Overview of boreholes

The Glasgow Observatory comprises arrays of research boreholes capable of observing changes in underground conditions. More details can be found within the borehole information packs and the site profiles .

Figure 1 Interpretive 3D block diagram illustrating the borehole geometry of nine boreholes at Cuningar Loop. No vertical exaggeration. BGS © UKRI 2021, from Monaghan et al. (2021).

Mine-water characterisation and monitoring boreholes

Five boreholes characterise the mined rock volume and mine-water hydrogeology. The boreholes penetrate two levels of mine workings at drilled depths of around 50 m and 85 m and are screened at water-filled voids, waste-filled mine workings and a fractured rock mass.

These boreholes allow scientists to study spatial and temporal connections and variability within a mined rock volume in unprecedented detail. The aim is to:

improve the scientific understanding of the subsurface mine-water environment (subsurface flow; heat and fluid connectivity; sustainability of mine-water resources; fluid/rock mass interactions) through the collection and interpretation of measured data such as mine-water temperature, geochemistry, levels, flow and ‘storativity’, etc.
characterise initial conditions and provide ongoing baseline monitoring of the mine-water hydrogeology, against which any future research results can be compared
provide continuous downhole data and cross-borehole imaging of geoelectrical and temperature properties of the subsurface, in order to monitor natural and induced properties and changes
provide open data in easily accessible formats
provide borehole infrastructure to test and optimise new technologies, sensors, performance and operational strategies

Location

Three pairs of mine-water characterisation and monitoring boreholes in the Cuningar Loop were planned. Due to construction problems, one of these boreholes, with geoelectrical and fibre-optic cabling installed, has been re-purposed for sensor testing.

The locations were chosen to penetrate two levels of mine workings in a triangular arrangement, to characterise the mine-water flow at a scale suitable for the heterogeneous geology. Small, mapped faults and a coal washout, all within the same larger fault block, are typical of the geology.

Figure 2 Borehole locations at Cuningar Loop along with the recorded extent of Glasgow Upper mine workings from BGS interpretation of mine plan records. Ordnance Survey Licence No. 100021290 EUL.

Borehole design and instrumentation

The mine-water boreholes:

are cased and screened over a worked coal seam, either the Glasgow Upper mine working or Glasgow Main mine working, to allow testing of aquifer properties, temperature and pressure monitoring and groundwater sampling of the particular mined seam
have an internal casing diameter of 248 mm with a 4 mm slotted screen
are fitted with a bladder pump for groundwater sampling
have electrical resistivity tomography and distributed temperature-sensing fibre-optic cables installed downhole between the rock wall and the borehole casing
measure hydraulic head and groundwater temperature and specific electrical conductance (SEC) via continuously recording, monthly downloaded pressure, temperature and conductivity transducers, supported by monthly manual groundwater level and conductivity measurements
two of the boreholes have a pump installed and operate as abstraction boreholes
two other boreholes are configured to be re-injection boreholes (details of the well head and geothermal infrastructure are provided in borehole packs)

Figure 3 Example of as-built mine-water borehole design, GGA07. (Please note that this diagram is schematic and not to scale.)

Figure 4 Example of a mine-water borehole log, GGA07, based on rock chip returns.

Environmental baseline characterisation and monitoring boreholes

The environmental baseline borehole array comprises five boreholes in Cuningar Loop. They have been drilled to provide baseline monitoring data for groundwater within superficial deposits and near the top of the bedrock. The aim is to:

improve the scientific understanding of the subsurface and near-surface groundwater environment, in particular the subsurface to surface interactions and potential risks associated with mine-water geothermal research activities, by collecting data relating to water geochemistry, temperature and level
characterise initial conditions and provide ongoing baseline monitoring of the non-mined subsurface hydrogeology, against which any future research results can be compared, to identify any environmental change(s) resulting from research activities
meet regulatory requirements for developing any future geothermal activities at the site and provide public assurance
provide open-access groundwater-monitoring data in easily accessible formats

Location

Three boreholes are screened across sand and gravel within the superficial deposits and two target unmined zones near the top of the bedrock (figures 1 and 2). This distribution of boreholes is designed to monitor groundwater interactions at the mine-water borehole sites and to the south of those sites, in the approximate direction of the predicted groundwater flow.

Borehole design and instrumentation

Each environmental baseline borehole has been completed with permanent casing, with a slotted screen across the target interval.

Two bedrock boreholes of around 40 m drilled depth have an internal casing diameter of 146 mm and a 3 mm slotted screen, with bladder pumps installed
Three superficial deposits boreholes of around 16 m drilled depth have an internal casing diameter of 103.8 mm and a 1 mm slotted screen
Hydraulic head, groundwater temperature and SEC are measured via continuously recording, monthly downloaded pressure, temperature andconductivity transducers, supported by monthly manual groundwater level and conductivity measurements

The boreholes have been designed to allow groundwater samples, levels and temperatures to be obtained and test pumping to be undertaken. They provide the opportunity to understand vertical variability in groundwater temperature, chemistry and groundwater head and to study the interactions between bedrock and superficial deposits.

Figure 5 Example of as-built environmental baseline borehole design, GGA03r. (Please note that this diagram is schematic and not to scale.)

Seismic monitoring and cored borehole

The seismic monitoring borehole at the Glasgow Observatory is designed to strengthen the national seismic monitoring network in the urban area, so that any felt earthquakes can be detected and located. Reliable characterisation of baseline levels of natural seismicity in the vicinity of the observatory allows discrimination of any future events that could erroneously be attributed to geothermal activities at the research site. Without this, in the unlikely event that there are any changes in the spatial or temporal behaviour of small seismic-magnitude events, these would be obscured by uncertainty.

The seismic monitoring borehole was cored during drilling to provide a record of the Scottish Coal Measures geology (no mine workings were encountered during drilling). This borehole was open-hole wireline logged to profile the bedrock geophysical parameters.

Location

The seismic monitoring borehole is located in Dalmarnock. The location was chosen due to the availability of power and broadband connections and being at some distance from motorways and railways.

Borehole design and instrumentation

The seismic borehole has an internal casing diameter of 76.6 mm, to 199 m drilled depth. It contains a string of five seismometers at drilled depths of 198 m, 156nbsp;m, 113nbsp;m, 71nbsp;m and 29nbsp;m. The core has a 102nbsp;mm diameter.

Further information and references

Drilling of the boreholes is synthesised in two publications.

Monaghan, A A, Starcher, V, Barron, H F, Shorter, K, Walker-Verkuil, K, Elsome, J, Kearsey, T, Arkley, S, Hannis, S, and & Callaghan, E. 2021. Drilling into mines for heat: geological synthesis of the UK Geoenergy Observatory in Glasgow and implications for mine water heat resources. Quarterly Journal of Engineering Geology and Hydrogeology, Vol. 55. DOI: https://doi.org/10.1144/qjegh2021-033
Starcher, V, Monaghan, A A, Barron, H F, Shorter, K, Walker-Verkuil, K, and Elsome, J. 2021. Method and key observations from constructing a mine water heat subsurface observatory in Glasgow UK. British Geological Survey Open Report OR/21/020. (Nottingham, UK: British Geological Survey.)