Mapping Ground Deformation from Space / / by Alessandro Novellino

Alessandro has been a Remote Sensing Geoscientist at the British Geological Survey since 2017. He specialises in the analysis and interpretation of spaceborne radar data for a better understanding of natural and anthropogenic hazards.

Located on the Italian island of Sicily, Mt. Etna is one of the world's most active volcanoes. In this image of the volcano in 2001, a plume of steam and smoke rising from the crater drifts over some of the many dark lava flows that cover its slopes.
Satellite imagery of Mount Etna

Interferometric Synthetic Aperture Radar (InSAR) is nowadays a widely used technique for measuring ground deformation from space with millimetric accuracy.

Over the last 25 years, it has been extensively used by earth scientists for mapping and modelling natural phenomena such as volcanoes and landslides.

I had the opportunity to do a PhD in Italy which, unfortunately or fortunately (it depending on your point of view! For a scientist, it’s a very interesting place to be), is a natural laboratory for such phenomena and hosts institutes at the forefront of InSAR research (e.g., University of Florence, National Institute of Geophysics and Volcanology - INGV).

A ground deformation chart of Campi Flegrei showing uplift between 2004 - 2007
Ground deformation observed with InSAR and GPS over the Campi Flegrei caldera (Italy) between 2004 - 2007. (Source: Bevilacqua et al, 2020)

On the other side, the UK is a country with limited experience of large natural disasters related to ground motion and therefore moving here has shifted my InSAR interests from natural-induced ground motion phenomena to human-induced ones such as groundwater extraction and groundwater rebound in mined areas. 

BGS has decades of experience in using InSAR through several international projects, such as PROTHEGO, PanGeo and the upcoming European Ground Motion Service where I am involved in the Task Force, which defines the technical requirements of the InSAR.

These works have enabled us a better understanding of the susceptibility and hazards associated to ground motion for Great Britain.

However, we are now in what I use to call, the ‘Golden Age’ for Earth Observation.

Latest satellite constellations such as Sentinel-1, funded by the European Commission and managed through the European Space Agency, are paving the way for mapping ground instabilities at unprecedented spatial and temporal resolution.

Such wealth of information is allowing to develop new and better solutions for building a more resilient society.

One example can be seen in a work recently published in the journal Remote Sensing.

The research has developed a model for groundwater rebound in recently abandoned coalfields in Nottinghamshire using InSAR which can be used, in turn, to enable predictions of surface discharges that can support mitigation strategies.

The work has been developed by David Gee, a PhD student funded by the GeoEnergy Research Centre of the University of Nottingham and co-supervised by myself and Luke Bateson along with staff from the Coal Authority.

Graph showing different discharge time in years from 0 - 5
Predicted time until discharge out of the Coal Measures Group between Derbyshire and Nottinghamshire (source Gee et al, 2020).

This work has a huge impact for a country like the UK where mining of coal dates back centuries. The extraction of large volumes of coal and adjacent rock, fracturing and collapse of in situ strata and generation of mining roadways and entry shafts has created a complex subsurface environment. Additionally, an extensive dewatering regime was required to artificially lower groundwater levels to maintain safe working conditions in working collieries. Following the decline of the mining industry and closure of deep mines, the systematic pumping of groundwater was no longer required and levels started to rebound to their assumed previous natural levels. The cessation of systematic dewatering can have a variety of detrimental impacts, including pollution of overlying aquifers and surface water, localised flooding and renewed mining subsidence and reactivation of geological faults.

Knowledge of the time-scales (i.e. the rate) of rebound is crucial to coalfield remediation strategies. The modelling concept developed here is able to map the change in groundwater with complete coverage, to fill in the measurement gaps between the boreholes. The data can be used by national bodies such as the Environment Agency which is responsible for managing hazards such as flooding, pollution and contaminated land, and the Coal Authority which has a mandate to manage the legacy and assets of underground coal mining in terms of public safety and subsidence. Next challenge for BGS and the wider InSAR community is to develop a (hardware and software) infrastructure system able to manage the huge volume of information (in the order of terabytes of data collected everyday over the UK only) and associated results (millions of points with ground displacement information every 6 days) needed to regularly monitor the Earth’s surface.

Thanks to BGS ODA and IFF funding, Ekbal Hussain and I have already developed an automatic InSAR processor able to reduce the user-interaction needed to process radar data (from weeks to days).We are now working on adapting statistical and machine learning techniques in order to quickly highlight areas characterised by anomalous patterns of ground deformation from InSAR results. These anomalies might be incipient signal of natural or anthropogenic-induced disasters.

For more information on the BGS InSAR research, please visit our website.