fracture fluid and saturation effects on the seismic attributes of rocks from the southern negros geothermal field philippines abstract seismic based geophysical methods are seeing increased usage in evaluating geothermal resources in order to maximize resource potential however interpreting geophysical data such as velocities and dynamic modulus and fracture density alignment generated from geothermal reservoirs remains difficult here we present the results of a new laboratory study measuring seismic attributes of fresh and hydrothermally altered rocks from a philippine geothermal field southern negros geothermal project sngp during triaxial deformation two types of rocks were obtained by sub coring samples of low porosity 1 andesite and higher porosity 10 volcaniclastic samples from the sngp samples were prepared with two offset drill holes to allow a natural fracture to permit fluid flow along the fracture an embedded array of acoustic emission ae sensors allows elastic wave and induced microseismic data to be collected we measure a significant reduction in elastic wave velocities and moduli with the exception of poisson s ratio after shear fracture development an initially pre fractured permeability of approximately 10 17 m2 is measured we find that the permeability decreases from 2 0 10 14 m2 to lower than 7 4 10 15 m2 as the confining pressure is increased from 5 mpa to 30 mpa a concomitant increase in p and s wave velocities dynamic bulk and young s moduli are also measured finally we simulate a geothermal venting situation by intentionally releasing the high pore fluid water pressure from 10 to 50 mpa to ambient pressure generating a swarm of ae that increases in duration with higher pore pressure we postulate that this is due to fluid phase change liquid to gas and movement along the natural fracture plane and damage zone

reconstruction of basal properties in ice sheets using iterative inverse methods inverse problems are used to estimate model parameters from observations many inverse problems are ill posed because they lack stability meaning it is not possible to find solutions that are stable with respect to small changes in input data regularization techniques are necessary to stabilize the problem for nonlinear inverse problems iterative inverse methods can be used as a regularization method these methods start with an initial estimate of the model parameters update the parameters to match observation in an iterative process that adjusts large scale spatial features first and use a stopping criterion to prevent the overfitting of data this criterion determines the smoothness of the solution and thus the degree of regularization here iterative inverse methods are implemented for the specific problem of reconstructing basal stickiness of an ice sheet by using the shallow shelf approximation as a forward model and synthetically derived surface velocities as input data the incomplete gauss newton ign method is introduced and compared to the commonly used steepest descent and nonlinear conjugate gradient methods two different stopping criteria the discrepancy principle and a recent improvement threshold are compared the ign method is favored because it is rapidly converging and it incorporates the discrepancy principle which leads to optimally resolved solutions

shear margins in glaciers and ice sheets analytical and numerical techniques are used to examine the flow response of a sloped slab of power law fluid power n subjected to basal boundary conditions that vary spatially across the flow direction as for example near an ice stream margin with planar basal topography the primary assumption is that basal shear stress is proportional to the basal speed time a spatially variable slip resistance the ratio of mean basal speed to the speed originating from shearing through the thickness denoted as r gives a measure of how slippery the bed is the principal conclusion is that a localized disturbance in slip resistance affects the basal stress and speed in a zone spread over a greater width of the flow in units of ice thickness h the spatial scale of spreading is proportional to a single dimensionless number r n r n 1 1 n 1 derived from n and r the consequence for a shear zone above a sharp jump in slip resistance is that the shearing is spread out over a boundary layer with a width proportional to r n for an ice stream caused by a band of low slip resistance with a half width of wh the margins influence velocity and stress in the central part of the band depending on r n in comparison to w three regimes can be identified which for n 3 are quantified as follows low r defined as r 3 1w for which the boundary layers from both sides bridge across the full flow width and the driving stress in the center is supported almost entirely by side drag intermediate r for which the driving stress in the center is supported by a combination of basal and side drag shear zones that are narrower than predicted on the basis of this theory r 3 would require localized softening of the ice to explain the concentration of deformation at a shorter scale

model experiments on the evolution and stability of ice streams this is the publisher s version also available electronically from http www igsoc org annals 23

fast recession of a west antarctic glacier satellite radar interferometry observations of pine island glacier west antarctica reveal that the glacier hinge line position retreated 1 2 0 3 kilometers per year between 1992 and 1996 which in turn implies that the ice thinned by 3 5 0 9 meters per year the fast recession of pine island glacier predicted to be a possible trigger for the disintegration of the west antarctic ice sheet is attributed to enhanced basal melting of the glacier floating tongue by warm ocean waters

sensitivity analysis of pine island glacier ice flow using issm and dakota assessing output errors of ice flow models is a major challenge that needs to be addressed if we are to increase our confidence level in projections of mass balance in antarctica and greenland major inputs to ice flow models include geometry ice thickness and surface elevation constitutive laws and boundary conditions geothermal flux basal drag coefficient surface temperature these inputs can be either measured in which case they carry errors due to instruments or inferred using inverse methods such as basal drag which is inverted using insar surface velocities in which case they carry additional errors generated by the inversion process itself in both cases these input errors will result in uncertainties that propagate throughout a forward model and that influence output diagnostics in order to estimate the resulting error margins on diagnostics such as mass flux we develop a new framework based on the design analysis kit for optimization and terascale applications dakota which we interface to the ice sheet system model issm we present results on the pine island glacier west antarctica for which we evaluate error margins of mass flux across the whole glacier given currently known error margins on ice thickness basal friction and ice hardness our results suggest errors in these inputs propagate linearly through the ice flow model providing a way to 1 calibrate measurement requirements for field campaigns collecting data such as bedrock or surface topography 2 quantify uncertainties in projections of mass balance and 3 assess the sensitivity of model outputs to input parameters this new error propagation model should help quantify confidence levels that we assign to model projections for the mass balance of antarctica and greenland which will ultimately improve our projections of future sea level rise in a warming climate copyright 2012 by the american geophysical union

role of transition zones in marine ice sheet dynamics 1 it has been suggested that rapid thinning of the west antarctic ice sheet is due to increased melting under ice shelves caused by a gradual ocean warming shepherd et al 2004 payne et al 2004 showed that such melting could lead to an acceleration of grounded ice flow in this paper we analyze the response of a marine ice sheet to different perturbations near the grounding line using a numerical ice sheet model that takes into account longitudinal stress coupling and grounding line migration at subgrid precision based on a novel technique results show that stress transmission or longitudinal coupling across the grounding line plays a decisive role the grounding line migration is a function of the length scale over which the basal conditions change from frozen to the bed to floating the transition zone we demonstrate that thinning of the ice shelf due to bottom melting has a negligible effect on the grounded ice mass only perturbations at the grounding line or reduction in buttressing of the ice shelf substantially thins the grounded ice sheet inclusion of lateral drag does not alter these results qualitatively marine ice sheets with large transition zones such as ice streams seem highly sensitive to such perturbations compared to ice sheets with small transition zones such as an abrupt ice sheet ice shelf junction

