SATELLITE MONITORING FOR A SAFER CONSTRUCTION ENVIRONMENT · 2 Satellite monitoring for a safer construction environment 1009 charge of the seismic network and data processing for

SATELLITE MONITORING

FOR A SAFER CONSTRUCTION ENVIRONMENT

S. F. BALAN1, V. PONCOS2, D. TELEAGA2, R. NICOLAE3, B.F. APOSTOL1

1 National Institute of Research and Development for Earth Physics, Magurele-Bucharest, Romania 2 Terrasigna, Bucharest, Romania

3 RATEN – Centre of Technology and Engineering for Nuclear Projects, Magurele-Bucharest, Romania

Received October 12, 2015

The results of Permanent Scaterrers Interferometry (PSI) technique is employed

herein in order to identify some seismic risk features in certain zones of interest, such

as Bucharest city area, capital of Romania, and the Nuclear Power Plant in

Cernavoda. A comparison is also made between in-situ and satellite monitoring. A

dense sampling of the structures in terms of temporal deformation profiles is provided

and further used to assess the stability and resilience of buildings. All these

information are corroborated with seismic hazard maps in terms of peak ground

accelerations (Bucharest case), highlighting areas with high-risk probability.

Advanced satellite interferometric techniques help locate certain regional or local

anomalies exemplified by ground uplifting or subsidence. These movements can be

general when large areas are involved, (Bucharest city case), or they may occur on

small areas, (Cernavoda city buildings) or those related to the zone of lake “Lacul

Morii”, consisting of artificial filling areas.

Key words: synthetic aperture radar, satellite, earthquake, settlement, microzoning.

1. INTRODUCTION

This paper presents applications of the InSAR (Interferometric Synthetic

Aperture Radar) technique to mitigate the seismic risk of two important zones in

Romania. The first zone is Bucharest Metropolis, the capital of Romania, and the

second zone is Cernavoda, the site of a nuclear power plant.

The results presented in this paper originate in the work conducted by

Terrasigna Ltd, National Institute of Research and Development for Earth Physics

(NIEP) and Centre of Technology and Engineering for Nuclear Projects (RATEN

CITON) – some of the participants in the project “Spaceborne Multiple Aperture

Interferometry and Sequential Patterns Extraction Techniques for Accurate

Directional Ground and Infrastructure Stability Measurements”. Terrasigna applied

the Persistent Scatterrer Interferometry (PSI) technique to extract the ground

deformation in Bucharest during July 2011 – January 2013, and at Cernavoda from

June 2013 to June 2014, from series of TerraSAR-X satellite images. NIEP is in

Rom. Journ. Phys., Vol. 61, Nos. 5–6, P. 1108–1119, Bucharest, 2016

2 Satellite monitoring for a safer construction environment 1009

charge of the seismic network and data processing for the whole Romania, using a

consistent number of seismic digital stations both in Bucharest and at Cernavoda

site; RATEN-CITON is responsible of the observations on Cernavoda grounds and

their data processing.

Both locations, Bucharest Metropolis and Cernavoda, are prone to strong

earthquakes from Vrancea region as those in the last century: November 10, 1940,

magnitude 7.4 on the Richter Scale; March 4, 1977, Mw = 7.4; August 30, 1986;

Mw = 7.1 and May 30, 1990, Mw = 6.9. In the seismic event of 1940 some several

hundred victims were reported; in 1977, about 1500 people died and major

building damage of both earthquakes were recorded, most of them in Bucharest.

2. GEOLOGICAL DATA ABOUT BUCHAREST AND CERNAVODA

Bucharest, the capital of Romania (Fig. 1), with more than 2.5 million

inhabitants, is considered, after Istanbul, the second-most earthquake-endangered

metropolis in Europe. It is identified as a natural disaster hotspot by a global study

of the World Bank and Columbia University [1]. All disastrous earthquakes, as

those presented above in the 20-th century, are generated within a small epicentral

area – the Vrancea region – about 150 km northeast of Bucharest, and about

250 km northwest of Cernavoda (Fig. 1).

