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

ava analysis reveals in situ sediment diagenesis at the costa rican decollement complete sediment subduction at the costa rica subduction zone makes this convergent margin an ideal place to investigate the effects of tectonic deformation in situ we present a seismic reflection study along a line located 3 km landward of the middle american trench and oriented parallel to the strike of the decollement the ocean bottom hydrophone obh seismic data include large offsets and incidence angles at the reflectors we derive the p and s waves velocity distribution below the decollement using a p wave analysis of amplitude with reflection angle the investigation shows that there are unexpected large lateral velocity variations at a scale of only a few 100 m the shear wave velocity in the uppermost subducted sediment varies between 300 and 700 m s while the variation of the compressional wave velocity is in a range of 1700 to 2000 m s the variation of the vp vs ratio between 2 8 and 5 2 can only be explained by variations of the pore fluid pressure the modelled velocities correspond to a normalised pore fluid pressure ratio in the range between 0 02 and 0 93 the most reasonable explanation for these observations is the localised presence of fluids which are released during diagenesis by smectite to illite transformation during this process which takes place in three discrete steps the interlayer water of the smectite is added to the pore fluid and the permeability of the sediment is decreased both effects lead to the formation of small overpressured cells

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

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

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

analysis of the black sea sediments by evaluating dsdp leg 42b drilling data for gas hydrate potential abstract many gas seepages temperature pressure salinity anoxic environment and high source gas potential of the black sea indicates that the black sea might have huge potentials for biogenic and thermogenic gas hydrates however the last important parameter to consider gas hydrate as an energy source is the type of sediments coarse marine sands are considered as good hydrate reservoirs because of high porosity and high permeability only very limited data is available related to the types of lithology of the black sea sediments hence in this study the literature data especially the drilling and coring data of dsdp leg 42b program about gas seepages temperature gradient pressure gradient salinity anoxic environment and high source gas potential and the types of the sediments in the black sea were investigated and analyzed although gas seepages temperature gradient pressure gradient salinity anoxic environment and high source gas potential of the black sea are appropriate for producible gas hydrate reservoirs the sediments of the black sea appear to be generally fine grained with high clay content sandy silt and silty sand layers in turbidites of the black sea might be potential producible hydrate reservoirs but these sediments are fine as well as turbidites separate thin sand layers might be potential gas hydrate reservoirs as an energy source in the black sea

gas hydrate occurrence on the continental slope of the northern south china sea high resolution multi channel seismic data and geological samples were collected during two research cruises of the r v fendou 4 in 1999 and 2000 studies on these data and samples together with results from sites 1143 1145 and 1148 of odp leg 184 suggest that the geological structure on the continental slope of the northern south china sea is favorable for the formation of gas hydrates bottom simulating reflectors bsrs and geochemical anomalies which indicate the existence of gas hydrates have been recognized in sediments of the xisha trough the dongsha rise and the accretionary wedge of the manila subduction zone these gas hydrates are generated by two different mechanisms depending on the tectonic regime and the seismic and geochemical characteristics the first applies to the passive continental margin of the nor them south china sea on the dongsha rise and in the xisha trough the gas hydrates are associated with diapiric structures active faults slumps and gravity flows as well as high late cenozoic sedimentation rates their seismic expression includes bsrs seismic blanking zones and velocity anomalies the second mechanism is operative on the active continental margin along the manila subduction zone especially in the accretionary wedge here gas hydrate occurrence is marked by widespread bsrs and acoustic pull down possibly related to the existence of free gas in the sediments beneath the bsr the thickness of the seismic blanking zones averages 250 m suggesting that the stable gas hydrate zone has about the same thickness c 2005 elsevier ltd all rights reserved

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

evidence for gas hydrate occurrences in the canadian arctic beaufort sea within permafrost associated shelf and deep water marine environments the presence of a wedge of offshore permafrost on the shelf of the canadian beaufort sea has been previously recognized and the consequence of a prolonged occurrence of such permafrost is the possibility of an underlying gas hydrate regime we present the first evidence for wide spread occurrences of gas hydrates across the shelf in water depths of 60 100 m using 3d and 2d multichannel seismic mcs data a reflection with a polarity opposite to the seafloor was identified 1000 m below the seafloor that mimics some of the bottom simulating reflections bsrs in marine gas hydrate regimes however the reflection is not truly bottom simulating as its depth is controlled by offshore permafrost the depth of the reflection decreases with increasing water depth as predicted from thermal modeling of the late wisconsin transgression the reflection crosscuts strata and defines a zone of enhanced reflectivity beneath it which originates from free gas accumulated at the phase boundary over time as permafrost and associated gas hydrate stability zones thin in response to the transgression the wide spread gas hydrate occurrence beneath permafrost has implications on the region including drilling hazards associated with the presence of free gas possible overpressure lateral migration of fluids and expulsion at the seafloor in contrast to the permafrost associated gas hydrates a deep water marine bsr was also identified on mcs profiles the mcs data show a polarity reversed seismic reflection associated with a low velocity zone beneath it the seismic data coverage in the southern beaufort sea shows that the deep water marine bsr is not uniformly present across the entire region the regional discrepancy of the bsr occurrence between the us alaska portion and the mackenzie delta region may be a result of high sedimentation rates expected for the central mackenzie delta and high abundance of mass transport deposits that prohibit gas to accumulate within and beneath the gas hydrate stability zone

geological and morphological setting of 2778 methane seeps in the dnepr paleo delta northwestern black sea abstract the dnepr paleo delta area in the nw black sea is characterized by an abundant presence of methane seeps during the expeditions of may june 2003 and 2004 within the eu funded crimea project detailed multibeam seismic and hydro acoustic water column investigations were carried out to study the relation between the spatial distribution of methane seeps sea floor morphology and sub surface structures 2778 new methane seeps were detected on echosounding records in an area of 1540 km 2 all seeps are located in the transition zone between the continental shelf and slope in water depths of 66 to 825 m the integration of the different geophysical datasets clearly indicates that methane seeps are not randomly distributed in this area but are concentrated in specific locations the depth limit for the majority of the detected seeps is 725 m water depth which corresponds more or less with the stability limit for pure methane hydrate at the ambient bottom temperature 8 9 c in this part of the black sea this suggests that where gas hydrates are stable they play the role of buffer for the upward migration of methane gas and thus prevent seepage of methane bubbles into the water column higher up on the margin gas seeps are widespread but accurate mapping illustrates that seeps occur preferentially in association with particular morphological and sub surface features on the shelf the highest concentration of seeps is found in elongated depressions pockmarks above the margins of filled channels on the continental slope where no pockmarks have been observed seepage occurs along crests of sedimentary ridges there seepage is focussed by a parallel stratified sediment cover that thins out towards the ridge crests on the slope seepage also appears in the vicinity of canyons bottom flanks and margins or near the scarps of submarine landslides where mass wasting breaches the fine grained sediment cover that acts as a stratigraphic seal the seismic data show the presence of a distinct gas front which has been used to map the depth of the free gas within the sea floor sediments the depth of this gas front is variable and locally domes up to the sea floor where the gas front approaches the seafloor gas bubbles were detected in the water column a regional map of the sub surface depth of the gas front emphasises this gas front versus seep relationship the integration of all data sets indicates that the spatial distribution of methane seeps in the dnepr paleo delta is mainly controlled by the gas hydrate stability zone as well as by stratigraphic and sedimentary factors