a tutorial on the use of control methods in ice sheet modeling control methods are recommended as an efficient means to estimate undetermined physical parameters and boundary conditions and in so doing to improve the fidelity of a given ice sheet flow model to specific ice sheet velocity observations to accomplish this task the underlying dynamics of the model are inverted this permits model tuning adjustments to be represented explicitly in terms of model observation misfit

force budget i theory and numerical methods a practical method is developed for calculating stresses and velocities at depth using field measurements of the geometry and surface velocity of glaciers to do this it is convenient to partition full stresses into lithostatic and resistive components the horizontal gradient in vertically integrated lithostatic stress is the driving stress and it describes the horizontal action of gravity the horizontal resistive stress gradients describe the reactions resistive stresses are simply related to deviatoric stresses and hence to strain rates through a constitutive relation a numerical scheme can be used to calculate stresses and velocities from surface velocities and slope and from ice thickness there is no mathematical requirement that the variations in these quantities be small

delineating fracture zones using surface tunnel surface seismic data p s and s p mode conversions a surface tunnel surface seismic experiment was conducted at the aspo hard rock laboratoryto study the seismic response of major fracture systems intersecting the tunnel a newly developedthree com

glaciology ice sheet advance in antarctica reliable forecasts of sea level rise depend on accurately modelling the dynamics of polar ice sheets a numerical framework that better reflects ice sheet basal drag adds greater realism to such models

ice stream basal conditions from block wise surface data inversion and simple regression models of ice stream flow application to bindschadler ice stream 1 widespread basal conditions controlling ice stream flows are still beyond the scope of direct observation thus knowledge of their magnitudes and variabilities comes from inversion of surface measurements ice velocities surface elevations and thicknesses we present a new approach to implement a widely accepted inverse method on regular 10 10 km blocks smaller than the whole domain to enhance the spatial resolution of calculated basal conditions inverted basal friction coefficients and calculated shear stress have sharp transitions and large variations in small areas overall the obtained basal shear stress is very small in regions of fast flowing ice the results of the inversion along with the surface variables are used to construct two simple regression models of bindschadler ice stream former ice stream d that reproduce 96 of observed velocity variations while highly idealized these regression models are sufficiently informative to be considered as parameterizations for ice stream flow in large scale ice sheet models which lack the spatial and temporal resolution necessary to simulate ice stream dynamics in detail

the basal roughness of pine island glacier west antarctica we assess basal roughness beneath pine island glacier pig west antarctica based on a recent airborne radio echo sounding dataset we identify a clear relationship between faster ice flow and decreased basal roughness in significant parts of pig the central portion and two of its tributaries are particularly smooth but the majority of the tributaries feeding the main trunk are rougher we interpret the presence of a smooth bed as being a consequence of the deposition of marine sediments following disappearance of the west antarctic ice sheet in the pliocene or pleistocene and conversely a lack of marine sedimentation where the bed is rough importantly we also identify a patchy distribution of marine sediments and thus a bed over which the controls on flow vary while there is a notable correspondence between ice velocity and bed roughness we do not assume a direct causal relationship but find that an indirect one is likely where low basal roughness results in low basal resistance to flow a lower driving stress is required to produce the flux required to achieve mass balance this in turn means that the surface in that area will be lower than surrounding areas with a rougher bed and this will tend to draw flow into the area with low bed roughness since our studies shows that bed roughness beneath the tributaries of the trunk varies substantially there is a strong likelihood that these tributaries will differ in the rate at which they transmit current velocity changes on the main trunk into the interior of the glacier basin

large scale ice flow over a viscous basal sediment theory and application to ice stream b antarctica recent seismic studies of ice stream b antarctica and field analysis of mid latitude glacial deposits suggest that deformable basal sediments e g water saturated till are important in determining ice sheet flow if the ratio of till viscosity to effective ice viscosity is small vertical shear associated with horizontal flow is confined to the deforming bed alone ice flow over a deformable bed is thus akin to that of floating ice shelves because ice shelves flow over inviscid seawater for some antarctic ice streams and possibly for portions of the late wisconsin ice sheets in north america and eurasia basal drag associated with deforming basal sediment does not induce significant vertical gradients of horizontal velocity instead basal drag affects the flow as if it were a horizontal body force balanced by longitudinal and transverse deviatoric stress gradients comparison of the observed flow of ice stream b to finite element simulations incorporating a viscous basal till suggest that a simple till theology is sufficient to explain the current velocity profile these simulations also highlight the importance of horizontal deviatoric stress in regions where driving stress and basal stress do not locally balance while bed deformation is critical to ice stream existence sensitivity tests suggest that ice shelf back pressure is still a crucial control affecting ice stream response to atmospheric and oceanic climate

α β phase transition in quartz monitored using acoustic emissions summary it is usually suggested that thermal cracking in a quartz bearing rock results from the anomalously high volumetric expansion coefficients of quartz e g simmons cooper 1978 it has also been recognized that thermal expansion mismatch and mineral anisotropy contribute to thermal cracking in materials that consist of a polycrystalline aggregate composed of several anisotropic minerals even in the absence of a temperature gradient friedrich wong 1986 experiments investigating thermal cracking in rocks commonly involve imaging and quantitative stereology of crack populations induced in rocks treated to various peak temperatures e g friedrich johnson 1978 homand etienne troalan 1984 atkinson mcdonald meredith 1984 meredith atkinson 1985 here we report on acoustic emission experiments that monitor the process of thermal cracking as it occurs during heating supported by measurements of crack surface area pore fluid permeability porosity and surface conductivity carried out on rock samples treated to various peak temperatures the acoustic emission measurements show a strong peak of microcracking at the phase transition temperature for quartz 573 c superimposed upon a background of microcracking due to thermal expansion there is also a clear peak of microcracking at higher temperatures 800 c that can be attributed to oxidation dehydroxylation reactions of hornblende and chlorite measurements of fluid permeability pore surface area porosity and electrical conductivity made on samples that have been heat treated to various maximum temperatures show increases associated with a major episode of cracking in the 500 600 c temperature range indicating that the new cracks form a well interconnected network this has been confirmed by sem and optical microscopy these results have implications for the electrical conductivity of the continental crust providing a mechanism enabling the high pore fluid connectivity needed to explain zones of high electrical conductivity at depth providing that cracks opened in this way remain open at the high pressures existing at depth it should be recognized however that th se measurements are limited in their direct application since they were obtained under initially dry conditions at laboratory pressures