Thick unconsolidated sedimentary layers in the area of Bucharest amplify the

seismic shear-waves which may cause severe destruction. [2, 3, 4] Thus, disaster

prevention and mitigation of earthquake effects is an issue of highest priority for

Bucharest and its population. Bucharest is located in the central part of Plain Vlasia

(Fig. 1), which is part of the Romanian Plain, and is approximately 165km from

Vrancea epicentral zone (Fig. 1). Plain Vlasia is considered by some authors [5, 6]

a transition zone between the northern piedmont plains and the Danube plain in the

South. It runs between Prahova valley north and Arges valley in the south and is a

continuation of the common alluvial cones of the rivers Ialomita and Dambovita.

Along these watercourses the altitude decreases slightly, with small fluctuations

from northwest to southeast and a relatively constant average slope value (0.5–

0.6 m/km). Cross-slopes have high values, especially on the right riverbank.

In the city, Dambovita valley looks like a long corridor approximately 22 km

long. Its width varies from 650 m opposite the Botanical Garden, to about 4 km at

the eastern end of the village Catelu (before being regularized in the last century).

There are lakes in the city, for example at the Park Carol and Youth Park, on

the right bank of Damboviţa, and Lake Cismigiu on the left bank. The lake “Lacul

Morii” (the Lake of the Mill) is an unusual feature, formed by the dam on river

Dambovita in the Ciurel zone, in the 20th century.

Geological, geotechnical and hydrogeological drillings in the city have made

it possible to know what is included in successive subsoil deposits.

1010 S.F. Balan et al. 3

Fig. 1 – Location map of Bucharest and Cernavoda in Romania.

Cernavoda area is located in the great geological, morphological, tectonic

and structural unit South Dobrogea. This structural unit is bounded on the North by

Ovidiu-Capidava fault, South by Shabla-Calarasi-Urziceni partially identified fault,

at West by the Danube fault and East by the Black Sea. The folded fundament of

this region consists of weak metamorphic sedimentary series of Proterozoic age,

known as green schists. These types of sediments have been encountered in a

borehole on the right side of Danube River, near Cernavoda city, at 1192 m depth.

From lithological point of view, the zone consists of an alternation of sandstones and

chlorites schist.

The sedimentary layer, discordant over the Palaeozoic fundament, consists

of deposits belonging to periods Jurassic, Cretaceous, tertiary and quaternary.


Quaternary is represented by inferior Pleistocene deposits which consist of

reddish clays with limestone concretions of no more than 5 m thickness, covered by

loess up to 45 m thickness, belonging to medium and inferior Pleistocene.

From the structural point of view, Dobrogea belongs to Wallachia-South

Dobrogea sector. In this unit there are two tectonic elements, namely a heavy folded

fundament and a sedimentary layer with slightly or even unfolded deposits.

From a morphological perspective, the site lies within the floodplain of Carasu

Valley, characterized by a flat relief, with low slopes, which does not encourage rapid

deployment of geomorphological processes.

3. SATELLITE MAPS AND PROCESSING

The development of spaceborne Synthetic Aperture Radar (SAR)

interferometric techniques started in 1992 after the launch of the ERS-1 mission of

the European Space Agency and since then it is continuously advancing and

gaining importance in geosciences. DInSAR (Differential InSAR) techniques have

been successfully applied for measuring the effects of a large spectrum of

phenomena, such as: deformation induced by volcanic activity, co-seismic and

post-seismic motions, glaciology, mining and groundwater related subsidence as

well as measurement of soil moisture.

However, since the SAR measurements may be affected by residual

topography and atmospheric perturbations, new techniques using large datasets

were developed. Permanent Scatterers Interferometry (PSI) [7] was the leap the

SAR technology needed to enter the domain of Geodesy where temporal

deformation profiles with millimetres accuracy are possible. TERRASIGNA

delivered InSAR-based results in terms of deformation maps showing the rate of

the displacement field with applications to landslide monitoring [8], mine tailing

ponds monitoring [9], urban site monitoring, or Danube Delta monitoring [10].

