measurements of the angle between the wind vector and wind stress vector in the surface layer over the north sea measurements of surface layer turbulence in addition to full meteorological and oceanography quantities over the north sea indicated that the stress vector on long time scales is often aligned with a direction slightly different from the mean wind flow when stratifications were near neutral the angle difference between the stress and wind exhibited a dependence on the heat and momentum fluxes in general the stress vector was observed to be to the left of the flow during stable stratifications while for unstable stratifications it was to the right this finding was consistent using two independent sets of wind stress data i e from marsen 1979 and the north sea platform winter exercise 1985

influence of recent depositional and tectonic controls on marine gas hydrates in trujillo basin peru margin abstract the development of bottom simulating reflectors bsrs and the gas hydrate stability zone ghsz in continental margins is related to recent depositional and tectonic processes this interrelation is important for understanding the potential resource of gas hydrate deposits the purpose of this study is to understand the influence of such activity on the marine gas hydrate system of the seaward extension of the trujillo basin peru margin here we analyze near seafloor heat flow probe data high resolution multichannel seismic mcs profiles swath bathymetry and ocean floor observation system ofos images based on our results we identify three main physiographical subregions in the study area 1 an area with turbidites continuous bsrs and low to moderate near seafloor heat flow 7 33 mw m 2 2 an area with sediment waves of turbidity origin a mix of continuous and patchy bsrs and moderate near seafloor heat flow 26 39 mw m 2 3 an area with extensional faulting patchy bsrs and moderate to high near seafloor heat flow 52 110 mw m 2 we conclude that sediment flow processes are the result of recent depositional controls and faulting is the result of recent tectonic activity near seafloor heat flow around chemoherms is moderate estimates of bsr derived heat flow show differences to near seafloor heat flow this difference points to advection of fluids occurring at the seafloor alive calyptogena and tube worms were observed in these venting zones based on our analysis we conclude that 1 recent depositional processes control the development of continuous bsrs and may restrict heat transfer through the ghsz in the form of fluid venting at the seafloor 2 recent tectonic processes control the development of patchy bsrs and allow heat transfer through the ghsz along faults which is manifested as fluid venting at the seafloor

joint analysis of p wave velocity and resistivity for morphology identification and quantification of gas hydrate abstract a knowledge of hydrate morphology is essential for assessing its resource potential understanding its formation and determining optimum strategies for exploitation previous studies have suggested two general morphologies of gas hydrates distributions in sediment settings pore and fracture filling however because of a lack of effective morphology identification methods it is currently difficult to quantitatively model gas hydrates here we propose a joint analysis of p wave velocity and resistivity to identify hydrate morphology and estimate hydrate saturation in a continuous depth profile first we perform numerical modeling to investigate the effects of hydrate morphology on the p wave velocity and resistivity properties of gas hydrate bearing sediments ghbs the results demonstrate that in the case of identical hydrate concentration fracture filling ghbs typically exhibit higher resistivity but lower p wave velocity than those of pore filling ghbs consequently the cross plot between these two properties are strikingly different for the two types of ghbs by comparing the cross plot of field measurements to the theoretical cross plot we hypothesize that hydrate morphology can be identified and hydrate saturations estimated we test and verify the hypothesis using the velocity and resistivity log data at different sites in china s second gas hydrate expedition in 2013 in the south china sea where the two morphologies were confirmed via core samples cross plots of field measurements agree closely with the theoretical results and gas hydrate morphologies are successfully identified the estimated hydrate saturations are comparable to measurements of pore water freshening in the majority of cases

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

preferential mode of gas invasion in sediments grain scale mechanistic model of coupled multiphase fluid flow and sediment mechanics 1 we present a discrete element model for simulating at the grain scale gas migration in brine saturated deformable media we rigorously account for the presence of two fluids in the pore space by incorporating forces on grains due to pore fluid pressures and surface tension between fluids this model which couples multiphase fluid flow with sediment mechanics permits investigation of the upward migration of gas through a brine filled sediment column we elucidate the ways in which gas migration may take place 1 by capillary invasion in a rigid like medium and 2 by initiation and propagation of a fracture we find that grain size is the main factor controlling the mode of gas transport in the sediment and we show that coarse grain sediments favor capillary invasion whereas fracturing dominates in fine grain media the results have important implications for understanding vent sites and pockmarks in the ocean floor deep subseabed storage of carbon dioxide and gas hydrate accumulations in ocean sediments and permafrost regions our results predict that in fine sediments hydrate will likely form in veins following a fracture network pattern and the hydrate concentration will likely be quite low in coarse sediments the buoyant methane gas is likely to invade the pore space more uniformly in a process akin to invasion percolation and the overall pore occupancy is likely to be much higher than for a fracture dominated regime these implications are consistent with laboratory experiments and field observations of methane hydrates in natural systems

distribution and characteristics of natural gas hydrates in the shenhu sea area south china sea abstract structure type gas composition and cage occupancy are important gas hydrate characteristics which play an important role in calculating gas hydrate stability and estimating the amount of global hydrate bound methane china conducted the fourth gas hydrate drilling campaign gmgs4 in the shenhu sea area ssa of the south china sea in 2016 during which comprehensive logging while drilling lwd data and natural gas hydrate samples were obtained the lwd data shows high electrical resistivity and high sound velocity at various depth indicating the occurrence of gas hydrate in this area coexistence of structure i si and structure ii sii gas hydrates in the south china sea was confirmed for the first time by using raman spectra analysis the methane cage occupancies of large cages and small cages in si hydrate are 0 95 and 0 96 respectively the hydration number of si hydrate is 6 03 gas composition analysis shows that methane 98 7 is the dominate gas with small amount of heavier hydrocarbon gases ethane 0 006 0 976 propane 0 033 0 216 isobutane 0 004 0 047 and other non hydrocarbon gases carbon dioxide

seismic anisotropy at hydrate ridge 1 p wave velocity increases in the presence of gas hydrates and decreases in the presence of free gas in the sediments making it an excellent means to investigate gas hydrate systems however seismic velocity is typically derived from surface seismic data without consideration of seismic anisotropy the presence of anisotropy in the hydrate bearing sediments adds an additional complexity in data analysis however anisotropy can help reveal the distribution of hydrates here we report on the evidence of seismic anisotropy at hydrate ridge along the cascadia convergent margin we find that the south summit is anisotropic while the basin side east of south summit is isotropic anisotropy is likely caused by the hydrate veins we interpret the anisotropy parameters in terms of the distribution and fabric of gas hydrates

