seismic anisotropy in the crystalline upper crust observations and modelling from the outokumpu scientific borehole finland summary seismic velocity anisotropy measurements are made of a fractured metamorphic formation from the 2 5 km deep international continental scientific drilling programme icdp borehole in outokumpu finland three component walk away vertical seismic profile vsp measurements are made along two source line azimuths at three receiver depths 1000 1750 and 2500 m and incidence angle dependent qp and qs wave velocities are extracted with a p method the highest qp wave anisotropy 13 6 per cent vfast 6160 m s 1 vslow 5370 m s 1 is measured between 1000 and 1750 m depth with anisotropy of up to 9 4 per cent vfast 6090 m s 1 vslow 5540 m s 1 measured between 1750 and 2500 m depth the top 1300 m of the region is composed of a homogeneous strongly intrinsically anisotropic biotite rich schist and is sampled by the shallowest walk away profile anisotropy of up to 11 1 per cent vfast 5950 m s 1 vslow 5320 m s 1 is measured by the walk away vsp between 50 and 1000 m depth along with shear wave splitting averaging 5 per cent 180 m s 1 laboratory derived intrinsic anisotropy of the schist cannot by itself explain the degree or orientation of the anisotropy measured in the walk away vsps however a model which modifies the intrinsic stiffnesses by the inclusion of a single set of dipping aligned cracks allows the observed in situ velocities to be reproduced forward modelling of the qp wave walk away vsp measurements from 50 to 1000 m depth is undertaken using an effective medium model to develop a 3 d velocity model of this region an orthorhombic medium is used to represent the intrinsic anisotropy of the biotite rich schist and a single set of aligned cracks is added to give a bulk elastic stiffness the resulting model predicts the schist to have an overall anisotropy of 16 8 per cent with qp wave velocities of up to 6315 m s 1 the accuracy of the model is assessed through its fit to the walk away vsp measurements as well as a comparison to known geology of the region

p wave velocity anisotropy related to sealed fractures reactivation tracing the structural diagenesis in carbonates abstract fracture properties are important in carbonate reservoir characterization as they are responsible for a large part of the fluid transfer properties at all scales it is especially true in tight rocks where the matrix transfer properties only slightly contribute to the fluid flow open fractures are known to strongly affect seismic velocities amplitudes and anisotropy here we explore the impact of fracture evolution on the geophysical signature and directional v p anisotropy of fractured carbonates through diagenesis for that purpose we studied a meter scale parallelepiped quarry block of limestone using a detailed structural and diagenetic characterization and numerous v p measurements the block is affected by two en echelon fracture clusters both being formed in opening mode mode 1 and cemented but only one being reactivated in shear we compared the diagenetic evolution of the fractures which are almost all 100 filled with successive calcite cements with the p wave velocities measured across this meter scale block of carbonate which recorded the tectonic and diagenetic changes of a south provence sedimentary basin we found that a directional v p anisotropy magnitude as high as 8 16 correlates with the reactivated fractures cluster dip angle which is explained by the complex filling sequence and softer material present inside the fractures that have been reactivated during the basin s tectonic inversion we show that although a late karstification phase preferentially affected these reactivated fractures it only amplified the pre existing anisotropy due to tectonic shear we conclude that v p anisotropy measurements may help to identify the fracture sealing opening processes associated with polyphased tectonic history the anisotropy being independent of the current stress state this case shows that velocity anisotropies induced by fractures resulted here from a cause that is different from how these features have often been interpreted selective reactivation of sealed fractures clusters rather than direction of currently open ones

effective viscoelastic behaviour of rock mass with double scale discontinuities summary effective viscoelastic behaviour of a rock mass with double scale discontinuities is investigated the rock mass is modelled as a system of microdefected rock traversed by macrojoints the effect of microdefects on the dynamic behaviour is evaluated by a viscoelastic equivalent medium model implicitly whereas the macrojoints in the rock mass are modelled explicitly as physical discontinuities therefore a homogenization discontinuity combination approach is proposed to investigate the effective viscoelastic behaviour of rock mass under dynamic loading the effective wave propagation coefficients and the effective viscoelastic complex moduli storage modulus and loss modulus are analytically derived the effects of the double scale discontinuities on the energy dissipation and the phase velocity are also discussed it is shown that the effective behaviour of the rock mass with double scale discontinuities is two phase viscoelastic microdefect and macrojoint have different frequency sensitive ranges and the effective viscoelastic behaviour is highly related to the stiffness of the discontinuities

validation of elastic wave measurements of rock fracture compliance using numerical discrete particle simulations we test various methods of quantifying the compliance of single and multiple rock fractures from synthetic ultrasonic data the data are generated with a 2d discrete particle scheme which has previously been shown to treat fractures in agreement with linear slip theory studying single fractures we find that delays derived from peak amplitudes do not correspond to group delays as might be expected this is due to waveform distortion caused by the frequency dependent transmission across the fracture instead the delays correspond to an expression for phase delays which we derive from linear slip theory phase delays are a unique function of compliance whereas group delays are non uniquely related to compliance we believe that this property of group delays has hindered the wider application of deriving fracture compliances from traveltimes we further show that transmission coefficients derived from waveform spectra yield more accurate fracture compliances than those obtained from ratios of signal peak amplitudes we also investigate the compliance of a set of parallel fractures fracture compliance can only be determined from transmission coefficients if the fracture spacing is so large that the first arriving pulse is not contaminated by reverberations in the case of contamination the direct measurement of group or phase delays is not practical however we demonstrate that in such cases of strong waveform distortion the coda wave interferometry method is very effective for determining relative fracture compliance first break delays in the fracture set data are related to those observed in single fracture simulations this means that fracture set compliance can be estimated from first break data if used together with numerical simulations

