natural gas hydrates on the southeast u s margin constraints from full waveform and travel time inversions of wide angle seismic data strong bottom simulating reflectors bsr have been mapped over a region of approximately 50 000 km2 on the southeastern u s margin and have been associated with possible abundance of natural gas hydrates in june 1992 coincident single channel seismic and wide angle ocean bottom seismic data were acquired in the region focusing on the blake ridge and the carolina rise wide angle reflections from the bsrs were clearly observed at offsets up to 6 km joint travel time inversion was conducted with wide angle and vertical incidence data in order to explore possible regional variation and the resultant two dimensional average velocity models imply higher background velocities on the carolina rise full waveform inversion was then performed to determine the seismic origin of the bsrs the best fit model shows a similar low velocity 1 4 km s beneath the bsr at both sites indicating trapped free gas with low saturation 10 the inversion results also indicate that a thin high velocity wedge with a maximum velocity of 2 3 km s is present just above the blake ridge bsr sediment reflectivities were also calculated and higher reflectivities are observed on the carolina rise an increase in reflectivity below the bsr seems to correspond to the gas bearing zone at both sites concentration of hydrates were estimated based on these velocity models whereas average hydrate concentration of 3 of the total sediment volume is suggested for the lower half of the hydrate stability zone at the blake ridge only a very low average concentration of hydrate can be expected at the carolina rise the hydrates seem to be concentrated near the base of the hydrate stability zone and the maximum hydrate concentration is estimated as 20 at the blake ridge and 7 at the carolina rise both of which are too high to be explained by in situ biogenic activity only and require some secondary accumulation mechanism it is suggested that hydrate recycling caused by the stability field migration may have effectively condensed hydrates at both sites additional enhancement by upward fluid expulsion may also be viable for the blake ridge

exploration for gas hydrates in the deepwater northern gulf of mexico part i a seismic approach based on geologic model inversion and rock physics principles this article presents the results of applying a five step process for using high quality seismic data to locate marine gas hydrates the process involved 1 reprocessing of seismic data for higher resolution 2 detailed stratigraphic evaluation and interpretation to locate possible hydrate bearing zones 3 seismic attribute analysis to further delineate these zones 4 seismic inversion to obtain appropriate elastic parameters of these zones in 3d and 5 quantitative estimation of gas hydrate saturation from seismic data using inversion and rock physics principles we used seismic data from keathley canyon 151 and atwater valley 14 in the northern gulf of mexico although careful analysis did indicate the presence of a bottom simulating reflector bsr our study mainly relied on a host of other seismic attributes e g gas chimneys seismically transparent zones other features associated with the petroleum system to characterize the occurrence of gas hydrates in these areas we tested and verified a viable rock model for hydrate bearing sediments using data from the mallik mckenzie delta canada and blake ridge odp leg 164 southeast u s atlantic margin wells and modified it for application in the current area we then used this model to estimate gas hydrate saturation in the host sediments of the northern gulf of mexico focus areas using estimates of p wave and s wave velocities from inversion of reflection seismic data in this paper we present the gas hydrate saturations predicted from the seismic processing methodology prior to 2005 drilling in the focus areas in frontier areas where no well data are available and lithologic heterogeneities are poorly understood implementing a seismic based technique like the one described here to identify potential gas hydrates can provide valuable pre drill information for site selection and for planning future characterization studies of gas hydrate bearing sediments

gas migration into gas hydrate bearing sediments on the southern hikurangi margin of new zealand we present multichannel seismic data from new zealand s hikurangi subduction margin that show widespread evidence for gas migration into the field of gas hydrate stability gas migration along stratigraphic layers into the hydrate system manifests itself as highly reflective segments of dipping strata that originate at the base of hydrate stability and extend some distance toward the seafloor the highly reflective segments exhibit the same polarity as the seafloor reflection indicating that localized gas hydrate precipitation from gas charged fluids within relatively permeable layers has occurred high density velocity analysis shows that these layer constrained gas hydrate accumulations are underlain by thick up to 500 m free gas zones which provide the source for focused gas migration into the hydrate layer in addition to gas being channeled along layers we also interpret gas migration through a fault zone into the field of hydrate stability in this case a low velocity layer within the hydrate stability zone extends laterally away from the fault which might indicate that gas charged fluids have also migrated away from the fault along strata at this site where both dipping strata and faulting seem to influence fluid migration we observe anomalously high velocities at the base of hydrate stability that we interpret as concentrated gas hydrates our results give insight into how shallow fluid flow responds to permeability contrasts between strata fault zones and perhaps also the gas hydrate system itself ultimately these relationships can lead to gas migration across the base of hydrate stability and the precipitation of concentrated hydrate deposits

gas hydrate reservoirs and gas migration mechanisms in the terrebonne basin gulf of mexico highlights identify new fine grained hydrate filled fracture units in the terrebonne basin identify new hydrate bearing thin sands mostly within fractured muds present detailed seismic amplitude maps of the new hydrate bearing units discuss methane migration mechanisms and hydrate formation in thin sands identify and discuss source reservoir relationships between thick muds and thin sands abstract the interactions of microbial methane generation in fine grained clay rich sediments methane migration and gas hydrate accumulation in coarse grained sand rich sediments are not yet fully understood the terrebonne basin in the northern gulf of mexico provides an ideal setting to investigate the migration of methane resulting in the formation of hydrate in thin sand units interbedded with fractured muds using 3d seismic and well log data we have identified several previously unidentified hydrate bearing units in the terrebonne basin two units are 100 m thick fine grained clay rich units where gas hydrate occurs in near vertical fractures in some locations these fine grained units lack fracture features and they contain 1 4 m thick hydrate bearing sands in addition several other thin sand units were identified that contain gas hydrate including one sand that was intersected by a well at the location of a discontinuous bottom simulating reflector using correlation of well log data to seismic data we have mapped and described these new units in detail across the extent of the available data allowing us to determine the variation of seismic amplitudes and investigate the distribution of free gas and or hydrate we present several potential source reservoir scenarios between the thick fractured mud units and thin hydrate bearing sands we observe that hydrate preferentially forms within thin sand layers rather than fractures when sands are present in larger marine mud units based on regional mapping showing the patchy lateral extent of the thin sand layers we propose that diffusive methane migration or short migration of microbially generated methane from the marine mud units led to the formation of hydrate in these thin sands as discontinuous sands would not be conducive to long range migration of methane from deeper reservoirs

new late jurassic to early cretaceous paleomagnetic results from north china and southern mongolia and their implications for the evolution of the mongol okhotsk suture

