calculation of ice velocities in the jakobshavn isbrae area using airborne laser altimetry abstract the ability to measure velocities of fast moving glaciers is essential to the understanding and interpretation of their dynamics and related processes this is especially important in the jakobshavn drainage basin of the greenland ice sheet where a significant portion of the ice sheet mass loss occurs and the iceberg calving rate is high in this article we present a method for using airborne laser altimetry data to determine surface velocities in portions of the jakobshavn drainage basin that have distinct elevation features by examining correlations between repeat elevation surveys the motion of these features the most distinct of which are crevasses is tracked results show that for areas with a standard deviation in elevation in excess of 1 m displacement of features can be tracked to a precision of 1 2 m and the corresponding velocities can be retrieved the maximum velocity of the jakobshavn isbrae at its calving front is found to be nearly 7 km yr 1 ice stream boundaries and flow patterns can also be observed in the derived velocity data

post stagnation behavior in the upstream regions of ice stream c west antarctica the region where two active tributaries feed into the now stagnant ice stream c isc west antarctica is thickening in this region we observe a correlation between faster ice flow the tributaries and elevated topography we conclude that stagnation of isc resulted in compression and thickening along the tributaries eventually forming a bulge on the ice sheet surface modern hydraulic potential gradients would divert basal meltwater from isc to ice stream b isb these gradients are primarily controlled by the bulge topography and so likely formed subsequent to trunk stagnation as such we argue against water piracy as being the cause for isc s stagnation kinematic wave theory suggests that thickness perturbations propagate downstream over time but that kinematic wave speed decreases near the stagnant trunk this and modest diffusion rates combine to trap most of the tributary fed ice in the bulge region using interferometric synthetic aperture radar velocity measurements we observe that half of the ice within isc s southern tributary flows into isb that flow pattern and other observations of non steady flow in the region likely result from stagnation induced thickening along upper isc combined with a longer period of thinning on upper isb if current trends in thickness change continue more ice from upper isc will be diverted to isb

monitoring ice sheet behavior from space satellite remote sensing has revolutionized ice sheet research a variety of instruments sensitive to different parts of the electromagnetic spectrum take what the human eye detects as a flat white desert and provide data sets rich in scientific information image based maps of ice sheets are becoming commonplace and have become an integral component of field work more than a pretty picture the digital character of the satellite data from these instruments has become fundamental to the production of elevation motion accumulation and reflectance data sets visible imagery shows the scientist a wealth of features that offer clues to the history and current behavior of the ice sheet radar and microwave imagery provide information from beneath the surface and have been used to estimate snow accumulation rates interferometry principles have recently been applied to measure surface topography and ice motion with unparalleled precision nonimaging instruments also keep a watchful eye monitoring the ice sheet for indications of growth or shrinkage further expansion of the uses of satellite data is anticipated in the future

cross hole seismic surveys applications for studying subsurface fracture systems at a hot dry rock geothermal site the use of cross hole seismic surveys for delineating the location and size of subsurface fracture systems is investigated the radiation pattern for p and s waves emitted by a seismic source in a borehole is derived experimental work in relatively homogeneous granite suggests that the derived relationship adequately describes the radiation pattern for both explosive sources and acoustic transducers placed in fluid filled boreholes using the above functional expressions for the s and p wave amplitudes we have developed a technique to estimate q and locate discrete fractures in crystalline rock that compose the hot dry rock geothermal reservoir at fenton hill new mexico to calculate q we measure the p and s wave amplitudes as a function of distance from the source compensate for the radiation pattern and geometrical spreading effects and match the relative attenuation to a function of the form exp fr qv using a least squares regression technique for undisturbed parts of the reservoir we obta

on angle domain common image gathers by wavefield continuation methods paul c sava and sergey fomel 2003 geophysics 68 1065 1074 sava and fomel 2003 present an original approach for the generation of angle domain common image gathers cigs by wave equation migration their procedure consists of two steps first local offset gathers are created during the migration second depth migrated angle gathers are generated by applying a slant stack to the local offset gathers angle domain gathers are very important for velocity analysis and amplitude versus offset avo studies unfortunately we believe that the procedure outlined in the article does not yield correct angle gathers when there are velocity errors in the following this is demonstrated for the simplest example of a single horizontal reflector embedded in a uniform velocity medium

