an experimental study of the relationship between joint spacing and layer thickness abstract two methods for measuring joint spacing are described and compared the area method is a constant for a given outcrop area and is not affected by joint distribution within that area in contrast the line method depends on the location of the linear traverse two kinds of joint sets are distinguished on bedding surfaces 1 a poorly developed set represents the early stages of development when typical joint lengths are less than typical spacing 2 a well developed set represents later stages when lengths are much greater than spacing the area method is applicable to both poorly and well developed sets whereas the line method produces inconsistent results for poorly developed joint sets surface textures on many joints indicate point fracture origins and propagation parallel to bedding whereas most numerical model studies of spacing assume linear origins and propagation perpendicular to bedding the laboratory experiments described in this paper do not suffer from these restrictions an important concept confirmed during these experiments is fracture saturation when the applied strain reaches a certain value fracture spacing stops evolving and remains nearly constant these fracture sets are well developed spacing at saturation is a function of layer thickness but is independent of strain whereas spacing before saturation varies strongly with applied strain thus plotting spacing vs thickness and comparing the slopes of lines fit to such data for poorly developed joint sets in different layers is unlikely to be a diagnostic test for differences in material properties on the other hand spacing may be a sensitive indicator of strain for layers with poorly developed joint sets assessing fracture saturation is a lirst order consideration when gathering spacing data

a pseudo genetic stochastic model to generate karstic networks summary in this paper we present a methodology for the stochastic simulation of 3d karstic conduits accounting for conceptual knowledge about the speleogenesis processes and accounting for a wide variety of field measurements the methodology consists of four main steps first a 3d geological model of the region is built the second step consists in the stochastic modeling of the internal heterogeneity of the karst formations e g initial fracturation bedding planes inception horizons etc then a study of the regional hydrology hydrogeology is conducted to identify the potential inlets and outlets of the system the base levels and the possibility of having different phases of karstification the last step consists in generating the conduits in an iterative manner using a fast marching algorithm in most of these steps a probabilistic model can be used to represent the degree of knowledge available and the remaining uncertainty depending on the data at hand the conduits are assumed to follow minimum effort paths in a heterogeneous medium from sinkholes or dolines toward springs the search of the shortest path is performed using a fast marching algorithm this process can be iterative allowing to account for the presence of already simulated conduits and to produce a hierarchical network the final result is a stochastic ensemble of 3d karst reservoir models that are all constrained by the regional geology the local heterogeneities and the regional flow conditions these networks can then be used to simulate flow and transport several levels of uncertainty can be considered large scale geological structures local heterogeneity position of possible inlets and outlets phases of karstification compared to other techniques this method is fast to account for the main factors controlling the 3d geometry of the network and to allow conditioning from available field observations

till beneath ice stream b 4 a coupled ice till flow model a nonsteady model of an ice stream flowing on deforming till shows that the system responds rapidly and in a stable manner to reasonable marginal perturbations for the model we require one dimensional flow and continuity of ice and till and we assume linear viscous till rheology and balance between the driving stress for ice flow and the resistive stress at the bed this allows us to write coupled equations for the time rate of change of the ice thickness and the till thickness in terms of these thicknesses and the till viscosity an analytic steady state solution for ice stream b west antarctica shows that till viscosity decreases slowly downstream probably in response to decreasing effective pressure downstream nonsteady numerical experiments with fixed ice thicknesses at the ends show that a marginal perturbation causes a wave of adjustment to travel the length of the ice stream in about 50 years with a new steady state in about 200 years changes in till thickness tend to moderate perturbations in the ice and thus stabilize the system

seismic measurements reveal a saturated porous layer beneath an active antarctic ice stream seismic reflection studies recently conducted on ice stream b part of the marine ice sheet of west antarctica show a metres thick layer immediately beneath the ice in which both compressional p and shear s wave speeds are very low these low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure from this and from arguments presented in an accompanying paper1 to the effect that the layer would be too weak to support the shear stress exerted by the overlying ice we conclude that the layer is deforming and that the ice stream probably moves principally by such deformation

estimating flow heterogeneity in natural fracture systems abstract examples of small to medium scale fault systems have been mapped in jurassic sedimentary rocks in north somerset england these examples include contractional and dilational strike slip oversteps as well as normal faults these maps form the basis of calculations performed to investigate heterogeneity in natural fracture systems with the aim of predicting fluid flow localisation in different fault styles as there is no way to measure fracture aperture directly we use vein thickness to represent an integrated flow path or palaeo aperture from which we derive a representation of the flow distribution three different methods are used to estimate flow heterogeneity based on 1 fracture density the ratio of fracture length to area 2 fracture aperture fracture porosity and 3 hydraulic conductance fracture permeability normalised to the pressure gradient and fluid properties our results show that fracture density and hydraulic conductance are poorly correlated and that fracture density does not fully represent the natural heterogeneity of fracture systems fracture aperture and hydraulic conductance indicate stronger degrees of flow localisation different types of structures also seem to display characteristic and predictable patterns of heterogeneity normal fault systems show the highest magnitude of localisation along the faults rather than in the relay ramps while contractional and dilational strike slip systems show very strong localisation in the faults and oversteps respectively in all cases the amount of damage in the oversteps can modify such patterns of heterogeneity

