changes in crack density and wave velocity in association with crack growth in triaxial tests of inada granite 1 a nondimensional second rank tensor fij called the crack tensor has successfully been introduced to deal with geometrical aspects of microcracks fabric such as anisotropy and crack density unfortunately however its usage for practical purposes is rather limited because its determination involves tedious and time consuming laboratory work we seek the possibility of using the directional change of longitudinal wave velocities to conquer the difficulty associated with the determination of crack tensors a new second rank tensor vij is introduced such that the directional change in the longitudinal wave velocities is represented in terms of the tensor and the crack tensor fij is then given as a function of vij on the basis of the analyses of the crack tensors for one intact and several damaged samples of inada granite we then discuss how microcracks grow through the whole inelastic process terminating at brittle failure the conclusions are summarized as follows the second rank symmetrical tensor vij or its inversion tensor vij 1 can be determined experimentally with sufficient accuracy from the directional change in the squared longitudinal wave velocity it is found that the tensor changes markedly so as to reflect the fabric of the damaged inada granite formed by open microcracks the principal axes of vij 1 are coaxial with the principal axes of fij so that both tensors are correlated in terms of their principal values fi and vi 1 four successive stages can be distinguished in regard to the crack growth as follows in stage 1 the rock behaves like an elastic solid in stage 2 microcracks start to grow so that inelastic volumetric strain is slowly accumulated along with microcracking however crack growth does not occur globally but rather is limited within some local zones probably in each grain in stage 3 microcracking is considerably accelerated suggesting that the micromechanism leading to crack growth changes substantially at the boundary stress between stages 2 and 3 in stage 4 the crack density as well as the dilatancy increases explosively in association with a drop of a few percent in the differential stress after the peak stress is reached interestingly this explosive increase is always associated with the development of a few fault zones experimental evidence seems to support the postulate that inada granite starts to collapse once the crack density f0 f the first invariant of fij attains a threshold value of 7 8 regardless of the applied confining pressure this can be a failure criterion in terms of the crack density and be an extended expression for the so called critical dilatancy for creep failure suggested by kranz and scholz 1977

sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

pore scale modeling of elastic wave propagation in carbonate rocks abstractthe relationship between p wave velocity and porosity in carbonate rocks shows a high degree of variability due to the complexity of the pore structure this variability introduces high uncertainties to seismic inversion amplitude variation with offset analysis porosity estimation and pore pressure prediction based on velocity data elastic wave propagation in porous media is numerically modeled on the pore scale to investigate the effects of pore structure on p wave velocities in carbonate rocks we built 2d models of porous media using pore structure information and the similarity principle then we simulated normal incidence wave propagation using finite element analysis finally the velocity was determined from received modeled signals by means of crosscorrelation the repeatability and accuracy of this modeling process was verified carefully based on the modeling results a simple formulation of sun s frame flexibility factor k aspect ratio ar the ratio of the major axis to the mi

on the magnetocrystalline anisotropy of greigite fe3s4 the ferrimagnetic mineral greigite cubic fe3s4 is well known as an intracellular biomineralization product in magnetic bacteria and as a widely occurring authigenic mineral in anoxic sediments due to the lack of suitable single crystal specimens the magnetic anisotropy parameters of greigite have remained poorly constrained to the point where not even the easy axis of magnetization is known here we report on an effort to determine the anisotropy parameters on the basis of ferromagnetic resonance fmr powder spectroscopy on hydrothermally synthesized chemically pure greigite microcrystals dispersed in a nonmagnetic matrix in terms of easy axis orientations the fmr data are consistent with or or less likely a more general type with a g factor of 2 09 the anisotropy field is about 90 mt and in some samples may reach 125 mt compared to 30 mt for cubic magnetite this confirms the dominating role of cubic anisotropy on the magnetic properties of greigite which we show to be responsible for large sirm k values k1 is in the range 15 23 j m3 or 10 15 kj m3 yielding upper limits of 44 or 34 nm for the superparamagnetic grain size respectively

low altitude aeromagnetic reconnaissance for petroleum in the arctic national wildlife refuge alaska variations in the earth s magnetic field arising from local concentrations of shallow subsurface magnetite were mapped in the arctic national wildlife refuge and elsewhere in northern alaska the anomalies were delineated with a magnetic horizontal gradiometer mounted on a low flying 300 ft or 90 m above ground fixed wing airplane limited data from stable carbon isotope and remanent magnetism measurements of rock cores from the cape simpson region strongly suggest that the magnetic anomalies result from the chemical reduction of iron oxides in the presence of seeping hydrocarbons the magnetic contrast between sedimentary rocks of normally low magnetic susceptibility and those locally enriched with this epigenetic magnetite results in distinctive high wavenumber and low amplitude total field anomalies magnetometers extended from each wing tip and in a tail stinger permit calculation of the resultant horizontal gradient vector relative to the flight path this calculation allows more meaningful interp

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

critical single domain grain sizes in chains of interacting greigite particles implications for magnetosome crystals magnetotactic bacteria contain chains of magnetically interacting crystals magnetosomes which aid navigation magnetotaxis to improve the efficiency of magnetotaxis magnetosome crystals which can consist of magnetite or greigite should be magnetically stable single domain sd particles larger particles subdivide into nonuniform multidomain md magnetic structures that produce weaker magnetic signals while small sd particles become magnetically unstable due to thermal fluctuations and exhibit superparamagnetic sp behavior in this study we determined the stable sd range as a function of grain elongation and interparticle separation for chains of identical greigite grains using fundamental parameters recently determined for greigite interactions significantly increase the stable sd range for example for cube shaped greigite grains the upper stable sd threshold size is increased from 107 nm for isolated grains to 204 nm for touching grains arranged in chains the larger critical sd grain size for greigite means that compared to magnetite magnetosomes greigite magnetosomes can produce larger magnetic signals without the need for intergrain interactions

magnetic properties of sedimentary greigite fe3s4 an update greigite fe3s4 is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction it can also grow at any time when dissolved iron and sulfide are available during diagenesis greigite is important in paleomagnetic environmental biological biogeochemical tectonic and industrial processes much recent progress has been made in understanding its magnetic properties greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites the crystallographic c axis is the easy axis of magnetization with magnetic properties dominated by magnetocrystalline anisotropy robust empirical estimates of the saturation magnetization anisotropy constant and exchange constant for greigite have been obtained recently for the first time and the first robust estimate of the low field magnetic susceptibility is reported here the curie temperature of greigite remains unknown but must exceed 350 c greigite lacks a low temperature magnetic transition on the basis of preliminary micromagnetic modeling the size range for stable single domain behavior is 17 200 nm for cubic crystals and 17 500 nm for octahedral crystals gradual variation in magnetic properties is observed through the pseudo single domain size range we systematically document the known magnetic properties of greigite at high ambient and low temperatures and with alternating and direct fields and illustrate how grain size variations affect magnetic properties recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records including widespread remagnetizations

