rheology of till beneath storglaciaren sweden in order to study in situ the rheology of a deforming subglacial till various instruments were emplaced in till beneath storglaciaren sweden boreholes were used to gain access to the till beneath about 100 m of ice tiltmeters provided an estimate of the shear strain rate in the till two other instruments yielded measures of till strength in addition water pressures were recorded in boreholes and in the till a computer controlled distance meter provided an effectively continuous record of the surface velocity and data from frequent surveys of a stake network were used to estimate the mean basal drag based on a force balance calculation tilt rates varied directly with effective pressure so decreases in water pressure apparently increased the coupling between the glacier and the bed surface speed was either out of phase with tilt or varied independently of tilt thus increases in speed were apparently a consequence either of longitudinal coupling or of reduced coupling between the glacier and the bed they were not a result of till deformation till strength varied directly with effective pressure which is consistent with it being a mohr coulomb or frictional material the devices measuring till strength are presumed to have been pulled through the till at a speed that varied in phase with the surface speed but till strength did not vary systematically with surface speed this implies that the residual strength of the till is insensitive to strain rate thus the appropriate constitutive equation for till rheology may be of the form e e k where k is a constant this is consistent with experimental data reported in the geotechnical literature

sliding versus till deformation in the fast motion of an ice stream over a viscous till the partitioning between till deformation and sliding in the fast flow of ice streams with active basal melting is examined assuming no adhesion of the till to the ice base and incompressible viscous fluid behavior for the till for deforming till thickness of 10 m or less the predicted contribution to basal motion by sliding is larger than shearing in the till unless there is short scale roughness with wavelengths less than order 0 1 m on the ice sole at such short scales strain heating within the till and focused melting on the ice sole would quickly eliminate the roughness thus fast flow over a till bed would be expected to be mostly by sliding over the subglacial till more realistic continuum behavior of the till including non linear and compressible deformation strengthens the conclusion if sliding is not dominant then there must be adhesion of the till to the ice base some mechanism that continuously generates short scale roughness on the ice till interface or very weak internal slip boundaries within the till

the attenuation of seismic shear waves in quaternary alluvium in santa clara valley california we used shear waves generated by an air powered source at the ground surface and recorded in a borehole to estimate the shear wave quality factor at strong motion station gilroy no 2 we find similar values of q using both the decay of the spectra with depth and the slope of the spectral ratio at two depths we find no evidence of a frequency dependence of q the mean value of q over the depth range 10 to 115 m is close to 10 the use of this value over the depth of the borehole and the observed travel time of 0 358 sec gives a cumulative attenuation factor t of 0 036 sec for the upper 180 m of the quaternary alluvium this is comparable to the differential decay between gilroy no 2 and a rock site 1 9 km away gilroy no 1 as measured from the decay of the high frequency spectra of accelerograms from large earthquakes plotted on a log linear scale t 0 05 0 04 and 0 03 sec for the 1979 coyote lake 1984 morgan hill and 1989 loma prieta earthquakes respectively the similarity between the attenuations measured from the low strain surface source and those from the larger amplitude earthquake sources suggests that increases of damping due to nonlinear wave propagation effects are limited

till beneath ice stream b 4 a coupled ice till flow model a nonsteady model of an ice stream flowing on deforming till shows that the system responds rapidly and in a stable manner to reasonable marginal perturbations for the model we require one dimensional flow and continuity of ice and till and we assume linear viscous till rheology and balance between the driving stress for ice flow and the resistive stress at the bed this allows us to write coupled equations for the time rate of change of the ice thickness and the till thickness in terms of these thicknesses and the till viscosity an analytic steady state solution for ice stream b west antarctica shows that till viscosity decreases slowly downstream probably in response to decreasing effective pressure downstream nonsteady numerical experiments with fixed ice thicknesses at the ends show that a marginal perturbation causes a wave of adjustment to travel the length of the ice stream in about 50 years with a new steady state in about 200 years changes in till thickness tend to moderate perturbations in the ice and thus stabilize the system

