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

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

dyke intrusion model for the 1989 seismovolcanic activity off ito central japan in the last 10 years extensive earthquake swarms have occurred intermittently in the region east of izu peninsula central japan in july 1989 an active swarm again took place off ito city and among all past activities it was nearest to land and shallowest this activity eventually became a weak submarine eruption the first of its kind in this region associated with this event a borehole tiltmeter at 92 m depth installed several kilometers from the source area detected distinct continuous tilt change with the following features 1 tilt of 20 rad was observed within 1 week preceding the eruption 2 tilt change showed a good correlation to swarm activity 3 no obvious coeruption tilt change was observed on the basis of the tilt record and swarm distribution we constructed a dislocation model to interpret the seismovolcanic activity the model consists of two tensile faults each of which corresponds to magma intrusions in may and july and a right lateral reverse fault representing the largest earthquake m5 5 at 1109 lt on july 9 this model can simulate not only the above tilt data but also the record of a nearby dilatometer the results from daily laser distance measurements and daily global positioning system monitoring as well as results from other conventional distance surveys and leveling surveys in the surrounding region from this model we conclude that the intersection of the shear fault with the tensile fault caused magma to come up into soft sedimentary layers at the seabottom creating a lava dome later an instability brought about a magma phreatic explosion which did not generate any substantial crustal deformations at deeper levels

bottom simulating reflectors seismic velocities and avo effects we obtain the wave velocities of ice and gas hydrate bearing sediments as a function of concentration and temperature unlike previous theories based on simple slowness and or moduli averaging or two phase models we use a biot type three phase theory that considers the existence of two solids grain and ice or clathrate and a liquid water and a porous matrix containing gas and water for consolidated berea sandstone the theory underestimates the value of the compressional velocity below 0 c including grain ice interactions and grain cementation yields a good fit to the experimental data strictly speaking water proportion and temperature are closely related fitting the wave velocity at a given temperature allows the prediction of the velocity throughout the range of temperatures provided that the average pore radius and its standard deviation are known the reflection coefficients are computed with a viscoelastic single phase constitutive model the analysis is carried out for the top and botto

new insights into the tectonic evolution of the andaman basin northeast indian ocean multibeam swath bathymetry data acquired over an area of about 30 000 km2 together with magnetic and single channel seismic data have been analyzed to understand the tectonic evolution of the andaman basin northeast indian ocean swath bathymetry data revealed several morphotectonic features that divide the basin into a complex western part comprising arc parallel seamount chains and n s trending fault systems and a relatively smooth eastern part a sw ne trending spreading ridge bisects the basin an offset of about 11 8 km at 94 21 e longitude divides the spreading ridge into sw and ne segments high resolution bathymetry and magnetic data enabled us to identify two finer scale segments over the sw part of the ridge that display distinct topographic fabric compared to the ne segment thin sediment cover and presence of axial seamounts characterize sw segments of the ridge whereas the ne segment has thick sediment cover and well maintained rift valley kinematics of basin evolution derived from the identification of the seafloor spreading magnetic anomalies suggest very slow initial opening rates of about 1 6 cm yr and an increase in the spreading rate to about 3 8 cm yr from anomaly 2 to present although this interpretation relies only on magnetic anomalies in the two westernmost segments it is suggested that the true seafloor spreading started about 4 ma in the andaman backarc basin rather than the 11 ma postulated previously we infer based on the integrated analysis of swath bathymetry magnetic and seismological data that the basin has evolved as a consequence of extrusive tectonics that prompted extension and rifting along the plane joining the sagaing and semengko fault systems extension by seafloor spreading during the past 4 myr resulted in the formation of the deep andaman backarc basin this phase has also experienced westward propagation of the spreading center

inferred gas hydrates and clay diapirs near the storegga slide on the southern edge of the voring plateau offshore norway this paper presents a new data set including single channel airgun seismic lines okean long range side scan sonar data and gravity cores acquired on the edge of the voring plateau and in the region of the storrega slide the data acquisition was part of the 8th training through research ttr 8 expedition with r v professor logachev the acoustic profiles clearly show two laterally separated zones characterised by the presence of bottom simulating reflectors bsrs at about 350 ms twt subbottom depth the lower bsr zone is located on the slope of the voring plateau in the immediate vicinity of the northern headwall of the storegga slide and in some places below the slide deposits this zone runs parallel to the general trend of the continental slope the spatial distribution of the upper bsr zone located upslope in an area where diapir like structures are found does not demonstrate any topographic control interpretation of high backscatter patches on the okean sonographs associates the observed structures with fluid escape features on the seabed most of them are pockmarks but in a few places diapirs are cropping out and form dome like elevations after analysing the behaviour of the bsr sometimes crosscutting and its acoustic characteristics reversed polarity and after applying seismic inversion processing to estimate the acoustic velocity change across the bsr this reflector is interpreted to represent the base of the local gas hydrate stability field ghsf this information was used to derive the regional geothermal gradient spatial variations of the inferred geothermal field appeared to be negligible the observation of two separated bsr zones suggests different environmental controls for the growth of the hydrates enhanced reflectors observed in the intermediate zone can be explained by the presence of strata bound free gas accumulations and migration combined with overlying permeability barriers therefore a model for gas hydrate formation in relation to directional fluid migration processes fluid escape features and the presence of diapiric structures is proposed in this paper our model is supported by sedimentological analyses of seafloor samples which demonstrate the presence of stiff clays with evidence of gas migration and by paleontological studies of the cores retrieved from the pockmarks the presence of a bsr in the sedimentary section within the slide scar area implies the repositioning of hydrates to newly established equilibrium conditions after the slide event this observation allows us to simply estimate an upper limit of the fluid migration velocity in the order of several cm year finally the effect of hydrate dissociation on slope instability is considered assuming that the in situ decomposition of hydrates due to instantaneous depressurisation is slow enough to permit the excess volume of released gas and water to be re distributed through the whole sedimentary section above the paleo bsr with consuming heat and increasing pore pressure dissociation tends to shift the pt conditions back to equilibrium values it appears that the dissociation of hydrates due to sliding would cause only 0 2 increase of the pore pressure which would hardly contribute to further slope instability