ultrasonic velocity and anisotropy of hydrocarbon source rocks an experimental study of the physical properties of black kerogen rich shales also including maturation analysis scanning electron microscope sem observations and physical modeling revealed fairly peculiar petrophysical parameters specifically these rocks have very low porosity and density but most importantly both p and s ultrasonic velocities normal to bedding are extremely low whereas they are much higher parallel to bedding giving rise to a strong anisotropy even at high confining pressures we found that these parameters primarily reflect kerogen content microstructure and maturation level of these rocks we found also that microcracks inferred from ultrasonic velocity measurements occur only in mature shales these microcracks are parallel to the bedding plane and further enhance strong intrinsic anisotropy notably at low effective pressure our results show that on a small scale kerogen rich shales are transversely isotropic rocks and can be effectively modeled using the thin laye

reductive diagenesis magnetite dissolution greigite growth and paleomagnetic smoothing in marine sediments a new view abstract in many anoxic sedimentary environments the onset of sulfate reduction and pyritization of detrital iron bearing minerals leads to a precipitous decline in magnetic mineral concentration during early diagenesis the usefulness of the surviving paleomagnetic record in such environments is usually argued to depend on how much of the primary detrital magnetic assemblage survives diagenetic dissolution detailed rock magnetic and electron microscope analyses of rapidly deposited 7 cm kyr latest pleistocene holocene sediments from the continental margins of oman 22 22 4 n 60 08 0 e and northern california 38 24 8 n 123 58 2 w demonstrate that pyritization during early diagenesis also leads to the progressive down core growth of the ferrimagnetic iron sulfide greigite greigite growth begins with nucleation of large concentrations of superparamagnetic sp nanoparticles at the inferred position of the sulfate methane transition which can explain the apparently paradoxical suggestion that diagenetically reduced sediments contain enhanced concentrations of sp particles looping of hysteresis parameters on a day plot records the dissolution of single domain sd titano magnetite and the formation of sp greigite which then slowly and progressively grows through its sd blocking volume and acquires a stable paleomagnetic signal this looping trend is also evident in data from several published records oregon margin korea strait japan sea niger fan argentine margin and the ontong java plateau indicating that these processes may be widespread in reducing environments our observations have profound implications for paleomagnetic records from sulfate reducing environments the paleomagnetic signal recorded by greigite is offset from the age of the surrounding sediments by 10 s of kyr and ongoing growth of greigite at depth results in smoothing of the recorded signal over intervals of 10 s to 100 s of kyr we therefore expect the presence of greigite to compromise paleomagnetic records in a wide range of settings that have undergone reductive diagenesis

stability of equidimensional pseudo single domain magnetite over billion year timescales interpretations of paleomagnetic observations assume that naturally occurring magnetic particles can retain their primary magnetic recording over billions of years the ability to retain a magnetic recording is inferred from laboratory measurements where heating causes demagnetization on the order of seconds the theoretical basis for this inference comes from previous models that assume only the existence of small uniformly magnetized particles whereas the carriers of paleomagnetic signals in rocks are usually larger nonuniformly magnetized particles for which there is no empirically complete thermally activated model this study has developed a thermally activated numerical micromagnetic model that can quantitatively determine the energy barriers between stable states in nonuniform magnetic particles on geological timescales we examine in detail the thermal stability characteristics of equidimensional cuboctahedral magnetite and find that contrary to previously published theories such nonuniformly magnetized particles provide greater magnetic stability than their uniformly magnetized counterparts hence nonuniformly magnetized grains which are commonly the main remanence carrier in meteorites and rocks can record and retain high fidelity magnetic recordings over billions of years

regional to global evolution of impacts of parameterized mountain wave drag in the lower stratosphere abstractmountain ranges are regional features on earth as are the regions of mountain wave drag mwd exerted by dissipating atmospheric gravity waves generated by flow over them however these r

azimuthally dependent anisotropic velocity model update abstractwe consider a case where a 3d depth migration has been performed in the local angle domain lad using rich azimuth seismic data e g conventional land surveys the subsurface geologic model is characterized by considerable azimuthally anisotropic velocity variations the background velocity field used for the migration can consist of azimuthally independent e g vertical transverse isotropy and or azimuthally dependent e g orthorhombic velocity layers the resulting 3d full azimuth reflection angle gathers generated by the lad migration represent in situ high resolution amplitude preserved reflectivities associated with opening angles between incident and reflected slowness vectors in the specular directions residual moveouts rmos automatically picked on these 3d image gathers along major horizons can indicate considerable residual periodic azimuthal variations this situation is typical in depth imaging applied to unconventional shale plays where the background velocity model does

three dimensional micromagnetic calculations for naturally shaped magnetite octahedra and magnetosomes abstract a three dimensional micromagnetic algorithm for modelling naturally shaped particles is presented it uses a fft accelerated calculation of the demagnetizing energy exchange energy calculation directly takes into account the micromagnetic boundary condition a rectangular grid enclosing the particle is chosen to economically fit the shape after verifying that the new program reliably reproduces previous calculations the program is used to determine room temperature magnetization structures and single domain to vortex transition sizes these calculations are performed for octahedral magnetite and magnetosomes of different elongations the least energy magnetization structures of octahedral particles are similar to the flower and vortex states found for cubic particles the critical size d 0 above which an inhomogeneous magnetization structure has lower energy than the homogeneous flower state as well as the grain size dependence of m rs m s closely resemble cubic particles however meta stable flower states persist up to much larger grain size in octahedral magnetite this explains previous observations of large trm arm peaks in hydrothermal magnetite in realistic magnetosome geometries the sd psd transition is shifted towards larger grain sizes as compared to previous estimates based on elongated rectangular grains for substantially elongated grains this is a result of the alignment of the particles long edge with the crystallographic easy axis for more equidimensional grains the rounded ends of magnetosomes reduce spin deflection at the top and bottom edges thereby the formation of nucleation centers is effectively inhibited even without additional stabilization by magnetostatic interaction within a magnetosome chain

putative greigite magnetofossils from the pliocene epoch greigite crystals of bacterial origin are widespread in modern sedimentary environments but their occurrence in the fossil record remains controversial grains from romanian pliocene aged sediments have now been identified as bacterial in origin tentatively placing them among the oldest known greigite magnetofossils

