ROCK QUALITY, SEISMIC
VELOCITY, ATTENUATION
AND ANISOTROPY
NICK BARTON
Published by: Taylor & Francis/Balkema
P.O. Box 447, 2300 AK Leiden, The Netherlands e-mail: Pub.NL@tandf.co.uk
www.balkema.nl, www.taylorandfrancis.co.uk, www.crcpress.com
Library of Congress Cataloguing-in-Publication Data
Barton, Nick, 1944–
Rock quality, seismic velocity, attenuation, and anisotropy/Nick Barton. p. cm.
ISBN 0-415-39441-4 (hardcover: alk. paper)
1. Soil-structure interaction. 2. Earthquake engineering. I. Title. TA711.5.B37 2006
624.151—dc22
2006005909
ISBN10: 0-415-39441-4 (Hbk) ISBN13: 978-0-415-39441-3 (Hbk) ISBN 0-203-96445-4 Master e-book ISBN
Preface XIII
Introduction XIX
The multi-disciplinary scope of seismic and rock quality XIX
Revealing hidden rock conditions XX
Some basic principles of P, S and Q XX
Q and Q XXI
Limitations of refraction seismic bring tomographic solutions XXII
Nomenclature XXIII
PART I
1 Shallow seismic refraction, some basic theory, and the importance of rock type 3
1.1 The challenge of the near-surface in civil engineering 3 1.2 Some basic aspects concerning elastic body waves 4
1.2.1 Some sources of reduced elastic moduli 5
1.3 Relationships between Vpand Vsand their meaning in field work 6
1.4 Some advantages of shear waves 7
1.5 Basic estimation of rock-type and rock mass condition, from shallow seismic P-wave velocity 9 1.6 Some preliminary conversions from velocity to rock quality 12 1.7 Some limitations of the refraction seismic velocity interpretations 13 1.8 Assumed limitations may hide the strengths of the method 16 1.9 Seismic quality Q and apparent similarities to Q-rock 17
2 Environmental effects on velocity 19
2.1 Density and Vp 19
2.2 Porosity and Vp 24
2.3 Uniaxial compressive strength and Vp 25
2.4 Weathering and moisture content 27
2.5 Combined effects of moisture and pressure 30
2.6 Combined effects of moisture and low temperature 32
3 Effects of anisotropy on Vp 35
3.1 An introduction to velocity anisotropy caused by micro-cracks and jointing 35
3.2 Velocity anisotropy caused by fabric 38
3.3 Velocity anisotropy caused by rock joints 40
3.4 Velocity anisotropy caused by interbedding 45
3.5 Velocity anisotropy caused by faults 47
4 Cross-hole velocity and cross-hole velocity tomography 49
4.1 Cross-hole seismic for extrapolation of properties 49
4.2 Cross-hole seismic tomography in tunnelling 52
4.3 Cross-hole tomography in mining 58
4.4 Using tomography to monitor blasting effects 61
4.5 Alternative tomograms 64
6 Deformation moduli and seismic velocities 97
6.1 Correlating Vpwith the ‘static’ moduli from deformation tests 97 6.2 Dynamic moduli and their relationship to static moduli 104
6.3 Some examples of the three dynamic moduli 109
6.4 Use of shear wave amplitude, frequency and petite-sismique 110 6.5 Correlation of deformation moduli with RMR and Q 111
7 Excavation disturbed zones and their seismic properties 117
7.1 Some effects of the free-surface on velocities and attenuation 117 7.2 EDZ phenomena around tunnels based on seismic monitoring 119 7.3 EDZ investigations in selected nuclear waste isolation studies 124
7.3.1 BWIP – EDZ studies 124
7.3.2 URL – EDZ studies 127
7.3.3 Äspö – EDZ studies 131
7.3.4 Stripa – effects of heating in the EDZ of a rock mass 133 7.4 Acoustic detection of stress effects around boreholes 136
8 Seismic measurements for tunnelling 139
8.1 Examples of seismic applications in tunnels 139 8.2 Examples of the use of seismic data in TBM excavations 148 8.3 Implications of inverse correlation between TBM advance rate and Vp 149
8.4 Use of probe drilling and seismic or sonic logging ahead of TBM tunnels 151 8.5 In-tunnel seismic measurements for looking ahead of the face 152 8.6 The possible consequences of insufficient seismic investigation due to depth limitations 154
9 Relationships between Vp, Lugeon value, permeability and grouting in jointed rock 159
9.1 Correlation between Vpand Lugeon value 159
9.2 Rock mass deformability and the Vp-L-Q correlation 162
9.3 Velocity and permeability measurements at in situ block tests 165 9.4 Detection of permeable zones using other geophysical methods 169 9.5 Monitoring the effects of grouting with seismic velocity 170 9.6 Interpreting grouting effects in relation to improved rock mass Q-parameters 172
PART II
10 Seismic quality Q and attenuation at many scales 181
10.1 Some basic aspects concerning attenuation and Qseismic 181
10.1.1 A preliminary discussion of the importance of strain levels 183 10.1.2 A preliminary look at the attenuating effect of cracks of larger scale 184 10.2 Attenuation and seismic Q from laboratory measurement 186 10.2.1 A more detailed discussion of friction as an attenuation mechanism 187
10.2.2 Effects of partial saturation on seismic Q 189
10.3 Effect of confining pressure on seismic Q 190
10.3.1 The four components of elastic attenuation 193 10.3.2 Effect on Qpand Qsof loading rock samples towards failure 195
10.4 The effects of single rock joints on seismic Q 197 10.5 Attenuation and seismic Q from near-surface measurements 202 10.5.1 Potential links to rock mass quality parameters in jointed rock 202 10.5.2 Effects of unconsolidated sediments on seismic Q 205 10.5.3 Influence of frequency variations on attenuation in jointed and bedded rock 207 10.6 Attenuation in the crust as interpreted from earthquake coda 209 10.6.1 Coda Qcfrom earthquake sources and its relation to rock quality Qc 209 10.6.2 Frequency dependence of coda Qcdue to depth effects 210 10.6.3 Temporal changes of coda Qcprior to earthquakes 212
10.6.4 Possible separation of attenuation into scattering and intrinsic mechanisms 213
10.6.5 Changed coda Q during seismic events 214
10.6.6 Attenuation of damage due to acceleration 218 10.6.7 Do microcracks or tectonic structure cause attenuation 219 10.6.8 Down-the-well seismometers to minimise site effects 221
10.6.9 Rock mass quality parallels 224
10.7 Attenuation across continents 226
10.7.1 Plate tectonics, sub-duction zones and seismic Q 226
10.7.2 Young and old oceanic lithosphere 228
10.7.3 Lateral and depth variation of seismic Q and seismic velocity 228 10.7.4 Cross-continent Lg coda Q variations and their explanation 230 10.7.5 Effect of thick sediments on continental Lg coda 231 10.8 Some recent attenuation measurements in petroleum reservoir environments 232 10.8.1 Anomalous values of seismic Q in reservoirs due to major structures 235 10.8.2 Evidence for fracturing effects in reservoirs on seismic Q 236 10.8.3 Different methods of analysis give different seismic Q 238
11 Velocity structure of the earth’s crust 241
11.1 An introduction to crustal velocity structures 241
11.2 The continental velocity structures 244
11.3 The continental margin velocity structures 254
11.3.1 Explaining a velocity anomaly 256
11.4 The mid-Atlantic ridge velocity structures 261 11.4.1 A possible effective stress discrepancy in early testing 263
11.4.2 Smoother depth velocity models 265
11.4.3 Recognition of lower effective stress levels beneath the oceans 266 11.4.4 Direct observation of sub-ocean floor velocities 267 11.4.5 Sub-ocean floor attenuation measurements 268 11.4.6 A question of porosities, aspect ratios and sealing 270
11.4.7 A velocity-depth discussion 271
11.4.8 Fracture zones 272
11.5 The East Pacific Rise velocity structures 273
11.5.1 More porosity and fracture aspect ratio theories 276 11.5.2 First sub-Pacific ocean core with sonic logs and permeability tests 277 11.5.3 Attenuation and seismic Q due to fracturing and alteration 279 11.5.4 Seismic attenuation tomography across the East Pacific Rise 281 11.5.5 Continuous sub-ocean floor seismic profiles 283
12.2.1 Reversal of Kotrends nearer the surface 299
12.3 Relevance to logging of borehole disturbed zone 301 12.4 Borehole in continuum becomes borehole in local discontinuum 302 12.5 The EDZ caused by joints, fractures and bedding-planes 306
12.6 Loss of porosity due to extreme depth 311
12.7 Dipole shear-wave logging of boreholes 312
12.7.1 Some further development of logging tools 315
12.8 Mud filtrate invasion 316
12.9 Challenges from ultra HPHT 320
13 Rock physics at laboratory scale 323
13.1 Compressional velocity and porosity 323
13.2 Density, Vsand Vp 324
13.3 Velocity, aspect ratio, pressure, brine and gas 326 13.4 Velocity, temperature and influence of fluid 328
13.5 Velocity, clay content and permeability 331
13.6 Stratigraphy based velocity to permeability estimation 332
13.6.1 Correlation to field processes 334
13.7 Velocity with patchy saturation effects in mixed units 335 13.8 Dynamic Poisson’s ratio, effective stress and pore fluid 337 13.9 Dynamic moduli for estimating static deformation moduli 339 13.10 Attenuation due to fluid type, frequency, clay, over-pressure, compliant minerals,
dual porosity 341
