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

velocity porosity relationships predictive velocity model for cemented sands composed of multiple mineral phases computer simulations are used to calculate the elastic properties of model cemented sandstones composed of two or more mineral phases two idealized models are considered a grain overlap clay quartz mix and a pore lining clay quartz mix unlike experimental data the numerical data exhibit little noise yet cover a wide range of quartz cement ratios and porosities the results of the computations are in good agreement with experimental data for clay bearing consolidated sandstones the effective modulus of solid mineral mixtures is found to be relatively insensitive to microstructural detail it is shown that the hashin shtrikman average is a good estimate for the modulus of the solid mineral mixtures the distribution of the cement phase is found to have little effect on the computed modulus porosity relationships numerical data for dry and saturated states confirm that gassmann s equations remain valid for porous materials composed of multiple solid constituents as noted previously the krief relationship successfully describes the porosity dependence of the dry shear modulus and a recent empirical relationship provides a good estimate for the dry rock poisson s ratio from the numerical computations a new empirical model which requires only a knowledge of system mineralogy is proposed for the modulus porosity relationship of isotropic dry or fluid saturated porous materials composed of multiple solid constituents comparisons with experimental data for clean and shaly sandstones and computations for more complex three mineral quartz dolomite clay systems show good agreement with the proposed model over a very wide range of porosities

bayesian inversion for effective pore fluid bulk modulus based on fluid matrix decoupled amplitude variation with offset approximation fluid indicators estimated from seismic dataplay important roles in reservoir characterization and prospect identification traditionally there are a variety of fluid indicators proposed but they are very likely to provide ambiguous results for fluid identification due to the fact that their sensitivity is dependent upon the mixed effect of pore fluid and rock porosity to raise the sensitivity of fluid indication we used the effective porefluid bulk modulus as a fluid indicator starting with the poroelastic amplitudevariation with offset avo theory and the corresponding rock physics model with the homogeneous sorting trend we derived a new avo approximation that allowed us toestimatethe effective pore fluidbulkmodulusina direct fashion the inversion for the fluid indicator is formulated inbayesian framework withthe cauchydistributionasa prior constraint we tested the method on synthetic data and analyzed the feasibility and stability of the inversion a field dataexampleshowsthatthe effective pore fluidbulkmodulus can reduce the ambiguity caused by the rock porosity and improve the quality of fluid discrimination in a clastic reservoir further research needs to be done on the reservoirs that do not fit the rock physics model without a sorting trend

method of estimating the amount of in situ gas hydrates in deep marine sediments abstract the bulk volume of gas hydrates in marine sediments can be estimated by measuring interval velocities and amplitude blanking of hydrated zones from true amplitude processed multichannel seismic reflection data in general neither velocity nor amplitude information is adequate to independently estimate hydrate concentration a method is proposed that uses amplitude blanking calibrated by interval velocity information to quantify hydrate concentrations in the blake ridge area of the us atlantic continental margin on the blake ridge blanking occurs in conjunction with relatively low interval velocities the model that best explains this relation linearly mixes two end member sediments hydrated and unhydrated sediment hydrate concentration in the hydrate end member can be calculated from a weighted equation that uses velocity estimated from the seismic data known properties of the pure hydrate and porosity inferred from a velocity porosity relationship amplitude blanking can be predicted as the proportions of hydrated and unhydrated sediment change across a reflection boundary our analysis of a small area near dsdp 533 indicates that the amount of gas hydrates is about 6 in total volume when the interval velocity is used as a criterion and about 9 5 when amplitude information is used this compares with a calculated value of about 8 derived from the only available measurement in dsdp 533

a new exploration tool quantitative core characterization we will describe a new laboratory system which was designed to be highly automated and portable while maintaining quality driving this design was the recognition of the temporal dependence of physical properties it becomes apparent that some sedimentary rocks particularly shales degrade and disaggregate so completely that mechanical or elastic properties cannot be measured this temporal dependence displays a time scale much shorter than normal weathering but greater than the time for stress relief a system was designed to permit field characterization of freshly recovered core material a benefit of automation and portability is a marked increase in measurement efficiency the attributes of this system permit rapid characterization of a large number of fresh cores in remote frontier exploration areas this feature can significantly reduce prospect evaluation time statistically significant rock property databases can be created in a short period of time

evaluation of liquefaction resistance for gravelly sands using gravel content corrected shear wave velocity abstractthe current practice of evaluating the liquefaction resistance of gravelly soils with shear wave velocity relies on the assumption that liquefiable gravelly soils behave as sandy ones recent laboratory tests on gap graded gravelly sands show that the cyclic resistance is dominated by the packing conditions of the sand matrix while the shear wave velocity is affected by the gravel content taking these findings into account the gravel content corrected shear wave velocity is derived based on the time average equation of lumped gravel particles and the sand matrix based on analyses of case histories a three step procedure which includes screening of gravelly sands computation of stress normalized gravel content corrected shear wave velocity and cyclic resistance evaluation using the modified correlation between the corrected shear wave velocity and the cyclic resistance is proposed for evaluating the cyclic resistance ratio of gravelly sands comparisons with case histories show that the p

influence of gas hydrate morphology on the seismic velocities of sands this paper reports the results of a series of resonant column tests on specimens where gas hydrate has been formed in sands using an excess water technique in these specimens the amount of hydrate formed is restricted by the amount of gas in the specimen and with an excess of water being present in the pore space results of resonant column tests carried out to determine compressional and shear wave velocities suggest that gas hydrate formed in this way are frame supporting in contrast the behavior observed in sands where the hydrate is formed from finite water where the remaining pore space is saturated with methane gas termed in this paper the excess gas method exhibits a cementing behavior while tetrahydrofuran hydrate sands or where the hydrate is formed from dissolved methane within the pore water exhibit a pore filling behavior for hydrate saturations less than 40 for sands where the hydrate is formed using the excess water method much larger volumes of hydrate are required before a significant increase in shear wave velocity occurs although increases in compressional wave velocity are seen at lower hydrate contents these results suggest that hydrate interaction with the sediment is strongly dependent on morphology and that natural hydrate may exhibit contrasting seismic signatures depending upon the geological environment in which it forms

propagation of dispersed compressional and rayleigh waves on the texas coastal plain during a refraction profile on the texas coastal plain a strong surface wave with predominant frequencies between 2 and 10 hz and group velocities near 1 9 km sec was observed to ranges as great as 64 km this dispersed wave with velocities near the compressional wave speed of near surface sediments corresponds to the leaky compressional or pl wave pl dispersion can be predicted from the theory of the normal modes of a layered liquid medium efficient propagation of the pl wave is related to the high poisson ratio of the unconsolidated sediments in the shallow subsurface and additional examples from the published literature show that the pl wave is commonly excited by shallow sources both on land and offshore in addition to the pl waves dispersed waves with group velocities between 0 3 and 0 7 km sec were observed at ranges less than 10 km these are identified as rayleigh waves lr smoothly varying p and s velocity structures for the upper 1 km are obtained by fitting theoretical dispersion cu

