Study on Ground Motions in Southwest Bulgaria based on in-Situ

and Satellite Data

Mila Atanasova-Zlatareva

, Hristo Nikolov

and Nikolay Dimitrov

Department of Geodesy, National Institute of Geophysics Geodesy and Geography,

Bulgarian Academy of Sciences, Sofia, Bulgaria

Institute of Space Research and Technology, Bulgarian Academy of Sciences, Sofia, Bulgaria

Keywords: Ground Movements, SAR Data, GNSS, Crustal Deformation.

Abstract: In the last decades data from satellites are being used more frequently to study the ground movements. This

fact is evidenced by the increased number of research papers and projects using freely provided data by

space agencies such ESA (European Space Agency) and JAXA (Japan Aerospace Exploration Agency) and

increased revisiting time of the new instruments on-board satellites. Other reason for this increase are the

latest developments in processing methods such as PSI (Persistent Scatterer Interferometry) and even

increasing number of cloud processing options provided by universities and research centres. Nevertheless

the information obtained by this manner has some drawbacks for example moderate spatial resolution. This

is why in-situ data from precise GNSS (Global Navigation Satellite System) measurements are essential. In

this study the authors used both kinds of data to study one of the regions of Bulgaria which is recognized to

be highly prone to seismic and geological hazards namely the Southwest region. For this research two

sources of data have been used – SAR (Synthetic Aperture Radar) data from Sentinel-1 mission of ESA and

in-situ acquired contemporary and older GPS (Global Positioning System) data. As a result of SAR data

processing produced were interferometric images from ascending and descending orbits to decrease the

effect of the mountainous topography, while the results from the GNSS measurements were used for

verification.

https://orcid.org/0000-0002-3105-3266

https://orcid.org/0000-0001-5764-1499

https://orcid.org/0000-0002-5369-5921

1 INTRODUCTION

The main objective of this research is monitoring of

the ongoing geodynamic processes by

complementary use of SAR and GNSS data. GNSS

data from permanent and local geodetic networks are

used for validation of the SAR derived information

concerning the study area. The study will give

reliable data for ongoing risky geo-processes for the

region of the Southwest Bulgaria.

Geodynamic processes and seismic activity are

considered to be the prime driver of horizontal and

vertical movements of the Earth's crust in the Balkan

Peninsula. One proven method for continuous

monitoring of ground deformations is the use of data

from active radar remote sensing. These data are the

basis for the creation of interferometric images

(IFIs) for quantitatively assessment the registered

ground movements of the Earth’s surface within a

fixed time interval. For this research a set of IFIs

was created for the areas surrounding the city of

Sofia.

2 PREVIOUS GPS

MEASUREMENTS, RESULTS

AND ANALYSIS

In 1996 a GPS geodynamic network for long-term

monitoring of the crustal movements is established

in the region within a joint project with the

Massachusetts Institute of Technology (Kotzev et

al., 2006). All points are stabilised with metal bolts

Atanasova-Zlatareva, M., Nikolov, H. and Dimitrov, N.

Study on Ground Motions in Southwest Bulgaria based on in-Situ and Satellite Data.

DOI: 10.5220/0010503101570164

In Proceedings of the 7th International Conference on Geographical Information Systems Theory, Applications and Management (GISTAM 2021), pages 157-164

ISBN: 978-989-758-503-6

157

in rocks after geological study. The points have been

placed on the main structural blocks in the area - the

Vitosha, Plana and Lozenska mountains. The

network includes the first order point of Cherni Vrah

peak and the Geodesic Observatory of the NIGGG

“Plana”, thus covering the main tectonic structures

in the area.

A complete measurement of the entire network

with processing and analysis of the results has so far

been performed only in two epochs 1997 and 2000

(Kotzev et al, 2001). Data from GPS measurements

conducted on the points of the geodynamic network

have been processed in the scientific software

GAMIT/GLOBK, which is designed specifically for

processing of GNSS measurements in geodynamic

networks, calculates the velocities on the points by

applying modern methods of Kalman filtering.

