precise evaluation of atmospheric loading effects on earth s time variable gravity field 1 new space gravity missions will provide gravity measurements with unprecedented accuracy and high spatial resolution to reveal the oceanic and hydrologic signals in monthly time variable gravity field from the gravity recovery and climate experiment grace satellite tapley et al 2004 entails the removal of the atmospheric contribution which in turn requires a precise knowledge of the atmospheric mass redistribution we reconstruct the three dimensional 3 d variations of air density from vertical profiles of pressure temperature and specific humidity provided by the national centers for environmental prediction ncep atmospheric model of a realistic topography we compare our results with those from the classical thin layer 2 d approximation and show that the differences between the complete 3 d and the 2 d computations are often nonnegligible in the presence of the expected grace sensitivity up to harmonic degrees of 15 20 corresponding to wavelengths of 2000 2500 km for actual computation we recommend the use of the sigma level atmospheric data with special attention to the latitude and altitude dependence of the earth s gravity we also examine and conclude the importance of the differences with previous study which assumed a constant surface gravity acceleration without a latitudinal dependence

decadal scale variations in ice flow along whillans ice stream and its tributaries west antarctica this is the publisher s version also available electronically from http www ingentaconnect com

variations of self potential and unsaturated water flow with time in sandy loam and clay loam soils abstract accurate assessment of soil water fluxes is essential in soil physics due to its direct implications in environmental agronomical or hydrological applications field estimations of soil water fluxes by classical hydraulic methods are often difficult to obtain moreover water fluxes are highly variable in space and time the obtainment of a reasonable estimate for this variable would require numerous measurement sites however such a requirement is rarely met thony et al cr acad sci paris earth planetary sci 325 1997 317 presented the experimental evidence of a linear relationship between the self potential sp and the unsaturated soil water flux therefore this relationship would allow the indirect assessment of the water flux using electrical measurements such an approach would appear much more flexible and easier to perform than the current hydraulic measurements the aim of this study is to experimentally investigate the existence and robustness of the flux sp relationship for different soil types and pedoclimatic conditions the soil water fluxes and the sp were monitored in a long term experiment involving two types of soils contrasting in hydraulic and electric properties the soils were placed in lysimeters which were instrumented with tensiometers and tdr probes for monitoring hydraulic heads and moisture content respectively unpolarizable sp electrodes temperature sensors and suction cups for collecting pore water were also installed in the lysimeters the sp and the fluxes were measured or calculated in the 30 40 cm depth section results show that the variations of the sp with time were clearly linked to both rainfall events and evaporation however in the long term the linear relationship between the unsaturated water flux and the sp evolves from strongly correlated to almost not correlated the slope sensitivity of the flux sp relationship varies with the soil type decreasing with more electrically conductive soil taking into account a varying soil electrode contact greatly improves the flux sp relationship at the scale of the rainfall event particularly when considering infiltration and drainage phases separately nevertheless at the scale of a year with alternated rainfalls and evaporation phases the robustness of the relationship decreases i e the coefficients of the relationship vary between events this variability could be related to time variations in electrical conductivity not so much to that of the soil water but rather to that of the water from the salted soil mud added to the sp electrodes at the time of installation this study points out methodological problems associated with the measurement of sp in shallow unsaturated soils over the long term and the need for designing specific electrodes for this purpose however in deep soils beneath the root zone environmental conditions generally vary slowly and lightly in comparison to surface horizons in this case and with our present set of sp measurement devices the flux sp relationship could be more stable than in the surface soil horizons and useful for examining aquifer recharge capillary rises or contaminant transfer

subsurface crevasse formation in glaciers and ice sheets 1 crevasses form in response to tensile stresses in glaciers and ice sheets it has been widely assumed that crevasses initiate at or near the surface of the ice from starter cracks up to a few centimeters long if the tensile stress is sufficient these cracks propagate downward into the ice to form a crevasse until the weight induced lithostatic stress prevents them penetrating deeper we present ground penetrating radar data acquired on the rutford ice stream antarctica which indicate that crevasses occur at depths of several meters beneath the ice surface and were formed in areas where surface crevassing is absent the data support the hypothesis that these are examples of subsurface crevasse formation using linear elastic fracture mechanics lefm we investigate the feasibility of crevasse initiation at depth we consider the initiation of an isolated crevasse from a subsurface crack subject to a dynamic tensile stress which results from deformation associated with ice movement and a weight induced lithostatic stress the lefm approach allows us to estimate a init the minimum length a crack must be before crack propagation will occur in earlier models of crevasse formation it was assumed that the dynamic tensile stress is constant with depth we consider a more realistic scenario where the dynamic tensile stress varies with depth in such a way that the tensile strain rate remains constant we show that in this scenario crevasse initiation from centimeter scale starter cracks is feasible at depths of 10 30 m as well as at the surface at present the formulation of a reliable predictive model is limited by an incomplete knowledge of the mechanical properties of firn in previous studies the depth of buried crevasses has been used to estimate the time elapsed since ice was exposed to higher stresses and different flow regimes in the light of the results presented here those estimates may need to be reviewed

seismic attenuation and velocity dispersion in heterogeneous partially saturated porous rocks summary using a numerical approach we explore wave induced fluid flow effects in partially saturated porous rocks in which the gas water saturation patterns are governed by mesoscopic heterogeneities associated with the dry frame properties the link between the dry frame properties and the gas saturation is defined by the assumption of capillary pressure equilibrium which in the presence of heterogeneity implies that neighbouring regions can exhibit different levels of saturation to determine the equivalent attenuation and phase velocity of the synthetic rock samples considered in this study we apply a numerical upscaling procedure which permits to take into account mesoscopic heterogeneities associated with the dry frame properties as well as spatially continuous variations of the pore fluid properties the multiscale nature of the fluid saturation is taken into account by locally computing the physical properties of an effective fluid which are then used for the larger scale simulations we consider two sets of numerical experiments to analyse such effects in heterogeneous partially saturated porous media where the saturation field is determined by variations in porosity and clay content respectively in both cases we also evaluate the seismic responses of corresponding binary patchy type saturation patterns our results indicate that significant attenuation and modest velocity dispersion effects take place in this kind of media for both binary patchy type and spatially continuous gas saturation patterns and in particular in the presence of relatively small amounts of gas the numerical experiments also show that the nature of the gas distribution patterns is a critical parameter controlling the seismic responses of these environments since attenuation and velocity dispersion effects are much more significant and occur over a broader saturation range for binary patchy type gas water distributions this analysis therefore suggests that the physical mechanisms governing partial saturation should be accounted for when analysing seismic data in a poroelastic framework in this context heterogeneities associated with the dry frame properties which do not play important roles in wave induced fluid flow processes per se should be taken into account since they may determine the kind of gas distribution pattern taking place in the porous rock

