APPLICATIONS OF SPACE GEODESY METHODS IN ROMANIA [304837]

APPLICATIONS OF SPACE GEODESY METHODS IN ROMANIA

Dr. Narvic Doru Mateciuc1, Dr. Andrei Bălă1

1[anonimizat]: [anonimizat], Romania,

ABSTRACT

The studies for Recent Crustal Movements by the aid of space geodesy techniques are integrated into the researches on the Earth’s [anonimizat] a reflection of the complex geodynamic phenomena that occur in the crust and in the subcrustal lithosphere. [anonimizat] a [anonimizat], to the determination of the soil stability degree in inhabited areas or in those in which large industrial targets and utilities are intended to be placed in. The knowledge of movements affecting the Earth’s [anonimizat], [anonimizat] a topical issue. [anonimizat]. There are presented some of the most significant results achieved in the framework of each research project together with the limitations imposed by the used acquisition technology. [anonimizat] 1994, in a [anonimizat]'s involvement in a [anonimizat] a [anonimizat]461, also continued after 2003 with the continuous help of the University of Delft (Nederland), a [anonimizat] a valuable support for the geodynamic studies. There are made brief references to the first application of the Finite Element Method in a GPS subnet from the Vrancea extended network together with some of the most important obtained results. [anonimizat] a [anonimizat], [anonimizat], to solve the very complex problem of the earthquake forecasting.

Keywords: [anonimizat], GPS, Earth’s crust.

INTRODUCTION

The most important objective of this paper is to discuss the main applications of geodetic measurements made by spatial geodesy techniques in Romania.

The applicability of GPS measurement has proved its usefulness in the determination of the Earth’s [anonimizat], [anonimizat], etc.

A particularly important emphasis is placed on the country's [anonimizat], for an information supplement in order to clarify the geodynamic context in which the crustal and subcrustal tensions in the region's lithosphere accumulate.

[anonimizat], which come into contact with this area located at the Oriental Carpathians curvature.

[anonimizat] are part of the Vrancea area and not only has been accomplished over the years through major international research projects, as they are below described.

GPS Studies in Romania – The Beginnings

The first applications using GPS technology on the Romanian territory were made in 1994 under the auspices of the U.S. National Geodetic Survey (NGS) – and with the participation of the Romanian Ministry of Agriculture (), former Institute of Geodesy, Cadastre, Photogrammetry and Cartography (ICGFC, formerly IGFCOT) and the Direction of Military Topography (DTM).

All observations were conducted in 7 locations (Constanța, Dealul Piscului, Moșnița, Osorhei, Sfântu Gheorghe, Sirca and Stănculești) considering the standard precision allowed for the NGS A order – (5 mm 1: 10,000,000). Geographical locations of the observatories were primarily made considering the uniformity distribution criterion in the territory; at each observation point there was also a first degree geodetic point. The main station is located in the proximity of the Military Astronomical Observatory  (Dealul Piscului – Bucharest) and was set up as the origin for the entire Romanian network for further GPS data processing.

The observation data processing was performed at NGS (Silver Spring, Maryland) using 4 different measuring time intervals (i.e. 26 September 1994, 0900 UTC on 27 September 1994, 0900 UTC) by the aid of a specialized S software installed on an HP 9000/700 working station [1]. All computed coordinates for the observation stations have been determined in the ITRF92 – 1994.7 system.

The coordinates of the network main station (Dealul Piscului) were determined by using the spatial positions of the three support points (ONSA, MADR, WETT – GPS permanent stations) for every time interval of the four observation measurements.

The four data sets average was considered to be the final set of coordinates in the ITRF92 – 1994.7 system. Once the coordinates of the Dealul Piscului point have been fixed, they were used to add more measurement points to the network by calculating the position of the other stations reported to the coordinates of the reference point.

The CERGOP Project

GPS geodetic satellite studies in Romania have been continued in 1995 as part of an international European project, OP (Central European Geodynamic Project) designed for geodynamic research in Central and Eastern Europe.

