correction to strong ferrimagnetic resonance signal and magnetic susceptibility of the nojima pseudotachylyte in japan and their implication for coseismic electromagnetic changes 1 pseudotachylyte is a glassy fault rock providing certain evidence of ancient seismic fault movement i examine the nojima pseudotachylyte found along the nojima fault which caused the 1995 kobe earthquake m 7 2 in japan using the electron spin resonance technique the nojima pseudotachylyte has a strong ferrimagnetic resonance fmr signal derived from bulky trivalent iron ions in ferrimagnetic iron oxides this fmr signal appears by heating the surrounding fault gouge that is the source material of the nojima pseudotachylyte furthermore the magnetic susceptibilities of the nojima pseudotachylyte are 10 2 10 3 times larger than those of the surrounding fault gouge and are proportional to the fmr signal intensities here i reconstruct the magnetization process of fault gouge during ancient seismic fault slip using the fmr signal the results indicate that the instantaneous magnetization of the nojima fault gouge may have induced instantaneous geomagnetic changes and that the instantaneous geomagnetic changes may have further induced geoelectric changes by electromagnetic induction

a physical basis for the frictional melting of some rock forming minerals abstract under conditions of coseismic slip 0 1 2 0 m s 1 within the upper crust differences in the shear yield strengths fracture toughnesses melting points and to a lesser extent thermal conductivities of rock forming minerals result in a hierarchy of friction melting susceptibilities in many so called pseudotachylytes it is observed that phyllosilicates and amphiboles are preferentially consumed to form the melt phase whilst quartz and feldspar tend to survive as clasts this can be explained in terms of the disequilibrium flash melting of those mineral phases possessing the lower shear yield strengths fracture toughnesses and thermal conductivities rather than by eutectic melting that would otherwise generate equilibrium minimum melts indicative of igneous processes at high strain rates typical of coseismic faulting the mechanical behaviour of minerals is primarily controlled by their fracture toughnesses with progressive comminution during slip leading to fusion the friction melting dependence on the mechanical properties of minerals indicates that there may be an important relationship between wall rock lithology and earthquake magnitude if frictional melting results in fault lubrication

anisotropy of vp and vs in an amphibolite of the deeper crust and its relationship to the mineralogical microstructural and textural characteristics of the rock abstract laboratory seismic measurements of compressional v p and shear wave v s velocities have been carried out on a deformed amphibolite from the ivrea zone at pressures and temperatures up to 600 mpa and 600 c respectively amphibolite is considered to be an important rock of the deeper crust from crystallographic orientations of the component minerals hornblende plagioclase and diopside and the respective elastic constants we calculated the average elastic constants voigt average for the rock specimen compressional and shear wave velocities in three orthogonal directions normal and parallel to foliation measured at high confining pressure 600 mpa are in good agreement with the calculated velocity surface this suggests that seismic wave velocity anisotropy is controlled by preferred orientations of minerals in rocks texture at high confining pressure where the effect of microcracks on seismic wave velocities is eliminated in the amphibolite sample the velocity anisotropy is dominated by the texture of hornblende because of the strong preferred orientation and large volume fraction of this mineral species a large s wave splitting is documented in the directions parallel to the foliation particularly in the lineation which strongly argues against the microcrack origin of s wave splitting at least at conditions of high effective pressure

specific surface area and pore size distribution in clays and shales one of the biggest challenges in estimating the elastic transport and storage properties of shales has been a lack of understanding of their complete pore structure the shale matrix is predominantly composed of micropores pores less than 2 nm diameter and mesopores pores with 2 50 nm diameter these small pores in the shale matrix are mainly associated with clay minerals and organic matter and comprehending the controls of these clays and organic matter on the pore size distribution is critical to understand the shale pore network historically mercury intrusion techniques are used for pore size analysis of conventional reservoirs however for unconventional shale reservoirs very high pressures 414 mpa 60 000 psi would be required for mercury to access the full pore structure which has potential pitfalls current instrumental limitations do not allow reliable measurement of significant portions of the total pore volume in shales nitrogen gas adsorption techniques can be used to characterize materials dominated by micro and mesopores 2 50 nm a limitation of this technique is that it fails to measure large pores diameter 200 nm we use a nitrogen gas adsorption technique to study the micro and mesopores in shales and clays and compare the results from conventional mercury porosimetry techniques our results on pure clay minerals and natural shales show that i they have a multiscale pore structure at different dimensions ii fine mesopores with a characteristic 3 nm pore size obtained with n2 gas adsorption are associated with an illite smectite group of clays but not with kaolinite iii compaction results in a decrease of pore volume and a reduction of pore size in the inter aggregate macropores of the illitesmectite clays while the fine intra tachoid mesopores are shielded from compaction iv for natural shales mineralogy controls the pore size distributions for shales and the presence of micropores and fine mesopores in natural shales can be correlated with the dominance of the illite smectite type of clays in the rock our assessment of incompressible 3 nm sized pores associated with illite smectite clays provides an important building block for their mineral modulus

intrinsic elasticity of a textured transversely isotropic muscovite aggregate comparisons to the seismic anisotropy of schists and shales 1 the anisotropic properties of phyllosilicate rocks are investigated through the modeling of the pore free intrinsic elasticity of a textured muscovite aggregate the elastic constants were calculated for textures as defined using orientation distribution functions odfs ranging from perfectly aligned to completely random the odfs were described using a gaussian distribution the width of which is controlled by the standard deviation of the orientations of the crystal c axis with respect to the sample foliation plane under the assumption that the sample has hexagonal transversely isotropic symmetry the values were obtained using the widely used voigt reuss hill averaging techniques as well as the geometric mean average gma the gma is more physically meaningful in that it requires that the elastic constants determined in either the stiffness or the compliance domains be invertible comparisons between these different averages indicate that strictly the voigt and reuss values do not necessarily provide the limiting bounds as is expected for the isotropic case despite this some characteristics of the elastic wave p wave anisotropy are only weakly dependent on the averaging procedure employed the calculated elastic constants are converted to the anisotropic and parameters in order to allow for comparison to existing measurements in the literature however these intrinsic results are not intended to provide a measure of the properties of phyllites per se but to provide a reference against which more sophisticated models that incorporate porosity layering and composition may be constructed

continuum discontinuum analysis of failure mechanisms around unsupported circular excavations in anisotropic clay shales abstract the stability of circular excavations in clay shales is a key issue in the drilling and tunnelling industries as well as in the field of deep geological waste storage a large body of experimental evidence indicates that the damaged zone around these cavities is influenced by strong mechanical anisotropy induced by the layered material structure the vast majority of numerical models adopted to date to analyse the stability of openings in layered rocks have been based on continuum mechanics principles using classic shear failure theory for elasto plastic materials however a number of experimental observations demonstrate that clay shales may fail in a brittle manner under low confinement conditions such as those characterizing the near field of the excavation therefore an alternative numerical approach based on non linear fracture mechanics principles and the discrete element method is adopted to gain new insight into the failure process of this class of geomaterials in order to account for the influence of clay shale microstructure on its mechanical behaviour a newly developed approach to capture the anisotropy of strength is proposed with this numerical approach the cohesive strength parameters of the fracture model are assumed to be a function of the relative orientation between the element bonds and the layering orientation the effectiveness of the numerical technique is quantitatively demonstrated by simulating standard rock mechanics tests on an indurated claystone namely opalinus clay emergent strength and deformation properties together with the simulated fracture mechanisms are shown to be in good agreement with experimental observations the modelling technique is then applied to the simulation of the excavation damaged zone edz around a circular tunnel in horizontally bedded opalinus clay the simulated fracturing process is mainly discussed in the context of the damage mechanisms observed at the mont terri url furthermore the influence of in situ stress on resulting edz geometry is analysed together with possible implications for ground support and tunnel constructability modelling results highlight the importance of shear strength mobilization along bedding planes in controlling the edz formation process in particular slippage of bedding planes is shown to cause rock mass deconfinement which in turn promotes brittle failure processes in the form of spalling the numerical technique is currently limited to two dimensional analyses without any thermo hydro mechanical coupling

fabric related seismic anisotropy in upper mantle xenoliths evidence from measurements and calculations the seismic properties and their directional dependence have been measured in four mantle peridotites from picritic tuffs vitim baikal region russia geothermobarometric data suggest a source depth for the xenoliths corresponding to the spinel garnet transition zone 40 90km the experimental data include the measurement of p and s wave velocities and densities at confining pressures to 600 mpa and temperatures to 600 c and the determination of velocity anisotropy and shear wave splitting the measured velocities are compared with velocity calculations based on lattice preferred orientation lpo of olivine orthopyroxene the modal composition of the rocks and the single crystal stiffness coefficients macroscopic velocity anisotropy is strongly controlled by the lpo of olivine measured 600 mpa room temperature and calculated p wave anisotropies compare fairly well although the experimentally determined velocities are markedly lower than the calculated velocities the correspondence between measured and calculated shear wave splitting data is less pronounced this is attributed to grain boundary effects retrogressive reactions interstitial glass incomplete crack closure and to the fact that the lpo of the cpx fraction has not been included in the calculation the magnitude of p wave anisotropy corresponds to that observed in mantle peridotites exposed at the surface ivrea zone northern italy ronda southern spain

esr isochron dating of the nojima fault gouge southwest japan using icp ms an approach to fluid flow events in the fault zone

investigation of the damage induced by desiccation and heating of tournemire argillite using digital image correlation abstract a set of samples taken from the tournemire site was submitted to a desaturation process in an oven at different temperature levels of 40 c 50 c 80 c 105 c 150 c and 200 c the resulting hydro mechanical behavior was studied by measuring the strain fields obtained from a non invasive method called digital image correlation dic after a few dozens of minutes from the beginning of the thermal loading the induced strains concentrated into narrow zones or bands indicating strain localization these narrow zones were precursors of desiccation cracks which were also detected from the first temperature level at 40 c or 50 c our results confirmed the fundamental role of the textural and sedimentological heterogeneities in the cracking process desiccation cracks appeared at the interface between clay matrix and macroscopic non clayey heterogeneity i e calcite concretion and were predominantly parallel to the bedding planes at a temperature level of 105 c the desiccation cracks were no longer observable and seemed to close this healing process was interpreted as a thermally driven process when the shrinkage limit was reached a thermal dilation of the water clay minerals system occurred leading to a gradual closure of desiccation cracks

parameterization of elastic stress sensitivity in shales stress dependency and anisotropy of dynamic elastic properties of shales is important for a number of geophysical applications including seismic interpretation fluid identification and 4d seismic monitoring using sayerskachanov formalism we developed a new model for transversely isotropic ti media that describes stress sensitivity behavior of all five elastic coefficients using four physically meaningful parameters the model is used to parameterize elastic properties of about 20 shales obtained from laboratory measurements and the literature the four fitting parameters namely specific tangential compliance of a single crack ratio of normal to tangential compliances characteristic pressure and crack orientation anisotropy parameter show moderate to good correlations with the depth from which the shale was extracted with increasing depth the tangential compliance exponentially decreases the crack orientation anisotropy parameter broadly increases with depth for most of the shales indicating that cracks are getting more aligned in the bedding plane the ratio of normal to shear compliance and characteristic pressure decreases with depth to 2500 m and then increases below this to 3600 m the suggested model allows us to evaluate the stress dependency of all five elastic compliances of a ti medium even if only some of them are known this may allow the reconstruction of the stress dependency of all five elastic compliances of a shale from log data for example