the problem of regionalization in the study of magnitude intensity correspondences in the mediterranean area a distribution free approach a distribution free statistical approach is proposed for tackling the problem of regionalization devoted to the study of magnitude intensity relationships in the mediterranean area the training set constitutes a compilation of more than 2000 earthquakes which occurred in the mediterranean region since the end of the nineteenth century when instrumental data became available with epicentral or maximum intensity vi and ms magnitude values firstly an empirical magnitude intensity correspondence has been computed for each intensity class by using the entire data set in the assumption of homogeneity at the regional scale residuals of such relation have been analyzed by a distribution free statistical approach in order to evaluate the opportunity of a regionalization able to locally improve the performances of magnitude intensity relationships the analysis indicates that data concerning larger earthquakes intensity vii do not suggest the opportunity of zonation and that unbiased estimates of macroseismic magnitude can also be obtained in the assumption that magnitude intensity correspondences are uniform over the entire mediterranean area therefore better constrained relations determined for the entire mediterranean region should be preferred to ill defined local ones as concerns smaller events intensity vi the procedure suggests that medium small scale lateral variations on a wavelength lower than 102 km should be taken into account if an efficient estimate of magnitudes from maximum observed intensity is pursued but that data presently available are not sufficient to suggest any reliable zonation of the area under study

seismic response of fractures by numerical simulation summary fractures occur on a wide range of scales and are important in the study of hydrocarbon reservoirs seismic simulation by finite difference modelling using an equivalent medium method can devise the elastic parameters of a cell intersected by a fracture as those of a medium with an equivalent seismic response numerical experiments confirm that diffractions from the fracture tips are a strong component of the total wavefield however a comparison of boxcar linear angular and elliptic tapering suggests that there is little dependence on shape because the energy involved in a single diffraction is much lower than the incident energy an open fluid filled fracture has stronger effect on the wavefield than the wet and dry multiple crack models because an open fracture would have a stronger dissimilarity to the background rock the density of microcracks within a fracture also has strong effect on the seismic response however the properties of those cracks are not significant to the overall seismic response considering a distribution of a large number of fractures even when the overall density of fractures is held constant longer fractures attenuate seismic energy more than smaller ones for the orientation effect fractures oriented in the direction of propagation seem to affect the wavefield more than those perpendicular because of the incident wave striking the fracture at an angle greater than the critical angle experiments on clustering of the fractures indicate that although clusters which are large compared to the wavelength may attenuate and shield more than for a uniform distribution smaller ones in fact attenuate less because of the healing effect these are important results when trying to characterize the fracture properties including density clustering size and orientation of a fractured reservoir from field seismic data

sh propagation in rocks with planar fractures i excess slowness summary a model for a fractured rock is a transversely isotropic solid containing randomly spaced plane parallel fractures with random compliance a fracture constitutive relation is the linear slip model in which the jump in motion across the fracture is proportional to the stress i study the propagation at normal and oblique angles of incidence of a shear wave with polarization parallel to the fractures obtaining the frequency dependent excess fracture normal slowness and attenuation i calculate these quantities using some new methods that are not restricted to weak anisotropy or to low frequency or to any particular fracture constitutive relation these are monte carlo mc averaged multiple imbedding ami path sum theory pst and a primary approximation pa finite frequency results from these dynamic theories are compared with results from quasi static theory for a variety of rocks all of which have the same quasi static anisotropy of 9 5 per cent i find that 1 quasi static theory works well at finite frequencies for high q fractures if the fractures are stiff and closely spaced but not if they are compliant and widely spaced 2 the primary approximation works well if the interfracture distance distribution is exponential but not if it is narrow periodic fractures or broad clumped fractures 3 reducing the q of the interfracture rock slightly increases the apparent anisotropy 4 reducing the q of the fracture filling material greatly reduces the apparent anisotropy and 5 increasing the variance of either the fracture spacing distribution or the fracture compliance distribution increases the attenuation a consequence of 4 is that estimates of fracturing derived from sh seismic data will be erroneously low if the fractures are low q elementary arguments suggest that overpressuring or lubrication increases the shear q of natural fractures thus making them more detectable with sh

colonization strategies of lithobiontic microorganisms on carbonate rocks biogenic weathering is caused by the action of lithobiontic organisms homogeneous carbonates are predominantly colonized by endolithic species that actively penetrate the rock substratum independent of already existing pores or fissures the organisms construct a system of ducts and cavities by active dissolution of the substratum a fresh noncolonized surface is penetrated by algae and ascomycetes in the first and second year after exposure to the environment the establishment of complex colonization patterns on and in the substratum by lichens takes several years in spite of the primary deteriorative effect on their substratum by the organisms long term endolithic growth also involves mechanisms that stabilize and preserve the rock surface morphology a tightly woven cellular network may strengthen the colonized stone this feature is of importance when natural and building stone are affected by biogenic weathering phenomena

