more time, these researches demonstrated the
reliability of the technology with small magnitude of surface deformation and are
available to bring new insight into the area which is subsiding for long-term, in
comparison with the ground based geodetic measurements.
On the other hand, we are known there is possible methods to improve the result
derived by InSAR with atmospheric correction besides advance techniques such as
Small Baseline, TCP InSAR or PSInSAR… The idea will be extreme important for
the region suffering high atmospheric effect and make the tool being robust.
Sentinel 1A exhibits its ability for integration with the reconnaissance tool by
providing comparable result with others data set from high resolution satellite as
TerraSAR-X, Envisat… Taking the advantage of open access basic data, it is potential
for our researchers working on remote sensing to perform more useful studies.

3. II. Methodology
One of the new process for the study of this period is consideration of geology
and hydrogeological for study area. The reason is that we already were aware of over
extractionof groundwater is the main cause leading to the subsidence in urban region
and for better analysis, other contributors is investigated to understand what causing
to the change of surface and behavior of triggering factors. These factors are aquifer
system, aquifer structure (coarse-grain sediments, median-grain, fine-grain),
extraction areas, compressible soil thickness and geology structure.
Small baseline InSAR is employed to bring higher coherence and more accurate
phase by small temporal and spatial baseline to select multi–master differential
interferograms. The slowly decorrelation filtered phase (SDFP) pixels were selected
by setting the threshold for the amplitude dispersion to generate the largest set of
candidate pixels. The phase unwrapping was applied and other techniques such as a
least-squares inversion method can be used to estimate displacement time series.
ERA-Interim (ERA-I) global atmospheric reanalysis was utilized to reduce
atmospheric error which is one of the most important factors producing noise. ERA-I
provides estimates of temperature, geopotential height and relative humidity at 37
pressure levels between 100hPa and 1hPa, on a 75 km x 75 km global grid, at
six-hourly intervals. The model cycle closest to the SAR acquisition time for each

4. satellite (within 1 h) and combine the model outputs with a digital surface model to
produce 2D maps of atmospheric delay. Differential maps of atmospheric delay are
then subtracted from the interferograms.
Furthermore, every step of InSAR technique is available for enhancement by
various methods. Besides that, the variety of software also changes the processing
chain of the tool which implies the diversity of science in general and particularly of
the popularity of SAR Interferometry.
III. Results and/or Discussion
The cumulative displacement in Beijing is 790 mm for the highest magnitude
during the period of 8 years. This deformation is comparable with in-situ data from
GPS and TerraSAR-X satellite. Besides that, the relationship between
over-exploitation of ground water and triggering factor was illuminated and providing
better understanding for the phenomenon. (Figure 1)
Perth Basin area is also recorded a large a region of subsidence in complex
environment with wet area near coastal sea. The subsidence reveals being adjacent
with uplift through both TerraSAR-X and new satellite Sentinal-1. (Figure 2)
These researches focus on the natural effect of deformation while the human
contributor is left open, which gives the idea of further study on the field.

5. IV. Conclusions and Future Work
InSAR Small Baseline changes the traditional method for surface observation
and benefits for the large scale of area. The results are qualified by and demonstrate
the reliability of the technique for ground displacement detection and monitoring.
Since the over-exploitation of ground water is popular in urban, the analysis of
its impact should be considered with geological triggering factors to prevent the
negative deformation of ground.
Sentinel-1 illustrated the comparable with InSAR technique and provided an
open chance for research on our planet in term of remote sensing. Since the human
impact was not considered comprehensively, which is one of the disadvantages for
those studies.
V. References
1. Parker, Amy L., Mick S. Filmer, and Will E. Featherstone. "First Results
from Sentinel-1A InSAR over Australia: Application to the Perth
Basin." Remote Sensing 9.3 (2017): 299.
2. Chen, M., Tomás, R., Li, Z., Motagh, M., Li, T., Hu, L., ... & Gong, X. (2016).
Imaging land subsidence induced by groundwater extraction in Beijing (China)
using satellite radar interferometry. Remote Sensing, 8(6), 468.
3. Tuan S. Le, Chung-Pai Chang, Xuan T. Nguyen and Akano Yhokha. (2016)
TerraSAR-X Data for High-Precision Land Subsidence Monitoring: A Case
Study in the Historical Centre of Hanoi, Vietnam.

