the temperature of europa s subsurface water ocean a 100 km deep liquid water ocean probably underlies the icy exterior of jupiter s satellite europa the long term persistence of a liquid ocean beneat ha n ice shell presents a thermal conundrum is the temperature of the ocean equal to the freezing point of water at the bottom of the ice shell or is it equal t o the somewhat warmer temperature at which water attains its maximum density we argue that most of the ocean is at the temperature of maximum density and that the bulk of the vigorously convecting ocean is separated from the bottom of the ice shell by a thin stratosphere of stably stratified water whi ch is at the freezing point and therefore buoyant if europa s subsurface water ocean is warm it could explain the widespread geologic evidence for app arent melt through events observed on its surface and may constrain the overall age of its surface

pilot scale field validation of the long electrode electrical resistivity tomography method a validation experiment carried out in a scaled field setting was attempted for the long electrode electrical resistivity tomography method in order to demonstrate the performance of the technique in imaging a simple buried target the experiment was an approximately 1 17 scale mock up of a region encompassing a buried nuclear waste tank on the hanford site the target of focus was constructed by manually forming a simulated plume within the vadose zone using a tank waste simulant the long electrode results were compared to results from conventional point electrodes on the surface and buried within the survey domain using a pole pole array both point and long electrode imaging techniques identified the lateral extents of the pre formed plume with reasonable fidelity but the long electrode method was handicapped in reconstructing vertical boundaries the pole dipole and dipole dipole arrays were also tested with the long electrode method and were shown to have the least favourable target properties including the position of the reconstructed plume relativetotheknownplumeandtheintensityoffalsepositivetargets thepoorperformance of the pole dipole and dipole dipole arrays was attributed to an inexhaustive and non optimal coverage of data at key electrodes as well as an increased noise for electrode combinations with high geometric factors however when comparing the model resolution matrix among the different acquisition strategies the pole dipole and dipole dipole arrays using long electrodes were shown to have significantly higher average and maximum values within the matrix than any pole pole array the model resolution describes how well the inversion model resolves the subsurface given the model resolution performance of the pole dipole and dipole dipole arrays it may be worth investing in tools to understand the optimum subset of randomly distributed electrode pairs to produce maximum performance from the inversion model

rift flank uplift at the gulf of california no requirement for asthenospheric upwelling continental rifts are commonly flanked by zones of high elevation but the cause of uplift remains controversial proposed uplift mechanisms include active and induced asthenospheric upwelling and isostatically driven lithospheric flexure discrimination between these hypotheses requires close constraint of the timing of rift flank uplift and crustal extension here we focus on the well preserved neogene gulf of california rift the western rift margin is characterized by a prominent east facing kilometer scale escarpment which bounds a west tilted topographically asymmetric rift flank we exploit west draining canyons incised into the rift flank to constrain the timing of uplift to between ca 5 6 and 3 2 ma using 40ar 39ar dating of lavas which show cut and fill relationships to the canyons rift flank uplift closely followed the onset of slip on the principal fault of the loreto rift segment at ca 8 6 ma the age of which we obtain from apatite u th he and fission track thermochronologic analysis of rift escarpment exhumation uplift was therefore coeval with lithospheric rupture and the onset of oceanic spreading between ca 6 and 3 ma but post dates a proposed asthenospheric upwelling event by 8 10 ma the timing of uplift is inconsistent with either active or induced upwelling as uplift mechanisms and we conclude that rift flank uplift was driven by the flexural response to lithospheric unloading

saline tracer visualized with three dimensional electrical resistivity tomography field scale spatial moment analysis 1 cross well electrical resistivity tomography ert was used to monitor the migration of a saline tracer in a two well pumping injection experiment conducted at the massachusetts military reservation in cape cod massachusetts after injecting 2200 mg l of sodium chloride for 9 hours ert data sets were collected from four wells every 6 hours for 20 days more than 180 000 resistance measurements were collected during the tracer test each ert data set was inverted to produce a sequence of 3 d snapshot maps that track the plume in addition to the ert experiment a pumping test and an infiltration test were conducted to estimate horizontal and vertical hydraulic conductivity values using modified moment analysis of the electrical conductivity tomograms the mass center of mass and spatial variance of the imaged tracer plume were estimated although the tomograms provide valuable insights into field scale tracer migration behavior and aquifer heterogeneity standard tomographic inversion and application of archie s law to convert electrical conductivities to solute concentration results in underestimation of tracer mass such underestimation is attributed to 1 reduced measurement sensitivity to electrical conductivity values with distance from the electrodes and 2 spatial smoothing regularization from tomographic inversion the center of mass estimated from the ert inversions coincided with that given by migration of the tracer plume using 3 d advective dispersion simulation the 3 d plumes seen using ert exhibit greater apparent dispersion than the simulated plumes and greater temporal spreading than observed in field data of concentration breakthrough at the pumping well

rheological properties of water ice applications to satellites of the outer planets 1 abstract the icy moons of the outer solar system have not been quiescent bodies in part because many have a substantial water component and have experienced significant internal heating we can begin to understand the thermal evolution of the moons and the rate of viscous relaxation of surface topography because we now have good constraints on how ice in several of its polymorphic forms flows under deviatoric stress at planetary conditions details of laboratory derived flow laws for pure polycrystalline ice are reviewed in detail one of the more important questions at hand is the role of ice grain size grain size may be a dynamic quantity within the icy moons and it may or may not significantly affect rheology one recent beneficiary of revelations about grain size sensitive flow is the calculation of the rheological structure of europa s outer ice shell which may be no thicker than 20 km

electrical resistivity tomography of vadose water movement cross borehole electrical resistivity tomography ert was used to image the resistivity distribution before and during two infiltration experiments in both cases water was introduced into the vadose zone and the change in resistivity associated with the plume of wetted soil was imaged as a function of time the primary purpose of this work was to study the capabilities and limitations of ert to image underground structure and ground water movement in the vadose zone a secondary goal was to learn specifics of unsaturated flow in a complex geologic setting tomographs of electrical resistivity taken before infiltration image coarser well drained soils sands and gravels as more resistive zones whereas finer grained soils silts and clays which hold more water by capillarity are imaged as more conductive images of changes in resistivity during infiltration show growth of the water infiltration plume with time that is consistent with known geology in the ert images we see the effects of capillary barriers and infer differences between capillary driven flow through fine sediments and gravity driven flow through very permeable sediments images are consistent with numerical flow simulations using hydrological parameter values consistent with soil types inferred from well logs ert can be a useful tool to monitor movement of circuitous moisture fronts in a heterogeneous field setting that would go undetected by borehole measurements