investigations of an ice plain in the mouth of pine island glacier antarctica we present newly acquired airborne radar data showing ice thickness and surface elevation for pine island glacier antarctica these data when combined with earlier measurements suggest the presence of a lightly grounded area immediately above the grounding line of pine island glacier we identify this region as a ice plain it lies close to the centre line of the glacier has an elevation above buoyancy of 28 km the upstream edge of the ice plain is defined by a coupling line the configuration of the ice plain implies that nearby thinning of the ice stream would result in substantial grounding line retreat we suggest that the grounding line retreat of pine island glacier observed between 1992 and 1996 probably commenced sometime after 1981

numerical modelling and data assimilation of the larsen b ice shelf antarctic peninsula in this study the flow and rheology of pre collapse larsen b ice shelf are investigated by using a combination of flow modelling and data assimilation observed shelf velocities from satellite interferometry are used to constrain an ice shelf model by using a data assimilation technique based on the control method in particular the ice rheology field and the velocities at the inland shelf boundary are simultaneously optimized to get a modelled flow and stress field that is consistent with the observed flow the application to the larsen b ice shelf shows that a strong weakening of the ice in the shear zones mostly along the margins is necessary to fit the observed shelf flow this pattern of bands with weak ice is a very robust feature of the inversion whereas the ice rheology within the main shelf body is found to be not well constrained this suggests that these weak zones play a major role in the control of the flow of the larsen b ice shelf and may be the key to understanding the observed pre collapse thinning and acceleration of larsen b regarding the sensitivity of the stress field to rheology the consistency of the model with the observed flow seems crucial for any further analysis such as the application of fracture mechanics or perturbation model experiments

fluids and frequency dependent seismic velocity of rocks summary compressional and shear velocities of rocks are dependent on frequency and this dispersion may be significant even within the seismic band the amount and position of dispersion will be largely a function of fluid properties distribution and motion in the pores velocities are thus directly coupled to rock permeability and pore compliance propagation often will be in the high frequency regime even at a few tens of hertz static or low frequency models such as gasmann will fail under such conditions in addition biot theory will not correctly model wave propagation in many cases

dependence of antarctic surface mass balance on temperature elevation and distance to open ocean the latest compilations of surface mass balance mean annual surface temperature and elevation for the antarctic ice sheet derived from data sets of approximately 1500 700 and 105 points respectively have been used to obtain areally integrated means for 24 ice drainage systems and 329 grid point values covering the whole ice sheet monthly summaries of remotely sensed sea ice data for 1973 1976 have been used to obtain mean annual distance to open ocean linear and second order regression analyses of surface balance on 1 temperature for the entire ice sheet 2 elevation for the conterminous grounded ice and 3 distance to the open ocean for the ice shelves including ice rises and attached islands have been made using both system means and grid point values these analyses show correlation coefficients of between 0 63 and 0 81 and they provide bases for descriptive models of the present antarctic ice sheet as well as for predictive models of the response of the ice sheet to air temperature changes and variations in meridional mass and energy transfers extrapolation to other ice sheets past and present may be possible but should be made cautiously linear models are recommended for paleoclimatic reconstructions of ice sheets in upper mid latitudes and second order models are recommended for those in high latitudes system means are shown to be reliable for these purposes incidental results are new estimates of the mean annual surface temperature for the whole ice sheet 36 c and mean surface elevation for the conterminous grounded ice 2290 m

characterisation and multifaceted anisotropy assessment of corvio sandstone for geological co2 storage studies we present a comprehensive characterisation of the physical mineralogical geomechanical geophysical and hydrodynamic properties of corvio sandstone this information together with a detailed assessment of anisotropy is needed to establish corvio sandstone as a useful laboratory rock testing standard for well constrained studies of thermo hydro mechanical chemical coupled phenomena associated with co2 storage practices and for geological reservoir studies in general more than 200 core plugs of corvio sandstone 38 1 and 50 mm diameters 2 1 length to diameter ratio were used in this characterisation study with a rock porosity of 21 7 1 2 dry density 2036 32 kg m 3 and unconfined compressive and tensile strengths of 41 3 28 and 2 3 0 14 mpa respectively geomechanical tests show that the rock behaves elastically between 10 and 18 mpa under unconfined conditions with associated young s modulus and poisson s ratio of 11 8 2 8 gpa and 0 34 0 01 gpa respectively permeability abruptly decreases with confining pressure up to 10 mpa and then stabilises at 1 md ultrasonic p and s wave velocities vary from about 2 8 3 8 km s 1 and 1 5 2 4 km s 1 respectively over confining and differential pressures between 0 1 and 35 mpa allowing derivation of associated dynamic elastic moduli anisotropy was investigated using oriented core plugs for electrical resistivity elastic wave velocity and attenuation permeability and tracer injection tests corvio sandstone shows weak transverse isotropy symmetry axis normal to bedding of 10 for velocity and 20 for attenuation

greenland ice sheet volume sensitivity to basal surface and initial conditions derived from an adjoint model we extend the application of control methods to a comprehensive three dimensional thermomechanical ice sheet model sicopolis simulation code for polythermal ice sheets lagrange multipliers i e sensitivities are computed with an exact efficient adjoint model that has been generated from sicopolis by rigorous application of automatic differentiation the case study uses the adjoint model to determine the sensitivity of the total greenland ice volume to various control variables over a 100 year period the control space has of the order 1 2 10 6 elements consisting of spatial fields of basal flow parameters surface and basal forcings and initial conditions reliability of the adjoint model was tested through finite difference perturbation calculations for various control variables and perturbation regions ascertaining quantitative inferences of the adjoint model as well as confirming qualitative aspects of ice sheet sensitivities e g expected regional variations we detect regions where model sensitivities are seemingly unexpected or counter intuitive albeit real in the sense of actual model behavior an example is inferred regions where sensitivities of ice sheet volume to basal sliding coefficient are positive i e where a local increase in basal sliding parameter increases the ice sheet volume similarly positive generally negative ice temperature sensitivities in certain parts of the ice sheet are found the detection of which seems highly unlikely if only conventional perturbation experiments had been used the object of this paper is largely a proof of concept available adjoint code generation tools now open up a variety of novel model applications notably with regard to sensitivity and uncertainty analyses and ice sheet state estimation or data assimilation