anisotropy of experimentally compressed kaolinite illite quartz mixtures the anisotropy of physical properties is a well known characteristic of many clay bearing rocks this anisotropy has important implications for elastic properties of rocks and must be considered in seismic modeling preferred orientation of clay minerals is an important factor causing anisotropy in clay bearing rocks such as shales and mudstones that are the main cap rocksofoilreservoirs thepreferredorientationofclaysdepends mostly on the amount of clays and the degree of compaction to studytheeffectoftheseparameters wepreparedseveralsamples compressingattwoeffectiveverticalstressesamixtureofclays illite and kaolinite and quartz silt with different clay quartz ratios the preferred orientation of the phases was quantified with rietveld analysis on synchrotron hard x ray images pole figures for kaolinite and illite display a preferred orientation of clay platelets perpendicular to the compaction direction increasing in strength with clay content and compaction pressure quartz particles have a random orientation distribution aggregate elastic properties can be estimated by averaging the singlecrystal properties over the orientation distribution obtained from the diffraction data analysis calculated p wave velocity anisotropy ranges from 0 pure quartz sample to 44 pure clay sample highly compacted but calculated velocities are much higher than measured velocities this is attributed to uncertainties about single crystal elastic properties and oriented micropores and limited grain contacts that are not accounted for in themodel inthiswork wepresentaneffectivemethodtoobtain quantitative data helping to evaluate the role of clay percentage and compaction pressure on the anisotropy of elastic properties ofclay bearingrocks

measurements of elastic and electrical properties of an unconventional organic shale under differential loading abstractwe have developed an experimental approach to simultaneously measure the stress dependence of ultrasonic wave velocities at 1 mhz and therefore the components of the undrained elastic stiffness tensor as well as the components of the complex conductivity tensor in the frequency range from 100 mhz to 10 hz we performed the experiments on a cylindrical core sample from the haynesville formation porosity of approximately 0 08 bound water excluded and clay content mostly illite approximately 30 40 wt we performed experiments under controlled confining and pore fluid pressures achieving differential pressure states representative of autochthonous reservoir conditions directional measurements were made using independent acquisition arrays piezoelectric crystals and nonpolarizing electrodes distributed azimuthally on the core sample external surface the bedding plane being along the axis of the cylindrical core sample ultrasonic waveforms were recorded on a high resolution oscilloscope

fabric anisotropy induced by primary depositional variations in the silt clay ratio in two fine grained slope fan complexes texas gulf coast and northern north sea article i nfo high resolution x ray texture goniometry hrxtg has been used to quantify the alignment anisotropy of phyllosilicate minerals in a series of diagenetically altered mudstone samples from the deep subsurface of the texas gulf coast and the northern north sea the sampled formations have undergone a simple burial history with no overprint of tectonic deformation that may have influenced the phyllosilicate orientation observations have been made in two constrained temperature windows 148 150 c north sea data and 169 210 c texas gulf coast wherein less than 20 smectite remains in the mixed layer phase illite smectite quantitative x ray powder diffraction xrpd shows quartz which is dominantly detrital and phyllosilicate content having an inverse relationship quantified illite smectite fabric intensity phyllosi licate alignments ranges from low alignment values of 2 42 m r d multiples of a random distribution to high alignment values 6 75 m r d and chlorite kaolinite ranges from 1 80 to 6 46 m r d we propose that the broad range of phyllosilicate fabric intensity observed reflects the range of quartz phyllosilicate ratios that are the imprint of depositional setting bedform truncation and initial grain size detrital quartz acts to disrupt fabric intensity by being non platy producing areas sheltered from effective stress pressure shadows and acting as a matrix support there is no evidence that bioturbation is a significant control on phyllosilicate alignment in these quartz phyllosilicate mixtures these findings have important implications for the role of grain scale alignment on seismic anisotropy in mudstone systems fabric intensity in post diagenetic samples can be predicted from 1 quartz content and 2 total clay content where

texture analysis of volcanic rock samples quantitative study of crystals and vesicles shape preferred orientation from x ray microtomography data abstract in the texture analysis of volcanic rocks the preferred orientation of the constituents can provide useful information for the interpretation of the processes involved in the rock formation we present here a new data analysis technique based on x ray microtomography measurements and on shape preferred orientation analysis to obtain the orientation distribution functions of the constituents of volcanic rocks this procedure proved to be very suitable for volcanic samples where diffraction based techniques developed for crystallographic preferred orientation studies are of limited utilization in addition to the fact that they cannot provide any information about vesicles or bubbles moreover the analysis performed directly in three dimensions 3d overcomes the problems that usually occur when employing stereological methods for the analysis of the images obtained via microscopy based techniques in this study two scoriae from stromboli and etna and a tube pumice from campi flegrei were measured via x ray microtomography and then the resulting volumes were analyzed following the proposed procedure results highlight little preferred orientation for the vesicles in the two scoria samples whereas the pumice shows a marked preferred orientation crystals also divided by mineral species were taken into account as well and in the two scoria samples there is no crystal preferred orientation in contrast with the pumice where crystal preferred orientation features are very similar to the ones found for the vesicles overall we found strong differences in preferred orientation weak for vesicles in scoriae showing an axial symmetry with the axis parallel to the elongation axis of the sample and a stronger and more complex orientation texture in the pumice sample for both crystals and vesicles the promising results obtained suggest that this procedure is potentially very useful for the analysis of preferred orientation in volcanic rocks and geomaterials in general

seismic anisotropy and shear wave splitting in lower crustal and upper mantle rocks from the ivrea zone experimental and calculated data to quantify the relationships between anisotropy s wave splitting and tectonics we determined the seismic properties of lower crustal and upper mantle rocks outcropping in the ivrea zone northern italy we obtained p and s wave seismic velocities by laboratory direct velocity measurements and or by calculations based on the modal compositions of the rocks the lattice preferred orientations lpos and the single crystal stiffness coefficients measured p and s wave velocities 6 0 7 5 km s 1 and 3 6 4 2 km s 1 are typical of the lower crust the p wave anisotropy is in the range 0 10 shear wave birefringence is in the range 0 0 0 6 km s 1 with typical values between 0 0 and 0 2 km s 1 in many cases the birefringence is clearly related to fabric elements foliation lineation mafic rocks such as anorthosite or pyroxene bearing gabbros exhibit low p wave anisotropies 5 and low shear wave birefringences less than 0 1 km s 1 in contrast the seismic properties of felsic rocks such as biotite bearing gneisses and mafic rocks such as amphibolites display high vp anisotropy 10 and strong birefringence 0 3 km s 1 biotite and amphibole preferred orientations clearly control seismic anisotropy and particularly shear wave splitting in these rocks maximum splitting is observed in directions parallel to the foliation with the fast split shear wave polarized parallel to the foliation plane to have an overview of the seismic properties of this lower crustal section at a broader scale we calculated from our data the anisotropic seismic properties of several hypothetical samples that are perhaps more representative of the regional anisotropy than each sample individually for instance the average lower crustal sample displays an anisotropy of 5 5 for p waves and a birefringence around 0 14 km s 1 for s waves propagating parallel to the foliation we observe little splitting for waves propagating at high angle to the foliation

creation of synthetic samples for physical modelling of natural shale natural shale samples particularly well preserved drilled core samples are extremely difficult to obtain for laboratory research multiple tests must be carried out on one sample and some samples are disposed after destructive tests therefore rarity and non reusability of samples strongly restrict shale studies in this study based on statistical data from the world s major shale block a new type of synthetic shale was physically constructed via a process of interfusion stuffing and compaction using quartz clay carbonate and kerogen as the primary materials according to statistical data from the world s major shale blocks further evaluation of the synthetic shale involved the use of scanning electron microscopy imagery and analysis of its anisotropic characteristics in comparison with natural shale the synthetic shale had a laminated microstructure similar to natural shale and its velocity anisotropy corresponded to thomsen s anisotropy of a transversely isotropic medium the results of tests for homogeneity and repeatability indicated that the construction process was stable and that several identical synthetic samples which were satisfactorily similar to natural shale could be produced for both iterative and destructive tests the composition of each mineral as well as the density porosity permeability and anisotropy of the samples were all variable therefore a series of synthetic samples could be obtained with properties set to meet the requirements of petrophysical experimentation moreover gas or oil saturation was also considered in the construction of the synthetic shale meaning that the characteristics of gas or oil saturation or the complete range of data from dry to saturated samples could be tested using the synthetic shale

applications of deterministic and stochastic rock physics modelling to anisotropic velocity model building modern anisotropic depth imaging requires transverse isotropy velocity models because the surface seismic experiment alone cannot uniquely determine all transverse isotropy parameters additional information is required traditional methods that use checkshots can only constrain anisotropy locally near the well and thus model building requires 3d extrapolation of well data we propose to use rock physics rock physics compaction modelling of shales and sandy shales can be used to constrain and predict anisotropic parameters of sediments whose depositional environment is governed by compaction in this study we present several applications of rock physics modelling to anisotropic velocity model building we show how both deterministic and stochastic rock physics modelling can be used for initial anisotropic velocity model building and as a constraint for anisotropic parameter estimation in all cases all predicted anisotropy models are physical and realizable in terms of effective medium theory and do not violate elasticity theory

effective medium modeling of gas hydrate filled fractures using the sonic log in the krishna godavari basin offshore eastern india 1 indian national gas hydrate program expedition 01 has established that clay rich marine sediment from the krishna godavari kg basin in the eastern indian margin hosts one of the richest gas hydrate deposits in the world resistivity at bit images and pressure cores reveal that the gas hydrate morphology in clay rich sediment varies from complex vein structures grain displacing to invisible pore filling existing rock physics models which relate acoustic data to in situ gas hydrate concentrations generally assume isotropic pore filling gas hydrate which yields misleading concentration estimates for fractured fine grained sediments the anisotropic kg basin sediment presents additional complications here we apply differential effective medium theory to incorporate grain displacing morphologies by which gas hydrate is included as vertical ellipsoids with aspect ratios ranging from those of thin veins up to those of nodules in an elastic anisotropic background we have estimated gas hydrate concentrations from sonic velocities at hole 10d in the kg basin considering three basic gas hydrate morphologies i pore filling ii grain displacing and iii a combination of grain displacing and pore filling average gas hydrate saturations for these three cases are 35 42 27 30 and 33 41 of the total porosity respectively in the depth range 60 140 m below seafloor mbsf saturation is highest at 67 mbsf for any morphology but the values differ between morphologies for the pore filling morphology the maximum gas hydrate saturation of 56 is 18 22 higher than the grain displacing morphology and 2 9 higher than the combined morphology estimates differ by 6 of the sediment volume with rotations of gas hydrate veins from vertical to horizontal

structure kinematics and composition of fluid controlled brittle faults and veins in lower cretaceous claystones lower saxony basin northern germany constraints from petrographic studies microfabrics stable isotopes and biomarker analyses abstract investigation of fault rocks is crucial for the evaluation of sealing properties of rock formations considered as oil and gas storage or for the disposal of heat generating radioactive waste even in tight rock formations fluid flow may concentrate along brittle faults the present study focuses on the sealing properties of faulted lower cretaceous clay and siltstones encountered in drill cores from the central and eastern lower saxony basin germany our investigations are based on a multidisciplinary approach including microstructural mineralogical geochemical stable isotope and biomarker analyses the new data suggest that preexisting discontinuities and or sedimentary heterogeneities played a significant role for the increase in fluid pressure and formation of mode i veins within low permeability claystones high strain domains of fault cores consist of fine grained fault gouge a scaly fabric of varying deformation intensity is present around larger faults calcite precipitation in faults and complex mineralized veins and the differences in trace element concentrations of various veins reflect several phases of palaeofluid activity calcite precipitation in the fault rocks led to elevated amounts of mn ba and or sr isotope signatures indicate a local carbonate source for the origin of thin calcite coatings of fault planes but a more complex palaeofluid interaction for the larger faults and for complex veins consistent with the microfabrics biomarker signatures reveal the presence of mature hydrocarbons pointing to hydrocarbon migration in sections where steep only weakly mineralized faults occur the new results suggest that faulted claystones represent critical zones with a potentially reduced barrier capability consistent with published data