the structures beneath submarine methane seeps seismic evidence from opouawe bank hikurangi margin new zealand the role of methane in the global bio geo system is one of the most important issues of present day research cold seeps where methane leaves the seafloor and enters the water column provide valuable evidence of subsurface methane paths within the new vents project we investigate cold seeps and seep structures at the hikurangi margin east of new zealand in the area of opouawe bank offshore the southern tip of the north island numerous extremely active seeps have been discovered high resolution seismic sections show a variety of seep structures we see seismic chimneys either characterised by high amplitude reflections or by acoustic turbidity and faults presumably acting as fluid pathways the bathymetric expression of the seeps also varies there are seeps exhibiting a flat seafloor as well as a seep located in a depression and small mounds the images of the 3 5 khz parasound system reveal the ear surface structure of the vent sites while highamplitude spots within the uppermost 50 m below the seafloor bsf are observed at the majority of the seep structures indicating gas hydrate and or authigenic carbonate formations with an accumulation of free gas underneath a few seep structures are characterised by the complete absence of reflections indicating a high gas content without the formation of a gas trap by hydrates or carbonates the factors controlling seep formation have been analysed with respect to seep location seep structure water depth seafloor morphology faults and gas hydrate distribution the results indicate that the revailing structural control for seep formation at opouawe bank is the presence of numerous minor faults piercing the base of the gas hydrate stability zone

quantitative interpretation for gas hydrate accumulation in the eastern green canyon area gulf of mexico using seismic inversion and rock physics transform gas hydrate can be interpreted from seismic data through observation of bottom simulating reflector bsr it is a challenge to interpret gas hydrate without bsr three dimensional qualitative and quantitative seismic interpretations were used to characterize gas hydrate distribution and concentration in the eastern green canyon area of the gulf of mexico where bsr is absent the combination of qualitative and quantitative interpretation reduces ambiguities in the estimation and identification of gas hydrate sandy deposition and faults are qualitatively interpreted from amplitude data the 3d acoustic impedance volume was interpreted in terms of high p impedance hydrate zones and low p impedance free gas zones gas hydrate saturation derived from p impedance is estimated by a rock physics transform we interpreted gas hydrate in the sand prone sediments with a maximum saturation of approximately 50 of the pore space sheet like and some bright spot gas hydrate accumulations are interpreted the interpr

mapping of elastic properties of gas hydrates in the carolina trough by waveform inversion gas hydrates are frozen methane gas that forms at appropriate pressure and temperature conditions they are found in the marine sediments along continental margins worldwide they have the economic potential of being tapped as a fuel source and also have the potential as a greenhouse agent after being freed into the atmosphere in seismic sections the occurrence of the base of gas hydrates in some areas is often marked by a bright amplitude reflection such reflections follow the sea floor topography and are called bottom simulating reflectors bsr the bsrs have negative polarity with respect to the sea floor reflection and in a common shot or a cdp gather the amplitude increases with offset the negative impedance contrast causing bsrs may be due to negative velocity contrast between hydrated sediments and normal sediment below or due to the presence of free gas at the base of the hydrates in this paper we carry out a prestack seismic waveform inversion of multichannel seismic data collected in

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

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

characterization of a stratigraphically constrained gas hydrate system along the western continental margin of svalbard from ocean bottom seismometer data the ongoing warming of bottom water in the arctic region is anticipated to destabilize some of the gas hydrate present in shallow seafloor sediment potentially causing the release of methane from dissociating hydrate into the ocean and the atmosphere ocean bottom seismometer obs experiments were conducted along the continental margin of western svalbard to quantify the amount of methane present as hydrate or gas beneath the seabed p and s wave velocities were modeled for five sites along the continental margin using ray trace forward modeling two southern sites were located in the vicinity of a 30 km long zone where methane gas bubbles escaping from the seafloor were observed during the cruise the three remaining sites were located along an e w orientated line in the north of the margin at the deepest northern site vp anomalies indicate the presence of hydrate in the sediment immediately overlying a zone containing free gas up to 100 m thick the acoustic impedance contrast between the two zones forms a bottom simulating reflector bsr at approximately 195 m below the seabed the two other sites within the gas hydrate stability zone ghsz do not show the clear presence of a bsr or of gas hydrate however anomalously low vp indicating the presence of free gas was modeled for both sites the hydrate content was estimated from vp and vs using effective medium theory at the deepest northern site modeling suggests a pore space hydrate concentration of 7 12 if hydrate forms as part of a connected framework and about 22 if it is pore filling at the two other northern sites located between the deepest site and the landward limit of the ghsz we suggest that hydrate is present in the sediment as inclusions hydrate may be present in small quantities at these two sites 4 5 of the pore space the variation in lithology for the three sites indicated by high resolution seismic profiles may control the distribution concentration and formation of hydrate and free gas

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

characterization of in situ elastic properties of gas hydrate bearing sediments on the blake ridge during ocean drilling program leg 164 shear sonic velocity and other geophysical logs were acquired in gas hydrate bearing sediments on the blake ridge to characterize the very distinct seismic signature of such formations anomalous low amplitudes overlying a strong bottom simulating reflector bsr a comparison of the bulk moduli derived from the logs to standard elastic consolidation models shows that the sediments are overconsolidated above the bsr at 440 meters below seafloor mbsf because of the presence of hydrates below the bottom of the thermodynamic hydrate stability zone at 520 mbsf the high compressibility of the formation and the attenuation of the monopole sonic waveforms are typical of sediments partially saturated with free gas between these two depths gas hydrate and free gas seem to coexist within the gas hydrate stability zone we estimate the amount of gas hydrates using different models based on theories for wave scattering in multiphase media and for grain cementation in close agreement with measurements made on discrete in situ samples the latter describes most accurately the interactions between the matrix the pore fluids and the hydrates this model indicates that 5 to 10 of the pore space is occupied by hydrates deposited uniformly on the surface of the grains the comparison with gassmann s model also show that the amount of free gas below the bsr never exceeds 5 of the pore space but is high enough to generate the bsr the coexistence of free gas and gas hydrates below the bsr may be explained by capillary effects in the smaller pores or by remaining crystalline structures after partial hydrate decomposition