75 plus years of anisotropy in exploration and reservoir seismics a historical review of concepts and methods the idea that the propagation of elastic waves can be anisotropic i e that the velocity may depend on the direction is about 175 years old the first steps are connected with the top scientists of that time people such as cauchy fresnel green and kelvin for most of the 19th century anisotropic wave propagation was studied mainly by mathematical physicists and the only applications were in crystal optics and crystal elasticity during these years important steps in the formal description of the subject were made at the turn of the 20th century rudzki stressed the significance of seismic anisotropy he studied many of its aspects but his ideas were not applied research in seismic anisotropy became stagnant after his death in 1916 beginning about 1950 the significance of seismic anisotropy for exploration seismics was studied mainly in connection with thinly layered media and the resulting transverse isotropy very soon it became clear that the effect of layer induced anisotropy on data ac

elastic anisotropy of core samples from the taiwan chelungpu fault drilling project tcdp direct 3 d measurements and weak anisotropy approximations summary the study of seismic anisotropy has become a powerful tool to decipher rock physics attributes in reservoirs or in complex tectonic settings we compare direct 3 d measurements of p wave velocity in 132 different directions on spherical rock samples to the prediction of the approximate model proposed by louis et al based on a tensorial approach the data set includes measurements on dry spheres under confining pressure ranging from 5 to 200 mpa for three sandstones retrieved at a depth of 850 1365 and 1394 metres in tcdp hole a taiwan chelungpu fault drilling project as long as the p wave velocity anisotropy is weak we show that the predictions of the approximate model are in good agreement with the measurements as the tensorial method is designed to work with cylindrical samples cored in three orthogonal directions a significant gain both in the number of measurements involved and in sample preparation is achieved compared to measurements on spheres we analysed the pressure dependence of the velocity field and show that as the confining pressure is raised the velocity increases the anisotropy decreases but remains significant even at high pressure and the shape of the ellipsoid representing the velocity or elastic fabric evolves from elongated to planar these observations can be accounted for by considering the existence of both isotropic and anisotropic crack distributions and their evolution with applied pressure

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

estimation of effective anisotropy simultaneously with locations of microseismic events passive seismic tomography in which the event locations and the velocity model are inferred simultaneously is seldom used to process microseismic surveys acquired in the oil and gas industry we discuss advantages of applying tomographic ideas to typical microseismic data recorded in a single nearly vertical well to monitor hydraulic stimulation of a shale gas reservoir microseismic events are conventionally located in the energy industry applications using a velocity model derived from sonic logs and perforation shots instead of fixing the model as is normally done we alter it while locating the events this added flexibility not only makes it possible to accurately predict traveltimes of the recorded p and s waves but also provides a convincing evidence for anisotropy of the examined shale formation while we find that velocity heterogeneity does not need to be introduced to explain the data acquired at each stage of hydraulic fracturing the obtained models are suggestive of possible time laps

stress induced seismic azimuthal anisotropy sand shale content and depth trends offshore north west australia abstractseismic azimuthal anisotropy is apparent when p wave velocities vary with source receiver azimuth and downward propagating s waves split into two quasi s waves polarized in orthogonal directions not accounting for these effects can degrade seismic image quality and result in erroneous amplitude analysis and geologic interpretations there are currently no physical models available to describe how azimuthal anisotropy induced by differential horizontal stress varies with sand shale lithology and depth we develop a model that does so in unconsolidated sand shale sequences offshore north west australia our method naturally introduces two new concepts critical anisotropy and anisotropic depth limit critical anisotropy is the maximum amount of azimuthal anisotropy expected to be observed at the shallowest sediment burial depth where the confining pressure and sediment compaction are minimal the anisotropic depth limit is the maximum depth where the stress induced azimuthal anisotropy is ex

impact of fabric microcracks and stress field on shale anisotropy summary few data are available on shales in terms of seismic to ultrasonic properties and anisotropy although all are important with regards to imaging problems often encountered in such lithologies additionally mechanisms causing changes in these properties are not well documented due to the fine grain size of such materials and time required for testing under controlled pore pressure conditions the results presented here are derived from a set of experiments run on muderong shale with pore pressure control in order to evaluate the effect of stress magnitude and stress anisotropy on ultrasonic response this shale was noted to have a linear velocitymean effective stress response and extremely high anisotropy both likely the result of the presence of fluid filled cracks in a low permeability porous medium changes in velocity and vp vs ratios are dependent on both stress and smectite content s wave velocity is significantly affected by the presence of smectite in this and other shales and at low stress 20 mpa is less sensitive to stress change than p wave velocity vp vs ratios are noted to increase in this shale up to 20 mpa effective stress then decrease slightly due to stress induced loss of interlayer water in smectite intrinsic anisotropy comes from composition a strong compaction fabric and the presence of microfractures changes to ultrasonic anisotropy are the result of the magnitude of the stresses their orientation with respect to the fractures and the degree of stress anisotropy

seismic anisotropy in sedimentary rocks part 2 laboratory data part one of this paper presents a method for measuring seismic velocities and transverse isotropy in rocks using a single core plug this method saves at least two thirds of the time for preparing core samples and measuring velocities in transversely isotropic ti rocks using this method we have measured velocity and anisotropy of many shale and reservoir rocks from oil and gas fields around the world we present some of the data in this paper which include seismic velocity and anisotropy in 17 brine saturated shale samples 1 gas and brine saturated coal sample 8 brine saturated sands 12 gas saturated sands 32 gas saturated carbonate samples and 25 brine saturated carbonate samples the results show that clays and fine layering in sedimentary rocks are the main causes of seismic anisotropy very little intrinsic anisotropy exists in unfractured reservoir rocks such as sands sandstones and carbonates under reservoir conditions in contrast all shales were found seismically anisotropic anisotr

seismic anisotropy in sedimentary rocks part 1 a single plug laboratory method a single plug method for measuring seismic velocities and transverse isotropy in rocks has been rigorously validated and laboratory tested the method requires only one sample to measure the velocities needed to derive the five independent elastic constants for transversely isotropic materials in this method piezoelectric transducers are fitted to the top bottom and sides of the cylindrical sample laboratory velocity and anisotropy can be measured as functions of pressure temperature fluid saturation and fluid displacement because this method uses a horizontal core plug that has much higher permeability than a vertical core plug it is especially suitable for low permeability shale measurements it reduces the sample preparation and velocity measurement time by more than two thirds

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

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

magnetite dissolution diachronous greigite formation and secondary magnetizations from pyrite oxidation unravelling complex magnetizations in neogene marine sediments from new zealand detailed rock magnetic and electron microscope analyses indicate that the magnetic signature of neogene marine sediments from the east coast of new zealand is dominated by the authigenic iron sulphide greigite the greigite is present as a mixed population of stable single domain and superparamagnetic grains which is consistent with authigenic growth from solution this growth can result from pyritization reactions soon after deposition which also leads to dissolution of most detrital magnetite however where constrained by field tests our data suggest that remanence acquisition can occur 1 myr after deposition and can vary in timing at the outcrop scale strong viscous overprints result from oxidation of the iron sulphides probably during percolation of oxic ground water this process can sometimes destroy any ancient remanent magnetization this complex magnetic behaviour particularly the presence of late forming magnetizations carried by greigite means that the remanence in new zealand cenozoic sediments and in similar sediments elsewhere cannot be assumed to be primary without confirmation by field tests the reversals test should be employed with caution in such sediments as patchy remagnetizations can lead to false polarity stratigraphies