borehole to surface electrical resistivity monitoring of a salt water injection experiment a field experiment was conducted at the university of california richmond field station to demonstrate the sensitivity of borehole to surface resistivity measurements in groundwater investigations a quantity of saline water was injected into a fresh water aquifer while the resistivity was monitored using a multichannel borehole to surface system two experiments were conducted using pole pole and pole dipole receiver electrode arrays the data from the pole pole experiment were superimposed to simulate a dipole pole array and the data from the pole dipole array were superimposed to simulate a dipole dipole array this superposition of the data was done to enhance the anomaly and facilitate interpretation a numerical modeling study was performed in conjunction with the field experiment in order to interprete the results a three dimensional modeling program was used to simulate the geological setting of the field experiment and the salt water injection this modeling revealed that an asymmetric displacem

hydrogeophysical tracking of three dimensional tracer migration the concept and application of apparent petrophysical relations 1 direct estimation of groundwater solute concentrations from geophysical tomograms has been only moderately successful because 1 reconstructed tomograms are often highly uncertain and subject to inversion artifacts 2 the range of subsurface conditions represented in data sets is incomplete because of the paucity of colocated well or core data and aquifer heterogeneity and 3 geophysical methods exhibit spatially variable sensitivity we show that electrical resistivity tomography ert can be used to estimate groundwater solute concentrations if a relation between concentration and inverted resistivity is used to deal quantitatively with these issues we use numerical simulation of solute transport and electrical current flow to develop these relations which we call apparent petrophysical relations they provide the connection between concentration or local resistivity and inverted resistivity which is measured at the field scale based on ert for media containing ionic solute the apparent petrophysical relations are applied to tomograms of electrical resistivity obtained from field measurements of resistance from cross well ert to create maps of tracer concentration on the basis of synthetic and field cases we demonstrate that tracer mass and concentration estimates obtained using these apparent petrophysical relations are far better than those obtained using direct application of archie s law applied to three dimensional tomograms from ert which gives severe underestimates

imaging and quantifying salt tracer transport in a riparian groundwater system by means of 3d ert monitoring abstractdetermining groundwater flow paths of infiltrated river water is necessary for studying biochemical processes in the riparian zone but their characterization is complicated by strong temporal and spatial heterogeneity we investigated to what extent repeat 3d surface electrical resistance tomography ert can be used to monitor transport of a salt tracer plume under close to natural gradient conditions the aim is to estimate groundwater flow velocities and pathways at a site located within a riparian groundwater system adjacent to the perialpine thur river in northeastern switzerland our ert time lapse images provide constraints on the plume s shape flow direction and velocity these images allow the movement of the plume to be followed for 35 m although the hydraulic gradient is only 1 43 the ert time lapse images demonstrate that the plume s center of mass and its front propagate with velocities of 2 10 4 m s and 5 10 4 m s respectively these velocities are compatible with groundwate

thermal equilibrium states of europa s ice shell implications for internal ocean thickness and surface heat flow abstract ice shell thickness and ocean depth are calculated for steady state models of tidal dissipation in europa s ice shell using the present day values of the orbital elements the tidal dissipation rate is obtained using a viscoelastic maxwell rheology for the ice the viscosity of which has been varied over a wide range and is found to strongly increase if an inviscid internal ocean is present to determine steady state values the tidal dissipation rate is equated to the heat transfer rate through the ice shell calculated from a parameterized model of convective heat transfer or from a thermal conduction model if the ice layer is found to be stable against convection although high dissipation rates and heat fluxes of up to 300 mwm 2 are in principle possible for europa these values are unrealistic because the states for which they are obtained are thermodynamically unstable equilibrium models have surface heat flows around 20 mwm 2 and ice layer thicknesses around 30 km which is significantly less than the total thickness of the h2o layer these results support models of europa with ice shells a few tens of kilometers thick and around 100 km thick subsurface oceans

three layered models of ganymede and callisto compositions structures and aspects of evolution abstract three layered structural models are determined for ganymede and callisto each consists of a rock core a mixed ice rock lower mantle and a pure ice upper mantle this structure results from differentiation subsequent to accretional melting attention is given to evaluating various candidates for the rock component and three alternatives representing various degrees of silicate hydration and oxidation are modeled and incorporated structures are calculated on the basis of a 250 k isotherm which is a reasonablev approximation to the gentle adiabats expected to occur in icy satellites differentiation of an ice rock satellite generally involves an increase in radius and the three layered approach allows this process to be examined in some detail it is determined that satellite expansion is most significant early in the process and much less so as differentiation proceeds to completion if tectonics are due to global expansion distinguishing on this basis between a completely differentiated satellite and one that is only partially differentiated is difficult the postaccretional global expansion of ganymede which may have left a tectonic record was probably limited to 1 in radius in agreement with observed limits useful quantities such as silicate mass and volume fraction uncompressed density j2 c22 binding energy and surface heat flow are also determined nonhydrostatic contributions to j2 and c22 are estimated and shown to be nonnegligible encounters with jupiter orbiting spacecraft are unlikely to determine callisto s degree of central condensation we conclude by calculating the relative likelihood of postaccretional melting caused by radiogenic heating three layered satellites have generally hotter interiors because additional thermal boundary layers divide the separately convecting upper and lower mantles inhibiting heat transport ganymedes and callistos that are less than about 1 3 differentiated by mass should experience a second episode of melting as these boundary layers are either above the level of the water ice minimum melting temperature or intersect the melting curve at deeper levels runaway differentiation to at least a depth corresponding to a pressure in the ice v stability field is likely the main point here is that if satellite tectonics are tied to differentiation by melting or its aftermath as in the instability following ocean closure of kirk and stevenson moderate or small amounts of accretional differentiation are unlikely to explain an absence of tectonics as in callisto because extensive differentiation ultimately occurs

electrical characterization of non fickian transport in groundwater and hyporheic systems 1 recent work indicates that processes controlling solute mass transfer between mobile and less mobile domains in porous media may be quantified by combining electrical geophysical methods and electrically conductive tracers whereas direct geochemical measurements of solute preferentially sample the mobile domain electrical geophysical methods are sensitive to changes in bulk electrical conductivity bulk ec and therefore sample ec in both the mobile and immobile domains consequently the conductivity difference between direct geochemical samples and remotely sensed electrical geophysical measurements may provide an indication of mass transfer rates and mobile and immobile porosities in situ here we present 1 an overview of a theoretical framework for determining parameters controlling mass transfer with electrical resistivity in situ 2 a review of a case study estimating mass transfer processes in a pilot scale aquifer storage recovery test and 3 an example application of this method for estimating mass transfer in watershed settings between streams and the hyporheic corridor we demonstrate that numerical simulations of electrical resistivity studies of the stream hyporheic boundary can help constrain volumes and rates of mobile immobile mass transfer we conclude with directions for future research applying electrical geophysics to understand field scale transport in aquifer and fluvial systems subject to rate limited mass transfer

