Ups and downs of the South Wales Coalfield

Satellite interferometric study of ground motions in the 1990s in the South Wales Coalfield, using the Intermittent SBAS (ISBAS) technique (Bateson et al., 2015). The ground motion velocity map is overlapped onto shaded relief of the SRTM V4 DEM produced by NASA (Jarvis et al. 2008).

BGS tested a new image processing method that can detect very small changes in the height of the land surface.

Using the South Wales Coalfield as one of their study areas, the BGS team was able to locate and study rates of historic land rise and its link to mining history and slope movements such as landslides.

Ground motions regularly affect the built infrastructure in the UK. For example, statistics on property insurance claims published by the Association of British Insurers (ABI) in September 2013 in the UK Insurance Key Facts Guide 2013 show that UK domestic property claims relating to subsidence cost over £130 million in 2012.

Ground movement can be relatively fast, but can also be very slow and hard to detect, and can be damaging over long time periods. It is therefore important to map the location and speed of land surface motion and discover its causes; information about stability, or instability, of the ground is important for planning and management of assets, as well as to local authorities, geodetic surveyors and geologists, etc.

New image processing method: ISBAS

Satellite sensors are frequently used for mapping and monitoring the Earth's surface to better understand land surface processes and dynamics.

BGS has successfully tested the new InSAR (interferometric synthetic aperture radar) image processing method named ISBAS (intermittent small baseline subset; Sowter et al., 2013), developed in collaboration with the University of Nottingham. The images used in the study were acquired by European satellites orbiting at approximately 800 km above the Earth during the 1990s.

By exploiting information acquired by radar sensors on board these satellite platforms, the new ISBAS technique can reveal very subtle, but potentially important, long-term changes in the land level that arise due to a variety of surface and sub-surface processes including:

  • mining related subsidence and uplift
  • slope instability and stress relief
  • peatland growth and decline
  • volume changes due to compression of sediments and shrink-swell processes in clays

ISBAS can reveal unprecedented spatial coverage of results across urban areas, but also rural and agricultural land covers, which are generally unfavourable for InSAR applications (Sowter et al., 2013; Cigna et al., 2014 (a-c); Bateson et al., 2015).

ISBAS identified over 375 000 natural targets across the South Wales Coalfield, with coverage of ground motion results around seven times higher than conventional satellite-based InSAR methods.

Minewater rebound

Observed uplift rates are as much as 1.3 cm per year and centred on the most recently exploited part of the South Wales Coalfield.

In this area, BGS geological interpretation reveals that the observed uplift is likely a result of minewater rebound, though the picture is complicated by slope movements.

Deep collieries required a great deal of water to be pumped to enable safe coal extraction. Following their closure, pumping activity ceased allowing the water levels to return to equilibrium, causing the ground to gradually spring back up, or swell.

Landsliding

Motions relating to landslides and other mass movements on steep sided valleys are also observed. The determination of the recent/present state of activity of landslide features is of critical importance to establish if deep block movements still contribute to landslide activation and reactivation, and the role of groundwater rebound in driving slope instability.

Research

The ISBAS ground motion study of the South Wales Coalfield has been published by Bateson et al. in the International Journal of Applied Earth Observation and Geoinformation in 2015, and is available here.

This research is supported by the European Space Agency with the provision of ERS-1/2 SAR imagery in the framework of the Category-1 Project id.13543: 'Enhancing landslide research and monitoring capability in GB using C-band satellite SAR imagery and change detection, InSAR and Persistent Scatterers techniques'.

References

Bateson, L, Cigna, F, Boon, D and Sowter, A.  2015.  The application of the Intermittent SBAS (ISBAS) InSAR method to the South Wales Coalfield, UK.  International Journal of Applied Earth Observation and Geoinformation, 34, 249–257.

Cigna, F, Bateson, L, Jordan, C, Dashwood, C.  2014 (a).  Simulating SAR geometric distortions and predicting Persistent Scatterer densities for ERS-1/2 and ENVISAT C-band SAR and InSAR applications: Nationwide feasibility assessment to monitor the landmass of Great Britain with SAR imagery.  Remote Sensing of Environment, 152, 441–466.

Cigna, F, Novellino, A, Jordan, C J, Sowter, A, Ramondini, M, Calcaterra, D.  2014 (b).  Intermittent SBAS (ISBAS) InSAR with COSMO-SkyMed X-band high resolution SAR data for landslide inventory mapping in Piana degli Albanesi (Italy). Proceedings of SPIE 9243, SAR Image Analysis, Modeling, and Techniques XIV, 92431B.

Cigna, F, Sowter, A, Jordan, C J, Rawlins, B G.  2014 (c).  Intermittent Small Baseline Subset (ISBAS) monitoring of land covers unfavourable for conventional C-band InSAR: proof-of-concept for peatland environments in North Wales, UK. Proceedings of SPIE 9243, SAR Image Analysis, Modeling, and Techniques XIV, 924305.

Jarvis, A, Reuter, H I, Nelson, A and Guevara, E.  2008.  Hole-filled seamless SRTM data V4, International Centre for Tropical Agriculture (CIAT), available from http://srtm.csi.cgiar.org

Sowter, A, Bateson, L, Strange, P, Ambrose, K and Syafiudin, M.  2013.  DInSAR estimation of land motion using intermittent coherence with application to the South Derbyshire and Leicestershire coalfield.Remote Sensing Letters, 4, 979–987.

Contact

For more information contact Dr Francesca Cigna, Luke Bateson or David Boon.