Establishing Surface Displacements along a Railway Route near

Mirovo Salt Deposit, NE Bulgaria

Mila Atanasova-Zlatareva

and Hristo Nikolov

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

Bulgarian Academy of Sciences, Sofia, Bulgaria

Remote Sensing Systems Department, Space Research and Technology Institute,

Bulgarian Academy of Sciences, Sofia, Bulgaria

Keywords: Ground Movements, SAR Data, GNSS Networks, Railway Line Deformations.

Abstract: Studying Earths’ surface motions using data acquired by active instruments such as satellite Synthetic

Aperture Radar (SAR) have become ubiquitous in the last years. This trend could be attributed to large

extent to the open data policy of ESA that provides such type data from Sentinel-1 mission at no cost from

several online repositories. On the other hand the results produced after processing them need to be

validated by data from other sources. In this paper a framework for SAR data processing is presented,

whose results are compared and analysed with results from GNSS networks. In order to increase the

reliability of the information provided by the radar data used in this research ascending and descending

orbits of the satellite were used in order to decrease the effect of the topography. Part of railway line which

passes through the town of Provadia and industrial area near it was selected as test site. This object was

chosen since surface deformations often occur in it caused by natural and anthropogenic activities in that

area.

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

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

1 INTRODUCTION

The main object of research is the railway line

passing through Provadia town and close to the salt

deposit, which is part of the route Sofia-Varna. It is

known that the subsidence registered in the railway

infrastructure located near the mine workings is

caused by deformations related to the lowering of

the upper layer of the mine workings. These

deformations lead to changes at the surface and are

the main reason for continuously maintenance works

along the railway line. As pointed out in (Dimitrova

et al., 2020) the seismic activities having different

origin as well as other geodynamical processes are

considered to be the main source for the registered

by different means horizontal and vertical surface

movements (Angelov А. 2017).

The prime focus of the current research is to

present a pilot study and a framework for constant

monitoring of recent motions of the Earths’ crust

combining data from satellite synthetic aperture

radar (SAR) and from local and continuously

operating reference stations GNSS networks. In the

framework, the data from SAR will provide the main

amount of information concerning the ground

motions while the GNSS data were used to provide

more precise information to validate it. The final

results are of importance to provide reliable

information for the surface changes close to the

railway line since this is one of the major routes in

Bulgaria for transportation of goods and

people.(Galve et al., 2015; Fengming Hu et al.,

2019)

The proposed framework for information

provision is based on well established and tested

method for processing SAR data, namely the

DInSAR technique, taking into account peculiarities

of the region and the specific object. One main

advantage of the adopted approach is that it could

provide information at much shorter intervals

(seasonal or monthly) compared to conventional

geodetic surveys usually made twice a year.

Atanasova-Zlatareva, M. and Nikolov, H.

Establishing Surface Displacements along a Railway Route near Mirovo Salt Deposit, NE Bulgaria.

DOI: 10.5220/0011075400003185

In Proceedings of the 8th International Conference on Geographical Information Systems Theory, Applications and Management (GISTAM 2022), pages 155-162

ISBN: 978-989-758-571-5; ISSN: 2184-500X

155

2 RESEARCHED REGION

The area of Provadia town is characterized by a

complex geological and tectonic environment (see

Figure 1), where a large number of fault structures

have been registered according to geophysical and

geological data. They are revealed north and south

of the town of Provadia the Provadia fault being the

most clearly delineated structure in the area of the

so-called salt body. The orientation of the fault is

from north-northwest to east-southeast. In the past

the eastern slope of the Provadia fault located south

of Provadia is lowered by 100 meters compared to

the western one. The rise of the salt body through

the Late Holocene took place along the fault. The

length of the Provadia fault is 19.88 km. The

analysis of the main fault structures in the Provadia

region that have been active during the neotectonic

stage shows that the decisive role in its youngest

development is played by the Provadia fault most

pro-nounced in the area of the Mirovo salt deposit,

where the salt body was raised during the Late

Holocene. (Dimitrov et al., 2016)

The above discussed fault lays the basis of

modern deformation processes on the slopes of the

Provadia River. A number of new north-south faults

have been observed in the upper part of the eastern

slope of the Provadia River (Zagorchev I. 2001);

Dimitrova et al., 2020). Landslides occur in

delineated sectors of the same eastern slope of the

river. They show that the development of faults

(regional and local) in the region of Provadia hasn’t

finished yet. Near the underground Mirovo salt

deposit is the route of the railway line Sofia-Varna,

which crosses this region.