gas hydrate reservoir and active methane venting province in sediments on 20 ma young oceanic crust in the fram strait offshore nw svalbard seafloor pockmarks are common indicators for vertical fluid flow and frequently associated with methane discharge through the gas hydrate stability zone ghsz the present day flux through these degassing systems is presumably at a low level on most rifted continental margins a pockmark field on the nw svalbard passive margin is located on young ocean crust 20 ma and shows evidence of ongoing episodic degassing new geophysical data from the vestnesa ridge 79 n a mounded and elongated sediment drift in the eastern fram strait reveal a gas hydrate free gas and venting system that is exceptionally more dynamic than documented elsewhere along the northeastern north atlantic margin the prominent bottom simulating reflection bsr about 200 mbsf separates anomalously high p wave velocities in the ghsz from a remarkable underlying low velocity zone indicating the presence of gas hydrate and gas in the pore space inversion of p wave velocity data using the differential effective medium theory yields a two dimensional concentration model of methane hydrate and free gas the model predicts saturations of up to 11 in the hydrate reservoir which due to the seafloor topography forms a large anticlinal permeability barrier below in the low velocity zone i e 1350 1500 m s up to 3 of free gas is predicted across the apex of the vestnesa ridge and in the immediate vicinity of extensional faults a conservative estimate indicates that 225 kg m2 of pure methane is stored in hydrate and gas in the upper 230 m of the sedimentary column an elongated pockmark field consisting of 100 individual pockmarks up to 600 m wide systematically aligns the apex of the vestnesa ridge active vigorous degassing from the topography controlled pressure valve system was evident from a 750 m high and 150 m wide gas flare observed in the water column during a cruise with r v jan mayen in october 2008 the gas flare documents dynamic degassing through the corresponding chimney which penetrates the entire ghsz and into the underlying free gas zone cruises in 2006 and 2007 did not detect active gas venting above the pockmark field accordingly vigorous degassing may operate in an episodic mode where hydrothermal circulation systems through young ocean crust may play a significant role

velocity structures imaged from long offset reflection data and four component obs data at jiulong methane reef in the northern south china sea abstract in this study p and s wave velocity models were built based on two pre stack depth migration psdm profiles of long offset reflection data and 25 four component ocean bottom seismometers obs data at jiulong methane reef off sw taiwan in the passive margin of the northern south china sea scs according to the velocity models the average p wave velocity and vp vs ratio of the free gas beneath the bottom simulating reflector bsr are 1 52 1 58 km s and 4 5 5 10 respectively the depth of the bsr is found at 80 300 m below the sea floor and the sedimentary thickness of the hydrate and the free gas are about 50 100 m and 70 100 m respectively p wave velocity of about 1 75 km s in the hydrate bearing sediment is southeastward increased to about 1 85 km s above the bsr similarly vp vs ratio of about 3 06 in the hydrate bearing sediment is increased southeastward to about 3 48 above the bsr based on the pseudo 3d map of gas hydrate saturation estimated from the psdm and obs models the average saturations of hydrate and free gas at jiulong methane reef are about 7 and 0 9 2 4 respectively the highest hydrate saturation 11 is located at 5 20 m above the bsr in the se portion of the jiulong methane reef on the other hand the highest gas saturation of about 2 is observed at 10 70 m below the bsr in the nw portion of the jiulong methane reef we suggested that several normal faults dipping southeastward beneath the continental slope provided conduits for gas migrating northwestward at jiulong methane reef therefore the highest gas saturation is observed below the anticline in the nw portion of the jiulong methane reef and the highest hydrate saturation high p wave velocity and high vp vs ratio is identified above the bsr in the se portion of the jiulong methane reef

velocity structure of a gas hydrate reflector seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs parallel to the seabed these are often interpreted as the base of a zone in which methane hydrate ice is stable waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a bsr and its thickness a test of this method at a drill site of the ocean drilling program predicts that sediment pores beneath the bsr contain free methane for approximately 30 meters the hydrate and underlying gas represent a large global reservoir of methane which may have economic importance and may influence global climate

visual observation of gas hydrate formation and dissociation in synthetic porous media by means of glass micromodels visual observation of gas hydrates at the microscopic scale in synthetic porous media provides unequivocal visual evidence that clathrates can form in systems without the presence of a free gas phase hydrates were formed from a soluble liquid hydrate former tetrahydrofuran c 4 h 8 o from free gas ch 4 and from dissolved gas co 2 clathrates were found to form within the center of pore spaces rather than on grain surfaces cementation of grains only occurred in regions of a small grain size or where a large proportion of pore space was filled with hydrate however even at high clathrate saturations a thin film of free water persisted on grain surfaces the results have important implications for the potential cementing effect of hydrates on sediments and thus for sediment permeability slope stability and seismic interpretation of hydrate bearing sediments

elasticity of marine sediments rock physics modeling we offer an effective medium model for the elastic moduli of high porosity ocean bottom sediments the elastic constants of the dry sediment frame depend on porosity elastic moduli of the solid phase and effective pressure the model connects two end points in the elastic modulus porosity plane the hertz mindlin modulus of a dense elastic sphere pack at critical porosity and zero at 100 porosity the elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation unlike the suspension model our model assigns non zero elastic constants to the dry sediment frame and can predict the shear wave velocity unlike various modifications of the travel time average equation it is first principle based and contains only physical parameters we justify this model by matching sonic data in shallow marine sediments and in an odp well

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

mechanical properties of sand silt and clay containing tetrahydrofuran hydrate 1 the mechanical behavior of hydrate bearing sediments subjected to large strains has relevance for the stability of the seafloor and submarine slopes drilling and coring operations and the analysis of certain small strain properties of these sediments for example seismic velocities this study reports on the results of comprehensive axial compression triaxial tests conducted at up to 1 mpa confining pressure on sand crushed silt precipitated silt and clay specimens with closely controlled concentrations of synthetic hydrate the results show that the stress strain behavior of hydrate bearing sediments is a complex function of particle size confining pressure and hydrate concentration the mechanical properties of hydrate bearing sediments at low hydrate concentration probably 50 of pore space the behaviorbecomesmoreindependentofstressbecausethehydratescontrolbothstiffnessand strength and possibly the dilative tendency of sediments by effectively increasing interparticle coordination cementing particles together and filling the pore space the cementation contribution to the shear strength of hydrate bearing sediments decreases with increasing specific surface of soil minerals the lower the effective confining stress the greater the impact of hydrate formation on normalized strength

gas migration pathways and slope failures in the danube fan black sea abstract a large geophysical dataset including bathymetry and 2d and 3d p cable seismic data revealed evidence of numerous gas flares near the s2 canyon in the danube fan northwestern black sea this dataset allows us to investigate potential relationships between gas migration pathways gas vents observed at the seafloor and submarine slope failures vertical gas migration structures as revealed in the seismics appear to be concentrated near submarine slope failure structures where these seismically defined features extend upwards to the seafloor they correlate with the location of gas flares however not all these structures reach the seafloor in some cases because they are capped by overlying sediments a strong correlation is inferred between gas migration pathways heterogeneous mass transport deposits and contacts between adjacent units of contrasting lithology although missing age constrains prevent a final judgement we discuss the potential relationship between submarine slope failures and gas migration in order to determine if gas migration is a precursor to failure or if the presence of slope failures and associated mass transport deposits facilitates the migration of gas our observations indicate that lithological heterogeneity mass transport deposits and minor sediment deformation control gas migration pathways and the formation of gas chimney like features gas migration is focused and gradual resulting in gas flares where the chimney like features extend to the seafloor with no evidence of erosive features such as pockmarks

