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Using radar satellites for monitoring underground construction

The challenge

Underground construction and civil engineering projects may cause displacements of the Earth’s surface. Frequently, extensive terrestrial surveying programmes are required to monitor surface movements related to construction activities such as the complex building project of a new underground line in the Hungarian capital Budapest.

The solution

In addition to the terrestrial surveillance, Airbus Defence and Space GEO-Information experts conducted a monitoring project using high-resolution TerraSAR-X imagery. By analysing >40 acquisitions recorded during 1.5 years, the experts detected significant surface displacements of up to 10 mm per year.

TerraSAR-X for monitoring subway construction in Budapest

Infrastructure constructions, excavations and underground engineering may cause displacements of the Earth’s surface. These typically happen very slowly yet may have sudden and far-reaching effects that endanger infrastructure or even human lives. Today underground civil engineering projects are particularly complex tasks when conducted in the densely populated areas of large cities.

Construction of Metro Line 4 in Budapest

© DBR Metro Project DirectorateThe Metro project

In the Hungarian capital Budapest a complex building project was initiated in 2006, when the founding stone was laid for the construction of a new Metro line almost crossing the entire town. Metro Line 4 is planned to connect South West Budapest with the North Western parts of the city, comprising 16 stations along a length of 12.7 km, to be completed by 2013.

Image: Construction of Metro Line 4 in Budapest, Hungary. © DBR Metro Project Directorate

Building methods

The tunnels of the new line are constructed using underground tunnel driving machines, while the stations are constructed by a top-down method. During the construction, an extensive terrestrial surveying programme was established in order to monitor surface movements related to the construction, which could potentially endanger not only human lives but also the historic heritage-protected buildings of the city.


The Airbus Defence and Space GEO-Information Services solution

Radar satellites can reliably monitor such surface movements and help in:

  • Supporting authorities in charge of the safe performance of engineering projects
  • Delivering timely information to prevent damage to infrastructure and human lives
  • Complementing established monitoring techniques

Satellite monitoring and surface-movement mapping

In addition to the terrestrial surveillance Airbus Defence and Space GEO-Information Services experts conducted a monitoring campaign using 43 TerraSAR-X StripMap scenes of the target area, acquired between 24 October 2008 and 16 April 2010.

The aim of the project was to confirm the suitability of satellite based surface motion monitoring to resolve surface movements related to underground construction works in urban areas. For the aim, results derived from TerraSAR-X data were compared to the terrestrial surveying.

Image: Artist view of TerraSAR-X in space. © Airbus Defence and Space GmbH

A sub-centimetre precision

The TerraSAR-X data was analysed using Persistent Scatterer Interferometry (PSI), a technique which determines movements of the ground surface with sub-centimetreprecision by analysing the change in the interferometric phase of the radar backscatter over time.

This interferometric technique makes use of the fact that the phase of the radar signal, transmitted by the sensor and reflected from the ground is proportional to the distance between the satellite and the point on the ground from which the signal is reflected. Thus a change in the interferometric phase of the reflected signal is proportional to the surface uplift or subsidence in that time frame.

Image (a) PSI Analysis in the wider area of Station Szent Gellért tér based on 43 TerraSAR-X satellite data sets; (b) focused analysis in the area of Station Szent Gellért tér based on 8 TerraSAR-X satellite data sets; (c) sample time series of surface displacements showing the highly non-linear displacements induced by temporarily restricted time of construction


The results

Surface displacements: up to 10 millimetres

Surface subsidence is made visible in the red colour scale, while surface uplifts are colour-coded in the blue colour scale. From the map it is clearly visible that surface movements have occurred along a line from North East to South West of the river Danube, which corresponds exactly to the track of the subway construction of the new Metro Line 4. Significant surface displacements of up to approx. 10 mm per year were detected especially in the vicinities of the underground stations.

In the comparison for single displacement measurements, a two millimetre accuracy with a standard deviation of about one millimetre was achieved.

Image: TerraSAR-X-based surface movement assessment performed during the construction of a new metro line in Budapest, Hungary


The Airbus Defence and Space GEO-Information offer

Reliable surface-movement monitoring services

Services provided by Airbus Defence and Space GEO-Information Services include:

  • Detailed analysis of expected surface movements
  • Client-tailored project design
  • Data acquisition and processing
  • Integration and interpretation of the results

Besides the monitoring of civil engineering projects, such surface movement monitoring services are used in the oil, gas and minerals industry and by public authorities worldwide.


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