effective medium theories for fluid saturated materials with aligned cracks there is general agreement between different theories giving expressions for the overall properties of materials with dry aligned cracks if the number density of cracks is small there is also very fair agreement for fluid filled isolated cracks however there are considerable differences between two separate theories for fluid filled cracks with equant porosity comparison with recently published experimental data on synthetic sandstones gives a good fit with theory for dry samples however although the crack number density in the laboratory sample is such that first order theory is unlikely to apply expressions correct to second order in the number density provide a worse fit it also appears that the ratio of wavelength to crack size is not sufficiently great for any detailed comparison with effective medium theories which are valid only when this ratio is large the data show dispersion effects for dry cracks and scattering neither of which will occur at sufficiently long wavelengths data from the water saturated samples indicate that the effect of equant porosity is significant although the two theories differ strongly as to just how significant once again and in spite of the reservations mentioned above a reasonable fit between theory and observation can be shown

subsurface fractures and their relationship to stress history in east texas basin sandstone abstract subsurface fractures were sampled with more than 400 m of core from the lower cretaceous travis peak formation a tabular sandstone and shale unit approximately 600 m thick that was deposited in the gradually subsiding gulf coast basin depth to the top of the formation in east texas now ranges between 1800 and 2900 m fractures in sandstone exhibit features that are due to intermittent fracturing and sealing crack seal which suggest that the fractures are the result of episodes of fluid pressure in excess of hydrostatic correlation of the vein mineral precipitation sequence with the diagenetic history and the results of previous 18 o studies of quartz cement indicates that fractures propagated at depths of between 900 and 1500 m during the migration of quartz precipitating fluids fracture permeability provides a mechanism for the passage of large fluid volumes recorded by the precipitation of extensive quartz cement in the formation fractures stopped propagating before maximum burial depth was achieved during approximately 1500 m of subsequent burial fractures either remained partly open or were sealed by postkinematic ankerite and clay minerals

inferring rock fracture evolution during reservoir stimulation from seismic anisotropy we have analyzed seismic anisotropy using shear wavesplitting measurements made on microseismic events recorded during a hydraulic fracture experiment in a tight gas reservoir in carthage east texas microseismic events were recorded on two downhole arrays of three component sensors the geometry of which provided good ray coverage for anisotropy analysis a total of 16 633 seismograms from 888 located events yielded 1545 well constrained shear wave splitting measurements manual analysis of splitting from a subset of this data set reveals temporal changes in splitting during fracturing inversion using the full data set allows the identification of fracture strike and density which is observed to vary during fracturing the recovered fracture strike in the rock mass is parallel to directions of regional borehole breakout but oblique to the hydraulic fracture corridor as mapped by the microseismic event we relate this to en echelon fracturing of preexisting cracks the magnitude of shear wave splitting shows a clear temporal increase during each pumping stage indicating the generation of cracks and fissures in a halo around the fracture corridor which thus increase the overall permeability of the rock mass our results show that shear wave splitting analysis can provide a useful tool for monitoring spatial and temporal variations in fracture networks generated by hydraulic stimulation

microcrack induced elastic wave anisotropy of brittle rocks the failure of brittle rocks during compression is preceded by the formation growth and coalescence of microcracks elastic wave velocities are reduced in the presence of open microcracks and fractures and may therefore be used to monitor the progressive damage of the rock in general these microcracks are not randomly oriented and the rock displays an elastic anisotropy the elastic anisotropy due to cracks can be expressed in terms of a second rank and fourth rank crack density tensor for open cracks the contribution of the fourth rank crack density tensor to the elastic wave velocities is small these results are compared with recent measurements of the ultrasonic compressional and shear wave velocities for propagation parallel and perpendicular to an increasing axial stress applied at constant confining stress to berea sandstone inversion of the velocity measurements indicates that the microcracks propagate parallel to the maximum compressive stress in agreement with current rock mechanics theory a reasonable fit to the data is obtained using only the second rank crack density tensor even though at high confining stress the cracks are expected to be in partial contact along their length this is consistent with the model of elastic wave propagation in a medium containing partially contacting fractures published by white however measurements of off axis wave velocities are required to fully quantify the contribution of the fourth rank crack density tensor