During the period 2001-2019, the network was

expanded with the stabilization of new points and

GPS campaigns were periodically conducted to

determine crustal movements in the area. (Georgiev

et. al, 2011). The horizontal crustal movement and

seismicity have been studied by GPS analysis in

numerous studies (McClusky et al., 2000, Georgiev

et al., 2013, Kotzev et al, 2008).

The region of South Bulgaria, especially

Southwest Bulgaria and Rhodope Mountains and

Northern Greece is an active tectonic and

seismotectonic area in the South Balkans with

proved recent active tectonic structures and crustal

motions. Based on these alleged boundaries,

geological structures and faults, (Atanasova M.,

2014) formed the contours of blocks structuring of

this research area. The GPS velocity fields are

estimated and they are used to estimate the block

rotation motions. The region of Southwest (SW)

Bulgaria has the most pronounced tectonic and

seismotectonic activity on the whole Bulgarian

territory (Shanov et al. 2001). The investigated area

exhibits diverse relief structures, which are subjected

to horizontal and vertical movements of various

intensity. Southwest Bulgaria falls within a zone of

contemporary extension of the Earth’s crust with

complex interaction between horizontal and vertical

movements of the geological structures (Zagorchev,

2001).

Figure 1: A network for GNSS measurements consisting of 30 points.

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GPS measurements of eight points in a region of

central western Bulgaria were carried out by

specialists from the Department of Geodesy at

National Institute of Geophysics, Geodesy and

Geography (NIGGG, Bulgarian Academy of

Science) in several periods between 1997 and 2006

is used in the estimation. Those points have been

stabilized on the main structural blocks in the area.

The results show that the general tendency of

movement of the points in the region of Central

West Bulgaria is in the south direction with respect

to the stable Eurasia, which is in line with the

extensive movement of southern Bulgaria and

northern Greece The existence of the recent tectonic

activity in the area is well justified (Dimitrov N.,

2019).

The area of Sofia is a moderately active

geodynamic area. However, it is exposed at a

significant geological hazard, based on the presence

of numerous active faults, seismicity, landslides,

rockfalls, etc. The results produced as output from

this research will contribute to the assessment of

natural risks and seismic hazards in the study area

and will have a positive impact for the sustainable

development of the region.

2.1 GNSS Network

At the end of 2019 the project "Monitoring of

geodynamic processes in the region of Sofia" was

launched by the Department of Geodesy, the

National Institute of Geophysics, Geodesy and

Geography at the Bulgarian Academy of Sciences,

funded by the Bulgarian National Science Fund

(Dimitrov et. al., 2020).

This project allowed for a new comprehensive

measurement of the geodynamic network (Fig. 1).

The geodetic GNSS measurements were performed

by four teams of specialists from the Department of

Geodesy of the National Institute of Geophysics,

Geodesy and Geography in the period from

13.07.2020 to 31.08.2020. CHC N71 receivers with

CHCA220GR antennas and V100 with HITV100

antennas were used. Point PLA1, is continuous

operating reference station for many years. To study

the modern movements of the Earth's crust, two-day

GNSS measurements of a network of 30 points were

performed. The measurements and analysis of the

results of the GNSS campaign in 2020 were

processed. The results of the processing and analysis

of the GNSS 2020 campaign show very good quality

(Dimitrov et al. 2020) and will be used for joint

processing with other, significant number of

previous and future GNSS measurements to obtain a

model of intraplate movements of the Earth's crust in

the study area.

3 SYNTHETIC APERTURE

RADAR INTERFEROMETRY

(InSAR)

Lately the interferometric approach was widely used

in geodetic studies. This method uses

interferometrically processed data from ground or

satellite based synthetic aperture radars. The aim of

the processing is to obtain the change of the phase

signal present in two radar images acquired at

different dates. Based on this difference information

on diverse type’s geophysical phenomena –

earthquakes, landslides and rockfalls etc. can be

monitored.

It should be noted that the SAR instrument is an

active radar imaging system which usually is

mounted on flying platforms in order to map the

Earth’s’ surface. In this instrument the main element

is the SAR antenna which emits and receives back

the radar signals (Pribičević et al, 2017).

The acquired data form a radar image which is

composed by two different components of the

electromagnetic wave – the amplitude and the phase.

