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

evolution and distribution of the conodonts sweetognathus and iranognathus and related genera during the permian and their implications for climate change abstract the conodont genus sweetognathus which is characterized by pustulose ornamentation on a wide flat topped carina originated in the earliest permian as s expansus from diplognathodus edentulus the asselian through artinskian part of the sweetognathus lineage is well represented in kansas by the successive evolution of s expansus s merrilli s aff s merrilli s whitei and s bucaramangus sweetognathus and iranognathus lineages were mainly confined to tropical areas from the kungurian onward the kungurian and guadalupian part of the sweetognathus lineage is well represented in south china by the successive evolution of s whitei artinskian s guizhouensis lower kungurian s subsymmetricus upper kungurian s iranicus hanzhongensis roadian and s fengshanensis capitanian an additional lineage in west texas is represented by the evolution of s sulcatus to s aff adjunctus to s adjunctus and to s bicarinum in the urals and northwestern pangea the late part of the sweetognathus lineage is represented by rare specimens of s bogoslovskajae upper artinskian lower kungurian during the late artinskian s whitei radiated into s toriyamai and s bucaramangus and also gave rise to neostreptognathodus pequopensis and probably pseudosweetognathus costatus neostreptognathodus exsculptus may have evolved from adetognathus paralautus through adetognathus telfordi sp nov iranognathus characterized by a narrow carina with poorly developed denticles and bearing subtle and smaller pustulose micro ornament replaced sweetognathus at or near the guadalupian lopingian boundary it probably also evolved from diplognathodus in the topmost guadalupian a lineage of iranognathus sp nov a to i erwini can be recognized for the guadalupian and lopingian boundary interval a lineage of iranognathus sp nov a to i movschovithschi to i sosioensis to i tarazi is found throughout the guadalupian and lopingian boundary interval and the wuchiapingian of south china sweetognathus and iranognathus tend to develop homeomorphic accessory nodes iteratively during lowstand periods of global sea level near the artinskian kungurian and guadalupian lopingian boundaries and in the late wuchiapingian sweetognathus and iranognathus seem to be equatorial warm water inhabitants distribution patterns reflect glaciation in gondwana during the asselian and sakmarian a warm climate during the artinskian cooling in north pangea during the kungurian and later permian slight amelioration during the guadalupian warming during the wuchiapingian and cooling during the changhsingian in the peri gondwana region definitions for most of the important sweetognathus species are refined three new species and subspecies iranognathus sp nov a adetognathus telfordi sp nov gullodus duani sp nov are described

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

hydrate distribution off vancouver island from multifrequency single channel seismic reflection data on the northern cascadia margin single channel seismic data clearly image the bottom simulating reflector bsr that marks the base of the hydrate stability zone airguns with three distinct source frequencies 30 75 and 150 hz were recorded along several coincident lines and at all frequencies the bsr appears as a single pulse bsr reflection coefficients are slightly larger for the 30 hz data than for the 75 or 150 hz data multi frequency analysis indicates that the bsr is due to a single sharp interface at the base of the hydrate zone whereas the velocity structure away from the interface must be gradationally varying so as to produce no separate reflection and no tuning effects at the 75 and 150 hz frequencies in contrast the seafloor reflection in some areas has significantly larger amplitudes for data recorded with the 75 hz source relative to the 30 hz source synthetic seismogram analysis of the two data sets shows that the high amplitudes can be produced by a 2 m thick high velocity layer at the seafloor with velocities and densities corresponding to those of carbonate using the 75 hz source seafloor and bsr amplitudes and reflection coefficients were measured over a tight grid of lines nominally spaced at 200 m maximum bsr reflection coefficients of 0 15 0 18 were observed beneath topographic highs while maximum seafloor reflection coefficients of up to 0 5 0 6 were found on the flanks of the topographic highs this suggests that topography provides a major control on the flow of methane bearing fluids amplitude measurements converted to values of velocity above the bsr were used to estimate hydrate concentrations which reaches a maximum of 15 18 of the total sediment volume with mean values near 5 10 over the 100 km2 survey area the volume of methane gas in the hydrate reservoir is estimated as 6 4 l010 m3 or 2 1 trillion cubic feet

