The fracture criticality of crustal rocks

SUMMARY The shear-wave splitting observed along almost all shear-wave ray paths in the Earth’s crust is interpreted as the effects of stress-aligned fluid-filled cracks, microcracks, and preferentially oriented pore space. Once away from the free surface, where open joints and fractures may lead to strong anisotropy of 10 per cent or greater, intact ostensibly unfractured crustal rock exhibits a limited range of shear-wave splitting from about 1.5 to 4.5 per cent differential shear-wave velocity anisotropy. Interpreting this velocity anisotropy as normalized crack densities, a factor of less than two in crack radius covers the range from the minimum 1.5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near-surface rocks. This narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4.5 and 10 per cent marking the boundary between essentially intact, and heavily fractured rock. When the level of fracture criticality is exceeded, cracking is so severe that there is a breakdown in shear strength, the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability. The range of normalized crack dimensions below fracture criticality is so small in intact rock, that any modification to the crack geometry by even minor changes of conditions or minor deformation (particularly in the presence of high pore-fluid pressures) may change rock from being essentially intact (below fracture criticality) to heavily fractured (above fracture criticality). This recognition of the essential compliance of most crustal rocks, and its effect on shear-wave splitting, has implications for monitoring changes in any conditions affecting the rock mass. These include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery, and in monitoring changes before and after earthquakes, amongst others.

[1]  L. Nicoletis,et al.  Shear-Wave Birefringence Measurement And Detection In the Paris Basin , 1989 .

[2]  J. C. Jaeger,et al.  The Mechanical Properties of an Interlocked Low-Porosity Aggregate , 1968 .

[3]  L. Thomsen,et al.  Reflection shear-wave data collected near the principal axes of azimuthal anisotropy , 1990 .

[4]  X. R. Shih,et al.  Observation of shear wave splitting from natural events: South Moat of Long Valley Caldera, California, June 29 to August 12, 1982 , 1990 .

[5]  S. Crampin,et al.  Shear-wave polarizations in a hot dry rock geothermal reservoir: Anisotropic effects of fractures , 1986 .

[6]  Stuart Crampin,et al.  A decade of shear-wave splitting in the Earth's crust: what does it mean? what use can we make of it? and what should we do next? , 1991 .

[7]  Mark A. Meadows,et al.  Shear-wave polarizations and subsurface stress directions at Lost Hills field , 1991 .

[8]  S. Kaneshima.  Origin of crustal anisotropy: Shear Wave splitting studies in Japan , 1990 .

[9]  S. Crampin,et al.  Observations of dilatancy-induced polarization anomalies and earthquake prediction , 1980, Nature.

[10]  Ray series modeling of seismic wave travel times and amplitudes in three‐dimensional heterogeneous anisotropic crystalline rock: Borehole vertical seismic profiling seismograms from the Mojave Desert, California , 1990 .

[11]  Bernard Budiansky,et al.  Seismic velocities in dry and saturated cracked solids , 1974 .

[12]  S. Kaneshima,et al.  An analysis of split shear waves observed above crustal and uppermost mantle earthquakes beneath Shikoku, Japan: Implications in effective depth extent of seismic anisotropy , 1989 .

[13]  Hisao Itô,et al.  Shear-wave splitting observed above small earthquakes in a geothermal area of Japan , 1988 .

[14]  S. Crampin,et al.  Paris Basin VSPs: case history establishing combinations of fine-layer (or lithologic) anisotropy and crack anisotropy from modelling shear wavefields near point singularities , 1991 .

[15]  Tianrun Chen,et al.  Shear-wave polarizations near the North Anatolian Fault – III. Observations of temporal changes. , 1987 .

[16]  P. Leary,et al.  Fractal fracture scattering origin of S-wave coda : spectral evidence from recordings at 2.5 Km , 1994 .

[17]  D. Winterstein,et al.  Shear-wave polarizations and subsurface stress directions at Lost Hills field , 1991 .

[18]  E. Shuck.  Azimuthal anisotropy analysis from shear VSPs , 1991 .

[19]  Iain Bush,et al.  Estimating the internal structure of reservoirs with shear‐wave VSPs , 1986 .

[20]  Y. Kuwahara,et al.  Comparison between natural fractures and fracture parameters derived from VSP , 1991 .

