neodymium isotope systematics of the mantle beneath the baltic shield evidence for depleted mantle evolution since the archaean abstract more than 300 published and unpublished nd isotopic analyses of mantle derived rocks from the baltic shield have been compiled the rocks range in age from archaean to phanerozoic within any given age interval the mantle derived rocks range in nd t from depleted mantle values at or above the growth curves of the global depleted mantle reservoirs of depaolo 1981 and depaolo et al 1991 to negative values initial neodymium isotopic compositions below the de paolo curve are best explained by interaction between depleted mantle derived magmas and local crustal contaminants the data now available lend no support to the existence of isolated less depleted or undepleted mantle domains beneath the baltic shield as was suggested by mearns et al 1986 and valbracht 1991a b

the effect of hydrate content on seismic attenuation a case study for mallik 2l 38 well data mackenzie delta canada 1 observations of velocities in sediments containing gas hydrates show that the strength of sediments increases with hydrate saturation hence it is expected that the attenuation of these sediments will decrease with increasing hydrate saturation however sonic log measurements in the mallik 2l 38 well and cross hole tomography measurements in the mallik field have shown that attenuation increases with hydrate saturation we studied a range of mechanisms by which increasing hydrate saturation could cause increased attenuation we found that a difference in permeability between the host sediment and the newly formed hydrate can produce the observed effect we modelled attenuation in terms of biot and squirt flow mechanisms in composite media we have used our model to predict observed attenuations in the mallik 2l 38 well mackenzie delta canada

attenuation of seismic waves in methane gas hydrate bearing sand compressional wave p wave and shear wave s wave velocities vp and vs respectively from remote seismic methods have been used to infer the distribution and volume of gas hydrate within marine sediments recent advances in seismic methods now allow compressional and shear wave attenuations q1p and q1s respectively to be measured however the interpretation of these data is problematic due to our limited understanding of the effects of gas hydrate on physical properties therefore a laboratory gas hydrate resonant column was developed to simulate pressure and temperature conditions suitable for methane gas hydrate formation in sand specimens and the subsequent measurement of both q1p and q1s at frequencies and strains relevant to marine seismic surveys 13 dry gas saturated sand specimens were investigated with different amounts of methane gas hydrate evenly dispersed throughout each specimen the results show that for these dry specimens both q1p and q1s are highly sensitive to hydrate saturation with unexpected peaks observed between 3 and 5 per cent hydrate saturation it is thought that viscous squirt flow of absorbed water or free gas within the pore space is enhanced by hydrate cement at grain contacts and by the nanoporosity of the hydrate itself these results show for the first time the dramatic effect methane gas hydrate can have on seismic wave attenuation in sand and provide insight into wave propagation mechanisms these results will aid the interpretation of elastic wave attenuation data obtained using marine seismic prospecting methods

geological evolution of the karelian granite greenstone terrain abstract the karelian craton 2 3 ga has been recognized within the baltic shield the basement of the craton is a late archaean granite greenstone terrain the greenstone belts from different regions of karelia show variations in age and composition of volcanic rocks two older belts 2 9 ga one with multimodal volcanism and the other with bimodal volcanism constitute the eastern part of the terrain central and eastern karelia two younger belts 2 8 2 6 ga with the same modal distributions of volcanic rocks make up the western part of the terrain western karelia and eastern finland according to magnetic anomaly patterns there are four types of granite gneiss areas that separate the greenstone belts three types represent the greenstone basement reworked at different degrees the fourth type corresponds to tonalites and granites surrounding the greenstone belts the emplacement and evolution of the tonalites and granites are related to the evolution of the greenstone belts geochronological data on the granites of the basement demonstrate a decrease in age from e to w late archaean low pressure granulite metamorphism took place almost synchronously 2 65 ga ago in both the western and eastern domains

