transient and steady state deformation of synthetic rocksalt abstract synthetic rocksalt with a porosity less than 2 5 and an average grain size of about 0 35 mm was made by warm pressing at 100 c 150 mpa and for 15 min cylinders of this material 25 mm in diameter by 50 mm long were deformed at strain rates of 0 1 ksec 1 at confining pressures of 20 50 100 and 200 mpa and at temperatures of 100 200 and 300 c the resulting stress time data show transient stress behavior before steady state stress occurs very little variation in the stress time data occurs for the above confining pressures at a constant temperature many of the tests reach steady state at 10 strain where all the experiments were terminated the differential stress at 10 varies from about 22 mpa at 100 c to about 6 mpa at 300 c these strengths are slightly less than those measured by heard 1972 also on synthetic polycrystalline rocksalt and are similar to those measured by wawersik and hannum 1980 and by hansen and mellegard 1980 in modified creep tests on coarse grained natural rocksalt under similar pressure and temperature conditions activation energies computed from these steady state stresses vary between 7 5 and 25 kcal mole and are consistent with those obtained by heard 1972 and by parrish and gangi 1977 1981 the stress time data were fit using one and two mechanism transient stress functions which assume independent mechanisms or processes the rms errors decrease from about 1 mpa for the one mechanism fits to about 0 2 mpa for the two mechanism fits indicating at least two mechanisms are operative in these tests similar one and two mechanism fits were made to creep tests performed by w wawersik et al on new mexico bedded rocksalt similar improvements in the fits were obtained for those tests that lasted long enough so that the effect of a second mechanism could be noted it was found that the steady state strain rate found in creep tests could be interpreted as the beginning of another mechanism this raises the question of whether the steady state phenomena exist at all or whether it is just an approximation to a mechanism with a time constant that is long compared to the length of the test

borehole stability analysis considering spatial variability and poroelastoplasticity abstract typically stability analysis of oil producing wells is carried out considering that both the hydraulic and mechanical parameters of the rock mass are deterministic when analyzing borehole stability two failure mechanisms are generally considered namely failure due to either tensile or compressive shear stresses these mechanisms are produced respectively by either too high or too low drilling fluid pressure mechanical and hydraulic properties of rock masses and in particular sedimentary rock masses may show a considerable degree of spatial variability this paper focuses on the evaluation of the limits for the internal pressure associated with a target probability of failure taking into account both the spatial variability of hydraulic and mechanical properties and the simple variability of the initial pore pressure and in situ stresses the analysis is performed with a finite element program that incorporates coupled fluid mechanical effects and elastoplastic behavior of the rock in this way the proposed borehole stability analysis can be set into a reliability based framework described in the paper examples are shown using stochastic data from the literature and conclusions are drawn regarding the effect of spatial variability on the borehole stability

compositional variation and the origin of deep crustal reflections abstract identifying the sources of crustal reflections is essential for deriving more geological information from deep crustal reflection profiles theoretical and model studies help place constraints on the role of compositional variation in producing deep crustal reflections analysis of laboratory derived velocities and densities of rock types typical for the continental crust indicates that reflection coefficients are generally small but significantly 17 of the possible reflection coefficients have magnitudes between 0 1 and 0 2 comparison between reflection coefficients derived from well logs and reflections observed in associated seismic profiles indicates that constructive interference associated with geological layering is at least as important as the magnitude of the reflection coefficients for producing detectable reflections constructive interference can increase reflection amplitude by two to three times but is limited to a relatively narrow range of layer thickness for a typical 10 40 hz seismic wavelet and typical crustal velocities of about 6 km s constructive interference occurs for layer thickness ranging between about 35 and 80 m layers thinner than 35 m interfere destructively if reflections result from compositional variation seismic models of hypothetical and observed geologic relations provide analogs for interpreting complex reflection patterns observed in deep crustal reflection profiles such models show reflection patterns similar to those observed in the reflection profiles the models indicate that the reflections could originate in the complexly deformed and intruded terranes that are common in the crystalline crust and it may not be necessary to appeal to unobserved phenomena such as special lamellae or fluid filled fractures to explain the reflections

structure and seismic response of extended continental crust stochastic analysis of the strona ceneri and ivrea zones italy a two dimensional stochastic seismic model of the crustal cross section exposed in the strona ceneri and ivrea zones northern italy was derived from geologic maps and petrophysical data with techniques used in microfabric analysis the model has a layered self affine structure which agrees with the intrinsic scale independence of geologic phenomena synthetic seismograms from this model replicate many pertinent features observed in deep seismic reflection data and allow insight into crustal structure 1 reflectivity depends on the lithological populations present and their distribution within a crustal unit 2 transparent zones within the reflective lower crust can arise from metamorphic reactions 3 the transition from upper to lower crust may be a change in small scale seismic fabric rather than a first order seismic discontinuity

polarization anisotropy and fine scale structure of the eurasian upper mantle we have observed shear wave splitting with tsh tsv tsv up to 1 5 on long period records of multiply reflected s waves bottoming in the upper mantle beneath the russian and siberian platforms the dispersion of love and rayleigh waves over these paths shows discrepancies of comparable or larger magnitude with respect to smooth isotropic si structures consistent with a model of the uppermost mantle having significant apparent vertical anisotropy although the splitting and dispersion data can be fit by smooth anisotropic sa models we have investigated the apparent anisotropy associated with fine scale rough structure beneath stable eurasia we fit the data with a rough isotropic ri model having an rms shear velocity fluctuation that varies from 14 in the uppermost mantle to zero at 400 km depth these fluctuations are larger than the variation in isotropically averaged parameters expected for even a diverse assemblage of upper mantle ultrabasic rocks which we take to be evidence for some sort of intrinsic local anisotropy

seismic properties of rock samples from the pikwitonei granulite belt god s lake domain crustal cross section manitoba laboratory measurements of compressional and shear wave velocity to confining pressures of 600 mpa for a suite of representative samples collected from the pikwitonei granulite belt and god s lake domain an archean crustal cross section in the northwestern superior province provide the basis of comparison of these terranes with the seismic characteristics of archean lower crust we found that felsic rocks in the pikwitonei granulite belt and god s lake domain which make up the bulk of these terranes have a similar average compressional wave velocity of 6 5 km s at 600 mpa indicating that felsic rocks show little velocity change across the amphibolite granulite facies transition compressional wave velocities for mafic rocks from each terrane are between 7 1 and 7 3 km s apparent poisson s ratio ranges from 0 24 to 0 26 and 0 26 to 0 28 for felsic and mafic rocks respectively these velocity data compare favorably with data for similar lithologies from the kapuskasing uplift using the relative abun

predicting the seismic implications of salt anisotropy using numerical simulations of halite deformation in the past the potential for seismic anisotropy in salt structures and its effect on their seismic imaging has received little attention we consider the plausibility of salt anisotropy through linked numerical studies of salt deformation and its seismic consequences numerical models are used to predict lattice preferred orientations lpos in halite polycrystalline aggregates subjected to axial extension and simple shear the elastic constants for the deformed polycrystalline aggregate are then calculated simple models representing a salt sill and the stem of a diapir are created using these elastic constants ray tracing is used to investigate the effects of halite lpo on the propagation of seismic waves the results suggest that salt anisotropy can cause significant traveltime effects and could lead to significant errors in seismic interpretation in salt environments if this anisotropy is ignored we also investigate potential amplitude variation with offset and azimuth avoa for the reflection from the top and bottom of an anisotropic salt sill ray paths with a shear wave leg within the salt display strong avoa effects with a clear four fold symmetry