use of s wave attenuation from perforation shots to map the growth of the stimulated reservoir volume in the marcellus gas shale abstract water filled fractures stimulated by hydraulic fracturing will severely attenuate s waves by comparing the p and s wave amplitudes generated from perforation shots in the marcellus gas shale one can infer the existence and constrain the location of stimulated hydraulic fractures the waveforms of the perforation shots in unstimulated gas shale are first analyzed to define the prestimulation radiation pattern then the change of perforation shot s wave waveform is examined for information about the development of hydraulic fractures s wave attenuation validates the existence of offstage hydraulic fractures and the invasion of treatment fluid from another neighbor stage this method might have potential use in designing an active seismic survey to more effectively monitor hydraulic fracture stimulation beyond the current microseismic mapping techniques

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

satellite imagery of the onset of streaming flow of ice streams c and d west antarctica five overlapping landsat multispectral scanner satellite images of the interior of the west antarctic ice sheet were enhanced with principal component analysis high pass filtering and linear contrast stretching and merged into a mosaic by aligning surface features in the overlap areas the mosaic was registered to geodetic coordinates to an accuracy of about 1 km using the five scene centers as control points the onset of streaming flow of two tributaries of ice stream c and one tributary of ice stream d is visible in the mosaic the onset appears to occur within a relatively short distance less than the width of the ice stream typically at a subglacial topographic feature such as a step or ridge the ice streams extend farther up into the interior than previously mapped ice stream d starts about 150 km from the ice divide at an altitude of about 1500 m approximately halfway up the convex upward dome shape of the interior ice sheet ice stream d is relatively much longer than ice stream c possibly because ice stream d is currently active whereas ice stream c is currently inactive the grounded portion of the west antarctic ice sheet is perhaps best conceptualized as an ice sheet in which ice streams are embedded over most of its area with slow moving ice converging into fast moving ice streams in a widely distributed pattern much like that of streams and rivers in a hydrologic basin a relic margin appears to parallel most of the south margin of the tributary of ice stream d separated from the active shear margin by about 10 km or less for a distance of over 200 km this means there is now evidence for recent changes having occurred in three of the five major ice streams which drain most of west antarctica b c and d two of which b and d are currently active

satellite image derived velocity field of an antarctic ice stream the surface velocity of a rapidly moving ice stream has been determined to high accuracy and spatial density with the use of sequential satellite imagery variations of ice velocity are spatially related to surface undulations and transverse velocity variations of up to 30 percent occur such large variations negate the concept of plug flow and call into question earlier mass balance calculations for this and other ice streams where sparse velocity data were used the coregistration of images with the use of the topographic undulations of the ice stream and the measurement of feature displacement with cross correlation of image windows provide significant improvements in the use of satellite imagery for ice flow determination

interaction of seismic waves with a viscous liquid layer abstract the interaction of seismic waves with a layer filled with a viscous fluid is studied this study is intended to provide the basis for designing active seismic experiments to detect the existence of hydraulically induced fractures in a hot dry rock geothermal energy system we compute transmission and reflection coefficients for p sv and sh waves incident at arbitrary angles onto a fluid layer embedded between two homogeneous half spaces we find that coefficients for sh waves at normal incidence can be written in terms of two dimensionless parameters one of these parameters i is the ratio of acoustic impedance of the fluid to the acoustic impedance of the solid the other dimensionless parameter d is a measure of the amount of energy the sh wave loses during one pass through the fluid layer we find that the amplitude of the sh wave transmitted through the fluid layer is equal to the amplitude of the incident sh wave if i is nearly 1 and d is small as d increases energy is lost due to viscous dissipation in the fluid and transmitted wave amplitude diminishes if i is not nearly 1 some energy is reflected by the fluid layer three dimensionless parameters are required for solution for the case of p waves normally incident on the fluid layer one parameter r is the ratio of the bulk modulus to effective shear modulus of the fluid the second parameter is the ratio of elastic impedance of the solid to the shear impedance of the fluid and the third is d we find that the value of r is a good measure of the importance of fluid viscosity on the propagation of p waves through the fluid detailed studies of the effects of fluid and solid physical properties on the amplitudes of transmitted and reflected waves when waves are normally incident provide valuable insight into the importance of physical properties in the calculations for non normal incidence finally reflection and transmission coefficients are computed for p and sv waves incident at arbitrary angle on a viscous fluid layer physical properties of fluid and rock are chosen to be those thought appropriate for the los alamos national laboratory hot dry rock geothermal test site located at fenton hill new mexico

