sea ice motion in response to geostrophic winds it is an old rule of thumb that sea ice moves with a speed of about 2 of the surface wind and about 45 to the right of the wind a similar relationship between the ice velocity and the geostrophic wind is examined here it is found that only about half of the long term several month average ice motion is directly related to the geostrophic wind the other half being due to the mean ocean circulation on shorter time scales and in all seasons more than 70 of the variance of the ice velocity in the central arctic ocean is explained by the geostrophic wind within about 400 km of the coasts the geostrophic wind is less successful in explaining the ice motion the spatial variations in ice velocity are also partly explained by the geostrophic wind about half of the variance in the large scale ice vorticity and shear are accounted for on the other hand none of the large scale ice divergence can be explained by the wind the long term average ocean current is estimated by subtracting the share of the ice motion caused by the wind from the total ice motion

sea ice models for climate study retrospective and new directions scientific observations of sea ice began more than a century ago but detailed sea ice models appeared only in the latter half of the last century the high albedo of sea ice is critical for the earth s heat balance and ice motion across the ocean s surface transports fresh water and salt the basic components in a complete sea ice model must include vertical thermodynamics and horizontal dynamics including a constitutive relation for the ice advection and deformational processes this overview surveys topics in sea ice modeling from the global climate modeling perspective emphasizing work that significantly advanced the state of the art and highlighting promising new developments

state of the art of modelling two phase flow in fractured rock two phase flow in fractured rock is an important phenomenon for a range of practical problems not the least of which is non aqueous phase liquid napl contamination of groundwater although multiphase systems have long been studied in the petroleum field in the hydrogeological field progress has only just reaches the point where models are being developed scale effect is one of the main issues of concern although models presented in this paper have the potential to provide useful predictions they can only be used to investigate a variety of possible scenarios with parameters being specified in the form of distribution of values the calibration and validation of all but the simplest of these models poses a formidable task with great demands on hydrogeologists and geophysicists to provide adequate data

simultaneous seismic and magnetic measurements in the low noise underground laboratory lsbb of rustrel france during the 2001 january 26 indian earthquake since the decommission of the underground launching control room of the ground based component of the french nuclear missile system the whole installation has been turned into a cross disciplinary underground laboratory the lsbb is a unique low noise underground laboratory because of its initial military conception and its location in the regional park of luberon far from large cities industry and heavy traffic the deepest point is 500 m below the surface at this depth a huge and non conventional shielded cylindrical capsule is installed with no metal 1268 m 3 in volume with a residual electromagnetic noise lower than 2 ft hz 1 2 above 10 hz as a result fluctuations of the earth s magnetic field under 10 hz can be recorded at a very low noise level with a low t c squid 3 d magnetometer taking advantage of the main gallery topology a broad band underground seismic array has been deployed since 2001 an analysis of data recorded simultaneously by the seismic underground array and by the magnetometer sensors during the indian earthquake of 2001 january 26 is presented evidence of a magnetic field perturbation induced by the seismic waves at teleseismic distance 6250 km is supported by a polarization analysis of seismic and magnetic signals spectral analysis shows specific frequency bands of perturbation related to physical processes such as ground water flow acceleration within the mountain structure

the quality of sea ice velocity estimates several aspects of observed and estimated velocities for arctic sea ice are explored the long term mean ice motion field and associated variance are derived using 15 years over 85 000 observations of daily buoy motions the mean field is separated into the part due to long term currents and internal ice stress and the part due to mean winds the velocity variance is partitioned into linear and nonlinear parts the linear response has no trend over the 15 years examined for a 5 year subset of the buoy data an ice ocean model with constant air drag parameters is used to estimate daily ice velocities at the buoy locations and then the linear relationship between the modeled velocities and the geostrophic winds is examined compared with buoy velocities the modeled velocities have a larger variance and larger linear response to the winds in a parameter study the effects of seasonally varying air drag coefficients increased ice strength and different ocean currents are examined seasonally varying air drag with smaller average magnitude than the standard value significantly improves the match of total linear and nonlinear variances between observed and modeled velocities the model simulations with varying air drag also have a smaller root mean square daily velocity error comparison of daily large scale deformations shows that the day to day deformations are not modeled well but generally have means and standard deviations within 50 of the observed values no one version of the model is consistently better than the others at matching the means and standard deviations of the observed daily divergence vorticity and shear ice thickness is sensitive to the model parameters varied with 5 year average thicknesses ranging from 2 2 to 3 4 m for the various simulations

integrating borehole image logs with core a method to enhance subsurface fracture characterization image logs provide critical data for characterizing fractured reservoirs however the nature of such logs coupled with borehole modified stresses creates challenges that may lead to erroneous fracture interpretations we developed a methodology to integrate image logs core and geomechanical models to obtain high confidence fracture data sets derived from image logs the methodology starts with a detailed comparison of overlapping image logs and core first to properly shift them into alignment core to log shift and second to determine which features interpreted as fractures on image logs picks can be validated as fractures in core then we derive guidelines for interpreting image logs without core control to reduce the likelihood of false fracture picks geomechanical modeling is used to assist with quality control and interpretation of image logs where drilling induced tensile fractures occur we use a fractured carbonate reservoir to demonstrate the application of this methodology comparison of image log fracture picks with core led to an average rejection rate of greater than 50 of fractures picked on the image logs these high rejection rates are mostly caused by overinterpretation of discontinuous sinusoids and misidentification of sedimentary features and induced fractures we derived rules for interpreting high quality fractures 1 only sinusoids with continuity greater than 50 should be interpreted and 2 sinusoids with dip angles less than 55 should be excluded from the interpretation application of this methodology has provided much higher quality fracture orientation and frequency data than that resulting from interpretation of borehole images alone the improved fracture data set provides a more reliable basis for testing hypotheses and deriving models of fracture distribution

numerical study of critical behaviour of deformation and permeability of fractured rock masses abstract the connectivity of fractures in rock masses is determined using a numerical simulation method there is a continuous fracture cluster throughout a fractured rock mass if fracture density d is at or above a threshold fracture density d c fractal dimension d f is used to describe the connectivity and compactness of the largest fracture clusters d f increases with increasing fracture density percolation theory is used to determine the universal law d f a f d d c f which describes the critical behaviour of connectivity of fractures in rock masses the results from numerical modeling show that the deformability of fractured rock masses increases greatly with increasing fracture density i e fractal dimension and the critical behaviour of deformability can be described by b s a d d f s also the overall permeability of a fractured rock mass occurs at or above a critical fracture density d c and increases with increasing fracture density the critical behaviour of permeability can be described by q a p d d c p the critical behaviour of connectivity and permeability of naturally fractured rock masses is examined using the universal forms

effects of stress on the two dimensional permeability tensor of natural fracture networks the effects of stress on the 2 d permeability tensor of natural fracture networks were studied using a numerical method universal distinct element code on the basis of three natural fracture networks sampled around dounreay scotland numerical modelling was carried out to examine the fluid flow in relation to the variations in burial depth differential stress and loading direction it was demonstrated that the permeability of all the networks decreased with depth due to the closure of aperture the permeability approached the minimum value at some depth below which little further variation occurred also differential stress had a significant effect on both the magnitude and direction of permeability the permeability generally decreased with increasing major horizontal stress for a fixed minor horizontal stress but the various networks considered showed different behaviours a factor termed the average deviation angle of maximum permeability am was defined to describe quantitatively the deviation degree of the direction of the major permeability component from the applied major stress direction for networks whose behaviour is controlled by set s of systematic fractures am is significantly greater than zero whereas those comprised of non systematic fractures have am close to zero in general fractured rock masses especially those with one or more sets of systematic fractures cannot be treated as equivalent porous media specification of the geometry of the network is a necessary but not sufficient condition for models of fluid flow knowledge of the in situ stress and the deformation it induces is necessary to predict the behaviour of the rock mass