a generic analytical solution for modelling pumping tests in wells intersecting fractures abstract the behaviour of transient flow due to pumping in fractured rocks has been studied for at least the past 80 years analytical solutions were proposed for solving the issue of a well intersecting and pumping from one vertical horizontal or inclined fracture in homogeneous aquifers but their domain of application even if covering various fracture geometries was restricted to isotropic or anisotropic aquifers whose potential boundaries had to be parallel or orthogonal to the fracture direction the issue thus remains unsolved for many field cases for example a well intersecting and pumping a fracture in a multilayer or a dual porosity aquifer where intersected fractures are not necessarily parallel or orthogonal to aquifer boundaries where several fractures with various orientations intersect the well or the effect of pumping not only in fractures but also in the aquifer through the screened interval of the well using a mathematical demonstration we show that integrating the well known theis analytical solution theis 1935 along the fracture axis is identical to the equally well known analytical solution of gringarten et al 1974 for a uniform flux fracture fully penetrating a homogeneous aquifer this result implies that any existing line or point source solution can be used for implementing one or more discrete fractures that are intersected by the well several theoretical examples are presented and discussed a single vertical fracture in a dual porosity aquifer or in a multi layer system with a partially intersecting fracture one and two inclined fractures in a leaky aquifer system with pumping either only from the fracture s or also from the aquifer between fracture s in the screened interval of the well for the cases with several pumping sources analytical solutions of flowrate contribution from each individual source fractures and well are presented and the drawdown behaviour according to the length of the pumped screened interval of the well is discussed other advantages of this proposed generic analytical solution are also given the application of this solution to field data should provide additional field information on fracture geometry as well as identifying the connectivity between the pumped fractures and other aquifers

physical conditions at the base of a fast moving antarctic ice stream boreholes drilled to the bottom of ice stream b in the west antarctic ice sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1 6 bars of the ice overburden pressure these conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick the mechanics of ice streaming plays a role in the response of the ice sheet to climatic change

flow bearing structures of fractured rocks insights from hydraulic property scalings revealed by a pumping test abstract a long duration pumping test conducted over 151 days in a fractured sandstone and shale formation displays a nonstandard drawdown response and anomalous pressure diffusion which cannot be properly interpreted using existing frameworks e g homogeneous double porosity boundary conditions and fractal models an alternative framework with simple geometry and more complex hydraulic properties is thus proposed to interpret such kind of drawdown responses the analytical development allows first to demonstrate all scaling relations in this interpretation framework then and most importantly the multi scale hydraulic test provides consistent scalings of transmissivity t to storativity s over distances ranging from 83 to 383 m in a faulted area drawdown analysis in several monitoring wells shows persistent decrease of transmissivity in highly channelized fracture flow structures in one structure the cubic dependency of transmissivity to storativity identifies a well defined fault and also demonstrates the validity of poiseuille flow at a scale rarely investigated in the other structure the linear dependency of transmissivity to storativity indicates that the flow bearing structure is the surrounding fracture network well designed pumping tests combined with scaling analysis driven by geological evidence thus provide essential information on flow bearing structures for site characterization and modeling tasks at least for moderate to low permeable fractured rocks the scaling of transmissivity to storativity appears to be more informative than any separate interpretation of hydraulic property scaling exponents

dual porosity fractal models for transient flow analysis in fissured rocks a dual porosity fractal model is derived that considers transient flow from block to fissure skin between the fissure and the block and storage capacity and skin effect on the pumping well type curves for different flow dimensions are provided they concern the pumping well and observation wells located either in the fissure system or in a matrix block the interpretation of a field case using this model shows good agreement between the theoretical drawdown and the observed data with consistent results from the pumping well and the observation well a flow dimension of 1 45 is obtained

the variability of antartic ice sheet response to the climatic signal high resolution numerical model experiments were carried out along two flowlines in dronning maud land antarctica one flowline passing through the central part of a coastal mountain range and one along a major continental ice stream shirase glacier results showed that ice sheet behaviour in response to the climatic signal locally differs response patterns are different for the inland ice sheet the coastal ice sheet and around marginal subglacial mountains modelled response time series were analyzed by lag correlation range and fractal analysis local differentiation in ice sheet response is primarily related to the sensitive interplay between surface accumulation patterns thermomechanical properties of the ice sheet and bedrock roughness

sedimentation beneath ice shelves the view from ice stream b abstract ice shelf development is favored by rapid flow of cold ice from outlet glaciers or ice streams into protected embayments with localized high spots basal melting of ice shelves is rapid near the ice front and may occur near the grounding line ice from outlet glaciers may contain significant englacial debris that is deposited as a dropstone diamicton in regions of basal melting englacial debris is sparse or absent in ice streams evidence from ice stream b draining into the ross ice shelf of west antarctica suggests that the rapid ice velocity arises from deformation of a several meter thick water saturated basal till layer that is eroding an unconformity on sediments beneath and that has deposited a till delta tens of meters thick and tens of kilometers long at the grounding line sea level fall would cause conveyor belt recycling of this till delta and grounding line advance across the ross sea to the edge of the continental shelf forming an ice sheet with a low ice stream profile resting on a several meter thick deforming till layer eroding an unconformity the modern ross sea is characterized by a regional unconformity overlain by a diamicton of probable latest pliocene pleistocene age measuring several meters to tens of meters thick we hypothesize that this diamicton is a deformed glacial till and that the ross sea sediments record one or more expansions of the till lubricated west antarctic ice sheet to the edge of the continental shelf

insights on pumping well interpretation from flow dimension analysis the learnings of a multi context field database abstract the flow dimension parameter n derived from the generalized radial flow model is a valuable tool to investigate the actual flow regimes that really occur during a pumping test rather than suppose them to be radial as postulated by the theis derived models a numerical approach has shown that when the flow dimension is not radial using the derivative analysis rather than the conventional theis and cooper jacob methods helps to estimate much more accurately the hydraulic conductivity of the aquifer although n has been analysed in numerous studies including field based studies there is a striking lack of knowledge about its occurrence in nature and how it may be related to the hydrogeological setting this study provides an overview of the occurrence of n in natural aquifers located in various geological contexts including crystalline rock carbonate rock and granular aquifers a comprehensive database is compiled from governmental and industrial sources based on 69 constant rate pumping tests by means of a sequential analysis approach we systematically performed a flow dimension analysis in which straight segments on drawdown log derivative time series are interpreted as successive specific and independent flow regimes to reduce the uncertainties inherent in the identification of n sequences we used the proprietary siren code to execute a dual simultaneous fit on both the drawdown and the drawdown log derivative signals using the stated database we investigate the frequency with which the radial and non radial flow regimes occur in fractured rock and granular aquifers and also provide outcomes that indicate the lack of applicability of theis derived models in representing nature the results also emphasize the complexity of hydraulic signatures observed in nature by pointing out n sequential signals and non integer n values that are frequently observed in the database