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

convective boundary layer structure in the presence of wind following swell the marine boundary layer is known to be influenced by fast long ocean swell waves travelling away from their generation area where they were initiated by momentum transferred to the ocean wave field during storms the atmospheric boundary layer during wind following swell and various stability states has been investigated using large eddy simulation les data the dominant energy containing motions in the near neutral atmospheric boundary layer over flat terrain are known to be dominated by near ground shear induced regions of high and low speed flow wind fields and momentum fluxes from les for swell dominated situations have been used to interpret field measurements suggesting that these motions are disrupted by effects related to the underlying wave field in the presence of swell waves statistical analysis and visualization are used to further describe the effects of stratification during swell for convective boundary layer winds and fluxes a mechanism for transport of momentum to the upper levels of the boundary layer is suggested from interpretation of les data coherent detached eddies from the directly wave induced motions near the surface are found to maintain an upward momentum transfer this mechanism is found to strengthen during stronger swell conditions and also during slightly convective conditions in this way it is argued that processes related to both the wave field and surface convection can have a significant influence on the global structure of neutral and convective boundary layers during swell this has implication for the turbulence length scales during wind following swell copyright 2012 royal meteorological society

the temporal and spatial distribution of volcanism in the south china sea region cenozoic magmatism in the south china sea the associated continental margins coastal south china and indochina is reviewed volcanism contemporaneous with rifting and sea floor spreading in the south china sea was very weak in the margins and adjacent areas there was only very limited magmatic activity most probably after the cessation of sea floor spreading the south china sea has non volcanic margins large basement relief caused by extension formed major basins and a continental oceanic transitional zone in the northern margin reflects high crustal rigidity during rifting and drifting the paucity of a magma supply and presence of rigid fault blocks are unfavorable to the mantle flow model previously proposed to account for the opening of the south china sea which was supposedly linked to the escape of indochina induced by indo eurasia collision c 2005 published by elsevier ltd

effective medium modeling of gas hydrate filled fractures using the sonic log in the krishna godavari basin offshore eastern india 1 indian national gas hydrate program expedition 01 has established that clay rich marine sediment from the krishna godavari kg basin in the eastern indian margin hosts one of the richest gas hydrate deposits in the world resistivity at bit images and pressure cores reveal that the gas hydrate morphology in clay rich sediment varies from complex vein structures grain displacing to invisible pore filling existing rock physics models which relate acoustic data to in situ gas hydrate concentrations generally assume isotropic pore filling gas hydrate which yields misleading concentration estimates for fractured fine grained sediments the anisotropic kg basin sediment presents additional complications here we apply differential effective medium theory to incorporate grain displacing morphologies by which gas hydrate is included as vertical ellipsoids with aspect ratios ranging from those of thin veins up to those of nodules in an elastic anisotropic background we have estimated gas hydrate concentrations from sonic velocities at hole 10d in the kg basin considering three basic gas hydrate morphologies i pore filling ii grain displacing and iii a combination of grain displacing and pore filling average gas hydrate saturations for these three cases are 35 42 27 30 and 33 41 of the total porosity respectively in the depth range 60 140 m below seafloor mbsf saturation is highest at 67 mbsf for any morphology but the values differ between morphologies for the pore filling morphology the maximum gas hydrate saturation of 56 is 18 22 higher than the grain displacing morphology and 2 9 higher than the combined morphology estimates differ by 6 of the sediment volume with rotations of gas hydrate veins from vertical to horizontal

geochemistry origin and deep water exploration potential of natural gases in the pearl river mouth and qiongdongnan basins south china sea more than 10 gas pools have been discovered since 1983 in the shallow water region of the pearl river mouth prm basin and the qiongdongnan qdn basin offshore south china sea gases produced from the qdn basin are characterized by high contents of benzene and toluene and relatively heavy 13c2 values 25 to 27 the associated condensates have a high abundance of bicadinanes and oleanane providing a good correlation with the coal bearing sequence of the oligocene yacheng formation in the basin in contrast the gases from the prm basin contain lower amounts of benzene and toluene and lighter 13c2 values 24 to 34 widely variable concentrations of bicadinane and oleanane were identifiied from the associated condensates which can be mostly correlated with the lower oligocene enping formation source rocks formed in a swamp to shallow lake environment oil cracked gases sourced from the eocene oil prone source rock may also provide some contribution to the prm basin gases the available geochemical data indicate that both the yacheng and enping formations contain mainly type iii and ii2 kerogens with dominant gas potential regional geological information indicates that the deep water regions of the two basins share the same hydrocarbon source sags with the shallow water areas and they developed massive sandstone reservoirs during the oligocene and miocene fluid flow modeling results show that the deep water regions were on the pathway of lateral migrating gases and the interpreted reservoirs in these zones have developed abundant seismic bright spots which may reflect the presence of gas the deep water regions of the offshore south china sea are believed to have great gas exploration potential

water salt and sodium dynamics in a natraquoll in argentina abstract the influence of soil and environmental factors on water and salt dynamics in a typic natraquoll of the center of the flooding pampa argentina was studied in phreatimeters the water table reached the soil surface during floods but in the soil profile it was never above the bottom of the b31 horizon 0 65 m this horizon and the over laying b22 one showed a permanent unsaturation the a1 b1 and b21 horizons however often became saturated with water the typical floods in the region are caused by the accumulation of rainwater on the previously saturated upper horizons simultaneously phreatic water undergoes an abrupt increase in electrical conductivity ecw these saline peaks ecw of up to 45 00 ds m 1 caused the immediate salinization of the deep horizons probably by diffusion the arrival of salts at the soil surface was episodic and by convection however salt leaching prevailed in the course of time in the upper horizons the soil saturation extracts showed a balanced proportion of anions but among cations na prevailed over mg ca and k soil alkalinity was low and with some significant variations in the surface horizons that depended on episodic salt rises or salt leaching processes in the deep horizons soil remained permanently natric this dynamic balance of water and salt showed that in this part of the region the soil profile exhibits two zones which are relatively independent of each other each represents different stages of the evolution as stated by gedroiz s theory the deep horizons undergo a continuous salinization and alkalinization process the upper horizons tend to be similar to zonal soils the slight halomorphism in this profile zone is due to the episodic salt rises

recharge characteristics of an unconfined aquifer from the rainfall water table relationship the determination of recharge levels of unconfined aquifers recharged entirely by rainfall is done by developing a model for the aquifer that estimates the water table levels from the history of rainfall observations and past water table levels in the present analysis the model parameters that influence the recharge were not only assumed to be time dependent but also to have varying dependence rates for various parameters such a model is solved by the use of a recursive least squares method the variable rate parameter variation is incorporated using a random walk model from the field tests conducted at tomago sandbeds newcastle australia it was observed that the assumption of variable rates of time dependency of recharge parameters produced better estimates of water table levels compared to that with constant recharge parameters it was observed that considerable recharge due to rainfall occurred on the very same day of rainfall the increase in water table level was insignificant for subsequent days of rainfall the level of recharge very much depends upon the intensity and history of rainfall isolated rainfalls even of the order of 25 mm day 1 had no significant effect on the water table levels

