the influence of fluid sensitive dispersion and attenuation on avo analysis summary analysis of seismic data suggests that hydrocarbon deposits are often associated with higher than usual values of attenuation but this is generally ignored during amplitude versus offset avo analysis the effect can be modelled with equivalent medium theory based on the squirt flow concept but the excess attenuation is associated with strong velocity dispersion consequently when we study reflections from the interface between such an equivalent medium and an elastic overburden we find that the reflection coefficient varies with frequency the impact of this variation depends on the avo behaviour at the interface class i reflections tend to be shifted to higher frequency while class iii reflections have their lower frequencies amplified we calculate synthetic seismograms for typical models using the reflectivity method for materials with frequency dependent velocities and attenuations and find that these effects are predicted to be detectable on stacked data two field data sets show frequency anomalies similar to those predicted by the analysis and we suggest that our modelling provides a plausible explanation of the observations

fracture characterization using frequency dependent shear wave anisotropy analysis of microseismic data summary the presence of fractures in hydrocarbon reservoirs can enhance porosity and permeability and consequently increase production the use of seismic anisotropy to characterize fracture systems has gained much interest in the last two decades however estimating fracture sizes from observations of seismic anisotropy has not been possible recent work has shown that frequency dependent anisotropy fda is very sensitive to the length scale of the causative mechanism for the anisotropy in this study we observe fda in a microseismic data set acquired from a carbonate gas field in oman the frequency dependent shear wave anisotropy observations are modelled using a poroelastic model which considers fluid communication between grain size pore spaces and larger scale fractures a grid search is performed over fracture parameters radius density and strike to find the model that best fits the real data the results show that fracture size varies from the microscale within the shale cap rocks to the metre scale within the gas reservoir to the centimetre scale within the non producing part of the carbonate formation the lateral variation in fracture density agrees with previous conclusions from ordinary shear wave splitting sws analysis cumulatively the results show the potential for characterizing fracture systems using observations of fda

comment on the 3d shear experiment over the natih field in oman the effects of fracture filling fluids on shear propagation by c m van der kolk w s guest and j h h m potters we find errors in theory and application in the paper by van der kolk et al geophysical prospecting 49 179 197 2001 that invalidate their arguments for the cause of the reduction in the velocity of the vertically propagating slower split shear wave in the gas cap over the natih field in oman we suggest that existing theories are adequate and can explain the anomaly

zircon lu hf systematics and the evolution of the archean crust in the southern superior province canada a combined lu hf and u th pb isotopic study was made of 25 zircons and 2 whole rocks from the late archean crust 2 888 2 668 ma in the southern superior province canada the relative abundances of u th lu and hf in zircons from the low grade michipicoten and gamitagama greenstone belts show variable patterns which in part reflect the bulk compositional differences of their parent rocks zircons from the high grade lower crustal regions adjacent to these belts kapuskasing structural zone are distinguished from the low grade zircons by their strong depletions of lu and hf the low hf contents imply that the growth of metamorphic zircon involves a significant fractionation of the zr hf ratio

paleomagnetism of archean granites and matachewan dikes in the wawa subprovince ontario reevaluation of the archean apparent polar wander path paleomagnetic measurements have been completed on 400 specimens from dated archean granites and matachewan dikes in the michipicoten and gamitagama greenstone belts in the western wawa subprovince

the 3d shear experiment over the natih field in oman reservoir geology data acquisition and anisotropy analysis this paper describes a large scale reservoir characterization experiment carried out in oman in 1991 which comprised the acquisition processing and interpretation of a 28 4 km 2 3d multicomponent seismic experiment over the natih field the objective of the survey was to obtain information on the fracture network present in the natih carbonates from shear wave anisotropy shear wave anisotropy in excess of 20 time splitting was encountered over a large part of the survey the seismic results are confirmed by geological and well data but provide additional qualitative information on fracturing where this was not available before regions of stronger and weaker shear wave anisotropy appear to be fault bounded the average well flow rates which are fracture dominated within such blocks correlate with the average anisotropy of the blocks the further observation that the anisotropy is largest in the fracture gas cap of the reservoir suggests that shear waves can provide a direct hydrocarbon indicator for fractured rock