It should be taken into account that the wavelength

and the phase of radar signal are mutually correlated.

For use the interferometric method two radar images

(often named interferometric pair - IFP) must be

processed in conjunction and in the resulting image

the phase difference of the backscattered signal

between both measurements is placed. This

difference is directly related with the changes (if

any) that have occurred on the Earth’s surface in the

time interval between the two acquisitions. It also

has to be accounted that the displacements registered

by this manner are in the line-of-sight (LOS) of the

antenna and can’t be directly interpreted as motion

in horizontal or vertical plane. In order to do this

additional calculations are needed.

3.1 DInSAR Technique in Measuring

the Ground Motions in SW

Bulgaria

DInSAR (Differential Interferometric Synthetic

Aperture Radar) approach used to produce the

results in this study is composed of following stages.

The first one is the data preparatory stage at which

based on the expertise of the authors from the

available online archives with SAR data maintained

Study on Ground Motions in Southwest Bulgaria based on in-Situ and Satellite Data

159

Figure 2: Area of study covered by the SAR images of ESA’s mission Sentinel-1 ascending and descending orbits tracks

102, 7, 29 and 80.

by different institutions the needed ones have been

obtained. In this process the perpendicular baselines

between all possible IFPs were calculated and the

most suitable ones for further processing at next

stage were selected. Further auxiliary information

(digital maps and other data) concerning the

topography of the studied region was collected too.

This information was used to minimize the

shadowing effect of the Vitosha, Rila Mountains in

the DInSAR processing carried out on the next

processing stage which results in layover and

foreshortening as well. Other consideration that was

taken into account to mitigate the mentioned effects

was to process data from ascending and descending

orbits and co-registers them. At the end of this stage

we ended up with 48 Sentinel 1A/B scenes for the

period 2015-2020 (see Fig. 2).

3.2 DInSAR Processing Procedure

In the second stage of this research implemented

was the procedure for DInSAR processing of every

IFP and it is considered to be the most important to

obtain the final results.

At this stage only data in Single look Complex

(SLC) format were processed. In this format both

data for the amplitude and phase signals are

registered. Additionally those data free of platform

and orbital errors, have good positional information

for latitude/longitude, and at the same time provide

spatial resolution which is relevant for the objectives

of this research. For creation of a single IFP the SAR

data were processed by SNAP software provided by

ESA into which a well-established methodology for

DInSAR is easily realized (see Fig.3).

In this study a set of 32 IFPs was created for the

time period between April 2015 and November

2020. It has to be clarified that the produced IFPs

cover a four-month interval which essential to

minimize the negative effects of radar signal

attenuation and decrease of coherence caused by the

vegetation present in the studied areas. Other

constrain that was accounted at the beginning of

processing was to maximize the modelled coherence

for each IFP. During the creation of the

interferometric image of importance is to select the

DEM that will provide the best possible spatial

resolution of the final product (Nikolov et. al. 2017).

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Since no better DEM was made available at the

moment of processing the 1arcsec SRTM raster was

used. To guarantee the high quality of the final IFPs

only precise orbital data for the co-registration of the

initial SLC images were used. This step is of

importance for single interferogram formation since

at it the registered amplitude and phase signals from

a single ground element in both range and azimuth

directions from both SCL images are matched.

Next step in the processing was to improve the

quality by high-pass filtering and multilooking to

produce square ground elements in the IFI. Since for

this research we narrowed the area of study it was

necessary to produce a smaller polygon by

subsetting the IFI. After this procedure the IFI can

be geocoded and exported to other software (e.g.

QGIS, Google Earth) for additional visualization and

analysis. The colours present in this IFI indicate the

degree of change of the phase signal. In case of

single smooth colour there is no phase change i.e. no

surface deformations are detected. In the areas

where “salt and paper” effect is visible it can be

concluded that the phase is highly decorrelated due

to low coherence in the single ground element. Only

for those areas in the IFI where fringes are present it

can be concluded that ground deformations were

reliable registered from phase signal change in LOS.