The organization structure of the CERGOP included a Steering Committee, an International Project Working Group (IPWG), study groups (CSG) and data processing centres. The CERGOP Study Groups (CSG) are [2]:

CSG1 – Investigation and tropospheric delays,

CSG2 – CERGOP site quality monitoring,

CSG3 – CERGOP reference frame,

CSG4 – Standardization of data and processing centres,

CSG5 – Permanent and epoch GPS CERGOP stations,

CSG6 – CEGRN and height determination,

CSG7 – CERGOP gravity network,

CSG8 – Geotectonic analysis of the region of the Central Europe,

CSG10 – Monitoring of recent crustal movements in Eastern Alps with GPS,

CSG11 – Three dimensional plate kinematics in Romania.

The CEGRN geodetic network (Central European GPS Geodynamic Reference Network) was materialized in 1994 and originally covered the territories of the ten countries – Austria, Czech Republic, Croatia, Germany, Italy, Poland, Slovakia, Slovenia, Ukraine and Hungary, consisting of 31 GPS benchmarks. Bulgaria participated as associated country. The number of observed GPS sites increased up to 45 sites (in 1997, including additional sites).

The pilot phase of the CERGOP started in 1994. Since 1995 CERGOP has got financial support from EC (European Community) in the frame of EC COPERNICUS Project [2] for three years (1995, 1996, 1997).

The main CERGOP Data Centre was located in Graz (Austria) where all the CEGRN GPS data were collected and disseminated by electronic access. In the same time a number of three (1994) to eight (1997) additional processing centres were established in participating countries. Four data processing standards were established. The software used in this project was Bernese (versions 3.4, 3.5, and 4.0), except for the group in Italy, which used MicroCosm software [2].

The main scientific goals of the CERGOP project were as follows:

geodynamic research interpretation in the tectonic context of Central European region,

investigating of the most profound geotectonic features in the Central European region, the Tornquist – Teisseire area structure, the Pannonic Basin, the Carpathians system, and their relation to the Alpine – Adria region,

creating a stable reference framework for subsequent geodynamic studies, local or across country borders for investigations and deformations studies,

establishment of extensive collaborative activities and exchanging data and information,

joint publishing of the results and distribution to more than 300 scientific institutions and research groups;

creation of a database for future geodynamic investigation projects and related researches of the CEGRN network.

Romania participated in the CEGRN campaigns in 1995, 1996 and 1997 with 8 GPS epoch stations [4]. The initial CEGRN network on the Romanian territory was formed of 5 benchmarks located along the Carpathian Orogen area (Tismana – TISM, Fundata – FUND, Vrancea – VRAN, Vatra Dornei – VATR and Gilău – GILA), one benchmark in the Northern Dobruja – Orogen (Macin – MACI), one in the Moldavian Platform (Iasi – IASI) and the last one in the Moesian Platform (Măgurele – MAGU, in the southern part of Bucharest city) (Fig. 1).

The planning of the GPS observation campaigns was performed by consensus of all the participants in the CERGOP, during special meetings organized before the beginning of each campaign. The duration of the observations was at least five full days.

The location of these stations were chosen by mutual agreement between the geologists, geophysicists and geodetic specialists from the Geological Institute of Romania (IGR), the National Institute for Earth Physics (INFP), the Faculty of Geodesy (Technical University of Civil Engineering, Bucharest – TUCE) and the Bundesamt für Kartographie und Geodäsie, Frankfurt am Main, Germany (BKG) [2].

The CEGRN network in Romania was designed to meet a number of mandatory requirements imposed by geodynamic research:

the locations must cover most of the geological units of the territory on which it is installed;

the locations must be installed outside the cities to minimize the noise;

the existence of radio obstacles located at more than 150 inclination to the horizontal plane is not allowed, for a good visibility of the GPS antenna;

there must be an access road for vehicles, open all year long;

there should be no problems regarding access to the property on which the GPS benchmark is installed throughout the year.

Fig. 1. The final CEGRN Geodetic Network in Romania, ▲- 1st phase observatories,

ș – new 2nd phase observatories, ș – permanent stations, ○ – towns.