quantitative determination of hydraulic properties of fractured rock using seismic techniques there have been significant advances over the last ten years in the use of the seismic anisotropy concept to characterize subsurface fracture systems measurements of seismic anisotropy are now used to deduce quantitative information about the fracture orientation and the spatial distribution of fracture intensity analysis of the data is based upon various equivalent medium theories that describe the elastic response of a rock containing cracks or fractures in the long wavelength limit conventional models assume scale frequency independence and hence cannot distinguish between micro cracks and macrofractures the latter however control the fluid flow in many oil gas reservoirs as the fracture size and spacing hence fracture storability are essential parameters for reservoir engineers recently a new equivalent medium theory for modelling of wave propagation in media with multi scale fractures has been presented the model predicts velocity dispersion and attenuation due to a squirt flow mechanism at two different scales the grain scale micro cracks and equant matrix porosity and formation scale fractures application of this model to field data shows that fracture density and fracture size can be inverted successfully from the frequency dependence of the time delay between split shear waves the derived fracture length matches independent observations from borehole data this paper presents the results of the latest development in the seismic characterization of natural fractures with an emphasis on the quantitative determination of fracture sizes

frequency dependent anisotropy due to meso scale fractures in the presence of equant porosity fractured rock is often modelled under the assumption of perfect fluid pressure equalization between the fractures and equant porosity this is consistent with laboratory estimates of the characteristic squirt flow frequency however these laboratory measurements are carried out on rock samples which do not contain large fractures we consider coupled fluid motion on two scales the grain scale which controls behaviour in laboratory experiments and the fracture scale our approach reproduces generally accepted results in the low and high frequency limits even under the assumption of a high squirt flow frequency we find that frequency dependent anisotropy can occur in the seismic frequency band when larger fractures are present shear wave splitting becomes dependent on frequency with the size of the fractures playing a controlling role in the relationship strong anisotropic attenuation can occur in the seismic frequency band the magnitude of the frequency dependence is influenced strongly by the extent of equant porosity with these results it becomes possible in principle to distinguish between fracture and microcrack induced anisotropy or more ambitiously to measure a characteristic fracture length from seismic data

modelling frequency dependent seismic anisotropy in fluid saturated rock with aligned fractures implication of fracture size estimation from anisotropic measurements measurements of seismic anisotropy in fractured rock are used at present to deduce information about the fracture orientation and the spatial distribution of fracture intensity analysis of the data is based upon equivalent medium theories that describe the elastic response of a rock containing cracks or fractures in the long wavelength limit conventional models assume frequency independence and cannot distinguish between microcracks and macrofractures the latter however control the fluid flow in many subsurface reservoirs therefore the fracture size is essential information for reservoir engineers in this study we apply a new equivalent medium theory that models frequency dependent anisotropy and is sensitive to the length scale of fractures the model considers velocity dispersion and attenuation due to a squirt flow mechanism at two different scales the grain scale microcracks and equant matrix porosity and formation scale fractures the theory is first tested and calibrated against published laboratory data then we present the analysis and modelling of frequency dependent shear wave splitting in multicomponent vsp data from a tight gas reservoir we invert for fracture density and fracture size from the frequency dependence of the time delay between split shear waves the derived fracture length matches independent observations from borehole data