calving processes and the dynamics of calving glaciers abstract calving of icebergs is an important component of mass loss from the polar ice sheets and glaciers in many parts of the world calving rates can increase dramatically in response to increases in velocity and or retreat of the glacier margin with important implications for sea level change despite their importance calving and related dynamic processes are poorly represented in the current generation of ice sheet models this is largely because understanding the calving problem involves several other long standing problems in glaciology combined with the difficulties and dangers of field data collection in this paper we systematically review different aspects of the calving problem and outline a new framework for representing calving processes in ice sheet models we define a hierarchy of calving processes to distinguish those that exert a fundamental control on the position of the ice margin from more localised processes responsible for individual calving events the first order control on calving is the strain rate arising from spatial variations in velocity particularly sliding speed which determines the location and depth of surface crevasses superimposed on this first order process are second order processes that can further erode the ice margin these include fracture propagation in response to local stress imbalances in the immediate vicinity of the glacier front undercutting of the glacier terminus by melting at or below the waterline and bending at the junction between grounded and buoyant parts of an ice tongue calving of projecting submerged ice feet can be regarded as a third order process because it is paced by first or second order calving above the waterline first order calving can be represented in glacier models using a calving criterion based on crevasse depth which is a function of longitudinal strain rate modelling changes in terminus position and calving rates thus reduces to the problem of determining the ice geometry and velocity distribution realistic solutions to the problem of modelling ice flow therefore depend critically on an appropriate choice of sliding law models that assume that basal velocities are controlled by basal drag can replicate much of the observed behaviour of calving glaciers with grounded termini but an important limitation is that they cannot be used to model floating glacier termini or ice shelves alternative sliding laws that parameterise drag from the glacier margins provide more flexible and robust ways of representing calving in ice sheet models such models can explain a remarkable range of observed phenomena within a simple unifying framework including downglacier increases in velocity and strain rates where basal and or lateral drag diminishes flow acceleration in response to thinning through time the tendency for glaciers to stabilise at pinning points in relatively shallow water or fjord narrowings the constraints on ice shelf stability and the contrasts in calving rates between tidewater and freshwater calving glaciers many unresolved issues remain however including the role played by the removal of backstress in the acceleration of retreating calving glaciers and the controls on melting at and below the waterline

glacier crevasses observations models and mass balance implications we review the findings of approximately 60 years of in situ and remote sensing studies of glacier crevasses as well as the three broad classes of numerical models now employed to simulate crevasse fracture the relatively new insight that mixed mode fracture in local stress equilibrium rather than downstream advection alone can introduce nontrivial curvature to crevasse geometry may merit the reinterpretation of some key historical observation studies in the past three decades there have been tremendous advances in the spatial resolution of satellite imagery as well as fully automated algorithms capable of tracking crevasse displacements between repeat images despite considerable advances in developing fully transient three dimensional ice flow models over the past two decades both the zero stress and linear elastic fracture mechanics crevasse models have remained fundamentally unchanged over this time in the past decade however multidimensional and transient formulations of the continuum damage mechanics approach to simulating ice fracture have emerged the combination of employing damage mechanics to represent slow upstream deterioration of ice strength and fracture mechanics to represent rapid failure at downstream termini holds promise for implementation in large scale ice sheet models finally given the broad interest in the sea level rise implications of recent and future cryospheric change we provide a synthesis of 10 mechanisms by which crevasses can influence glacier mass balance

deformation induced melting in the margins of the west antarctic ice streams flow of glacial ice in the west antarctic ice sheet localizes in narrow bands of fast flowing ice streams bordered by ridges of nearly stagnant ice but our understanding of the physical processes that generate this morphology is incomplete here we study the thermal and mechanical properties of ice stream margins where flow transitions from rapid to stagnant over a few kilometers our goal is to explore under which conditions the intense shear deformation in the margin may lead to deformation induced melting we propose a 2 d model that represents a cross section through the ice stream margin perpendicular to the downstream flow direction we limit temperature to the melting point to estimate melt rates based on latent heat using rheology parameters as constrained by laboratory data and observations we conclude that a zone of temperate ice is likely to form in active shear margins

time variability of the earth s gravity field hydrological and oceanic effects and their possible detection using grace the grace satellite mission scheduled for launch in 2001 is designed to map out the earth s gravity field to high accuracy every 2 4 weeks over a nominal lifetime of 5 years changes in the gravity field are caused by the redistribution of mass within the earth and on or above its surface grace will thus be able to constrain processes that involve mass redistribution in this paper we use output from hydrological oceanographic and atmospheric models to estimate the variability in the gravity field i e in the geoid due to those sources we develop a method for constructing surface mass estimates from the grace gravity coefficients we show the results of simulations where we use synthetic grace gravity data constructed by combining estimated geophysical signals and simulated grace measurement errors to attempt to recover hydrological and oceanographic signals we show that grace may be able to recover changes in continental water storage and in seafloor pressure at scales of a few hundred kilometers and larger and at timescales of a few weeks and longer with accuracies approaching 2 mm in water thickness over land and 0 1 mbar or better in seafloor pressure

