Shear-Wave Splitting in a Critical Crust: the Next Step

Abstract

On pourrait avancer que l'anisotropie dans la biréfringence des ondes transversales n'a pas répondu à ses promesses initiales, à savoir ouvrir une nouvelle voie dans la compréhension des phénomènes de fissures et de contraintes dans la croûte terrestre. Dans cet article sont présentés deux développements révélés récemment, qui paraissent raviver ces premiers espoirs et apportent des opportunités nouvelles pour le contrôle, la modélisation et même la prévision des déformations (avant fracture) dans les roches microfracturées et saturées de fluides.Ainsi, un modèle de poroélasticité (APE) développé récemment concerne l'évolution sous contraintes des roches microfracturées et saturées en fluide et reproduit une large gamme de phénomènes, qui seraient autrement inexpliqués ou dissociés, et semble être une bonne approximation au premier ordre de l'évolution des roches microfracturées et saturées en fluide. Puisque les paramètres qui contrôlent à petite échelle la déformation (avant fracture) contrôlent aussi la biréfringence des ondes transversales, il apparaît que l'évolution des roches microfracturées et saturées peut être aussi directement contrôlée par cette biréfringence et que la réponse à des changements futurs peut être prédite par l'APE.Le bon usage de la modélisation de l'APE et des observations de la biréfringence des ondes transversales implique que la plupart des roches soient proches d'un stade de fracturation critique associé à une percolation limite, situation où la résistance aux contraintes transversales disparaît et où les fractures transversales peuvent se propager. Ceci corrobore une autre hypothèse concernant la mise en situation critique spontanée des roches in situ. La conséquence de cette identification est que la physique à petite échelle, qui contrôle l'ensemble du phénomène, peut maintenant être associée aux contraintes générées par les fluides baignant les fissures intergranulaires. Ceci a probablement l'avantage unique, parmi les systèmes critiques, de donner la possibilité de contrôler en chaque point les détails des déformations avant fracture et l'approche du seuil critique (dans ce cas l'approche d'une fracturation), au moyen d'observations adéquates de la biréfringence des ondes transversales.Cet article passe en revue ces développements et commente leurs conséquences et leurs applications, en particulier les conséquences sur la mise en situation critique des roches spontanément. L'étape suivante sera d'exploiter ces techniques pour modéliser, contrôler et prédire les effets de changements de conditions sur la déformation de la masse rocheuse. Copyright © 1998, Institut français du pétrole understanding cracks and stress in the crust. This paper reviews two recent related developments which appear to renew these initial hopes and provide new opportunities for monitoring, modelling, and even predicting, the (pre-fracturing) deformation of fluid-saturated microcracked rock. SHEAR-WAVE SPLITTING INA recently developed model of anisotropic poro-elasticity (APE) for the stress-induced ev...

A review of shear-wave splitting in the compliant crack-critical anisotropic Earth

“…However, macrofractures lead to seismic reflections and refractions and cause shear-wave splitting only when they are numerous enough and then they lead to significant attenuation and deterioration of the slower split shear-wave [51]. In addition, microcracks are sensitive to the contemporary Table 2 Some practical implications of critical crack systems with SOC for fluid-rock interactions within the Earth's crust (references in [4]) (a) General implications (1) Fluid-saturated crack distributions are highly compliant and crack geometry responds to small nearly negligible changes of stress, pressure, and physical properties of the pore fluids (2) Since fluid-rock properties vary with time, and vary from place to place, measured fluid-rock properties are only strictly valid at the place and time they are measured. Hence, the need for measurements with single-well imaging if accurate specifications are required (3) Since fluid-rock interactions have a dominant effect on almost all physical and chemical behaviour within the crust and mantle (see a2, above), these various effects apply to a huge range of geophysical phenomena, particularly those associated with any deformation, including almost all processes during hydrocarbon recovery (4) Behaviour of stress-aligned fluid-saturated crack distributions appears to be remarkably uniform (within certain limits) even in very heterogeneous structures (5) Pre-fracturing deformation of any given fluid-rock configuration can be monitored by observations of shear-wave splitting (6) Pre-fracturing deformation can be modelled by anisotropic poro-elasticity (APE) (7) Response of fluid-rock systems to known changes can be calculated by APE (8) Response to calculated changes (a6, above) can be monitored by shear-wave splitting (a5, above), and the response controlled by feedback by adjusting changes to optimise the response (b) Specific implications Implications for hydrocarbon exploration and production (1) Reservoir properties may change from place to place (2) Reservoir properties may change with time, even without production processes (3) Relevant properties need to be measured at the place and time they are needed (4) Response to known changes can be calculated and predicted (5) Response of a reservoir can be controlled, in the sense of a8, above (6) Possibility of long-range and long-time correlations across and between reservoirs (7) There is a limit to the temporal and spatial resolution of any particular measurement (c) Implications for earthquake geophysics (1) Deterministic prediction of time, magnitude, and place of large earthquakes is likely to be impossible (2) With sufficient source seismicity or appropriate cross-hole SMS observations, times and magnitudes of future large earthquakes can be stress-forecast.…”

“…The underlying reason for the universality is that the fluid-saturated microcrack distributions are so closely spaced that they are critical systems possessing self-organised criticality [4][5][6]36]. Table 1 summarises the match of APE-modelling to observations.…”

“…The reasons for this sensitivity is thought to be that the fluid-saturated cracks are critical systems verging on fracture-criticality with all the sensitivity, universality, and calculability that criticality implies [4,5,7,[34][35][36]. This criticality means that there are spatial and temporal heterogeneities so the detailed behaviour and properties of much of the crust and upper mantle cannot be extrapolated from place to place or from time to time.…”

“…This means that changes in shear-wave splitting directly monitor changes in rock mass deformation. We suggest that the ability to monitor low-level deformation with shear-wave splitting is probably the most fundamental advance in solid Earth geophysics for several decades [4][5][6][7]. Thus monitoring shear-wave propagation in anisotropic rocks is opening a new window into our understanding of rock deformation and the evolution of crustal and mantle properties.…”

Preminary Results of the iSTEP Program on Integrated Search for Taiwan Earthquake Precursors

“…Changes in shear wave splitting: Shear wave splitting can be used to monitor the effects of stress build-up before earthquakes, and hopefully to forecast future large earthquakes Crampin and Zatsepin 1997;Crampin 1998). The goal of this study is to systematically examine possible anisotropy changes before earthquakes and look for possible earthquake precursors.…”