research on snow and ice this review unlike its predecessor martinelli et al 1979 is an intentionally selective survey of recent advances in selected areas of snow and ice research i have chosen to concentrate on studies relating to large scale aspects of the cryosphere and to consider their wider significance for other disciplines results obtained by new measurement techniques and the application of remote sensing methods are also stressed at the same time the broad lines of advance in glaciological research as represented in books conference themes and major field or laboratory programs are noted and a detailed bibliography is appended

a coupled marine ice stream ice shelf model theorie des variations climatiques du pleistocene a aujourd hui liees a la dynamique des couvertures marines de glace couplee aux glaces libres flottantes on neglige les effets thermodynamiques et on etudie les processus de flottaison en relation avec les rapports de densite entre la place et l eau de mer

analytical 1d dual porosity equivalent solutions to 3d discrete single continuum models application to karstic spring hydrograph modelling abstract one dimensional analytical porosity weighted solutions of the dual porosity model are derived providing insights on how to relate exchange and storage coefficients to the volumetric density of the high permeability medium it is shown that porosity weighted storage and exchange coefficients are needed when handling highly heterogeneous systems such as karstic aquifers using equivalent dual porosity models the sensitivity of these coefficients is illustrated by means of numerical experiments with theoretical karst systems the presented 1d dual porosity analytical model is used to reproduce the hydraulic responses of reference 3d karst aquifers modelled by a discrete single continuum approach under various stress conditions simulation results show the relations between the dual porosity model coefficients and the structural features of the discrete single continuum model the calibration of the equivalent 1d analytical dual porosity model on reference hydraulic responses confirms the dependence of the exchange coefficient with the karstic network density the use of the analytical model could also point out some fundamental structural properties of the karstic network that rule the shape of the hydraulic responses such as density and connectivity

basal mechanics of ice stream b west antarctica 2 undrained plastic bed model based on the results of our studies of the physical conditions beneath ice stream b we formulate a new analytical ice stream model the undrained plastic bed model henceforth the upb model mathematically the upb model is represented by a non linear system of four coupled equations which express the relationships among ice sliding velocity till strength water storage in till and basal melt rate we examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant less than hundreds of years temporal variability is introduced into the upb model only by the direct dependence of till void ratio changes e t on the basal melting rate m r since till strength b e and ice stream velocity u b b change as long as till void ratio varies the first condition for ice stream stability is that of constant till water storage 0 the second condition for ice stream stability arises from the feedback between ice stream velocity till strength and the basal melting rate which depends on shear heating m r u b b this is the weak till condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress b n 1 1 d the salient feature of the upb model is its ability to produce two thermo mechanically controlled equilibrium states one with a strong bed and slow ice velocities ice sheet mode and one with a weak bed and fast ice velocities ice stream mode this bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in west antarctica basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes the upb model demonstrates that ice streams may be prone to thermally triggered instabilities during which small perturbations in the basal thermal energy balance grow leading to generation or elimination of the basal conditions which cause ice streaming

till beneath ice stream b 3 till deformation evidence and implications most of the velocity of ice stream b near the upstream b camp upb west antarctica appears to arise from deformation of a seismically detected subglacial till layer that averages 6 m thick available evidence indicates that the entire thickness of this till layer is deforming and is eroding subjacent bedrock into flutes parallel to ice flow and hundreds of meters across the resulting till flux beneath upb is equivalent to an average erosion rate of about 0 4 mm yr 1 in the catchment area and suggests that till deltas tens of kilometers long have been deposited at the grounding line during the holocene such deltas should be characterized by partial ice till decoupling across a water film and by a small ice air surface slope they may have been discovered by recent geophysical work

flow behavior in a dual fracture network a model that incorporates a pseudo random process controlled by mechanical rules of fracturing is used to generate 3d orthogonal joint networks in tabular stratified aquifers the results presented here assume that two sets of fractures each with different conductivities coexist this is the case in many aquifers or petroleum reservoirs that contain sets of fractures with distinct hydraulic properties related to each direction of fracturing constant rate pump tests from partially penetrating wells are simulated in synthetic networks the transient head response is analyzed using the type curve approach and plots as a function of time of pressure propagation in the synthetic network are shown the hydrodynamic response can result in a pressure transient that is similar to a dual porosity behavior even though such an assumption was not made a priori we show in this paper that this dual porosity like flow behavior is in fact related to the major role of the network connectivity especially around the well and to the aperture contrast between the different families of fractures that especially affects the earlier hydrodynamic response flow characteristics that may be interpreted as a dual porosity flow behavior are thus related to a lateral heterogeneity large fracture or small fault accordingly when a dual porosity model matches well test data the resulting reservoir parameters can be erroneous because of the model assumptions basis that are not necessarily verified finally it is shown both on simulated data and well test data that such confusion in the interpretation of the flow behavior can easily occur well test data from a single well must therefore be used cautiously to assess the flow properties of fractured reservoirs with lateral heterogeneities such as large fractures or small faults