atmospheric pressure and gravity summary gravity green s functions for a column load of a model atmosphere on a spherical elastic earth are presented and they are used to evaluate the contribution of global atmospheric pressure variations to local gravity the green s functions are found to be relatively insensitive to the details of the model atmosphere but they are dependent on the temperature at the base of the column and on the relative height difference between the base of the column and the gravity station the total signal that global pressure systems contribute to gravity is about 30 gal of which about 90 per cent is produced by the atmosphere within 50 km of the gravity station a zone between 50 and 1000km from the gravity station contributes a couple of gal as does the remainder of the globe this pattern the coherence scale of pressure fluctuations the time and spatial scales appropriate to the hydrostatic approximation and the distance of the gravity station from the oceans suggest a division of the globe into local regional and global zones data requirements processing details and the reliability of the computed signal are different in each zone the local zone is within about 50 km of the gravity station within this zone pressure changes rapidly in time but is spatially coherent so that hourly observations of pressure and temperature at the gravity site alone are sufficient to compute an accurate correction except when a front is passing through the regional zone extends from the edge of the local zone to between several hundred and a thousand kilometres the signal from this zone is small and is only weakly coherent with the signal from the central zone so that a rather sparse array of hourly samples of pressure and temperature are required the gravity signal from the global zone can reach about a gal it varies on a time scale of days and is influenced by the response of the oceans to pressure variations previously reported observations that the admittance between local pressure and gravity residuals depends on epoch frequency or site are most probably due to incorrect modelling a proper local regional temperature and global correction can adequately account for the gravity signal from the atmosphere to within a few tens of ngal in the diurnal band and about 100 ngal in the days to seasonal band except during extreme weather conditions the application of the local correction lowers the power spectral density of the gravity residuals in every band from seasonal to hourly the regional global and temperature corrections lower the residual noise in the seasonal and synoptic bands but are not consistently effective at periods less than about half a day

comment on dependence of shear wave seismoelectrics on soil textures a numerical study in the vadose zone by f i zyserman l b monachesi and l jouniaux

velocity of ice streams b and c antarctica the essential difference between ice stream and inland ice and the cause of ice streaming are not known in order to study these problems velocities have been measured on and near ice streams b and c by repeated tracking of transit satellites the results confirm the difference in mode of flow between fast on ice stream b and slow on the interstream ice ridges also as expected ice stream c is nearly stagnant which with other evidence of formerly fast flow confirms its recent slowing and current thickening the drainage of ice stream b has a negative balance equivalent to thinning by 0 15 0 05 m yr 1 over the entire catchment the lower portion of the ice stream is thickening at about 2 m yr 1 but the overall behavior is thinning this general thinning is probably partly due to progressive extension of the ice stream into inland ice the transition from inland ice to ice stream seems to occur irregularly both spatially and in time with rafts of inland ice being carried into the ice stream and slowly incorporated

new and improved determinations of velocity of ice streams b and c west antarctica measurements of velocity have been made on and next to ice streams b and c west antarctica the results are more precise than previous work and constitute a 93 increase in the number of values these velocities are used to describe the confluence of flow into the ice streams and the development of fast ice stream flow the onset of fast streaming flow occurs in many separate tributaries that coalesce down glacier into the major ice streams for those inter stream ridges that have been studied the flow is consistent with steady state along ice stream b gradients in longitudinal stress offer little resistance to the ice flow the transition from basal drag control to ice shelf flow is achieved through reduced drag at the glacier base and increased resistance associated with lateral drag velocities in the trunk of ice stream c are nearly zero but those at the up glacial head are similar to those at the head of ice stream b

reduction of surface gravity data from global atmospheric pressure loading summary besides solid earth and ocean tides atmospheric pressure variations are one of the major sources of surface gravity perturbations as shown by previous studies merriam 1992 mukai et al 1995 boy et al 1998 the usual pressure correction with the help of local pressure measurements and the barometric admittance a simple transfer function between pressure and gravity both measured locally does not allow an adequate estimation of global atmospheric loading we express the response of the earth to pressure forcing using a green s function formalism farrell 1972 the atmosphere acts on surface gravity through two effects first a direct gravitational attraction by air masses which is sensitive to regional about 1000 km around the gravimeter pressure variations second an elastic process induced by the earth s surface deformation and mass redistribution which is sensitive to large scale pressure variations wavelengths greater than 4000 km we estimate atmospheric loading using green s functions and global pressure charts provided by meteorological centres we introduce different hypotheses on the atmospheric thickness and atmospheric density variations with altitude for the modelling of the direct newtonian attraction all computations are compared to gravity data provided by superconducting gravimeters of the ggp global geodynamics project network we show the improvement by modelling global pressure versus the local estimates in terms of reduction of the variance of gravity residuals we can also validate the inverted barometer ib hypothesis as the oceanic response to pressure forcing for periods exceeding one week the non inverted barometer nib hypothesis is shown to be definitely an inadequate assumption for describing the oceanic response to atmospheric pressure at seasonal timescales

theory of transient streaming potentials in coupled unconfined aquifer unsaturated zone flow to a well a semianalytical solution is presented for transient streaming potentials associated with flow to a pumping well in an unconfined aquifer taking into account the effect of flow in the unsaturated zone above the water table flow in the unsaturated zone is modeled with a linearized form of richards equation using an exponential model for soil moisture retention and unsaturated hydraulic conductivity archie s law is invoked for unsaturated electrical conductivity the unsaturated electrokinetic coupling coefficient is modeled with a decaying exponential where the maximum value is at and below the water table the coupled flow and electrokinetic problem is solved using laplace and hankel transforms the results of the model predicted behavior are presented and compared to that observed in laboratory simulations of pump ing tests the early time polarity reversal predicted the model is observable in the experiments other non monotonic streaming potential behaviors predicted by the model are also evident in experimental measurements the model is used to estimate hydraulic parameters from sp data and these compare well to those obtained from drawdown data for example a hydraulic conductivity of 3 6 3 10 24 m s is obtained from sp data compared to 3 4 3 10 24 m s from drawdown data

induced polarization of shaly sands equations have been developed that relate induced polarization ip in shaly sands to measurable petrophysical parameters the induced polarization process has been modeled in terms of two mechanisms clay counterion displacement and membrane blockage the resulting equations can be used to determine shaliness brine conductivity and oil saturation from in phase and out of phase conductivities laboratory measurements have confirmed the ip dependence on these variables as well as on temperature

streaming potentials at hydrocarbon reservoir conditions we have examined the behavior of the streaming potential under multiphase conditions and under conditions of varying temperature and salinity to evaluate the feasibility of using downhole streaming potential measurements to determine fluid distributions in a reservoir using new insights into the pore scale distribution of fluids and of electric charge we found that the saturation dependence of the streaming potential coupling coefficient is important in determining the resulting streaming potential through examination of the four independent physical parameters which comprise the coupling coefficient we developed an understanding of the behavior of the coupling coefficient under conditions of elevated temperature and brine salinity we found that although increasing salinity substantially reduces the magnitude of the coupling coefficient and therefore also the magnitude of the predicted streaming potential increasing temperature has only a small effect showing about a 10 change between 25 c and