determining the dilation factor in 4d monitoring of compacting reservoirs by rock physics models hydrocarbon depletion and fluid injection cause compaction and stretching of the reservoir and overburden layers 4d prestack seismic data can be used to detect these changes because compaction stretching causes changes in traveltimes and seismic velocities we show that by using two different petro elastic models at varying effective pressures a good approximation is to assume that the fractional changes in layer

correlations between compressional and shear wave velocities and corresponding poisson s ratios for some common rocks and sulfide ores abstract the correlations between v p and v s and corresponding poisson s ratios are important in modeling and interpreting seismic data in terms of chemistry and lithology data from high pressure laboratory measurements of compressional and shear wave velocities v p and v s are analyzed for 12 common categories of rocks amphibolite anorthosite basalt diorite eclogite felsic gneiss gabbro diabase granite intermediate gneiss limestone mafic gneiss and peridotite and 4 types of massive sulfide ores chalcopyrite pyrite sphalerite and pyrrhotite the analysis reveals that the linear correlation provides good descriptions for the v s v p and ln v s ln v p relationships poisson s ratio is linearly correlated with v s v p shear modulus g and young s modulus e for these rocks and sulfide ores a decrease in poisson s ratio is associated with increases in v s g and e however the variation of poisson s ratio with v p depends on the logarithmic ratio r s p i e ln v s ln v p poisson s ratio increases or decreases with v p when r s p 1 r s p varies systematically with lithology 0 300 for granite 0 573 for diorite 0 602 for felsic gneiss 0 631 for intermediate gneiss 0 721 for gabbro diabase 0 768 for mafic gneiss 0 866 for eclogite 0 890 for amphibolite and 1 391 for peridotite it is suggested that r s p can be used as a proxy for the composition of the deep continental crust and the upper mantle

seismic velocity studies of a gas hydrate bottom simulating reflector on the northern cascadia continental margin amplitude modeling and full waveform inversion on the northern cascadia subduction margin the multichannel seismic amplitude versus offset avo behavior of a bottom simulating reflector bsr suggests a p wave velocity change from high velocity hydrate bearing sediment to lower velocity sediment containing a small amount of free gas the observed nonlinear avo behavior constant or slightly decreasing amplitudes at near to mid offsets and a large amplitude increase at far offsets can be reproduced in the models if an s wave velocity enhancement is assumed as expected from hydrate cementation the avo behavior of the hydrate bsr is found not to be as useful as was earlier thought for determining the amount of free gas below the bsr this is because poisson s ratio change below the bsr due to gas is likely very small in high porosity unconsolidated sediments the uncertainty in the models is large as there is no reliable s wave velocity information for the sediments containing hydrate or gas avo modeling alone is not sufficient to distinguish different velocity models across the bsr our interpretation of the bsr amplitude behavior is that a p wave velocity increase above the bsr is the main cause of the bsr reflection amplitude increase at large incidence angles caution must be taken in applying avo analysis as little is known about s wave velocities however subtle differences in bsr amplitude behavior and reflection waveform can provide constraints through very careful full waveform inversion a well defined reference velocity depth profile is also required to represent water saturated sediment unaffected by either hydrate or free gas using full waveform velocity inversion a high resolution velocity model for the hydrate bsr has been derived the best fit model for the seismic data near the ocean drilling program odp sites 889 890 consists of a high velocity zone above the bsr and a thin low velocity layer below in agreement with the odp downhole velocity data

influence of seismicity on the lusi mud eruption earthquakes trigger the eruption of mud and magmatic volcanoes and influence ongoing eruptive activity one mechanism that could trigger an eruption is clay liquefaction here we model the propagation of seismic waves beneath the lusi mud eruption east java indonesia using available seismic velocity and density models to assess the effect of subsurface structure on the amplification of incident seismic waves we find that using an updated subsurface density and velocity structure there is no significant amplification of incident seismic energy in the upper kalibeng formation the source of the erupting solids hence the hypothesis that the lusi eruption was triggered by clay liquefaction appears unlikely to be correct independent constraints from gas chemistry as well as analyses of drilling activities at the nearby banjar panji 1 gas exploration well and an analysis of the effects of other earthquakes all favor a drilling trigger

sensitivity of reflection seismic data to oil column height in high porosity sandstones tuning is the effect of interference between the reflections from the top and bottom of a thin layer on the amplitude of the composite reflection for a homogeneous sandstone reservoir containing an oil column overlying brine interference between the reflection from the top reservoir and the oil water contact is a function of the height of the oil column if the properties of the sandstone do not vary across the oil water contact the ss ps and sp reflection coefficients from the oil water contact are small in comparison to the pp reflection coefficient this allows analytic expressions for the effective pp and ps reflection coefficients from the reservoir to be derived that include all p wave multiples within the oil column for a given source receiver offset the component of the wavevector inside the oil column normal to the interface is larger for the pppp reflection than for the ppps reflection due to the asymmetry in the raypath for the ppps reflection the ppps reflection is therefore useful for determining oil column heights larger than that discriminated by the pppp reflection especially when used at wider offsets a convenient classification of the avo response of hydrocarbon bearing sandstone reservoirs overlain by shale is the scheme of rutherford and williams class 1 sands have higher acoustic impedance for normal incidence than the overlying shale class 2 sands have nearly the same acoustic impedance as the shale and class 3 sands have lower acoustic impedance synthetic shot gathers calculated for these three classes as a function of oil column height show that a combination of the pppp and the ppps amplitudes can be plotted as a tuning trajectory which can be used to determine the oil column height this method is most sensitive for reservoirs that belong to avo classes 1 and 2 and therefore may be useful in avo analysis of class 1 and 2 reservoirs where the traditional avo indicators developed for class 3 reservoirs do not work very well these results demonstrate the usefulness of shear waves recorded in the marine environment at wide offsets

rock properties of smectite and ooze rich claystones abstractparameters measured by well logs define rock properties and seismic reflections at lithology interfaces parameters from standard embedding claystones are normally used when calculating the top sand amplitude variation with offset avo response but this might give erroneous results when the real claystone rock properties deviate from the standard using cuttings and high quality wireline logs from well 34 8 a 33 h above the visund field in the northern north sea we evaluated rock properties of the pleistocene glaciomarine claystones lower miocene and upper oligocene oozy claystones lower oligocene and eocene smectite rich claystones and two interbedded sands glaciomarine claystones fit best with the greenberg castagna equation and have the highest measured velocities even though they are the shallowest buried sediments environmental scanning electron microscope analysis proves the lower miocene and upper oligocene claystones to be oozy the amount of low density oozy material causes signif