vadose zone flow model parameterisation using cross borehole radar and resistivity imaging cross borehole geoelectrical imaging in particular electrical resistivity tomography ert and transmission radar tomography can provide high resolution images of hydrogeological structures and in some cases detailed assessment of dynamic processes in the subsurface environment through appropriate petrophysical relationships these tools offer data suitable for parameterising and constraining models of groundwater flow this is demonstrated using cross borehole radar and resistivity measurements collected during a controlled vadose zone tracer test performed at a field site in the uk sherwood sandstone both methods show clearly the vertical migration of the tracer over a 200 h monitoring period by comparing first and second spatial moments of changes in moisture content predicted from a numerical simulation of vadose zone flow with equivalent statistics from two and three dimensional ert and cross borehole radar profiles the effective hydraulic conductivity is estimated to be approximately 0 4 m d such a value is comparable to field estimates from borehole hydraulic tests carried out in the saturated zone at the field site and provides valuable information that may be utilised to parameterise pollutant transport models of the site

friction of ice the frictional strength of ice seems to be well below that for all other rocks triaxial testing of cylinders of pure water ice containing a 45 inclined sawcut at temperatures of 77 t 115 k and confining pressures 0 1 p 250 mpa reveals the frictional laws 0 20 n 8 3 mpa for p 10 mpa and 0 55 n 1 0 mpa for p 5 mpa where and n are the shear and normal stresses on the sawcut surface friction is independent of t and over the one order of magnitude tested of average sliding velocity the sliding behavior is invariably stick slip examination of sliding surfaces shows only minor gouge development a curious arrangement of densely packed short vertical fractures occurs primarily in samples with anomalously low strength these results apply to models of near surface tectonic activity on jupiter s three large icy moons europa callisto and ganymede they indicate for example that a global expansion on ganymede of 3 linear percent will cause extensional movement on preexisting faults at depths to 7 3 km

meteoric smoke fallout over the holocene epoch revealed by iridium and platinum in greenland ice an iridium anomaly at the cretaceous tertiary boundary layer has been attributed to an extraterrestrial body that struck the earth some 65 million years ago1 it has been suggested that during this event the carrier of iridium was probably a micrometre sized silicate enclosed aggregate2 or the nanophase material of the vaporized impactor3 but the fate of platinum group elements such as iridium that regularly enter the atmosphere via ablating meteoroids remains largely unknown here we report a record of iridium and platinum fluxes on a climatic cycle timescale back to 128 000 years ago from a greenland ice core4 we find that unexpectedly constant fallout of extraterrestrial matter to greenland occurred during the holocene whereas a greatly enhanced input of terrestrial iridium and platinum masked the cosmic flux in the dust laden atmosphere of the last glacial age we suggest that nanometre sized meteoric smoke particles5 6 formed from the recondensation of ablated meteoroids in the atmosphere at altitudes 70 kilometres are transported into the winter polar vortices by the mesospheric meridional circulation7 and are preferentially deposited in the polar ice caps this implies an average global fallout of 14 5 kilotons per year of meteoric smoke during the holocene

comment on superplastic deformation of ice experimental observations by d l goldsby and d l kohlstedt knowledge of the rheological properties of ice at devia toric stresses lower than 0 1 mpa is of great interest whenmodeling the flow of glaciers and ice sheets in spite of thedifficulty of obtaining convincing results in the laboratory thereis now agreement on a flow law with a stress exponent lowerthan 2 mellor and testa 1969 duval 1973 alley 1992 duval and castelnau 1995 this result is also supported byborehole deformation measurements dahl jensen and gun destrup 1987 and bubbly ice densification modeling lipenkovet al 1997

a model for the interior structure evolution and differentiation of callisto abstract the recently measured dimensionless moment of inertia moi factor for callisto of 0 3549 0 0042 anderson et al 2001 icarus 153 157 161 poses a problem its value cannot be explained by a model in which callisto is completely differentiated into an ice shell above a rock shell and an iron core such as its neighboring satellite ganymede nor can it be explained by a model of a homogeneous undifferentiated ice rock satellite we show that callisto may be incompletely differentiated into an outer ice rock shell in which the volumetric rock concentration is close to the primordial one at the surface and decreases approximately linearly with depth an ice mantle mostly depleted of rock and an about 1800 km rock ice core in which the rock concentration is close to the close packing limit the ice rock shell thickness depends on uncertain rheology parameters and the heat flow and can be roughly 50 to 150 km thick we show that if callisto accreted from a mix of metal bearing rock and ice and if the average size of the rocks was of the order of meters to tens of meters then callisto may have experienced a gradual but still incomplete unmixing of the two components an ocean in callisto at a depth of 100 200 km is difficult to obtain if the ice is pure h2o and if the ice rock lithosphere is 100 km or more thick a water ocean is more plausible for ice contaminated by ammonia methane or salts or for pure h2o at a depth of 400 600 km

the application of time lapse electrical tomography in groundwater studies recently various techniques of electrical tomography have been refined and fast efficient commercial systems are now available new inversion algorithms produce electrical images that more accurately model a two dimensional or three dimensional subsurface than ever before over the past three years electrical tomography surveys in germany the u k and the u s have produced remarkable results

influence of grain size evolution on convective instability 1 grain size one of the most important microstructural properties of materials evolves during creep deformation to minimize the free energy of polycrystalline aggregates we apply a model of grain size evolution to the study of convective instability of cooling boundary layers the grain size evolution model is coupled to a composite rheology where the deformation rate is the sum of that due to dislocation creep and grain size sensitive diffusion creep the onset of convection is sensitive to grain growth rates and the initial grain size the formation of convective instabilities is enhanced by stresses induced by plate motions therefore small scale convection is more likely to occur beneath fast moving plates in finite amplitude convection grain size evolution leads to high viscosity in regions where convective stresses are low and can induce viscosity contrasts exceeding one order of magnitude such viscosity contrasts are sufficient to influence the dynamics of convection often leading to domains which remain isolated from the well mixed convecting fluid a composite viscosity including diffusion creep which has a lower activation energy than dislocation creep reduces the effective temperature dependence of viscosity

