on the analysis of longitudinal stress in glaciers analyse des contraintes et des deformations dans un glacier lorsque les contraintes longitudinales deviatrices sont comparables aux contraintes de cisaillement a la base on etudie aussi le cas ou la composante du poids vers l aval n est pas equilibree par la contrainte de cisaillement basale on obtient les expressions de la contrainte de cisaillement a la base en fonction de l epaisseur de la glace de la pente du gradient de deformation en surface et des proprietes de la glace

a numerical study of plane ice sheet flow l ecoulement bidimensionnel stationnaire d une nappe de glace reposant sur le sol est analyse numeriquement au moyen du modele simplifie de morland et hutter la glace se comporte comme un fluide visqueux non lineaire dependant de la temperature la nappe de glace est supposee longue et mince

on the use of combined geophysical methods to assess water content and water conductivity of near surface formations abstract we propose to deepen the interpretation of combined geophysical methods georadar seismic and electric to assess physical properties characterizing the near surface porous formations especially the influence of water velocity analysis of multioffset georadar data are used together with seismic methods to estimate lateral and vertical ground water fluctuations this enables us to identify transitions from non saturated to fully saturated porous layers with certainty furthermore the accurate knowledge of seismic velocities helps to estimate the porosity of the ground water formations finally we show how the radar technique may be useful in solving the problem of trade off between bulk resistivity which is deduced from electrical measurements and that of water ionic conductivity and water content these theoretical considerations are illustrated using various measurements conducted at the same test site our interpretation is compared with a few laboratory measurements on water and soil samples this study displays the impact of combined geophysical approaches for providing models of water and ionic transfers down to a depth of several metres

the shallow ice approximation for anisotropic ice formulation and limits ice sheet flow modeling is generally based on the shallow ice approximation sia developed for isotropic ice we extend this approximation to anisotropic ice and check its validity for both the isotropic and anisotropic cases by comparing the results of up to second order sia with those obtained from a model solving the full set of mechanical equations the theory is developed for a constitutive relation describing newtonian and non newtonian behavior but numerical results are shown only for a newtonian behavior the results are compared for plane flow of isothermal ice under steady state conditions sia gives an excellent representation of the newtonian flow of an isotropic ice sheet and a good description for the anisotropic case the zero order approximation is sufficient to describe ice flow precisely over flat bedrock for the anisotropic and isotropic cases even close to an ice divide for uneven bedrock the second order sia gives excellent results for isotropic ice and acceptable results for anisotropic ice finally we quantify the error introduced by using an enhancement factor to represent the anisotropy when longitudinal stresses are taken into account

using dc resistivity method to characterize the geometry and the salinity of the plioquaternary consolidated coastal aquifer of the mamora plain morocco the direct current resistivity method was applied to the consolidated coastal plioquaternary aquifer of mamora plain located on the atlantic coast of morocco the aim is to determine the depth of the base of the saturated zone in the aquifer and to help in imaging lateral and vertical distribution of groundwater salinity the geoelectric survey showed four geoelectric formations with the following electrical resistivities from top to bottom 20 80 200 2000 200 300 and 5 70 ohm m the latter designates the basement of the aquifer constituted of marls and sandy marls the mean resistivity of 250 ohm m designates the aquifer formation it decreases to less than 25 of its initial values for the soundings near the shore reflecting the oceanic impact on the aquifer formation resistivity the contour map shows that the basement of the saturated zone in the aquifer is deeper in the northwest near the sebou river estuary with values up to 70 m below sea level this results in a larger thickness of the saturated zone of the aquifer leading to a consequent hydraulic potential on the other hand it has been deduced that the extent of marine intrusion inside the continent can be governed by human activities natural properties and substratum geometry of the aquifer as well as by ecological factors an optimal network of electrical soundings has been proposed for the monitoring of saltwater intrusion