response of subglacial sediments to basal freeze on 1 theory and comparison to observations from beneath the west antarctic ice sheet 1 we have constructed a high resolution numerical model of heat water and solute flows in sub ice stream till subjected to basal freeze on the model builds on quantitative treatments of frost heave in permafrost soils the full version of the clapeyron equation is used hence ice water phase transition depends on water pressure osmotic pressure and surface tension the two latter effects can lead to supercooling of the ice base this supercooling in turn induces hydraulic gradients that drive upward flow of pore water which feeds the growth of segregation ice onto the freezing interface this interface may progress into the till and form ice lenses if supercooling is sufficiently strong hence the ice segregation process develops a stratified basal ice layer in our model a high basal temperature gradient 0 054 c m 1 triggers ice stream stoppage and the loss of basal shear heating leads to relatively high basal freeze on rates 3 5 mm a 1 in response the subglacial till experiences comparatively rapid consolidation till porosity can decrease from 40 to 120 kpa approximately within one century basal supercooling arising from redistribution of solutes and ice water interfacial effects amounts to ca 0 35 c below the pressure melting point fine grained till is in our model associated with widely spaced thick ice lenses coarse grained till yields thinner ice lenses that are more closely spaced our model results compare favorably although not in all details with available observational evidence from borehole studies of west antarctic ice streams

rheological nonlinearity and flow instability in the deforming bed mechanism of ice stream motion contrary to what has recently been assumed in modeling the proposed deforming bed mechanism for the rapid motion of antarctic ice streams the rheology of water saturated till is probably highly nonlinear according to information from soil mechanics and preliminary experiments on till from the base of ice stream b the equivalent flow law exponent n is probably as high as 100 and the nonlinearities of the shear stress and effective pressure dependences are closely linked the high nonlinearity has important consequences for the deforming bed mechanism a flow system operating by this mechanism can be unstable as a result of feedback from the generation of basal water by shear heating of basal till the short term feedback effect is analyzed for a perturbation in a model ice stream in which the basal meltwater is transported through a distributed system of narrow gap conduits at the ice till interface although the analysis is approximate and some of the system parameters are poorly known the results suggest that the deforming bed mechanism is unstable for n 20 the apparent lack of such an instability in the currently active ice streams implies that their motion is controlled not by the deforming bed mechanism but by some other as yet unidentified mechanism

a numerical model of interactions between a marine ice sheet and the solid earth application to a west antarctic ice stream a time dependent numerical model has been constructed that simulates retreat of a west antarctic ice stream from the edge of the continental shelf during the holocene period of rising sea level this paper describes a method for computing the deformation of the solid earth caused by changes in ice and water loading during retreat of the ice stream the relative sea level changes caused by earth deformation are incorporated as a feedback mechanism in the ice stream model elastic and viscous uplift of the earth caused by thinning of the ice stream and its catchment area is found to delay retreat of the grounding line relative to computed retreat when the ice stream was assumed to be resting on a rigid earth computed retreat of the grounding line began very slowly at 15 000 b p because of rising eustatic sea level retreat accelerated after about 30 km at 13 000 b p because of increasing water depth and a seabed sloping down toward the ice sheet interior by 8000 b p the feedback effects of earth deformation caused retreat of the grounding line to be delayed by about 1000 years relative to computed retreat on a rigid earth the elastic and viscous earth response exerted a moderating influence on the computed retreat rate because uplift at the grounding line partially counteracted the effect of increasing water depth computed retreat of the grounding line slowed and gradually stopped near the present position of the ice stream grounding line uplift of the seafloor caused the total retreat distance to be reduced by 80 km relative to retreat computed on a rigid earth a readvance of 20 km of the grounding line occurred between 3000 b p and the present because continuing viscous uplift caused sea depth to decrease resistance from the ross ice shelf was found to be of primary importance in bringing retreat of the grounding line to a stop in the ross embayment as suggested by earlier investigators within the context of a given ice shelf retreat history the feedback effects of earth deformation caused a reduction of the grounding line retreat rate a reduction of the total computed retreat distance and a readvance of the grounding line after eustatic sea level stopped rising

mechanism for joint saturation in mechanically layered rocks an example from southern israel abstract over 700 joint spacings were measured along a 190 m scanline within a single limestone bed of the turonian gerofit formation southern israel by investigating joints belonging to a single systematic set confined to a bed of uniform thickness one can factor out the major influences of bed thickness lithology and mechanical properties on joint set development joint spacing distributions and fracture spacing ratios fsr vary dramatically along the length of the scanline and correlate directly with throughgoing fracture zones belonging to the same tectonic stress regime regions cross cut by fracture zones display narrower median joint spacings fsr of 1 3 and unimodal joint spacing distributions where fracture zones are absent joint spacings have wider medians fsr of 0 8 and multimodal distributions because fracture zones most likely represent areas of high strain and laboratory experiments demonstrate a systematic decrease in fracture spacing with increasing strain regions of the bed cross cut by fracture zones are likely at a more advanced stage of joint set development and hence closer to saturation than other regions joint saturation in the gerofit formation may be achieved by a shift in scale from single layer to multi layer jointing as the development of throughgoing joints prevents the formation of additional bedding confined joints

evidence for a frozen bed byrd glacier antarctica ice thickness computed within the fjord region of byrd glacier on the assumptions that byrd glacier is in mass balance equilibrium and that ice velocity is entirely due to basal sliding are on average 400 m less than measured ice thicknesses along a radio echo profile we consider four explanations for these differences 1 active glacier ice is separated from a zone of stagnant ice near the base of the glacier by a shear zone at depth 2 basal melting rates are some 8 m yr 3 internal shear occurs with no basal sliding in much of the region above the grounding zone or 4 internal creep and basal sliding contribute to the flow velocity in varying proportions above the grounding zone large gradients of surface strain rate seem to invalidate the first explanation computed values of basal shear stress 140 to 200 kpa provide insufficient factional heat to melt the ice demanded by the second explanation both the third and fourth explanations were examined by making simplifying assumptions that prevented a truly quantitative evaluation of their merit nevertheless there is no escaping the qualitative conclusion that internal shear contributes strongly to surface velocities measured on byrd glacier as is postulated in both these explanations