13.10.1 Comparison of velocity and attenuation in the presence of gas or brine 341 13.10.2 Attenuation when dry or gas or brine saturated 341 13.10.3 Effect of frequency on velocity and attenuation, dry or with brine 342 13.10.4 Attenuation for distinguishing gas condensate from oil and water 343 13.10.5 Attenuation in the presence of clay content 345 13.10.6 Attenuation due to compliant minerals and microcracks 346 13.10.7 Attenuation with dual porosity samples of limestones 348 13.10.8 Attenuation in the presence of over-pressure 350 13.11 Attenuation in the presence of anisotropy 351 13.11.1 Attenuation for fluid front monitoring 352 13.12 Anisotropic velocity and attenuation in shales 354 13.12.1 Attenuation anisotropy expressions , and 356 13.13 Permeability and velocity anisotropy due to fabric, joints and fractures 357 13.13.1 Seismic monitoring of fracture development and permeability 359 13.14 Rock mass quality, attenuation and modulus 365
14 P-waves for characterising fractured reservoirs 369
14.2 Anisotropy and heterogeneity caused by inter-bedded strata and jointing 372
14.2.1 Some basic anisotropy theory 373
14.3 Shallow cross-well seismic tomography 374
14.3.1 Shallow cross-well seismic in fractured rock 377 14.3.2 Cross-well seismic tomography with permeability measurement 377 14.3.3 Cross-well seismic in deeper reservoir characterization 378
14.4 Detecting finely inter-layered sequences 379
14.4.1 Larger scale differentiation of facies 380 14.5 Detecting anisotropy caused by fractures with multi-azimuth VSP 382 14.5.1 Fracture azimuth and stress azimuth from P-wave surveys 382 14.5.2 Sonic log and VSP dispersion effects and erratic seismic Q 386 14.6 Dispersion as an alternative method of characterization 386 14.7 AVO and AVOA using P-waves for fracture detection 388 14.7.1 Model dependence of AVOA fracture orientation 391 14.7.2 Conjugate joint or fracture sets also cause anisotropy 392
14.7.3 Vpanisotropy caused by faulting 394
14.7.4 Poisson’s ratio anisotropy caused by fracturing 394 14.8 4C four-component acquisition of seismic including C-waves 394
14.9 4D seismic monitoring of reservoirs 397
14.9.1 Possible limitations of some rock physics data 397 14.9.2 Oil saturation mapping with 4D seismic 397 14.10 4D monitoring of compaction and porosity at Ekofisk 398 14.10.1 Seismic detection of subsidence in the overburden 400 14.10.2 The periodically neglected joint behaviour at Ekofisk 401 14.11 Water flood causes joint opening and potential shearing 402
14.12 Low frequencies for sub-basalt imaging 403
14.13 Recent reservoir anisotropy investigations involving P-waves and attenuation 404
15 Shear wave splitting in fractured reservoirs and resulting from earthquakes 407
15.1 Introduction 407
15.2 Shear wave splitting and its many implications 408 15.2.1 Some sources of shear-wave splitting 410
15.3 Crack density and EDA 411
15.3.1 A discussion of ‘criticality’ due to microcracks 412 15.3.2 Temporal changes in polarization in Cornwall HDR 413 15.3.3 A critique of Crampin’s microcrack model 415
15.3.4 90°-flips in polarization 415
15.4 Theory relating joint compliances with shear wave splitting 416 15.4.1 An unrealistic rock simulant suggests equality between ZNand ZT 417
15.4.2 Subsequent inequality of ZNand ZT 419
15.4.3 Off-vertical fracture dip or incidence angle, and normal compliance 419 15.4.4 Discussion of scale effects and stiffness 421 15.5 Dynamic and static stiffness tests on joints by Pyrak-Nolte 422 15.5.1 Discussion of stiffness data gaps and discipline bridging needs 424
15.5.2 Fracture stiffness and permeability 425
15.6 Normal and shear compliance theories for resolving fluid type 425 15.6.1 In situ compliances in a fault zone inferred from seismic Q 427
15.7 Shear wave splitting from earthquakes 428
15.7.1 Shear-wave splitting in the New Madrid seismic zone 428 15.7.2 Shear-wave splitting at Parkfield seismic monitoring array 429
15.8.2 Classification of fractured reservoirs 440 15.8.3 Crack density and shearing of conjugate sets at Ekofisk might enhance splitting 442 15.8.4 Links between shear wave anisotropy and permeability 445 15.8.5 Polarization-stress alignment from shallow shear-wave splitting 447 15.8.6 Shear-wave splitting in argillaceous rocks 450 15.8.7 Time-lapse application of shear-wave splitting over reservoirs 451 15.8.8 Temporal shear-wave splitting using AE from the Valhall cap-rock 454 15.8.9 Shear-wave splitting and fluid identification at the Natih field 455 15.9 Dual-porosity poro-elastic modelling of dispersion and fracture size effects 459 15.9.1 A brief survey of rock mechanics pseudo-static models of jointed rock 460 15.9.2 A very brief review of slip-interface, fracture network and poro-elastic crack models 461 15.9.3 Applications of Chapman model to Bluebell Altamont fractured gas reservoir 471
15.9.4 The SeisRox model 475
15.9.5 Numerical modelling of dynamic joint stiffness effects 476
15.9.6 A ‘sugar cube’ model representation 479
15.10 A porous and fractured physical model as a numerical model validation 480
16 Joint stiffness and compliance and the joint shearing mechanism 483
16.1 Some important non-linear joint and fracture behaviour modes 483 16.2 Aspects of fluid flow in deforming rock joints 486 16.2.1 Coupled stress-flow behaviour under normal closure 487 16.2.2 Coupled stress-flow behaviour under shear deformation 488 16.3 Some important details concerning rock joint stiffnesses Knand Ks 492
16.3.1 Initial normal stiffness measured at low stress 494 16.3.2 Normal stiffness at elevated normal stress levels 495 16.4 Ratios of Knover Ksunder static and dynamic conditions 497 16.4.1 Frequency dependence of fracture normal stiffness 497 16.4.2 Ratios of static Knto static Ksfor different block sizes 498
16.4.3 Field measurements of compliance ZN 499
16.4.4 Investigation of normal and shear compliances on artificial surfaces in limestones 501 16.4.5 The Worthington-Lubbe-Hudson range of compliances 503 16.4.6 Pseudo-static stiffness data for clay filled discontinuities
and major shear zones 505
16.4.7 Shear stress application may apparently affect compliance 506 16.5 Effect of dry or saturated conditions on shear and normal stiffnesses 507
16.5.1 Joint roughness coefficient (JRC) 508
16.5.2 Joint wall compression strength (JCS) 509 16.5.3 Basic friction angle band residual friction angle r 509
16.5.4 Empirical equations for the shear behaviour of rock joints 511 16.6 Mechanical over-closure, thermal-closure, and joint stiffness modification 513
16.6.2 Thermal over-closure of joints and some implications 515
16.6.3 Mechanical over-closure 517
16.7 Consequences of shear stress on polarization and permeability 517 16.7.1 Stress distribution caused by shearing joints, and possible consequences for shear
wave splitting 518
16.7.2 The strength-deformation components of jointed rock masses 520 16.7.3 Permeability linked to joint shearing 523 16.7.4 Reservoir seismic case records with possible shearing 525 16.7.5 The apertures expected of highly stressed ‘open’ joints 526 16.7.6 Modelling apertures with the BB model 531 16.7.7 Open joints caused by anisotropic stress, low shear strength, dilation 534 16.8 Non-linear shear strength and the critical shearing crust 536 16.8.1 Non-linear strength envelopes and scale effects 536 16.9 Critically stressed open fractures that indicate conductivity 541 16.9.1 The JRC contribution at different scales and deformations 544 16.9.2 Does pre-peak or post-peak strength resist the assumed crustal shear stress? 545 16.10 Rotation of joint attributes and unequal conjugate jointing may explain azimuthal
deviation of S-wave polarization 548
16.11 Classic stress transformation equations ignore the non-coaxiality of stress and displacement 552 16.12 Estimating shallow crustal permeability from a modified rock quality Q-water 554 16.12.1 The problem of clay-sealed discontinuities 555
17 Conclusions 559
Appendix A – The Qrockparameter ratings 615
The six parameters defined 615
Combination in pairs 615
Definitions of characterization and classification as used in rock engineering 615
Notes on Q-method of rock mass classification 615
Appendix B – A worked example 625
References 627
Index 655
This book traces an accelerating path through an important part of the earth sciences, describing seismic behaviour and rock mechanics interpretation at many scales, to illuminate what lies beneath the earth’s immediate surface. Although geophysics, and the rock mechanics and engineering geology of discontinuous media share the same medium, they have had a mostly separate development – with little cross-referencing in the multitude of journals. Regrettably, we seldom see geophysics colleagues at our rock conferences. This book attempts to bridge this void in strategic locations.