fluid dependent shear wave splitting in fractured media azimuthal anisotropy in rocks can result from the presence of one or more sets of partially aligned fractures with orientations determined by the stress history of the rock a shear wave propagating in an azimuthally anisotropic medium splits into two components with different polarizations if the source polarization is not aligned with the principal axes of the medium for vertical propagation of shear waves in a horizontally layered medium containing vertical fractures the shear wave splitting depends on the shear compliance of the fractures but is independent of their normal compliance if the fractures are not perfectly vertical the shear wave splitting also depends on the normal compliance of the fractures the normal compliance of a fluid filled fracture decreases with increasing fluid bulk modulus for dipping fractures this results in a decrease in shear wave splitting and an increase in shear wave velocity with increasing fluid bulk modulus the sensitivity of the shear wave splitting to fluid bulk modulus depends on the interconnectivity of the fracture network the permeability of the background medium and on whether the fracture is fully or partially saturated

experimental verification of the fracture density and shear wave splitting relationship using synthetic silica cemented sandstones with a controlled fracture geometry we present laboratory ultrasonic measurements of shear wave splitting from two synthetic silica cemented sandstones the manufacturing process which enabled silica cementation of quartz sand grains was found to produce realistic sandstones of average porosity 29 7 0 5 and average permeability 29 4 11 3 md one sample was made with a regular distribution of aligned penny shaped voids to simulate meso scale fractures in reservoir rocks while the other was left blank ultrasonic shear waves were measured with a propagation direction of 90 to the coincident bedding plane and fracture normal in the water saturated blank sample shear wave splitting the percentage velocity difference between the fast and slow shear waves of 0 5 was measured due to the bedding planes or layering introduced during sample preparation in the fractured sample shear wave splitting corrected for layering anisotropy of 2 72 0 58 for water 2 80 0 58 for air and 3 21 0 58 for glycerin saturation at a net pressure of 40 mpa was measured analysis of x ray ct scan images was used to determine a fracture density of 0 0298 0 077 in the fractured sample this supports theoretical predictions that shear wave splitting sws can be used as a good estimate for fracture density in porous rocks i e sws 100 f where f is fracture density regardless of pore fluid type for wave propagation at 90 to the fracture normal

a shear experiment over the natih field in oman pilot seismic and borehole data an experimental multicomponent three dimensional 3d seismic survey has been carried out over the natih field in oman this paper describes the small scale two dimensional experiment carried out beforehand and how the results obtained from this pilot were used to assess the feasibility of a nine component three dimensional 9c3d operation as well as to determine the field parameters for the field scale 3d survey it also describes the two vsps and a wireline shear log acquired in conjunction with the pilot experiment and the importance of such borehole data for establishing the correct time to depth relationship for the seismic data and for providing an independent check on the seismic interpretation the observation of cusps in the offset vsp indicated the strong anisotropy of the fiqa shales overlying the natih reservoir

rock physics developments in seismic exploration a personal 50 year perspective i present an historical overview of the major developments in rock physics over the past half century key developments are described for both modeling and laboratory experiments and how they have been applied in seismic exploration reservoir characterization and monitoring in particular i chronicle the development of laboratory methods for determining the velocity and attenuation of elastic waves in rock samples at sonic and ultrasonic frequencies this is followed by a description of the validity of theories originally developed by gassmann and biot in the 1950s and employed for describing the propagation of elastic waves in fluid saturated porous media i outline problems and solutions in the use of ultrasonic measurements to predict velocities and attenuation at seismic frequencies field and laboratory studies are described in which a range of frequencies seismic through sonic and ultrasonic was covered and successfully modeled i describe procedures developed during the 1950s and 1960s for interpreting laboratory measurements in terms of pore structure and degree of saturation based on self consistent and scattering theories laboratory equipment used for determining the elastic properties of anisotropic materials is described along with a discussion of the results obtained including the correlation between both fluid permeability and prediction of in situ state of stress with the degree of shear wave splitting finally i discuss the application of rock physics developments to reservoir characterization and monitoring in particular to time lapse seismic monitoring