induced microearthquake patterns and oil producing fracture systems in the austin chalk abstract microearthquakes collected during hydraulic stimulation allowed us to study fracture zones in austin chalk oil reservoirs at two sites in the giddings field texas we deployed three component downhole geophone tools in production wells at depths of 2100 m and greater one near cook s point and two on the matcek lease near caldwell at cook s point we collected 482 microseismic events during a 4000 m 3 25 000 bbl hydraulic stimulation in an offset well we collected 770 events during a similar operation on the matcek lease many seismograms contained reflected phases that constrained location depths to the production zone at the base of the austin chalk by restricting all microearthquake locations to production depths we located 20 of the cook s point events and over 60 of the matcek events at both sites we observed only the fracture wing closest to the observation stations locations formed elongated patterns extending up to 1 km from the stimulation well and trending n60 e parallel to the known regional fracture trend the cook s point seismic zone measured over 100 m in width while long stretches of the matcek seismic zone narrowed to 30 m or less we believe that the width of the seismic zone reflected the density of conductive fractures and thus the volume of the reservoir accessed by the stimulation indeed production rates in the first year following stimulation were much higher at cook s point where we observed the wider of the two seismic zones

measuring snow and glacier ice properties from satellite satellite remote sensing is a convenient tool for studying snow and glacier ice allowing us to conduct research over large and otherwise inaccessible areas this paper reviews various methods for measuring snow and glacier ice properties with satellite remote sensing these methods have been improving with the use of new satellite sensors like the synthetic aperture radar sar during the last decade leading to the development of new and powerful methods such as sar interferometry for glacier velocity digital elevation model generation of ice sheets or snow cover mapping some methods still try to overcome the limitations of present sensors but future satellites will have much increased capability for example the ability to measure the whole optical spectrum or sar sensors with multiple polarization or frequencies among the methods presented are the satellite derived determination of surface albedo snow extent snow volume snow grain size surface temperature glacier facies glacier velocities glacier extent and ice sheet topography in this review emphasis is put on the principles and theory of each satellite remote sensing method an extensive list of references with an emphasis on studies from the 1990s allows the reader to delve into specific topics

beyond the dots in the box microseismicity constrained fracture models for reservoir simulation summary monitoring of induced microseismic events usually results in locations for these events and a geometrical interpretation of these dots in the box in this study we show how additional information obtained from observed microseismic events namely the source mechanisms were used to generate a discrete fracture network using the wide aperture of a surface star like array fracstar allows inversion for both shear and non shear source mechanisms both volumetric and shear only source mechanism inversion was carried out on microseismic events from the treatment of a shale gas reservoir in the continental us during the same hydraulic fracture stimulation treatment both dip slip and reverse faulting sources were active in this reservoir the source mechanisms revealed fracture orientations more accurately than could be inferred from microseismic event locations alone the activity associated with different mechanisms is interpreted as indicating reactivation of existing fractures in the rock as well as suggesting generation of new fractures failure analysis using source mechanisms on individual events allows an integrated understanding of the complex fracture interactions taking place in the reservoir and also provides a more complete understanding of the stress conditions in the reservoir during the treatment fracture orientations locations and failure mechanisms are translated into discrete fracture network dfn models that can be used to verify the extent and character of the fractures created or reactivated during the fracture treatment and may ultimately be used to generate fracture flow properties for reservoir simulation

