OR/18/054 Introduction

Bateson, L, and Novellino, A. 2018. Glasgow Geothermal Energy Research Field Site - Ground motion survey report. British Geological Survey Internal Report, OR/18/054.

It remains a research question as to whether changes in underground water levels, pressure and temperature caused by mine water geothermal energy production activities can lead to surface subsidence/uplift. Ground motions have been observed following shallow geothermal drilling through anhydrite-bearing formations at Staufen im Breisgau (Germany; Lubitz et al., 2013) and for the enhanced geothermal field in Pohang (South Korea; Grigoli et al., 2018). However, these examples are for geothermal sites where borehole-related changes in hydrogeological flow led to swelling of anhydrite or are under pressure and hence drilling has disturbed the equilibrium. In mine water geothermal systems the existing anthropogenic hydrogeological pathways are re-used and the mine system is not pressurised, and so whilst ground motion is not anticipated as a result of future geothermal research at the Glasgow site, it is important to establish a pre-drilling baseline.

In order to address this question, monitoring is required to both assess the baseline of naturally occurring ground motion and surface-level impacts of geothermal abstraction and re-injection research activities. Because we cannot assume that the area is stable prior to the GGERFS (Glasgow Geothermal Energy Research Field Site) operations, the baseline survey is conducted in order to determine the pre-existing ground stability conditions due to either natural or induced factors and the occurrence of any displacement at the site.

Once this baseline is established further, ground motion investigations following the onset of GGERFS activities can be compared to understand if there is any impact derived from the geothermal research activities. With this regard, InSAR provides insights into the evolution of the geothermal system by regularly monitoring transient stress related to abstraction and re-injection activities over large areas, and can also be used to assess the stability, and therefore safety, of the above ground infrastructure.

In this study, we present results from geodetic radar surveys using Interferometric Synthetic Aperture Radar (InSAR) technique applied over the city of Glasgow using satellite imagery from June 1995 to December 2017. The investigation is designed to monitor the land stability condition at the GGERFS during phase 1, namely the baseline study before the geothermal infrastructure will be installed to study the low-temperature geothermal energy from the flooded mine workings below eastern Glasgow.

Using InSAR results provides the following benefits:

• Archive radar data acquired since 1990s permitting retrospective investigation.
• Data from currently-orbiting satellites such as Sentinel-1 (S-1) are regularly acquired and available free of charge.
• Time-effective and cost-saving solution compared to traditional techniques such as Global Navigation Satellite System (GNSS) where displacement can be retrieved at regional scale rather than at a point location.
• Non-invasive analysis so no need for permissions is required and remote/difficult to access locations can be investigated.

The investigation is independent of any monitoring carried out by the industry or the regulators, and information collected will be freely available to the public at https://www.bgs.ac.uk/research/energy/esios/glasgow/home.html.

In this report we describe the different satellite imagery (i.e., ERS, ENVISAT and Sentinel-1) and processing methodologies (i.e., ISBAS and SqueeSAR™) adopted for the SAR imagery in section 2. In the Results (section 3), we present the maps of the surface displacement around the planned geothermal field site in Glasgow for the different time spans. We then present geological interpretations for the baseline ground motions observed for the past 3 decades.