oceans in the icy galilean satellites of jupiter abstract equilibrium models of heat transfer by heat conduction and thermal convection show that the three satellites of jupiter europa ganymede and callisto may have internal oceans underneath ice shells tens of kilometers to more than a hundred kilometers thick a wide range of rheology and heat transfer parameter values and present day heat production rates have been considered the rheology was cast in terms of a reference viscosity 0 calculated at the melting temperature and the rate of change a of viscosity with inverse homologous temperature the temperature dependence of the thermal conductivity k of ice i has been taken into account by calculating the average conductivity along the temperature profile heating rates are based on a chondritic radiogenic heating rate of 4 5 pw kg 1 but have been varied around this value over a wide range the phase diagrams of h2o ice i and h2o 5 wt nh3 ice have been considered the ice i models are worst case scenarios for the existence of a subsurface liquid water ocean because ice i has the highest possible melting temperature and the highest thermal conductivity of candidate ices and the assumption of equilibrium ignores the contribution to ice shell heating from deep interior cooling in the context of ice i models we find that europa is the satellite most likely to have a subsurface liquid ocean even with radiogenic heating alone the ocean is tens of kilometers thick in the nominal model if tidal heating is invoked the ocean will be much thicker and the ice shell will be a few tens of kilometers thick ganymede and callisto have frozen their oceans in the nominal ice i models but since these models represent the worst case scenario it is conceivable that these satellites also have oceans at the present time the most important factor working against the existence of subsurface oceans is contamination of the outer ice shell by rock rock increases the density and the pressure gradient and shifts the triple point of ice i to shallower depths where the temperature is likely to be lower then the triple point temperature according to present knowledge of ice phase diagrams ammonia produces one of the largest reductions of the melting temperature if we assume a bulk concentration of 5 wt ammonia we find that all the satellites have substantial oceans for a model of europa heated only by radiogenic decay the ice shell will be a few tens of kilometers thinner than in the ice i case the underlying rock mantle will limit the depth of the ocean to 80 100 km for ganymede and callisto the ice i shell on top of the h2o nh3 ocean will be around 60 to 80 km thick and the oceans may be 200 to 350 km deep previous models have suggested that efficient convection in the ice will freeze any existing ocean the present conclusions are different mainly because they are based on a parameterization of convective heat transport in fluids with strongly temperature dependent viscosity rather than a parameterization derived from constant viscosity convection models the present parameterization introduces a conductive stagnant lid at the expense of the thickness of the convecting sublayer if the latter exists at all the stagnant lid causes the temperature in the sublayer to be warmer than in a comparable constant viscosity convecting layer we have further modified the parameterization to account for the strong increase in homologous temperature and therefore decrease in viscosity with depth along an adiabat this modification causes even thicker stagnant lids and further elevated temperatures in the well mixed sublayer it is the stagnant lid and the comparatively large temperature in the sublayer that frustrates ocean freezing

performance analysis and simulation of a two well tracer test at the mobile site during the late summer of 1984 a two well tracer test was performed which involved travel distances of 38 3 m to 90 m a bromide tracer slug was injected at one well while groundwater was simultaneously pumped from the other well the two well test which required 32 days to complete was then simulated by geotrans inc using a three dimensional advection dispersion model with a longitudinal dispersivity of 0 15 m a transverse dispersivity of 0 05 m and the permeability distribution being measured independently in a single well test single well test results which involved a travel distance of 5 5 m indicated that the hydraulic conductivity varied by about a factor of 4 throughout the aquifer thickness the two well test simulation made without knowledge of the test results and without any calibration was remarkably good local hydrodynamic dispersion was negligible so that the concentration versus time breakthrough at the withdrawal well could be simulated just as well with a quasi three dimensional advection model having zero hydrodynamic dispersion this implies that over the travel distances applicable to the two well test the spreading of the slug in the aquifer depended mainly on macroscopic velocity variations that were quantifiable in terms of the inferred permeability distribution this result which pertains to an aquifer in a fluvial terrace deposit of quaternary age is similar to that observed by pickens and grisak in a sandy glaciofluvial aquifer near the chalk river nuclear laboratories in canada such flow systems can result in fully three dimensional plumes with high concentrations of contaminant moving over large horizontal distances in the higher permeability zones

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

fully coupled hydrogeophysical inversion of a laboratory salt tracer experiment monitored by electrical resistivity tomography 1 we perform a salt tracer experiment monitored by time lapse electrical resistivity tomography in a quasi two dimensional sandbox with the aim of determining the hydraulic conductivity distribution in the domain we use sodium chloride as a tracer together with cochineal red for visual monitoring the time series of observed resistance for each electrode configuration is characterized by its temporal moments we invert the mean arrival time of electrical potential perturbations and a few steady state hydraulic head measurements using the fully coupled hydrogeophysical approach recently introduced by pollock and cirpka 2010 this is the first application of the approach to experimental data the results obtained show a reasonable agreement between the estimated hydraulic conductivity field and the pattern of the actual sandbox filling using this estimation a transient simulation is performed to compute the propagation of the salt tracer plume through the sandbox the latter is compared to pictures taken during the experiment these results show an even better agreement indicating that the lenses of different sand types are not entirely homogeneous and some unexpected preferential flow paths are present we conclude that temporal moments of potential perturbations obtained during salt tracer tests provide a good basis for inferring the hydraulic conductivity distribution by fully coupled hydrogeophysical inversion

reply to comment by p duval and m montagnat on superplastic deformation of ice experimental observations 1 in a recent paper we reported the results of a detailed experimental investigation of the rheological properties of ice goldsby and kohlstedt 2001 one of the innovative aspects of our investigation was the use of fine grained samples that enabled us to explore deformation not only by grain size insensitive mechanisms involving grain matrix dislocation processes but also by grain size sensitive flow mechanisms involving grain boundary sliding gbs an exciting discovery in our research was an extensive creep regime in which gbs contributes substantially to the flow of ice most importantly this deformation regime in which basal slip is accommodated by gbs referred to in our paper as the superplastic flow regime governs the flow of ice over a wide range of temperature grain size and stress conditions found in glaciers ice sheets and icy planetary interiors 2 in their comment on our paper duval and montagnat 2002 repeat the objection that they had raised previously montagnat and duval 2000 concerning our conclusion that superplastic deformation controls the flow of glaciers and ice sheets they assert that gbs as a significant creep mechanism is not compatible with observations on the development of fabrics and microstructures in ice sheets special emphasis is placed instead on the role of grain boundary migration as a recovery process in glacier ice they then continue by arguing that basal slip accommodated by grain boundary migration gbm rather than by gbs is the dominant deformation mechanism in ice 3 in this reply we extend the results presented in detail by goldsby and kohlstedt 1997 and goldsby and kohlstedt 2001 to further demonstrate that gbs is a very important deformation process in natural ice bodies our reasoning is based on three points 1 the microstructures reported for samples from large natural ice bodies are remarkably similar to those observed in samples deformed in the laboratory in the superplastic flow regime 2 our constitutive equation for ice is in excellent agreement with the flow behavior determined from field measurements on glaciers and ice sheets 3 contrary to the assertion of montagnat and duval 2000 and duval and montagnat 2001 fabric development is fully compatible with deformation in the superplastic regime described in our paper in addition we note that while gbm associated with dynamic recrystallization is an important recovery mechanism in ice it is not a deformation mechanism and hence cannot accommodate basal slip