the gassmann burgers model to simulate seismic waves at the earth crust and mantle the upper part of the crust shows generally brittle behaviour while deeper zones including the mantle may present ductile behaviour depending on the pressure temperature conditions moreover some parts are melted seismic waves can be used to detect these conditions on the basis of reflection and transmission events basically from the elastic plastic point of view the seismic properties seismic velocity and density depend on effective pressure and temperature confining and pore pressures have opposite effects on these properties such that very small effective pressures the presence of overpressured fluids may substantially decrease the p and s wave velocities mainly the latter by opening of cracks and weakening of grain contacts similarly high temperatures induce the same effect by partial melting to model these effects we consider a poro viscoelastic model based on gassmann equations and burgers mechanical model to represent the properties of the rock frame and describe ductility in which deformation takes place by shear plastic flow the burgers elements allow us to model the effects of seismic attenuation velocity dispersion and steady state creep flow respectively the stiffness components of the brittle and ductile media depend on stress and temperature through the shear viscosity which is obtained by the arrhenius equation and the octahedral stress criterion effective pressure effects are taken into account in the dry rock moduli using exponential functions whose parameters are obtained by fitting experimental data as a function of confining pressure since fluid effects are important the density and bulk modulus of the saturating fluids water and steam are modeled using the equations provided by the nist website including supercritical behaviour the theory allows us to obtain the phase velocity and quality factor as a function of depth and geological pressure and temperature as well as time frequency we then obtain the ps and sh equations of motion recast in the velocity stress formulation including memory variables to avoid the computation of time convolutions the equations correspond to isotropic anelastic and inhomogeneous media and are solved by a direct grid method based on the runge kutta time stepping technique and the fourier pseudospectral method the algorithm is tested with success against known analytical solutions for different shear viscosities an example shows how anomalous conditions of pressure and temperature can in principle be detected with seismic waves

wide angle seismic reflections from two dimensional random target zones we have analyzed the shot gather seismic response of a buried zone of isotropic random velocity fluctuations scattering analysis predicts that events at offsets greater than the depth of the target zone will appear more continuous and coherent than events at zero and small offsets due to an effective dip filter inherent to the source target receiver geometry the scattering theory is confirmed with finite difference synthetic seismograms calculated for an idealized crustal model the closely spaced synthetic seismograms show bright events having the appearance of refraction arrivals which suggest a horizontally layered structure in the target zone these effects appear to be independent of the magnitude of the velocity fluctuations provided that the magnitude of the fluctuations is small 3 0 of the mean velocity we have made a second finite difference calculation of a crustal scale model to reinterpret previously published wide angle data from the black forest germany sandmeier and wenzel 1986 the response of the lower crust seen in the conventionally recorded field data can be explained by a lower crustal model of isotropic small amplitude random velocity fluctuations we present this in contrast to a laterally homogeneous model of a finely layered lower crust of alternating high and low velocities proposed by sandmeier and wenzel 1986 the synthetic studies and the scattering analysis demonstrate that the geometry of the wide angle experiment produces signals which appear to be from layered target zones even where no layering exists they also underscore the importance of conducting seismic surveys which make use of closely spaced receivers and which integrate vertical incidence and wide angle recording

small scale heterogeneity and large scale velocity structure of the continental crust since conrad 1925 first identified a midcrustal refraction event in central europe seismologists have developed layered models of the continental crust with velocity steps at various levels modern analysis of crustal refraction data including automated inversion relies on ray theoretical techniques or wave theory for plane layered media which generally parameterize the earth as a series of grossly horizontal layers the bias toward layered models is in part based on physical and geologic intuition but it is to a large measure a result of available interpretation methods for example layer based reflectivity and ray tracing methods in contrast reflection seismology has produced a picture of a highly heterogeneous crust with wavelength scale variations in velocity existing at different levels of the crust in different tectonic regimes finite difference modeling shows that zones of small amplitude random velocity fluctuations not only produce short discontinuous reflection segments as observed on deep seismic reflection data but also produce strong wide angle reflections some of the events identified as crustal wide angle reflections in field data may in fact be the result of scattering from wavelength scale heterogeneities and need not result from reflection from a first or second order velocity discontinuity significant changes in the large scale velocity structure associated with zones of random velocity fluctuations produce relatively small changes in the dynamic properties of the backscattered wave field this observation is in disagreement with the common credo that the seismic refraction method is primarily sensitive to changes in the large scale velocity structure as a consequence the ray theoretical interpretation of such wide angle reflections as first order discontinuities leads to crustal velocity structures that are nonunique and may be erroneous in detail while correctly predicting average crustal velocity and thickness we are not suggesting that crustal velocity does not change with depth on the large scale rather we are proposing that crustal seismic events observed at intermediate and large offsets may be the result of scattering from wavelength scale velocity fabric rather than specular reflection or refraction from first order discontinuities

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

stochastic characterization and seismic response of upper and middle crustal rocks based on the lewisian gneiss complex scotland summary we have developed statistical models of upper and middle crustal seismic velocity heterogeneity based on geologic maps and petrophysical data from the lewisian gneiss complex scotland the level of heterogeneity we have measured is relevant to seismic exploration of the crystalline crust using conventional reflection and refraction techniques we digitized two 1 10560 geologic maps of laxfordian archean age lewisian rocks on a 26 8m grid both maps are believed to be representative of the upper and middle crust in north western scotland and both are believed to provide cross sectional views of parts of the crust the digital maps were characterized by the statistics of their lithologic populations and by their 2 d spatial autocorrelation functions different lithologies were assigned seismic velocities appropriate for the mid crust using petrophysical data three lithologies are dominant silicic gneisses vp 6 2 km s 1 mafic amphibolites vp 6 75 kms 1 and intermediate composition schists vp 6 5kms 1 both maps have self affine spatial fabrics the first map covers the core of a syncline its autocorrelation function defines a medium with a fractal dimension of 2 78 a horizontal characteristic length of about 244m and a vertical correlation of about 133m aspect ratio is 2 1 it has an essentially trimodal velocity lithology population consisting of 37 per cent silicicgneiss 43 per cent mafic amphibolites and 20 per cent schists this map is representative of 30 40 per cent of laxfordian rocks the second map is a plan view which can be rotated 90 about an axis perpendicular to strike to give a cross section this map is characterized by a fractal dimension of 2 55 with a horizontal correlation length of about 111m and a vertical correlation of about 38m aspect ratio 3 1 it has a nearly bimodal population consisting of 77 per cent silicic gneisses and 22 per cent mafic amphibotites it is representative of 60 70 per cent of laxfordian rocks lastly we examine the seismic response of an upper to middle crust based on our statistical models using acoustic and elastic 2 d finite difference synthetic seismograms short offset shot records demonstrate that a lewisian upper crust produces scattered waves which significantly disrupt signals reflecting from deeper levels measurements of transmission scattering q and coda decay rates confirm that seismic scattering in lewisian type crust is strong the migrated cmp response of a lewisian crustal model shows the characteristic salt and pepper pattern often observed in the upper crust and described incorrectly as transparent we suggest that translucent is a more appropriate descriptor

