We study the physical behaviour of ice and rocks that make up the surface and interior of the Earth, and other solid bodies in the solar system, so as to constrain the dynamic, tectonic and environmental processes of planetary evolution. Our research is nationally unique and multi-disciplinary, being based on experiment and theory. Our main research themes include:
In the Rock Physics Laboratory, our aim is to investigate the mechanical, physical and transport properties of crustal rocks. We perform deformation experiments on rocks under conditions that simulate the pressure, temperature, deviatoric stress, and pore pressure environment of the upper (seismogenic) crust. Importantly, we also measure changes in a range of key properties (porosity, permeability, wave velocities, microseismic emissions and electrical conductivity and potential) simultaneously with deformation.
These are the properties that are routinely measured on the reservoir or crustal scale. Thus, we are able directly to link the changes in physical and transport properties to the state of stress and deformation in the rock under carefully controlled conditions. This cannot be done at the reservoir or crustal scale. Thus, we are able to provide insights into improved interpretation of reservoir and crustal scale geophysical data.
Major Research themes:
- Time-dependent rock deformation driven by rock-fluid interactions and chemo-mechanical processes.
- Deformation under true triaxial conditions replicating stress states in the crust.
- Ultra-high strain rate rock deformation and pulverization.
- Permeability evolution and anisotropy in crack–damaged rocks.
- Physical, mechanical and transport properties of shales and mudstones.
- Response of bentonite to chemo-mechanical conditions related to long term storage of radioactive waste.
- Deformation of rocks across the brittle-ductile transition under lower crustal P/T conditions to 1 Gpa and 1000º C.
Water-saturated rocks are ubiquitous in Earth’s upper crust and the chemistry of water-rock interactions leads to time-dependent deformation through the mechanism of stress corrosion that allows rocks to fail over extended periods of time at stresses far below their short-term strength. Recent theoretical models, based on mean-field damage mechanics, suggest that the growth and interaction of cracks leads to acceleration to failure once some critical damage threshold has been reached. Recent experimental observations in the Rock Physics laboratory at UCL (Baud & Meredith, 1997).support this approach.
We are therefore investigating this process by conducting constant stress, brittle creep experiments in our triaxial deformation apparatus. Because the process is highly non-linear, it is necessary to run experiments lasting from a few tens on minutes to a few tens of days in the laboratory.
In order to extend the range of observation even further, we would need to conduct experiments with durations of months or even years. Clearly, such experiments are not practicable in a normal laboratory. We have therefore constructed an apparatus to allow us to conduct ultra-long-term experiments at 2km depth in a deep-sea observatory in the Ionian Sea off Sicily. The main reason for considering the deep-sea environment is its stability; constant pressure and temperature throughout the year. The constant pressure at depth provides both the confining pressure and the constant creep stress (via a pressure intensifier).
To date, we have run experiments up to 6 months duration and achieved strain rates in the range 10-11 s-1; some two orders of magnitude lower than our slowest laboratory experiments. Importantly, these rates encompass the time-scales typical of the periods of precursory activity that precede major crustal failure events, and they bridge the gap between laboratory strain rates and those typical of the crust.
Influence of pore space anisotropy on the development of compaction bands
Compaction bands are newly recognized but ubiquitous feature of the deformation of high porosity sedimentary rocks. They are generally manifested as densely packed, poorly sorted, low porosity bands in otherwise well sorted, high porosity rocks. They are important because they occur as low permeability bands in generally high permeability rocks, and thus act to compartmentalize reservoirs and aquifers.
Figure indicating Differential stress vs Effective mean stress /Mpa
Anisotropy of pore space is also a ubiquitous feature of sedimentary rocks, which leads to an anisotropy of the physical, transport and mechanical properties. We are therefore studying compaction band development in Diemelstadt sandstone; a bedded, anisotropic sandstone with an initial porosity of abut 23% and a mean grain size of about 0.3 mm .
We have quantified the pore-space anisotropy of Diemelstadt sandstone by measuring radial elastic S and P wave velocities as a function of azimuth around cores samples taken in three orthogonal directions. Fluid permeability has also been measured along these three principal directions and the technique of pAMS (Benson et al., 2003) has been used to quantify the anisotropic pore space geometry. We find a velocity anisotropy of around 10% for P-waves and 5% for S-waves, lower permeability normal to bedding, and a mean pore space geometry approximating to an oblate spheroid.
With the pore-space anisotropy established, we characterised its influence on mode of failure by performing triaxial deformation experiments on samples cored normal and parallel to bedding. Both orientations demonstrated a transition from brittle faulting at low effective pressure to the growth of discrete compaction bands at higher effective pressure. We found that the compactive yield envelope for the bedding-normal samples expanded more towards higher stress values than the envelope for bedding-parallel samples.