geostatistical traveltime tomography in elliptically anisotropic media in geological materials anisotropy may arise due to different mechanisms and can be found at different scales neglecting anisotropy in traveltime tomographic reconstruction leads to artefacts that can obscure important subsurface features in this paper a geostatistical tomography algorithm to invert cross hole traveltime data in elliptically anisotropic media is presented the advantages of geostatistical tomography are that the solution is regularized by the covariance of the model parameters that known model parameters can be used as constraints and fitted exactly or within a prescribed variance and that stochastic simulations can be performed to appraise the variability of the solution space the benefits of the algorithm to image anisotropic media are illustrated by two examples using synthetic georadar data and real seismic data the first example confirms suspected electromagnetic anisotropy in the vadose zone caused by relatively rapid water content variations with respect to wavelength at georadar frequencies the second presents how sonic log data can be used to constrain the inversion of cross well seismic data and how geostatistical simulations can be used to infer parameter uncertainty results of both examples show that considering anisotropy yields a better fit to the data at high ray angles and reduces reconstruction artefacts

ultrasonic velocities in cretaceous shales from the williston basin compressional and shear wave velocities were measured in the laboratory from 1 bar to 4 kbar confining pressure for wet undrained samples of cretaceous shales from depths of 3200 and 5000 ft in the williston basin north dakota these shales behave as transversely isotropic elastic media the plane of circular symmetry coinciding with the bedding plane for compressional waves the velocity is higher for propagation in the bedding plane than at right angles to it and the anisotropy is greater for the 5000 ft shale for shear waves the sh wave perpendicular to bedding and the sv wave parallel to bedding propagate with the same speed which is about 25 percent lower than that for the sh wave parallel to bedding in general compressional and shear velocities are higher for the indurated 5000 ft shale than for the friable 3200 ft shale all velocities increase with in increasing confining pressure to 4 kbar the 3200 ft shale exhibits velocity hysteresis as a function of pressure whereas this effect is a

characteristics of anisotropy and dispersion in cracked medium abstract in upper crustal conditions where effective pressure is not high enough to close them all rocks contain cracks understanding the effect of cracks on both the elastic anisotropy and the elastic waves frequency dependence dispersion is of major interest to underground storage monitoring reservoir monitoring and fault mechanics the challenge is to extract from geophysical data the maximum information with a minimal number of assumptions and model parameters this is possible if reference data exist at the laboratory scale and if extrapolation from laboratory to field scale is not biased in order to check that investigating few basic idealized cases can provide useful guidelines let us consider an isotropic rock matrix and assume that dispersion is the result of squirt flow in this paper we will not consider crack to pore but rather focus on crack to crack squirt flow three basic cracks configurations are quantitatively investigated for which the elastic symmetry is transverse isotropy ti our aim is to check if from first principles high frequency laboratory data can be used for field data interpretation in these three cases the first case is that of identical cracks aligned in the same direction a sufficiently strong deviatoric stress field in a given plane is expected to induce such a distribution at the field scale 1 2 3 we show that it results in a maximum anisotropy but with virtually no dispersion because there is no dispersion high frequency data obtained in the laboratory mhz ultrasonic range are directly applicable to field data hz to khz range interpretation thomsen parameters provide then a direct proxy for crack density a second case is that of identical cracks with random orientations isotropic distribution such a situation could exist at the field scale if cracks result from the superposition of several independent tectonic episodes associated with different directions of principal stresses in that case 1 2 3 this configuration could also result from thermal cracking in an initially isotropic metamorphic rock obviously there is no overall anisotropy then but we show that dispersion may be large due to dispersion ultrasonic laboratory data are not directly applicable to field data interpretation unless the medium is dry a third intermediate case is that of cracks randomly distributed in zone with a given axis the corresponding symmetry is that of a so called axi symmetric triaxial test which in fact is biaxial it is of interest because it represents a classical laboratory experiment situation it can also exist at the field scale if one principal stress is much larger than both others 1 2

the midlatitude lower stratospheric mountain wave valve layer abstractthe vertical propagation and attenuation of mountain waves launched by new zealand terrain during the deep propagating gravity wave experiment deepwave field campaign are investigated new zealand mountain waves were frequently attenuated in a lower stratospheric weak wind layer between z 15 and 25 km this layer is termed a valve layer as conditions within this layer primarily minimum wind speed control mountain wave momentum flux through it analogous to a valve controlling mass flux through a pipe this valve layer is a climatological feature in the wintertime midlatitude lower stratosphere above the subtropical jet mountain wave dynamics within this valve layer are studied using realistic weather research and forecasting wrf model simulations that were extensively validated against research aircraft radiosonde and satellite observations locally wave attenuation is horizontally and vertically inhomogeneous evidenced by numerous regions with wave induced low richardson numbers an

magnetic vortex effects on first order reversal curve forc diagrams for greigite dispersions abstract first order reversal curve forc diagrams are used increasingly in geophysics for magnetic domain state identification the domain state of a magnetic particle is highly sensitive to particle size about which forc diagrams provide valuable information however the forc signal of particles with nonuniform magnetisations which are the main carrier of natural remanent magnetisations in many systems is still poorly understood in this study the properties of non interacting randomly oriented dispersions of greigite fe3s4 in the uniform single domain sd to non uniform single vortex sv size range are investigated via micromagnetic calculations signals for sd particles 50 nm are found to be in excellent agreement with previous sd coherent rotation studies a transitional range from 50 nm to 80 nm is identified for which a mixture of sd and sv behaviour produces complex forc diagrams particles 80 nm have purely sv behaviour with the remanent state for all particles in the ensemble in the sv state it is found that for sv ensembles the forc diagram provides a map of vortex nucleation and annihilation fields and that the forc distribution peak should not be interpreted as the coercivity of the sample but as a vortex annihilation field on the path to saturation

gyroremanent magnetization and the magnetic properties of greigite bearing clays in southern sweden summary the acquisition of a gyroremanent magnetization grm by single domain sd greigite particles during alternating field af demagnetization is demonstrated previous palaeomagnetic studies failed to identify the presence of authigenic greigite in the glacio marine clays studied these clays formed the subject of an earlier debate about the validity of a late weichselian geomagnetic excursion the gothenburg flip in southern sweden the greigite carries a stable chemical remanent magnetization crm which coexists with a detrital remanent magnetization drm carried by magnetite af demagnetization could not isolate the primary remanence in the sediments where magnetite and greigite coexist due to the overlapping coercivity spectra of the two minerals and the inability to determine the time lag between sediment deposition and crm formation thermal demagnetization removed the crm at temperatures below 400 dc but this method was hindered by the unconsolidated nature of the sediments and the formation of secondary magnetic minerals at higher temperatures the results suggest that the low coercivity drm carried by magnetite was mistaken for a viscous component in the earlier studies hence the former debate about the record of the gothenburg flip may have been based on erroneous palaeomagnetic interpretations or non reproducible results af demagnetization procedures applied to samples suspected of bearing sd magnetic particles such as greigite should be carefully selected to recognize and account for grm acquisition

