At over 1000 km long and 40 km deep, the Great Glen Fault is the largest geological fault structure in the UK. As part of ground investigations for SSE Renewables’ proposed pumped hydro storage scheme at the Coire Glas site on the shores of Loch Lochy in the Highlands, deep drill core was extracted from beneath the Great Glen. BGS scientists were granted a unique opportunity to study the newly drilled fault rocks that are part of the Great Glen Fault Zone. These ‘first of their kind’ core samples have lived up to their billing, with experts claiming that they give unprecedented insight into the inner workings and behaviour of crustal-scale faults worldwide.

The Great Glen Fault formed around 400 million years ago in a massive mountain-building event, as the ancient continental plates ofÌýLaurentia (North America and Scotland) and Baltica (Scandinavia, England, Wales and Europe)Ìýcollided. This tectonic event is known as theÌý. The fault stretches from Ireland, all the way through Scotland, to Norway. Today, the fault underlies the major valley of the Great Glen, which crosses the whole of Scotland and was scoured out by glaciers during the last ice age. Generally, rocks associated with the Great Glen Fault Zone remain mostly hidden to the human eye by the waters of Loch Ness, Loch Oich and Loch Lochy, along with ice age deposits along the valley floor.

The new drill core from the Coire Glas Project offers the tantalising prospect of furthering our understanding of how these fault systems work and how fluids emerging from deep within the Earth crust change the properties of the rock. Over 1500 m of core were recovered, reaching depths of 650 m below ground level. Core was drilled on the shore of Loch Lochy and from within an underground tunnel at the base of the mountain. Drilling geological core is expensive and is normally only justifiable to such extensive depths as part of major energy or infrastructure projects. The added difficulty in relation to the Great Glen Fault is that, in addition to being located in remote parts of the Highlands, fault rock can be very weak and presents a technical challenge to drill successfully.

The new geological samples provide an opportunity to understand the geological processes happening deep in the Earth crust. This is also relevant for understanding other major crustal faults, such as the San Andreas and Anatolian faults. Several key questions remain:

• does this fault connect all the way to the Earth’s mantle, thought to be at more than 30 km depth?
• what is the source of fluids in crustal fault zones?
• how do hot fluids interact and change the mechanical properties of the rocks in a fault zone?
• how many times has the fault moved in its long geological history?
• how have the hundreds of earthquakes that likely made the fault zone changed the properties of the rock?

I have had the privilege to study the first samples of these Great Glen Fault rocks using state-of the-art microscope facilities at BGS. Our findings give strong clues as to how ancient deformation processes and fluid/rock chemical reactions caused the fault to initially weaken associated with displacements of hundreds of kilometres. Remarkably, it then appears to have been cemented following later tectonic movements that channelled deeply sourced carbonate mineralisation. Much more remains to be discovered, but it is clear that these cores have the potential to elevate the Great Glen Fault to one of the great natural laboratories for fault zone studies worldwide.

Professor Bob Holdsworth, Durham University

Newly drilled core from the Coire Glas site has provided a unique opportunity to study fundamental geological processes occurring in the UK biggest fault zone. The storage of the Coire Glas core at BGS will allow access for the scientific community and will ensure that these rocks are preserved for future generations.

Romesh Palamakumbura, BGS Geologist

SSE Renewables is delighted to support the advance in scientific understanding of the Great Glen Fault and similar structures worldwide, thanks to the core that was recovered during the ground investigation for Coire Glas.ÌýAs well as being of scientific value, the recovered core has been critical for understanding ground conditions and managing ground risk as the project progresses towards a final investment decision.

SSE Renewables

The Coire Glas core will be stored and made available for future research purposes at the 51ÁÔÆæ National Geological Repository, a bespoke facility that is publicly funded through 51ÁÔÆæ and houses the UK foremost collection of geological samples. This will enable long-term preservation of the core, allowing scientists to study and attempt to unlock its secrets long into the future.

