origin of natural gases and associated gas hydrates in the shenhu area northern south china sea results from the china gas hydrate drilling expeditions abstract geochemical data for hydrate gases acquired from the gmgs3 and gmgs4 gas hydrate drilling expeditions conducted by the guangzhou marine geological survey gmgs are used to explore the origin of hydrate gases and their relationship to deep hydrocarbon reservoirs and to evaluate the contribution of different genetic types of gases to the formation and accumulation of gas hydrates in the shenhu area of the northern part of the south china sea scs compositionally methane is the dominant gas 90 in the void gas and pressure core gas in addition as much as 3 of the gas is composed of c2 hydrocarbons including ethane propane iso butane butane iso pentane and n pentane the 13c ch4 and d ch4 values indicate a mixed biogenic thermogenic origin for the hydrate forming gas the methane isotope correlation indicates that the source of the hydrate gas is closely related to the deep conventional gas reservoirs discovered in the baiyun sag panyu low uplift area both the hydrate gases and the deep reservoir gases are sourced from the hydrocarbon kitchens in the baiyun sag revealing a paragenetic relationship within the same petroleum system the composition of the hydrocarbons and the isotopic variation of methane with depth suggest that the thermogenic gas was likely affected by compositional and isotopic fractionation during the long distance migration from the deep source rocks to the shallow gas hydrate stability zone ghsz the impact of biodegradation on a solely thermogenic gas could also affect the final composition of the hydrate forming gas analysis of the ghsz based on gas hydrate compositions suggests that the occurrence of thermogenic gas could also indicate the coexistence of structure i si and structure ii sii gas hydrates in the shenhu area with the sii hydrates accumulating in or below the lower part of the si ghsz the confirmed presence of sii hydrates in the shenhu area relocated the base of the ghsz deeper than was indicated by the bottom simulating reflector which warrants further study in future explorations for gas hydrates in the shenhu area

scaling laws between seismo electric magnetic fields and earthquake magnitude the mechanisms generating the charges potentials and abnormal electromagnetic radiation that accompany earthquakes have been discussed based on observational theoretical and laboratory studies in particular over the last few decades we previously conducted stick slip tests using precut granite cores with a contact electrode on the sample side and proposed that surface charges released from charge trapping centres on sheared asperities are a possible source of the above mentioned charges or potentials here we discuss seismo electric and magnetic fields formed by the surface charges and derive their scaling laws with the earthquake magnitude at a distance according to our model detectability of these fields at a point on the ground surface depends strongly on wet dry condition at a fault zone sheared asperities

painting a picture of gas hydrate distribution with thermal images 1 a study of cold thermal anomalies identified in continuous down core records of thermal structure acquired by an infrared ir camera during ocean drilling program odp leg 204 to southern hydrate ridge is combined with core based observations of structure and lithology to present new insights into the influence of lithology on hydrate distribution in an accretionary margin hydrate distribution is spatially associated with 0 5 cm thick laterally discontinuous sand horizons that compose 1 of the sedimentary section at all sites proximity to the fractured ridge crest determines the extent to which individual sand horizons host hydrate with 70 of the horizons hosting hydrate at the crest and only 29 in the basin this trend is consistent with focused up dip migration of fluids along permeable lithologic conduits and shows that lateral flow may be as important as vertical diffusion and fracture flow in determining gas hydrate distribution at hydrate ridge

frictional discharge at fault asperities origin of fractal seismo electromagnetic radiation abstract to investigate the relation between the rock friction and the fractal electromagnetic radiation before the main shock of earthquakes we conducted a friction experiment simulating the motion of an asperity on a fault plane and observed photon emissions due to electric discharge by dielectric breakdown of ambient gases from friction contacts between rock minerals this indicates that frictional discharges plasma generations could occur locally at microscopic asperities on fault surfaces from concepts on the fractal size distribution and temporal evolution of fault asperities the frictional discharge occurring at asperities on the fault plane can be one of origins of the fractal electromagnetic radiation benioff electromagnetic radiation prior to earthquakes

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

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

sand ridges off sarasota florida a complex facies boundary on a low energy inner shelf environment abstract the innermost shelf off sarasota florida was mapped using sidescan sonar imagery seismic reflection profiles surface sediment samples and short cores to define the transition between an onshore siliciclastic sand province and an offshore carbonate province and to identify the processes controlling the distribution of these distinctive facies the transition between these facies is abrupt and closely tied to the morphology of the inner shelf a series of low relief nearly shore normal ridges characterize the inner shelf stratigraphically the ridges are separated from the underlying pleistocene and tertiary carbonate strata by the holocene ravinement surface while surficial sediment is fine to very fine siliciclastic sand on the southeastern sides of the ridges and shell hash covers their northwestern sides the cores of these holocene deposits are a mixture of both of these facies along the southeastern edges of the ridges the facies boundary coincides with the discontinuity that separates the ridge deposits from the underlying strata the transition from siliciclastic to carbonate sediment on the northwestern sides of the ridges is equally abrupt but it falls along the crests of the ridges rather than at their edges here the facies transition lies within the holocene deposit and appears to be the result of sediment reworking by modern processes this facies distribution primarily appears to result from south flowing currents generated during winter storms that winnow the fine siliciclastic sediment from the troughs and steeper northwestern sides of the ridges a coarse shell lag is left armoring the steeper northwestern sides of the ridges and the fine sediment is deposited on the gentler southeastern sides of the ridges this pronounced partitioning of the surficial sediment appears to be the result of the siliciclastic sand being winnowed and transported by these currents while the carbonate shell hash falls below the threshold of sediment movement and is left as a lag the resulting facies boundaries on this low energy sediment starved inner continental shelf are of two origins which both are tied to the remarkably subtle ridge morphology along the southeastern sides of the ridges the facies boundary coincides with a stratigraphic discontinuity that separates holocene from the older deposits while the transition along the northwestern sides of the ridges is within the holocene deposit and is the result of sediment redistribution by modern processes

stratigraphy and genesis of a modern shoreface attached sand ridge peahala ridge new jersey abstract we investigated the genesis and evolution of peahala ridge a modern shoreface attached sand ridge 1 km 6 km in size and 4 7 m in bathymetric relief through an integrated sedimentologic and stratigraphic study involving vibracoring box coring grab samples high resolution seismic paleontology radiocarbon dating and oceanographic measurements near surface strata of peahala ridge include six important stratigraphic units modern shoreface upper ridge sand lower ridge sand swale inlet fill middle holocene back barrier and late pleistocene strandplain radiocarbon dating and determination of the stratigraphic relationships indicates that peahala ridge formed initially from an ebb tidal delta associated with a tidal inlet so thwestward migration of the inlet channel a vector resultant of landward coastal retreat and southerly longshore drift cut and then filled the swale separating peahala ridge from long beach island following inlet closure peahala ridge developed its present form as a shoreface attached shoreline oblique bathymetric feature hydrodynamic processes have played a major role in evolution of the ridge including considerable growth and accretion this combination of long term eustatic and short term hydrodynamic factors is the best explanation for the present morphology and internal stratigraphy of peahala ridge

magnetic and electric fields associated with changes in high pore pressure in fault zones application to the loma prieta ulf emissions we determined the electric and magnetic fields generated during failure of faults containing sealed compartments with pore pressures ranging from hydrostatic to lithostatic levels exhumed fault studies and strain measurement data limit the possible size of these compartments to less than 1 km in extent rupture of seals between compartments produces rapid pore pressure changes and fluid flow and may create fractures that propagate away from the high pressure compartment along the fault face nonuniform fluid flow results from pressure decrease in the fracture from crack generated dilatancy partial blockage by silica deposition and clearing as pressure increases a direct consequence of this unsteady fluid flow may be associated transient magnetic signals caused by electrokinetic piezomagnetic and magnetohydrodynamic effects models of these processes for fault geometries with 1 km high pressure compartments show that electrokinetic effects are several orders of magnitude larger than the other mechanisms the electrokinetic signals produced by this unsteady flow are comparable in magnitude and frequency to the magnetic signals observed prior to the ml 7 1 loma prieta earthquake of october 18 1989 provided fracture lengths are less than 200 m