first paleomagnetic data for the new siberian islands implications for arctic paleogeography abstract in this paper we present new paleomagnetic and paleontological data from the ordovician and silurian carbonate rocks of kotelny island the anjou archipelago and from the ordovician turbidities of bennett island the de long archipelago it is assumed that both archipelagos belong to the nsi new siberian islands terrane a key tectonic element in the arctic region ages of the studied rocks have been established by paleontological data and lithological correlations our new data on conodonts combined with those from previous studies of ordovician and silurian fauna indicate a biogeographic similarity between the shelves of the siberian paleocontinent and the nsi in the early paleozoic three new paleomagnetic poles for the nsi 48 9 n 13 8 e a95 18 1 for 475 ma 45 5 n 31 9 e a95 11 0 for 465 ma and 33 7 n 55 7 e a95 11 0 for 435 ma fall between the south eastern part of central europe and the zagros mountains the similarity of paleomagnetic directions from kotelny and bennet islands confirms that both the anjou and de long archipelagos belong to the same terrane calculated paleolatitudes indicate that in ordovician silurian times this terrane has been located between 30 and 45 possibly in the northern hemisphere based on this observation we suggest a linkage between the nsi and the kolyma omolon superterrane comparison of apparent polar wander paths apwps of the nsi siberia and other cratons terranes suggests that the nsi drifted independently we demonstrate that the structural line between svyatoy nos peninsula and great lyakhovsky island is the continuation of the kolyma loop suture on the arctic shelf and expect that the continuation of the south anyui suture is to be found east of the nsi

estimating the amount of gas hydrate and free gas from marine seismic data marine seismic data and well log measurements at the blake ridge offshore south carolina show that prominent seismic bottom simulating reflectors bsrs are caused by sediment layers with gas hydrate overlying sediments with free gas we apply a theoretical rock physics model to 2 d blake ridge marine seismic data to determine gas hydrate and free gas saturation high porosity marine sediment is modeled as a granular system where the elastic wave velocities are linked to porosity effective pressure mineralogy elastic properties of the pore filling material and water gas and gas hydrate saturation of the pore space to apply this model to seismic data we first obtain interval velocity using stacking velocity analysis next all input parameters to the rock physics model except porosity and water gas and gas hydrate saturation are estimated from geologic information to estimate porosity and saturation from interval velocity we first assume that the entire sediment does not contain gas hydrate o

the effect of methane hydrate morphology and water saturation on seismic wave attenuation in sand under shallow sub seafloor conditions a better understanding of seismic wave attenuation in hydrate bearing sediments is needed for the improved geophysical quantification of seafloor methane hydrates important for climate change geohazard and economic resource assessment hence we conducted a series of small strain 10 6 seismic frequency 50 550 hz laboratory resonant column experiments on synthetic methane hydrate bearing sands under excess water seafloor conditions the results show a complex dependence of p and s wave attenuation on hydrate saturation and morphology p and s wave attenuation in excess water hydrate bearing sand is much higher than in excess gas hydrate bearing sand and increases with hydrate saturation between 0 and 0 44 the experimental range theoretical modelling suggests that load bearing hydrate is an important cause of heightened attenuation for both p and s waves in gas and water saturated sands while pore filling hydrate also contributes significantly to p wave attenuation in water saturated sands a squirt flow attenuation mechanism related to microporous hydrate and low aspect ratio pores at the interface between sand grains and hydrate is thought to be responsible for the heightened levels of attenuation in hydrate bearing sands at low hydrate saturations 0 44

analysing sand dominated channel systems for potential gas hydrate reservoirs using an avo seismic inversion technique on the southern hikurangi margin new zealand abstract gas hydrates have recently been recognised as a class of unconventional petroleum resource and the economic viability of gas production from hydrates is now being viewed as a realistic possibility within the next decade therefore potential offshore hydrate accumulations in the world class endowed gas hydrate province the hikurangi margin new zealand represent a significant medium to long term opportunity to meet the country s future energy requirements in this paper we delineate a potential gas hydrate reservoir in the east coast basin new zealand and quantitatively estimate its gas hydrate concentrations from 2d seismic data with no well information available the target is interesting for exploration since it shows evidence for gas hydrate bearing sands in particular buried channel systems we use a combined analysis of high resolution velocity analysis amplitude versus offset avo attribute and avo inversion to investigate whether we can identify regions that are likely to contain highly concentrated gas hydrates and whether they are likely to be sand dominated to estimate hydrate concentrations we apply a rock physics model our results indicate the presence of several up to 200 m thick zones that are likely to host gas hydrates with one location predicted to consist of high permeable channel sands and an inferred gas hydrate saturation of 25 these findings suggest significant amounts of gas hydrates may be present in high quality reservoirs on this part of the margin

natural gas hydrate investigations by synchrotron radiation x ray cryo tomographic microscopy srxctm 1 we report the 3d microstructure analyses of natural gas hydrates sampled from gulf of mexico the samples were characterized by synchrotron radiation x ray cryo tomographic microscopy srxctm using the tomcat beam line at the swiss light source sls the srxctm demonstrates its applicability to unlock some microscopic features of the marine hydrates in particular of crystallite size and grain boundary network the gas hydrate domains are surrounded by a network of pores of typically a few micrometers which are largely due to decomposition out of the srxctm data the porosity total volume of the voids the void surface area and number of the total gas filled voids have been calculated the results reveal the capability of srxctm to access the 3d microstructure which is of fundamental importance to model the petrophysical properties of natural gas hydrates

seismic investigation of a bottom simulating reflector and quantification of gas hydrate in the black sea a bottom simulating reflector bsr which marks the base of the gas hydrate stability zone has been detected for the first time in seismic data of the black sea the survey area is in the northwestern black sea at 44 45 n and 31 5 32 5 e in this paper seismic wide angle ocean bottom hydrophone obh and ocean bottom seismometer obs data are investigated with the goal to quantify the gas hydrate and free gas saturation in the sediment an image of the subsurface is computed from wide angle data by using kirchhoff depth migration the image shows the bsr at 205 270 m depth below the seafloor and six to eight discrete layer boundaries between the seafloor and the bsr the top of the hydrate layer and the bottom of the gas layer cannot be identified by seismic reflection signals an analysis of traveltimes and reflection amplitudes leads to 1 d p wave velocity depth and density depth models an average s wave velocity of 160 m s 1 between the seafloor and the bsr is determined from the traveltime of the p to s converted wave the normal incidence pp reflection coefficient at the bsr is 0 11 where the p wave velocity decreases from 1840 to 1475 m s 1 velocities and density are used to compute the porosity and the system bulk modulus as a function of depth the gassmann equation for porous media is used to derive explicit formulae for the gas hydrate and free gas saturation which depend on porosity and on the bulk moduli of the dry and saturated sediment a gas hydrate saturation depth profile is obtained which shows that there is 38 10 per cent hydrate in the pore space at the bsr depth where the porosity is 57 per cent obs 24 this value is derived for the case that the gas hydrate does not cement the sediment grains a model that is supported by the low s wave velocities there is 0 9 or 0 1 per cent free gas in the sediment below the bsr depending on the model for the gas distribution in the sediment the free gas layer may be more than 100 m thick as a result of a zone of enhanced reflectivity which can be identified in the subsurface image