The main factor for the concentration of

earthquakes in the area of the Mirovo salt deposit

and the increase in the number of weak earthquakes

in the last 50 years is due to the intensive

exploitation of salt by leaching (injection of high

pressure water into the earth layers) which forms

huge under-ground chambers. This method of

extraction accelerates the ground deformation

processes around the salt body since as a result it

becomes lighter at its top part and leads to tensions

that are released through weak earthquakes. (Knoll,

1996)

In the framework of ground deformations

monitoring by geodetic methods in the area of the

Mirovo salt deposit high-precision methods and

modern GPS technology have been applied, which

provide quantitative data for displacements within

millimeters (mm) range. The GNSS data registered

by the a CORS station in the town of Provadia and

those in its vicinity within a radius of 50 km, namely

the stations in Varna, Shumen, Shkorpilovtsi, Aytos

and Dobrich are considered in regard with and for

comparison to the stable speed of the time series

from Eurasia'2005 from ETRS89 coordinates. The

geodetic monitoring showed almost identical values

for the movements of the mentioned GNSS stations

the Provadia station being the only exception

providing a velocity of 1.9 ± 0.5 mm/yr in direction

southwest and a significant subsidence with a

velocity of 7.9 ± 1.6 mm/yr. Based on these

observations, it can be concluded that most probably

the observed seismicity in the area of Provadia is an

induced seismicity as a result of intensive

exploitation of salt production in the nearby (3-4 km

away) Mirovo salt deposit. (Kostyanev et al. 2010;

Dimitrova et al., 2010)

A local geodetic network was built in 1988 for

monitoring the ground deformations in the area of

the Mirovo salt deposit. This a network for

horizontal determinations and currently consists of

26 points, which include 6 existing points from the

State Geodetic Network of the Republic of Bulgaria

and 20 new points that have been purposely built.

The new levelling network includes one century-old

benchmark from the first-class levelling network of

Bulgaria, one benchmark from the second-class

levelling network, several landmarks from the state

network, 3 newly built century-old landmarks, 37

leveling marks with special construction attached to

the drilling columns, 13 depth benchmarks with

fundaments 5 m below the surface, 123 surface

benchmarks at a depth of 2 m and 26 benchmarks

stabilized to the foundations of the points of the

horizontal network. The first cycle of geodetic

measurements was carried out in May 1990. The

changes in the coordinates of the points of the

geodynamic network show displacement velocities

that reach up to 35 mm/yr, with a mean square

deviation of ± 1 mm/yr. For measuring points 4, 11,

14, 16, 17, 18 are those for which maximum speeds

of 12.6-18.5 [mm/year] and maximum significant

subsidence for point 13 with a velocity of the order

of 24.3 mm/year (Atanasova-Zlatareva 2015).

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156

Figure 1: Local seismological network (green triangles), local geodetic network (red dots) and local geodetic network for

railway monitoring (red stars) in the area of the Mirovo salt deposit (red contour).

In (Valev et. al., 2015) it was underlined that “It

cannot be denied that the deformations (in the area

of the salt deposit) are mostly caused by technogenic

activities, but they would hardly have manifested

themselves to such an extent if there had not been

lateral tectonic pressure. This statement is justified

by the tangible values of deformations outside the

deposit.” In the same paper one more conclusion

was drawn that “… whether some of the boreholes

are exploited or not the velocities of vertical and

horizontal movements remain almost constant.

There is no relationship between the cessation of

drilling and the dynamics of relocations and after

several years of suspension of production in some of

the chambers”. In (Kostyanev et al., 2010) for

ongoing geodynamic processes in the same region it

was stated that “…it is necessary to determine what

part of the total value of deformation is the one of

technogenetic character and what is the part, which

is of tectonic origin”.

Figure 2: The geodynamic network for the area of the

Mirovo salt deposit - general scheme (Valev et. al., 2015).

Since for this particular case the main object of

research is the railway line passing through Provadia

Establishing Surface Displacements along a Railway Route near Mirovo Salt Deposit, NE Bulgaria

157

town and close to the salt deposit, which is part of

the route Sofia-Varna we will briefly comment on

the geodetic measurements performed on it in order

to guarantee its reliable functioning. In the last few

years (after 2010) an additional geodetic network of

36 points has been purposely built to monitor the

surface deformations along the railway line. On this

network levelling and GNSS measurements have

been performed at regular basis. Its points are

measured with greater frequency due to the higher

importance of this infrastructure site since it

concerns the public safety. The traffic on the railway

line is considered as dangerous dynamic load, which

can cause depressurization of the boreholes near it,

as well as there are problems with the safety of

trains. From the geodetic measurements made it was

concluded that the vertical and horizontal

displacements in the area of the railway line have

significant values. On October 7, 2010 an

earthquake event with magnitude 4.5 on the Richter

scale 4 , caused (or was caused by) fault movements,

probably led to a general relocation of the

benchmarks along the railway line and accelerated

the ground movements in direction towards the

central part of the salt deposit through which the

main fault passes. It is known that the subsidence

registered in the railway infrastructure located near

the mine workings is caused by tensile and

compressive deformations related to the lowering of

the upper layer of the mine workings. These

deformations lead to changes in tensions inside the

continuously repaired railway line.