Seismic velocity, seismic quality (the inverse of attenuation), and anisotropy are some of the very basics of geo-physics, and they depend absolutely on the rock and fluid properties, the rock mass structures, the jointing, the frac-turing, the microcracks and the other pore space. These are some of the fundamentals of earth science. All contribute to the resultant dynamic stiffnesses, and to the fluid pressure micro-flow reactions, whether at dam foun-dation depths, tunnel depths, reservoir-well depths, or earthquake depths. All components of the anisotropic, dynamic, stiffness-velocity-permeability half-space, respond together in a logical pattern. Attempting to understand this pattern is a major objective of this book.
The assumed ‘shared earth’ response is revealing itself with increasing speed. Despite the very small strains and dis-placements involved in seismic wave loading there are inevitable, encouraging parallels, to the rock mechanics of larger strains and displacements. This makes seismic response more understandable and more logical for a wider group of pro-fessionals, with contributing areas of expertise.
In synthetic modelling in geophysics, there is now much interest in the rock joint or rock fracture compliances that may hold part of the secret of fractured reservoir description. These same properties, when inverted, are used over much larger displacements, in rock stability and deformation modelling. Remarkably, the dynamic compliance and static stiffness of fractures and joints have mostly had a compartmentalized development in the different disciplines. A dynamic, micro-strain-based normal compliance of 1013m/Pa1derived from shear-wave anisotropy measurement in
the sub-surface, is of recognisable magnitude when inverted, to compare with the pseudo-static ‘macro-strain’ joint
nor-mal stiffness (i.e. 10,000 MPa/mm or 10 MPa/micron) obtained from incremental loading tests on similar rock joints
at similar high stress levels.
The level of rock stress, the joint wall roughness, and the joint wall compressive strength, which are also important components of aperture and permeability, provide estimates of these physical properties, not just the diagonal mem-bers of a stiffness matrix. Here we have a classic reason for a disconnect between part of the earth sciences, which can be bridged with advantage.
Attenuation and rock quality, another area of disconnect, can also be linked, but not quite so simply as taking the inverse of attenuation and calling it seismic quality. The universally used seismic quality Q of geophysics, that we will often call Qseis, shows some qualitative and quantitative connections to rock mass quality, also called Q, and widely used in rock engineering since the 1970’s. The rock mass quality (Q), which we will often call Qrock, is
com-posed of several attenuation-causing parameters, that are directly equivalent to block size, inter-block friction and a rough measure of effective stress and permeability.
There are clear, broad links between Qrockand Qseis, due to the discovery of a mutual connection to the
empir-ically derived and stress-dependent deformation modulus of rock masses. This connection is despite the fact that only micro-strains, micro-displacements, and micro-flows (squirt) occur with the passage of dynamic waves. Rock mass behaviour is non-linear and scale-dependent. Load-deformation curves have different gradients at differ-ent stress levels. Dynamic waves seem to sense this non-linearity, and they appardiffer-ently sense some of the scale effect too.
This book is dedicated to making some of these cross-discipline empirical connections, in a simple non-mathe-matical way, so that the people who see a lot of rock in their daily endeavours (geologists, engineering geologists, rock mechanics and rock engineers), and those who see, and interpret, and model complex seismic results, from
physical interpretation of the near-surface, the sub-ocean, and the seismic shallow crust. Their dedication and inter-esting publications have made this book a possibility. This volume is a well-illustrated documentation of just some of their excellent work. The journey through their contributions has been one of increasing excitement.
Efforts have been made to reproduce the physical essence of reviewed work with suitable choice of author’s fig-ures. Ricardo and Marcelo Abrahão have excelled in the expert redrawing of such figures, and are sincerely thanked for their painstaking work. The writer’s summaries of key aspects of reviewed work are interspersed with personal and rock mechanics based interpretations with which authors need not be in full agreement.
Material contributions, in the form of inaccessible articles, figures and data, and some valuable discussions and improved insight, have kindly been provided by Dr. Enru Liu, Dr. Eda Quadros, Dr. Baotang Shen, Dr. Axel Makurat, Prof. Stavros Bandis, Dr. Karstein Monsen, Prof. Michael King, Dr. Stuart Crampin, Dr. Heloise Lynn, Harald Westerdahl, Dr. Sonja Maultzsch, Dr. Paul Chapman, Dr. Rudi Lubbe, Dr. Tor Arne Johansen, Dr. Barry New, Dr. Saul Denekamp and Dr. Tore Lasse By, who enthusiastically introduced the writer to cross-hole seismic tomography in 1986.
Part I of this book was mostly completed while the writer was Visiting Professor in the University of São Paulo Polytechnic (USP). The writer’s kind neighbour in the Mining Department, Prof. Lineu Ayres da Silva, was indir-ectly responsible for the five years extension involved in starting and completing Part II of this book. A recently pur-chased volume by Kearey and Vine, 1996 lay open on his desk. A plate tectonics section of a plunging sub-ducting crust with labels ‘low Q’, ‘high Q’ caught the writer’s rock-engineering attention. What did this ‘Q’ mean? Some of the complex answers, and a simple one showing promise, will be found in Part II.
My final acknowledgements are firstly to Pat Coughlin, who has ensured a smooth-running and expert manuscript production over a long period of endeavour. This started with the deciphering of handwriting and ended with countless explanations of Microsoft’s hidden logic. The enthusiastic team at Taylor & Francis, Germaine Seijger and Lukas Goosen and the Charon Tec team have produced a work to be proud of. The reader can be the judge of this. Finally my thanks and apologies to a tolerant and loving wife Eda, who also ensured some key insights into rock-fluid interactions.
Permissions to Reproduce Figures
The nature of this book, specifically a wide-reaching literature review, involving some 830 references from some forty different journals and publishing houses, has made obtaining permissions to reproduce figures a daunting and sometimes impossible task regarding author-permissions, due to the several hundreds of first authors, and thousands of multiple authorships. There are instances where we have been unable to trace or contact the copyright holder. If notified, the publisher will be pleased to rectify any errors or omissions at the earliest opportunity. Many key authors are retired, regrettably some have died, including Bengt Sjögren, who’s published work from 1979, 1984 and 2000 was an important source for key figures in several chapters of Part I. The most prominent authors have kindly given permission for multiple reproduction of figures from my limited selection from their important contributions. All publishers as listed below, have kindly given their permission for multiple reproduction of the numerous figures reproduced in this reference volume. Their joint permissions, and those of contacted authors, and the contribution of all authors that could not be contacted for whatever reason, are gratefully acknowledged. Their excellent work, reproduced in this book, is a sincere acknowledgement of their contributions to geophysics.