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

predicted geoacoustic properties of gas hydrate saturated marine sediments natural gas hydrates are clathrates a class of compounds in which an expanded crystalline structure of water forms about a host gas molecule gas hydrates found in the marine environment are generally composed of methane combined with varying amounts of other hydrocarbon gases such as ethane and propane the occurrence of gas hydrates within the seafloor may significantly alter elastic properties of the bulk sediment resulting in the possibility of identifying hydrated sediments by acoustic remote sensing the biot poroelastic theory r d stoll geophysics 42 715 725 1977 is used to gain insight into the acoustic response of an unconsolidated marine sediment with included gas hydrates available geoacoustic parameter measurements for hydrated sediments and their constituents are of limited extent therefore physical sediment model parameters are estimated both theoretically and empirically for varying concentrations of gas hydrate saturation within the sediment these geoacoustic modeling result

compressional velocity structure and poisson s ratio in marine sediments with gas hydrate and free gas by inversion of reflected and refracted seismic data south shetland islands antarctica abstract a bottom simulating reflector bsr on the south shetland margin antarctic peninsula was surveyed within the progetto nazionale di ricerche in antartide pnra high resolution multichannel mcs reflection profiles were acquired and a three component ocean bottom seismograph obs was deployed where the bsr was particularly strong the compressional velocity field associated to the bsr was determined by the inversion of travel times of reflected mcs data and refracted obs data events information on shear wave velocities was extracted from the horizontal components of the obs records the analysis indicates the presence of velocity anomalies both above and below the bsr that are not related to the lithologic characteristics of the sedimentary column but can be associated to gas hydrate and free gas presence in pore space amplitude versus offset avo analysis was performed on common depth point gathers which correspond to the obs position the study determined reflection coefficient trends that provided qualitative information about gas hydrate and free gas abundance a theoretical model for elastic porous media was applied to quantify amounts of gas hydrate and free gas the resulting theoretical velocity curves were compared to interval velocity obtained by seismic inversion a quantitative estimation of free gas and gas hydrates trapped in the sediment pore spaces was made by interpreting velocity anomalies with respect to reference velocity curves normally consolidated marine terrigenous sediments the comparison of the results obtained by these independent methods seismic inversion avo analysis and theoretical model gives consistent values of poisson s ratio and gas hydrate and free gas estimation

gas hydrates in ocean bottom sediments gas hydrates belong to a special category of chemical substances known as inclusion compounds an inclusion compound is a physical combination of molecules in which one component becomes trapped inside the other in gas hydrates gas molecules are physically trapped inside an expanded lattice of water molecules the pressures and temperatures beneath arctic water depths greater than 1 100 ft 335 m and subtropical water depths greater than 2 000 ft 610 m are suitable for the formation of methane hydrate theoretical depths to the base of a gas hydrate layer in ocean bottom sediments are determined by assuming 1 a constant hydrostatic pressure gradient 2 two typical hydrothermal gradients 3 variable geothermal gradients and 4 pure methane hydrated with conna e seawater in addition to pressure and geothermal gradient other variables affecting the stability of gas hydrate are examined these variables are hydrothermal gradient sediment thermal conductivity heat flow hydrate velocity gas composition and connate water salinity if these variables are constant in a lateral direction and the above assumptions are valid a local geothermal gradient can be determined if the depth to the base of a gas hydrate is known the base of the gas hydrate layer is seen on seismic profiles as an anomalous reflection nearly parallel to the ocean bottom cross cutting geologic bedding plane reflections and generally increasing in sub ocean bottom time with increasing water depth the acoustic impedance is a result of the relatively fast velocity hydrate layer overlying slower velocity sediments in addition free gas may be trapped beneath the hydrate thereby enhancing the reflection

multicomponent ocean bottom cable data in gas hydrate investigation offshore of norway 1 a multicomponent seismic ocean bottom cable obc line from the northern flank of the storegga slide offshore of norway is the first obc line ever acquired over a gas hydrate related bottom simulating reflector bsr the seismic data are of very good quality and display strong p s converted wave reflections for the whole sediment column all p s reflections follow the sediment layers and are therefore interpreted to be caused by increased shear modulus and density due to sediment compaction no p s reflections are associated with the bsr along this line indicating that the gas hydrate bearing sediments at the base of the hydrate stability zone are not stiff enough to increase the shear modulus of the sediments to produce p s converted wave reflections seismic interpretation and seismic stratigraphic modeling suggest that the bsr in this area is a contrast in compressional wave velocity only mainly caused by small amounts of free gas confined to dipping sediment layers beneath the bsr gas hydrate saturation above the bsr tentatively estimated to be about 1 1 5 of the sediment pore space is most likely too low to distinguish between different models for microscale gas hydrate distribution

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

acoustic measures of partial gas saturation in tight sandstones the acoustic properties of tight sandstones are very sensitive to partial gas water saturation measurements at 5 khz show that compressional velocity drops sharply with undersaturation extensional and shear losses at 5 khz are large in both fully and partially saturated sands the shear loss qs 1 declines linearly with increasing gas content the extensional loss qe 1 peaks at 0 80 water saturation q 1 is frequency dependent the attenuation is well explained by hydrodynamic relaxation in the grain contacts the measurements indicate velocity dispersions consistent with this mechanism ultrasonic velocities at 500 khz are roughly 10 25 higher than acoustic velocities at 5 khz the ultrasonic velocities exhibit a weak dependence on saturation frame moduli are unrelaxed at these frequencies

a single channel seismic reflection method for quantifying lateral variations in bsr reflectivity abstract results of seismic inversion techniques and logs of deep sea bore holes indicate that bottom simulating reflectors bsrs which exhibit high reflection amplitudes are underlain by a thin layer of free gas often however bsrs exhibit relatively low amplitudes and display significant lateral variability in these cases the structure is not well understood and remains a topic of research waveform inversion has been used to investigate the distribution of propagation speeds in the vicinity of bsrs but the technique is not practical in some situations because it requires multi channel data sets that include large offset distances between sources and receivers such data are not available in many instances so it has become attractive to consider other methods of achieving the same end a method that is applicable to single channel short offset data is discussed here it was originally developed to help characterize shallow submarine sediments for engineering and environmental purposes of course no single channel method can provide information concerning speeds of propagation such as is available from multi channel methods in this case the single channel method has an advantage however in that it is self calibrating that allows it to provide after correction for wave front divergence true reflection amplitudes without considering source characteristics or referencing to a known or inferred propagation parameter such as speed or density these true amplitudes then yield reflection coefficients by correcting for transmission losses use of the method is illustrated with the help of synthetic data it is demonstrated that the accuracy of results is improved by using a rapid digitizing rate during data acquisition the method is then applied to a set of real data that previously had been analyzed by full waveform inversion the results are noisy largely due to the data having been digitized at a rather slow rate and the length of recording being too short however average values of reflection coefficients at the sea floor and the bsr compare well with average values obtained by the inversion procedure it is concluded that the single channel method provides reasonable values for reflection coefficients this suggests that with judicious constraints on density variations single channel data could provide information on the structure of propagation speed in the vicinity of bsrs moreover it would involve substantially less effort than is required for full waveform inversion