To obtain surface displacements in LOS in

metric units one more procedure is needed which is

known as phase unwrapping. In fact this is a process

of reconstruction of the phase difference by adding

the correct integer multiple of 2π to the

interferometric fringes (Veci L. 2016). This

procedure for phase unwrapping can be performed

by several software products, but in this study we

used Snaphu module (Snaphu, 2020).

To study the ongoing geodynamic processes in

the region of SW Bulgaria created was a local

repository with SAR data from Sentinel-1 mission.

Using the above described method produced was a

set of IFIs at intervals of 4, 8 and 12 months and

when specific ground motion event had to be

studied. The validation of the information produced

from the IFIs was done using the data from the

GNSS network (see sec. 2 and Fig. 4). (Larsen Y. et

al., 2020)

The third and a final phase comprises the

analysis of the information produced from DInSAR

processing and is based on the produced deformation

maps in metric units for the corresponding period. It

needs to be underlined first that the registered

surface displacements are in the LOS of the antenna

and provide information if the points move toward

Figure 3: Radar data processing.

or away from it, and second they are relative to a

point with known deformation. One more practical

consideration when interpreting LOS deformations

is that the real three dimensional ground motion

(North, East, Up) is an estimation and should made

carefully.

Study on Ground Motions in Southwest Bulgaria based on in-Situ and Satellite Data

161

Figure 4: Sample IFI created from two images dated Nov 12 2016 (master) and Apr 5 2017 (slave).

On Fig. 5 an example of the occurred ground

displacements in LOS is presented. Those were

calculated from the unwrapped phase signal which is

color-coded. It can be noted that for the period of 4

months for 2016-2017 the calculated displacements

are in the range between 2 (uplift) and -25

(subsidence) cm. The resulting IFIs reveal that the

registered deformations are concentrated in some

areas exhibiting non-uniform pattern. From them a

map of the concentration of deformations of the

Earth's crust was created as well.

4 COMBINED USE OF GNSS AND

InSAR RESULTS

The combination of GPS/GNSS and DInSAR results

from the project "Monitoring of geodynamic

processes in the region of Sofia" will provide a more

detail insight in about the ongoing ground surface

deformations for the period 2015÷2023. The

information gathered through several GNSS

campaigns has ensured the precise determination of

horizontal and vertical ground displacements on the

discrete geodetic points while the information

produced after DInSAR reflects surface

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Figure 5: Subset of displacement map created from two images: Nov/12/2016 (mast) and Apr/5/2017 (slave).

displacements for thousands of ground targets but

only in LOS direction. The velocity model obtained

for the GNSS points is of high accuracy and for this

reason is considered as referent while from DInSAR

processing much larger coverage was obtained.

Therefore, with the combination of these two

techniques, more accurate and detailed investigation

of the ongoing geodynamic processes in the area is

produced. (Pribičević et al, 2017). The final result of

the combined use of these two independent geodetic

techniques for the determination of the ground

displacements in the studied region of SW Bulgaria

is depicted in Fig. 5.

5 CONCLUSIONS

This research demonstrated the potential and

capability of radar satellite data and DInSAR time-

series technique to investigate and monitor the

ground-surface deformations, and also to measure

their variations in LOS with centimetre accuracy

over time using freely available data and software.

The DInSAR can be regarded as an attractive

technique and operational tool for geological hazard

tasks such as detection and monitoring of ground-

surface deformations. It needs to be underlined that

the surface deformations produced from satellite-

based data require precise calculations and are

complementary to the field measurements and could

direct them, but can’t replace them.

Final products from the DInSAR processing were

surface displacement maps showing the average

surface displacements in LOS direction. One more

advantage of the DInSAR results is that they

provided dense spatial information in non-vegetated

areas only while in the highly vegetated areas where

the coherence is low an interpolation was done. The

combination of GNSS and DInSAR increased the

quality of the information regarding the current

geodynamic processes and surface motions.

Study on Ground Motions in Southwest Bulgaria based on in-Situ and Satellite Data

163

ACKNOWLEDGEMENTS

This paper has been made available with the

financial support provided by National Science

Fund, Bulgaria, call identifier “Competition for

financial support of basic research projects – 2019”

Project “Monitoring of geodynamic processes in the

area of Sofia”. Contract No KП-06-H 34/1.

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