permeability anisotropy and its relations with porous medium structure 1 the complete permeability tensor of 18 porous rock cores was determined by means of x ray tomography monitoring during the displacement of a salty tracer to study the effect of the pore space geometry on the anisotropy of permeability we compared the three dimensional shape of the invasion front with the x ray porosity maps obtained before injection the samples clean and shale bearing sandstones limestones and volcanic rocks belong to a broad variety of granulometry and pore space geometry their porosity ranges from 12 to 57 and their permeability ranges from 1 5 10 14 to 4 10 12 m2 for sandstones the permeability anisotropy is well correlated with the presence of bedding for volcanic rocks it is clearly related to the orientation of vesicles or cracks however for limestones no evident link between the geometry of the porous network and the permeability anisotropy appears probably because of the influence of the nonconnected porosity that does not contribute to the hydraulic transport this systematic work evidences the ability and the limitations of the tracer method to characterize the anisotropy of permeability in the laboratory in a simple and rapid way

fracture detection using p wave and s wave vertical seismic profiling at the geysers in a pilot vertical seismic profiling study p wave and cross polarized s wave vibrators were used to investigate the potential utility of shear wave anisotropy measurements in characterizing a fractured rock mass the caprock at the geysers geothermal field was found to exhibit about an 11 percent velocity variation between sh waves and sv waves generated by rotating the s wave vibrator orientation to two orthogonal polarizations for each survey level in the well the effect is generally consistent with the equivalent anisotropy expected from the known fracture geometry

anisotropy in seismic velocities and amplitudes from multiple parallel fractures many rock structures include multiple near parallel planar discontinuities such as bedding planes or joints the effects of these nonwelded interfaces on seismic wave propagation are often analyzed using effective moduli in terms of which seismic wave propagation is independent of frequency and without loss unless the moduli include imaginary terms an alternative approach is to treat these interfaces as a boundary condition in the seismic wave equation across which seismic stress is continuous but seismic particle displacements are discontinuous the ratio of the stress to displacement is called the specific stiffness of the interface and characterizes the elastic properties of a fracture for a completely elastic system this results in frequency dependent reflection and transmission coefficients for each interface as well as a frequency dependent group time delay using multiple parallel displacement discontinuities and ignoring converted and reflected waves expressions derived for transmitted wave amplitudes and group velocities show that these depend on frequency angle of incidence and polarization in the case of shear waves measurements on a laminated steel block show that shear pulses propagating parallel to the laminations and polarized parallel and perpendicular to the plane of the laminations both travel at the velocity for solid steel although the spectra of these pulses differ considerably however the energy of the pulse polarized perpendicular to the laminations may propagate as an interface wave between each pair of laminations predictions of the displacement discontinuity model have features quite distinct from many crustal observations to date we suggest that we are able to model dense populations of coplanar cracks that cannot be treated by effective moduli methods which require a dilute concentration of cracks

seismic characterization of vertical fractures described as general linear slip interfaces fluid flow in many hydrocarbon reservoirs is controlled by aligned fractures which make the medium anisotropic on the scale of seismic wavelength applying the linear slip theory we investigate seismic signatures of the effective medium produced by a single set of general vertical fractures embedded in a purely isotropic host rock the generality of our fracture model means the allowance for coupling between the normal to the fracture plane stress and the tangential jump in displacement and vice versa despite its low triclinic symmetry the medium is described by just nine independent effective parameters and possesses several distinct features which help to identify the physical model and estimate the fracture compliances and background velocities for example the polarization vector of the vertically propagating fast shear wave s1 and the semi major axis of the s1 wave normal moveout nmo ellipse from a horizontal reflector always point in the direction of the fracture strike moreover for the s1 wave both the vertical velocity and the nmo velocity along the fractures are equal to the shear wave velocity in the host rock analysis of seismic signatures in the limit of small fracture weaknesses allows us to select the input data needed for unambiguous fracture characterization the fracture and background parameters can be estimated using the nmo ellipses from horizontal reflectors and vertical velocities of p waves and two split s waves combined with a portion of the p wave slowness surface reconstructed from multi azimuth walkaway vertical seismic profiling vsp data the stability of the parameter estimation procedure is verified by performing non linear inversion based on the exact equations

measurements of seismic anisotropy and fracture compliances in synthetic fractured media ultrasonic velocities were measured on a stack of synthetic material plexiglass at low 90 120 khz long wavelength range and high 480 khz short wavelength range frequencies plates were pressed together with uniaxial normal stress these measurements were repeated under different uniaxial pressures for two cases 1without circular rubber inclusions between synthetic fractures and 2with circular rubber inclusions between fractures calculation of anisotropy parameters and also normal and shear interface fracture compliances using linear calculus show reasonable variations anisotropy and compliances decreases as overburden pressure increases suggesting that anisotropy is caused by compliances at higher frequencies synthetic fractures seem to be stiffer than at lower frequencies for the long and short wavelength ranges similar variations were observed for anisotropy parameters and fracture compliances vs measurements in different directions show that for the case without rubber inclusions there is a good approximation of vti medium whereas for the case with inclusions there is a significant difference between vs travelling in directions parallel and perpendicular to the plates

