structural and stratigraphie features and ers 1 synthetic aperture radar backscatter characteristics of ice growing on shallow lakes in nw alaska winter 1991 1992 changes in ers 1 c band synthetic aperture radar sar backscatter intensity from ice growing on shallow tundra lakes at three locations in nw alaska are described ice core analysis shows that at all lakes on the coast at barrow the ice whether floating or frozen to the bottom includes an inclusion free layer overlying a layer of ice with tubular bubbles oriented parallel to the direction of growth the clear ice may also be overlain by a discontinuous layer of bubbly snow ice backscatter is low 16 to 22 db at the time of initial ice formation probably due to the specular nature of the upper and lower ice surfaces causing the radar pulse to be reflected away from the radar as the ice thickens during the autumn backscatter rises steadily once the ice freezes to the lake bottom regardless of the presence of forward scattering tubular bubbles low backscatter values of 17 to 18 db are caused by absorption of the radar signal in the lake bed for ice that remains afloat all winter the ice water interface and the tubular bubbles combine presumably via an incoherent double bounce mechanism to cause maximum backscatter values of the order of 6 to 7 db the saturates at 6 to 7 db before maximum ice thickness and tubular bubble content are attained a simple ice growth model suggests that the layer of ice with tubular bubbles need be only a few centimeters thick midway through the growth season to cause maximum backscatter from floating ice during the spring thaw a previously unreported backscatter reversal is observed on the floating and grounded portions of the coastal lakes but not on the lakes farther inland this reversal may be related to the ice surface topography and wetness plus the effects of a longer cooler melt period by the coast time series of backscatter variations from shallow tundra lakes are a record of 1 the development of tubular bubbles in the ice and by association changes in the gas content of the underlying water and 2 the freezing of ice to the bottoms of the lakes and therefore lake bathymetry and water availability sar is also able to detect the onset of lake ice growth in autumn and the initiation of the spring thaw and thus has potential for monitoring high latitude lake ice growth and decay processes in relation to climate variability

recent progress in snow and ice research a review of snow and ice research in 1987 1990 is presented focusing on the effects of layers in seasonal snow covers ice mechanics on fresh water and sea ice and remote sensig of polar ice sheets these topics provide useful examples of general needs in snow and ice research applicable to most areas such as better representation in models of detailed processes controlled laboratory experiments to quantify processes and field studies to provide the appropriate context for interpretation of processes from remote sensing

glaciological studies on nelson island south shetland islands antarctica the ice cap on nelson island in the south shetland islands west antarctica was studied between 1985 and 1989 the ice cap has an average thickness of 120 m it is temperate exists under the sub antarctic maritime climate and almost completely covers the island owing to intense percolation of meltwater and to some extent liquid precipitation the snow firn layer is in the soaked facies with a firn ice transition at a depth of 25 26 m at the summit a force balance model suggests that the ice is almost linearly viscous but has a high viscosity the model further suggests that basal sliding makes a larger contribution to the ice movement than does ice deformation from 1970 to 1988 the average accumulation rate was 120 kg m 2 a 1 at the centre and between 1985 and 1989 the equilibrium line elevation averaged 110 m a s l analysis of chemical impurities in the surface snow suggests that the precipitation source is mainly local marine air masses and that human activity has already exerted a detectable influence on the local environment

10 be in ice at vostok antarctica during the last climatic cycle we have previously reported accelerator mass spectrometer measurements of the cosmogenic isotope 10be half life 1 5 myr in an 906 m ice core recovered at dome c antarctica1 in addition to an enhanced concentration during a period of reduced solar activity maunder minimum ad 1645 1715 2 we found substantially increased concentrations during that part of the core corresponding to the last ice age both effects have been confirmed and extended in greenland ice cores3 4 the recovery of a 2 083 m ice core at vostok antarctica5 together with an extended oxygen isotope chronology reported elsewhere in this issue6 permits an extension of 10be studies over the whole of the last climatic cycle 125 kyr we report here such measurements which show an excellent correlation with the oxygen isotope and thus presumably climate record over the whole of this period the results imply that precipitation rates in the antarctic during the last interglacial were similar to those of the holocene but were roughly halved during the last glaciation

rapid wavelet based bathymetry inversion method for nearshore x band radars abstract a wavelet based method for bathymetry retrieval from x band radar images is proposed the method combines traditional fast fourier transform techniques for retrieving peak frequency maps by evaluating the spectral peaks in the time domain and a localized 2d continuous wavelet transform for retrieving the corresponding peak wavenumbers the main improvements of the new method compared to conventional fft based methods are as follows a the wavelet based approach is localized naturally fitting the inhomogeneous conditions typically found in the nearshore environment b it is continuous and uses wave phase information providing smooth bathymetry maps with good accuracy rmse in the range 2 6 6 6 of the maximum water depth c it requires relatively small number of successive images without the limitation of requiring a uniform time step in order to verify the proposed method 2d simulations of wave shoaling and refraction were performed for different sea conditions and over several bottom topographies a radar image model including tilt and shadowing modulations together with speckle noise was applied to the simulated surface elevations to provide synthetic radar images the method s ability to reconstruct the original bathymetry is shown to be robust in intermediate to shallow water depths kph

surface elevation contours of greenland and antarctic ice sheets surface elevations of the ice sheets are contoured at 50 m intervals for the region of greenland covered by seasat radar altimetry south of 72 n and at 100 m intervals for a region of east antarctica north of 72 s the surface elevations were obtained from computer retracking of the radar altimeter waveforms which were recorded at 0 1 s intervals corresponding to 662 m spacings on the surface the precision of the elevation measurements before adjustment for radial orbit errors is 1 9 m as shown by analysis of elevation differences at orbital crossover points this precision is partly determined by radial errors of approximately 1 0 m in orbit determination and partly by noise due to ice surface irregularities adjustment of the radial components of the orbits to minimize the differences in elevations at crossovers over a small relatively flat region reduces the rms difference to 0 25 m which is indicative of the optimum precision obtainable over the ice sheets however the precision degrades as the slope of the surface or amplitude of the undulations increases yielding an overall precision of 1 6 m the preliminary contour maps are not corrected for slope induced displacements a 2 m contour map in a region of highest data density illustrates the three dimensional characteristics of some surface undulations

seismic measurements reveal a saturated porous layer beneath an active antarctic ice stream seismic reflection studies recently conducted on ice stream b part of the marine ice sheet of west antarctica show a metres thick layer immediately beneath the ice in which both compressional p and shear s wave speeds are very low these low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure from this and from arguments presented in an accompanying paper1 to the effect that the layer would be too weak to support the shear stress exerted by the overlying ice we conclude that the layer is deforming and that the ice stream probably moves principally by such deformation

