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

p wave anisotropy stress and crack distribution at coso geothermal field california a new inversion method for p wave anisotropy wu and lees 1999a has been applied to high precision microseismic traveltime data collected at coso geothermal region california direction dependent p wave velocity and thus its perturbation are represented by a symmetric positive definite matrix a instead of a scalar the resulting anisotropy distribution is used to estimate variations in crack density stress distribution and permeability within the producing geothermal field a circular dome like structure is observed at the southwestern part of the geothermal region southwest of sugarloaf mountain using a linear stress bulk modulus relationship deviatoric stress is estimated to be 3 6 mpa at geothermal production depths 1 2 km assuming all the anisotropy is related to stress the stress field is compressional nne ssw and dilational wnw ese coinciding with a previous independent study using earthquake focal mechanisms following a theory on flat elliptic cracks residual crack density estimated from p anisotropy is 0 0078 assuming crack aspect ratios 1 60 and is 0 041 when crack aspect ratios are close to 1 60 residual crack orientation distribution is related to velocity anisotropy on the basis of anisotropic part of crack density distribution function the anisotropic part of permeability distribution may be calculated by a statistical approach via simple parallel fluid flow along cracks

model based amplitude versus offset and azimuth inversion for estimating fracture parameters and fluid content abstractthe normal to shear fracture compliance ratio is commonly used as a fluid indicator in the seismic frequency range the fluid indicator lies between the values for isolated fluid filled fractures and dry fractures and it is not easy to discriminate the fluid content assuming that the fracture surfaces are smooth we use 1 n ndry with n and ndry representing the normal fracture weakness of the saturated and dry rock to indicate fluid types and to define a fluid influencing factor the fluid influencing factor is sensitive to the fluid properties the aspect ratio of the fractures and the frequency conventionally the amplitude versus offset and azimuth avoa inversion is formulated in terms of the contrasts of the fracture weaknesses across the interface assuming that the fractures are vertical with the same symmetry axis we consider fractures with arbitrary azimuths and develop a method to estimate fracture parameters from wide azimuth seismic data the proposed avoa inversion algo

ultrasonic moduli for fluid saturated rocks mavko jizba relations rederived and generalized mavko and jizba propose a quantitative model for squirt dispersion of elastic wave velocities between seismic and ultrasonic frequencies in granular rocks their central results are the expressions for the so called unrelaxed frame bulk and shear moduli computed under an assumption that the stiff pores are drained or dry but the soft pores are filled with fluid mavko jizba expressions are limited to liquid saturated rocks but become inaccurate when the fluid bulk modulus is small e g for gas saturated rocks we have derived new expressions for unrelaxed moduli of fluid saturated porous rocks using sayers kachanov discontinuity formalism the derived expressions generalize the established mavko jizba relations to gas saturated rocks reduce to mavko jizba results when the pore fluid is liquid and yield dry moduli when fluid bulk modulus tends to zero we tested this by comparing our model and the model of mavko and jizba against laboratory measurements on a sample of westerly granite

compressional and shear wave anisotropy in the oceanic lithosphere the ngendei seismic refraction experiment summary synthetic seismogram modelling of seismic refraction data from the 1983 ngendei expedition to the south pacific indicate seismic anisotropy both within the top kilometre of oceanic crust and in the uppermost mantle we calculate average p and s wave velocity versus depth functions for two orthogonal azimuths n3ooe approximately aligned with the fast mantle direction and n120 e close to the fast crustal direction probable lateral heterogeneities at the ngendei site prevent perfect matching of data and synthetic waveforms crustal anisotropy is indicated by a 28 pattern of both p and s wave travel times as a function of azimuth and is consistent with an approximate 0 2 km s difference in p wave velocities and 0 1 km s difference in s wave velocities upper mantle anisotropy is characterized by pn velocity differences of 7 95 8 4 km s l and a nearly uniform s velocity of 4 65 km s we use these velocity profiles and traveltime data to calculate bounds on the properties of the elastic constants of the crust and upper mantle the crustal anisotropy can be explained by a model involving aligned cracks parallel to the original spreading ridge resulting in a fast direction perpendicular to the fossil spreading direction the upper mantle anisotropy is consistent with aligned olivine crystal models in which the fast direction is parallel to the fossil spreading direction for both of these models we use bounds from the ngendei data to place constraints on the physical properties of the lithosphere