finite difference simulations of seismic scattering implications for the propagation of short period seismic waves in the crust and models of crustal heterogeneity synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two dimensional media with random spatial variations in seismic velocity the results of this study provide important insights about the propagation of short period 5 the self similar medium is characterized by a scattering q that is constant with frequency whereas theory predicts that the apparent q in an exponential medium is proportional to frequency these alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal q at frequencies greater than about 1 hz assuming that scattering is responsible for most of the attenuation at these frequencies measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation scattering q 200 in these cases q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission q of the medium the cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium the self similar random medium proposed here for the crust produces waveform variations at high frequencies 15 30 hz similar to those reported for actual small scale seismic arrays with apertures of hundreds of meters

large scale anisotropy within the crust in the basin and range province seismic reflection records of the 1986 nevada passcal seismic experiment in the roughly east west direction dip line show a highly reflective crust with a broad diffuse moho which is characterized at normal incidence by strong subparallel laterally discontinuous multicyclic events lacking observable diffractions recordings in the roughly north south direction strike line show a transparent crust with short scattered events and a strong continuous moho elastic finite difference modeling indicates that 1 effects due to the topography of the reflecting interfaces and or the existence of heterogeneities increase the travel times thus lowering the velocity estimates and 2 the seismic response of a layered structure can be disrupted from a strong laterally continuous event to a series of short multicyclic events by increasing the dimensions of the overlying heterogeneities finite difference modeling demonstrates that large scale anisotropy achieved by roughly east west alignment of strongly elongated lenses boudins located at mid and lower crustal depths can explain the observed differences in the seismic response along the perpendicular lines

an approach to estimate gas hydrate saturation from 3 d heterogeneous resistivity model a study from krishna godavari basin eastern indian offshore abstract a method is proposed to estimate gas hydrate saturation from three dimensional 3 d heterogeneous model of resistivity simulated using resistivity log pure gas hydrates are highly resistive compared to the host sediments and their presence in the pore space of sediments increase the resistivity of the formation the anomalous increase of resistivity is used as a proxy for the delineation of gas hydrates using the resistivity log a 3 d heterogeneous resistivity model has been constructed from one dimensional resistivity log in krishna godavari kg basin eastern indian offshore the simulated model contains all small scale variation in resistivity of the reservoir and maintains all properties associated with covariance like root mean square fluctuation characteristic scales and fractal dimension of the observed log we have estimated volumetric hydrate saturation using the three dimensional simulated model the porosity used for estimating hydrate saturation is calculated from the simulated density field generated using the observed density log estimated average gas hydrate saturation is about 9 84 of the pore volume over a 1000 m 1000 m x 131 m cubic meters area

non transparent uppermost mantle in the island arc region of japan abstract in japan the crust and uppermost mantle seismic character is yet unimaged although many refraction surveys have been recorded the longest seismic profiles are analyzed a remarkable feature a long duration coda wave after the pmp wave reflected wave at the moho boundary is observed on the record sections several possible models are considered to explain the long duration coda wave the model with many scatterers located in the uppermost mantle explains the observed data well while the undulating moho and continuous layering models do not account for some aspects of the observed data the scatterer distributed uppermost mantle is not consistent with that of continental region which is often characterized as transparent we estimate the scattering coefficient of the uppermost mantle and crust using simulations the scattering coefficients obtained for upper crust lower crust and uppermost mantle are 0 01 0 02 and 0 025 respectively the scattering coefficient of the uppermost mantle is slightly larger than that of lower crust which is characterized as being reflective the many scatterers in the uppermost mantle might be related to magmatism in japan this will be one of the important observations for understanding formation processes of the moho boundary and uppermost mantle in the island arc environment

pore pressure estimation in reservoir rocks from seismic reflection data a method is used to obtain pore pressure in shaly sandstones based upon an acoustic model for seismic velocity versus clay content and effective pressure calibration of the model requires log data porosity clay content and sonic velocities to obtain the dry rock moduli and the effective stress coefficients as a function of depth and pore pressure the seismic p wave velocity derived from reflection tomography is fitted to the theoretical velocity by using pore pressure as the fitting parameter this approach based on a rock physics model is an improvement over existing pore pressure prediction methods which mainly rely on empirical relations between velocity and pressure the method is applied to the tune field in the viking graben sedimentary basin of the north sea we have obtained a high resolution velocity map that reveals the sensitivity to pore pressure and fluid saturation in the tarbert reservoir the velocity map of the tarbert reservoir and the inverted pressure distribution agree with the structural features of the tarbert formation and its known pressure compartments

psv wave propagation in a medium with local heterogeneities a hybrid formulation and its application summary hybrid methods which combine so called numerical and analytical methods are a successful tool for computing synthetic seismograms for media with localized heterogeneous regions each of the two methods is applied in that part of the medium in which it can be employed efficiently this paper deals with a combination of a finite difference algorithm applied in the neighbourhood of the heterogeneity and a frequency wavenumber filter for the psv wave equation applied outside this method is an alternative to the use of green s theorem as a wavefield continuation method e g van den berg 1984 in the hybrid method presented here absorption can be incorporated additionally for different models with first and second order discontinuities the hybrid method was tested successfully against the reflectivity method the hybrid method was applied to study the effects of a heterogeneous lower crust on the wavefield this application has been inspired by recent deep seismic reflection profiling which often reveals a highly reflecting lower crust the scattering models have been divided into three groups horizontal vertical and isotropic scatterers the comparison of the reflection responses of these models shows that thin horizontal or small isotropic scatterers yield high reflected energy coherent over a few kilometres in the time window of lower crustal reflections vertical structures characterized by a small correlation length in horizontal direction have less energy in this time window but they can generate a complex moho reflection with a strong moho coda it is also shown that the acoustic approximation is mostly not sufficient to describe the wave propagation in such complex media

constraints of crustal heterogeneity and q f from regional 4 hz wave propagation for the 2009 north korea nuclear testconstraints of crustal heterogeneity and q f from regional 4 hz wave propagation

gas hydrate saturations estimated from fractured reservoir at site nghp 01 10 krishna godavari basin india 1 during the indian national gas hydrate program expedition 01 nghp 01 one of the richest marine gas hydrate accumulations was discovered at site nghp 01 10 in the krishna godavari basin the occurrence of concentrated gas hydrate at this site is primarily controlled by the presence of fractures assuming the resistivity of gas hydrate bearing sediments is isotropic the conventional archie analysis using the logging while drilling resistivity log yields gas hydrate saturations greater than 50 as high as 80 of the pore space for the depth interval between 25 and 160 m below seafloor on the other hand gas hydrate saturations estimated from pressure cores from nearby wells were less than 26 of the pore space although intrasite variability may contribute to the difference the primary cause of the saturation difference is attributed to the anisotropic nature of the reservoir due to gas hydrate in high angle fractures archie s law can be used to estimate gas hydrate saturations in anisotropic reservoir with additional information such as elastic velocities to constrain archie cementation parameters m and the saturation exponent n theory indicates that m and n depend on the direction of the measurement relative to fracture orientation as well as depending on gas hydrate saturation by using higher values of m and n in the resistivity analysis for fractured reservoirs the difference between saturation estimates is significantly reduced although a sizable difference remains to better understand the nature of fractured reservoirs wireline p and s wave velocities were also incorporated into the analysis