anisotropic damage mechanics as a novel approach to improve pre and post failure borehole stability analysis anisotropic damage distribution and evolution have a profound effect on borehole stress concentrations damage evolution is an irreversible process that is not adequately described within classical equilibrium thermodynamics therefore we propose a constitutive model based on non equilibrium thermodynamics that accounts for anisotropic damage distribution anisotropic damage threshold and anisotropic damage evolution we implemented this constitutive model numerically using the finite element method to calculate stress strain curves and borehole stresses the resulting stress strain curves are distinctively different from linear elastic brittle and linear elastic ideal plastic constitutive models and realistically model experimental responses of brittle rocks we show that the onset of damage evolution leads to an inhomogeneous redistribution of material properties and stresses along the borehole wall the classical linear elastic brittle approach to borehole stability analysis systematically overestimates the stress concentrations on the borehole wall because dissipative strain softening is underestimated the proposed damage mechanics approach explicitly models dissipative behaviour and leads to non conservative mud window estimations furthermore anisotropic rocks with preferential planes of failure like shales can be addressed with our model

mapping submarine sand waves with multiband imaging radar 2 experimental results and model comparison on august 16 1989 and on july 12 1991 experiments were performed to study the mapping of submarine sand waves with the airborne imaging radar a polarimetric and in 1991 interferometric airborne p l and c band synthetic aperture radar system the experiments took place in an area 30 km off the coast of the netherlands where the bottom topography is dominated by sand waves with a height between 2 and 6 m and a crest to crest distance of about 400 m at an average depth of 22 m ground measurements were recorded on a nearby platform and on a ship in the test area which also acted as a position fix on august 16 1989 the wind was 5 m s directed toward the northeast while the surface current velocity was around 0 5 m s directed toward the southwest one overflight was made with the flight direction parallel to the sand wave crests and the radar looking upwind at p band the sand waves are clearly visible as dark bands as predicted by theory while at l band the sand waves show up as sawtooth shaped modulations on july 12 1991 wind and surface current had the same opposite directions as in 1989 though the wind was much higher 10 m s three flights were made with the radar pointing upwind cross wind and downwind the upwind and downwind images are very similar despite the high wind speed the sand waves are clearly visible as sawtooth shaped modulations at p band and vaguely visible at l band at c band only wind streaks can be seen all cross wind images show the sand waves as dark bands now with the highest modulations at c band the wind streaks that dominated the upwind and downwind images at c band are much less pronounced in the cross wind images the images are compared with predictions from a new model of the imaging mechanism which includes contribution to the radar cross section of waves moving both from and to the radar wave blocking or wave reflection is treated in an approximate manner for the radar looking upwind or downwind the predicted modulations at p and l band agree well with the observations while those at c band are too high for the radar looking cross wind the model severely underestimates the modulations it is questioned whether a local relaxation source term can describe such a situation the interferogram shows some structure caused by bottom induced surface current variations

short pulse radar detection of groundwater in the sagavanirktok river floodplain in early spring short pulse radar operating in the uhf band was used to search for unfrozen water beneath ice blisters and within the gravels of the sagavanirktok river floodplain near prudhoe bay alaska the investigations were carried out in early april 1991 when daily mean air temperatures still were below 20 c the radar pulse spectrum was centered near 400 mhz and the radar time range was set to cover about 10 m depth the flat snow covered surface allowed good antenna ground coupling and the ice and gravels provided a low loss propagation medium that allowed detail of the gravel structure to be seen in the data grids of several traverses were profiled at three sites all within an area of about 0 5 km2 unfrozen water was inferred from the strength and polarization of the radar reflections one site contained a large icing blister beneath which an extensive reservoir was mapped and seen to extend several tens of meters beyond the mound data from two other sites indicated taliks 5 7 m deep near a frozen mound and within the gravels one of the profiles extended 700 m across the floodplain and revealed other high reflectivity targets which were presumed to be water within about 7 m of the surface a deeper sounding confirmed this depth limit it is concluded that taliks exist in the floodplain throughout the winter and are a probable source for the numerous icing mounds seen along most arctic rivers east of prudhoe bay

a signal processing algorithm for the extraction of thin freshwater ice thickness from short pulse radar data a radar data processing algorithm is presented which can estimate freshwater ice thicknesses both greater than and less than the normal minimum resolvable thickness the basis for the algorithm is a model of the geophysical system and the resulting phase signature of the transfer function the algorithm attempts to isolate this signature and estimate the relevant parameters in principle the method can be used to detect layering in other dielectric media the algorithm was based on a simple model for scattering from a thin ice layer and refined using radar returns taken over a range of thin ice thicknesses in a laboratory environment the method was also applied to data obtained with two different radar frequencies and over a wide range of thickness and surface roughness the results were compared to thickness estimates from classical deconvolution methods

a cross anisotropic elastoplastic model applied to sedimentary rocks abstract strength and deformability properties of sedimentary rocks are difficult to characterize as these geomaterials show anisotropic mechanical behavior which cannot be adequately evaluated by an isotropic constitutive model this essay proposes a cross anisotropic elastoplastic model to evaluate stress strain relationships of sedimentary rocks the approach couples the twelve parameter lade kim isotropic model with a non uniform scaling of the stress tensor and includes two scaling parameters that link the orientation of the bedding planes with the loading direction the anisotropic model parameters can be conveniently determined using bayesian analysis we illustrate our method using triaxial tests on a variety of confining stresses and bedding plane orientations for a travertine and a tournemire shale our results demonstrate that the predicted maximum deviatoric stresses and the simulated stress strain curves are shown to be in good agreement with the measured data

static and dynamic pressure sensitivity anisotropy of a calcareous shale optimizing the productivity of nonconventional low permeability shale reservoirs requires detailed knowledge of the mechanical properties of such materials these rocks elastic anisotropy is acknowledged but usually ignored due to difficulties in obtaining such information here we study in detail the dynamic and static elastic properties of a suite of calcareous mudstones from the nonconventional duvernay reservoir of alberta canada the complete set of transversely isotropic elastic constants is obtained from strategically oriented ultrasonic transducers to confining pressures of 90 mpa wave speed anisotropies of up to 35 are observed at even the highest confining pressures furthermore the stress sensitivity of the wave speeds and hence moduli is itself highly dependent on direction with speeds taken perpendicular to the bedding plane being highly nonlinearly dependent on pressure whereas those along the bedding plane show unexpectedly nearly no pressure dependence these observations are in qualitative agreement with the preferentially oriented porosity and minerals seen in scanning electron microscope images these results may be significant to the interpretation of sonic logs and azimuthal amplitude versus offset for principal stress directions for the concentration of stress within such formations and for estimation of static engineering moduli from sonic log wave speeds