measurement of glacier geophysical properties from insar wrapped phase a method is presented for calculating longitudinal glacier strain rates directly from the wrapped phase of an interferometric synthetic aperture radar insar interferogram assuming the ice flow path is known this technique enables strain rates to be calculated for scenes lacking any velocity control points or areas within a scene where the phase is not continuously unwrappable from a velocity control point the contributions to the error in the estimate of the strain rate are evaluated and recommendations for appropriate sar and insar parameters are presented an example using radarsat 1 insar data of an east antarctic ice stream demonstrates the technique for calculating longitudinal strain rate profiles and estimating tensile strength of ice 186 215 kpa from locations of crevasse initiation the strain rate error was found to be 17 corresponding to a tensile strength of ice error of 5 3

automated detection and temporal monitoring of crevasses using remote sensing and their implications for glacier dynamics detailed studies on temporal changes of crevasses and their linkage with glacier dynamics are scarce in the himalayan context observations of temporally changing surficial crevasse patterns and th

experimental and theoretical fracture mechanics applied to antarctic ice fracture and surface crevassing recent disintegration of ice shelves on the antarctic peninsula has highlighted the need for a better understanding of ice shelf fracture processes generally in this paper we present a fracture criterion incorporating new experimental fracture data coupled with an ice shelf flow model to predict the spatial distribution of surface crevassing on the filchner ronne ice shelf we have developed experiments that have enabled us to quantify for the first time quasi stable crack growth in antarctic ice core specimens using a fracture initiation toughness kinit for which crack growth commences the tests cover a full range of near surface densities 560 871 kg m 3 10 9 75 7 m depth results indicate an apparently linear dependence of fracture toughness on porosity such that kinit 0 257 80 7 predicting a zero porosity toughness of ko 155 kpa m1 2 we have used this data to test the applicability to crevassing of a two dimensional fracture mechanics criterion for the propagation of a small sharp crack in a biaxial stress field the growth of an initial flaw into a larger crevasse which involves a purely tensile crack opening depends on the size of the flaw the magnitude of kinit and the nature of the applied stress field by incorporating the criterion into a stress map of the filchner ronne ice shelf derived from a depth integrated finite element model of the strain rate field we have been able to predict regions of potential crevassing these agree well with satellite imagery provided an initial flaw size is assumed in the range 5 50 cm

a comparison of the gravity field over central europe from superconducting gravimeters grace and global hydrological models using eof analysis we analyse data from seven superconducting gravimeter sg stations in europe from 2002 to 2007 from the global geodynamics project ggp and compare seasonal variations with data from grace and several global hydrological models gldas wghm and era interim our technique is empirical orthogonal function eof decomposition of the fields that allows for the inherent incompatibility of length scales between ground and satellite observations ggp stations below the ground surface pose a problem because part of the attraction from soil moisture comes from above the gravimeter and this gives rise to a complex mixed gravity response the first principle component pc of the eof decomposition is the main indicator for comparing the fields although for some of the series it accounts for only about 50per cent of the variance reduction pcs for grace solutions rl04 from csr and gfz are filtered with a cosine taper degrees 20 40 and a gaussian window 350km significant differences are evident between grace solutions from different groups and filters though they all agree reasonably well with the global hydrological models for the predominantly seasonal signal we estimate the first pc at 10 d sampling to be accurate to 1 gal for ggp data 1 5 gal for grace data and 1 gal between the three global hydrological models within these limits the cnes grgs solution and ground ggp data agree at the 79per cent level and better when the ggp solution is restricted to the three above ground stations the major limitation on the ggp side comes from the water mass distribution surrounding the underground instruments that leads to a complex gravity effect to solve this we propose a method for correcting the sg residual gravity series for the effects of soil moisture above the station

response of subglacial sediments to basal freeze on 2 application in numerical modeling of the recent stoppage of ice stream c west antarctica 1 ross ice streams supply over 90 of the ice volume flowing out of the ross sector of the west antarctic ice sheet wais stoppage of ice stream c isc ca 150 years ago appears to have pushed this sector of wais from negative into positive mass balance joughin and tulaczyk 2002 we propose an explanation for the unsteady behavior of isc using a new numerical ice stream model which includes an explicit treatment of a subglacial till layer when constrained by initial conditions emulating prestoppage geometry dynamics and mass balance of isc the model yields a rapid 100 years stoppage of the main ice stream trunk the stoppage is triggered by basal freeze on which consolidates and strengthens the subglacial till our numerical simulations produce results consistent with a number of existing observations for example continuing activity of the two tributaries of isc the model always yields rapid stoppage unless we specify ice stream width that is smaller than its prestoppage values maximum of 80 km we conjecture that if isc was active for at least a few thousand years before slowdown its width was significantly smaller than today to sustain the long active phase ice stream width is a key control that helps determine whether ice stream flow is sustainable over a long term our work indicates that the recent stoppage of ice stream c could have been part of inherent ice stream cyclicity and it leaves open the possibility that other active ice streams may evolve in the future toward rapid shutdowns

self potential modeling from primary flows this paper discusses a new method for the investigation of self potentials sp based on induced current sources the induced current sources are due to divergences of the convection current which is driven in turn by a primary flow either heat or fluid as a result of using this approach there is a shift in emphasis toward the vector flow field and its interaction with current cross coupling structure when compared with the total potential approach of nourbehecht 1963 which emphasized the primary flow potential and the voltage cross coupling this shift in emphasis is advantageous because it is analogous to the actual physical processes for example fluid flow in the ground gives rise to drag convection currents and the interaction of the convection currents with the electrical structure gives rise to the electrical potentials sp this simple physical picture should aid in developing a better intuitive understanding of the generation of sp effects the convective current approach is easily ada