palynofacies analysis of the permian triassic transition in the amb section salt range pakistan implications for the anoxia on the south tethyan margin the uppermost chhidru formation and the lower part of the mianwali formation were sampled in the amb valley salt range pakistan for the study of the particulate organic matter pom content in order to evaluate the depositional environment during the permian triassic transition the pom content was assigned to four distinct palynofacies palynofacies a d palynofacies a recovered from siltstone within the white sandstone unit of the upper permian chhidru formation indicates a shallow marine oxic shelf setting recorded from the siltstone intercalations in the kathwai member of the basal mianwali formation the griesbachian palynofacies b is characterised by abundant acritarchs indicating a transgressive event palynofacies c recovered from the siltstone of the lowest ceratite marls middle dienerian is dominated by terrestrial organic particles and indicates shallowing of the depositional environment whereas 40 cm above palynofacies d represents transgressive oxygen depleted conditions the comparison with sections from the australian tethyan margin shows that oxygen depleted conditions occurred during the griesbachian in the perth basin while in the bonaparte basin oxygenated conditions prevailed hence oxygen depleted facies do not correspond to a single synchronous permian triassic oceanic anoxic event but depend on local geography and bathymetry

three dimensional seismic characterization of a venting site reveals compelling indications of natural hydraulic fracturing based on a three dimensional high resolution seismic survey off vancouver island canada we show that natural hydraulic fracturing is an efficient process to create permeable pathways for focused fluid upflow at submarine venting sites the pockmark structure examined in this study is located on top of an accreted ridge where the low permeability base of the gas hydrate stability field is also elevated and fluid overpressure will be induced tectonically elongated hydraulic fractures reveal decreased reflection amplitudes and are oriented along the maximum principal stress direction perpendicular to nearby fault planes a surface expression is created by fluid and material expulsion forming an irregular noncircular rim along the axis of the upflow zone our findings confirm a relationship between lateral variations in seismic reflectivity and sediment distribution regional tectonics episodic fluid flow and the formation and dissociation of gas hydrate they explain the presence of gas hydrate at the top of the upflow zone which requires transport of large volumes of gas through the gas hydrate stability field since hydraulic fracturing is not restricted to compressional regimes it is a likely explanation for the nature shape and orientation of upflow zones as well as for seismic blanking widely observed at both active and passive continental margins

detecting hydrate and fluid flow from bottom simulating reflector depth anomalies methane hydrates ice like compounds that consist of water and methane represent a potentially enormous unconventional methane resource that may play a critical role in climate change and ocean acidification however it remains unclear how much hydrate exists here using a newly developed three dimensional 3 d thermal technique we reveal a novel method for detecting and quantifying methane hydrate the analysis reveals where fluids migrate in three dimensions across a continental margin and is used to quantify hydrate with meter scale horizontal resolution our study located at hydrate ridge offshore oregon united states suggests that heat flow and hydrate concentrations are coupled and that 3 d thermal analysis can be used to constrain hydrate and fluid flow in 3 d seismic data hydrate estimates using this technique are consistent with 1 d drilling results but reveal large previously unrecognized swaths of hydrate rich sediments that have gone undetected due to spatially limited drilling and sampling techniques used in past studies the 3 d analysis suggests that previous hydrate estimates based on drilling at this site are low by a factor of approximately three