west antarctic ice streams draining into the ross ice shelf configuration and mass balance extensive airborne radar soundings closely tied to accurately located ground stations made over a portion of the west antarctic inland ice sheet and the ross ice shelf have enabled us to map the boundaries of the ice streams and their flow bands on the ross ice shelf the surfaces of the active ice streams a and b are heavily crevassed but there are no visible crevasses on ice stream c however the existence of numerous crevasses at a depth of about 35 m implies that ice stream c ceased to be active about 250 years ago there is a complex zone at the head of ice stream b that suggests that ice stream b is currently widening and advancing toward the interior of the ice sheet the grounding lines between the inland ice and the ice shelf were mapped from the surface elevations using the requirement of hydrostatic balance and the character of the radar echoes the inland ice is more extensive than previously mapped and includes a large weakly grounded area downstream of ice streams a and b where the surface elevations are barely above hydrostatic equilibrium crary ice rise extends 70 km upstream of the previously mapped position to relate surface elevation data from satellite observations to sea level three geoidal models were tested we conclude that the gem 10c model is the best of the three for this section of antarctica a new map of surface elevation shows deep marginal troughs and longitudinal ridges along ice streams a and b but not on ice stream c whose surface contours show signs of stagnation ice stream c shows a strongly positive and ice stream b a significantly negative net balance rate ice streams a and d also show a significantly negative net flux whereas ice streams e and f do not the overall net balance of this part of the west antarctic inland ice is suggestively negative 23 15 km3 yr 1 from the measured flux into the ross ice shelf and previous measurements we calculate an average basal melt rate from beneath the ice shelf of 0 12 0 03 m yr 1

microwave backscatter from arctic lake ice and polarimetric implications polarimetric synthetic aperture radar satellite and ground based ku and x band scatterometer measurements are used to explore the scattering mechanism for ice in shallow arctic lakes wherein strong radiometric responses are seen for floating ice and low returns are evident where the ice has grounded scatterometer measurements confirm that high backscatter is from the ice water interface whereas polarimetric decomposition suggests that the dominant scattering mechanism from that interface is single bounce using fresnel equations a simple model for surface bounce from the ice water interface is proposed and its predictions are supported by experimental parameters such as co pol phase difference co pol ratio and the results of rigorous numerical modeling despite early research suggesting double bounce scattering from columnar air bubbles and the ice water interface as the dominant scattering mechanism in shallow lakes this paper strongly supports a single bounce model

use of c band ground penetrating radar to determine backscatter sources within glaciers the question of penetration of synthetic aperture radar sar signals at c band frequency into polar glaciers is addressed by comparing ground penetrating radar gpr and sar backscatter signatures profiles of the kongsvegen glacier svalbard were obtained with a c band gpr the received signal is converted to the equivalent radar cross section using the standard radar equation thus mapping the effective scattering sources within the glacier at this frequency the depth of the observed scattering sources is greatest in the superimposed ice where layers are clearly seen to a depth of approximately 14 m the very high scattering properties of the upper firn layers preclude layers deeper than approximately 6 m from being imaged integrating the radar cross sections over the depth gives a single backscatter value that we compare with the backscatter coefficient scattering cross section per unit area of the processed sar data for the same profile the comparison indicates that for coincidentally acquired gpr and sar data the radar cross section measured by the gpr does represent the features that contribute to the sar signal

an improved coherent radar depth sounder the university of kansas developed a coherent radar depth sounder during the 1980s this system was originally developed for glacial ice thickness measurements in the antarctic during the field tests in the antarctic and greenland we found the system performance to be less than optimum the field tests in greenland were performed in 1993 as a part of the nasa program for arctic climate assessment parca we redesigned and rebuilt this system to improve the performance the radar uses pulse compression and coherent signal processing to obtain high sensitivity and fine along track resolution it operates at a center frequency of 150 mhz with a radio frequency bandwidth of about 17 mhz which gives a range resolution of about 5 m in ice we have been operating it from a nasa p 3 aircraft for collecting ice thickness data in conjunction with laser surface elevation measurements over the greenland ice sheet during the last 4 years we have demonstrated that this radar can measure the thickness of more than 3 km of cold ice and can obtain ice thickness information over outlet glaciers and ice margins in this paper we provide a brief survey of radar sounding of glacial ice followed by a description of the system and subsystem design and performance we also show sample results from the field experiments over the greenland ice sheet and its outlet glacicrs

mass balance and ice flow law parameters for east antarctica on compare les donnees de vitessses et d epaisseurs obtenues sur des profils de la zone etudiee entre 90 e et 135 e au nord de 80 s aux debits d equilibre calcules spri on determine le rapport entre la vitesse moyenne et la vitesse basale les variations de la vitesse de deformation les debits montrent que cette region n est pas significativement desequilibree

seismic anisotropy in sedimentary rocks part 2 laboratory data part one of this paper presents a method for measuring seismic velocities and transverse isotropy in rocks using a single core plug this method saves at least two thirds of the time for preparing core samples and measuring velocities in transversely isotropic ti rocks using this method we have measured velocity and anisotropy of many shale and reservoir rocks from oil and gas fields around the world we present some of the data in this paper which include seismic velocity and anisotropy in 17 brine saturated shale samples 1 gas and brine saturated coal sample 8 brine saturated sands 12 gas saturated sands 32 gas saturated carbonate samples and 25 brine saturated carbonate samples the results show that clays and fine layering in sedimentary rocks are the main causes of seismic anisotropy very little intrinsic anisotropy exists in unfractured reservoir rocks such as sands sandstones and carbonates under reservoir conditions in contrast all shales were found seismically anisotropic anisotr

helicopter borne ground penetrating radar investigations on temperate alpine glaciers a comparison of different systems and their abilities for bedrock mapping abstracton the basis of a large data set comprising approximately 1200 km of profile lines acquired with different helicopter borne ground penetrating radar gpr systems over temperate glaciers in the western swiss alps we have analyzed the possibilities and limitations of using helicopter borne gpr surveying to map the ice bedrock interface we have considered data from three different acquisition systems including 1 a low frequency pulsed system hanging below the helicopter bgr 2 a stepped frequency system hanging below the helicopter radar systemtechnik gmbh rst and 3 a commercial system mounted directly on the helicopter skids geophysical survey systems incorporated gssi the systems showed considerable differences in their performance the best results were achieved with the bgr system on average the rst and gssi systems yielded comparable results but we observed significant site specific differences a comparison with ground based gpr data found that the quality of helicopter

reflection profiling of arctic lake ice using microwave fm cw radar x and c band fm cw radar reflection profiles were obtained across frozen lakes in northern alaska using a single elevated high gain antenna clear returns were obtained from the air snow snow ice ice water and ice ground interfaces surface to bottom signal intensity ratios are within ranges predicted by plane wave reflection theory use of which also gives plausible permittivity values for the ice rich bottom silts scattering losses are interpreted for the x band ice bottom signals but evidence of increased volumetric scattering loss with increasing ice thickness may have been masked by changes in bottom dielectric contrasts the results suggest that this type of radar is superior to conventional transient gpr systems for this application and ice grounded to almost any type of bottom sediments could be profiled from an airborne platform such a system could help inventory wetland water volume in northern regions

