Assessing ground motion from space...by Kieran Parker

Diagram showing satellite image acquisition process to enable multiple
 images to be assessed. Source http://trussty-jasmine.blogspot.co.uk

Ground movement is an issue of global concern and one that regularly grabs the attention of the media due to its impact to public safety, property and infrastructure networks often necessitating expensive remedial action.

In Northern Ireland, ground movement is closely associated with slope instability, most notably on the margin and valley slopes of the Antrim plateau as well as surface subsidence in areas of historic mining. While most of the movement is natural there is also the human influence which exasperates or creates instability through social development and extractive legacy. The extent and form of surface motions can vary dramatically from location to location with a number of controlling factors. These movements are traditionally monitored with the placement of instrumentation around sites which have been causing persistent problems however these methods are costly, time consuming while also limited by resources only enabling a number of areas to be assessed over the long term.

Subsequently, there is a clear need for accurate assessment of ground motion for land use planning and development across areas suspected of being susceptible to movement along with a better understanding of the instigating factors and potentially the development of tools that will enable early warning to a catastrophic movement event.

Throughout 2016, the BGS Earth and Planetary Observation and Monitoring (EPOM) Team, Shallow Geohazards and Risks (SGR) Team and the Geological Survey of Northern Ireland (GSNI), together with Queen's University Belfast (QUB) have been working on a research study to analyse the benefits of using satellite radar interferometry  (InSAR) techniques to remotely assess risk to infrastructure associated with ground movements in Northern Ireland. The project is analysing historical radar data available for 1992-2010 obtained from the European Space Agency (ESA) operated ERS1/2 and ENVISAT satellites.

The methodology works by processing of numerous images collected by the satellites during each repeat pass. Stacked together these images allows for the extraction of reflective targets and measure, to millimetre precision, surface displacement. A total of 127 images will be analysed throughout the project.

InSAR techniques have the capability to remotely monitor large areas which would enable a step change in techniques currently used by organisations to analyse risk to their infrastructure network.

Distribution of radar reflectors identified by processing satellite radar data aquired
from ERS 1/2 (left) from 1992-2000 and ENVISAT (right) from 2002-20.

The project team is working with five major stakeholders TransportNI (TNI), Northern Ireland Railway (NIR), Department for the Economy (DfE), Arup and Belfast City Council (BCC) to examine areas of slope instability and subsidence which have proven to be problematic in the past while also aiding the identification of others areas potentially at risk.

While the data coverage takes in an area of 3,000 km², the project will focus closely at problematic sites identified by the stakeholders:

• Site 1: North Belfast – A densely populated urban location, this area has been subject to shallow translation landslides with evidence of movement can be seen at Ligoniel Park and Throne Bend on the Antrim Road. 
• Site 2: Belfast-Bangor Railway line – This section of rail line is positioned within steep sided cuttings prone to instability, particularly after periods of heavy and prolonged rainfall.
• Site 3: Carrickfergus – The residential town contains eight abandoned salt mines which display continual subsidence. Over the past two decades a number of crown holes have appeared at various locations as a result of mine collapses resulting in the permanent closure of two public roads.
• Site 4: Straidkilly, Antrim Coast Road (A2) – Positioned at the base of the Antrim Plateau, the A2 is a scenic route used extensively by the many coastal towns and villages as well as a high number of tourists. This section of road cuts through soft Jurassic clays and debris from the slide area has frequently reached the road increasing the risk to users and also leading to road closures.

Aerial photo of crown hole collapse at abandoned Maidenmount salt mine,
Carrickfergus 2001. The collapse generated a hole >100 meters in diameter
with 8 metre vertical displacement.  © Crown Copyright

Preliminary InSAR results display variable movements in many of the known landslide areas while also highlighting motions associated within areas of historic mining activity. The initial results have also identified a number of areas of interest which are displaying subsidence and surface heave potentially as a result of water abstraction, soil compaction and shrink swell processes. 

Newtownards, Co.Down showing significant subsidence
within the centre of the town. Raw ERS-1/2 satellite
data provided by ESA under grant id.32627.

These results were presented at a workshop held at Queens University Belfast in September 2016 where the stakeholders were given the opportunity to have a close look at the data in areas where their assets may be affected by ground motion. With each reflective point representing average annual motion over 100m x 100m ‘parcels’, the workshop enabled stakeholders to identify several sites each of greatest interest where the team will process the data further to increase the resolution by reducing the size of each parcel thus providing more precise results of ground movement leading to a more accurate assessment of the risk to the infrastructure network. Crucial for the stakeholder will be the identification of trends leading up to major movement.

The project team is currently working through the time series data from reflective points in areas of interest to analyse the variations of motion across the areas while also validating it with previously collected terrestrial and airborne data obtained by the stakeholders.

The potential outcome will be an enhanced capability to monitor and assess hazards associated with ground motion across the infrastructure network and for the stakeholders to implement regional scale hazard mapping using satellite technology to compliment terrestrial monitoring. This could see huge benefits in mapping and understanding geo-hazards allowing better informed engineering techniques to be considered, better targeting of sites while reducing the risk to people monitoring on unstable ground. Further outcomes from the project will be the capability to communicate the risk posed by ground movement and the development of an early warning system.

The project started in February 2016 and will run for 18 months, until July 2017.

Project: InSAR for geotechnical infrastructure: enabling stakeholders to remotely assess environmental risk and resilience (NERC Grants: NE/N013018/1 and NE/N013042/1)

Project Team:
Queen’s University Belfast (QUB)
Dr David Hughes, Dr Jenny McKinley, Dr Shane Donohue, Conor Graham

British Geological Survey (BGS) / Geological Survey of Northern Ireland (GSNI)
Dr Francesca Cigna, Dr Vanessa Banks, Kieran Parker, Alex Donald

The project is funded by NERC under the Environmental Risks to Infrastructure Innovation Programme (ERIIP). ERS-1/2 and ENVISAT raw satellite data is provided by ESA under grant id.32627. For further details contact Dr David Hughes at Queen's University Belfast or Dr Francesca Cigna.