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

coupled thermomechanical response of an axisymmetric cold ice sheet this study proposes a detailed model for ice sheets having a radially symmetric geometry governing relations are derived from first principles of physics and the interactive effects of mechanical and thermal energy are taken into account the resulting model is a nonlinear partial differential equation relating the temperature velocity and stress fields boundary conditions of accumulation rate geothermal flux sliding laws and surface temperatures having been specified and the model determines a steady state response in terms of ice sheet profile and internal temperatures velocities and stresses we describe the computational methodology for numerically determining solutions from which one can assess the credibility of the model itself and to the extent that it is credible examine effects such as perturbing of external temperatures accumulation rates and so forth piloting examples illustrate this

the reconstruction of former ice sheets and their mass balance characteristics using a non linearly viscous flow model on a utilise un modele d ecoulement visqueux non lineaire pour etudier l influence des caracteristiques du bilan de masse des conditions aux limites sur le fond rocheux et de la topographie sous glaciaire sur l etendue et la forme des calottes glaciaires le but est de rendre possible l utilisation des traces geologiques de l extension glaciaire passee pour reconstituer les paleoclimats

electrical resistance tomography electrical resistance tomography ert is a method that calculates the subsurface distribution of electrical resistivity from a large number of resistance measurements made from electrodes for in situ applications ert uses electrodes on the ground surface or in boreholes it is a relatively new imaging tool in geophysics the basic concept was first described by lytle and dines as a marriage of traditional electrical probing introduced by the schlumberger brothers and the new data inversion methods of tomography development of both the theory and practice of ert was confined mostly to the late 1980s and the 1990s tomographic inversion added important new capabilities as it was more general accurate and rigorous at spatial imaging of geophysical electrical resistance data than earlier pseudosection or curve fitting methods

a comparison of peak and plate electrodes in electrical resistivity tomography application to the chalky groundwater of the beauvais aquifer northern part of the paris basin france electrical resistivity tomography was used in order to explore an experimental site of the lasalle beauvais polytechnic institute france the test was conducted along a profile line of 315 m length using 64 electrodes deployed at an inter electrode spacing of 5 m and the data were recorded using gradient wenner and pole dipole arrays the performance of plate electrodes non conventional flat based is compared with the performance of peak electrodes conventional spike the hydrogeophysical investigation of the chalk aquifer system of beauvais shows that the performance of plate electrodes is satisfactory leading to inversions of small root mean square errors peak and the plate electrodes were tested before and after injection of a salt tracer in the piezometer of the experimental site the study demonstrates the usefulness of plate electrodes efficient less time consuming and the possibility of aquifer characterization by a salt tracer copyright 2010 john wiley sons ltd

a glacier flow model incorporating longitudinal deviatoric stresses le modele d ecoulement de glacier developpe introduit les effets de la composante longitudinale du deviateur des contraintes dans les equations les hypotheses sous jacentes peuvent etre appliquees au developpement de modeles pour des cas de figures variees ici elles sont envisagees pour des glaciers glissant en ecoulement plan a l etat stationnaire et non stationnaire le modele se ramene a une generalisation adequate de la theorie pseudo hydrostatique en ecoulement plan si la contrainte longitudinale deviatrice est negligee devant la contrainte de cisaillement basale

rapid least squares inversion of apparent resistivity pseudosections by a quasi newton method1 a fast inversion technique for the interpretation of data from resistivity tomography surveys has been developed for operation on a microcomputer this technique is based on the smoothness constrained least squares method and it produces a two dimensional subsurface model from the apparent resistivity pseudosection in the first iteration a homogeneous earth model is used as the starting model for which the apparent resistivity partial derivative values can be calculated analytically for subsequent iterations a quasi newton method is used to estimate the partial derivatives which reduces the computer time and memory space required by about eight and twelve times respectively compared to the conventional least squares method tests with a variety of computer models and data from field surveys show that this technique is insensitive to random noise and converges rapidly this technique takes about one minute to invert a single data set on an 80486dx microcomputer