the elastic anisotrophy of shales shales constitute about 75 of the clastic fill sedimentary basins and have a decisive effect on fluid flow and seismic wave propagation because of their low permeability and anisotropic microstructure the elastic stiffnesses of a shale with partially oriented clay particles is expressed in terms of the coefficients w sub lmn in an expansion of the clay particle orientation distribution function in generalized legendre functions application is made to the determination of the anellipticity of shales for transverse isotrophy the anellipticity quantifies the deviation of the p wave slowness curve from an ellipse and is shown to depend on a single coefficient w sub 400 in the expansion of the clay particle orientation distribution function if w sub 400 is small the anellipticity may be neglected as is apparently the case for a near surface late tertiary shale studied by winterstein and paulson strongly aligned clay particles result in a positive value of w sub 400 and a positive anellipticity in agreement with the majority of the field measurements however less well ordered shales could have a significantly positive second moment w sub 200 but only a small positive or even negative value of w sub 400 for such shales the anellipticity would be small or negative despite amore preferred alignment of clay particles in the bedding plane numerical examples of clay particle orientation distribution functions leading to zero or negative anellipticity are given less

sea surface current features observed by doppler radar a coherent x band radar was operated from the shore to measure velocities at the sea surface the test site at a tidal inlet lister tief in the german bight lay protected from swell and long wind waves a vertical vertical antenna was used to obtain the backscattered radar signal caused by bragg scattering to cover a sector of the circle the antenna was rotated in 1dstrok steps at each step the data acquisition was performed to obtain a time series of the backscattered radar signal the maximum range is 1905 m with a range resolution of 7 5 m and an azimuthal resolution of 3 5diam maps of the averaged backscattered radar power and radial doppler velocities are obtained the radar backscatter maps show the modulation of the radar cross section due to changes in the surface roughness the radar doppler velocity maps show the modulations of the surface scatterer speed the maps show many features for example the impact of subsurface sand waves eddies current shears slicks ship wakes atmospheric effects and small scale current changes behind sheet pile walls as the radar backscatter is caused by short wind waves the wind influence on the doppler signal was calculated for a first attempt to obtain true current motions the results were compared with in situ current measurements by subsequently measuring the surfaces current from two radar positions and performing the wind correction the current vector fields are assessed

river discharge measurements by using helicopter mounted radar 1 the united states geological survey and the university of washington collaborated on a series of initial experiments on the lewis toutle and cowlitz rivers during september 2000 and a detailed experiment on the cowlitz river during may 2001 to determine the feasibility of using helicopter mounted radar to measure river discharge surface velocities were measured using a pulsed doppler radar and river depth was measured using ground penetrating radar surface velocities were converted to mean velocities and horizontal registration of both velocity and depth measurements enabled the calculation of river discharge the magnitude of the uncertainty in velocity and depth indicate that the method error is in the range of 5 percent the results of this experiment indicate that helicopter mounted radar can make the rapid accurate discharge measurements that are needed in remote locations and during regional floods

ultrasonic velocities in cretaceous shales from the williston basin compressional and shear wave velocities were measured in the laboratory from 1 bar to 4 kbar confining pressure for wet undrained samples of cretaceous shales from depths of 3200 and 5000 ft in the williston basin north dakota these shales behave as transversely isotropic elastic media the plane of circular symmetry coinciding with the bedding plane for compressional waves the velocity is higher for propagation in the bedding plane than at right angles to it and the anisotropy is greater for the 5000 ft shale for shear waves the sh wave perpendicular to bedding and the sv wave parallel to bedding propagate with the same speed which is about 25 percent lower than that for the sh wave parallel to bedding in general compressional and shear velocities are higher for the indurated 5000 ft shale than for the friable 3200 ft shale all velocities increase with in increasing confining pressure to 4 kbar the 3200 ft shale exhibits velocity hysteresis as a function of pressure whereas this effect is a

borehole stability analysis accounting for anisotropies in drilling to weak bedding planes abstract borehole instabilities pose significant challenges to drilling and completion operations particularly in regions with weak bedding planes and pre existing fractures where formations have strong anisotropies the bedding planes rock anisotropy and their impacts on horizontal stresses are considered in the proposed model to improve borehole stability modeling this improved model enables to calculate borehole failures and minimum mud weight along borehole trajectories with various drilling orientations versus bedding directions laboratory test data of rock compressive strengths are analyzed and a new correlation is developed to allow for predicting uniaxial compressive strengths in weak rocks from sonic velocities time dependent rock compressive strength is also examined to analyze the wellbore failure evolution with time the slip failure gradient in the weak planes is derived which can be used to model wellbore sliding shear failure in the planes of weakness the mud weight applied to prevent borehole shear failures in both intact rocks and ones with weak bedding planes can be obtained from the proposed model

hydrological discharges and motion of fels and black rapids glaciers alaska u s a implications for the structure of their drainage systems characteristics of the hydrology and motion of black rapids and fels glaciers alaska were observed from 1986 to 1989 hydrological measurements included stage electrical conductivity and suspended sediment concentration in the discharge stream of each glacier and were made at 0 5 1 h intervals continuously through most of the melt seasons variations in the glacier speed were monitored through the full year at a number of locations along the length of each glacier using time lapse photography 1 d time resolution strain meters 0 5 1 h resolution and seismometers set up to count acoustic emissions both glaciers show similar seasonal diurnal and short term event changes in hydrological discharges and ice speed the hydrological behavior is analyzed in terms of a fast sub system composed of surface streams moulins and large tunnels with discharge that responds rapidly and a slow sub system composed of heterogeneous small passageways through the ice and distributed over the bed that maintain approximately uniform discharge over a day the timing and amplitude of water discharge in the diurnal cycle indicate that roughly 10 40 of the water is routed directly into the fast system the remaining 90 60 of the water enters the slow system dilution of the solute discharged from the slow system by the variable discharge in the fast system results in changes in water discharge and solute concentration that are approximately equal in relative amplitude and inversely related a small time lag from discharge maximum minimum to solute minimum maximum suggests that the fast system is confined to roughly the lowermost 30 40 of the full glacier length the residence time of water in the fast system is short compared to 1 d the slow system contains both short and long residence time passages characteristics of the diurnal cycles are somewhat variable through the melt season but no systematic evolutionary patterns were discerned even though large changes in the mean discharges of water and solutes occur which suggests parallel evolution of the variables that control the response of the fast system events were characterized by contemporaneous increases in suspended sediment concentration in the discharge water and distinct changes in straining on the glaciers events caused by increases in melt or precipitation related to weather and events related to release from reservoirs internal to the glaciers could be distinguished based on the changes in electrical conductivity of the discharge water the correlated changes in sediment discharge and motion of the glaciers indicate that the events were associated with temporary modifications of the slow passages distributed over the bed that allowed enhanced sliding and access of basal water flow to erosion products hydrological differences between black rapids and fels glaciers can be explained by differences in the size of the glaciers if there is a difference in bed structure that explains the difference in dynamics surge black rapids glacier versus non surge fels glacier it does not affect the hydrological parameters that were observed