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

landscape resources and people during the mesolithic and neolithic times in ne iberia the arba de biel basin abstract the upper arba de biel constitutes a small valley in north eastern spain where a detailed study of five archaeological sites pena 14 legunova rambla valcervera and paco pons has been performed defining the recurrent prehistoric occupation from the late magdalenian until the chalcolithic the aim of this study focuses on exploring the main drivers of long term human persistence on a particular humid shelter located in the pre pyrenean area the combination of a multiproxy dataset evidenced by the detailed synthesis of lithic tools and pottery faunal remains pollen and charcoal assemblages has been integrated with a gis approach and a regional cultural and palaeoenvironmental contextualization after an occupation gap that coincides with the second half of the younger dryas and the first holocene millennia synchronous to other archaeological records found in ne iberia the arba de biel valley was recurrently visited by small hunter gatherer groups along the mesolithic and by herders during the neolithic these people profited this territory independently of environmental changes because the easy access to a wide spectre of economic resources flint nodules diverse vegetation supplies varied preys etc in a heterogeneous mosaic type landscape the valley main habitation spots pena 14 legunova rambla and valcervera could have been occupied at the same time by small groups that did not interfere each other the use of the fifth rockshelter paco pons seems to be related to the exploitation of copper mineral outcrops in the neolithic and for metallurgical activities during the chalcolithic these last prehistoric visits to the valley reflect a notable shift in the human employment of the shelters they cease to be living points to be used as funerary deposits

natural gas hydrates on the continental slope off pakistan constraints from seismic techniques summary in october 1997 new vertical incidence wide angle reflection and refraction data were obtained on the convergent continental margin ov pakistan the most prominent feature across the entire margin into the gulf of oman is a bottom simulating reflector bsr this reflection generally coincides with the depth predicted for the base of the gas hydrate stability field on the accretionary prism the bsr is a clear reflection with inverse polarity and strata beneath the bsr show enhanced reflectivities suggesting that gas is trapped between stratigraphic layers joint inversion of wide angle and refraction data yielded a 200 m thick low velocitiy zone at bsr depth using the born expansion band limited impedance logs were calculated from deconvolved and scaled single channel data suggesting that velocity at the bsr drops by 200 m s 1 some 40 km towards the coast cdp4400 of minshull white 1989 yielded a velocity inversion of about 600 m s 1 indicating that bsr characteristics vary significantly across the continental margin ov pakistan beneath the gulf the bsr was sampled at a similar subbottom depth where seismic bright spots occur however its reflection amplitude is unusually low and seismic stratigraphy beneath the bsr did not show enhanced reflectivities joint traveltime inversion of wide angle and refraction data revealed continuously increasing velocities suggesting that an adequate supply of methane for forming free gas and gas hydrate is mainly confined to sediments incorporated into the accretionary prism we believe that these variations are related to an evolutionary process due to tectonic uplift and ongoing sedimentation as sediments are incorporated into the accretionary prism sedimentation and uplift produce an upward migration of isotherms and therefore lead to a dissociation of gas hydrates which releases methane gas and thus causes an enhancement of the bsr our survey imaged bsr properties of the abyssal plain and within the first slope basin at water depths of 3300 m and 2490 m respectively cdp4400 was located farther landwards at a depth of 1730 m within a buried and uplifted and hence more evolved slope basin we therefore suggest that in the accretionary wedge tectonic uplift and ongoing sedimentation cause hydrate recycling by upward migration of the stability field in addition tectonic dewatering may accumulate hydrate at the base of the stability field by advection of methane from depth

combined interpretation of marine controlled source electromagnetic and reflection seismic data in the german north sea a case study this paper focuses on the combined analysis and interpretation of controlled source electromagnetic csem and multichannel reflection seismic mcs data along one profile in the german north sea with the goal to reduce ambiguities in interpretation the shallow water environment of the north sea is characterized by a complex geological development which includes rifting several ice age cycles a propagating shelf margin mass transport deposits and salt dome formation seismic and electromagnetic methods are sensitive to different physical properties of the seabed and therefore complement each other we analyse the mcs data with a migration velocity tomography and an amplitude variation with offset analysis and discuss seismic velocities and densities for true amplitude recovery the amplitude distortions are calibrated with in situ logging data the csem data are analysed in 2 d for which for the first time data were included that were acquired while the instrument was towed on the seafloor in addition to the stationary sites the csem inversions are constrained by seismic horizons the joint interpretation focuses on two seismic reflectors one can be interpreted as an unconformity marking a lithological change from fresh water bearing glacial deposits to compacted sediments below and the other one as a layer of fine grained deposits potentially capping patchy shallow gas occurrences this exemplary case study shows how the combination of both methods can benefit by interpreting complex geology and eliminating ambiguous explanations

hydrate occurrence in europe a review of available evidence abstract large national programs in the united states and several asian countries have defined and characterised their marine methane hydrate occurrences in some detail but european hydrate occurrence has received less attention the european union funded project marine gas hydrate an indigenous resource of natural gas for europe migrate aimed to determine the european potential inventory of exploitable gas hydrate to assess current technologies for their production and to evaluate the associated risks we present a synthesis of results from a migrate working group that focused on the definition and assessment of hydrate in europe our review includes the western and eastern margins of greenland the barents sea and onshore and offshore svalbard the atlantic margin of europe extending south to the northwestern margin of morocco the mediterranean sea the sea of marmara and the western and southern margins of the black sea we have not attempted to cover the high arctic the russian ukrainian and georgian sectors of the black sea or overseas territories of european nations following a formalised process we defined a range of indicators of hydrate presence based on geophysical geochemical and geological data our study was framed by the constraint of the hydrate stability field in european seas direct hydrate indicators included sampling of hydrate the presence of bottom simulating reflectors in seismic reflection profiles gas seepage into the ocean and chlorinity anomalies in sediment cores indirect indicators included geophysical survey evidence for seismic velocity and or resistivity anomalies seismic reflectivity anomalies or subsurface gas escape structures various seabed features associated with gas escape and the presence of an underlying conventional petroleum system we used these indicators to develop a database of hydrate occurrence across europe we identified a series of regions where there is substantial evidence for hydrate occurrence some areas offshore greenland offshore west svalbard the barents sea the mid norwegian margin the gulf of cadiz parts of the eastern mediterranean the sea of marmara and the black sea and regions where the evidence is more tenuous other areas offshore greenland and of the eastern mediterranean onshore svalbard offshore ireland and offshore northwest iberia we provide an overview of the evidence for hydrate occurrence in each of these regions we conclude that around europe areas with strong evidence for the presence of hydrate commonly coincide with conventional thermogenic hydrocarbon provinces

