OR/18/054 Results

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

Within this section we report the general characteristics for each InSAR dataset as derived from the two different processing techniques. We then present the overall patterns of ground motion within Glasgow for the three time periods covered by the datasets. Following this we examine any ground motions in the GGERFS site and then go on to examine specific ground motions within the wider Glasgow area.

A velocity threshold has been set to ±3 mm/yr considering the standard deviation (σ) of the radar targets in each dataset and the creation of a common arrangement to compare different datasets processed with different techniques (Table 2).

Table 2    Main characteristics of each InSAR dataset used over the AoI.
dataset geometry technique Number of targets Average σ [mm/yr]
ERS descending ISBAS 155 614 1.1
Envisat descending ISBAS 28 323 1.2
Sentinel-1 descending ISBAS 201 689 1.7
ERS descending SqueeSAR™ 191 701 0.3
Envisat descending SqueeSAR™ 114 989 0.38
Sentinel-1 ascending SqueeSAR™ 247 656 0.69
Sentinel-1 descending SqueeSAR™ 256 914 0.67

Finally, the geolocation precision and relative accuracies associated to the radar target location (in a projected coordinate system) are in the order of a few meters.

ERS[edit]

The SqueeSAR™ and ISBAS results from ERS-1/2 data indicates that the majority of the area, including the GGERFS site, was stable in the 1990’s (Figure 2).

Figure 2    ERS results from TRE ALTAMIRA’s SqueeSAR™ (a) and GVL’s ISBAS (b) processing showing average rates of ground motion for the measured points. Location of the reference point is indicated by a star. Contains © Geomatic Ventures Limited 2017 data, © TRE ALTAMIRA 2018 data, Ordnance Survey data © Crown copyright and database right 2018, license number 100021290 EUL. Contains British Geological Survey materials © UKRI 2019.

Although Glasgow appears stable overall within the ERS time period (1995–2001) there are small areas, limited to spatially isolated radar targets, exhibiting relatively small rates of subsidence which can be either explained by anthropogenic activities or noise in InSAR results.

ENVISAT[edit]

The SqueeSAR™ and ISBAS results from ENVISAT data indicates that the GGERFS site and the majority of the wider area was stable between 2002 and 2010.

Figure 3    ENVISAT results from SqueeSAR™ (left) and ISBAS (right) processing showing average rates of ground motion for the measured points. Location of the reference point is indicated by a star. Contains © Geomatic Ventures Limited 2017 data, © TRE ALTAMIRA 2018 data, Ordnance Survey data © Crown copyright and database right 2018, license number 100021290 EUL. Contains British Geological Survey materials © UKRI 2019.

Sentinel-1[edit]

TRE ALTAMIRA (Figures 4a–b) and GVL (Figure 4c) results reveal that Glasgow is, overall, relatively stable over the 2015–2017 time period, as processed with Sentinel 1 data. There are notable areas of ground motion, which are addressed in the following sections of this report.

Figure 4    Sentinel-1 ascending (a) and descending (b) results from SqueeSAR™; Sentinel-1 descending results from ISBAS (c). Location of the reference point is indicated by a star. Contains © Geomatic Ventures Limited 2017 data, © TRE ALTAMIRA 2018 data, Ordnance Survey data © Crown copyright and database right 2018, license number 100021290 EUL.

Thanks to the availability of both ascending and descending S-1 SqueeSAR™ results over the AOI (see Figures 4a–b), it was possible to combine the single geometry results in order to obtain 2D measurements, along the vertical (Figure 5a) and the horizontal east–west (Figure 5b) directions, given the assumption of negligible motion in the north-south direction.

Figure 5    Sentinel-1 horizontal (a) and vertical (b) components from SqueeSAR™ InSAR results. Contains © TRE ALTAMIRA 2018 data, Ordnance Survey data © Crown copyright and database right 2018, license number 100021290 EUL.

Motion characteristics of the GGERFS site[edit]

Figure 6    Central portion of the GGERFS site with TRE ALTAMIRA SqueeSAR™ ascending InSAR data. The blue dots are borehole locations, note that the group of three in the north of the Cuningar Loop are no longer being constructed. Contains © TRE ALTAMIRA 2018 data. Contains British Geological Survey materials © UKRI 2019.