three dimensional tomographic inversion of combined reflection and refraction seismic traveltime data summary a tomographic inversion method is presented for the determination of 3 d velocity and interface structure from a wide range of body wave seismic traveltime data types it is applicable to refraction wide angle reflection normal incidence and multichannel seismic data and is best suited to a combination of these that provides good independent constraints on seismic velocities and interface depths the inversion process seeks a layer interface minimum structure model that is able to explain the given data satisfactorily by inverting to minimize data misfit and model roughness norms simultaneously this regularized inversion and the use of smooth functions to describe velocities and depths allows the highly non linear tomographic problem to be approximated as a series of linear steps the inversion process begins by optimizing the fit to the data of a highly smoothed initial model in each subsequent step structure is allowed to develop in the model with successively greater detail evolving until a satisfactory fit to the data is obtained parameter uncertainties for the final model are then estimated using an a posteriori covariance matrix analysis smooth layer interface models are parametrized using regular grids of velocity and depth nodes from which spline interpolated interface surfaces and velocity fields are defined forward modelling is achieved using ray perturbation theory and a two point ray tracing method that is optimized for a large number of closely spaced shot or receiver points the method may be used to generate 1 and 2 d models from for example vertical seismic profile data or 2 d surveys in which the 3 d geometry of a survey is correctly accounted for the ability of the method to resolve typical target structures is tested in a synthetic salt dome inversion from a set of noisy traveltime data the model converges quickly to a well resolved final model from different starting models the application of this method to real data is demonstrated with a combined 3 d inversion of refraction and reflection data which provide p wave velocity constraints on the methane hydrate stability zone in the cascadia margin offshore vancouver island

origins of porosity and velocity variations at cascadia accretionary prism at cascadia accretionary prism log based porosities exhibit complex lateral changes in response to variations in cementation horizontal compression and fracturing porosities at the prism toe are lower than in reference sediments whereas porosities landward of the toe are slightly higher than those of reference sediments ideally a porosity velocity relationship can be used to determine two dimensional porosity structure from seismic velocities porosity is the major control on velocity at cascadia but variations in grain framework strength frame bulk modulus cause deformed and undeformed rocks to have quite different velocity porosity relationships within the prism cementation can raise velocities substantially above those for unconsolidated reference sediments of the same porosity also compression induced compaction and fracturing may affect this velocity porosity transform because these factors vary both within and between prisms velocity porosity transforms are local not universal

heterogeneity of conodont faunas in the cache creek terrane canada significance for tectonic reconstructions of the north american cordillera abstract the oceanic cache creek terrane has long been interpreted to be exotic relative to the neighbouring stikine and quesnel terranes on the basis of its unusual fossil faunas current tectonic models for the amalgamation of these three terranes have to account for the position of the exotic cache creek terrane between two volcanic island arc terranes and competing models invoke either terrane entrapment or strike slip motion to explain this configuration although previous paleobiogeographic studies have attempted to assess the degree of faunal variability among terranes of the north american cordillera none have yet measured the amount of internal faunal heterogeneity and considered the effect that this may have on tectonic reconstructions the use of multivariate statistics demonstrates that the late paleozoic to triassic conodont faunas from the cache creek terrane show distinct heterogeneity the degree of heterogeneity among faunas from different regions of the terrane varies with time and does not correlate with temporal changes in depositional environment conodont ecology or species endemism instead it is proposed that the cache creek terrane may consist of a number of sub terranes which moved independently of each other prior to the final accretion of the intermontane terranes periods of low faunal heterogeneity may correspond to times when the sub terranes were close together whereas periods of high heterogeneity may indicate that the sub terranes had moved further apart this interpretation requires that tectonic models for the formation of this part of the cordillera emphasise lateral movement both within and between terranes prior to accretion