on convection in ice i shells of outer solar system bodies with detailed application to callisto it has been argued that the dominant non newtonian creep mechanisms of water ice make the ice shell above callisto s ocean and by inference all radiogenically heated ice i shells in the outer solar system stable against solid state convective overturn conductive heat transport and internal melting oceans are therefore predicted to be or have been widespread among midsize and larger icy satellites and kuiper belt objects alternatively at low stresses where non newtonian viscosities can be arbitrarily large convective instabilities may arise in the diffusional creep regime for arbitrarily small temperature perturbations for callisto ice viscosities are low enough that convection is expected over most of geologic time above the internal liquid layer for plausible ice grain sizes a few mm the alternative for early callisto a conducting shell over a very deep ocean 450 km is not compatible with callisto s present partially differentiated state moreover if convection is occurring today the stagnant lid would be quite thick 100 km and compatible with the lack of active geology nevertheless callisto s steady state heat flow may have fallen below the convective minimum for its ice i shell late in solar system history in this case convection ends the ice shell melts back at its base and the internal ocean widens considerably the presence of such an ocean of order 200 km thick is compatible with callisto s moment of inertia but its formation would have caused an 0 25 radial expansion the tectonic effects of such a late slow expansion are not observed so convection likely persists in callisto possibly subcritically ganymede due to its greater size rock fraction and full differentiation has a substantially higher heat flow than callisto and has not reached this tectonic end state titan if differentiated and triton should be more similar to ganymede in this regard pluto like callisto may be near the tipping point for convective shutdown but uncertainties in its size and rock fraction prevent a more definitive assessment

electrical properties of partially saturated sandstones novel computational approach with hydrogeophysical applications 1 electric and electromagnetic geophysical tools are suitable methods for investigating moisture content transport and storage properties of the subsoil with noninvasive surveys interpretation of field data is still based on empirical or semiempirical equations which must be calibrated with complex time consuming laboratory measurements in this paper we develop pore scale models suitable for computing the effective electric conductivity and the effective permittivity of partially saturated porous media on the basis of simple structural parameters grain size distribution and porosity we compute the electrical conductivity with three approaches that model the conductivity of the shell of coating clay around solid grains of shaly sandstones using different assumptions we compare the simulated results with experimental data from sherwood sandstone uk and discuss the effect of these assumptions on the effective conductivity simulations performed using a modeling approach that accounts for interaction between volume and surface conduction show an excellent agreement with experimental data we also model the effective permittivity of the same porous medium at several saturation values and obtain a good match with laboratory measurements

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

viscosity of rock ice mixtures and applications to the evolution of icy satellites theory and experiments are used to establish lower and upper bounds on the ratio of actual viscosity to pure ice viscosity for a suspension of rock particles in a water ice matrix for typical conditions encountered in icy satellites this ratio is of order ten or possibly larger depending on unknown factors such as the particle size distribution it is shown that even this modest increase in viscosity may be enough to have caused a failure of solid state convective self regulation early in the evolution of a homogeneous rock water ice satellite provided the satellite is large enough and sufficiently silicate rich the criteria for this failure are satisfied by ganymede and are marginal for callisto if the silicates are hydrated failure of self regulation means that the viscosity is too high for the interior to remain completely solid and eliminate the heat production of long lived radioisotopes by solid state convection partial melting of the ice then occurs it is further shown that satellites of this size may then undergo runaway differentiation into a rock core and almost pure ice mantle because the gravitational energy release is sufficient to melt nearly all the ice and the rayleigh taylor instability time scale is short although the high pressure phases of ice melt the resulting water quickly refreezes at a higher level it is conjectured that these results explain the striking surface dissimilarity of ganymede and callisto if these satellites accreted cold and undifferentiated ganymede may have gone supercritical melted and differentiated because of a failure of self regulation whereas callisto remained undifferentiated to the present day like all proposed explanations for the ganymede callisto dichotomy this conjecture cannot be quantified with confidence because of inadequate or incomplete observations theory and experimental data

convective instability in europa s floating ice shell models of the tidally heated floating ice shell proposed for the jovian satellite europa generally find shell thicknesses less than 30 km past parameterized convection models indicated that such shells are stable against convective overturn which otherwise ostensibly leads to freezing of the ocean underneath here i apply the temperature dependent viscosity convection scaling developed by solomatov and coworkers to the europan ice shell the temperature dependent properties of ice are linearized about 260 k as any convective interior should be close to this temperature with the colder ice forming an essentially passive stagnant lid ice shells 10 km thick are found to be unstable to convection at their base for melting point viscosities of 1013 pa s as linearized by tidal stresses if the ice deforms by superplastic creep but such low viscosities require small grain sizes 1 mm this requirement may be met if grain sizes observed in terrestrial polar glaciers can be strain rate scaled to europa regardless convection at the base of the ice shell if initiated may not freeze the ocean because of tidal heating a stagnant lid regime ice shell is much more dissipative than a conductive shell of the same thickness such a shell should thin not thicken and the potential exists for further thermal instabilities and runaways

a tracer test in a shallow heterogeneous aquifer monitored via time lapse surface electrical resistivity tomography we illustrate a case study of a saline tracer test in a shallow highly heterogeneous aquifer monitored by means of surface time lapse ert the test was aimed at identifying the system s hydraulic properties some of the expected limitations of the method particularly caused by the strong decrease in ert resolution with depth and the consequent problems with mass balance and moment calculation could be partly balanced by the use of direct measurements of groundwater electrical conductivity and tracer concentration at one selected location the vast heterogeneity of the system ranging in lithology from clay to gravel at a scale of meters to tens of meters reflects itself in the tracer migration and distribution over time the tracer is trapped in the low permeability regions and from these it is slowly released over time high resolution surface ert proves effective at picturing this system behavior over time the extreme heterogeneity is also a challenge in the attempt to translate bulk electrical