preferred orientation of phyllosilicates effects of composition and stress on resedimented mudstone microfabrics abstract development of a preferred orientation of clay minerals is investigated in response to changes in vertical effective stress and composition in resedimented material boston blue clay boston blue clay bbc an illitic glacio marine clay composed of 57 clay sized particles

creep and time dependent damage in argillaceous rocks abstract this paper presents the results of laboratory tests on the time dependent behaviour of three rocks characterized by a high proportion of clay particles the viscosity of these sedimentary rocks was studied under different loading conditions in uniaxial compression static or cyclic creep tests and quasistatic tests low loading strain rate were performed across various orientations of fabric planes the quasistatic tests showed similarities in the mechanical response of these three argillaceous rocks a late phase of dilation and a linear development of volumetric deformation before the beginning of unstable crack propagation the development of secondary and tertiary creep phases during the creep tests highlighted the existence of a deviatoric stress threshold below which only primary creep is observed long term creep tests also showed that the volumetric variation is not constant during the development of viscoplastic deformations a microstructural analysis of thin sections extracted from specimens after the tests gave evidence of cataclastic and granular creep damage to the argillaceous matrix occurs and no cracks were observed in the quartz and carbonate grains this evidence was also demonstrated for tests with loading at a high strain rate finally this study highlights the significant viscoplasticity of argillaceous rocks the mechanical properties deteriorate rapidly when crack propagation becomes unstable and the viscoplastic strains seem to be due to clay particle slips known as granular creep

thin crust at the western iberia ocean continent transition and ophiolites western iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments the age of breakup decreases from south to north seismic refraction and reflection profiles and magnetic and gravity data from each segment show a consistent pattern of geophysical observations across the ocean continent transition oct zone which is a few tens of kilometers wide we emphasize here the discovery of thin 2 4 km oceanic crust underlain by 7 6 km s 1 material within the oct the available evidence favors the suggestion that the 7 6 km s 1 layer is serpentinized peridotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup this thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities

pseudotachylyte controversy fact or friction high speed slip experiments performed on westerly granite using friction welding apparatus reveal that comminution is an essential precursor to melting by friction observations of slip surfaces via analytical scanning electron microscopy sem document the following sequence of events occurring in 2 s with increasing velocity up to 2 m s fracture progressive comminution surface melting of mineral fragments fragment to fragment adhesion and finally production of a fragment laden melt supported suspension explosive dehydration and melting of the epidote group mineral allanite indicates that temperatures of at least 1000 c were realized at the interface this is corroborated by calculation of the temperature rise for the known operating conditions contrary to earlier proposals these results show that comminution and frictional melting are complementary and not mutually exclusive processes depending on the velocity shear stress displacement relations prevailing during frictional slip rocks produced in seismogenic zones can be predominantly comminuted wall rock or fragment melt mixes pseudotachylytes

sequence stratigraphy and tectonosedimentary history of the upper jurassic of the eastern paris basin lower and middle oxfordian northeastern france in the present study the difficulty which lies in the identification of sea level fall discontinuities in deep depositional environments led the authors to use the transgressive surfaces i e the most noticeable surfaces in the present case to determine depositional cycles four 3rd order lower and middle oxfordian cycles were identified s1 s2 s3 and s4 in the eastern paris basin these four cycles can be organised into two lower frequency cycles so i and so ii which comprise the s1 s2 and the s3 s4 cycles respectively during the time intervals of the s1 s2 and s3 cycles sedimentation occurred along a southward dipping carbonate siliciclastic ramp prograding from the northern ardennes area the s4 cycle shows the development of a reefal distally steepened ramp which subsequently evolved into a flat topped platform as a result of the compensation infill of the available accommodation space by the carbonate production during a climatic warming in this case reef growth isopach and facies maps suggest synsedimentary activities of hercynian faults coevals with the floodings of the so i and so ii cycles one of these events generated a diachronism of the maximum flooding plicatilis zone during the so ii cycle between the northwestern and southeastern parts of the studied area the depositional patterns found in the eastern paris basin and the swiss jura show great similarities for the early middle oxfordian both regions were probably connected and recorded the same tectonosedimentary evolution in contrast a tectonic control certainly generated differences between the sequence stratigraphic framework of the eastern paris basin and the eustatic chart

the elastic anisotrophy of shales shales constitute about 75 of the clastic fill sedimentary basins and have a decisive effect on fluid flow and seismic wave propagation because of their low permeability and anisotropic microstructure the elastic stiffnesses of a shale with partially oriented clay particles is expressed in terms of the coefficients w sub lmn in an expansion of the clay particle orientation distribution function in generalized legendre functions application is made to the determination of the anellipticity of shales for transverse isotrophy the anellipticity quantifies the deviation of the p wave slowness curve from an ellipse and is shown to depend on a single coefficient w sub 400 in the expansion of the clay particle orientation distribution function if w sub 400 is small the anellipticity may be neglected as is apparently the case for a near surface late tertiary shale studied by winterstein and paulson strongly aligned clay particles result in a positive value of w sub 400 and a positive anellipticity in agreement with the majority of the field measurements however less well ordered shales could have a significantly positive second moment w sub 200 but only a small positive or even negative value of w sub 400 for such shales the anellipticity would be small or negative despite amore preferred alignment of clay particles in the bedding plane numerical examples of clay particle orientation distribution functions leading to zero or negative anellipticity are given less

seismic and mechanical properties of opalinus clay comparison between sandy and shaly facies from mont terri switzerland for the safe disposal of high level radioactive waste different host rocks are currently being considered the favorable properties of clay are low permeability some retention capacity concerning radionuclides and the ability to self seal cracks and fissures e g by swelling or time dependent compaction creep in switzerland the jurassic opalinus clay is envisaged as a potential host rock which at mont terri is subdivided into the sandy shaly and carbonate rich facies the latter being less abundant for long term safety assessments the understanding of the relations of properties e g mineralogical composition and microstructure and performance e g mechanical behavior of clays and claystones is essential in the case of the sandy opalinus clay the mechanical strength increases with increasing carbonate content because carbonates form the matrix the mineralogical investigation of a set of sandy facies samples proved a significantly larger carbonate content 20 40 mass when compared to the shaly facies 10 20 mass the carbonates of the shaly opalinus clay on the other hand are mostly localized fossils aligned parallel to the bedding acting as predetermined breaking points image analysis of sem images of polished sections proved the determined microstructural differences in addition carbonate particles of the sandy facies are mostly isometric whereas carbonate particles of the shaly facies cover a greater range of shapes the mechanical tests were accompanied by investigations of the p and s wave velocities which revealed that the anisotropy of the sandy facies is less pronounced than sedimentological analyses would suggest the mechanical strength which for the first time presents results of real triaxial tests of the sandy facies the samples of the sandy facies exhibit a failure strength of eff b approximately twice as high as was found for the shaly facies considering the deformation axis parallel to the bedding similar values were obtained when measuring perpendicularly to the bedding

moisture expansion as a deterioration factor for sandstone used in buildings the expansion processes that develop in building stones upon changes of moisture content may be an important contributing factor for their deteriorations until recently few data could be found in the literature concerning this parameter and weathering processes moreover the processes that may be responsible for the moisture related expansion of natural building stones are not yet completely understood to further elucidate this process extensive mineralogical petrophysical and fabric investigations were performed on eight german sandstones in order to obtain more information regarding the weathering process and its dependence on the rock fabric the analysed sandstones show a wide range of pore size distributions and porosities a positive correlation with the fabric and the pore space can be found for all studied petrophysical parameters the intensity of the expansion and related swelling pressure cannot be attributed only to the swelling of clay minerals the investigations suggest that the micropores and the resulting disjoining pressure during wet dry cycles also play an important role the results obtained suggest that the mechanism is related to the presence of liquid water within the porous material

effect of grain scale alignment on seismic anisotropy and reflectivity of shales the elastic properties and anisotropy of shales are strongly influenced by the degree of alignment of the grain scale texture in general an orientation distribution function odf can be used to describe this alignment which in practice can be characterized by two legendre coefficients we discuss various statistical odfs that define the alignment by spreading from a mean value in particular the gaussian fisher and bingham distributions we compare the statistical models with an odf resulting from pure vertical compaction no shear strain of a sediment the compaction odf may be used to estimate how the elastic properties and anisotropy evolve due to burial of clayey sediments our study shows that the three statistical odfs produce almost identical correspondence between the two legendre coefficients as a function of the spreading parameter so that the spreading parameter of one odf can be converted to the spreading parameter of another odf in most cases it is then sufficient to apply the spreading parameter for the odf instead of the two legendre coefficients the effect of compaction on the odf gives a slightly different correspondence between the two legendre coefficients from that for the other models in principle this opens up the possibility of distinguishing anisotropy effects due to compaction from those due to other processes we also study reflection amplitudes versus angle of incidence ava for all wave modes where shales having various odfs overlie an isotropic medium the ava responses are modelled using both exact and approximation formulae and their intercepts and gradients are compared the modelling shows that the s wave velocity is sensitive to any perturbation in the spreading parameter while the p wave velocity becomes increasingly sensitive to a perturbation of a less ordered system similar observations are found for the ava of the p p and p sv waves modelling indicates that a combined use of the amplitude versus offset of p p and p sv reflected waves may reveal certain grain scale alignment properties of shale like rocks

anisotropic effective medium modeling of the elastic properties of shales shales are complex porous materials normally consisting of percolating and interpenetrating fluid and solid phases the solid phase is generally comprised of several mineral components and forms an intricate and anisotropic microstructure the shape orientation and connection of the two phases control the anisotropic elastic properties of the composite solid we develop a theoretical framework that allows us to predict the effective elastic properties of shales its usefulness is demonstrated with numerical modeling and by comparison with established ultrasonic laboratory experiments the theory is based on a combination of anisotropic formulations of the self consistent sca and differential effective medium dem approximations this combination guarantees that both the fluid and solid phases percolate at all porosities our modeling of the elastic properties of shales proceeds in four steps first we consider the case of an aligned biconnected clay fluid composite composed of ellipsoidal inclusion

interpretation of sks waves using samples from the subcontinental lithosphere abstract the seismic properties of five south african kimberlite nodules have been calculated from petrofabric measurements these garnet lherzolite and harzburgite composition nodules 56 80 olivine are known from previous studies to have originated at depths of 120 170 km in the subcontinental lithosphere their in situ seismic properties have been calculated by extrapolating the elastic constants to the appropriate temperature 900 1050 c and pressure 3 0 3 5 gpa conditions the average of the five samples has a maximum s wave anisotropy v s of 3 7 which is sufficiently high to explain previously reported teleseismic sks delay times t in continental shield areas of up to 1 7 s the biggest delay time is observed when the foliation is vertical the lineation horizontal and the fastest s wave polarized parallel to the lineation i e an orientation for transcurrent motion delay times of over 1 s can only be generated by this orientation a strong mantle crust mechanical coupling is suggested for such situations the potential use of seismic anisotropy as an indicator of strain in the lithosphere has been investigated using experimental and simulated fabric data for olivine the most abundant phase 70 in the upper mantle the v p and v s seismic anisotropy increases with fabric strength and finite strain recrystallization tends to reduce fabric strength and seismic anisotropy resulting in saturation values for experimental and simulated fabrics which correspond to approximately 50 60 strain or 8 9 v s for pure olivine aggregates a survey of 25 naturally deformed peridotites of oceanic origin suggests an average maximum s wave anisotropy for the olivine component of 9 or about 8 for a lherzolite or harzburgite rock composition when the orthopyroxene component is taken into account the ophiolite samples are twice as anisotropic as the kimberlite nodules if an average anisotropy value is representative of the lithosphere then the sks delay times represent variations in anisotropic layer thickness for delay times over 1 s as the orientation is constrained to be foliation vertical and lineation horizontal the magnitude of s wave anisotropy is less sensitive than v p to variations in olivine volume fraction in range 50 100 and to deformation or fabric intensity further suggesting that s wave anisotropy is particularly apt for the determination of the anisotropic lithosphere thickness for smaller delay times there is some trade off between structural orientation and thickness