seismic attenuation in faroe islands basalts we analysed vertical seismic profiling vsp data from two boreholes at glyvursnes and vestmanna on the island of streymoy faroe islands to determine the magnitude and causes of seismic attenuation in sequences of basalt flows the work is part of seifaba a major project integrating data from vertical and offset vsp surface seismic surveys core samples and wireline log data from the two boreholes values of effective seismic quality factor q obtained at glyvursnes and vestmanna are sufficiently low to significantly degrade the quality of a surface reflection seismic image this observation is consistent with results from other vsp experiments in the north atlantic region we demonstrate that the most likely cause of the low values of effective q at glyvursnes and vestmanna is a combination of 1d scattering and intrinsic attenuation due to seismic wave induced fluid flow within pores and microcracks tests involving 3d elastic wave numerical modelling with a hypothetical basalt model based on field observations indicate that little scattering attenuation is caused by lateral variations in basalt structure

layer stripping of shear wave splitting in marine ps waves summary the properties of split s waves can be used to infer 1 the state of stress and strain in the earth 2 the directional dependence of hydraulic conductivity and 3 small changes in pore fluid pressure in the rock mass that occur in response to dynamic processes such as the earthquake cycle measurements of split s waves are particularly useful in shallow 1000 m subseabed marine sediments where s wave splitting from an azimuthal elastic anisotropy is typically produced by the presence of near vertical aligned cracks here we present a method of measuring small amounts of s wave splitting in marine p to s mode converted waves and illustrate the technique with data from an ocean bottom seismometer obs deployed on the west svalbard continental slope the analysis applies a modified version of the alford rotation and layer stripping technique developed for zero offset s wave sources and treats ps waves that undergo mode conversion at reflectors that are close to the seabed in comparison with the overlying water depth when the seismic record contains coherent signal on both the in plane and out of plane components the layer stripping technique is capable of decoupling the swave splitting from the effects of p wave velocity anisotropy and reflector dip that influence the downgoing p wave part of the ray path the amount of s wave splitting in the data is small however and we find that this causes a greater practical problem for the analysis than the known theoretical limitations of the layer stripping theory such as use of a finite offset source for the analysis of the example data we develop a number of procedures that are necessary to mitigate the low signal to noise levels these include using a wide range of shot receiver azimuths to generate data redundancy methods of identifying and rejecting poor measurements and a predictive layer stripping approach that minimizes the propagation of errors through the analysis that arise from scatter in the layer by layer results with the ps waves of the example data which have a dominant period about 30 ms we find the technique is capable of measuring s wave splitting to a precision of about 0 5 ms for 4 or 5 layers the number of layers successfully treated would increase if the amount of s wave splitting were larger than in these data for which the total cumulative s wave splitting was about 10 ms over a 450 m depth interval the orientation of the fast split s wave was measured with a precision of about 15 our results give an s wave velocity anisotropy of 1 2 per cent in the shallowest 25 30 m subseabed that implies the presence of a differential horizontal stress at or very close to the seabed the s wave splitting accumulated throughout the investigated section at a rate that was consistent with predictions made for a single set of parallel fluid filled cracks with crack density about 0 015 the fast s wave was found to be oriented at 75 15 for the uppermost 150 m before drifting clockwise to an azimuth of 190 210 the clockwise drift in the fast s wave polarization direction implies a transfer of dominance between one set of cracks in the near surface probably related to the slope of the seabed and a different set in deeper sediments of tectonic origin a zone of azimuthal isotropy where the mechanisms that produce the elastic anisotropy may cancel each other out occupies the interval between the two almost orthogonal sets of cracks from interpretation

elastic piezosensitivity of porous and fractured rocks understanding the stress dependencies of seismic velocities is important for interpreting a variety of seismic data ranging from amplitude versus offset avo and velocity analysis to overpressure prediction and 4d seismic monitoring of reservoirs sometimes rather complex forms of these dependencies based on specific models of porous space geometry are used for example spherical contacts models duffy and mindlin 1957 merkel et al 2001 duffy merkel 2001 and crack contacts models gangi and carlson 1996 carcione and tinivella 2001 have been used in different studies gangi96 however usually the pore pressure velocity dependence along with the velocity dependence on the confining pressure are phenomenologically described by the following simple relationship zimmerman et al 1986 eberhart phillips et al 1989 freund 1992 jones 1992 prasad and manghnani 1997 khaksar et al 1999 carcione and tinivella 2001 kirstetter and macbeth 2001

