The Heat Beneath Our Feet

Around 300 million years ago, this part of Yorkshire looked very different. Dense tropical swamps covered the landscape, home to early amphibians and giant dragonflies. Over time, layers of sand and finer sediment were deposited by the ancient rivers here, that flowed across a vast river delta. The sand and sediment built up, were buried, hardened into rock and formed the ‘Elland Flags’— a wide band of sandstone beneath Leeds and West Yorkshire. 

Geologists can read traces of this ancient landscape in 'outcrops' — places where rock is exposed at the Earth’s surface — and in local quarries where stone has been extracted for building and paving. By observing how rock layers fit together, geologists can investigate how the landscape formed and how its structure has changed over time. 

At the Elland Edge quarry, shown in the photograph below, the sandstone preserves the shapes of these ancient river channels. Several channels can be seen stacked on top of one another, recording how rivers shifted from side to side and carved new routes through the landscape.

Researchers Joe Kelly and Arka Dyuti Sarkar used measurements of the outcrop, photography, and field sketches to study the Elland Flags aquifer (a body of rock that holds groundwater). They examined the size and texture of the sand grains within the rock, identified different rock types and recorded how the layers fit together.

The darker bands in the photograph above, are mudstone and claystone layers, formed from much smaller sediment grains that are packed more tightly together than the sandstone. Because water moves through these layers less easily, they can block groundwater flow. Fractures provide vertical passages down through the sandstone, but if they are blocked at the mudstone layers, the movement of water will come to a stop.

By studying these features, geologists build a picture of the underground architecture — the arrangement of rock layers and fractures that influences where groundwater might travel.

Boreholes are narrow holes drilled into the ground so scientists can investigate the rocks and water below. The boreholes drilled for the University of Leeds Geothermal Campus project reached a depth of almost 250 metres. That's as far as 10 lengths of a swimming pool!

Researchers can lower cameras and specialist instruments into boreholes to explore what lies hidden underground. Cameras provide photographs of the rock wall, revealing its natural colours, layers and visible fractures.

Imagine peeling the label off a baked bean tin and laying it flat: that’s how scientists turned images of the borehole wall into a long, flat picture of the rock underground.

Another tool uses ultrasound to investigate features deeper inside the rock, that a camera couldn’t capture. It sent sound pulses through the water in the borehole. These bounced back from the surrounding rock, creating echoes that researchers could analyse to build a picture of the rock’s structure. This can help them identify cracks or fractures in the rock that influence how water moves underground. It’s a bit like giving the underground its own medical scan.

Geologists also study long cylinders of rock, called cores, which are extracted from underground and brought to the surface. During drilling on the Leeds Geothermal Campus, scientists brought more than 500 metres of core up to the surface, revealing the rock formations beneath the University campus. That’s roughly the length of twelve Diplodocus dinosaurs lined up nose to tail!

Sedimentologists like Richard Collier studied the cores and recorded their features in sedimentary logs to piece together how the landscape changed over time. Understanding what rocks were being deposited in these ancient environments, and what processes they’ve since undergone, helps researchers make predictions about water flow today.

The cores also contain fossils that offer further evidence of the ancient environment and the plants and animals that lived here millions of years ago. Palaeontologist Paul Wignall helped interpret these clues, including the fossils brought to life in our Light Night projection.

Fossilised plants such as ferns and now-extinct trees called calamites, reveal a landscape of lush, tropical vegetation, very different from Yorkshire today. In swampy forests, dead plants accumulated as peat, which was eventually buried and transformed into coal over millions of years. These coal deposits later became an important fuel during the Industrial Revolution. Because burning coal releases carbon dioxide, a greenhous gas that drives climate change, we now need cleaner and more sustainable energy sources to help reduce emissions and move towards a low-carbon future.

Look closely at these fan-shaped shells found in one of the cores brought up from below the University campus. What are they doing beneath Leeds, so far from the sea? They belonged to a thin-shelled, now extinct, marine animal called Dunbarella. Their presence, along with spiral-shelled marine creatures called goniatites, tell us that the sea periodically flooded this region, transforming the landscape once again.

A core sample gives researchers a real piece of rock from deep underground. But what if we could look inside it without breaking it open?

Researchers at the University of Leeds use microscopes to examine very thin slices of rock, revealing the tiny grains that make it up and the pore spaces between them.

They also use a technology called X-ray computed tomography, or XCT. It works like the modern baggage scanners at an airport that can see inside your bag without you opening it. It takes hundreds of X-ray images from different angles to build a detailed, three-dimensional picture of the inside of a rock.

Researchers Sam Allshorn and Renato Diamanti use these scans to explore how water might travel through rock. Water can trickle through tiny spaces between grains or find its way through cracks and fractures. The more connected pathways there are, the more routes water may have to flow through.

This water movement is important because when water flows it carries heat with it. This naturally occurring heat can be used to heat our buildings in Yorkshire.

But how do we go about working out how much heat might be hidden deep underground in the rocks and water beneath our homes?

Imagine lifting up Leeds like the top of a giant cake to reveal its layers. That’s what a three-dimensional geological model allows researchers to do! Mohamed Gouiza and his colleagues combined geological maps, records from more than 60 boreholes, information about historic coal mines, rock studies and temperature measurements to create a digital picture of the rocks beneath Leeds and estimate how much heat they might contain. Research focused on Leeds is helping build knowledge about the geothermal potential of the wider region.

The model identified several potential geothermal resources. The Lower Millstone Grit emerged as the largest potential heat resource in the study. These deeper rocks could offer access to higher temperatures, but the shallower Elland Flags may provide an easier, less costly option for local heating schemes. Flooded coal mines, in places like Wakefield, could also have a new purpose: mine water can provide low-temperature heat, and old mine workings may offer opportunities to store heat and recover it later.