fast inversion of formation permeability from stoneley wave logs using a simplified biot rosenbaum model this paper describes a fast algorithm for estimating formation permeability from stoneley wave logs the procedure uses a simplified biot rosenbaum model formulation the input to the inversion is the stoneley wave spectral amplitudes at each receiver position the bore hole fluid properties velocity and density the borehole caliper log the formation density and porosity from log data and the compressional and shear velocities for the interval of interest the model uses the borehole caliper and elastic properties to compute the stoneley wave excitation that is predicted amplitude without permeability effects as a function of frequency and the porosity and permeability to compute the fluid flow amplitude reduction this method also uses a reference depth of known permeability and compares amplitude variations at other depths relative to the reference depth the permeability value obtained from the inversion represents the best fit over all receivers and all relevant frequencies a processing example is shown to demonstrate the ability of this technique to extract formation permeability from stoneley wave logs

rock physics based relations for density and s velocity versus p velocity in deepwater subsalt gulf of mexico shales which is commonly used to estimate s wave velocity s from p wave velocity p where the units of velocity are km s and the empirical relations for estimating the shear wave velocity from the compressional wave velocity given by greenberg and castagna 1992 figure 1 compares the relationship between density and sonic velocity for two deepwater subsalt wells in the green canyon area of the gulf of mexico to gardner s predic tion assuming values for parameters a and b given by castagna et al 1993 for sands and shales while the gardner estimate is reasonable for the sands the shales at any given density indicate a significantly lower velocity than that predicted by gardner s relation similarly figure 2 compares p wave versus s wave ve locity data for the same two wells to the prediction of equation 2 and of the greenberg castagna relation for shales it is seen that these relations tend to overestimate s wave velocity for the shales at a given p wave velocity in this paper these discrepancies are investigated using a rock physics model this model not only gives a good descrip tion of the data but also allows the effective pore aspect ratio to be estimated rock physics model to model the elastic moduli of shales the shales were considered as a mixture of clay and quartz with properties taken as those given by carmichael 1989 for quartz and gulf clays han et al 1986 the properties used are listed in table 1 pore spaces were assumed filled by brine having a bulk mod ulus of 3 4 gpa appropriate for the pressure and temperature in the depth range of interest since quartz grains in shales are embedded within a softer continuous clay matrix the elastic moduli of the solid phase were estimated using the hashin sh trikman lower bound hashin and shtrikman 1963 the up per and lower hashin shtrikman bounds for the elastic moduli in table 1 are plotted in figure 3

acoustic and elastic characterization of marine sediments by analysis modeling and inversion of ultrasonic p wave transmission seismograms ultrasonic p wave transmission seismograms recorded on sediment cores have been analyzed to study the acoustic and estimate the elastic properties of marine sediments from different provinces dominated by terrigenous calcareous and diatomaceous sedimentation instantaneous frequencies computed from the transmission seismograms are displayed as gray shaded images to give an acoustic overview of the lithology of each core centimeter scale variations in the ultrasonic waveforms associated with lithological changes are illustrated by wiggle traces in detail cross correlation multiple filter and spectral ratio techniques are applied to derive p wave velocities and attenuation coefficients s wave velocities and attenuation coefficients elastic moduli and permeabilities are calculated by an inversion scheme based on the biot stoll viscoelastic model together with porosity measurements p and s wave scatter diagrams are constructed to characterize different sediment types by their velocity and attenuation porosity relationships they demonstrate that terrigenous calcareous and diatomaceous sediments cover different velocity and attenuation porosity ranges in terrigenous sediments p wave velocities and attenuation coefficients decrease rapidly with increasing porosity whereas s wave velocities and shear moduli are very low calcareous sediments behave similarly at relatively higher porosities foraminifera skeletons in compositions of terrigenous mud and calcareous ooze cause a stiffening of the frame accompanied by higher shear moduli p wave velocities and attenuation coefficients in diatomaceous ooze the contribution of the shear modulus becomes increasingly important and is controlled by the opal content whereas attenuation is very low this leads to the opportunity to predict the opal content from nondestructive p wave velocity measurements at centimeter scale resolution

multimode migration of scattered and converted waves for the structure of the hikurangi slab interface new zealand reflectivity imaging of local earthquake seismograms has revealed the structure of the hikurangi subduction interface at the location of two strong earthquakes that occurred in 1990 the earthquakes originated within the continental plate of the north island of new zealand and below in the subducting pacific slab we used seismograms from 500 well located events in two earthquake sequences recorded by a small temporary seismograph deployment to directly image the structure and multiphase reflectivity of the plate interface synthetic tests of the imaging method show the effects of the poor 3 d geometric coverage afforded by the seismometer array kirchhoff summation image sections computed from synthetics show accurate depth imaging of backscattering interfaces phase converting interfaces imaged with forward scattered waves are smeared by poor ray coverage to 5 km depth inaccuracy and are only imaged over a small range of their horizontal extent from the data we computed image sections for p p p s s p and s s scattering we mitigated imaging artifacts due to poor ray coverage with an obliquity factor an antialiasing criterion and enhancement by resampling statistics imaging used a sharply layered velocity model we tested for the effects of imaging with first arriving headwaves by imaging through smoothly varying velocity models for our ray geometry early arrival headwaves contribute little to the images the plate interface appears as a 3 5 km thick p p and possibly s s backscatterer with 5j nw dip offset 5 km down to the nw above a normal fault in the slab when illuminated from below a wedge of the interface on the downdip side of the slab fault forms a very prominent p p forward scatterer the edges of the wedge forward scatter some s p and s s energy but an order of magnitude less than the p p forward scattering the imbalances between forward scattering of p and s energy suggest a wedge of subducted sediment retaining significant porosity but with rigidity close to that of surrounding rocks d 2002 elsevier science b v all rights reserved

high resolution passive seismic tomography for 3d velocity poisson s ratio ν and p wave quality qp in the delvina hydrocarbon field southern albania we have studied using traveltimes of p and s waves and initial seismic pulse rise time measurements from natural microearthquakes to derive 3d p wave velocity vp information mostly structural as well as p and s wave velocity vp vs and p wave quality factor qp information mostly lithologic in a known hydrocarbon field in southern albania during a 12 month monitoring period 1860 microearthquakes were located at a 50 station seismic network and were used to obtain the above parameters the data set included earthquakes with magnitudes ranging from 0 1 to 3 0 r richter scale and focal depths typically occurring between 2 and 10 km kohonen neural networks were implemented to facilitate the lithological classification of the passive seismic tomography pst results the obtained results which agreed with data from nearby wells helped delineate the structure of the reservoir two subregions of the investigated area one corresponding to an oil field and one to a gas field were correlated with the

model based shear wave velocity estimation versus empirical regressions modelling of avo signatures for reservoir characterization requires v s estimation from other available logs when shear wave data are not available we tested various models for predicting v s from p wave velocity porosity and shale volume measured in well logs effective medium models which characterize the pore space in terms of ellipsoidal inclusions were compared with statistical v p v s regressions the inclusion models were calibrated by non linear minimization of the difference between model predicted velocities and actual measured velocities the quality of the v s prediction was quantified in terms of the rms error by comparison with shear wave data in wells where both v p and v s were measured the linear regressions were found to be more robust and the rms error in the prediction was comparable to effective medium model based predictions

full waveform processing and interpretation of kilohertz cross well seismic data high frequency cross well seismic data from the midale oil field of southeastern saskatchewan are analyzed for direct and reflected energy the goal of the analysis is to produce interpretable sections to assist in enhanced oil recovery activities co2 injection in this field direct arrivals are used for velocity information while reflected arrivals are processed into a reflection image raw field data show a complex assortment of wave types that includes direct compressional and shear waves and reflected shear waves a traveltime inversion technique layer stripping via ray tracing is used to obtain p and s wave interval velocities from the respective direct arrivals the velocities from the cross well inversion and the sonic log are in reasonable agreement the subsurface coverage of the cross well geometry is investigated it covers zones extending from the source well to the receiver well and includes regions above and below the source receiver depths upgoing and downgoing primary reflections