regime of the filchner ronne ice shelves antarctica seismic radio echo twerle balloon and oceanographic data of the filchner ronne ice shelves are analysed a large thin area in the central ronne ice shelf is found to differ from the morphology of the ross and filchner ice shelves this area is partly filled by basal saline ice which can be attributed to regional regelation caused by oceanic circulation

comparison of radio echo sounding 30 1000 mhz and high resolution borehole temperature measurements at finsterwalderbreen southern spitsbergen svalbard

radial profiling of the three formation shear moduli and its application to well completions near wellbore alteration in shear stiffnesses in the three orthogonal planes can be described in terms of radial variations of the three shear moduli or slownesses the three shear moduli are different in formations exhibiting orthorhombic or lower degree of symmetry as is the case in deviated wellbores in triaxially stressed formations these shear moduli are affected by factors such as overbalanced drilling borehole stress concentrations shale swelling near wellbore mechanical damage and supercharging of permeable formations the two vertical shear moduli c44 and c55 in an anisotropic formation with a vertical x3 axis are obtained from crossed dipole sonic data whereas the horizontal shear modulus c66 is estimated from borehole stoneley data the effective shear modulus c66 is smaller than the vertical shear moduli c44 or c55 in a poroelastic formation exhibiting high horizontal fluid mobility consequently analyses of radial profiling of the three shear moduli in a reasonably uniform lithology

flow of ice sheets in the vicinity of subglacial peaks although changes of surface slope across a subglacial peak can be explained at least semi quantitatively on a two dimensional basis in terms of gradients of longitudinal and shear stresses along a flow line we lack appreciation of flow in the third dimension which may tend to cause lower layers to flow around rather than over a sharp peak this paper collects and surveys evidence relevant to flow in the third dimension and discusses the processes involved particularly the evidence from radio echo layering and also discusses possible reasons for the lack of radio echo layers near the base of the antarctic ice sheet

comparison between analytical and 3d finite element solutions for borehole stresses in anisotropic elastic rock we present a rigorous validation of the analytical amadei solution for the stress concentration around an arbitrarily orientated borehole in general anisotropic elastic media first we revisit the theoretical framework of the amadei solution and present analytical insights that show that the solution does indeed contain all special cases of symmetry contrary to previous understanding provided that the reduced strain coefficients b11 and b55 are not equal it is shown from theoretical considerations and published experimental data that the b11 and b55 are not equal for realistic rocks second we develop a 3d finite element elastic model within a hybrid analytical numerical workflow that circumvents the need to rebuild and remesh the model for every borehole and material orientation third we show that the borehole stresses computed from the numerical model and the analytical solution match almost perfectly for different borehole orientations vertical deviated and horizontal and for several cases involving isotropic transverse isotropic and orthorhombic symmetries it is concluded that the analytical amadei solution is valid with no restriction on the borehole orientation or the symmetry of the elastic anisotropy

quiescent phase evolution of a surge type glacier black rapids glacier alaska u s a black rapids glacier a surge type glacier in the alaska range most recently surged in 1936 37 and is currently in its quiescent phase mass balance ice velocity and thickness change have been measured at three to ten sites from 1972 to 1994 the annual speed has undergone cyclical fluctuations of as much as 45 about the mean speed ice thickness and surface slope did not change enough to cause the speed fluctuations through changes in ice deformation which indicates that they are being driven by changes in basal motion the behavior of black rapids glacier during this quiescent phase is significantly different from that variegated glacier another well studied surge type glacier in alaska the present medial moraine configuration of black rapids glacier indicates that a surge could occur at any time however ice velocity data indicate that the next surge may not be imminent we believe that there is little chance that the next surge will cross and dam the delta river

1 4 ghz radar penetration and evidence of drainage structures in temperate ice black rapids glacier alaska u s a we have tested the ability of a 1 12 1 76 ghz bandwidth airborne frequency modulation continuous wave fm cw radar with an effective pulse duration of 3 ns to penetrate temperate ice of the ablation zone of black rapids glacier central alaska u s a we used high gain horn antennas to suppress clutter and tested over cold and nearly ideal surface conditions englacial horizons dipping to at least 60 m depth were found along three sections of one axial profile more narrow band 1 21 1 29 ghz low resolution 24 ns pulse duration profiles from a fourth section detected events at about 100 150 m depth comparative profiles recorded with a 100 mhz short pulse type radar reproduce the horizons of two of the sections and verify the penetration in all cases all horizons are composed of diffractions we interpret voids from the phase of the 100 mhz diffractions within one of the horizons the diffraction nature of the horizons the void interpretation and the proximity to a nearby and up glacier pothole field lead us to conclude that the horizons within two of the sections are meandering drainage channels a more complex branching structure with near surface horizons profiled within the third section much farther down glacier may also be a complex drainage system fed by near surface melting the fm cw signal to clutter noise ratios of some of the targets predict that they could be detected at 200 m depth in the 1 2 ghz range significant performance improvements at maximum vertical resolution could be achieved with higher gain antennas

high resolution radar mapping of internal layers at the north greenland ice core project existing accumulation maps with reported errors of about 20 are determined from sparsely distributed ice cores and pits a more accurate accumulation rate might be obtained by generating continuous profiles of dated layers from high resolution radar mapping of near surface internal layers in the ice sheet isochrones to generate such profiles we designed and developed an ultrawideband radar for high resolution mapping of internal layers in the top 200 m of ice and tested it at the north greenland ice core project drill site reflection profiles of 2 and 10 km length reveal horizons that we correlate with electrical conductivity measurement ecm recordings our results show that the radar determined depth of internal layers is within 2 m of that in an ice core collected at a nearby location preliminary frequency analyses of layer reflections reveal that the reflections are strongest at the 500 1000 mhz frequency range long term accumulation rate computed from radar data is within 5 of that obtained from snow pits

the dynamics of ice sheet outlets a comparison of data from aircraft altimetry landsat imagery and radia echo sounding has shown characteristic surface topographies associated with sheet and stream flow the transition between the two is abrupt and occurs at a step in the subglacial topography this marks the onset of basal sliding and high velocities caused by subglacial water it results in crevassed amphitheatre like basins round the head of outlet glaciers it is also the zone of maximum driving stress beyond which values decline rapidly as velocities increase this abrupt transition appears to be topographically controlled since basal temperatures are at the pressure melting point well inland of the change in regime the marie byrd land ice streams exhibit qualitative differences from other ice sheet outlets however the change to lower driving stresses is much more gradual and occurs several hundred kilometres inland such ice streams have particularly low surface slopes and appear in form and flow regime to resemble confined ice shelves rather than grounded ice the repeated association of the transition to rapid sliding with a distinct subglacial feature implies a stabilizing effect on discharge through outlet glaciers acceleration of the ice is pinned to a subglacial step and propagation of high velocities inland of this feature seems improbable rapid ice flow through subglacial trenches may also ensure a relatively permanent trough through accentuation of the feature by erosion this is concentrated towards the heads of outlet glaciers up stream of the region where significant basal decoupling occurs this may be a mechanism for the overdeepening of fjords at their inland ends and the development of very steep fjord headwalls