potential of electrical resistivity tomography to infer aquifer transport characteristics from tracer studies a synthetic case study 1 with time lapse electrical resistivity tomography ert transport processes in the subsurface can be imaged and monitored the information content of obtained spatiotemporal data sets opens new ways to characterize subsurface spatial variability difficulties regarding a quantitative interpretation of the imaged transport may arise from the fact that data inversion used in ert is generally underdetermined and that ert data acquisition is often limited to two dimensional 2 d image planes to address this problem we conducted a numerical tracer experiment in a synthetic heterogeneous aquifer with preset variability and spatial correlation of the hydraulic conductivity and monitored the tracer breakthrough in a 2 d image plane perpendicular to the mean flow direction using time lapse ert the tracer breakthrough patterns in the image plane were interpreted using equivalent transport models an equivalent convection dispersion equation to characterize the spatially averaged breakthrough and a stream tube model to characterize local breakthrough curves equivalent parameters derived from simulated and from ert inverted concentrations showed a good agreement which demonstrates the potential of ert to characterize subsurface transport using first order approximate solutions of stochastic flow and transport equations equivalent model parameters and their spatial variability were interpreted in terms of the local scale dispersion and the spatial variability of the hydraulic conductivity the spatial correlations of the stream tube velocity and of the hydraulic conductivity were found to be closely related consequently the hydraulic conductivity spatial correlation may be inferred from stream tube velocity distributions which can be observed with sufficiently high spatial resolution using ert

assessment of local hydraulic properties from electrical resistivity tomography monitoring of a three dimensional synthetic tracer test experiment 1 in recent years geophysical methods have become increasingly popular for hydrological applications time lapse electrical resistivity tomography ert represents a potentially powerful tool for subsurface solute transport characterization since a full picture of the spatiotemporal evolution of the process can be obtained however the quantitative interpretation of tracer tests is difficult because of the uncertainty related to the geoelectrical inversion the constitutive models linking geophysical and hydrological quantities and the a priori unknown heterogeneous properties of natural formations here an approach based on the lagrangian formulation of transport and the ensemble kalman filter enkf data assimilation technique is applied to assess the spatial distribution of hydraulic conductivity k by incorporating time lapse cross hole ert data electrical data consist of three dimensional cross hole ert images generated for a synthetic tracer test in a heterogeneous aquifer under the assumption that the solute spreads as a passive tracer for high peclet numbers the spatial moments of the evolving plume are dominated by the spatial distribution of the hydraulic conductivity the assimilation of the electrical conductivity 4d images allows updating of the hydrological state as well as the spatial distribution of k thus delineation of the tracer plume and estimation of the local aquifer heterogeneity can be achieved at the same time by means of this interpretation of time lapse electrical images from tracer tests we assess the impact on the performance of the hydrological inversion of i the uncertainty inherently affecting ert inversions in terms of tracer concentration and ii the choice of the prior statistics of k our findings show that realistic ert images can be integrated into a hydrological model even within an uncoupled inverse modeling framework the reconstruction of the hydraulic conductivity spatial distribution is satisfactory in the portion of the domain directly covered by the passage of the tracer aside from the issues commonly affecting inverse models the proposed approach is subject to the problem of the filter inbreeding and the retrieval performance is sensitive to the choice of k prior geostatistical parameters

a multi disciplinary study of deformation of the basaltic cover over fine grained valley fills a case study from eastern sardinia italy the pliocene to early pleistocene volcanic activity which generated the basaltic plateau of the orosei dorgali area in eastern sardinia led to the disruption of the local hydrographic network by damming some tracts of the fluvial valleys incised in the granite basement this resulted in the formation of lacustrine basins whose fine grained fills were partly interfingered and eventually covered by younger lava flows in the sw part of the plateau close to the galtelli village a number of unknown depressions locally named paules were formed in order to reconstruct their subsurface structure two electrical resistivity tomography surveys were carried out across these depressions the geophysical results which demonstrate the existence of a disrupted layered system were used to build a numerical geomechanical model that suggest the depressions originated by local collapses of the basaltic cover due to the compaction of the underlying fine grained valley fills

time lapse mise a la masse measurements and modelling for tracer test monitoring in a shallow aquifer abstract the main goal of this study is to evaluate the reliability of the mise a la masse malm technique associated with saline tracer tests for the characterization of groundwater flow direction and velocity the experimental site is located in the upper part of the alento river alluvial plain campania region southern italy in this paper we present the hydrogeological setting the experimental setup and the relevant field results subsequently we compare those data against the simulated results obtained with a 3d resistivity model of the test area coupled with a model describing the advection dispersion equation for continuous tracer injection in particular we calculate a series of 3d forward solutions starting from a reference model all derived from electrical tomography results but taking into consideration different values of mean flow velocity and directions each electrical resistivity 3d model is used to produce synthetic voltage maps for malm surveys finally the synthetic malm voltage maps are compared with the ones measured in the field in order to assess the information content of the malm dataset with respect to the groundwater field characteristics the results demonstrate that the information content of the malm data is sufficient to define important characteristics of the aquifer geometry and properties this work shows how a combination of three dimensional time lapse modeling of flow tracer transport and electrical current can substantially contribute towards a quantitative interpretation of malm measurements during a saline tracer test this approach can thus revive the use of malm as a practical low cost field technique for tracer test monitoring and aquifer hydrodynamic characterization

investigations into the relationship between changes in internal moisture regimes and rock surface deterioration in cavernous sandstone features cavernous features commonly develop in sandstone but their development over time remains enigmatic it has been suggested that moisture movements within the rock mass play a key role in the location development and dynamics of cavernous features in this research the role of internal moisture movement is tested through monitoring moisture and surface deterioration dynamics in april 2008 and april 2009 within two large cavernous features mega tafoni in the golden gate highlands national park south africa gghnp data are presented from surveys of internal moisture using electrical resistivity tomography ert surface moisture using a protimeter and deterioration using surface hardness as recorded with an equotip as a proxy for surface deterioration across five 2 45 m long transects in addition a high resolution temperature record is presented to assess the influence of micro climates within the caverns the results indicate consistency in the gross spatial pattern of moisture flow within the rock mass over a one year period but significant changes in mean moisture contents and in the fine detail of moisture patterning some noticeably weakened areas had developed within the central parts of the cavernous features often linked to wetter subsurface conditions supporting the theory that core softening is a main driver of cavernous feature formation however in some areas surface hardening is also found to be associated with wetter subsurface conditions supporting the theory that case hardening is a main driver of cavernous feature formation in addition the presence of well established biofilms suggests an even more complex interaction between moisture surface development and biological activity a model is presented therefore which integrates this paradox by proposing a non linear relationship between moisture dynamics facilitation of biofilm formation and deterioration within cavernous features