alleviation of a systematic westerly bias in general circulation and numerical weather prediction models through an orographic gravity wave drag parametrization systematic westerly biases in the northern hemisphere wintertime flow of the meteorological office 15 layer operational model and 11 layer general circulation model are described evidence that the failure to parametrize subgrid scale orographic gravity wave drag may account for such biases is presented this evidence is taken from aircraft studies surface pressure drag measurements and studies of the zonally averaged momentum budget a parametrization scheme is described in which the surface stress is proportional to the near surface wind speed and static stability and to the variance of subgrid scale orography the stress is absorbed in the vertical by considering the influence of such gravity wave activity on static stability and vertical wind shear a richardson number dependent wave breaking formulation is devised and the vertical stress profile determined by a saturation hypothesis whereby the breaking waves are maintained at marginal stability it is shown that wave breaking preferentially occurs in the boundary layer and in the lower stratosphere results from a simple zonally symmetric model show how the adjustment to thermal wind balance with a wave drag in the stratosphere warms polar regions by adiabatic descent and decelerates the mean westerlies in the troposphere the influence of the parametrization scheme on integrations of the 11 layer model is described and found to be generally beneficial in a discussion of the reasons why this problem has only recently emerged it is suggested that the satisfactory northern hemisphere winter circulations of previous coarser general circulation models were due to a compensation implied by underestimating both the surface drag and the horizontal flux of momentum by explicitly resolved large scale eddies

comparison between analytical and 3d finite element solutions for borehole stresses in anisotropic elastic rock we present a rigorous validation of the analytical amadei solution for the stress concentration around an arbitrarily orientated borehole in general anisotropic elastic media first we revisit the theoretical framework of the amadei solution and present analytical insights that show that the solution does indeed contain all special cases of symmetry contrary to previous understanding provided that the reduced strain coefficients b11 and b55 are not equal it is shown from theoretical considerations and published experimental data that the b11 and b55 are not equal for realistic rocks second we develop a 3d finite element elastic model within a hybrid analytical numerical workflow that circumvents the need to rebuild and remesh the model for every borehole and material orientation third we show that the borehole stresses computed from the numerical model and the analytical solution match almost perfectly for different borehole orientations vertical deviated and horizontal and for several cases involving isotropic transverse isotropic and orthorhombic symmetries it is concluded that the analytical amadei solution is valid with no restriction on the borehole orientation or the symmetry of the elastic anisotropy

microcrack induced versus intrinsic elastic anisotropy in mature hc source shales recent experimental studies of ultrasonic velocity anisotropy in kerogen rich shales indicate that these rocks are characterized by a very strong anisotropic response related to a very fine bedding parallel lamination of organic matter and preferred orientation of clay particles in the rock matrix vernik and nur 1992 this intrinsic anisotropy is further enhanced in thermally mature shales by bedding parallel microcracks caused by the processes of hydrocarbon generation vernik paper in preparation however the potential of recognizing mature source rocks in situ using downhole sonic crosshole seismic or vsp data clearly depends on our ability to discriminate between these two major causes of elastic anisotropy remove the effect of the intrinsic anisotropy and estimate crack density and crack porosity from velocity measurements

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

formation of magnetic minerals at hydrocarbon generation conditions in this paper we report the pyrolysis and formation of magnetic minerals in three source rock samples from the wessex basin in dorset southern england the experimental conditions in the laboratory recreated the catagenesis environment of oil source rocks magnetic analysis of both the heated and the unheated samples at room temperature and at very low temperatures 5 k coupled with transmission electron microscopy imaging and x ray analysis revealed the formation of nanometre sized 10 nm magnetic particles that varied across the rock samples analysed but more importantly across the pyrolysis temperature range magnetic measurements demonstrated the formation of these magnetic minerals peaked at 250 c for all rock samples and then decreased at 300 c before rising again at 320 c the newly formed magnetic minerals are suggested to be primarily pyrrhotite though magnetite and greigite are also thought to be present the sizes of the magnetic minerals formed suggest a propensity to migrate together with oil potentially explaining the magnetic anomalies observed above and within oil fields

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

magnetic domain state and coercivity predictions for biogenic greigite fe3s4 a comparison of theory with magnetosome observations the discovery of bacteria that precipitate greigite within intracellular organelles magnetosomes offers new evidence about the origin of greigite in natural environments unlike magnetite only scarce information is available about the magnetic characteristics of greigite for this reason and the present inability to grow these microorganisms in pure culture it is not known whether or not the magnetosomes in the newly discovered greigite precipitating bacteria are of single domain sd size as are the magnetosomes from magnetite precipitating bacteria the hypothesis of natural selection for magnetotactic behavior predicts that the greigite bearing magnetosomes should also be single magnetic domains using previously reported magnetic properties and crystallographic features for greigite we have calculated the size and shape boundaries expected for sd and superparamagnetic spm behavior in this mineral for further characterization of the greigite crystals we analyzed the domain state at various length width ratios assuming crystal shapes of parallelepipeds and prolate spheroids magnetite was used as control for the current theories supporting these calculations we also present a simple algorithm to calculate the upper size limit of single domain grains our results show that the crystals of bacterial greigite characterized so far are located in the region close to the single domain superparamagnetic boundary and should have relatively low coercivity if these crystals contribute to the magnetization of sediments remanence produced by bacterial greigite could be mistaken for large multidomain magnetite in alternating field demagnetization studies

sulfidization and magnetization above hydrocarbon reservoirs geochemical and rock magnetic studies of strata over cement oil field anadarko basin oklahoma simpson oil field north slope basin alaska and the edwards deep gas trend south texas coastal plain document changes in original magnetizations caused by postdepositional iron sulfide minerals that are or may be related to hydrocarbon seepage at cement ferrimagnetic pyrrhotite fe sub 7 s sub 8 formed with pyrite and marcasite in permian red beds the fe s minerals contain isotopically heavy abiogenic sulfur derived from thermal degradation of petroleum and or isotopically light sulfur derived from sulfate reducing bacteria fed by leaking hydrocarbons at simpson ferrimagnetic greigite fe sub 3 s sub 4 dominates magnetizations in upper cretaceous nonmarine beds that contain biodegraded oil sulfur isotopic data are consistent with but do not prove a genetic link between the greigite delta sup 34 s gt 20 per mil and seepage in middle tertiary sandstones of southeast texas pyrite and marcasite formed when abiogenic h sub 2 s migrated upward from deep reservoirs or when h sub 2 s was produced at shallow depths by bacteria that utilized organic material dissolved in migrating water from depth the sulfide minerals replaced detrital magnetite to result in a systematic decrease in magnetic susceptibility toward faults that connect deep petroleum reservoirs tomore shallow sandstone the authors results show that abiologic and biologic mechanisms can generate magnetic sulfide minerals in some sulfidic zones of hydrocarbon seepage the magnetizations in such zones are diminished most commonly by replacement of detrital magnetic minerals with nonmagnetic sulfide minerals or are unchanged if such detrital minerals were originally absent less

the magnetic structure and palaeomagnetic recording fidelity of sub micron greigite fe3s4 abstract we present the results of a finite element micromagnetic model of 30 nm to 300 nm greigite fe3s4 grains with a variety of equant morphologies this grain size range covers the magnetic single domain sd to pseudo single domain psd transition and possibly also the psd to multi domain md transition the sd psd threshold d 0 is determined to be 50 nm d 0 56 nm depending on grain shape the nudged elastic band method was used to determine the room temperature energy barriers between stable states and thus the blocking volumes it is found that in the absence of interparticle magnetostatic interactions the magnetisation of equant sd greigite is not stable on a geological scale and only psd grains 70 nm can be expected to carry a stable magnetisation over billion year timescales i e all non interacting sd particles are essentially superparamagnetic we further identify a mechanism for the psd to multi domain md transition which is of a continuous nature from psd nucleation up to 300 nm when structures typical of md behaviour like closure domains begin to form