changes in the modeled ice thickness distribution near the surface heat budget of the arctic ocean sheba drifting ice camp 1 in the polar oceans the ice thickness distribution controls the exchange of heat between the ocean and the atmosphere and determines the strength of the ice the surface heat budget of the arctic ocean sheba experiment included a year long field program centered on a drifting ice station in the beaufort and chukchi seas in the arctic ocean from october 1997 through october 1998 here we use camp observations and develop methods to assimilate ice thickness and open water observations into a model in order to estimate the evolution of the thickness distribution in the vicinity of the camp a thermodynamic model is used to simulate the ice growth and melt and an ice redistribution model is used to simulate the opening and ridging processes data assimilation procedures are developed and then used to assimilate observations of the thickness distribution assimilated observations include those of the thin end of the distribution determined by aircraft surveys of the surface temperature and helicopter photographic surveys and aircraft microwave estimates of the open water fraction the deformation of the ice was determined primarily from buoy and radarsat geophysical processor system rgps measurements of the ice velocity because of the substantial convergence and ridging observed in the spring and summer the estimated mean ice thickness increases by 59 from 1 53 to 2 44 m over the year in spite of a net thermodynamic ice loss for most multiyear ice

hydromechanical interactions in a fractured carbonate reservoir inferred from hydraulic and mechanical measurements hydromechanical coupled processes in a shallow fractured carbonate reservoir rock were investigated through field experiments coupled with analytical and numerical analyses the experiments consist of hydraulic loading unloading of a water reservoir in which fluid flow occurs mainly inside a heterogeneous fracture network made up of vertical faults and bedding planes hydromechanical response of the reservoir was measured using six pressure normal displacement sensors located on discontinuities and two surface tiltmeters a dual hydraulic behavior was characterized for low permeability bedding planes well connected to highpermeability faults displacement responses show high variability nonlinear changes sometimes with high frequency oscillations and a large scattering of magnitudes initial normal stiffnesses and effective normal stresses along fault planes were estimated in the field by interpreting pressure normal displacement relations with a nonlinear function between effective normal stress and normal displacement two dimensional discontinuum modeling with transient fluid flow was performed to fit measurements during hydraulic loading tests results show that the hydromechanical behavior of the reservoir is restored if a high stiffness contrast is allocated between low and high permeability discontinuities thus a dual permeability network of discontinuities will likely also be a contrasting stiffness network in which the deformation of major flow conducting discontinuities is significantly influenced by the stiffness of the surrounding less permeable discontinuities

metamorphic and structural setting of archaean amphibolitehosted gold deposits near southern cross southern cross province yilgarn block western australia abstract the southern cross archaean greenstone belt comprises dominantly low to mid amphibolite facies high strain domains which show evidence of a complex deformational and metamorphic history aeromagnetic field and petrographic data suggest that this complex structural metamorphic history was related to a single progressive deformation event which was broadly synchronous with granitoid emplacement between 2 68 and 2 62 ga lode gold deposits at corinthia hopes hill fraser s and polaris south occur in sequence from north to south along the fraser s corinthia shear zone in the southern cross greenstone belt corinthia and hopes hill are situated along a sheared granitoid greenstone contact whereas fraser s and polaris south occur within the greenstones about 200 m west of the contact gold mineralisation along the fraser s corinthia shear zone is associated predominantly with variably deformed quartz pyrrhotite pyrite plagioclase and or pyroxene calcite quartz plagioclase apatite veins with zoned alteration haloes of quartz plagioclase pyroxene calcite apatite and amphibole biotite chlorite there are accessory amounts of scheelite chalcopyrite arsenopyrite and galena with the fe sulphides bismuth and gold a variety of evidence including structural timing p t conditions deduced from critical metamorphic 550 c 2 5 3 7 kbars and metasomatic alteration assemblages 520 c 3 4 kbars the high thermodynamic variance of the metasomatic mineral assemblages and the zoning of individual alteration minerals indicates that gold related wallrock alteration occurred during or immediately after peak metamorphism there is some evidence for a sympathetic decline in metamorphic grade and the temperature of gold related alteration from south polaris south to north corinthia along the mineralised shear zone the shear zone acted as a major conduit for ore fluids at a depth of about 12 14 km 3 3 5 kbars lithostatic pressure in the crust gold mineralisation along the shear zone is related to large and small scale extensional crenulation cleavages the gold deposits might thus represent a relatively deep section of the crustal continuum of lode gold deposits proposed for the yilgarn block

the relationship of the degree of interconnection to permeability in fracture networks the problem of determining the permeability of a rock mass containing a system of finite fractures is highly dependent on both the degree of interconnection between fractures and the heterogeneity of individual fracture characteristics this paper examines how degree of interconnection affects both magnitude and nature of the fracture permeability the interconnection between given fracture sets is a complex function of 1 fracture density i e the number of fractures per unit volume and 2 the fracture extent or size unfortunately neither the density nor the extent of fractures is easily determined from borehole data however fracture frequency can be directly measured in a borehole because it is simply the number of fractures intersected per unit length of the borehole the frequency is a measure of the product of fracture density and size because the probability of a fracture intersecting a borehole is proportional to this product the effect of the degree of interconnection was investigated by numerically simulating flow in fracture networks where fracture size and density varied inversely while the product of these two parameters was held fixed directional permeabilities of a number of such networks were determined and the hydraulic behavior of each fracture system was compared to that of an ideal porous medium the permeability of the rock matrix between the fractures was assumed to be low enough to be negligible the results show that as fracture length increases the degree of interconnection increases thus for a given fracture frequency as measured in a borehole the permeability of the system increases as fracture length is increased and density is proportionally decreased also fracture systems with shorter but more dense fractures behave less like porous media than do systems with longer but less dense fractures knowledge of fracture frequency and orientation alone is inadequate when determining permeability or deciding the important question of whether a given system behaves like a porous medium for many cases where these parameters can be measured in the borehole it is critical to know fracture length in order to predict the degree of fracture network interconnection however for certain cases where fractures are larger than some critical size it may not be necessary to know fracture lengths exactly

observation and modelling of fault zone fracture seismic anisotropy i p sv and sh travel times summary three component vsp borehole seismograms taken in the vicinity of an active normal fault in california show strong systematic shear wave splitting that increases with proximity to the fault using cerven s method of characteristics for ray tracing in anisotropic heterogeneous media and hudson s formulation of elastic constants for media bearing aligned fractures we have fitted a suite of p sv and sh hanging wall and foot wall travel times with a simple model of aligned fractures flanking the fault zone the dominant fracture set is best modelled as parallel to the fault plane and increasing in density with approach to the fault the increase in fracture density is non uniform power law or gaussian with respect to distance to the fault although the hanging wall and the foot wall rock are petrologically the same unit the fracture halo is more intense and extensive in the hanging wall than in the foot wall upon approach to the fault plane the fracture density or fracture density gradient becomes too great for the seismic response to be computed by hudson cerven procedures the maximum fracture density that can be modelled is about 0 08 within this 25 m fracture domain it appears more useful to model the fault and near field fractures as a low velocity waveguide we observe production of trapped waves within the confines of the intense fracture interval

effective stress history and the potential for seismicity associated with hydraulic fracturing of shale reservoirs the influence of effective stress history on reservoir stability and the response to hydraulic fracturing are simulated for two hypothetical shale reservoirs it is numerically demonstrated that the effective stress history influences the present day stability of faults and natural fractures any assumption of a homogeneous stress state is shown to be unrealistic in the majority of fractured shale reservoirs the simulations demonstrate that depending upon the effective stress history shear displacements can be induced by stress changes associated with stimulation without a change of fluid pressure in the destabilized fractures elastoplastic shale and other fractured reservoirs are shown to have a memory of past geomechanical states these numerical findings demonstrate the value of interpreting reservoir effective stress history when planning fluid injection a previous case history is summarized to illustrate the overwhelming dependence of stress history analyses on a wide spectrum of earth science disciplines it also illustrates the application of stress history analyses to activities other than hydraulic fracturing the current inexperience of effective stress history description means that new approaches must be identified to optimize the necessary multidisciplinary investigations supplementary material details of the simulations are available at www geolsoc org uk sup18743