microstructural evolution of an incipient fault zone in opalinus clay insights from an optical and electron microscopic study of ion beam polished samples from the main fault in the mt terri underground research laboratory abstract slickensided shear surfaces are ubiquitous in many fault zones however the internal structure the micromechanics and the evolution of these structures are not fully understood and the contributions of crystal plasticity grain boundary sliding microfracturing solution precipitation and mineral transformation under different conditions are subject of debate we studied well preserved core samples from the main fault an up to 3 m wide zone of approximately 10 m offset in the mont terri underground research laboratory ch a site to evaluate long term safety of radioactive waste disposal the drill core breaks easily along many slickensided shear surfaces indicating reverse slip which form an anastomosing network connected by branch lines broad ion beam polishing and scanning electron microscopy shows that the slickensides are invariably revealed by fracture of the drill core along a few m thick shear zone which acts as a crack guide for fracturing the samples in this zone a complex set of processes is inferred leading to extreme localization of strain development of strong particle preferred orientation the formation of nanoparticles and local precipitation of calcite veins in releasing sections in lenses between shear zones homogeneous gouge is formed with a well developed oblique foliation and removal of calcite grains by pressure solution we infer that with progressive deformation the number and density of slickensided shear surfaces increases generating tectonically derived scaly clay and more homogeneous gouge in all deformed elements of the main fault porosity is much smaller than in the undeformed opalinus clay an interesting observation is the almost complete absence of cataclastic microstructures transmission electron microscopy tem of focused ion beam lamellae of this micron wide shear zone shows a strong preferred orientation of clay minerals including nano sized illite particles in tem the shear zones envelop hard particles and confirm an almost complete loss of porosity compared to the protolith we propose that inter and transgranular microcracking pressure solution clay neoformation crystal plasticity and grain boundary sliding are important micro scale processes during the early stages of faulting in opalinus clay and thus need to be considered in extrapolating laboratory results to long term mechanical behavior

influence of carbonate content and micro fabrics on the failure strength of the sandy facies of the opalinus clay from mont terri underground rock laboratory abstract the relationship between micro fabric failure strength and mineralogical composition is studied on representative samples of the sandy facies of the opalinus clay mont terri an approximately 7 m long drill core of the bore hole blt a2 sandy facies mont terri was investigated on different scales mainly with respect to carbonate distribution image analysis showed that the carbonates of the sandy facies mainly occur as i matrix which in turn acts as cement ii in the fine sand fraction and iii as biogenic carbonates being present in minor amounts mechanical testing indicated a clear relation the failure strength increased with increasing carbonate content the carbonates of the sandy facies therefore appear to be similar to the carbonates of the callovo oxfordian clay with respect to their possible influence on failure strength

fluidization and melting of fault gouge during seismic slip identification in the nojima fault zone and implications for focal earthquake mechanisms 1 the record of physical processes that occur during seismic slip events is well preserved in fault rocks from the active nojima fault in japan the fault rocks formed at about 3 km depth and comprise thin alternating layers of very fine gouge and pseudotachylyte derived from granite each layer is thinner than a few millimeters and corresponds to one seismic slip event the very thin slip zone width suggests that some mechanisms of slip weakening operated and our studies of the fault rocks suggest that fluidization and melting of gouge were particularly important fluidized and nonfluidized gouges were distinguished using the detection probability of fragmented counterparts method it is known from granular material science that the phase transition from a grain friction regime to fluidization of granular materials can occur only by a very small decrease in volume fraction of solid grains once fluidization occurs the frictional resistance decreases abruptly to nearly zero even before thermal pressurization reaches its extreme state when fault gouge is melted frictional resistance is governed by the viscosity of melt which depends mainly on temperature chemical composition h2o concentration and the volume fraction of unmelted grains for each pseudotachylyte layer we estimated the temperature of melt using various temperature indices and measured volume fraction of unmelted grains we synthesized these data to reconstruct the change in viscosity during seismic slip events the melt viscosity is very high 107 9 pas during initial melting due to the combined effect of low temperature 750 800 c and large volume fraction of solid grains thus seismic slip is inevitably restrained instantaneously once this mechanical barrier is overcome the viscosity reduces continuously and dramatically as slip increases at 1000 c the viscosity reduces to 104pas and eventually to 10pas at 1280 c thus stress drops almost completely and rupturing tends to run away

elastic properties of anisotropic synthetic calcite muscovite aggregates 1 a set of synthetic aggregates containing mixtures of calcite and muscovite were compacted with varying uniaxial loads ranging from 20 mpa up to 400 mpa their elastic properties have been measured using compressional and shear waves in confining hydrostatic pressures up to 475 mpa measured seismic velocities are shown to depend on 1 the ratio of calcite to muscovite 2 the uniaxial load used during sample manufacturing and 3 the porosity amount prior to velocity measurements the matrix framework may also affect the seismic velocities but this effect is not easily quantifiable in general measured seismic velocities decrease with increasing muscovite content and porosity elastic properties have been calculated based on texture measurements which were obtained by neutron diffraction the calculated velocities are based on the calcite and muscovite single crystal elastic tensors their orientation distribution functions and their modal fractions a large discrepancy is apparent between calculated and measured velocities where the former always overestimate the actual velocities shear waves display less of a difference between calculated and measured values than do compressional waves indicating that waves propagating by shearing are less affected by sample porosity and matrix framework a better agreement between measured and calculated seismic velocities arises when the initial porosity is taken into account using a differential effective medium model seismic anisotropy is evident from both measured and calculated velocities and is low for samples containing pure calcite but becomes prominent as the muscovite concentration increases the intensity of anisotropy further depends on the uniaxial load used during sample compaction and the initial porosity

preferred orientation of phyllosilicates comparison of fault gouge shale and schist abstract samples of fault gouge from the san andreas fault drill hole safod a shale from the north sea sedimentary basin and schists from metamorphic rocks in the alps have been analyzed with high energy synchrotron x rays to determine preferred orientation of mica and clay minerals the method relies on obtaining 2d diffraction images which are then processed with the crystallographic rietveld method implemented in the software maud allowing for deconvolution of phases and extraction of their orientation distributions it is possible to distinguish between detrital illite muscovite and authigenic illite smectite kaolinite and chlorite and muscovite and biotite with strongly overlapping peaks in the diffraction pattern the results demonstrate that phyllosilicates show large texture variations in various environments where different mechanisms produce the rock microfabrics fault gouge fabrics are quite weak and asymmetric with maxima for 001 in the range of 1 5 2 5 multiples of random distribution m r d this is attributed to heterogeneous deformation with randomization as well as dissolution precipitation reactions shale fabrics have maxima ranging from 3 to 9 m r d and this is due to sedimentation and compaction the strongest fabrics were observed in metamorphic schists 10 14 m r d and developed by deformation as well as recrystallization in a stress field in the analyzed samples fabrics of co existing quartz are weak all phyllosilicate textures can be explained by orientation of 001 platelets with no additional constraints on a axes

t matrix approach to shale acoustics summary the t matrix approach of quantum scattering theory is used here to place many long wavelength equivalent medium approximations for porous composites polycrystals and cracked media on a common footing and to indicate their limitations but also to derive some new results based on two point statistics in this way we have obtained an insight into the difficult problem of elastic inclusions at finite concentrations which is of foremost relevance when estimating the effective material parameters of porous cracked shales involving stacks of more or less horizontally aligned clay platelets mixed together with more rounded silt minerals and with fluid filling the spaces a rather involved perturbative analysis of the effects of interactions between or structural correlations among the various inclusions minerals and cavities making up a real shale of hexagonal symmetry was performed in an attempt to obtain a better match between theoretical predictions based on a combination of coherent and optical potential approximations and experimental results recovered from ultrasonic wave speeds for the effective elastic stiffness tensor for the particular data set considered in this study the t matrix approach was able to match the data better than the approach of hornby et al but the match was not completely satisfactory further progress in theoretical shale modelling may come from a better knowledge of the elastic properties of pure clay minerals a more detailed knowledge of the microstructure of shales the incorporation of constraints obtained from comparisons between theoretical predictions and experimental results as well as a continuing development of the t matrix approach numerical results also for the effect of bedding parallel microcracks on the elasticity of such a real shale have value in illustrating the importance of taking into account the effects of spatial distribution when trying to deal with non dilute mixtures of highly contrasting material properties

grain size dependence of the rock magnetic properties for a natural maghemite the natural maghemite from the robe river mining district australia is shown to be thermally very stable the maghemite hematite inversion occurs at 650 c for sized fractions 250 m to 5 m enabling determination of t c for maghemite at 610 c the maghemite is intimately intergrown with hematite and j s of the maghemite corrected for the hematite content is 64 am 2 kg j rs h cr and h c range between 7 46 10 18 am 2 kg 16 90 23 30 mt and 6 38 9 00 mt respectively indicating psd maghemite with grain sizes between 1 and 20 m the grain size dependence of the rock magnetic parameters of the maghemite is less prominent than those of magnetite of nominally the same size it is suggested that the lt behavior of arm can be diagnostic of the presence of maghemite upon cooling to 196 c it shows no change upon rewarming a gradual decrease in intensity occurs starting at 120 c

compaction of quartz kaolinite mixtures the influence of the pore fluid composition on the development of their microstructure and elastic anisotropy abstract shales play important roles in sedimentary basins acting as both seals and reservoir rocks and knowledge of their anisotropic velocity trends is of practical importance for correct seismic image processing and inversion and for seismic to well tie whereas velocity depth trends are extensively studied for conventional reservoirs and critical factors that control their compaction are well understood little work has been done on shales compaction trends of shaly formations are controlled by a number of parameters such as clay mineralogy silt fraction and depositional environment this large number of parameters which are difficult if not impossible to control in naturally deposited shales make the study of shale compaction a statistically complex multivariate and non unique problem in such a situation laboratory experiments that allow precise planning of mixture mineralogy and chemical compositions of pore fluids as well as an accurate control of microstructural changes seems to be an attractive alternative in this work we have conducted two sets of compaction experiments on quartz kaolinite mixtures with 100 75 and 60 of kaolinite powders dry weight in the first set the use of a kcl brine solution to saturate the mixtures has led to the aggregation of the existing clay particles with each other and with silt particles in the other set the mixtures have been treated with a dispersant to separate existing clay aggregates into individual platelets the mixtures have been further compacted in an oedometric cell at uniaxial stresses up to 30 mpa compressional in vertical and horizontal directions and shear in vertical direction ultrasonic wave velocities have been measured during these compaction experiments and the p wave anisotropy has been estimated the effect of the chemical composition of the pore fluid on shale microstructure has been studied using the micro ct and sem image analysis neutron diffraction experiments have been conducted to understand the orientation of clay platelets at the final stage of the compaction our research shows that the chemical composition of pore fluid significantly affects the microstructure of the clay component and via this the elastic properties of the artificial shales the samples with the dispersed clay microstructure dcm have shown larger elastic anisotropies at the same porosity and significantly larger anisotropies at the same stress the dispersed clay platelets have orientated preferably parallel to bedding plane while the clay platelets in the samples with the aggregated clay microstructure acm show a wider angle distribution in the absence of quartz particles 100 kaolinite samples in the case of 40 of quartz fraction though the clay particles are more aligned in the acm sample than in the dcm one