In this geodetic network satellite observations during the years 1995, 1996, 1997, 1998 and 2001 have been performed. All observation equipment – SSE and SSI Trimble GPS receivers – were made available by partners of the German Federal Agency for Cartography and Geodesy (Frankfurt am Main) [3]. Field measurements were carried out by German specialists from the University of Karlsruhe and the Romanian specialists from the Faculty of Geodesy and the National Institute for Earth Physics in Măgurele. The Romanian CEGRN network was completed in 1997, 2000, 2002 and 2003 in the frame of CERGOP – 2 Project, targeting the movements monitoring the Vrancea seismogene area in the first decade of the 21st century.

As the CEGRN network is a global, international GPS network, it is obvious that it cannot meet all the specific requirements for highlighting the appropriate geodynamic behavior of a restricted, small area, such as the Vrancea seismogenic zone, where the Vrâncioaia observation point (VRAN) is only located.

Here it can be concluded that the calculated relative speeds are very small, on the order of 3 mm/year, with the remarkable exception of IAS3 and TIS3 locations, but here the approximately high observed values could be explained by the influence of the eccentricities which have seriously affected the measurements. Regarding the vertical movements’ calculations there were compared by using a Helmert transformation the computed campaigns solutions with an average solution according to the 1995.89 epoch. It can be easily seen that the movements recorded on the vertical component have generally low values, on the order of ± 5 mm for the considered time span.

However, these values are comparable to the noise level affecting the vertical component and, even if it might highlight certain tendencies of movement, they are difficult to confirm because of the relatively short period of time for which the analysis was carried out. The estimated noise level, about 10 mm, was surpassed only in the case of VRAN and VATR stations.

It can be concluded from these determinations that both the horizontal movements and the vertical ones are highly comparable to the noise level and are not significant [5].

GPS Studies in the Vrancea Area

Since 2001 the GPS measurements in the Vrancea network were carried out within the framework of the research project SUBDUCT (Surface Behaviour and Dynamical Units of the Southern Carpathians Tectonics) initiated by the Dutch Centre for integrated research on Solid Earth Science (ISES) and the University of Delft (Netherlands) [6].

The goals of the project have been fulfilled in collaboration by the Romanian experts belonging to the Faculty of Geology and Geophysics from the University of Bucharest and the National Institute for Earth Physics. The main objective of the project was related to the monitoring, analysis, and interpretation of surface movements caused by the dynamics of the lithosphere crust – active in the region of Vrancea (South Eastern Carpathians) [7].

The SUBDUCT project was accomplished in collaboration with the Institute of Geodesy in Karlsruhe (Germany), which led another international programme, namely 461 – "Strong Earthquakes: A Challenge for Geosciences and Civil Engineering"; this program involved GPS data acquisition for the region under investigation, starting from 1997, when 28 GPS points were installed, covering an area of 350 x 350 km2 located in eastern part of Romania, centered on the Romanian most important seismic area (Fig. 2, Table 1).

In 1997 the designed geodetic network was centered on the Vrancea seismic area; it has originally consisted of 28 points, which has also to cover some of the neighbouring regions of the concerning zone. In this network configuration have been included 5 special points belonging to the CEGRN European network [6], points measured from 1995 in the framework of the CERGOP International Project at whose achievement Romania has also participated.

The situation of the installed GPS observation points in the Vrancea 2000 network, in its extensive variant, is presented in Table 1 and Fig. 2. We must notice that only 30 of the 35 GPS observatories of the Vrancea 2000 geodetic network have been presented in Table 1, points which will be subject of the main further discussion.

Fig. 2. The Vrancea GPS network (1997-2004), ●,●- GPS observatory belonging to Vrancea network, ș- GPS observatory belonging to CEGRN network, ♦ – GPS observatories not included in Vrancea network, ○ – towns.

In 2002 the Delft Institute for Earth – Oriented Space Research (DEOS) joined the team and 19 new stations were added to the existing CEGRN network, including five permanent stations [8].

The final Vrancea network was composed of 54 GPS measurement points, of which 6 permanent stations, measured between 2001 and 2004.

This geodetic network has been measured over the period of time 1997 – 2004 by the Dutch specialists of the University of Delft, as well as by the specialists of the German Institute of Geodesy from Karlsruhe University and specialists of the Romanian National Institute for Earth Physics.