frequency and fluid effects on elastic properties of basalt experimental investigations 1 in order to investigate the effects of fluid and frequency on the elastic properties we performed hydrostatic experiments on an icelandic basalt specimen under both dry and saturated conditions this basalt is characterized by a bimodal porosity i e cracks and equant pores the elastic properties bulk moduli in our case were investigated under high pressure through two experimental methods 1 a classical one using ultrasonic p and s waves velocities frequency 106 hz 2 and a new one using oscillation tests frequency 10 2 hz in dry condition experimental data show no significant difference between high hf and low lf frequency bulk moduli however in saturated conditions two effects are highlighted a physico chemical effect emphasized by a difference between drained and dry moduli and a squirt flow effect evidenced by a difference between hf and lf undrained moduli

anisotropy of elastic moduli p wave velocities and thermal conductivities of asan gneiss boryeong shale and yeoncheon schist in korea abstract this paper presents the anisotropic characteristics of the elastic moduli p wave velocities and thermal conductivities of three types of anisotropic rocks i e asan gneiss boryeong shale and yeoncheon schist occurring in korea the experiments were conducted on rock samples that show clear evidence of transverse isotropy cylindrical core samples with different anisotropy angles were prepared by coring at 15 degree intervals from the transversely isotropic plane using the laboratory directional coring system established for this study elastic moduli p wave velocities and thermal conductivities were determined along the sample axis for different anisotropy angles the anisotropy ratio is defined as the ratio of the properties parallel to the transversely isotropic plane to those perpendicular to the plane and the anisotropy ratios for the thermal conductivities k 90 k 0 of asan gneiss boryeong shale and yeoncheon schist were 1 4 2 1 and 2 5 respectively the p wave velocity anisotropy ratios v p 90 v p 0 for asan gneiss boryeong shale and yeoncheon schist were 1 2 1 5 and 2 3 respectively the elastic moduli p wave velocities and thermal conductivities that were obtained were compared with theoretical predictions by mean prediction error mpe the correlations between the measured properties were evidently correlated with some minor scatter in the data the degree of anisotropy measured in this study suggests that ignoring anisotropy in rock properties may mislead to erroneous results the application of tensorial transformation evaluations revealed that the three types of rocks chosen for this study can be modeled effectively by a transversely isotropic model

microfracture analysis of fault growth and wear processes punchbowl fault san andreas system california abstract fabric and timing relations of mode i microfractures are used to test current hypotheses for the origin of damage along large displacement faults by the processes of fault growth and wear oriented samples 0 075 m to 1 km from the punchbowl fault surface i e ultracataclasite layer document an increase in development of preferred orientation and increase in density of microfractures towards the ultracataclasite layer defining a zone of fault related microfracture damage about 100 m thick a distinct microfracture set that is perpendicular to the slip direction of the fault is present throughout the damage zone this implies that the average orientation of the maximum principal compressive stress within the damage zone was nearly normal to the fault surface two additional microfracture sets are present one oriented at low angles to the fault within meters of the ultracataclasite layer and another low angle set that occurs in the outermost damage zone the preferred orientations and timing relations are most consistent with local damage accumulation from stress cycling associated with slip on a geometrically irregular relatively weak fault surface low angle microfractures nearest the ultracataclasite layer also may record wear associated with the passage of earthquake ruptures and those in the outermost damage zone may be consistent with andersonian fault formation and subsequent fault weakening

evaluation of ice stream model sensitivities for parameter estimation abstract large ensemble perturbed parameter forward ice flow modeling can provide useful insights to uncertainties in inversions for basal drag or other ice flow parameters inversion and data assimilation provide estimates of poorly known parameters that are essential for accurate prognostic modeling because ice flow depends on many such parameters with their associated uncertainties which may interact in nonlinear ways full uncertainty assessment for parameter estimates is challenging with rising computational power it is increasingly practicable to explore co dependencies and sensitivities here we use a well characterized higher order flowline model configured for a well lubricated shelfy ice stream to run large ensembles perturbing the magnitude and spatial pattern of basal drag basal topography and input flux we find for steady state that ice stream velocity and thickness along the entire domain are especially correlated to drag at the downstream end but with greater local correlation during transients the modeled effects of basal topographic perturbations on velocity and ice thickness are primarily local perturbations of input ice fluxes interact with the others in interesting ways these insights point to the value of inversions informed by concentrated observations during forced transients such as lake drainage events accumulation rate fluctuations or ice shelf losses and to the care needed when interpreting local results of some inversions for basal drag parameters

basal friction of ice stream e west antarctica we use surface velocity derived from sequential landsat imagery and a control method to estimate the basal friction distribution of a major west antarctic ice stream the area averaged basal stress is approximately 1 4 10 4 pa or about 29 of the area averaged driving stress of 4 9 10 4 pa uncertainty of the derived area averaged basal stress is difficult to assess and depends primarily on spatial variation of the flow law rate factor in the constitutive law spatial variation associated with depth averaged temperature variation gives an uncertainty of approximately 10 3 pa approximately 60 of the ice stream has a basal stress magnitude less than 10 4 pa and approximately 30 has less than 10 3 pa these characteristics suggest the presence of a mechanically weak water charged subglacial till small scale sticky spots where basal friction exceeds the area averaged driving stress are scattered irregularly across the subglacial regime and comprise approximately 15 of the ice stream area sticky spots cluster in regions where landsat imagery suggests structural features in the underlying bedrock