evolution em signals before earthquakes in terms of mesomechanics and complexity abstract mhz to khz electromagnetic em anomalies have been detected worldwide over time intervals ranging from a few days to a few hours prior to near surface earthquakes eqs that have occurred on land with magnitude approximately six or larger the mhz radiation systematically appears earlier than the khz laboratory experiments on rock samples reveal a similar change in the frequency content during progressive deformation i e the emissions exhibit a frequency shift from mhz to khz just before failure of the samples herein we attempt to associate these consecutively emerging mhz and khz modes of the pre fracture em emission with successive distinct stages of the fracture preparation process and the associated fracture mechanisms we base our approach on synergetic principles of physical mesomechanics a universal principle of physical mesomechanics is that the plastic deformation of a solid is related to its loss of shear stability at the micro meso and macroscale levels global fracture is determined by the mechanism controlling the development of macroscale bands of localized deformation associated with meso and microscale relaxation processes the major result we obtain is that the shift from mhz to khz em activity may signal the transition of plastic flow localization from the mesoscale to the macroscale culminating in global fracture a complexity measure approximate entropy and a persistency anti persistency measure hurst exponent also give evidence of a two stage model of eq preparation process significant complexity decrease and accession of persistency can be confirmed in the strong khz em activity that emerges in the tail of the pre seismic em activity laboratory experiments on rock samples reveal similar symptoms of an imminent global instability

investigation of in situ salinity and methane hydrate dissociation in coarse grained sediments by slow stepwise depressurization abstract slow 8 15 day stepwise degassing of synthesized methane hydrate bearing sand packs was used to determine in situ salinity to within 0 5 wt rebounds in pressure during shut in intervals between degassing steps record gradual hydrate dissociation during successive dissociation steps and pressure rebounds the pressure decreases more than predicted for a sample with homogenous salinity and temperature at thermodynamic equilibrium we interpret that hydrate dissociation is limited by local pore water freshening and endothermic cooling around the dissociating hydrate these results suggest that during production hydrate dissociation in a reservoir will occur along the freshwater phase boundary even though the bulk salinity of the system may be higher pore pressure rebound during shut in is more rapid in freshwater experiments than in brine experiments which indicates that salt diffusion plays a role in limiting recovery to homogenous bulk equilibrium during hydrate dissociation we demonstrate that it is possible to interpret the in situ salinity from the temperature and pressure at the onset of dissociation this approach can be applied to actual hydrate bearing pressure core samples and the results will assist our understanding of how these hydrates form depressurization experiments with controlled shut in periods provide insight into the role of salt and heat diffusion on dissociation in coarse grained sediments particularly relevant to pauses in production and in regions of a hydrate reservoir near the hydrate phase boundary

effect of wave topography interactions on the formation of sand ridges on the shelf 1 the role of wave topography interactions in the formation of sand ridges on microtidal inner shelves is investigated with an idealized morphodynamic model the latter uses the two dimensional shallow water equations to describe a storm driven flow on an inner shelf with an erodible bottom and a transverse slope both bed load and suspended load sediment transport are included new are the incorporation of a wave module based on physical principles and a critical shear stress for erosion a linear stability analysis is used to study the initial growth of bed forms by analyzing the initial growth of small perturbations evolving on an alongshore uniform basic state which describes a storm driven flow on a microtidal inner shelf model simulations show that wave topography interactions cause the ridges to become more trapped to the coast both growth and migration of the ridges are controlled by suspended load transport the physical mechanism responsible for ridge growth is related to transport by the stormdriven current of sediment that is entrained due to wave orbital motions induced by bed forms this new mechanism even acts in absence of a transverse bottom slope the orientation spacing and shape of the modeled ridges agree well with field observations from different shelves

estimation of the gas hydrate resource volume in a small area of the ulleung basin east sea using seismic inversion and multi attribute transform techniques we estimated the volume of gas hydrate and in place gas in a small 37 km 58 km area of the ulleung basin east sea from a dense grid of 2 d seismic and logging while drilling lwd data using seismic inversion and multi attribute transform techniques multi attribute transform finds the relationship between any measured or calculated logs and the combination of the seismic attributes and the acoustic impedance computed from inversion we assumed that the bottom simulating reflector marks the base of the gas hydrate stability zone ghsz first the pore space gas hydrate saturation at the wells was estimated from the simplified three phase biot type equation then the porosity and the pore space gas hydrate saturation along the seismic lines were predicted from multi attribute transform the ghsz was divided into ten layers of the equal time thickness the time thickness of each layer was converted into depth using the time depth relationship constructed from seismic to well tie at the wells and gridded at 500 m cell size the average porosity and pore space gas hydrate saturation were computed for each layer and multiplied to obtain the average total gas hydrate saturation which was gridded with the same cell size as the thickness grid thus each 2 d cell is represented by a rock volume and an average total gas hydrate saturation the gas hydrate volume for each cell was computed by the multiplication of the cell rock volume and the average total gas hydrate saturation finally the total gas hydrate volume was computed by summing the gas hydrate volumes of all cells the estimated gas hydrate and in place gas volumes in the study area are about 3 43 109 m3 1 21 1011 ft3 and about 4 50 1011 m3 1 59 1013 ft3 respectively the more conservative estimates excluding the top three layers that comprise about 50 m of the near seafloor sediments where the lwd data are often unreliable are 3 03 109 m3 1 07 1011 ft3 for gas hydrate and about 3 97 1011 m3 1 40 1013 ft3 for gas

complex resistivity measurements of confined rock complex resistivity was measured for samples of westerly granite and berea sandstone saturated with 0 01 m kclaq a four electrode technique was employed to make measurements over a frequency range of 10 3 to 106 hz and a confining pressure range of 0 2 to 200 mpa when the electrical response was analyzed in terms of equivalent conductivity and relative permittivity k both of these quantities varied more with confining pressure for the granite samples than they did for the sandstone in addition both rock types exhibited large low frequency dispersion of permittivity while previous studies have measured the response of near dc conductivity to changes in pressure measurements of pressure dependence of the full complex resistivity provide greater insight into earthquake related phenomena as well as induced polarization measurements and other remote sensing techniques

high resolution swath sonar investigation of sand ridge dune and ribbon morphology in the offshore environment of the new jersey margin abstract sand ridges on the northeast us atlantic shelf form in the near shore environment most likely in response to storm driven flows as the holocene transgression has continued the ridges have been transferred to an offshore hydrodynamic regime where currents are not constrained by the coast and storms do not influence bottom currents as frequently or as strongly here we qualitatively and quantitatively investigate the morphology of offshore sand ridges and smaller scale features in an effort to place constraints on bedform development in these deeper waters a recent high resolution swath sonar survey mapped a portion of the new jersey shelf from 20 m water depth to the shelf break 120 m imaging both sand ridges and smaller scaled dunes and ribbons in far greater detail than has been previously possible using a robust statistical analysis we find that the gross morphology of ridges height width length does not change with depth beyond 20 m water depth and changes in ridge orientation generally mirror changes in regional contour orientation hence ridges have not continued to grow since transgression has brought them into the offshore hydrodynamic regime however on the inner shelf 20 m water depth to the mid shelf shore we do find evidence in the ridge shape which has an asymmetry opposite to that seen near shore and in the complex backscatter response that some important modifications to ridges are taking place at these water depths in contrast on the mid shelf from the mid shelf shore to the franklin shore ridges tend to have higher backscatter at the crests implying that these are largely winnowed relict features lineated smaller scale 100 500 m wavelength