heat flow anomalies in the gulf of cadiz and off cape san vincente portugal heat flow anomalies provide critical information in active tectonic environments the gulf of cadiz and adjacent areas are affected by the plate convergence between africa and europe causing widespread deformation and faulting active thrust faults cause lateral movement and advection of heat that produces systematic variations in surface heat flow in december 2003 new heat flow data were collected during the research vessel sonne cruise so175 in the gulf of cadiz over two sites of recent focused research activity i the gulf of cadiz sedimentary prism and ii the marques de pombal escarpment both features have also been discussed as potential source areas of the great lisbon earthquake and tsunami of 1755 background heat flow at the eastern terminus of the horseshoe abyssal plain is about 52 59 mw m2 over the gulf of cadiz prism heat flow decreases from 57 mw m2 to unusually low values of 45 mw m2 roughly 120 km eastward such low values and the heat flow trend are typical for active thrusting supporting the idea of an east dipping thrust fault slip rates are 10 5 mm per year assuming that the fault dips at 2 a fault dipping at 5 however would result into slip rates of 1 5 5 mm per year suggesting that subduction has largely ceased based on seismic data the marques de pombal fault is interpreted as part of an active fault system located 100 km westward of cape san vincente heat flow over the fault is affected by refraction of heat caused by the 1 km high escarpment thermal models suggest that the slip rate along the fault must either be small or shear stresses acting on the fault are rather high with respect to other fault zones however it is reasonable to assume that the fault s slip rate is small

effect of thermal non equilibrium seafloor topography and fluid advection on bsr derived geothermal gradient abstract the seafloor and bottom simulating reflectors bsrs are interpreted from the 3d seismic data acquired in krishna godavari kg offshore basin in the vicinity of sites drilled cored during national gas hydrate program nghp expedition 01 the shallow structures such as inner toe thrust fault system regional and local linear fault systems and mass transport deposits are inferred from attributes of seafloor time structure as well as from the seismic profiles the geothermal gradient is estimated from the depths and temperatures of the seafloor and the bsr the temperature at the bsr depth is estimated from the methane hydrate and seawater salinity phase boundary assuming that the bsr represents the base of the gas hydrate stability zone the spatial variations in geothermal gradient gtg show a strong correlation with seafloor topography in the kg basin the gtg decreases by 13 30 over the topographic mounds formed due to inner toe thrust faults and recent mass transport deposits the gtg decreases by only 5 10 over the mounds likely due to defocusing of heat flux based on one dimensional topographic modeling hence the gtg perturbation due to topography alone cannot explain the observed gtg anomaly the temperature profile beneath these mounds may not be in equilibrium with the surroundings either due to the recent upliftment of sediments along the inner toe thrust faults or rapid deposition of sediments due to slumping sliding in contrast an increase in gtg by 10 15 is observed in the vicinity of major fault systems we presume that the likely mechanism for the increase in gtg is fluid advection from a deeper part of the basin a detailed thermal modeling involving the effect of surface topography high sedimentation rates fluid advection and sediment thickening due to tectonics is required to understand the thermal profile in kg offshore basin

coseismic crustal deformations of 1994 northridge california earthquake detected by interferometric jers 1 synthetic aperture radar coseismic deformations of the january 17 1994 northridge california earthquake are mapped using interferometric combination of synthetic aperture radar images acquired by the japanese earth resource satellite jers 1 on april 30 1993 and june 14 1994 we make an interferogram in a rectangular area with a size of 18 6 km n s by 29 9 km e w although coherence is not perfect in some areas fringes in flat and inhabited areas clearly show the coseismic movements to remove ambiguity of the fringes due to inaccurate orbital information we use a least squares adjustment to minimize the difference between the radar and the global positioning system gps measurements the difference between the radar and gps measurements is less than 3 5 cm the observed fringe patterns agree with a gps based theoretical fault model within 10 cm we also find a lobed pattern that suggests up to 20 cm of subsidence within a small area the results suggest that the l band 23 5 cm radar used by jers 1 is more robust to spatial and temporal decorrelation than the c band radar used by the ers 1 and radarsat satellites

observation and tentative interpretation of a double bsr on the nankai slope seismic data collected during the french japanese kaiko tokai cruise of r v l atalante on the upper slope of the eastern nankai margin reveal the simultaneous presence at two distinct depths below the seafloor of two bottom simulating reflector bsr type reflectors the upper bsr is traced as a continuous reflector over about 10 km as water depth decreases from 850 m to 550 m its depth below seafloor decreases from 200 m to 40 m the lower bsr is traced at 50 100 m below the upper one the two bsrs end abruptly near the summit of the daichii tenryu knoll into an area where the 3 5 khz record suggests active gas expulsion through the seabed the observed depth of the upper bsr fits the predicted one for the base of the methane gas hydrate stability zone as estimated from present temperature and pressure conditions at the seafloor and in the slope sediments thus we interpret the upper bsr as an active methane hydrate bsr we further suggest that the lower bsr is a residual hydrate related bsr this could have followed a recent migration of the base of the methane hydrate stability zone from the lower bsr to the upper one as possible causes for this migration we discuss sea bottom warming and tectonic uplift the bsr migration could have occurred as a response to a 1 2 c sea bottom warming or with an equivalent effect an event of fast uplift of the seafloor by about 90 m we do not discard other interpretations of the lower bsr such as an active hydrate related bsr formed from a mixture of gases

multi measurement approach for establishing the base of gas hydrate occurrence in the krishna godavari basin for sites cored during expedition nghp 02 in the offshore of india abstract the 2015 national gas hydrate program of india s second expedition nghp 02 acquired logging and coring datasets for constraining the base of the gas hydrate occurrence zone deepest gh and the theoretical base of gas hydrate stability zone bghs these data are used here for two primary goals to constrain the deepest occurrence of gas hydrate relative to predicted stability limits and the observed bsr and to characterize the nature of the contact between gas hydrate bearing sediment and the underlying gas hydrate free sediment a consensus depth for the deepest gh is derived for each nghp 02 coring site from downhole indicators of gas hydrate occurrence obtained from well log electrical resistivity and acoustic data pressure core compressional wave velocity measurements and conventional core measurements of anomalously low temperatures to establish the theoretical bghs models of gas hydrate phase stability with depth are compared with downhole temperature profiles derived from 1 assuming a constant geothermal gradient consistent with downhole temperature measurements and 2 assuming constant heat flow using a geotherm through the downhole temperature measurements and incorporating thermal conductivity calculated from borehole logging data although the deepest nghp 02 gh occurrences are controlled at several sites by a lithologic boundary most sites have deepest gh occurrences within a single coarse grained lithology cutoffs within a single coarse grained lithology which occur for the primary nghp 02 area b gas hydrate reservoir will inhibit pore pressure drawdowns used to extract methane from gas hydrate as an energy resource