seismic quality factors across a bottom simulating reflector in the makran accretionary prism arabian sea the hydrate bearing sediments above the bottom simulating reflector bsr are associated with low attenuation or high quality factor q whereas underlying gas bearing sediments exhibit high attenuation hence estimation of q can be important for qualifying whether a bsr is related to gas hydrates and free gas this property is also useful for identifying gas hydrates where detection of bsr is dubious here we calculate the interval q for three submarine sedimentary layers bounded by seafloor bsr one reflector above and another reflector below the bsr at three locations with moderate strong and no bsr along a seismic line in the makran accretionary prism arabian sea for studying attenuation q 1 characteristics of sediments interval q for hydrate bearing sediments layer b above the bsr are estimated as 191 11 223 12 and 117 5 whereas interval q for the underlying gas bearing sediments layer c are calculated as 112 7 107 8 and 124 11 at moderate strong and no bsr locations respectively the large variation in q is observed at strong bsr thus q can be used for ascertaining whether the observed bsr is due to gas hydrates and for identifying gas hydrates at places where detection of bsr is rather doubtful interval q of 98 4 108 5 and 102 5 respectively at moderate strong and no bsr locations for the layer immediately beneath the seafloor layer a show almost uniform attenuation

geochronology of an archaean tonalitic gneiss dome in northern finland and its relation with an unusual overlying volcanic conglomerate and komatiitic greenstone archaean gneiss greenstone relationships are still unresolved in many ancient cratonic terrains although there is growing evidence that most of the late archaean greenstone assemblages were deposited on older tonalitic crust

sonic waveform attenuation in gas hydrate bearing sediments from the mallik 2l 38 research well mackenzie delta canada 1 the mallik 2l 38 research well was drilled to 1150 m under the mackenzie delta canada and penetrated a subpermafrost interval where methane hydrate occupies up to 80 of the pore space a suite of high quality downhole logs was acquired to measure in situ the physical properties of these hydrate bearing sediments similar to other hydrate deposits resistivity and compressional and shear sonic velocity data increase with higher hydrate saturation owing to electrical insulation of the pore space and stiffening of the sediment framework in addition sonic waveforms show strong amplitude losses of both compressional and shear waves in intervals where methane hydrate is observed we use monopole and dipole waveforms to estimate compressional and shear attenuation comparing with hydrate saturation values derived from the resistivity log we observe a linear increase in both attenuation measurements with increasing hydrate saturation which is not intuitive for stiffening sediments numerical modeling of the waveforms allows us to reproduce the recorded waveforms and illustrate these results we also use a model for wave propagation in frozen porous media to explain qualitatively the loss of sonic waveform amplitude in hydrate bearing sediments we suggest that this model can be improved and extended allowing hydrate saturation to be quantified from attenuation measurements in similar environments and providing new insight into how hydrate and its sediment host interact

nd isotopic constraints on crustal formation in the north china craton abstract recent tectonic analysis suggests that the north china craton consists of two archean continental blocks called the eastern and western blocks separated by the paleoproterozoic trans north china orogen although the published geochronological data are not sufficient to constrain the detailed tectonothermal evolution of the craton the available nd isotopic data show some important differences in nd model ages between the tectonic units the eastern block shows two main nd model age peaks one between 3 6 and 3 2 ga and the other between 3 0 and 2 6 ga limited nd isotopic data from the western block show a large range of model ages between 3 2 and 2 4 ga these differences are consistent with the recently proposed model the nd isotopic data from mantle derived mafic rocks indicate that the mantle beneath the north china craton was depleted in the archean consistent with major crustal growth during this period in the paleoproterozoic however the mantle derived mafic rocks show negative end t values implying crustal contamination this may have resulted from subduction and collision between the eastern and western block implying that the mechanisms of crustal formation and evolution may have been different between the archean and paleoproterozoic the north china craton was re activated by addition of mantle derived magma into the lower crust in the late mesozoic resulting in rejuvenation of the lower crust this indicates that underplating is also an important mechanism for continental addition although in this case it may not equate to crustal growth since it was preceded by removal of lithospheric mantle and possible some lower crust