evaluation of anisotropy by shear wave splitting the polarizations of three component shear wavetrains carry unique information about the internal structure of the rock through which they pass specifically commonly observed shear wave splitting may contain information about the orientation of crack distributions this information cannot usually be recovered from shear waves recorded at the free surface because of interference with the interaction of the shear wave with the surface even for nearly vertical incidence however shear wave splitting in synthetic three component vertical seismic profiles in some cases may be interpreted directly in terms of the direction of strike of vertical cracks because the human eye is not skilled at identifying the phase relationships between three component seismograms played out conventionally as parallel time series the polarizations are displayed in orthogonal sections of the particle displacements to facilitate recognition and evaluation of the shear wave splitting estimating the orientations of cracks an

effective elasticity of fractured rocks production related changes in subsurface stress are known to cause velocity variations that can be detected from time lapse seismic data and may provide valuable information for reservoir surveillance it has been long recognized that the stresses primarily influence the most compliant part of a rock by preferentially opening or closing the fractures walsh 1965 therefore examining wave propagation in fractured formations might give important clues about the stresses acting upon them as the length of seismic waves is usually much larger than that of the fractures deformation of cracked rocks can be treated as static

fluid substitution effects on seismic anisotropy we derive equations for hti and orthorhombic symmetries to analyze fluid substitution effects in porous fractured media the derivations are based on the anisotropic gassmann equation and linear slip theory we assess the influence of fluid substitution gas brine and oil on elastic moduli velocities anisotropy and azimuthal amplitude variations we find that in the direction normal to fractures p wave moduli increase as much as 56 and p wave velocity increases up to 19 for gas to brine substitution for the direction parallel to fractures p wave velocity remains almost constant when porosity is low 5 but can increase up to 4 if porosity is high 25 since p waves in two different directions have different sensitivities to fluids and fractures the thomsen s parameters defined for hti and orthorhombic symmetries and are sensitive to fluid types and fractures we also found that is sensitive to porosity for liquid saturation but insensitive to porosity for the case of gas saturation gassmann assumes and as has been observed that shear modulus does not depend on fluids and we observe no changes in shear wave splitting for different fluids the azimuthal amplitude variation is dependent on fluid types fractures and porosity we observe up to 12 increase in azimuthal amplitude variation for low porosity gas sands after brine saturation and 6 decrease for high porosity gas sands we find that the percentage changes in gas to oil substitution are about half that of the gas to brine case the equations we have derived provide a useful tool to quantitatively evaluate the effects of fluid substitution on seismic anisotropy

reflection moveout inversion for horizontal transverse isotropy accuracy limitation and acquisition horizontal transverse isotropy hti is the simplest azimuthally anisotropic model used to describe vertical fracturing in an isotropic matrix assuming that the subsurface is laterally homogeneous and using the elliptical variation of p wave nmo velocity with azimuth measured in at least three different source to receiver orientations we can estimate three key parameters of hti media the vertical velocity anisotropy and the azimuth of the symmetry axis such parameter estimation is sensitive to the angular separation between the survey lines in 2 d acquisition or equivalently to source to receiver azimuths in 3 d acquisition and the set of azimuths used in the inversion procedure the accuracy in estimating the azimuth in particular is also sensitive to the strength of anisotropy while the accuracy in resolving vertical velocity and anisotropy is about the same for any strength of anisotropy to maximize the accuracy and stability in parameter estimation it is best to have the azimuths for the

measurements of seismic anisotropy in synthetic rocks with controlled crack geometry and different crack densities seismic anisotropy can help to extract azimuthal information for predicting crack alignment but the accurate evaluation of cracked reservoir requires knowledge of degree of crack development which is achieved through determining the crack density from seismic or vsp data in this research we study the dependence of seismic anisotropy on crack density using synthetic rocks with controlled crack geometries a set of four synthetic rocks containing different crack densities is used in laboratory measurements the crack thickness is 0 06 mm and the crack diameter is 3 mm in all the cracked rocks while the crack densities are 0 00 0 0243 0 0486 and 0 0729 p and s wave velocities are measured by an ultrasonic investigation system at 0 5 mhz while the rocks are saturated with water the measurements show the impact of crack density on the p and s wave velocities our results are compared to the theoretical prediction of chapman j app geophys 54 191 202 2003 and hudson geophys j r astron soc 64 133 150 1981 the comparison shows that measured velocities and theoretical results are in good quantitative agreement in all three cracked rocks although chapman s model fits the experimental results better the measured anisotropy of the p and s wave in the four synthetic rocks shows that seismic anisotropy is directly proportional to increasing crack density as predicted by several theoretical models the laboratory measurements indicate that it would be effective to use seismic anisotropy to determine the crack density and estimate the intensity of crack density in seismology and seismic exploration