effective q determination using frequency methods on birps data abstract an investigation to determine q for compressional waves from marine reflection seismic records is presented the spectral ratio method is used to determine values for the lower crust results from spectral modelling from a known synthetic source wavelet are used to give minimum estimates for the whole crust the effective q for the reflective lower crust around britain is found to be 500 200 which is consistent with laboratory results for dry rocks and is probably too large to allow significant amounts of fluid to be present

composition of the lower crust in west central arizona from three component seismic data new p and s wave seismic data from the basin and range province colorado plateau transition zone allow a better determination of the composition of the lower crust than would be possible from p wave data alone the data were collected in 1987 by recording large seismic refraction shots into stationary reflection spreads during the us geological survey s pacific to arizona crustal experiment significant seismic observations about the lower crust include 1 strong p wave reflections at its top 2 moderate strength s wave reflections from the entire lower crust 3 an average p wave velocity of 6 6 plus minus 0 15 km s sup minus 1 and 4 an average poisson s ratio of sigma 0 27 plus minus 0 02 using these constraints and physical property measurements from laboratory rock samples the authors conclude that the average composition of the lower crust in this region is intermediate to mafic and that mafic underplating has been minor consequently the cenozoic age mafic and ultramafic xenoliths that are found in the transition zone must have resided in the upper mantle or in a thin transition zone at the base of the crust since these rock have higher p wave velocities and poisson s ratio than observed seismically

modeling of the influence of gas hydrate content on the electrical properties of porous sediments the geophysical investigation of gas hydrate accumulations within marine sediments and the sediments of permafrost regions suffer from a lack of information on the influence of the hydrate content on the physical sediment properties the estimation of the hydrate content using downhole electrical measurements based on archie s law requires the knowledge of the saturation exponent the saturation exponent is an empirical parameter that includes influences from the internal rock structure such as pore shape connectivity and constrictivity of the pore network and the distribution of the conducting phase based on different models that account for the different morphological forms of gas hydrates found during gas hydrate sampling in various research wells the influence of gas hydrate content on the electrical properties of the hydrate bearing sediment was investigated for all studied forms of hydrate occurrence disseminated in the pore space nodular and layered the saturation exponent depends on the sediment properties and on saturation itself the growth of gas hydrate nodules lenses and layers is a process that is assumed to result in the displacement and compaction of the surrounding sediment because of this change of sediment properties during hydrate generation the saturation exponent for these forms of hydrate occurrence depends strongly on the relationship between porosity and formation resistivity factor expressed in the form of archie s cementation exponent for the case that hydrate occurs disseminated in the pore space and the assumption that capillary effects are important for hydrate generation the saturation exponent depends on grain size and grain size sorting for the parameters chosen for these model calculations the saturation exponent varies between 0 5 and 4 the use of a constant mean value for the saturation exponent of approximately 2 can result in both underestimation and overestimation of the hydrate content

gas hydrate versus geological features the south shetland case study abstract a gas hydrate reservoir is hosted in marine sediments of an accretionary prism located offshore the south shetland islands antarctic peninsula and affected by widespread deformations to analyse gas hydrate distribution and fluid circulation inside sediments available velocity models were used seismic velocities are translated in terms of hydrate porosity which is the difference between the reference porosity i e the porosity without gas hydrate and the effective porosity i e the porosity reduced by the gas hydrate presence the pre stack depth migration sections underlined the presence of several geological features such as gentle and open folds fractures and faults in this paper we observed a relationship between syncline anticline structures and hydrate presence in particular a relationship is underlined between the hydrate porosity values and the distance from the hinge of the anticline the hydrate porosity increases toward the limbs of anticline the micro fracturing model supports the idea that the syncline favours the hydrate formation while the anticline favours the free gas accumulation below the bottom simulating reflector

gas hydrate saturation in the krishna godavari basin from p wave velocity and electrical resistivity logs abstract during the indian national gas hydrate program nghp expedition 01 a series of well logs were acquired at several sites across the krishna godavari kg basin electrical resistivity logs were used for gas hydrate saturation estimates using archie s method the measured in situ pore water salinity seafloor temperature and geothermal gradients were used to determine the baseline pore water resistivity in the absence of core data arp s law was used to estimate in situ pore water resistivity uncertainties in the archie s approach are related to the calibration of archie coefficient a cementation factor m and saturation exponent n values we also have estimated gas hydrate saturation from sonic p wave velocity logs considering the gas hydrate in frame effective medium rock physics model uncertainties in the effective medium modeling stem from the choice of mineral assemblage used in the model in both methods we assume that gas hydrate forms in sediment pore space combined observations from these analyses show that gas hydrate saturations are relatively low s h 15 20 in two zones between 168 and 198 mbsf site nghp 01 05 s h 35 38 in two discrete zone between 70 and 90 mbsf and site nghp 01 07 shows the gas hydrate saturation more than 25 in two zones between 75 and 155 mbsf a total of 10 drill sites and associated log data regional occurrences of bottom simulating reflectors from 2d and 3d seismic data and thermal modeling of the gas hydrate stability zone were used to estimate the total amount of gas hydrate within the kg basin average gas hydrate saturations for the entire gas hydrate stability zone seafloor to base of gas hydrate stability sediment porosities and statistically derived extreme values for these parameters were defined from the logs the total area considered based on the bsr seismic data covers 720 km 2 using the statistical ranges in all parameters involved in the calculation the total amount of gas from gas hydrate in the kg basin study area varies from a minimum of 5 7 trillion cubic feet tcf to 32 1 tcf

the present day deformation of the central zagros from gps measurements 1 in 1997 and in 2000 we measured the distances between 14 geodetic benchmarks across the central zagros mountain belt the results show that about 10 mm yr of shortening in the central zagros is distributed across the mountain belt this shortening corresponds to roughly 50 of the total convergence between arabia and eurasia and is consistent in direction the persian gulf does not deform significantly the main zagros reverse fault is not an active kinematic boundary the internal deformation of the folded belt is rather homogeneous at the scale of our survey which does not allow us to detect any individual active blind fault however the strain pattern suggests that n s dextral strike slip faults may accommodate part of the deformation

coincident seismic reflection refraction studies of the continental lithosphere a global review nearly 50 coincident seismic reflection refraction studies to depths of at least the moho provide an improved understanding of the continental lithosphere some conclusions include the following 1 a transparent upper crust a common observation on vertical reflection profiles cannot generally be correlated with velocity gradients or low velocity zones rather a commonly transparent upper crust may be explained by short wavelength steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source generated noise 2 the reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity 6 6 7 3 km s and to consist of laminated high and low velocity layers with typical thicknesses of 100 200 m 3 landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high and low velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin 4 the crust mantle boundary may differ sufficiently when imaged with vertical incidence and wide angle data to justify the retention for the present of the concept of separate reflection and refraction mohos while there is good evidence that these features are coincident within measurement uncertainties in most regions recently recorded data from the basin and range admit the possibility for noncoincidence in that area 5 upper mantle reflections which cannot be migrated into the lower crust remain rare despite isolated unequivocal examples thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust the wide angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come 6 there appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes 7 no truly three dimensional coincident experiment i e including three dimensional migration has been conducted but some three dimensional data have been collected using both methods measurements of attenuation poisson s ratio and anisotropy within the crust using coincident data sets remain frontiers