geological features controlling factors and potential prospects of the gas hydrate occurrence in the east part of the pearl river mouth basin south china sea abstract logging while drilling lwd and wireline log cwl data were acquired during china s second gas hydrate drilling expedition gmgs 2 in the east of pearl river mouth basin south china sea disseminated and massive gas hydrates deposits were found at different sites gas hydrate bearing lithologies identified from the sample coring included the fine grained sediments and coarse grained sediments lwd logs from site gmgs2 08 indicate significant gas hydrate in clay bearing sediments including two layers with massive gas hydrate with a bulk density near to 1 08 g cm3 high electrical resistivities with a range of 2 5 2000 0 m and high p wave velocities are simultaneously observed in the hydrate bearing sediments the average gas hydrate saturation estimated from the pore water freshing analysis ranges from 45 to 55 of the pore space buried carbonate layers above the massive gas hydrate deposit discovered at sites gmgs2 08 indicate that the formations are likely to have formed initially at the surface and then were buried significant high amplitude seismic anomalies discontinuous bottom simulating reflection bsr and blanking zone are detected in the drilling zone the hydrate bearing sediments predominantly consist of silty clay and limestone grains in which the gas hydrates are deposited primarily in the form of laminated massive veins or nodule the gas hydrates occurrences are subjected to the sediment lithology new tectonic activities migration of fluid and gas and also the factors such as heat flow salinity and time which affect the nucleation of gas hydrates its natural morphologies present massive laminated nodular nugget and disseminated of which the former four often formed in shallow fault inter layer s weak cementation zone and on the seabed the buried gas hydrates with high saturation are good zones for gas hydrate exploitation

acoustic imaging of gas hydrate and free gas at the storegga slide 1 analysis of p wave velocities of an ocean bottom cable obc data set demonstrates the existence of gas hydrates and free gas on the northern flank of the storegga slide the distribution and concentration of gas hydrates and free gas show significant variation over the length of the obc average gas hydrate saturation of pore space is 5 when modeled by effective medium theory with hydrates as a sediment frame component average gas saturation is 0 45 assuming homogeneous distribution the bottom simulating reflector bsr is generally identified as termination of enhanced reflection but in some places appears as a reflection proper amplitude anomalies related to the bsr are primarily caused by the gas and not the hydrates the gas hydrates at the storegga slide develop from gas rich fluids migrating into the gas hydrate stability zone ghsz from the sediments below development of polygonal faults and the related expulsion of formation water might drive the fluid flow in the area the physical and geological properties of the sediments control the distribution and concentration of gas which migrates predominantly along strata rather than along the base of the ghsz this migration mechanism controls the flux of gas rich fluids into the ghsz and hence the distribution of gas hydrates and the variation in gas hydrate concentration

gas hydrate saturations estimated from fractured reservoir at site nghp 01 10 krishna godavari basin india 1 during the indian national gas hydrate program expedition 01 nghp 01 one of the richest marine gas hydrate accumulations was discovered at site nghp 01 10 in the krishna godavari basin the occurrence of concentrated gas hydrate at this site is primarily controlled by the presence of fractures assuming the resistivity of gas hydrate bearing sediments is isotropic the conventional archie analysis using the logging while drilling resistivity log yields gas hydrate saturations greater than 50 as high as 80 of the pore space for the depth interval between 25 and 160 m below seafloor on the other hand gas hydrate saturations estimated from pressure cores from nearby wells were less than 26 of the pore space although intrasite variability may contribute to the difference the primary cause of the saturation difference is attributed to the anisotropic nature of the reservoir due to gas hydrate in high angle fractures archie s law can be used to estimate gas hydrate saturations in anisotropic reservoir with additional information such as elastic velocities to constrain archie cementation parameters m and the saturation exponent n theory indicates that m and n depend on the direction of the measurement relative to fracture orientation as well as depending on gas hydrate saturation by using higher values of m and n in the resistivity analysis for fractured reservoirs the difference between saturation estimates is significantly reduced although a sizable difference remains to better understand the nature of fractured reservoirs wireline p and s wave velocities were also incorporated into the analysis

impact of swell on the marine atmospheric boundary layer a model that describes the impact of swell on the marine atmospheric boundary layer is proposed the model is based on the two layer approximation of the boundary layer the near surface inner region and the outer region above the swell induced momentum and energy fluxes are confined within the inner region swell loses energy to the atmosphere and enhances the turbulent kinetic energy in the inner region the transfer of momentum results in acceleration or deceleration of the airflow near the surface following wind swell accelerates the flow which for a very low wind results in a swell driven wind opposite wind swell decelerates the airflow which for a steep swell could cause the reverse airflow the sea drag in the case of opposite wind swell is considerably enhanced as compared with the following wind swell case cross wind swell causes the rotation of the wind velocity vector with height which leads to the deviation of the turbulent stress vector from the wind velocity vector the impact of swell becomes more pronounced when the wind speed decreases or when the swell phase velocity increases in fact it is the wave age of the swell that characterizes the swell impact because the dimensionless wave induced fluxes of energy and momentum are proportional to the wave age parameter both fluxes are also proportional to the swell slope so that the swell impact is stronger for a steeper swell the model reproduces qualitatively and quantitatively the main experimental findings for the ocean swell the impact of swell on the sea drag is very pronounced for opposite wind swell is less pronounced for cross wind swell and is only at low wind speeds

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

gas hydrate systems at hydrate ridge offshore oregon inferred from molecular and isotopic properties of hydrate bound and void gases abstract we report and discuss molecular and isotopic properties of hydrate bound gases from 55 samples and void gases from 494 samples collected during ocean drilling program odp leg 204 at hydrate ridge offshore oregon gas hydrates appear to crystallize in sediments from two end member gas sources deep allochthonous and in situ as mixtures of different proportions in an area of high gas flux at the southern summit of the ridge sites 1248 1250 shallow 0 40 m below the seafloor mbsf gas hydrates are composed of mainly allochthonous mixed microbial and thermogenic methane and a small portion of thermogenic c2 gases which migrated vertically and laterally from as deep as 2 to 2 5 km depths in contrast deep 50 105 mbsf gas hydrates at the southern summit sites 1248 and 1250 and on the flanks of the ridge sites 1244 1247 crystallize mainly from microbial methane and ethane generated dominantly in situ a small contribution of allochthonous gas may also be present at sites where geologic and tectonic settings favor focused vertical gas migration from greater depth e g sites 1244 and 1245 non hydrocarbon gases such as co2 and h2s are not abundant in sampled hydrates the new gas geochemical data are inconsistent with earlier models suggesting that seafloor gas hydrates at hydrate ridge formed from gas derived from decomposition of deeper and older gas hydrates gas hydrate formation at the southern summit is explained by a model in which gas migrated from deep sediments and perhaps was trapped by a gas hydrate seal at the base of the gas hydrate stability zone ghsz free gas migrated into the ghsz when the overpressure in gas column exceeded sealing capacity of overlaying sediments and precipitated as gas hydrate mainly within shallow sediments the mushroom like 3d shape of gas hydrate accumulation at the summit is possibly defined by the gas diffusion aureole surrounding the main migration conduit the decrease of gas solubility in shallow sediment and refocusing of gas by carbonate and gas hydrate seals near the seafloor to the crest of the local anticline structure