results of the sea ice model intercomparison project evaluation of sea ice rheology schemes for use in climate simulations a hierarchy of sea ice rheologies is evaluated on the basis of a comprehensive set of observational data the investigations are part of the sea ice model intercomparison project simip four different sea ice rheology schemes are compared a viscous plastic rheology a cavitating fluid model a compressible newtonian fluid and a simple free drift approach with velocity correction the same grid land boundaries and forcing fields are applied to all models as verification data there are 1 ice thickness data from upward looking sonars uls 2 ice concentration data from the passive microwave radiometers smmr and ssm i 3 daily buoy drift data obtained by the international arctic buoy program iabp and 4 satellite derived ice drift fields based on the 85 ghz channel of ssm i all models are optimized individually with respect to mean drift speed and daily drift speed statistics the impact of ice strength on the ice cover is best revealed by the spatial pattern of ice thickness ice drift on different timescales daily drift speed statistics and the drift velocities in fram strait overall the viscous plastic rheology yields the most realistic simulation in contrast the results of the very simple free drift model with velocity correction clearly show large errors in simulated ice drift as well as in ice thicknesses and ice export through fram strait compared to observation the compressible newtonian fluid cannot prevent excessive ice thickness buildup in the central arctic and overestimates the internal forces in fram strait because of the lack of shear strength the cavitating fluid model shows marked differences to the statistics of observed ice drift and the observed spatial pattern of ice thickness comparison of required computer resources demonstrates that the additional cost for the viscous plastic sea ice rheology is minor compared with the atmospheric and oceanic model components in global climate simulations

marginal ice zone rheology comparison of results from continuum plastic models and discrete particle simulations computer simulations of ice behaviour in the marginal ice zone miz are described two cases were studied a constant velocity wind blowing at certain angle with respect to the coast in what can be considered a simplified miz problem and a vortex wind force field the viscous plastic approach of hibler has been used to model the ice rheology we study the effects of adopting two different rheologies cavitating fluid and mohr coulomb the introduction of shear strength effects through a mohr coulomb yield criterion plays an important role in determining ice drift velocities in the miz the effects that ice floe collisions can have in determining the ice rheological behavior have been tested by using discrete particle simulations in order to do so simulations for the miz were performed using different mean floe sizes as well as different miz widths and wind conditions the predictions of the discrete particle simulations are in general agreement with the results obtained by using the mohr coulomb rheology in the viscous plastic model the continuum models were solved by using a fully lagrangian method known as smoothed particle hydrodynamics sph the method has the advantage of the lagrangian techniques in avoiding problems of numerical diffusion an additional advantage of sph in comparison with classical lagrangian approaches is that sph does not need grids and this makes it well suited to handle problems of large deformations

fracture analysis of a volcanogenic massive sulfide related hydrothermal cracking zone upper bell river complex matagami quebec application of permeability tensor theory located in the matagami mining district of the abitibi greenstone belt quebec the bell river complex is a 5 km thick tholeiitic layered gabbro and or anorthosite body 2724 6 2 5 1 9 mau pb which likely acted as the heat source that drove hydrothermal convection and volcanogenic massive sulfide vms hydrothermal mineralization in the area abundant fractures and veins crosscutting the western lobe of the bell river complex formed over a range of temperatures from 250 to 700 c the 250 to 400 c assemblage quartz epidote sericite chlorite plagioclase is the most widespread and occurs as orthogonal anastomosing and random vein sets vein densities average between 15 and 25 veins per m2 locally reaching as high as 40 to 60 veins per m2 these veins typically 1 to 3 mm wide are interpreted to represent thermal cracking associated with hydrothermal fluid mineralization in the district furthermore they crosscut earlier higher temperature pyroxene plagioclase 600 c and magnetite rich 300 600 c veins 1 10 mm wide densities commonly 0 5 veins per m2 detailed field measurements of quartz epidote vein geometries coupled with permeability tensor theory have produced a first order approximation of the maximum model permeability structure veins unfilled of the hydrothermal cracking zone representative district wide values indicate maximum model bulk permeabilities of 10 10 to 10 8 m2 for the hydrothermal cracking zone similar to permeabilities calculated for the fractured sheeted dike complexes of the semail and troodos ophiolites however a high flow zone located within the central parts of the hydrothermal cracking zone is characterized by a maximum model permeability of 10 7 m2 locally 1 m2 the high flow zone reaches maximum model permeability values as high as 10 6 to 10 5 m2 where two or more veins occur with apertures in excess of 2 cm mapping has shown that the hydrothermal cracking zone is confined to an 350 m thick interval located within a strongly layered gabbro and or anorthosite horizon of the layered zone upper bell river complex the base of this interval is 1 000 m below the top of the bell river complex furthermore in and around the town of matagami the dominant orientation of quartz epidote veins parallels the orientation of the layering 110 15 this parallelism is further reflected in the orientation of the calculated quartz epidote vein permeability tensors 94 30 1 n 71 the parallelism suggests that heterogeneities associated with the layer contacts provided planes of low tensile strength along which the hydrothermal cracks preferentially developed such an interpretation further explains why the hydrothermal cracking zone appears to be restricted to the strongly layered zone there are many more low tensile strength planes zones of quartz epidote veining approximately orthogonal to layering are also present the orthogonal veins are usually less continuous and commonly truncated but not crosscut by layer parallel veins the orthogonal subset of veins is interpreted to represent short pathways that allowed fluids to travel between adjacent layer parallel veins however locally longer layering orthogonal quartz epidote veins showing offsets of 10 to 30 cm normal to the layering of the bell river complex may represent initial conduits that allowed fluids to travel into overlying stratigraphy and through to the paleosea floor although volumetrically small in comparison with the permeability structure of the bulk sea floor the significantly higher permeability of the hydrothermal cracking zone indicates its importance in controlling the flow paths and fluxes of fluids deep within the hydrothermal system

optimization of a sea ice model using basinwide observations of arctic sea ice thickness extent and velocity a stand alone sea ice model is tuned and validated using satellite derived basinwide observations of sea ice thickness extent and velocity from the years 1993 to 2001 this is the first time that basin scale measurements of sea ice thickness have been used for this purpose the model is based on the cice sea ice model code developed at the los alamos national laboratory with some minor modifications and forcing consists of 40 yr ecmwf re analysis era 40 and polar exchange at the sea surface poles data three parameters are varied in the tuning process ca the air ice drag coefficient p the ice strength parameter and the broadband albedo of cold bare ice with the aim being to determine the subset of this three dimensional parameter space that gives the best simultaneous agreement with observations with this forcing set it is found that observations of sea ice extent and velocity alone are not sufficient to unambiguously tune the model and that sea ice thickness measurements are necessary to locate a unique subset of parameter space in which simultaneous agreement is achieved with all three observational datasets

interpretation by numerical modelling of changes of fracture system hydraulic conductivity induced by fluid injection distinct element modelling of naturally fractured reservoirs is used to investigate changes of hydraulic conductivity induced by fluid injection general conclusions are drawn relating to the effects of uniform changes of pore pressure on conductivity anisotropy in connected joint systems the influence of cross cutting discontinuous joints on the conductivity of the joints against which they terminate is emphasized results from a two dimensional model are related to the observed performance of a granitic geothermal reservoir on emploie le modele a elements distincts des reservoirs fractures de facon naturelle pour etudier les changements dans la conductivite induits par l injection de fluide des conclusions generales sont tirees concernant les effets de changements uniformes de pression interstitielle dans des systemes de joints relies sur l anisotropie de conductivite on accentue l influence exercee sur la conductivite des joints par des joints discontinus transversaux des resultats obtenus a parti