experimental study of a van network in the french alps abstract in 1989 a telluric and magnetotelluric network was installed in the northern part of the french alps by the laboratoire de detection et de geophysique ldg the purpose of this experiment was to check the applicability of the greek van method to earthquake prediction in a different geotectonic and seismic context from the greek one the sites were selected according to several criteria for geology and potential noise sources a complete station consists of six telluric non polarisable dipoles of petiau type 2 magnetometers of mosnier type and an acquisition unit run by solar energy the interesting signals are not correlated with the geomagnetic field variations anthropogenic noise electrochemical variations and atmospheric storms are not listed synthetic graphs sum up the seismic and telluric activities during 3 months and 2 weeks for the correlation between earthquakes and potential telluric precursors we consider first the multiple epicentres then the near field ones and finally the most significant events some encouraging results are presented this experiment must be continued over a longer period and the network completed with low frequency radio electric receiving stations

lithology mapping of a mixed siliciclastic carbonate evaporite system using 3d seismic and well data lower triassic jialingjiang formation sichuan basin southwestern china abstract high quality three dimensional 3d seismic data acquired in the central sichuan basin southwestern china offer an opportunity to map complex lithologies in a mixed siliciclastic carbonate evaporite system in the lower triassic jialingjiang t1j formation the formation consists of siliciclastics limestone dolostone anhydrite and salt the lithologies consist several source reservoir cap assemblages in the area lithologies in the t1j formation change rapidly in the vertical direction forming different interbed patterns in thin layers in the mean time the lateral extend of each lithology is complex this vertical and lateral distribution makes it difficult to predict lithology by single seismic attribute therefore principle component analysis pca was applied to tens of seismic attributes to extract useful information the first three components contain most 83 02 99 85 of the lithology information preserved in seismic attributes which were used to correlate with lithology content calculated by core calibrated wireline logs correlation coefficients of the three seismic components with lithologies are significantly higher 0 37 0 79 than those of individual seismic attributes near zero to 0 58 different assemblies of end member lithologies were selected from anhydrite siliciclastics tight dolostone limestone and salt to perform pca in different sequences lithologic content distribution of individual end members was shown by color blending method to map the lithology mixture sedimentary history in the t1j formation was reconstructed based on lithology mixture maps and regional geology background in the study area eight 4th order sequences were interpreted in the t1j formation only the second sequence was influenced by siliciclastic input forming a complete mixed siliciclastic carbonate evaporite system open platform and or restricted evaporative environment dominated the other seven sequences forming a mixed carbonate evaporite system

physical properties of the variations of the electric field of the earth preceding earthquakes ii determination of epicenter and magnitude abstract as reported in the preceding paper a transient change of the electric field of the earth seismic electric signal hereafter called ses appears many hours before an earthquake eq by measuring this change in a given direction and dividing it with a suitable relative effective resistivity one obtains a quantity that reflects the current density in this direction measurements in two directions e w and n s give the relative signal intensity jrel at the station under consideration by measuring jrel at a number of stations and considering that it attenuates according to a 1 r law the epicenter can be determined with an accuracy usually around 100 km once the epicenter has been determined the product jrel r can be evaluated so that the magnitude m can be estimated by resorting to an empirical log jrel r versus m plot the uncertainty of m is around 0 5 units following sobolev 1975 and for the statistics to be beyond any doubt predictions were officially documented before the eq occurrence for 23 earthquakes with a magnitude equal or greater than ms 5 0 two events were missed the present method is compared to other electrical methods used in china japan and soviet union a number of problems concerning the origin of the effect its directivity and the attenuation with distance remain open for further studies

electrical properties of granite with implications for the lower crust the electrical properties of granite appear to be dominantly controlled by the amount of free water in the granite and by temperature minor contributions to the electrical properties are provided by hydrostatic and lithostatic pressure structurally bound water oxygen fugacity and other parameters the effect of sulfur fugacity may be important but is experimentally unconfirmed in addition to changing the magnitude of electrical properties the amount and chemistry of water in granite significantly changes the temperature dependence of the electrical properties with increasing temperature changes in water content retain large but lessened effects on electrical properties near room temperature a monolayer of water will decrease the electrical resistivity by an order of magnitude several weight percent water may decrease the electrical resistivity by as much as 9 orders of magnitude and decrease the thermal activation energy by a factor of 5 at elevated temperatures just below granitic melting a few weight percent water may still decrease the resistivity by as much as 3 orders of magnitude and the activation energy by a factor of 2 above the melting temperature 650 to 1100 c depending upon water pressure a few weight percent water will decrease the resistivity by less than an order of magnitude and will barely change the activation energy remarkably the few weight percent water must be present as free water experiments with hydrated hornblende schist with structural water indicate an electrical resistivity very similar to that for dry granite the implications of these results together with the findings of deep magnetic sounding and magnetotelluric surveys suggest much more free water than is commonly associated with the lower crust and possibly into the upper mantle

origin evolution and distribution of shoreface sand ridges atlantic inner shelf u s a abstract a computer mapping system was employed to document the location of 259 shoreface attached and detached sand ridges in water although shoreface attached sand ridges appear to have several different modes of formation a two step process for the development of most shoreface attached and detached sand ridges along u s atlantic barrier island and cape coastlines is proposed 1 sand is deposited as ebb tidal deltas or river deltas along the lower shoreface and or inner continental shelf prior to or during transgression followed by 2 further transgression which reworks the deltaic sand bodies into linear sand ridges at the base of the shoreface by shelf processes the best developed shoreface sand ridge fields along the u s atlantic shelf lie adjacent to shorelines characterized by all of the following 1 transgression 2 mixed energy wave dominated barrier islands and 3 laterally migrating tidal inlet systems tidal inlet systems are natural sediment sinks that capture sand carried by longshore sediment transport ebb tidal delta deposits associated with these migrating tidal inlets provide the initial sand body for the development of shoreface attached sand ridges the oblique orientation and linear form of shoreface attached sand ridges appear to be a function of shoreline transgression lateral inlet migration and wave reworking of ebb tidal delta deposits concentrated along an ebb tidal delta retreat path shelf processes act as modifying agents in the evolution of sand ridges during and after ebb tidal delta deposition in general shoreface attached sand ridges are poorly developed or absent along eroding headlands spits and barrier island shorelines characterized by naturally stabilized tidal inlets these latter inlets and shoreline types are dominated or influenced by one or all of the following 1 antecedent topography 2 higher tidal ranges 3 larger tidal prisms 4 lower wave energies or 5 finer grain sizes the coupling of shoreline and shallow marine sedimentary processes during a transgression is critical to the origin evolution and distribution of shoreface sand ridges in the study area modern shoreface attached sand ridges are also known to occur in different coastal and shelf settings where large amounts of sediment were supplied to the shoreface and inner shelf during or immediately before transgression commonly observed vertical and lateral interrelationships of holocene shoreface attached sand ridges and tidal inlets or distributary channels have important ramifications for the development of shelf sandstone facies models in addition the geometric relationship documented in this study provides predictive petroleum and hard mineral exploration models of the spatial and temporal distribution of shoreface and shelf sand ridges

evolution of a classic sand ridge field maryland sector north american inner shelf the ridge and swale topography of the middle atlantic bight is best developed on the delaware maryland inner shelf here sand ridges can be seen in all stages of formation several aspects of the ridge field are pertinent to the problem of ridge genesis the first is ridge morphology there is a systematic morphologic change from shoreface ridges through nearshore ridges to offshore ridges which reflects the changing hydraulic regime as successively more seaward ridges are examined maximum side slope decreases the ratio of maximum seaward slope to maximum landward slope decreases and the cross sectional area increases these changes in ridge morphology with depth and distance from shore appear to be equivalent to the morphologic changes experienced by a single ridge during the course of the holocene transgression a second aspect is the change in bottom sediment characteristics that accompanies these large scale morphologic changes megaripples sand waves and mud lenses appear in the troughs between nearshore and offshore ridges these changes indicate that the storm flows which maintain ridges are less frequently experienced in the deeper sector and that the role of high frequency wave surge becomes less important relative to the role of the mean flow component in shaping the sea floor a third aspect is the systematic relationship of grain size to topography grain size is 90 out of phase with topography so that the coarsest sand lies between the axis of the landward trough and the ridge crest while the finest sand lies between the ridge crest and the axis of the seaward trough this relationship is characteristic of large scale bedforms finally flow was measured and transport calculated on the same ridge during a one month period november 1976 threshold was exceeded only during storm events mean transport was southerly and a little seaward with respect to both the ridge crest and the shoreline these flow measurements are in conformity with the pattern of smaller bedforms a 43 year time series of bathymetric change for this ridge reveals a systematic pattern of landward flank erosion seaward flank deposition and seaward crest migration sand ridges are considered the consequence of constructive feedback between an initial topography and the resulting distribution of bottom shear stress the relationship between grain size and topography supports this model but does not account directly for the oblique angle of the ridge with respect to the coastline this feature may be due to a more rapid alongshore migration rate of the inshore edge of the ridge than the offshore edge and the relationship between this migration rate and the rate of shoreface retreat