implementation of 3 d prestack kirchhoff migration with application to data from the ouachita frontal thrust zone a 3 d prestack kirchhoff migration is implemented and successfully applied to a 3 d data set from the ouachita frontal thrust zone in southeastern oklahoma the algorithm uses data decomposition and operates efficiently both in multiprocessor vector and parallel environments image time maps are precomputed once for each survey point regardless of how many times it is a source or receiver and stored in random access disk files these files and the data traces are read asynchronously during migration image time computation is done by two point ray tracing to avoid interpolation between irregularly spaced rays or points on wavefronts the same software can be used to depth migrate all input data geometries stacked sections common source or common receiver or common offset or common midpoint gathers the output images from the ouachita field data show structural closure on folded thrust sheets of wapanucka limestone and repetition of lithology caused by lateral displacement on the thrusts

sedimentary processes at the base of a west antarctic ice stream constraints from textural and compositional properties of subglacial debris samples of sediments from beneath ice stream b at camp upb west antarctica provide the first opportunity to study the relationship between sediment properties and physical conditions in a sub ice stream environment piston coring in holes bored by hot water drilling yielded five 1 3 m long undisturbed subglacial sediment cores we analyzed granulometry composition and particle morphology in these cores the upb cores are composed of a clay rich unsorted diamicton containing rare marine diatoms sedimentary particles in these cores bear no evidence of the recent crushing or abrasion that is common in other subglacial sedimentary environments the presence of reworked diatoms and their state of preservation as well as the relative spatial homogeneity of this diamicton suggest that the upb cores sampled a several meter thick till layer and not in situ glacimarine sediments the till does incorporate material recycled from the subjacent poorly indurated tertiary glacimarine sediments of the ross sea sedimentary basin which extends beneath this part of the west antarctic ice sheet we propose that the lack of significant comminution in the upb till is ultimately due to its setting over these easily erodible clay rich source sediments the resulting fine grained till matrix inhibits glacial comminution because it facilitates buildup of high pore water pressures and hinders interparticle stress concentrations our observations are consistent with the conjecture that subglacial deformation of weak fine grained tills does not produce significant comminution of till debris elson 1988 based on our findings we hypothesize that extensive layers of weak till may develop preferentially where ice overrides preexisting poorly indurated fine grained sediments since such weak till layers create a permissive condition for ice streaming sub glacial geology may have an indirect but strong control over the location extent and basal mechanics of ice streams

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

deforming beds viscous and plastic scales of deformation small scale field and laboratory observations of sub glacial sediment deformation suggest that failure is the predominant method of sediment deformation while large scale modelling studies using a viscous model of till deformation have been reasonably successful in predicting the geological consequences of ice sheet action this suggests that the cross over scale between viscous and plastic deformation occurs at a much larger scale than the grain grain interaction scale previously thought to be appropriate to deforming till sediment deformation and the drainage of sub glacial areas are considered at different scales and how these interact to produce failure and viscous behaviour is discussed a fluctuation length scale is proposed whose horizontal dimensions are of the order of the depth of the base of deformation 1 10 m on this length scale the glacier deforming bed system is highly variable in time as well as in space till deformation occurring at this length scale is plastic happening though failure events three other specific scales are proposed whose different properties depend on the relative spatial variability of static and seepage pressures and topography the key theoretical problem which has yet to be solved is how multiple small scale failure events combine into a viscous type flow this is discussed in the context of self organised criticality it is argued that the length scales appropriate for some drainage features are not appreciably larger than the scale for plastic failure events implying that modelling sub glacial drainage channels by balancing erosion rates against viscous closure of the channel is misconceived the outlines of an alternative model involving percolation theory are presented certain key questions which can only be settled through observations by glaciologists and glacial geologists are discussed it is suggested that the difference between lodgement tills and deformation tills reflects the rate of motion of till by deformation and that lodgement from the ice is unimportant

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

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

tidal forcing of basal seismicity of ice stream c west antarctica observed far inland the seismicity rate beneath the downglacier 85 km of ice stream c west antarctica is modulated by the tide the tide beneath the ross ice shelf modifies the force balance of the ice stream basal environment enough to change the rate of basal microearthquake generation by an order of magnitude this tidal forcing travels up the ice stream as an attenuating wave at approximately 1 6m s 1 and is detectable 85km from the grounding line we successfully model this behavior as an elastic ice stream underlain by a viscous substrate of viscosity and thickness hb and calculate that the substrate has an apparent stiffness hb of o 108 pa s m 1 this finding suggests that the conditions of the till layer at the bed of ice stream c are similar to those of ice stream b and that the reason for the recent stagnation of ice stream c is other than loss of till we further find that the ice stream at the grounding line is more strongly affected by ice shelf processes than by the basal shear stress

