Monitoring of Land Subsidence using PSInSAR: A Case Study in

Nanjing Urban Area

Miao Hou

, Yongping Xu

, Huaiqian Xiao

, Houjun Yan

, Xing Yang

and Songgan Weng

1 d

Jiangsu Hydraulic Research Institute, 97 Nanhu Road, Nanjing 210017, China

Jiangsu Administration of Huaihe, Shuhe and Xinhe Rivers, 8 Shenzhen Road, Huaian 223005, China

Keywords:

Land subsidence, Nanjing, PSInSAR, Sentinel-1, Urban area

Abstract:

Urban land subsidence is mainly induced by human activities, such as ground-water overdraft, mining of

mineral resources and load of constructions. The geodetic-based monitoring systems utilizing leveling have

been used to study land subsidence in many urban subsidence regions for a long time. But many

benchmarks established for leveling have been destroyed or are unstable due to human activities or land

subsidence. In addition, conventional leveling in urban areas is costly and time-consuming, often taking

months or years to complete land subsidence measurement. So, it is of great significance to adopt new

measurement technology in urban land subsidence monitoring. Nanjing city with rapid urbanization, a large

city in China, has suffered subsidence problems in the past 30 years. This paper uses the Permanent

Scatterers Synthetic Aperture Radar Interferometry (PSInSAR) methodology with Synthetic Aperture Radar

(SAR) images acquired from the Sentinel-1 between 2014 and 2019 to characterize the subsidence of

Nanjing city, which provides high-resolution assessment of deformations for sluices, bridges, high-rise

buildings, historical buildings and so on. The analysis shows that PSInSAR is very efficient space-borne

technique for monitoring subsidence phenomena in urban area.

a https://orcid.org/0000-0002-0924-615X

b https://orcid.org/0000-0002-2583-8534

c https://orcid.org/0000-0002-5512-1817

d https://orcid.org/0000-0003-2122-8576

1 INTRODUCTION

As a global disastrous natural hazard, land

subsidence may occur in large urban areas over the

world and usually caused by human activities, such

as ground-water overdraft, mining of mineral

resources and load of constructions. However, the

conventional geodetic technique utilizing leveling is

costly and time-consuming in the large-scale urban

land subsidence monitoring. Interferometric

synthetic-aperture radar (InSAR) is a modern

space-borne technology for monitoring earth surface

deformation. InSAR techniques (e.g. DInSAR,

PSInSAR, SqueeSAR) can provide centimeter- to

millimeter-level accuracy in large-scale land

subsidence monitoring without much time and high

cost. Also, InSAR techniques have the advantages of

all-weather adaptability (Yu et al., 2021; Yang et al.,

2020; Alani et al., 2020). InSAR techniques have

been successfully applied in different deformation

research work, such as glacier drift (Yan et al., 2016),

surface subsidence (zhou et al., 2020; Smith &

Knight, 2019), landslide (Guo et al., 2021; Fobert et

al., 2021), volcanic eruption (Liang et al., 2021;

Hooper, 2008), etc.

Wright et al. (2004) used InSAR to measure

surface displacement across the western Tibetan

plateau. Pritchard & Simons (2013) used InSAR to

analyze the deformations of 900 remote volcanos in

the central Andes to determine which one might have

magma moving at depth. Baer et al. (2002) used 16

SAR images obtained by the European Remote

Sensing ERS-1 and ERS-2 satellites from 1992 to

1999 to research land subsidence along the Dead Sea

shores. Rivera et al. (2017) used COSMO-SkyMed

InSAR to obtain time-dependent ground deformation

268

Hou, M., Xu, Y., Xiao, H., Yan, H., Yang, X. and Weng, S.

Monitoring of Land Subsidence using PSInSAR: A Case Study in Nanjing Urban Area.