mixed gas hydrate structures at the chapopote knoll southern gulf of mexico abstract in underwater hydrocarbon seepage environments gas hydrates are considered to play a significant role as shallow gas reservoirs and buffers for light hydrocarbon expulsion here we report on mixed hydrate structures from the chapopote knoll in the southern gulf of mexico and discuss several options on how a mixture of structure i si and structure ii sii gas hydrate may occur in nature locally resolving microscopic methods are needed to characterize the coexistence of different hydrate structures at geological hydrate deposits we used raman spectroscopy x ray diffraction and gas chromatography for our investigations gas hydrates were found within the matrix and pores of the asphalts extruded at the seafloor two of the three hydrate pieces investigated comprised only si formed mostly from methane in contrast one piece comprised an intimate mixture of both si and sii with sii representing ca 25 wt and si ca 75 wt of the hydrate present the two structures were closely associated within individual grain agglomerates the crystallites of sii were significantly larger than of si suggesting differences in the nucleation density or different crystallite ages the structural coexistence may be a result of one or more processes i de mixing into two hydrate structures during the growth from the gas phase which provides an additional degree of freedom for lowering the free energy in the system ii fractionated crystallization with a subsequently changing molecular composition iii crystallization from separated gas bubbles with different hydrocarbon compositions and water and iv partial transformation from sii to si after hydrate nucleation ceasing when a thermodynamically stable state was reached the presented work will affect future assessments of natural hydrate deposits at thermogenic hydrocarbon systems as it shows that both hydrate types i and ii can be present at a certain geological site and may provide a lingering strength to the system if conditions fall below the si stability limit this suggests that present worldwide hydrate occurrences are likely to be underestimated

morphology lateral migration and internal structures of shoreface connected ridges sable island bank nova scotia canada an extensive complex of shoreface connected ridges is present around sable island near the outer edge of the canadian east coast shelf two orders or ridges occur the larger first order ridges are longer wider and higher than ridges along the east coast of the us whereas the superimposed second order ridges are smaller all of the ridges converge eastward with the shoreface at angles up to 90 sup 0 sediment grain size becomes finer eastward over each ridge the finest sediment occurring at the base of the eastern flank landward facing straight crested megaripples and sandwaves are present in the troughs whereas hummocky megaripples occur on the ridge crests eastward migration of the ridges will produce a 3 20 m thick upward coarsening sequence that rests on an upward fining basal lag cross bedding predominates throughout the sequence although the upper part may contain hummocky cross stratification the differences between the us and the canadian ridges are due to a more intense storm climate and to obliquely onshore rather than obliquely offshore geostrophic flows in the sable island area nevertheless the sedimentological patterns over the ridges in both areas are similar 17 references 5 figures

chemical and structural characteristics of gas hydrates from the haima cold seeps in the qiongdongnan basin of the south china sea abstract crystalline structure cage occupancy and gas composition are important gas hydrate characteristics which can be used to calculate the gas hydrate s stability and to estimate the energy potential in may and october of 2015 a guangzhou marine geological survey team investigated two sites in the haima cold seeps in the qiongdongnan basin northern south china sea they recovered massive gas hydrate samples by conventional gravity coring after the shenhu sea area and dongsha sea area the qiongdongnan basin became a new area on the northern slope of the south china sea where gas hydrates have been found in order to reveal the structural and geochemical characteristics of the natural gas hydrates samples were analyzed by micro raman spectroscopy and x ray diffraction under ambient pressure and low temperature conditions the results indicate that the gas hydrate samples from the haima cold seeps are typical structure i hydrates with a hydration number of 6 12 6 19 in addition trace amounts of h2s trapped in the hydrate were identified based on its characteristic vibrational signature the gas composition and 13c ch4 of the hydrate bound gas samples were analyzed for gas source correlation all of the gas samples are dominated by methane with small amounts of ethane and propane and had relatively light 13c ch4 indicating that all of the hydrate bound gases are mixtures of biogenic and thermogenic gas the thermogenic gas is inferred to be mainly derived from the coal layers of the late miocene pliocene yacheng formation in the northern lingshui sag

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

long term changes in morphology and textural sediment characteristics in response to energy variation on shoreface connected ridges off the east frisian barrier island coast southern north sea abstract patterns of surficial sediment distribution across shoreface connected sand ridges located along the barrier island coast of the southern north sea generally show a strong affinity between textural parameters and ridge morphology comparable to that observed in other parts of the world thus the sediment of the upper shoreface and the inner ridge crest consists of fine sand water depths 12 m in order to assess the mobility of the sediments and the consistency of the observed textural patterns two high resolution multi beam bathymetric surveys were carried out along selected corridors across a ridge and trough system off spiekeroog island located along the southern north sea coast of germany one in 2003 the other in 2007 the bathymetric data of the two surveys show only small scale topographic changes these were produced mainly by local changes in the bedform patterns on the inner ridge the bathymetric surveys therefore document a remarkable morphological stability of the ridges over time periods of several years spatial and temporal variations in the distribution of standard textural parameters were assessed on the basis of 95 sediment samples recovered on the same grid in 1989 and 2005 these data show that for example the distribution of coarse sand 0 1 phi became enhanced along the axis of the outer trough while expanding onto the lower ridge flanks current measurements in fair weather demonstrate that the coarse sediments cannot be moved by the local tidal currents the observed changes in the distribution of coarse sand and in particular its expansion onto the adjacent ridge flanks must therefore be associated with wave action probably in combination with tidal currents this is supported by an assessment of the entrainment potential of coarse sand d 1 mm by typical waves during storms t 8 s h 2 5 m and fair weather t 4 s h 0 7 m the results demonstrate that the effective wave base for coarse sand during fair weather conditions is limited to a water depth of 4 8 m whereas during storms it exceeds a depth of 26 4 m the mobility of the coarse sands in the outer trough at water depths 16 m and with it the morphodynamics of the ridges as a whole is thus clearly controlled by the superimposition of storm waves on the tidal current regime

a mechanism for the formation and maintenance of shore oblique sand ridges on storm dominated shelves a remarkable set of shore oblique sand ridges occupies the inner portion of the storm dominated middle atlantic and south atlantic bights off the east coast of the united states of america a mechanism for present day formation and growth of these distinctive geological features follows from a linearized analysis of the stability of alongshore periodic topographical perturbations based on a depth averaged fluid dynamical model with a simple representation of sediment transport the analysis indicates exponential growth of shore oblique features as a result of offshore deflection of storm driven alongshore flows at ridge crests which leads to a convergence of sediment flux because the effective carrying capacity decreases with increasing distance offshore the analysis indicates a set of conditions required for ridge growth model solutions for topography under conditions favorable for ridge growth are strikingly similar to the observed sand ridges

the west central florida inner shelf and coastal system a geologic conceptual overview and introduction to the special issue abstract this paper provides an overview for this special publication on the geologic framework of the inner shelf and coastal zone of west central florida this is a significant geologic setting in that it lies at the center of an ancient carbonate platform facing an enormous ramp that has exerted large scale control on coastal geomorphology the availability of sediments and the level of wave energy in order to understand the holocene geologic history of this depositional system a regional study defined by natural boundaries north end of a barrier island to the apex of a headland was undertaken by a group of government and university coastal geologists using a wide variety of laboratory and field techniques it is the purpose of this introductory paper to define the character of this coastal inner shelf system provide a historical geologic perspective and background of environmental information define the overall database present the collective objectives of this regional study and very briefly present the main aspects of each contribution specific conclusions are presented at the end of each paper composing this volume