poroelastic model to relate seismic wave attenuation and dispersion to permeability anisotropy a transversely isotropic model with a horizontal axis of symmetry based on the biot and squirt flow mechanisms predicts seismic waves in poroelastic media the model estimates velocity dispersion and attenuation of waves propagating in the frequency range of crosswell and high resolution reverse vertical seismic profiling vsp 250 1250 hz for vertical permeability values much greater than horizontal permeability parameters the model assumes the principal axes of the stiffness constant tensor are aligned with the axes of the permeability and squirt flow tensors in addition the unified biot and squirt flow mechanism bisq model is adapted to simulate cracks in permeable media under these conditions the model simulations demonstrate that the preferential direction of fluid flow in a reservoir containing fluid filled cracks can be determined by analyzing the phase velocity and attenuation of seismic waves propagating at different azimuth and incident angles as a result the fast compressional wave

cdp and dss data along the uchta kem profile the baltic shield abstract detailed common depth point cdp and deep seismic sounding dss observations were carried out along the w e trending kem uchta profile between the white sea and the russian finnish border the cross sections obtained from wide angle and near vertical reflections show different features in the cdp cross section several inclined boundaries are traced from the surface down to 25 30 km depth they correlate with the well known fault zone between the belomorian mobile belt and the korelian protocraton the dss data also show these inclined reflectors as well as a near horizontal boundary k1 velocity of 6 3 versus 6 4 km s depth of 10 15 km under a low velocity zone the boundary generates high amplitude wide angle reflections which cross the cdp inclined reflections a lower crustal boundary k2 velocity of 6 7 versus 6 8 km s depth of 30 km has no clear expression in the cdp reflectivity pattern as well strong reflections from moho pmp coincide with the boundary between reflective lower crust and transparent upper mantle on the cdp section depth 40 km this suggests that the near horizontal crustal boundaries and the moho are transition zones with high velocity gradients and not sharp discontinuities the data analysis suggests the crustal layering to be a result of rock transformation metamorphism change in mechanical properties after consolidation of the crust tectonic features of orogenic periods were overprinted by the metamorphic processes in the lower crust in the middle crust a destruction low velocity zone and the k1 boundary were created but the initial tectonic structure of the upper crust was preserved the moho is a young feature that was formed during the platform stage of the crustal evolution

an effective medium inversion algorithm for gas hydrate quantification and its application to laboratory and borehole measurements of gas hydrate bearing sediments the presence of gas hydrate in marine sediments alters their physical properties in some circumstances gas hydrate may cement sediment grains together and dramatically increase the seismic p and s wave velocities of the composite medium hydrate may also form a load bearing structure within the sediment microstructure but with different seismic wave attenuation characteristics changing the attenuation behaviour of the composite here we introduce an inversion algorithm based on effective medium modelling to infer hydrate saturations from velocity and attenuation measurements on hydrate bearing sediments the velocity increase is modelled as extra binding developed by gas hydrate that strengthens the sediment microstructure the attenuation increase is modelled through a difference in fluid flow properties caused by different permeabilities in the sediment and hydrate microstructures we relate velocity and attenuation increases in hydrate bearing sediments to their hydrate content using an effective medium inversion algorithm based on the self consistent approximation sca differential effective medium dem theory and biot and squirt flow mechanisms of fluid flow the inversion algorithm is able to convert observations in compressional and shear wave velocities and attenuations to hydrate saturation in the sediment pore space we applied our algorithm to a data set from the mallik 2l 38 well mackenzie delta canada and to data from laboratory measurements on gas rich and water saturated sand samples predictions using our algorithm match the borehole data and water saturated laboratory data if the proportion of hydrate contributing to the load bearing structure increases with hydrate saturation the predictions match the gas rich laboratory data if that proportion decreases with hydrate saturation we attribute this difference to differences in hydrate formation mechanisms between the two environments