joint interpretation of hydrological and geophysical data electrical resistivity tomography results from a process hydrological research site in the black forest mountains germany the use of electrical resistivity tomography ert non intrusive geophysical technique was assessed to identify the hydrogeological conditions at a surface water groundwater test site in the southern black forest germany a total of 111 ert transects were measured which adopted electrode spacings from 0 5 to 5 m as well as using either wenner or dipole dipole electrode arrays the resulting two dimensional 2d electrical resistivity distributions are related to the structure and water content of the subsurface the images were interpreted with respect to previous classical hillslope hydrological investigations within the same research basin using both tracer methods and groundwater level observations a raster grid survey provided a quasi 3d resistivity pattern of the floodplain strong structural heterogeneity of the subsurface could be demonstrated and non connectivities between surface and subsurface bodies were mapped through the spatial identification of likely flow pathways and source areas of runoff the deep groundwater within the steeper valley slope seems to be much more connected to runoff generation processes within the valley floodplain than commonly credited in such environmental circumstances further there appears to be no direct link between subsurface water bodies adjacent to the stream channel deep groundwater sources are also able to contribute towards streamflow from exfiltration at the edge of the floodplain as well as through the saturated areas overlying the floodplain itself such exfiltrated water then moves towards the stream as channelized surface flow these findings support previous tracer investigations which showed that groundwater largely dominates the storm hydrograph of the stream but the source areas of this component were unclear without geophysical measurements the work highlighted the importance of using information from previous complementary hydrochemical and hydrometric research campaigns to better interpret the ert measurements on the other hand the ert can provide a better spatial understanding of existing hydrochemical and hydrometric data copyright 2009 john wiley sons ltd

imaging and characterisation of subsurface solute transport using electrical resistivity tomography ert and equivalent transport models abstract we assess the usefulness of electrical resistivity tomography ert in imaging and characterising subsurface solute transport in heterogeneous unconfined aquifers a field tracer experiment was conducted at the krauthausen test site germany the spatial and temporal evolution of the injected nabr solute plume was monitored in a 2d ert image plane located downstream of the injection well for 90 days since ert maps changes in bulk electrical conductivity the reconstructed images at selected time intervals are first converted to solute concentration maps by postulating a linear relation the concentration maps are then analysed using an equivalent convection dispersion model cdm which conceptualises the aquifer as a homogeneous medium with a uniform mean flow velocity as demonstrated by associated synthetic model studies ert resolution in terms of recovered equivalent dispersivities is limited due to spatial smoothing inherent to the imaging algorithm since for heterogeneous media local concentrations within the plume deviate from those predicted by the equivalent cdm we also interpret the ert derived pixel breakthrough curves in terms of an equivalent stream tube model stm the stm represents transport in the aquifer by a set of 1d convection dispersion processes allowing the degree of mixing and the heterogeneity of transport within the plume to be quantified we believe that the observed tracer plume is satisfactorily described by the equivalent cdm probably because the tracer plume was small relative to the heterogeneity scale of the aquifer even though application of the stm revealed some deviation from the ideal homogeneous case the equivalent dispersivity in the stm matches the longitudinal dispersivity of the cdm closely consistent with predominantly homogeneous mixing however the stm analysis illustrates how ert results can be used to quantify the variability of parameters relevant to flow and transport in heterogeneous aquifers

characterization of seasonal local recharge using electrical resistivity tomography and magnetic resonance sounding a groundwater recharge process of heterogeneous hard rock aquifer in the moole hole experimental watershed south india is being studied to understand the groundwater flow behaviour significant seasonal variations in groundwater level are observed in boreholes located at the outlet area indicating that the recharge process is probably taking place below intermittent streams in order to localize groundwater recharge zones and to optimize implementation of boreholes a geophysical survey was carried out during and after the 2004 monsoon across the outlet zone magnetic resonance soundings mrs have been performed to characterize the aquifer and measure groundwater level depletion the results of mrs are consistent with the observation in boreholes but it suffers from degraded lateral resolution a better resolution of the regolith bedrock interface is achieved using electrical resistivity tomography ert ert results are confirmed by resistivity logging in the boreholes ert surveys have been carried out twice before and during the monsoon across the stream area the major feature of recharge is revealed below the stream with a decrease by 80 of the calculated resistivity the time lapse ert also shows unexpected variations at a depth of 20 m below the slopes that could have been interpreted as a consequence of a deep seasonal water flow however in this area time lapse ert does not match with borehole data numerical modelling shows that in the presence of a shallow water infiltration an inversion artefact may take place thus limiting the reliability of time lapse ert a combination of ert with mrs provides valuable information on structure and aquifer properties respectively giving a clue for a conceptual model of the recharge process infiltration takes place in the conductive fractured fissured part of the bedrock underlying the stream and clayey material present on both sides slows down its lateral dissipation