no frictional heat along the san gabriel fault california evidence from fission track thermochronology large earthquakes generate frictional heat and the magnitude of heating is related to the slip magnitude the applied effective normal stress and the frictional strength of the fault we looked for evidence of this heating in apatite fission track age and track length distributions of samples from adjacent to and within the san gabriel fault zone in southern california the fault is thought to be an abandoned major trace of the san andreas fault system active from 13 to 4 ma and has since been exhumed from depths of 2 5 km at our sample locality as much as 40 km of total slip is thought to have accumulated along a localized ultracataclasite layer just 1 8 cm thick we see no evidence of a localized thermal anomaly in either fission track ages or track lengths even in samples within just 2 cm of the ultracataclasite because of the absence of any measurable impact on fission tracks we have been able to use forward modeling of heat generation heat transport and fission track annealing to constrain the frictional properties of the fault we find that either there has never been an earthquake with 4 m of slip at this locality or the average apparent coefficient of friction must have been 0 4

the mont terri rock laboratory a new international research project in a mesozoic shale formation in switzerland abstract in many countries argillaceous formations are being considered as potential host rocks for repositories of radioactive waste therefore in 1995 several organisations decided to start an international research project in the reconnaissance gallery of the mont terri motorway tunnel in north western switzerland in a mesozoic shale formation the opalinus clay aalenian this project is under the patronage of the swiss national hydrological and geological survey the following organisations are also partners in the project andra and ipsn france bgr germany enresa spain nagra switzerland jnc and obayashi japan sckcen belgium the aims of the project are to analyse the hydrogeological geochemical and rock mechanical properties of an argillaceous formation the changes of these properties induced by the excavation of galleries and to evaluate and improve appropriate investigation techniques in january 1996 eight niches were excavated for the project and 15 experiments were started in boreholes of no more than 30 m in length in the winter of 1997 1998 a new research gallery was excavated to host further experiments different excavation drilling and investigation techniques have been tested and improved significant results of the different experiments are already available some of those dealing with hydrochemistry porewater pressure measurements and the evolution of the excavation disturbed zone edz are reported in this paper

analysis of acoustic emissions recorded during a mine by experiment in an underground research laboratory in clay shales abstract during a mine by experiment performed at the mont terri underground research laboratory located at the transition between the sandy and the shaly facies of the opalinus clay formation excavation induced micro acoustic events were recorded in the so called gallery 08 the ez g experiment a first cluster of events occurred in the vicinity of the eastern sidewall of gallery 08 and a second cluster was observed ahead of the advancing tunnel face for each recorded micro acoustic event ae all located in the sandy facies of the opalinus clay formation the total stresses associated with the onset of inelastic deformations were estimated using a three dimensional numerical model the numerical analysis is based on the assumption that the rock mass behavior in the vicinity of the excavation is essentially elastic before the stress redistribution causes damage evidenced by the triggered ae activity for the cluster located at the tunnel sidewall the source mechanism analysis reveals the predominance of extensional failure tensile events the numerical analysis of each individual micro acoustic event suggests that the differential stresses at the onset of damage range between 3 and 10 mpa these values are in reasonable agreement with crack initiation thresholds obtained in the laboratory from samples of the various sub facies types of the sandy facies in the opalinus clay formation which range between 2 and 18 mpa for the cluster located ahead of the tunnel face the source mechanism analysis indicates the predominance of local shear failure events this is consistent with observed shear dislocations on bedding planes within weak beds in the sandy facies that have similar strength properties as the shaly facies thus the modelled shear and total normal stresses acting across the average bedding plane orientation at each event location were modelled and used to estimate the in situ shear strength along the bedding planes the model suggests a friction angle of 33 4 and a cohesion of 0 43 mpa the results are overall consistent with the bedding plane strength obtained through undrained direct shear tests on specimens of the shaly facies suggesting that the observed weak beds with a strength similar to the shaly facies govern the behavior at the tunnel face

the role of texture cracks and fractures in highly anisotropic shales organic shales generally have low permeability unless fractures are present however how gas oil and water flows into these fractures remains enigmatic the alignment of clay minerals and the alignment of fractures and cracks are effective means to produce seismic anisotropy thus the detection and characterization of this anisotropy can be used to infer details about lithology rock fabric and fracture and crack properties within the subsurface we present a study characterizing anisotropy using s wave splitting from microseismic sources in a highly anisotropic shale we observe very strong anisotropy up to 30 with predominantly vti vertical transverse isotropy symmetry but with evidence of an hti horizontal transverse isotropy overprint due to a ne striking vertical fracture set parallel to the maximum horizontal compressive stress we observe clear evidence of a shear wave triplication due to anisotropy which to our knowledge is one of only a very few observations of such triplications in field scale data we use modal proportions of minerals derived from x ray fluorescence data combined with realistic textures to estimate the contribution of intrinsic anisotropy as well as possible contributions of horizontally aligned cracks we find that aligned clays can explain much of the observed anisotropy and that any cracks contributing to the vertical transverse isotropy vti must have a low ratio of normal to tangential compliance zn zt typical of isolated cracks with low hydraulic connectivity subhorizontal cracks have also been observed in the reservoir and we propose that their reactivation during hydraulic fracturing may be an important mechanism to facilitate gas flow

review of major shale dominated detachment and thrust characteristics in the diagenetic zone part ii rock mechanics and microscopic scale abstract characterising large shale thrust zone behaviour down to the diagenetic metamorphic boundary is both simple and complex the task in critical taper analogue and numerical models has been successfully approximated using simple material parameters yet the weakness of shales thrusts on a case by case basis show great variability in key factors across a wide range of scales understanding of these variables and their relationships to different tectonic settings is patchy due to the different objectives of workers studying the various systems e g seismic hazards hydrocarbon exploration structural geology research and the types of available geological and geophysical data early research recognized that megathrusts feature creeping and locked patches which may be described using the rate and state formalism in addition to temperature differential stress and strain rate key controlling factors on thrust weakness are mineralogy amount of weak phase present structure localization mechanisms grain size and shapes porosity permeability and pore fluid pressure the smectite illite transition has been a focus of seismic hazard research due to its coincidence with the top of the seismogenic zone while there may be a relationship other factors such as pore fluid pressure variations or structure localization are also important the critical taper model is a simple means of determining whether basic rock mechanics data e g frictional strength pore fluid pressure is appropriate for natural wedges for example aseismic basal detachments of gravity driven systems smaller critical taper wedges appear to retain higher pore fluid pressures along the basal detachment than seismogenic basal detachments higher critical taper wedges in many continental and deltaic fold and thrust belts high organic carbon content is a very important factor in shale weakness due to 1 the overall more ductile and well cleaved nature of the shale when organic content is high 2 the presence of high overpressures due to maturation of organic material and 3 metamorphism of carbon to low friction graphite this multifaceted influence of organic carbon content is just one example of the diversity of potential influences on shale thrust zone weakness which enable shales to be weak despite considerable lateral deposition related and vertical burial related changes in composition some key variations between different types of fold and thrust belt gravity driven accretionary prism andean himalayan type lie in systematic variations in clay mineralogy magnitudes and origins of overpressures seismic versus aseismic detachments and different structural localization mechanisms further research is required to explore the viability of such distinctions and their impact on structural styles

anisotropy in shale from mont terri anisotropy of shales is the subject of this report and we use an example of the jurassic opalinus clay from mont terri switzerland that is being investigated in the context of radioactive waste disposal the study is targeted at the geomechanical characterization of shale by laboratory testing the overall aim is to improve the constitutive material laws and their application in numerical models

the magnitude symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonites summary seismic anisotropy within the upper mantle originates from the preferred orientation of highly anisotropic single crystals the symmetry and magnitude of anisotropy depend upon 1 the volume percentages of the minerals constituting the upper mantle 2 the degree and symmetry of preferred orientation of each mineral and 3 the alignment of the minerals crystallographic axes relative to one another the nature of upper mantle anisotropy can be examined by studying mineral orientations within ultramafic rocks which were once part of the mantle petrofabric data for olivine and pyroxene have been used to obtain velocity anisotropy patterns over large regions of ultramafic rocks from the samail ophiolite oman the troodos ophiolite cyprus the bay of islands ophiolife newfoundland the twin sisters ultramafic washington usa the dun mountain ophiolite new zealand the red hills ophiolite new zealand and the red mountain ophiolite new zealand the compressional wave anisotropy calculated for these massifs ranges from 3 to 8 per cent in excellent agreement with observed seismic anisotropy in the upper continental and oceanic mantle the symmetry varies from orthorhombic to axial with the axial symmetry axis corresponding to the olivine a axes maxima and subparallel to spreading directions in oceanic upper mantle pyroxene a b and c axes maxima generally parallel olivine b c and a axes respectively and anisotropy decreases with increasing pyroxene content shear wave splitting is predicted for all propagation directions within the upper mantle symmetry is also orthorhombic or axial with the minimum difference in velocity between the two shear waves parallel to the maximum compressional wave velocity

elastic anisotropy modeling of kimmeridge shale 1 anisotropy of elastic properties in clay rich sedimentary rocks has been of long standing interest these rocks are cap rocks of oil and gas reservoirs as well as seals for carbon sequestration elasticity of shales has been approached by direct velocity measurements and by models based on microstructures here we are revisiting the classical kimmeridge shale studied by hornby 1998 by first quantifying microstructural features such as phase volume fractions grain shapes and grain orientations and pore distributions with advanced analytical methods and then using this information in different models to explain bulk elastic properties it is shown that by application of a self consistent algorithm based on eshelby s 1957 model of inclusions in a homogeneous medium it is possible to explain most experimental elastic constants though some discrepancies remain which may be due to the interpretation of experimental data using a differential effective medium approach an almost perfect agreement with experimental stiffness coefficients can be obtained though the physical basis of this method may be questionable the influence of single crystal elastic properties grain shapes preferred orientation and volume and shapes of pores on elastic properties of shale is explored

linking preferred orientations to elastic anisotropy in muderong shale australia the significance of shales for unconventional hydrocarbon reservoirs nuclear waste repositories and geologic carbon storage has opened new research frontiers in geophysics among many of its unique physical properties elastic anisotropy had long been investigated by experimental and computational approaches here we calculated elastic properties of cretaceous muderong shale from australia with a self consistent averaging method based on microstructural information the volume fraction and crystallographic preferred orientation distributions of constituent minerals were based on synchrotron x ray diffraction experiments aspect ratios of minerals and pores determined from scanning electron microscopy were introduced in the self consistent averaging our analysis suggested that phyllosilicates i e illite mica illite smectite kaolinite and chlorite were dominant with 70 vol the shape of clay platelets displayed an average aspect ratio of 0 05 these platelets were aligned parallel to the bedding plane with a high degree of preferred orientation the estimated porosity at ambient pressure was 17 vol and was divided into equiaxial pores and flat pores with an average aspect ratio of 0 01 our model gave results that compared satisfactorily with values derived from ultrasonic velocity measurements confirming the validity and reliability of our approximations and averaging approach