3 DINSAR TECHNIQUE IN

MEASURING THE GROUND

MOTIONS IN PROVADIA

In the last decade, the interferometric approach has

been widely used in modern geodynamic research.

The method uses interferometrically processed data

from terrestrial or satellite radars with a synthetic

aperture. The purpose of the processing is to obtain

the change of the phase signal present in two radar

images obtained on different dates. Based on this

difference, information can be obtained about the

horizontal and vertical movements of the Earth's

crust and thus the dynamical behaviour of various

geophysical phenomena - earthquakes, landslides,

sink holes, etc.

To use the interferometric technique for SAR

data processing two radar images (often called the

interferometric pair - IFP) must be processed

together and the resulting height difference is

inferred from the difference of the phase component

of the backscattered signal of the two measurements.

This difference is directly related to the changes (if

any) that have occurred on the Earth's surface during

the time interval between the two acquisitions. It

should also be borne in mind that the movements

recorded in this way are in the line of sight (LOS) of

the antenna and cannot be interpreted directly as

movement in a horizontal or vertical plane.

The DInSAR (Differential Synthetic Aperture

Radar) approach used to obtain the results presented

below is based on data obtained from the Sentinel

1A/B mission for the period 2015-2021 (see Figure

3). Based on their expertise the authors used data

from online available SAR data archives maintained

by various institutions selecting the IFPs best suited

for further processing based on values perpendicular

baselines between all possible data sets for the

considered time intervals. Additional information

from digital maps and other data concerning the

topography of the study area was used to minimize

the negative effects (shadows, layover,

foreshortening) caused by the hilly landscape found

in the researched region during the DInSAR

processing.

Figure 3: Area of study covered by the used SAR images

of ESA’s mission Sentinel-1 ascending and descending

orbits tracks 36 and 58.

Another consideration that was taken into

account to mitigate the mentioned effects was to

process data from ascending and descending orbits

and to register the results from both together. To

create single interferometric image (IFI) data from

previously formed IFP is processed with DInSAR

method using the freely provided by ESA software

SNAP.

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For the study presented here a set of IFIs for the

late autumn/early spring periods for the years 2015

to 2021 was produced. This period was selected to

decrease the temporal decorrelation due to

vegetation present in the studied area, which is

predominantly agricultural land, leading to loss of

coherence. Other parameter we experimented with

were the DEMs used in DInSAR processing as

provided by the software (Nikolov et. al. 2017) in

order to increase the spatial resolution of the final

results. To improve the co-registration of the initial

single look complex SAR data from the primary and

secondary SAR data sets only precise information

for the satellite orbits at the time of their acquisition

were used.

4 RESULTS AND DISCUSSION

In order to facilitate the processing first a local

archive was created with more than 300 SAR data

sets from both types orbits of the Sentinel-1 mission

aquired in IW mode and stored in SLC format for

the area including the town of Provadia. (see figure

3) From this repository selected were only those that

met the criteria for short perpendicular baseline

between data in a single IFP – less than 20m. This

was an essential requirement since the smaller the

baseline is the better results concerning the

deformations are obtained (Vassileva, 2017). After

creating an interferogram from each IFP extracted

was smaller area that covers only the studied region

(see figures 1 and 3). This was done because this

way the next processing steps are performed in less

time and the results after unwrapping procedure are

more reliable. The latter is of particular importance

since at that step the interferometric phase is

transformed from (-π; π) to metric units thus

delivering values for the surface motions detected by

DInSAR. It was established in (Larsen, 2020, p106)

that the decomposition of the derived after DInSAR

phase signal in LOS into E-W, N-S and vertical

components is not trivial task being an ill posed

problem to derive three unknowns from a single

equation. To this end we used a simplified version of

the said equation making the assumption that the

dominant movement is subsidence and a formula as

(1) will provide reasonable results.

𝑑𝑖𝑠𝑝𝑙





𝑢𝑛𝑤 𝑝ℎ𝑎𝑠𝑒 ∗ 0.056

4∗𝜋∗𝑟𝑎𝑑𝑐𝑜𝑠



𝑖𝑛𝑐 𝑎𝑛𝑔𝑙𝑒







𝑚



(1)

Despite this simplification the interpretation of

the results should be made supported by as much

additional information as possible. For the studied

object it was not possible to acquire more data from

in-situ measurements and for this reason the authors

decided to use only the LOS information to assess

the displacements of the points and not to obtain

their exact velocities.