the 3d shear experiment over the natih field in oman the effect of fracture filling fluids on shear propagation this is the final paper in a series on the 3d multicomponent seismic experiment in oman in this experiment a 3d data set was acquired using three component geophones and with three source orientations the data set will subsequently be referred to as the natih 9c3d data set we present for the first time evidence demonstrating that shear waves are sensitive to fluid type in fractured media two observations are examined from the natih 9c3d data where regions of gas are characterized by slow shear wave velocities one is that the shear wave splitting map of the natih reservoir exhibits much larger splitting values over the gas cap on the reservoir this increase in splitting results from a decrease in the slow shear wave velocity which senses both the fractures and the fracture filling fluid using a new effective medium model it was possible to generate a splitting map for the reservoir that is corrected for this fluid effect secondly an anomaly was encountered on the shear wave data directly above the reservoir the thick fiqa shale overburden exhibits a low shear wave velocity anomaly that is accompanied by higher shear reflectivity and lower frequency content no such effects are evident in the conventional p wave data this feature is interpreted as a gas chimney above the reservoir a conclusion supported by both effective medium modelling and the geology with this new effective medium model we show that introduction of gas into vertically fractured rock appears to decrease the velocity of shear waves s2 polarized perpendicular to the fracture orientation whilst leaving the vertical compressional wave velocity largely unaffected this conclusion has direct implications for seismic methods in exploration appraisal and development of fractured reservoirs and suggests that here we should be utilizing s wave data as well as the conventional p wave data as a direct hydrocarbon indicator

fluid dependent shear wave splitting in a poroelastic medium with conjugate fracture sets the dependence of shear wave splitting in fractured reservoirs on the properties of the filling fluid may provide a useful attribute for identifying reservoir fluids if the direction of wave propagation is not perpendicular or parallel to the plane of fracturing the wave polarized in the plane perpendicular to the fractures is a quasi shear mode and therefore the shear wave splitting will be sensitive to the fluid bulk modulus the magnitude of this sensitivity depends upon the extent to which fluid pressure can equilibrate between pores and fractures during the period of the deformation in this paper we use the anisotropic gassmann equations and existing formulations for the excess compliance due to fracturing to estimate the splitting of vertically propagating shear waves as a function of the fluid modulus for a porous medium with a single set of dipping fractures and with two conjugate fracture sets dipping with opposite dips to the vertical this is achieved using two alternative approaches in the first approach it is assumed that the deformation taking place is quasi static that is the frequency of the elastic disturbance is low enough to allow enough time for fluid to flow between both the fractures and the pore space throughout the medium in the second approach we assume that the frequency is low enough to allow fluid flow between a fracture set and the surrounding pore space but high enough so that there is not enough time during the period of the elastic disturbance for fluid flow between different fracture sets to occur it is found that the second approach yields a much stronger dependence of shear wave splitting on the fluid modulus than the first approach this is a consequence of the fact that at higher wave frequencies there is not enough time for fluid pressure to equilibrate and therefore the elastic properties of the fluid have a greater effect on the magnitude of the shear wave splitting

pressure dependent seismic response of fractured rock effects of external stress and pore pressure variations on the seismic signature of fractured rocks remain of interest to geoscientists and practicing geophysicists commonly the effects are modeled theoretically assuming fracture faces to be rough surfaces contacting each other via the surface asperities the model proposed here differs from other models of this kind in that 1 fracture roughness is described by a single parameter and 2 a controlled degree of hydraulic connectivity between fractures and equant pores is introduced this adds to the model s convenience and makes it applicable to a wide variety of reservoirs the model predictions of seismic velocities in fractured rock at variable stress are consistent with experimental data for fixed effective stress the model predictions coincide with those obtained using the model with ellipsoidal fractures of certain average aspect ratio and the same fracture porosity apart from known effects the model introduced predicts an amplification of th