gas hydrate occurrences and their relation to host sediment properties results from second ulleung basin gas hydrate drilling expedition east sea the second ulleung basin gas hydrate drilling expedition ubgh2 recovered various forms of gas hydrate bearing sediments from 10 drill sites in the lower slope and basin floor of the ulleung basin to characterize the gas hydrate occurrences and the properties of the host sediments whole round core samples were taken from portions of recovered cores determined to be hydrate bearing based on infrared ir scanning these samples were further characterized by a variety of shipboard experiments such as imaging of the sediments with hand held ir and visual cameras measurements of pore water chlorinity within and around ir inferred cold regions in the core and grain size analysis of pore water squeeze cakes sediment compositions of selected samples were further characterized by x ray diffraction and scanning electron microscopes during post cruise analysis the shipboard and post cruise analysis results collectively indicate that the recovered gas hydrates mainly occur as 1 pore filling type bounded by discrete silty sand to sandy silt layers 2 fracture filling veins and nodules or 3 disseminated type in silt in addition minor but significant variation in gas hydrate concentrations were observed in diatomaceous silt where gas hydrates occur as pore filling material in layers dominated by intact diatom frustules gas hydrate accumulations of fracture filling type occur predominantly in regions where acoustic blanking features in the seismic record suggest gas migration from below the gas hydrate stability zone results from the ubgh2 core studies along with the analysis of similar samples from other expeditions including those executed by the ocean drilling program the integrated ocean drilling program and the first ulleung basin gas hydrate drilling expedition greatly improved our understanding of lithologic controls on marine gas hydrate occurrences

global methane emission through mud volcanoes and its past and present impact on the earth s climate a comment the role of mud volcanoes mvs as a source of methane ch4 flux to the atmosphere and the ocean has been increasingly recognised in the last several years milkov 2000 dimitrov 2002 2003 etiope and klusman 2002 kopf 2002 2003 milkov et al 2003 etiope and milkov 2004 in one of the most recent papers kopf 2003 claims to report a reliable estimate of the global ch4 emission from mvs however the significance and usefulness of the estimate presented by kopf 2003 are rather poor the used dataset is smaller than in previous studies although the author makes a reverse claim and some previously published works are misquoted and misinterpreted numerous arithmetic mistakes made during simple calculations and data manipulations lead to confusing results and conclusions in this comment we highlight some of the most significant problems with the estimates published by kopf 2003

the experimental results on the actual measurement of energy transmission loss of magnetic field component across the tunnel abstract since 1980 we have detected impulsive noise bursts of seismogenic emissions at 82 khz 1525 and 36 hz with a multipoint detection network around tokyo region this system has recorded em signals prior to several earthquake events and two volcanic eruptions at mt mihara 1986 and at mt unzen 1991 by our statistical analysis of these emission characteristics using the last 29 events over the last decade impulsive magnetic energy generated with rock crash at focus area is carried along the magnetic flux from focus to epicenter and this magnetic intensity variation seems to be induced the electromagnetic waves at the ground surface the results of our underground measurement in the tunnel support above explanation

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

a comparison of earthquake damage areas as a function of magnitude across the united states this study investigates the relationship between earthquake magnitude and the size of damage areas in the eastern and western united states to quantify damage area as a function of moment magnitude m 149 mmi vi and vii areas for 109 earthquakes 88 in the western united states 21 in the eastern united states and canada were measured regression of isoseismal areas versus m indicated that areas in the east were larger than those in the west at both intensity levels by an average 5 in the m 4 5 to 7 5 range in terms of radii for circles of equivalent area these results indicate that damaging ground motion from shocks of the same magnitude extend 2 the epicentral distance in eastern north america compared to the west to determine source and site parameters consistent with the above results response spectral levels for eastern north america were stochastically simulated and compared with response spectral ordinates derived from recorded strong ground motion data in the western united states stress drop values of 200 bars combined with a surficial 2 km thick low velocity sedimentary layer over rock basement produced results that are compatible with the intensity observations i e similar response spectral levels in the east at approximately twice their epicentral distance in the western u s distance these results suggest that ground motion modeling in eastern north america may need to incorporate source and site parameters different from those presently in general use the results are also of importance to eastern u s hazard assessments as they require allowance for the larger damage areas in preparedness and mitigation programs

characterization of natural gas hydrate recovered from pearl river mouth basin in south china sea abstract china carried out a drilling campaign in the eastern part of the pearl river mouth prm basin and successfully obtained natural gas hydrate samples in 2013 laboratory based studies for the characteristics of gas hydrate were carried out on one of the hydrate bearing sediment sample the gas hydrate sample shows nodular occurrence with host matrix predominantly composed of silty sand and clay on the basis of the raman spectroscopic and x ray diffraction results the gas hydrate demonstrates a typical structure i si with cage occupancy of methane is more than 99 5 in large cage and 91 4 in small cage respectively corresponding to a hydration numbers of 5 90 by thermodynamic calculation the molecular compositions of hydrate bound gas suggest that the guest molecules are predominantly methane 99 9 with trace amount of ethane 0 04 and propane 0 01 the isotopic analysis indicates that the methane of hydrate bound gas of the studied drilling well in prm basin is a typical microbial origin

how strong is an asperity a recent study of repeating earthquakes on the san andreas fault in central california by nadeau and johnson 1998 found that the smallest events occurred on patches having a linear dimension of the order of 0 5 m displacements of about 2 cm and stress drops of the order of 2000 mpa roughly 10 times larger than rock strengths measured in the laboratory the stress drop for larger events was observed to decrease as a power law of the seismic moment reaching the commonly observed value of 10 mpa at about magnitude 6 these large strengths are shown here to be consistent with laboratory data if the preexisting microcracks are all healed a hierarchical fractal asperity model is presented which is based on recent laboratory observations of contact distributions in sliding friction experiments this cantor dust model is shown to be consistent with the observed power law decrease in stress drop and increase in displacement with increasing event size the spatial distribution of hypocenters in the parkfield area is shown to be consistent with this simple fractal model and with a hierarchical clustering of asperities having a fractal dimension of d 1 and discrete rescaling factor of about 20

estimation of the global amount of submarine gas hydrates formed via microbial methane formation based on numerical reaction transport modeling and a novel parameterization of holocene sedimentation this study provides new estimates for the global offshore methane hydrate inventory formed due to microbial ch4 production under quaternary and holocene boundary conditions a multi 1d model for particular organic carbon poc degradation gas hydrate formation and dissolution is presented the novel reaction transport model contains an open three phase system of two solid compounds organic carbon gas hydrates three dissolved species methane sulfates inorganic carbon and one gaseous phase free methane the model computes time resolved concentration profiles for all compounds by accounting for chemical reactions as well as diffusive and advective transport processes the reaction module builds upon a new kinetic model of poc degradation which considers a down core decrease in reactivity of organic matter various chemical reactions such as organic carbon decay anaerobic oxidation of methane methanogenesis and sulfate reduction are resolved using appropriate kinetic rate laws and constants gas hydrates and free gas form if the concentration of dissolved methane exceeds the pressure temperature and salinity dependent solubility limits of hydrates and or free gas with a rate given by kinetic parameters global input grids have been compiled from a variety of oceanographic geological and geophysical data sets including a new parameterization of sedimentation rates in terms of water depth we find prominent gas hydrate provinces offshore central america where sediments are rich in organic carbon and in the arctic ocean where low bottom water temperatures stabilize methane hydrates the world s total gas hydrate inventory is estimated at 0 82 x 10sup13 m3 2 10 x 10sup15 m3 ch4 at stp conditions or equivalently 4 18 995 gt of methane carbon the first value refers to present day conditions estimated using the relatively low holocene sedimentation rates the second value corresponds to a scenario of higher quaternary sedimentation rates along continental margins our results clearly show that in situ poc degradation is at present not an efficient hydrate forming process significant hydrate deposits in marine settings are more likely to have formed at times of higher sedimentation during the quaternary or as a consequence of upward fluid transport at continental margins