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

establishing the link between the chesapeake bay impact structure and the north american tektite strewn field the sr nd isotopic evidence the chesapeake bay impact structure which is about 35 ma old has previously been proposed as the possible source crater of the north american tektites nat here we report major and trace element data as well as the first sr nd isotope data for drill core and outcrop samples of target lithologies crater fill breccias and post impact sediments of the chesapeake bay impact structure the unconsolidated sediments cretaceous to middle eocene in age have srt 35 7 ma of 54 to 272 and ndt 35 7 ma ranging from 6 5 to 10 8 one sample from the granitic basement with a tndchur model age of 1 36 ga yielded an srt 35 7 ma of 188 and an ndt 35 7 ma of 5 7 the exmore breccia crater fill can be explained as a mix of the measured target sediments and the granite plus an as yet undetermined component the post impact sediments of the chickahominy formation have slightly higher tndchur model ages of about 1 55 ga indicating a contribution of some older materials newly analyzed bediasites have the following isotope parameters 104 to 119 srt 35 7 ma 5 7 ndt 35 7 ma 0 47 ga tsrur and 1 15 ga tndchur which is in excellent agreement with previously published data for samples of the nat strewn field target rocks with highly radiogenic sr isotopic composition as required for explaining the isotopic characteristics of deep sea drilling project dsdp site 612 tektites were not among the analyzed sample suite based on the new isotope data we exclude any relation between the na tektites and the popigai impact crater although they have identical ages within 2 errors the chesapeake bay structure however is now clearly constrained as the source crater for the north american tektites although the present data set obviously does not include all target lithologies that have contributed to the composition of the tektites

cause of the rupture and distribution of broken submarine carbonate chimneys in the gulf of cadiz southwestern spain abstract during the 2000 and 2001 r v cornide de saavedra cruises a vast field of hydrocarbon derived carbonate chimneys was discovered and sampled along the cadiz contourite channel and guadalquivir diapiric ridge photographs taken on board using an underwater camera revealed a spectacularly high density of pipe like chimneys lying on the sea floor though some of them were also found protruding from muddy sediment the major axes of the fallen chimneys which can be as long as 1 m exhibit a regular spatial distribution showing a conspicuous nw se trend furthermore their basal morphology shows a characteristic angular breakage associated to a flexo traction process both the uniform distribution of the major axis and the basal morphology of the chimneys imply that the cause of the rupture could be related either to strong bottom sea currents or to inertia forces associated with oscillatory ground motion due to seismic shaking the preferential orientation of the broken chimneys their mechanical properties the geological and topographical conditions of the area in relation to seismic ground motion amplification and the flow speed of the mediterranean outflow water mow along the cadiz contourite channel suggest that the most probable cause of the chimneys rupture was an earthquake the minimum magnitude of this seismic event is estimated to be more than 7 0 the causative fault being located directly under the chimney field or as far as 30 60 km away the chimneys rupture took place between ca 5 ka to present

prediction of elastic wave velocities in sandstones using structural models elastic wave propagation in fluid saturated sandstones depends upon two sets of rock features 1 the volume fractions and elastic constants of the rock constituents quartz clay water etc and 2 microstructural geometry grain contacts pore aspect ratios while the former data are usually obtainable the latter are relatively inaccessible we present a new method for determining microstructural data using idealized but physically representative models of sandstone the key to the method is the simulation of certain depositional and diagenetic processes in a manner that completely specifies the geometry of the resulting models hence the geometric features of the grain space and void space required for various theories of elastic propagation can be calculated directly from the models we find good agreement between predictions and measurements of compressional and shear wave velocities in both clean and clay bearing saturated sandstones in contrast with previous efforts at predicting velocitie

seismic velocity structure at a gas hydrate reflector offshore western colombia from full waveform inversion seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs which have been interpreted as being formed at the base of a methane hydrate stability field such reflectors might arise either from an impedance contrast between high velocity partially hydrated sediments and water saturated sediments or from a contrast with partially gas saturated sediments these alternatives may be hard to distinguish by conventional amplitude versus offset or waveform modeling approaches here we investigate the origin of a high amplitude bsr in the accretionary wedge offshore of western colombia by seismic waveform inversion the inversion procedure consists of three steps firstly determination of root mean square velocities and hence estimates of the interval velocities between major reflectors by a global grid search for maximum normalized energy along elliptical trajectories in the intercept time slowness domain secondly determination of accurate interval velocities between these reflectors by a monte carlo search for maximum energy and thirdly a waveform fit in the frequency slowness domain using differential reflectivity seismograms and a conjugate gradient optimization algorithm to minimize the sample by sample waveform misfit between data and synthetic at two locations near a structural high we find a 30 m thick low velocity zone beneath the bsr with the properties of a partially gas saturated zone while at a third location where the bsr amplitude is lower we find no evidence for anomalously low velocities the preferential development of the bsr in structures that would tend to intercept fluid flow or migrating gas and the presence of free gas beneath the bsr indicate a mechanism of bsr formation in which free methane gas migrates upward into the hydrate stability field or is carried there in advecting pore water

southeast papuan crustal tectonics imaging extension and buoyancy of an active rift southeast papua hosts the world s youngest ultra high pressure uhp metamorphic rocks these rocks are found in an extensional setting in metamorphic core complexes competing theories of extensional shear zones or diapiric upwelling have been suggested as driving their exhumation to test these theories we analyze the cdpapua temporary array of 31 land and 8 seafloor broadband seismographs seismicity shows that deformation is being actively accommodated on the core complex bounding faults offset by transfer structures in a manner consistent with overall north south extension rather than radial deformation rayleigh wave dispersion curves are jointly inverted with receiver functions for crustal velocity structure they show crustal thinning beneath the core complexes of 30 50 and very low shear velocities at all depths beneath the core complexes on the rift flanks velocities resemble those of normal continents and increase steadily with depth there is no evidence for velocity inversions that would indicate that a major density inversion exists to drive crustal diapirs also low density melt seems minor within the crust together with the extension patterns apparent in seismicity these data favor an extensional origin for the core complexes and limit the role of diapirism as a secondary exhumation mechanism although deeper mantle diapirs may be undetected a small number of intermediate depth earthquakes up to 120 km deep are identified for the first time just northeast of the d entrecasteaux islands they occur at depths similar to those recorded by uhp rocks and similar temperatures indicating that the modern seismicity occurs at the setting that generates uhp metamorphism