multi frequency seismic study of gas hydrate bearing sediments in lake baikal siberia abstract in this paper we present and discuss the frequency dependent behaviour of the acoustic characteristics of methane hydrate bearing sediments in lake baikal siberia five different types of seismic sources airgun array two types of single airguns watergun and sparker are used encompassing a frequency bandwidth from 10 up to 1000 hz on low frequency airgun array data the base of the hydrate stability zone hsz is observed as a high amplitude bottom simulating reflection bsr with reversed polarity the amplitude and continuity of the bsr decrease or even disappear on medium to high frequency data a feature explained in terms of vertical and horizontal resolution the increasing reflection amplitude of the bsr with increasing offset the calculated reflection coefficient of the bsr and the occurrence of enhanced reflections below the bsr suggest the presence of free gas below the hsz the observation of some enhanced reflections extending above the bsr may be interpreted as an indication for free gas co existing with hydrates within the hsz amplitude blanking above the bsr is highly variable while the bsr itself appears to act as a low pass frequency filter for medium to high frequency data new single channel airgun profiles provide the first seismic information across the baikal drilling project bdp 97 deep drilling site at which hydrate bearing sediments were retrieved at about 200 m above the base of the local hsz at the drilling site there are no seismic characteristics indicative of the presence of hydrates combination of the drilling and seismic information has allowed us to make a rough estimation of the volume of hydrates and carbon stored in the sediments of lake baikal which lead us to conclude that the lake baikal gas hydrate reservoirs do not form a prospective energy resource

assessment of gas hydrate and free gas distribution on the south shetland margin antarctica based on multichannel seismic reflection data summary processing and interpretation of a grid of intermediate resolution multichannel seismic reflection profiles collected on the ne sector of the south shetland continental margin allowed us to map the lateral extent of a bottom simulating reflector bsr the margin an accretionary wedge located off the northern tip of the antarctic peninsula consists of two distinct and superimposed tectonic regimes an older regime is related to mesozoic middle cenozoic subduction related tectonism a younger one is associated with a mainly extensional tectonic phase and related to the oligocene development of the western scotia sea the occurrence of the bsr appears to be controlled by the geological structure of the margin the bsr lacks continuity near basement structures main geological discontinuities and faults on the other hand the amplitude and continuity of the bsr are not affected by the presence of folded structures and undeformed sedimentary layering we found that the bsr is mostly confined to the ne sector of the south shetland margin where propagation of faulting associated with the shackleton fracture zone may have driven migration of natural gas towards the surface and created the conditions for a bsr to appear the application of reflection tomography techniques allowed us to reconstruct the averaged seismic velocity field between the seafloor and bsr in order to map the depth of bsr by averaging the observed velocity structure above and below the bsr and applying a theoretical model of elastic wave propagation in porous media we attempted as rigorous a quantitative assessment as possible of the natural gas present as gas hydrate above the bsr and as free gas between the bsr and the base of gas reflector bgr

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

the effect of the extent of freezing on seismic velocities in unconsolidated permafrost we develop a model to relate velocities of seismic waves in unconsolidated permafrost is idealized as an assemblage of spherical quartz grains imbedded in a matrix composed of spherical water inclusions in ice the theory of kuster and toksoz based on wave scattering considerations is used to determine the effective elastic moduli and hence the wave speeds the hasin shtrikman theoretical bounds on the elastic moduli of heterogeneous materials and considerations establish the plausibility of the model the model predicts vp and vs to be decreasing functions of both the porosity and the water to ice ratio and the ratio vp vs to be an increasing function of these two parameters the theory is then applied to laboratory measurements of shear and compressional wave velocities in 23 permafrost samples from different sites in the beaufort sea mackenzie river valley and canadian arctic islands although no direct measurements were made of the extents of freezing in these samples the data are consistent w

gas hydrate fluid flow and free gas formation of the bottom simulating reflector abstract gas hydrate in continental margins is commonly indicated by a prominent bottom simulating seismic reflector bsr that occurs a few hundred metres below the seabed the bsr marks the boundary between sediments containing gas hydrate above and free gas below most of the reflection amplitude is caused by the underlying free gas gas hydrate can occur without a bsr however and the controls on its formation are not well understood here we describe two complementary mechanisms for free gas accumulation beneath the gas hydrate stability zone ghsz the first is the well recognised hydrate recycling mechanism that generates gas from dissociating hydrate when the base of the ghsz moves upward relative to hydrate bearing sediment the second is a recently identified mechanism in which the relationship between the advection and diffusion of dissolved gas with the local solubility curve allows the liquid phase to become saturated in a thick layer beneath the ghsz when hydrate is present near its base this mechanism for gas production called the solubility curvature mechanism is possible in systems where the influence of diffusion becomes important relative to the influence of advection and where the gas water solubility decreases to a minimum several hundred metres below the ghsz we investigate a number of areas in which gas hydrate occurs to determine where gas formation is dominated by the solubility curvature mechanism and where it is dominated by hydrate recycling we show that the former is dominant in areas with low rates of upward fluid flow such as old rifted continental margins low rates of seafloor uplift and high geothermal gradient and or pressure conversely free gas formation is dominated by hydrate recycling where there are rapid rates of upward fluid flow and seabed uplift such as in subduction zone accretionary wedges using these two mechanisms to investigate the formation of free gas beneath gas hydrate in continental margins we are able to resolve a number of problems these include why sub bsr free gas zones are generally thin in convergent margins and thick in passive margins and why areas such as the gulf of mexico and mackenzie delta do not have regional bsrs finally we use our understanding of how bsrs are formed to show that regional gas hydrate is unlikely to exist in substantial quantities in passive continental margins outside areas with an observable bsr