dispersion and anisotropy of elastic waves in cracked rocks 1 rocks in the earth s crust contain variable amount of cracks depending on the deviatoric and confining stress levels pore pressure and temperature conditions crack damage results in effects that have been investigated for a long time in particular a decrease in elastic wave velocity and the development of anisotropy in this paper we focus on cracked rocks and develop a method to calculate both the elastic wave anisotropy and the dispersion in a fluid saturated cracked rock we show that analytic expressions can be derived for both properties in terms of the crack density tensor components two fundamental theoretical tools are used in this purpose the first one is the effective medium theory and the second one is the anisotropic poroelastic theory using both it is possible from first principles to predict anisotropy and dispersion results are shown to be valid up to a total crack density of 0 5 the failure threshold appears close to a total crack density of 1 between 0 5 and 1 the model still gives reasonably good results anisotropy can increase up to 60 and dispersion up to 30 depending on the crack distribution these results point out important differences that could exist in fluid saturated rocks between laboratory high frequency data and field low frequency results

seismic anisotropy of shales the seismic properties and preferred clay mineral orientation of a suite of shales are investigated using laboratory velocity measurements as a function of confining pressure x ray diffraction techniques and electron microprobe backscatter bse imaging the velocity measurements indicate that these shales are transversely isotropic with the main symmetry axis perpendicular to bedding anisotropy at elevated pressures caused mainly by preferred orientation of clays illite parallel to bedding ranges from 20 vp and 19 vs for a sample of new albany shale to 30 vp and 35 vs for a sample of chattanooga shale the degree of clay mineral alignment in the shales is constrained by orientation indices produced using simple x ray diffraction techniques a strong positive correlation is found between the degree of preferred orientation as expressed in the orientation indices and seismic anisotropy bse images of the shale fabrics confirm in a qualitative manner the results of the x ray study to investigate wave propagation in the shales elastic constants of each sample are calculated and used to produce phase and group wave velocity surfaces which describe variation in velocity as a function of angle to the bedding normal the calculated velocity surfaces constrained by independent velocity measurements display a lack of shear wave splitting at near normal incidence in even the most anisotropic shales for the highly anisotropic chattanooga shales group velocity surfaces differ significantly from corresponding phase velocity surfaces

hybrid fourier finite difference 3d depth migration for anisotropic media when a subsurface is anisotropic migration based on the assumption of isotropy will not produce accurate migration images we develop a hybrid wave equation migration algorithm for vertical transversely isotropic vti media based on a one way acoustic wave equation using a combination of fourier finite difference ffd and finite difference fd approaches the hybrid method can suppress an additional solution that exists in the vti acoustic wave equation and it offers speed and other advantages over conventional ffd or fd methods alone the algorithm is tested on a synthetic model involving log data from onshore eastern saudi arabia including estimates of both intrinsic and layer induced vti parameters results indicate that vti imaging in this region offers some improvement over isotropic imaging primarily with respect to subtle structure and stratigraphy and to image continuity these benefits probably will be overshadowed by perennial land seismic data issues such as near surface distortions and multiples

diagenetic formation of greigite and pyrrhotite in gas hydrate marine sedimentary systems mineral magnetic results and electron microscope observations from gas hydrate bearing marine sediments cored at southern hydrate ridge during ocean drilling program leg 204 sites 1244 to 1252 cascadia margin offshore oregon demonstrate that authigenic greigite and pyrrhotite formed as a byproduct of microbially mediated diagenetic reactions in the sulphate the anaerobic oxidation of methane aom and the methanic gas hydrate zones geochemical conditions favourable for formation and preservation of greigite and pyrrhotite appear to be a limited source of sulphide whether it derives from microbially driven sulphate reduction in the sulphate zone in the aom zone or in deep sediments undergoing aom so that pyritization reactions are not driven to completion our results indicate that rock magnetic identification of greigite and pyrrhotite should be useful for detecting ancient gas hydrate systems in the marine sedimentary record because it can enable rapid screening of ancient sediments for potential horizons where methane and disseminated gas hydrates might have occurred formation of authigenic greigite and pyrrhotite at different depths within the gas hydrate stability zone also implies that the magnetization of the host sediments will have been acquired at variable times which is likely to compromise paleomagnetic results from greigite and pyrrhotite bearing marine sediments

transient mountain waves and their interaction with large scales abstract the impact of transient mountain waves on a large scale flow is examined through idealized numerical simulations of the passage of a time evolving synoptic scale jet over an isolated 3d mountain both the global momentum budget and the spatial flow response are examined to illustrate the impact of transient mountain waves on the large scale flow additionally aspects of the spatial response are quantified by potential vorticity inversion nearly linear cases exhibit a weak loss of domain averaged absolute momentum despite the absence of wave breaking this transient effect occurs because over the time period of the large scale flow the momentum flux through the top boundary does not balance the surface pressure drag moreover an adiabatic spatial redistribution of momentum is observed in these cases which results in an increase decrease of zonally averaged zonal momentum south north of the mountain for highly nonlinear cases the zonally averaged momentum field shows a region of flow de

diagenetic formation of ferrimagnetic iron sulphide minerals in rapidly deposited marine sediments south island new zealand abstract detailed magnetostratigraphic studies of late neogene siliciclastic sediments of the awatere group south island new zealand 41 45 s 174 05 e have revealed a wide range of palaeomagnetic behaviour examination of rock magnetic properties was undertaken using conventional palaeomagnetic techniques and thermomagnetic x ray diffraction and electron microprobe analyses these analyses indicate that the ferrimagnetic iron sulphide minerals greigite and pyrrhotite are responsible for a stable and intense magnetic remanence in fine grained sediments whereas titanomagnetite is the only remanence bearing mineral identified in coarser grained sediments which are less strongly and less stably magnetised than the fine grained sediments detrital titanomagnetite grains are likely to have undergone dissolution during early diagenesis as a result of iron sulphide formation which occurs commonly in rapidly deposited anoxic sediments that support active sulphate reduction and h 2 s formation preservation of greigite and pyrrhotite is inferred to result from the arrest of the pyritisation process probably due to the low permeability of the fine grained sediments and consumption of available h 2 s before full reaction to pyrite occurred relative palaeomagnetic instability and weak remanence intensities in coarser grained sediments is likely to be due to low titanomagnetite concentrations resulting from titanomagnetite dissolution

a laboratory study of seismic velocity and attenuation anisotropy in near surface sedimentary rocks the laboratory ultrasonic pulse echo method was used to collect accurate p and s wave velocity 0 3 and attenuation 10 data at differential pressures of 5 50 mpa on water saturated core samples of sandstone limestone and siltstone that were cut parallel and perpendicular to the vertical borehole axis the results when expressed in terms of the p and s wave velocity and attenuation anisotropy parameters for weakly transversely isotropic media q q show complex variations with pressure and lithology in general attenuation anisotropy is stronger and more sensitive to pressure changes than velocity anisotropy regardless of lithology anisotropy is greatest over 20 for velocity over 70 for attenuation in rocks with visible clay organic matter laminations in hand specimens pressure sensitivities are attributed to the opening of microcracks with decreasing pressure changes in magnitude of velocity and attenuation anisotropy with effective pressure show similar trends although they can show different signs positive or negative values of q q we conclude that attenuation anisotropy in particular could prove useful to seismic monitoring of reservoir pressure changes if frequency dependent effects can be quantified and modelled