seismic rheological model and reflection coefficients of the brittle ductile transition it is well established that the upper cooler part of the crust is brittle while deeper zones present ductile behaviour in some cases this brittle ductile transition is a single seismic reflector with an associated reflection coefficient we first develop a stress strain relation including the effects of crust anisotropy seismic attenuation and ductility in which deformation takes place by shear plastic flow viscoelastic anisotropy is based on the eigenstrain model and the zener and burgers mechanical models are used to model the effects of seismic attenuation velocity dispersion and steady state creep flow respectively the stiffness components of the brittle and ductile media depend on stress and temperature through the shear viscosity which is obtained by the arrhenius equation and the octahedral stress criterion the p and s wave velocities decrease as depth and temperature increase due to the geothermal gradient an effect which is more pronounced for shear waves we then obtain the reflection and transmission coefficients of a single brittle ductile interface and of a ductile thin layer the pp scattering coefficient has a brewster angle a sign change in both cases and there is substantial ps conversion at intermediate angles the pp coefficient is sensitive to the layer thickness unlike the ss coefficient thick layers have a well defined brewster angle and show higher reflection amplitudes finally we compute synthetic seismograms in a homogeneous medium as a function of temperature

sensitivity of the lateral correlation function in deep seismic reflection data in the absence of multiple scattering the lateral correlation function of the reflected wavefield can be related to the lateral correlation function of the medium we have analyzed the lateral correlation function of a series of synthetic seismograms from stochastic models of reflective lower crust with the structural and petrophysical statistics of the ivrea zone our results show that in the presence of ivrea type lower crustal heterogeneity the lateral correlation function of the backscattered wavefield is insensitive to the lateral correlation function of the scattering medium which suggests that multiple scattering is important in the reflective lower crust this interpretation is consistent with lower crustal reflections commonly being shorter than a fresnel zone the absence of clear diffractions and the poor performance of standard migration algorithms on deep seismic reflection data

birps deep seismic reflection studies of the british caledonides the western isles north channel winch traverse fig 1 an extension of the successful moist profile1 was recorded in 1982 at sea along the west coast of britain for birps british institutions reflection profiling syndicate by the geophysical company of norway geco the purpose of the winch traverse was to study crustal structure of the caledonian foreland to cross the caledonian orogen and to establish the three dimensional geometry of mantle reflectors originally seen on moist we describe here the data first emphasizing british caledonian structures and then discussing features of the deep crust of wider significance the data are of very good quality and contain clear moho and upper mantle reflections the lower crust is surprisingly reflective deeply penetrative thrusts are observed and there is firm evidence that several of the mesozoic basins round the shores of the united kingdom were formed by reactivation of older features

generalized dix equation and analytic treatment of normal moveout velocity for anisotropic media despite the complexity of wave propagation in anisotropic media reflection moveout on conventional common midpoint cmp spreads is usually well described by the normal moveout nmo velocity defined in the zero offset limit in their recent work grechka and tsvankin showed that the azimuthal variation of nmo velocity around a fixed cmp location generally has an elliptical form i e plotting the nmo velocity in each azimuthal direction produces an ellipse and is determined by the spatial derivatives of the slowness vector evaluated at the cmp location this formalism is used here to develop exact solutions for the nmo velocity in anisotropic media of arbitrary symmetry for the model of a single homogeneous layer above a dipping reflector we obtain an explicit nmo expression valid for all pure modes and any orientation of the cmp line with respect to the reflector strike the contribution of anisotropy to nmo velocity is contained in the slowness components of the zero offset ray along with the derivatives of the vertical slowness with respect to the horizontal slownesses quantities that can be found in a straightforward way from the christoffel equation if the medium above a dipping reflector is horizontally stratified the effective nmo velocity is determined through a dix type average of the matrices responsible for the interval nmo ellipses in the individual layers this generalized dix equation provides an analytic basis for moveout inversion in vertically inhomogeneous arbitrarily anisotropic media for models with a throughgoing vertical symmetry plane i e if the dip plane of the reflector coincides with a symmetry plane of the overburden the semi axes of the nmo ellipse are found by the more conventional rms averaging of the interval nmo velocities in the dip and strike directions modelling of normal moveout in general heterogeneous anisotropic media requires dynamic ray tracing of only one zero offset ray remarkably the expressions for geometrical spreading along the zero offset ray contain all the components necessary to build the nmo ellipse this method is orders of magnitude faster than multi azimuth multi offset ray tracing and therefore can be used efficiently in traveltime inversion and in devising fast dip moveout dmo processing algorithms for anisotropic media this technique becomes especially efficient if the model consists of homogeneous layers or blocks separated by smooth interfaces the high accuracy of our nmo expressions is illustrated by comparison with ray traced reflection traveltimes in piecewise homogeneous azimuthally anisotropic models we also apply the generalized dix equation to field data collected over a fractured reservoir and show that p wave moveout can be used to find the depth dependent fracture orientation and to evaluate the magnitude of azimuthal anisotropy

slowness domain offset and traveltime approximations in layered vertical transversely isotropic media considering horizontally layered transversely isotropic media with vertical symmetry axis and all types of pure mode and converted waves we present a new wide angle series approximation for the kinematical characteristics of reflected waves horizontal offset intercept time and total reflection traveltime as functions of horizontal slowness the method is based on combining gluing both zero offset and large finiteoffset series coefficients the horizontal slowness is bounded by the critical value characterised by nearly horizontal propagation within the layer with the highest horizontal velocity the suggested approximation uses five parameters to approximate the offset six parameters to approximate the intercept time or the traveltime and seven parameters to approximate any two or all three kinematical characteristics overall the method is very accurate for pure mode compressional waves and shear waves polarised in the horizontal plane and for converted waves the application of the method to pure mode shear waves polarised in the vertical plane is limited due to cusps and triplications to demonstrate the high accuracy of the method we consider a synthetic multi layer model and we plot the normalised errors with respect to numerical ray tracing

creep of rocksalt abstract a review is presented of the fundamental flow properties and processes in experimentally deformed natural and synthetic halite single crystals and polycrystalline aggregates included in the summary are discussions of a microstructures induced during steady state creep b creep rupture of rocksalt c experiments associated with project salt vault and more recent field studies and d brine migration a representative steady state flow law determined for natural aggregates and maximum natural deviatoric stresses deduced from subgrain sizes are applied briefly to considerations of creep in waste repositories and of salt dome dynamics while the mechanical behavior of rocksalt is probably better understood than for all other rock types further investigations especially on load path stress history and creep rupture are clearly mandated furthermore additional investigations of brine migration and of bench and field scale deformations are needed the latter incorporating realistic rocksalt flow properties into numerical simulations of natural rock mass response