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

origin of bottom simulating reflectors geophysical evidence from the cascadia accretionary prism vertical seismic profile vsp data from two drill sites on the cascadia margin show low velocity zones indicative of free gas be neath a bottom simulating reflector bsr offshore oregon at ocean drilling program odp site 892 velocities drop from an average of 1750 m s above the bsr to less than 1250 m s below it sonic logs confirm that seismic velocity in the sediments adjacent to the borehole is less than that of water for at least 50 m beneath the depth of the bsr at this site similarly at odp site 889 offshore vancouver velocities range from 1700 to 1900 m s in the 100 m above the bsr and drop abruptly to 1520 m s in the 15 m just beneath it the low velocities observed beneath the bsr are strong evidence for the presence of 1 5 free gas by volume the bsr at these two sites results from the contact between gas free sediments containing a small quantity of hydrate above the bsr and a low velocity free gas zone beneath it although the bsr is associated with the base of the hydrate stability field hydrate appears to account for relatively little of the velocity contrast that produces the bsr velocity above the bsr at site 889 is only about 100 m s greater than that expected for sediments of similar porosity sediments above the bsr at site 892 appear to have normal velocity for their porosity and may contain little hydrate

comments on some permian conodont faunas reported from southeast asia and adjacent areas and their global correlation abstract some important conodont faunas reported from southeast asia excluding south china and japan are re evaluated on the basis of the improved taxonomy and resulting newly established provincialism these faunas are representative of either the equatorial warm water province ewwp or the peri gondwana cool water province gcwp except that the fauna from jilin north china represents the north cool water province ncwp the gcwp is marked by vjalovognathus and or merrillina in the cisuralian vjalovognathus merrillina and mesogondolella in the guadalupian and vjalovognathus and mesogondolella sheni or merrillina in the lopingian the ewwp is characterized by the absence of gondolelloides and vjalovognathus in the early and middle cisuralian abundance of sweetognathus and pseudosweetognathus in the kungurian late cisuralian dominance of jinogondolella and sweetognathus in the guadalupian and dominance of clarkina and iranognathus in the lopingian mixed faunas are recognized in regions on the border between the ewwp and gcwp including western timor during the artinskian southeast pamirs during the kungurian and the salt range during the guadalupian and lopingian

a refined succession of changhsingian and griesbachian neogondolellid conodonts from the meishan section candidate of the global stratotype section and point of the permian triassic boundary abstract a detailed study of new conodont collections from the changxing formation at the meishan section has resulted in taxonomic refinement of several important neogondolellid species most of the previously erected species are much more strictly redefined mainly based on the denticulation of the holotypes and the stratigraphic ranges attributed to key conodont taxa are modified three new species and two new subspecies all of which are form species are tentatively erected and described mainly for the purpose of taxonomic explanation as a result of the taxonomic refinement six neogondolellid conodont zones are recognized for the changxing formation and the permian triassic boundary interval

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

the assessment of marine gas hydrates through electrical remote sounding hydrate without a bsr marine gas hydrates prevalent in offshore sediments in canada and japan are a possible hydrocarbon resource a hazard to drilling and a source of a major greenhouse gas quantitative estimates of hydrate concentrations in deep sea sediment are difficult to obtain by conventional methods we have sought novel techniques specifically designed for assessment such as transient electric dipole dipole electromagnetics the latter method is based on the assumption of a reduced electrical conductivity in hydrate rich zones field trails of new apparatus on the cascadia margin have proven successful excellent data were collected and analysed using a differential phase method to reduce systematic error apparent resistivities collected on three lines demonstrate that the resistivity of the seafloor is remarkably uniform over the whole survey area the average hydrate concentration deduced with the aid of a reference model based on the electrical logs of odp holes 888 and 889 is about 17 26 of pore space 9 13 of sediment volume in the 100 m interval above the bsr the values are consistent with those obtained by other analyses further the presence of hydrate is predicted in a region to the east of 889b where there is no visible bsr