characterization and rheology of krishna godavari basin sediments abstract the krishna godavari offshore basin to the east of india is a proven reserve rich in natural gas hydrate the impact of sediment characteristics on the hydrate formation has already been well established in open literature in the present study we have attempted to investigate the mineralogy of sediments collected from kg basin by elemental analysis energy dispersive spectroscopy and x ray diffraction various physicochemical characteristics of the clayey sediments such as tds salinity ph conductivity and resistivity were analysed at various concentrations and compared together also this paper highlights the rheological behavior of the sediment samples with concentrations of 20 35 and 50 wt at different experimental temperatures 278 15 k 283 15 k and 288 15 k viscosity measurements were performed for a wide range of shear rates for all concentrations and comparative studies have been conducted on their exhibited behavior the viscosity of sediment sample were found to be varying from 76 3 to 0 003 pa s depending on the sediment concentration temperature and shear rate in addition viscoelastic measurements were carried out at various angular frequencies for all the sediment samples the work aims to characterise the sediments of kg basin and analyse the rheological behavior of sediment solution which has not yet been reported in open literature this will provide vital information for possible methane recovery from kg basin hydrate reservoir by manipulating the host sediment behavior

smnd evidence for late archaean crust formation in the lapland kola mobile belt kola peninsula russia and norway abstract a sm nd model age traverse across the kola peninsula has been undertaken to gain insight into the major episodes of crustal growth and to test the possible presence of early archaean saamian crust in the region granitoid gneisses and subordinate metasediments have been sampled from the main archaean geological units i e from northeast to southwest the murmansk domain model ages of 2 68 2 96 ga central kola domain including samples from the superdeep borehole 2 74 2 92 ga belomorian domain including the eastern part of the tersk area 2 70 2 93 ga the narrow range of depleted mantle model ages from 2 68 to 2 96 ga for archaean granitoid orthogneisses close to the oldest crystallization ages from the region indicate a major episode of late archaean crustal growth and reveal no indication of ancient saamian material in keeping with other geochronological evidence similar results 2 86 to 2 93 ga for archaean metasediments support this conclusion proterozoic model ages for supracrustals from the western part of the tersk area 2 16 to 2 23 ga indicate that the crust in this area is much younger than previously recognized initial nd values for late archaean samples with reasonable age constraints fall within a range of 2 3 to 2 5 two well dated samples from the belomorian domain have the highest initial nd values nd 2 81 ga 2 0 and nd 2 78 ga 2 3 and suggest that a depleted mantle source was present in the kola region in late archaean times dated samples from the central kola domain have lower initial nd between 2 5 and 1 2 suggesting a possible regional difference in mantle character in contrast to the depleted signature of the archaean orthogneisses between 2 7 ga and 2 8 ga previous studies of earliest palaeoproterozoic rift related majic intrusives in the kola and karelia regions report negative nd values at 2 4 2 5 ga possibly these data reflect a change to more enriched mantle values

early proterozoic palaeomagnetism new results from the intrusives and related rocks of the karelian belomorian and kola provinces eastern fennoscandian shield the palaeomagnetic data that are available for the eastern fennoscandian shield are too sparse to allow any palaeomagnetic constraints to be determined for intracratonic horizontal movements in this part of the shield to address this issue we have obtained new palaeomagnetic data from mafic and ultramafic plutons sills dykes and basement country rocks in the kola belomorian and karelian provinces of the fennoscandian shield a number of natural remanent magnetization nrm components were isolated and we associate these with various events of precambrian tectonomagmatic activity within the eastern fennoscandian shield 36 palaeomagnetic poles were obtained 12 poles with ages ranging from 1700 ma to 2450 ma satisfy the accepted reliability criteria corresponding to the b category of briden duff 1981 of these six might be classified as reference poles for the kola and belomorian provinces there are poles from the main range gabbro dolerites 16 7 s 304 8 e a95 8 3 u pb age of 2436 25 ma the voche lambina ultramafics and amphibolites 12 9 n 314 1 e a95 8 6 rb sr age of 2198 172 ma and the kolvitsa peridotites and schists 44 2 n 232 6 e a95 5 4 assigned magnetization age of 1900 ma for the karelian province reference poles are presented for the burakovka gabbro norites 19 5 s 262 6 e a95 7 2 sm nd age of 2439 22 ma the matozero doleritic dykes 20 6 s 308 5 e a95 12 3 u pb age of 2150 60 ma and the ropruchey sill gabbro dolerites 40 5 n 229 8 e a95 8 1 u pb age of 1770 12 ma although some nrm components of other rocks are not as precisely dated a number of palaeomagnetic poles are in good internal agreement forming two tracks of the lower proterozoic apparent polar wander path apwp a preliminary version of the pre svecofennian apwp tracks for the karelian and kola belomorian provinces is proposed our results confirm the general drift pattern of fennoscandia in the proterozoic and support the idea of the absence of large scale block movements within the fennoscandian shield since jatulian times 2300 2100 ma to help constrain the relative positions of the various tectonic blocks of the eastern fennoscandian shield during pre jatulian times more new palaeomagnetic and radiometric data need to be added to the available palaeomagnetic database