[21]  Jer-Ming Chiu,et al.  Temporal changes in shear wave splitting during an earthquake swarm in Arkansas , 1990 .

[22]  T. Davis,et al.  Shear‐wave birefringence: A new tool for evaluating fractured reservoirs , 1987 .

[23]  M. Meadows,et al.  Changes in shear-wave polarization azimuth with depth in Cymric and Railroad Gap oil fields , 1991 .

[24]  Peter Leary,et al.  Deep borehole log evidence for fractal distribution of fractures in crystalline rock , 1991 .

[25]  Enru Liu,et al.  Behaviour of shear waves in rocks with two sets of parallel cracks , 1993 .

[26]  Clive McCann,et al.  Seismic velocities in fractured rocks: An experimental verification of Hudson`s theory , 1994 .

[27]  M. Savage,et al.  Shear wave anisotropy and stress direction in and near Long Valley Caldera, California, 1979–1988 , 1990 .

[28]  R. M. Alford,et al.  Shear data in the presence of azimuthal anisotropy: Dilley, Texas , 1986 .

[29]  K. Heffer,et al.  Scaling Relationships in Natural Fractures: Data, Theory, and Application , 1990 .

[30]  S. Crampin,et al.  Shear-wave anisotropy in the Austin chalk from multi-offset VSP data , 1992 .

[31]  Shear wave polarization anisotropy observed in a rift zone in Japan , 1989 .

[32]  R. Stoneley,et al.  The propagation of surface elastic waves in a cubic crystal , 1955, Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences.

[33]  G. Buchbinder.  Shear wave splitting and anisotropy from the aftershocks of the Nahanni, Northwest Territories, Earthquakes , 1990 .

[34]  J. B. Fletcher,et al.  Shear wave splitting in the Anza Seismic Gap, southern california: Temporal Variations as possible precursors , 1988 .

[35]  Hisao Ito,et al.  Comparison between natural fractures and fracture parameters derived from VSP : Geophys J V107, N3, Dec 1991, P475–483 , 1992 .

[36]  S. Crampin,et al.  Shear-wave polarizations near the North Anatolian Fault – I. Evidence for anisotropy-induced shear-wave splitting , 1985 .

[37]  M. C. Mueller,et al.  Prediction of lateral variability in fracture intensity using multicomponent shear-wave surface seismic as a precursor to horizontal drilling in the Austin Chalk , 1991 .

[38]  D. Prior,et al.  Thrust‐related mudstone fabrics from the Barbados Forearc: A backscattered scanning electron microscope study , 1990 .

[39]  D. E. Powley.  Pressures and hydrogeology in petroleum basins , 1990 .

[40]  K. Gledhill.  Evidence for shallow and pervasive seismic anisotropy in the Wellington Region, New Zealand , 1991 .

[41]  S. Crampin,et al.  Limits to Crack Density: The State of Fractures In Crustal Rocks , 1993 .

[42]  G. West,et al.  Seismic tomography at a fire-flood site , 1989 .

[43]  Stuart Crampin.  Effective anisotropic elastic constants for wave propagation through cracked solids , 1984 .

[44]  S. Crampin,et al.  Shear-wave splitting showing hydraulic dilation of pre-existing joints in granite , 1989 .

[45]  J. Fredrich,et al.  Micromechanics of the brittle to plastic transition in Carrara marble , 1989 .

[46]  Jon B. Fletcher,et al.  Changes in shear wave splitting at Anza near the time of the North Palm Springs Earthquake , 1990 .

[47]  M. Meadows,et al.  Changes in shear-wave polarization azimuth with depth in Cymric and Railroad Gap oil fields , 1991 .

[48]  A. Deschamps,et al.  Evidence of shear-wave anisotropy in the upper crust of central Italy , 1989 .

[49]  S. Crampin,et al.  Neotectonics of the Marmara Sea region of Turkey , 1986, Journal of the Geological Society.

[50]  Peter M. Shearer,et al.  Quantitative measurements of shear wave polarizations at the Anza Seismic Network, southern California: Implications for shear wave splitting and earthquake prediction , 1990 .

[51]  J. Walsh,et al.  The prediction of small-scale faulting in reservoirs , 1992 .