Based on the ground deformation satellite map, we can identify some seismic

risk (hazard and vulnerability) in zones of interest, in our case Bucharest

Metropolis and Cernavoda. The map reveals local ups and downs on the surface,

which could have different causes.

If there is a large settlement and structural cracks would appear, even almost

unnoticed, and an earthquake of magnitude more than 7 would come, (3 of such

events were in Romania in the XX-th century: in the year 1940 with magnitude

MW = 7.6; year 1977, with magnitude MW = 7.4, year 1986, with magnitude MW =

= 7.1) then the buildings could suffer damages.


This is why it is very useful to know that a large settlement have occurred,

because it could affect structure of the buildings and, in the case of a strong

earthquake, may increase the risk of damage.

Fig. 2 – Persistent Scatterrer Interferometry displacement map of Bucharest

(July 2011–January 2013).

From Persistent Scatterrer Interferometry (PSI) displacement map of

Bucharest surface (Fig. 2) we could notice that the N – W part tends to lift and the

S – E part tends to lower. These trends cannot be attributed to earthquakes, being

of moderate magnitude and in small numbers in the time range considered: July

2011–January 2013.

The trends seen in Fig. 2 are due to basement lithology and geological

composition of Bucharest.


Fig. 3 – Map of mean deformation rate for 395 radar points located on Nuclear Power Plant

Cernavoda (NPP-Cernavoda) Office Building.

Fig. 4 – Displacement profiles for 7 radar points corresponding to leveling mark P2.

In the case of Cernavoda site, satellite monitoring is more complete, being

accompanied also by in-situ monitoring.

For exemplification we present such a study for an office building,

belonging to Cernavoda compound.

In Figure 3 radar measurements are presented (395 points), in Figure 4

displacement profiles are shown for 7 radar points corresponding to leveling mark

P2, and in Figure 5 in-situ measurements are included (8 leveling marks) for an

office building in the Nuclear Power Plant (NPP) Cernavoda compound during the

period June 2013–June 2014. Level mark P2 could be observed in both satellite and

in-situ measurements.


Fig. 5 – Vertical displacements measured on 8 levelling marks from the Nuclear Power Plant

Cernavoda Office Building (left) during May 2013–May 2014.

Another example is the comparison between the measurements obtained with

satellite-radar and in-situ in a point on Interim Radioactive Waste Storage Building

(DIDR) of the NPP Cernavoda. In Figure 6 we can see the building on the upper

right part and in Figure 7 we could observe the temporal displacement profiles for a

point (D10) on the building obtained from in-situ and radar measurements during

2002–2009.

Fig. 6 – Interim Radioactive Waste Storage Building (DIDR) of the NPP Cernavoda

(upper-right part of the figure).


Fig. 7 – Temporal displacement profiles for the point D10 on the building obtained from in-situ and

radar measurements during 2002–2009.

In the map of Figure 2 we may notice red colored areas near river Colentina. The red colored areas according to figure caption are areas which are subsiding in the time of measurement more than those in their immediate vicinity. According to our interpretation (Fig. 8), in these areas the settlement phenomenon on relatively soft soil is noticed, mainly areas with artificial fillings, near actual or former riverbeds. The artificial filling areas are also noted in the seismic microzonation map of Mandrescu et al., 2007 [11], (marked by a black contour line) but now their instability is clearly proved by the Persistent Scatterrer Interferometry result (Fig. 8).

Fig. 8 – PSI deformation during 2011–2014 of the central part of Bucharest; this zone is overlaid

with a layer of old artificial fillings areas (in Bucharest, near river Colentina).


On Cernavoda site we could observe also by satellite a slow-subsidence (red)

and risk of land-slides (blue) (Fig. 9).

A particularly interesting situation seen in Figure 10 is an area near lake

“Lacul Morii”, city of Bucharest, where a portion of land is sinking in an area that

rises. The diagram made by Terrasigna Ltd. shows that this portion sunk with 10

mm in the period July 2011–January 2013 compared to surrounding areas. In this

situation satellite measurements are of particular importance, highlighting a local

area with risk due to descending movements in which it is involved.

