Adaptive geophysics boost UK pumped storage development

Project overview

The proposed Cruachan 2 pumped storage scheme is a major infrastructure project, led by our client Drax. It aims to expand the existing hydroelectric facility with a new 600 MW underground power station, which has the potential to power over one million homes in the UK. To support design and construction of the scheme, we conducted a comprehensive geophysical investigation across three distinct areas (Lots 1, 2, and 3), where we used seismic refraction tomography and electrical resistivity tomography to map subsurface conditions.

Working in difficult terrain

The Cruachan site presented extreme logistical and environmental challenges for a site investigation. The terrain of the three survey areas comprised steep slopes, areas of dense vegetation, boulders fields and scree, areas of peat and marsh and existing sensitive infrastructure. The Lot 1 study area was located adjacent to the existing upper intake reservoir and along the proposed hydro tunnel route. The Lot 2 study area was located at the base of the valley in steep terrain adjacent to an A road and railway infrastructure. The Lot 3 survey was within the upper reservoir itself.

In Lot 1, many survey lines traversed slopes with limited or no vehicle access, requiring manual transport of equipment and hand-prepared shot holes. In Lot 2, access was further complicated by dense vegetation and steep gradients, requiring sensible vegetation clearance and careful planning around existing critical transport infrastructure prior to surveying to ensure safety and data quality.

In Lot 3, the investigation was carried out over water within the Cruachan Reservoir. Together with our partners, we built a temporary access track and a floating ScaffFloat work platform to complete the seismic surveys. Using an airgun as the source of the signal and a hydrophone streamer underwater microphone, to receive the echoes from the reservoir floor, we could understand the subsurface conditions from the floating platform.

Working in an active pumped reservoir

With the Cruachan reservoir remaining operational throughout our investigation, there were additional challenges and considerations that needed to be made. The reservoir’s water level fluctuated daily (sometimes by tens of metres) due to power generation activities releasing water through the dam. This posed a challenge for accessibility to the survey areas. The floating platform had to be launched from a temporary beach area and dynamically positioned during acquisition to ensure that we could collect high quality data safely and as efficiently as possible.

Despite these challenges, five seismic lines totalling c. ~800 m were successfully surveyed, providing valuable insight into subsurface conditions beneath the reservoir.

Ecological and safety constraints

Safety was a top priority throughout the project, particularly where work was conducted close to the existing dam and rail infrastructure. To make sure these structures weren’t impacted, the team used vibration monitoring during seismic data acquisition. For this, vibration monitoring equipment was set up on the surrounding assets to ensure that these assets were not exposed to excess vibration levels which could damage the structure. All activities followed strict safety rules and involved close coordination with Drax operations staff managing the Cruachan hydroelectric power station.

Close collaboration was also required with the project ecological team to ensure that the geophysical survey operations were sensitive and not impactful to the habitat and well-being of birds and mammals within the Cruachan area.

Innovative use of shallow shot holes and small charges

To overcome the access challenges that prevented use of drilling equipment, we collaborated with EPC-UK to develop a low-impact, mobile alternative seismic source that could be safely implemented across difficult terrain without the need for heavy machinery. A novel method using 25 g PETN explosive charges deployed in shallow, hand-prepared shot holes was developed that was suitable for the difficult terrain without compromising the seismic data quality.

Thorough testing (offsite and onsite) was carried out to ensure the suitability and effectiveness of the method for the Cruachan project and to establish a clear and practical safe system of work. Additional measures, such as the packing of the shallow holes with sand and matting for containment, were put in place to minimise environmental impact and noise during operations.

Concluding with clarity beneath the surface

The Cruachan 2 geophysical investigation shows how technical innovation, environmental sensitivity, and rigorous planning can overcome the challenges of working in remote, ecologically sensitive, and operationally constrained terrain.

Our investigation and innovative use of shallow explosive charges and overwater techniques delivered 2D and 3D models of the subsurface, identifying key geological features to help Drax develop the design of the underground infrastructure of one of the UK’s most ambitious pumped storage projects. Our innovative techniques, developed with EPC-UK, allowed us to deliver efficient, high-quality data to Drax whilst also prioritising the safety of our staff and minimising the impact on the surrounding environment.