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

3d simulation of seismic wave propagation in fractured media using an integral method accommodating irregular geometries fractures play an important role in most carbonate and unconventional reservoirs precise identification of fractures and their associated properties from seismic data have a significant impact on reservoir management and hydrocarbon recovery traditional seismic methods for fracture identification and characterization include shear birefringence and amplitude variations with offset and azimuth these techniques are based on equivalent medium theory assuming that fracture dimensions and spacing are small relative to the seismic wavelength hence the overall population of fractures is equivalent to a homogeneous anisotropic medium we focus on discrete fracture sets with spacing on the order of the seismic wavelength where individual fractures have large areas but small apertures such fracture sets scatter seismic waves and they are important to characterize because they affect fluid flow in the subsurface analyzing scattering induced wave patterns observed in seismograms can provide crucial guidanc

effects of sand shale anisotropy on amplitude variation with angle ava modelling the sawan gas field pakistan as a key case study for south asia s sedimentary basins amplitude variation with angle ava is a technique widely used in the characterisation of hydrocarbon reservoirs and assumes the earth s crust to be an isotropic medium yet anisotropy is ubiquitous in stratigraphic sequences and has first order effects on seismic ava responses when investigating subsurface prospects this work analyses the effects of anisotropic strata on ava responses using the lower goru formation middle indus basin pakistan as a case study in the study area shale intervals are interbedded with reservoir sands of the sawan gas field shales in this field form laminae or are dispersed within reservoir sands making the lower goru formation an example of a vertically transversely isotropic vti medium in this work we calculate the effective saturated mechanical properties of the lower goru formation based on rock physics templates the backus 1962 average typically designed for layered media combined with the empirical relations of brown and korringa 1975 and wood 1955 the input data used in our rock physics modelling is based on detailed petrophysical analyses of well data using the saturated effective mechanical properties of the lower goru formation we generate angle dependent reflection coefficient curves and seismic ava responses based on exact and approximate solutions for both isotropic and anisotropic reservoir scenarios our results suggest that the effects of lithological anisotropy are more pronounced in places with thick shale beds within reservoir sands conversely angle dependent reflection curves and seismic ava responses based on isotropic or anisotropic cases give similar solutions in the presence of thin shale beds as a corollary of this work we present a bayesian inversion method for the estimation of porosity in vti media

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

an investigation of seismic anisotropy due to cracks in the upper oceanic crust at 45 n mid atlantic ridge summary a detailed seismic refraction survey using three 3 component ocean bottom seismographs over young 1 1 3 4 ma old mid atlantic ridge at 45 n recorded compressional wave velocity anisotropy in the upper crust of about 5 per cent and associated particle motion deviations of up to 8 the magnitudes and azimuths of these anisotropic variations suggest that they are generated by sets of subparallel near vertical water filled cracks in layer 2 the directions of the cracks cannot be determined uniquely but they lie between the seafloor spreading and the ridge axis directions in the range 30 60 with respect to the ridge axis we explain this unexpected crack orientation by the occurrence of cracking and fracturing induced in the upper crust as it moves away from the spreading axis the principal horizontal stresses responsible for this fracturing are parallel and perpendicular to the ridge axis the apparently negligible seismic influence exerted by ridge parallel sets of shallow cracks observed at the ridge axis is explained by their infilling by sediment and hydrothermal precipitates and by their closure under ridge normal compression we find a normal oceanic crustal velocity structure that can be explained by decreasing numbers of open cracks with increasing depth through the upper 2 km of the crust lateral variability across the 25 km square survey is limited in magnitude typically 0 05 s variations in delay times and the observed travel times are consistent with a model of the crust in which the isovelocity surfaces at depth are a subdued copy of the basement topography