predictive modelling of hydraulic head responses to dipole flow experiments in a fractured karstified limestone aquifer insights from a comparison of five modelling approaches to real field experiments summary five modelling approaches were tested against a well studied limestone aquifer constituting the hydrogeological experimental site hes of poitiers france the modelling exercise consisted of predicting the drawdown responses of a series of observation wells for two dipole pumping injection flow experiments involving two distinct well pairs at the hes the differences between model predictions appear to be mainly related to the hydraulic datasets used for model parameterisation and to a much lesser extent to the conceptual modelling approach equivalent porous medium vs discrete fracture networks the model dimensionality two dimensional vs three dimensional and or the parameterisation approach forward vs inverse despite the abundance and diversity of calibration parameterisation data all of the models failed to predict the drawdowns with a reasonable degree of accuracy only the order of magnitude of the drawdowns was correctly predicted by three of the five models whereas all models failed to predict the drawdown behaviour at both intermediate and late times the primary source of error is attributed to a lack of information in the single well pumping test data which did not capture the multi permeability structure of the aquifer as revealed a posteriori by the responses to dipole flow experiments this highlights the need to develop novel or improved approaches to characterise the hydraulic properties of limestone aquifers

numerical simulations of major ice streams in western dronning maud land antarctica under wet and dry basal conditions we study the presence and effect of subglacial water on the motion of the inland ice in western dronning maud land a full stokes model including three routing schemes for a thin film of subglacial water and a modification of a weertman type sliding relation to account for higher sliding velocities under wet basal conditions were used to perform 200 ka spin up simulations on a 2 5 km grid subsequent 30 ka simulations with wet and dry basal conditions were analysed for the effects of sliding on the thermal regime and velocities the occurrence of the major ice streams in this area is mainly controlled by the ice and bedrock geometry smaller glaciers only appear as pronounced individual glaciers when subglacial water is taken into account the thermal regime is affected by creep instabilities produced by an ice rheology including a microscopic water content leading to a cyclic behaviour on millennial time scales of the ice flow and occurrence of temperate ice at the base

water pressure coupling of sliding and bed deformation ii velocity depth profilesr b alley 130 139water pressure coupling of sliding and bed deformation iii application to ice stream b antarctica

basal hydraulic system of a west antarctic ice stream constraints from borehole observations pressure and tracer measurements in boreholes drilled to the bottom of ice stream b west antarctica are used to obtain information about the basal water conduit system in which high water pressures are developed these high pressures presumably make possible the rapid movement of the ice stream pressure in the system is indicated by the borehole water level once connection to the conduit system is made on initial connection here also called breakthrough to the basal water system the water level drops in a few minutes to an initial depth in the range 96 117 m below the surface these water levels are near but mostly somewhat deeper than the flotation level of about 100m depth water level at which basal water pressure and ice overburden pressure are equal which is calculated from depth density profiles and is measured in one borehole the conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed the thickness of the gap has to be about 2 mm to account for the water level drop on breakthrough and about 4 mm to fit the results of a salt tracer experiment indicating downstream transport at a speed of 7 5 mm s 1 the above gap conduit model is however ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole and also by the large hole to hole variation in measured basal pressure which if present in a gap conduit system with 2 or 4 mm would result in unacceptably large local water fluxes an alternative model that avoids the e objections called the gap opening model involves opening a gap as injection proceeds starting with a thin film the injection of water under pressure lifts the ice mass a round the borehole creating a gap 3 or 4 mm wide at the ice bed interface evaluated quantitatively the gap opening model accounts for the volume of water that the basal watery system accepts on breakthrough which obviates the gap conduit model in order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits of which the most promising type is the canal conduit proposed theoretically by walder and fowler 1994 flat low conduits incised into the till 0 1 m deep and perhaps 1 m wide with a flat ice roof the basal water pressure data suggest that the canals are spaced 50 300 m apart much closer than r tunnel would be the deepest observed water level 117 m i the most likely to reflect the actual water pressure in the canals corresponding to a basal effective pressure of 1 6 bar in this interpretation the shallower water levels are affected by loss of hydraulic head in the narrow passageway s that connect along the bed from borehole to canal s once a borehole has frozen up and any passageways connecting with canals have become closed a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till given enough time for pressure equilibration this pressure varies considerably with time over the equivalent water level range from 100 to 113 m basal pressure sensors 500 m apart report uncorrelated variations whereas sensors in boreholes 25 m apart report mostly but not entirely well correlated variations of unknown origin in part of the record remarkable anticorrelated variations are interspersed with positively correlated one and there are rare abrupt excursions to extreme water levels as deep as 125m and as shallow as 74 m a diurnal pressure fluctuation intermittently observed may possibly be caused by the ocean tide in the ross sea the lack of any observed variation in ice stream motion when large percentagewise variations in basal effective pressure were occurring according to our data suggests that the observed pressure variations are sufficiently local and so randomly variable from place to place that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed

theoretical analysis of cross joint geometries and their classification one commonly observed bedrock joint pattern consists of cross joints that terminate against an older set of longer systematic joints dyer 1988 showed that unique cross joint geometries may develop in response to a combination of remote stresses and local perturbations of the stress field in the vicinity of preexisting joints we expand upon dyer s analysis by providing the general solutions for cross joint geometry as a function of remote principal stress orientations and ratios specific cross joint geometries are determined by solving for the local cross joint angle as a function of distance from the preexisting joint on the basis of theoretical derivations cross joint geometries are grouped into five main categories curving parallel curving perpendicular quasi curving parallel quasi curving perpendicular and noncurving geometries furthermore by discussing the stress state in the vicinity of the preexisting joint a general expression for the dimension of compressive zone is derived and a more detailed classification of cross joint geometry is provided results from this study provide a method to estimate the relative magnitudes of remote principal stresses simply based on the observed cross joint geometry which may help constrain the tectonic history of a region in addition knowledge of precise cross joint curvature and intersection geometries may provide important insights into their connectivity and fluid flow characteristics

study of fracture induced anisotropy from discrete fracture network simulation of well test responses different conceptualizations of fracture networks have been generated in order to simulate well tests in fractured reservoirs these models have two purposes firstly to investigate whether the behaviour of realistic fracture networks can be emulated with more simple models and secondly to assess whether different sources of anisotropy have characteristic and recognizable influences on the pressure derivative the anisotropy of the fracture networks was increased by decreasing the angle between two originally orthogonal fracture sets and by increasing the permeability contrast between two orthogonal fracture sets results indicate that simple models can capture the first order behaviour of more realistic examples however because early time data are strongly influenced by the connectivity of the fracture network the degree of anisotropy can only be assessed at later times in a test increasing anisotropy results in increasing heterogeneity and compartmentalization and permeability anisotropy in an orthogonal system can only partially replicate the behaviour of a geometrically anisotropic system

fluid involvement in the active helike normal fault gulf of corinth greece abstract rock fabric and mineralogical composition from the fault core and the unaffected protolith have been used to define the role of the segmented helike fault to fluid flow sixty samples were investigated by xrd sem observation and sem eds microanalyses detrital smectite calcite and quartz represent the mineral assemblage at the crest of the footwall block in foniskaria sampling site in this site smectite is enriched at the rims of the fault core all other sampling sites located at the base of the fault scarp are characterized by detrital and newly formed minerals detrital minerals include plagioclase quartz calcite and illite in nikolaiika sampling site and smectite illite kaolinite quartz calcite in selinous sampling site in the latter sampling site erionite and cerussite are newly formed minerals with erionite considered as the hydrothermal alteration product of fluids at 80 100 c at the eastern fault segment illite quartz and calcite t13 site corresponds to detrital minerals mineralogy in the fault core reflects its high permeability to down flowing meteoric water and weak hydrothermal alteration the rock fabric suggests mineral alignment parallel to the fault plane mineralogy indicates that the aigion helike and pyrgaki faults in the gulf of corinth host hydrothermal activity at shallow levels

analogue simulation of natural orthogonal joint set formation in brittle varnish abstract two orthogonal joint sets are commonly observed in exposed sedimentary rocks with a wide variety of abutting and cross cutting relationships brittle varnish analogue models are carried out in conjunction with field studies in order to classify the different orthogonal fracture patterns and constrain the mechanical basis of orthogonal joint development the results suggest that the stress which creates the second joint set can result from i relaxation effects ii slight tension due to warping of the bands defined by the first parallel fractures and iii local and or regional reversals between 2 and 3 a ladder pattern formed by the combination of an initial set of long parallel joints and associated non cross cutting joints of the second set is obtained if the shear strength of the initial joints is low during the development of the second set a grid pattern where both sets mutually cross cut occurs when the shear strength of the initial joint set is high possibly the result of a high normal stress or healing and could result from two independent stress events an intermediate pattern comprising cross cutting and abutting orthogonal cross joints can form if the shear strength of the initial joints was intermediate and or variable during the development of the second set mutually abutting joint sets are observed within each pattern and could result from stress reversals or low differential stress during the final stages of joint development the classical presentation of joint data using rose diagrams cannot distinguish between the wide variety of orthogonal joint patterns maps of fracture intersections should compliment the orientation data we discuss methods to estimate the joint pattern where intersection data are absent

can electrical conductivity data from a single pumping test provide information about the location of a neighboring mixing zone between two aquifers an example from aix les bains marlioz savoie france summary pumping tests were first developed to interpret observed drawdown in wells analyses of drawdown and temperature profiles now help petroleum geologists and geothermal energy specialists improve production rates and numerous hydrogeological studies have combined drawdown data with measures of electrical conductivity or a chemical parameter often a pollutant the present study used electrical conductivity data from a pumping test in a single well to obtain information about the position of the hydrothermal plume that feeds aix les bains thermes de marlioz spa applying this data to a 3d model of an equivalent porous medium showed that the plume at the bottom of the subsurface aquifer must be downstream from the well in the present study combining drawdown data for a single well with electrical conductivity measurements provided an efficient method for determining the position of a mixing zone near pumped wells this method may well be generalizable to other situations

towards a hydrological model for computerized ice sheet simulations ice sheet modelling typically uses grid cells 10km or more on a side so any hydrological and sliding model must average or parameterize processes that vary over shorter distances than this observations and theory suggest that basally produced water remains in a distributed high pressure system unless it encounters low pressure channels fed by surface melt such distributed systems appear to exhibit increasing water storage water transmission and water lubrication of sliding with increasing water pressure a model based on these assumptions successfully simulates some aspects of the non steady response of mountain glaciers to externally forced channel pressure variations it merits testing in ice sheet modelling