vostok ice core provides 160 000 year record of atmospheric co2 direct evidence of past atmospheric co2 changes has been extended to the past 160 000 years from the vostok ice core these changes are most notably an inherent phenomenon of change between glacial and interglacial periods besides this major 100 000 year cycle the co2 record seems to exhibit a cyclic change with a period of some 21 000 years

subaerial salt extrusions in iran as analogues of ice sheets streams and glaciers abstract ice h20 and salt halite nacl share many physical properties and resemble each other in hand specimens and subaerial gravity driven flows however while most significant bodies of ice accumulate in cold highlands and gravity spread where and soon after they form most significant bodies of salt accumulate in tropical marine basins and have to be buried by 1 km of other rocks before they flow buried salt is driven by differential loading into various categories of piercing structures known as diapirs many diapirs extrude onto the surface as sheets of allochthonous out of place salt thousands of sheets of allochthonous salt have been interpreted in over 35 basins worldwide in the last 25 years mainly in the toes of passive continental margins and in orogenic belts where some are 103 km2 in area most former salt sheets are now submarine or subsurface but several active examples are beautifully exposed in iran these were compared to ice glaciers soon after they were introduced to western science a comparison that has been neglected since here we update this analogy and use modern understanding of flowing ice and salt to examine the similarities and differences that might be mutually beneficial to both fields of study as well as to extraterrestrial scientists the profiles internal structures and fabrics in flowing bodies of ice and salt are sensitive gauges of the histories of their budgets of supply and loss however whereas snow merely compacts where it accumulates salt sheets are fed from below by already deformed salt when salt diapirs first emerge on land they extrude domes that mature to the profiles of viscous fountains that often feed glacier like flows known as namakiers after locally exhausting their deep source layers salt fountains spread to the profiles of viscous droplets normal for ice caps ice typically deforms at 80 usually 90 of its absolute melting temperature while most salt deforms at

significant wave height measured by coherent x band radar significant wave height is one of the most important parameters for characterizing ocean waves and essential for coastal protection shipping as well as off shore industry operations within this paper a robust method is introduced for retrieving significant wave heights from doppler speed measurements acquired with a coherent on receive marine radar the doppler velocity is caused by the surface scattering in the line of site of the radar to a huge extent its periodic component is induced by the orbital motions associated with surface waves the proposed methodology is based on linear wave theory accounts for projection effects caused by the fixed antenna look direction and was applied to a coherent on receive radar operating at x band with vertical polarization in transmit and receive to show the overall performance of the method a data set consisting of approximately 100 days of radar measurements was analyzed and used to retrieve significant wave heights comparisons to wave measurements collected by a wave rider buoy resulted in a root mean square rms error of 0 21 m and a bias of 0 m without any calibration parameters needed to further improve the accuracy of significant wave height a calibration factor needs to be accounted for which improves the rms error to 0 15 m with a negligible bias of 0 01 m

vostok ice core a continuous isotope temperature record over the last climatic cycle 160 000 years a continuous deuterium profile along the 160 000 year vostok ice core antarctica is interpreted in terms of atmospheric temperature changes this climatic record is the awaited terrestrial complement of the deep sea records supporting the existence of a relation between the pleistocene climate and orbital forcing

freezing rate effects on the physical characteristics of basal ice formed by net adfreezing a number of theoretical and empirical studies have indicated that many individual characteristics of ice formed by processes of net basal adfreezing may be sensitive to the rate of propagation of the freezing front through the reservoir concerned the effects of freezing rate on the the stable isotope chemistry and crystallography of ice in addition to the disposition and character of included debris and gas are reported unidirectional freezing through a cylindrical reservoir containing various water sediment mixtures has been conducted in the laboratory and the resulting cores analysed for debris and gas disposition and ice crystal size and fabric the data lend support to inferences drawn from studies concerned with specific ice properties and an idealized suite of characteristics is developed which may be diagnostic of basal ice formed by net adfreezing

internal geometry and evolution of moulins storglaciaren sweden depart ment of physical geog raphy university of stoc kholm s 106 91 stockholm sweden abstract the initial conditions needed for formation of moulins are crevasses and a supply of melt water water pouring into a crevasse may fill it until it ove rflows at the lowest point which is normally near the margin however as the crev asse deepens it intersects englacial chann els through which the water can drain these chann els may be finger tip tributaries in a dendritic system such as that described by shreve 1972 and observ ed by raymond and harrison 1975 when the cre vasse closes heat in the melt water keeps the conne ction open and a moulin is formed the englacial channel enlarges rapidly by melt ing utilizing mechanical energy released by the descending water descents into moulins and mapping of stru ctures exposed at the surface after many years of melting demonstrate that the drainage channels leading down from the bottoms of the moulins have incli nations of 0 450 from the vertical these channels trend in the dire ction of the original crevasse but appear to be deeper than the expected depth of the crevasse they have not

airborne river ice thickness profiling with helicopter borne uhf short pulse radar l objet de cette etude est d evaluer la capacite d un radar aeroporte dans la mesure d epaisseur de glace de riviere

target interaction with stratigraphy beneath shallow frozen lakes quarter wave resonances within gpr profiles resonances within ground penetrating radar gpr reflection profiles recorded over shallow frozen lakes located onanartilleryandbombingrangeininterioralaskaoriginate near ice bottom and freeze thaw interface horizons and are similar to those caused by shallow utilities the transmitted pulse spectra were centered near 280 mhz low pass filteringdifferentiatestheresonancesfrombackgroundscattering thetransittimesbetweentheironsetandthatoftheoverlying interface reflections correspond with spectral peaks predicted for layers with thickness of one in situ quarter wavelength which we call quarter wave resonance the peaks are below 160 mhz which is well below the transmitted pulse bandwidth and may occur as a modulation of a reflection sequence although the transit times cannot resolve both layer thickness and permittivity reasonable values of permittivity give overburden layers above the targets on the order of 10 cm which is consistent with the persistence of the resonances numerical models of metal objects beneath an interface between materials of strong permittivity contrast reveal that significant energy at the predicted frequencies requires cylinderdiameterstoolargetobeordnance however strong resonances at the predicted frequencies result fromflat metal sheets of 0 5 1 m dimension and so we interpret the resonances to be caused by nonhazardous targets late winter is an ideal time to survey for targets in this area because the ice affords repeatable positioning for the antennas and thermal high dielectric contrast interfaces are then well developed thebandwidthofthepulseweusedappearsideallysuitedfor bothresolutionoftheshallowstrataanddetectionoftheresonances thereby allowing the resonances to be checked againstquarter wavetheory