gas hydrate stability and the assessment of heat flow through continental margins summary a prominent feature across some continental margins is a bottom simulating reflector bsr this seismic reflection generally coincides with the depth predicted for the base of the gas hydrate stability field because the occurrence of gas hydrates is controlled by temperature and pressure conditions it has been suggested that bsrs mark an isotherm and they have therefore been used to estimate the heat flow through continental margins crucial parameters are the temperature at bsr depth and at the seafloor and the thermal conductivity structure between the bsr and the seabed however very often the required parameters are not available and therefore they have been derived from models for gas hydrate stability and empirical relationships to obtain thermal conductivities from seismic velocities here we use downhole temperature thermal conductivity porosity and logging data from 10 ocean drilling program odp sites drilled into and through the gas hydrate field to investigate the quality of estimates our analyses and application of constraints to the makran margin off pakistan indicate the following i the temperature at bsr depth could be approximated by a seawater methane system although capillary forces chemical impurities or non equilibrium conditions can lower or increase the temperature if calibration by heat probe measurements is possible errors of geothermal gradients are less than 10 per cent otherwise uncertainties of 20 per cent or even higher may arise in addition seasonal variations of bottom water temperature have to be considered because they may affect thermal gradients by up to y10 per cent ii the impact of typical quantities of low thermal conductivity gas hydrate on the bulk thermal conductivity is insignificant iii the thermal conductivity profile between the bsr and the seabed can generally be approximated by a mean value thus iv seabed measurements should be used instead of empirical relationships which may produce errors of 5 30 per cent consequently in addition to high quality seismic data a prerequisite should be a large data set of thermal conductivities and oceanographic data heat probe measurements are recommended to constrain geothermal gradients in this case the uncertainty of heat flow is 5 10 per cent of the estimated heat flow if these data are not available errors uncertainties can reach 50 60 per cent of the calculated value

age and origin of early paleozoic and mesozoic granitoids in western yunnan province china geochemistry shrimp zircon ages and hf in zircon isotopic compositions abstractthis work presents evidence for early paleozoic magmatism in the baoshan block and mesozoic magmatism in the tengchong block and along the gaoligong fault system in the tethyan belt western yunnan province southwestern china on the basis of shrimp dating of zircons from 12 granitoid samples deformed granitoids with gneissic textures contain zircons with magmatic oscillatory zoning that define weighted mean 206pb 238u ages ranging from 481 3 to 493 3 ma and are interpreted as the crystallization time of the host rocks the granitoids are characterized by enriched light rare earth elements high th and u contents and negative nb anomalies on a primitive mantle normalized spider diagram which is indicative of crustal melting the peraluminous silica rich compositions high initial 87sr 86sr ratios as well as negative end t and ehf t values also suggest that the granitoid rocks originated through crustal reworking zircons of 216 226 ma late triassic s type granites of the tengchong blo

atmospheric effects in interferometric synthetic aperture radar surface deformation and topographic maps interferogram images derived from repeat pass spaceborne synthetic aperture radar systems exhibit artifacts due to the time and space variations of atmospheric water vapor other tropospheric variations such as pressure and temperature also induce distortions but the effects are smaller in magnitude and more evenly distributed throughout the interferogram than the wet troposphere term spatial and temporal changes of 20 in relative humidity lead to 10 cm errors in deformation products and perhaps 100 m of error in derived topographic maps for those pass pairs with unfavorable baseline geometries in wet regions such as hawaii these are by far the dominant errors in the spaceborne imaging radar c and x band synthetic aperature radar sir c x sar interferometric products the unknown time delay from tropospheric distortion is independent of frequency and thus multiwavelength measurements such as those commonly used to correct radar altimeter and global positioning system gps ionospheric biases cannot be used to rectify the error in the topographic case the errors may be mitigated by choosing interferometric pairs with relatively long baselines as the error amplitude is inversely proportional to the perpendicular component of the interferometer baseline for the sir c x sar hawaii data we found that the best longest baseline pair produced a map supporting 100 m contouring whereas the poorest baseline choice yielded an extremely noisy topographic map even at this coarse contour interval in the case of deformation map errors the result is either independent of baseline parameters or else very nearly so here the only solution is averaging of independent interferograms so in order to create accurate deformation products in wet regions many multiple passes may be required rules for designing optimal data acquisition and processing sequences for interferometric analyses in nondesert parts of the world are 1 to use the longest radar wavelengths possible within ionospheric scintillation and faraday rotation limits 2 for topography maximize interferometer baseline within decorrelation limits and 3 for surface deformation use multiple observations and average the derived products following the above recipe yields accuracies of 10 m for digital elevation models and 1 cm for deformation maps even in very wet regions such as hawaii