In Proceedings of the 7th International Conference on Water Resource and Environment (WRE 2021), pages 268-274

ISBN: 978-989-758-560-9; ISSN: 1755-1315

data over Cotopaxi volcano. Koning et al. (2020)

carried out the land subsidence monitoring using

InSAR in a dike strengthening project and obtained

the land subsidence rate. Xu et al. (2018) reviewed

the applications of remote sensing technology, such

as InSAR, in urban flood simulation.

Nanjing city with rapid urbanization, a large city

in China, had suffered serious subsidence problems

in the past 30 years. Geological hazards caused by

land subsidence, such as the building tilt, ground

collapse and embankment deformations, have long

been a problem affecting the development of

Nanjing and resulted in increased flood risk in some

urban subsidence regions, especially in Hexi area of

Nanjing city, where the soft soil layer is widely

distributed. Therefore, urban land subsidence

monitoring in Nanjing is of great significance. This

paper takes Nanjing as an example, and four types of

buildings (i.e. sluice, bridge, historic building and

high-rise building) are selected as research cases to

carry out the application research of InSAR

technology in urban land subsidence monitoring.

This paper is structured as follows: At first, the

research background is presented in Section 1. Then,

the study area and InSAR data is introduced in

Section 2. Section 3 presents the InSAR method.

And, the results are summarized in Section 4. Finally,

main conclusions are presented in Sections 5.

2 STUDY AREA AND DATA

Nanjing city, the capital of Jiangsu Province, is

located in the lower reaches of the Yangtze River and

in the southwest of Jiangsu Province (Figure 1). The

city covers an area of 6 587.02 km

, with 11

municipalities under its jurisdiction and a permanent

population of 9.31 million. Nanjing has a subtropical

monsoon climate with an average annual rainfall of 1

106 mm. Nanjing has four distinctive seasons (i.e.

spring, summer, autumn and winter), with the

temperature above 30 ℃ in summer and 2 - 10 ℃ in

winter. Nanjing had grown rapidly during the past 30

years. Land subsidence occurred in different regions

of Nanjing. In this paper, InSAR methodology with

SAR images acquired from the Sentinel-1 between

2014 and 2019 has been used to characterize

subsidence in Nanjing. Four types of buildings,

including sluice (i.e. Qinhuai new river sluice),

bridge (i.e. Shuiximen bridge), historic building (i.e.

Sun Yat-sen's Mausoleum) and high-rise building

(i.e. Greenland Square Zifeng Tower), are selected

as monitoring cases and their locations are shown in

Figure 1.

Figure 1: Location of Nanjing.

The European Space Agency (ESA) Sentinel-1 is

a day-and-night, all-weather, polar-orbiting radar

imaging mission for global monitoring of

environment and security services at C-band. The

Sentinel-1A and B are the two satellites that

composes the Sentinel-1 radar mission, and were

launched in April 2014 and April 2016, respectively.

The Sentinel-1 SAR instrument can acquire data in

four exclusive modes, namely Strip map (SM),

Interferometric Wide swath (IW), Extra Wide swath

(EW) and Wave (WV) modes. Table 1 shows the

selected 72 C-band VV-polarizatio ascending SAR

images acquired by sentinel-1 in this study, covering

the period October 8, 2014 to September 25, 2019.

Monitoring of Land Subsidence using PSInSAR: A Case Study in Nanjing Urban Area

269

Table 1: SAR data in monitoring area.

Parameter Number

Satellite type; Date band; Spatial resolution Sentinel-1; C-bond (5.6 cm); 20 m

Up/down mode; Polarization mode; Side view Ascending; VV; 39.92°

Acquisition time; Number of image

Date level

20141008-20190925; 72

SLC data (single view and multiple view)

Date of image

20141008, 20141102, 20141118, 20141220, 20150117 20150222,

20150306, 20150310, 20150322, 20150411 20150513, 20150614,

20150712, 20150716, 20150817 20150902, 20150918, 20151004,

20151016, 20151105 20151117, 20151207, 20151219, 20160108,

20160120 20160312, 20160328, 20160413, 20160429, 20160515

20160531, 20160616, 20160702, 20160714, 20160730 20160904,

20161002, 20161107, 20161209, 20170207 20170227, 20170416,

20170518, 20170603, 20170721 20170822, 20170919, 20171005,

20171110, 20171212 20180113, 20180214, 20180314, 20180403,

20180419 20180517, 20180622, 20180708, 20180805, 20180910

20181012, 20181113, 20181211, 20190112, 20190213 20190305,

20190418, 20190520, 20190621, 20190723 20190823, 20190925

3 RESEARCH METHOD

The Permanent Scatterers Synthetic Aperture Radar

Interferometry (PSInSAR) is a InSAR technique.