airborne radar stratigraphy and electrical structure of temperate firn bagley ice field alaska u s a helicopter borne 135 mhz short pulse radar profiles of the bagley ice field southern alaska u s a have been analyzed to determine the nature of radio wave propagation through crevassed stratified wet and draining firn diffractions characterize the firn horizons along the trunk and reflections characterize those of overlying snow fields dynamic stacking was used to form and determine firn depths and unstacked diffraction analysis to determine firn layer properties refractive indices range from 4 1 to 4 5 for the near surface from 1 7 to 2 1 at about 13 17 m depth along the main trunk and to 2 6 to 58 m depth within a snowfield average trunk values correspond with volumetric water contents of about 0 09 which agree with values measured for other glaciers the analysis of the airborne recorded diffractions is confirmed by deriving the approximate refractive index of water from a supraglacial lake bottom diffraction the lack of snowfield diffractions suggests that crevassing and not firn structure caused the trunk diffractions the reasonable values of the indices imply that the diffractions originated from single points or edges orthogonal to the profile transect and they predict low interlayer transmission losses the snowfield penetration suggests that several hundred meters might be penetrated in uncrevassed accumulation zones with improved system design

radar sensing of thin surface layers and near surface buried objects a robust ground penetrating radar gpr signal processing approach is developed and applied to the sensing of surface soil and ice water layers as well as near surface buried objects the principal technique relies on a reference set of waveforms which are tested for optimal matching with measured radar reflections to be analyzed in principle the reference set can be based on measurements as well as on model output with the latter employed even when layers are quite thin relative to the incident wavelength or pulse the approach provides accurate information on layer characteristics particularly thickness the method assumes a well defined transmit signal and hence is best used with antennas elevated shove the surface in tests using uhf pulses on lab and field ice and on thawing soil the system offers approximately an order of magnitude improvement in layer resolution relative to more traditional methods also in the process of its signal matching calculations the procedures provide a numerical indication of the reliability of each result in application to thawing soil simulations suggest that one can address a wide variety of conditions using quite a limited set of reference signals by detecting the thin layer effect from soil over a buried metallic object the system also locates near surface targets when their reflections cannot be separated in time from ground surface returns an alternative system also succeeds in detecting near surface objects under the same conditions by detecting wavelet dispersion this is done without reliance on specific details of transmit wavelet or reflected signals a mine like target is detectable in a wet clay soil when the ground is frozen but not when it is thawed

thickness profiling of freshwater ice using a millimeter wave fm cw radar a prototype broadband millimeter wave 26 5 to 40 ghz fm cw radar employing digital signal processing techniques has been developed for profiling the thickness of freshwater ice the radar was tested at elevations of up to 7 m above ice surfaces and at speeds up to 40 km h both from a surface vehicle and a helicopter the thickness of pond and river ice sheets between 3 cm and 35 cm thick with and without fresh snow cover and minimal surface roughness showed direct correlation with borehole thickness measurements losses due to volume scattering by imbedded air bubbles did not significantly affect system capability to discern the air ic and ice water scattering boundaries

investigation of borehole cross dipole flexural dispersion crossover through numerical modeling crossover of the dispersion of flexural waves recorded in borehole cross dipole measurements is interpreted as an indicator of stress induced anisotropy around a circular borehole in formations that are isotropic in the absence of stresses we have investigated different factors that influence flexural wave dispersion through numerical modeling we determined that for a circular borehole surrounded by an isotropic formation that is subjected to an anisotropic stress field the dipole flexural dispersion crossover is detectable only when the formation is very compliant this might happen only in the shallow subsurface or in zones having high pore pressure however we found that dipole dispersion crossover can also result from the combined effect of formation intrinsic anisotropy and borehole elongation we found that a small elongation on the wellbore and very weak intrinsic anisotropy can result in a resolvable crossover in flexural dispersion that might be erroneously interpreted as borehole stress induced anisotropy a thorough and correct interpretation of flexural dispersion crossover thus has to take into account the effects of stress induced and intrinsic anisotropy and borehole cross sectional geometry

evolution of stress induced borehole breakout in inherently anisotropic rock insights from discrete element modeling the aim of this study is to better understand the mechanisms controlling the initiation propagation and ultimate pattern of borehole breakouts in shale formation when drilled parallel with and perpendicular to beddings a two dimensional discrete element model is constructed to explicitly represent the microstructure of inherently anisotropic rocks by inserting a series of individual smooth joints into an assembly of bonded rigid discs both isotropic and anisotropic hollow square shaped samples are generated to represent the wellbores drilled perpendicular to and parallel with beddings at reduced scale the isotropic model is validated by comparing the stress distribution around borehole wall and along x axis direction with analytical solutions effects of different factors including the particle size distribution borehole diameter far field stress anisotropy and rock anisotropy are systematically evaluated on the stress distribution and borehole breakout propagation simulation results reveal that wider particle size distribution results in the local stress perturbations which cause localization of cracks reduction of borehole diameter significantly alters the crack failure from tensile to shear and raises the critical pressure rock anisotropy plays an important role on the stress state around wellbore which lead to the formation of preferred cracks under hydrostatic stress far field stress anisotropy plays a dominant role in the shape of borehole breakout when drilled perpendicular to beddings while a secondary role when drilled parallel with beddings results from this study can provide fundamental insights on the underlying particle scale mechanisms for previous findings in laboratory and field on borehole stability in anisotropic rock

a 150 000 year climatic record from antarctic ice during much of the quaternary the earth s climate has undergone drastic changes most notably successive glacial and interglacial episodes the past 150 kyr includes such a climatic cycle the last interglacial the last glacial and the present holocene interglacial a new climatic time series for this period has been obtained using 18 o data from an antarctic ice core

short pulse radar wavelet recovery and resolution of dielectric contrasts within englacial and basal ice of matanuska glacier alaska u s a wavelets transmitted by short pulse radar are recovered from continuous profiles and used to determine interfacial dielectric contrasts within the englacial and basal ice at the terminus area of matanuska glacier alaska u s a the field studies were in the ablation region where radar horizons could at some point be identified with interfaces between clear ice and air water or basal ice and were performed in early spring before drainage fully developed the profiles usedclosely spaced antennas with bandwidths centered near 50 and 400 mhz transmitted wavelets reflected from interfaces of known dielectric contrasts are used to establish a phase reference for other events from interfaces between unknown contrasts migration and fourier transform filtering are then applied to the profiles and shown to recover these wavelets from diffractions and reflections interfacial dielectric contrasts are determined from the relative phase of the wavelets near the terminus lake some basal ice events above 8 m depth are interpreted as voids further up glacier most englacial events are interpreted as voids but deeper localized reflectors and horizons to 90 m depth and within the basal zone are interpreted as voids water or debris phase cannot be determined for the basal substrate transition reflections recommendations are made for improving the wavelet recovery process and the quality of gpr migration