interaction of faults and perturbation of slip influence of anisotropic stress states in the presence of fault friction and comparison between wallace bott and 3d distinct element models abstract two decades after their birth the validity of fault slip inversion methods is still strongly debated these methods are based upon a very simplified mechanical background the wallace bott hypothesis following previous studies the 3d distinct element method 3dec software is used to explore the effect of varying stress anisotropy i e the shape ratio on slip perturbation along pairs of faults two end member configurations are modelled in taking into account fault friction and internal deformation of faulted blocks the first model deals with a relatively simple case where two nonintersecting conjugate normal faults are reactivated in an oblique normal stress regime the second one simulates an extreme situation where two perpendicular intersecting faults are submitted to oblique extension the average direction of fault slip predicted by 3dec models is compared to the corresponding slip predicted by the simplified wallace bott model for the two simulated cases it is shown that results from 3dec and wallace bott models are mutually consistent and argue for the validity of fault slip data inversion methods consistency remains even if slip is significantly deviated near the intersection line of faults these deviations depend on the degree of anisotropy of applied stresses in presence of fault friction furthermore modelling results suggest that for intersecting faults with convergent slip directions consideration of fault friction in the models leads to reduction of slip perturbation in other words modelling results lead to the nonintuitive conclusion that the validity of the simplified wallace bott model is strengthened when 3dec model s complexity i e the number of parameters incorporated increases

the role of microcracking and grain boundary dilation during retrograde reactions department of geology university of southampton southampton oceanography centre southampton so14 3zh uk email ajb1 soc soton ac ukveined metamorphic rocks are common and give evidence of fracture controlled fluid flow however the processesinvolved in fluid infiltration in domains between fractures is rarely well established reaction products concentrated atgrain boundaries give an indication that diffusion of elements along grain boundaries may be important and linearfluid inclusion trails provide evidence of grain scale microcracking despite these common observations the relativeimportance of grain boundary processes and microcracking at different metamorphic grades is not well characterised etheridge et al 1983 developed a metamorphic porosity model comprising grain boundary bubbles and tubules ofwhite white 1981 but also incorporating dilatant grain boundaries and intra grain tensile cracks variations in thelocal granular stress state will initiate transient micro cracking and grain boundary dilatancy promoting enhancedporosity permeability and grain scale fluid infiltration this in turn causes retrogression the extent to which it devel ops being influenced by the degree of connectivity of the dilatant cracks and grain boundaries and whether for particu lar p t x conditions the infiltrating fluid is at equilibrium or not with the host being infiltrated with reference to examples of greenschist facies retrogression from troms norway and south harris scotland this study establishes that the processes of microcracking and grain boundary dilatancy are highly heterogeneous atthin section scale in examples where reaction products are not related to mutual contact of specific phases it is shownthat the development of reaction products varies according to grain boundary orientation and density of microcracks an example from norway shows a quartz rich garnet mica schist where a centimetre scale brittle fracture has in duced extensive microcracking and fluid infiltration in the adjacent matrix fluid inclusions defining healed microcracks perpendicular to schistosity have h

response of fractured rock subject to fluid injection part ii characteristic behaviour abstract distinct element modelling has been used to investigate the mechanics of naturally fractured reservoirs subject to a constant rate of fluid injection constant stress boundary conditions were imposed with magnitudes typical of those pertaining at approximately 3 km depth in some sedimentary environments fluid leak off through fracture walls and fracture dilation during shear were modelled the progressive nature of the changes of stress and displacement occurring within the coupled fluid rock system were examined for four combinations of uniform fracture geometry and applied stress the effects of a uniform change of pore pressure on the directional nature of hydraulic conductivity in a fractured medium were characterised the influence of variation of injection rate pore fluid compressibility and leak off rate was examined effects associated with contrasts of shear stiffness between fractures and blocks and with inelastic deformation were interpreted

moho topography beneath the corinth rift area greece from inversion of gravity data summary our aim is to understand better the rifting process by imaging the moho depth variation beneath corinth and evvia we present here the results of a gravity inversion analysis in the region of the corinth and evvia rift system and compare them to those obtained independently from teleseismic tomography and receiver function analyses the results of these different studies appear to be consistent and show 1 a 10 km crustal thickening in the western part of the area beneath the hellenides mountains 2 nw se trending periodic crustal thinning and 3 a maximum crustal thinning north of the gulf of corinth this 4 km thinning is unlikely to be the result of the rifting alone which seems to have been reactivated since only 1 ma we propose here a geodynamical scenario in two major steps to explain the evolution of corinth area aegean miocene extension involving boudinage resulted in periodic crustal thinning consistent with observations these lithospheric instabilities could have favoured rupture initiation in particular areas especially near the city of corinth then the reactivation of the corinth rift extension 1 myr ago led to westward rift propagation the offset observed between the maximum crustal thinning and the gulf of corinth could be accommodated by a lowangle normal fault at about 10 15 km depth the corinth rift is thus asymmetrical and was initiated in places of crustal weakness due to miocene lithospheric instabilities

fracture porosity creation and persistence in a basement involved laramide fold upper cretaceous frontier formation green river basin usa fracture hosted porosity and quartz distribution along with crack seal texture and fluid inclusion assemblage sequences in isolated bridging quartz deposits show that open fractures can persist through protracted burial and uplift in foreland basins fractures oriented at a high angle to current maximum compressive stress remain open and were weak mechanical discontinuities for millions of years even at great depth upper cretaceous frontier formation sandstones in the basement involved laramide table rock anticline eastern greater green river basin wyoming sampled by two horizontal wells cut parallel or nearly parallel to bedding and at a high angle to steeply dipping fractures have 41 5 m of rock in four cores at depths of 4538 4547 m cores intersect older e striking set 1 fractures are abutted by or locally cross cut by n striking set 2 fractures both sets contain quartz and porosity sequenced using quartz crack seal cement texture maps set 1 fluid inclusion assemblage fia trapping temperatures increase progressively from 140 to 165 c then decrease to c 150 c compatible with fracture opening over c 15 ma during rapid burial followed by uplift in eocene oligocene time set 2 opened at c 160 c probably near maximum burial after a period of quiescence set 2 reopened at c 5 ma at c 140 c on a cooling trajectory intermittent set 2 movement could reflect local basement involved fault movement followed after a pause by further set 2 reactivation in the modern stress field during uplift interpretations are sensitive to available burial thermal histories which have considerable uncertainty

scale up of two dimensional conductivity tensor for heterogenous fracture networks abstract an approach is developed to scale up the two dimensional conductivity tensor for highly heterogenous fracture networks based on a numerical modelling approach that was proposed previously by zhang et al j rock mech min sci 33 17 37 1996 in that approach it was assumed that the fractured rock mass was statistically uniform and only a unit square of sample rock was needed to work out the conductivity tensor hence its application was limited where the heterogenous characteristics of a fracture network was not negligible in the present paper the local conductivity tensors both magnitude and direction either calculated based on small fracture networks or samples measured on separate exposures are scaled up to estimate the overall nature of conductivity for a relatively large region the approach developed is used to estimate the two dimensional conductivity tensor of naturally fractured rock masses for two types of fracture networks constant aperture and variable aperture under stress the effects of sample number and sample size of local sub areas were examined against the totally sampled area finally field data were used to predict the overall conductivity tensor of a large region based on four fracture networks sampled on separate exposures within the region