The core has the potential to help us answer fundamental geological questions about the history of the Earth as well as better understand major crustal-scale faults in seismically active regions elsewhere. It will also enable us to understand rock properties that are important for major renewable infrastructure projects, energy storage and geothermal targets. These cylinders of rock truly are one-of-a-kind windows back into our distant geological past.

In a warehouse just outside Nottingham, vast racks of geological core are carefully curated and stored in climate-controlled conditions. Part of the collections held within BGS National Geological Repository (NGR), this core is quietly energising the UK’s economy, supporting the nation growth agenda and energy transition aspirations.

Understanding our subsurface environment requires both direct observation, through samples such as drill core, and indirect observation, through sensors and monitoring. These observations are the basis on which we build models that constrain and test hypotheses explaining the Earth, its composition and its many processes. Such knowledge is critical for determining how society is affected by or can safely interact with the ground beneath our feet.

Saving cost, reducing risk and accelerating project timelines

Drilling new core is expensive. The cost of drilling just one new offshore borehole can be in the region of £20 to £30 million, around 20 times more than the annual operational costs for the NGR. Access to existing core can therefore significantly streamline the process for new infrastructure projects; it allows both public and private sector project managers to plan with a greater degree of certainty and better mitigate risk. More informed planning can result in drilling fewer new boreholes and a shorter project timeline. This not only saves significant costs; it also reduces any associated environmental impact.

Digital scanning has unlocked new opportunities … with limitations

Digitisation of rock samples and core is a powerful tool for the modern-day geologist. Improvements in analytical techniques, including core scanning and 3D imagery, allow cores to be re-studied and preserve a record of the original material prior to sampling. These advancements are providing scientists with better opportunities to investigate changes in physical properties such as porosity (the free space inside a rock that fills with fluids).

An array of scanning technologies, including X-rays and hyperspectral imaging, allows scientists to extract more data than ever before from samples. Collectively, this data, sourced from different analytical techniques, can be compiled into digital geo-specimens that enable exciting opportunities through machine learning and artificial intelligence tools. However, there is a cost associated with scanning and digitisation. With the size of the core archive, these activities need to be targeted to deliver the largest benefit to the UK.

Although geological observations and digital samples have significant long-term value, they are limited by the context in which they were collected and the technologies available at the time. Discarding physical samples after digitising risks losing the ability to re-examine them with new techniques and technologies as they emerge in future. Digitisation enhances the samples, makes them more discoverable, and increases their value, but is not a replacement for holding physical specimens.

Safeguarding for the future

What society needs from the subsurface changes over time. The academic and commercial relevance of core varies and does so in ways that can be hard to predict. Many of the reservoir cores from the Southern North Sea gas fields, which were drilled in the 1960s and 1970s, are now being re-studied to assess their potential for carbon dioxidestorage. Sites that were once prized for their coal reserves are now being revisited for geothermal potential. These uses were almost certainly never envisaged when the core was originally drilled.

In some cases, the core may be unique and irreplaceable, especially where land has since been developed or reclassified (for example, as a Site of Special Scientific Interest). Maintaining a reference library of boreholes enables future research to take place using new techniques, saving time, reducing costs and limiting the environmental impact. Crucially, it also supports reproducible and repeatable science.

Physical space within the NGR is always a consideration. It is not possible to retain every specimen we are offered. Material is selected based on its value to inform the geological record. Sometimes, materials may be discounted or discarded where there is an abundance of material from a particular area or where samples have deteriorated, but such instances are rare. BGS is actively exploring funding opportunities to expand this national facility, so that we can continue to ingest materials critical to the UK economy.

Over the last two decades, it is estimated that the NGR has saved the UK economy at least  Â£1.5 billion in avoided drilling and analysis costs alone. The importance of this facility can only increase as we maximise the potential of geological ‘super regions’ for renewable energy technologies.

As demand for natural resources grows and the effects of climate change intensify, so does the need for geological data to address the economic and societal challenges. All indications are that the most important phase of the NGR is yet to come.