the biogenic gas potential of the submarine canyon systems of plio peistocene foreland basin southwestern taiwan abstract biogenic gas was accidentally discovered and produced from the plio pleistocene formation of the hsinying gas field in southwestern taiwan in 1989 a stratigraphic trapping mechanism related to the evolution of submarine canyon systems in the plio peistocene foreland basin has been proposed in a previous study which explained underestimated recoverable gas reserve before drilling to verify this shallow gas exploration hypothesis and to systematically evaluate the biogenic gas generation and entrapment potential of the submarine canyon systems seismic interpretation high resolution sequence stratigraphic interpretation seismic attribute analysis and geochemical analysis were performed and integrated in this study twenty nine submarine canyons mapped mainly trend in a ne direction except the nw trending eurchungchi submarine canyons located near the chiali paleo high bright seismic amplitudes were often observed at the incised valley heads of the canyon systems the shales located near the incised valley heads and deposited during flooding stage possess the highest biogenic gas generation potential as canyon fill reveals the second highest due to the high sediment accumulation rate in the foreland basin organic matter in such a depositional environment tends to become diluted a class iii avo anomaly inverted impedance lower than 4 7 e 6 kg m 3 m s and a b the ratio between the target horizon amplitude and the rms amplitude from the background strata greater than 1 78 were identified as valid direct gas indicators as sand is buried shallower than 1000 m class iv avo anomaly and a b greater than 1 4 were concluded to be the indicators of gas sand in the case that sand is buried deeper than 1600 m based on the results of sequence stratigraphic interpretation and the consistency between spatial geometries of seismic attributes and those of the submarine canyons a stratigraphic trap associated with the incised valley heads was concluded to be the original gas entrapment style of the hsinying and the kuantian gas fields biogenic gas migrated after being trapped startigraphically hence contributing to the present day gas field structure due to the prevalent erosional features of the submarine canyons on the time structural maps different types of stratigraphic traps formed in combination with faults and submarine canyons can be recognized easily

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

gas hydrate and associated free gas in the dongsha area of northern south china sea abstract a large ongoing cold seep named jiulong methane reef was discovered in the dongsha area of northern south china sea scs in 2004 to understand links among seafloor methane seepage subsurface gas hydrate origin of gas source and structural styles controlling the formation and accumulation of gas hydrate high resolution multi channel seismic data were interpreted in this region bottom simulating reflectors bsrs known as the base of gas hydrate stability zone were identified and mapped additionally seismic attributes interval velocity model and acoustic p wave impedance were obtained to better delineate the spatial distribution of gas hydrate hydrate bearing sediments are characterized by weak reflection i e blanking zone above bsrs high velocities and relatively small acoustic impedance differences while enhanced reflection low frequency and low velocities beneath bsrs suggest the presence of underlying free gas beneath solid hydrate a number of faults mud diapirs and a possible submarine landslide were observed in seismic profiles both faults and mud diapirs are useful conduits allowing methane gas to migrate upward and linking free gas zones beneath bsrs hydrate zones and seafloor methane seepage a submarine landslide with a length scale of 10 km is observed in a seismic profile the landslide may trigger dissociation of gas hydrate accounting for the occurrence of jiulong methane seepage seismic observations and estimated heat flow coupled with geochemical analyses suggest that gas source of methane in this region probably originates from mixture of biogenic and thermogenic methane this study would not only help to understand gas hydrate system in the dongsha area but also provide insights to geological hazards associated with the dissociation of gas hydrate

a case study of air sea interaction during swell conditions air sea interaction data from a situation with pronounced unidirectional swell have been analyzed measurements of turbulence at three levels 10 18 and 26 m above mean sea level together with directional wave buoy data from the site ostergarnsholm in the baltic sea were used the situation which lasted for 48 hours appeared in the aftermath of a gale the wind direction during the swell situation turned slowly within a 90 sector both during the gale phase and the swell phase the over water fetch was 150 km the wind speed during the swell phase was typically 4 m s 1 during the swell phase a wind maximum near or below the lowest wind speed measuring level 10 m was observed the net momentum flux was very small resulting in cd values 0 7 10 3 throughout the lowest 26 m covered by the tower measurements turbulence intensities in all three components remained high despite the low value of the kinematic momentum flux u w resulting in a reduction of the correlation coefficient for the longitudinal and vertical velocity from its typical value around 0 35 to between 0 2 and 0 and with some positive values at the higher measuring levels appearing abruptly at wave age c0 u10 equal to 1 2 turbulence spectra of the horizontal components were shown not to scale with height above the water surface in contrast to vertical velocity spectra for which such a variation was observed in the low frequency range in addition spectral peaks in the horizontal wind spectra were found at a frequency as low as 10 3 hz from a comparison with results from a previous study it was concluded that this turbulence is of the inactive kind being brought down from the upper parts of the boundary layer by pressure transport

review of wave turbulence interactions in the stable atmospheric boundary layer flow in a stably stratified environment is characterized by anisotropic and intermittent turbulence and wavelike motions of varying amplitudes and periods understanding turbulence intermittency and wave turbulence interactions in a stably stratified flow remains a challenging issue in geosciences including planetary atmospheres and oceans the stable atmospheric boundary layer sabl commonly occurs when the ground surface is cooled by longwave radiation emission such as at night over land surfaces or even daytime over snow and ice surfaces and when warm air is advected over cold surfaces intermittent turbulence intensification in the sabl impacts human activities and weather variability yet it cannot be generated in state of the art numerical forecast models this failure is mainly due to a lack of understanding of the physical mechanisms for seemingly random turbulence generation in a stably stratified flow in which wave turbulence interaction is a potential mechanism for turbulence intermittency a workshop on wave turbulence interactions in the sabl addressed the current understanding and challenges of wave turbulence interactions and the role of wavelike motions in contributing to anisotropic and intermittent turbulence from the perspectives of theory observations and numerical parameterization there have been a number of reviews on waves and a few on turbulence in stably stratified flows but not much on wave turbulence interactions this review focuses on the nocturnal sabl however the discussions here on intermittent turbulence and wave turbulence interactions in stably stratified flows underscore important issues in stably stratified geophysical dynamics in general

electrical anisotropy due to gas hydrate filled fractures in 2006 the indian national gas hydrate program expedition01 ornghp 01 discoveredgashydrateasfillinnearvertical fractures in unconsolidated sediments at several drillingsitesontheindiancontinentalmargins thesegashydrate filled fractures were identified on logging while drilling resistivity images the gas hydrate filled fracture intervals coincide with high measured resistivity at the nghp 01 sites high measured resistivity translates into high hydrate saturationsviaarchie sequation however thesehighsaturations contradict lower gas hydrate saturations determined from pressure core and chlorinity measurements also in intervalswithnear verticalgashydrate filledfractures thereis considerable separation between phase shift and attenuation resistivity logs with 2 mhz resistivity measurements being significantly higher than 400 khz resistivity measurements we modeled the sensitivity of the propagation resistivity measurements in the gas hydrate filled fracture intervals at nghp 01 sites 5 and 10 near vertical hydrate filled fracturescancausetheabnormallyhighresistivitymeasurements in vertical holes due to electrical anisotropy the model suggests the gas hydrate saturations in situ are usually significantlylowerthanthosecalculatedfromarchie sequation in addition these modeled gas hydrate saturations generally agree with the lower gas hydrate saturations obtained from pressure core and chlorinity measurements at nghp 01 sites5and10