environmental changes recorded in the upper pleistocene along the desert boundary southern israel abstract most paleoclimatic studies in the middle east and africa suggest large changes in climate during the last 100 000 years a specific area of interest is the record of environmental changes along the desert boundary this study was conducted in such a boundary in the northern negev israel sedimentological evidence and the 18 o 16 o 13 c 12 c and 14 c content of carbonate nodules formed within the sediments were examined data obtained in two study areas 35 km apart suggest that while soil formation was occurring in the northwest only fluviatile and lake deposition occurred to the southeast the results of this study suggest that during the late pleistocene the boundary was much steeper than in the holocene the period before 35 000 yr b p periodically was wetter than at present with freshwater lakes and soil covered by c 3 plants a change to a more arid condition occurred during the last glacial maximum in europe 18 000 yr b p a second wet period not as wet as the earlier one occurred with the retreat of the glaciers in europe at this time decrease of loess supply and formation of the top soil occurred at both study locations in the southern area this wet period ended with highly evaporated sabkha deposits about 11 000 yr b p the holocene record suggests progressive aridity which was accompanied in the southeast location by covering the area with dunes

a subglacial water flow model for west antarctica the current generation of continental scale ice sheet models cannot successfully reproduce the complex ice water sediment interactions of the west antarctic ice sheet wais in a physically meaningful manner the potential of a thin film based subglacial water flow model for incorporation into a continental scale coupled ice water flow model of the wais is evaluated in this paper the subglacial water flow model is applied to the ross sea sector of the wais in both a steady state and time dependent form to derive the equilibrium water depth for the present day configuration the potential for coupling the model to an ice flow model is then demonstrated using a variable sliding parameter that is a function of the subglacial water depth a coupled ice water flow model using the parameterization tested in this paper could have the potential for reproducing the surface elevation velocity and thermal regime of the wais successfully these requirements are crucial in modelling the evolution of the wais and must be addressed before reliable continental scale predictive models can be utilized

the dynamics of ice sheet outlets a comparison of data from aircraft altimetry landsat imagery and radia echo sounding has shown characteristic surface topographies associated with sheet and stream flow the transition between the two is abrupt and occurs at a step in the subglacial topography this marks the onset of basal sliding and high velocities caused by subglacial water it results in crevassed amphitheatre like basins round the head of outlet glaciers it is also the zone of maximum driving stress beyond which values decline rapidly as velocities increase this abrupt transition appears to be topographically controlled since basal temperatures are at the pressure melting point well inland of the change in regime the marie byrd land ice streams exhibit qualitative differences from other ice sheet outlets however the change to lower driving stresses is much more gradual and occurs several hundred kilometres inland such ice streams have particularly low surface slopes and appear in form and flow regime to resemble confined ice shelves rather than grounded ice the repeated association of the transition to rapid sliding with a distinct subglacial feature implies a stabilizing effect on discharge through outlet glaciers acceleration of the ice is pinned to a subglacial step and propagation of high velocities inland of this feature seems improbable rapid ice flow through subglacial trenches may also ensure a relatively permanent trough through accentuation of the feature by erosion this is concentrated towards the heads of outlet glaciers up stream of the region where significant basal decoupling occurs this may be a mechanism for the overdeepening of fjords at their inland ends and the development of very steep fjord headwalls

identifying flow networks in a karstified aquifer by application of the cellular automata based deterministic inversion method lez aquifer france the distributed modeling of flow paths within karstic and fractured fields remains a complex task because of the high dependence of the hydraulic responses to the relative locations between observational boreholes and interconnected fractures and karstic conduits that control the main flow of the hydrosystem the inverse problem in a distributed model is one alternative approach to interpret the hydraulic test data by mapping the karstic networks and fractured areas in this work we developed a bayesian inversion approach the cellular automata based deterministic inversion cadi algorithm to infer the spatial distribution of hydraulic properties in a structurally constrained model this method distributes hydraulic properties along linear structures i e flow conduits and iteratively modifies the structural geometry of this conduit network to progressively match the observed hydraulic data to the modeled ones as a result this method produces a conductivity model that is composed of a discrete conduit network embedded in the background matrix capable of producing the same flow behavior as the investigated hydrologic system the method is applied to invert a set of multiborehole hydraulic tests collected from a hydraulic tomography experiment conducted at the terrieu field site in the lez aquifer southern france the emergent model shows a high consistency to field observation of hydraulic connections between boreholes furthermore it provides a geologically realistic pattern of flow conduits this method is therefore of considerable value toward an enhanced distributed modeling of the fractured and karstified aquifers

water pressure coupling of sliding and bed deformation iii application to ice stream b antarctica geophysical studies and glaciological analyses suggest strongly that ice stream b west antarctica moves primarily by pervasive deformation of a meters thick subglacial till analysis of the longitudinal profile of the ice stream up stream of the ice plain suggests that basal sliding is slow everywhere that effective pressure decreases slowly down stream and that the strain rate of pervasive shear is proportional to the basal shear stress and inversely proportional to the square or cube of the effective pressure discrete shearing may occur beneath the pervasively deforming zone these and other hypotheses which build on the analyses of the first two papers in this series can be tested in the field

a numerical model of interactions between a polar ice stream and the ocean application to ice stream e west antarctica a time dependent numerical model has been developed to study the dynamics of a polar ice stream grounded below sea level the model was applied to ice stream e one of the two most active ice streams draining the ross sea sector of the west antarctic ice sheet the retarding force or back stress from the floating ross ice shelf needed to hold the model grounding line in dynamic equilibrium was found to compare closely with retarding force mapped from field data by earlier investigators this implies that the grounding line of ice stream e is in fact close to dynamic equilibrium neither advancing nor retreating rapidly if the ice stream and its catchment area are approximately in mass balance sensitivity tests showed that the model ice stream is more sensitive to changes in back stress from the ice shelf than to changes in the accumulation rate changes in back stress can be caused by changes in the average thickness of the ice shelf the model was used to simulate retreat of the ice stream from the edge of the continental shelf in the ross sea during the holocene period of rising sea level the sea floor was assumed to be isostatically depressed but to remain rigid during retreat the effect of the assumed retreat history of the ross ice shelf on the computed timing of grounding line retreat was investigated by considering several alternative ice shelf histories the results support the conclusion of earlier investigators that the ross ice shelf caused the grounding line to stop retreating near its present position after extensive retreat during the holocene in addition the timing of retreat of the calving front of the ross ice shelf exerted major influence on the timing of retreat of the ice stream grounding line