estimates of specific yield with the geoelectric resistivity method in glacial aquifers abstract specific yield is the volume of water that drains from a saturated soil due to gravity relative to its total volume methods to determine this hydrogeologic parameter depend on well tests and well sample analyses low cost surface geoelectrical depth sounding after schlumberger is suggested as an alternative method to estimate specific yield this method is based on a determination of the layer resistivities for the saturated and unsaturated zone and on the water resistivity porosity and the degree of saturation are estimated from the electrical resistivity according to archie s law specific yields estimated with the suggested geophysical method agree with estimates after preuss and todd which are based on porosity and median grain size data uncertainties in the relationship between resistivities and porosity and saturation exist due to generalized assumptions on material constants in archie s law for the range of specific yield estimates between 18 and 36 the constants seem to be valid the method after preuss and todd requires a detailed evaluation of porosity and median grain size it produces best results for d 50 values that are larger than 0 3mm these values can vary considerably vertically as well as horizontally in the glacial aquifers derived mainly from fluvial outwash of the receding glaciers in southern new england

basal sliding relations deduced from ice sheet data l analyse de l etat d equilibre isotherme d une masse glaciaire proposee par morland et johnson 1980 1982 est appliquee a des profils de surface connus au groenland et a la calotte glaciaire de l ile devon avec les distributions correspondantes des bilans de masse pour determiner la force tractrice tangentielle b la pression au fond b et la vitesse tangentielle b dans chaque cas on utilise en retour ces estimations sur ce fond pour construire par correlation aux moindres carres des representations polynomiales pour une fonction de la profondeur b dans la forme adoptee pour la loi de glissement b b b 1 m avec m 1

asymptotic analysis of flow near a glacier terminus the non linearly viscous ice flow in the vicinity of a glacier terminus an observation region depends crucially on the upstream flow as well as on the local surface and bed conditions the former requires a likely complex solution of the balance laws and boundary conditions for the complete glacier however if the profile and downstream surface tangential velocity in the observation region are measured at an observation time t 0 and a two dimensional flow approximation is satisfactory the complete stress and velocity fields satisfying local reduced model equations in the observation region at time t 0 can be determined by asymptotic expansions in upstream distance from the moving terminus thus the full strain rate and stress tensors are determined without prescribing the basal conditions the terminus velocity is determined in terms of the net accumulation or melt flux and surface velocity at the terminus with bounds for advance or retreat the analysis and illustration are presented for a plane flow approximation

mechanisms of water storage and flow in the unsaturated zone of the chalk aquifer abstract it is known that the volumes of water draining from some chalk catchments in recessions are significantly greater than can be explained by gravity drainage from fissure porosity the most likely source of this water is slow drainage from the matric porosity above the water table this paper suggests that the location of this storage is in the irregularities on the fissure surfaces studies of resin impregnated blocks of chalk failed to reveal the presence of hydrologically significant microfissures or macropores that could be the source of additional drainage ultrasonic measurements at varying values of effective stress show no indication of changes in the acoustic transmission properties that could be attributed to closure of microfissures drainage and mercury intrusion measurements indicate that there is significant storage on block surfaces the drainage measurements confirm that the storage is of the order of magnitude about 0 3 needed to explain the slow drainage this additional storage explains why the water table is slow to respond after recharge events and why the chalk is resilient to drought the presence of storage in the unsaturated zone implies that the position of the water table may not be an accurate indicator of groundwater resources the concept of filling and draining of irregularities on fissure surfaces also leads to a new model for the generation of fissure flow in the unsaturated zone of the chalk this suggests that fissure flow can be generated at any depth in the profile and in a sequence of uniform vertical permeability is likely to originate near the water table rather than high in the unsaturated zone

shallow karst exploration using mt vlf and dc resistivity methods1 a geophysical test was carried out over a well located and fairly embedded karstic conduit the mt vlf method was selected because of its high resolution and its ability to provide a resistivity parameter sensitive to water and clay this method was used together with dc resistivity methods which allow vertical adjustment of the vlf data and show consistency between the investigation and target depths after correcting the deformations due to the polarization of the primary field the mt vlf data show clearly in the central part of the site that the conduit does not coincide with an anomaly axis but coincides with the boundary between a conductive area and a resistive area 2d modelling confirms that direct detection of the conduit is not feasible and that the conduit is located close to a conductive zone corresponding to a completely clay filled fractured zone this situation was observed on the whole site and the conduit seemed systematically to thread its way between the conductive zones to join the outlet the distribution of the conductive fractured zones and the direction of the hydraulic gradient were two important elements in predicting the location of the conduits a 3d approach would increase the probability of finding the conduits in such a case