an equivalent viscoelastic model for rock mass with parallel joints an equivalent viscoelastic medium model is proposed for rock mass with parallel joints a concept of virtual wave source vws is proposed to take into account the wave reflections between the joints the equivalent model can be effectively applied to analyze longitudinal wave propagation through discontinuous media with parallel joints parameters in the equivalent viscoelastic model are derived analytically based on longitudinal wave propagation across a single rock joint the proposed model is then verified by applying identical incident waves to the discontinuous and equivalent viscoelastic media at one end to compare the output waves at the other end when the wavelength of the incident wave is sufficiently long compared to the joint spacing the effect of the vws on wave propagation in rock mass is prominent the results from the equivalent viscoelastic medium model are very similar to those determined from the displacement discontinuity method frequency dependence and joint spacing effect on the equivalent viscoelastic model and the vws method are discussed

crosswell seismic imaging in a contaminated basalt aquifer multiple seismic crosswell surveys have been acquired and analyzed in a fractured basalt aquifer at idaho national engineering and environmental laboratory most of these surveys used a high frequency 1000 10 000 hz piezoelectric seismic source to obtain p wave velocity tomograms the p wave velocities range from less than 3200 m s to more than 5000 m s additionally a new type of borehole seismic source was deployed as part of the subsurface characterization program at this contaminated groundwater site this source known as an orbital vibrator allows simultaneous acquisition of p and s waves at frequencies of 100 to 400 hz and acquisition over larger distances the velocity tomograms show a relationship to contaminant transport in the groundwater zones of high contaminant concentration are coincident with zones of low velocity and high attenuation and are interpreted to be fracture zones at the boundaries between basalt flows the orbital vibrator data show high vp vs values from 1 8 to 2 8 in

wave propagation across non linear rock joints based on time domain recursive method summary studying wave propagation across joints is crucial in geophysics mining and underground construction limited analyses are available for oblique incidence across non linear joints in this paper the time domain recursive method tdrm proposed by li et al is extended to analyse wave propagation across a set of non linear joints the barton bandis model b b model and the coulomb slip model are adopted to describe the non linear normal and shear properties of the joints respectively with the displacement discontinuity model and the time shifting function the wave propagation equation is established for incident longitudinal p or transverse s wave across the joints with arbitrary impinging angles comparison between the results from the tdrm and the existing methods is carried out for two specific cases to verify the derived wave propagation equation the effects of some parameters such as the incident angle the joint spacing the amplitude of incidence and the joint maximum allowable normal closure on wave propagation are discussed

seismic modelling for geological fractures based on knowledge of a commutative group calculation of the rock stiffness and on some geophysical assumptions the simplest fractured medium may be regarded as a fracture embedded in an isotropic background medium and the fracture interface can be simulated as a linear slip interface that satisfies nonwelded contact boundary conditions the kinematic displacements are discontinuous across the interface whereas the dynamic stresses are continuous across the interface the finite difference method with boundary conditions explicitly imposed is advantageous for modelling wave propagation in fractured discontinuous media that are described by the elastic equation of motion and nonwelded contact boundary conditions in this paper finite difference schemes for horizontally vertically and orthogonally fractured media are derived when the fracture interfaces are aligned with the boundaries of the finite difference grid the new finite difference schemes explicitly have an additional part that is different from the conventional second order finite difference scheme and that directly describes the contributions of the fracture to the wave equation of motion in the fractured medium the numerical seismograms presented to first order show that the new finite difference scheme is accurate and stable and agrees well with the results of previously published finite difference schemes the coates and schoenberg method the results of the new finite difference schemes show how the amplitude of the reflection produced by the fracture varies with the fracture compliances later comparisons with the reflection coefficients indicate that the reflection coefficients of the fracture are frequency dependent whereas the reflection coefficients of the impedance contrast interface are frequency independent in addition the numerical seismograms show that the reflections of the fractured medium are equal to the reflections of the background medium plus the reflections of the fracture in the elastic fractured medium this article is protected by copyright all rights reserved

wave propagation and diffraction through non persistent rock joints an analytical and numerical study abstract the presence of joints persistent and non persistent ones within rock masses has a significant influence on wave propagation however previous studies were limited to wave propagation across persistent joints to understand p wave propagation and diffraction across non persistent jointed rock masses theoretical and numerical analyses were carried out in this paper a theoretical method was introduced to investigate p wave propagation and diffraction across a single non persistent joint by combining the displacement discontinuity method and the angular diffraction theory with this proposed theoretical method analytical solutions for wave propagation and diffraction across a single non persistent joint were derived after validation through comparison with theoretical solutions fem based numerical simulations were carried out to study wave propagation and diffraction across multiple parallel non persistent joints results showed that the wave field after passing through a single non persistent joint is two dimensional which is different from plane wave propagation across the persistent joint obvious enhancement regions and suppression regions exist which are determined by the combined effect of wave transmission across the joint segments and wave diffraction at the rock bridges in addition spatial parameters including rock bridge length joint segment length joint spacing propagation distance and joint dislocation have a remarkable influence on wave propagation across multiple non persistent joints the findings in this paper could fill in the gap between wave propagation across persistent and non persistent joints and thus facilitate a better understanding of wave propagations across discontinuous rock masses