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

widespread occurrence of greigite in the sediments of lake pannon implications for environment and magnetostratigraphy abstract lake pannon was a large lake at the southern margin of the european plate which separated from the central paratethys at the sarmatian pannonian boundary and was filled completely with sediment during the pliocene the separation produced an endemic fauna that makes classical geological correlation problematic with sediments outside the lake magnetostratigraphic correlation could potentially solve the problem and several long cores from deep drillings have been analysed for polarity information in the course of these studies primary magnetite was assumed to carry the natural remanent magnetization nrm later when outcrops of dark grey clays and marls of lake pannon were sampled for palaeomagnetic investigations of tectonic processes greigite was discovered as the carrier of the nrm at several localities this motivated a systematic study of the fine grained sediments of lake pannon in order to identify the principal carriers of the nrm to relate them to depositional environments and to assess the importance of neoformation of magnetic minerals the concentration of magnetic minerals is low in the fine grained sediments of lake pannon this made extraction of the magnetic minerals impossible so they were identified with magnetic methods magnetic experiments suggest that greigite is the most common magnetic mineral in the lake pannon sediments magnetite when it occurs is often accompanied by pyrite haematite is rare the studied sediments represent three time intervals 11 9 3 ma 8 7 ma and 6 4 ma in the 11 9 3 ma sediments greigite occurs as an original early diagenetic magnetic mineral during this period the climate was warm and humid the lake was fairly deep and stratified high organic productivity resulted in oxygen depleted bottom waters burial was rapid due to intensive erosion from the surrounding carpathian and alpine mountains magnetite bearing sediments deposited during this period do not carry a coherent palaeomagnetic signal which suggests that the magnetite was neoformed the 8 7 ma sediments were deposited in sublittoral upper delta plain and swamp environments in humid conditions they are also greigite bearing during the third investigated time period 6 4 ma the climate was arid the lake was of low salinity and was restricted to the southern part of the pannonian basin these sediments were deposited in a distal environment low sedimentation rate the characteristic magnetic mineral of this period is magnetite which most probably has a detrital origin widespread greigite which is always of diagenetic origin suspected neoformation of magnetite in the older parts of lake pannon and occasionally opposite polarities connected to greigite and magnetite when they occur together in the present study are important for magnetostratigraphic interpretation they imply that detailed magnetic mineralogical investigations and documentation of the consistency of the nrms by working on fully oriented samples are essential in magnetostratigraphic studies of molasse sediments

the impact of mountain waves on an idealized baroclinically unstable large scale flow abstractthe feedback of mountain waves and low level blocking on an idealized baroclinically unstable wave passing over an isolated ridge is examined through numerical simulation theoretical analy

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

structure and dynamics of an alpine potential vorticity banner a detailed analysis is undertaken of the primary alpine shear zone that occurred on 1 october 1999 during the mesoscale alpine programme this shear zone emanates from the south western alpine tip in north westerly flow conditions develops in response to the alpine scale flow splitting and separates the northerly mistral wind to the south west of the alps from the quasi stagnant air within the alpine wake the study is based on in situ flight level and dropsonde data from two research aircraft the data are used for the construction of an objective analysis in a cross section perpendicular to the shear zone and for diagnostic computations of the potential vorticity pv flux between vertically stacked flight legs the observed flow structure is compared with simulation results of the swiss model and the non hydrostatic canadian mesoscale compressible community model mc2 operating at horizontal resolutions of 14 and 3 km respectively immediately downstream of the topography the observed shear zone has a surprisingly quasi steady structure that becomes evident provided a slow westward migration is accounted for further downstream the shear zone shows increased signs of transience near the surface it has an overall width of 150 km but narrows to about 25 km near the top of the mistral inversion at a height of 2 km detailed analysis shows that the shear zone has a complex substructure consisting of at least three positive and three negative pv filaments these shear lines are characterized by a pronounced vertical coherence in the mixed layer and throughout most of the inversion layer one of the banners is also evident in low level tracer constituents diagnostic computations of the pv flux along the banners using the generalized bernoulli theorem reveal that most of the associated pv flux is due to vertically oriented vorticity within the inversion layer within the shear zone there is a spanwise circulation of appreciable strength it consists of a deformation flow with the axes of deformation oriented perpendicular to the shear zone this deformation flow in effect squeezes the pv banners and thereby makes them resilient with respect to dissipation and barotropic instabilities detailed intercomparison with numerical model results shows that models with a grid spacing below 15 km are able to capture the overall structure of the shear zone the mc2 model in addition credibly replicates some of the finer scale sub structure of the shear zone in particular at low levels the high predictability and steadiness of the pv banners supports the idea that individual banners are generated by flow separation and or gravity wave breaking events upstream it is argued that the overall width of the shear zone is thus in effect determined by the geometry of the upstream topography and the dynamical processes that govern the downstream advection of the pv banners copyright 2003 royal meteorological society

anisotropy estimate for the horn river basin from sonic logs in vertical and deviated wells abstract shale anisotropy must be quantified to obtain reliable information about reservoir fluids lithology and pore pressure from seismic data failure to account for anisotropy in seismic processing can lead to errors in normal moveout nmo dip moveout dmo migration time to depth conversion and amplitude variation with offset avo analysis kriged estimates of density vertical compressional wave and shear wave velocities were derived from vertical wells in an area of interest in the horn river resource play in northeastern british columbia the kriged predictions were compared with measured logs along the trajectory of a deviated well whereas the density comparison provided a blind test of kriging accuracy because density is scalar and independent of well deviation the same comparison for sonic velocities revealed that they are systematically higher than the kriged vertical velocities this difference was used to estimate anisotropy parameters at the location of the deviated well the fac