assimilation of surface velocities acquired between 1996 and 2010 to constrain the form of the basal friction law under pine island glacier in ice sheet models slip conditions at the base between the ice and the bed are parameterized by a friction law the most common relation has two poorly constrained parameters c and m the basal slipperiness coefficient c depends on local unobserved quantities and is routinely inferred using inverse methods while model results have shown that transient responses to external forcing are highly sensitive to the stress exponent m no consensus value has emerged with values commonly used ranging from 1 to depending on the slip processes by assimilation of pine island glacier surface velocities from 1996 to 2010 we show that observed accelerations are best reproduced with m 5 we conclude that basal motion in much of the fast flowing region is governed by plastic deformation of the underlying sediments this implies that the glacier bed in this area can not deliver resistive stresses higher than today making the drainage basin potentially more sensitive to dynamical perturbations than predicted with models using standard values m 1 or 3

causes of pre collapse changes of the larsen b ice shelf numerical modelling and assimilation of satellite observations satellite observations revealed that beside a rapid thinning the larsen b ice shelf lbis was undergoing a significant acceleration before its collapse in 2002 this paper investigates the ice shelf acceleration between 1995 and 1999 using a combination of data assimilation and numerical modelling based on a flow model adjusted to the 1995 insar velocities perturbation experiments are performed such as ice front retreat thinning increase in tributary flow and rheological weakening furthermore an inversion for ice shelf rheology and tributary flow velocity is performed for both the 1995 and the 1999 insar velocities the perturbation experiments together with the inversion strongly suggest that the acceleration cannot solely be explained by the retreat of the ice shelf front but relies on a further significant rheological weakening of the already weak shear zones within the lbis minor tributary acceleration is found to be an effect rather than a cause of the ice shelf acceleration furthermore the observed acceleration cannot be explained by the observed recent thinning we conclude that for smaller ice shelves such as the lbis such weak shear margins play a crucial role in controlling their dynamics and are the key to understand changes in the future finally we compare the dynamic thinning likely to be associated with the observed acceleration with the observed thinning for the ice shelf as a whole this thinning accounts for 20 of the observed value which implies that factors such as enhanced basal melt were the primary cause of the observed thinning

nonlinear viscoelastic behavior of sedimentary rocks part ii hysteresis effects and influence of type of fluid on elastic moduli uniaxial stress cycling experiments were conducted on dry brine saturated and hexadecane saturated berea sandstone samples to observe in detail the hysteresis in stress strain diagrams and to understand the influence of different fluids on the strain amplitude dependence of elastic moduli and attenuation cycling experiments were also conducted with sandstone samples saturated with ctab a cationic surfactant that renders the mineral surfaces hydrophobic hexadecane and ctab were selected so as to investigate the relative contributions of adhesion hysteresis and stick slip sliding on attenuation in sedimentary granular rocks young s moduli and poisson s ratios obtained from the cycling tests show a significant dependence on strain amplitude on dry as well as water and hexadecane saturated samples bow tie shaped diagrams are obtained when loading and unloading tangent moduli are plotted against strain the type of fluid in the pore space and at the grain contacts has a large influence on the hysteresis observed in the stress strain diagrams

the effects of transverse bed topography variations in ice flow models 1 a suite of ice flow models is subject to a performance test designed to investigate treatment of flow over variable basal topography using transfer functions developed by gudmundsson 2003 and numerical models of various complexity this study demonstrates that a widely used flowband model has strong limitations associated with its underlying assumptions and thus should be applied only in specific geometrical settings its performance can be significantly improved by using the laterally averaged basal topography instead of centerline topography in applications where spatial variability of flow fields is important hybrid ice flow models can be a viable alternative to flowband models in addition analysis of horizontal distributions of the various ice flow characteristics e g surface elevation velocity and horizontal stress components shows that field observations that focus on assessing these parameters only along the centerline of ice flow could be misleading these results also suggest that such spatial variability needs to be taken into account when designing field surveys of ice flow

seismic wave attenuation and dispersion resulting from wave induced flow in porous rocks a review one major cause of elastic wave attenuation in heterogeneous porous media is wave induced flow of the pore fluid between heterogeneities of various scales it is believed that for frequencies below 1 khz the most important cause is the wave induced flow between mesoscopic inhomogeneities which are large compared with the typical individual pore size but small compared to the wavelength various laboratory experiments in some natural porous materials provide evidence for the presence of centimeter scale mesoscopic heterogeneities laboratory and field measurements of seismic attenuation in fluid saturated rocks provide indications of the role of the wave induced flow signatures of wave induced flow include the frequency and saturation dependence of p wave attenuation and its associated velocity dispersion frequency dependent shear wave splitting and attenuation anisotropy during the last four decades numerous models for attenuation and velocity dispersion from wave induced flow have been developed with varying degrees of rigor and complexity these models can be categorized roughly into three groups according to their underlying theoretical framework the first group of models is based on biot s theory of poroelasticity the second group is based on elastodynamic theory where local fluid flow is incorporated through an additional hydrodynamic equation another group of models is derived using the theory of viscoelasticity though all models predict attenuation and velocity dispersion typical for a relaxation process there exist differences that can be related to the type of disorder periodic random space dimension and to the way the local flow is incorporated the differences manifest themselves in different asymptotic scaling laws for attenuation and in different expressions for characteristic frequencies in recent years some theoretical models of wave induced fluid flow have been validated numerically using finite difference finite element and reflectivity algorithms applied to biot s equations of poroelasticity application of theoretical models to real seismic data requires further studies using broadband laboratory and field measurements of attenuation and dispersion for different rocks as well as development of more robust methods for estimating dissipation attributes from field data

the evolution of elastic moduli with increasing crack damage during cyclic stressing of a basalt from mt etna volcano article i nfo volcanic edifices such as mt etna italy are commonly subject to repeated cycles of stress over time due to the combination of magma emplacement from deep reservoirs to shallow depths and superimposed tectonic stresses such repeated stress cycles lead to anisotropic deformation and an increase in the level of crack damage within the rocks of the edifice and hence changes to their elastic moduli which are a key parameter for reliable modelling of deformation sources we therefore report results of changes in elastic moduli measured during increasing amplitude cyclic stressing experiments on dry and water saturated samples of etna basalt in all experiments the young s modulus decreased by approximately 30 over the total sequence of loading cycles and the poisson s ratio increased by a factor of approximately 3 0 5 microseismicity in terms of acoustic emission ae output was also recorded throughout each experiment our results demonstrate that ae output only re commences during any loading cycle when the level of stress where ae ceased during the unloading portion of the previous cycle is exceeded a manifestation of the kaiser stress memory effect in cycles where no ae output was generated we also observed no change in elastic moduli this result holds for both mechanical and thermal stressing our results are interpreted in relation to measurements of volcano tectonic seismicity and deformation at mt etna volcano