Acoustical Society of America (ASA): Journal of the Acoustical Society of America: Figure 13.42 American Association of Petroleum Geologists (AAPG): Figure 15.36
American Geophysical Union (AGU): Journal of Geophysical Research: Figures 2.8, 3.1, 5.33, 5.34, 5.35, 10.14,
10.21, 10.25, 10.27, 10.28, 10.33, 10.37, 10.38, 10.41, 10.43, 10.44, 10.47, 10.48, 10.52, 10.53, 10.55, 10.58–10.60, 11.1, 11.6, 11.7, 11.8, 11.9ab, 11.10–11.21, 11.24–11.30, 11.31a, 11.32, 11.33, 11.35, 11.36, 11.38, 11.40–11.42, 11.48, 11.49, 11.52, 11.54–11.64, 11.66–11.71, 12.11, 12.22, 12.23, 13.2, 13.5a, 13.25, 13.29, 13.32, 13.33, 13.46, 14.16, 14.25, 14.26, 15.8, 15.11, 15.14, 15.18, 15.63. Figure Part II; Tables: 10.5, 10.6, 11.2, 15.2, 15.3, 16.5, 16.6
American Institute of Mining, Metallurgical and Petroleum Engineers (AJME): 16.42, 16.68 American Institute of Physics (AIP): Figure 10.21
American Physical Society (APS): Physical Review E: Figure 10.64
American Society of Civil Engineering (ASCE): Journal of Geotechnical Engineering: Figure 2.15
American Society of Mechanical Engineers (ASME): Transactions of the American Society of Mechanical Engineering:
12.6; Journal of Applied Mechanics: 2.9
Blackwell Publishing: Geophysical Prospecting: Figures 1.3, 1.5, 1.7, 1.8, 1.10, 1.11, 3.9, 4.3, 5.2–5.4, 5.10, 5.11,
6.11, 6.17, 8.12, 9.2, 10.65, 10.67, 13.24, 13.25, 13.36–13.41, 13.44, 13.48, 13.61, 14.15, 15.5, 15.6, 15.22, 15.28, 15.39, 15.40–15.42, 15.47, 15.48, 15.51–15.53, 15.55, 16.20–16.22; Geophysical Journal International (Geophys. J. Int.): 10.22–10.24, 15.1a, 15.3, 15.4, 10.67; Other sources: Figures Part II, 11.1, 11.2, 11.18; Table 11.1
Cambridge University Press: Figures 11.3, 13.1, 13.2, 13.5 and 14.4 Centek Publishers, Luleå: Figure 16.13
Comité Francais de Géologie de l’Ingénieur et de l’Environnement (CFGI): Paris: Figures 5.6, 5.7, 8.5; Tables 8.1, 8.2 Coyne et Bellier: Figures 7.7, 6.19, 6.21
Elsevier: International Journal of Rock Mechanics and Mining Sciences and Geomechanics Abstracts: Figures 2.1,
3.2, 3.8, 4.7ab, 4.13, 4.14, 4.17, 4.20, 5.29, 5.30ab, 6.9, 6.20, 7.18, 7.20, 7.25, 7.26, 7.31, 7.32, 8.2–8.4, 9.6, 15.9, 13.53–13.55, 13.58, 15.17, 16.2, 16.4, 16.6, 16.7, 16.9, 16.12, 16.16, 16.17, 16.44, 16.46, 16.69, 16.73, 16.74; Table 4.1; Engineering Geology: Figures 5.17, 5.19, 15.26, 14.39; Journal of Applied Geophysics: 14.15, 15.5a, 15.56, 15.57; Table 16.8; Tectonophysics: Figures 11.31b, 11.53, 16.64, 16.65, 16.76. Other sources: Figures 1.1, 1.6, 2.18, 4.12, 4.21, 5.13, 10.57, 11.5, 11.34, 15.23; Tables 2.2, 5.2, 11.1
European Association of Geoscientists and Engineers (AEGE): First Break: Figure 15.31; Other sources: Figures 10.2,
10.3, 10.10; 10.20, 10.21, 10.31, 10.36, 13.24, 14.37, 14.38, 15.27, 15.31, 15.37, 15.38, 15.43, 15.45, 15.46, 15.54; Table 13.2
Geophysical Research Letters: Figures: 4.9, 10.52, 11.51, 11.56, 15.44, 16.19; Other sources: Figures 9.7, 12.7, 12.8, 11.39, 11.46, 11.54, 13.11, Table 11.3
Geological Society of America (GSA): Geology: Figures 3.13, 10.6, 16.11, 16.56, 16.63; Figure 1.4
Geological Society: The Quarterly Journal of Engineering Geology: Figures 3.7, 3.10, 5.15, 5.16; Other sources: 2.12,
11.47, 13.56, 13.57, 15.16, 16.23
Imperial College, London: Figure 16.6
Imprime Adosa, Madrid: Figure 3.3, 5.1, 5.8, 5.9, 8.16
Institut du Bâtiment et des Travaux Publics; Annales d’ITBTP: Figure 6.20
Kansas Society of Petroleum Engineering: Figures 13.2, 13.5ab
Laboratório Nacional de Engenharia Civil (LNEC), Lissabon: Figures 2.2, 6.1, 6.15, 6.22, 6.23 Nagra; Nagra Bulletin: Figure 9.1
National Academy Press, Washington: Figures 6.2, 6.3, 6.8, 7.1, 7.8, 9.11; Table 6.2 Norwegian Petroleum Society (NFP): Figures 14.29, 14.30, 15.36
Office of Nuclear Waste Isolation (ONWI), Columbus: 16.10, 16.14, 16.15, 16.29–16.33, 16.46, 16.67 Österreichischen Gesellschaft für Geomechanik (ÖGG), Felsbau: Figure 6.4
Oyo Corporation: Figure 2.12
Royal Astronomical Society (RAS): Quarterly Journal of the Royal Astronomical Society: Figures 11.5, 11.37, 11.50 Schlumberger: Oilfield Reviews: Figures 12.24–12.26, 14.15, 15.1b, 15.19, 15.36, 15.1b; Other sources: 4.10 Seismological Society of America (SSA): Bulletin of the Seismological Society of America: Figures: 10.39, 10.40,
10.46, 10.52, 10.61, Table 10.7
SGE Editoriali, Padova: Figure 2.13 SKB, Stockholm: Figure 7.23
Society for Mining, Metallurgy and Exploration (SME): Various sources: Figures 2.4–2.7, 5.1a, 6.12, 7.12–7.15, 7.28,
15.7, 15.25, 16.27, 16.75
Society of Exploration Geophysicists (SEG): Geophysics: Figures 2.11, 2.19, 2.21, 3.11, 3.16, 4.15, 7.27, 10.1,
10.4–10.11, 10.13, 10.15ab, 10.16–10.19, 10.29, 10.30, 10.34, 10.35, 10.52, 10.64, 10.66, 10.68–10.72, 11.22, 11.23ab, 11.43, 11.48, 12.27, 13.3, 13.4, 13.6–13.8, 13.11–13.13, 13.17–13.23, 13.26–13.31, 13.34, 13.35, 13.42–13.45, 13.50–13.52, 14.1–14.3, 14.6–14.15, 14.18–14.24, 14.28, 14.31, 15.11, 15.29, 15.30, 15.60, 16.64. Tables: 10.10, 14.1–14.3, 15.1; The Leading Edge: Figures: 12.1a–d, 12.2a–d, 13.10, 13.14a–b, 13.15, 13.16, 14.33–14.36, 14.38, 15.15, 15.24, 15.35, 15.60; Canadian Journal Exploration Geophysics: Figures 10.63, 15.12–15.14, 15.32; Other sources: Figures 12.3, 12.30, 14.1, 15.5c, 15.10, 15.29, 15.44, 15.54, 15.65abc, 15.66, 16.6
Society of Petroleum Engineers (SPE): SPE Journal: Figures 13.2, 13.5ab, 14.32, 14.33; Other sources: Figures 12.12,
12.13, 12.29
Southern Africa Institute of Mining and Metallurgy (SIAMM): Figure 15.46
Springer Science and Business Media: Rock Mechanics: Figures 2.10, 16.10, 16.26, 16.41, 16.54ab, 16.57ab, 16.58;
Pure and Applied Geophysics – Pageophysik: 7.22ab, 10.12, 10.49ab, 10.50ab, 10.51, 10.52, 10.54; Other Sources: Figures 10.42, 13.1, 16.60; Table 3.1
Swedish National Science Council: Figure 1.45 Tapir Academic Press, Trondheim: Figure 15.25
Thomas Telford: Geotechnique: Figures 12.5, 12.9, 12.10, 15.2, 16.2, 16.8, 16.53, 16.75
Other sources: Figure 4.4, Tables 1.2, 1.3
University of California Berkeley: Figure 16.46
Wilmington: Tunnel & Tunnelling International: Figure 9.13
PhD Theses:
S. Bandis, 1980, University of Leeds (Fig. 16.3?, 16.16, 16.18, 16.40, 16.47, 16.52, 16.66, Tbl. 16.2, 16.3); T. Cadoret, 1993, University of Paris (Fig. 13.2, 13.5ab, 13.20);
D. Han, 1986, Stanford University (Fig. 13.2, 13.4, 13.5e); K. Iwai, 1976, University of California Berkeley (Fig. 16.46); D.L. Jizba, 1991, Stanford University (Fig. 13.5d);
Y.-Q. Liu, 2003, University of Edinburgh (Fig. 14.15b); R. Lubbe, 2005, Oxford University (Fig. 16.20, 16.23); N. Lucet, 1989, University of Paris (Fig. 13.2, 13.5ab); E. Quadros, 1982 (Msc), University of São Pualo (Fig. 16.6); A. Shakeel, 1995, Imperial College, Univ. London (Fig. 13.58); J.C. Sharp, 1970, University of London (Fig. 16.6);
C. Slater, 1997, University of Edinburgh (Fig. 15.20, 15.34, 15.35); S.R. Tod, 2002, University of Cambridge (Fig. 15.44);