diagnosing monthly mean boundary layer properties from reanalysis data using a bulk boundary layer model abstract the mixed layer approach to modeling the planetary boundary layer pbl is particularly well suited to inversion topped pbls such as the stratocumulus topped boundary layer found off the west coast of america in the subtropical pacific ocean at northern and southern latitudes however a strong temperature inversion near 850 hpa the trade wind inversion is not confined to the stratocumulus regimes but has been observed over most parts of the subtropical tropical pacific ocean in this paper the authors test the ability of a simple bulk boundary layer model bblm to diagnose entrainment velocity cumulus mass flux and surface latent heat flux from monthly mean reanalysis data the pbl depth is estimated from geoscience laser altimeter system data the model is based on the conservation equations for mass total water mixing ratio and moist static energy the bblm diagnoses entrainment velocities between 1 and 8 mm s 1 in the stratocumulus and trade wind regions with increasing rates towar

beyond the dots in the box microseismicity constrained fracture models for reservoir simulation summary monitoring of induced microseismic events usually results in locations for these events and a geometrical interpretation of these dots in the box in this study we show how additional information obtained from observed microseismic events namely the source mechanisms were used to generate a discrete fracture network using the wide aperture of a surface star like array fracstar allows inversion for both shear and non shear source mechanisms both volumetric and shear only source mechanism inversion was carried out on microseismic events from the treatment of a shale gas reservoir in the continental us during the same hydraulic fracture stimulation treatment both dip slip and reverse faulting sources were active in this reservoir the source mechanisms revealed fracture orientations more accurately than could be inferred from microseismic event locations alone the activity associated with different mechanisms is interpreted as indicating reactivation of existing fractures in the rock as well as suggesting generation of new fractures failure analysis using source mechanisms on individual events allows an integrated understanding of the complex fracture interactions taking place in the reservoir and also provides a more complete understanding of the stress conditions in the reservoir during the treatment fracture orientations locations and failure mechanisms are translated into discrete fracture network dfn models that can be used to verify the extent and character of the fractures created or reactivated during the fracture treatment and may ultimately be used to generate fracture flow properties for reservoir simulation

microseismic growth born from success over the last several years microseismic ms monitoring has quickly grown as a technology to map various reservoir processes imaging hydraulic fracture stimulations is the most common application especially within north america as indicated in the introduction to this special section initial interest in ms was primarily in the engineering community as the method was pushed forward by the engineers geophysicists within operating companies began to get pulled in to support the engineering projects as application of the method gathered steam interest expanded in the geophysical community and into academia as seen by the growth of research consortia looking at the technique while ms is a seismic technique it is sufficiently novel that most geophysicists are largely uninformed about technology challenges nevertheless over the last few years the geophysical community has been both educating itself and taking stewardship of the technology ms remains a unique geophysical technology although the tech

characterizing production induced anisotropy of fractured reservoirs having multiple fracture sets since natural fractures in petroleum reservoirs play an important role in determining fluid flow during production knowledge of the orientation and density of fractures is required to optimize production this paper outlines the underlying theory and implementation of a fast and efficient algorithm for upscaling a discrete fracture network dfn to predict the fluid flow elastic and seismic properties of fractured rocks potential applications for this approach are numerous and include the prediction of fluid flow elastic and seismic properties for fractured reservoirs model based inversion of seismic amplitude versus offset and azimuth avoa data and the optimal placement and orientation of infill wells to maximize production given that a single fracture network may comprise hundreds of thousands of individual fractures the sheer size of typical dfns has tended to limit their practical applications this paper demonstrates that with efficient algorithms the utility of discrete fracture networks can be extended far beyond mere visualization

tunnel engineering in the iron age geoarchaeology of the siloam tunnel jerusalem abstract the siloam tunnel st is the best identified biblical structure that can be entered today we use geological structural and chemical features of st and its internal deposits to show that it is an authentic engineering project without any pre existing natural conduit that could have guided its excavators radiometrically and historically dated to 700 bce st pinpoints the technological advance in leveling techniques that was essential for the construction of such a long tunnel without intermediate shafts a combination of geological and archaeological evidence demonstrates that the circuitous route of st and the final meeting of the two excavating teams are associated with continuous modifications of the plan to allow acoustic communication between hewers and the surface teams hydraulic plaster was applied throughout the tunnel in order to seal voids of dissolution and tectonic origin organic material accidentally entrapped in the plaster was carbon 14 dated and speleothems were dated by u th both corroborating the historic and epigraphic evidence ascribing the engineering advance in tunneling techniques to the judahite king hezekiah