fracture parameter inversion from passive seismic shear wave splitting a validation study using full waveform numerical synthetics abstract fractures are pervasive features within the earth s crust and they have a significant influence on the multi physical response of the subsurface the presence of coherent fracture sets often leads to observable seismic anisotropy enabling seismic techniques to remotely locate and characterise fracture systems since fractures play a critical role in the geomechanical and fluid flow response there has been significant interest in quantitatively imaging in situ fractures for improved hydro mechanical modelling in this study we assess the robustness of inverting for fracture properties using shear wave splitting measurements we show that it is feasible to invert shear wave splitting measurements to quantitatively estimate fracture strike and fracture density assuming an effective medium fracture model although the sws results themselves are diagnostic of fracturing the fracture inversion allows placing constraints on the physical properties of the fracture system for the single seismic source case and optimum receiver array geometry the inversion for strike has average errors of between 11 and 25 whereas for density has average errors between 65 and 80 for the single fracture set and 30 and 90 for the double fracture sets for real microseismic datasets the range in magnitude of microseismicity i e frequency content spatial distribution and variable source mechanisms suggests that the inversion of fracture properties from sws measurements is feasible

an improved equivalent viscoelastic medium method for wave propagation across layered rock masses abstract the behavior of waves propagating through a rock mass is complicated by the discontinuous nature posed by the joints in the rock mass however at large scale the jointed rock mass can be treated as an equivalent continuous medium the purpose of this paper is to establish a method to determine the equivalent viscoelastic property of rock masses with different seismic quality factors and to analyze seismic wave propagation across jointed rock masses each rock mass contains uniformly distributed joint sets with arbitrary orientation the equivalent viscoelastic parameters are calculated for a rock mass with different seismic quality factors the wave propagation equation is then derived for seismic wave across layered rock masses where the parallel layer interfaces are assumed to be welded comparison between the improved equivalent viscoelastic medium method evmm and the existing analytical methods is carried out for two specific cases reflection at free surface and transmission across a discontinuous interface between two media the effects of some parameters such as the incident angle the thickness of the layer and the rock mass seismic quality factor on wave propagation are discussed

modelling seismic waves around underground openings in fractured rock the potential for large excavation induced seismic events may be recognised even if the timing of an event may be inherently unpredictable in this case modelling the wave propagation from a potential event could allow the dynamic motions around an excavation to be projected and for areas of danger to be anticipated however the above and other potential applications require accurate models of wave interaction with the openings as well as with the fractured rock which surrounds such excavations this paper considers real recorded waveforms and how well these waveforms are modelled by explicit mechanical models of the source the medium and the excavation models of experiments at three different scales of the problem are presented small and large amplitude waveforms recorded around a deeplevel mining tunnel in a synthetic rockburst experiment waveforms from laboratory experiments of waves through plates of steel representing fractures waveforms from active pulses in an acoustic emission experiment in a small volume of fractured rock at the surface of an underground excavation the results show that elastic wave propagation around an excavation was a first approximation for small amplitude waves but was less successful for modelling large amplitude waves and more fractured rock fractures in the models were represented explicitly with displacement discontinuities waveforms through known fracture geometries were particularly well reproduced and indicate the importance of fracture stiffness the in situ stress state and stress dependence of the fractures in such models overall the models are sufficiently successful at representing recorded behaviour to be encouraging for the goal of representing accurate wave motions around excavations

a rapid and nondestructive method to determine normal and shear stiffness of a single rock joint based on 1d wave propagation theory abstractdetermination of joint stiffness one of the most important mechanical properties of rock joints is of great significance however joint stiffness is often difficult to complete and accurate determine because of the 3d nature of joints and limited budgets and visible exposure wave attenuation in a rock mass is mainly attributed to the presence of joints based on 1d plane wave theory a rapid and nondestructive method namely the rapid evaluation method is proposed to calculate normal and shear stiffness of joints the joint is viewed as a boundary condition modeled by the displacement discontinuity model by solving wave equations with reasonable approximation joint stiffness can be inversely solved which is dependent on the seismic impedance transmission or reflection coefficient and dominant frequency of transmitted waves ultrasonic laboratory tests were carried out to record incident reflected and transmitted waveforms as input to the rapid evaluation method it was found that the

stress wave interaction with a nonlinear and slippery rock joint keywords deformational behavior parallel fractures single fractures shear strength elastic waves transmission propagation attenuation boundary model reference epfl article 171633doi 10 1016 j ijrmms 2010 11 013view record in web of science record created on 2011 12 16 modified on 2016 08 09