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

quantitative detection of methane hydrate through high resolution seismic velocity analysis a laterally extensive high resolution travel time velocity analysis and acoustic wave form inversion were used to quantitatively determine methane hydrate content in deep water sediments of the blake ridge off the southeast u s coast the interval acoustic velocity vp analyses were performed in the p domain by interactively picking the p trajectories of prominent reflections in each of 50 plane wave decomposed common midpoint gathers the reflections correspond to seismic stratigraphic boundaries so that lateral vp changes due to lithology changes are mitigated and vp changes due to changing hydrate content are enhanced two separate interval vp analyses were performed one with thick 0 4 km layers which yielded lower uncertainty but also lower resolution and one with thinner layers 0 1 km yielding higher resolution but slightly larger uncertainties results show no correlation between low sediment reflectivity and vp however in the areas exhibiting a bottom simulating reflector bsr a high vp interval 2 0 km s and 0 15 km thick is seen immediately above the bsr where the bsr is strongest a 256 layer least squares acoustic wave form inversion reveals the bsr to be caused by a vp decrease from 2 0 to 1 5 km s with little or no change in density the inversion also reveals a thin 0 025 km layer of anomalously low vp lying immediately below the bsr two models of methane hydrate distribution are tested each indicating that the volume of methane hydrate in the intervals of elevated vp is up to 25 of the total volume

a three dimensional seismic tomographic study of the gas hydrate stability zone offshore vancouver island 1 methane hydrate bottom simulating reflectors bsrs are widespread on the northern cascadia margin offshore vancouver island we conducted a three dimensional tomographic seismic study of the hydrate stability zone in an area around ocean drilling program site 889 using two deployments of five ocean bottom hydrophones and air gun shots along a series of closely spaced profiles in various orientations further constraints on reflector geometry come from coincident single channel reflection profiles travel times of reflected and refracted phases were inverted with a regularized three dimensional inversion using perturbation ray tracing through smooth isotropic media for the forward step the seismic data allow us to constrain the velocity structure in a 6 km2 area around the drill site mean velocities range from 1 50 km s 1 at the seabed to 1 84 km s 1 at the bsr and velocities at site 889 match well those measured using a vertical seismic profile at equivalent depths below the seafloor velocities vary laterally by typically 0 15 km s 1 close to the seafloor velocities may be controlled primarily by lithology but close to the bsr we infer hydrate contents of up to 15 of the pore space from effective medium modeling the mean hydrate saturation in the well constrained volume of the velocity model is estimated to be 2 2 there is no correlation between the seismic velocity above the bsr and the reflection coefficient at the bsr so the latter is likely controlled primarily by the distribution of free gas beneath the hydrate stability zone

direct geometrical simulation of pore space evolution through hydrate dissociation in methane hydrate reservoirs abstract the identification of methane hydrate behavior in porous media is one of the most challenging yet rewarding pore level visualization and simulation tasks the hydrate morphology influences the physical characteristics of the host sediments as during the hydrate formation and dissociation processes the pore space and flow pathways constantly evolve here a direct three phase pore morphological simulation approach is proposed verified and utilized to simulate hydrate deformities and predict fluid occupancies and absolute and effective permeabilities of hydrate bearing geological formations the proposed technique simulates capillary dominant displacements by applying a set of geomaterial rules directly to the pixels of pore level porous media images the case studies are sandy microstructures generated based on the particle size distributions of the mallik gas hydrate deposit the fluid occupancy profiles absolute permeability and hydraulic tortuosity curves are comparable with the experimental datasets the sensitivity analysis shows that although the gas relative permeability is less sensitive to the hydrate specifications the porosity grain size distribution and hydrate content and occupancy remarkably influence the rock absolute permeability

focussed fluid flow on the hikurangi margin new zealand evidence from possible local upwarping of the base of gas hydrate stability the southern hikurangi subduction margin is characterized by significant accretion with predicted high rates of fluid expulsion bottom simulating reflections bsrs are widespread on this margin predominantly occurring beneath thrust ridges we present seismic data across the porangahau ridge on the outer accretionary wedge the data show high amplitude reflections above the regional bsr level based on polarity and reflection strength we interpret these reflections as being caused by free gas we propose that the presence of gas above the regional level of bsrs indicates local upwarping of the base of gas hydrate stability caused by advective heatflow from upward migrating fluids although we cannot entirely rule out alternative processes simplified modelling of the increase of the thermal gradient associated with fluid flow suggests that funnelling of upward migrating fluids beneath low permeability slope basins into the porangahau ridge would not lead to the pronounced thermal anomaly inferred from upwarping of the base of gas hydrate stability focussing of fluid flow is predicted to take place deep in the accretionary wedge and or the underthrust sediments above the high amplitude reflections sediment reflectivity is low a lack of lateral continuity of reflections suggests that reflectivity is lost because of a destruction of sediment layering from deformation rather than gas hydrate related amplitude blanking structural permeability from fracturing of sediments during deformation may facilitate fluid expulsion on the ridge a gap in the bsr in the southern part of the study area may be caused by a loss of gas during fluid expulsion we speculate that gaps in otherwise continuous bsrs that are observed beneath some thrusts on the hikurangi margin may be characteristic of other locations experiencing focussed fluid expulsion