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

self affine growth pattern of earthquake rupture zones data from 41 moderate and large earthquakes have been used to derive a scaling law for fault parameters fault lengthl widthw and areas are empirically related byl s andw s where 0 6 0 7 0 3 0 4 and 1 these relations indicate that the growth pattern of earthquake rupture zones is statistically self affine it is also found that these relations are similar to the relation derived from a diffusion limited aggregation dla model with anisotropic sticking probability this suggests that a modified dla model could describe the evolution of earthquake rupture zones

late holocene estuarine inner shelf interactions is there evidence of an estuarine retreat path for tampa bay florida abstract the purpose of this study was to determine if and how a large modern estuarine system situated in the middle of an ancient carbonate platform has affected its adjacent inner shelf both in the past during the last post glacial sea level rise and during the present an additional purpose was to determine if and how this inner shelf seaward of a major estuary differed from the inner shelves located just to the north and south but seaward of barrier island shorelines through side scan sonar mosaicking bathymetric studies and ground truthing using surface grab samples as well as diver observations two large submarine sand plains were mapped one being the modern ebb tidal delta and the other interpreted to be a relict ebb tidal delta formed earlier in the holocene the most seaward portion of the inner shelf studied consists of a field of lobate bathymetrically elevated fine sand accumulations which were interpreted to be sediment starved 3d dunes surrounded by small 2d dunes composed of coarse molluscan shell gravel additionally exposed limestone hardbottoms supporting living benthic communities were found as well this modern shelf sedimentary environment is situated on a large buried shelf valley which extends eastward beneath the modern tampa bay estuary these observations plus the absence of an incised shelf valley having surficial bathymetric expression and the absence of sand bodies normally associated with back tracking estuarine systems indicate that there was no cross shelf estuarine retreat path formed during the last rise in sea level instead the modern tampa bay formed within a mid platform low relief depression which was flooded by rising marine waters late in the holocene with continued sea level rise in the late holocene this early embayment was translated eastward or landward to its present position whereby a larger ebb tidal delta prograded out onto the inner shelf extensive linear sand ridges common to the inner shelves to the north and south did not form in this shelf province because it was a low energy open embayment lacking the wave climate and nearshore zone necessary to create such sand bodies the distribution of bedforms on the inner shelf and the absence of seaward oriented 2d dunes on the modern ebb tidal delta indicate that the modern estuarine system has had little effect on its adjacent inner shelf

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

the characteristics of gas hydrates recovered from the mount elbert gas hydrate stratigraphic test well alaska north slope abstract systematic analyses have been carried out on two gas hydrate bearing sediment core samples hypv4 which was preserved by ch 4 gas pressurization and hyln7 which was preserved in liquid nitrogen recovered from the bpxa doe usgs mount elbert stratigraphic test well gas hydrate in the studied core samples was found by observation to have developed in sediment pores and the distribution of hydrate saturation in the cores imply that gas hydrate had experienced stepwise dissociation before it was stabilized by either liquid nitrogen or pressurizing gas the gas hydrates were determined to be structure type i hydrate with hydration numbers of approximately 6 1 by instrumentation methods such as powder x ray diffraction raman spectroscopy and solid state 13 c nmr the hydrate gas composition was predominantly methane and isotopic analysis showed that the methane was of thermogenic origin mean 13 c 48 6 and d 248 for sample hyln7 isotopic analysis of methane from sample hypv4 revealed secondary hydrate formation from the pressurizing methane gas during storage

results of statistical analysis of low frequency seismogenic em emissions as precursors to earthquakes and volcanic eruptions abstract since 1980 we have detected impulsive noise bursts of seismogenic emissions at 82 khz 1525 hz and 36 hz with a multipoint detection network around the tokyo region and the izu peninsula in earlier times this system has recorded em signals prior to several earthquake events and it has also recorded volcanic eruptions on 15 and 21 november 1986 at mt mihara on ohshima island on 12 july 1989 in itoh bay in the izu peninsula region and since june 1991 at mt unzen on the shimabara peninsula to obtain a clear explanation of the source mechanism of these emissions we are progressing with statistical analyses of these emission characteristics using the data of 29 events over the last 5 years

electric potential changes associated with slip failure of granite preseismic and coseismic signals electric potential changes were measured for stick slip events in granite samples with a three block direct shear arrangement at 8 mpa normal stress two electrodes were mounted on the left and right hand blocks and the electric potential difference between each electrode and the ground was measured with a high input impedance recording system of frequency range from dc to 100 hz as well as coseismic electric signals of about 1 5 v which appeared the moment of the dynamic slip event preseismic signals were detected just before the slip event the coseismic signal rises stepwise with opposite polarities at the two electrodes and exponentially decays with a time constant of e s where e is the permittivity and s is the conductivity of the rock sample we conducted a simple test of rapid stress drop without slipping and observed almost the same electric signal as the coseismic signal this suggests that the electric signal is generated by the piezoelectric effect we proposed a generation model based on the piezoelectric effect and the resultant relaxation process and obtained a theoretical frequency response which is in agreement with experimental data the preseismic signal appears about 2 3 s before the dynamic event with an amplitude of about 50 mv the local strains along two sliding surfaces were also measured to monitor the growth of the rupture nucleation zone when the growth of the rupture nucleation zone occurred on the left sliding surface a clear preseismic signal was detected at the electrode mounted on the left granite block when the growth occurred on the right hand surface a signal was detected at the electrode on the right block this shows that the preseismic electric signal is caused by stress change in the rupture nucleation zone these preseismic and coseismic signals were also detected with an antenna which was placed away from the sample surface

on the formation of sand banks of finite extent we consider a model where the fluid depth depends on both horizontal coordinates quasi steady depth uniform non divergent fluid flow is governed by inertial pressure and bottom frictional forces sand transport is proportional to the cube of the instantaneous current but augmented by a down slope component and by wind wave action and sand is conserved it is found that low parallel banks grow fastest so that in an extensive spatially uniform sea previous calculations for linear banks are appropriate the inclination of banks to the tidal currents can be interpreted in terms of similarly inclined deposition bands resulting from vorticity generation and advection in flow over a small isolated hump a small bump can evolve to an equilibrium bank typically after an initial rapid extension across the tidal currents provided that sand is sufficiently restricted and particularly if some wind wave action prevents growth up to the sea surface sand banks are likely to be in a late stage of evolution when the main change is a slow lengthening as the net current and transport along the bank side slows and turns around the bank end with net deposition the equilibrium is apparently stable except when there is an overall bed slope in the direction of the tidal currents or when sand is abundant

patterns and controls of surface sediment distribution west central florida inner shelf abstract the west central florida inner shelf represents a transition between the quartz dominated barrier island system and the carbonate dominated mid outer shelf surface sediments exhibit a complex distribution pattern that can be attributed to multiple sediment sources and the ineffectiveness of physical processes for large scale sediment redistribution the west florida shelf is the submerged extension of the florida carbonate platform consisting of a limestone karst surface veneered with a thin unconsolidated sediment cover a total of 498 surface sediment samples were collected on the inner shelf and analyzed for texture and composition results show that sediment consists of a combination of fine quartz sand and coarse biogenic carbonate sand and gravel with variable but subordinate amounts of black phosphorite rich sand the carbonate component consists primarily of molluskan fragments the distribution is patchy and discontinuous with no discernible pattern and the transition between sediment types is generally abrupt quartz rich sediment dominates the inner 15 km north of the entrance into tampa bay but south of the bay is common only along the inner 3 km elsewhere carbonate rich sediment is the predominate sediment type except where there is little sediment cover in which cases black phosphorite rich sand dominates sediment sources are likely within or around the periphery of the basin fine quartz sand is likely reworked from coastal units deposited during pleistocene sea level high stands carbonate sand and gravel is produced by marine organisms within the depositional basin the black phosphorite rich sand likely originates from the bioerosion and reworking of the underlying strata that irregularly crop out within the study area the distribution pattern contains elements of both storm and tide dominated siliciclastic shelves but it is dictated primarily by the sediment source similar to some carbonate systems other systems with similar sediment attributes include cool water carbonate sediment starved and mixed carbonate siliciclastic systems this study suggests a possible genetic link among the three systems

microstructures of structure i and ii gas hydrates from the gulf of mexico gas hydrate samples from various locations in the gulf of mexico gom differ considerably in their microstructure distinct microstructure characteristics coincide with discrete crystallographic structures gas compositions and calculated thermodynamic stabilities the crystallographic structures were established by x ray diffraction using both conventional x ray sources and high energy synchrotron radiation the microstructures were examined by cryo stage field emission scanning electron microscopy fe sem good sample preservation was warranted by the low ice fractions shown from quantitative phase analyses gas hydrate structure ii samples from the green canyon in the northern gom had methane concentrations of 70 80 and up to 30 of c2 c5 of measured hydrocarbons hydrocarbons in the crystallographic structure i hydrate from the chapopote asphalt volcano in the southern gom was comprised of more than 98 methane fairly different microstructures were identified for those different hydrates pores measuring 200 400 nm in diameter were present in structure i gas hydrate samples no such pores but dense crystal surfaces instead were discovered in structure ii gas hydrate the stability of the hydrate samples is discussed regarding gas composition crystallographic structure and microstructure electron microscopic observations showed evidence of gas hydrate and liquid oil co occurrence on a micrometer scale that demonstrates that oil has direct contact to gas hydrates when it diffuses through a hydrate matrix

appearance and preservation of natural gas hydrate from hydrate ridge sampled during odp leg 204 drilling abstract the state of preservation of natural gas hydrate samples recovered from 6 sites drilled during odp leg 204 at southern summit of hydrate ridge oregon margin has been investigated by x ray diffraction xrd and cryo scanning electron microscopy cryo sem techniques a detailed characterization of the state of decomposition of gas hydrates is necessary since no pressurized autoclave tools were used for sampling and partial dissociation must have occurred during recovery prior to the quench and storage in liquid nitrogen samples from 16 distinct horizons have been investigated by synchrotron x ray diffraction measurements at hasylab hamburg a full profile fitting analysis rietveld method of synchrotron xrd data provides quantitative phase determinations of the major sample constituents such as gas hydrate structure i si hexagonal ice ih and quartz the ice content ih in each sample is related to frozen water composed of both original existing pore water and the water from decomposed hydrates hydrate contents as measured by diffraction vary between 0 and 68 wt in the samples we measured samples with low hydrate content usually show micro structural features in cryo sem ascribed to extensive decomposition comparing the appearance of hydrates at different scales the grade of preservation seems to be primarily correlated with the contiguous volume of the original existing hydrate the dissociation front appears to be indicated by micrometer sized pores in a dense ice matrix

the characteristics of gas hydrates recovered from shenhu area in the south china sea abstract for the first time gas hydrate samples were recovered in the south china sea during a drilling campaign implemented in shenhu area in june 2007 laboratory based studies for the characteristics of gas hydrate were carried out on two hydrate bearing sediment samples observations found that gas hydrates occurred in the pores of fine grained sediments the hydrates were structure i based on the raman spectroscopic results the guest gas molecules in hydrate cages were predominantly methane 99 3 and trace amount of ethane 0 4 and propane 0 04 as determined with the method of gas chromatography as estimated from the intensities of raman peaks the cage occupancy rates were more than 99 in large cage and 86 in small cage respectively corresponding to a hydration number of 5 9

bedforms of the canadian eastern seaboard a comparison with global occurrences this paper presents a compilation of sedimentary bedforms identified from acoustic and seismic records collected over the southeastern canadian continental shelf in order to make the compilation it was found necessary to firstly review and synthesise the various terminologies used to describe the bedforms found and thereafter to relate the subsequent terminology to any diagnostic generic characteristic identifiable from the records sand waves are considered to be those forms that are composite in nature and are generated solely by the action of superimposed megaripples these latter features are identifiable adopting the classification of dalrymple et al 1978 which proves adequate for the acoustic differentiation of 2 d and 3 d types though we observed a greater coherence in bedform spacing of the 2 d types than reported hitherto sand ridges were adequately defined and recognised using the description of swift et al 1978 with respect to storm generated ridges and that of off 1963 and others with respect to tidal current ridges such ridges appear not to be uniquely described by ratios of their metrics the bedforms on the southeastern canadian continental shelf are abundant and wide spread virtually all types of bedforms recognisable on the u s shelf are present to the north the majority of these bedforms appear to be storm generated and are considered intermittently active many of the sand waves s s have no measureable relief though retaining their distinctive spatial geometry both active and moribund sand ridges were found of these ridges both tidal and storm generated types were identified the storm generated ridges were further broken down into shoreface connected ridges and offshore ridges a metrical analysis of the various bedforms show that a differentiation into stable bed phases can be made using height length ratios provided the bedforms are active and fully developed there is some overlap between megaripples and small sand waves and indeed between 2 d and 3 d megaripples yet the distinction is good between sand waves and sand ridges even moribund types sand ribbons in general can be confused with moribund sand waves and megaripples a detailed morphological examination is required to distinguish each feature data from sand ridges of the eastern canadian shelf combined with data from similar features found globally shows that ridge heights tend to cluster into three distinctive modes these modes are at heights of 7 13 and 27 m the first mode is correlated with storm generated ridges the second with tidal current ridges and the third possibly with submerged barrier islands

seafloor response to flow in a southern hemisphere sand ridge field argentine inner shelf abstract the inner continental shelf adjacent to buenos aires province argentina is characterized by a series of sand ridges averaging 4 7 m in relief and spaced about 2 7 km apart the ridges trend north south forming a 20 to 35 southward opening angle with the northeast southwest trending coast although the adjacent coast is of depositional strand plain origin it is presently undergoing erosional retreat and the formation of the ridges appears to be part of the retreat process seafloor grain size distributions and bedform arrays are responses to the same velocity field that has formed the ridges along transects normal to the ridges grain size variation is 90 out of phase with the topography the coarsest sediment is not at ridge crests or trough axes but instead occurs on the landward slope finest sands are on the seaward slope side scan sonar reveals bedforms aligned obliquely to both ridges and the coast these are inferred to be sand waves the grain size and bedform patterns suggest that the formative flows are northward currents with an offshore flow component near the bottom although flows observed during the study period were dominantly north to south the grain size gradients and bedform orientations are equivalent to those characteristic of the atlantic shelf of north america with a north south inversion the comparison suggests that inner shelf transport directions in response to wind events can be predicted from the regional geometry and climate of the shelf

frictional discharge plasma and seismo electromagnetic phenomena abstract although several mechanisms of precursory electromagnetic emissions of earthquakes have been proposed there has been no in situ observation of electromagnetic phenomena occurring at the frictional contacts during rock frictions from an experiment simulating the motion of an asperity on a fault surface we report the generation of photon emissions around frictional contacts between natural rock minerals spectroscopic analysis clarified that the photon emissions are caused by dielectric breakdown of ambient gases electric discharge plasma due to frictional electrification the plasmas were found to occur under normal stress of 4 mpa and sliding speed of 10 2 m s this indicates that plasma generations do not require high normal stresses needed to fracture rocks and sliding speeds as high as seismic rates 1 m s thus frictional discharges could occur at microscopic asperities at the onset of the slips of earthquakes and might be one of the sources of the seismo electromagnetic emission