lattice preferred orientation and seismic anisotropy in sedimentary rocks summary although it is well known that sedimentary rocks can be seismically anisotropic there have been few detailed investigations of the underlying cause of such anisotropy here we investigate anisotropy due to the preferred orientation of minerals or lattice preferred orientation lpo in a suite of sedimentary rocks seismic properties are predicted by averaging single crystal elastic constants of minerals according to their crystal orientation and modal volume fraction in the rock aggregate both electron backscattered diffraction ebsd and x ray texture goniometry xtg are tested as quantitative techniques for measuring the lpo of sedimentary rocks although ebsd has promise for future lpo measurements in polymineralic sedimentary rocks problems currently remain in measuring low symmetry phases e g feldspars and very small clay or mica particles however the lpo of very fine grained phyllosilicates can be measured using xtg and the lpo of low symmetry minerals can be measured using manual ebsd pattern analysis here we use such a hybrid approach to estimate lpo in a suite of sedimentary samples the seismic properties calculated from lpo data show anisotropy values for p waves ranging from 1 5 per cent in sandstones to over 3 5 per cent in a siltstone to 12 per cent in a shale the effect of thin multilayering on long wavelength propagation in a siltstone is predicted by applying backus type averaging the layering does not enhance the anisotropy because of the small differences in density and in elasticity between the two layer types the lpo of phyllosilicates and to a lesser extent dolomite and siderite seem to contribute significantly to the seismic anisotropy of phyllosilicate rich rocks siltstones and shales the weak lpo of quartz in sandstones causes a few per cent anisotropy cumulatively our results suggest that field observations of seismic anisotropy have the potential to be used as an indicator of rock lithology

multiple mechanisms of remagnetization involving sedimentary greigite fe3s4 sedimentary greigite fe3s4 is being increasingly implicated as the carrier of late diagenetic remagnetizations in fine grained marine and terrestrial sediments we have conducted detailed scanning electron microscope observations on polished sections from such sediments coupled with elemental microanalysis to identify greigite and to characterize the mode of occurrence and the relationships between the observed greigite and other authigenic and detrital mineral phases this detailed observational work in conjunction with recently published work has enabled identification of several mechanisms for remagnetization involving greigite 1 neoformation of greigite on the surfaces of early diagenetic framboidal and nodular pyrite 2 greigite growth within cleavages of detrital sheet silicate grains 3 neoformation of greigite on the surfaces of authigenic clays smectite illite 4 greigite nucleation on the surfaces of siderite 5 greigite growth on the surface of gypsum that resulted from earlier oxidation of nodular pyrite many other mechanisms for greigite neoformation are conceivable the key variables are appropriate redox conditions for generation of sulphide and availability of iron from the dissolution of a wide range of possible reactive detrital and authigenic iron bearing minerals documentation of a wide range of mechanisms for neoformation of greigite provides compelling evidence that sediments containing greigite should be routinely suspected of remagnetization which will complicate or compromise studies of environmental magnetism and geomagnetic field behaviour

velocity anisotropy in shales a petrophysical study using ultrasonic velocity and anisotropy measurements on a variety of shales with different clay and kerogen content clay mineralogy and porosity at a wide range of effective pressure we find that elastic anisotropy of shales increases substantially with compaction the effect is attributed to both porosity reduction and smectite to illite transformation with diagenesis a means of kerogen content mapping using velocity versus porosity crossplot for shales is shown matrix anisotropy of shales dramatically increases with kerogen reaching the maximum values of about 0 4 at total organic carbon toc 15 20 a strong chemical softening effect was found in shales containing even minor amounts of swelling smectite clay when saturated with aqueous solution this effect results in a significant p wave anisotropy reduction as compared to dry and oil saturated shales since mature black shales are normally oil wet this effect can only have a local significance restricted to the wellbore wall accurate measurements of phase velocities including velocities at a 45 direction to the bedding plane allow us to immediately calculate elastic stiffnesses and anisotropic parameters intrinsic high pressure properties of shales display an e 0 relation introduction of the bedding parallel microcracks in overpressured shales results in a decrease when fully fluid saturated and a increase when partially gas saturated with a characteristic effect on the shape of the p wave velocity surface at small angles of incidence filtering the contribution of the intrinsic anisotropy of shales it is possible to estimate the pore fluid phase microcrack density and aspect ratio parameters using seismic anisotropy measurements

comparison of the anisotropic behaviour of undeformed sandstones under dry and saturated conditions abstract this article presents a systematic analysis of the anisotropic behaviours of the bentheim and rothbach sandstones using ultrasonic p wave velocity electrical conductivity and magnetic susceptibility measurements for each sandstone the data were obtained from three core samples drilled perpendicularly to each other and tested in dry and water saturated conditions for acoustic and magnetic investigations the same statistical analysis was applied in order to present the data on comparable stereoplots surprisingly the bentheim sandstone which appeared homogeneous at macroscopic scale showed a stronger elastic and electrical anisotropy than the rothbach sandstone in which cross laminations were clearly identified as confirmed by a sedimentary magnetic fabric a discussion on the velocity contrasts between dry and saturated samples led us to consider two different origins for the observed anisotropies first by comparing electrical and acoustic properties in the bentheim sandstone we conclude that the nature of the anisotropic behaviour is linked to the anisotropy of pore shape the inclusion model developed by kachanov kachanov m 1993 elastic solids with many cracks and related problems advances in applied mechanics vol 30 academic press boston ma pp 259 445 accounts for our observations if one considers that the pore space is made of parallel flat pores with moderate pore aspect ratio second acoustic electrical and magnetic properties indicate that the observed anisotropy in the rothbach sandstone can be attributed to the matrix and more specifically to cementation we modified the dvorkin and nur geophysics 61 5 1996 1363 model of cemented granular media by introducing a spatially variable contact length and the model suggests that a very small variability of cemented contact length is enough to account for the observed p wave velocity anisotropy we emphasise the fact that combining several kinds of measurements is of great help in capturing the nature of the anisotropic behaviour of porous rocks

greigite detected as dominating remanence carrier in late pleistocene sediments lisan formation from lake kinneret sea of galilee israel summary a rock magnetic investigation of three sedimentary cores of lisan formation of late pleistocene age from lake kinneret sea of galilee northern israel demonstrates that the magnetization of these sediments is controlled by various degrees of a secondary chemical remanent magnetization crm carried by greigite fe3s4 this crm is superimposed on a primary detrital remanent magnetization drm that resides in ti magnetite this finding is independently confirmed by x ray diffraction xrd measurements performed on magnetic extracts of the sediments the domain state of the greigite is largely single domain behaviour sd thus dominating the magnetization therefore the magnetic record retrieve from of these sediments is not reflecting geomagnetic variations but rather chemical rock magnetic properties resulting from diagenetic processes the results of our study suggest that paleomagnetic record of greigite bearing sediments should be interpreted with caution because of the following reasons 1 geomagnetic secular variations can be biased due to large coercivity overlap between magnetite and greigite 2 alternating field af demagnetization can produce erroneous directions due to vector distortion by acquisition of a gyro remnant magnetization grm 3 estimation of relative paleointensity can be hampered by large scale variations in natural remnant magnetization nrm intensity caused by the acquisition of a secondary crm of unknown age and unknown extent superimposed on the drm the precipitation of greigite requires reducing conditions at the sediment water interface and or interstitial water associated with the presence of sulphur and iron oxides reducing conditions are typical of stratified lakes and other stratified water bodies such as the dead sea paleo lake lisan temporarily the eastern mediterranean sea the caspian sea the black sea and many other marine basins according to our findings paleomagnetic records from such environments carried dominantly by greigite therefore should be treated with care

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