fluid flow monitoring in oil fields using downhole measurements of electrokinetic potential downhole measurements of electrokinetic potential are a promising new technology for hydrocarbon reservoir monitoring usinga3dfinite elementmodelcombiningbothmultiphase flow and electrokinetic components we investigated the behavior of electrokinetic streaming potential during oil production in a range of reservoir environments we found that streamingpotential signals originate at fluid fronts and at geologic boundaries where fluid saturation changes as water encroaches on an oil production well the streaming potential signal associated with the water front encompasses the well even when the front is up to 100 m away so the potential measured at the well starts to change significantly relative to a distant reference electrode variations in the geometry of the encroaching water front can be characterized using an array of electrodes positioned along the well but a good understanding of the local reservoir geology is required to identify signals caused by the front the streaming potential measured at a well is maximized in low permeability reservoirsproducedatahighrateandinthickreservoirswithlow shale content however considerable uncertainties remain particularly relating to the nature of electrokinetic coupling at high salinity and during multiphase flow our results suggest that the streaming potential at low salinity 10 3 10 4 mol l is large 100 1000 mv but might become too small to resolve 0 1 mv at high salinity 0 5 2 mol l depending on how the available data for the electrokinetic coupling at low salinity are extrapolated into the high salinity domain more work remains to determine the behavior of electrokinetic coupling andthereforetheutilityofthistechniqueathighsalinity

measurement of streaming potential coupling coefficient in sandstones saturated with high salinity nacl brine 1 we present measurements of the streaming potential coupling coefficient in intact sandstone samples saturated with nacl brines at concentrations up to 5 5 moll 1 the values we record at low salinity are consistent with those reported previously as brine salinity increases the coupling coefficient decreases in magnitude but is still measureable up to the saturated concentration limit the magnitude of the zeta potential also decreases with increasing salinity but approaches a constant value at high salinity this behaviour is not captured by current models of the electrical double layer our results suggest that streaming potential measurements may be used to monitor flow in saline subsurface environments such as deep saline aquifers and hydrocarbon reservoirs however they were obtained at laboratory temperature future work will focus on the effect of elevated temperatures at high salinity

ice stream related patterns of ice flow in the interior of northeast greenland ice flow in the interior of the ne quadrant of the greenland ice sheet is focused on the large ice stream draining the north side of the summit dome the rapid ice flow in the stream is apparent in the surface features in the stream and at the margins in the broad scale topography that drives ice flow and in satellite derived ice motion information the patterns of ice flow in the upper half of the stream are remarkable for their level of organization simple geometry and effects on local surface topography the stream begins less than 100 km from the ice divide as a current 15 km wide and then broadens symmetrically downstream by the addition of ice from the sides to a width of more than 60 km elevation data and visible band imagery show that the stream has marginal troughs tens of meters deep in its upper reach which are coincident with regions of high shear strain rate the topography of the margins and undulating surface of the stream is generated by the ice flow the surface undulations in the stream are fixed in location and shape over the 5 year period from 1994 to 1999 the enhanced flow presents a challenge for researchers trying to understand the history of ice discharge from a significant area in the interior of the ice sheet

principles of electrography applied to self potential electrokinetic sources and hydrogeological applications 1 the electrical potential field passively recorded at the ground surface of the earth and termed self potential can be analyzed to determine the shape and the depth of the piezometric surface the coupling between hydraulic flow and electrical current density is electrokinetic in nature the electrokinetic coupling coefficient entering into the integral equation relating the depth of the water table to self potential signals is analyzed for various types of porous materials it is simply related to the electrical conductivity of the pore water in steady state conditions each element of the water table can be seen as an elementary dipole with an inclination locally perpendicular to the water table and strength proportional to the water table elevation then we propose three methods to obtain the shape and range of possible depths of the water table from the study of the self potential distribution recorded at the ground surface the nonuniqueness of the solution is removed if one knows either the electrokinetic coupling coefficient or the water table at one location and under the assumption of the spatial homogeneity of the electrokinetic coupling coefficient two field cases are discussed to show the success of the proposed methods for estimating the shape and depth of the water table at two different scales of investigations they concern the study of self potential signals associated with the shape of the water table in the vicinity of a pumping well and in the flank of the kilauea volcano

structural evolution of variegated glacier alaska u s a since 1948 an excellent photographic database comprising 12 sets of vertical aerial photographs taken between 1948 and 1983 has enabled the structural glaciology of surge type variegated glacier alaska u s a to be analyzed for a period that spans two surge cycles and includes the effects of three surges the glacier wide patterns of crevasse development at the end of each of these surges which occurred in 1947 48 1964 65 and 1982 83 were remarkably similar despite differences in surge intensities and extents at the end of each surge a three fold longitudinal zonation of crevasse orientation characterised that part of the glacier that had been affected by surge activity this zonation comprises i transverse crevasses in a narrow upper zone ii superimposed longitudinal and transverse crevasses in an extensive middle zone and iii longitudinal crevasses in a narrow lower zone the main difference between patterns produced by successive surges was small variations in the extent of the three zones during quiescence the most intense crevasse development occurs in the upper glacier in the latter part of quiescence and is a function of increasingly steep down glacier velocity gradients developing in the buildup to the impending surge some crevassing also develops in the central part of the glacier in mid quiescence that is not related to the surge nature of the glacier but to normal flow processes over bedrock there is no structural evidence to indicate that any of the tributaries of variegated glacier surge this observation suggests that the bulb like loops in the medial moraines at variegated glacier form during quiescence as a result of the steady flow of the tributaries into the stagnant trunk glacier rather than as a result of tributary surges

evidence for a recently abandoned shear margin adjacent to ice stream b2 antarctica from ice penetrating radar measurements satellite imagery of the area between ice streams b1 and b2 antarctica shows a lineation on the surface of the ice sheet of uncertain origin ice motion in the area 2 m yr 1 is 2 orders of magnitude slower than that of the surrounding streams and shows no significant variation that could explain the feature a low power high resolution radar system was used to image the upper 80 m of the ice sheet between the two ice streams the survey shows that the lineation is associated with what is likely an abandoned shear margin the radar data show that a set of chaotic diffractors lies buried beneath two thirds of the area while the remaining one third is undisturbed to 80 m depth the chaotic ice ends abruptly along a boundary that is parallel to but offset 2 5 km from the surface lineation also isolated linear diffractors are commonly observed in the otherwise undisturbed ice immediately adjacent to the boundary the depth location orientation and curved form of the diffractors strongly suggest they are the tops of crevasses that were active at the time the chaotic ice was being strained and that they were formed by left lateral shear this is the same sense of shear that is presently active in the b2 margin 6 km away the depth to the chaotic diffractors suggests that the shear margin abandoned its prior position 190 years b p the burial depth decreases toward the b2 margin and suggests a migration rate of 100 m yr 1 in addition a separate high power radar system was used to image the entire thickness of the ice sheet 1 km in the same area these data show numerous linear diffractors near the base of the ice sheet it is very likely that at least some of these diffractors are entrained morainal debris others may be bottom crevasses or zones of wet reflective ice that developed in the high strain environment of the now abandoned shear margin