geophysical properties within the san andreas fault zone at the san andreas fault observatory at depth and their relationships to rock properties and fault zone structure 1 we examine the relationships between borehole geophysical data and physical properties of fault related rocks within the san andreas fault as determined from data from the san andreas fault observatory at depth borehole geophysical logs cuttings data and drilling data from the region 3 to 4 km measured depth of the borehole encompass the active part of the san andreas fault the fault zone lies in a sequence of deformed sandstones siltstone shale serpentinite bearing block in matrix rocks and sheared phyllitic siltstone the borehole geophysical logs reveal the presence of a low velocity zone from 3190 to 3410 m measured depth with vp and vs values 10 30 lower than the surrounding rocks and a 1 2 m thick zone of active shearing at 3301 3303 m measured depth seven low velocity excursions with increased porosity decreased density and mud gas kick signatures are present in the fault zone geologic data on grain scale deformation and alteration are compared to borehole data and reveal weak correlations and inverse relationships to the geophysical data in places vp and vs increase with grain scale deformation and alteration and decrease with porosity in the fault zone the low velocity zone is associated with a significant lithologic and structural transition to low velocity rocks dominated by phyllosilicates and penetratively foliated sheared rocks the zone of active shearing and the regions of low sonic velocity appear to be associated with clay rich rocks that exhibit fine scale foliation and higher porosities that may be a consequence of the fault related shearing of foliated and fine grained sedimentary rocks

estimation of pressure saturation changes for unconsolidated reservoir rocks with high vp vs ratio abstractwe find that conventional time lapse seismic pressure saturation discrimination methods become unstable for high vp vs ratios using first order approximations in the amplitude variation with offset avo gradient and intercept changes and in the rock physics models increase the inaccuracy in pressure saturation changes estimates we propose a new method based on a stepwise linear approximation to the intercept and gradient reflectivity changes to estimate pressure and saturation changes the applicability of the new method is tested on synthetic data over a range of 0 50 gas saturation and 0 3 5 mpa pore pressure changes the new method is more consistent and provides better estimates compared with conventional methods in the presence of random noise up to 15 the estimates are reasonably better for the new method we use this new method to investigate the feasibility of pressure saturation discrimination for a shallow unconsolidated sand reservoir in which an underground blowout occurred

predicting lithology and transport properties from acoustic velocities based on petrophysical classification of siliciclastics based on the recently developed petrophysical classification of siliciclastics which takes into account the amount of the volumetric clay content c and textural position of clay it is shown that acoustic velocities can be fairly accurate tools in predicting lithology porosity and ultimately transport properties of these rocks four major petrophysical groups of carbonate and organic poor siliciclastics are distinguished 1 clean arenites c 35 percent the compressional velocity versus porosity relation for consolidated rocks in each of these groups is found to be linear with very high correlation coefficients this allows for remarkably accurate porosity estimates or lithology prediction in consolidated siliciclastics from acoustic velocities compared to the widely used time average wyllie equation or its improved modification raymer equations both of which neglect textural factors

seafloor compliance imaging of marine gas hydrate deposits and cold vent structures 1 seafloor compliance the transfer function between pressure induced by surface gravity waves and the associated seafloor deformation can address outstanding questions about the nature of seismic blank zones identified as cold vents offshore vancouver island near ocean drilling program odp site 889 and the recent integrated ocean drilling program iodp expedition 311 despite extensive seismic studies it is not known whether free gas or gas hydrate is primarily responsible for the seismic blanking nor are the concentration and distribution of either component conclusively determined here we use the formation s shear modulus to distinguish between gas hydrate which can strongly alter the shear modulus and free gas which cannot the depth dependent gas hydrate distribution and pore space saturation are estimated from the frequency variation of seafloor compliance measurements sensitive to the formation s shear modulus compliance data are consistent with pervasive moderate hydrate concentration over a broad region and significantly higher cylindrical concentrations associated with the cold vents perhaps as much as 9 106 m3 in the 400 m diameter bullseye vent in agreement with controlled source electromagnetic data compliance data can help distinguish between different models for the structure of such gas hydrate bearing cold vents

subsidence derived volumetric strain models for mapping extensional fissures and constraining rock mechanical properties in the san joaquin valley california large scale subsidence due to aquifer overdraft is an ongoing hazard in the san joaquin valley subsidence continues to cause damage to infrastructure and increases the risk of extensional fissures here we use insar derived vertical land motion vlm to model the volumetric strain rate due to groundwater storage change during the 2007 2010 drought in the san joaquin valley central valley california we then use this volumetric strain rate model to calculate surface tensile stress in order to predict regions that are at the highest risk for hazardous tensile surface fissures we find a maximum volumetric strain rate of 232 microstrain yr at a depth of 0 to 200 m in tulare and kings county california the highest risk of tensile fissure development occurs at the periphery of the largest subsiding zones particularly in tulare county and merced county finally we assume that subsidence is likely due to aquifer pressure change which is calculated using groundwater level changes observed at 300 wells during this drought we combine pressure data from selected wells with our volumetric strain maps to estimate the quasi static bulk modulus k a poroelastic parameter applicable when pressure change within the aquifer is inducing volumetric strain this parameter is reflective of a slow deformation process and is one to two orders of magnitude lower than typical values for the bulk modulus found using seismic velocity data the results of this study highlight the importance of large scale high resolution vlm measurements in evaluating aquifer system dynamics hazards associated with overdraft and in estimating important poroelastic parameters

range equation weights for avo duren 1991 presented a detailed development of the seismic range equation showing how to calculate a wavelet s voltage amplitude spectrum and phase spectrum and how to use the resulting wavelets in forming model seismic gathers these same wavelets can then be used to process model and seismic gathers via prestack deconvolution to reveal a target s amplitude behavior with offset

waveform tomography of two dimensional three component seismic data for hti anisotropic media reservoirs with vertically aligned fractures can be represented equivalently by horizontal transverse isotropy hti media but inverting for the anisotropic parameters of hti media is a challenging inverse problem because of difficulties inherent in a multiple parameter inversion in this paper when we invert for the anisotropic parameters we consider for the first time the azimuthal rotation of a two dimensional seismic survey line from the symmetry of hti the established wave equations for the hti media with azimuthal rotation consist of nine elastic coefficients expressed in terms of five modified thomsen parameters the latter are parallel to the thomsen parameters for describing velocity characteristics of weak vertical transverse isotropy media we analyze the sensitivity differences of the five modified thomsen parameters from their radiation patterns and attempt to balance the magnitude and sensitivity differences between the parameters through normalization and tuning factors which help to update the model parameters properly we demonstrate an effective inversion strategy by inverting velocity parameters in the first stage and updates the five modified thomsen parameters simultaneously in the second stage for generating reliably reconstructed models