Further, we have used a seismic hazard map of Bucharest Metropolitan Area

for peak ground accelerations (PGA) (cm/s2) from Marmureanu et al., 2010 [12].

The earthquake used to generate this map was a synthetically one, with magnitude

MGR = 7.5, from Vrancea source, with strong and well-defined acceleration in the

area. We chose this option, the strongest design earthquake, because this is the only

way we can get an image of the high acceleration areas of the Bucharest

metropolitan area.

Fig. 9 – Deformation map of Nuclear Power Plant Cernavoda area (June 2013–June 2014).

This acceleration map from Marmureanu et al., 2010 [12] is superimposed

on Fig. 2 resulting in Fig. 11.

What could be seen on this overlap of maps are large accelerations, between

250–280 cm/s2 and uplifting areas (up to ~2mm/year), situated in the north-western

part of the city, and between 270–290 cm/s2, in areas going down between 2 to 3.9

mm/year, in the south-eastern part, in the former floodplains of Dambovita. The


amplifications of the seismic signal, generated by these soft soils, may increase the

risk of damage to buildings in the area, in the case of strong earthquakes.

Fig. 10 – Detail from deformation map in Fig. 2, with temporal displacement profile representative

for a local subsidence in the northern region of the Lake Lacul Morii.

Fig. 11 – Acceleration map from Marmureanu et al., 2010 [12] superimposed

on radar deformation map from Fig. 2.


4. CONCLUSIONS

The points discussed in the paper show the great importance of satellite

observations, which identify areas with hazard potential generating seismic risk.

The seismic risk considered here includes local seismic hazard places, with high

acceleration and vulnerability of buildings in Bucharest [13] and Cernavoda area.

For zones showing subsidence, especially if their spatial extent is well

delimited, a settlement might have occurred, because of natural soft land or man-

made causes (artificial fillings). It is very important to locate these spots and to

know about these trends because the soil strata move there. It is also important to

know what kind of buildings are located in the area, what kind of structure they

have, if they were affected before by repairs or previous earthquakes. These soil

movements can produce extra stress in the building structure and during an

earthquake, this extra stress can develop some extra damage (cracks in different

structure elements, displacements of interior walls, damage in floors and interior

installations, etc.). If this damage is non-structural it will be repaired quickly, but if

the deterioration is in structural elements, it can endanger the stability of the

building.

The vulnerability of buildings could emerge if they are subjected to stress-

strain induced by local settlements on which overlap stress-strains from strong

earthquakes. High resolution InSAR data, as those used in the Cernavoda

monitoring, are useful to identify non-uniform settlements of individual buildings.

We have made in this paper a comparison between in-situ and satellite

monitoring. Conventional methods of monitoring are limited to visual inspection

and topo-geodesic measurements through geometric leveling method, in only a few

points from the structure.

By applying Persistent Scatterrer Interferometry technique a very dense

sampling of the structure could be provided. These observations can be used to

assess the stability and resilience of structures, leading to a better risk assessment.

Advanced InSAR interferometric techniques help locate certain regional or

local anomalies exemplified by ground uplifting or subsidence. The satellite

observations certified some previous known artificial fillings areas, identified by

Mandrescu et al., 2007 [11]. These movements can be general when large areas are

involved, (Bucharest case), or they may occur on small areas, (Cernavoda

buildings) or those related to the zone of Lake Lacul Morii, consisting of artificial

filling areas.

Acknowledgements. This work was supported by the Romanian Executive Agency for Higher

Education, Research, Development and Innovation Funding (UEFISCDI) through the project

Spaceborne Multiple Aperture and Sequential Patterns Extraction Techniques for Accurate

Directional Ground Control and Infrastructure Stability Measurements (DGI-SAR), contract no.

200/2012, Partnerships Program, PN-II-PT-PCCA-2011-3.2-1448.


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SATELLITE MONITORING FOR A SAFER CONSTRUCTION ENVIRONMENT · 2 Satellite monitoring for a safer construction environment 1009 charge of the seismic network and data processing for