Data processing and analyses was accomplished by using different processing strategies by the Karlsruhe University and the Delft Institute groups, [8] and [9]. Since the expected vertical and horizontal displacement signal is in the order of 1-5 mm/year, the errors bars of the GPS stations might be larger than the signal.

The Vrancea 2000 network consists of two sectors, separated by the Trotuș Fault, which are very different from the geodynamic behaviour point of view.

The northern sector, materialized in Vrancea Nord subnet, where the GPS observatories: Babușa (BABU – 3), Tazlău (TAZL – 13), Moinești (MOIN – 23), Pogana (POGA – 4), Vatra Dornei (VTRA, not included in the network, due to the destruction of the main benchmark), Feldioara (FELD – 15), Berești (BERE – 5), Potoci (POTO – 2) can be included, characterized by small to very small vertical displacements, in the order of 0.9 mm / year.

The southern sector, materialized in the Vrancea Sud subnet where the GPS observatories: Zăbala (ZABA – 27), Cheia (CHEI – 19), Independența (INDE – 6), Mănăstirea Cașin (MANA – 24), Vrâncioaia (VRAN – 25) ), Mihăilești (MIHA – 18), Măcin (MACI – not included in the network due to the destruction of the main benchmark), Voșlobeni (VOSL – 22), Cleja (CLEJ – 14), Garoafa (GARO – 16), Tușnad (TUSN – 28), Gura Văii (GURA – 26), Balta Albă (BALT – 17), Iazu (IAZU – 7), Fundata (FUND – not included in the extended network due to the destruction of the main bechmark) can be included, where vertical displacements are much higher than in the northern sector.

Tab. 1. The Vrancea 2000 GPS network.

Figures 3 (a, b, c) illustrates the position of the Northern Vrancea subnet, Southern Vrancea subnet, and Extended Vrancea 2000 network, highlighting the heights of the measurement points.

In all measurement campaigns Leica XRS – 1000 GPS receivers with antennas AT – 504 were installed. There were performed seven measurements campaigns (1997, 1998, 2000, 2002, 2003, 2004, and 2006) until 2006 using Leica receptors 300 & 500 and the corresponding antennas.

The GPS studies carried out during 1995 – 2004 have provided useful information on local tectonic movements in the Carpathians Arc bend, characterized by lower values, less than 1 mm/year for the horizontal component and 3 mm/year for the vertical component.

The interpretation work accomplished by [8] in 2005 displays the horizontal GPS displacement with respect to a fixed Eurasian plate. A certain movement of the block between the Intramoesian fault and the Peceneaga – Camena fault towards south-east is identified.

The data processing and interpretation performed by a group from Karlsruhe University in 2007 evidenced no clear movement of this block toward south-east [10]. Most of the GPS stations within this block in the foreland show displacements in the order of 1-2 mm/year. Exceptions are two stations in the centre of the Vrancea area, located above the subducting slab, that show a south-eastward displacement of 4-5 mm/year. The vertical GPS observations are partly in agreement with the results from other methods such as leveling and geomorphology. They show a normal subsidence in the Focsani Basin, in which sedimentary complex is down to 18 km depth, and uplift in the Vrancea region, i.e. in the Neogene flysch nappes (accretionary wedge) of the SE-Carpathians [10].

The idea that the block between the Intramoesian fault and the Peceneaga – Camena fault is having any recent horizontal movements is rejected also by Romanian group [11] and [12]. Working on the revised catalogue of earthquake mechanisms for the events occurred in Romania until the end of XX century (REFMC), the authors have established that the earthquakes are not generally occurring along the major faults in Moesian platform and if present, the crustal earthquakes do not show a strike-slip regime, so there are not enough evidence that the block have a current horizontal movement, after [11] and [12].

The data have revealed some movements, along the Intramoesian fault, with a rate of 1 – 2 mm/year, in accordance with the values obtained from geological studies concerning the Pliocene – Quaternary formations kinematics.

A significant change in the horizontal direction of the movements occurs near the Southern boundary of the Moldavian Platform (the Eastern sector of East – European Platform), besides the Trotuș Fault.