6. Figure 1. Spatio-temporal evolution of InSAR-derived accumulated LOS displacement
in Beijing from Envisat-ASAR images.

7. Figure 2. Deformation rate-maps for Sentinel-1A & TerraSAR-X. (C) Zoomed in
sections showing subsidence in three wetland areas. (D) Time-series of cumulative
vertical displacement measured at GPS stations.

8. Appendix I
Groundwater is the main water source for industrial, agricultural and
household activities. However, the rapid urban growth is leading to the increasing
of water demand in cities of developing countries such as Beijing (China). As a
result of over-exploitation groundwater, this city is suffering land subsidence and
more seriously, the rate and extent of land subsidence shows an increasing trend.
The study employed Small Baseline (SBAS) Interferometric Synthetic
Aperture Radar technique to investigate land subsidence in the Beijing region. This
work aims to use TerraSAR-X (X-band) stripmap images collected from 2010 to
2011 and Envisat ASAR (C-band) images acquired between 2003 and 2010 to
provide a new insight into the spatial and temporal distribution characteristics and
main conditioning and triggering factors of land subsidence in the Beijing plain.
The joint cross analysis of radar-derived deformation data and existing
geo-information will help to improve the knowledge of the mechanisms that govern
land subsidence to be used for the development of suitable groundwater
management policies in this area.
The results reveal that the Beijing region has experienced significant ground
subsidence from 2003 to 2010 with a maximum accumulative displacement of 790
mm. The SBAS results have been validated by GPS measurements with a mean
difference of 2.41 ± 1.84 mm/year.
Keywords: Small Baseline InSAR, SBAS, land subsidence, Envisat ASAR,
TerraSAR-X.
References: Chen, M., Tomás, R., Li, Z., Motagh, M., Li, T., Hu, L., ... & Gong, X.
(2016). Imaging land subsidence induced by groundwater extraction in Beijing (China)
using satellite radar interferometry. Remote Sensing, 8(6), 468.

9. Appendix II
Traditional ground-based geodetic technologies in the Perth Basin, Australia,
record small-magnitude subsidence (up to 7 mm/y), but are limited to discrete points
or traverses across parts of the metropolitan area. To continuously monitor this
geo-hazard, Sentinal-1A data was employed to investigate in board area and this is
also the first application of the satellite in Australia.
The duration of the study is short (0.7 y), as dictated by the new satellite -
Sentinel-1A data. Though this limited observation period, Sentinel-1A result was
verified with continuous GPS and independent TerraSAR-X, providing new insights
into the deformation field of the Perth Basin. The displacements observed by each
satellite are in agreement, identifying broad (>5 km wide) areas of subsidence at rates
up to 15 mm/y. Subsidence at rates greater than 20 mm/y over smaller regions (∼2 km
wide) is coincident with wetland areas, where displacements are temporally correlated
with changes in groundwater levels in the unconfined aquifer. Longer InSAR time
series are required to determine whether these measured displacements are
representative of long-term deformation or (more likely) seasonal variations.
The high similar result between datasets illustrates the ability of Sentinel-1A
to detect small-magnitude deformation over different spatial scales (from 2 km–10 s
of km) in the Perth Basin. In particularly, the intervention of geology to this
phenomenon should be considered in other subsiding area.
Keywords: InSAR, land subsidence, Sentinal-1A, TerraSAR-X.
References: Parker, Amy L., Mick S. Filmer, and Will E. Featherstone. "First
Results from Sentinel-1A InSAR over Australia: Application to the Perth
Basin." Remote Sensing 9.3 (2017): 299.