on the compliance method and the assessment of three dimensional seafloor gas hydrate deposits summary marine gas hydrates prevalent in offshore sediments in canada and japan are a possible hydrocarbon resource a hazard to drilling and the source of a major greenhouse gas quantitative estimates of hydrate concentrations in deep sea sediment are difficult to obtain by conventional methods our group has sought novel techniques specifically designed for assessment such as the compliance method where ocean surface gravity waves are used as a source compliance is the transfer function between the vertical displacement of the seafloor and the corresponding pressure expressed as a function of wavelength it is sensitive to the elastic parameters of the underlying sediments particularly the shear modulus which is probably increased in zones containing hydrate due to cementation our group has demonstrated a connection between a compliance measurement and the amount of hydrate present in the hydrate stability zone for layered models here we develop a 3 d numerical finite difference code using control volume discretization to predict the compliance response over non layered structures among the features of the algorithm are an ability to handle sharp contrasts in elastic moduli with a low usage of computer memory the compliance anomaly over such structures has a signature and resolution not unlike the corresponding gravity anomaly the average response over heterogeneous structures is sensitive to the bulk hydrate content but not the connectivity pattern gravity waves with different polarizations crossing markedly anisotropic structures produce statistically the same average stiffness value

the inverse correlations between methane content and elastic parameters of coal bed methane reservoirs we investigated three coal bed methane cbm reservoirs in which there exist when we are careful about data quality and reduce environmental variations inverse correlations between methane content and the elastic properties of the coal beds such inverse correlations may exist in general in high and medium rank cbm reservoirs there is evidence that supports the generalization of such inverse correlations and factors that distort them these distorting factors may prevent identification of such inverse correlations in certain cbm prospects however they cannot negate the underlying inverse correlations such inverse correlations may play a role in cbm amplitude versus offset avo similar to the role played by gassmann biot theory gassmann 1951 biot 1956 1962 in avo technology for conventional gas reservoirs this may lay the first brick for a theoretical basis of cbm avo

shear wave logging to enhance seismic modeling in an effort to understand better the amplitude variation with offset for reflections from an oil sand and the sensitivity of the avo response to shear wave velocity variations i studied synthetic and field gathers collected from an onshore field in the gulf of mexico basin a wave equation based modeling program generated the synthetic seismic gathers using both measured and estimated shear wave velocities the measured shear wave velocities came from a quadrupole sonic tool the estimated shear wave velocities were obtained by applying published empirical and theoretical equations which relate shear wave velocities to measured compressional wave velocities i carefully processed the recorded seismic data with a controlled amplitude processing stream comparison of the synthetic gathers with the processed field data leads to the conclusion that the model containing the measured shear wave velocities matches the field data much better than the model containing the estimated shear wave velocities therefo

relationships between compressional wave and shear wave velocities in clastic silicate rocks new velocity data in addition to literature data derived from sonic log seismic and laboratory measurements are analyzed for clastic silicate rocks these data demonstrate simple systematic relationships between compressional and shear wave velocities for water saturated clastic silicate rocks shear wave veloci ty is approximately linearly related to compressional wave velocity and the compressional to shear velocity ratio decreases with increasing compressional velocity laboratory data for dry sandstones indicate a nearly constant compressional to shear velocity ratio with rigidity approximately equal to bulk modulus ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed water saturated and dry trends for dry rigidity equal to dry bulk modulus gassmann s equations predict velocities in close agreement with data from the water saturated

wave propagation modelling of induced earthquakes at the groningen gas production site gas extraction from the groningen natural gas field situated in the netherlands frequently induces earthquakes in the reservoir that cause damage to buildings and pose a safety hazard and a nuisance to the local population due to the dependence of the national heating infrastructure on groningen gas the short term mitigation measures are mostly limited to a combination of spatiotemporal redistribution of gas production and strengthening measures for buildings all options become more effective with a better understanding of both source processes and seismic wave propagation detailed wave propagation simulations improve both the inference of source processes from observed ground motions and the forecast of ground motions as input for hazard studies and seismic network design the velocity structure at the groningen site is relatively complex including both deep high velocity and shallow low velocity deposits showing significant thickness variations over relatively small spatial extents we performed a detailed 3 d wave propagation modelling study for an induced earthquake in the groningen natural gas field using the spectral element method we considered an earthquake that nucleated along a normal fault with local magnitude of ml 3 we created a dense mesh with element size varying from 12 to 96 m such that with a dominant source frequency of 7 hz waveforms with a frequency content up to 10 hz could be accurately modelled the velocity density model is constructed using a 3 d geological model of the area including both deep high velocity salt deposits overlying the source region and shallow low velocity sediments present in a deep and narrow tunnel valley the results show that the 3 d density velocity model in the groningen area clearly plays a large role in the wave propagation and resulting surface ground motions and causes significant lateral variations in site response the highvelocity salt deposits have a dispersive effect on the radiated wavefield reducing the seismic energy reaching the surface near the epicentre in turn the presence of low velocity tunnel valley deposits can locally cause a significant increase in peak ground acceleration here we study induced seismicity on a local scale and use specfem3d to conduct full waveform simulations and show how local velocity variations can affect seismic records the author s 2018

a mechanism for the formation of methane hydrate and seafloor bottom simulating reflectors by vertical fluid expulsion bottom simulating reflectors bsr are observed commonly at a depth of several hundred meters below the seafloor in continental margin sedimentary sections that have undergone recent tectonic consolidation or rapid accumulation they are believed to correspond to the deepest level at which methane hydrate clathrate is stable we present a model in which bsr hydrate layers are formed through the removal of methane from upward moving pore fluids as they pass into the hydrate stability field in this model most of the methane is generated below the level of hydrate stability but not at depths sufficient for significant thermogenic production the methane is primarily biogenic in origin the model requires either a mechanism to remove dissolved methane from the pore fluids or disseminated free gas carried upward with the pore fluid the model accounts for the evidence that the hydrate is concentrated in a layer at the base of the stability field for the source of the large amount of methane contained in the hydrate and for bsrs being common only in special environments strong upward fluid expulsion into the hydrate stability field does not occur in normal sediment depositional regimes so bsrs are uncommon upward fluid expulsion does occur as a result of tectonic thickening and loading in subduction zone accretionary wedges and in areas where rapid deposition results in initial undercconsolidation in these areas hydrate bsrs are common the most poorly quantified aspect of the model is the efficiency with which methane is removed and hydrate is formed as pore fluids pass into the hydrate stability field the critical boundary in the phase diagram between the fluid plus hydrate and fluid only fields is not well constrained however the amount of methane required to form the hydrate and limited data on methane concentrations in pore fluids from deep sea boreholes suggest very efficient removal of methane from rising fluid that may contain less than the amount required for free gas production in most fluid expulsion regimes the quantity of fluid moved upward to the seafloor is great enough to continually remove the excess chloride and the residue of isotope fractionation resulting from hydrate formation thus as observed in borehole data there are no large chloride or isotope anomalies remaining in the local pore fluids the differences in the concentration of methane and probably of co2 in the pore fluid above and below the base of the stability field may have a significant influence on early sediment diagenetic reactions