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

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

fine structure of methane hydrate bearing sediments on the blake outer ridge as determined from deep tow multichannel seismic data high resolution deep tow multichannel seismic data are used to investigate the detailed structure of sediments containing methane hydrate these data support thick laterally extensive layers of methane hydrate bearing sediment underlain by a bottom simulating reflector bsr and spatially discontinuous zones of hydrate within the sediments above the bsr depth where no bsr is present these data resolve normal faults which extend from the surface through the bsr with apparent offsets of up to 20 m a phase inversion identified at the top of the bsr shows that the material immediately beneath the bsr has anomalously low velocity consistent with a layer of sediment containing free methane gas the fault offsets along the bsr are consistent with a pressure change of 200 kpa 2 bars across the fault zone

analysis of remotely sensed long period wave motions synthetic aperture radar has been used to image long period 15 200 s gravity waves in the nearshore region of lake michigan these long period waves are a response of the sea surface to forcing by a nonmonochromatic wind generated surface wave field the synthetic aperture radar data were successfully compared with an in situ wave gauge record both one and two dimensional fast fourier transforms were generated from near and offshore regions of synthetic aperture radar data the synthetic aperture radar derived near and offshore spectral estimates exhibited both low and high frequency wave components classical bathymetrically controlled wave refraction was observed for both the short as well as long wave components of the sea surface this paper demonstrates the ability of x band synthetic aperture radar to detect low amplitude long period signals the signals appear to correspond to a surf beat generated by the incident wind wave field

a method for determining gas hydrate or free gas saturation of porous media from seismic measurements the occurrence of gas hydrate or free gas in a porous medium changes the medium s elastic properties explicit formulas for gas hydrate or free gas saturation of pore space on the basis of the frenkel gassmann equations describe the elastic moduli and seismic velocities of a porous medium for low frequencies a key assumption of the model is that either gas hydrate or free gas is present in the pore space in addition to water under this assumption the method uses measured p and s wave velocities and bulk density along with estimates of the moduli and densities of the solid and fluid phases present to determine whether gas or hydrate is present the method then determines the saturation level of either the gas or the hydrate i apply the method to published velocity and density data from seismic studies at the antarctic shetland margin and at the storegga slide offshore norway and to borehole log and core data from ocean drilling program odp leg 164 at blake ridge offshore south carolina a sensitivity analysis reveals that the standard deviations of the gas hydrate and free gas saturations reach 30 70 of the saturations if the standard deviations of the p and s wave velocities and of the bulk density are 50m s

time average equation revisited expressions that relate velocity to porosity and to pore fluid compressibility are among the most important deliverables of rock physics such relations are used often as additional controls for inferring porosity from well logs as well as in situ indicators of pore fluid type the oldest and most popular is the wyllie et al 1956 equation p s f where p is the measured traveltime of a p wave s is the traveltime expected in the solid phase material and f is the traveltime expected in the pore fluid it follows from equation 1 that 1vp 1 vps vpf where is porosity vp is the measured p wave velocity and vps and vpf are the p wave velocities in the solid and in the pore fluid phases respectively

estimating the free gas content in baltic sea sediments using compressional wave velocity from marine seismic data a 2 d high resolution velocity field was obtained from marine seismic data to quantify free gas content in shallow muddy sediments at in situ pressure and temperature the velocities were acquired applying migration velocity analysis on prestack time migrated data compressional wave velocities are highly sensitive to free gas as very small amounts of gas can cause a significant decrease in the medium velocity the analyzed profile crosses a depression filled with organic rich holocene mud in the bornholm basin baltic sea the interval velocity field reveals two low velocity patches which extend from the reversed polarity reflections marking the top of the gassy sediment layer down to the base of the holocene mud average interval velocities within the gassy mud are lower than the seafloor migration velocity by up to 500 m s this decrease using a geoacoustic model is caused by an average 0 046 gas volume fraction the interval velocities in individual cells of the velocity field are reduced to 200 m s predicting up to 3 4 gas content the velocity field is limited in resolution due to velocity determination at and between reflections however together with the stratigraphic interpretation geological units containing free gas could be identified shallow gas occurs vertically throughout most of the holocene mud in the gassy area comparison with biogeochemical studies at other baltic sea sites suggests that the distribution of free gas is likely to be patchy in the sediment but the gas concentration may peak below the sulfate methane transition zone and gradually decrease below

a laboratory investigation into the seismic velocities of methane gas hydrate bearing sand remote seismic methods which measure the compressional wave p wave velocity vp and shear wave s wave velocity vs can be used to assess the distribution and concentration of marine gas hydrates in situ however interpreting seismic data requires an understanding of the seismic properties of hydrate bearing sediments which has proved problematic because of difficulties in recovering intact hydrate bearing sediment samples and in performing valid laboratory tests therefore a dedicated gas hydrate resonant column ghrc was developed to allow pressure and temperature conditions suitable for hydrate formation to be applied to a specimen with subsequent measurement of both vp and vs made at frequencies and strains relevant to marine seismic investigations thirteen sand specimens containing differing amounts of evenly dispersed hydrate were tested the results show a bipartite relationship between velocities and hydrate pore saturation with a marked transition between 3 and 5 hydrate pore saturation for both vp and vs this suggests that methane hydrate initially cements sand grain contacts then infills the pore space these results show in detail for the first time using a resonant column how hydrate cementation affects elastic wave properties in quartz sand this information is valuable for validating theoretical models relating seismic wave propagation in marine sediments to hydrate pore saturation

new middle cambrian and middle ordovician palaeomagnetic data from siberia llandelian magnetostratigraphy and relative rotation between the aldan and anabar angara blocks abstract whether siberia was a coherent tectonic unit throughout the phanerozoic or whether significant rotations occurred in early palaeozoic times between the anabar angara and aldan blocks is still a matter of debate addressing this problem a detailed palaeomagnetic study of sections of mid cambrian and llandeillian age has been carried out along the river lena river maya river kulumbe and river stolbovaya with a total of 403 collected hand samples a section of llandeilian age was also investigated on the russian platform near st petersburg the studied rocks are carbonates or fine grained terrigenous sediments of reddish and greyish colors stepwise thermal demagnetization experiments reveal a rather simple directional behavior in most samples after removal of a secondary magnetizations at low to intermediate temperatures a high temperature component is isolated which is carried by magnetite and or hematite positive reversal tests and fold tests clearly demonstrate the primary character of the latter component while only reversed polarity directions are obtained in the lower two thirds of the llandeilo six magnetozones of normal polarity are found in the upper llandeilo which mark the end of the moyero reverse superchron together with previously published paleomagnetic data our new results show rather systematic difference in declinations of 20 between the aldan and anabar angara blocks this argues in favor of a relative rotation between these two blocks with an euler pole of rotation located at latitude 62 n and longitude 117 our interpretation is further supported by existing results from deep sounding seismic experiments yielding a very similar pole of rotation