deep borehole log evidence for fractal distribution of fractures in crystalline rock summary sonic velocity and electrical resistivity logs run to a depth of 3 5 km in crystalline rock near the san andreas fault at cajon pass in southern california correlate over scale lengths both small sub metre and large tens to hundreds of metres no such correlations are seen with the more lithologically sensitive natural gamma intensity log the correlation between the sonic velocity and electrical resistivity logs suggests that a non lithologic property of the crystalline rock controls fluctuations in situ fracture intensity is a logical candidate for the controlling rock property the fluctuations of the individual sonic velocity and electrical resistivity logs are examined with the hurst rescaled range parameter over borehole log intervals 1 5 m l 1500 m for log fluctuations arising from a scale invariant physical process the hurst rescaled range scales with data interval as lh 0 h 1 a purely random sequence of in situ fractures produces a scaling exponent h 0 50 fluctuations in the cajon pass sonic velocity and electrical resistivity logs yield h 0 70 evidence that in situ fracture sets tend to occur in clusters rather than at purely random intervals the tendency for fracture clustering over log intervals 1 5 m l 1500 m suggests that fracture formation is a fractal process independent of length scale in which larger fracture intervals form from clustering of numerous smaller fracture intervals seismic reflectivity derived from the borehole sonic velocity log is also scale independent over the range of data intervals 1 5m l 1500 m with a hurst exponent h 0 21 if we associate fracture clustering with crustal fault formation the cajon pass borehole sonic velocity and electrical resistivity logs predict that crustal faults scale fractally with fractal dimension d 2 30 the equivalent b value for earthquake size distribution is b d 2 1 15 on this hypothesis observed b values 1 15 indicate a tendency for earthquakes to cluster on existing weak faults a scale independent mechanics for crystalline rock fracture formation in which larger scale fractures form as clusters of smaller scale fractures provides a mechanical link between the small pervasive stress aligned flaws cracks and microfractures which can impart anisotropy to crystalline rock and the larger scale fractures associated with finite strain and faulting thus in crustal regions of active but low strain fracture anisotropy is observed to be aligned with the inferred maximum principal stress while in actively faulted crust fracture anisotropy is observed to be more nearly fault parallel as if the fractures are aligned by the finite strain faulting process

compressional waves guided between parallel fractures abstract we have performed experiments on the laboratory scale to image seismic wavefronts of compressional waves that are guided between parallel fractures the wavefronts are detected using water coupled 1 mhz piezoelectric transducers the stiffness of the fractures is controlled by applying radial stress in a hydraulic stress apparatus that leaves the observation face of the fractured sample open to measurement the two fractures form a waveguide in isotropic homogeneous aluminum and act as parallel displacement discontinuity boundary conditions separated by a distance d 2 the compressional wave is a leaky guided mode that sheds energy into the medium beyond the fractures however the fractures provide sufficiently strong energy confinement inside the central waveguide for a sample length l of approximately l 15 to observe the guided wave fractures with low fracture specific stifffness produce strong energy confinement in the guided compressional mode wave

shear wave propagation in jointed rock state of stress the mechanical characteristics and orientation of joints determine the behaviour of rock masses including the strength the stiffness and all forms of conduction and diffusion furthermore the preferential orientation of joints renders the medium anisotropic in turn these properties affect the velocity and damping of propagating elastic waves the purpose of this study is to investigate the propagation of long wavelength shear waves through a rock mass with open joints filled with softer gouge and subjected to low confinement these conditions are selected in view of near surface geophysical characterisation studies for geotechnical engineering applications such as foundations tunnels and slopes experimental data are gathered with a slender column made of discrete elements with gouge material in the joints the column is excited in the first torsional vibration mode to gather accurate measurements of velocity and damping for wave propagation direction normal to the joints data are analysed with m

when do fractured media become seismically anisotropic some implications on quantifying fracture properties fractures are pervasive features within the earth s crust and they have a significant influence on the multi physical response of the subsurface the presence of coherent fracture sets often leads to observable seismic anisotropy enabling seismic techniques to remotely locate and characterise fracture systems in this study we confirm the general scale dependence of seismic anisotropy and provide new results specific to shear wave splitting sws we find that sws develops under conditions when the ratio of wavelength to fracture size s d is greater than 3 where rayleigh scattering from coherent fractures leads to an effective anisotropy such that effective medium model emm theory is qualitatively valid when 1 s d 3 there is a transition from rayleigh to mie scattering where no effective anisotropy develops and hence the sws measurements are unstable when s d 1 we observe geometric scattering and begin to see behaviour similar to transverse isotropy we find that seismic anisotropy is more sensitive to fracture density than fracture compliance ratio more importantly we observe that the transition from scattering to an effective anisotropic regime occurs over a propagation distance between 1 and 2 wavelengths depending on the fracture density and compliance ratio the existence of a transition zone means that inversion of seismic anisotropy parameters based on emm will be fundamentally biased more importantly we observe that linear slip emm commonly used in inverting fracture properties is inconsistent with our results and leads to errors of approximately 400 in fracture spacing equivalent to fracture density and 60 in fracture compliance although emm representations can yield reliable estimates of fracture orientation and spatial location our results show that emm representations will systematically fail in providing quantitatively accurate estimates of other physical fracture properties such as fracture density and compliance thus more robust and accurate quantitative estimates of in situ fracture properties will require improvements to effective medium models as well as the incorporation of full waveform inversion techniques

fracture detection using crosswell and single well surveys we recorded high resolution 1 to 10 khz crosswell and single well seismic data in a shallow 15 to 35 m water saturated fractured limestone sequence at conoco s borehole test facility near newkirk oklahoma our objective was to develop seismic methodologies for imaging gas filled fractures in naturally fractured gas reservoirs the crosswell 1 4 m receiver spacing 50 to 100 m well separation surveys used a piezoelectric source and hydrophones before during and after an air injection that we designed to displace water from a fracture zone our intent was to increase the visibility of the fracture zone to seismic imaging and to confirm previous hydrologic data that indicated a preferred pathway for the single well seismic imaging a piezoelectric source and an eight element hydrophone array at 1 4 m spacing we also recorded data before and after the air injection the crosswell results indicate that the air did follow a preferred pathway that was predicted by hydrologic modeling in addition