dynamic accumulation of gas hydrates associated with the channel levee system in the shenhu area northern south china sea abstract research on the formation and distribution of submarine channel systems and associated gas bearing fluids is of great significance for gas hydrate exploration disseminated gas hydrates with high saturation up to 65 were recovered from a submarine ridge equivalent to the levee of the channel levee system in the shenhu area northern south china sea sedimentary deposits in the submarine ridge were dominated by fine grained silt and clay rich silt gas hydrates with relatively high saturation preferentially accumulated in coarser sediments with less clay content although abundant foraminifera fossils may have increased reservoir pore space their presence was not a necessary condition for high saturation hydrates higher levels of pyrite appeared in the reservoirs corresponding to high saturation hydrates which suggests that the reducing environment caused by sufficient methane provided adequate gas to form higher saturation hydrates because of the migration of the channel levee system different channels formed their respective depositional systems composed of channel filling buried channel filling erosion grooves and slumped turbidities relatively coarse grained deposits were identified in the channel fillings and levees and the accumulation of hydrates was affected by the lithological features of the sediments and their spatial coupling with the gas hydrate stability zone ghsz ghsz modeling based on in situ measurements indicated that erosion and sedimentation as well as variations of the geothermal gradient resulted in the upward downward migration of bottom simulating reflectors bsrs on the erosion flank of the channel the strata thinned and rapid erosion was likely to destroy the shallower bsr causing gas hydrate decomposition and methane release and may have caused turbidite slumping and seepage whereas the strata thickened on the deposition flank of the channel the bsr in the channel levee system would gradually move toward the new ghsz eventually forming a new bsr parts of the bsr that formed under the original p t conditions have remained and double bsrs occurred in the seismic profile the thermal fluid that moved upward through a gas chimney may also have caused the migration of the ghsz resulting in the emergence of double bsrs during the lateral migration of the channel and the vertical migration of the gas bearing fluid there was a dynamic adjustment relationship between the ghsz and the erosion deposition process of the channel resulting in the dynamic accumulation of hydrates in the shenhu area a model to demonstrate the relationship between channel migration and variation of the bsr was established which is of great significance for understanding the formation and accumulation mechanisms of gas hydrates

formation and evolution of the ionospheric plasma density shoulder and its relationship to the superfountain effects investigated during the 6 november 2001 great storm this study investigates the 6 november 2001 great storm s impact on the topside ionosphere utilizing data from the onboard topex poseidon nasa altimeter defense meteorological satellite program special sensor ions electrons and scintillation instruments and ace interplanetary observatory a set of field aligned profiles demonstrate the storm evolution caused by the precursor and promptly penetrating interplanetary eastward electric e fields and strong equatorward winds reducing chemical loss during the long duration negative bz events at daytime evening the forward fountain experienced repeated strengthening as the net eastward e field suddenly increased the resultant symmetrical equatorial anomaly exhibited a continuous increase while the energy inputs at both auroral regions were similar in both hemispheres by progressing poleward a midlatitude shoulder exhibiting increased plasma densities a plasma density dropoff steep gradient and a plasma depletion appeared these features were maintained while the reverse fountain operated at the dropoff elevated temperatures indicated the plasmapause consequently the plasma depletion was the signature of plasmaspheric erosion in each hemisphere an isolated plasma flow supplying the minimum plasma was detected at the shoulder plasmaspheric compression due to the enhanced e fields could trigger this plasma flow exhibiting strong longitudinal variation at evening nighttime the shoulder increased 306 over the southeastern pacific where the nighttime weddell sea anomaly wsa appeared before the storm there the shoulder indicated the storm enhanced equatorward section of the quiet time wsa owing to the substantial equatorward plasmapause movement a larger poleward section of the quiet time wsa eroded away leaving a large depletion behind this study reports first these northern southern plasma flows and dramatic storm effects on a nighttime wsa

characteristics and interpretation of fracture filled gas hydrate an example from the ulleung basin east sea of korea abstract through the use of 2 d and 3 d seismic data a total of thirteen sites were selected and drilled in the east sea of korea in 2010 a suite of logging while drilling lwd logs was acquired at each site lwd logs from the ubgh2 3a well indicate significant gas hydrate in clay bearing sediments including several zones with massive gas hydrate with a bulk density less than 1 0 g m 3 for depths between 5 and 103 m below the sea floor the ubgh2 3a well was drilled on a seismically identified chimney structure with a mound feature at the sea floor average gas hydrate saturations estimated from the isotropic analysis of ring resistivity and p wave velocity logs are 80 13 and 47 16 respectively whereas they are 46 17 and 45 16 respectively from the anisotropic analysis modeling indicates that the upper part of chimney between 5 and 45 m below sea floor mbsf is characterized by gas hydrate filling near horizontal fractures 7 dip and the lower part of chimney between 45 and 103 mbsf is characterized by gas hydrate filling high angle fractures on the basis of ring resistivity and p wave velocity the anisotropic analysis using p40h resistivity phase shift resistivity at 32 mhz with 40 inch spacing and the p wave velocity yields a gas hydrate saturation of 46 15 and 46 15 respectively similar to those estimated using ring resistivity and p wave velocity but with quite different fracture dip angles differences in vertical resolution depth of investigation and a finite fracture dimension relative to the tool separation appear to contribute to this discrepancy forward modeling of anisotropic resistivity and velocity are essential to identify gas hydrate in fractures and to estimate accurate gas hydrate amounts

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

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

estimating pore space gas hydrate saturations from well log acoustic data relating pore space gas hydrate saturation to sonic velocity data is important for remotely estimating gas hydrate concentration in sediment in the present study sonic velocities of gas hydrate bearing sands are modeled using a three phase biot type theory in which sand gas hydrate and pore fluid form three homogeneous interwoven frameworks this theory is developed using well log compressional and shear wave velocity data from the mallik 5l 38 permafrost gas hydrate research well in canada and applied to well log data from hydrate bearing sands in the alaskan permafrost gulf of mexico and northern cascadia margin velocity based gas hydrate saturation estimates are in good agreement with nuclear magneto resonance and resistivity log estimates over the complete range of observed gas hydrate saturations

effect of gas hydrates melting on seafloor slope instability we present a theoretical study of the thermodynamic chemical equilibrium of gas hydrate in soil by taking into account the influence of temperature pressure pore water chemistry and the mean pore size distribution the model uses a new formulation based on the enthalpy form of the law of conservation of energy the developed model shows that due to a temperature and pressure increase hydrates may dissociate at the top of the hydrate occurrence zone to ensure a chemical equilibrium with the surrounding bulk water this original result confirms what has been already shown through experiments the second part of the paper presents an application of the model through a back analysis of the giant storegga slide on the norwegian margin two of the most important changes during and since the last deglaciation hydrostatic pressure due to the change of the sea level and the increase of the sea water temperature were considered in the calculation simulation results show that melting of gas hydrate due to the change of the gas solubility can be at the origin of a retrogressive failure initiated at the lower part of the storegga slope once again the developed model leads to predictions which are supported by laboratory experiment results but contradictory to previous interpretations and beliefs considering that hydrate dissociation occurs only at the bottom of the gas hydrate stability zone