microseismic growth born from success over the last several years microseismic ms monitoring has quickly grown as a technology to map various reservoir processes imaging hydraulic fracture stimulations is the most common application especially within north america as indicated in the introduction to this special section initial interest in ms was primarily in the engineering community as the method was pushed forward by the engineers geophysicists within operating companies began to get pulled in to support the engineering projects as application of the method gathered steam interest expanded in the geophysical community and into academia as seen by the growth of research consortia looking at the technique while ms is a seismic technique it is sufficiently novel that most geophysicists are largely uninformed about technology challenges nevertheless over the last few years the geophysical community has been both educating itself and taking stewardship of the technology ms remains a unique geophysical technology although the tech

microseismicity beyond dots in a box introduction this special section of geophysics is a consequence of a very successful seg workshop titled microseismicity beyond dots in a box the workshop was held 22 october 2010 following the seg annual meeting that was held in denver more than 120 people attended the meeting and nine oral papers and 22 posters were presented small microseismic events or acoustic emissions occur naturally and a result of anthropogenic influences in reservoirs sudden stress release leads to elastic rock failure which serves as an effective seismic source these microearthquakes may be the result of production or hydraulic stimulation but they also may be a consequence of natural tectonic activity they are usually detectable using only sensitive sensors and after careful data processing such passive seismic monitoring has been used in mining settings for more than 100 years but its application in petroleum setting is relatively new as such it is a rapidly advancing field of technology where the challenges are manifold and span issues associated with data acquisition processing and interpretation much can be learned from methods and case studies developed for microseismic monitoring in volcanological geothermal and mining settings but one of the key advantages in oilfield monitoring is that a great deal is ordinarily already known about the reservoir comparatively good velocity models exist and production or injection information is available the same is obviously not true for a volcano the microearthquake or microseismic event location is useful in itself clusters of events delineate faults and fracturing highlighting reactivation or the generation of new zones of failure this has been very effective in helping assess the efficacy of hydraulic stimulation i e frac monitoring longer term monitoring will no doubt provide an early warning system for detecting top seal leakage and fault reactivation in co 2 sequestration projects for example until

a thermomechanical model of ice flow in west antarctica this study uses a three dimensional thermo mechanical model to investigate the internal flow dynamics of the west antarctic ice sheet wais the model allows ice thickness flow and temperature to interact freely its domain is prescribed as that of the present day grounded wais realistic present day climatic and topographical boundary conditions are employed the analysis of a series of experiments pays particular attention to the location and dynamics of concentrations of ice flow ice streams underlying topographic troughs are crucial in determining the strength and location of these concentrations of flow the flow pattern generated by subglacial troughs is made more distinct by the inclusion of ice flow temperature coupling the inclusion of sliding leads to the generation of limit cycles in the ice flow they are concentrated around the present day ice streams b and c of the siple coast and have a period of 5 to 10 ky there appears to be competition between several preferred ice flow pathways in this area the two end members of the flow regime are a strong ice stream c with a weakened ice stream a b complex and strong ice streams a and b with a dormant ice stream c ice streams appear to require ice discharges above a certain threshold in order to maintain frictional heat generation and fast flow individual ice streams can therefore interact through changes in catchment area size a reduction in catchment area reduces the volume of ice entering a stream and can cause stagnation as the amount of frictional heating falls

changes in west antarctic ice stream velocities observation and analysis 1 we have produced a map of velocity covering much of the siple coast ice streams the map confirms earlier estimates of deceleration on whillans ice stream comparison with bed elevation data indicates that subglacial topography and the location of consolidated sediment play a strong role in determining the location of the tributaries feeding the ice streams force balance estimates based on these data indicate that the tributaries have beds nearly an order of magnitude stronger than those beneath many of the ice streams we have used a theoretical analysis to examine the controls on fast flow this analysis suggests that ice plains very wide ice streams are inherently unstable this instability may be responsible for the current deceleration on the ice plain of whillans ice stream and the shutdown of ice stream c 150 years ago thinning induced reductions in driving stress may also explain some of the observed deceleration particularly in upstream areas the active portions of ice stream c coincide well with the areas where we estimate that melt should be taking place current topography and inferences of large thickening following a shutdown suggest the upstream migration of a stagnation front that initiated at the ice plain uncertainty remains about the basal conditions on ice stream d while the basal resistance on ice stream e is large enough to ensure basal melting