fracture toughness and subcritical crack growth during high temperature tensile deformation of westerly granite and black gabbro abstract the double torsion testing method has been used to determine catastrophic and subcritical crack propagation parameters for pre cracked specimens of westerly granite and black gabbro under a number of environmental conditions the critical stress intensity factor for catastrophic crack propagation fracture toughness of granite and gabbro has been measured at temperatures from 20 to 400 c in a vacuum at 20 c the fracture toughness of westerly granite was 1 79 0 02 mpa m 1 2 and for two blocks of black gabbro it was 3 03 0 08 mpa m 1 2 and 2 71 0 15 mpa m 1 2 respectively these values are very close to those reported by other investigators for tests conducted in air of ambient humidity at room temperature for both rocks fracture toughness at first increased slightly and then decreased steadily on raising the temperature above ambient conditions this behaviour is explained in terms of the density and distribution of thermally induced microcracks as determined by quantitative optical microscopy subcritical crack growth behaviour has been studied at temperatures up to 300 c and under water vapour at pressures of 0 6 to 15 kpa both the load relaxation and incremental constant displacement rate forms of the double torsion testing method were utilised to generate stress intensity factor crack velocity diagrams crack growth was measured over the velocity range 5 10 3 to 10 7 m s 1 increasing both temperature and water vapour pressure resulted in substantially higher crack growth rates the overall effect of raising the temperature over the range studied here 20 300 c was to increase the crack growth rate in granite and gabbro by 5 and 7 orders of magnitude respectively at constant stress intensity factor and vapour pressure of water for both rocks the slopes of stress intensity factor crack velocity curves were sensitive to changes in both temperature and water vapour pressure at low values of the latter parameter slopes fell substantially on raising the water vapour pressure but were relatively insensitive to changes in temperature at these higher pressures no subcritical crack growth limit was encountered estimates of the uncertainty in our experimental data are given from the results of multiple load relaxation experiments on westerly granite specimens we estimate the uncertainty in position of stress intensity factor crack velocity curves along the stress intensity axis to be c 10 of the fracture toughness and the uncertainty in slope of such curves to be c 12 problems associated with the extrapolation of our experimental data to regions of higher effective confining pressure in the earth s crust are discussed

geometry and dynamics of an east antarctic ice sheet outlet glacier under past and present climates 1 the behavior of east antarctic ice sheet eais outlet glaciers both at present and during the recent geological past is of considerable interest both from the point of view of understanding the mechanics of contemporary glacier dynamics and also with regard to epoch scale ice sheet stability during plio pleistocene climate transitions here we use a glacier flowline model that incorporates the effects of longitudinal stresses to numerically simulate ferrar glacier first under present day environmental conditions and subsequently under both colder and warmer climate regimes representing the last glacial maximum lgm and mid pliocene peak warmth respectively using airborne radar profiles insar derived surface velocities ice core and geological data for empirical constraint we present a diagnostic simulation that uses an iterative method to closely reproduce observed dynamics our model suggests that the glacier is largely cold based under present conditions flows predominantly by way of internal deformation and cascades over bedrock ridges due to the combined action of changes in cross sectional valley geometry local steepening of the glacier surface and the non local effects of longitudinal coupling time dependent evolutionary simulation of a lower profile glacier under a colder drier lgm climate predicts flow velocities lower than present with minimal bedrock erosion or basal till flux conversely the warmer than present mid pliocene climate produces a more dynamic glacier that is warm based and sliding along much of its bed we propose that eais outlet glaciers such as the ferrar respond dynamically along their length in response to changing environmental forcings with most significant changes taking place in their lower reaches adjustment to perturbations in upper catchments is more muted

inland thinning of pine island glacier west antarctica the pine island glacier pig transports 69 cubic kilometers of ice each year from 10 of the west antarctic ice sheet wais it is possible that a retreat of the pig may accelerate ice discharge from the wais interior satellite altimetry and interferometry show that the grounded pig thinned by up to 1 6 meters per year between 1992 and 1999 affecting 150 kilometers of the inland glacier the thinning cannot be explained by short term variability in accumulation and must result from glacier dynamics

inference of velocity pattern from isochronous layers in firn using an inverse method this study investigates the suitability of a kinematic approach to nd the velocity eld from dated internal layer architecture in rn internal layers are isochrones and the depositional age of a layer particle is treated as a tracer the forward problem uses two dimensional steady state advection of age and conservation of mass to predict layer ar chitecture dieren t combinations of constraints on horizontal or vertical velocity properties are added d the inverse problem can be formulated as the solution of underdetermined and overdetermined systems of equations e the systems are solved using singular value decomposition allowing analysis of the singular value spectrum model d resolution e and data resolution d solutions of the inverse problem e are evaluated by comparing the velocity eld solutions with synthetic input velocity data compared to conventional accumulation estimates d the new approach takes lateral advection into account e enabling improved separation of spatial and temporal variations in accumulation d two glaciological applications are presented the determination of the migration velocity of a spa tially non stationary accumulation pattern and reconstruction of past accumulation and its stationarity over time e

sensitivity of grounding line dynamics to the choice of the friction law basal slip accounts for a large part of the flow of ice streams draining ice from antarctica and greenland into the ocean therefore an appropriate representation of basal slip in ice flow models is a prerequisite for accurate sea level rise projections various friction laws have been proposed to describe basal slip in models here we compare the influence on grounding line gl dynamics of four friction laws the traditional weertman law and three effective pressure dependent laws namely the schoof tsai and budd laws it turns out that even when they are tuned to a common initial reference state the weertman budd and schoof laws lead to thoroughly different steady state positions although the schoof and tsai laws lead to much the same result in particular under certain circumstances it is possible to obtain a steady gl located on a reverse slope area using the weertman law furthermore the predicted transient evolution of the gl as well as the projected contributions to sea level rise over a 100 year time horizon vary significantly depending on the friction law we conclude on the importance of choosing an appropriate law for reliable sea level rise projections and emphasise the need for a coupling between ice flow models and physically based subglacial hydrological models