deformation structures and implications for fluid flow at the costa rica convergent margin odp sites 1040 and 1043 leg 170 abstract during odp leg 170 five sites were drilled and sampled off the costa rica pacific margin two of them site 1040 and 1043 yielded material from a wedge of deformed sediments the main decollement zone and the underthrusting sedimentary sequence of the subducting cocos plate detailed mesoscopic and microscopic analyses of the deformation features characteristic of each domain help to define four different structural hydrologic regimes unexpectedly the wedge of deformed hemipelagic sediments does not represent an accretionary wedge because little or no transfer of material from the subducting plate has occurred the deformed sedimentary wedge records periods of tectonic bulk strain in which fluid pressure intermittently rises and induces fracturing alternating with periods of gravitational bulk strain the last stage of this alternating strain regime is represented by the growth of thick sub horizontal shear zones the sediments caught up in the decollement zone show distinctly different deformation features reflecting different mechanical behaviour and a different response to fluid flow but again transient pulses of increased pore pressure are interpreted as the driving mechanism of dewatering the underthrust pelagic and hemipelagic section is also characterised by dewatering features but only in the uppermost part the basal sediments of this section in contact with underlying gabbro intrusions have a completely different structural history and hydrologic regime structures here are interpreted as related to near ridge processes and the hydrologic system is not linked to the upper sediment dewatering regime but rather seems to have been fed by a seawater source

age tectonic setting and provenance of ostfold marstrand belt supracrustals westward crustal growth of the baltic shield at 1760 ma the ostfold marstrand paragneiss belt is the oldest part of the sw scandinavian domain of the baltic shield in the swedish part of the belt minor mafic volcanics now amphibolites in the metagreywacke dominated stora le marstrand slm supracrustals are chemically similar to primitive modern oceanic arcs or back arc regions a wide compositional range is attributed mainly to fractional crystallisation yielding fe rich andesitic amphibolites diverging lree patterns between different localities are consistent with variations in the degree of partial melting in the source a nine point sm nd errorchron of 1758 78 ma mswd 9 4 for the amphibolites is interpreted to date the volcanism and coeval sedimentation and hence directly dates crustal growth in this part of the baltic shield the exclusion of two pillow lava samples which may have been contaminated yields an isochron with an identical though more precise age an nd value of 4 3 indicates a depleted mantle source similar to that of canada sw u s a s greenland and parts of finland but rather more depleted than that reported hitherto from eastern sweden at the same time nd isotopic data suggest a two component provenance for the slm sediments involving acid derivatives from the mafic volcanics and a continental source similar to the exposed 2 0 1 75 ga old svecofennian crust of eastern sweden with minimal contribution from substantially older crust the mafic volcanism took place during subduction of oceanic crust outboard of the svecofennian part of the baltic shield after substantial addition of 1650 ma old granitoids accretion to the baltic shield possibly took place at 1580 ma