anisotropic amplitude variation of the bottom simulating reflector beneath fracture filled gas hydrate deposit 1 for the first time we report the amplitude variation with angle ava pattern of bottom simulating reflectors bsrs beneath fracture filled gas hydrate deposits when the effective medium is anisotropic the common depth point cdp gathers of two mutually perpendicular multichannel seismic profiles located in the vicinity of site nghp 01 10 are appropriately processed such that they are fit for ava analysis ava analysis of the bsr shows normal incidence reflection coefficients of 0 04 to 0 11 with positive gradients of 0 04 to 0 31 indicating class iv pattern the acoustic properties from isotropic rock physics model predict class iii ava pattern which cannot explain the observed class iv ava pattern in krishna godavari basin due to the anisotropic nature of fracture filled gas hydrate deposits we modeled the observed class iv ava of the bsr by assuming that the gas hydrate bearing sediment can be represented by horizontally transversely isotropic hti medium after accounting for anisotropic wave propagation effects on bsr amplitudes the effective medium properties are estimated using backus averaging technique and the ava pattern of bsrs is modeled using the properties of overlying hti and underlying isotropy hti media with or without free gas anisotropic ava analysis of the bsr from the inline seismic profile shows 5 30 gas hydrate concentration equivalent to fracture density and the azimuth of fracture system fracture orientation with respect to the seismic profile is close to 45 free gas below the base of gas hydrate stability zone is interpreted in the vicinity of fault system f1

the nanogranular acoustic signature of shale a multiscale micromechanics model has been developed for the prediction of anisotropic acoustic properties of shale the model is based on the recently identified nanogranular mechanical response of shale through indentation experiments it recognizes the dominant role of the anisotropic elastic properties of compacted clay in the anisotropic elasticity of shale at different length scales compared to contributions of shape and orientation of particles following a thorough validation at multiple length scales using mineral elasticity data nanoindentation experiment results and ultrasonic pulse velocity tests the model predictions compare adequately with measurements on kerogen free and kerogen rich shales and shaley sandstones the acoustic signature of shale thus is found to be controlled by two volumetric parameters that synthesize the porosity and mineralogy information the clay packing density and the silt inclusion volume fraction through a series of dimensionless isoparametric plots the micromechanics model predicts trends of increasing elastic anisotropy with increasing clay packing density or decreasing porosity which correspond to the intrinsic mechanical response of unfractured shale and quantifies the stiffness reduction induced by the presence of kerogen

texture and anisotropy analysis of qusaiba shales scanning and transmission electron microscopy synchrotron x ray diffraction microtomography and ultrasonic velocity measurements were used to characterize microstructures and anisotropy of three deeply buried qusaiba shales from the rub alkhali basin saudi arabia kaolinite illite smectite illite mica and chlorite show strong preferred orientation with 001 pole figure maxima perpendicular to the bedding plane ranging from 2 4 6 8 multiples of a random distribution m r d quartz feldspars and pyrite crystals have a random orientation distribution elastic properties of the polyphase aggregate are calculated by averaging the single crystal elastic properties over the orientation distribution assuming a nonporous material the average calculated bulk p wave velocities are 6 2 km s maximum and 5 5 km s minimum resulting in a p wave anisotropy of 12 the calculated velocities are compared with those determined from ultrasonic velocity measurements on a similar sample in the ultrasonic experiment which measures the effects of the shale matrix as well as the effects of porosity velocities are smaller p wave maximum 5 3 km s and minimum 4 1 km s the difference between calculated and measured velocities is attributed to the effects of anisotropic pore structure and to microfractures present in the sample which have not been taken into account in the matrix averaging

elastic properties of hydrate bearing sediments using effective medium theory accurate and detailed models of the seismic velocity structure of gas hydrate bearing sediments may be determined by careful analysis of controlled source seismic data however interpretation of these velocities in terms of hydrate saturation of the pore space has hitherto relied on semiempirical formulas and or simple effective medium theory we develop a rigorous theoretical scheme to relate the seismic properties of a clay rich hydrate bearing sediment to its porosity mineralogy microstructural features and hydrate saturation we consider separately the two possible end members for the distribution of hydrate in the pore space 1 hydrates are unconnected and located in the pore voids without appreciable grain contact and 2 connected hydrates are forming cement binding around the grains the scheme is transversely isotropic to allow for anisotropy due to alignment of clay platelets and is based on a combination of a self consistent approximation a differential effective medium theory and a method of smoothing for crystalline aggregates we have applied the scheme to lithological and seismic velocity data from ocean drilling program site 995 on the blake ridge southeastern u s continental margin to make estimates of the hydrate saturation it was found that the hydrates are probably unconnected and their volume concentration varies between approx 0 at 100 m below the seabed and approx 9 at 400 m depth just above the bottom simulating reflector if the clay platelet orientation distribution resembles the function we have used

texture development in naturally compacted and experimentally deformed silty clay sediments from the nankai trench and forearc japan the petrophysical properties of fine grained marine sediments to a large extent depend on the microstructure and crystallographic preferred orientations cpos in this contribution we show that rietveld based synchrotron texture analysis is a new and valuable tool to quantify textures of water saturated fine grained phyllosilicate rich sediments and assess the effects of compaction and tectonic deformation we studied the cpo of compositionally almost homogeneous silty clay drillcore samples from the nankai accretionary prism slope and the incoming philippine sea plate offshore sw japan basal planes of phyllosilicates show bedding parallel alignment increasing with drillhole depth thus reflecting progressive burial and compaction in some samples calcite and albite display a cpo due to crystallographically controlled non isometric grain shapes or nannofossil tests consolidated undrained experimental deformation of a suite of thirteen samples from the prism slope shows that the cpos of phyllosilicate and calcite basal planes develop normal to the experimental shortening axis there is at least a qualitative relation between cpo intensity and strain magnitude scanning electron micrographs show concurrent evolution of preferred orientations of micropores and detrital illite flakes normal to axial shortening this indicates that the microfabrics are sensitive strain gauges and contribute to anisotropic physical properties along with the cpo

microfabric and rock properties of experimentally compressed silt clay mixtures abstract oedometric mechanical compaction tests were performed on brine saturated synthetic samples consisting of silt clay mixtures to study changes in microfabric and rock properties as a function of effective stress the silt consisted of crushed quartz 100 with grain size range between 4 and 40 m whereas the clay consisted of 81 kaolinite 14 mica illite and 5 microcline of grain size between 0 4 and 30 m five sample pairs ranging in composition from pure silt to pure clay were compacted to 5 and 50 mpa effective stress respectively sem studies were carried out to investigate microfabric changes in the mechanically compacted silt clay mixtures the degree of alignment of the different minerals present quartz mica illite and kaolinite were computed by using an image analysis software experimental compaction have measured the changes in the rock properties such as porosity and velocity as a function of effective stress for different mixtures of clay and silt clay rich samples showed a higher degree of mineral orientation and lower porosity compared to silt dominated samples as a function of effective stress pure clay sample had 11 porosity at 50 mpa effective stress whereas the pure silt sample retained about 29 porosity at the same effective stress the experiments showed that low porosity down to 11 is possible by mechanical compaction only a systematic increase in strain was observed in the silt clay mixtures with increasing clay content but the porosity values found for the 50 50 silt clay mixture were lower than that of 25 75 silt clay mixture no preferential mineral orientation is expected before compaction owing to the high initial porosity suggesting that the final fabric is a direct result of the effective stress both p and s wave velocities increased in all silt clay mixtures with increasing effective stress the maximum p and s wave velocities were observed in the 25 75 silt clay mixture whereas the minimum vp and vs were recorded in the pure silt mixture at 50 mpa effective stress p wave velocities as high as 3 km s resulted from experimental mechanical compaction alone the results show that fine grained sediment porosity and velocity are dependent on microfabric which in turn is a function of grain size distribution particle shape sediment composition and stress at 5 mpa effective stress quartz orientation increased as a function of the amount of clay indicating that clay facilitate rotation of angular quartz grains adding clay from 25 to 75 in the silt clay mixtures at 50 mpa effective stress decreased the quartz alignment the clay mineral orientation increased by increasing both the amount of clay and the effective stress the mica illite fabric alignment being systematically higher than that of kaolinite even small amount of silt 25 added to pure clay reduced the degree of clay alignment significantly this study demonstrates that experimental compaction of well characterized synthetic mudstones can be a useful tool to understand microfabric and rock properties of shallow natural mudstones where mechanical compaction is the dominant process

geochemical and petrophysical characteristics of permian shale gas reservoirs of raniganj basin west bengal india abstract thick shale horizons of permian age emerged as the potential source of gas through primary investigation and resulted into the first indian pilot scale gas recovery demonstration project in the raniganj basin however the relation of geochemical properties with shale pore matrix porosity and permeability mechanism are yet to be evaluated for better dealing the phenomena of diffusion transport and gas recovery in this study the analyses like proximate rock eval toc vro low pressure n2 sorption porosity permeability sem edx and image processing have been carried out the values of s1 s2 toc and tmax indicated fair to excellent source rock potential of the shales having type iii iv kerogens prone to thermal gas genesis the low pressure n2 desorption curve of raniganj formation shales shows the type h3 hysteresis comprising slit shaped pores the barren measures shales have type h2 and h3 hysteresis patterns specifying to ink bottle shaped pores and slit shaped pores whereas the oldest barakar formation shales are having ink bottle shaped pores caused due to blocking effect the slit shaped pores favour the pore network and characteristically excellent for the flow of gas the results of sem edx are indicating alteration stability in the order of o the increasing tmax values with a centric decrease in porosity and permeability specifying the role of devolatilization disintegration and blocking of pore spaces openings is a function of the thermal gradient the linear evolution of multipoint bet surface area with increasing porosity suggests that the porosity values from 2 0 to 6 5 mainly corresponds to pore size of 3 0 11 0 nm similarly the inverse relationship between average pore size and porosity attributed to a greater contribution of smaller pores in total porosity the pore network model derived through the sem image processing has shown two types of connectivity i various pore sizes of diverse pore throats with dual opening directions and ii interlinked large and small pores obeying similar normal distributions the barren measures shales have shown higher pore connectivity than the raniganj and barakar formation shales

the clay fabric of shales is a strain gauge abstract shales with their large clay fraction are prone to deform in an almost ductile way under tectonic loading here we propose to monitor and quantify the evolution of strained shales clay fabric in the field of illite diagenesis by using magnetic fabric in the footwall of a south pyrenean thrust 281 shales samples have been collected along a 2 km long exceptional exposure the cleavage evolves from pencil structures at 1 km of the thrust to pencil cleavage at 850 m and slaty cleavage at 170 m the average magnetic foliation of illite remains nearly parallel to bedding over a distance of 600 m despite development of pencil cleavage it is at 600 m of the thrust that illite foliation shifts gradually from bedding to cleavage by contrast the magnetic fabric scalar data reflect a more continuous evolution of the deformation the application of a rigid rotation model of the clays allows to quantify the strain magnitude using eigen values of tensor this strain magnitude is consistent with distance to the thrust and conforms favorably with a trishear modelling of this thrust damage we demonstrate therefore that the magnetic fabric of illite rich shales is a strain gauge