viscoelastic models of tidal heating in enceladus abstract tidal heating in enceladus is investigated using multilayered viscoelastic models of the satellite dissipation in a homogeneous maxwell model of enceladus at the current eccentricity can be as large as 920 gw nearly a factor of 2000 greater than previously thought reflecting the fact that the dynamic love number can approach unity the elastic love number 10 3 the theoretical tidal heating maximum violates an upper bound based on saturn s q and models with this much heating are unreasonable plausible multilayered models are shown to be thermally stable at the current eccentricity with 10 km thick lithosphere and surface heat flow 5 mw m 2 the models require a combination of near surface insulation subsurface solar heating or anomalously low lithosphere conductivity to simultaneously fit dynamic and geologic constraints a two layer model with a conductive lithosphere over a maxwell mantle has mantle viscosity of about 10 13 pa sec and mantle temperature about 210 k a three layer model with an internal ocean of accumulated nh 3 h 2 o eutectic melt that decouples the lithosphere from a maxwell core has slightly lower internal viscosity and slightly higher temperature than the two layer model dissipation in the ocean layer may be significant if the tidally forced flow is turbulent the models suggest that enceladus thermal and dynamical evolution may have been relatively simple involving only dione in a 2 1 resonance the current eccentricity could have been maintained since resonance onset with recurring surface activity due to a thermally active interior

time lapse imaging of saline tracer transport in fractured rock using difference attenuation radar tomography 1 accurate characterization of fractured rock aquifer heterogeneity remains one of the most challenging and important problems in groundwater hydrology we demonstrate a promising strategy to identify preferential flow paths in fractured rock using a combination of geophysical monitoring and conventional hydrogeologic tests cross well differenceattenuation ground penetrating radar was used to monitor saline tracer migration in an experiment at the u s geological survey fractured rock hydrology research site in grafton county new hampshire radar data sets were collected every 10 min in three adjoining planes for 5 hours during each of 12 tracer tests an innovative inversion method accounts for data acquisition times and temporal changes in attenuation during data collection the inverse algorithm minimizes a combination of two functions the first is the sum of weighted squared data residuals second is a measure of solution complexity based on an a priori space time covariance function subject to constraints that limit radarattenuation changes to regions of the tomograms traversed by high difference attenuation ray paths the time series of tomograms indicate relative tracer concentrations and tracer arrival times in the image planes from these we infer the presence and location of a preferential flow path within a previously identified zone of transmissive fractures these results provide new insights into solute channeling and the nature of aquifer heterogeneity at the site index terms 0910 exploration geophysics data processing 0915 exploration geophysics downhole methods 1829 hydrology groundwater hydrology 1832 hydrology groundwater transport 1894 hydrology instruments and techniques keywords radar tomography fractured rock ground penetrating radar geophysics hydrogeophysics

stagnant lid convection on venus the effect of strongly temperature dependent viscosity on convection in the interior of venus is studied systematically with the help of finite element numerical models for viscosity contrasts satisfying experimental constraints on the rheology of rocks venus is likely to be in the regime of stagnant lid convection this regime is characterized by the formation of a slowly creeping very viscous lid on top of the mantle venusian lithosphere and is in agreement with the tectonic style observed on venus stagnant lid convection explains large geoid to topography ratios on venus by the thermal thinning of a thick lithosphere the thickness of the lithosphere can be as large as 400 550 km for beta regio and 200 400 km on average geoid and topography data and experimental data on the rheology of rocks provide constraints on the viscosity of the mantle 10 20 10 21 pa s the convective stresses in the interior 0 2 0 5 mpa the stresses in the lid 100 200 mpa the velocity in the interior 0 5 3 cm yr 1 and the heat flux beneath the lithosphere 8 16 mw m 2 parameterized convection calculations of thermal history of venus are difficult to reconcile with a thick present day lithosphere however a sufficiently thick lithosphere can be formed if a convective regime with mobile plates was replaced by stagnant lid convection around 0 5 b y ago one of the possible explanations for the cessation of venusian plate tectonics is that during the evolution of venus stresses in the lid dropped below the yield strength of the lithosphere this model predicts a drastic drop in the heat flux thickening of the lithosphere and suppression of melting and is consistent with the hypothesis of cessation of resurfacing on venus around 0 5 b y ago

onset of convection in the icy galilean satellites influence of rheology 1 ice i exhibits a complex rheology at temperature and pressure conditions appropriate for the interiors of the ice i shells of europa ganymede and callisto we use numerical methods and existing parameterizations of the critical rayleigh number to determine the conditions required to trigger convection in an ice i shell with each of the stress temperature and grain size dependent rheologies measured in laboratory experiments by goldsby and kohlstedt 2001 the critical rayleigh number depends on the ice grain size and the amplitude and wavelength of temperature perturbation issued to an initially conductive ice i shell if the shells have an assumed uniform grain size 0 4 mm deformation during initial plume growth is accommodated by newtonian volume diffusion if the ice grain size is between 0 4 mm and 3 cm deformation during plume growth is accommodated by weakly non newtonian grain boundary sliding where the critical ice shell thickness for convection depends on the amplitude of temperature perturbation to the 0 5 power if the ice grain size exceeds 2 cm convection can not occur in the ice i shells of the galilean satellites regardless of the amplitude or wavelength of temperature perturbation if the grain size in a convecting ice i shell evolves to effective values greater than 2 cm convection will cease if the ice shell has a grain size large enough to permit flow by dislocation creep the ice is too stiff to permit convection even in the thickest possible ice i shell consideration of the complex ice rheology implies that estimates of the grain size in the satellites and knowledge of their initial thermal states are required when judging the convective instability of their ice i shells

subsurface oceans on europa and callisto constraints from galileo magnetometer observations magnetic field perturbations measured during galileo flybys of europa and callisto are consistent with dipole fields induced by the temporal variations of the ambient jovian magnetospheric field these fields are close to those expected for perfectly conducting moons we investigate the implications of these observations for the electrical structure of the moon s interiors using a simple shell model it is found that europa and callisto must possess regions where the conductivity exceeds 0 06 and 0 02 s m at a depth of less than 200 and 300 km below the surface respectively this conductivity is unattainable in ice or silicates unless the ice layer is at least partially molten or very large temperature gradients can be maintained below the ice an ionosphere or a cloud of pick up ions are probably also insufficiently conductive global earth like oceans under the surface of both moons could account for the observations provided they are at least a few kilometers thick c 2000 academic press