stochastic modeling of the reflective lower crust petrophysical and geological evidence from the ivera zone northern italy crustal scale seismic wave equation modeling demonstrates that wavelength scale velocity fluctuations can dominate the seismic response over a wide range of offsets and that a wide range of velocity distributions qualitatively can explain deep seismic data this range can be narrowed by studying the detailed petrophysical and structural properties of middle and lower crustal exposures the ivrea zone in the southern alps is a well studied sliver of extended lower continental crust brought into its present upright position as a result of late alpine lithospheric shortening the high degree of structural and petrophysical complexity and in particular the limited exposure of the ivrea zone favor a statistical rather than deterministic approach to develop a two dimensional crustal scale seismic model for this purpose we digitized two 1 25 000 geological maps from the central ivrea zone on a 60 m grid we find that the ivrea zone is characterized by a bimodal velocity distribution and a band limited self affine structure having a fractal dimension of 2 7 a characteristic horizontal scale of 500 to 1000 m and an aspect ratio around 4 0 assuming statistical stationarity a realization of this stochastic model was used to develop synthetic seismograms typical of ivrea type lower continental crust our results indicate that 1 ivrea type lower crust can explain qualitatively commonly observed lower crustal reflectivity 2 relatively minor changes in bulk composition and or metamorphic grade may lead to dramatic changes in reflectivity which cannot be explained by current generic models on the origin of lower crustal reflections and 3 travel time interpretation of the synthetic wide angle data may lead to erroneous large scale velocity structures

rational interpolation of qp traveltimes for semblance based anisotropy estimation in layered vti media the p domain is the natural domain for anisotropy parameterestimationinhorizontallylayeredmedia theneedto transformthedatatothe p domainortopicktraveltimesin thet xdomainis however apracticaldisadvantage toovercomethis wecombine p derivedtraveltimesandoffsetsin horizontally layered transversely isotropic media with a verticalsymmetryaxisvtiwitharationalinterpolationprocedureappliedinthet xdomain thiscombinationresultsinan accurate and efficient t x based semblance analysis for anisotropy parameter estimation from the moveout of qpwaves in horizontally layered vti media the semblance analysisisappliedtothemoveouttosearchdirectlyfortheinterval values of the relevant parameters to achieve this the method is applied in a layer stripping fashion we demonstrate the method using synthetic data examples and show thatitisrobustinthepresenceofrandomnoiseandmoderate statics

statistical approaches to interpretation of seismic reflection data abstract statistical approaches offer a potentially powerful tool for interpretation of the complex seismic reflection patterns typically recorded from the crystalline crust these methods focus on characterization of the complexity scaling characteristics and spatial variability of the wave field with the ultimate aim of recovering information on the geometric variation of the geology in this paper we examine the potential and limitations of statistical approaches focusing on the issues of quality of the statistical estimator how noise and migration affect spatial statistics and the potential for both qualitative and quantitative approaches to statistics based interpretation the limited spatial bandwidth of typical seismic data presently inhibits complete quantitative recovery of the spatial properties of the geology migration which should increase the spatial bandwidth has to date not proven particularly effective on complex wave fields mapping of relative variation in the spatial properties of the wave field reveals subtle differences that are directly associated with the geometric character of the geology such mapping allows identification and mapping of variations in the geologic fabric at scales significantly larger than the seismic wavelength and objective access to scaling information that has previously been unavailable

full azimuth subsurface angle domain wavefield decomposition and imaging part i directional and reflection image gathers we present a new subsurface angle domain seismic imaging systemforgeneratingandextractinghigh resolutioninformation about subsurface angle dependent reflectivity the system enables geophysicists to use all recorded seismic data in a continuousfashiondirectlyinthesubsurfacelocalangledomainlad resulting in two complementary full azimuth common imageangle gather systems directional and reflection the complete setofinformationfrombothtypesofanglegathersleadstoaccurate high resolution reliable velocity model determination and reservoir characterization the directional angle decomposition enables the implementation of specular and diffraction imaging in real 3d isotropic anisotropic geological models leading to simultaneous emphasis on continuous structural surfaces and discontinuous objects such as faults and small scale fractures structural attributes at each subsurface point e g dip azimuth andcontinuity canbederiveddirectlyfromthedirectionalangle gathers the reflection angle gathers display reflectivity as a function of the opening angle and opening azimuth these gathers are most meaningful in the vicinity of actual local reflecting surfaces wherethereflectionanglesaremeasuredwithrespectto the derived background specular direction the reflection angle gathers are used for automatic picking of full azimuth angle domainresidualmoveoutsrmowhich togetherwiththederived background orientations of the subsurface reflection horizons provide a complete set of input data to isotropic anisotropic tomography the full azimuth angle dependent amplitude variations are used for reliable and accurate amplitude versus angle andazimuthavazanalysisandreservoircharacterization the proposed system is most effective for imaging and analysis below complex structures such as subsalt and subbasalt high velocity carbonate rocks shallow low velocity gas pockets and others in addition it enables accurate azimuthal anisotropic imaging and analysis providing optimal solutions for fracture detectionandreservoircharacterization

deep crustal extensional faulting in the ivrea zone of northern italy abstract the ivrea zone in northern italy consists of amphibolite and granulite facies metasedimentary metabasic and ultrabasic rocks and is thought to represent a section through the deep continental crust detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones which after the effects of later low temperature faulting and folding have been removed appears to accommodate extension of the sequence the shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks and locally show evidence for development under prograde conditions these observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting in this model the effects of shear heating and displacement of cool hanging wall rocks against hotter footwall rocks are calculated these observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust especially when basic rocks are the dominant components of the rock pile a restored section through the extended lower crust as observed in the ivrea zone is presented and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed

physical properties of shale at temperature and pressure deformational properties p wave and s wave velocities and electrical resistivity were measured for three north sea malm shales in the laboratory under pressures to 800 bars and temperatures to 100 c these data were used to evaluate how factors such as mineralogy microstructure compaction and pore fluid conductivity affect a shale s seismic and electrical responses deformation in the shales is dominated by inelastic processes which cause time dependent changes in velocity resistivity and pore pressure overall shales are less sensitive to pressure changes as compared to sandstones of similar porosity however changes in temperature result in large changes in physical properties as compared to sandstones or shaly sands p wave and s wave velocities may decrease by as much as 10 percent over the temperature range studied and calculated activation energies for surface conduction are nearly twice those observed in shaly sands these comparisons emphasize fundamental differences in fabric among the

the crust as a heterogeneous optical medium or crocodiles in the mist abstract based on petrophysical data geologic maps and a well log we present statistical descriptions of likely upper middle and lower crustal rocks to characterize the fine scale heterogeneity observed in crustal exposures and inferred from deep crustal seismic data the statistical models developed for granitic and metamorphic upper crust and for an extended metamorphic lower crust are used to construct whole crustal models of seismic velocity heterogenity we present finite difference synthetic cmp data from several models which compare favorably with field data the statistical models also permit classification of the seismic reflection experiment and the crustal heterogeneity according to scattering regime the optical or scattering properties of importance for classification are the velocity fluctuation intensity the horizontal and vertical correlation lengths of the medium the correlation function of the medium and the velocity population function for the crustal properties we measured the bandwidth of a typical deep crustal experiment overlaps from the weak to the strong scattering regime with implications for crustal seismic data processing and imaging notably deep crustal signals are likely to have experienced multiple scattering making common seismic imaging techniques of questionable value moreover the details of the unmigrated cmp stacked section bears little resemblance to the underlying medium