can seismic observations of bed conditions on ice streams help constrain parameters in ice flow models we investigate correlations between seismically derived estimates of basal acoustic impedance and basal slipperiness values obtained from a surface to bed inversion using a stokes ice flow model using high resolution measurements along several seismic profiles on pine island glacier pig we find no significant correlation at kilometer scale between acoustic impedance and either retrieved basal slipperiness or basal drag however there is a stronger correlation when comparing average values along the individual profiles we hypothesize that the correlation appears at the length scales over which basal variations are important to large scale ice sheet flow although the seismic technique is sensitive to the material properties of the bed at present there is no clear way of incorporating high resolution seismic measurements of bed properties on ice streams into ice flow models we conclude that more theoretical work needs to be done before constraints on mechanical conditions at the ice bed interface from acoustic impedance measurements can be of direct use to ice sheet models

dynamic and static moduli static and dynamic bulk moduli ks and kd are measured as continuous functions of pressure from zero to 2 3 kilobars for two sandstones a tuff limestone granite and oil shale results for the sandstones and granite are in good agreement with previously reported data with ks kd varying from about 0 5 at atmospheric pressure to close to unity at pressures 2 kilobars and above for rocks behaving elastically under static loading the ks kd ratio is inversely related to the microcrack density for the limestone time dependent deformation associated with pore collapse results in ks kd ratios approaching 0 1 at high pressure upon unloading while initially high 1 0 at high pressures ks kd becomes lower than values obtained during loading at low pressures 1 kilobar due to opening of microcracks generated during pore collapse for the oil shale in which few microcracks exist ks kd remains relatively constant with pressure at a value of about 0 7

the effect of fracture density and stress state on the static and dynamic bulk moduli of westerly granite elastic properties are key parameters during the deformation of rocks they can be measured statically or dynamically but the two measurements are often different in this study the static and dynamic bulk moduli kstatic and kdynamic were measured at varying effective stress for dry and fluid saturated westerly granite with controlled fracture densities under isotropic and differential stress states isotropic fracturing of different densities was induced in samples by thermal treatment to 250 450 650 and 850 c results show that fluid saturation does not greatly affect static moduli but increases dynamic moduli under isotropic loading high fracture density and or low effective pressure results in a low kstatic kdynamic ratio for dry conditions kstatic kdynamic approaches 1 at low fracture densities when the effective pressure is high consistent with previous studies stress induced anisotropy exists under differential stress state that greatly affects kstatic compared to kdynamic as a result the kstatic kdynamic ratio is higher than that for the isotropic stress state and approaches 1 with increasing axial loading the effect of stress induced anisotropy increases with increasing fracture density a key omission in previous studies comparing static and dynamic properties is that anisotropy has not been considered the standard methods for measuring static elastic properties such as poisson s ratio young s and shear modulus involve subjecting the sample to a differential stress state that promotes anisotropy our results show that stress induced anisotropy resulting from differential stress state is a major contributor to the difference between static and dynamic elasticity and is dominant with high fracture density

elastic moduli evolution and accompanying stress changes with increasing crack damage implications for stress changes around fault zones and volcanoes during deformation summary the elastic moduli of rock in areas susceptible to crack damage such as within fault zones or volcanic edifices can be subject to large modifications knowledge of how elastic moduli may vary in such situations is important for both the reliable modelling of volcano deformation and stability and for linear and non linear elastic crack models for earthquake rupture furthermore it has previously been shown that changes in elastic moduli can induce changes in the stress field surrounding faults here we report both uniaxial experimental measurements of changes in elastic moduli during increasing amplitude cyclic stressing experiments on a range of different rock types basalts sandstones and granite and the results of modelled stress modifications the trend in elastic moduli evolution with increasing damage was remarkably similar for each rock type with the exception of essentially crack free intrusive basalt that exhibited very minor changes in general young s modulus decreased by between 11 and 32 per cent and poisson s ratio increased by between 72 and 600 per cent over the total sequence of loading cycles these changes are attributed to an increasing level of anisotropic crack damage within the samples our results also show that acoustic emission ae output during any loading cycle only commenced when new crack damage was generated this corresponded to the level of stress where ae ceased during the unloading portion of the previous cycle using the multilayer elastic model of faulkner et al we demonstrate that the damage induced changes in elastic moduli also result in significant decreases in differential stress increases in mean stress and rotation of the applied greatest principal stress relative to the orientation of the mechanical layering the similar trend in the evolution of the elastic moduli of all the rocks tested suggests that stress modification in the damage zone of faults might take the same form regardless of the lithology through which the fault runs these observations are discussed in terms of their applicability to both fault zones and deformation at volcanoes

calibrated prediction of pine island glacier retreat during the 21st and 22nd centuries with a coupled flowline model a flowline ice sheet model is coupled to a box model for cavity circulation and configured for the pine island glacier an ensemble of 5000 simulations are carried out from 1900 to 2200 with varying inputs and parameters forced by ocean temperatures predicted by a regional ocean model under the a1b business as usual emissions scenario comparison is made against recent observations to provide a calibrated prediction in the form of a 95 confidence set predictions are for monotonic apart from some small scale fluctuations in a minority of cases retreat of the grounding line over the next 200 yr with huge uncertainty in the rate of retreat full collapse of the main trunk of the pig during the 22nd century remains a possibility

determining the mean large scale circulation of the atlantic with the adjoint method abstract a new model approach based on the adjoint formalism and aimed at assimilating large sets of hydrographic data is presented the goal of the model calculations is to obtain the mean large scale ocean circulation together with coefficients of iso and diapycnal mixing and air sea heat and freshwater fluxes the final optimal solution is required to be consistent with the principle of geostrophy and must predict realistic distributions of temperature and salinity the model covers the entire atlantic and has realistic topography model resolution is nonuniform and ranges from 2 5 to 10 horizontally and from 60 m to 500 m in the vertical model velocities are initialized with geostrophic flows calculated from original hydrographic station data initial air sea heat fluxes and mixing coefficients are taken from the literature mass heat and salt budgets are satisfied exactly and for given boundary conditions and model flows the model temperature and salinity distributions are calculated then h