glacial and glaciofluvial deposits in the interlobate areas of the scandinavian ice sheet abstract rapid flow within ice streams took place in a broad marginal zone of the scandinavian ice sheet during deglaciation ice streams and interlobate zones have been characterised by analysis of glacial flow patterns and examination of sedimentary features of interlobate glacial and glaciofluvial deposits reconstructed glaciodynamic systems are compared with ice streams of modem ice sheets wide interstream areas of inactive ice were left between some adjacent ice streams where they separated during the retreat of the margin these areas are characterised by morainic hummocks composed of both till and glaciofluvial sediment and sections show indications of subglacial deformation observations from fennoscandia suggest that these interstream areas were sites of net accumulation of glacial and glaciofluvial sediment during deglaciation glaciofluvial sediment was deposited in subglacial tunnels or cavities simultaneously with till deformation and deposition resulting from subglacial melting substantial glaciofluvial complexes were formed in narrow interlobate joints where adjacent ice lobes coalesced a case study made in southern finland shows converging flow patterns of two ice lobes and intervening glaciofluvial deposits concentration of meltwater in interlobate zones was a consequence of ice sheet configuration thinner ice in interlobate zones caused convergence of supraglacial and subglacial drainage the crevassed and strain softened ice may also have allowed surface meltwater to penetrate down to the ice bed interface close to the ice margin steep ice velocity gradients in interlobate zones would have increased subglacial melting rates and this environment was favourable for establishment of a subglacial drainage network and deposition of glaciofluvial sediment

ice dynamics at the mouth of ice stream b antarctica field data collected at the mouth of ice stream b show that the flow dynamics of this region are distinctly different than either the major portion of the ice stream upstream or the ice shelf downstream surface slopes in this region are as low as ice shelf surface slopes yet with the exception of patches of ice which may be floating the ice is grounded basal shear stress is negligible the resistance to flow being partitioned between shear at the sides and longitudinal gradients of longitudinal and transverse stress the surface is generally crevasse free features similar to ice rises are observed upstream of the grounding line their origin is uncertain but they move at velocities comparable to the surrounding ice the flow is laterally extensive and longitudinally compressive but there are large local variations of the strain rate from the regional trends the boundary between the two major tributaries to ice stream b followed with the radar is characterized by a band of strain rates much smaller than average detailed measurements at the downstream b network highlight this local variability of strain rates but confirm that there is a strong correlation between surface topography and strain rates the strain rates indicate that the undulating topography is locally generated the lower elevation ice is thicker and moves faster a velocity profile across the crevassed northern margin shows that the decrease of velocity toward the edge is nearly linear a calculation of ice stream discharge at this location agrees closely with two rather rough estimates of balance flux and is considerably larger than a third estimate the discharge of ice stream b does not appear to be significantly out of balance with published estimates of total ice accumulation within the present catchment basin

streaming potential during drainage and imbibition total are thanked for partially funding jackson under the total chairs programme at imperial college london and for funding vinogradov shell are thanked for funding leinov the department of earth science and engineering at imperial college london are thanked for supporting zhang with a janet watson scholarship damien jougnot frederic perrier and an anonymous reviewer are thanked for their careful and thorough reviews the data for this paper are available in supporting information or by contacting the corresponding author m d jackson imperial ac uk

crevasse patterns and the strain rate tensor a high resolution comparison values of the strain rate tensor represented at a 20 m length scale are found to explain the pattern and orientation of crevasses in a 0 13 km 2 reach of worthington glacier alaska u s a the flow field of the reach is constructed from surveyed displacements of 110 markers spaced 20 30 m apart a velocity gradient method is then used to calculate values of the principal strain rate axes at the nodes of a 20 m 20 m orthogonal grid crevasses in the study reach are of two types splaying and transverse and are everywhere normal to the trajectories of greatest most tensile principal strain rate splaying crevasses exist where the longitudinal strain rate x is 0 and transverse crevasses are present under longitudinally extending flow i e x 0 the orientation of crevasses changes in the down glacier direction but the calculated rotation by the flow field does not account for this change in orientation observations suggest that individual crevasses represent local values of the regional flow field and are transient on the time scale of 1 2 years they are not persistent features that are translated and rotated by flow crevasse patterns are thus found to be a useful tool for mapping the strain rate tensor in this reach of a temperate valley glacier

elevation changes and dynamic provinces of jakobshavn isbrae greenland derived using generalized spatial surface roughness from icesat glas and atm data

crevasse squeeze ridge corridors diagnostic features of late stage palaeo ice stream activity a 200 km long and 10 km wide linear assemblage of till filled geometrical ridges on the bed of the maskwa palaeo ice stream of the late wisconsinan southwest laurentide ice sheet are interpreted as crevasse squeeze ridges csr developed during internal flow unit reorganization immediately prior to ice stream shutdown ridge orientations are predominantly orientated wnw ese with a subordinate wsw ene alignment both indicative of ice fracture development transverse to former ice stream flow as indicated by nne ssw aligned msgl subglacial till injection into basal and or full depth mode i and ii crevasses occurred at the approximate centreline of the ice stream in response to extension and fracturing landform preservation indicates that this took place during the final stages of ice streaming immediately prior to ice stream shutdown this linear zone of ice fracturing therefore likely represents the narrowing of the fast flowing trunk similar to the plug flow identified in some surging valley glaciers lateral drag between the final active flow unit and the slower moving ice on either side is likely recorded by the up ice bending of the csr limbs the resulting csr corridor here related to an individual ice stream flow unit constitutes a previously unreported style of crevasse infilling and contrasts with two existing csr patterns 1 wide arcuate zones of csrs related to widespread fracturing within glacier surge lobes and 2 narrow concentric arcs of csrs and recessional push moraines related to submarginal till deformation at active temperate glacier lobes