topography climate and ice masses a review topography acts as a filter between regional climate and the consequent response of a glacier or ice sheet it influences the mass and energy inputs and modifies the ice dynamics the evolution of the ice sheet surface and over longer time scales of the bed topography modulates the climatic forcing until we have a better understanding of these topographic linkages the use of geophysical signals from former ice masses to determine palaeoclimates must be regarded as fundamentally flawed

interaction between shallow groundwater saline surface water and contaminant discharge at a seasonally and tidally forced estuarine boundary this paper presents findings from a 2 year field investigation of a dissolved hydrocarbon groundwater plume flowing towards a tidally and seasonally forced estuarine river system in perth western australia samples collected from transects of multiport wells along the riverbank and into the river enabled mapping of the fine scale 0 5 m vertical definition of the hydrocarbon plume and its longitudinal extent spear probing beneath the river sediments and water table and transient monitoring of multiport wells electrical conductivity was also carried out to define the zone of mixing between river water and groundwater the hyporheic zone and its variability the results showed that groundwater seepage into the estuarine surface sediments occurred in a zone less than 10 m from the high tide mark and that this distance and the hyporheic transition zone were influenced by tidal fluctuations and infiltration of river water into the sediments the dissolved btexn benzene toluene ethylbenzene the xylene isomers and naphthalene distributions indicated the behaviour of the hydrocarbon plume at the groundwater surface water transition zone to be strongly influenced by edge focussed discharge monitoring programs and risk assessment studies at similar contaminated sites should therefore focus efforts within the intertidal zone where contaminants are likely to impact the surface water and shallow sediment environments

geoelectrical exploration for groundwater in al maha forest ain jouhra morocco an area of about 30 km2 located in ain jouhra south of rabat morocco was the subject of a geoelectric resistivity investigation the main goal of the investigation was the assessment of the groundwater potential of the uppermost aquifer the aquifer conditions such as depth thickness and boundaries were also investigated the obtained apparent resistivity curves were first analysed qualitatively and classified using simple curve shapes thereafter the data were converted to resistivity and thickness pairs semi quantitatively by means of master curves and then quantitatively by computer modelling using ato and winsev software zohdy 1989 zohdy and bisdrof 1989 lithological control from the available single well with a stratigraphic log aided in the correlation of the resistivity values to different rock units three different ab spacing iso resistivity maps an isopach map of the main groundwater bearing horizon the depth to the aquifer substratum map and five geoelectric cross sections were constructed the interpretation of these soundings indicates the presence of an unconfined to semi confined sandy aquifer with relatively important extent and varying thickness the maximal thickness of the aquifer is recorded in the central part of the investigated area and is thinning southwards to pinch out farther to the south geophysical as well as field data indicate a hydraulic connection between the upper and deeper aquifers indeed the two aquifers are separated from each other by a marly substratum that is indicated throughout the area by the lowest values of the interpreted true resistivity the value of this resistivity varies laterally most likely due to the lateral variation in the shale to sand ratio the altitude of the substratum decreases towards the north and increases southwards regarding the availability of the groundwater in the study area zones with high potential are theoretically expected to occur in the central part where the transversal resistance is greatest however sufficient water supply and high flow rates from wells intended to produce restrictively from the most upper aquifer are not likely to exist this conclusion which seems to be very pessimistic is evidenced from two real field and experimental observations the first is the rapid fall of the level of gharnoug lake despite the ongoing feeding by three wells hence the amount of water level drop cannot be accounted for by the evaporation alone that means that the deeper aquifer is continuously draining the upper aquifer at a high flow rate very low rates are recorded in all the wells that penetrated only the upper aquifer the exception being the well that reached deeper into the lower aquifer the flow rate in this lower aquifer measured 18 litre s 1 copyright 2007 john wiley sons ltd