HR: 09:00h
AN: MR31B-05 INVITED     [Abstracts]
TI: Micromechanics of Brittle Rock Deformation Revealed Through Measurements of Crack Damage Evolution
AU: * Meredith, P
EM: p.meredith@ucl.ac.uk
AF: University College London, Department of Earth Sciences, London, WC1E 6BT United Kingdom
AB: Traditionally, the mechanical properties of brittle rocks have been represented via stress-strain curves for short-term deformation or strain-time curves for long-term deformation. However, such curves generally represent globally averaged deformational properties and do not necessarily provide information about the micromechanisms responsible. Here we report multiple internal measurements of changes in rock physical and transport properties during deformation that act as proxies for estimating both the state and evolution of crack damage. Ultrasonic wave speeds are highly susceptible to the total number and size of cracks and their measurement can therefore provide information about the instantaneous state of damage. By contrast, the output of acoustic emission energy is related only to the rate of active cracking and therefore provides us with information about the evolution of crack damage with strain or time. If we further relate the size of individual acoustic emission events to the size of individual increments of crack growth, we can calculate a proxy crack damage parameter and thereby reconstruct stress-strain curves from the acoustic data. However, this reconstruction tends to break down once cracks start to interact and coalesce. This is not helpful, since this is the part of the deformation cycle that is commonly of most interest. The problem can be overcome by applying the same technique to measurements of transport properties such as fluid permeability or electrical conductivity which are inherently related to the pathways provided by crack linkage processes. Essentially, all of these measurements provide partial but complementary information. Our understanding of crack damage evolution is therefore best served by simultaneous measurement of several different damage parameters during deformation experiments. For example, we report simultaneous measurements of strain, acoustic emission energy and internal void volume during constant stress creep experiments. All three measures of damage evolve very similarly with time. Furthermore, the onset of mechanical instability that leads to fault generation occurs at the same level of damage for each parameter regardless of the time-to-failure. This implies that a critical level of damage is required for instability to occur. We have techniques for monitoring the damage evolution, but we will only be able to predict the onset of failure if we can define the initial state of damage. This has not generally been done in most reported rock deformation studies, but we consider that is should be an essential component of current and future studies. We therefore present methodologies for both characterizing and controlling the initial state of damage in experimental rock samples.
DE: 1822 Geomechanics
DE: 3902 Creep and deformation
DE: 5102 Acoustic properties
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005