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Mapping without the sun
Zhang, Jixian

Z. Li, C. Xie, Q. Chen
State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing Application, Chinese Academy of
Sciences, P.O.Box 9718, Datun 3, Beijing, 100101, China, zli@irsa.ac.cn
KEY WORDS: Interferometry, Detection, Subsidence, Permanent frozen area, Qinhai-Tibetan Plateau, Railway
The surface displacement by seasonally freezing bulge and thawing subsidence are main hazards for engineering construction in
permafrost regions, especially for the Qinghai-Tibet railway. For detecting the distortion at permafrost area, we try to study the
interferometric method of monitoring the deformation at permafrost area with time-series EnviSat ASAR data. In this paper, the
coherence characteristics are analyzed for different baseline, time interval with or without season change and different classes (rock,
bare soil, vegetation, water), used 13 time-series ASAR data from Jan. 2004 to June 2006 at the Beiluhe test area in the Qinghai-
Tibetan Plateau. The results showed that the coherence coefficients is lower in summer and fall than in other two seasons as the
freezing and thawing phenomenon. For pairs of crossed different season, such as from spring to summer or from summer to fall, the
coherence coefficient decrease for bare soil and vegetation cover, little decrease for rock cover. The deformation of railway roadbed,
detected by Permanent Scatters Interferometry, mainly appears in the way of thawing subsidence from May to November every year
with 25 mm maximum subsidence in the two years.
The permafrost and the seasonal frozen ground cover
respectively 1,272,709 km2 and l,146,399km2 in the Qinghai-
Tibetan Plateau, and the highway and railway that passes
through permafrost areas about 550km. Extensive areas of
frozen ground hold large quantities of ice. The seasonally
thawing layer is highly sensitive to temperature changes, and
thawing and temperature rising has a great influence on railway
stability. One of the main problems is how to monitor the
frozen ground’s displacement (Ma , 2006).
The traditional methods to monitor deformation of plateau
frozen ground is burying settlement meter, such as inclinometer,
or constructing time-serial GPS observation station. Using these
methods, the acquired information of plateau frozen ground’s
deformation can only be gotten at some limited locations due to
the limitation of observation condition, especially in the Tibet
Plateau, although which is with high precision and continuity.
Differential Synthetic Aperture Radar Interferometry (DInSAR)
has also been widely used in recent years for monitoring
ground’s deformation (Li, 2004). DInSAR analysis the phase
information of radar echo signal, and extract dense deformation
information in a relative large spatial domain. But temporal and
geometrical decorrelation often prevents traditional SAR
interferometry from being an operational tool for surface
deformation monitoring in the area of frozen ground. To get
long term deformation information, scientists presented the
methods of Permanent Scatters (PS) and Small Baseline Subset
(SBAS), and have successfully used them for monitoring
subsidence in urban area (Ferretti, 2000; Berardino, 2002). But
in the area of frozen ground, ground surface changes very
quickly with time, so the coherence is relative lower in the area
of frozen ground than in the area of urban. To use the method of
PS or SBAS to analyze surface subsidence history in the area of
frozen ground, characteristics of coherence at permanent frozen
area need to be firstly analyzed. With the analysis of the
characteristics of coherence, it can be recognized what’s the
factors that influent object’s coherence, and also be made clear
when the factors work and what degree they influent the
object’s coherence. Based on the analysis of coherence
characteristics, we can determine the SAR data choice for
detecting the surface deformation and the choice of stable
There are a lot of factors that influence freezing and thawing of
the frozen ground, e.g. season, vegetation, topography, snow
cover, water, lithology, and moisture. In the zone of meadow in
the Qinghai-Tibetan plateau, vegetation reduces surface
temperature and influent thawing and freezing of frozen ground.
The influence of topography to the frozen ground is mainly
dues to elevation, gradient and direction of slopes.
The active process in active layer in the permafrost region of
the Qinghai-Tibetan Plateau is composed of main four sections
(Zhao, 2000):
(1) The process of active layer’s thawing in summer begins at
the end of April and completes at the middle of
September. During the process, the frozen ground thaws
downwards from the surface until reaching the maximum
(2) The process of freezing begins at the middle of September
and the frozen ground freeze slowly upwards from the
bottom. From the middle of September to the middle of
October, the frozen ground freezes from two directions
(i.e. downwards and upwards). At the end of October, the
process of freezing ends.
(3) When the process of freezing completes thoroughly, the