comparison of water table fluctuation and chloride mass balance methods for recharge estimation in a tropical rainforest climate a case study from kelantan river catchment malaysia estimated groundwater recharge for selected locations in northeast peninsular malaysia north kelantan river catchment was determined using the water table fluctuation wtf and chloride mass balance cmb methods the wtf method recharge estimates were compared to the cmb method estimates to see if results are similar and the methods can be applied in a humid tropical region such as malaysia effective specific yields of 0 18 and 0 16 were obtained for sites which are much lower than values in the literature for use with the wtf method the wtf gives mean recharge values of 447 and 319 mm year at pc61 and wakaf bharu wb respectively the difference between sites may be attributed to the difference in lithology whereby less water table fluctuation is observed for finer grained lower permeability soil or sediment and higher water table fluctuation observed for more granular larger grain soil or sediment for the site with lower permeability the time lag between the storm events and water level rise in a well is longer and water does not reach the water table for small storms recharge from chloride profile study was estimated through measurement of enrichment of chloride concentration in soil rain fed groundwater direct recharge ranged from 263 3 to 627 7 mm year in wb and averaged 691 8 mm year for pengkalan chepa recharge estimates from unsaturated zone chloride strongly correlate with soil texture generally and is greater in the coastal area due to a higher percentage of sand content parts of the study area located toward the interior of the peninsula showed lower recharge due to the presence of fine grained sediment comparison of methods from different sites indicates that lithologic setting has a significant role in controlling natural recharge rate the inconsistency observed between recharge value estimated by the wtf and cmb method may in part be due to adsorption desorption process and ion exchange in the unsaturated soil which affects the initial assumption that chloride is a conservative tracer

evaluation of groundwater replenishment coefficients from the record of a borehole penetrating the unsaturated zone the coefficients of groundwater replenishment from surficial sources are the most critical parameters in assessing the exploitable yield of an aquifer so far the methods used for estimating these coefficients failed to yield plausible results either because of crude models of the system or because of a simultaneous estimation of many parameters requiring long and diverse data records this paper proposes a method for obtaining the in situ chronological sequence of the coefficients of replenishment at a given site independently from the estimates of other parameters it is based on the profiles of water content and concentrations of tritium and chloride in a single borehole penetrating the unsaturated zone a combination of these data with the historical record of their inputs at the surface of the ground enables dating of the moisture content in each depth interval this yields a series of seasonal replenishment coefficients from both natural and anthropogenic sources the application of the method is illustrated at two sites in the central and southern parts of israel

wave induced wind in the marine boundary layer recent field observations and large eddy simulations have shown that the impact of fast swell on the marine atmospheric boundary layer mabl might be stronger than previously assumed for low to moderate winds blowing in the same direction as the waves swell propagates faster than the mean wind the momentum flux above the sea surface will then have two major components the turbulent shear stress directed downward and the swell induced stress directed upward for sufficiently high wave age values the wave induced component becomes increasingly dominant and the total momentum flux will be directed into the atmosphere recent field measurements have shown that this upward momentum transfer from the ocean into the atmosphere has a considerable impact on the surface layer flow dynamics and on the turbulence structure of the overall mabl the vertical wind profile will no longer exhibit a logarithmic shape because an acceleration of the airflow near the surface will take place generating a low level wave driven wind maximum a wind jet as waves propagate away from their generation area as swell some of the wave momentum will be returned to the atmosphere in the form of wave driven winds a model that qualitatively reproduces the wave following atmospheric flow and the wave generated wind maximum as seen from measurements is proposed the model assumes a stationary momentum and turbulent kinetic energy balance and uses the dampening of the waves at the surface to describe the momentum flux from the waves to the atmosphere in this study simultaneous observations of wind profiles turbulent fluxes and wave spectra during swell events are presented and compared with the model in the absence of an established model for the linear damping ratio during swell conditions the model is combined with observations to estimate the wave damping for the cases in which the observations showed a pronounced swell signal and almost no wind waves the agreement between observed and modeled wind profiles is remarkably good the resulting attenuation length is found to be relatively short which suggests that the estimated damping ratios are too large the authors attribute this at least partly to processes not accounted for by the model such as the existence of an atmospheric background wind in the model this extra momentum must be supplied by the waves in terms of a larger damping ratio

winds near the surface of waves observations and modeling abstractthe concept of a constant flux layer is usually employed for vertical profiling of the wind measured at some elevation near the ocean surface the surface waves however modify the balance

jiulong methane reef microbial mediation of seep carbonates in the south china sea chemoherm carbonates as well as numerous other types of methane seep carbonates were discovered in 2004 along the passive margin of the northern south china sea lithologically the carbonates are micritic containing peloids clasts and clam fragments some are highly brecciated with aragonite layers of varying thicknesses lining fractures and voids dissolution and replacement is common mineralogically the carbonates are dominated by high magnesium calcites hmc and aragonite some hmcs with mgco3 contents of between 30 38 mol extreme hmc occur in association with minor amounts of dolomite all of the carbonates are strongly depleted in 13c with a range from 35 7 to 57 5 pdb and enriched in 18o 4 0 to 5 3 pdb abundant microbial rods and filaments were recognized within the carbonate matrix as well as aragonite cements likely fossils of chemosynthetic microbes involved in carbonate formation the microbial structures are intimately associated with mineral grains some carbonate mineral grains resemble microbes the isotope characteristics the fabrics the microbial structure and the mineralogies are diagnostic of carbonates derived from anaerobic oxidation of methane mediated by microbes from the succession of hmcs extreme hmc and dolomite in layered tubular carbonates combined with the presence of microbial structure and diagenetic fabric we suggest that extreme hmc may eventually transform into dolomites our results add to the worldwide record of seep carbonates and establish for the first time the exact locations and seafloor morphology where such carbonates formed in the south china sea characteristics of the complex fabric demonstrate how seep carbonates may be used as archives recording multiple fluid regimes dissolution and early transformation events

gas hydrate identified in sand rich inferred sedimentary section using downhole logging and seismic data in shenhu area south china sea downhole wireline log dwl data was acquired from eight drill sites during china s first gas hydrate drilling expedition gmgs 1 in 2007 initial analyses of the acquired well log data suggested that there were no significant gas hydrate occurrences at site sh4 however the re examination of the owl data from site sh4 indicated that there are two intervals of high resistivity which could be indicative of gas hydrate one interval of high resistivity at depth of 171 175 m below seafloor mbsf is associated with a high compressional wave p wave velocities and low gamma ray log values which suggests the presence of gas hydrate in a potentially sand rich low clay content sedimentary section the second high resistivity interval at depth of 175 180 mbsf is associated with low p wave velocities and low gamma values which suggests the presence of free gas in a potentially sand rich low clay content sedimentary section because the occurrence of free gas is much shallower than the expected from the regional depth of the bottom simulating reflector bsr the free gas could be from the dissociation of gas hydrate during drilling or there may be a local anomaly in the depth to the base of the gas hydrate stability zone in order to determine whether the low p wave velocity with high resistivity is caused by in situ free gas or dissociated free gas from the gas hydrate the surface seismic data were also used in this analysis the log analysis incorporating the surface seismic data through the construction of synthetic seismograms using various models indicated the presence of free gas directly in contact with an overlying gas hydrate bearing section the occurrence of the anomalous base of gas hydrate stability at site sh4 could be caused by a local heat flow conditions this paper documents the first observation of gas hydrate in what is believed to be a sand rich sediment in shenhu area of the south china sea c 2014 elsevier ltd all rights reserved