determination of the principal directions of azimuthal anisotropy from p wave seismic data in an azimuthally anisotropic medium the principal directions of azimuthal anisotropy are the directions along which the quasi p and the quasi s waves propagate as pure p and s modes when azimuthal anisotropy is induced by oriented vertical fractures imposed on an azimuthally isotropic background two of these principal directions correspond to the directions parallel and perpendicular to the fractures s waves propagating through an azimuthally anisotropic medium are sensitive to the direction of their propagation with respect to the principal directions as a result primary or mode converted multicomponent s wave data are used to obtain the principal directions apart from high acquisition cost processing and interpretation of multicomponent data require a technology that the seismic industry has not fully developed anisotropy detection from conventional p wave data on the other hand has been limited to a few qualitative studies of the amplitude variation with offset avo for different azimutha

high grade metamorphism in the chapleau foleyet area ontario archean rocks superior province kapuskasing structural zone in migmatitic mafic gneiss paragneiss dioritic and tonalitic rocks a lower grade garnet clinopyroxene plagioclase gt cpx pl zone and patchy higher grade orthopyroxene zones are distinguished based on mineral melt equilibria minimum conditions for the gt cpx pl zone are 750 degrees c 6 kbar a h 2 o 0 5 0 7 and for the orthopyroxene zone 770 degrees c 6 kbar a h 2 o 800 degrees c pressure values are 5 4 8 4 kbar and define a nne trending area of relatively high p in the kapuskasing zone supporting the interpretation of a tilted crustal section modified journal abstract

precise u pb zircon ages from the gamitagama greenstone belt southern superior province volcanism in the gamitagama lake greenstone belt 50 km south of wawa ontario on the east shore of lake superior took place ago at the same time as similar volcanism in the much larger abitibi greenstone belt to the east two post orogenic intrusions in the metavolcanic metasedimentary terrain are coeval at and and could very well be comagmatic as suggested by ayres a granitic sill in a metasedimentary granitic migmatite terrain that borders the greenstone belt to the southwest has an age of 2615 10 ma and is much younger than the post orogenic plutons to the northeast this migmatite region represents either an uplifted deep zone that remained hot long after volcanism or a region that accumulated heat as a result of its sedimentary cover

elastic wave propagation in media with parallel fractures and aligned cracks a model of parallel slip interfaces simulates the behaviour of a fracture system composed of large closely spaced aligned joints the model admits any fracture system anisotropy triclinic the most general monoclinic orthorhombic or transversely isotropic and this is specified by the form of the 3 3 fracture system compliance matrix the fracture system may be embedded in an anisotropic elastic background with no restrictions on the type of anisotropy to compute the long wavelength equivalent moduli of the fractured medium requires at most the inversion of two 3 3 matrices when the fractures are assumed on average to have rotational symmetry transversely isotropic fracture system behaviour and the background is assumed isotropic the resulting equivalent medium is transversely isotropic and the effect of the additional compliance of the fracture system may be specified by two parameters in addition to the two isotropic parameters of the isotropic background dilute systems of flat aligned microcracks in an isotropic background yield an equivalent medium of the same form as that of the isotropic medium with large joints i e there are two additional parameters due to the presence of the microcracks which play roles in the stress strain relations of the equivalent medium identical to those played by the parameters due to the presence of large joints thus knowledge of the total of four parameters describing the anisotropy of such a fractured medium tells nothing of the size or concentration of the aligned fractures but does contain information as to the overall excess compliance due to the fracture system and its orientation as the aligned microcracks which were assumed to be ellipsoidal with very small aspect ratio are allowed to become non fiat i e have a growing aspect ratio the moduli of the equivalent medium begin to diverge from the standard form of the moduli for flat cracks the divergence is faster for higher crack densities but only becomes significant for microcracks of aspect ratios approaching 0 3