compressional and shear wave velocities from gas hydrate bearing sediments examples from the india and cascadia margins as well as arctic permafrost regions abstract shear wave velocity data have been acquired at several marine gas hydrate drilling expeditions including the india national gas hydrate program expedition 1 nghp 01 the ocean drilling program odp leg 204 and integrated ocean drilling program iodp expedition 311 x311 in this study we use data from these marine drilling expeditions to develop an understanding of general grain size control on the p and s wave properties of sediments a clear difference in the downhole trends of p wave vp and s wave vs velocity and the vp vs ratio from all three marine regions was observed the northern cascadia margin iodp x311 shows the highest p wave and s wave velocity values overall and those from the india margin expedition nghp 01 are the lowest the southern cascadia margin odp leg 204 appears to have similar low p wave and s wave velocity values as seen off india s wave velocity values increase relative to the sites off india but they are not as high as those seen on the northern cascadia margin such regional differences can be explained by the amount of silt sand or lack thereof occurring at these sites with northern cascadia being the region of the highest silt sand occurrences this grain size control on p wave and s wave velocity and associated mineral composition differences is amplified when compared to the arctic permafrost environments where gas hydrate predominantly occurs in sand and silt dominated formations using a cross plot of gamma ray values versus the vp vs ratio we compare the marine gas hydrate occurrences in these regions offshore eastern india margin offshore cascadia margin the ignik sikumi site in alaska and the mallik 5l 38 site in the mackenzie delta the log data from the arctic permafrost regions show a strongly linear vp vs relationship similar to the previously defined empirical relationships by greenberg and castagna 1992 p and s wave velocity data from the india margin and odp leg 204 deviate strongly from these linear trends whereas data from iodp x311 plot closer to the trend of the arctic data sets and previously published relationships three new linear relationships for different grain size marine sediment hosts are suggested a mud dominated mahanadi basin odp leg 204 nghp 01 17 vs 1 5854 vp 2 1649 b silty mud kg basin vs 0 8105 vp 1 0223 c silty sand iodp x311 vs 0 5316 vp 0 4916 we investigate the relationship of gas hydrate saturation determined from electrical resistivity on the vp vs ratio and found that the sand dominated arctic hosts show a clearly decreasing trend of vp vs ratio with gas hydrate saturation though limited due to lower overall gh saturations a similar trend is seen for sites from iodp x311 and at the ash dominated nghp 01 17 sediment in the andaman sea gas hydrate that occurs predominantly in fractured clay hosts show a different trend where the vp vs ratio is much higher than at sand dominated sites and remains constant or increases slightly with increasing gas hydrate saturation this trend may be the result of anisotropy in fracture dominated systems where p and s wave velocities appear higher and archie based saturations of gas hydrate are overestimated gas hydrate concentrations were also estimated in these three marine settings and at arctic sites using an effective medium model combining p and s wave velocities as equally weighted constraints on the calculation the effective medium approach generally overestimates s wave velocity in high porosity clay dominated sediments but can be accurately used in sand rich formations

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

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

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

fluid flow and origin of a carbonate mound offshore vancouver island seismic and heat flow constraints a 1 5 km long 1 km wide and 70 80 m high carbonate mound was identified on the mid slope region of the subduction accretionary sedimentary prism offshore vancouver island 3 5 km west of ocean drilling program odp site 889 and integrated ocean drilling program iodp site u1327 seabed video images show the presence of seafloor carbonate as well as chemosynthetic communities a high resolution single channel seismic survey with close line spacing recording coherent reflectivity down to about 400 m beneath the seafloor provided acoustic images of this mound and of the gas hydrate bottom simulating reflector bsr beneath it the mound is interpreted to have developed as a structural topographic high in the hanging wall of a large reverse fault formed at the base of the current seaward slope the fault zone provides pathways for fluids including gas to migrate to the seafloor where diagenetic carbonate forms and cements the near surface sediments to examine the thermal effect of possible upward fluid flow beneath the mound heat flow at the mound and in the neighbouring region was calculated from the depth of the bsr below the seafloor these data were combined with heat flow calculated over a broader region from previous multi channel seismic data heat flow within the flattest portion of the surrounding 4 km by 8 km region averages 74 mw m2 taking this value to represent the regional or background heat flow a simple 2d analytical method was used to calculate theoretical heat flow variations due to topography across the mound most of the variability is explained by topographic effects including a local 6 mw m2 negative anomaly over the central mound and a large 20 mw m2 positive anomaly over the mound steep side slope however just south of the mound there is a 6 7 mw m2 positive anomaly in a 2 km long band that has predominantly flat seafloor most of this anomaly is probably unrelated to topographic effects but rather likely due to warm upward fluid flow along faults or fracture zones

crustal deformation measurements using repeat pass jers 1 synthetic aperture radar interferometry near the izu peninsula japan we have examined the precision of interferometric synthetic aperture radar sar measurements of surface deformation of the earth using 24 cm wavelength data acquired by the japanese earth resources satellite 1 jers 1 spacecraft over the izu peninsula japan radar images acquired from 1993 to 1994 were made into 11 independent interferograms apparent water vapor signatures with equivalent path delay up to 16 cm preclude reliable estimates of small deformation from only one interferogram averaging of interferograms improves the measurement such that residuals between precise leveling data and corresponding radar observations are less than 1 cm rms there is a clear correlation between cloud cover extent on a given day and the spectral power of the water vapor signature in an interferogram formed from the corresponding day s sar image also several water vapor features associated with local topography are consistent with accumulation of moisture on the windward side of an on shore flow analysis shows that decorrelation in mixed conifer and deciduous forest is a weak function of the time between observations imposing a roughly constant level of additional decorrelation relative to scatter stable urban areas furthermore in high relief areas correlation is high when the baseline is tens of meters but drops rapidly with increasing baseline length with strict control of the interferometric baseline length adequate correlation in interferograms over very long time periods is possible even in forested high relief regions observed inflation over ito city is consistent with a localized magma source at depth suggesting a different mechanism from the previous episode in 1989

accuracy of topographic maps derived from ers 1 interferometric radar an interferometric radar technique for topographic mapping of surfaces promises a high resolution approach to the generation of digital elevation models the authors present analyses of data collected by the synthetic aperture radar instrument on board the ers 1 satellite on successive orbits use of a single satellite in a nearly repeating orbit is attractive for reducing cost and spaceborne hardware complexity also it permits inference of changes in the surface from the correlation properties of the radar echoes the data have been reduced to correlation maps and digital elevation models the correlation maps show that temporal correlation decreases significantly with time but not necessarily at a constant well defined rate likely depending on environmental factors when correlation among passes remains high however it is possible to form digital elevation models analyses of noise expected in ers 1 interferometric data collected over alaska and the southwestern united states indicate that maps with relative errors less than 5 m rms are possible in some regions however orbit uncertainties imply that tie points are required in order to reduce absolute height errors to a similar magnitude the authors find that about 6 tie points per 40 spl times 40 km scene with 5 m rms or better height accuracy are needed to keep systematic map height errors below 5 m rms the performance of the ers 1 radar system for topographic applications though useful for a variety of regional and local discipline studies may be improved with respect to temporal decorrelation errors and absolute height acuity by modifying the orbit repeat period and incorporating precise orbit determination techniques the resulting implementation will meet many but not all objectives of a global mapping mission