J. Yan, 2003, University of Edinburgh (Fig. 13.14); J. Yuan, 2001, University of Edinburgh (Fig. 14.27).
The multi-disciplinary scope of seismic and rock quality
Seismic, sonic and ultrasonic measurements are utilised by a large number of science, engineering and geo-resource disciplines. Their use is so widespread in the earth-sciences, that it should be of no surprise to us that such techniques are also used to register such diverse subjects as osteoporosis in cows, and the control of ‘crispiness in breaded fried chicken nuggets’. The latter was a thesis in Biological Systems Engineering.
Since rock engineers tackle different problems from petroleum engineers and geophysicists, who in turn tackle different problems from tectonophysicists, there has been an understandable yet regrettable compartmentalisation between the disciplines. Both practitioners and researchers in each of these major fields, generally go to different conferences and read and publish in different journals, as there are ‘too many’ choices of each in each discipline, even in each speciality where we earn our living. The luxury of cross-discipline interaction, occasionally experienced with great interest and resulting stimulation, is usually defeated by time, cost and also in part, by technical-language bar-riers, and even mathematics.
An interesting example of partial ‘compartmentalization’ is stiffness and compliance. Each have followed almost separate development since the late 1960s in rock mechanics, and since the early 1980s in geophysics. Each are essential to each subject; for numerical modelling of stability and deformability in rock engineering; for improved interpretation of attenuation, anisotropy and shear wave splitting in the geophysics of fractured petroleum reser-voirs. Yet the dynamically measured, micro-deformation fracture compliances in geophysics (in the normal and shear directions), are numerically close to the inverse of incrementally-loaded joint stiffnesses in rock mechanics, at least when rock quality is high.
The frequently illustrated material in this book has been assembled as a result of an interest in a variety of civil, mining, petroleum, geophysics and earth-science fields. The common denominator has been rock mass and rock joint behaviour as presumably impacting the seismic interpretation. An interesting and very large selection of seis-mic velocity and seisseis-mic quality related data, from practitioners working in widely varied disciplines, has been assembled. Much has obviously been left out or not yet seen. Much is still under development.
The chapters of Part I are mostly civil engineering related with strong links to the interpretation of rock condi-tions at both laboratory and field-scale, with their impact on engineering of tunnels and dams and planned nuclear waste repositories. The chapters of Part II go deeper both figuratively and literally, and consider much larger scale uses of seismic attributes, from hydrocarbon reservoirs and the use of multiple dynamic energy sources, to the inter-pretation of mid-ocean spreading-ridges, to crustal conditions interpreted from natural earthquake hypocentres.
The phenomen of seismic anisotropy, known already in the nineteenth century to give lower stiffness perpen-dicular to layering than parallel, is now in widespread use for investigating fractured rock at depth. Features of the rock mass, though of sub-seismic-wave size, can be detected at many kilometers depth, due to shear wave splitting, giving polarization parallel and perpendicular to dominant jointing. Different time delays for the fast and slow shear wave components vary with fracture properties and with frequency, giving frequency-dependent anisotropy.
Efforts have been made to seek out and to reproduce in brief, with helpful figures, the seismic measurements and interpretations which have a clear or potential rock quality content, at whatever scale. Clearly the term ‘rock quality’ conceals various techniques and scales of measurement, and varied interests in ‘rock quality’ per se. A rock mass with
high velocity and high rock quality (i.e., exhibiting low attenuation) would make life less profitable for machine bored
tunnellers due to slow progress and frequent cutter-changes. Aggregate producers would need more drilling and explosives per ton, and would seek other quarries. The very existence of hydrocarbon reservoirs and their product-ivity would be severely prejudiced if either ‘rock quality’ or ‘seismic quality’ was too high. Others would welcome good ‘rock quality’ characteristics, for example producers of dimension stone and clients expecting cheap drill-and-blasted tunnels requiring little rock support.
seismic structure of the earth’s crust, and further again to depths of 5000 km or more, to the solid iron core of the earth, as a result of global-station analyses following large earthquakes.
Sjøgren, 1984, gave the civil engineering (near-surface) profession a particularly useful guide in the use of shallow seis-mic refraction techniques for those involved in shallow sub-surface projects. The fundamental principle is that seisseis-mic waves propagate with significantly different velocities in different near-surface geotechnical and geological strata, due to the seismic visibility of weathered, low-stressed materials in general. This also means that the velocities tend to increase rapidly with depth, which must not be misinterpreted as meaning better quality per se. Intermediate high-speed layers, or hidden low velocity layers obviously disturb this simplified picture, and velocity anomalies and incorrect depth inter-pretations result unless separate analysis i.e., downhole vertical seismic profiling (VSP), or coring is performed.
Fundamental difficulties in the context of rock engineering (and in all other disciplines too) are that the means of access, superficial or along boreholes, are often limited by the geometry of the problem, by the (urban or sub-sea) location, and by the cost. The freedom to choose optimal experimental layouts is therefore limited. As pointed out by Cosma, 1995, this may cause blind zones, even in the immediate vicinity of the observation points.
In the case of soil or weathered rock horizons, seismic velocity interpretation readily distinguishes the water table from a lithological boundary by inspection of the shear or transverse wave velocity (Vs). If this remains constant
across the region of changing water content, while Vpchanges, a groundwater surface is indicated, since the shear waves do not respond to changing water content due to the lack of shear stiffness. If Vsalso changes, a geotechnical
or geological layer will have been crossed. Typical ranges of Vpfor a variety of near-surface sediments and rocks are reviewed in Chapter 1.
One of the historic and important applications of refraction seismic in civil engineering, has been at dam sites, which were investigated in great numbers, especially in the 1950s, 60s and 70s. Rock quality, permeability, and deformation modulus were of fundamental importance. Associated hydropower tunnels such as headrace and tail-race tunnels have been the subject of countless thousands of kilometres of seismic refraction spreads, not to men-tion all the power house foundamen-tions and high pressure penstock locamen-tions.
The seismic spreads at the ground surface should if possible be set out in optimal directions to investigate sus-pected sub-surface anomalies. Since the ray paths are essentially following sub-horizontal paths, steeply dipping or vertical features such as faults or deeply weathered zones can be readily located and given a characteristic seismic sig-nature. Localised P-wave velocities of 2 or 3 or 5 km/s have distinct engineering implications for near-surface tun-nelling or foundation stripping. Their interpretation in relation to rock type (uniaxial strength and porosity) and in relation to the depth of measurement, or to stress level and stress-induced anisotropy, will be reviewed in detail in this book, with the help of a quantitative rock mass quality description.