experimental observation of water saturation effects on shear wave splitting in synthetic rock with fractures aligned at oblique angles fractured rocks are known to exhibit seismic anisotropy and shear wave splitting sws sws is commonly used for fractured rock characterization and has been shown to be sensitive to fluid type the presence of partial liquid gas saturation is also known to affect the elastic properties of rocks the combined effect of both fractures and partial liquid gas saturation is still unknown using synthetic silica cemented sandstones with aligned penny shaped voids we conducted laboratory ultrasonic experiments to investigate the effect fractures aligned at an oblique angle to wave propagation would have on sws under partial liquid gas saturation conditions the result for the fractured rock shows a saturation dependence which can be explained by combining a fractured rock model and a partial saturation model at high to full water saturation values sws decreases as a result of the fluid bulk modulus effect on the quasi shear wave this bulk modulus effect is frequency dependent as a result of wave induced fluid flow mechanisms which would in turn lead to frequency dependent sws this result suggests the possible use of sws for discriminating between full liquid saturation and partial liquid gas saturation

pore fluid viscosity effects on p and s wave anisotropy in synthetic silica cemented sandstone with aligned fractures ultrasonic 500 khz p and s wave velocity and attenuation anisotropy were measured in the laboratory on synthetic octagonal shaped silica cemented sandstone samples with aligned penny shaped voids as a function of pore fluid viscosity one control blank sample was manufactured without fractures another sample with a known fracture density inline image measured from x ray ct images velocity and attenuation were measured in four directions relative to the bedding fabric introduced during packing of successive layers of sand grains during sample construction and the coincident penny shaped voids fractures both samples were measured when saturated with air water viscosity 1 cp and glycerin 100 cp to reveal poro visco elastic effects on velocity and attenuation and their anisotropy the blank sample was used to estimate the background anisotropy of the host rock in the fractured sample the bedding fabric was found to show transverse isotropy with shear wave splitting sws of 1 45 1 18 i e for s wave propagation along the bedding planes in the fractured rock maximum velocity and minimum attenuation of p waves was seen at 90 to the fracture normal after correction for the background anisotropy the fractured sample velocity anisotropy was expressed in terms of thomsen s weak anisotropy parameters a theory of frequency dependent seismic anisotropy in porous fractured media was able to predict the observed effect of viscosity and bulk modulus on and in water and glycerin saturated samples and the higher and values in air saturated samples theoretical predictions of fluid independent are also in agreement with the laboratory observations we also observed the predicted polarisation cross over in shear wave splitting for wave propagation at 45 to the fracture normal as fluid viscosity and bulk modulus increases

use of avoa data to estimate fluid indicator in a vertically fractured medium microstructural attributes of cracks and fractures such as crack density aspect ratio and fluid infill determine the elastic properties of a medium containing a set of parallel vertical fractures although the tangential weakness t of the fractures does not vary with the fluid content the normal weakness n exhibits significant dependence on fluid infill based on linear slip theory we used the ratio g n t termed the fluid indicator as a quantitative measure of the fluid content in the fractures with g representing the square of the ratio of s and p wave velocity in the unfractured medium we used a born formalism to derive the sensitivity to fracture weakness of pp and ps reflection coefficients for an interface separating an unfractured medium from a vertically fractured medium our formulae reveal that the pp reflection coefficient does not depend on the 2d microcorrugation surface roughness with ridges and valleys parallel to the fracture strike whereas the ps reflection coefficien

analytical solution to the seismic response of visco elastic soil layers the response of a multi layered soil deposit subjected to a horizontal earthquake ground motion is obtained in a form which is exact in the sense that both the starting transients and the steady state parts are included in the solution and can be evaluated to any degree of accuracy the solution is compared with that obtained as a steady state one for harmonic excitations it is shown that the transient part of the response which is required for the solution to satisfy initial conditions and which does not appear in the steady state may be important particularly for long period layers subjected to short period acceleration trains for small values of the radiation coefficients radiation damping may be introduced in the solution in the form of equivalent viscous damping la reponse d un sol multicouches soumis a des mouvements sismiques horizontaux est presentee sous une forme exacte puisque l etat transitoire du depart et l etat permanent sont tous deux pris en compte dans la solution et peuvent etre