incorporating sinuous connectivity into stochastic models of crustal heterogeneity examples from the lewisian gneiss complex scotland the franciscan formation california and the hafafit gneiss complex egypt stochastic models are valuable and sometimes essential tools for investigating the behavior of complex phenomena in seismology stochastic models can be used to describe velocity heterogeneities that are too small or too numerous to be described deterministically where analytic approaches are often infeasible synthetic realizations of such models can be used in conjunction with finite difference algorithms to systematically investigate the response of the seismic wave field to complex heterogeneity this paper represents a continuing effort at formulating a complete and robust stochastic model of lithologic heterogeneity within the crust and the means of generating synthetic realizations complete implies that the model is flexible enough to describe all types of random heterogeneity within the crust while robust implies sufficiently constrained parameterization that an inversion problem may be well posed as a basis for investigation we use geologic maps of crustal exposures and petrophysically inferred velocities earlier efforts at stochastic modeling have focused on characterization of the univariate probability density function which is typically modal i e binary ternary etc and the covariance function which is typically fit with a von karman function here we provide a means of characterizing the property of sinuous connectivity and for generating realizations that possess this property sinuous connectivity is the tendency for individual lithologic units to be continuous over long and highly contorted paths there is no means in the earlier modeling of either characterizing or synthesizing this property we generate sinuously connective realizations by mapping regions encompassed by two contours in a gaussian distributed surface into the two values of the binary field this operation is nonunique as one can choose in many ways the values for the contours

tectono sedimentary framework of the gachsaran formation in the zagros foreland basin abstract continental collision between iranian and arabian plates resulted in the formation of the zagros fold thrust belt and its associated foreland basin during convergence pre existing faults in the basement were reactivated and the sedimentary cover was shortened above two different types of basal decollement viscous frictional this led to heterogeneous deformation which segmented not only the zagros fold thrust belt but also its foreland basin into different compartments resulting in variation in facies thickness and age of the sediment infill based on this concept a new tectono sedimentary model is proposed for one of the most important syn tectonic sedimentary unit the gachsaran salt in the zagros foreland basin in this proposed model it is argued that differential propagation of the deformation front above decollements with different mechanical properties viscous versus frictional results in along strike irregularity of the zagros deformation front whereas movement along pre existing basement faults leads to development of barriers across the zagros basin the irregularity of the deformation front and the cross basin barriers divided the zagros foreland basin into six almost alternating sub basins where gachsaran salt and its non salt equivalents are deposited in the salt sub basins two different processes were responsible for the deposition of gachsaran salt 1 evaporation and 2 dissolution of extruding hormuz salt and its re precipitation as gachsaran salt re precipitation was probably the most significant process responsible for the huge deposit of gachsaran salt in the extreme south east part of the zagros foreland basin

stochastic modeling of seafloor morphology inversion of sea beam data for second order statistics at scale lengths less than 100 km or so statistical descriptions of seafloor morphology can be usefully employed to characterize ridge crest processes off ridge tectonics and vulcanism sedimentation and postdepositional transport we seek to develop methods for the estimation of seafloor statistics that take into account the finite precision resolution and sampling obtained by actual echo sounding systems in this initial paper we restrict our attention to the problem of recovering second order statistics from data sets collected by multibeam devices such as sea beam the seafloor is modeled as a stationary zero mean gaussian random field completely specified by its two point covariance function we introduce an anisotropic two point covariance function that has five free parameters describing the amplitude orientation characteristic wave numbers and hausdorff fractal dimension of seafloor topography we formulate the general forward problem relating this model to the statistics of an ideal multibeam echo sounder in particular the along track autocovariance functions of individual beams and the cross covariance functions between beams of arbitrary separation using these second moments as data functionals we then pose the inverse problem of estimating the seafloor parameters from realistic noisy data sets with finite sampling and beamwidth and we solve this inverse problem by an iterative linearized least squares method the inversion method is applied to sea beam transit data from both the pacific and atlantic oceans sea beam system noise stands out as a sharp spike on the along track autocovariance function and can be modeled as a white noise process whose amplitude generally increases with beam angle the five parameters in our second order model can be estimated from the inversion of data sets comprising 100 200 km of track length in general the cross track wave number is the most poorly determined although uncertainties in the assumed sea beam response may bias the values of the fractal dimension using the assumed beamwidth the measured noise values and the seafloor parameters recovered from the inversion we generate sea beam synthetics whose statistical character can be directly compared with raw sea beam data for most of the track segments we have processed thus far the synthetics are similar to the data in the case of one atlantic profile however the comparison clearly indicates the necessity of incorporating higher order statistics the space domain procedures described in this paper can be extended for this purpose

traveltime approximation in vertical transversely isotropic layered media the well known asymptotic fractional four parameter traveltime approximation and the five parameter generalised traveltime approximation in stratified multi layer transversely isotropic elastic media with a vertical axis of symmetry have been widely used for pure mode and converted waves the first three parameters of these traveltime expansions are zero offset traveltime normal moveout velocity and quartic coefficient ensuring high accuracy of traveltimes at short offsets the additional parameter within the four parameter approximation is an effective horizontal velocity accounting for large offsets which is important to avoid traveltime divergence at large offsets the two additional parameters in the above mentioned five parameter approximation ensure higher accuracy up to a given large finite offset with an exact match at this offset in this paper we propose two alternative five parameter traveltime approximations which can be considered extensions of the four parameter approximation and an alternative to the five parameter approximation previously mentioned the first three short offset parameters are the same as before but the two additional long offset parameters are different and have specific physical meaning one of them describes the propagation in the high velocity layer of the overburden nearly horizontal propagation in the case of very large offsets and the other characterises the intercept time corresponding to the critical slowness that includes contributions of the lower velocity layers only unlike the above mentioned approximations both of the proposed traveltime approximations converge to the theoretical asymptotic linear traveltime at the limit case of very large infinite offsets their accuracy for moderate to very large offsets for quasi compressional waves converted waves and shear waves polarised in the horizontal plane is extremely high in cases where the overburden model contains at least one layer with a dominant higher velocity compared with the other layers we consider the implementation of the proposed traveltime approximations in all classes of problems in which the above mentioned approximations are used such as reflection and diffraction analysis and imaging

gas hydrates saturation using geostatistical inversion in a fractured reservoir in the krishna godavari basin offshore eastern india a reservoir of gas hydrates filling fractures was discovered in the krishna godavari kg basin during the indian national gas hydrate program nghp expedition 01 at site nghp01 10 the existing methods for estimating gas hydrate saturation from seismic data rely on establishing an empirical relation between acoustic impedance and density porosity from well logs assuming isotropic pore filling gas hydrate this method however yields a misleading saturation for fractured clay dominated sediments here we present a methodology to estimate gas hydrate saturation from seismic data based on geostatistical inversion it integrates the verticals detail of well log data with the lateral details from seismic data to produce highly detailed estimates of gas hydrate saturation first gas hydrate saturation is calculated from p wave velocities assuming anisotropic distribution at the well site then probability density functions pdfs between acoustic impedance and the calculated gas hydrate saturation at the well site are analyzed a markov chain monte carlo method is employed to integrate well logs with the seismic data to produce acoustic impedance finally crossplots and histograms at the well site are used to estimate gas hydrate saturations along the seismic line from inverted acoustic impedance the spatial distribution of gas hydrate varies both laterally and vertically along the line with an average saturation of 22 5 the estimate matches reasonably with the value at the wells crown copyright c 2013 published by elsevier ltd all rights reserved