the stress dependent elastic properties of thermally induced microfractures in aeolian rotliegend sandstone the impact of thermally induced microfractures on the stress sensitive elastic wave properties of aeolian rotliegend sandstone samples is analysed it is found that to identify the effects of the microfracture contribution accurately a correction must first be made to account for water loss representing a mass loss of 4 6 from the pore throats and clays due to the heating process despite care being taken to ensure that the thermally fractured samples re adsorb room moisture both the original and thermally fractured rocks are stress sensitive at the ultrasonic wave frequencies of the laboratory however a distinct shift in the estimated distribution of internal rock compliance indicates that the population of thermal microfractures differs in nature from that caused solely by core plug extraction damage in particular the ratio of normal to tangential compliance is observed to be higher for the thermally generated microfractures than for the broken grain grain contacts created by extraction unloading this can be explained by the intragranular thermal fracture surfaces being smoother when compared to the intergranular boundaries mechanical hysteresis is observed between the up and downgoing test cycles for both the original and to a greater extent the thermally fractured rock this indicates that there is compaction induced movement of the fractures in the samples during application of stress in the laboratory

measurement of the normal tangential fracture compliance ratio zn zt during hydraulic fracture stimulation using s wave splitting data we develop a method to invert s wave splitting sws observations measured on microseismic event data for the ratio of normal to tangential compliance zn zt of sets of aligned fractures we demonstrate this method by inverting for zn zt using sws measurements made during hydraulic fracture stimulation of the cotton valley tight gas reservoir texas when the full sws data set is inverted we find that zn zt 0 74 0 04 windowing the data by time we were able to observe variations in zn zt as the fracture stimulation progresses most notably we observe an increase in zn zt contemporaneous with proppant injection rock physics models and laboratory observations have shown that zn zt can be sensitive to 1 the stiffness of the fluid filling the fracture 2 the extent to which this fluid can flow in and out of the fracture during the passage of a seismic wave and 3 the internal architecture of the fracture including the roughness of the fracture surfaces the number and size of any asperities and the presence of material filling the fracture these factors have direct implications for modelling the fluid flow properties of fractures consequently the ability to image zn zt using sws will provide useful information about fractured rocks and allow additional constraints to be placed on reservoir behaviour

in situ monitoring of rock fracturing using shear wave splitting analysis an example from a mining setting summary changing stress conditions are well known to cause rupturing of rock this is well constrained on a small scale from laboratory experiments and inferred on a much larger scale from tectonic earthquakes here we present a study of rock fracturing induced by changes in stress state during block caving operations in an australian mine this intermediate scale study provides further evidence of the scalability of processes involved in rock fracturing and thus helps to link laboratory and seismological observations we analyse the temporal evolution of rock fracturing during a production cycle using the analysis of fracture induced anisotropy fracturingoftherockmassismonitoredusingevidenceofseismicanisotropyfromestimates of shear wave splitting and their subsequent inversion for fracture parameters the data set consists of more than 40 000 three component seismograms recorded by an array of sensors which provides excellent ray coverage we applied a novel automatic quality assessment technique to handle this large data set and find that anisotropy and thus fracturing correlates strongly with the excavation process we then perform a grid search over a series of synthetic models based on rock physics to invert the splitting parameters for fracture orientation and density finally by applying a sliding window on our results we are able to identify production related fracture evolution during production the fracture density increases with horizontal fracture density being stronger than the vertical fracture density this can be explained by the removal of the supporting rock during caving during short intervals of reduced production the horizontal fracture density decreases whereas vertical fracture density increases we relate this to change in stress regime with reducing overburden mass during cavity collapse this scenario is similar to a collapsing caldera or the inverse of an inflating magma chamber

a successfully stress forecast earthquake summary a m 5 earthquake in iceland has been successfully stress forecast by using variations in time delays of seismic shear wave splitting to assess the time and magnitude at which stress modified microcracking reaches fracture criticality within the stressed volume where strain is released local investigations suggested the approximate location of the forecast earthquake we report the criteria on which this stress forecast was based