basal conditions for pine island and thwaites glaciers west antarctica determined using satellite and airborne data we use models constrained by remotely sensed data from pine island and thwaites glaciers west antarctica to infer basal properties that are difficult to observe directly the results indicate strong basal melting in areas upstream of the grounding lines of both glaciers where the ice flow is fast and the basal shear stress is large farther inland we find that both glaciers have mixed bed conditions with extensive areas of both bedrock and weak till in particular there are weak areas along much of pine island glacier s main trunk that could prove unstable if it retreats past the band of strong bed just above its current grounding line in agreement with earlier studies our forward ice stream model shows a strong sensitivity to small perturbations in the grounding line position these results also reveal a large sensitivity to the assumed bed sliding or deforming model with non linear sliding laws producing substantially greater dynamic response than earlier simulations that assume a linear viscous till rheology finally comparison indicates that our results using a plastic bed are compatible with the limited observational constraints and theoretical work that suggests an upper bound exists on maximum basal shear stress

influence of debris rich basal ice on flow of a polar glacier at taylor glacier a cold based outlet glacier of the east antarctic ice sheet observed surface speeds in the terminus region are 20 times greater than those predicted using glen s flow law for cold 17 c thin 100 m ice rheological properties of the clean meteoric glacier ice and the underlying deformable debris rich basal ice can be inferred from surface velocity and ablation rate profiles using inverse theory here with limited data we use a two layer flowband model to examine two end member assumptions about the basal ice properties 1 uniform softness with spatially variable thickness and 2 uniform thickness with spatially variable softness we find that the basal ice contributes 85 98 to the observed surface velocity in the terminus region we also find that the basal ice layer must be 10 15 m thick and 20 40 times softer than clean holocene age glacier ice in order to match the observations because significant deformation occurs in the basal ice our inverse problem is not sensitive to variations in the softness of the meteoric ice our results suggest that despite low temperatures highly deformable basal ice may dominate flow of cold based glaciers and rheologically distinct layers should be incorporated in models of polar glacier flow

application of control methods for modelling the flow of pine island glacier west antarctica the distribution of basal traction on a transect along pine island glacier west antarctica is estimated by inverting observed surface velocities using a control method and a simple numerical stream flow model this model calculates the horizontal flow along a transect based on the assumptions that the horizontal flow is independent of ice depth and that the driving stresses are balanced by resistive forces at the glacier bed and margin and by gradients in longitudinal stress basal traction is assumed to be linearly related to the basal velocity for the lateral shear traction a parameterization based on an inversion of glen s flow law is used the applicationof the control method allowsus to calculate the set of model parameters e g the basal friction coefficient that gives the best fit betweenmodelled andobserved surface velocities themodel is used to investigate the stress regime of pine island glacier in particular to estimate the importance of basal lateral and longitudinal stresses relative to each other in the flat region just behind the grounding line basal drag lateral drag and the longitudinal stress gradient are the same order of magnitude in the steep region up glacier from the grounding line the driving stresses are highest and balanced predominantly by basal resistive stresses further upstream in the trunk of the glacier lateral and basal drag predominate

fluid mobility and frequency dependent seismic velocity direct measurements the influence of fluid mobility on seismic velocity dispersion is directly observed in laboratory measurements from seismic to ultrasonic frequencies a forceddeformation system is used in conjunction with pulse transmission to obtain elastic properties at seismic strain amplitude 10 7 from 5 hz to 800 khz varying fluid types and saturations document the influence of pore fluids the ratio of rock permeability to fluid viscosity defines mobility which largely controls pore fluid motion and pore pressure in a porous medium high fluid mobility permits pore pressure equilibrium either between pores or between heterogeneous regions resulting in a low frequency domain where gassmann s equations are valid in contrast low fluid mobility can produce strong dispersion even within the seismic band here the low frequency assumption fails since most rocks in the general sedimentary section have very low permeability and fluid mobility shales siltstones tight limestones etc most rocks are not in the lowfrequency domain even at seismic frequencies only those rocks with high permeability porous sands and carbonates will remain in the low frequency domain in the seismic or sonic band

flow speed within the antarctic ice sheet and its controls inferred from satellite observations accurate dynamical models of the antarctic ice sheet with carefully specified initial conditions and well calibrated rheological parameters are needed to forecast global sea level by adapting an inverse method previously used in electric impedance tomography we infer present day flow speeds within the ice sheet this inversion uses satellite observations of surface velocity snow accumulation rate and rate of change of surface elevation to estimate the basal drag coefficient and an ice stiffness parameter that influences viscosity we represent interior ice motion using a vertically integrated approximation to incompressible stokes flow this model represents vertical shearing within the ice and membrane stresses caused by horizontal stretching and shearing combining observations and model we recover marked geographical variations in the basal drag coefficient relative changes in basal shear stress are smaller no simple sliding law adequately represents basal shear stress as a function of sliding speed low basal shear stress predominates in central east antarctica where thick insulating ice allows liquid water at the base to lubricate sliding higher shear stress occurs in coastal east antarctica where a frozen bed is more likely examining thwaites glacier in more detail shows that the slowest sliding often coincides with elevated basal topography differences between our results and a similar adjoint based inversion suggest that inversion or regularization methods can influence recovered parameters for slow sliding and finer scales on broader scales we recover a similar pattern of low basal drag underneath major ice streams and extensive regions in east antarctica that move by basal sliding

computing the volume response of the antarctic peninsula ice sheet to warming scenarios to 2200 the contribution to sea level to 2200 from the grounded mainland antarctic peninsula ice sheet apis was calculated using an ice sheet model initialized with a new technique computing ice fluxes based on observed surface velocities altimetry and surface mass balance and computing volume response using a linearized method volume change estimates of the apis resulting from surface mass balance anomalies calculated by the regional model racmo2 forced by a1b and e1 scenarios of the global models echam5 and hadcm3 predicted net negative sea level contributions between 0 5 and 12mm sea level equivalent sle by 2200 increased glacier flow due to ice thickening returned 15 of the increased accumulation to the sea by 2100 and 30 by 2200 the likely change in volume of the apis by 2200 in response to imposed 10 and 20km retreats of the grounding line at individual large outlet glaciers in palmer land southern antarctic peninsula ranged between 0 5 and 3 5mm sle per drainage basin ensemble calculations of apis volume change resulting from imposed grounding line retreat due to ice shelf break up scenarios applied to all 20 of the largest drainage basins in palmer land covering 40 of the total area of apis resulted in net sea level contributions of 7 16mm sle by 2100 and 10 25mm sle by 2200 inclusion of basins in the northern peninsula and realistic simulation of grounding line movement for ap outlet glaciers will improve future projections