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

estimating the thickness of the free gas zone beneath hydrate ridge oregon continental margin from seismic velocities and attenuation recent ocean drilling program odp results in the oregon accretionary prism and on the blake ridge indicate that the zone containing free gas beneath the hydrate bearing near surface sediments is considerably thicker than previously thought in this paper we present results from travel time inversion of refracted seismic waves that show very low 100 in the normal accretionary complex sediments west of hydrate ridge these results suggest that refraction seismic techniques are a powerful way to constrain the depth to which free gas is present in sediments beneath the hydrate stability zone defining the extent of the free gas zone is an important factor for estimating the total volume of gas present and for evaluating its impact on slope stability and potential contribution to global climate change

amplitude versus offset modeling of the bottom simulating reflection associated with submarine gas hydrates abstract a bottom simulating seismic reflection bsr that parallels the sea floor occurs worldwide on seismic profiles from outer continental margins the bsr coincides with the base of the gas hydrate stability field and is commonly used as indicator of natural submarine gas hydrates despite the widespread assumption that the bsr marks the base of gas hydrate bearing sediments the occurrence and importance of low velocity free gas in the sediments beneath the bsr has long been a subject of debate this paper investigates the relative abundance of hydrate and free gas associated with the bsr by modeling the reflection coefficient or amplitude variation with offset avo of the bsr at two separate sites offshore oregon and the beaufort sea the models are based on multichannel seismic profiles seismic velocity data from both sites and downhole log data from oregon odp site 892 avo studies of the bsr can determine whether free gas exists beneath the bsr if the saturation of gas hydrate above the bsr is less than approximately 30 of the pore volume gas hydrate saturation above the bsr can be roughly estimated from avo studies but the saturation of free gas beneath the bsr cannot be constrained from the seismic data alone the avo analyses at the two study locations indicate that the high amplitude bsr results primarily from free gas beneath the bsr hydrate concentrations above the bsr are calculated to be less than 10 of the pore volume for both locations studied

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

sources of hydrocarbon gases in mud volcanoes from the sorokin trough ne black sea based on molecular and carbon isotopic compositions abstract molecular and stable carbon isotope properties of hydrocarbon gases methane through pentanes and sometimes hexanes from seven sediment cores collected from five mud volcanoes mvs in the sorokin trough ne black sea suggest that these gases are initially derived from the comparable hydrocarbon pools and are likely initial products of non microbial oil cracking processes our results state that dry characteristics of gas and 13 c depleted signatures of methane are result of a high admixture of secondary microbial gas formed due to subsequent microbial anaerobic degradation of redeposited hydrocarbons in the shallow reservoirs the wet gas components in all mvs and gas hydrates are related to each other the compositional variations in c 2 content appear to result from a complex of secondary processes such oil cracking in the deep subsurface migration and mixing of resulted gaseous and liquid hydrocarbons biodegradation of possibly redeposited hydrocarbons forming shallow reservoirs and additional alterations of hydrocarbon gases in the surface sediments due to currently active microbial processes such as aom c 2 consumption etc our data show that the most unaltered gas is in the mud breccia from the kazakov mv the gas mixture possibly represents the original properties of the hydrocarbons trapped in the deep subsurface of the sorokin trough analysis of the hydrocarbon gas data complemented with published maturity characteristics of organic matter from maycopian rock clasts and mud breccia matrix implies that the original source of gases is likely to be located below the maycopian shale formation

meteorological conditions over antarctic blue ice areas and their influence on the local surface mass balance this paper addresses the causes of the prevailing meteorological conditions observed over an antarctic blue ice area and their effect on the surface mass balance over blue ice areas net accumulation is zero and ablation occurs mainly through sublimation sublimation rates are much higher than over adjacent snowfields the meteorological conditions favourable for high sublimation rates warm dry and gusty are due to the specific orographic setting of this blue ice area with usually a steep upwind mountainous slope causing strong adiabatic heating diabatic warming due to radiation and entrainment of warm air from aloft into the boundary layer augment the warming the prevailing warm dry conditions explain roughly 50 of the difference in sublimation and the different characteristics of blue ice mainly its lower albedo the other 50 most of the annual sublimation 70 takes place during the short summer mainly in daytime with winter ablation being restricted to occasional warm dry fohn like events the additional moisture is effectively removed by entrainment and horizontal advection which are maximum over the blue ice area low frequency turbulent motions induced by the upwind mountains enhance the vertical turbulent transports strong gusts and high peak wind speeds over blue ice areas cause high potential snowdrift transports which can easily remove the total precipitation thereby maintaining zero accumulation

thermal instability of gas hydrate bearing sediments some issues abstract the overall stability of marine strata holding gas hydrates is dependent on their shear strength characteristics these characteristics in turn are dependent on thermal flux that is imposed for dissociation of the hydrates for the safe and efficient extraction of methane gas from the hydrate bearing sediments due to the imposition of thermal flux on these sediments their fabric structure and pore space hydrate saturation changes which impacts the overall stability of the sea bed estimating stability conditions in such a multiphase and dynamic system necessitates collection of undisturbed samples without compromising their in situ thermodynamic conditions this is a daunting task given the huge cost of procuring samples and the challenge of maintaining an undisturbed sample with in situ thermodynamic conditions till it is brought to the laboratory synthesizing hydrate bearing sample sediments in laboratory for conducting studies to identify heat migration mechanisms and thermal property measurements and linking them to the shear strength characteristics provides an affordable solution to this problem with this in view a critical review of the available literature dealing with laboratory synthesis of hydrate bearing sediments their thermal and strength characteristics the coupled phenomenon of heat and fluid migration and its impact on the overall stability of marine sediments has been conducted and presented in this paper this will facilitate understanding the factors governing and the mechanism of heat transfer in a multiphase system the changes in the system brought about by the hydrate dissociation front and the overall impact on the stability of seabed

role of naturally occurring gas hydrates in sediment transport naturally occurring gas hydrates have the potential to store enormous volumes of both gas and water in semi solid form in ocean bottom sediments and then to release that gas and water when the hydrate s equilibrium conditions are disturbed therefore hydrates provide a potential mechanism for transporting large volumes of sediments under the combined low bottom water temperatures and moderate hydrostatic pressures that exist over most of the continental slopes and all of the continental rises and abyssal plains hydrocarbon gases at or near saturation in the interstitial waters of the near bottom sediments will form hydrates the gas can either be autochthonous microbially produced gas or allochthonous catagenic gas from deeper sediments equilibrium conditions that stabilize hydrated sediments may be disturbed for example by continued sedimentation or by lowering of sea level in either case some of the solid gas water matrix decomposes released gas and water volume exceeds the volume occupied by the hydrate so the internal pressure rises drastically if large volumes of hydrate are decomposed part of the once rigid sediment is converted to a gas and water rich relatively low density mud when the internal pressure due to the presence of the compressed gas or to buoyancy is sufficiently high the overlying sediment may be lifted and or breached and the less dense gas cut mud may break through such hydrate related phenomena can cause mud diapirs mud volcanos mud slides or turbidite flows depending on s diment configuration and bottom topography