GPS data have shown a senestral sliding domain, also highlighted in geological and neotectonic studies; the latter have revealed the existence of a NW – SE orientated corridor inside that senestral deformations took place in the period of time associated with upper Miocene – Quaternary [8].

a).

b).

Fig. 3. a). Northern Vrancea GPS network map, the finite elements mesh which will be used to compute the strain parameters, ● – Northern Vrancea network nodes; ● – CLEJ GPS measurement point which is part of Southern Vrancea network;

rightside – height scale.

b). Southern Vrancea GPS network map, the finite elements mesh; ● – Southern Vrancea network nodes; rightside – height scale.

The above mentioned corridor is separating the stable or raising areas belonging to the Scythic domain and the East European Platform from the subsidence and the SSE movements areas belonging to the Focsani depression.

From the vertical movements point of view it has been revealed the existence of alternative lifting and sinking areas (subsidence) in the south – eastern part of the Eastern Carpathians, with NW to SE orientations.

Fig. 3. c). Extended Vrancea 2000 GPS network map, the finite elements mesh, ● – extended Vrancea 2000 network nodes, ● – GPS observatories, not included in the network; rightside – height scale.

The subsidence areas overlap on the Focșani and Brașov basins with considerable accumulation of Pliocene – Quaternary sediments [13].

The most significant subsidence observed in the Focșani depression has indicated vertical speeds in the range of 2 – 3 mm/year.

The subsidence trend is common to the eastern sector of the Moesian Platform, but with much reduced values compared to those observed in the Focșani – Odobești area. The Perșani Mountains area, located in the South – Eastern Carpathians, which corresponds with the most significant lifting movements domain, during Pliocene – Quaternary, is highlighted between all lifting areas.

The high velocities recorded within the vertical movements revealed at the Carpathians curvature probably indicates the lowering of the litospheric plate in this region, whose displacement is responsible for the subcrustal activity in the Vrancea region.

A significant change in velocity is recorded at the limit between the Dobrogean domain and the East European platform near the Trotus Fault, indicating a sinistral strike-slip zone.

This is compatible with geological and neotectonic studies which suggest increased late Miocene-Quaternary sinistral transtensional deformation along a NW – SE oriented corridor limited southwards by the Trotus Fault [7]. This zone accommodates and separates the horizontal stable and uplifting East European/Scythian domain from subsidence and SSEward movement of Moesia in the area of the Focsani basin. For the Transylvanian domain, most of the velocities indicate small residual motions with respect to stable Eurasia.

According to [7], it appears to be possible that the time series analysis might be too short for a realistic interpretation of the geodynamic results; future campaigns will improve performances through an increase in the accuracy with which both horizontal and vertical velocities are determined from this very important tectonic area of the Carpathians.

In the PhD thesis elaborated by Mateciuc (2010) the crustal strain analysis in an extensive network based on GPS measurements carried out in all campaign measurements have been discussed in [14].

The geodetic network was made up of 35 benchmarks located both in the Orogenic area (16) and in the Vorland area (19), in the Carpathian Foredeep, Moesian Platform, Moldavian Platform and Transylvanian Basin). This analysis was performed using the Finite Element Method (MEF).

In the case of geodetic measurements the finite element type has been chosen as the triangle shape, as close as possible to the equilateral triangle which represents a theoretical constraint, defined in a two – dimensional space represented by the land area between the considered GPS benchmarks.

The computing of the deformation parameters (strains) inferred only from geodetical measurements shall be made on the basis of the knowledge of the analyzed points position at least in two different epochs or by knowing of the movement speeds of the finite elements’ nodes.

In the extended Vrancea network the strain parameters for all those 42 finite elements which forms the network’s mesh were computed; the section under investigation covered an area marked out by Bucharest – Pitesti – Tg. Mures – Galați – Bucharest cities.

For each finite element the directions for the main maximum and minimum strains have been mapped out. It should be noted here that always, by convention, positive values are assigned to extensions (dilatations) and negative values to compressions.

The computation of the strain parameters’ values have enabled the development of two crustal strain maps of the area, namely the Vrancea extended network. Thus, one may notice that 90% of the values of maximum principal strain 1 are positive, extensional, and those belonging to the minimum principal strain 2 are mainly negative, with a compressional behavior [15].