multi chronometric ages and origin of archaean tonalitic gneisses in finnish lapland a case for long crustal residence time the tojottamanselka gneisses of the koitelainen region northern finland have been dated by the sm nd and the common pb methods the sm nd data of seven samples from a small area 100 m 100 m define an isochron oft 3 06 0 12 2 ga with correspondingind 0 50848 9 2 orend t 3 7 1 8 this age is in good agreement with the zircon u pb discordia age 3 1 ga reported by kroner et al 1981 and is interpreted as the time of magmatic emplacement the distinctly negativeend t value is found for the first time for archean tonalitic gneisses and implies derivation of these magmas by remelting of continental material with a long 200 500 ma crustal residence time a few samples on the other hand possessend t values close to zero hence they are thought to be derived by partial melting of basaltic sources with nearchondritic ree distribution patterns

origin of continental crust of 1 9 1 7 ga age nd isotopes in the svecofennian orogenic terrains of sweden initial nd isotopic ratios are reported for 57 samples representing all crustal components in the central and southern parts of the 1 9 1 7 ga old svecofennian orogenic terrains of sweden and the 1 8 1 7 ga old transscandinavian granite porphyry belt transscandinavian igneous belt u pb zircon ages are determined where the magmatic ages were not sufficiently constrained the nd isotopic results show that there are no archaean crustal blocks within the studied 1 9 1 7 ga old crustal segment this is consistent with results from finland the main input of recycled archaean crust to the svecofennian was in the form of sediments which are concentrated in the central svecofennian subprovince of north central sweden this is consistent with the vicinity of this subprovince to the archaean craton of the baltic shield these turbidite sediments contain up to 40 archaean material but s type granitoids associated with them have a smaller archaean component around 20 most of the quantitatively predominant i type granitoids and volcanics as well as all the basaltic rocks studied have nd from 1 to 3 and consist of 90 material newly derived from the mantle 1 9 1 7 ga ago we argue that the archaean material was introduced in the form of sediments but we cannot distinguish between pelagic sediments delivered on subducting oceanic crust and turbidite accumulations like the ones seen in north central sweden in either case an overall 15 archaean contribution for the swedish svecofennian was reasonably well homogenized with mantle derived magmatic products for this to occur an active near surface tectonic environment such as a complex of subduction zones must have been necessary

elastic properties of hydrate bearing sediments using effective medium theory accurate and detailed models of the seismic velocity structure of gas hydrate bearing sediments may be determined by careful analysis of controlled source seismic data however interpretation of these velocities in terms of hydrate saturation of the pore space has hitherto relied on semiempirical formulas and or simple effective medium theory we develop a rigorous theoretical scheme to relate the seismic properties of a clay rich hydrate bearing sediment to its porosity mineralogy microstructural features and hydrate saturation we consider separately the two possible end members for the distribution of hydrate in the pore space 1 hydrates are unconnected and located in the pore voids without appreciable grain contact and 2 connected hydrates are forming cement binding around the grains the scheme is transversely isotropic to allow for anisotropy due to alignment of clay platelets and is based on a combination of a self consistent approximation a differential effective medium theory and a method of smoothing for crystalline aggregates we have applied the scheme to lithological and seismic velocity data from ocean drilling program site 995 on the blake ridge southeastern u s continental margin to make estimates of the hydrate saturation it was found that the hydrates are probably unconnected and their volume concentration varies between approx 0 at 100 m below the seabed and approx 9 at 400 m depth just above the bottom simulating reflector if the clay platelet orientation distribution resembles the function we have used

seismic wave attenuation in methane hydrate bearing sediments vertical seismic profiling data from the nankai trough exploratory well offshore tokai central japan 1 data from two vertical seismic profiles vsps from the nankai trough exploratory well offshore tokai central japan are used to estimate compressional attenuation in methane hydrate mh bearing sediments at seismic frequencies of 30 110 hz we compare spectral ratio and centroid frequency shift methods for measuring attenuation to isolate intrinsic attenuation from total attenuation attenuation is computed from multiples using one dimensional synthetic vsp data from sonic and density logs the use of two different measurement methods and two vsps recorded at just 100 m separation provides an opportunity to validate the attenuation measurements no significant compressional attenuation was observed in mh bearing sediments at seismic frequencies macroscopically the peaks of highest attenuation in the seismic frequency range correspond to low saturation gas zones in contrast high compressional attenuation zones in the sonic frequency range 10 20 khz are affected by the presence of methane hydrates in the same well locations thus this study demonstrated the frequency dependence of attenuation in mh bearing sediments mh bearing sediments cause attenuation in the sonic frequency range rather than the seismic frequency range