evolution of permian conodont provincialism and its significance in global correlation and paleoclimate implication the main components of asselian through artinskian conodont faunas found around the world are basically the same and the provincialism is indicated only by less common endemic elements such as gondolelloides and new genus a henderson in north pangea sweetognathus bucaramangus around the equator and vjalovognathus in eastern gondwana provincialism is marked by differences at the species level of mesogondolella neostreptognathodus and sweetognathus during the kungurian and becomes very distinct with differences at the genus level during the guadalupian and lopingian three provinces of permian conodonts referred to as the north cool water province ncwp the equatorial warm water province ewwp and the peri gondwana cool water province gcwp are recognized the ncwp is marked by gondolelloides in the early cisuralian dominance of neostreptognathodus and no or rare sweetognathus in the late cisuralian dominance of merrillina and mesogondolella and absence of sweetognathus in the guadalupian and dominance of merrillina and mesogondolella and absence of iranognathus in the lopingian the ewwp is characterized by the absence of gondolelloides and vjalovognathus in the cisuralian abundance of sweetognathus and pseudosweetognathus in the kungurian late cisuralian jinogondolella and sweetognathus in the guadalupian and clarkina and iranognathus in the lopingian the gcwp is marked by vjalovognathus merrillina in the cisuralian vjalovognathus merrillina and mesogondolella in the guadalupian and vjalovognathus and merrillina in the lopingian mixed faunas are recognized in regions bordering between the ewwp and gcwp including western timor during the artinskian pamirs during the kungurian and the salt range during the guadalupian and lopingian three different conodont zonations are proposed one for each of the three conodont provinces four potential horizons for inter provincial correlation of permian conodonts are recognized they are in ascending order 1 the first appearance of sweetognathus whitei which is closely related to the last occurrence of carboniferous type conodonts such as streptognathodus and adetognathus 2 the first appearance of neostreptognathodus pequopensis 3 the base of the jinogondolella nankingensis zone and 4 the base of the clarkina postbitteri iranognathus erwini zone the spatial and temporal distribution pattern of permian conodonts suggests that temperature is the primary controlling factor evolution of permian conodont provincialism reveals a glaciation during the asselian and sakmarian a global warming during the artinskian a climate cooling in north pangea during the kungurian a continuation of kungurian climate trends in the guadalupian a relatively minor warming during the wuchiapingian a returned cooling in the changhsingian and lower and middle griesbachian and a global warming in the late griesbachian which ended the permian conodont lineage

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

high resolution deep towed multichannel seismic survey of deep sea gas hydrates off western canada a multichannel seismic survey was carried out using the high resolution deep towed acoustics geophysics system dtags to image the structure of deep sea gas hydrates on the continental slope off vancouver island and to determine the velocity profile of the hydrated sediments the high frequency dtags data provide the means to estimate the frequency response of the bottom simulating reflector bsr that defines the base of the hydrate stability field in these sediments over a broad frequency band from 15 to 650 hz the dtags sections resolved fine scale layering as thin as a few meters within the hydrated zone and below the bsr and they revealed small scale faults and vertically oriented zones of very low acoustic reflectivity that may represent channels for upward migration of fluids or gas interval velocities determined from the dtags data indicate uniformly low values of about 1500 m s to depths of 100 m below sea floor mbsf increasing to about 1850 m s at the bsr 250 mbsf the reflection from

gas hydrate within the winona basin offshore western canada in western canada gas hydrates have been thought to exist primarily in the cascadia accretionary prism off southern vancouver island british columbia bc we present evidence for the existence of gas hydrate in folds and ridges of the winona basin up to 40 km seaward from the foot of the continental slope off northern vancouver island the occurrence of a bottom simulating reflector bsr observed in a number of vintage seismic reflection profiles is strongly correlated to faulted and folded sedimentary ridges and buried folds the observed tectonic structures of the winona basin are within the rapidly evolving juan de fuca cascadia queen charlotte triple junction off bc re processing of multi channel data imaged mildly to strongly deformed sediments the bsr is confined to sediments with stronger deformation changes in the amplitude character of sediment reflections above and below the depth of the base of gas hydrate stability zone were also used as an indicator for the presence of gas hydrate additionally regional amplitude and frequency reduction below some strong bsr occurrences may indicate free gas accumulations gas hydrate formation in the winona basin appears strongly constrained to folds and ridges and thus correlated to deeper routed fluid advection regimes methane production from in situ microbial activities as a source of gas to form gas hydrates as proposed to be a major contributor for gas hydrates within the accretionary prism to the south appears to be insufficient to produce the widespread gas hydrate occurrences in the winona basin potential reasons for the lack of sufficient in situ gas production may be that sedimentation rates are 5 100 times higher than those in the accretionary prism so that available organic carbon moves too quickly through the gas hydrate stability field the confinement of bsrs to ridges and folds within the winona basin results in an areal extent of gas hydrate occurrences that is a factor of five less than what is expected from regional gas hydrate stability field mapping using water depth pressure as the only controlling factor only