Compared with Differential Interferometric

Synthetic Aperture Radar (DInSAR), it can

overcome the influence of spatio-temporal coherence

and atmospheric delay on SAR radar accuracy, and

aims at ground deformation mapping with

millimetric precision. Therefore, this paper adopts

PSInSAR technology. The basic flow of PSInSAR

data processing is shown in Figure 2.

PSInSAR identifies coherent radar targets (PS

points) to monitor ground deformation. Hooper et al.

(2004) proposed the identification algorithm of PS

based on phase characteristics, which includes

registration, radiometric calibration, PS detection

and interference processing for single-view complex

SAR images. After that, the differential interference

phase set of each PS point in each differential

interferogram can be obtained.

The phase residual of the PS point x in the scene i

interferogram is shown as follows:

𝜑

,

𝜑

,,

𝜑

,,

𝜑

,,

𝜑

,,

𝑛

,

(1)

where 𝜑

,

is the phase value of the pixel x of the

scene i interferogram, 𝜑

,,

is the deformation

phase in radar line of sight direction, 𝜑

,,

is the

delay phase difference of satellite transit atmosphere

at different times, 𝜑

,,

is the phase due to error

in the DEM, 𝜑

,,

is the DEM error phase, 𝑛

,

the phase of noise.

If 𝜑



,𝜑



and 𝜑



are spatially correlated over

distances of a specified length scale L, and that 𝜑



and 𝑛 are uncorrelated over the same distance, with

the mean of Zero. Then, the mean value of each

phase in the circular region with pixel x as the center

of the circle and pixel L as the radius can be

expressed as:

𝜑

,

𝜑

,,

𝜑

,,

𝜑

,,

(2)

Subtracting formula (2) from formula (1) leads to:

𝜑

,

𝜑

,

𝜑

,,

𝑛

,

𝑛′



,

(3)

where 𝑛′



𝜑



𝜑







𝜑



𝜑





𝜑





𝜑



.

The DEM error phase 𝜑

,,

and the vertical

WRE 2021 - The International Conference on Water Resource and Environment

270

baseline 𝐵

,,

are proportional to each other, the

formula (3) also can be represented as follows:

𝜑

,

𝜑

,

𝐵

,,

𝐾

,

𝑛

,

𝑛′



,

(4)

where 𝐾

,

is a proportionality constant.

Then, in all available interferogram, the smallest

square method is used to estimate 𝐾

,

𝛾



1/𝑁|

∑

𝑒𝑥𝑝𝑗𝜑

,

𝜑

,

𝜑

,,







| (5)

where, 𝛾



is a measure of the temporal coherence

based on pixel x, 𝑁 is the number of available

interference patterns and 𝜑

,,

is our estimate of

𝜑

,,

Figure 2: The basic flow chart of PSInSAR data processing.

4 RESULTS

In this paper, 72 radar interference images in

Nanjing city from October 8, 2014 to September 25,

2019 had been obtained to analyze the deformations

of the selected sluice, bridge, historical building and

high-rise building. Figure 3 shows the uplift or

settlement deformations of PS points in study area.

The results indicate that the annual average ground

motion velocity (mm/yr) was mainly in the range of

-18 mm/yr to 18 mm/yr.

Monitoring of Land Subsidence using PSInSAR: A Case Study in Nanjing Urban Area

271

Figure 3: Map of ground deformation in study area.