comment on streaming potential dependence on water content in fontainebleau sand by v allegre l jouniaux f lehmann and p sailhac summary allegre et al recently presented new experimental data regarding the dependence of the streaming potential coupling coefficient with the saturation of the water phase such experiments are important to model the self potential response associated with the flow of water in the vadose zone and the electroseismic seismoelectric conversions in unsaturated porous media however the approach used to interpret the data is questionable and the conclusions reached by allegre et al likely incorrect

seismoelectric wave propagation numerical modelling in partially saturated materials to better understand and interpret seismoelectric measurements acquired over vadose environments both the existing theory and the wave propagation modelling programmes available for saturated materials should be extended to partial saturation conditions we propose here an extension of pride s equations aiming to take into account partially saturated materials in the case of a water air mixture this new set of equations was incorporated into an existing seismoelectric wave propagation modelling code originally designed for stratified saturated media this extension concerns both the mechanical part using a generalization of the biot gassmann theory and the electromagnetic part for which dielectric permittivity and electrical conductivity were expressed against water saturation the dynamic seismoelectric coupling was written as a function of the streaming potential coefficient which depends on saturation using four different relations derived from recent laboratory or theoretical studies in a second part this extended programme was used to synthesize the seismoelectric response for a layered medium consisting of a partially saturated sand overburden on top of a saturated sandstone half space subsequent analysis of the modelled amplitudes suggests that the typically very weak interface response ir may be best recovered when the shallow layer exhibits low saturation we also use our programme to compute the seismoelectric response of a capillary fringe between a vadose sand overburden and a saturated sand half space our first modelling results suggest that the study of the seismoelectric ir may help to detect a sharp saturation contrast better than a smooth saturation transition in our example a saturation contrast of 50 per cent between a fully saturated sand half space and a partially saturated shallow sand layer yields a stronger ir than a stepwise decrease in saturation

multiphase streaming potential in sandstones saturated with gas brine and oil brine during drainage and imbibition 1 we have measured the streaming potential in water wet sandstones during both drainage and imbibition and demonstrate that it behaves differently in oil brine and gas brine displacements during drainage the streaming potential remains significantly greater than zero at the irreducible water saturation in oil brine displacements but falls to zero in gas brine displacements during imbibition the streaming potential at partial saturation in oil brine displacements exceeds that measured at saturation but not in gas brine experiments our results have application to streaming potential measurements in the vadose zone hydrocarbon reservoirs contaminated aquifers and during co2 sequestration

hydrogeological investigations at the membach station belgium and application to correct long periodic gravity variations 1 a comprehensive hydrogeological investigation regarding the influence of variations in local and regional water mass on superconducting gravity measurements is presented for observations taken near the geodynamic station of membach belgium applying a regional water storage model the gravity contribution due to the elastic deformation of the earth was derived in addition the newtonian gravity effect induced by the local water mass variations was calculated using soil moisture observations taken at the ground surface about 48 m above the gravimeters the computation of the gravimetric effect is based on a digital elevation model with spatially discretized rectangular prisms the obtained results are compared with the observations of a superconducting gravimeter sg we find that the seasonal variations can be reasonably well predicted with the regional water storage model and the local newtonian effects shorter period effects depend on the local changes in hydrology this result shows the sensitivity of sg observations to very local water storage changes

an assessment of surge induced crevassing and the formation of crevasse squeeze ridges 1 evidence for extensive crevassing is preserved on the deglaciated forelands of many surging glaciers as crevasse squeeze ridges csrs at some point these crevasses make direct connection with the bed in order to become sediment filled and full depth connections have been inferred from turbid water up wellings in crevasses and the formation of concertina eskers the dynamics of seven surging glaciers are assessed using a linear elastic fracture mechanics approach to determine the likely directions of fracture and controlling parameters for mode i crevasses extensional surface strain rates are insufficient to promote top down full depth penetration for small crevasse spacing 5 m surface strain rates are sufficient for top down crevassing to depths of 4 12 m explaining the extensive surface crevassing associated with glacier surging as has been shown in other settings top down full depth crevassing is only possible when water is added and approaches 97 of the crevasse depth the provision of sufficient meltwater to facilitate this is problematic due to the extensive surface crevassing unless water can move along connected crevasses to a dominant water capturing crevasse for ice thicknesses greater than 200 m basal water pressures in excess of 80 90 of flotation are required for full depth bottom up crevassing field evidence suggests that this is the default for surging glaciers and that on occasion water pressures may even become artesian csrs found across many surging glacier forelands and ice margins most likely result from the infilling of basal crevasses driven for the most part bottom up by high basal water pressures

seismoelectric effects due to mesoscopic heterogeneities 1 while the seismic effects of wave induced fluid flow due to mesoscopic heterogeneities have been studied for several decades the role played by these types of heterogeneities on seismoelectric phenomena is largely unexplored to address this issue we have developed a novel methodological framework which allows for the coupling of wave induced fluid flow as inferred through numerical oscillatory compressibility tests with the pertinent seismoelectric conversion mechanisms simulating the corresponding response of a water saturated sandstone sample containing mesoscopic fractures we demonstrate for the first time that these kinds of heterogeneities can produce measurable seismoelectric signals under typical laboratory conditions given that this phenomenon is sensitive to key hydraulic and mechanical properties we expect that the results of this pilot study will stimulate further exploration on this topic in several domains of the earth environmental and engineering sciences

crevasse patterns at the onset to ice stream b west antarctica sequential satellite imagery and modeling are used to investigate crevasse patterns at the head of ice stream b tributary blb the crevasses informally called the chromosomes form at the upstream limit to blb s northern shear margin and chaotic crevasse zone we find that the onset to crevasse formation and by inference the onset to streaming flow has migrated upstream over time at a mean rate of 230 16 m a 1 a possible cause for that migration is changes in net basal friction due to changes in basal water production rate and storage

weak bands within ice stream b west antarctica kilometer scale variations in ice velocity and surface topography are used to investigate the style of glacier deformation in the main body of ice stream b west antarctica the pattern is very different from that reported for other glaciers for the 250 km 2 area studied on ice stream b most of the observed deformation occurs within two narrow bands in which there is large across flow compression and slow lateral shearing the bands underlie valleys in the ice surface topography measured upward displacement of ice adjacent to the rapidly compressing bands appears to be linked to the creation of the ice stream s topography the most likely cause for the observed pattern of strain rates and surface topography and their changes over time is deformation guided by longitudinal bands of ice with an aligned crystal fabric