long wavelength p wave and s wave propagation in jointed rock masses field data suggest that stress level and joint condition affect shear wave propagation in jointed rock masses however thestudyoflong wavelengthpropagationinajointedrock mass is challenging in the laboratory and limited data are available under controlled test conditions long wavelength p wave and s wave propagation normal to joints using an axially loaded jointed column device reproduces a range of jointconditions theeffectsofthenormalstress loadinghistory joint spacing matched surface topography i e joint roughness joint cementation e g after grouting joint opening and plasticity of the joint filling on the p wave and s wavevelocitiesandons waveattenuationarenotable the ratio vp vs in jointed rock masses differs from that found in homogeneous continua the concept of poisson s ratio as a functionof vp vsisunwarranted and vp vscanbeinterpreted in terms of jointed characteristics analytic models that consider stress dependent stiffness and frictional loss in joints as well as stress independent properties of intact rocks can model experimental observations properly and extract joint properties from rock mass test data thus joint propertiesandnormalstresshaveaprevalentroleinpropagationvelocityandattenuationinjointedrockmasses

elastic interface waves along a fracture non welded interfaces can be treated as a displacement discontinuity characterized by elastic stiffnesses applying this boundary condition to a generalized rayleigh wave it is shown that a fast and a slow dispersive wave can propagate along the fracture even when the seismic properties of the rock on each side are identical

fracture toughness anisotropy in shale the use of hydraulic fracturing to recover shale gas has focused attention on the fundamental fracture properties of gas bearing shales but there remains a paucity of available experimental data on their mechanical and physical properties such shales are strongly anisotropic so that their fracture propagation trajectories depend on the interaction between their anisotropic mechanical properties and the anisotropic in situ stress field in the shallow crust here we report fracture toughness measurements on mancos shale determined in all three principal fracture orientations divider short transverse and arrester using a modified short rod methodology experimental results for a range of other sedimentary and carbonate rocks are also reported for comparison purposes significant anisotropy is observed in shale fracture toughness measurements at ambient conditions with values as high as 0 72mpam1 2 where the crack plane is normal to the bedding and values as low as 0 21mpam1 2 where the crack plane is parallel to the bedding for cracks propagating non parallel to bedding we observe a tendency for deviation towards the bedding parallel orientation applying a maximum energy release rate criterion we determined the conditions under which such deviations are more or less likely to occur under more generalized mixed mode loading conditions we find for mancos shale that the fracture should deviate towards the plane with lowest toughness regardless of the loading conditions

a time domain recursive method to analyse transient wave propagation across rock joints the present investigation is concerned with transient wave propagation in a rock mass with a set of parallel joints by using a recursive method according to the displacement field of a rock mass with a set of parallel joints the interaction between four plane waves two longitudinal waves and two transverse waves and a joint is analysed first with the displacement discontinuity model and the time shifting function the wave propagation equation based on the recursive method in time domain for obliquely longitudinal p or transverse s waves across a set of parallel joints is established the joints are assumed linearly elastic the analytical solution obtained by the proposed method is compared with the existing results for some special cases including oblique incidence across a single joint and normal incidence across a set of parallel joints by verification it is found that the solutions by the proposed method match very well with the existing methods the applicability and limitations of the new method are then discussed for incident waves with different properties

seismic fracture anisotropy in the earth s crust an overview fluid filled fractures and microcracks are the most stress sensitive component of crustal rock and often determine the pathways and volume of crustal fluid movement if we are to comprehend the role of fractures and fluids in tectonic processes to formulate an accurate hypothesis for phenomena precursory to catastrophic seismic failure or to monitor hydrocarbon and geothermal reservoirs for the presence or absence of major fluid pathways we must understand how seismic waves interact with the fracture crack and microcrack structures within the rock mass a salient feature of fracture related rock structure is the effective seismic anisotropy of aligned fractures and microcracks with the advent of large volume digital seismic data sets it is feasible to use polarized shear waves to explore the anisotropic fracture properties of the crust crustal fracture structures which may be monitored by polarized shear waves range in dimensions from the 10 100 km scale of crustal stress orientation and preseismic stress buildup through the 100 1000 m scale of fluid reservoir structure and characterization and the 1 10 scale of mining and geoengineering rock strain monitoring to millimeter and micrometer or possibly smaller fluid filled inclusions determining the elastic response of the intact rock mass to focus developments in seismic anisotropic studies of crustal fracture phenomena the american geophysical union and the society of exploration geophysicists sponsored a chapman conference on seismic anisotropy in the earth s crust held may 31 to june 4 1988 at the lawrence berkeley laboratory of the university of california this was the third international workshop on seismic anisotropy 3iwsa nineteen conference papers are presented in this special section of the journal of geophysical research

seismic focusing by a single planar fracture 1 a single plane fracture with an axially symmetric stress distribution behaves as a seismic lens that focuses seismic energy to a beam waist at a focal plane both phase and amplitude effects on a seismic wave propagating across the fracture contribute to the lensing behavior radial gradients in the fracture specific stiffness cause wave refraction through a radially varying group time delay while the fracture transmission amplitude approximates a fresnel zone plate this work demonstrates that a two dimensional planar fracture contrasted with three dimensional geologic structures such as basins and domes can focus seismic waves focusing of seismic waves by fractures should be considered in the interpretation of seismic data from fractured strata with heterogeneous stress distributions