Figure 4 and Table 2 shows four deformation

record examples (i.e. Qinhuai new river sluice,

Shuiximen bridge, Sun Yat-sen's Mausoleum and

Greenland Square Zifeng Tower). The negative

value in the picture represents settlement and

positive value represents uplift. It can be seen from

the picture that in the past 5 years, (i) The annual

average motion velocity of the sluice, bridge,

historic building and high-rise building range from

-3.3 mm/yr to -0.5 mm/yr, -0.1 mm/yr to 1.3 mm/yr,

-0.6 mm/yr to 1.0 mm/yr and -0.2 mm/yr to 1.0

mm/yr, respectively, (ii) The most of the cases were

relatively stable within a limited deformation range,

and their cumulative deformation is -4.4 mm, -8.3

mm, -3.7 mm and 2.6 mm respectively, (iii) Their

average deformation is -2.4 mm, -4.6 mm, 1.1 mm

and 1.3 mm respectively, so the two buildings are in

the more stable state than the rests.

Figure 4: Four deformation record examples: (a) Qinhuai new river sluice, (b) Shuiximen bridge, (c) Sun Yat-sen's

Mausoleum, (d) Greenland Square Zifeng Tower.

Table 2: The deformation data of four cases.

Cases

The annual avera

e motion velocit

(mm/

The cumulative

deformation(mm)

The average

deformation(mm)

min max

Case 1 -3.3 -0.5 -4.4 -2.4

Case 2 -0.1 1.3 -8.3 -4.6

Case 3 -0.6 1.0 -3.7 1.1

Case 4 -0.2 1.0 2.6 1.3

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5 CONCLUSIONS

In this paper, the Qinhuai new river sluice,

Shuiximen bridge, Sun Yat-sen's Mausoleum, and

Greenland Square Zifeng Tower in Nanjing are

selected as study cases. SAR images acquired by

sentinel-1 and PSInSAR technology have been used

to characterize subsidence in Nanjing. The main

conclusions are as follows:

1) In the past 5 years, most of the study area was

relatively stable within a limited deformation range.

In all cases, the maximum annual average settlement

velocity and uplift velocity were 3.3 mm/yr and 1.3

mm/yr respectively, the maximum cumulative

settlement and uplift were only 8.3 mm and 2.6 mm

respectively, and the maximum average settlement

and uplift were only 4.6 mm and 1.3 mm

respectively. Qinhuai new river sluice and

Shuiximen bridge existed a slight settlement trend,

while Sun Yat-sen's Mausoleum and Greenland

Square Zifeng Tower tended to be a stable state with

small scale fluctuation.

2) Due to the lack of leveling results from the

above four cases, the PSInSAR technology proposed

by Hooper et al. (2004) has not been verified in this

study. The distribution of PS points is related to the

objects (e.g. roofs, road surfaces, rocks, etc) and

landforms of the research area, and the position of

PS point is random. Therefore, the manually

arranged leveling point and the PS point identified

by PSInSAR technology are unlikely to be in the

same position. How to compare the displacement

results of PS points and leveling points to verify the

reliability of PSInSAR in this study is worthy of

further research.

3) Some factors such as soft soil layer with high

compressibility and low bearing capacity, falling

groundwater levels, and surcharge loads (e.g.

high-rise buildings, subways) can result in urban

land subsidence in Nanjing city. The results showed

there was no obvious uplift trend and settlement

trend for most of the study area. The reasons for the

land uplift in some areas may be related to stress

release of foundation soil, the rise of groundwater

level and the decrease of load. But the real reason

needs to be further studied.

4) PSInSAR technique can provide

millimeter-level accuracy in urban land subsidence

monitoring without much time and high cost, and it

is worthy of further discussion in other deformation

research work based on PSInSAR technology.

ACKNOWLEDGMENTS

This work was supported by the Water Resources

Science and Technology Project of Jiangsu Province

(Grant No. 2019022), and the Science and

Technology Project of Jiangsu Province (Grant No.

BM2018028).

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