time dependent fracture formation around a borehole in a clay shale abstract opalinus clay a mesozoic clay shale has been chosen as host rock formation for the disposal of radioactive waste in switzerland for this study borehole damage zones were utilized as a proxy for an excavation damage zone that forms around a circular mechanically excavated tunnel in intact opalinus clay pilot boreholes were resin impregnated and over cored at different times after drilling to provide insight into the time dependent formation of fractures on both the micro and the macro scale observed fractures were characterized in terms of failure mode their relation to the rock anisotropy and the in situ stress tensor the analyses show that fractures that form in the short term initiate as shear fractures at the pilot borehole wall and propagate parallel to bedding typically a dominant shear fracture tangential to the pilot borehole wall was observed upon propagation of these shear fractures wing cracks horsetail splays and second order shears form sub parallel and sub perpendicular to bedding planes forming a complex fracture network which extends a quarter pilot borehole diameter into the rock mass in the longer term tangential shear fractures tend to propagate in a direction opposite to the initial propagation direction in addition new bedding parallel fractures deeper in the rock develop leading to the formation of thin slabs buckling of the slabs when unsupported and eventually progression of the buckling zone deeper into the rock mass buckling is associated with the formation of extensional fractures normal to bedding in the center and lateral to the buckling zone the zone of buckled rock slabs was found to have an extension of more than one borehole diameter at the time of preservation with resin in the short term the axis connecting the maximum failure depth on opposing sides of the borehole is parallel to the minimum stress direction in a plane normal to the borehole axis in the long term this axis rotates significantly towards the maximum stress direction primarily as a consequence of tangential shear fracture propagation slab formation and buckling dissipation of excess pore pressures may be the key process underpinning longer term fracture propagation and formation

modeling of elasticity effects of sandstone compaction using coated inclusions compaction of siliciclastic sediments leads to an increase in their stiffness parameters and seismic velocities although mechanical compaction implies a reduction of porosity and closing of compliant pores chemical compaction may alter the mineral properties the cementing of grain contacts and the pore volume the ability of rock physics models to quantify such effects on seismic observables will aid hydrocarbon exploration a framework was designed for modeling compaction effects by use of a so called coated inclusion model that eliminates the need of using a hybrid approach through combining different theories a basic feature of the model is that the inclusion is defined by a kernel representing the pore which is surrounded by shells that may individually have different elastic properties from those of the pore filling material and the background matrix the modeling can be designed to explore seismic effects of various texture perturbations including contact cementing and pore filling processes the numerical modelings seem to be consistent with the results obtained from other rock physics models the model allows for the possibility of including small scale heterogeneities within the rock texture and estimating frequency dispersion together with attenuation due to pore fluid flow a basic weakness of the method is the relatively large number of parameters needed to describe a porous rock which will always limit its practical usage however its basic physical foundation may provide a reference for understanding the qualitative and quantitative effects of various cementation scenarios on seismic parameters

microstructure and composition of brittle faults in claystones of the mont terri rock laboratory switzerland new data from petrographic studies geophysical borehole logging and permeability tests abstract claystones are considered as a geological barrier however the properties of claystone may be modified if these are cut by brittle faults and fractures investigations of fault rocks are therefore crucial to evaluate the barrier properties of clay rich formations the present study is dealing with the characterization of naturally and artificially deformed opalinus clay of the mont terri rock laboratory in nw switzerland complete core sections covering the artificially excavation damaged zone edz and several tectonic fault zones have been studied using a multidisciplinary approach consisting of geophysical geotechnical mineralogical geochemical and lithological structural data the fault zones encountered are characterized by a high density of planar discontinuities which often show slickenside striations under the microscope the fault zones turned out to be pervasively deformed resulting in open veins and pore space now filled with calcite and celestine fault zone reactivation led to very fine grained cohesionless fault gouge and fragmentation of the previously formed calcite veins packer tests in boreholes reveal excavation induced enhanced permeabilities up to 3 m borehole depth 1 9 m to gallery floor seismic borehole measurements indicate that seismic attributes which are typical for undisturbed opalinus clay are not reached until 7 5 m borehole depth 4 8 m to gallery floor which is larger than in the usual gallery configuration we interpret the anomalies in geophysical measurements as well as the elevated permeabilities measured in the main fault 3 6 m to 4 2 m to gallery floor to result from an enlarged edz influenced by the presence of the brittle fault structures the up to two orders of magnitude higher permeability compared to the intact claystone most probably results from excavation induced stress and a local reactivation of fault planes the results suggest that the presence of a fault zone can alter the extent of the edz significantly and thereby affect the rock integrity at least in the near field of a repository tunnel

calibration of shear wave splitting in the subcontinental upper mantle beneath active orogenic belts using ultramafic xenoliths from the canadian cordillera and alaska abstract despite the abundance of measurements of shear wave splitting around the world some fundamental questions about its geological interpretation have not yet been answered in order to constrain 1 i the orientation and magnitude of s wave anisotropy 2 ii the thickness of the anisotropic layer and 3 iii the possible variation of the anisotropy with depth in the subcontinental upper mantle beneath active orogenic belts we have carried out a systematic investigation on shear wave properties of mantle xenoliths from recent alkaline basalts of the canadian cordillera and alaska it is found that the polarization direction of the fast s wave as well as the time delay t between the two arrivals are strongly dependent on the propagation direction with respect to the structural frame at a single station the variations of measured and t values for sks phases from different events can result from this dependence because the average grain size of olivine in upper mantle samples is commonly larger than 0 5 mm dislocation creep prevails over a thickness of at least 250 km and results in olivine lpo and hence anisotropy and shear wave splitting a single thick anisotropic layer in mountain parallel subvertical shear zones may cause high values of tgd while multiple subhorizontal layers will produce multiple splitting making the measurement and interpretation of s wave splitting difficult our results also suggest that the s wave splitting observed in cold old stable cratons such as the canadian shield is likely dominated by fossil anisotropy whereas that observed in hot active orogenic regions such as the northwest american cordillera and the tibetan plateau may be dominated by modern tectonic deformation in the lithosphere and flow in the asthenosphere

velocity and attenuation anisotropy in deep sea carbonate sediments laboratory studies have been carried out to determine the causes of velocity and attenuation anisotropy in pelagic carbonate sediments from deep sea drilling project site 288 on the ontong java plateau compressional velocity vp was measured under independently controlled pore pressure and confining pressure velocity anisotropy for compressional waves horizontal velocity vertical velocity is found to decrease with increasing effective pressure i e with closure of pores previous studies have suggested that velocity anisotropy in pelagic carbonate sequences is due largely to preferred vertical orientation of calcite c axes the present results indicate that preferred horizontal orientation of grain contacts and flat pores is also a significant cause of the observed anisotropy for many samples significant anisotropy in ultrasonic attenuation for compressional qp 1 and shear qs 1 waves is observed such that the losses are greatest for propagation modes that can generate relative motion across horizontal grain contacts or flat horizontal pores the results are discussed in terms of the possible mechanisms of dissipation

phased array compaction cell for measurement of the transversely isotropic elastic properties of compacting sediments sediments undergoing compaction typically exhibit transversely isotropic ti elastic properties we present a new experimental apparatus the phased array compaction cell for measuring the ti elastic properties of clay rich sediments during compaction this apparatus uses matched sets of p and s wave ultrasonic transducers located along the sides of the sample and an ultrasonic p wave phased array source together with a miniature p wave receiver on the top and bottom ends of the sample the phased array measurements are used to form plane p waves that provide estimates of the phase velocities over a range of angles from these measurements the five ti elastic constants can be recovered as the sediment is compacted without the need for sample unloading recoring or reorienting this paper provides descriptions of the apparatus the data processing and an application demonstrating recovery of the evolving ti properties of a compacting marine sediment sample

compaction fabrics of pelites experimental consolidation of kaolinite and implications for analysis of strain in slate abstract compaction of clay and shale results in large reductions in volume as pore water is expelled preferred orientation of the platy minerals increases with compaction strain and loss of porosity according to the march owens model this relationship has been studied quantitatively by experimentally consolidating kaolinite clay from slurries and analyzing the resulting fabrics with the x ray pole figure goniometer and scanning electron microscope sem initial porosity corresponds to the onset of the strain recorded by the preferred orientation and the values of 0 78 for dispersed slurries and 0 76 for flocculated slurries reflect the electrostatic forces between the clay platelets initial porosities of recently deposited fine silt and clay are in the range of 0 60 0 90 and are a function of grain size and mineralogy loss of this initial porosity has a large effect on the subsequent development of slaty cleavage matrix methods were used to model deformation paths for slates in the welsh slate belt preferred orientation of mica and ellipsoidal shapes of reduction spots were simulated for one locality by loss of a 0 60 initial porosity a 6 tilt of the beds and horizontal shortening involving plane strain strain determinations for shales and slates should include the large reduction in volume

compaction trend versus seismic anisotropy in shaly formations shales comprise more than 60 of sedimentary rocks and form natural seals above hydrocarbon reservoirs their sealing capacity is also used for storage of nuclear wastes the world s most important conventional oil and gas reservoirs have their corresponding source rocks in shale furthermore shale oil and shale gas are the most rapidly expanding trends in unconventional oil and gas shales are notorious for their strong elastic anisotropy i e so called vertical transverse isotropy this vertical transverse isotropy characterised by a vertical axis of invariance is of practical importance as it is required for correct surface seismic data interpretation seismic to well tie and amplitude versus offset analysis a rather classical paradigm makes a clear link between compaction in shales and the alignment of the clay platelets main constituent of shales this would imply increasing anisotropy strength with increasing compaction our main purpose is to check this prediction on two large databases in shaly formations more than 800 samples from depths of 0 6 km by extracting the major trends in the relation between seismic anisotropy and compaction the statistical analysis of the database shows that the simultaneous increase in density and velocity a classical compaction signature is quite weakly correlated with the anisotropy strength as a consequence compaction can be excluded as a major cause of seismic anisotropy at least in shaly formations also the alignment of the clay platelets can explain most of the anisotropy measurements of both databases finally a method for estimating the orientation distribution function of the clay platelets from the measurement of the anisotropy parameters is suggested

melt origin of fault generated pseudotachylytes demonstrated by textures a petrographic and electron microprobe study of fault generated pseudotachylytes from the outer hebrides thrust zone scotland demonstrates that the textures have resulted from the primary crystallization of a clast laden melt rather than the devitrification of a glass or by crushing and cataclasis the observations are consistent with these pseudotachylytes having formed by frictional fusion followed by rapid quenching implying faulting at shallow crustal depths

the continent ocean crustal transition across the southwest greenland margin this paper examines the complete crustal transition across the nonvolcanic southwest greenland continental margin of the labrador sea using wide angle and coincident vertical incidence seismic profiles six ocean bottom seismometers and a sonobuoy record p and s wave first and multiple arrivals from the crust and upper mantle which are analyzed by two dimensional dynamic ray tracing and one dimensional reflectivity modeling the resulting seismic velocity model requires that the preexisting 30 km thick continental crust is thinned abruptly to 3 km across the continental slope primarily by removal of the lower crust farther seaward the crust thickens to 6 km primarily through the addition of a high velocity 7 0 7 6 km s layer in the lower crust this lower crustal layer is 4 5 km thick extends for a horizontal distance of 80 km and is interpreted as partially serpentinized upper mantle it is overlain by a low velocity 4 0 5 0 km s upper layer which is interpreted as highly fractured upper continental crust our model suggests that seafloor spreading did not start until chrons 27 28 13 ma younger than previously suggested this interpretation is supported by two dimensional modeling of gravity and magnetic data along the refraction line our results are consistent with a simple shear mechanism for the initial rifting with the sw greenland margin as the upper plate however a full characterization of the rifting mechanism must await comparison with a seismic model for the conjugate margin east of labrador