geochronology of ophiolites of the newfoundland appalachians precise u pb zircon ages 2 errors are reported for four ophiolite complexes from the newfoundland appalachians zircons from trondhjemite from blow me down mountain bay of islands complex give an age of five points those from gabbro from blow me down brook give a minimum 207pb 206pb age of 480 ma one point confirming that this is the age of the main plutonic episode these data refute previously determined latest cambrian u pb zircon and sm nd mineral rock isochron ages of 504 10 and 505 ma for the complex gabbro from the betts cove complex yielded zircon that gives an age of four points this refutes the interpretation of an earlier u pb zircon age of 463 6 ma for a trondhjemite dyke as the age of the ophiolite it was inconsistent with the presence of arenigian graptolites in the overlying snooks arm group two zircon ages from a pegmatitic and medium grained trondhjemite of the annieopsquotch complex are five points and two points zircon from trondhjemite of the pipestone pond c

u pb zircon ages and the evolution of the michipicoten plutonic volcanic terrane of the superior province ontario the archean michipicoten greenstone belt of the superior province in ontario is made up of supracrustal rocks divided into lower middle and upper metavolcanic rocks with associated metasedimentary rocks the belt has been intruded by granitic rocks and is also surrounded by granitic terranes based on u pb zircon geochronology it appears that volcanism in the area extended from at least 2749 to 2696 ma and plutonism and tectonic activity extended from at least 2888 to 2615 ma the various granitic and also one gabbroic plutons both internal and external to the greenstone belt were emplaced concomitantly with the three volcanic cycles as well as before and after the formation of the volcanic rocks zircon ages reported here together with previously published ages show that the area evolved in six major volcanic and plutonic events i 2888 ma plutonism ii 2743 ma volcanism and plutonism iii 2717 ma volcanism and plutonism iv 2696 ma volcanism and plutonism v 2668 ma plutonism and vi

seismic structure of the continental crust based on rock velocity measurements from the kapuskasing uplift the michipicoten greenstone belt mgb wawa gneiss terrane wgt and kapuskasing structural zone ksz in ontario canada are regarded as a 25 km thick partial cross section through the crust of the superior province we measured compressional wave velocities vp at confining pressures up to 600 mpa for representative rock samples from these terranes vp for mgb is variable 6 1 7 km s and depends on composition and anisotropy compressional wave velocities for low anisotropy quartzofeldspathic gneisses and intrusive rocks from the wgt and the ksz show little variation over a wide range of silica content but mafic gneisses and anorthositic rocks show a wide velocity range over a narrower silica range a trend corroborated by data for other high grade rocks a model of the continental crust based on the laboratory data shows three megalayers with differing seismic characteristics the shallowest levels mgb are seismically heterogeneous with high average velocities 6 6 km s at 100 mpa because of the dominance of metabasalts the balance of the upper crust wgt is seismically homogeneous as it is dominated by a variety of isotropic quartzofeldspathic rocks with average velocities of 6 35 km s at 100 mpa and 6 6 km s at 600 mpa the uppermost lower crust ksz is layered seismically heterogeneous and transversely isotropic the average velocity at 600 mpa is 6 84 km s the boundary between wgt and ksz is marked by a velocity increase of between 0 2 and 0 3 km s an increase similar to that commonly associated with midcrustal discontinuities i e conrad in shield areas here this boundary corresponds to the amphibolite granulite facies transition and to a change from a tonalitic middle crust to a lithologically heterogeneous uppermost lower crust refraction results show an anomalous high velocity zone in the upper crust under the ksz that on the basis of laboratory velocity data can be correlated with ksz lithologies these data indicate that ksz rocks can be traced to the west to depths as great as 20 km levels deeper than 20 km not exposed at the surface in the ksz are characterized by velocities greater than 7 0 km s and may be dominated by mafic and anorthositic lithologies or by more garnet rich rocks the velocity model of the superior province crust based on geological reconstruction and laboratory velocity data is in general agreement with the velocity structure to depths of about 25 km determined for the crust below the mgb from refraction experiments the velocity structure of this model is also generally similar to the velocity structure of shield areas