evidence for a thick free gas layer beneath the bottom simulating reflector in the makran accretionary prism abstract seismic reflection data from the makran continental margin indicate the presence of a strong and widespread bottom simulating reflector bsr we apply a nonlinear full waveform inversion technique to multichannel reflection data from this area to investigate the detailed velocity structure and hence the origin of the bsr our result shows an abrupt decrease in the compressional wave velocity from 2 2 to 1 3 km s at a depth of 500 m below the sea bed the low velocity zone is unusually thick 200 350 m and may contain large quantities of free gas similar to some of the recently drilled blake ridge sites of odp leg 164 the voluminous free gas may have been generated by the dissociation of gas hydrates as a consequence of the upward movement of the base of gas hydrate stability field relative to the sediment column due to uplift and sedimentation in the accretionary wedge

seismic offset balancing the ability to successfully predict lithology and fluid content from reflection seismic records using avo techniques is contingent upon accurate pre analysis conditioning of the seismic data however all too often residual amplitude effects remain after the many offset dependent processing steps are completed residual amplitude effects often represent a significant error when compared to the amplitude variation with offset avo response that we are attempting to quantify we propose a model based offset dependent amplitude balancing method that attempts to correct for these residuals and other errors due to sub optimal processing seismic offset balancing attempts to quantify the relationship between the offset response of back ground seismic reflections and corresponding theoretical predictions for average lithologic interfaces thought to cause these background reflections it is assumed that any deviation from the theoretical response is a result of residual processing phenomenon and or suboptimal

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

anisotropic amplitude variation of the bottom simulating reflector beneath fracture filled gas hydrate deposit 1 for the first time we report the amplitude variation with angle ava pattern of bottom simulating reflectors bsrs beneath fracture filled gas hydrate deposits when the effective medium is anisotropic the common depth point cdp gathers of two mutually perpendicular multichannel seismic profiles located in the vicinity of site nghp 01 10 are appropriately processed such that they are fit for ava analysis ava analysis of the bsr shows normal incidence reflection coefficients of 0 04 to 0 11 with positive gradients of 0 04 to 0 31 indicating class iv pattern the acoustic properties from isotropic rock physics model predict class iii ava pattern which cannot explain the observed class iv ava pattern in krishna godavari basin due to the anisotropic nature of fracture filled gas hydrate deposits we modeled the observed class iv ava of the bsr by assuming that the gas hydrate bearing sediment can be represented by horizontally transversely isotropic hti medium after accounting for anisotropic wave propagation effects on bsr amplitudes the effective medium properties are estimated using backus averaging technique and the ava pattern of bsrs is modeled using the properties of overlying hti and underlying isotropy hti media with or without free gas anisotropic ava analysis of the bsr from the inline seismic profile shows 5 30 gas hydrate concentration equivalent to fracture density and the azimuth of fracture system fracture orientation with respect to the seismic profile is close to 45 free gas below the base of gas hydrate stability zone is interpreted in the vicinity of fault system f1

constraints on gas hydrate distribution and morphology in vestnesa ridge western svalbard margin using multicomponent ocean bottom seismic data source at https doi org 10 1029 2018jb016574 2019 american geophysical union all rights reserved

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

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

gas hydrate associated with mud diapirs in southern okinawa trough many mud diapirs have been recognized in southern okinawa trough by a multi channel seismic surveying on r v kexue i in 2001 gas hydrates have been identified by the seismic reflection characteristics the velocity analysis and the impedance inversion geothermal heat flow around the central of the mud diapir has been determined theoretically by the bottom simulating reflectors bsrs comparing the bsr derived and the measured heat flow values we infer that the bsr immediately at the top of the mud diapirs indicate the base of the saturated gas hydrate formation zone bsghfz but not as we ordinarily know the base of the gas hydrate stability zone bghsz which could be explained by the abnormal regional background heat flow and free gas flux associated with mud diapirs as a result it helps us to better understand the generation mechanism of the gas hydrates associated with mud diapirs and to predict the gas hydrate potential in the southern okinawa trough

seismic characterization of hydrates in faulted fine grained sediments of krishna godavari basin full waveform inversion in fine grained faulted sediments both stratigraphic and fault induced structural variations can simultaneously determine the gas hydrate distribution insights into hydrate distribution can be obtained from p wave velocity vp and attenuation qp 1 character of the gas hydrate stability zone ghsz in this paper we apply frequency domain full waveform inversion fwi to surface towed 2d multichannel seismic data from the krishna godavari kg basin india to image the fine scale 100 x 30 m vp and qp 1 variations within the ghsz we validate the inverted vp model by reconciling it with a sonic log from a nearby 250 m well the vp model shows a patchy distribution of hydrate away from the faults dominated parts of the profile hydrates demonstrate stratigraphic control which appears to be permeability driven the qp 1 model suggests that attenuation is relatively suppressed in hydratesbearing sediments elevated attenuation in non hydrate bearing sediments could be driven by the apparent pore fluid immiscibility at seismic wavelengths the vp and the qp 1 models also suggest that fault zones within the ghsz can be hydrate or free gas rich depending on the relative supply of free gas and water from below the ghsz

theory for the effects of free gas in subsea formations on tidal pore pressure variations and seafloor displacements loading of the seafloor by regional scale pressure variations such as those imposed by ocean tides is supported by both the rock matrix and interstitial fluid the nature of the partitioning of the support between the two depends primarily on the compressibility of the fluid and the compressibility and fluid transport properties of the rock matrix in this paper we examine theoretically the influence of free gas on pore fluid compressibility on the nature of time dependent load partitioning and on the consequent vertical rock deformation and seafloor displacement an example is the gas trapped below deep sea gas hydrate we have derived an expression for the steady state compressibility of pore fluid considering the influence of gas solubility in water the effect of gas solubility is seen to be important at low such as tidal loading frequencies and thus must be included when observations of tidally induced pore fluid pressure variations or seafloor displacements are used to constrain gas content for very low gas concentrations ng much less than 0 1 the steady state fluid compressibility can be enhanced by gas solution dissolution over the loading cycle by several factors at high ambient pressure and more than an order of magnitude at low ambient pressures 2 the fluid compressibility increases sensitively with ng and greatly affects the tidal response of the pore fluid pressure regardless of the solubility thus with careful experimental design tidally induced pore pressure variations may be used to detect very small amounts of free gas and constrain the quantity if ng 2 this method is complementary to using acoustic velocity to constrain the quantity of free gas which works well in the ng 0 2 2 range we have also given an expression for the vertical deformation of subsea formations and hence of the seafloor displacement under tidal loading the presence of free gas enhances tidally induced seafloor displacement but the maximum effect is limited by the compressibility of the matrix frame given relatively low frame compressibility tidally induced seafloor displacement is small of the order of 1 mm which is presently difficult to detect at tidal frequencies