numerical modeling of cenozoic stress patterns in the mid norwegian margin and the northern north sea numerical modeling of cenozoic stress patterns in the northern north sea and the mid norwegian margin is presented and the sense of potential slip along major fault planes belonging to the two areas is restored we assume that the main regional source of stresses is the atlantic ridge push as demonstrated by previous studies furthermore we also assume a nearly consistent nw se strike for the far field stress from continental breakup between greenland and norway earliest eocene to present day first we applied the commercial two dimensional distinct element method udec to simulate cenozoic stress and displacement patterns in the study area variations in rheology and major fault zones were introduced into the model the more trondelag fault complex and its inferred continuation into the shetland platform forms the major mechanical discontinuity in the model second we used the sortan method developed at the university of paris vi to predict the sense of potential slip along major fault planes the input for the sortan model was constrained by the geometry of the selected fault planes and local principal stress directions extracted from the udec modeling our results show that the more trondelag fault complex and its inferred continuation into the shetland platform act as a weak fault zone this fault zone divides the study area into two different stress provinces the continental margin and the northern north sea this result agrees well with the observed differences in cenozoic structural evolution of the two areas compressive structures are observed along the continental margin whereas relative tectonic quiescence characterizes the northern north sea during the tertiary the restored stress patterns in the northern north sea and the mid norwegian margin also agree well with the observed present day stress configuration our analysis demonstrates a method to reconstruct the sense of slip on major fault planes by combining two complementary numerical tools udec and sortan as a result it is demonstrated that oblique slip motions are mainly expected in particular strike slip and reverse dip slip faulting are simulated

forecasting bering sea ice edge behavior a coupled ice ocean dynamics model is developed to provide arctic offshore operators with 5 to 7 day forecasts of ice motions ice conditions and ice edge motions an adaptive grid is introduced to follow the ice edge and the grid may move independently of the ice motion the grid can be lagrangian or eulerian at different locations away from the ice edge ice stress is described using an elastic plastic model with strength determined by the ice conditions the ocean dynamics model describes time dependent three dimensional behavior including wind driven currents and barotropic and baroclinic flows the thermal energy budget of the ice cover is coupled to the ocean with mass and salt interchange accompanying freezing or melting near the marginal ice zone miz surface winds determined by reducing and turning the geostrophic winds are enhanced to reflect observed behavior the model was tested by simulating ice edge motions observed during the 1983 marginal ice zone experiment west and during drilling of the 1983 north aleutian shelf continental offshore stratigraphic test well simulations of ice edge movement in the bering sea compare with observed data to within about 5 km d the model correctly describes mixed layer evolution in the marginal ice zone as fresh meltwater ismore mixed downward by turbulence along edge baroclinic flows due to density gradients across the ice edge are simulated by the model in agreement with observations increased ice drift speeds generate higher melt rates due to increased turbulence levels with the result that ice edge advance is moderated in spite of higher ice drift speeds less

numerical methods in rock mechanics the purpose of this civilzone review paper is to present the techniques advances problems and likely future development directions in numerical modelling for rock mechanics and rock engineering

assimilation of ice motion observations and comparisons with submarine ice thickness data 1 aided by submarine observations of ice thickness for model evaluation we investigate the effects of assimilating buoy motion data and satellite ssm i 85 ghz ice motion data on simulation of arctic sea ice the sea ice model is a thickness and enthalpy distribution model and is coupled to an ocean model ice motion data are assimilated by means of optimal interpolation assimilating motion data particularly from drifting buoys significantly improves the modeled ice motion reducing the error to 0 04 m s 1 from 0 07 m s 1 and increasing the correlation with observations to 0 90 from 0 66 without data assimilation the modeled ice moves too slowly with excessive stoppage assimilation leads to more robust ice motion with substantially reduced stoppage which in turn leads to strengthened ice outflow at fram strait and enhanced ice deformation everywhere enhanced deformation doubles the production of ridged ice to an arctic ocean average of 0 77 m yr 1 and raises the amount of ridged ice to half the total ice volume per unit area of 2 58 m assimilation also significantly alters the spatial distribution of ice mass and brings the modeled ice thickness into better agreement with the thickness observed in four recent submarine cruises reducing the error to 0 66 m from 0 76 m and increasing the correlation with observations to 0 65 from 0 45 buoy data are most effective in reducing model errors because of their small measurement error ssm i data because of their more complete spatial coverage are helpful in regions with few buoys particularly in coastal areas assimilating both ssm i and buoy data combines their individual advantages and brings about the best overall model performance in simulating both ice motion and ice thickness

response of fractured rock subject to fluid injection part i development of a numerical model abstract the selection and development of a numerical model used to investigate the response of fractured rock to injection are described the distinct element method was chosen on the basis of its ability to represent explicitly the fractures and to couple the mechanical deformation to the fluid flow the numerical model was used to simulate fluid injection from a single well into each of four basic model reservoirs the results of these simulations form the basis of the interpretation given in parts ii and iii

permeability of fractured rock effect of fracture size and data uncertainties equations are derived for equivalent darcy permeability of a fracture system that consists of nonextensive fractures having arbitrary orientations with respect to the rock block considered it is shown that in the case of nonextensive fractures one cannot proceed to the limiting process where the volume of the bounding rock block can be reduced to an arbitrarily small value therefore the dimensions of the block along with the dimensions of the fractures appear in the equations for the nine components of permeability this leads to a situation where the equivalent permeability matrix is not only nonsymmetric but also it does not follow the tensorial rules of axis rotation this leads to the conclusion that in the case of nonextensive fractures the equivalent permeability cannot be defined as a characteristic intrinsic property of the medium as is possible in the case of porous medium or in a fractured medium that has extensive fractures to analyze the effect of uncertainties in the field data on the estimation of equivalent permeability a second order statistical analysis is proposed the method is quite general and can be used to analyze the propagation of parameter uncertainties in models a numerical example using preliminary fracture data frommore the columbia river basalt in washington state is presented the mean and standard deviation of the equivalent permeability of a 5 x 5 m block is estimated assuming that each component of the equivalent permeability has log normal distribution the probability of their assuming values different from the mean is found to be significant this points out the uncertainty that would be inherent in the flow simulation if the mean value of permeability is used less

strike slip fault bridge fluid pumping mechanism insights from field based palaeostress analysis and numerical modelling abstract we present a finite element study of stress perturbation in evolving compressive and extensional strike slip fault bridges the results are compared with a fracture study of a compressive bridge at st donats uk horizontally interbedded calcareous mudstone and bioclastic calcilutite at st donats have a distinct vertical permeability anisotropy this sedimentary sequence behaves as a set of horizontal aquifers the fluid flow in these aquifers is sensitive to mean stress gradients paleostress analysis of field fracture data verified by finite element modelling indicates a rotation of 1 towards parallelism with boundary faults inside the growing compressive bridge boundary faults and bridge faults recorded numerous fluid flow events the modelled mean stress pattern shows a regional maximum within the bridge and local maxima minima pairs at boundary fault tips finite element modelling of an extensional bridge indicates that 3 rotates towards parallelism with boundary faults the mean stress pattern is similar to the pattern in compressive bridge but with maxima and minima locations interchanged the stress patterns are reestablished by each stress build up preceding the rupturation of the boundary faults throughout the development stages of strike slip fault bridges mean stress gradients developed pre failure control the fluid flow in fractures of the strike slip fault system at and after the end of each stress build up and the fluid flow in boundary faults post failure fracture reactivation and new fracture generation within an evolving bridge is a process consisting of multiple successive events that retain the storage capacity of the bridge rupture and sealing of the main bounding faults is a step wise process that opens and closes fluid conduits between areas with different pressures

coupled htm modelling of cold water injection in fractured hydrocarbon reservoirs abstract this paper presents a numerical modelling approach for analyzing the coupled hydro thermo mechanical htm behaviour of fractured hydrocarbon reservoirs the coupled htm approach is applied to generic models of five spot reservoirs with different fracture patterns the presence of fractures was shown to have important effects on cold water injectivity and sweep efficiency due to high fluid velocities along fractures local areas particularly close to fracture intersections cannot be saturated with cold water the volume of oil by passed by the displacing water was found to be strongly dependent on fracture geometry and was generally larger if the fracture conductivity enhancement due to fracture deformation is taken into consideration the thermo poro elastic stress changes due to cold water injection were found to induce significant normal and shear displacements and conductivity changes along the fractures another important result of the numerical modelling is the possibility of creating secondary tensile fracturing close to existing fractures the secondary fractures are generally oriented perpendicular to the natural fractures and are expected to help in draining the oil from the rock matrix towards the fractures