observations of fluid dependent shear wave splitting in synthetic porous rocks with aligned penny shaped fractures p and s wave velocity and attenuation coefficients accurate to 0 3 and 0 2 db cm respectively were measured in synthetic porous rocks with aligned penny shaped fractures using the laboratory ultrasonic pulse echo method shear wave splitting was observed by rotating the s wave transducer and noting the maximum and minimum velocities relative to the fracture direction a block of synthetic porous rock of fracture density 0 0201 0 0068 and fracture size 3 6 0 38 mm measured from image analysis of x ray ct scans was sub sampled into three 20 30 mm long 50 mm diameter core plugs oriented at 0 45 and 90 to the fracture normal transversely isotropic symmetry axis full waveform data were collected over the frequency range 500 1000 khz for both water and glycerin saturated cores to observe the effect of pore fluid viscosity at 1 cp and 100 cp respectively the shear wave splitting observed in the 90 core was 2 15 0 02 for water saturated and 2 39 0 02 for glycerin saturated in agreement with the theory that suggests that the percentage splitting should be 100 times the fracture density and independent of the saturating fluid in the 45 core by contrast splitting was 0 00 0 02 for water saturation and 0 77 0 02 for glycerin saturation this dependence on fracture orientation and pore fluid viscosity is consistent with the poro visco elastic theory for aligned meso scale fractures in porous rocks the results suggest the possible use of shear or converted wave data to discriminate between fluids on the basis of viscosity variations

modeling the effect of multiple sets of mesoscale fractures in porous rock on frequency dependent anisotropy this study models the seismic response of porous rock containing two fracture sets with different orientations sizes and connectivities modeling demonstrates frequency dependent anisotropy controlled by two characteristic frequencies that are defined by fluid mobility and the length scales of the fractures fracture related dispersion and attenuation typically occur over a wider frequency band than is seen in the case of a single fracture set when one set of fractures is modeled as being sealed the azimuthal variation of velocity and attenuation can be different with the azimuth of minimum attenuation coinciding with the strike direction of the open fracture set the results should support attempts to differentiate between open and closed fractures on the basis of seismic measurements

ion microprobe u th pb isotopic studies of zircons from three early precambrian areas in the u s s r abstract ion microprobe u th pb dating of zircons from three polymetamorphic archaean terranes in the soviet union has provided some precise estimates of protolith and metamorphic ages which confirm that the ages inferred previously from conventional zircon u pb analysis are broadly correct the gneisses of the novopavlovsk complex intersected by drilling in the eastern ukrainian shield contain pyroxenite enclaves zircons in one pyroxenite give 3450 8 ma and in a nearby enderbite give 3441 10 ma two pyroxenites from another drill hole contain zircons as old as 3633 16 ma and 3640 11 ma respectively in addition to clearly metamorphic zircons interpreted to be 2901 34 ma and 2664 68 ma old in one and 2595 92 ma old in the other the tonalitic orthogneiss host to these two enclaves is 3609 5 ma old and was metamorphosed at 2826 16 ma the zircons in the pyroxenites are probably not primary but grew when the pyroxenites were incorporated into the magmatic precursors of the orthogneisses zircons in a leucocratic orthogneiss from the aulandjha block in the omolon massif soviet far east are interpreted to be mostly 3481 18 ma old but a minor metamorphic population gives 2667 29 ma a recently reported age of 3900 ma from the daldin series in the anabar shield in central siberia was not confirmed the zircon age of an enderbite from the daldin series is interpreted to be 3164 32 ma