Another significant observation that could be made is the correlation between the area of intense crustal seismic activity and the strong anomalies emphasized in the crustal strains.

The short time period of only 2 years which was the subject of the study has limited the quality of the analysis, highlighted by the absence of any spectacular phenomena in the crustal strain analysis; more conclusive results will be obtained from tests carried out for longer periods of time.

GNSS PERMANENT Stations Installed after 2012

The permanent stations network in Romania was initiated in 2001 by the first GPS Observatory installed in Lăcăuți. The network was designed with respect to the idea of constantly adding of new stations, for national general targets; 24 stations are in use so far (Fig. 4) and other stations are planned to be installed in the next future.

The main purpose of the measurements is related to the three-dimensional monitoring of crustal movements with high accuracy required by fundamental geodynamic research necessary for the national territory where are not highlighted significant displacements. As the newly installed stations are in operation for only a short time, data analysis is still in the beginning, as it requires a longer working time to ensure that the data provided will enter the required precision.

Fig. 4. Romania's NIEP permanent stations network,

● – GNSS operational observatories, ● – old GNSS observatories (not in use), ○ – towns.

Most stations are installed on a concrete pillar monument, embedded in the non-weathered rock, where possible. As far as possible one has been tried to avoid the installation of antennas on buildings or other type of man-made constructions, due to the possibility of the seasonal oscillation phenomenon.

The stations of the INFP network have mixed equipment, the majority being those produced by Leica, GRX1200GGPro and GRX1200 + GNSS receivers, and the antenna models used are LEIAT504, LEIAT504GG, and LEIAR10. The network also has a mobile Leica GX1230 station with AX1203 + GNSS antenna, and recently received a GR10 Professional receiver also provided with a LEIAT504GG antenna. At the same time, three stations operate with equipment manufactured by Septentrio, AsteRx2HDC receivers and PolaNt-x MF antennas.

Data acquisition is done in real time, in RAW DATA and RINEX format using the Leica GNSS Spider and Septentrio Rx software, and the analysis part is performed with the Spider QC program.

CONCLUSIONS

After this retrospective of the activities which involve the use of the space geodesy techniques in Romania it can be concluded that their development was very strong, starting from experimental works using a small number of observatories, to networks of tens, even hundreds of measurement points, in the most diverse types of activities.

The National Institute for Earth Physics (NIEP) operates a strong regional GPS/GNSS reference network for monitoring the Carpathian-Danubian-Pontic area deformations and the impact of local earthquakes. The network development started in 2001 when the first permanent station was installed on the Lacauti peak in the mountainous zone of the Carpathian Bending Zone, west of the Vrancea epicentral area. The network was established as a result of an international research project based on a strategic partnership between: the National Institute for Earth Physics (NIEP), the Faculty of Geology and Geophysics – University of Bucharest (FGG), Delft University of Technology, the University of Utrecht and the Netherlands Research Center for Integrated Solid Earth Sciences (ISES). Starting with 2013, the GPS network is maintained and developed by NIEP.  With a focus on Vrancea seismic area and the Eastern Carpathian Bend (Romania), the network consists of 30 operational stations and others to be installed in the near future. This will provide observations of the crustal motions in order to better understand the surface-to-depth interconnections for intermediate deep earthquakes, improved, reliable and high-accuracy environmental measurements for global weather forecasts, climate monitoring, earthquake precursors (ionospheric studies), co-seismic studies, GNSS positioning and navigation, and other types of complementary research. Nowadays, advances in GNSS receiver’s technology and computational algorithms such as 20 Hz acquisition rate (and even more), that are commonly available, make us search worldwide for systems & algorithms that would make possible a real-time estimation of waveforms and co-seismic displacements.

ACKNOWLEDGMENTS

This paper was carried out within Nucleu Program, Nucleu project PN 19080102 (Advanced researches of the factors that contribute to the seismic risk in Romania), supported by MCI – Romania; project no. PN19080201 and Project no. 18PCCDI/2018, supported by UEFISCDI – Romania.

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