dynamics and contribution of karst groundwater to surface flow during mediterranean flood this paper describes the role of groundwater contribution to surface flow at the causse d aumelas a karst system near montpellier france which is traversed by an intermittent river the coulazou a first hydrologic model integrating a digital terrain model shows the inability of a standard rainfall runoff model to replicate recorded flood hydrographs while the flood peaks are routed through the karstic system along the coulazou without a phase lag the peak magnitude is somewhat modified these results indicate an initial karst system recharge followed by a significant contribution to surface flow a hydrodynamic analysis of ground water flow confirms these results the karst system first absorbs part of the rainfall which induces a general water table rise within the aquifer and then contributes to surface flow in the coulazou

deformation of till beneath ice stream b west antarctica the behaviour and possible instability of the west antarctic ice sheet depend fundamentally on the dynamics of the large ice streams which drain it model calculations show that most ice stream velocity arises at the bed1 2 and radar sounding has shown the bed to be wet3 but the basal boundary condition is not well understood seismic evidence from the upstream b camp upb on the siple coast of west antarctica4 shows that the ice stream there rests on a layer of unconsolidated sediment averaging 5 or 6 m thick in which the water pressure is only 50 kpa less than the overburden pressure because this thin layer occurs well inland beneath an active ice sheet and rests on a surface showing flutes4 characteristic of glacial erosion5 we presume that it is glacial till we propose here that deformation within the till is the primary mechanism by which the ice stream moves and we discuss implications of this hypothesis

variations of ice bed coupling beneath and beyond ice streams the force balance 1 a geometrical force balance that links stresses to ice bed coupling along a flow band of an ice sheet was developed in 1988 for longitudinal tension in ice streams and published 4 years later it remains a work in progress now gravitational forces balanced by forces producing tensile compressive basal shear and side shear stresses are all linked to ice bed coupling by the floating fraction of ice that produces the concave surface of ice streams these lead inexorably to a simple formula showing how varies along these flow bands where surface and bed topography are known ho hi with ho being ice thickness hi at x 0 for x horizontal and positive upslope from grounded ice margins this captures the basic fact in glaciology the height of ice depends on how strongly ice couples to the bed it shows how far a high convex ice sheet 0 has gone in collapsing into a low flat ice shelf 1 here captures ice bed coupling under an ice stream and ho captures ice bed coupling beyond ice streams

pleistocene paleoclimate of the arid region of israel as recorded in calcite deposits along regional transverse faults and in veins abstract the 18o and 13c values of the calcites associated with e w and ne sw transverse faults in the negev israel indicate that calcite was deposited from meteoric water a regional change in the 18o and 13c values was observed the 18o content in the calcite increases from the southwestern 18 o 17 8 to the northeastern 18 o 2 9 part of the region the 13c values show the opposite trend of the 13c content decrease from 2 in the south to 10 in the northeast these trends had to reflect changes in regional paleoclimate suggesting a change in the isotopic composition of the solution from which the calcite was deposited in different periods the variations in the 18o values reflect shifts in the 18o values of precipitation and are associated with a change in the source of moist air masses which came from the equatorial atlantic in the early pleistocene and from the mediterranean during a later period variations in 13c values reflect changes from humid to arid conditions two modes of calcite deposition are suggested 1 precipitation of calcite minerals in the unsaturated zone following the dissolution in the soil or 2 calcite deposition that occurred as co2 was lost during emergence of paleogroundwater from lower cretaceous and jurassic aquifers

unified pipe network method for simulation of water flow in fractured porous rock abstract rock masses are often conceptualized as dual permeability media containing fractures or fracture networks with high permeability and porous matrix that is less permeable in order to overcome the difficulties in simulating fluid flow in a highly discontinuous dual permeability medium an effective unified pipe network method is developed which discretizes the dual permeability rock mass into a virtual pipe network system it includes fracture pipe networks and matrix pipe networks they are constructed separately based on equivalent flow models in a representative area or volume by taking the advantage of the orthogonality of the mesh partition numerical examples of fluid flow in 2 d and 3 d domain including porous media and fractured porous media are presented to demonstrate the accuracy robustness and effectiveness of the proposed unified pipe network method results show that the developed method has good performance even with highly distorted mesh water recharge into the fractured rock mass with complex fracture network is studied it has been found in this case that the effect of aperture change on the water recharge rate is more significant in the early stage compared to the fracture density change

antarctic ice streams a review an ice stream is a part of an inland ice sheet that flows rapidly through the surrounding ice the distinction between an ice sheet and an outlet glacier is clear in principle but muddy in practice many antarctic glaciers are intermediate in character the ross ice streams which flow through the west antarctic inland ice into the ross ice shelf are distinct in character differing even from other ice streams in the marine ice sheet of west antarctica their surface elevation profiles are low their bed slopes are low and smooth and their driving stresses diminish monotonically downglacier in transverse profile they are broader in relation to ice thickness and exhibit shallower subglacial troughs than other ice streams many models for the fast sliding of glaciers have been applied to the ross ice streams most have included in some form a reduction in basal drag resulting from a lesser effective than glaciostatic pressure at the bed most models are only semi physical at best so need to be tuned by fitting to assumed parameters at some place or places in the real ice sheet the recent discovery of a very small effective pressure beneath one ice stream consequently has led to some gross errors in the velocities predicted by the models the difficulty may be resolved if it is true as recent experiments suggest that ice stream b and by extrapolation other ross ice streams as well slide on a deforming bed that absorbs most or all of the differential motion between the ice and the bedrock