heat flow and gas hydrate saturation estimates from andaman sea india gas hydrate was recovered in the andaman sea along the eastern coast of the andaman islands during the india national gas hydrate program nghp expedition 01 at site nghp 01 17 coring confirmed gas hydrate occurs predominantly in discrete volcanic ash layers pore water chemistry electrical resistivity and p wave velocity logs are used to estimate gas hydrate saturations at site nghp 01 17 gas hydrate saturation estimated from chloride concentrations shows values up to 85 of the pore space for distinct ash layers from 270 m below seafloor to the base of gas hydrate stability zone bghsz gas hydrate saturations estimated from the electrical resistivity and acoustic velocity logs using standard empirical relations and modeling approaches are comparable to each other but saturations are only 20 of the pore space on average this much lower gas hydrate saturation estimate from the log data is a result of overall reduced resolution of the logging tools relative to the typically 20 30 cm thick hydrate bearing ash layers available 2d multi channel seismic data were also analyzed and a bottom simulating reflector bsr was imaged along several seismic profiles the depth of the bsr is more than 600 m along the seismic line crossing site nghp 01 17 which makes this one of the deepest bsrs observed worldwide to understand the unusual depth of the bsr we mapped its depth and estimated heat flow from the bsr depth using a simple conductive model bsr derived heat flow values range from 12 to 41 5 mw m2 from the study area and follow the bathymetry trend of dominant north south ridges and can be explained with the east ward trending increase in heat flow toward the current seafloor spreading center we also modeled the bghsz to analyze the linkage between gas hydrate occurrences in the andaman sea and its relation to the tectonic activity our analysis suggests an extensively variable bghsz in the andaman sea controlled mainly by overall low geothermal gradients consistent local minor variations were observed with lower heat flow values over prominent topographic highs and higher values in valleys troughs due to focusing and defocusing effects of the topography

atmospheric limitations to repeat track radar interferometry in its recent radar imaging mission the shuttle imaging radar satellite sir c devoted three days to repeat track interferometry we have analyzed the data from a test site in the mojave desert of california although good topography 10 m on 21 m postings was obtained most of the error was caused by turbulent water vapor in the lower atmosphere spatial structure of 6 km and all smaller sizes was observed the rms one way time delay was found to be 0 24 cm essentially identical results were obtained at two wavelengths 24 and 5 7 cm

seismic character of bottom simulating reflectors examples from the mid norwegian margin three classes of bottom simulating reflectors bsr cross cut the post breakup sediments of the mid norwegian margin the first class is caused by free gas at the base of the pressure and temperature dependent gas hydrate stability zone the second class of bsr is caused by the diagenetic transition from opal a to opal ct the third class of bsr is always observed underneath the opal a opal ct transition but heat flow data and the amplitude characteristics of this arrival exclude one of the known silicate diagenetic transitions or gas hydrates as the explanation for this reflector odp site 643 drilling results from the voring plateau suggest two possible processes as the reason for this third bsr a smectite illite conversion or b a sudden increase in the abundance of authigenic carbonates the genesis of both is pressure and temperature dependent and could potentially result in a cross cutting seismic reflector the data are not conclusive as to which process is causing the third class of observed bsr

seismic indicators of gas hydrate and associated gas in the ulleung basin east sea japan sea and implications of heat flows derived from depths of the bottom simulating reflector abstract analysis of multi channel seismic reflection data from the ulleung basin east sea japan sea reveals various seismic indicators of gas hydrate and associated gas including the bottom simulating reflector bsr enhanced reflections below the bsr and seismic chimneys the recent recovery of massive gas hydrate by drilling together with these seismic indicators strongly suggests favorable conditions for the formation of gas hydrate in the area the bsr is most common and of a wide range of amplitude and continuity seismic chimneys characterized by columnar zones of amplitude reduction terminate mostly within the gas hydrate stability zone ghsz but a few extend to pockmarks or mounds on the seafloor seismic chimneys with seafloor expressions probably represent vertical vents for gas originating from below the ghsz the increase in the degree of amplitude reduction in seismic chimneys with increasing seismic frequency may be due to seismic attenuation by gas bubbles trapped in hydrate filled coated fractures in the ghsz bsr derived heat flows range from about 65 to over 115 mw m2 comparable to those from direct measurements heat flows in the northern part of the study area are very high for the age of the basin suggesting hotter than normal mantle temperature the area of high 105 mw m2 heat flows also coincides largely with the distribution of the presumed incipient oceanic crust formed during the earliest stage of seafloor spreading

gas hydrate saturation from acoustic impedance and resistivity logs in the shenhu area south china sea during the china s first gas hydrate drilling expedition 1 gmgs 1 gas hydrate was discovered in layers ranging from 10 to 25 m above the base of gas hydrate stability zone in the shenhu area south china sea water chemistry electrical resistivity logs and acoustic impedance were used to estimate gas hydrate saturations gas hydrate saturations estimated from the chloride concentrations range from 0 to 43 of the pore space the higher gas hydrate saturations were present in the depth from 152 to 177 m at site sh7 and from 190 to 225 m at site sh2 respectively gas hydrate saturations estimated from the resistivity using archie equation have similar trends to those from chloride concentrations to examine the variability of gas hydrate saturations away from the wells acoustic impedances calculated from the 3 d seismic data using constrained sparse inversion method were used well logs acquired at site sh7 were incorporated into the inversion by establishing a relation between the water filled porosity calculated using gas hydrate saturations estimated from the resistivity logs and the acoustic impedance calculated from density and velocity logs gas hydrate saturations estimated from acoustic impedance of seismic data are similar to 10 23 of the pore space and are comparable to those estimated from the well logs the uncertainties in estimated gas hydrate saturations from seismic acoustic impedances were mainly from uncertainties associated with inverted acoustic impedance the empirical relation between the water filled porosities and acoustic impedances and assumed background resistivity c 2011 elsevier ltd all rights reserved