relationship between anisotropy of p and s wave velocities and anisotropy of attenuation in serpentinite and amphibolite velocities and q values of p and s waves as functions of pressure and temperature at 100 and 600 mpa are presented for a serpentinite and an amphibolite both rocks exhibit a strong lattice preferred orientation lpo of the major mineral phases antigorite and hornblende respectively velocities and q values increase with pressure the rate of increase is different in the three orthogonal directions normal and parallel to foliation and lineation and closely related to progressive closure of microcracks increasing temperature decreases velocities and q values only slightly as long as thermal cracking is prevented by the applied confining pressure substantial anisotropy of velocities and q in p and s waves is observed in both rocks but is found to be different in origin anisotropy of p and s wave velocities is highest at low pressure and basically caused by constructive interference of effects related to oriented microcracks and to the lpo of major minerals increasing confining pressure decreases velocity anisotropy at a smaller and smaller rate the residual anisotropy of p and s wave velocities shear wave splitting at high confining pressure is mainly a result of preferred mineral orientation by contrast anisotropy of q is very low at low confining pressure and markedly enhanced as pressure is increased at high confining pressure substantial anisotropy of q in p waves is apparent but reversed from that of p wave velocities qp is highest in the direction normal to the foliation plane whereas vp and vs is lowest in this direction the generation of a pronounced anisotropy of qp by increasing pressure is due to a directionally dependent increase of contact areas on the oriented grain boundaries of the platy minerals defining the foliation the increase of q with pressure in the direction normal to foliation is mainly caused by the decrease of energy loss due to compressive strain relative to shear strain the reverse is true for the x and y directions serpentinite and x direction amphibolite parallel to the foliation plane

frictional stability permeability relationships for fractures in shales there is wide concern that fluid injection in the subsurface such as for the stimulation of shale reservoirs or for geological co2 sequestration gcs has the potential to induce seismicity that may change reservoir permeability due to fault slip however the impact of induced seismicity on fracture permeability evolution remains unclear due to the spectrum of modes of fault reactivation e g stable versus unstable as seismicity is controlled by the frictional response of fractures we explore friction stability permeability relationships through the concurrent measurement of frictional and hydraulic properties of artificial fractures in green river shale grs and opalinus shale ops we observe that carbonate rich grs shows higher frictional strength but weak neutral frictional stability the grs fracture permeability declines during shearing while an increased sliding velocity reduces the rate of permeability decline by comparison the phyllosilicate rich ops has lower friction and strong stability while the fracture permeability is reduced due to the swelling behavior that dominates over the shearing induced permeability reduction hence we conclude that the friction stability permeability relationship of a fracture is largely controlled by mineral composition and that shale mineral compositions with strong frictional stability may be particularly subject to permanent permeability reduction during fluid infiltration

magnetic properties of fault rocks from the yingxiu beichuan fault constraints on temperature rise within the shallow slip zone during the 2008 wenchuan earthquake and their implications abstract rock magnetic measurements were conducted on fault rocks from the dagou village outcrop at shuiguan town pingwu county sichuan province china which is located on the yingxiu beichuan fault the main fault ruptured in the 2008 wenchuan m s 8 0 earthquake results revealed that magnetic behavior of fault gouge is possibly dominated by paramagnetic siderite and lepidocrocite as well as pyrrhotite thermomagnetic analysis indicated that these fe bearing minerals are readily decomposed once temperature over 250 400 c and form newly ferrimagnetic magnetite maghemite that would increase the magnetization of fault rocks consequently the paramagnetic nature and relatively low magnetization of fresh fault gouge suggest that the shallow fault plane has not been subjected to thermal event over 250 400 c during the wenchuan earthquake this inference can be confirmed by a kinetic model analysis these findings proposed that thermal pressurization was not likely to occur during the wenchuan earthquake at least at the shallow part of slip zone onset of an elastohydrodynamic lubrication mechanism was alternatively discussed using a model in which fault zone containing viscous gouge results indicated that the lubrication pressure could reach up to 30 2 mpa which can widen the gap between the fault surfaces and reduce the area of asperity contact this prediction well matches the reported injection veins within the coseismic shear zone and the largest ground motions associated with lower predominant frequencies and large stress drop during the wenchuan earthquake

a comparative study of representative 2d microstructures in shaly and sandy facies of opalinus clay mont terri switzerland inferred form bib sem and mip methods abstract a combination of broad ion beam bib polishing and scanning electron microscopy sem has been used to study qualitatively and quantitatively the microstructure of opalinus clay in 2d high quality 2d cross sections ca 1 mm 2 belonging to the shaly and sandy facies of opalinus clay were investigated down to the nanometre scale in addition mercury intrusion porosimetry mip and x ray powder diffraction experiments have been used to extend characterization of the microstructure to the mm cm scale on bulk volume sample material interestingly both end member samples of the opalinus clay show qualitatively similar mineralogy and pore characteristics as well as a comparable pore size distribution and pore morphology within the different mineral phases and mineral aggregates differences between the facies are mainly due to variations in mineral size and mineral amount present in the alternating layers of the different facies six different porous mineral phases have been identified and the pores have been subdivided into ten different pore types pores visible in the sem images are most abundant in the clay matrix and these seem to follow a power law distribution with a power law exponent of ca 2 25 independent of the sample location furthermore all common mineral grains show characteristic porosity pore shape and pore size distribution in 2d and are proposed to be considered as elementary building blocks for opalinus clay combined these homogeneous elementary building blocks make up the heterogeneous fabric of the different facies of opalinus clay based on extrapolation of the power law size distribution in the clay matrix below sem resolution results in a porosity of 10 25 for clay rich layers 60 90 of clay matrix whereas sand and carbonate layers show an extrapolated porosity of 6 14 these extrapolated porosities are in agreement with water loss and physical porosity measurements performed on bulk material of comparable samples

influence of carbonate microfabrics on the failure strength of claystones claystones are currently being investigated as a potential host rock for radioactive waste disposal the relevant parameters are hydraulic conductivity swelling and shrinkage water uptake rheological and mechanical behavior much attention is paid to the opalinus clay opa in switzerland and the callovo oxfordian clay cox in france one of the scientific challenges is to develop numerical models to predict the mechanical behavior of claystones under repository like conditions recent studies have demonstrated the influence of carbonates on selected mechanical parameters e g failure strength in this study we show that opa and cox behave differently with respect to their failure strength the failure strength of opa decreases with increasing carbonate content whereas it increases with increasing carbonate content in case of the cox the different behavior is caused by microstructural variations results of carbonate microfabric investigations verify that opa mainly contains coarse grained shell fragments while cox shows a more homogeneous distribution of fine grained mainly non biogenic carbonates in conclusion not only the carbonate content but in addition their grain size distribution their shape and the spatial distribution control the mechanical behavior of both clays

elastic and resistivity anisotropy of shale during compaction and diagenesis joint effective medium modeling and field observations abstractshales are anisotropic materials which have been observed to have both electrical and elastic anisotropy shales also comprise most of the sedimentary column in clastic basins and thus the ability to model shale response to electrical and seismic fields may improve our ability to better resolve anisotropy by using different measurements compacting shale effective media modeling which captures the geological process of porosity reduction and geochemical compaction was formulated in this research in microstructural parameters associated with particle alignment and pore shape deformation the state of compaction captured by the porosity of the soft sediment governs the pore aspect ratio and the amount of particle alignment diagenetic processes are captured using temperature dependent mineral elasticity and conductivity the model uses the same microstructural parameters and equivalent effective medium approximation scheme to predict electrical and elastic anisotropy in situ measurements of e

revisiting elastic anisotropy of biotite gneiss from the outokumpu scientific drill hole based on new texture measurements and texture based velocity calculations abstract a sample of biotite gneiss from the outokumpu deep drilling project in finland was investigated by kern et al 2008 for crystal preferred orientation and elastic anisotropy considerable differences between measured acoustic velocities and velocities calculated on the basis of texture patterns were observed measured p wave anisotropy was 15 1 versus a voigt average yielding 7 9 here we investigate the same sample with different methods and using different averaging techniques analyzing time of flight neutron diffraction data from dubna skat and lansce hippo diffractometers with the rietveld technique much stronger preferred orientation for biotite is determined compared to conventional pole figure analysis reported previously the comparison reveals important differences hippo has much better counting statistics but pole figure coverage is poor skat has better angular resolution using the new preferred orientation data and applying a self consistent averaging method that takes grain shapes into account close agreement of calculated and measured p wave velocities is observed 12 6 this is further improved by adding 0 1 vol flat micropores parallel to the biotite platelets in the simulation 14 9

anisotropic velocity analysis1 results from walkaway vsp and shale laboratory experiments show that shale anisotropy can be significantly anelliptic heterogeneity and anellipticity both lead to non hyperbolic moveout curves and the resulting ambiguity in velocity analysis is investigated for the case of a factorizable anisotropic medium with a linear dependence of velocity on depth more information can be obtained if there are several reflectors the method of dellinger et al for anisotropic velocity analysis in layered transversely isotropic media is examined and is shown to be restricted to media having relatively small anellipticity a new scheme based on an expansion of the inverse squared group velocity in spherical harmonics is presented this scheme can be used for larger anellipticity and is applicable for horizontal layers having monoclinic symmetry with the symmetry plane parallel to the layers the method is applied to invert the results of anisotropic ray tracing on a model sand shale sequence for transversely isotropic media with small anisotropy the scheme reduces to the method of byun et al and byun and corrigan the expansion in spherical harmonics allows the p phase slowness surface of each layer to be determined in analytic form from the layer parameters obtained by inversion without the need to assume that the anisotropy is weak

experimental laboratory determination of the dynamic elastic properties of wet drained shales the ultrasonic anisotropic elastic properties of drained saturated shales were measured as a function of confining pressure two shales were characterized in this study a jurassic outcrop shale retrieved under the sea in a saturated state and a kimmeridge clay shale cut from a north sea borehole both shales were highly anisotropic both texturally as revealed by scanning electron microscopy analysis and elastically as measured ultrasonically in the laboratory the strongest anisotropy was seen in the kimmeridge clay shale where up to 38 compressional wave anisotropy thomsen s parameter e and up to 58 shear wave anisotropy thomsen s parameter were observed in addition for both shales e and were found to decrease as a function of increasing confining pressure with the pore pressure drained to atmosphere while the anellipticity deviation of the slowness surfaces from ellipses was found to be positive and decreased as a function of increasing confining pressure accompanying these changes in elastic properties was a decrease in porosity with increasing confining pressure from 10 5 to 8 5 for the jurassic shale the decrease in overall anisotropy of the shales with increasing confining pressure was found to be consistent with theoretical modeling of shale properties where the shale anisotropy and anellipticity were predicted to decrease as a function of decreasing fluid filled porosity

esr detection of seismic frictional heating events in the nojima fault drill core samples japan abstract we have analyzed the nojima fault nied 1800 m drill core samples by esr electron spin resonance to detect seismic frictional heating events especially during the 1995 kobe earthquake dark gray fault gouge with foliation 10 cm away from the fault plane at about 1140 m in depth which was produced by ancient fault movements has a fmr ferrimagnetic resonance signal heating experiments show that this fmr signal is derived from ferrimagnetic trivalent ion oxides fe 2 o 3 maghemite with imperfect crystallinity which is produced by thermal dehydration of feooh lepidocrocite or fe oh 3 limonite the existence of the fmr signal means that dry heating such as frictional heating once occurred and that the frictional heat temperature along the dark gray fault gouge may have risen to over 350 c during ancient seismic fault slip in order to detect frictional heating events in fault zones the increase of the fmr signal and the color change of fault gouge into dark gray or black are important indexes on the other hand no fmr signal is detected from the fault gouges just on two fault planes at about 1140 m and 1300 m in depth which are considered to be possible main fault planes in the 1995 kobe earthquake these two fault planes may not have played an important role of fault slip in the earthquake