response of fractured rock subject to fluid injection part iii practical application abstract the results of the distinct element modelling are used to interpret field observations of injection pressures two additional examples of the qualitative use of the model are presented the sensitivity of reservoirs to even brief periods of high rate injection and the value of using dynamic coupled models to interpret field performance are emphasised

critical stress localization of flow associated with deformation of well fractured rock masses with implications for mineral deposits fluid flow and deformation of fractured rock masses is investigated using numerical models based on distinct element methods udec which couple mechanical and hydraulic behaviour a series of numerical experiments is presented in which diffuse flow through fracture networks changes to highly localized flow at a critical stress state slip on parts of the fracture network causes the opening up of some fractures often at fracture intersections thus a change in the stress state results in a sudden localization of deformation and fluid flow a series of plots of hydraulic conductivity v differential stress are presented for some natural fracture networks to demonstrate the influence of fracture network geometry and in situ stress on the critical phase boundary between diffuse and localized flow the onset of localized deformation and fluid flow is a critical point phenomenon resulting in fractal properties fractal and multifractal techniques are used to characterize heterogeneity of the flow in order to generalize the results of the modelling of discrete systems to a continuum based description of the flow the significance of the models and their implications for crustal behaviour are discussed in relation to hydrothermal mineralization and natural vein systems

field examples and numerical modelling of oversteps and bends along normal faults in cross section abstract oversteps and bends along cross sections through small scale numerical modelling distinct element analysis has been carried out to simulate the patterns of deformation at oversteps and bends area increase pull apart development and localised stress concentration occur at extensional oversteps and bends stress concentration and a tendency for compaction are typical of contractional oversteps and bends such results are compatible with the structures observed in the field a sensitivity analysis of the modelling has been carried out to investigate the effects of variations in mechanical parameters on deformation although models with weaker properties are more deformable the overall geometry of deformation varies little with variations in mechanical parameters

study of the crustal thickness and the subducting lithosphere in greece from gravity data the area of greece has experienced a complex tectonic history dominated by the subduction of the african plate beneath eurasia in this study the variations of the crustal thickness in the area of greece were obtained by means of the multiple source werner deconvolution mswd method applied to gravity data thicknesses of 40 49 km are estimated beneath the hellenides mountain belt to the west eastward thinning of the crust to thicknesses ranging from 25 km in the north to 30 km in the south is seen in the aegean region these results are in good agreement with recent seismological results demonstrating that the mswd method successfully treated the problem using the crustal model we derived we computed the gravity effect of the crust and extracted it from the bouguer anomaly we also extracted the gravity effect of the subducting lithosphere from the bouguer anomaly producing a residual map where most of the original gravity variation has been successfully removed the remaining anomalies appear related to near surface features and an area of low velocity mantle in the central aegean sea

on the mesoscale interaction of lead ice and floes the plasticity of the arctic ice pack depends on its granular nature in particular on the size and distribution of areas of thin ice and open water surrounding multiyear ice floes the paper begins with construction of a mesoscale 10 100 km granular model of the central arctic ice pack the mesoscale model is based on a dynamic particle simulation in which individual multiyear ice floes and surrounding parcels of first year ice are explicitly modeled as discrete convex polygons in a two dimensional domain deformation of the domain produces areas of localized failure and areas of open water the areas of localized failure are modeled as pressure ridging events using the results of numerical experiments performed with a computer simulation of the ridging process the paper focuses on the results of numerical experiments performed with the mesoscale model in the experiments the model ice pack is biaxially deformed at constant strain rates the principal strain rates are varied to create deformation states ranging from pure shear to uniform compression the results define the shape and magnitude of the plastic yield surface the strain rate vectors associated with points on the yield surface the partition of energy dissipation between ridging and in plane sliding and the changes in the ice thickness distribution associated with various deformation states

thinning of the arctic sea ice cover comparison of sea ice draft data acquired on submarine cruises between 1993 and 1997 with similar data acquired between 1958 and 1976 indicates that the mean ice draft at the end of the melt season has decreased by about 1 3 m in most of the deep water portion of the arctic ocean from 3 1 m in 1958 1976 to 1 8 m in the 1990s the decrease is greater in the central and eastern arctic than in the beaufort and chukchi seas preliminary evidence is that the ice cover has continued to become thinner in some regions during the 1990s

characterization of stress perturbations near major fault zones insights from 2 d distinct element numerical modelling and field studies jura mountains abstract we consider and discuss the presence of discontinuities in the crust as a major source of stress perturbations based on 2 d distinct element modelling we reconstruct the local stress field around a vertical discontinuity in various geological contexts the resulting stress distribution reveals that major directional stress changes occur near the tips of the discontinuity so that stress deviations can reach values as large as 50 we establish simple relationships controlling stress changes around a pre existing fault zone as a function of 1 the remote differential stress magnitude 1 3 2 the friction coefficient on the discontinuity and 3 the strike of the discontinuity relative to the far field stress as a geological example we present the morez fault zone in the internal jura paleostress reconstruction in forty two sites indicates that the trends of the mio pliocene compression are n110 on average near the fault whereas they are n130 in the surrounding areas a comparison between the results of the tectonic study and those of theoretical modelling suggests that the 20 counterclockwise deviation is directly related to the reactivation of this large weak zone we thus evaluate the role of mechanical decoupling along pre existing zones of weakness especially with consideration to the accommodation of the alpine deformation in the jura belt

gravity modelling of the hellenic subduction zone a regional study abstract the hellenic subduction zone is clearly expressed in the arc shaped distribution of earthquake epicenters and gravity anomalies which connect the peloponnesos with crete and anatolia in this region oceanic crust of the african plate collides northward with continental crust of the aegean microplate which itself is pushed apart to the south west by the anatolian plate and at the same time is characterised by crustal extension the result is an overall collision rate of up to 4 cm year and a retreating subduction process recent passive and active seismic studies on and around crete gave first but not in all details consistent structural results useful for supporting gravity modelling this was undertaken with the aim of presenting the first 3d density structure of the entire subduction zone gravity interpretation was based on a bouguer map newly compiled using data from land marine and satellite sources the anomalies range from 170 mgal cretan sea to 10 mgal mediterranean ridge 3d gravity modelling was done applying the modelling software igmas the computed bouguer map fits the low frequency part of the observed one which is controlled by variations in moho depth less than 20 km below the cretan sea and extending 30 km below crete and the extremely thick sedimentary cover partly up to 18 km of the mediterranean ridge the southernmost edge of the eurasian plate with its more triangular shaped backstop area was traced south off crete only 50 to 100 km further to the south the edge of the african continent was traced as well in between these boundaries there is african oceanic crust which has a clear arc shaped detachment line situated at the eurasian continental edge the subduction arc is open towards the north its slab separates hotter mantle material lower density below the updoming moho of the cretan sea from colder one higher density in the south subjacent to the upper continental crust of crete is a thickened layer of lower crust followed by the subducted oceanic crust with some mantle material as intermediate layer the depth of the oceanic moho below crete is 50 km the presence and structure of subducted or underplated sediments remains uncertain

three dimensional structure and constraints on the nature of the coupled subduction spreading process in the aegean area abstract p wave travel time residuals from stations of the athens observatory network are inverted by a tomographic technique to investigate the velocity structure of the lithosphere in the aegean region both local and teleseismic data are used the velocity anomalies are best resolved in the mantle lithosphere and indicate a markedly heterogeneous velocity structure these results described elsewhere in more detail are correlated in this paper with other geophysical fields in the area the velocity heterogeneity observed in the upper mantle is very well correlated with thermal data and other geophysical fields in the aegean in particular the present day active extension in the aegean takes place behind the volcanic arc where in the mantle lithosphere velocity anomalies lower than normal are observed consisted with a stretched lithosphere in the back arc the area associated with the strongest bouguer and free air anomalies occurs in the southern aegean sea of crete above a zone of relatively higher velocity anomalies