neural network analysis of crosshole tomographic images the seismic signature of gas hydrate bearing sediments in the mackenzie delta nw canada 1 crosshole seismic experiments were conducted to study the in situ properties of gas hydrate bearing sediments ghbs in the mackenzie delta nw canada seismic tomography provided images of p velocity anisotropy and attenuation self organizing maps som are powerful neural network techniques to classify and interpret multi attribute data sets the coincident tomographic images are translated to a set of data vectors in order to train a kohonen layer the total gradient of the model vectors is determined for the trained som and a watershed segmentation algorithm is used to visualize and map the lithological clusters with well defined seismic signatures application to the mallik data reveals four major litho types 1 ghbs 2 sands 3 shale coal interlayering and 4 silt the signature of seismic p wave characteristics distinguished for the ghbs high velocities strong anisotropy and attenuation is new and can be used for new exploration strategies to map and quantify gas hydrates

seismic attenuation effects of pore fluids and frictional sliding seismic wave attenuation in rocks was studied experimentally with particular attention focused on frictional sliding and fluid flow mechanisms sandstone bars were resonated at frequencies from 500 to 9000 hz and the effects of confining pressure pore pressure degree of saturation strain amplitude and frequency were studied observed changes in attenuation and velocity with strain amplitude are interpreted as evidence for frictional sliding at grain contacts since this amplitude dependence disappears at strains and confining pressures typical of seismic wave propagation in the earth we infer that frictional sliding is not a significant source of seismic attenuation in situ partial water saturation significantly increases the attenuation of both compressional p and shear s waves relative to that in dry rock resulting in greater p wave than s wave attenuation complete saturation maximizes s wave attenuation but causes a reduction in p wave attenuation these effects can be interpreted in term

attenuation modes from vertical seismic profiling and sonic waveform in a carbonate reservoir abu dhabi united arab emirates in this study we derived accurate and high resolution attenuation profiles using spectral ratio centroid frequency shift and seismic interferometry methods we utilized high quality vertical seismic profiling and sonic waveform data acquired in a carbonate reservoir located in abu dhabi united arab emirates the scattering profile of vertical seismic profiling data contributes significantly to wave attenuation that can be explained by high heterogeneity of the carbonate rocks the scattering profile also correlates well with the reservoir lithology and fractured zones imaged by the formation microimager a tar mat zone occurs within the lower part of arab d reservoir this zone corresponds with a decrease in scattering attenuation the tar mat may have filled the pores and made this zone less heterogeneous therefore a decrease in scattering attenuation can be considered a potential parameter for tar mat detection after removing the scattering effect nonphysical negative intrinsic attenuation values still exist at certain depths the most probable explanation for this is the three dimensional scattering effect which is not taken into account in this paper and short period upgoing waves seismic interferometry is less sensitive to the remaining scattered upgoing wave which is why seismic interferometry method shows fewer negative values than the spectral ratio and centroid frequency shift methods compared with vertical seismic profiling attenuation scattering attenuation estimated from sonic waveforms recorded in the reservoir zones is insignificant and the intrinsic attenuation is almost equivalent to the total attenuation we attribute this underestimation of the scattering attenuation to the sparse spatial sampling of the sonic logging data at 0 1524 m which is not sufficient to appropriately estimate the scattering effect in heterogeneous media the cross plots between sonic attenuation and various petrophysical properties show slight dependence between the sonic attenuation and neutron porosity and resistivity in the reservoir zones however we can highlight from these plots two zones belonging to the arab reservoirs the lower zone corresponds to arab d reservoir and displays higher sonic intrinsic attenuation than the upper zone arab a c reservoirs due to higher oil saturation this highlights the sensitivity of the intrinsic attenuation to the oil saturation