dynamic recrystallization and texture development in ice as revealed by the study of deep ice cores in antarctica and greenland the preferred c axis orientation of ice from polar ice sheets develops essentially as a result of intracrystalline slip but dynamic recrystallization appears to alter the kinetics of the development of deformation textures and is at high temperature at the origin of recrystallization textures the purpose of this work is to obtain a better understanding of recrystallization processes that occur in polar ice sheets and to clarify the relationship between dynamic recrystallization and textures the study was based on two deep ice cores from greenland and antarctica the greenland ice core project grip and vostok ice cores the structure along the grip core displays normal grain growth in the first 100 m of the ice sheet and rotation recrystallization and migration recrystallization near the bottom only grain growth and rotation recrystallization appear to occur in the vostok ice core the transition between these recrystallization regimes was studied estimating for interglacial ice the evolution with depth of the dislocation density this calculation has shown the efficiency of grain boundary migration for the absorption of dislocations at vostok the highest value of the dislocation density is found at a depth of about 1000 m and the continuous decrease in the dislocation density below this depth is related to the increase of the grain boundary migration rate it is shown that the driving force required to initiate migration recrystallization is not reached in interglacial ice at vostok the observed textures were compared with those predicted by the self consistent approach recrystallization textures are interpreted by assuming that the less stressed grains i e the best oriented for basal slip are favored by the size advantage of subgrains the recrystallization textures are compared with those of other materials

non newtonian stagnant lid convection and magmatic resur facing on venus abstract mantle convection on venus is likely to occur in the regime known as stagnant lid convection we investigate this regime for internally heated convection with temperature and pressure dependent power law viscosity dislocation creep scaling relationships obtained for large aspect ratio convection are different from steady state square box calculations but agree well with scaling theory and boundary layer stability analysis results for arrhenius viscosity and pressure dependent viscosity show that the efficiency of heat transport is sensitive to the viscosity function at the bottom of the lid new scaling relationships are applied to parameterized convection calculations of the thermal history of venus assuming that plate tectonics could not occur during evolution the onset of convection beneath the lid is delayed even for initial potential temperatures near the solidus during the conductive regime melting is suppressed due to the development of a thick cold lid at the surface after convection begins the lid becomes thinner and the planet undergoes a period of widespread melting and volcanism the timing of the beginning and ending of melting depends on various factors such as the initial conditions and mantle rheology the episode of melting predicted by the models can be reconciled with the cessation of global resurfacing on venus 300 800 myr ago the models yield present day lithospheric thicknesses around 200 km which is similar to previously suggested estimates

quantitative assessment of electrical resistivity tomography for monitoring dnapls migration comparison with high resolution light transmission visualization in laboratory sandbox abstract real time monitoring of dense non aqueous phase liquids dnapls migration and distribution is essential for the decision of an effective remediation strategy light transmission visualization ltv has shown its accuracy and efficiency for measuring dnapls saturation and water content in the laboratory but it cannot be implemented in three dimensional sandbox or field scale sites recently developed electrical resistivity tomography ert has been applied in monitoring the migration and distribution of dnapls in bench and field scale studies however the evaluation of the ability of ert for monitoring dnapls migration by a direct comparison of ert with high resolution techniques such as ltv within an experimental system is still lacking two sandbox experiments with different permeability conditions are conducted to quantitatively assess the capability of ert for monitoring the dnapls migration during the injections ltv method is used to visualize the dnapls migration and provide high resolution saturation data while ert method is applied to capture the change of resistivity the results from the comparison between ltv and ert methods show that ert is successful in detecting the accumulation and flow bypassing phenomenon around the low permeability lenses as well as the penetration through the high permeability lenses there is a fair correlation between the resistivity and saturation with overall correlation coefficients above 0 6 except at last stage however using classical regularization techniques based on smoothness the area of dnapls plume determined by ert is commonly overestimated compared to the plume around the low permeability lenses the plume around the high permeability lenses estimated by ert is more extensive due to larger resistivity contrasts in addition ert measurements indicate that the resistivity increase caused by the low saturation dnapls is not apparent enough which is likely to be covered up under the changing hydrogeochemical environments in field investigations

field study of dispersion in a heterogeneous aquifer 1 overview and site description results are presented for a large scale natural gradient tracer experiment conducted in a heterogeneous alluvial aquifer at a site near columbus mississippi the study was initiated with a 48 hour pulse injection of 10 m3 of groundwater containing bromide and three organic tracers pentaflourobenzoic acid o trifluoromethylbenzoic acid and 2 6 diflourobenzoic acid over a 20 month period seven comprehensive samplings of the tracer plume were performed at approximately 1 to 4 month intervals using an extensive three dimensional sampling well network the dominant feature of the tracer plume that evolved during the study was the highly asymmetric concentration distribution in the longitudinal direction this asymmetry was produced by accelerating groundwater flow along the plume travel path that in turn resulted from an approximate 2 order of magnitude increase in the mean hydraulic conductivity between the near field and far field regions of the site the columbus study is distinct from previous natural gradient experiments because of the extreme heterogeneity of the aquifer the large scale spatial variations in groundwater velocity and the extensive set of hydraulic conductivity measurements for the aquifer

large scale natural gradient tracer test in sand and gravel cape cod massachusetts 1 experimental design and observed tracer movement a large scale natural gradient tracer experiment was conducted on cape cod massachusetts to examine the transport and dispersion of solutes in a sand and gravel aquifer the nonreactive tracer bromide and the reactive tracers lithium and molybdate were injected as a pulse in july 1985 and monitored in three dimensions as they moved as far as 280 m down gradient through an array of multilevel samplers the bromide cloud moved horizontally at a rate of 0 42 m per day it also moved downward about 4 m because of density induced sinking early in the test and accretion of areal recharge from precipitation after 200 m of transport the bromide cloud had spread more than 80 m in the direction of flow but was only 14 m wide and 4 6 m thick the lithium and molybdate clouds followed the same path as the bromide cloud but their rates of movement were retarded about 50 relative to bromide movement because of sorption onto the sediments

the geology of callisto the geology of callisto is not boring although cratered terrain dominates callisto a key end member of the jovian satellite system a number of more interesting features are apparent cratered terrain is broken into irregular mappable bright and dark subunits that vary in albedo by a factor of 2 and several relatively smooth units are depleted of small craters some of these areas may have been volcanically resurfaced lineaments including parallel and radial sets may be evidence for early global tectonism frost deposition occurs in cold traps and impact scars have formed from tidally disrupted comets geologic evidence suggests that callisto does have a chemically differentiated crust central pit and central dome craters and palimpsests are common the preferred interpretation is that a relatively ice rich material at depths of 5 km or more has been mobilized during impact and exposed as domes or palimpsests the close similarity in crater morphologies and dimensions indicates that the outermost 10 km or so of callisto may be as differentiated as on ganymede the geology of cratered terrain on callisto is simpler than that of cratered terrain on ganymede however orbital evolution and tidal heating may provide the answer to the riddle of why callisto and ganymede are so different malhotra 1991 we should expect a few surprises and begin to answer some fundamental questions when callisto is observed by galileo in late 1996