azimuthal variation in p wave signatures due to fluid flow the favored dominant mechanism for attenuation in the upper crust at seismic frequencies is intracrack fluid flow in cracked media the azimuthal attenuation of the p wave amplitudes arising from such flow is predicted to be quite substantial the consequence of this variation in azimuth is a modification in the amplitude behavior of the base event from a cracked reservoir due to transmission through the attenuative layer indeed the effect is of sufficient strength to exacerbate diminish or reverse variations that arise solely due to the reflectivity coefficient thus although this attenuation is always greatest perpendicular to the crack strike the direction of the dimming or brightening of the base reservoir event will depend upon the exact attenuation law and the crack properties the combination of these factors contributes to a wave behavior that can provide a more adequate discrimination between conditions of brine and oil fill than an interpretation assuming the reflection coefficient alone

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

elastic anisotropy due to aligned cracks in porous rock all theoretical expressions which relate the characteristics of saturated aligned cracks to the associated elastic anisotropy are restricted in some important way for example to the case of stiff pore fluids or of the absence of equant porosity or of a moderately high frequency band because of these restrictions previous theory is not suitable for application to the upper crust where the pore fluid is brine kf k820 the equant porosity is often substantial p 0 1 and the frequency band is sonic to seismic this work removes these particular restrictions recognizing in the process an important mechanism of dispersion a notable feature of these more general expressions is their insensitivity at low frequency to the aspect ratio of the cracks only the crack density is critical an important conclusion of this more general model is that many insights previously achieved concerning the shear wave splitting due to vertical aligned saturated cracks are sustained however conclusions on crack orientation or crack aspect ratio which were derived from p wave data or from shear wave critical angles may need to be reconsidered further the non linear coupling between pores and cracks due to pressure equalization effects means that the linear schoenberg muir calculus may not be applied to such systems the theory receives strong support from recent data by rathore et al on artificial samples with controlled crack geometry

cosmic gps observations of northern hemisphere winter stratospheric gravity waves and comparisons with an atmospheric general circulation model 1 cosmic satellite temperature data are used to derive the 2006 07 winter mean stratospheric northern hemisphere potential energy ep from gravity waves with vertical wavelengths less than 7 km in grid cells of size 10 5 and to study longitudinal and latitudinal variability in cells of size 20 5 7 days large ep at 17 23 km is mostly associated with the sub tropical jet and shows significant longitudinal variability some contribution to total ep from local orographic sources may occur above the canadian rockies scandinavia and northern japan but not above the himalayas due to the background wind conditions many of the waves are likely to have low ground based phase speeds as observed by filtering around the 0 10 m s 1 background zonal wind cosmic results are compared with a t106l60 agcm confirming sub tropical jet related generation upward propagation and low phase speeds of the observed gravity waves

analysis of dynamic fracture compliance based on poroelastic theory part ii results of numerical and experimental tests in part i a dynamic fracture compliance model dfcm was derived based on the poroelastic theory the normal compliance of fractures is frequency dependent and closely associated with the connectivity of porous media in this paper we first compare the dfcm with previous fractured media theories in the literature in a full frequency range furthermore experimental tests are performed on synthetic rock specimens and the dfcm is compared with the experimental data in the ultrasonic frequency band synthetic rock specimens saturated with water have more realistic mineral compositions and pore structures relative to previous works in comparison with natural reservoir rocks the fracture pore geometrical and physical parameters can be controlled to replicate approximately those of natural rocks p and s wave anisotropy characteristics with different fracture and pore properties are calculated and numerical results are compared with experimental data although the measurement frequency is relatively high the results of dfcm are appropriate for explaining the experimental data the characteristic frequency of fluid pressure equilibration calculated based on the specimen parameters is not substantially less than the measurement frequency in the dynamic fracture model the wave induced fluid flow behavior is an important factor for the fracture wave interaction process which differs from the models at the high frequency limits for instance hudson s un relaxed model

