In March 2023, researchers from geology and geography gathered in the Department of Earth, Ocean, and Ecological Sciences to explore new cross-disciplinary applications of cutting-edge lab facilities in the department.
Led by Dr Janine Kavanagh (https://www.liverpool.ac.uk/environmental-sciences/staff/janine-kavanagh/), the Mechanical and Geological Model Analogues (MAGMA) Laboratory (https://www.liverpoolmagmalab.org/) specialises in using laser imaging and scaled analogue models to explore magma-driven propagation in volcanic systems. Using specific scaling parameters, understanding of fracture occurring in laboratory materials (e.g., water or oil through gelatine) can be transferred to understanding of subsurface events.
However, fluid flow through open fractures is a phenomenon that occurs across the natural world. In glaciers and ice sheets, fluid-driven fracture can lead to the drainage of supraglacial lakes and the calving of icebergs, whilst in geothermal energy applications, maintenance of open rock fractures is critical for transfer of liquid and heat to the surface. However, scaled analogue models have had extremely limited applications to glaciology or geothermal systems where, similar to volcanology, direct observations of processes are difficult or impossible to obtain. This is despite the fundamental theory of fluid-filled crack propagation now commonly used in volcanology analogue experiments (a ‘Weertman crack’) originating from a study in glaciology in the 1970’s.
Funded by a NERC Cross-Disciplinary Research for Environmental Sciences Discovery grant, Dr Kavanagh hosted researchers from across disciplinary boundaries to explore the opportunities that analogue modelling can provide. At Liverpool, Dr David McNamara (https://www.liverpool.ac.uk/environmental-sciences/staff/david-mcnamara/research/) studies fluid flow through the Earth’s crust with a specific interest in fractured geothermal resources and Dr James Lea (https://www.liverpool.ac.uk/environmental-sciences/staff/james-lea/) specialises in the stability of marine-terminating glaciers. Visiting from the Department of Geography at Durham University, Dr Tom Chudley (https://www.durham.ac.uk/staff/thomas-r-chudley/) explores controls on the transfer of water through ice sheets.
In the first half of the week, the team explored the stress distribution surrounding active fracture systems using a combination of dyed water as the intruding liquid and gelatine as the solid medium. Using polarising filters, stress distribution can be visualised within the transparent gelatine due to its photoelasticity and as a fluid-filled fracture passes through it. The team were introduced to the basics of the lab experiments, and on the second day were able to observe how stress could be transferred across pre-made fractures when dry, but not when filled with water.
This has implications in glaciological applications, where fractures can close (‘heal’) or remain active when filled with water. From a geothermal perspective this is interesting as it may help us understand how reservoirs become compartmentalised and how stress distribution may evolve with time in a reservoir as open fractures becomes closed due to mineral growth.
In the second half of the week, work moved to the state-of-the-art MEDUSA Laser Imaging facility. Here, the intruding fluid is seeded with luminescent passive-tracer particles, which, when illuminated by the MEDUSA laser, can be tracked in sequential imagery using Particle Image Velocimetry (PIV) to explore the way that fluid flows within the fracture. The team explored the buoyant propagation of vegetable oil through gelatine, simulating ‘upside down’ the gravity-driven propagation of a water-filled crevasse in ice. The particle movement showed distinctive circulating flow-patterns transition to upward buoyant propagation when the liquid stopped being pumped into the experiment and continued to fracture on its own.
Alongside exploring the capabilities of the equipment, the team worked towards developing appropriate scaling parameters for new applications. Scaling parameters allow for the lab scale to be converted into a real-world approximation – for instance, by calculating the amount of ‘natural’ time represented by one second of experiment time. Scaling factors exist for time, space, the velocity of fracture propagation, and material strength (Young’s Modulus).
As the natural material rheology and spatial-temporal scales differ widely for geothermal and glaciological propagation compared to magmatic applications, the team outlined potential changes to lab materials that could allow for the technique to be transferred from one scenario to another. This included the use of denser fluids or stronger solid mediums. The team also produced prototype programming scripts that could allow for the rapid modelling of new materials and their suitability to different natural problems.
After a productive week, the team are ready to move forward with a new appreciation of laser-imaging and scaled analogue modelling and aim to produce a paper on their preliminary experiences and scaling work. In the future, further cross-disciplinary grant applications could be made to allow the development of the technique into more complex geophysical environments.
The team are grateful for funding from the NERC Cross-Disciplinary Research for Environmental Sciences Discovery scheme, and toDr Caitlin Chalk and Amanda Valentine-Baars for their tireless work during the week setting up and running the lab experiments.
Portrait of an Earth scientist – Rob Duller
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