west antarctic ice sheet collapse charles r bentley perspectives 21 feb p 1077 1 suggests that a useful estimate of the probability of a west antarctic ice sheet collapse in the next 100 years can be obtained by assuming it to be a random event occurring once every 100 000 years he bases this view on results of a single

ring shear studies of till deformation coulomb plastic behavior and distributed strain in glacier beds a ring shear device was used to study the factors that control the ultimate steady strength of till at high shear strains tests at a steady strain rate and at different stresses normal to the shearing direction yielded ultimate friction angles of 26 3 and 18 6 for tills containing 4 and 30 clay sized particles respectively other tests at steady normal stresses and variable shear strain rates indicated a tendency for both tills to weaken slightly with increasing strain rate this weakening may be due to small increases in till porosity these results provide no evidence of viscous behavior and suggest that a coulomb plastic idealization is reasonable for till deformation however viscous behavior has often been suggested on the basis of distributed shear strain observed in subglacial till we hypothesize that deformation may become distributed in till that is deformed cyclically in response to fluctuations in basal water pressure during a deformation event transient dilation ofdiscrete shear zones should cause a reduction in internal pore water pressure that should strengthen these zones relative to the surrounding till a process called dilatant hardening consequent changes in shear zone position when integrated over time may yield the observed distributed strain

three dimensional finite difference viscoelastic wave modelling including surface topography summary i have undertaken 3 d finite difference fd modelling of seismic scattering fromfree surface topography exact free surface boundary conditions for arbitrary 3 d topographies have been derived for the particle velocities the boundary conditions are combined with a velocity stress formulation of the full viscoelastic wave equations a curved grid represents the physical medium and its upper boundary represents the free surface topography the wave equations are numerically discretized by an eighth order fd method on a staggered grid in space and a leap frog technique and the crank nicholson method in time i simulate scattering from teleseismic p waves by using plane incident wave fronts and real topography from a 60 60 km area that includes the noress array of seismic receiver stations in southeastern norway synthetic snapshots and seismograms of the wavefield show clear conversion from p to rg short period fundamental mode rayleigh waves in areas of rough topography which is consistent with numerous observations by parallelization on fast supercomputers it is possible to model higher frequencies and or larger areas than before

irregular oscillations of the west antarctic ice sheet model simulations of the west antarctic ice sheet suggest that sporadic perhaps chaotic collapse complete mobilization of the ice sheet occurred throughout the past one million years the irregular behaviour is due to the slow equilibration time of the distribution of basal till which lubricates ice sheet motion this nonlinear response means that predictions of future collapse of the ice sheet in response to global warming must take into account its past history and in particular whether the present basal till distribution predisposes the ice sheet towards rapid change

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

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

geologic and topographic controls on fast flow in the laurentide and cordilleran ice sheets ice streams are fast flowing arteries which play a vital role in the dynamics and mass balance of present day ice sheets although not fully understood fast flow dynamics are intimately coupled with geologic topographic thermal and hydrologic conditions of the underlying bed these are difficult observables beneath contemporary ice sheets hindering elucidation of the processes which govern ice streaxn behavior for past ice sheets the problem is antithetic geologic evidence of former ice streams exists but spatim and temporal histories are uncertain however detailed knowledge of bed geology and topography is available in many places we take advantage of this information to compile terrain characteristics relevant to fast flow dynamics in the laurentide and cordilleran ice sheets using seed points where fast flowing wisconsinan ice has been geologically inferred discriminant analysis of a suite of north american geologic and topographic properties yields a concise measure of ice bed coupling strength our analysis suggests that the interior plains and continental shelf regions of north america have low basal coupling relative to areas of variable relief or exposed bedrock in the cordillera and on the canadian shield we conclude that the interior plains and continental shelves are both topographically and geologically predisposed to large scale basal flows i e ice streaxns or surge lobes this result holds independent of whether the mechanism of fast flow is sediment deformation or decoupled sliding over the bed

