School of Earth and Environmental Sciences
Rock Mechanics Laboratory
The research of our group focuses on how rocks deform, under what conditions of pressure and temperature, and how the physics of the process generate signals such as seismicity that be used to better understand our natural world. Earthquakes, whether small or large, are a ubiquitous method for monitoring rock deformational processes across a wide scale, from deep megathrust earthquakes such as the great Tohuku earthquake of 2011 (Japan), to volcano-tectonic earthquakes that commonly precede eruptions, to small, localised, tremors associated with mining, fluid injection, and geothermal activity. By understanding the physics behind these processes, the research of the Rock Mechanics Laboratory seeks to better link seismic and elastic wave velocity data to the pressure conditions and failure characteristics of the rock mass. To achieve this, we use the latest technology in high pressure rock deformation machines, equipped with systems for controlling stress, strain, temperature and fluid flow, and instrumented with advanced Acoustic Emission recording technology – the laboratory analogue of a natural earthquake.
As well as cross-group research within the school, in particular with colleagues in CAG and crustal evolution, we collaborate widely with other groups within and outside the UK. Our group comprises five full time academic staff (Philip Benson, Nick Koor, Carmen Solana, Gareth Swift, Derek Rust), a dedicated laboratory technician and engineer (Emily Butcher), and three research (PhD) students. The laboratory has recently benefitted from a major refit thanks to significant investment by the University of Portsmouth, as well as grant funding from the European Union (FP7).
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Rock mechanics: Hydro-fracture in the laboratory and its use in linking permeable fracture networks to seismic data
Stephan Gehne, Philip Benson, Nick Koor
Hydrofracturing is a key process in many areas of pure and applied geosciences, such as the intentional hydraulic fracturing of impermeable rock formations in the hydrocarbon and geothermal energy industries. This new project investigates the dependence and fracture mechanics behaviour of the fluid driven mechanical fracture process to assess the competition between permeability and overpressure upon the derived fracture pattern. In addition, new data is being generated to test the link between the measured seismicity and the accuracy of the fracture patters with the aim of calculating the permeability of the fracture network remotely, as well as using the seismicity as a forecasting tool to mitigate risk.
Friction and slip controls on Mt Etna, and the influence of the Pernicana fault system.
Angela Castagna (Leicester), Philip Benson, Pete Rowley.
Mount Etna volcano, Sicily, sits atop a structurally complex sedimentary basement continuously subjected to tectonic deformation. Carbonate rocks represent a major component of this sedimentary basement, spread as both continuous strata and discontinuous lenses (eg, Comiso Limestone) in the foreland. To investigate how the combined effects of fluid pressure and temperature controls the stability of the edifice, this research combines analogue gauge experiments with rock physics data to better characterize the mechanical strength. New data suggests that thermal damage to carbonate units acts as a stiffener in the edifice, slowing creep and arresting large scale collapse.
Stress-induced fracture and thermo-mechanical-hydraulic coupling in crustal rocks, influence on underground excavations
Philip Benson, James Lawrence (ICL), Gareth Swift, Nick Koor
To better understand the design and rock engineering challenges facing the next generation of engineered deep geological structures, this project tests scaled-structures in a controlled laboratory environment, subjecting these to stress fields in the presence of temperature and active fluids over laboratory timescales. Data from these tests will build a new understanding of the short-term and long-term stability of rock caverns by comparing these data to numerical models and simulations that seek to extend the timescale to design times for thousands a of years, using a combination of advanced continuum / dis-continuum models together with existing finite element modes to investigate the stability of crack tips, and how these stresses are influenced by the newly generating cavern or structure.
Rock Physics: Low-frequency events generated by poorly consolidated volcanic rocks.
Chris Bean (DiaS), Pete Rowley, Philip Benson, Marco Fazio
This project tests a new hypothesis whereby the fracture of volcanic materials – especially poorly cemented/consolidated type – may provide an alternative mechanism for the generation of Low Frequency (LF) events. Conventional thinking suggests that the LF type seismic events are primarily generated due to fluid movement and resonance inside cracks, faults and volcanic conduits and there is ample evidence to support this. To investigate whether other sources (that is, besides fluid) may produce these characteristic LF events, this collaborative project seems to use synthetic samples to correlate the competency of the samples to the recorded AE data.
Physical Properties of Rocks and Minerals: mapping rock fluids in 3D via seismic tomography methods in dehydrating systems.