earthquake statistics at parkfield 1 stationarity of b values 1 in this paper paper 1 we quantitatively show that the heterogeneous pattern of b values of the gutenberg richter relation in the parkfield segment of the san andreas fault is to a high degree stationary for the past 35 years this prepares the grounds for paper 2 where we test the hypothesis that our model of spatially varying b values forecasts future seismicity more accurately than the approach in which one assumes a constant b value equal to the average regional value the method we develop to measure stationarity in the presence of spatial heterogeneity consists of the following steps 1 determine the optimal dimensions of the sampling volume by mapping b values with a wide range of radii and selecting the largest radius that gives the most detailed resolution of the b value heterogeneity along the selected fault segment the high data density permits the definition of the dominant dimensions of the seismotectonic fabric which is about 8 10 km 2 map the difference in b value between two periods selecting numerous possible catalog divisions 3 identify significant changes of b values by the utsu test utsu 1992 along the studied fault segment of 110 km length only one patch of radius 5 km showed a significant increase in b from below average to above as a function of time this change in b initiates around 1993 and thus correlates in space and time with a well documented episode of creep at depth using the derived spatial variable b value distributions we find that the highest probability for earthquakes with magnitude m 6 is in the middle mountain asperity where the 1966 parkfield earthquake nucleated and where all m 4 5 events in the data set occurred in contrast if only the regional average b value of 0 92 is used to predict future seismicity the creeping segment north of parkfield should produce major earthquakes most frequently a conclusion that contradicts the observations

p and s wave anisotropy of a synthetic sandstone with controlled crack geometryl1 a method is reported for the production of synthetic porous sandstones containing cracks of known dimensions and geometry with respect to the matrix a synthetic sandstone was manufactured from sand cemented with an epoxy glue the cracks of known geometry were introduced into the material in the manufacturing stage by emplacing thin metallic discs in the sand epoxy matrix these discs were chemically leached out of the consolidated porous sandstone acoustic anisotropy and shear wave splitting were observed in the synthetic sandstones for the dry sample the observed angular dependence of the p and s wave velocities at 100 khz compares well qualitatively with the theoretical models of hudson and of thomsen quantitatively however the experimental data fits hudson s model better for the case of a saturated sample the experimental results are in excellent agreement with thomsen s model hudson s model on the other hand predicts a different angular dependence for p waves this demonstrates that the concept of fluid transfer between cracks and the ambient porosity can be a significant process the results reported here are from the first successful experiment in which the theoretical models were tested on a porous material containing a known crack geometry

a comparison of cross hole electrical and seismic data in fractured rock cross hole anisotropic electrical and seismic tomograms of fractured metamorphic rock have been obtained at a test site where extensive hydrological data were available a strong correlation between electrical resistivity anisotropy and seismic compressional wave velocity anisotropy has been observed analysis of core samples from the site reveal that the shale rich rocks have fabric related average velocity anisotropy of between 10 and 30 the cross hole seismic data are consistent with these values indicating that observed anisotropy might be principally due to the inherent rock fabric rather than to the aligned sets of open fractures one region with velocity anisotropy greater than 30 has been modelled as aligned open fractures within an anisotropic rock matrix and this model is consistent with available fracture density and hydraulic transmissivity data from the boreholes and the cross hole resistivity tomography data however in general the study highlights the uncertainties that can arise due to the relative influence of rock fabric and fluid filled fractures when using geophysical techniques for hydrological investigations

stress dependent seismic anisotropy of shales a simple theory for the stress dependent seismic anisotropy of shales can be obtained in terms of a second rank tensor and a fourth rank tensor that depend on the orientation distribution of contacts between clay platelets the theory allows the normal and shear stiffness of the contact regions between clay platelets to be obtained as a function of stress from measurements of seismic p and s wave velocities for shales the ratio of the normal to shear compliance bn bt of the contact regions between clay particles is found to be sensitive to the saturation state of the shale inversion of velocity measurements for fully saturated shales shows a low value of bn bt when compared with measurements on air dry shales consistent with the expected reduction in normal compliance in a fluid saturated low permeability rock for all shales considered bn bt is found to be less than unity the contacts between clay particles are therefore more compliant in shear than in compression

seismic wave propagation in cracked porous media summary the movement of interstitial fluids within a cracked solid can have a significant effect on the properties of seismic waves of long wavelength propagating through the solid we consider three distinct mechanisms of wave induced fluid flow flow through connections between cracks in an otherwise non porous material fluid movement within partially saturated cracks and diffusion from the cracks into a porous matrix material in each case the cracks may be aligned or randomly oriented leading respectively to anisotropic or isotropic wave speeds and attenuation factors in general seismic velocities exhibit behaviour that is intermediate between that of empty cracks and that of isolated liquid filled cracks if fluid flow is significant in the range of frequencies for which considerable fluid flow occurs there is high attenuation and dispersion of seismic waves fluid flow may be on either a wavelength scale or a local scale depending on the model and whether the cracks are aligned or randomly oriented resulting in completely different effects on seismic wave propagation a numerical analysis shows that all models can have an effect over the exploration seismic frequency range