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

anisotropy in the oceanic lithosphere theory and observations from the ngendei seismic refraction experiment in the south west pacific summary p wave travel time data from a seismic refraction experiment on the 1983 ngendei expedition to the south pacific indicate anisotropy at two levels in the oceanic lithosphere in the upper mantle p wave velocities vary between 8 0 and 8 5 km s 1 with the fast direction at n30 e crustal anisotropy within layer 2 is characterized by azimuthal p wave velocity differences of about 0 2 0 4 km s 1 with the fast direction at n120 e orthogonal to the upper mantle anisotropy the observed anisotropy is consistent with a model in which aligned olivine crystals cause anisotropy in the upper mantle and aligned cracks within layer 2 cause anisotropy in the crust we examine a model of p wave propagation from a point source in an isotropic layer above an anisotropic half space and show that under these conditions anisotropy can cause seismic ray paths to deviate from the vertical plane connecting source and receiver this deviation has a negligible effect on seismic travel times but must be taken into account in modelling p wave polarization anomalies p wave polarization anomalies within the anisotropic half space are the sum of the particle motion deviation from the ray path and ray path deviation from the source receiver azimuth and are typically range dependent p wave polarization anomalies will also occur in the overlying isotropic layer purely as a result of the ray path deviation with greater deviations at long ranges a simple model of upper mantle anisotropy 6kms 1 crust above 8 0 8 6 km s 1 anisotropic mantle produces surface polarization anomalies of 1 5 3 5 in practice p wave polarization anomalies are difficult to observe because they are small compared to observed scatter in polarization data and are sensitive to differences in horizontal seismometer gain levels we examine the implications of anisotropic ray path deviations on travel times and p wave polarizations for the ngendei data as well as some previous marine seismic refraction experiments

effective elasticity of fractured rocks a snapshot of the work in progress exploration and development of naturally fractured reservoirs rely on understanding and interpretation of certain signatures associated with seismic waves propagating through cracked rocks this understanding comes primarily from the effective media theories that predict an overall elastic behavior of a solid containing many inhomogeneities cracks in particular whose sizes are too small to be seen individually by the waves to model seismic responses of fractured formations a geophysicist typically has a choice between the effective media schemes of hudson and schoenberg while the two predictions usually deviate slightly for liquid filled cracks the differences are significant when the fractures are dry explaining the origin of these differences and selecting a more accurate scheme is the first goal of this tutorial our second more challenging task is to prove that simply adding the compliance contributions of cracks as if they were isolated and noninteracting remains sufficiently accurate even for fractures that grossly violate the basic theoretical assumption of penny shaped cracks real fractures have notoriously irregular shapes might be partially closed and often form interconnected networks yet these details of fracture microgeometry turn out to be unimportant for the effective elasticity given a typical noise level in seismic data no closed form theory exists for irregular fracture shapes however take into account finite element simulations on so called digital rocks demonstrate which features of the crack geometry have to be taken into account because they influence propagation of long compared to the size of fractures seismic waves and which features can be ignored

qvoa analysis p wave attenuation anisotropy for fracture characterization this paper investigates q anisotropy for characterizing fractured reservoirs specifically the variation of the seismic quality factor q versus offset and azimuth qvoa w e derive an analytical expression for p wave attenuation in a transversely isotropic medium with horizontal symmetry axis hti and provide a method qvoa for estimating fracture direction from azimuthally varying q in pp wave reflection data the qvoa formula is similar to ruger s approximation for pp wave reflection coefficients the theoretical basis for amplitude variation with angle offset avoa analysis the technique for qvoa analysis is similar to azimuthal avo analysis we introduce two new seismic attributes q versus offset qvo gradient and intercept qvo gradient inversion not only indicates fracture orientation but also characterizes q anisotropy we relate the q anisotropy parameter q to fractured medium parameters and invert the qvo gradient to estimate q the attenuation parameter q and thomsen style anisotropy parameter v are found to be interdependent the attenuation anisotropy magnitude strongly depends on the host rock s vs vp parameter whereas the dependence on fracture parameters is weak this complicates the qvo gradient inversion for the fracture parameters this result is independent of the attenuation mechanism to illustrate the qvoa method in synthetic data we use hudson s first order effective medium model of a dissipative fractured reservoir with fluid flow between aligned cracks and random pores as a possible mechanism for p wave attenuation