Ricardo Tomas, Philip Benson, Craig Storey, John Wheeler (Liverpool)
New methods for non-destructive laboratory imaging are now widespread, including X-Ray Computed Tomography and micro-seismic tomography. In field settings, such methods have greatly elucidated our knowledge of global geophysics such as subduction slab dynamics and the role that fluids play in subduction zone seismicity. It is thought that such ‘deep’ earthquakes are largely as a result of dehydrating minerals that liberate water, supported by seismic such as anomalously low Vp/Vs ratios. This research aims to test these ideas directly by simulating the system in the laboratory using a triaxial cell in which the conditions of pressure and temperature are used to stimulate the dehydration gypsum, permitting comparisons/validations with the large scale seismic interpretations, and links to pore fluid chemistry using ICP/OES analyses.
Integrating multiscale tomography techniques for determining the physical properties of the Earth from laboratory experiments to field scale:
Thomas King, Sergio Vinciguerra (UNITO), Luca DeSiena (Aberdeen)
Recent scattering and attenuation tomography studies have highlighted their effectiveness at imaging potential fluid and gas rich regions within volcanic provinces. To build on the current understanding of measurements, such as the coda quality factor Q (figure below) , this new research will combine new models of attenuation data with datasets generated from rock physics laboratory experiments. This combined approach will generate new knowledge of how routine P-Wave and S-wave velocity surveys in and around active volcanic zones, may be mapped to the movement of subsurface fluid and gas, and linked to the risk ad hazards these pose to vulnerable populations.
Instron 60 tonne uniaxial press: The workhorse of the laboratory is a 600 kN uniaxial loading frame equipped with ancillary pressure systems to provide confining pressure via a simple Hoek-type cell. Force and strain are digitally logged, and a 2 channel AE setup allows fracturing to be measured.
Controls shear box: Digitally logged hydraulic shear box for the testing of rock samples up to 115 x 125 mm.
400 MPa Large Bore “Harwood 1” Hydrostatic pressure vessel: This hydrostatic pressure vessel is one of thee rock physics vessels that were acquired from the university of Wyoming where they were previously installed and used as the laboratory of David Fountain. The largest of these can accommodate a large sample of 100mm diameter and 250mm long up to pressures of 400 MPa (approximately 16 km). The sample assembly is equipped with P-wave and S-wave transducers for measuring elastic wave velocity and is plumbed for the application of pore fluid/pressure.
600 MPa / 400C “Autoclave” Hydrostatic pressure vessel: Our mid-size hydrostatic pressure vessel is fitted with an external clamshell furnace that that heat the pressure vessel (and sample) to 400°C for measuring P-wave/S-wave velocities and AE at pressures of up to 600 MPa, approximately 24 km. This cell is used for measuring P/S ratios under conditions similar to shallow subduction and volcano-tectonics.
1400 MPa: Ultra-high pressure “Harwood 2” rock physics cell: The largest of our hydrostatic pressure vessels trades a smaller sample size in return for ultra-high pressure capability that can reach simulated depths to ~56 km. This allows us to research the deepest crustal processes such as the lower levels of subduction zones.
100 MPa Triaxial “rock physics” cell: This new triaxial cell (Sanchez engineering) is the latest addition to the laboratory, and is equipped with the latest instrumentation for dynamic permeability and elastic wave velocity measurements up to 4km simulated depths and 200°C. The maximum stress (across a 40 mm diameter) of 700 MPa allows the deformation of most rock types in the shallow crust for basic research into earthquake and rock physics.
ASC “Milne” and “Richter” advanced Acoustic Emission system: This state-of-the art Acoustic Emission recorder (Applied Seismology Consultants) can measure 12 channels of AE data simultaneously, and locate the locations of the earthquakes in quasi-real time for display and analysis at data rate of up to 100 events/second. The instrument can also be used in a active mode, ‘pinging’ successive sensors to generate a dense P-wave raypath network for tomography. The recorder is used during triaxial deformation experiments to investigate the fracturing and cracking processes, due to applied stress and pore pressure, and with reference to fluid resonances.
The lab facilities are available for commercial contract work, as well as collaborative research projects. For more information see our Scientific Testing and Research Facilities website, or contact Pete Rowley.
- Frederico Barata
- Angela Castagna (Leicester)
- Marco Fazio
- Stephan Gehne
- Thomas King (UNITO)
- Helen Lacey
- Ricardo Tomas
- Dr. Richard Bakker (TU Delft)
- Prof. Chris Bean (DiAS, Dublin)
- Dr. Hamed Ghaffari (University of Texas)
- Prof. Philip Meredith (UCL)
- Prof. Sergio Vinciguerra (University of Turin)
- Dr. James Lawrence (Imperial college London)
- Dr. Luca DeSiena (Aberdeen)
- Dr. John Browning (UCL)