moment tensor inversion of microseisms from the b sand propped hydrofracture m site colorado abstract we apply a moment tensor inversion algorithm to microseismic events generated by hydrofracturing our dataset consists of microseismic events associated with a hydraulic fracturing experiment conducted at the gri doe m site near rifle colorado the event recordings were from a vertical string of 3 component accelerometer pods that are cemented in a borehole the accelerograms clearly show p and s waves which can be rotated into the sh and sv components to invert the p and s amplitudes for moment tensors we use just the first two peaks after the first arrival using this amplitude information and the average pulse width we formed a symmetric stylized pulse for integration and we inverted for the moment tensor resolution tests revealed that with the source microseism to receiver geometry we have at the m site where all receivers are in one borehole and at the same azimuth from the source we can extract the five deviatoric components of the moment tensor using combinations of p sh and sv amplitudes for example a minimum data set of 3 p wave 2 sv wave and 1 sh wave amplitudes to obtain the sixth component which is the isotropic component requires amplitude information from receivers at another azimuth we studied seven high quality events in detail and tested their results against two alternative source geometries these microseismic events tend to have a major double couple nodal plane aligned parallel to the main hydrofracture plane rather than have their tension t axes aligned parallel to the regional minimum principal compressive stress direction the events have seismic moments m0 on the order of 10 5 nm and moment magnitudes mw of about 2 5

field measurements of normal and shear fracture compliance abstract field estimates of the normal and shear compliance of water saturated fractures were obtained from a seismic experiment carried out on a wave cut platform of upper caithness flagstone on the north coast of scotland the rocks are cut by two orthogonal sets of vertical fractures vertical geophones were glued to the rock surface and seismic waves were generated by striking the surface with a sledge hammer first arrival traveltimes were obtained for source receiver distances up to 30 m differences of compressional velocity with direction across the platform were interpreted as resulting from the compliance of the fractures intersected by the transmitted waves the average normal compliance for fractures at this site is 4 0 10 12 1 0 10 12 m pa there is much less certainty about the precise value of shear compliance however the normal shear compliance ratio is determined to be 0 1

the effect of microstructure and nonlinear stress on anisotropic seismic velocities recent work in hydrocarbon reservoir monitoring has focused on developing coupled geomechanical fluid flow simulations to allow production related geomechanical effects such as compaction and subsidence to be included in reservoir models to predict realistic time lapse seismic signatures generation of appropriate elastic models from geomechanical output is required these elastic models should include not only the fluid saturation effects of intrinsic shape induced and stress induced anisotropy but also should incorporate nonlinear stress dependent elasticity to model nonlinear elasticity we use a microstructural effective medium approach in which elasticity is considered as a function of mineral stiffness and additional compliance is caused by the presence of low aspect ratio displacement discontinuities by jointly inverting observed ultrasonic p and s wave velocities to determine the distribution of such discontinuities we assessed the appropriateness of modeling them as simple planar penny shaped features by using this approximation we developed a simple analytical approach to predict how seismic velocities will vary with stress we tested our approach by analyzing the elasticity of various sandstone samples from a united kingdom continental shelf ukcs reservoir some of which display significant anisotropy as well as two data sets taken from the literature

transmission of seismic waves across single natural fractures fractures and other nonwelded contacts are important mechanical and hydrological features of rock masses their effects on seismic wave propagation can be modeled as a boundary condition in the seismic wave equation seismic stress is continuous across such a boundary but seismic particle displacement and seismic particle velocity are not the complete solutions for seismic wave reflection conversion and transmission across a displacement and velocity discontinuity between two half spaces with different densities and elastic properties are derived for all angles of the incident wave the ratio between the seismic stress across this boundary and the seismic particle displacement and velocity are described by a specific stiffness and a specific viscosity respectively a displacement discontinuity results in frequency dependent reflection and transmission coefficients and a frequency dependent group time delay the velocity discontinuity results in frequency independent coefficients and zero delay results of laboratory experiments on compressional and shear wave transmission across three different natural fractures in a quartz monzonite are described measurements were made at different effective stresses under dry and saturated conditions at room temperature it is shown that the effect of these fractures on the spectral amplitudes for compressional and shear pulses transmitted across these fractures are described well by a displacement discontinuity for compressional pulses under dry and saturated conditions and by a combined displacement and velocity discontinuity for shear wave pulses under dry and saturated conditions values of specific stiffness and specific viscosity vary between fractures and increase with increasing effective stress as does the static specific stiffness of these fractures changes in the spectral amplitudes of transmitted pulses are also analyzed in terms of attenuation using the seismic quality factor q which is found to be a function of frequency