sunda java trench kinematics slab window formation and overriding plate deformation since the cretaceous the kinematics and time dependence of back arc extension or compression is one of the most poorly understood aspects of plate tectonics and has nearly exclusively been studied from snapshots of present day observations here we combine absolute and relative plate motions with reconstructions of now subducted ocean floor to analyse subduction kinematics and upper plate strain from geological observations since 80 ma along the 3200 km long sunda java trench one of the largest subduction systems on earth combining plate motions and slab geometries enables us to reconstruct a time dependent slab window beneath sundaland formed through wharton spreading ridge subduction we find that upper plate advance and retreat is the main influence on upper plate strain but subduction of large bathymetric ridges and slab window effects also play a significant and at times dominant role compression in the sundaland back arc region can be linked to advance of the upper plate extension of the sundaland backarc region correlates with two patterns of upper plate motion a retreat of the upper plate and b advance of the upper plate combined with more rapid advance of the sundaland margin due to hinge rollback subduction of large bathymetric ridges causes compression in the upper plate especially wharton ridge subduction underneath sumatra over the period 15 0 ma our reconstructions unravel the evolving geometry of a slab window underlying the java south sumatra region and we propose that decreased mantle wedge viscosities associated with this slab window exacerbated palaeogene extension in the java sea region via active rifting and enabled sumatran continental extension to continue at 50 35 ma when upper plate advance would otherwise have led to compression

surface displacement of the 17 may 1993 eureka valley california earthquake observed by sar interferometry satellite synthetic aperture radar sar interferometry shows that the magnitude 6 1 eureka valley earthquake of 17 may 1993 produced an elongated subsidence basin oriented north northwest parallel to the trend defined by the aftershock distribution whereas the source mechanism of the earthquake implies a north northeast striking normal fault the 3 millimeter accuracy of the radar observed displacement map over short spatial scales allowed identification of the main surface rupture associated with the event these observations suggest that the rupture began at depth and propagated diagonally upward and southward on a west dipping north northeast fault plane reactivating the largest escarpment in the saline range

heat flow variations from bottom simulating reflectors on the cascadia margin abstract a bottom simulating reflector bsr representing the base of the gas hydrate stability field was observed over a closely spaced grid of seismic lines on the continental slope of the northern cascadia margin a simple conductive model was used to calculate heat flow from the depth of the bsr a regional trend was observed in which heat flow decreased landward across the margin from an average of 80 mw m2 at a distance of 15 km from the deformation front to 65 mw m2 at a distance of 40 km this trend reflects the processes of tectonic thickening of accretionary wedge sediments and subduction of the juan de fuca plate and contrasts with the near constant heat flow across the oregon margin where the dip of the subducting plate and thus the rate of sediment thickening are slightly smaller consistent local variations were also observed notably low heat flow values over prominent topographic highs and high heat flow values over the flanks of the topographic highs at some localities over a horizontal distance of 1 2 km heat flow increased by as much as 50 from typical values of 65 to 100 mw m2 much of this variation may be due to focusing and defocusing effects of the topography alone and indicates the importance of carrying out topographic corrections to heat flow in regions of significant relief such as the continental slope however a component of the local heat flow patterns may be due to dynamic effects including the displacement of isotherms by thrust faulting and the upward migration of warm fluids along permeability channels such as faults

on the derivation of coseismic displacement fields using differential radar interferometry the landers earthquake we present a map of the coseismic displacement field resulting from the landers california june 28 1992 earthquake derived using data acquired from an orbiting high resolution radar system we achieve results more accurate than previous space studies and similar in accuracy to those obtained by conventional field survey techniques data from the ers 1 synthetic aperture radar instrument acquired in april july and august 1992 are used to generate a high resolution wide area map of the displacements the data represent the motion in the direction of the radar line of sight to centimeter level precision of each 30 m resolution element in a 113 km by 90 km image our coseismic displacement contour map gives a lobed pattern consistent with theoretical models of the displacement field from the earthquake fine structure observed as displacement tiling in regions several kilometers from the fault appears to be the result of local surface fracturing comparison of these data with global positioning system and electronic distance measurement survey data yield a correlation of 0 96 thus the radar measurements are a means to extend the point measurements acquired by traditional techniques to an area map format the technique we use is 1 more automatic 2 more precise and 3 better validated than previous similar applications of differential radar interferometry since we require only remotely sensed satellite data with no additional requirements for ancillary information the technique is well suited for global seismic monitoring and analysis

assessment of gas hydrate stability zone and geothermal modeling of bsr in the andaman sea abstract wide spread bottom simulating reflectors bsrs are observed along available multichannel seismic profiles covering an area of about 290 km 2 in the andaman sea the seismic data shows that the bsr occurs at places where water depth exceeds 1000 m and is identified by cross cutting relationships with the dipping reflectors the bsr that represents the base of gas hydrate stability field can be used to infer the gas hydrate stability thickness which ranges between 518 m to 861 m depending on water depths in situ measurement at site 17 during the indian national gas hydrate program nghp expedition 01 shows very low geothermal gradient 19 2 c km a conductive model was used to determine geothermal gradients from bsrs which is calculated and varying between 10 c km to 40 c km the low geothermal gradient is responsible for the deepest bsr or gas hydrate stability zone ghsz in the andaman region and in the world the geothermal modeling shows a close match of the predicted base of the gas hydrate stability zone with the observed bsr depths

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

multiple bottom simulating reflections in the black sea potential proxies of past climate conditions abstract a previously unknown pattern of multiple bottom simulating reflections bsrs occurs on high resolution reflection seismic data in the danube deep sea fan associated with acoustic features indicating free gas our study provides evidence that this pattern is developed in relation with the architecture of distinct channel levee systems of the danube fan channel levee systems hosting multiple bsrs act as relatively sealed gas bearing systems whose top is situated above the base of the gas hydrate stability zone bghsz inside these systems free gas accumulates below the bghsz under a combined lithological structural and stratigraphical control the uppermost bsr marks the current equilibrium bghsz for a gas composition of more than 99 methane model derived depths of the bghsz for different gas compositions and pressure temperature conditions show that multiple bsrs would correspond to the bghsz either for 1 layers of gas hydrates with high contents of heavy hydrocarbons or hydrogen sulphide or 2 stable climatic episodes with temperatures between glacial values and the present day conditions as the gas hydrate compositions required by hypothesis 1 are in sharp contradiction with the general background of the gas composition in the study area we suggest that multiple bsrs are most probably relics of former positions of the bghsz corresponding to successive steps of climate warming in this case they can provide sea bottom paleotemperature values for these episodes and hence they are potential new proxies for deciphering past climate conditions