the gravity field of a dipping plate in greece summary the surface gravity anomalies for a three dimensional density model of a dipping lithospheric plate under the aegean island arc and the aegean sea in greece in the eastern mediterranean sea have been calculated such a dipping plate has been suggested in geophysical investigations the calculated gravity maximum over the dipping plate is located in the area of the largest observed bouguer anomalies inside the island arc it is suggested that this should be taken into account in studies of crustal structure and gravity in the area

a new lagrangian model of arctic sea ice a new lagrangian sea ice model for the arctic ocean has been developed to accommodate the assimilation of integral measures of ice displacement over periods of days or weeks the model is a two layer thick ice and open water dynamic model with 600 700 cells spaced at roughly 100 km the force balance equation is solved for each cell with standard wind and water stress terms a coriolis term and an internal ice stress term the internal stress is found using a viscous plastic rheology and an elliptical yield curve the strain rate is determined by the smoothed particle hydrodynamics formalism which determines the spatial derivatives of the velocity by a weighted summation of the velocities of adjacent cells a length scale of 150 km is used the model is driven with observed geostrophic winds and climatological mean ocean currents ice growth and melt are found from a seasonal and thickness based lookup table because the current focus is on the model dynamics the model ice velocity simulations are compared to observed buoy motion over the 5 yr period 1993 97 the correlation of the daily averaged model velocity with buoy velocities is r 5 0 76 n 5 42 553 rms difference 5 0 072 m s21 speed bias 51 0 009 m s21 angle bias 5 8 08 this compares favorably with the correlation of a state of the art eulerian coupled ice ocean model where r 5 0 74 in the summer and 0 66 in the winter over the same 5 yr period

geophysics of sea ice in the baltic sea a review abstract with improved observation methods increased winter navigation and increased awareness of the climate and environmental changes research on the baltic sea ice conditions has become increasingly active sea ice has been recognized as a sensitive indicator for changes in climate although the inter annual variability in the ice conditions is large a change towards milder ice winters has been detected from the time series of the maximum annual extent of sea ice and the length of the ice season on the basis of the ice extent the shift towards a warmer climate took place in the latter half of the 19th century on the other hand data on the ice thickness which are mostly limited to the land fast ice zone basically do not show clear trends during the 20th century except that during the last 20 years the thickness of land fast ice has decreased due to difficulties in measuring the pack ice thickness the total mass of sea ice in the baltic sea is however still poorly known the ice extent and length of the ice season depend on the indices of the arctic oscillation and north atlantic oscillation sea ice dynamics thermodynamics structure and properties strongly interact with each other as well as with the atmosphere and the sea the surface conditions over the ice covered baltic sea show high spatial variability which cannot be described by two surface types such as ice and open water only the variability is strongly reflected to the radiative and turbulent surface fluxes the baltic sea has served as a testbed for several developments in the theory of sea ice dynamics experiences with advanced models have increased our understanding on sea ice dynamics which depends on the ice thickness distribution and in turn redistributes the ice thickness during the latest decade advance has been made in studies on sea ice structure surface albedo penetration of solar radiation sub surface melting and formation of superimposed ice and snow ice a high vertical resolution has been found as a prerequisite to successfully model thermodynamic processes during the spring melt period a few observations have demonstrated how the river discharge and ice melt affect the stratification of the oceanic boundary layer below the ice and the oceanic heat flux to the ice bottom in general process studies on ice ocean interaction have been rare in the future increasingly multidisciplinary studies are needed with close links between sea ice physics geochemistry and biology

seismicity induced by hydraulic fracturing in shales a bedding plane slip model passive seismic monitoring of microseismic events induced in oil or gas reservoirs is known as microseismic monitoring microseismic monitoring is used to understand the process of hydraulic fracturing which is a reservoir stimulation technique we use a new geomechanical model with bedding plane slippage induced by hydraulic fractures within shale reservoirs to explain seismicity observed in a typical case study of hydraulic fracturing of a shale gas play in north america microseismic events propagating from the injection point are located at similar depths within the uncertainty of their locations and their source mechanisms are dominated by shear failure with both dip slip and strike slip senses of motion the prevailing dip slip mechanisms have one nearly vertical nodal plane perpendicular to the minimum horizontal stress axis while the other nodal plane is nearly horizontal such dip slip mechanisms can be explained by slippage along bedding planes activated by the aseismic opening of vertical hydraulic fractures the model explains the observed prevailing orientation of the shear planes of the microseismic events as well as the large difference between seismic and hydraulic energy

numerical experiments on permeability tensor and its application to jointed granite at stripa mine sweden rock masses containing a large number of geological discontinuities called joints are treated as homogeneous anisotropic porous media a permeability tensor hydraulically equivalent to a flow network formed by joints is formulated in terms of two tensors and a nondimensional scalar both depending only on the geometrical aspects of joints such as the spacing size orientation and aperture two dimensional numerical analyses on seepage flow through the networks support the validity of the present theory the information about the geometry of joints which is obtained from the ventilation drift at stripa mine sweden is interpreted to give the corresponding permeability tensor the permeability tensor is compared with the result of the large scale hydraulic conductivity test with the conclusion that it provides a reasonable prediction for the hydraulic properties of stripa jointed granite

study on three dimensional fracture network connectivity path of rock mass and seepage characteristics based on equivalent pipe network nuclear waste repositories have extremely stringent requirements for geological environment however natural fractures in rock mass can be potential channels for nuclide migration therefore the influence of fractures on the permeability of rock mass must be assessed in this paper a well research was conducted on well exposed granite outcrops in the xinchang site the chinese high level radioactive waste repository the high precision three dimensional model of a typical outcrop is built to obtain fracture information combined with field measurement and then the three dimensional fracture network model is generated using the relevant parameters by monte carlo method to obtain more comprehensive fracture connectivity while avoiding the traditional method of searching the connectivity path in the complicated 3d fracture model taking up a lot of storage space and costing a lot of time this paper presents an approach using matlab cell array instead of traditional adjacency matrix to search and store fracture network connectivity paths in dfn model the fracture disc with certain thickness is equivalent to three dimensional pipe network model epnm with variable diameter and the equivalent path permeability coefficient epc is proposed to objectively study the permeability characteristics of the seepage path in fractured rock mass based on that especially noteworthy is that some fractures in a certain strike range belong to open type while those in another range belong to cemented closed fractures when fresh fractures were exposed by cutting off the surface rock to a certain depth the calculation of epc under different conditions shows that the order of magnitude of epc mean value is 1e 7m s and 1e 3m s respectively when fractures are cemented and not partly on this basis the size of the representative elementary volume rev of the fractured rock mass in the study area is determined to be about 25 m by rotating the matrix in model the spatial permeability tensor of the region including permeability principal value and main direction is obtained which is within the range of borehole data the predicted results may provide some reference for the related projects in the future

scaling and geometric properties of extensional fracture systems in the proterozoic basement of yemen tectonic interpretation and fluid flow implications abstract multi scale mappings of fracture systems in the crystalline basement of yemen are presented fracture datasets are described through statistical analyses of direction length spacing density and spatial distribution results are combined with field observations and can be directly used to model the geometry of the fracture networks in analog basement rocks from multi kilometric to decametric scales the fractured reservoir analog is defined with a dual porosity model in which tectonic and joint systems correspond to the basement reservoir backbone and matrix respectively these two end members reveal contrasting geometrical reservoir and scaling properties in tectonic systems multi scale geometries are self similar the fracture network shows fractal behavior power law length distribution and clustered spacing and fault zones show hierarchical organization of geometrical parameters such as length thickness and spacing in joint systems the fracture network is scale dependent with exponential length distribution and shows anti clustered spacing however these two end members have both well connected properties with fault zones acting as main drain and joint systems acting as the fluid supply