Some basic principles of P, S and Q
The P-wave is a longitudinal wave, in which the direction of particle motion coincides with the wave propagation. It is often termed the first arrival or compressional wave. By contrast, the lower velocity transverse S-wave has par-ticle motion in the plane perpendicular to the direction of wave propagation. An S-wave is of two possible basic types: the SH-wave in which particle motion is parallel to a boundary, usually the ground surface, and the SV-wave which has particle motion perpendicular to both the wave propagation direction, and to the particle motion of the SH-wave.
When passing through anisotropically fractured petroleum reservoirs, a shear wave will likely split into fast (qS1) and slower (qS2) polarized components, giving clues about the fracturing character and perhaps the principal stress direc-tion. The latter coupling may be more complex than convention suggests however, due to adverse stress-closure-per-meability behaviour in reservoir rocks, unless they are strong enough to tolerate tens of megapascals of effective normal stress across their ‘open’ joints or fractures. Slight shearing and dilation may actually be needed on conju-gate joint or fracture sets, to explain permeability and production from fractures in weaker reservoir rocks, and to explain the ‘surprising’ maintenance of permeability deep into the crust.
There is a ‘problem’ of frequency dependence for all the component velocities of P- and S-waves, but in fact in the problem lies the more accurate interpretation. There are exciting current developments in these dispersive, frequency-dependent interpretations of velocities and attenuation, and in their relation to anisotropy, where rock mechanics knowledge of ‘joint stiffnesses’, or their dynamic micro-strain-based near-inverses: the geophysicist’s ‘fracture com-pliances’, are proving extremely important supplements to the earlier focus on the elliptic aspect ratios of micro-cracks, and the larger-scale – and smaller magnitudes – of the aspect ratios of almost closed fractures.
Q and Q
Seismologists have had a long tradition of utilising a quality factor Q-seismic (with numerous sub-sets such as the basic Qp, Qs, and Qc , the latter from the coda or tail-end of a dynamic wave sequence). Q-seismic was popularized by a famous Knopoff, 1964 paper with the briefest possible title: ‘Q’. We will see the possibility of a Q-seismic rela-tion with another quality descriptor called the ‘Q-value’, from rock engineering, not directly, but via a mutual apparent relation to the stress-dependent pseudo-static deformation modulus: surprisingly not to the dynamic modu-lus, at least not in the top kilometre or so.
Q-seismic is a dimensionless factor whose inverse (Q1
seis) indicates, if simply stated, the percentage loss of energy
of a single wave length due to various (and sometimes disputed) mechanisms of attenuation in the rock mass at many possible scales. Reduction in wave amplitude is the most obvious effect. The attenuation is caused by scatter-ing from geo-structures of different scales, and by absorption in intrinsic micro-mechanisms like normal and shear displacements across microcracks and joints, therefore involving friction to some degree, and relative micro-movement of fluids between the pore-space, the micro-cracks and the jointing or fracturing.
As a result of the passage of the very slightly deforming seismic waves there will be a lot of references to ‘squirt flow’ losses in Part II of this book, in connection with anisotropic attenuation, which is one of several properties of the fluid conducting structures of fractured or naturally jointed hydrocarbon reservoirs.
In parallel but previously almost unrelated endeavours, a prominent engineering geologist (Deere, 1964) developed a simple empirical rock quality factor RQD, related with the degree of jointing or fracturing in drill-core. In the 1970s, with no knowledge of Qseis, the rock quality Q-value was developed, which includes RQD as one of the six parameters. The rock engineering rock quality Q-value describes the degree of jointing (as relative block size) and important ‘internal’ joint properties like roughness and clay-filling (giving the inter-block friction coefficient). It also incorporates estimates of the permeability and the stress-to-strength ratio.
Frequent use will be made of the Barton et al., 1974 and Barton 2002 rock quality Q-value and Qc-value in
vari-ous parts of this book. It provides a simple link to seismic velocity, and it probably has the potential for explaining some attenuation mechanisms as well. The rock quality Q-value has a six orders of magnitude scale of quality (from 0.001 to 1000), and it predicts a two to three orders of magnitude range of deformation modulus. Completely unjointed, massive rock masses, with Q 1000, will clearly show almost no attenuation. At many kilometres depth, Qseis values are of similar magnitude. Completely decomposed, near-surface, faulted rock with Q 0.01–0.001 will obviously give complete attenuation (i.e. effectively lower than the theoretically lowest possible Qseisand highest possible Q1
seis– each probably beyond measurement limits).
It is expected that future graphs of Q (seismic quality factor) versus Q (rock quality factor) in rock masses (as opposed to lab-samples), can show strong correlations in the future, when geophysics data is reported in parallel with rock quality data. Each of the ‘Q-factors’ will be described in greater detail later in this book. We will also see the ‘problem’ of frequency-dependence, and the ‘problem’ of anisotropy, but both these problem areas are obviously
takes in interpretation, due to such features. However, as with most limitations, there are various solutions, and geophysicists have been extremely creative, and also willing to modify and apply techniques from other well-funded fields like medicine.
While P-wave and S-wave measurement between two points can be expressed as average wave velocities (or give a rather unhelpful ‘average’ picture of a patients brain), there is the possibility of using more comprehensive mul-tiple source and receiver positions in separate mulmul-tiple-boreholes, thereby giving positional (2D or 3D) tomo-graphic imaging. A tumour in an unfortunate patient, and real-time scanning of brain-wave activity, as illuminated in medicine, have their engineering-scale equivalents. A fault zone delaying a tunnel, and four-dimensional fluid-migration-imaging in a producing reservoir would be approximate, large-scale geophysics equivalents. The most basic imaging analogy has been practiced for many years by geophysicists, who use earthquake sources and global monitoring stations to deduce the structure of the whole earth. So perhaps geo-physicists actually helped to inspire medical imaging of the human body?
In intermediate-scale, near-surface civil engineering, the strategic positioning of pairs of boreholes across complex zones or faults can be used for optimal characterization of these features, if they appear to be a threat to progress of a tunnel, or to dam foundation integrity. In special cases cross-hole tomography measurements may lead to the avoid-ance of collapse, as more reliable decisions can be taken concerning the need for strengthening by pre-grouting, or the need for special pre-installed ground support, or perhaps even ground-freezing. Tunnels with inadequate overburden or severe water leakage potential such as inundation by rivers or lakes, or local inflows that would allow pore pressure draw-down compaction in soft clays beneath important buildings, can also benefit greatly from seismic-based deci-sions for special treatment of the ground.
Part I which occupies the first third of this book, will be found to contain mostly civil-engineering and tunnel
engi-neering treatments of the velocity-quality links that are helpful when interpreting near-surface conditions.The com-plementary laboratory testing that has often accompanied geophysics investigations of the near-surface, will also have emphasis on lower stress. Because of this, the effect of weathering and alteration and excavation on seismic attributes, will each be emphasised. Despite the obvious challenges of seismic interpretation in fractured and faulted petroleum reservoirs at many kilometers depth, or of mid-ocean ridge investigations beneath three kilometers of ocean, many geophysicists insist that obtaining high resolution images from ground level to just 50 m depth, is still one of the major challenges of modern geophysics. This happens to be the layer of the subsurface closest to most of our civil engineering endeavours, from tunnels, to dams, to the foundations for high buildings.