[52]  J. Hudson.  Wave speeds and attenuation of elastic waves in material containing cracks , 1981 .

[53]  R. J. Greaves,et al.  Three-dimensional seismic monitoring of enhanced oil recovery project , 1983 .

[54]  Michael C. Mueller.  Prediction of lateral variability in fracture intensity using multicomponent shear-wave surface seismic as a precursor to horizontal drilling in the Austin Chalk , 1991 .

[55]  William Rizer,et al.  An integrated study of seismic anisotropy and the natural fracture system at the Conoco Borehole Test Facility, Kay County, Oklahoma , 1990 .

[56]  R. Evans,et al.  Earthquake prediction: a new physical basis , 1984 .

[57]  Gordon M. Holmes,et al.  PRELIMINARY ANALYSIS OF SHEAR-WAVE SPLITTING IN GRANITE AT THE UNDERGROUND RESEARCH LABORATORY, MANITOBA , 1993 .

[58]  Du Xingxin.  On shear-wave splitting in the Los Angeles basin , 1990 .

[59]  P. Bak,et al.  Earthquakes as a self‐organized critical phenomenon , 1989 .

[60]  Geoffrey C. P. King,et al.  The accommodation of large strains in the upper lithosphere of the earth and other solids by self-similar fault systems: the geometrical origin of b-Value , 1983 .

[61]  Peter M. Shearer,et al.  High-frequency borehole seismograms recorded in the San Jacinto Fault zone, Southern California. Part 1. Polarizations , 1991, Bulletin of the Seismological Society of America.

[62]  S. Crampin,et al.  Estimating crack parameters from observations of P-wave velocity anisotropy , 1980 .

[63]  S. Crampin,et al.  Fracture detection using crosshole surveys and reverse vertical seismic profiles at the Conoco Borehole Test Facility, Oklahoma , 1991 .

[64]  J. B. Fletcher,et al.  Comment on “Quantitative measurements of shear wave polarizations at the Anza Seismic Network, southern California: Implications for shear wave splitting and earthquake prediction” by Richard C. Aster, Peter M. Shearer, and Jon Berger , 1991 .

[66]  Heloise B. Lynn,et al.  Reflection Shear-Wave Data Along the Principal Axes of Azimuthal Anisotropy , 1987 .

[67]  Terrance J. Fulp,et al.  AAPG Memoir 42 and SEG Investigations in Geophysics, No. 9, Chapter 9 (Case Histories of Three-Dimensional Seismic Surveys) -- Case History 4: Three-Dimensional Seismic Monitoring of An Enhanced Oil Recovery Process , 1987 .

[68]  D. Michon,et al.  Anisotropy survey for reservoir definition , 1991 .

[69]  J. Hudson.  Overall properties of a cracked solid , 1980, Mathematical Proceedings of the Cambridge Philosophical Society.

[70]  R. Aster,et al.  Shear-wave anisotropy of active tectonic regions via automated S-wave polarization analysis , 1989 .

[71]  J. A. H. Deparrment.  A higher order approximation to the wave propagation constants for a cracked solid , 1986 .

[72]  David C. Booth,et al.  Shear-wave polarizations on a curved wavefront at an isotropic free surface , 1985 .

[73]  S. Crampin.  SUGGESTIONS FOR A CONSISTENT TERMINOLOGY FOR SEISMIC ANISOTROPY , 1989 .

[74]  Stuart Crampin,et al.  A review of wave motion in anisotropic and cracked elastic-media , 1981 .

[75]  T. V. McEvilly,et al.  Shear-wave anisotropy in the Parkfield Varian well VSP , 1990, Bulletin of the Seismological Society of America.

[76]  T. Davis,et al.  Three-dimensional multicomponent imaging of reservoir heterogeneity, Silo Field, Wyoming : Geophysics V56, N12, Dec 1991, P2048–2056 , 1991 .

[77]  Stuart Crampin,et al.  Seismic-wave propagation through a cracked solid: polarization as a possible dilatancy diagnostic , 1978 .

[78]  K. Aki,et al.  Observation and modelling of fault‐zone fracture seismic anisotropy ‐ I. P, SV and SH travel times , 1987 .

[79]  S. Crampin,et al.  Shear-wave splitting above small earthquakes in the Kinki district of Japan , 1987 .