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

porosity and elastic anisotropy of rocks under tectonic stress and pore pressure changes elastic properties of rocks depend on tectonic stress using the theory of poroelasticity as a constraint we analyze features of these dependencies related to changes in rock pore space geometry we develop a formalism describing elastic moduli and anisotropy of rocks as nonlinear functions of confining stress and pore pressure this formalism appears to agree with laboratory observations to a first approximation elastic moduli and seismic velocities as well as porosity depend only on the difference between the confining tectonic stress and pore pressure however in general both the confining stress tensor and the pore pressure must be taken into account as independent variables the stress dependent geometry of the pore space fully controls the stress induced changes in elastic moduli and seismic velocities specifically the compliant porosity plays the most important role despite the fact that in many rocks the compliant porosity is a very small part of total porosity changes in compliant porosi

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

seismic anisotropy in the upper oceanic crust seismic anisotropy in the upper oceanic crust is observed in borehole data obtained at deep sea drilling project dsdp site 504b on dsdp leg 92 particle motion analysis of converted shear wave arrivals from explosive sources at various azimuths reveals a set of patterns which is indicative of hexagonally isotropic structure with a horizontal symmetry axis there are four diagnostic patterns 1 along symmetry axes where vertically polarized shear waves sv are generated but horizontally polarized shear waves sh are not generated the particle motions are purely vertical 2 for azimuths at which both sv and sh are generated and the sh velocity is significantly faster than sv a cruciform pattern with horizontal first motion is observed 3 for azimuths at which both are generated and the sv velocity is significantly faster than sh a cruciform pattern with vertical first motion is observed and 4 for azimuths at which both are generated and sv and sh velocities are similar elliptical particle motions are observed the shear wave particle motions and compressional wave travel times from a 2 km radius circle are consistent with an anisotropic model with hexagonal symmetry the compressional wave velocity has a two theta azimuthal variation between 4 0 and 5 0 km s the symmetry axis is horizontal with an azimuth of n20 w 10 the spreading direction at the site 6 m y age is north south the observed seismic anisotropy is most probably caused by the preferred orientation of large scale fractures and fissures in upper layer 2 which were created in the early stages of crustal development by near axis extensional processes and normal block faulting

shear wave splitting and reservoir crack characterization the coso geothermal field abstract this paper aims to improve current understanding of the subsurface fracture system in the coso geothermal field located in east central california the coso reservoir is in active economic development so that knowledge of the subsurface fracture system is of vital importance for an accurate evaluation of its geothermal potential and day to day production to detect the geometry and density of fracture systems we applied the shear wave splitting technique to a large number of high quality seismograms from local microearthquakes recorded by a permanent 16 station down hole 3 component seismic array running at 480 samples s the analysis of shear wave splitting seismic birefringence provides parameters directly related to the strike of the subsurface fractures and their density number of cracks per unit volume and consequently is an important technique to outline zones of high permeability three major fracture directions n10 30w n0 20e and n40 50e of which the first and the second are the most prominent were identified from the seismograms recorded by the 16 station down hole array all orientations are consistent with the known strike of local sets of faults and fractures in local wells and at the surface as well as with previous analyses of seismic anisotropy in the region the high quality of the recordings has allowed us to launch an unprecedented investigation into the characteristics of the temporal variations in crack polarization and crack density in a producing geothermal environment preliminary results point to significant temporal changes in shear wave time delays probably influenced by temporal changes in crack density within a period of 5 years 1996 2000 they are tentatively interpreted as due to a local 3 increase in shear wave velocity in the southwestern part of the field during 1999

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

azimuthally anisotropic elastic impedance inversion for fluid indicator driven by rock physics the normal to tangential fracture compliance ratio is usually used as a fracture fluid indicator ffi for fluid identification in fractured reservoirs with a new parameterization for fracture weaknesses we define a new ffi based on azimuthally anisotropic elastic impedance ei inversion and fractured anisotropic rock physics models first we derived a new azimuthally anisotropic ei equation with a similar expression for the isotropic and anisotropic ei parts to remove the exponential correction of ei which is attributable to weak anisotropy then we built a fractured anisotropic rock physics model used for the estimation of well log parameters for the normal and tangential fracture weaknesses which builds the initial background low frequency trend of fracture weaknesses finally based on the azimuthally anisotropic ei inversion method with the cauchy sparse and low frequency information regularization we estimated a ffi applied to fluid identification in fractured reservoirs tests on both synthe