upward shifts in the southern hydrate ridge gas hydrate stability zone following postglacial warming offshore oregon 1 high resolution three dimensional 3 d seismic reflection data acquired on the r v thomas g thompson in 2000 reveal a pair of bottom simulating reflections bsrs across a broad region of southern hydrate ridge offshore oregon the primary bsr bsrp is a regionally extensive reflection that lies 120 150 m below seafloor and exhibits typical characteristics of a gas hydrate bsr we also imaged a second weaker bsr bsrs 20 40 m below bsrp with similar characteristics bsrs is interpreted as a remnant of a bsr that probably formed during the last glacial maximum 18 000 years ago when the base of the gas hydrate stability zone ghsz was deeper an increase in bottom water temperatures of 1 75 2 25 and a corresponding sea level rise of 120 m could have produced the bsr shift the preservation of bsrs for at least 5000 years which is the time since subseafloor temperatures stabilized following ocean warming after the last glacial maximum implies very slow upward advective and diffusive flow of methane 1 m 1000 years in the vicinity of bsrs bsrs appears where there are no resolvable steeply dipping faults and fractures consistent with very low advective flow rates and has dispersed where vertical fractures are visible free gas released by the shift in the bsr either migrates so slowly that it remains stable beneath the ghsz or is directed upward along fractures to reform as hydrate in the ghsz there is no evidence for release of this free gas into the ocean or atmosphere

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

trapping and migration of methane associated with the gas hydrate stability zone at the blake ridge diapir new insights from seismic data abstract the blake ridge diapir is the southernmost of a line of salt diapirs along the carolina trough diapirs cause faulting of the superjacent sediments creating pathways for migration of fluids and gas to the seafloor we analyzed reflection seismic data from the blake ridge diapir which is located in a region with known abundant gas hydrate occurrence a striking feature in these data is a significant shallowing of the base of gas hydrate stability bghs over the center of the diapir the seafloor is warped up by about 100 m above the diapir from about 2300 m to about 2200 m the bghs as indicated by a bottom simulating reflection bsr is about 4 5 s off the flanks of the diapir rising to about 4 15 s at the center above the diapir a fault system appears to rise vertically from the bghs to about 0 05 s below the seafloor 40 50 m it then diverges into several steeply dipping faults that breach the seafloor and cover an area 700 m in diameter other secondary faults diverge from the main fault or emerge directly from the bghs near the crest of the diapir gas and other fluids may migrate upward through the faults we performed complex trace analysis to compare the reflection strength and instantaneous frequency along individual reflections a low frequency anomaly over the center of the diapir indicates high seismic attenuation this is interpreted to be caused by migration of fluids probably methane along fault pathways the migration of gas i e probably mainly methane through the gas hydrate stability zone is not yet understood we speculate that pore fluids in the faults may be too warm and too salty to form gas hydrate even at depths where gas hydrate is stable away from the diapir alternatively gas hydrates may seal the fault walls such that water supply is too low to transform all the gas into gas hydrates the shallowing of the bsr may reflect increased heatflow above the diapir either caused by the high thermal conductivity of the underlying salt or by advective heat transport along with fluids high pore water salinity shifts the gas hydrate stability to lower temperatures and may also play a significant role in bsr shallowing we therefore investigated the possible effect of pore water salinity on shallowing of the bsr we found that bsr shallowing may theoretically be entirely caused by increased salinity over the diapir although geologically this would not be reasonable this observation demonstrates the potential importance of pore water salinity for lateral variations of bsr depths in particular above salt structures

heat flow and thermal structure of the washington oregon accretionary prism a study of the lower slope heat flow measurements across the washington oregon convergent margin show several characteristic features a well defined peak at the foot of the overthrust a gradual decrease landward across the lower slope reaching a minimum at the ridge and a gradual rise further landward constrained by geological and seismic evidence on the rate of stacking and the dimension of stacked thrust sheets and by drilling results on sediment properties we use a two dimensional time dependent finite element procedure to model the stacking of overthrust slices of sediments and to calculate the thermal structure beneath the lower slope of the accretionary prism the calculated heat flow profile is in good agreement with the measurements advective heat transfer due to stacking of overthrust sheets and topographic changes are shown to play a dominant role in controlling the surface heat flow and the thermal state of the accretionary prism

an analysis of a seismic reflection from the base of a gas hydrate zone offshore peru seismic reflection data recorded near odp site 688 offshore peru exhibit a persistent bottom simulating reflector bsr from a depth corresponding to the theoretical base of the gas hydrate stability field although gas hydrate has been recovered in cores from scientific drilling in both the pacific and the atlantic drilling in areas exhibiting a bsr has usually been avoided because of the perceived risk of releasing free gas trapped beneath the hydrated sediments to carry out a quantitative analysis of the bsr the seismic data were reprocessed using signature deconvolution and true amplitude recovery techniques synthetic seismograms that were created using acoustic parameters extracted from the seismic data and physical properties from site 688 borehole measurement were compared to the observed seismic data to estimate the thickness of the free gas zone results indicate the bsr is discontinuous laterally where the bsr is of high amplitude free gas in a zone 5 5 17 m thick beneath the hydrated sediments provides the observed waveform where the bsr amplitude is low the free gas zone is much thinner than 5 5 m or is entirely absent

estimation of marine sediment bulk physical properties at depth from seafloor geophysical measurements a combination of geophysical techniques can yield estimates of several physical properties versus depth an example is presented for a thick sequence of sediments in middle valley near the present spreading axis of the juan de fuca ridge we have computed the porosity temperature and thermal conductivity versus depth by combining the results of seafloor electrical and heat flow surveys the electrical resistivity of the sediments is empirically related to the porosity of the medium and to the pore fluid electrical conductivity which is in turn sensitive to the temperature the temperature profile is determined from the heat flow and the thermal conductivity the latter is determined from calculated porosity and estimates of the constituent mineral and pore fluid thermal conductivities the measured surface heat flow is assumed to be and the resistivity has been determined to be approximately constant with depth the computation of the variation of temperature porosity and thermal conductivity with depth yields physically realistic and reasonable results other physical properties void ratio bulk density water content compressional wave velocity are estimated from the porosity the shear wave velocity is calculated empirically from the compressional wave velocity and poisson s ratio is then computed from the compressional and shear wave velocities the results are in good agreement with values derived for similar rapidly deposited sediment sequences elsewhere but the model calculations indicate that deep sea drilling project measurements may be biased toward higher porosity samples