seismic anisotropy of shales shales are a major component of sedimentary basins and they play a decisive role in fluid flow and seismic wave propagation because of their low permeability and anisotropic microstructure shale anisotropy needs to be quantified to obtain reliable information on reservoir fluid lithology and pore pressure from seismic data and to understand time to depth conversion errors and non hyperbolic moveout a single anisotropy parameter thomsen s parameter is sufficient to explain the difference between the small offset normal moveout velocity and vertical velocity and to interpret the small offset avo response the sign of this parameter is poorly understood with both positive and negative values having been reported in the literature is sensitive to the compliance of the contact regions between clay particles and to the degree of disorder in the orientation of clay particles if the ratio of the normal to shear compliance of the contact regions exceeds a critical value the presence of these regions acts to increase and a change in the sign of from the negative values characteristic of clay minerals to the positive values commonly reported for shales may occur misalignment of the clay particles can also lead to a positive value of for transverse isotropy the elastic anisotropy parameters can be written in terms of the coefficients w 200 and w 400 in an expansion of the clay particle orientation distribution function in generalized legendre functions for a given value of w 200 decreasing w 400 leads to an increase in while for fixed w 400 increases with increasing w 200 perfect alignment of clay particles with normals along the symmetry axis corresponds to the maximum values of w 200 and w 400 given by w max 200 10 8 2 and w max 400 3 2 8 2 a comparison of the predictions of the theory with laboratory measurements shows that most shales lie in a region of the w 200 w 400 plane defined by w 400 w 200 w max 400 w max 200

dielectric constant and layer thickness interpretation of helicopter borne short pulse radar waveforms reflected from wet and dry river ice sheets the objective of the study described was to demonstrate that radar ice thickness profiling is possible under some wet conditions radar data for both wet and dry ice were extracted from surveys performed from a helicopter operating at an altitude of about 2 7 m and a speed of about 5 m s the radar used a broadband wavelet of several nanoseconds duration at a center frequency of about 500 mhz by use of plane wave theory individual waveforms within particular echo scans are analyzed to interpret a value for the solid ice dielectric constant and thus ice thickness the latter values of which correlate with some very limited observations given this dielectric constant the theory is then used to reproduce the observed effects of the melt upon the radar reflections and this to predict the range of melt conditions over which ice thickness may still be interpreted

borehole evidence for a thick layer of basal ice in the central ronne ice shelf the ronne ice shelf floating between berkner island and the antarctic peninsula is the biggest ice shelf in antarctica it drains an estimated 1 2 106km2 of the antarctic ice sheet much of it resting on bedrock below sea level consequently the balance and dynamics of this ice shelf is of importance to antarctic glaciology especially with regard to the integration of this part of the cryo sphere into the global processes that control the climate of the earth extensive radio echo sounding res by robin and others revealed reflections in the central part of the ronne ice shelf at the relatively shallow depth of 100 200 m below surface1 the interpretation of these echoes which varied in strength was ambiguous and the possibility of internal reflecting horizons was thoroughly discussed but after surface elevation measurements by radar altimeter from drifting balloons appeared to fit the presence of thin ice it was decided to base a thickness map of the ronne ice shelf on these res echoes1 4 we now present direct observational evidence from boreholes that the total ice thickness is much greater than mapped and that the shallow res reflections therefore do come from internal horizons

longitudinal variations in glacial flow theory and test using data from the byrd station strain network antarctica application de la theorie de budd 1970 des effets du lit sur les variations d ecoulement du glacier aux observations de la calotte de glace a la station byrd on utilise une rheologie visqueuse lineaire et on resout une equation biharmonique des variations des efforts et des vitesses de deformation les conditions aux limites comportent une topographie du fond avec ondulations glissements et cisaillements

experimental laboratory determination of the dynamic elastic properties of wet drained shales the ultrasonic anisotropic elastic properties of drained saturated shales were measured as a function of confining pressure two shales were characterized in this study a jurassic outcrop shale retrieved under the sea in a saturated state and a kimmeridge clay shale cut from a north sea borehole both shales were highly anisotropic both texturally as revealed by scanning electron microscopy analysis and elastically as measured ultrasonically in the laboratory the strongest anisotropy was seen in the kimmeridge clay shale where up to 38 compressional wave anisotropy thomsen s parameter e and up to 58 shear wave anisotropy thomsen s parameter were observed in addition for both shales e and were found to decrease as a function of increasing confining pressure with the pore pressure drained to atmosphere while the anellipticity deviation of the slowness surfaces from ellipses was found to be positive and decreased as a function of increasing confining pressure accompanying these changes in elastic properties was a decrease in porosity with increasing confining pressure from 10 5 to 8 5 for the jurassic shale the decrease in overall anisotropy of the shales with increasing confining pressure was found to be consistent with theoretical modeling of shale properties where the shale anisotropy and anellipticity were predicted to decrease as a function of decreasing fluid filled porosity

strong spatial variability of snow accumulation observed with helicopter borne gpr on two adjacent alpine glaciers the spatial distribution of snow accumulation plays an essential role for the mass balancedistribution on alpine glaciers traditional point measurements snow pits and probes are labour intensive and interpolation in between the points causes uncertainties airborne radar measurements have already been used for snow mapping innon glacierized terrain but not on alpine glaciers to enhance our understanding ofthe spatial distribution of accumulation and pin down reasons for observed variations we have conducted high resolution helicopter borne radar measurements on the temperatedglacier findel and neighbouring adler glacier in southwestern switzerland the radar sensor was mounted underneath a helicopter and operated at a centre frequencyof 500 mhz with a bandwidth of 400 mhz the results were validated withextensive ground based profiling of the snow cover the radar data allows a clearrecognition of the snow cover 6 of the total profile length of 10 km did not allowinterpretation due to missing or disturbed layering and agreed well with the groundbased measurements r2 0 85 reduced accumulation has been observed in allcrevassed zones statistical analysis of the correlation between observed accumulationand terrain characteristics have been performed in a gis environment revealing differingaccumulation patterns on the lower part of findel glacier accumulation showsa clear altitudinal trend while the upper part is dominated by strongly varying snowdepth without an altitudinal trend the accumulation characteristic on adler glacieris similar to the upper part of findel glacier but despite of their close vicinity accumulationis reduced by 40 compared to the same elevation on findel glacier thisstudy reveals a large potential of helicopter borne snow profiling for measurements ofaccumulation distribution on alpine glaciers

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr