transient seepage model for saturated unsaturated soil systems a geotechnical engineering approach a two dimensional finite element model is proposed to simulate transient seepage for complex groundwater flow systems the complete soil system is treated as a continuum encompassing flow in both saturated and unsaturated zones in the unsaturated zone the air phase is assumed to be continuous and open to atmospheric pressure the coefficient of permeability of the unsaturated soil is assumed to be a function of pore water pressure the governing differential equation is derived within a framework familiar to geotechnical engineers the stress state variables and the constitutive relationships for an unsaturated soil are used in the derivation the finite element solution to the governing differential equation is based on the galerkin weighted residual method the nonlinearity of the equation is solved by iterative procedures the finite element formulation is implemented into a computer model named trasee the model can be applied to a wide variety of problems involving complex boundary conditions and geo

mechanical properties of hydrate bearing turbidite reservoir in the first gas production test site of the eastern nankai trough abstract the study of mechanical properties of marine sediments is essential for the prediction of the occurrence of geohazards e g subsea landslides and seafloor subsidence and the design of submarine structures for offshore industry in this study triaxial compression tests of gas hydrate bearing sandy sediments and clayey silty sediments were conducted the sediments were recovered by pressure coring in the eastern nankai trough the area of the first japanese offshore production test soil index properties were measured and revealed porosity of 40 50 with porosity decreasing gradually with greater depth below the seafloor the mean particle size was less than 10 m for clayey silty sediments and approximately 100 m for sandy sediments permeability estimated by a consolidation process of triaxial testing and with x ray diffraction analysis depended on the content of fines which consisted chiefly of mica kaolinite and smectite the results of undrained compression tests for clayey silty sediments showed positive excess pore pressure under all test conditions this mechanical behavior indicates that the core samples are normally consolidated sediments drained compression tests showed that the strength and stiffness of sandy sediments increase with hydrate saturation furthermore the volumetric strain of hydrate bearing sediments changed from compression to dilative this result was obtained for hydrate saturation values sh of more than 70 the shear strength of hydrate bearing turbidite sediments of the eastern nankai trough is shown to be a function of the confining pressure

hydrogeology of two saskatchewan tills i fractures bulk permeability and spatial variability of downward flow groundwater flow patterns were compared and contrasted in two deposits of clayey till of different thicknesses both of which overlie regional aquifers in a flat upland recharge area analysis of 67 field and laboratory hydraulic conductivity determinations indicated that conductive fractures in the unoxidized zone of the thinner till dalmeny site give rise to an isotropic bulk permeability of approximately 5 10 9 ms 1 which is two orders of magnitude greater than the bulk permeability of unoxidized till in the thicker deposit warman site vertical pore water velocities estimated using hydraulic and isotopic evidence vary greatly at dalmeny and appear to be of the order of 8 m in 10 000 yr at warman the association of vertical groundwater flux depth to water table and depth of oxidized zone at a given location is referred to as the flow regime there spatially detailed water level monitoring and test drilling indicated substantial variation of the flow regime at dalmeny on a horizontal scale of tens of meters no such variation exists at warman the differentiation of the flow regime at dalmeny is due to higher till permeability which permits a spatially variable response to microtopographically focussed infiltration of surface water recognition of the causes and symptoms of flow regime variability beneath flat till landscapes has implications for environmental assessment procedures agricultural practices wetland management and waste disposal

strength behavior of methane hydrate bearing sand in undrained triaxial testing abstract gas hydrates represent a potential future energy source as well as a considerable geohazard in order to assess both the benefits and risks that gas hydrate bearing sediments pose fundamental information about their physical properties is required in this study the undrained shear strength of methane hydrate bearing sand was investigated the experimental program required modifications to an existing triaxial apparatus and accurate determination of the hydrate saturation lead to the use of two methods for comparison of the saturation calculations strength results indicated that the presence of gas hydrate will increase the sediment s undrained shear strength and corresponding stiffness the relative contribution of cohesion and friction angle was observed to be a function of the hydrate saturation for this particular hydrate formation methodology

the structure of methane gas hydrate bearing sediments from the krishna godavari basin as seen from micro ct scanning abstract the indian national gas hydrate program nghp expedition 1 of 2006 cored through several methane gas hydrate deposits on the continental shelf around the coast of india the pressure coring techniques utilized during the expedition hyacinth and pcs enabled recovery of gas hydrate bearing fine grained sediment cores to the surface after initial characterization core sections were rapidly depressurized and submerged in liquid nitrogen preserving the structure and form of the hydrate within the host sediment once on shore high resolution x ray ct scanning was employed to obtain detailed three dimensional images of the internal structure of the gas hydrate using a resolution of 80 m the detailed structure of the hydrate veins present in each core could be observed and allowed for an in depth analysis of orientation width and persistence of each vein hydrate saturation estimates could also be made and saturations of 20 30 were found to be the average across the core section with some portions showing highs of almost 60 saturation the majority of hydrate veins in each core section were found to be orientated between 50 and 80 to the horizontal analysis of the strikes of the veins suggested a slight preferential orientation in individual sample sections although correlation between individual sections was not possible due to the initial orientation of the sections being lost during the sampling stage the preferred vein orientation within sample sections coupled with several geometric features identified in individual veins suggest that hydraulic fracturing by upward advecting pore fluids is the main formation mechanism for the veined hydrate deposits in the k g basin

visual observation of gas hydrate formation and dissociation in synthetic porous media by means of glass micromodels visual observation of gas hydrates at the microscopic scale in synthetic porous media provides unequivocal visual evidence that clathrates can form in systems without the presence of a free gas phase hydrates were formed from a soluble liquid hydrate former tetrahydrofuran c 4 h 8 o from free gas ch 4 and from dissolved gas co 2 clathrates were found to form within the center of pore spaces rather than on grain surfaces cementation of grains only occurred in regions of a small grain size or where a large proportion of pore space was filled with hydrate however even at high clathrate saturations a thin film of free water persisted on grain surfaces the results have important implications for the potential cementing effect of hydrates on sediments and thus for sediment permeability slope stability and seismic interpretation of hydrate bearing sediments

major influences on the evolution of the 2 67 2 1 ga transvaal basin kaapvaal craton abstract the transvaal is one of three structural basins of the transvaal supergroup preserved on the kaapvaal craton the evolution of the neoarchaean palaeoproterozoic transvaal basin is ascribed predominantly to magmatism palaeoclimate and eustasy with plate tectonics playing a sporadic role the supergroup comprises basal protobasinal rocks followed by the black reef formation chuniespoort group and uppermost pretoria group immature siliciclastic and bimodal volcanic rocks of the protobasinal unit reflect a wide zone of rifting related to the c 2 7 ga ventersdorp supergroup mantle plume individual protobasinal successions were laid down in separate fault bounded basins controlled at least partially by greenstone belt orientations in the kaapvaal basement post magmatic post rifting thermal subsidence accommodated black reef fluvial sheet sandstones and the subsequent thick carbonate bif epeiric platform succession of the chuniespoort group subordinate mechanical subsidence accompanied this long lived thermal relaxation intense weathering due to neoarchaean atmosphere composition greatly reduced clastic sedimentation and the greenhouse palaeoclimate further encouraged carbonate sedimentation globally enhanced sea levels due to enhanced mid ocean ridge growth consequent upon either global magmatic events or supercontinent break up also played a pivotal role in chuniespoort epeiric basin evolution pretoria group sedimentation is ascribed to two cycles of rifting and subsequent thermal subsidence the first cycle appears to reflect plate tectonically induced rifting with an epeiric sea drowning the rift basin despite reduced sea levels due to the first major global glaciation the second pretoria cycle is most likely related to a major continental flood basalt event with thermal subsidence allowing a second and probably larger epeiric sea to advance onto the northern kaapvaal craton evaluation of the inferred relatively minor importance of plate tectonics in the evolution of the essentially intracratonic transvaal basin depends to a large degree on the age assigned to the limpopo orogeny during which the zimbabwe and kaapvaal cratons collided recent evidence for a c 2 0 ga collision supports the concept that catastrophic global magmatic events gradually gave way to plate tectonics during transvaal basin evolution

influence of gas hydrate morphology on the seismic velocities of sands this paper reports the results of a series of resonant column tests on specimens where gas hydrate has been formed in sands using an excess water technique in these specimens the amount of hydrate formed is restricted by the amount of gas in the specimen and with an excess of water being present in the pore space results of resonant column tests carried out to determine compressional and shear wave velocities suggest that gas hydrate formed in this way are frame supporting in contrast the behavior observed in sands where the hydrate is formed from finite water where the remaining pore space is saturated with methane gas termed in this paper the excess gas method exhibits a cementing behavior while tetrahydrofuran hydrate sands or where the hydrate is formed from dissolved methane within the pore water exhibit a pore filling behavior for hydrate saturations less than 40 for sands where the hydrate is formed using the excess water method much larger volumes of hydrate are required before a significant increase in shear wave velocity occurs although increases in compressional wave velocity are seen at lower hydrate contents these results suggest that hydrate interaction with the sediment is strongly dependent on morphology and that natural hydrate may exhibit contrasting seismic signatures depending upon the geological environment in which it forms

mechanical properties of sand silt and clay containing tetrahydrofuran hydrate 1 the mechanical behavior of hydrate bearing sediments subjected to large strains has relevance for the stability of the seafloor and submarine slopes drilling and coring operations and the analysis of certain small strain properties of these sediments for example seismic velocities this study reports on the results of comprehensive axial compression triaxial tests conducted at up to 1 mpa confining pressure on sand crushed silt precipitated silt and clay specimens with closely controlled concentrations of synthetic hydrate the results show that the stress strain behavior of hydrate bearing sediments is a complex function of particle size confining pressure and hydrate concentration the mechanical properties of hydrate bearing sediments at low hydrate concentration probably 50 of pore space the behaviorbecomesmoreindependentofstressbecausethehydratescontrolbothstiffnessand strength and possibly the dilative tendency of sediments by effectively increasing interparticle coordination cementing particles together and filling the pore space the cementation contribution to the shear strength of hydrate bearing sediments decreases with increasing specific surface of soil minerals the lower the effective confining stress the greater the impact of hydrate formation on normalized strength

physical properties and rock physics models of sediment containing natural and laboratory formed methane gas hydrate this paper presents results of shear strength and acoustic velocity p wave measurements performed on 1 samples containing natural gas hydrate from the mallik 2l 38 well mackenzie delta northwest territories 2 reconstituted ottawa sand samples containing methane gas hydrate formed in the laboratory and 3 ice bearing sands these measurements show that hydrate increases shear strength and p wave velocity in natural and reconstituted samples the proportion of this increase depends on 1 the amount and distribution of hydrate present 2 differences in sediment properties and 3 differences in test conditions stress strain curves from the mallik samples suggest that natural gas hydrate does not cement sediment grains however stress strain curves from the ottawa sand containing laboratory formed gas hydrate do imply cementation is present acoustically rock physics modeling shows that gas hydrate does not cement grains of natural mackenzie delta sediment natural gas hydrates are best modeled as part of the sediment frame this finding is in contrast with direct observations and results of ottawa sand containing laboratory formed hydrate which was found to cement grains waite et al 2004 it therefore appears that the microscopic distribution of gas hydrates in sediment and hence the effect of gas hydrate on sediment physical properties differs between natural deposits and laboratory formed samples this difference may possibly be caused by the location of water molecules that are available to form hydrate models that use laboratory derived properties to predict behavior of natural gas hydrate must account for these differences

observations related to tetrahydrofuran and methane hydrates for laboratory studies of hydrate bearing sediments the interaction among water molecules guest gas molecules salts and mineral particles determines the nucleation and growth behavior of gas hydrates in natural sediments hydrate of tetrahydrofuran thf has long been used for laboratory studies of gas hydrate bearing sediments to provide close control on hydrate concentrations and to overcome the long formation history of methane hydrate from aqueous phase methane in sediments yet differences in the polarizability of thf polar molecule compared to methane nonpolar molecule raise questions about the suitability of thf as a proxy for methane in the study of hydrate bearing sediments from existing data and simple macroscale experiments we show that despite its polar nature thf s large molecular size results in low permittivity prevents it from dissolving precipitated salts and hinders the solvation of ions on dry mineral surfaces in addition the interfacial tension between water and thf hydrate is similar to that between water and methane hydrate the processes that researchers choose for forming hydrate in sediments in laboratory settings e g from gas liquid or ice and the pore scale distribution of the hydrate that is produced by each of these processes likely have a more pronounced effect on the measured macroscale properties of hydrate bearing sediments than do differences between thf and methane hydrates themselves

volume change associated with formation and dissociation of hydrate in sediment 1 gas hydrate formation and dissociation in sediments are accompanied by changes in the bulk volume of the sediment and can lead to changes in sediment properties loss of integrity for boreholes and possibly regional subsidence of the ground surface over areas where methane might be produced from gas hydrate in the future experiments on sand silts and clay subject to different effective stress and containing different saturations of hydrate formed from dissolved phase tetrahydrofuran are used to systematically investigate the impact of gas hydrate formation and dissociation on bulk sediment volume volume changes in low specific surface sediments i e having a rigid sediment skeleton like sand are much lower than those measured in high specific surface sediments e g clay early hydrate formation is accompanied by contraction for all soils and most stress states in part because growing gas hydrate crystals buckle skeletal force chains dilation can occur at high hydrate saturations hydrate dissociation under drained zero lateral strain conditions is always associated with some contraction regardless of soil type effective stress level or hydrate saturation changes in void ratio during formation dissociation decrease at high effective stress levels the volumetric strain during dissociation under zero lateral strain scales with hydrate saturation and sediment compressibility the volumetric strain during dissociation under high shear is a function of the initial volume average void ratio and the stress dependent critical state void ratio of the sediment other contributions to volume reduction upon hydrate dissociation are related to segregated hydrate in lenses and nodules for natural gas hydrates some conditions e g gas production driven by depressurization might contribute to additional volume reduction by increasing the effective stress

glacial lake saskatchewan and lake agassiz deltas in east central saskatchewan with special emphasis on the nipawin delta the nipawin delta extends eastward from the town of nipawin in east central saskatchewan the prince albert and fort a la corne deltas formed in lake saskatchewan at the mouths of the south and north saskatchewan rivers whereas the nipawin and mossy vale deltas formed in lake agassiz at the mouth of the saskatchewan river these deltas became lower in elevation and younger eastward clays beneath the nipawin and mossyvale deltas can be traced beneath the prince albert and fort a la corne deltas with little change in thickness suggesting that most of the clays beneath the nipawin and mossyvale deltas were deposited during the formation of the prince albert and fort a la corne deltas the nipawin delta formed between 10 200 and 9500 bp the campbell strandline emerson phase was the shoreline of the nipawin delta the mossyvale delta formed about 9500 bp the surface of the mossyvale delta mccauleyville level is about 40 m below the surface of the nipawin delta campbell level

gas production from a cold stratigraphically bounded gas hydrate deposit at the mount elbert gas hydrate stratigraphic test well alaska north slope implications of uncertainties abstract as part of an effort to identify suitable targets for a planned long term field test we investigate by means of numerical simulation the gas production potential from unit d a stratigraphically bounded class 3 permafrost associated hydrate occurrence penetrated in the bpxa doe usgs mount elbert gas hydrate stratigraphic test well on north slope alaska this shallow low pressure deposit has high porosities 0 4 high intrinsic permeabilities k 10 12 m2 and high hydrate saturations sh 0 65 it has a low temperature t 2 3 2 6 c because of its proximity to the overlying permafrost the simulation results indicate that vertical wells operating at a constant bottomhole pressure would produce at very low rates for a very long period horizontal wells increase gas production by almost two orders of magnitude but production remains low sensitivity analysis indicates that the initial deposit temperature is by the far the most important factor determining production performance and the most effective criterion for target selection because it controls the sensible heat available to fuel dissociation thus a 1 c increase in temperature is sufficient to increase the production rate by a factor of almost 8 production also increases with a decreasing hydrate saturation because of a larger effective permeability for a given k and is favored to a lesser extent by anisotropy

hydrate morphology physical properties of sands with patchy hydrate saturation 1 the physical properties of gas hydrate bearing sediments depend on the volume fraction and spatial distribution of the hydrate phase the host sediment grain size and the state of effective stress determine the hydrate morphology in sediments this information can be used to significantly constrain estimates of the physical properties of hydrate bearing sediments including the coarse grained sands subjected to high effective stress that are of interest as potential energy resources reported data and physical analyses suggest hydrate bearing sands contain a heterogeneous patchy hydrate distribution whereby zones with 100 pore space hydrate saturation are embedded in hydrate free sand accounting for patchy rather than homogeneous hydrate distribution yields more tightly constrained estimates of physical properties in hydrate bearing sands and captures observed physical property dependencies on hydrate saturation for example numerical modeling results of sands with patchy saturation agree with experimental observation showing a transition in stiffness starting near the series bound at low hydrate saturations but moving toward the parallel bound at high hydrate saturations the hydrate patch size itself impacts the physical properties of hydrate bearing sediments for example at constant hydrate saturation we find that conductivity electrical hydraulic and thermal increases as the number of hydrate saturated patches increases this increase reflects the larger number of conductive flow paths that exist in specimens with many small hydrate saturated patches in comparison to specimens in which a few large hydrate saturated patches can block flow over a significant cross section of the specimen

erratum the gronlid crossing saskatchewan provincial highway no 6 at the saskatchewan river a traffic connection between regina and the northeastern part of saskatchewan required a crossing of the saskatchewan river north of gronlid slope instability developed in lacustrine and deltaic sands on the approaches to a ferry site installed in the early 1960s alternative ferry sites were considered but a river hydraulics study showed that other sites would not offer any improvement the creation of a reservoir by projected dam at nipawin and potential mining development in the area added incentive for the construction of a bridge to replace the original ferry installation valuable information was obtained from the ferry site for locating the new bridge and designing the approach roadways it was possible to locate the bridge and approaches to minimize seepage foundation and slope instability problems in a deposit of over 100 m of saturated interbedded deltaic sand lacustrine silt and clay a major artesian groundwater system was found in the bedrock sand of the swan river group whereas groundwa

mechanical behavior of hydrate bearing pressure core sediments visualized under triaxial compression abstract geomechanical and geotechnical properties are essential for evaluating the stability of deep seabed and subsea production systems for gas hydrate extraction from marine sediments in this study natural gas hydrate bearing sediment was subjected to triaxial compression tests shearing using a newly developed triaxial testing system tactt to investigate the geomechanical behavior of sediments recovered from below the seafloor in the eastern nankai trough where the first japanese offshore production test was conducted in 2013 the sediments were recovered using a hybrid pressure coring system with pressure cores cut using onboard pressure core analysis tools the pressure cores were subsequently transferred to our shore based laboratory and subsampled using pressure core non destructive analysis tools pnats for the tactt system pressure and temperature conditions were maintained within the hydrate stability boundary during coring and laboratory testing an image processing technique was used to capture deformation of the sediment sample within the transparent acrylic test cell and digital photographs were obtained for each 0 1 strain level experienced by the sample during the triaxial compression test analysis of the digitized images showed that sediments with 63 hydrate saturation exhibited brittle failure whereas hydrate free sediments exhibited ductile failure the increase in shear strength with increasing hydrate saturation in natural gas hydrates is in agreement with previous data from sediments containing synthetic gas hydrates

unraveling the record of successive high grade events in the central zone of the limpopo belt using pb single phase dating of metamorphic minerals abstract dating of relic metamorphic assemblages can provide important information about the timing and character metamorphic grade and or pt evolution of early high grade episodes in polymetamorphic provinces using pb stepwise leaching of metamorphic silicates we have dated multiple granulite facies metamorphic episodes in the central zone cz of the limpopo belt ages of 2 52 ga were obtained from sillimanite and cogenetic garnet and ages of about 2 01 ga from titanite garnet and clinopyroxene together with new and published conventional age data from accessory phases and in the context of combined petrological and structural data these results lead us to a reinterpretation of the tectono metamorphic history of the cz three distinct high grade events at about 3 2 3 1 ga 2 65 2 52 ga and 2 0 0 05 ga are recognized each of these is suggested to correspond to a tectonic episode of distinct character a for the 3 2 ga event magmatic activity can mainly be identified best represented for example by the sand river gneisses or the messina layered intrusion the field relationships concerning the tectonometamorphic history of this early archean event are largely erased by at least two high grade metamorphic overprints b late archean 2 6 2 52 ga low pressure granulite facies metamorphism was associated with voluminous granitic and charnockitic plutonism the anticlockwise p t evolution of these granulites probably reflects deep crustal processes associated with magmatic underplating or in plating contemporaneous with vertical crustal growth of the zimbabwe craton around 2 6 ga c during the proterozoic event 2 05 1 95 ga tectonic thickening was caused by the collision of the kaapvaal and zimbabwe cratons the cz was squeezed between these two cratons and as a consequence underwent high pressure granulite facies metamorphism with a clockwise p t evolution the structural metamorphic and geochronological data can be best explained with a tectonic model that describes this final event as a dextral transpressive orogeny

ice thrust features and the maymont landslide in the north saskatchewan river valley the battleford glacier created shearing and softening of bedrock clays contributing to widespread slope instability along the valley of the north saskatchewan river the battleford glacier flowed southeastward down the valley of the north saskatchewan river during its final stage in west central saskatchewan it created flutings crevasse fillings folding and faulting in ice thrust ridges and extensive horizontal gouge zones filled with soft malleable clay extensive drilling and testing for the foundations of the maymont bridge provided definitive data outlining at least three gouge zones the shallowest gouge zone at an elevation of 465 m defines the base of the most severe glacial folding and deformation of the bedrock in at least one of the ice thrust ridges the deeper gouge zones are at about 433 and 438 m elevation postglacial landslides slumps along the southern side of the north saskatchewan river are controlled by the gouge zones which are low friction slide surfaces along which large ho

the petrofka landslide saskatchewan both abutments and all piers of the petrofka bridge are on a landslide the bridge has performed satisfactorily for 23 years indicating the factor of safety of the petrofka landslide is greater than unity this stability is abnormal because landslide slopes on bedrock clays shale in this region are notoriously unstable a dormant landslide is difficult to analyze because the input parameters must represent a condition of limiting equilibrium the petrofka landslide is 70 m high 880 m long 3000 m wide and up to 100 m thick the shear zone is in highly plastic montmorillonitic clays in an unnamed formation of probable tertiary age the upper cretaceous lea park judith river and bearpaw formations and the unnamed formation underly the landslide and are restricted to a collapse structure resulting from dissolution of devonian evaporites a retrogressive mechanism developed as valley downcutting progressed the pore water pressures could be estimated from the hydraulic head in the artesian aquifer of th

the measurement of soil stiffness in the triaxial apparatus this paper describes a simple technique for accurately measuring the mean local axial strains of triaxial samples over a central gauge length the technique makes use of an axial displacement gauge which is a development of one devised by burland symes 1982 which makes use of electrolytic levels the device can resolve to less than 1 m over a range of 15 mm is simple to mount on the specimen and is not damaged when the sample is taken to failure the results of undrained triaxial tests are presented for a wide spectrum of soil types ranging from sands through intact reconstituted and remoulded low plasticity till undisturbed london clay to intact unweathered chalk the test results show that conventional external measurements of displacement contain errors which are frequently so large that their use in the determination of soil stiffness at working levels of stress is invalid the errors mainly result from tilting of the sample bedding at the end platens and the effects of compliance in the app

movement and stability analysis of the beaver creek landslide saskatchewan canada this paper presents a case history of a multiblock landslide where the blocks move at varying rates along a common horizontal slip surface which follows the contact between stratified drift and underlying till movement measurements indicate that the blocks towards the toe move at a higher rate than blocks towards the scarp stability analyses show that the entire slide mass can be analyzed as a single unit as opposed to considering each block separately this finding is compared with the analysis of other multiblock slides the friction angle mobilized along the horizontal slip surface falls within the range of residual values measured in the laboratory key words landslides stability analysis translational slides residual strength

laboratory formation of noncementing hydrates in sandy sediments natural hydrate bearing sediment hbs predominantly exists in noncementing habit and its limited availability for use in laboratory studies demands a time effective and repeatable laboratory process for forming representative samples with natural accumulation habit this study reports on a three step laboratory process for forming noncementing methane hydrate in sandy sediments 1 initial hbs formation under excess gas conditions 2 slow saline water 5 wt cacl2 injection under strictly controlled pressure temperature p t conditions and 3 a temperature warming cooling cycle changes in compressional wave velocity vp of sediment as well as p t condition were monitored throughout the tests the evolution of vp in good agreement with rock physics model calculations suggested that the transition from cementing hydrate into noncementing hydrate occurs during saline injection as well as temperature warming cooling cycle the proposed process appeared to be an efficient and consistent substitute for the existing methods to form noncementing hydrate habit in sandy sediments

capillary controls on methane hydrate distribution and fracturing in advective systems 1 we simulate 1 d steady advective flow through a layered porous medium to investigate how capillary controls on solubility including the gibbs thomson effect in fine grained sediments affect methane hydrate distribution in marine sediments we compute the increase in pore fluid pressure that results from hydrate occluding the pore space and allow fractures to form if the pore fluid pressure exceeds a fracture criterion we apply this model to hydrate ridge and northern cascadia two field sites where hydrates have been observed preferentially filling cm scale coarse grained layers we find that at hydrate ridge hydrate forms in the coarse grained layers reaching saturation of 90 creating fractures through intervening fine grained layers after 2000 years at northern cascadia hydrate forms preferentially in the coarse grained layers but 2 105 years are required to develop the observed hydrate saturations 20 60 suggesting that hydrate formation rates may be enhanced by an additional source of methane such as in situ methanogenesis we develop expressions to determine the combinations of sediment physical properties and methane supply rates that will result in hydrate filled coarse grained layers separated by hydrate filled fine grained layers the conditions necessary to fracture the fine grained layers and the conditions that will lead to complete inhibition of hydrate formation as pore space is constricted this work illustrates how sediment physical properties control hydrate distribution at the pore scale and how hydrate distribution affects fracturing behavior in marine sediments

a transient analysis of slope stability following drawdown after flooding of a highly plastic clay the stability of slopes at bridge abutments across the carrot river in east central saskatchewan was not influenced significantly by drawdown after flooding in the spring of 1995 traditional metho

microstructural evolution of gas hydrates in sedimentary matrices observed with synchrotron x ray computed tomographic microscopy the formation process of gas hydrates in sedimentary matrices is of crucial importance for the physical and transport properties of the resulting aggregates this process has never been observed in situ at submicron resolution here we report on synchrotron based microtomographic studies by which the nucleation and growth processes of gas hydrate were observed at 276 k in various sedimentary matrices such as natural quartz with and without admixtures of montmorillonite type clay or glass beads with different surface properties at varying water saturation both juvenile water and metastably gas enriched water obtained from gas hydrate decomposition was used xenon gas was employed to enhance the density contrast between gas hydrate and the fluid phases involved the nucleation sites can be easily identified and the various growth patterns are clearly established in sediments under saturated with juvenile water nucleation starts at the water gas interface resulting in an initially several micrometer thick gas hydrate film further growth proceeds to form isometric single crystals of 10 20 m size the growth of gas hydrate from gas enriched water follows a different pattern via the nucleation in the bulk of liquid producing polyhedral single crystals a striking feature in both cases is the systematic appearance of a fluid phase film of up to several micron thickness between gas hydrates and the surface of the quartz grains these microstructural findings are relevant for future efforts of quantitative rock physics modeling of gas hydrates in sedimentary matrices and explain the anomalous attenuation of seismic sonic waves

synchrotron x ray computed microtomography study on gas hydrate decomposition in a sedimentary matrix in situ synchrotron x ray computed microtomography with sub micrometer voxel size was used to study the decomposition of gas hydrates in a sedimentary matrix xenon hydrate was used instead of methane hydrate to enhance the absorption contrast the microstructural features of the decomposition process were elucidated indicating that the decomposition starts at the hydrate gas interface it does not proceed at the contacts with quartz grains melt water accumulates at retreating hydrate surface the decomposition is not homogeneous and the decomposition rates depend on the distance of the hydrate surface to the gas phase indicating a diffusion limitation of the gas transport through the water phase gas is found to be metastably enriched in the water phase with a concentration decreasing away from the hydrate water interface the initial decomposition process facilitates redistribution of fluid phases in the pore space and local re formation of gas hydrates the observations allow also rationalizing earlier conjectures from experiments with low spatial resolutions and suggest that the hydrate sediment assemblies remain intact until the hydrate spacers between sediment grains finally collapse possible effects on mechanical stability and permeability are discussed the resulting time resolved characteristics of gas hydrate decomposition and the influence of melt water on the reaction rate are of importance for a suggested gas recovery from marine sediments by depressurization this article is protected by copyright all rights reserved

a laboratory investigation into the seismic velocities of methane gas hydrate bearing sand remote seismic methods which measure the compressional wave p wave velocity vp and shear wave s wave velocity vs can be used to assess the distribution and concentration of marine gas hydrates in situ however interpreting seismic data requires an understanding of the seismic properties of hydrate bearing sediments which has proved problematic because of difficulties in recovering intact hydrate bearing sediment samples and in performing valid laboratory tests therefore a dedicated gas hydrate resonant column ghrc was developed to allow pressure and temperature conditions suitable for hydrate formation to be applied to a specimen with subsequent measurement of both vp and vs made at frequencies and strains relevant to marine seismic investigations thirteen sand specimens containing differing amounts of evenly dispersed hydrate were tested the results show a bipartite relationship between velocities and hydrate pore saturation with a marked transition between 3 and 5 hydrate pore saturation for both vp and vs this suggests that methane hydrate initially cements sand grain contacts then infills the pore space these results show in detail for the first time using a resonant column how hydrate cementation affects elastic wave properties in quartz sand this information is valuable for validating theoretical models relating seismic wave propagation in marine sediments to hydrate pore saturation

sampling disturbance in hydrate bearing sediment pressure cores nghp 01 expedition krishna godavari basin example sampling natural sediments causes unavoidable disturbance as recovered sediments experience changes in stress and strain during drilling core recovery transportation handling and early stages of testing in hydrate bearing sediments the potential for sampling disturbance may be aggravated since pressure and temperature changes can lead to hydrate dissociation and gas exsolution pressure core technology attempts to recover and characterize hydrate bearing sediments while preserving them under in situ pressure and temperature conditions which is an essential requirement to assess the mechanical physical chemical and biological properties of natural hydrate bearing sediments previous studies on near surface sampling effects are extended in this study to evaluate additional sampling disturbances relevant to hydrate bearing sediments 1 hydrate dissociation due to mechanical extension 2 negative pore pressure generation during unloading mandelecryer effect 3 secondary hydrate formation 4 changes in hydrate mass as a function of changes in pressure and temperature within the stability field 5 hydrate anomalous preservation and its benefits for pressure core handling and testing and 6 relaxation aging following sampling results provide valuable insight to sampler design coring and operation procedures high pressure chamber design and pressure core testing techniques

the hepburn landslide an interactive slope stability and seepage analysis mechanisms of active landslides are continuous processes involving the dynamics of slope failure interacting with the groundwater regime this process is simulated in phases by combining stability calculations interactively with a seepage analysis to determine the factor of safety for a dormant landslide near hepburn saskatchewan the landslide is a multiple retrogressive failure with two parallel slip surfaces at different elevations the slip surfaces are in originally overconsolidated cretaceous and tertiary clays softened by shear from glaciation an artesian aquifer is present at the base of the slip surface causing saline springs at the base of the valley slope a residual effective friction angle of 6 7 with zero cohesion was found to best characterize the shear strength of the clays in the failure zone potential nets and head profiles from the seepage analysis illustrate the strong influence of changing topography on the groundwater flow system the present day factor of safety for the dormant

measured acoustic wave velocities of r11 ccl3f hydrate samples with and without sand as a function of hydrate concentration we report acoustic laboratory measurements on initially unconsolidated sand packs with hydrate formation taking place in the pore space both p and s wave velocities were measured hydrate was formed from water and the refrigerant r11 ccl3f at 2 c and atmospheric pressure measurements were performed on two types of initially unconsolidated sand packs with average grain size 100 m and 280 m respectively p wave velocities varied from about 1700 m s at low hydrate content to 3810 m s at a calculated hydrate fraction in the pore space equal to 0 52 s wave velocities were only detectable at hydrate fractions larger than 0 35 where s wave velocities up to 2230 m s were measured our measurements indicate that acoustic measurements are not sensitive to small amounts of hydrate in a system of initially unconsolidated sand the data show a very distinct change in behavior when the hydrate fraction exceeds 0 35 the concentration at which hydrate cementation of the sand grains is believed to occur measurements for samples without sand samples containing water r11 and hydrate are also reported for hydrate without sand p wave velocities ranged from about 1400 m s at very low hydrate content to about 2500 m s at a measured hydrate fraction equal to 0 68 for these samples reliable s wave data were not detected

red deer hill a drumlinized glaciotectonic feature near prince albert saskatchewan canada red deer hill is 5 km long 3 km wide and 70 m high the hill is a glaciotectonic structure formed by a single 106 m thick thrust block presumably derived from the 80 m deep upglacier holmes depression the thrust block became dislodged by displacement along a presheared horizontal gouge zone where the shearing resistance of the clay was reduced to a residual state by the first mennon glaciation the hill depression structure was formed by the battleford glacier which drumlinized red deer hill before depositing a veneer of till over the structure the holmes depression was filled with glacial lake saskatchewan lacustrine and deltaic sediments horizontal displacement along a decollement in the direction of the activating force resulted in a stress environment changing from extension to compression in the zone of extension holmes depression an active rankine state developed and beds stretched and thinned in the zone of compression red deer hill conversely a passive rankine state developed an

deformation of the deer creek bridge by an active landslide in clay shale two parallel concurrently active slip surfaces of a landslide in clay shale of the cretaceous lea park formation are causing deformation of a bridge structure across the north saskatchewan river n

stratigraphy and structure of a late wisconsinan salt collapse in the saskatoon low south of saskatoon saskatchewan canada an update the saskatoon low is a collapse structure that formed as a result of dissolution of salt from the middle devonian prairie evaporite formation in this study the collapse has affected the upper cretaceous lea park judith river and bearpaw formations of the montana group the early and middle pleistocene mennon dundurn and warman formations of the sutherland group and the late pleistocene floral battleford and haultain formations of the saskatoon group locally the collapse is about 180 m which is about equal to the thickness of the salt the first phase of collapse took place after deposition of the ardkenneth member of the bearpaw formation and before glaciation or during a pre illinoian glaciation the second phase of collapse occurred during the battleford glaciation late wisconsinan prior to deposition of the battleford formation the saskatoon low was glacially eroded removing the sutherland group and the floral formation after the glacial erosion up to 110 m of soft till of the battle

methane hydrate formation in partially water saturated ottawa sand bulk properties of gas hydrate bearing sediment strongly depend on whether hydrate forms primarily in the pore fluid becomes a load bearing member of the sediment matrix or cements sediment grains our compressional wave speed measurements through partially water saturated methane hydrate bearing ottawa sands suggest hydrate surrounds and cements sediment grains the three ottawa sand packs tested in the gas hydrate and sediment test laboratory instrument ghastli contain 38 1 porosity initially with distilled water saturating 58 31 and 16 of that pore space respectively from the volume of methane gas produced during hydrate dissociation we calculated the hydrate concentration in the pore space to be 70 37 and 20 respectively based on these hydrate concentrations and our measured compressional wave speeds we used a rock physics model to differentiate between potential pore space hydrate distributions model results suggest methane hydrate cements unconsolidated sediment when forming in systems containing an abundant gas phase

triaxial compressive properties of artificial methane hydrate bearing sediment 1 knowledge of the mechanical properties of gas hydrate bearing sediments is essential for simulating the geomechanical response to gas extraction from a gas hydrate reservoir in this study drained triaxial compression tests were conducted on artificial methane hydrate bearing sediment samples under hydrate stable temperature pressure conditions toyoura sand average particle size d50 0 230 mm number 7 silica sand d50 0 205 mm and number 8 silica sand d50 0 130 mm were used as the skeleton of each specimen axial loading was conducted at an axial strain rate of 0 1 min 1 at a constant temperature of 278 k the cell and pore pressures were kept constant during axial loading we found that the strength and stiffness of the hydrate sand specimens increased with methane hydrate saturation and with the effective confining pressure and the secant poisson s ratio decreased with the effective confining pressure the stiffness depends on the type of sand forming the skeleton of the specimens although the strength has little dependence on the type of sand according to an earlier work hydrate sand specimens are thought to contract in the early stage of axial loading before starting to expand owing to the dilatancy effect as is the case for many other geological materials the test results in this study are discussed in relation to the deformation mechanism proposed in an earlier work

the effects of hydrate cement on the stiffness of some sands cementing of sediment occurs naturally in many soils and weak rocks during both the early and late stages of diagenesis this paper reports the results of a series of resonant column tests carried out on a range of sand sized geomaterials to explore the effects of different hydrate cement morphologies on the very small strain stiffness of the materials it is shown that the proportion of void space filled by hydrate cement cement location sand size and grain shape all have a significant effect on shear modulus the change in stiffness between different host materials is affected by the density specific surface and grading of the geomaterial

influence of fines content on the mechanical behavior of methane hydrate bearing sediments methane hydrate bearing sediments with different amounts of fines content and at three densities were artificially prepared under controlled temperature and pressure conditions the void ratios of specimens after isotropic consolidation tend to decrease with a rise in fines content the fines particles enter into the pore space between sand grains and densify the specimens a series of triaxial compression tests were performed to systematically investigate the influences of fines content and density on the shear properties of hydrate free sediments and methane hydrate bearing sediments the test results demonstrate that a rise in fines content within methane hydrate bearing sediments significantly enhances peak shear strength and promotes dilation behavior these influences are particularly prominent for specimens at loose packing state a decrease in void ratio increases the shear strength and stiffness of hydrate free sediments and methane hydrate bearing sediments containing fines content of 0 and 8 9 it is noted that the formation of methane hydrate in samples with varying amounts of fines content increases the stress ratios at the critical state the addition of fines particles into coarse grained sand grains alters the internal microstructure of sand matrix and the hydrate formation pattern in the pore space between sand grains and fines particles

anatomy of a fossil landslide from the pleistocene of se spain abstract pleistocene landslide failures are rarely recorded in the literature due to difficulties in recognition in the field an adaptation of standard landslide classification based on activity is used to identify and examine the significance of a fossil pleistocene landslide from the mocatan catchment of the sorbas basin se spain recognition of the landslide is based on identifying key but subtle anomalies in the geology and drainage the fossil landslide at mocatan developed principally in response to a rapid increase in valley incision rates associated with capture induced base level changes in the master drainage this was facilitated by the weak sand and silt lithology of the study area the landslide was a significant morphological modifier of the environment holocene drainage follows the original landslide morphology whilst more recent gullying and piping exploits planes of weakness developed within the internal body of the landslide feature in addition the landslide removed much of the pleistocene divide facilitating future river capture

elastic wave velocity in marine sediments with gas hydrates effective medium modeling we offer a first principle based effective medium model for elastic wave velocity in unconsolidated high porosity ocean bottom sediments containing gas hydrate the dry sediment frame elastic constants depend on porosity elastic moduli of the solid phase and effective pressure elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation to model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions a hydrate modifies the pore fluid elastic properties without affecting the frame b hydrate becomes a component of the solid phase modifying the elasticity of the frame the goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data we apply the model to sonic and vsp data from odp hole 995 and obtain hydrate concentration estimates from assumption b consistent with estimates obtained from resistivity chlorinity and evolved gas data

a mechanism for the formation of methane hydrate and seafloor bottom simulating reflectors by vertical fluid expulsion bottom simulating reflectors bsr are observed commonly at a depth of several hundred meters below the seafloor in continental margin sedimentary sections that have undergone recent tectonic consolidation or rapid accumulation they are believed to correspond to the deepest level at which methane hydrate clathrate is stable we present a model in which bsr hydrate layers are formed through the removal of methane from upward moving pore fluids as they pass into the hydrate stability field in this model most of the methane is generated below the level of hydrate stability but not at depths sufficient for significant thermogenic production the methane is primarily biogenic in origin the model requires either a mechanism to remove dissolved methane from the pore fluids or disseminated free gas carried upward with the pore fluid the model accounts for the evidence that the hydrate is concentrated in a layer at the base of the stability field for the source of the large amount of methane contained in the hydrate and for bsrs being common only in special environments strong upward fluid expulsion into the hydrate stability field does not occur in normal sediment depositional regimes so bsrs are uncommon upward fluid expulsion does occur as a result of tectonic thickening and loading in subduction zone accretionary wedges and in areas where rapid deposition results in initial undercconsolidation in these areas hydrate bsrs are common the most poorly quantified aspect of the model is the efficiency with which methane is removed and hydrate is formed as pore fluids pass into the hydrate stability field the critical boundary in the phase diagram between the fluid plus hydrate and fluid only fields is not well constrained however the amount of methane required to form the hydrate and limited data on methane concentrations in pore fluids from deep sea boreholes suggest very efficient removal of methane from rising fluid that may contain less than the amount required for free gas production in most fluid expulsion regimes the quantity of fluid moved upward to the seafloor is great enough to continually remove the excess chloride and the residue of isotope fractionation resulting from hydrate formation thus as observed in borehole data there are no large chloride or isotope anomalies remaining in the local pore fluids the differences in the concentration of methane and probably of co2 in the pore fluid above and below the base of the stability field may have a significant influence on early sediment diagenetic reactions

pcats triaxial a new geotechnical apparatus for characterizing pressure cores from the nankai trough japan abstract understanding the physical nature and mechanical behaviour of hydrate bearing sediments is of fundamental importance in assessing the resource potential of methane gas hydrates advances in pressure coring techniques and associated processing equipment have enabled intact samples to be recovered under in situ pressures however testing of these samples under the in situ stress conditions has not been possible to help address this issue the pcats triaxial apparatus was developed to enable the physical properties of such samples to be measured the apparatus was deployed for the first time during the jogmec funded site investigation of the eastern nankai trough in the summer of 2012 prior to a planned hydrate production test in 2013 a number of pressurized core were recovered and sub samples successfully tested in pcats triaxial to determine a range of geomechanical properties including small strain stiffness from resonance testing stress strain properties triaxial shear tests and permeability samples tested included fine grained soils with no appreciable hydrate sands with hydrate saturation greater than 20 and one sample that had a combination of both materials testing showed an increase in stiffness and undrained shear strength with increasing grain size hydrate saturation and applied effective stress permeability was significantly reduced for hydrate bearing sands compared to clayey samples with no hydrate present

the impact of ice on the tensile strength of unconsolidated sand a model for gas hydrate bearing sands abstract tensile strength is an important parameter when it comes to predictions of potential fracturing of sediments by natural processes such as the emplacement of ice or gas hydrate lenses as well as anthropogenic fracturing or else the stability of engineering constructions such as boreholes yet tensile strength measurements of unconsolidated ice bearing or gas hydrate bearing sands are scarce and affected by a large variability in the course of the sugar project we successfully used ice as a model for pore filling and load bearing gas hydrate in sand to determine compressional wave velocity we were thus able to verify comparable formation characteristics and morphologies of ice and gas hydrate within the pore space as these are important values for the tensile strength of ice hydrate bearing sands ice was also used as a model for hydrate bearing sands despite differences in the mechanical behavior and strength of pure ice and gas hydrate water saturated sand cores with ice saturations sice between 0 and 100 were tested at 6 8 c the varying sice were a result of the freezing point depression caused by saline solutions of different concentrations the was directly determined using a sleeve fracturing test with an internal pressure that was created within the frozen samples the setup was also adapted to fit a pressure vessel for tests using confining pressure the correlation of sice shows an exponential increase of with sice whereas at sice 80 in conjunction with the change in strength the viscoelastic behavior changes a clear peak strength occurs at sice 80 we conclude that given 60 80 the frame gains strength while the amount of residual water decreases tensile failure and cracking now exceed grain boundary sliding as the prevailing failure mode the ice morphology in the sand is non cementing and comparable to a gas hydrate sand mixture

physical properties of hydrate bearing sediments 1 methane gas hydrates crystalline inclusion compounds formed from methane and water are found in marine continental margin and permafrost sediments worldwide this article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate bearing sediments formation phenomena include pore scale habit solubility spatial variability and host sediment aggregate properties physical properties include thermal properties permeability electrical conductivity and permittivity small strain elastic p and s wave velocities shear strength and volume changes resulting from hydrate dissociation the magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate bearing sediments to changes in mechanical thermal or chemical boundary conditions these predictions are vital for mitigating borehole local and regional slope stability hazards optimizing recovery techniques for extracting methane from hydrate bearing sediments or sequestering carbon dioxide in gas hydrate and evaluating the role of gas hydrate in the global carbon cycle

experimental verification of a prediction model for hydrate bearing sand this paper presents an experimental verification of a prediction model for the mechanical properties of hydrate bearing sand the model is examined using experimental drained triaxial test results of three independent data sets which are associated with different hydrate formations and testing conditions for each data set an optimization process is applied based on numerical modeling of the testing conditions in order to evaluate the pure sand properties based on these properties the model forecasts the stress stain curves for different hydrate saturations based on an advanced hydrate simulator although the model does not show a fair forecast with ice seeding hydrate formation samples and for specimens tested under gas saturation it predicts the mechanical behavior of samples with partly saturated hydrate formation tested under water saturation

instrumented pressure testing chamber for characterizing sediment cores recovered at in situ hydrostatic pressure marine sediments retrieved with pressure coring systems such as those used by the integrated ocean drilling program maintain samples at in situ hydrostatic pressure throughout recovery such pressure cores are particularly important for the study of gas hydrate bearing sediments which must be maintained within the gas hydrate stability field during shipboard core characterization until now there has been no device capable of directly measuring a suite of physical properties on sediments contained in pressure cores without first depressurizing them this study describes the design construction and deployment of the instrumented pressure testing chamber iptc which was first used to measure the physical properties of pressure cores recovered 1530 m below the sea surface during 2005 drilling in the gulf of mexico the iptc permits drilling through the plastic liner while the core is under pressure and the sequential measurement of p and s wave velocities undrained strength and electrical conductivity preliminary results indicate that the seismic velocities measured on a pressure core with the iptc are more representative of in situ seismic velocities at comparable depths than are velocities measured on conventional cores from these same depths the flexibility of the iptc design allows future modifications to account for restoration of effective not just hydrostatic stress the addition of new types of measurements sampling of pore fluids and sediments under pressure and use of the device as a laboratory reactor for gas hydrates studies

the denholm landslide saskatchewan canada an update the lower block of the denholm landslide has moved 370 m over alluvium deposited by the north saskatchewan spillway and river during the last 11 500 years at an average rate of 32 mm year these values must be considered minimal because erosion of the toe of the landslide is required for the formation of retrogressive landslides the shear strength for the bedrock clay shale of the lea park formation was back calculated to be assuming zero cohesion key words retrogressive landslide clay shale residual strength movement rates geological age inclinometer

stratigraphy and paleomagnetism of late pliocene and pleistocene sediments in the wellsch valley and swift current creek areas southwestern saskatchewan canada the late pliocene and pleistocene are represented in southern saskatchewan by sequences of preglacial glacial and nonglacial deposits these have been studied in surface exposures and bore cores and have been subdivided and correlated on the basis of their lithologies in this study new observations of magnetic polarity are presented they reveal a lower preglacial sequence empress group with reversed polarity and an upper normally magnetized sequence which contains glacial deposits of pre illinoian illinoian and wisconsinan age and which are correlative with the saskatoon and sutherland groups of central saskatchewan the reversed preglacial sequence is referred to the matuyama zone late pliocene to early pleistocene 2 58 0 78 ma and the normal glacial sequence to the brunhes zone middle to late pleistocene and holocene 0 78 ma to present in southern saskatchewan there is no evidence of glaciation during the late pliocene and early pleistocene matuyama chron which is in marked contrast w

formation of gas hydrate from dissolved gas in natural porous media abstract migration of methane bearing fluids from the depth is commonly proposed to explain the formation of marine gas hydrate this formation mechanism is viable only if the gas hydrate can nucleate and grow directly from an aqueous solution while phase equilibrium calculations suggest that the gas hydrate should crystallize from the dissolved gas barriers associated with nucleation may impede or even prevent hydrate formation at present experimental support for the phase equilibrium calculations is insufficient to confirm whether the hydrate can form in the sea floor from the dissolved gas here we report on a set of experiments in which the hydrate is crystallized from dissolved co 2 in natural porous media the experiments demonstrate that the hydrate crystals can nucleate in the absence of free gas providing important experimental justification for models of hydrate formation that depend on fluid migration our results are also relevant for other formation models that depend on in situ methane generation from biogenic sources experimental evidence for hydrate formation at low gas concentrations implies less stringent requirements on the biogenic sources of methane compared with some previous estimates