Part II of this book tackles greater depths, greater scales, and more subtle geophysical detail, as benefits this rapidly
developing field. Geophysics has been in ‘rapidly developing’ phases many times in the past. The latest phase is due to many parallel developments, not least an acceptance of the benefits of three-dimensional surveys, of monitoring reservoir changes over time (4D), each requiring the ever-developing power of modern computers for the complex processing of huge amounts of digital data. Investment in geophysics is growing further, due to the inestimable advan-tages of improved information. The continued search for reliable earthquake precursors, and the pressures to find more hydrocarbons in more heterogeneous reservoirs, and improve the recovery from those already being depleted, are each driving the developments in this remarkable field. In the future, more geophysical investments may also be used to aid in the search for potable water, which already far exceeds the price of gasoline in many locations.
angle subtended between a discontinuity and the major principal stress s1 rock mass density (t/m3)
shear-wave anisotropy parameter
change in value (e.g. e, E applying to changes in joint or fracture apertures) v vertical component of deformation
h horizontal component of deformation
m1 frequency of joints (or fractures) per meter (also F m1)
shear modulus
c uniaxial compression strength (MPa) h min minimum horizontal component of stress
H max maximum horizontal component of stress
r radial stress around an excavation in rock v vertical component of principal stress 1 2 3 principal stresses
tangential stress around a (circular) opening
max maximum tangential stress min minimum tangential stress shear stress (in a direct shear test)
friction angle of joint, fracture, filled discontinuity, fault (geomechanics) fractional porosity (rock physics)
b basic friction angle, flat unweathered surfaces, low stress c critical state line defining s1= 3s3
peak peak friction angle of a joint, fracture
r residual friction angle of a joint, fracture, fault axial modulus
ANDRA Agence Nationale pour la gestion des Déchets Radioactifs AR advance rate (TBM, actual weekly, monthly rate)
AVO amplitude variation with offset
AVOA amplitude variation with offset and azimuth
BB Barton-Bandis constitutive model for rock joints, used with UDEC as UDEC-BB BEM boundary element method of numerical modelling
BGS British Geological Survey BHA bottom hole assembly
BHC borehole compensated sonic logging tool BHTV borehole televiwer
BISQ Biot and squirt flow model BP British Petroleum
BWIP Basalt Waste Isolation Project, Hanford, Washington, USA c cohesion of intact rock, joint, fracture, or rock mass CBTF Conoco Borehole Test Facility
CC cohesive component of rock mass (from Q-value) CDR compensated dual resistivity log
FL dyn (as Edyn, lab-scale, based on ultrasonic measurements, shortened to EL)
Emass pseudo-static modulus of deformation (also D, Edand M) from loading stiffness of rock mass e hydraulic aperture of a joint or fracture (kintrinsiclaminar flow, defined as e2/12)
E mean physical aperture of joint or fracture (empirical JRC-estimated, or BB-model) EDA extensive dilatancy anisotropy
EDZ excavation disturbed/damaged zone ( typically around tunnels)
Mini-EDZ ‘alteration zone’ typically around boreholes or wells
EOR enhanced oil recovery
F m1 frequency of fractures (or joints) per meter
FEM finite element method of numerical modelling FC frictional component of rock mass (from Q-value)
FLAC two-dimensional continuum code for modelling small or large deformations in rock or soil FLAC3D 3D continuum code for modelling small or large deformations in rock or soil
FM, FMS formation micro-scanner
FRACOD fracture mechanics boundary element code for modelling fracturing process in rock FZI flow zone indicator
GRM generalized reciprocal method HDR hot dry rock
HPHT high pressure high temperature (well)
HRSN high resolution seismic network, Parkfield, California HSP horizontal (in-tunnel) seismic reflection profiling
HTI as TIH, transversely isotropic, horizontal axis of symmetry HTM hydro-thermal-mechanical (coupling) (also MHT)
i with or implies dilation or contraction when loaded in shear I50 point load index for 50 mm size samples
IPT Institute of Technological Research (S~ao Paulo) ISONIC sonic while drilling tool
ISRM International Society of Rock Mechanics
Ja rating for joint alteration, discontinuity filling in Q-calculation JCS joint wall compression strength (MPa)
Jn rating for number of joint sets in Q-calculation Jr rating for joint surface roughness in Q-calculation
JRC joint roughness coefficient (dimensionless: range 0 to 20) Jv volumetric joint count (sum of frequencies for different sets)
Jw rating for water softening, inflow and pressure effects in Q-calculation K,k permeability (intrinsic: units of length2, engineering: units of m/s)
K bulk modulus (also Kbulk)
Kint intermediate principal permeability
Kmax maximum principal permeability
Kmin minimum principal permeability
Kn dyn dynamic normal stiffness (of joint or fracture) Ko ratio of rock stresses sh min/sv
KS shear stiffness (of joint or fracture: non-linear, sample dependent, scale dependent) Ks dyn dynamic shear stiffness (of joint or fracture)
L Lugeon unit of water injection (l/min/m of borehole/1MPa excess pressure 107m/s) Lg coda waves, tail of seismogram
LOFS life of field seismic LSS long-spaced sonic tool LWD logging while drilling
M deformation modulus (pseudo-static loading stiffness: plate load test. Also Emass, D)
M1,2 dynamic elastic moduli at frequencies f1and f2 MAR mid-Atlantic ridge
MHF massive hydraulic fracturing
MIT Massachusets Institute of Technology MPBX multiple position borehole extensometer MWD measurement while drilling
n effective stress coefficient (Biot) n% porosity of matrix
NAFZ North Anatolian Fault Zone, Turkey ND natural directivity
NGI Norwegian Geotechnical Institute, Oslo, Norway NMO normal moveout
NPF Norsk Petroleumsforening (Norwegian Petroleum Society) OC over-closure of joints, mechanical or thermal
O/R open/rock-to-rock sections of shearing joint, opposite rotation OBC ocean bottom cable
OBS ocean bottom seismometers P volumetric stress
Pg direct (P-) wave (crustal scale studies) Pn refracted (P-) wave (crustal scale studies)
Pr support pressure, radial capacity of support in a tunnel PR penetration rate (TBM, uninterrupted boring)
Q rock mass quality rating (‘Q-value’ range 103to 103, dimensionless)
Qrock rock mass quality rating, distinguish from Qseis, seismic quality, inverse of attenuation Qc seismic quality of coda wave
QE seismic quality in extensional resonance mode
Qe seismic quality component (Young’s mode of elastic excitation)
Qk seismic quality component (bulk mode of elastic excitation)
Qo seismic quality, Lg coda at 1 Hz
QP seismic quality of P-wave (through given medium)
Qs seismic quality component (shear mode of elastic excitation)
QS seismic quality of S-wave (through given medium)
Qc rock mass quality rating (Q or Qrocknormalized by c/100)
Qo Q (or Qcor Qrock) calculated with RQDo, oriented in the loading or measurement direction Qseis, seismic quality factor (‘Q’), inverse of attenuation, also for QPor QS, or the coda wave Qc Qtbm rock-machine quality factor for TBM tunnel boring machines based partly on Q-value QVO Q(seismic) versus offset
r,R Schmidt hammer rebound % on wet joint surfaces, dry intact samples, respectively REV representative elemental volume
TBM tunnel boring machine
3DEC three-dimensional distinct element code for modelling jointed rock masses TIH transversely isotropic, horizontal axis of symmetry (also HTI)
TIV transversely isotropic, vertical axis of symmetry TSP (in-tunnel) seismic reflection profiling
TSX tunnel sealing experiment
UCS uniaxial compressive strength of rock cylinder
UDEC universal distinct element code, for modelling jointed, fractured rock in 2D
(3DEC) three-dimensional distinct element code, for modelling jointed, fractured rock in 3D) URL Underground Research Laboratory, Manitoba, Canada
Vp P-wave seismic velocity (km/s) VS S-wave seismic velocity (km/s)
VSP vertical seismic profiling WAP wide aperture profile
WIPP Waste Isolation Pilot Plant, New Mexico w.r.t. with respect to (index only)
ZEDEX Zone of EXcavation Disturbance Experiment, SKB project, Äspö, Sweden ZN dynamic compliance (of joint or fracture) ( 1/Kn dyn)
ZT dynamic compliance (of joint or fracture) ( 1/Ks dyn)
Cross-discipline differences and connections
• effective stress total stress minus pore pressure in geomechanics
• differential stress shear stress caused by 1– 3application in geomechanics • differential pressure confining pressure minus pore pressure in rock physics • compliance (dynamic stiffness)1, compliance (pseudo-static stiffness)1
1
some basic theory, and the
importance of rock type
‘Nature has left us an incomplete and often well-concealed record of her activities, and no ‘as con-structed’ drawings!’ (Stapledon and Rissler, 1983)
‘Tenders for the Tay pipeline crossing did not allow time for boreholes to locate bedrock. Seismic refraction took one day to confirm that the trench would not encounter rock. The pipeline was laid in sediments.’ (Gardener, 1992)
‘The time may come when the various relations between geophysical parameters and rock properties can be usefully combined into a single classification system.’ (Darracott and Orr, 1976)
1.1 The challenge of the near-surface in civil engineering
Refraction seismics is by far the oldest method used in exploration seismology, with its origin traced to R. Mallet from 1848. Shallow refraction seismic measurements using first arrival, compressional P-wave velocities close to the surface often give a remarkable picture of near surface conditions due to some fortuitous interactions of physical phenomena. Firstly, weathering and the usual lack of significant stress near the surface has allowed joint systems, shear zones and faults to be exaggerated in both their extent and severity. Secondly, stress levels are low enough to allow joints and discontinuities to be seismic-ally visible due to their measurable apertures.