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

a seismic study of methane hydrate marine bottom simulating reflectors multichannel seismic reflection data have been analyzed from an area of clear bottom simulating reflectors bsrs on the northern cascadia subduction zone margin off vancouver island the reflector at a depth of about 300 m subbottom is interpreted to represent the base of a layer of methane hydrate or clathrate the shallow water depth of 1300 m and the 3600 m long hydrophone array have allowed bsr amplitude versus offset and high resolution velocity analysis as well as modelling of vertical incidence data the results of all three types of analysis can be best explained by a 10 to 30 m thick high velocity layer located immediately above the bsr about 300 m below the seafloor having a sharp base and transitional top in the layer about one third of the sediment pore spaces must be filled with hydrate ice there is no seismically detectable free gas beneath the bsrs these results put important constraints on models for the distribution and formation of bsr hydrate

p wave and s wave velocity structure of northern cascadia margin gas hydrates summary single channel seismic and wide angle reflection data collected in september 2005 were analysed along a 2 d profile of 10 ocean bottom seismometers obss on the continental slope region off vancouver island near odp site 889 and iodp site u1327 the objectives were to determine the shallow p wave and s wave velocity structure associated with marine gas hydrates and to estimate the hydrate concentration and distribution in the sediment pore space combined traveltime inversion of single channel and obs data produced a p wave velocity model down to the depth of the bottom simulating reflector bsr at 230 5 m below the seafloor mbsf mean velocities which increased from 1 50 km s 1 at the seafloor to 1 88 km s 1 at the bsr are in good agreement with the sonic log data from sites 889 and u1327 the increase in p wave velocity of the hydrate bearing sediments relative to a background no hydrate velocity was utilized to estimate the hydrate concentration by using effective medium theory an average concentration of 13 per cent in the interval from 120 230 mbsf was estimated from the p wave velocity model lateral continuity of the model data confirms that these average hydrate concentrations are also found around the drillsites out to distances of a few kilometres forward modelling of s waves was carried out using the data from the obs horizontal components above the bsr s wave velocities are higher than a background velocity profile based on a rock physics model and on global averages for unconsolidated sediments this increase in velocity suggests that the hydrate is distributed as part of the load bearing matrix to increase the rigidity of the sediment

a geomagnetic polarity timescale for the permian calibrated to stage boundaries the reverse polarity kiaman superchron has strong evidence for at least three or prob ably four normal magnetochrons during the early permian normal magnetochrons are during the early asselian base ci1r 1n at 297 94 0 33 ma late artinskian ci2n at 281 24 2 3 ma mid kungurian ci3n at 275 86 2 0 ma and roadian ci3r an at 269 54 1 6 ma the mixed polarity illawarra superchron begins in the early wordian at 266 66 0 76 ma the wordian capitanian interval is biased to normal polarity but the basal wuchiapingian begins the beginning of a significant reverse polarity magnetochron lp0r with an overlying mixed polarity interval through the later lopingian no significant magnetostratigraphic data gaps exist in the permian geomagnetic polarity record the early cisuralian magnetochrons are calibrated to a succession of fusulinid zones the later cisuralian and guadalupian to a conodont and fusulinid biostratigraphy and lopingian magnetochrons to conodont zonations age calibration of the magnetochrons is obtained through a bayesian approach using 35 radiometric dates and 95 confidence intervals on the ages and chron durations are obtained the dating control points are most numerous in the gzhelian asselian wordian and changhsingian intervals this significant advance should provide a framework for better correlation and dating of the marine and non marine permian