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

new paleomagnetic paleolatitudes for the omulevka terrane of northeast russia a comparison with the omolon terrane and the eastern siberian platform abstract paleomagnetic measurements were made on samples from silurian devonian carboniferous and permian aged rocks from the omulevka terrane northeast russia the omulevka terrane is a fragment of continental margin that forms a major component of the kolyma omolon superterrane a superterrane that constitutes the core of northeast russia all samples were collected from sites along the taskan river and all showed a very strong secondary overprint and in some cases more than one with detailed thermal demagnetization a component of magnetization that decayed towards the origin when viewed as an orthogonal plot was commonly present and showed better clustering of magnetization directions in stratigraphic coordinates all of the sites sampled were tilted but none had enough variation in bedding attitude to perform fold tests comparing data between localities was used as a pseudo fold test and indicates that in all but one case the characteristic remanent magnetization found represents a magnetization acquired very early in the history of the rock interpretation of these data in terms of both virtual pole positions and paleolatitude changes with time shows that the omulevka terrane and the adjacent omolon terrane were together at low latitudes in silurian and devonian time and until late devonian their latitude was consistent with them being closely related to the siberian craton between devonian and permian times the paleolatitudes of the terranes decreased and when compared with equivalent reference paleolatitudes for the nearby siberian platform show a clear latitudinal separation from permian to jurassic time the omulevka and omolon terranes move steadily northwards reducing the relative latitudinal separation from siberia by late jurassic to early cretaceous time the observed and equivalent siberian paleolatitudes are identical indicating the accretion of the kolyma omolon superterrane to the eastern siberian continent

quantitative characterization of gas hydrate bearing sediment using elastic electrical rock physics models abstract successful evaluation of reservoir resources in gas hydrate bearing sediments depends on the accurate estimation of reservoir parameters such as hydrate saturation porosity and clay content elastic and electrical measurements are affected by several rock and fluid properties and their joint interpretation can provide complementary information and improve the accuracy of reservoir characterization results in this work we propose a new rock physics inversion workflow based on 3d rock physics template of elastic and electrical attributes for the direct estimation of reservoir parameters we first conduct a quantitative analysis of the available well log data and core data to understand the morphology of gas hydrate within the marine sediment and choose the appropriate elastic rock physics model then we construct the 3d rock physics template by discretizing the 3d space of reservoir parameters and implementing the suitable elastic and electrical models to calculate the poisson s ratio p wave impedance and electrical resistivity for each combination of values of rock and fluid properties finally we superimpose the measurements on the templates and use a grid searching method to find the nearest gridded node to the data point and minimize the objective function of the mismatch between data and model predictions to investigate the feasibility of our proposed inversion scheme we test a rock physics model based on effective medium theory and a modified archie s equation and invert for reservoir parameters of gas hydrate bearing sediments at well 1250f and 1247b in hydrate ridge the good match between inversion results and core data suggests that the proposed inversion scheme is applicable to real data and allows combining elastic and electrical data for reservoir parameter estimation of gas hydrate bearing sediments

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

characterization of seismic wave velocity and attenuation and interpretation of tetrahydrofuran hydrate bearing sand using resonant column testing abstract understanding the geophysical properties of hydrate bearing sediments is critical for hydrate utilization such as prediction of hydrate distribution and concentration as well as safety evaluation during hydrate production here a customized resonant column is employed to test tetrahydrofuran thf hydrate bearing sandy specimens the results show that wave velocities of all tested specimens increase exponentially with increasing effective confining stress v whereas wave velocity evolution becomes almost stress independent when critical hydrate saturation sh 0 53 in this study is reached which transforms the hydrate morphology from non cementing to cementing type in addition regardless of structure differences between tested specimens an increased factor and decreased exponent follows a uniform equation 0 61 0 2 log m s measured wave velocity differences between hydrate bearing and hydrate free specimens confirm the coexistence of various hydrate morphologies based on matching results of theoretical predictions from rock physics models and interpretation of volumetric proportions of hydrates with different morphologies in tested specimens wave attenuation of tested specimens generally decreases with increasing effective confining stress and increase with increasing hydrate saturation the relationship between wave attenuation and hydrate saturation qs 1 0 015 0 17 sh is confirmed to be a hydrate saturation indicator considering data from field seismic surveys this type of monotonous relationship appears to be less sensitive for hydrate morphology nevertheless through the combined interpretation of wave velocities and attenuations it is theoretically feasible to not only predict hydrate saturation but also illustrate the evolution of hydrate morphology our study contributes to a better understanding of stress related geophysical properties of hydrate bearing sediments which can be useful for the characterization of hydrate reservoirs

sediments with gas hydrates internal structure from seismic avo we interpret amplitude variation with offset avo data from a bottom simulating reflector bsr offshore florida by using rock physics based synthetic seismic models a previously conducted velocity and avo analysis of the in situ seismic data showed that the bsr separates hydrate bearing sediments from sediments containing free methane the amplitudes at the bsr are increasingly negative with increasing offset this behavior was explained by p wave velocity above the bsr being larger than that below the bsr and s wave velocity above the bsr being smaller than that below the bsr we use these avo and velocity results to infer the internal structure of the hydrated sediment to do so we examine two micromechanical models that correspond to the two extreme cases of hydrate deposition in the pore space 1 the hydrate cements grain contacts and strongly reinforces the sediment and 2 the hydrate is located away from grain contacts and does not affect the stiffness of the sediment frame only the second model can qualitatively reproduce the observed avo response thus inferred internal structure of the hydrate bearing sediment means that 1 the sediment above the bsr is uncemented and thereby mechanically weak and 2 its permeability is very low because the hydrate clogs large pore space conduits the latter explains why free gas is trapped underneath the bsr the seismic data also indicate the absence of strong reflections at the top of the hydrate layer this fact suggests that the high concentration of hydrates in the sediment just above the bsr gradually decreases with decreasing depth this effect is consistent with the fact that the low permeability hydrated sediments above the bsr prevent free methane from migrating upwards

seafloor nuclear magnetic resonance assay of methane hydrate in sediment and rock 1 proton nuclear magnetic resonance nmr of liquid water can be used to indirectly detect and assay methane hydrate in opaque earth materials it is also potentially useful for quantifying pore size control of hydrate formation and for estimating in situ hydraulic permeability of hydrate affected earth formations the method is quantitative nondestructive and volumetrically averaging over a spatially selected region in the first controlled validation of this measurement we have used nmr to measure changes in the liquid water content of sediment and rock resulting from methane hydrate formation hydrates were artificially formed at the seafloor by introducing methane into tubes containing sediments or rock saturated with seawater after several weeks the samples were visited by a remotely operated vehicle on which nmr equipment was mounted reduction of the nmr signal indicated hydrate had formed in the sample within a well defined measurement volume hydrate content determined by nmr quantitatively predicted the volume of methane gas evolved during subsequent dissociation

estimation of gas hydrate and free gas saturation concentration and distribution from seismic data gas hydrates contain a major untapped source of energy and are of potential economic importance the theoretical models to estimate gas hydrate saturation from seismic data predict significantly different acoustic elastic properties of sediments containing gas hydrate we do not know which to use thus we develop a new approach based on empirical relations the water filled porosity is calibrated using well log data to acoustic impedance twice one calibration where gas hydrate is present and the other where free gas is present the water filled porosity is used in a combination of archie equations with corresponding parameters for either gas hydrate or free gas to estimate gas hydrate or free gas saturations the method is applied to single channel seismic data and well logs from ocean drilling program leg 164 from the blake ridge area off the east coast of north america the gas hydrate above the bottom simulating reflector bsr is estimated to occupy 3 8 of the pore space 2 6 by volume free