a prestack basis pursuit seismic inversion abstractresolving thin layers and clearly delineating layer boundaries in inverted seismic sections are very important goals for exploration and production many seismic inversion methods based on a least squares optimization approach with tikhonov type regularization can lead to unfocused transitions between adjacent layers a basis pursuit inversion bpi algorithm based on the l1 norm optimization method can however resolve sharp boundaries between layers we have formulated a bpi algorithm for amplitude versus angle inversion and investigated its potential to improve contrasts between layers like the bpi for poststack case the sparse layer constraint rather than the sparse spike constraint is used to construct the model space as a wedge dictionary all the elements of the dictionary are bed reflectivities which include solutions consisting of thin beds as well with this dictionary we use an l1 norm optimization framework to derive three reflectivities namely rp rs and r although bpi doe

acoustic and petrophysical relationships in low shale sandstone reservoir rocks this paper describes the measurements of the acoustic and petrophysical properties of two suites of low shale sandstone samples from north sea hydrocarbon reservoirs under simulated reservoir conditions the acoustic velocities and quality factors of the samples saturated with different pore fluids brine dead oil and kerosene were measured at a frequency of about 0 8 mhz and over a range of pressures from 5 mpa to 40 mpa the compressional wave velocity is strongly correlated with the shear wave velocity in this suite of rocks the ratio v p v s varies significantly with change of both pore fluid type and differential pressure confirming the usefulness of this parameter for seismic monitoring of producing reservoirs the results of quality factor measurements were compared with predictions from biot flow and squirt flow loss mechanisms the results suggested that the dominating loss in these samples is due to squirt flow of fluid between the pores of various geometries the contribution of the biot flow loss mechanism to the total loss is negligible the compressional wave quality factor was shown to be inversely correlated with rock permeability suggesting the possibility of using attenuation as a permeability indicator tool in low shale high porosity sandstone reservoirs

prestack inversion of a gulf of thailand ocean bottom cable obc data set seismic waveform inversion can be an important tool for reservoir characterization because high resolution within the seismic frequency band images of elastic parameters are obtainable from good quality seismic data a detailed description of the velocity field with both high and low frequency variations is essential for delineating elastic properties of the medium for direct detection of hydrocarbon lithologic discrimination and estimation of fluid contents conventionally amplitude variation with offset avo analysis determines fractional changes in p impedance and poisson s ratio from a linear fit of normal moveout nmo corrected p reflection amplitude to a two or three term approximation of reflection coefficients although avo analysis gives a useful estimation of elastic properties of the subsurface from avo attributes its limitation is well recognized for example it deals with primaries only models and is inadequate to account for mode converted waves and internal multiples in spite of several sources of error such as random and coherent noise and a 1d earth model assumption a prestack seismic waveform inversion is generally desirable for more accurate mapping of the variation of the elastic properties of the subsurface however practical implementation of such an inversion scheme on a routine basis becomes discouragingly difficult as 1 the inverse problem is highly nonlinear 2 the error function a measure of data misfit in general possesses multimodality 3 the inverse problem is extremely computer intensive and 4 inversion needs better data acquisition systems and data processing steps to preserve true amplitude in order to get precise estimates of elastic properties of the medium recently sen and roy 2003 developed a regularized gradient descent based waveform inversion algorithm that is highly robust and addresses many of the problems encountered in wavefrom inversion this paper is a sequel to that paper in that we demonstrate the applicability of the technique to a field data set from a gas field in the gulf

shear wave reflectivity anisotropies poisson s ratios and densities of a southern appalachian paleozoic sedimentary sequence abstract the physical properties of a sequence of paleozoic sedimentary rocks have been examined in detail with an emphasis on laboratory measurements of density shear wave velocity shear wave splitting and vp vs ratios seismic properties of 147 cores from 49 rock samples collected from the thorn hill sedimentary sequence of eastern tennessee are examined in terms of implications for future seismic studies in the southern appalachians the shear wave velocities of these rocks are strongly influenced by the relatively high shear wave velocity of quartz shear wave velocity anisotropy is present in most of the lithologic groups it is highest in the shales while being almost insignificant in the dolostones the related phenomenon of shear wave splitting occurs to some degree in all of the lithologies studied and at high pressures originates from mineral orientation splitting due to preferred mineral orientation is most prominent in the shales but also is significant in some of the carbonates compressional to shear velocity vp vs ratios of approximately 1 82 dolostones and 1 95 limestones effectively characterize the carbonates while other lithologies display wider ranges of vp vs primarily due to the influence of accessory minerals such as quartz densities of the sample suite range from 2 34 g cm3 shale to 2 86 g cm3 dolostone using average data for each lithologic group to calculate normal incidence reflection coefficients significant differences in the shear and compressional reflectivity of hypothetical interfaces are found normal incidence shear and compressional wave synthetic seismograms approximating the subsurface reflectivity of the entire thorn hill section indicate that three zones of high amplitude reflections would be seen on reflection records obtained over this 3327 m thick sequence overall the shear and compressional synthetics are similar in appearance differences are seen at some interfaces in the mississippian devonian interval which are more reflective to shear waves and in the ordovician martinsburg formation which appears more reflective to compressional waves

the nature and distribution of bottom simulating reflectors at the costa rican convergent margin summary bottom simulating reflectors bsrs at the base of the hydrate stability zone are often observed in marine seismic reflection data the compressional p wave velocity structure of a bsr offshore costa rica was analysed by applying a full waveform inversion technique the resulting velocity profile indicates that the bsr is caused by a thin low velocity layer suggesting the presence of at least a small amount of free gas in the sediment pore space in undisturbed sediment sections bsrs offshore costa rica are observed over much of the continental margin they frequently occur at shallow depths beneath the seafloor usually 100 400 m at some locations bsrs appear to intersect the seafloor this was interpreted as an indication that hydrates in the study area form partly from methane which was produced beneath the hydrate stability zone and migrated upwards in the study area the vertical movement of the hydrate stability zone relative to a given point in the sediment column one of the potential factors leading to bsr formation or suppression is controlled by both sedimentation and vertical tectonism both processes may hence play a role in controlling bsr distribution bsrs are absent in areas which have been affected by slumping except where the sedimentary section above the bsr remained intact during slumping this indicates that slope failure can cause the destruction of bsrs

localizing co2 at sleipner seismic images versus p wave velocities from waveform inversion abstractthe presence of injected co2 in the utsira sand at the sleipner site norway is associated with a high negative p wave velocity anomaly that is a low postinjection velocity and a strong seismic response time lapse seismic imaging of co2 injection at sleipner is thus a viable monitoring tool of the injected co2 the work flow usually involves conventional seismic processing including stacking and results in seismic images multiple reflections interference effects such as tuning and the velocity pushdown effect due to co2 injection render these seismic images ambiguous in terms of the localization and the quantification of the co2 in the utsira sand nonetheless seismic images often form the basis for analyses that aim to quantify the injected co2 we employed elastic 2d full waveform inversion to invert prestack seismic sleipner data from preinjection 1994 and postinjection 1999 and compared the resulting postinjection p wave velocity model with the corresponding seismic image we fou