We find that compaction bands formed both normal and parallel to bedding act as barriers to flow and reduce bulk permeability, consistent with previous work (Vajdova et al., 2004). Microstructural comparisons of the discrete compaction bands formed parallel vs normal to bedding show that those formed normal are more tortuous and less extensive than those formed parallel.
- Permeability Modelling
Modelling the permeability evolution of micro-cracked rocks from elastic wave velocity inversion at elevated isostatic pressure.
A key consequence of the presence of microcracks within rock is their significant influence upon elastic anisotropy and transport properties. In this study, two microcracked rock types (a basalt and a granite) with known but contrasting microstructures have been investigated using an advanced experimental setup to measure sample porosity, P-wave velocity, S-wave velocity and permeability contemporaneously for effective pressures up to 100 MPa (Rock Fluids Laboratory).
Using the Kachanov (1994) non-interactive effective medium theory, the laboratory measured elastic wave velocities are inverted using a least square fit, allowing us to evaluate the evolution of crack density and crack aspect ratio with increasing isostatic pressure. Overall, the agreement between measured and predicted velocities is good, with average error less than 0.05 km/s. At larger scales above the percolation threshold, macroscopic fluid flow also depends on the crack density and aspect ratio. Using the permeability model of Guéguen & Dienes (1989) and the crack density and aspect ratio recovered from the elastic wave velocity inversion, we successfully predict the evolution of permeability with pressure for direct comparison to the laboratory permeability results. In addition, we calculate the influence of the crack porosity element, also based upon crack density and evolution, for direct comparison to the experimentally measured porosity change with isostatic pressure. These combined modelling/experimental results illustrate the importance of understanding the details of how the rock microstructure can change in response to an external stimulus in order to predict the common evolutions of rock physical properties.
Ice & Climate Physics
- Arctic Ocean Dynamics:
Geophysical Scale Sea Ice Rheology from Laboratory Experiments
The UK will be the largest population concentration to be affected by Arctic climate change in the coming decades and understanding the Arctic cryosphere is now a key objective in UK science. Our general aim is to improve the representation of sea ice dynamics in GCMs. We will do this by relating sea ice rheology on climatically important length scales to the material properties of sea ice as measured in the laboratory. But we will focus on sea ice friction.
Sea ice is notably brittle. Ice deformation in the Arctic Ocean, driven by wind shear, causes formation of thick ice through pressure ridging and thin ice through creation of open-water leads. To improve current models it is essential to incorporate brittle-discontinuous processes into rheological models. The aim of this research project is to establish a geophysical scale sea ice rheology from laboratory experiments.
In collaboration with UCL CPOM we have been doing laboratory experiments in the Ice Physics Lab on ice friction and large scale ice floe friction simulations in the environmental ice basins at Helsinki University of Technology and the HSVA Hamburg Ship Testing Ice Basin. We have demonstrated that a new friction law is required for sea ice dynamics. The next step is to incorporate this into large scale models of sea ice dynamics.
- Antarctic Ice Sheet
Flow of the Antarctic Ice Sheet
We are studying the anistropic flow of the Antarctic ice sheet with the aim of improving models of ice sheet response to climate change.
Steve Boon, Neil Hughes and Peter Sammonds have designed and built a new unique triaxial deformation and tomographic imaging cell for ice, funded by the Paul Instrument Fund. We are now testing EPICA ice from the European deep boreholes in Antarctica at Dome C and Dronning Maud Land. We are collaborating with the Alfred-Wegener Institut, Bremerhaven and the British Antarctic Survey in the EPICA programme.
It has been recognised, rather than treating glacier motion as being a combination of internal deformation and sliding over bedrock alone, that as unlithified sediments underlie many glaciers, deformation of these subglacial sediments makes a significant contribution to overall glacier motion. Crucial to the deforming bed model of glacier motion are the mechanical properties of these sediments. Beatrice Baudet (Civil Engineering, UCL) and Peter Sammonds are now measuring the rheological properties of glacial sediments from Langjokull ice cap in Iceland. This will enable us to model the response of small ice caps to climate change.
High Pressure Petrology
The Haskel Multi-Anvil Laboratory conducts research into the effects of high pressure and high temperature on igneous rock/mineral/magma systems using a solid media high pressure apparatus.
Areas of research include understanding the origins of mantle derived melts and minerals (silicate and non silicate, but especially carbonate systems) , and results are incoporated into undergraduate teaching courses.
- High Temperature Fracture
Mechanics of Dome Lava [Standard NERC grant]
Volcanoes that erupt highly-viscous lava domes are amongst the most hazardous, intensely studied and newsworthy of geological phenomena, as the past twenty-five years of activity at Mount St Helens, USA have demonstrated. A key challenge in geological science is how to predict reliably the sudden changes in behaviour that are typical of such domes, from gentle lava effusion to devastating explosive eruptions. Much progress has been made in recent years in studying eruptive behaviour, with the realisation that the material response of magma to applied stresses largely controls eruption mechanisms and the rheology of dome lava changes greatly during shallow degassing and crystallisation. A goal of the current generation of models for dome growth is to identify thresholds in behaviour (e.g. explosive/effusive eruptions, endogenous/exogenous dome growth) that are closely related to the fundamental physical behaviour of whether deforming magma will fracture or flow.