combined vertical incidence and wide angle seismic study of a gas hydrate zone blake ridge wide angle ocean bottom seismic data and single channel seismic reflection data were collected in june 1992 over an area where gas hydrates are thought to be extensive on blake ridge offshore south carolina wide angle reflections were observed on four woods hole oceanographic institution ocean bottom hydrophones at offsets up to 15 km results from traveltime inversion show that the bottom simulating reflector bsr that marks the base of the hydrate stability field lies 400 500 m below the seafloor and is overlain by a 200 to 300 m thick layer of average velocity 1 9 km s 1 there is no evidence for significant lateral velocity variation associated with lateral changes in bsr character ray tracing calculations show that the observation of the bsr out to large offsets 7 km constrains the maximum vertical velocity gradient to about 0 5 s 1 amplitude versus offset avo analysis was performed for ranges up to about 6 km and incidence angles of more than 70 the high amplitude of the bsr in the normal incidence data can be explained by two models both of which include a strong negative impedance contrast the hydrate wedge model and the free gas layer model synthetic seismogram calculation for the wedge model using the reflectivity method shows a strong increase in the bsr amplitudes at offsets between 3 and 5 km because of postcritical reflections and diving waves within the wedge this amplitude increase is clearly not observed in the data which shows only a relatively modest increase in amplitude with offset this observation agrees better with the calculated avo for the gas layer model the lack of a distinct reflection from the base of the gas layer implies that the gas layer is not thicker than 25 m and or that the base of the gas layer is a gradational boundary

multiple episodes of fluid flow in the sw barents sea loppa high evidenced by gas flares pockmarks and gas hydrate accumulation abstract gas and gas hydrates have gained considerable attention in the last 20 yr due to their possible impact on global climate change from methane escalations our study area the sw barents sea is unique in its evolution due to the effect of glaciations which removed thick layers of sediment from the seabed unloading due to erosion and deglaciation resulted in opening of pre existing faults and creation of new ones facilitating fluid migration and eventually escape into the water from the subsurface expressions of hydrocarbon gas accumulation and fluid flow such as gas hydrates and pockmarks are widely distributed in the barents sea several gas flares some of them 200 m high in echograms occur along a segment of the ringvassoy loppa fault complex indicating open fractures and active fluid flow observation of gas flares along regional fault complexes outside pockmark regions indicates that present gas escape occurs along faults only the pockmark formation and gas flares belong to different episodes of fluid flow the pockmarks are absent outside soft sediment depocentres indicating that their existence is related to the presence of recording medium the thin sediment cover in pockmarks and their penetration down into glaciomarine sediments indicate that they formed after deposition of these sediments and that fluid escape was active to the very recent past methane hydrate stability zone mhsz modelling shows that after the last glacial maximum 18 000 20 000 14c years ago the mhsz has thinned from 600 m to zero in most parts of the sw barents sea the fluid expulsion probably happened after retreat of the glaciers causing release of methane from dissociation of methane hydrates through fluid escape processes which lasted until recently bottom simulating reflectors bsr observed are probably due to structure ii hydrates formed from gas containing higher order hydrocarbon components such as ethane propane etc along with methane the study area falls close to the new gigantic oil discoveries skrugard and havis and north of the snohvit hydrocarbon field

the pore scale distribution of sediment hosted hydrates evidence from effective medium modelling of laboratory and borehole seismic data much of our knowledge of hydrate distribution in the subsurface comes from interpretations of remote seismic measurements a key step in such interpretations is an effective medium theory that relates the seismic properties of a given sediment to its hydrate content a variety of such theories have been developed these theories generally give similar results if the same assumptions are made about the extent to which hydrate contributes to the load bearing sediment frame we have further developed and modified one such theory the self consistent approximation differential effective medium approach to incorporate additional empirical parameters describing the extent to which both the sediment matrix material clay or quartz and the hydrate are load bearing we find that a single choice of these parameters allows us to match well both p and s wave velocity measurements from both laboratory and in situ datasets and that the inferred proportion of hydrate that is load bearing varies approximately linearly with hydrate saturation this proportion appears to decrease with increasing hydrate saturation for gas rich laboratory environments but increase with hydrate saturation when hydrate is formed from solution and for an in situ example