diagnostics of seismic time lapse effects of sandstones based on laboratory data we evaluated a viscoelastic modeling of p and s wave velocity dispersion attenuation pressure and fluid effects for a set of siliciclastic rock samples our analysis used a published laboratory data set of 63 sandstones with a wide range of compositional heterogeneities we observed a notable correlation between the velocity and attenuation pressure sensitivity and the abundance lack of quartz in the samples we included compliant pores low aspect ratio proportionally to the content of secondary minerals to account for the differential sensitivity to pressure the observed velocity and attenuation were well reproduced by the applied viscoelastic modeling we found that pores of significantly different scale required pore fluid relaxation time constants of proportionally different magnitudes to reproduce the velocity and attenuation measurements the relaxation time constant of crack sized pores can be one order of magnitude smaller than the constant of mesopores moreover the velocity dispersion and attenuation signatures revealed that a pore textural model dependent on lithological composition is critical in the prediction of time lapse fluid and pressure responses

seismic velocities for hydrate bearing sediments using weighted equation a weighted equation based on the three phase time average and wood equations is applied to derive a relationship between the compressional wave p wave velocity and the amount of hydrates filling the pore space the proposed theory predicts accurate p wave velocities of marine sediments in the porosity range of 40 80 and provides a practical means of estimating the amount of in situ hydrate using seismic velocity the shear s wave velocity is derived under the assumption that the p to s wave velocity ratio of the hydrated sediments is proportional to the weighted average of the p to s wave velocity ratios of the constituent components of the sediment in the case that all constituent components are known a weighted equation using multiphase time average and wood equations is possible however this study showed that a three phase equation with modified matrix velocity compensated for the clay content is sufficient to accurately predict the compressional wave velocities for the marine sediments this theory was applied to the laboratory measurements of the p and s wave velocities in permafrost samples to infer the amount of ice in the unconsolidated sediment the results are comparable to the results obtained by repeatedly applying the two phase wave scattering theory the theory predicts that the poisson s ratio of the hydrated sediments decreases as the hydrate concentration increases and the porosity decreases in consequence the amplitude versus offset avo data for the bottom simulating reflections may reveal positive negative or no avo anomalies depending on the concentration of hydrates in the sediments

explicit use of the biot coefficient in predicting shear wave velocity of water saturated sediments predicting the shear wave s wave velocity is important in seismic modelling amplitude analysis with offset and other exploration and engineering applications under the low frequency approximation the classical biot gassmann theory relates the biot coefficient to the bulk modulus of water saturated sediments if the biot coefficient under in situ conditions can be estimated the shear modulus or the s wave velocity can be calculated the biot coefficient derived from the compressional wave p wave velocity of water saturated sediments often differs from and is less than that estimated from the s wave velocity owing to the interactions between the pore fluid and the grain contacts by correcting the biot coefficients derived from p wave velocities of water saturated sediments measured at various differential pressures an accurate method of predicting s wave velocities is proposed numerical results indicate that the predicted s wave velocities for consolidated and unconsolidated sediments agree well with measured velocities

geomechanical and geometric seismic attributes in an interpretation workflow for characterization of unconventional reservoirs conventional seismic attributes often provide satisfactory results especially in the characterization of conventional reservoirs such as sandstones this is not so evident with more compact low permeability and apparently uniform reservoirs which comprise substantial reserves in amounts unthinkable a few decades ago and whose development seemed almost impossible until very recently unconventional reservoirs considered in this article are located in a predominantly jurassic cretaceous rifted depositional basin the basin shifted into an early sag phase during the late jurassic and early cretaceous resulting in the accumulation of a thick series of carbonates and shales tithonian berriasian and valanginian rocks compose the segment of the geologic column under consideration the studied sedimentary sequence is a highly prolific oil prone source rock with total organic carbon toc up to 8 wt and amorphous organic matter type i ii kerogen deposited in a euxinic paleoenvironment which generated s

estimation of gas hydrate concentration from multi component seismic data at sites on the continental margins of nw svalbard and the storegga region of norway high resolution seismic experiments employing arrays of closely spaced four component ocean bottom seismic recorders were conducted at a site off western svalbard and a site on the northern margin of the storegga slide off norway to investigate how well seismic data can be used to determine the concentration of methane hydrate beneath the seabed data from p waves and from s waves generated by p s conversion on reflection were inverted for p and s wave velocity vp and vs using 3d travel time tomography 2d ray tracing inversion and 1d waveform inversion at the nw svalbard site positive vp anomalies above a sea bottom simulating reflector bsr indicate the presence of gas hydrate a zone containing free gas up to 150 m thick lying immediately beneath the bsr is indicated by a large reduction in vp without significant reduction in vs at the storegga site the lateral and vertical variation in vp and vs and the variation in amplitude and polarity of reflectors indicate a heterogeneous distribution of hydrate that is related to a stratigraphically mediated distribution of free gas beneath the bsr derivation of hydrate content from vp and vs was evaluated using different models for how hydrate affects the seismic properties of the sediment host and different approaches for estimating the background velocity of the sediment host the error in the average vp of an interval of 20 m thickness is about 2 5 at 95 confidence and yields a resolution of hydrate concentration of about 3 if hydrate forms a connected framework or about 7 if it is both pore filling and framework forming at nw svalbard in a zone about 90 m thick above the bsr a biot theory based method predicts hydrate concentrations of up to 11 of pore space and an effective medium based method predicts concentrations of up to 6 if hydrate forms a connected framework or 12 if hydrate is both pore filling and framework forming at storegga hydrate concentrations of up to 10 or 20 were predicted depending on the hydrate model in a zone about 120 m thick above a bsr with seismic techniques alone we can only estimate with any confidence the average hydrate content of broad intervals containing more than one layer not only because of the uncertainty in the layer by layer variation in lithology but also because of the negative correlation in the errors of estimation of velocity between adjacent layers in this investigation an interval of about 20 m thickness equivalent to between 2 and 5 layers in the model used for waveform inversion was the smallest within which one could sensibly estimate the hydrate content if lithological layering much thinner than 20 m thickness controls hydrate content then hydrate concentrations within layers could significantly exceed or fall below the average values derived from seismic data

stochastic inversion by matching to large numbers of pseudo wells abstractwe have developed a 1d stochastic algorithm for estimating reservoir properties based on matching large numbers of pseudo wells to seismic angle stacks the pseudo wells are part deterministic and part stochastic 1d stratigraphic profiles with consistent elastic and reservoir properties pseudo wells are sampled from a prior distribution defined by the geological interpretation a rock physics model and a model for the vertical statistics that provides close control of the lithofacies proportions a new set of pseudo wells typically 103 104 tied to the local stratigraphy is constructed for each seismic trace synthetics derived from the pseudo wells using extended elastic impedance are matched to either one or two seismic angle stacks and the best matches are selected and averaged to provide a joint estimate of reservoir properties and impedances and the associated uncertainties the algorithm has been tested on a number of data sets and validated by blind well ties the algorithm is 1d with