estimation of rev size and three dimensional hydraulic conductivity tensor for a fractured rock mass through a single well packer test and discrete fracture fluid flow modeling abstract a new methodology is presented to determine the representative elementary volume rev size and three dimensional 3 d hydraulic conductivity tensor for a fractured rock mass first a 3 d stochastic fracture network model was built and validated for a gneissic rock mass based on the fracture data mapped from scanline surveys at the site this validated fracture network model was combined with the fracture data observed on a borehole to generate a stochastic deterministic fracture network system in a cubic block around each packer test conducted at a different depth region in the same borehole each packer test was simulated numerically applying a developed discrete fracture fluid flow model to estimate the influenced region or effective range for the packer test a cubic block of size 18 m with the packer test interval of length about 6 5 m located at the centre of this block was found to be suitable to represent the influenced region using this block size the average flow rate per unit hydraulic gradient defined as the transmissivity multiplied by mean width of flow paths field for fractures was calibrated at different depth regions around the borehole by numerically simulating the packer tests conducted at different depth regions the average flow rate per unit hydraulic gradient of the fractures that intersect the borehole was considered to be quite different to the average flow rate per unit hydraulic gradient of the fractures that do not intersect the borehole a relation was developed to quantify the ratio between these two parameters by studying the directional hydraulic conductivity behaviour of different cubic block sizes having the validated stochastic fracture network and calibrated hydraulic parameters a rev for the hydraulic behaviour of the rock mass was estimated to be a block size of 15 m the hydraulic conductivity tensor in 3 d computed through regression analysis using the calculated directional hydraulic conductivity values in many directions was found to be significantly anisotropic the principal directions of the hydraulic conductivity tensor were found to be agreeable with the existing fracture system of the site further the geometric hydraulic conductivity calculated was found to be comparable to the hydraulic conductivity estimated through the radial flow assumption in continuum porous media

using the material point method to model sea ice dynamics 1 the material point method mpm is a numerical method for continuum mechanics that combines the best aspects of lagrangian and eulerian discretizations the material points provide a lagrangian description of the ice that models convection naturally thus properties such as ice thickness and compactness are computed in a lagrangian frame and do not suffer from errors associated with eulerian advection schemes such as artificial diffusion dispersion or oscillations near discontinuities this desirable property is illustrated by solving transport of ice in uniform rotational and convergent velocity fields moreover the ice geometry is represented by unconnected material points rather than a grid this representation facilitates modeling the large deformations observed in the arctic as well as localized deformation along leads and admits a sharp representation of the ice edge mpm also easily allows the use of any ice constitutive model the versatility of mpm is demonstrated by using two constitutive models for simulations of wind driven ice the first model is a standard viscous plastic model with two thickness categories the mpm solution to the viscous plastic model agrees with previously published results using finite elements the second model is a new elastic decohesive model that explicitly represents leads the model includes a mechanism to initiate leads and to predict their orientation and width the elastic decohesion model can provide similar overall deformation as the viscous plastic model however explicit regions of opening and shear are predicted furthermore the efficiency of mpm with the elastic decohesive model is competitive with the current best methods for sea ice dynamics

active deformation of taiwan from gps measurements and numerical simulations using a two dimensional distinct element model we evaluate the relationships between plate kinematics and present day deformation in taiwan where active collision occurs in particular the distribution of velocity fields calculated in our models is compared with the actual velocity field revealed by the most recent geodetic data gps obtained in taiwan and the surrounding islands of the philippine sea plate and the eurasian shelf the main aim of this paper is to produce a mechanically consistent 2 d model that accounts for the observed velocity field taken as whole within the limits of acceptable rheological parameters and reasonable boundary displacement conditions we evaluate how the active deformation of taiwan is influenced by the presence of strong and weak zones such as the structural highs in the foreland and subduction zones with accretionary prisms respectively major mechanical discontinuities such as the main fault zones in the mountain belt and the opening of the okinawa trough particular attention is paid to the role of preexisting discontinuities since the presence of mechanical decoupling along major faults strongly affects the distribution of the velocity and stress patterns we show that despite parameter uncertainties several tectonic factors the presence of the strong kuanyin and peikang highs in contrast to the weak subduction zone to the south the weak active regional shear zones and the opening of the back arc okinawa trough concur to provide an acceptable mechanical model for this regional deformation these sources are related not only to the geometry of the plate boundary the direction of plate convergence and the shape of the chinese margin but also to the presence of major zones of relative weakness and mechanical decoupling such as the longitudinal valley fault zone and the western thrust belt of taiwan

image processing based characterisation of coal cleat networks abstract characterisation of the cleat network serves as the basis for estimating the hydraulic and mechanical seam properties which in turn are fundamental for flow and geomechanical modelling in the context of underground coal mining cleat and cleat network geometry can be described as a function of frequency aperture size orientation relative to in situ stresses connectivity and porosity with mineralised and un mineralised cleats occurring to describe these properties ct scans of core samples of a bowen basin coal in central queensland australia are analysed a unique image processing workflow method is introduced to extract the key statistical parameters of perpendicular butt and face cleats present in a two dimensional image as face and butt cleats have different characteristics the presented method distinguishes face cleats and butt cleats by direction and present detailed data for both cleat types the results comprise cleat length apertures sizes intensities densities shape parameter spacing orientation and connectivity and are therefore more comprehensive than previous cleat descriptions three generally different cleat geometries are considered within this study one sample shows perpendicular face and butt cleats the second two sets of face cleats intersected by butt cleats and the third parallel face cleats only

investigation of the effects of fractured porous media on hydraulic tests an experimental study at laboratory scale using single well methods abstract in this paper a study of detailed pneumatic tests at laboratory scale is presented the study comprises two different test methods which were conducted on an unsaturated fractured sandstone block of about 1 m 3 volume first a steady state flow field with constant gas injection pressure and consequently constant gas flow rates was applied to the fractured sandstone block via a vertical borehole the discharge of the injected gas was measured at the block surface second a constant gas pressure was injected over the borehole and the transient pressure buildup was recorded at the block surface it was the objective of the study to investigate the effects of the fractured porous system on pneumatic measurements and to provide an insight into processes occurring during flow and pressure buildup in fractured porous media this is an essential prerequisite for a reliable validation of different modeling approaches and it can further assist to improve techniques for the determination of the heterogeneity of hydraulic parameters in fractured porous media the analysis of the distribution of the flow field and the temporal and spatial evolution of pressure buildup during the hydraulic tests shows that the direction and contribution of the flow field is highly depending on the spatial distribution and the characteristics of the fracture network as well as on the position of the observation points with respect to highly conductive structures finally the introduced test methods are suitable tools for the characterization of the heterogeneous nature of fractured porous media and for the interpretation of the effects of the heterogeneous system on hydraulic tests

a three dimensional model to simulate jointnetworks in layered rocks modelling the discontinuity network of fractured reservoirs may be addressed 1 by purely stochastic means 2 with a fractal approach or 3 using mechanical parameters describing the spatial organisation of fracture systems our paper presents an approach where the geometrical properties of the fracture networks are incorporated in the form of both statistical and mechanical rules this type of approach is particularly suitable to model stratified fractured rock masses comprising two orthogonal families of joints and a family of sedimentary discontinuities their geometrical arrangement is governed by two kinds of rules based on 1 statistical parameters such as the mean standard deviation of joint length and of bed thickness both determined by field observations and 2 geometrical parameters that result from genetic processes inferred from field observations and analogue experiments on the nucleation and propagation mechanisms of joints using these parameters we generate realistic networks in