The GGERFS site itself is being developed south of a woodland park at Cuningar Loop. The Cuningar Loop and surrounding area were redeveloped for the 2014 Commonwealth Games. In the past the, ~35 hectare area within the loop has been used as a waterworks, and for disposal of building demolition rubble and coal mine waste. The creation of the woodland park, within the Cuningar Loop, including paths and play structures has introduced some objects which act as suitable radar targets and therefore become Persistent Scatterers. Therefore in the TRE ALTAMIRA SqueeSAR™ S-1 results (post 2015) (Figure 6) there are a few measurement points whereas in the ERS and Envisat results (pre-development) there are no InSAR points within the Cuningar Loop (Figure 12).

The TRE ALTAMIRA SqueeSAR™ results provide full time series for persistent scatterers, ascending and descending datasets provide more measurement points than the vertical measurements (which are derived through the integration of ascending and descending observations) and hence give an opportunity to see the detailed 2015–2017 ground motions in this area. Examination of the time series for the S-1 ascending data indicate that the area is subsiding by approximately 30 mm over the 2.5 years processed. Time series for the points highlighted in Figure 6 are shown in Figure 7. These indicate that the motion is largely linear in nature.

Figure 7    Time series for the points highlighted in Figure 6. Left is the upper point (number 1) and the right plot is the lower point (number 2).

InSAR time series for the former Athletes Village (the area to the west of the Cuningar Loop) indicate that, although not immediately obvious in the average velocity, this area is also subsiding, although at a lower rate of approximately 15 mm over the 2.5 years processed (Figure 8). This area was developed for the Commonwealth Games Athletes village, development has seen a substantial change in land use. Prior to 2009 two high-rise tower blocks of flats stood in open areas of land (see older aerial photography in Figure 12). These were demolished and smaller accommodation buildings built between 2012 and 2014. We therefore interpret the small rates of linear subsidence seen here relate to settling of the development and loading of the superficial deposits during the widespread development of small buildings.

Figure 8    Western portion of the GGERFS site with TRE ALTAMIRA SqueeSAR™ ascending InSAR data. The blue dots are borehole locations, note that the group of three in the north of the Cuningar Loop are no longer being constructed. Contains © TRE ALTAMIRA 2018 data. Contains British Geological Survey materials © UKRI 2019.
Figure 9    Time series for TRE ALTAMIRA InSAR results for the area to the west of the Cuningar Loop (white circle in Figure 8).

Given the past land use, mapped artificial ground and indications of 35–40 m of superficial deposits on and around the site (Figure 10) it is likely that the subsidence observed here is a result of the development activity and subsequent settling of the infilled ground and/or the superficial deposits. The analysis of the vertical time series from SqueeSAR™ confirms that most of the targets in this area have a discontinuous movement with a slight deceleration of the rate located sometime between October 2015 and November 2017.

Figure 10    Superficial deposit thickness and artificial ground in the Cuningar Loop. Contains British Geological Survey materials © UKRI 2019. Aerial photography © UKP/Getmapping Licence No. UKP2006/01.

The GVL Sentinel 1 data for the GGERFS site indicates that it is mostly stable; there are two small areas with approximately 2–3 mm per year of subsidence within the Cuningar Loop (Figure 11). Although the lack of time series associated with these data makes it difficult to fully understand the indicated motion, they are in the same location as subsiding points in the SqueeSAR™ results and relate to settlement of the built environment.

Figure 11    GVL ISBAS average motions within the GGERFS site 2015–2017. The blue dots are borehole locations, note that the group of three in the north of the Cuningar Loop and in the far east of this image are no longer being constructed. Contains © Geomatic Ventures Limited 2017 data.

Note the area of subsidence in the lower left corner of Figure 11, motion rates of up to 10 mm per year are observed in both GVL and TRE ALTAMIRA Sentinel 1 results for this area of the M74 motorway. The radar scatterers appear to be related to the motorway gantry but the motion is likely to relate to instability in the embankment supporting the motorway at this location.

ERS and ENVISAT data for the GGERFS site[edit]

Although there are no measurement points within the Cuningar Loop, the ERS and ENVISAT InSAR results provided by both TRE ALTAMIRA and GVL indicate the GGERFS and surrounding area to be stable during the 1990’s and 2000’s.

Figure 12    Ground motions for the GGERFS site during the ERS (1990’s) and ENVISAT (2000’s) as derived by the SqueeSAR™ and ISBAS processing techniques. Contains © Geomatic Ventures Limited 2017 data, © TRE ALTAMIRA 2018 data. Aerial photography © UKP/Getmapping Licence No. UKP2006/01.