seismic investigations of a vent field associated with gas hydrates offshore vancouver island 1 a three dimensional seismic reflection survey was carried out over a gas hydrate field offshore vancouver island in the area of ocean drilling program leg 146 site 889 890 the area of investigation is located on the accretionary prism of the cascadia margin in water depths of 1200 1500 m several seismic blank zones were observed in the seismic data over a frequency range from 30 hz to 3 5 khz where the degree of blanking increases with seismic frequency time slices and selected horizon slices were used to define the spatial distribution of the blank zones the zones range from 80 m to several hundred meters in width one blank zone almost circular with a diameter of 400 m has a distinct seafloor expression it shows the characteristics of a mud carbonate mound and is probably associated with free gas expulsion hydrate was found at four sites with a piston corer within this blank zone at depths of 3 8 m below the seafloor we present a tectonic model that explains the blank zones to be a result of differential uplift along thrust faults the blank zones represent conduits for fluids and gas migrating upward blanking of the seismic energy is believed to be mainly the result of increased hydrate formation in lenses within the faults the blanking effect is enhanced locally because of scattering at carbonates formed at the seafloor

geophysical and geochemical signatures associated with gas hydrate related venting in the northern cascadia margin this paper presents a comprehensive multidisciplinary study of cold vents associated with near seafloor gas hydrate several cold vents characterized by seismic blank zones have been identified on the northern cascadia margin near ocean drilling program odp site 889 890 the most prominent vent site bullseye vent has been the subject of intense geophysical and geochemical studies including two and three dimensional 2d 3d seismic imaging heat flow measurements piston coring with measurements of sediment physical properties and pore fluid geochemistry seafloor video observation and sampling with the unmanned submersible ropos the main seismically derived constraining observations are 1 blanking increases with seismic frequency 2 at low frequencies layers can be traced through the zones 3 blank zones widen with depth 4 blank zones are underlain by a bottom simulating reflector bsr and 5 no velocity anomalies were detected across the vents constraints from piston core and thermal probe analyses are 1 massive hydrate was recovered just below the seafloor at bullseye vent and 2 chemical alteration of sediments was observed by reduced magnetic susceptibility increased thermal conductivity and an elevated sulfate methane interface additional constraints are 1 no thermal anomaly was observed 2 widespread carbonates and active chemosynthetic communities were found and 3 elevated levels of methane were detected in the water column above bullseye vent we present a model for the seismic blanking at bullseye vent that honors the constraints from all observations the cold vents represent channels or networks of filamentous fractures containing hydrate and or free gas free gas can be present within the hydrate stability field only in fractures which may be coated with hydrate that prevents the inflow of water the overall concentration of hydrate or gas within the vent must be small because there was no observable velocity anomaly

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

geology and geophysics of the underground research laboratory site lac du bonnet batholith manitoba the lease area of the atomic energy of canada limited underground research laboratory covers 3 8 km2 and is located 2 5 km north of the south contact of the lac du bonnet batholith a shaft to 255 m and 130 boreholes up to 1100 m deep expose the third dimension the underlying granite is largely of two types i pink porphyritic which may be biotite rich gneissic and or xenolithic and ii grey homogeneous and equigranular composition layering including xenolith rich zones outlines domes along an antiform trending north northeast through the western part of the lease area the southeast dipping flank underlies the eastern half of the site including the shaft axes of folding trend 065 and 140 homogeneous grey granite being relatively fresh and unfractured is associated with a magnetic field that is about 100 nt higher and with a resistivity that is up to 5000 m higher than those of other units a pattern of highs in the magnetic field caused by the high magnetite content of some xenolith

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

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