further testing of the bedding plane slip model for hydraulic fracture opening using moment tensor inversions abstractmoment tensors are calculated by using the p wave first motion peak amplitudes of 59 microseismic events with high signal to noise ratio these events are from a surface microseismic data set gathered during hydraulic fracture stimulation of the marcellus gas shale in washington county pennsylvania usa the majority of these 59 events have a horizontal nodal plane a few degrees characteristic of a dip slip horizontal slip moment tensor if the horizontal nodal plane is an auxiliary the vertical nodal plane has a pure dip slip motion which is inconsistent with the opening motion for vertical hydraulic fractures that enables proppant loading this points to slip on horizontal nodal planes with the auxiliary vertical nodal planes aligned with the local maximum horizontal stress orientation as indicated by drilling induced fractures in nearby vertical wells these 59 microseismic events are caused by slip on horizontal mechanical discontinuities such as bedding planes during the opening of ver

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

scattering and dip angle decomposition based on subsurface offset extended wave equation migration abstractan angle dependent reflection coefficient is recovered by seismic migration in the angle domain we have developed a postmigration technique for computing scattering and dip angle common image gathers cigs from seismic images extended by the subsurface offset based on wave equation migration methods our methodology suggests a system of radon transform operators by introducing local transform relations between the subsurface offset image and the angle domain components in addition to the commonly used decomposition of the scattering angle the methodology associates the wave equation migration with dip domain images as well the same postmigration subsurface offset image is used to decompose scattering and dip angle cigs individually or to decompose a multiangle cig by showing simultaneously both angles on the gather s axis we show that the dip angle response of seismic reflections is a spot like signature focused at the specular dip of the subsurface reflector it differs from the well stu

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

performance of landsat 8 operational land imager for mapping ice sheet velocity landsat imagery has long been used to measure glacier and ice sheet surface velocity and this application has increased with increased length and accessibility of the archive the radiometric characteristics of landsat sensors however have limited these measurements generally to only fast flowing glaciers with high levels of surface texture and imagery with high sun angles and cloud free conditions preventing wide area velocity mapping at the scale achievable with synthetic aperture radar sar the operational land imager oli aboard the newly launched landsat 8 features substantially improves radiometric performance compared to preceding sensors enhancing performance of automated repeat image feature tracking rift for mapping ice flow speed in order to assess this improvement we conduct a comparative study of oli and the landsat 7 enhanced thematic mapper plus etm performance for measuring glacier velocity in a range of surface and atmospheric conditions to isolate the effects of radiometric quantization and noise level we construct a model for simulating etm imagery from oli and compare rift results derived from each we find that a nonlinearity in the relationship between etm and oli radiances at higher brightness levels results in a particularly large improvement in rift performance over the low textured interior of the ice sheets as well as improved performance in adverse conditions such as low sun angles and thin clouds additionally the reduced noise level in oli imagery results in fewer spurious motion vectors and improved rift performance in all conditions and surfaces we conclude that oli imagery should enable large area ice sheet and glacier mapping so that its coverage is comparable to sar with a remaining limitation being image geolocation

the relationship between microseismicity and high pore pressures during hydraulic stimulation experiments in low permeability granitic rocks several hundred microearthquakes with local magnitudes ranging between 4 and 2 were recorded during hydraulic stimulation experiments at the hot dry rock demonstration site fenton hill new mexico these events are probably caused by shear failure induced by high pore fluid pressures since the event locations seem to cluster in a narrow band near the hydraulic fracture we were able to use micro seismic techniques to locate the hydraulic fracture and monitor its growth we calculate the minimum pore pressure increase necessary to induce rock failure using a simple model of slippage on preexisting fractures by comparing a pore pressure distribution calculated using a one dimensional diffusion model with the distribution of event foci we demonstrate that high pore pressures are a probable cause for these microearthquakes stress drops for these microearthquakes calculated from spectral parameters increased by a factor of 3 during the course of the experiment the stress drop increase correlates with a migration of seismic activity away from the injection well bore which suggests relatively low in situ shear stresses near the well bore caused by strain release in this region during previous pressurizations