frequency dependent anisotropy of porous rocks with aligned fractures naturally fractured reservoirs are becoming increasingly important for oil and gas exploration in many areas of the world because fractures may control the permeability of a reservoir it is important to be able to find and characterize fractured zones in fractured reservoirs the wave induced fluid flow between pores and fractures can cause significant dispersion and attenuation of seismic waves for waves propagating normal to the fractures this effect has been quantified in earlier studies here we extend normal incidence results to oblique incidence using known expressions for the stiffness tensors in the low and high frequency limits this allows us to quantify frequency dependent anisotropy due to the wave induced flow between pores and fractures and gives a simple recipe for computing phase velocities and attenuation factors of quasi p and sv waves as functions of frequency and angle these frequency and angle dependencies are concisely expressed through dimensionless velocity anisotropy and attenuation anisotropy parameters it is found that although at low frequencies the medium is close to elliptical which is to be expected as a dry medium containing a distribution of penny shaped cracks is known to be close to elliptical at high frequencies the coupling between p wave and sv wave results in anisotropy due to the non vanishing excess tangential compliance

seismic markers of the messinian salinity crisis in an intermediate depth basin data for understanding the neogene evolution of the corsica basin northern tyrrhenian sea the messinian salinity crisis msc which widely affected the whole mediterranean basin induced rapid and spectacular palaeoenvironmental changes it led to a major erosion of the onshore areas and the upper parts of the continental shelves and slopes as well as important evaporitic deposits in the offshore deep basins the corsica basin belongs to the so called intermediate depth basin type because of its location between the coastal plain shelf area of east corsica and deeper marine basins in the northern tyrrhenian sea the msc event took place in the corsica basin during a period of active regional tectonics contemporaneous with the opening of the tyrrhenian sea as well as during the development of the corsica basin and the elba pianosa ridge based on the interpretation of high resolution seismic reflection profiles and a new 1 250 000 scale synthetic geological map we establish that the msc is recorded in the corsica basin through the occurrence of two sedimentary formations bounded by three remarkable surfaces these seismic markers suggest local and temporal variations in relative water level and associated depositional environments that differ between the north and south of the corsica basin during the msc the northern sector golo basin was emerged whereas the southern sector orbo basin was often flooded or submerged in the orbo basin the deposits record both an episode of intra msc climate change and regional tectonic events during the msc the corsica basin was a perched lake isolated from the deeper tyrrhenian basins the connection with the tyrrhenian basins was re established during the final stages of the msc before the catastrophic pliocene reflooding initiated by retrogressive erosion during a relative lowstand an extensive network of incised valleys was thus established in the orbo basin allowing the drainage of the corsica basin southwards into the deeper tyrrhenian basins these markers record the active regional tectonic context throughout the msc the orbo basin was strongly subsident along its western rim while its eastern rim was uplifted in turn the golo basin subsided from middle pliocene times onwards while the northern eastern and southern edges of the corsica basin were deformed and uplifted

extensive dilatancy anisotropy beneath mount hood oregon and the effect of aspect ratio on seismic velocities through aligned cracks the azimuthal variations of p wave velocity measured in u s geological survey usgs seismic refraction experiments around mount hood oregon are modeled by propagation through the effective anisotropy of parallel vertical cracks the three refractors at 1 3 7 and 8 5 km depth show different behavior the shallow refractor at 1 km displays p wave velocity variations typical of the parallel water filled vertical microcracks found throughout much of the upper continental crust the arrivals from the refractor at 3 7 km display velocity variations consistent with either dry cracks or more probably with cracks filled with supercritical water as one interpretation of the heat flow around mount hood suggests that temperatures at 3 7 km would be above the critical temperature for water at in situ pressures the velocity variations at the deepest refractor at 8 5 km depth are poorly constrained but again suggest liquid filled cracks the possibility of high temperatures suggests that at this depth the cracks could be thin skins of melt along grain boundaries the vertical cracks in all three refractors strike in the northwest to southeast quadrants which is consistent with the compressional quadrants in the mechanisms of local earthquakes shear wave splitting reported elsewhere indicates that there are stress aligned water filled microcracks extensive dilatancy anisotropy or eda throughout at least the brittle upper 10 or 20 km of the continental crust the high heat flow in the high cascades may confine the range of crack behavior usually spread over the whole thickness of the crust to the top 10 km beneath mount hood thus the p wave velocity variations beneath mount hood may indicate that eda cracks occur throughout the whole of the continental crust the large number of shot to recorder travel times measured by the usgs allows extensive dilatancy anisotropy to be recognized from the analysis of p wave velocity variations for the first time