To meet these goals, Dr Rosanna Smith is conducting experiments in the fracture mechanics laboratory withProf Peter Sammonds, Dr Hugh Tuffen, Prof Philip Meredith, Mr Neil Hughes and Mr Steve Boon in collaboration with the Cascades Volcano Observatory. They are deforming lava dome material from the recent eruption of Mt St Helens under simulated volcanic conditions (stress, pressure, temperature, and pore fluid conditions) whilst recording acoustic emissions (AE) and stress-strain relationships. These experiments address the role of fracture, friction and the brittle-ductile transition during volcanic lava dome growth and in magma conduit dynamics. Characteristics of AEs recorded during these laboratory experiments are compared to those of seismic events recorded during the emplacement of the lava dome from which the samples were taken in order to aid interpretation of volcanic seismicity. These laboratory studies will provide direct input into the development of the current generation of models of volcanic dome behaviour, which hinge upon whether deforming magma will flow, slip or fracture.
This project builds on earlier work on rates of rock fracture before eruptions, seismogenic fracture of obsidian magma, and fracturing of basalt lava flows.
- Hazard Prediction
Experimental reconstruction and characterisation of Long-Period harmonics with application to volcanic hazard prediction: The laboratory volcano [EU Marie-Curie Outgoing International Research Fellowship (EU FP6)]
Dr Philip Benson(RIPL, UCL and Lassonde Institute, University of Toronto) with Prof Philip Meredith (Co-PI), Dr Sergio Vinciguerra (INGV, Rome), and Dr Chris Kilburn (Benfield UCL Hazard Research Centre).
Europe includes some of the most volcanically active regions on Earth, hosting about 6% of the 600 volcanoes known to have erupted in historical time and, of those, 2-3 are normally in eruption each year. Some 4-5 million people live within sight of an active European volcano, and ~10% of the EU population is economically vulnerable. Seismicity and ground deformation are the precursory phenomena most frequently seen before eruption, as the Earth's crust is distorted by magma moving to the surface, and as fluids (magma / gas / hydrothermal fluid) move within faulted rock. Final approach to eruption is commonly preceded by accelerating rates in the rate of low magnitude volcano-tectonic (VT) earthquakes and of long-period (LP) events (seismic signals unique to volcanoes and associated with fluid movement). Although the association of LP events with volcanic activity is not new, the specific mechanisms for LP generation is poorly understood. This project is, for the first time under in-situ conditions, generating unique, well-constrained laboratory data under simulated volcanic conditions of stress and temperature. Using state-of-the-art acoustic emission recording systems, microseismic events due to brittle failure and fluid movement (in a manner analogous to LP events at field scale on a volcanic edifice) have been recorded in the rock physics laboratory. By comparing this data to published field monitoring and theoretical data, this work is contributing to improved methods for investigating short-term precursors before volcanic eruptions.
Recent results include the full-waveform recording and event location of Etna basalt deformation in real-time, and – for the first time – without the need to artificially slow the failure process using AE-feedback servo control. In addition, frequency analysis of brittle deformation AE events shows marked differences to pore fluid movement and decompression events. Future experimental plans will investigate these data with reference to field seismic data, and expand the laboratory deformation studies to true-triaxial conditions in order to explore the complex stress components acting during volcanic flank collapse.
- Recent Research Projects
Dr Rosanna Smith studied fracturing rates before volcanic eruptions for her PhD with Dr Chris Kilburn and Prof Peter Sammonds in the UCL Hazard Research Centre and Rock & Ice Physics Laboratory. This involved triaxial, uniaxial, and tensile deformation of andesites under simulated volcanic conditions in the fracture mechanics laboratory. Acoustic emission rates during these experiments were compared with rates of volcano-tectonic earthquakes before volcanic eruptions.
Dr Hugh Tuffen deformed obsidian at temperature and strain rates within the brittle-ductile transition in the fracture mechanics laboratory whilst recording acoustic emission rates (2005-2006). This work identified the mechanical properties of obsidian in the glass transition in compressive stress regimes, highlighting that the behaviour is different to that expected from extrapolating results from tensile experiments on synthetic glasses (often used in volcanology). The high AE rates at conditions very close to the glass transition also showed that earthquakes can occur in hot magma, with such small changes in extrinsic conditions necessary for the transition from ductile flow to seismogenic fracture that they may occur concurrently.
Dr Valentina Rocchi studied fracture of basalts under simulated lava flow conditions for her PhD with Prof Peter Sammonds and Dr Chris Kilburn in the Rock & Ice Physics Laboratory and the Benfield UCL Hazard Research Centre . The high temperature triaxial deformation cell was designed as part of this project.