comparison of surface and borehole locations of induced seismicity monitoring of induced microseismic events has become an important tool in hydraulic fracture diagnostics and understanding fractured reservoirs in general we compare microseismic event and their uncertainties using data sets obtained with surface and downhole arrays of receivers we first model the uncertainties to understand the effect of different acquisition geometries on location accuracy for a vertical array of receivers in a single monitoring borehole we find that the largest part of the final location uncertainty is related to estimation of the backazimuth this is followed by uncertainty in the vertical position and radial distance from the receivers for surface monitoring the largest uncertainty lies in the vertical position due to the use of only a single phase usually p wave in the estimation of the event location in surface monitoring results lateral positions are estimated robustly and are not sensitive to the velocity model in this case study we compare event location solutions from two catalogues of microseismic events one from a downhole array and the second from a surface array of 1c geophone our results show that origin time can be reliably used to find matching events between the downhole and surface catalogues the locations of the corresponding events display a systematic shift consistent with a poorly calibrated velocity model for downhole dataset for this case study locations derived from surface monitoring have less scatter in both vertical and horizontal directions

the integration of jers 1 and ers sar in differential interferometry for measurement of complex glacier motion complex glacier motion in the grove mountains region antarctica is measured using four pass differential synthetic aperture radar sar interferometry insar the components of the motion vector field are resolved using a 44 day separation japanese earth resources satellite 1 jers 1 insar pair and a european remote sensing satellite 1 2 ers 1 2 tandem insar pair the 44 day temporal baseline provides the sensitivity required to observe the range of ice motion around 8 10 m a 1 and the 1 day short baseline provides the best choice for glacier digital elevation model reconstruction it is remarkable that the scattering field of the jers 1 pair remained coherent over the long time interval and the interferometric fringes are clear the overall ice flow is from east to west downslope and towards lambert glacier the regional flow is obstructed by nunataks extending north south with two wide gaps two narrow glaciers flow past the nunataks and rejoin each other at the downstream end regional morphology and the resolved flow in the grove mountains area suggests that ice flow is channeled throughout this eastern flank of lambert glacier

remote sensing of ice motion in antarctica a review abstract the antarctic ice sheet holds 91 of the global ice mass making it the biggest potential contributor to global sea level rise in order to evaluate the ice sheet s present and future response to global climate change detailed information about spatial and temporal ice mass dynamics is required while ice sheet surface mass balance can be estimated from regional climate models ice discharge is often determined from spaceborne measurements of ice thickness and ice motion at selected flux gates the retrieval of ice motion is not only critical for analysing the rate of ice transport into the ocean but also for investigating the spatial and temporal pattern or the driving forcings of antarctic ice flow dynamics this study provides a comprehensive review of state of the art spaceborne ice velocity measurements in antarctica based on 201 scientific papers published over the last three decades our analysis revealed a dominant interest in local and regional scale investigations yet the launch of the ers 1 2 and radarsat 1 sar satellites in the 1990s dramatically increased data coverage over the antarctic continent and enabled the first circum antarctic velocity product to be published in the late 2000s in recent years the improved imaging capabilities of landsat 8 led to a shift towards using mainly optical satellite data and enabled great advances in circum antarctic velocity mapping data from the currently operating sentinel 1 2 missions was applied in fewer studies but will play a crucial role in future ice motion studies e g as part of esa cci projects in addition we found a growing interest in high temporal resolution velocity change analyses based on the reviewed studies we moreover summarized the highly dynamic and locally variable flow pattern over the antarctic continent being in accordance with varying local and regional driving mechanisms despite the great advances in spaceborne ice velocity mapping in antarctica we report some major drawbacks and discuss future challenges and requirements to start with our study uncovered strong variations regarding the spatial availability of investigations while we found an aggregation of studies over selected glaciers we report a major lack of studies in all three antarctic sectors additionally many velocity datasets were created with strongly heterogeneous output parameters and were provided at coarse temporal resolution or at point based measurement locations only future studies should consequently aim at creating more uniform ice motion products at defined spatial and temporal coverage and resolution standards and at analysing particularly the least studied glaciers