elastic wave velocity in marine sediments with gas hydrates effective medium modeling we offer a first principle based effective medium model for elastic wave velocity in unconsolidated high porosity ocean bottom sediments containing gas hydrate the dry sediment frame elastic constants depend on porosity elastic moduli of the solid phase and effective pressure elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation to model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions a hydrate modifies the pore fluid elastic properties without affecting the frame b hydrate becomes a component of the solid phase modifying the elasticity of the frame the goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data we apply the model to sonic and vsp data from odp hole 995 and obtain hydrate concentration estimates from assumption b consistent with estimates obtained from resistivity chlorinity and evolved gas data

the character and amplitude of discontinuous bottom simulating reflections in marine seismic data abstract bottom simulating reflections bsrs identified in seismic data are well documented and are commonly interpreted to indicate the presence of gas hydrates along continental margins as well as to estimate regional volumes of gas hydrate a bsr is defined as a reflection that sub parallels the seafloor but is opposite in polarity and cross cuts dipping sedimentary strata bsrs form as a result of a strong negative acoustic impedance contrast bsrs however are a diverse seismic phenomena that manifest in strikingly contrasting ways in different geological settings and in different seismic data types we investigate the characteristics of bsrs using conventional and high resolution 2d and 3d seismic data sets in three locations the terrebonne and orca basins in the gulf of mexico and blake ridge on the us atlantic margin the acquisition geometry and frequency content of the seismic data significantly impact the resultant character of bsrs as observed with depth and amplitude maps of the bsrs furthermore our amplitude maps reinforce the concept that the bsr represents a zone over which the transition from hydrate to free gas occurs as opposed to the conventional model of the bsr occurring at a single interface our results show that a bsr can be mapped in three dimensions but it is not spatially continuous at least not at the basin scale rather a bsr manifests itself as a discontinuous or patchy reflection and only at local scales is it continuous we suggest the discontinuous nature of bsrs is the result of variable saturation and distribution of free gas and hydrate acquisition geometry and frequency content of the recorded seismic data the commonly accepted definition of a bsr should be broadened with careful consideration of these factors to represent the uppermost extent of enhanced amplitude at the shallowest occurrence of free gas trapped by overlying hydrate bearing sediments

tectonic setting of the plutonic belts of yakutia northeast russia based on 40ar 39ar geochronology and trace element geochemistry samples from plutonic belts and individual intrusions in the sakha republic yakutia northeastern russia were analyzed using 40ar 39ar geochronology and trace element geochemistry four phases of activity were identified a subduction event along the southwest margin of the kolyma omolon superterrane with apparent ages of ca 160 140 ma a superposed collisional event dated at 143 138 ma a subduction event along the northern margin of the superterrane dated at 130 123 ma with a slightly older to contemporaneous extensional event to the west and intrusions associated with east west extension between 106 and 92 ma these phases are interpreted to represent 1 subduction under and 2 accretion to asia of kolyma omolon followed by local extension 3 subduction along the northern edge of this superterrane and 4 regional extension following closure of the south anyui suture and or the start of subduction associated with the okhotsk chukotka volcanic belt

characterization of hydrate and gas reservoirs in plate convergent margin by applying rock physics to high resolution seismic velocity model abstract gas hydrates are widely distributed in the kumano forearc basin which is located above accretionary prism in the nankai margin off the kii peninsula japan bottom simulating reflector bsr at the base of gas hydrate stability zone has been imaged as a strong acoustic impedance contrast on the reflection seismic profiles in order to better define the accumulations of gas hydrates and free gases we performed a high resolution seismic velocity analysis to 3d seismic data using a method of conventional semblance spectra via automatic velocity picking algorithm the results revealed that gas hydrate bearing sediments above the bsr and free gas bearing sediments below the bsr are characterized by p wave velocities of 1900 2500 m s and 1000 1800 m s respectively then the velocity model was converted into gas hydrate and free gas saturation using rock physics approaches the results indicated that saturation of gas hydrates ranges from 0 to 45 in the pore space and highly concentrated around the outer ridge where faults are densely developed additionally concentrations of free gas ranging from 0 to 20 in the pore space are widely distributed below bsrs and are considerably high above ridge structure generated by displacement of large fault splayed from the deep plate boundary decollement based on these results we suggest that the gas hydrates concentrated due to the free gas influx which migrated upward through the steeply dipping strata and faults or fractures cutting through the basin the accumulations of gas and or hydrates are further controlled by fault movements in the accretionary prism beneath the forearc basin therefore these factors generated by intensive tectonic movements in the plate subduction zone control the distribution and saturation pattern of gas hydrate and free gas formations

local seismic quantification of gas hydrates and bsr characterization from multi frequency obs data at northern hydrate ridge high resolution multi frequency and multi component seismic data were acquired at northern hydrate ridge in the accretionary complex of the cascadia subduction zone to quantify the amount of hydrate and free gas in the sediment we present a detailed local analysis of four component 4c ocean bottom seismometer obs data and show the importance of multi frequency and shear wave data for determining hydrate reservoir properties a detailed model of the elastic parameters at the bottom simulating reflector bsr is developed by using synthetic seismogram modelling the main focus in this study is an amplitude versus offset avo analysis of shear waves which originate by ps conversion at the bsr in 73 m below the seafloor bsf the avo analysis enables the determination of the shear wave velocity above the bsr a velocity of 400 m s indicates that the presence of gas hydrate in the pore space significantly increases the shear modulus of the sediment above the bsr information about the attenuation and the shape of the bsr transition zone is obtained from the frequency dependent reflection amplitudes of the bsr the bsr is shown to be a gradual type transition zone of 1 5 2 5 m thickness average q factors of qp 150 for p waves and of qs 35 for s waves are determined within the gas hydrate stability zone ghsz the low qs factor points to a pronounced attenuation of s waves in the uppermost sediments from rock physics modelling the hydrate concentration is estimated to vary locally between 3 12 of the pore space below the bsr free gas concentrations of 0 5 and 8 are determined for homogeneous and patchy distributed gas respectively

geology of the east siberian sea russian arctic from seismic images structures evolution and implications for the evolution of the arctic ocean basin 1 the kinematics and history of the opening of the amerasia basin are closely linked to the geology of the wide shelves surrounding the arctic ocean in this context multichannel seismic reflection data from the virtually unexplored shelf of the east siberian sea russian arctic are discussed in combination with potential field data three seismic marker horizons were defined and mapped their ages were linked to main tectonic and regional events and to onshore findings the data reveal that there is no continuation of the large rift basins from the laptev shelf onto the east siberian shelf and there are no indications for the previously defined several hundred kilometers wide blagoveshchensk basin the east siberian shelf is best described as an epicontinental platform that synsedimentarily subsided continuously since late cretaceous times with stronger subsidence to the northeast resulting in the formation of a large depocenter some form of extensional transtensional stresses affected the area and created relatively small ese wnw striking basins within this depocenter these basins are filled with 6 km thick late cretaceous to tertiary sediments the general dip of the platform of the east siberian shelf toward the northeast may be explained by dip slip movements along a major transform fault that is proposed by the rotation model for the opening of the amerasia basin for the evolution of small sag shaped basins within the east siberian depocenter we suggest a link to the opening of the eurasia basin instead of to the opening of the makarov basin

