Shear-Wave Splitting in a Critical Crust: II - Compliant, Calculable, Controllable, Fluid-Rock Interactions
Stuart Crampin, Sebastien Chastin, 2001. "Shear-Wave Splitting in a Critical Crust: II - Compliant, Calculable, Controllable, Fluid-Rock Interactions", Anisotropy 2000: Fractures, Converted Waves, and Case Studies, L. Ikelle, A. Gangi
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This paper argues that the pervasive distributions of closely-spaced stress-aligned fluid-saturated microcracks in almost all rocks are a critical system close to fracture criticality and loss of shear strength. New evidence includes three examples in which observations and modelling directly imply non-linear interactive critical systems with some form of self-organised criticality (SOC). These are a direct calibration of anisotropic poro-elasticity (APE) by monitoring and modelling the response of a reservoir to a high-pressure injection. Monitoring and modelling velocity and attenuation dispersion in a rock physics laboratory. Monitoring the effect of the build-up of stress before earthquakes and volcanic eruptions, including the successful stress forecast of the time and magnitude of an ML=5 (MS ≈ 6) earthquake in southwest Iceland.
These new results from three very different fields strongly suggest that the earth's crust is a critical interactive non-linear system with self-organised criticality (SOC). Some effects are subtle and easily ignored. Others are so common and familiar that we have developed one-off explanations in terms of conventional deterministic physics to describe their behaviour and occurrence. We suggest that the identification of the sub-critical physical processes is one reason for the success of APE-modelling.
Recognition of (crack) criticality leads to a new understanding of low-level (pre-fracturing) deformation that has massive implications for almost all dynamic processes in the crust. These include reservoir characterisation, hydrocarbon recovery, monitoring the progress of fluid-fluid fronts, and the build-up of stress before fracturing, faulting, and earthquakes, and the movement of magma before volcanic activity. The implications will be discussed and the arguments presented.
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“This volume contains 25 papers that represent most of the best work in seismic anisotropy in 1998 and 1999. Fracture characterizations and processing of converted waves are the two main topics covered in this volume. They are addressed from both theoretical and practical viewpoints. Also included are papers describing the historical roots of seismic anisotropy.”