regional stratigraphic framework linking continental shelf and coastal sedimentary deposits of west central florida abstract a regional study of the holocene sequence onlapping the west central florida platform was undertaken to merge our understanding of the barrier island system with that of the depositional history of the adjacent inner continental shelf key objectives were to better understand the sedimentary processes sediment accumulation patterns and the history of coastal evolution during the post glacial sea level rise in the subsurface deformed limestone bedrock is attributed to mid cenozoic karstic processes this stratigraphic interval is truncated by an erosional surface commonly exposed that regionally forms the base of the holocene section the holocene section is thin and discontinuous and north or south of the tampa bay area is dominated by low relief sand ridge morphologies depositional geometries tend to be more sheet like nearshore and mounded or ridge like offshore sand ridges exhibit 0 5 4 m of relief with ridge widths on the order of 1 km and ridge spacing of a few kilometers the central portion of the study area is dominated nearshore by a contiguous sand sheet associated with the tampa bay ebb tidal delta sedimentary facies in this system consist mostly of redistributed siliciclastics local carbonate production and residual sediments derived from erosion of older strata hardground exposures are common throughout the study area regional trends in holocene sediment thickness patterns are strongly correlated to antecedent topographic control both the present barrier island system and thicker sediment accumulations offshore correlate with steeper slope gradients of the basal holocene transgressive surface proposed models for coastal evolution during the holocene transgression suggest a spatial and temporal combination of back stepping barrier island systems combined with open marine low energy coastal environments the present distribution of sand resources reflects the reworking of these earlier deposits by the late holocene inner shelf hydraulic regime

possible correlation between geoelectric potential change in izu oshima island and the earthquake swarm off the east izu peninsula japan abstract since may 1987 we have monitored the geoelectric potential change in izu oshima island using three long span dipoles and ntt nippon telegraph and telephone corporation facilities occasionally anomalous potential changes which were different from those induced by geomagnetic variations have been observed their possible correlation with nearby seismic activity has been examined with the result that the daily number of appearances of one type of anomalous potential change increased shortly before m 5 events in all three earthquake swarms that have occurred since 1987 in the area off the east izu peninsula whether or not this correlation is valid and useful for prediction purpose is still not certain

rates of aftershock decay and the fractal structure of active fault systems abstract aftershock activity on frequency decay against time is characterized by a power law the modified omori formula of an exponent p which differs with each aftershock sequence a theoretical study suggested that p which is a rate constant of aftershock decay is related to the fractal dimension of a pre existing fault system this has however never been checked aftershock activity on size distribution is also characterized by an exponential distribution against magnitude the gutenberg richter relation with a slope b although p is expected to be related to b which is related to the partitioning rate of earthquake energy the relationship has never been established here the relation between the p values and the fractal dimensions of the pre existing fault systems and that between the p values and the b values are explored using natural seismicity data and data of the observable fault systems the p and b values were estimated for fifteen aftershock sequences which occurred in japan in this paper aftershocks were identified on the basis of a phenomenological definition in the seismicity data the fractal capacity dimensions d0 are estimated for the pre existing active fault systems observed on the surface in the aftershock regions in the present paper the standard box counting method was adopted to get the d0 negative correlations between 1 p and d0 and 2 p and b were observed with some scattering observation 1 shows that the rate of aftershock decay p decreases systematically with increasing occupancy rate of the pre existing active fault system d0 and suggests that aftershock decay dynamics is constrained by the pre existing fracture field observation 2 shows that p certainly has a relation with b moreover we offer possible interpretation on these negative correlations and some scatters in both observations the scatters are interpreted as the scatter of the difference of two fractal dimensions between 3 d fracture construction in the crust and 2 d cross sectional surface observed active fault system supported by further tests this paper strongly suggests that the scaling for a natural fracture system is self affine with different fractal scalings in different directions rather than self similar which would be a manifestation of regional anisotropy of the fracture system and that the seismic parameters p and b depend on the 3 d construction of the fracture system in the crust

architecture and long term evolution of a tidal sandbank the middelkerke bank southern north sea abstract the internal structure of the middelkerke bank one of the flemish banks located in the southern north sea off the coast of oostende belgium has been studied in the framework of the marine science and technology mast program co funded by the european community a dense grid of high and very high resolution seismic profiles has been used as well as several vibrocorings seven major seismic units can be identified in the quaternary sediments bounded by major discontinuities correlated across the whole study area the lower units clearly appear as being deposited during periods of relative low sea level channel infillings shoreface estuarine and or ebb tidal delta deposits the present shape of the bank results partly from recent erosional processes reworking the underlying deposits thus the lower part of the bank as a morphological feature does not consist of offshore tidal sands the master bedding of the upper part of the bank consists of inclined reflectors dipping at an angle of about 5 in the same direction as the bank s steep face these reflectors very similar to those described by houbolt 1968 are interpreted as being the result of alternating periods of deposition and erosion related to the episodic combination of tidal currents and storms

physical properties of the variations of the electric field of the earth preceding earthquakes i abstract in an earlier paper we described the physical properties of the preseismic transient variations in the electrotelluric field the present letter clarifies a few points that were not properly emphasized in our previous publications furthermore recent results like that of the destructive kalamata earthquake on september 13 1986 indicate that the duration of a preseismic electric signal may reach considerably high values lasting up to several hours

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

thermogenic gas controls high saturation gas hydrate distribution in the pearl river mouth basin evidence from numerical modeling and seismic anomalies abstract we integrate three dimensional 3d seismic data well log and two dimensional 2d modeling to understand the accumulation and saturation of gas hydrates and their relationships with biogenic and thermogenic gases in the pearl river mouth basin south china sea sites sh2 sh5 w17 and w19 the model based on physical geochemical and geological parameters from shallow gas hydrate sites deep oil sites and seismic interpretation and it is tied to well log and seismic quantitative interpretation to predict the main controlling factors for gas hydrate distribution the modeling results show that the biogenic methane generation is distributed within the strata shallower than 1500 mbsf meters below the seafloor whereas thermogenic gas generation ranges from 2300 mbsf to 6000 mbsf the erosion surfaces of migrating canyons represent important pathways for upward migration of biogenic gas resulting in average hydrate saturation 40 at the base of gas hydrate stability zone bghsz is related to the focused migration of thermogenic gas along faults and throughout permeable carrier beds although gas chimneys are widely identified from the coherence attribute extracted from 3d seismic data their impact on the obviously higher locally up to 60 gas hydrate saturations in the fine grained sediments appears site dependent and associated with fault activity by comparing well log derived gas hydrate saturations our modeling shows that about 80 of the methane forming hydrate is thermogenically sourced at sites w19 and w17 the integrated modeling effort presented in this work based on provides a better understanding of the different contributions from different gas sources for gas hydrate accumulation along the northern slope of the south china sea

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

evidence of gas hydrate from downhole logging data in the ulleung basin east sea the gas hydrate research and development organization ghdo of korea successfully accomplished both coring hydraulic piston and pressure coring and logging logging while drilling lwd and wireline logging to investigate the presence of gas hydrate during the first deep drilling expedition in the ulleung basin east sea of korea referred to as ubgh1 in 2007 the lwd data from two sites ubgh1 9 ubgh1 10 showed elevated electrical resistivity 80 m and p wave velocity 2000 m s values indicating the presence of gas hydrate during the coring period the richest gas hydrate accumulation was discovered at these intervals based on log data the occurrence of gas hydrate is primarily controlled by the presence of fractures the gas hydrate saturation calculated using archie s relation shows greater than 60 as high as 90 of the pore space although archie s equation typically overestimates gas hydrate saturation in near vertical fractures the saturation of gas hydrate is also estimated using the modified biot gassmann theory bgtl by lee and collett 2006 the saturation values estimated rom bgtl are much lower than those calculated from archie s equation based on log data the hydrate bearing sediment section is approximately 70 m ubgh1 9 to 130 m ubgh1 10 in thickness at these two sites this was further directly confirmed by the recovery of gas hydrate samples and pore water freshening collected from deep drilling core during the expedition lwd data also strongly support the interpretation of the seismic gas hydrate indicators e g vent or chimney structures and bottom simulating reflectors which imply the probability of widespread gas hydrate presence in the ulleung basin