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

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

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

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

new evidence on the earliest human presence in the urban area of genoa liguria italy a multi proxy study of a mid holocene deposit at the mouth of the bisagno river the article presents the results of a multi proxy study investigating an alluvial deposit located in the coastal plain of genoa liguria northwest italy a very deep core was sampled from 15 to 2

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

broadband seismic amplitude variation with offset inversion abstractseismic amplitude variation with offset avo inversion is well known as a popular and pragmatic tool used for the prediction of elastic parameters in the geosciences low frequencies missing from conventional seismic data are conventionally recovered from other geophysical information such as well log data for estimating the absolute rock properties which results in biased inversion results in cases of complex heterogeneous geologic targets or plays with sparse well log data such as marine or deep stratum broadband seismic data bring new opportunities to estimate the low frequency components of the elastic parameters without well log data we have developed a novel avo inversion approach with the bayesian inference for broadband seismic data the low frequency components of the elastic parameters are initially estimated with the proposed broadband avo inversion approach with the bayesian inference in the complex frequency domain because seismic inversion in the complex frequency domain is he

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

environmental history of european high mountains this volume brings together 16 papers which investigate various aspects of high mountain areas primarily in europe dietre et al investigated the influence of settlement in the silv retta alps switzerland austria festi et al combined an extensive archaeological survey and pollen analyses in the high altitudes of the otztal alps to elucidate the palaeo environmental and past cultural implications that triggered the onset and development of seasonal transhumance and alpine summer farming branch and marini analyzed radiocarbon dated palaeoecologi cal records from the upland zone of the northern apennines spanning the mid late holocene last 7000 years to detect anthropogenic impact walsh et al investigated the nature of humaneenvironment interactions in the southern ecrins french alps from the mesolithic to the post medieval period py et al investigated the effect of silverelead mining in the upper durance valley of the french southern alps using an archaeological palynological geochemical anthracological and dendrochronological studies lopez saez et al present a review of the available holocene pollen records from the spanish central system countrasts bog mont loz ere medium mountain was the subject of an interdisciplinary study by servera vives et al combining mul tiproxy analyses which includes pollen non pollen palynomorphs npp macrocharcoal particles sedimentology geochemistry archaeological and archaeobotanical data orengo et al conducted a multidisciplinary micro regional landscape research project in the madriu perafita claror valleys of the eastern pyrenees andorra martin et al identified carnivore consumption in holocene horizons of el mirador cave sierra de atapuerca spain including domestic dogs wild cats badgers and foxes garcia medrano et al analyzed the oldest acheulean lithic assemblages from the galeria site dated to ca 503 95 ka to produce a detailed technological characterization of the earliest acheulean presence in atapuerca navazo and carbonell undertook an archaeological survey in sierra de atapuerca oliva et al integrate and summarize all the studies focused on the reconstruction of the palaeoenvironmental history in the sierra nevada of spain since the last glaciation jurickova et al investigated the holocene mollusc and vertebrate fauna of the sudetes mountains zebre and stepisnik investigated pleistocene glaciations in the dinaric alps ebrahimi and seif undertook morphometric analysis using dem on the main cirque like features in the zardkuh mountains central zagros mountain range iran alexandrowicz et al conducted detailedmalacological analysis on calcareous tufa occurring in the podhale basin carpathians southern poland

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

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

geological and geophysical features of and controls on occurrence and accumulation of gas hydrates in the first offshore gas hydrate production test region in the shenhu area northern south china sea abstract integrated three dimensional seismic logging sediment cores and geochemical testing data were collected from guangzhou marine geological survey 3 and 4 hydrate drilling expeditions and used in this study for a comprehensive investigation of the geological and geophysical features and accumulation mechanism of hydrates in the first offshore gas hydrate production test region ghptr in the shenhu area of the south china sea seismic signatures indicative of disseminated hydrates and free gas include the bottom simulating reflector bsr gas chimney and mud diapir associated with enhanced seismic reflections acoustic blanking masking and chaotic appearance have been observed the acoustic travel time responses density and compensated neutron three porosity log analysis high precision grid tomography inversion analysis and constrained sparse spike inversion confirm the presence of free gas below the gas hydrate bearing zone ghbz free gas bearing zones have significantly different p wave impedances and low velocity anomalies than the overlying ghbz and surrounding strata these anomalous zones are controlled by the structural attitude of the reservoir strata which are characterized as inter bedded stratigraphic units variations in the type and geological characteristics of the hydrocarbon migration pathways were observed between sites w18 and w19 on the western ridge and sites w11 and w17 on the eastern ridge in the gmgs study area the efficiency of gas migration in the western ridge may be higher than that in the eastern ridge resulting in variations in hydrate gas types thickness of the ghbz and gas migration flux and accumulation except for site w11 hydrates were recovered below the structure i inferred bsr at sites w17 w18 and w19 the gas hydrate stability zone calculations reveal that the structure i hydrate stability zone differs from the bsr depth and is generally shallower than the base of the logging anomaly indicating the coexistence of structure i and ii hydrates the bsr is not indicative of the bghsz it is rather regarded as a transitional indicator of structure i and ii gas hydrates in the ghptr the appearance of free gas and hydrates below the structure i inferred bsr indicates that the shenhu area is characterized by a complex hydrate formation and accumulation system resulting from the supply of biogenic and thermogenic gases despite fine grained host sediments predominating the ghptr the coupling of favorable conditions including efficient hydrocarbon generation sufficient gas supply multiple pathways for gas migration and relatively high reservoir porosity have led to the development of highly saturated gas hydrate accumulations within relatively thick sedimentary sections which demonstrates a significant resource potential

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

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

absolute reflection coefficients from deep seismic reflections abstract many deep seismic reflection profiles show bright sub horizontal layered reflections in the lower continental crust accurate estimates are obtained for normal incidence reflection coefficients for the brightest of these reflections by using a marine seismic acquisition system that has been calibrated using deep water sea bottom multiples minimum bounds are placed upon these reflection coefficients by assuming minimum losses due to scattering anelastic attenuation and geometric spreading reflection coefficients of around 0 1 are obtained from the layered lower crust and around 0 15 from the moho it is argued that constructive interference between successive reflections from a layered sequence of reflectors is often unimportant consequently these coefficients can be used to place useful limits on the absolute acoustic impedance contrasts present at depth