the scattering of seismic waves through a crust and upper mantle with random lateral and vertical inhomogeneities abstract the propagation of seismic waves through earth models with slightly random lateral and vertical inhomogeneities superimposed on one composed of layers with vertical velocity gradients was investigated the maximum deviation of velocity from a mean value at a given depth and a correlation distance derived from a two dimensional smoothing filter were two parameters used to vary the amplitude and size of the velocity anomalies the resulting models show short discontinuous reflectors scattered about at various depths throughout the model and are thus in agreement with many deep seismic reflection experiments on the other hand numerical experiments using ray tracing techniques showed that the effect of the lateral and vertical velocity anomalies is to scatter the energy and break up the continuous travel time lines from vertical gradient models into travel time segments with different slopes similar to those observed in many long range seismic refraction experiments and to those resulting from layering effects in the media many of the numerical experiments which modelled the random crust produced a pg segment and a p segment with an apparent conrad discontinuity at a depth of 10 20 km this apparent depth being related to the correlation distance when a seismic wave propagates through a heterogeneous earth the amount of its energy which is converted into scattered energy will be a function of the inhomogeneous characteristics of the medium through which it has passed if a ray passes through a homogeneous earth the energy arriving at an array station should be relatively coherent whereas if the ray encounters lateral and vertical inhomogeneities its energy will be incoherent and much more complex a series of coherency measurements done on array recordings of earthquakes at various distances showed that large lateral and vertical variations in complexity exist for different ray paths through the earth with the region below the 650 km discontinuity in the mantle tending to be much simpler than the region just below the lithosphere

features of the water mass front in the northwestern north pacific analysis of the hydrographic data taken in the northern north pacific has revealed a water mass front defined as the abrupt change of mixing rates between the two typical water masses typical water masses are situated above the main thermocline in the subtropical and subpolar gyres this front termed water mass front in the present study can be regarded as the boundary between the subtropical gyre and the subpolar gyre the index for detecting the frontal position from the temperature field is given as the 5 c isotherm at the 300 m level comparisons with data from previous studies are also made and this front appears to correspond to the subarctic front in almost every instance

seismic attenuation in the continental crust sw of england summary significant variability in crustal attenuation exists off england s sw coast as shown by birps deep seismic reflection lines the presence of the thick cornubian batholith with no sedimentary cover acting as a low attenuating region compared to normal upper crust with about 300 m of sedimentary cover can account for the observed differences in the strength of signal returning from the lower crust application of the spectral amplitude ratio method to determine the effective quality factor q of the lower crust beneath the granite yields a value of 400 200 a modification of the spectral amplitude ratio method to measure relative q s at different locations indicates that the average q of the upper and middle crust containing the granite is twice that elsewhere although the hypothesis that the lower crustal composition differs significantly beneath the granite cannot be ruled out geological and seismic observations suggest that differential attenuation is the main factor causing variation in the observed reflectivity thus thick granites act as low attenuating windows to the lower crust

a stochastic view of lower crustal fabric based on evidence from the ivrea zone despite its complicated history the ivrea zone is considered to be a representative surface exposure of extended continental crust we have digitized two standard 1 25 000 geological maps from this area and evaluated their structural statistics because of the subvertical orientation of the ivrea zone these maps can be considered as small scale cross sections through the lower continental crust the autocorrelation functions of the digitized lithologies measured from these maps show a clear self similar or fractal rather than a gaussian or deterministic trend we found that an anisotropic von karman correlation function with an aspect ratio around 4 and a hurst number of 0 3 corresponding to a fractal dimension of 2 7 matches the observed data our results represent an explicit confirmation of previous indirect evidence for the fractal nature of lithospheric heterogeneities and provide the means to construct realistic crustal scale seismic models of ivrea type lower continental crust

assessment of gas hydrate accumulations using velocities derived from vertical seismic profiles and acoustic log data in krishna godavari basin india abstract the second indian national gas hydrate program expedition nghp 02 was executed in 2015 in four areas termed as areas a b c and e during this expedition twenty five research sites were drilled and or cored in krishna godavari kg and mahanadi basins eastern indian offshore during nghp 02 zero offset vertical seismic profile vsp data were acquired at three sites sites nghp 02 17 19 and 22 in area b of the kg basin in this study we focus on the three sites in area b of the kg basin where zero offset vsp and downhole acoustic log data are used to assess and characterize the gas hydrate deposits zero offset vsp data are correlated with wireline log surface seismic and synthetic seismic data to characterize and delineate gas hydrate accumulations in the kg basin low velocities ranging from 1500 to 1650 m s are observed in the unconsolidated shallow sedimentary section above gas hydrate bearing units whereas very high velocities are observed in the acquired acoustic log and vsp data in the gas hydrate bearing sedimentary sections the vsp derived interval velocities range from 2000 to 3000 m s in the depth interval between 267 and 287 m below sea floor mbsf at site nghp 02 17 between 1650 and 1750 m s in the depth interval between 306 and 366 mbsf at site nghp 02 19 and between 1700 and 1800 m s in the depth interval between 198 and 290 mbsf at site nghp 02 22 gas hydrates are distributed both as pore filling and fracture filling at all three sites however high concentrations are only observed as pore filling morphology we estimate the amount of gas hydrate considering both isotropic pore filling and anisotropic fracture filling acoustic reservoir models our estimations of gas hydrate saturation match well with the available pressure core data and suggest that high gas hydrate concentrations to nearly 85 of pore space are distributed in a load bearing morphology

lower crustal reflectivity patterns in wide angle seismic recordings many features of wide angle reflection data have been attributed to finely layered structure in the lower crust reflections with similar character however can be produced by small amplitude 2 d random velocity variations finite difference seismograms for a lower crustal model with isotropic random velocity variations match field data recorded at the black forest germany at source receiver offsets beyond about twice the target depth waves scattered from a random zone produce the appearance of horizontal layering in wide angle data lower crustal reflectivity patterns can thus be explained by models that have lateral and vertical random fluctutations rather than fine scale layering

theory of borehole stability when drilling through salt formations we present a mathematical analysis of borehole stability when drilling through rock salt first we consider an elastic transversely isotropic medium and find the optimal mud weight as a function of the vertical overburden and horizontal tectonic stresses then the zener and maxwell mechanical models are used to model the effects of transient and steady state creep flow respectively in isotropic media under certain conditions such as the absence of dilatational anelasticity the burger model can be used to describe the steady state flow including transient creep effects the type of creep is regulated by critical octahedral stress values that depend on temperature and pressure a typical drilling results in conditions of plane strain whose solution is given by kirsch s equations in this case the borehole is subject to minimum and maximum horizontal stresses which differ from the vertical stress the analysis provides expressions for the shape of the borehole cross section the boreholewall closure time and the optimal mud weight to avoid wall collapse or expansion it is shown that an anisotropic state of tectonic stress may require mud pressures exceeding the overburden stress and that the calculation should consider the joint optimization of the shape and area of the borehole cross section

absolute reflection coefficients from deep seismic reflections abstract many deep seismic reflection profiles show bright sub horizontal layered reflections in the lower continental crust accurate estimates are obtained for normal incidence reflection coefficients for the brightest of these reflections by using a marine seismic acquisition system that has been calibrated using deep water sea bottom multiples minimum bounds are placed upon these reflection coefficients by assuming minimum losses due to scattering anelastic attenuation and geometric spreading reflection coefficients of around 0 1 are obtained from the layered lower crust and around 0 15 from the moho it is argued that constructive interference between successive reflections from a layered sequence of reflectors is often unimportant consequently these coefficients can be used to place useful limits on the absolute acoustic impedance contrasts present at depth