estimating aquifer recharge due to rainfall abstract an existing dynamic model of the annual water balance is modified to account for two phenomena peculiar to arid climates i e variable rainy season length and soil dessication during the dry season model parameter values are estimated for two locations in the central zone of saudi arabia and the cumulative distribution function of annual recharge from precipitation is found for two of the primary sedimentary aquifers outcripping in this zone results quantify the large uncertainty of annual recharge in such climates and confirm the accepted notion that these aquifers are being mined

gas hydrates associated with gas chimneys in fine grained sediments of the northern south china sea drilling for gas hydrates at the northern continental margin of the south china sea scs provides a unique insight into the formation of the gas hydrate system in fine grained sediments even though most high concentrations of hydrates are found in coarse sediments detailed studies of 3d seismic data from a deep water basin baiyun depression of this region reveal the presence of overpressured fluid flow mostly manifested in gas chimneys gas chimneys are commonly characterized by disrupted reflections dr dim amplitude anomalies da and enhanced reflections er on conventional seismic profiles gas chimneys are also characterized by chaotic low continuity and low frequency on instantaneous frequency and instantaneous amplitude profiles gas hydrates and a prominent bsr were discovered in the region of gas chimney occurrence logging data and sample analyses from drill holes over the chimney structures indicate close relationships between active fluid flow and the formation and accumulation of gas hydrates our results document that gas chimneys consisting of a connected network of fractures provide a passageway along which fluids ascend beneath the gas hydrate stability zone ghsz for hydrate formation in fine grained sediments of the northern south china sea fluids accumulate in uppermost part of the slope where they are trapped beneath the prograding submarine delta sequence which is not permeable enough for fluids to migrate through gas charged fluids may originate from deep seated hydrocarbon reservoirs which are indicated by the molecular and isotopic signatures of gases in gas hydrates occurrence zone we suppose that the fractures extend into the ghsz creating a space for hydrate condensation in the fine grained sediments of the slope gas chimneys may account for the overpressured fluid activity and most likely the gas hydrate enrichment in the northern scs c 2012 elsevier b v all rights reserved

estimating infiltration recharge using a response function model abstract rainfall infiltration influences both the quantity and quality of groundwater systems the knowledge of the process of infiltration recharge is of great importance to the management of groundwater systems and the hydraulically connected streams in this study a response function model is developed to estimate soil water flux at the water table or the process of infiltration recharge from rainfall and evaporation data the gamma probability density function is modified to represent the travel time distribution of infiltrating water by rainfall events the travel time distribution is used to determine the recharge process according to the effective infiltration of a rainfall event the effective infiltration is the amount of soil water pushed down into groundwater by infiltrating water of a rainfall event and is evaluated based on the vadose zone soil water balance superposition is employed to compute the recharge rate produced by a sequence of rainfall events estimations of soil water flux at different groundwater depths are compared with lysimetric observations in the field good agreements between the estimated and observed results confirm the reliability of the response function model for predicting infiltration recharge processes

observed directional characteristics of the wind wind stress and surface waves on the open ocean sonic anemometer data were taken during the surface waves processes program swapp in march 1990 in the north pacific measurements of the wind stress vector span several strong wind events significant angles between the wind stress vector and the mean wind vector are seen simultaneous measurements of the directional wave field were made with a surface scanning doppler sonar the data suggest that the wind stress direction is influenced by the direction of the surface waves especially for stronger winds overall the stress vector lies between the mean wind and the mean wave directions at the higher wind speeds over 8 6 m s there is a nonzero correlation between the variations in wave directions and stress directions as well finally the stress and wave component directions have similar frequency dependence over the frequency band where wave energy is nonnegligible suggesting a dynamic link

air sea fluxes and surface layer turbulence around a sea surface temperature front the observed effects of sharp changes in sea surface temperature sst on the air sea fluxes surface roughness and the turbulence structure in the surface layer and the marine atmospheric boundary layer are discussed in situ flux and turbulence observations were carried out from three aircraft and two ships within the fasinex framework three other aircraft used remote sensors to measure waves microwave backscatter and lidar signatures of cloud tops descriptions of the techniques intercomparison of aircraft and ship flux data and use of different methods for analyzing the fluxes from the aircraft data are described changing synoptic weather on three successive days yielded cases of wind direction both approximately parallel and perpendicular to a surface temperature front for the wind perpendicular to the front wind over both cold to warm and warm to cold surface temperatures occurred model results consistent with the observations suggest that an internal boundary layer forms at the sst

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

analysis of rainfall recharge relationships abstract understanding the recharge process and its relationship with rainfall is of critical importance to the management of groundwater systems and enormous effort has been made to estimate the amount and process of recharge by infiltration using precipitation data in the present research in situ lysimeter experiments and numerical simulations were used to study the relationships between rainfall and recharge by infiltration at different groundwater depths four lysimeters 1 5 3 4 5 and 5 m deep were installed in the field to measure the infiltration recharge rates under conditions of fixed water tables at the bottoms of the soil columns annual infiltration recharges at different groundwater depths were obtained through the lysimeter measurements the effect of rainfall pattern and annual rainfall distribution on infiltration recharge was simulated at various groundwater depths using a numerical model based on soil water dynamics and comparisons of the simulated and observed recharges showed very good agreement relationships between rainfall and infiltration recharge at different groundwater depths were investigated using the lysimeter measurements and computer simulations groundwater systems were classified into three categories i e shallow water table intermediate water table and deep water table based on the relationship between rainfall and infiltration recharge at different groundwater depths

identification of gas hydrate dissociation from wireline log data in the shenhu area south china sea wireline logs were acquired in eight wells during china s first etas hydrate drilling expedition gmgs 1 in april june of 2007 well logs obtained from site sh3 indicated gas hydrate was present in the depth range of 195 206 in below seafloor with a maximum pore space gas hydrate saturation calculated from pore water freshening of about 26 assuming gas hydrate is uniformly distributed in the sediments resistivity calculations using archie s equation yielded hydrate saturation trends similar to those from chloride concentrations however the measured compressional p wave velocities decreased sharply at the depth between 194 and 199 mbsf dropping as low as 1 3 km s indicating the presence of free gas in the pore space possibly caused by the dissociation of gas hydrate during drilling because surface seismic data acquired prior to drilling were not influenced by the in situ gas hydrate dissociation surface seismic data could be used to identify the cause of the low p wave velocity observed in the well log to determine whether the low well log p wave velocity was caused by in situ free gas or by gas hydrate dissociation synthetic seismograms were generated using the measured well log p wave velocity along with velocities calculated assuming both gas hydrate and free gas in the pore space comparing the surface seismic data with various synthetic seismograms suggested that low p wave velocities were likely caused by the dissociation of in situ gas hydrate during drilling