dynamic poroelasticity a unified model with the squirt and the biot mechanisms the velocities and attenuation of seismic and acoustic waves in rocks with fluids are affected by the two most important modes of fluid solid interaction 1 the biot mechanism where the fluid is forced to participate in the solid s motion by viscous friction and inertial coupling and 2 the squirt flow mechanism where the fluid is squeezed out of thin pores deformed by a passing wave traditionally both modes have been modeled separately with the biot mechanism treated in a macroscopic framework and the squirt flow examined at the individual pore level we offer a model which treats both mechanisms as coupled processes and relates p velocity and attenuation to macroscopic parameters the biot poroelastic constants porosity permeability fluid com pressibility and viscosity and a newly introduced microscale parameter a characteristic squirt flow length the latter is referred to as a fundamental rock property that can be determined experimentally we show that the squirt flow mechanism dominates the biot mechanism and is responsible for measured large velocity dispersion and attenuation values the model directly relates p velocity and attenuation to measurable rock and fluid properties therefore it allows one to realistically interpret velocity dispersion and or attenuation in terms of fluid properties changes e g viscosity during thermal enhanced oil recovery eor or to link seismic measurements to reservoir properties as an example of the latter transformation we relate permeability to attenuation and achieve good qualitative correlation with experimental data

late archaean smnd model ages from the voche lambina area implications for the age distribution of archaean crust in the kola peninsula russia abstract although an early archaean age for the saamian gneisses has been generally accepted in the former soviet union partly on the basis of their commonly granulite facies metamorphic grade and partly on account of their structural complexity and complex intrusion history these rocks have yielded no precise isotopic ages in excess of 2 9 ga sm nd isotopic analyses are used to test the possible presence of early archaean 3 5 ga saamian gneisses at voche lambina in the kola peninsula here tonalitic trondhjemitic and granodioritic gneisses presumed to be early archaean saamian in age are overlain by a sequence of late archaean lopian supracrustals a sample of the structurally oldest trondhjemitic gneiss with a minimum age of 2 64 ga has a depleted mantle model age tdm of 2875 ma tonalitic gneiss also assumed on structural grounds to be early archaean has a tdm age of 2808 ma identical to its u pb zircon age of 2807 9 ma this suggests that a depleted mantle model may be appropriate for at least this part of the kola region in the late archaean lopian rhyodacite with a u pb zircon age of 2800 ma and a rb sr isochron age of 2540 83 ma has a tdm age of 2756 ma in each case the close agreement between the sm nd model age and the time of intrusion or eruption leaves little scope for a significantly older crustal history for these rocks the data provide no evidence for a saamian component in this part of the kola peninsula

a theory of compressional wave attenuation in noncohesive sediments published reviews indicate that attenuation coefficients of compressional waves in noncohesive water saturated sediments vary linearly with frequency biot s theory which accounts for attenuation in terms of the viscous interaction between the solid particles and pore fluid predicts in its presently published form variation proportional to f2 at low frequencies and f1 2 at high frequencies a modification of biot s theory which incorporates a distribution of pore sizes is presented and shown to give excellent agreement with new and published attenuation data in the frequency range 10 khz to 2 25 mhz in particular a linear variation of attenuation with frequency is predicted in that range

physical properties of hydrate bearing sediments 1 methane gas hydrates crystalline inclusion compounds formed from methane and water are found in marine continental margin and permafrost sediments worldwide this article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate bearing sediments formation phenomena include pore scale habit solubility spatial variability and host sediment aggregate properties physical properties include thermal properties permeability electrical conductivity and permittivity small strain elastic p and s wave velocities shear strength and volume changes resulting from hydrate dissociation the magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate bearing sediments to changes in mechanical thermal or chemical boundary conditions these predictions are vital for mitigating borehole local and regional slope stability hazards optimizing recovery techniques for extracting methane from hydrate bearing sediments or sequestering carbon dioxide in gas hydrate and evaluating the role of gas hydrate in the global carbon cycle

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