subglacial sediments as a control on the onset and location of two siple coast ice streams west antarctica laterally continuous subglacial sediments are a necessary component for ice streaming in the modern onset regions of the ice streams draining the siple coast of west antarctica on the basis of new seismic data combined with previous results we present geophysical results from seismic reflection and refraction experiments in the upper reaches of ice streams c and d that highlight continuous sedimentary basins within and upstream of the current onset regions of both ice streams with streaming ice overlying these sedimentary packages the subglacial environment changes from no sediment to discontinuous sediment to continuous sediment cover along a longitudinal profile from the ice sheet to tributary c1b along this same profile we observe a speedup of ice flow and then full development of the ice stream tributary ice stream d flows above a thick sedimentary package with an uppermost low seismic velocity zone indicative of soft till and the upglacier and lateral extensions of ice stream d are tightly constrained by the extent of continuous sediments the inland termination of these sediments suggests that future migration of high velocity low shear stress ice flow in these regions appears unlikely

uncertainties in passive seismic monitoring the use of passive seismic techniques to monitor oil field completion and production processes is on the rise stress changes induced by such reservoir activities as hydraulic fracturing water injection or fluid extraction will often result in failure of the rocks with a concurrent release of seismic energy in the form of compressional p and shear s waves passive seismic monitoring is based on recording these emitted waves and then using their arrival times to estimate the location of the failure events

simultaneous inversion of multiple microseismic data for event locations and velocity model with bayesian inference abstractwe have applied bayesian inference for simultaneous inversion of multiple microseismic data to obtain event locations along with the subsurface velocity model the traditional method of using a predetermined velocity model for event location may be subject to large uncertainties particularly if the prior velocity model is poor our study indicated that microseismic data can help to construct the velocity model which is usually a major source of uncertainty in microseismic event locations the simultaneous inversion eliminates the requirement for an accurate predetermined velocity model in microseismic event location estimation we estimate the posterior probability density of the velocity model and microseismic event locations with the maximum a posteriori estimation and the posterior covariance approximation under the gaussian assumption this provides an efficient and effective way to quantify the uncertainty of the microseismic location estimation and capture the correlation between the velo

surface morphology and internal layer stratigraphy in the downstream end of kamb ice stream west antarctica satellite images of kamb ice stream formerly ice stream c west antarctica reveal several long curved linear features lineations oriented sub parallel to the ice flow direction we use ground based radar to characterize the internal layer stratigraphy of these lineations and the terrains that they bound some lineations are relict ice stream shear margins identified by hyperbolic diffractors near the surface interpreted to be buried crevasses and highly disturbed internal layers at depth satellite images show another set of lineations outside the relict margins that wrap around the ends of the surrounding inter ice stream ridges internal layers beneath these lineations are downwarped strongly into a syncline shape the internal stratigraphy of the terrain between these lineations and the relict margins is characterized by deep hyperbolic line diffractors our preferred hypothesis for the origin of this terrain is that it was floating sometime in the past the deep hyperbolas are interpreted to be basal crevasses and the strongly downwarped internal layers mark the position of a relict grounding line our study shows that lineations and intervening terrains have different internal layer characteristics implying different origins differentiation between these features is not possible using satellite images alone

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

stagnation of ice stream c west antarctica by water piracy the dynamic behavior of the west antarctic ice sheet is of interest because of the possibility that it may change and cause rapid sea level rise attention is focused on the fast moving and rapidly responding ice streams that drain the ice sheet one of these ice stream c largely stopped about a century ago and some models for this shutdown postulate negative feedbacks that would tend to stabilize the ice sheet here new data are presented indicating that the slowdown of the ice stream is restricted to its lower part and occurred because of loss of lubrication on localized sticky spots at the bed of the ice stream the increased friction probably arises from a topographic accident of the glacier bed that has directed lubricating water to the neighboring ice stream b together with slow drawdown of the ice sheet rather than from any general stabilizing feedbacks