Further removed were the pixels (ground

elements) having low coherence which were

considered as influenced by temporal decorrelation

(see figure 4). The last step performed in SNAP was

the geocoding of the final results in geographic

coordinate system WGS84 which is needed to use

the results as images in external software products

for analysis and visualization.

Using the above mentioned steps a set of IFIs was

created at intervals of 4-5 months in order to establish

the deformations along the studied route of the

railway line as in (Atanasova,Nikolov 2016). The

validation of the information received from the

produced IFIs was performed using the data from the

local and CORS GNSS networks (figures 1 and 2).

The third and final phase of the study involves

the analysis of the information obtained after

DInSAR processing and is based on the obtained

deformation maps in metric units for the respective

period. Firstly, it should be emphasized that the

registered surface displacements are in the LOS of

the antenna and give information whether the points

are moving towards or moving away from it, and

secondly, they are relative to a point with precisely

measured deformation. Another practical

consideration

in interpreting LOS movements is that the actual

three-dimensional motion of the Earth (North, East,

Up) is an estimate and should interpreted with care.

For the pixels laying along the railway line from

six presented images that contain displacements in

LOS we calculated some statistics that allowed

conclusions to be drawn. First it was made clear

based on information from the coherence band of

each image that not for all said pixels the

information produced from SAR data could be

considered reliable since only the pixels in the

displacement band that have coherence above 0.3

could be trusted. In this case the largely varying

Establishing Surface Displacements along a Railway Route near Mirovo Salt Deposit, NE Bulgaria

159

Figure 4: Subset of displacement map for the period Oct 2018-10 March 2019 and geodetic points from railway route.

Table 1: Statistics for the pixels along the railway line (see figures 4 and 5) for the three studied periods.

Track 58 ascendin

Track 36 descendin

Nov2018

r2019

Nov2019

r2020

Nov2020

r2021

Oct2018

March2019

Nov2019

March2020

Nov2020

March2021

Valid

ixels 953 1338 1283 1266 2299 732

Statistics for the displacements in LOS [m]

min value -0.088 -0.055 -0.088 0.010 -0.047 0.023

max value -0.048 0.003 -0.020 0.067 0.019 0.122

range 0.041 0.058 0.067 0.057 0.066 0.099

Pixels that have data in all six ima

–

203

min value -0.088 -0.036 -0.086 0.017 -0.026 0.072

max value -0.075 -0.005 -0.053 0.031 -0.018 0.120

ran

e 0.013 0.031 0.033 0.014 0.008 0.048

Stddev 0.003 0.006 0.007 0.003 0.001 0.014

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

Oct 2018-10 March 2019 05 Nov 2019-28 March 2020 11 Nov 2020-23 March 2021

Nov2018

r2019 Nov2019

r2020 Nov2020

r2021

Figure 5: Subset of displacement maps created from SAR data satellite orbits 36 and 58 (black line is the monitored railway

line).

number of valid pixels could be attributed to the

length of acquisition periods and to the weather

conditions at the date of the acquisition. It could be

stated that based on the signs for every valid pixel

the overall movement could be produced – if the

signs from both geometries are the same the

dominant movement is vertical otherwise the

movement is in east-west direction (Vassileva,

2017). From the table provided it is made clear that

the seasonal movements, regardless of its type, don’t

exceed 0.1 m. In order to make proper comparison

between the results produced from ascending and

descending orbits of the satellite only the pixels that

had valid values for all six periods were considered.

It is seen in the same table that since values the

range and the standard deviation of the detected

movements for this pixels only is small which leads

to the conclusion that the movement is reliably

registered.

5 CONCLUSIONS

From the all above presented the main conclusion is

that the presented framework delivers reliable

information with regard to the ground movements in

the area of the Provadia town to all interested

stakeholders. Also it needs to be underlined the

possibility of regular monitoring of the region with

less financial and human investment.

ACKNOWLEDGEMENTS

This study was supported by the Contract No D01-

404/18.12.2020 (project “National Geoinformation

Center (NGIC)” financed by the National Roadmap

for Scientific Infrastructure 2020-2027. The authors

would like to express their gratitude to European

Establishing Surface Displacements along a Railway Route near Mirovo Salt Deposit, NE Bulgaria

161

Space Agency for providing Sentinel-1 SAR data

and geospatial processing software SNAP at no cost.

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