measurement of temperature in a margin of ice stream b antarctica implications for margin migration and lateral drag ice temperature was measured in and around the chaotically crevassed south margin of ice stream b antarctica from 1992 to 1994 the temperatures at 30 m depth in the chaotic zone are about 12 k lower than in the adjacent uncrevassed ice due to the ponding of cold winter air at depths greater than 150 m there is clear evidence of internal heating of the ice due to the large shear deformation rate in the marginal zone analysis of the depth of cooling below the crevasses and of the internal heating gives two pieces of information first over the last half century the lateral shear stress averaged 2 0 x 10 5 pa in the top third of the margin and second the margin moved outward at an average rate of 7 3 m a these values do not involve any assumptions about the flow law of ice the uncertainties are roughly 20 the value of lateral shear stress indicates that the most of the drag on the ice stream is along its sides

reply to comment on dependence of shear wave seismoelectrics on soil textures a numerical study in the vadose zone by f i zyserman l b monachesi and l jouniaux by revil a

changes in the west antarctic ice sheet the portion of the west antarctic ice sheet that flows into the ross sea is thinning in some places and thickening in others these changes are not caused by any current climatic change but by the combination of a delayed response to the end of the last global glacial cycle and an internal instability the near future impact of the ice sheet on global sea level is largely due to processes internal to the movement of the ice sheet and not so much to the threat of a possible greenhouse warming thus the near term future of the ice sheet is already determined however too little of the ice sheet has been surveyed to predict its overall future behavior

bering glacier surge 2011 analysis of laser altimeter data the bering glacier bagley icefield system in alaska is currently surging 2011 large scale elevation changes and small scale elevation change characteristics are investigated to understand surge progression especially mass transport from the pre surge reservoir area to the receiving area and propagation of the kinematic surge wave as manifested in heavy crevassing characteristic of rapid brittle deformation this analysis is based on airborne laser altimeter data collected over bering glacier in september 2011 results include the following 1 maximal crevasse depth is 60 m reached in a rift that separates two deformation domains indicative of two different flow regimes otherwise surge crevasse depth reaches 20 30 m 2 characteristic parameters of structural provinces are derived by application of geostatistical classification parameters include significance and spacing of crevasses surface roughness and crevasse edge curvature indicative of crevasse age a classification based on these parameters serves to objectively discriminate structural provinces indicative of surge progression down glacier and up glacier 3 elevation changes from 2011 and 2010 altimetry show 40 70 m surface lowering in the reservoir area in lower central bering glacier and 20 40m thickening near the front in tashalich arm combining elevation changes with results of crevasse profilometry and pattern analysis the rapid progression of the surge can be mathematically physically reconstructed

time variation of the european gravity field from superconducting gravimeters summary ground based gravity observations have the potential to add useful information to the interpretation of data from the new satellite gravity missions champ grace goce we examine 4 5 yr of data from eight superconducting gravimeters sgs associated with the global geodynamics project ggp from 1997 to 2001 and simulate the time variations that might be seen by a satellite signals that are removed from the gravity data before spatial averaging are the solid earth and ocean tides a global atmospheric loading using a vertical perfect gas law for the atmosphere international earth rotation service iers polar motion and instrument drift the 1 d gravity residuals form the basis of an interpolated minimum curvature grid that we spatially average and analyse using both surface polynomials and empirical orthogonal functions eofs a clear annual component is present that if truly regional should be easily detectable by a satellite such as grace the signal is consistent with expected continental water storage which provides some interest for the future comparison of ground and satellite data

an analytical study of seismoelectric signals produced by 1 d mesoscopic heterogeneities the presence of mesoscopic heterogeneities in fluid saturated porous rocks can produce measurable seismoelectric signals due to wave induced fluid flow between regions of differing compressibility the dependence of these signals on the petrophysical and structural characteristics of the probed rock mass remains largely unexplored in this work we derive an analytical solution to describe the seismoelectric response of a rock sample containing a horizontal layer at its center that is subjected to an oscillatory compressibility test we then adapt this general solution to compute the seismoelectric signature of a particular case related to a sample that is permeated by a horizontal fracture located at its center analyses of the general and particular solutions are performed to study the impact of different petrophysical and structural parameters on the seismoelectric response we find that the amplitude of the seismoelectric signal is directly proportional to the applied stress to the skempton coefficient contrast between the host rock and the layer and to a weighted average of the effective excess charge of the two materials our results also demonstrate that the frequency at which the maximum electrical potential amplitude prevails does not depend on the applied stress or the skempton coefficient contrast in presence of strong permeability variations this frequency is rather controlled by the permeability and thickness of the less permeable material the results of this study thus indicate that seismoelectric measurements can potentially be used to estimate key mechanical and hydraulic rock properties of mesoscopic heterogeneities such as compressibility permeability and fracture compliance

height and gravity variations by continuous gps gravity and environmental parameter observations in the southern po plain near bologna italy abstract during 1996 a global positioning system gps receiver and a superconducting gravimeter sg were installed at medicina italy in order to monitor ground deformation and signals related to global regional processes and local environmental effects first results provided by this combined observing strategy are presented by focusing on common patterns as well as on the differences observed in the two time series a marked annual signal of increasing amplitude present in both series is becoming quite recognizable towards the end of 1997 seasonal loading effects induced by air pressure the ocean and surficial water table were estimated and modeled for both data sets for the gravity series 12 h data of balloon radio sounding launches have been used to estimate the attraction effect of the seasonal vertical density distribution of the air pressure column above the station mass effects due to the surficial water table and the ocean were also estimated and accounted for the comparison between the observed and modeled series both in the case of gps heights and gravity shows a remarkable agreement and provides a convincing explanation for the observed seasonal fluctuations in the july september 1997 time frame a sudden gravity increase occurred this anomaly is in the order of 3 gal and is present in the data series after accounting for the seasonal fluctuation this step like feature is responsible for most of the estimated positive linear trend present in the data series the anomalous increase in gravity has been interpreted as due to mass density variations likely associated with the uprising of deep seated waters in relation with local stress field changes during the second half of 1997 concurrent seismic activity occurred both in the southern po valley and in central italy umbria seismic crisis