quantifying bedrock fracture patterns within the shallow subsurface implications for rock mass strength bedrock landslides and erodibility 1 the role of bedrock fractures and rock mass strength is often considered a primary influence on the efficiency of surface processes and the morphology of landscapes quantifying bedrock characteristics at hillslope scales however has proven difficult here we present a new field based method for quantifying the depth and apparent density of bedrock fractures within the shallow subsurface based on seismic refraction surveys we examine variations in subsurface fracture patterns in both fiordland and the southern alps of new zealand to better constrain the influence of bedrock properties in governing rates and patterns of landslides as well as the morphology of threshold landscapes we argue that intense tectonic deformation produces uniform bedrock fracturing with depth whereas geomorphic processes produce strong fracture gradients focused within the shallow subsurface additionally we argue that hillslope strength and stability are functions of both the intact rock strength and the density of bedrock fractures such that for a given intact rock strength a threshold fracture density exists that delineates between stable and unstable rock masses in the southern alps tectonic forces have pervasively fractured intrinsically weak rock to the verge of instability such that the entire rock mass is susceptible to failure and landslides can potentially extend to great depths conversely in fiordland tectonic fracturing of the strong intact rock has produced fracture densities less than the regional stability threshold therefore bedrock failure in fiordland generally occurs only after geomorphic fracturing has further reduced the rock mass strength this dependence on geomorphic fracturing limits the depths of bedrock landslides to within this geomorphically weakened zone

frequency dependent seismic anisotropy in fractured rock to examine the effects of multiple aligned fractures in rock we have developed a two dimensional elastic finite difference code for fractured media fractures are incorporated into the model explicitly as displacement discontinuity boundary conditions the wavefield is computed using a 4th order staggered grid scheme simulations were performed for a broadband explosion point source center frequency 374 hz located at the center of the model the model consisted of 90 horizontal fractures spaced approximately 1 8 of a wavelength apart the normal and shear fracture stiffnesses were selected such that the normal incidence transmission coeficient is 0 6 the simulations show strong scattering attenuation of the p wave in the vertical direction to the fractures and channeling of guided waves in the horizontal direction ii to the fractures the same code was also used to model wave propagation in an anisotropic medium with equivalent effective moduli for the 90 fracture system significant differences between the amplitudes velocities and frequency content of the waves in the explicit and equivalent medium fracture models were observed these differences result from frequency dependent time delays and filtering across each fracture and channeling along fractures that are not included in the zero frequency effective medium description these effects are especially interesting because they illustrate that the dynamic properties of fractured rock include significant amplitude anisotropy that may prove useful in the characterization of fractured rock

laboratory scale study of field of view and the seismic interpretation of fracture specific stiffness the effects of the scale of measurement i e the field of view on the interpretation of fracture properties from seismic wave propagation was investigated using an acoustic lens system to produce a pseudo collimated wavefront the incident wavefront had a controllable beam diameter that set the field of view at 15 mm 30 mm and 60 mm on a smaller scale traditional acoustic scans were used to probe the fracture in 2 mm increments this laboratory approach was applied to two limestone samples each containing a single induced fracture and compared to an acrylic control sample from the analysis of the average coherent sum of the signals measured on each scale we observed that the scale of the field of view affected the interpretation of the fracture specific stiffness many small scale measurements of the seismic response of a fracture when summed did not predict the large scale response of the fracture the change from a frequency independent to frequency dependent fracture stiffness occurs when the scale of the field of view exceeds the spatial correlation length associated with fracture geometry a frequency independent fracture specific stiffness is not sufficient to classify a fracture as homogeneous a nonuniform spatial distribution of fracture specific stiffness and overlapping geometric scales in a fracture cause a scale dependent seismic response which requires measurements at different field of views to fully characterize the fracture

shales at all scales exploring coupled processes in mudrocks abstract fine grained sedimentary rocks namely mudrocks including their laminated fissile variety shales make up about two thirds of all sedimentary rocks in the earth s crust and a quarter of the continental land mass organic rich shales and mudstones are the source rocks and reservoirs for conventional and unconventional hydrocarbon resources mudrocks are relied upon as natural barriers for geological carbon storage and nuclear waste disposal consideration of mudrock multi scale physics and multi scale spatial and temporal behavior is vital to address emergent phenomena in shale formations perturbed by engineering activities unique physical characteristics of shales arise as a result of their layered and highly heterogeneous and anisotropic nature low permeability fabric compositional complexity and nano scale confined chemical environments barriers of lexicon among geoscientists and engineers impede the development and use of conceptual models for the coupled thermal hydraulic mechanical chemical biological thmcb processes in mudrock formations this manuscript reviews the thmcb process couplings resulting emergent behavior and key modeling approaches we identify future research priorities in particular fundamental knowledge gaps in understanding the phase behavior under nano scale confinement coupled chemo mechanical effects on fractures the interplay between physical and chemical processes and their rates and issues of non linearity and heterogeneity we develop recommendations for future research and integrating multi disciplinary conceptual models for the coupled multi scale multi physics behavior of mudrocks consistent conceptual models across disciplines are essential for predicting emergent processes in the subsurface such as self focusing of flow time dependent deformation creep fracture network development and wellbore stability