an overview of the sedimentary geology of the bengal basin in relation to the regional tectonic framework and basin fill history abstract the bengal basin in the northeastern part of indian subcontinent between the indian shield and indo burman ranges comprises three geo tectonic provinces 1 the stable shelf 2 the central deep basin extending from the sylhet trough in the northeast towards the hatia trough in the south and 3 the chittagong tripura fold belt due to location of the basin at the juncture of three interacting plates viz the indian burma and tibetan eurasian plates the basin fill history of these geo tectonic provinces varied considerably precambrian metasediments and permian carboniferous rocks have been encountered only in drill holes in the stable shelf province after precambrian peneplanation of the indian shield sedimentation in the bengal basin started in isolated graben controlled basins on the basement with the breakup of gondwanaland in the jurassic and cretaceous and northward movement of the indian plate the basin started downwarping in the early cretaceous and sedimentation started on the stable shelf and deep basin and since then sedimentation has been continuous for most of the basin subsidence of the basin can be attributed to differential adjustments of the crust collision with the various elements of south asia and uplift of the eastern himalayas and the indo burman ranges movements along several well established faults were initiated following the breakup of gondwanaland and during downwarping in the cretaceous by eocene because of a major marine transgression the stable shelf came under a carbonate regime whereas the deep basinal area was dominated by deep water sedimentation a major switch in sedimentation pattern over the bengal basin occurred during the middle eocene to early miocene as a result of collision of india with the burma and tibetan blocks the influx of clastic sediment into the basin from the himalayas to the north and the indo burman ranges to the east rapidly increased at this time and this was followed by an increase in the rate of subsidence of the basin at this stage deep marine sedimentation dominated in the deep basinal part while deep to shallow marine conditions prevailed in the eastern part of the basin by middle miocene with continuing collision events between the plates and uplift in the himalayas and indo burman ranges a huge influx of clastic sediments came into the basin from the northeast and east throughout the miocene the depositional settings continued to vary from deep marine in the basin to shallow and coastal marine in the marginal parts of the basin from pliocene onwards large amounts of sediment were filling the bengal basin from the west and northwest and major delta building processes continued to develop the present day delta morphology since the cretaceous architecture of the bengal basin has been changing due to the collision pattern and movements of the major plates in the region however three notable changes in basin configuration can be recognized that occurred during early eocene middle miocene and plio pleistocene times when both the paleogeographic settings and source areas changed the present basin configuration with the ganges brahmaputra delta system on the north and the bengal deep sea fan on the south was established during the later part of pliocene and pleistocene and delta progradation since then has been strongly affected by orogeny in the eastern himalayas pleistocene glacial activities in the north accompanied sea level changes in the bay of bengal

detection and estimation of gas hydrates using rock physics and seismic inversion examples from the northern deepwater gulf of mexico natural gas hydrates composed primarily of water and methane are solid crystalline ice like substances found in permafrost areas and deepwater basins around the world as the search for oil and gas extends into ever deeper waters particularly within the northern gulf of mexico gas hydrates are becoming more of a focus in terms of both safety and as a potential energy resource

seismic and heat flow constraints from the gas hydrate system in the krishna godavari basin india abstract high resolution multi channel seismic reflection data acquired in the offshore domain of the krishna godavari kg basin provide seismic indicators of gas hydrate including bottom simulating reflectors bsr and high reflectivity zones below the bsr as an indication of free gas the recent recovery of gas hydrate by drilling across various sites in the kg basin together with the seismic indicators suggests overall widespread favourable gas hydrate stability conditions the bsrs provide indirect estimates of geothermal gradients and heat flow a simple conductive model is used to calculate heat flow from the depth of the bsr bsr derived heat flow values range from about 30 to 102 mw m2 across the kg basin we also modeled the base of gas hydrate stability zone bghsz to analyse the linkage between gas hydrate occurrences in the basin and its relation to the fault systems in the basin the influence of bottom water temperature pore water salinity and geothermal gradient variation on gas hydrate stability zone ghsz thickness is analysed in relation to both geological formations and tectonics our analysis suggests a highly variable ghsz in the kg basin controlled by local variations in the parameters of stability condition and faulting the bsr derived heat flow increases towards deeper water depths across the basin consistent local variations were also observed with low heat flow values over prominent topographic highs and high heat flow values over the flanks of the topographic highs this variation may be due to focusing and defocusing effects of the topography however local bsr depths patterns may also be due to dynamic effects including the displacement of isotherms by thrust faulting additionally recent mass wasting events and normal faulting resulted in the bghsz being too shallow relative to the predicted model which assumed equilibrium conditions

assessment of gas hydrate saturation in marine sediments from resistivity and compressional wave velocity log measurements in the mahanadi basin india abstract gas hydrate was recovered in the mahanadi basin along the eastern continental margin of india during the india national gas hydrate program nghp expedition 01 in 2006 infrared imaging of recovered core confirmed gas hydrate occurs predominantly in discrete layers pore water chemistry electrical resistivity and acoustic velocity down hole logs were used to estimate gas hydrate saturations at three of the sites in the mahanadi basin sites nghp 01 08 09 and 19 gas hydrate saturation estimated from pore water chloride concentrations shows values up to 10 of the pore space at 200 m below seafloor just above the base of the gas hydrate stability zone bghsz gas hydrate saturations estimated from electrical resistivity and acoustic velocity logs using standard relations and modeling approaches are comparable to each other and saturations are 10 15 of the pore space seismic reflection data were also analyzed for the evidence of gas hydrate and a bottom simulating reflector bsr was imaged along the seismic profiles in the study area the depth of the bsr is varying from 200 m to 300 m below seafloor depending on water depth in the mahanadi basin the occurrence of gas hydrate was observed to be associated with deep water channel and levee complexes especially at site nghp 01 19 based on the regional seismic data but the cored logged section at each site lacked any significant sand fraction which does not allow for higher gas hydrate saturations as identified from seismic time slice data all sites drilled in the mahanadi basin are within the steeper slope region of the channel system and any sand bypassed this region significant sand deposition would occur further down slope where typical fan type deposits can be inferred from the seismic data and thus higher accumulations of gas hydrate would be expected

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