theoretical modeling insights into elastic wave attenuation mechanisms in marine sediments with pore filling methane hydrate the majority of presently exploitable marine methane hydrate reservoirs are likely to host hydrate in disseminated form in coarse grain sediments for hydrate concentrations below 25 40 disseminated or pore filling hydrate does not increase elastic frame moduli thus making impotent traditional seismic velocity based methods here we present a theoretical model to calculate frequency dependent p and s wave velocity and attenuation of an effective porous medium composed of solid mineral grains methane hydrate methane gas and water the model considers elastic wave energy losses caused by local viscous flow both i between fluid inclusions in hydrate and pores and ii between different aspect ratio pores created when hydrate grows the inertial motion of the frame with respect to the pore fluid biot s type fluid flow and gas bubble damping the sole presence of pore filling hydrate in the sediment reduces the available porosity and intrinsic permeability of the sediment affecting biot s type attenuation at high frequencies our model shows that attenuation maxima due to fluid inclusions in hydrate are possible over the entire frequency range of interest to exploration seismology 1 106 hz depending on the aspect ratio of the inclusions whereas maxima due to different aspect ratio pores occur only at sonic to ultrasound frequencies 104 106 hz this frequency response imposes further constraints on possible hydrate saturations able to reproduce broadband elastic measurements of velocity and attenuation our results provide a physical basis for detecting the presence and amount of pore filling hydrate in seafloor sediments using conventional seismic surveys

particle size effect on the saturation of methane hydrate in sediments constrained from experimental results except for those occurring at seafloor most of natural gas hydrate form in sediments and are subject to the influence of sediment among these factors the particle size effect on hydrate saturation level in sediment have been studied with a series of silica sands with various sizes and the results obtained clearly indicate that particle size does play an important role in affecting the saturation level of hydrate in sediments the proton relaxation times of water confined in the same series of silica sands which were determined with nmr measurement show logarithmic relationship with particle size by comprehensive consideration of the results of hydrate saturation and water proton relaxation times the particle size effect observed is tentatively explained by the water availability for hydrate formation in sediments

the connection between natural gas hydrate and bottom simulating reflectors bottom simulating reflectors bsrs on marine seismic data are commonly used to identify the presence of natural gas hydrate in marine sediments although the exact relationship between gas hydrate and bsrs is undefined to clarify this relationship we compile a data set of probable gas hydrate occurrence as appraised from well logs of 788 industry wells in the northern gulf of mexico we combine the well log data set with a data set of bsr distribution in the same area identified from 3 d seismic data we find that a bsr increases the chances of finding gas hydrate by 2 6 times as opposed to drilling outside a bsr and that the wells within a bsr also contain thicker and higher resistivity hydrate accumulations even so over half of the wells drilled through bsrs have no detectable gas hydrate accumulations and gas hydrate occurrences and bsrs do not coincide in most cases

seismic studies of a bottom simulating reflection related to gas hydrate beneath the continental margin of the beaufort sea the upper continental margin of the beaufort sea north of alaska is underlain by a strong bottom simulating reflection bsr that lies 300 to 700 m beneath the seafloor and corresponds to the phase boundary between interstitial water and natural gas below and solid gas hydrate above bsrs of similar origin are common worldwide where they are usually interpreted to mark the base of gas hydrate bearing clastic sediment with or without underlying free gas in the sediment surprisingly little is known about the origin of these strong reflections in this paper we analyze the contrasting physical properties which produce bsrs by comparing synthetic bsr amplitudes and waveforms for varying source receiver offsets with multichannel seismic reflection data across the well developed bsr of the beaufort sea in order to discriminate whether free gas is present under the bsr or not it was necessary to supplement near vertical incidence data with prestack offset data the amplitude versus offset avo analysis indicates that the bsr is produced mainly by the existence of free gas in the clastic sediments beneath the bsr the zone of free gas is based on vertical incidence synthetics estimated to be thinner than 11 16 m it is possibly thicker than 16 m if the gas concentration decreases with depth saturation of gas hydrate in the sediment above the bsr is tentatively estimated from the avo modeling to be less than 10 of the pore volume

seismic velocity increase and deep sea gas hydrate concentration above a bottom simulating reflector on the northern cascadia continental slope the amount of gas hydrates in the accretionary wedge sediments of the northern cascadia subduction zone off vancouver island has been estimated from multichannel seismic mcs and ocean drilling program odp data derailed semblance velocity analyses and full waveform inversion of mcs data combined with previously published odp sites 889 890 sonic log and vertical seismic profile vsp data show that sediment velocities increase downward more rapidly than the no hydrate no gas reference profile from about 1500 m s near the seafloor to a maximum of 1900 m s just above the bottom simulating reflector bsr at a subbottom depth of 224 m immediately below the bsr the mcs velocities drop to 1700 m s a low velocity of 1500 m s from the vsp data probably represents a thin layer 20 m containing free gas the difference between the reference and observed velocities is used to estimate hydrate concentration which reaches a maximum of 20 30 of the pore space above the bsr a simple interpretation of the drill core chlorinity dilution data at the odp site yields a similar hydrate concentration of 35 the estimated hydrate concentration with subbottom depth represents about 7 m 3 of hydrate per square meter of seafloor the total methane gas at stp including hydrate above the bsr and the small amount of free gas below is about 800 m 3 per square meter of seafloor or 200 tcf trillion cubic feet on the vancouver island continental slope application of the same method in the blake bahama region of the eastern u s margin indicates that the velocity enhancement and inferred hydrate concentration are very similar

the direction correction tilt test an all purpose tilt fold test for paleomagnetic studies abstract the tilt or fold test in paleomagnetism is used to infer whether paleomagnetic remanence was acquired before or after tectonic tilting while several tilt test formulations have been proposed none fully satisfies the requirements of statistical validity applicability to all bedding geometries and ease of use this paper introduces the direction correction dc test which examines the relationship between the paleomagnetic site mean directions and their corresponding bedding tilt corrections the dc test is similar to mcfadden s correlation test because both test whether or not the site directions contain information about the bedding tilts however the dc test gives greater weight to sites with greater counter bedding the dc test is similar to watson s numerical tilt tests because they determine the degree of untilting which gives optimal concentration of site directions however the dc test uses analytical rather than numerical methods graphical output aids the researcher in recognizing problem sites using both real and simulated data the dc test is demonstrated to be more discriminating than other tilt test formulations for all bedding geometries the simulations show that the power of the tilt test is inversely proportional to the 95 confidence interval 95 of the overall mean as a rule of thumb paleomagnetists should attempt to sample sufficient sites to obtain an 95 less than 1 6 of the bedding attitude difference