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

escape of methane gas through sediment waves in a large methane hydrate province despite paleoceanographic evidence that large quantities of methane have escaped from marine gas hydrates into the oceans the sites and mechanisms of methane release remain largely speculative new seismic data from the blake ridge a hydrate bearing drift deposit in the western atlantic show clear evidence for methane release and suggest a new mechanism by which methane gas can escape without thermal or mechanical disruption of the hydrate bearing layer rapid post 2 5 ma formation of large sediment waves and associated seafloor erosion created permeable pathways connecting free gas to the seafloor allowing methane gas expulsion the amount of missing methane 0 6 gt is equivalent to 12 of total present day atmospheric methane our results imply that significant amounts of methane gas can bypass the hydrate stability zone and escape into the ocean mechanisms of tapping methane directly from the free gas zone such as widespread seafloor erosion should be considered when seeking the causes of large negative carbon isotope excursions in the geological record

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

porewater geochemical and lipid biomarker signatures for anaerobic methane oxidation in the hydrate bearing system from the taixinan basin the south china sea abstract anaerobic oxidation of methane aom is one of the most important biogeochemical processes associated with hydrate dissociation and carbon cycling though aom has been extensively investigated the variations of its microorganisms and lipid biomarkers are still not fully understood in this study the porewater geochemical and lipid biomarker evidence for aom was systematically investigated in the hydrate bearing system from the taixinan basin the south china sea aiming to better expound the vertical variations of aom related biomarker signatures and causes of the weak lipid biomarker patterns accordingly the sulfate methane interface smi depth of 1 000 cmbsf and onset of aom were determined on the basis of the vertical profiles of porewater parameters the occurrence of 13c depleted 2 6 10 15 19 pentamethylicosane pmi with the 13c ratios of 77 7 to 131 4 near the smi are in accord with the porewater parameters despite the absence of archaeol and sn2 hydroxyarchaeol and low abundances of pmi meanwhile the strongly 13c enriched iso c15 0 and anteiso c15 0 fatty acids suggest that non methane hydrocarbons or oil derivatives are speculated as the probable carbon sources for sulfate reducing bacteria due to the presence of unresolved complex mixtures ucms and odd even predominance oep values about 1 0 the weak in situ signal of aom related biomarkers may be attributed to the low flux methane supply and dilution effect from the high input of allochthonous and autochthonous organic matters the great difference in biomarkers between the taixinan basin and previously reported sites should be further clarified on the basis of more extensive studies in various aom related settings

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

fracture networks and hydrate distribution at hydrate ridge oregon abstract analysis of resistivity at the bit rab images acquired during ocean drilling program odp leg 204 to southern hydrate ridge oregon reveals changes in the distribution and orientation of resistive fractures between the different hydrologic provinces sampled fractures on the ridge crest at sites 1247 1248 and 1250 are typically high angle 60 with isotropically distributed strikes while those on the ridge flanks at sites 1244 1246 have lower dips 20 60 and strike parallel to the trend of the ridge the eastern slope basin site 1251 lacks significant fracturing these differences indicate that the minimum principal stress 3 direction rotates to horizontal at the ridge crest as a result of topographically driven extension and gravitational collapse of the ridge chaotic fracturing on the ridge crest results from the interaction of the stress state and free gas migration which is confined to high permeability sediments and fracture zones by capillary pressure constraints in contrast on the ridge flanks well ordered fractures that agree with stress orientations determined from borehole breakouts result from an obliquely dipping 3 and the predominance of aqueous rather than gaseous flow

large eddy simulations and observations of atmospheric marine boundary layers above nonequilibrium surface waves abstract winds and waves in marine boundary layers are often in an unsettled state when fast running swell generated by distant storms propagates into local regions and modifies the overlying turbulent fields a large eddy simulation les model with the capability to resolve a moving sinusoidal wave at its lower boundary is developed to investigate this low wind fast wave regime it is used to simulate idealized situations with wind following and opposing fast propagating waves swell and stationary bumps les predicts momentum transfer from the ocean to the atmosphere for wind following swell and this can greatly modify the turbulence production mechanism in the marine surface layer in certain circumstances the generation of a low level jet reduces the mean shear between the surface layer and the pbl top resulting in a near collapse of turbulence in the pbl when light winds oppose the propagating swell turbulence levels increase over the depth of the boundary layer and the surface drag increases

effects of wind trend and gustiness on the sea drag lake george study 1 the air sea coupling is usually parameterized in terms of the drag coefficient cd but the scatter of experimental data around such dependences is very significant and has not improved noticeably over some 30 years in the paper which is meant to be the first in a series a complex approach to the problem is suggested multiple mechanisms contributing into the sea drag are to be singled out studied separately evaluated and then reunited in a joint parameterization for cd dependences of the drag coefficient on the wind speed and sea state and effects of wind trends and gustiness are investigated in detail gustiness is found to be responsible for the most distant outliers our approach also combines an experimental lake george study with theoretical investigations conducted by means of the wowc wind over waves coupling model overall agreement of the model with measured wind stresses is quite good within 20 for the bulk of the data lower envelopes of the drag dependences are an important result of the paper they provide some lake george ideal relationships for the sea drag almost any deviation from such conditions causes the drag to increase it is suggested that decrease of the drag with respect to the ideal conditions which exhibits itself in a number of known open ocean data sets would be caused by a momentum flux back from the waves to the wind due to long waves outrunning the wind behavior of the charnock parameter in terms of wave age is also considered

gas hydrates saturation using geostatistical inversion in a fractured reservoir in the krishna godavari basin offshore eastern india a reservoir of gas hydrates filling fractures was discovered in the krishna godavari kg basin during the indian national gas hydrate program nghp expedition 01 at site nghp01 10 the existing methods for estimating gas hydrate saturation from seismic data rely on establishing an empirical relation between acoustic impedance and density porosity from well logs assuming isotropic pore filling gas hydrate this method however yields a misleading saturation for fractured clay dominated sediments here we present a methodology to estimate gas hydrate saturation from seismic data based on geostatistical inversion it integrates the verticals detail of well log data with the lateral details from seismic data to produce highly detailed estimates of gas hydrate saturation first gas hydrate saturation is calculated from p wave velocities assuming anisotropic distribution at the well site then probability density functions pdfs between acoustic impedance and the calculated gas hydrate saturation at the well site are analyzed a markov chain monte carlo method is employed to integrate well logs with the seismic data to produce acoustic impedance finally crossplots and histograms at the well site are used to estimate gas hydrate saturations along the seismic line from inverted acoustic impedance the spatial distribution of gas hydrate varies both laterally and vertically along the line with an average saturation of 22 5 the estimate matches reasonably with the value at the wells crown copyright c 2013 published by elsevier ltd all rights reserved

some evidence of colinear wind stress and wave breaking abstract data collected during the surface waves and processes program are employed to investigate a possible interrelation between wind stress and surface wave breaking from comparison of data from 15 half hour long time segments the directions of the wind stress and the whitecap motion are observed to be generally colinear with both lying between the mean wind and the swell as well a nondimensionalized whitecap speed is found to correlate with the drag coefficient these results suggest that the magnitude and direction of the wind stress might be estimated from wave breaking information