So-called acoustic closure occurs at greater depths than those usually penetrated by conventional hammer seis-mic, unless rock strengths are rather low (e.g., New and West, 1980; Hudson et al., 1980). (At this juncture, we need to differentiate between two ‘J.A. Hudson’ authors, one in geophysics, the other in rock engineering, and both very prominent in their chosen fields. We will occasionally refer to ‘rock’ Hudson in Part I, and later in Part II to ‘seismic’ Hudson).
Since micro-fractures and rock joints are sensitive to stress levels, the more closed state of the discontinuities
that are perpendicular to the major stress, and the more open state of those that are parallel will give the rock mass anisotropic stiffness. Consequently the rock mass will frequently display anisotropic seismic velocities. By implications, hydraulic conductivities and deformation moduli that show anisotropic distributions will be, at least in part, detectable by seismic measurements. Anisotropy will also be caused by layered inter-beds, foliation and schistocity, and of course by a dominant joint set. Simple examples of (azimuthal) anisotropy, applicable in civil engineering, will be given in Chapter 3, while larger-scale examples of anisotropy detection will be described in much greater detail, and from various fields of the earthsciences, in Chapters 13, 14 and 15 in Part II.
Despite the obvious challenges of seismic interpret-ation in fractured and faulted petroleum reservoirs at many kilometers depth, or of mid-ocean ridge investi-gations beneath three kilometers of ocean, many geo-physicists insist that obtaining high resolution images from ground level to just 50 m depth, is still one of the major challenges of modern geophysics. This happens to be the layer of the subsurface closest to most of our civil engineering endeavours, from tunnels, to dams, to the foundations for high buildings.
Undoubtedly, the ‘0 to 50 m’ challenge is mainly due to the extreme variability of the near-surface, resulting from the contrasting geological materials and weathering
There are an infinite number of challenges in the near-surface. Some of the worse may be karst phenomena in limestones, or the ‘inverse’ problems of core-stone anom-alies in the case of sparsely jointed but deeply weathered granites and gneisses. These features have caused tun-nelling surprises in numerous countries, with nearly as numerous arbitrations as a result. Although completely weathered Grade V is an expected feature beneath the Grade VI soil in tropical terrains, Grade V saprolite sometimes confusingly swaps places with the usually deeper, and almost unjointed Grade I or II. (Saprolite is a weak, water sensitive, weathered in-place, some-times beautifully structured and coloured relic of the rock).
If this reversal of weathering grades appears in a tunnel arch beneath massive, high velocity core-stones, or if there is a generally very undulating rock surface, with fre-quent tunnel penetrations into weathered materials, there can be major delays. A tunnel collapse is difficult to avoid when water is present, unless preparations have been made, as a result of the more frequent exploratory drilling demanded when seismic anomalies such as these are suspected.
Pre-injection ahead of the tunnel face, and heavier tunnel support, would be the very basic requirements in a drill-and-blasted tunnel. (This is one of the purposes of the ‘Q-system’ of rock mass characterization and tunnel support selection). In the case of a TBM (tunnel boring machine) excavation, a change to a closed mode in the case of a hybrid machine with earth-pressure-balance (EPB) would be needed, especially if the wea-thered depressions in the bedrock contained water, as is usually the case.
Best advice of all, as a direct result of a seismic
refrac-tion survey, would be to drive a deeper tunnel from the
start. It is easy to imagine subway station construction under such heterogeneous conditions. It could be extremely time-consuming, and even dangerous. The cost of deeper access to the stations, via longer escal-ators, would be a small price to pay for much reduced tunnelling and station costs.
A detailed determination of the velocity distribu-tion in the main refractor.
● An estimate of the uncertainty of the velocity and
depth determinations.
● An analysis of the (velocity-) depth structure. ● An assessment of velocities in vertical and lateral
directions in relation to the geology.
● Seismic results in relation to results from other
investigations, if available.
● Conclusions and recommendations resulting from
the investigation that are of importance to the project.
Although reflection methods have eventually dom-inated the field of exploration seismics due to the various needs involved with deeper exploration, there is ‘univer-sal’ use of shallow refraction seismic in sub-surface inves-tigations for civil engineering projects around the world, due to its apparent simplicity and low cost. Further-more, refraction seismics can be used to remove (from the more deeply focussed reflection data), the ‘adverse’ effect of the first meters or tens of meters of the hetero-geneous weathered layer, where differences in the ori-ginal rock quality may cause tens of meters of sub-surface ‘topography’ in the case of on-land exploration.
1.2 Some basic aspects concerning elastic body waves
It is usually assumed that the strains associated with the passage of a seismic wave are of minute, sub-micron mag-nitude, and except in the neighbourhood of the source, the strains are generally assumed to be elastic. Based on this assumption, the velocities of propagation of seismic waves are determined by the appropriate elastic moduli and densities of the materials passed through. The general form of the classic equations linking these three quan-tities is V (E/)1⁄
2. Compressional bodywaves (primary
or P-waves) propagate by alternating compression and dilation (Figure 1.1 a) in the direction of the waves.
The oscillating uniaxial strain involved in the case of a confined body, means that the axial modulus () con-trols the velocity of propagation, thus:
(1.1)
Shear bodywave waves, termed secondary, transverse or S-waves propagate by a sinusoidal pure shear strain (Figure 1.1 b) in a direction perpendicular to the direc-tion of the waves. The shear modulus (), which is given by the ratio of shear stress ( ) divided by the shear strain (tan ), will therefore control the (lower) velocity of propagation, thus:
(1.2)
The third important elastic modulus influencing the conversion between dynamic properties is the bulk modu-lus (K), defined as the ratio of the volumetric stress (P) and the volumetric strain (v/v). Since the three mod-uli are linked by the equation K 4/3, it follows that Vpcan also be expressed as:
(1.3) This equation therefore demonstrates the fundamen-tally faster nature of Vpin relation to Vs. The ratio of these two dynamic properties are also linked by the dynamic Poisson’s ratio for the material, as will be shown in the next section, which contains some standard equations.
1.2.1 Some sources of reduced elastic moduli
In the case of micro-cracked, fractured, or jointed rock masses, there is a correspondingly reduced set of moduli in relation to the undisputed elastic nature of the intact matrix, because of micro (and probably elastic) displace-ments in normal and/or shear directions across and/ or along the micro-cracks, fractures or joints. These repre-sent an important part of the source of attenuation of the seismic waves in the dry state, due both to various scales of wave scattering and due to the intrinsic deformations. Added losses are incurred if these micro-or macro-discontinuities are partly saturated, since there is communication with the pores and eventual pore fluid, and minute flows may be initiated to equilibrate pressures. These micro-imbalances will only be equili-brated when the frequency is sufficiently low.
The above mechanisms mean that dynamic proper-ties, such as the velociproper-ties, Poisson’s ratio and attenuation tend in practice to be dispersive, or frequency depend-ent. They are also of course rock quality and
environment-dependent, in the broadest possible meanings of these
words. As rock quality declines, or the surface is approached, there develops a serious discrepancy between the dynamic or elastic properties of the intact matrix and the dynamic properties of the (partly discontinu-ous) medium. The ratio between the dynamic proper-ties of the (partly discontinuous) medium and the static deformation properties, such as the (rock mechanics) deformation moduli and joint stiffnesses (the inverse of compliances), may rise into double figures in this
Vp K 4 3 1 2 / Vs 1 2 Vp 1 2
Figure 1.1 Elastic deformations and particle motions associated with the propagation of body waves: a) P-wave, b) S-wave. Based on Bott, 1982.