a ring shear device for the study of till deformation tests on tills with contrasting clay contents abstract deformation of subglacial sediment may have had a profound influence on the geomorphic effects and dynamics of pleistocene ice sheets but data that bear on the rheology of such sediment are few and contradictory a means of studying systematically the mechanical properties of glacial tills is provided by a ring shear device that shears a large annular specimen to high strains at variable rates special features of the device allow continuous observation of the distribution of shear strain measurement of local stresses normal to the direction of shear and isolation of wall effects thus far a linear viscous putty and two tills with different clay contents 4 and 32 by weight have been tested the experiments with till indicate as expected that the presence of dispersed clay minerals reduces markedly both the residual strength and hydraulic diffusivity of till clay also reduces spatial variations in normal stress associated with grain bridges the walls bounding the specimen support only 9 17 of the total resistance to shearing strain was localized in both tills but not in the linear viscous putty strain localization in the tills indicates that their theology departs significantly from linear and bingham viscous models

the role of lateral drag in the dynamics of ice stream b antarctica the partitioning of resistive force between the bed and sides of ice stream b antarctica is obtained for three large areas that have been measured using repeat aerial photogrammetry problems associated with data errors and local variations in ice strength and velocity are reduced by considering the areally averaged budget of forces for each photo block results indicate that the bed under ice stream b must be very weak and unable to provide muche resistance mechanical control on this ice stream emanates almost entirely from the lateral margins

viscoelastic seismic responses of 2d reservoir models the use of relaxation mechanisms has recently made it possible to simulate viscoelastic q effects accurately in time domain numerical computations of seismic responses as a result seismograms may now be synthesized for models with arbitrary spatial variations in compressional and shear wave quality factors q sub p and q sub s as well as in density rho and compressional and shear wave velocities v sub p and v sub s reflections produced by q contrasts alone may have amplitudes as large as those produced by velocity contrasts q effects including their interaction with v sub p v sub s and rho contribute significantly to the seismic response of reservoirs for ban limited data at typical seismic frequencies the effects of q on reflectivity and attenuation are more visible than those on dispersion synthetic examples include practical applications to reservoir exploration evaluation and monitoring q effects are clearly visible in both surface and offset vertical seismic profile data thus avo analyses that neglect q may produce erroneous conclusions

2d surface topography boundary conditions in seismic wave modelling new formulations of boundary conditions at an arbitrary two dimensional 2d free surface topography are derived the top of a curved grid represents the free surface topography while the grid s interior represents the physical medium the velocity stress version of the viscoelastic wave equations is assumed to be valid in this grid however the rectangular grid version attained by grid transformation is used to model wave propagation in this work in order to achieve the numerical discretization we show the detailed solution of the particle velocities at the free surface resulting from discretizing the boundary conditions by second order finite differences fds the resulting system of equations is spatially unconditionally stable the fd order is gradually increased with depth up to eighth order inside the medium staggered grids are used in both space and time and the second order leap frog and crank nicholson methods are used for time stepping we simulate point sources at the surface of a homogeneous medium with a plane free surface containing a hill and a trench applying parameters representing exploration surveys we present examples with a randomly realized surface topography generated by a 1d von karman function of order 1 viscoelastic simulations are presented using this surface with a homogeneous medium and with a layered randomized medium realization all generating significant scattering

3 d viscoelastic anisotropic modeling of data from a multicomponent multiazimuth seismic experiment in northeast texas three component seismic data were acquired along six azimuths at a site in northeast texas sources were inclined impacts combined to produce equivalent vertical and crossline horizontal components the data were analyzed by iterative modeling using a new 3 d staggered grid finite differencing scheme the 3 d model is defined in terms of spatially variable anisotropic viscoelastic parameters p velocity s velocity density qp and qs and converted to elastic tensor components and relaxation times for computation simultaneous fitting of amplitudes of near surface reverberations surface waves converted waves and deeper reflections gives an anisotropic viscoelastic model that provides constraints on earth properties the assumption of transverse isotropy in previous experiments is supported by the results of this experiment the final model consists of a stack of flat layers each of which has isotropic p velocity qp and qs the wills point and kincaid shales are strongly transversely isotropic

how glaciers entrain and transport basal sediment physical constraints abstract simple insights from the physics of ice water and sediment place constraints on the possible sediment transport behavior of glaciers and ice sheets because glaciers concentrate runoff streams generated by glaciers transport much sediment and may erode bedrock rapidly deforming glacier beds also can transport much sediment particularly in marginal regions rapid sediment entrainment producing thick debris rich basal zones may occur by regelation into subglacial materials and by freeze on from rising supercooled waters numerous other mechanisms may be important but primarily near ice margins especially those of advancing or fluctuating glaciers several sediment entrainment mechanisms may be active beneath a single glacier but one process is likely to be dominant at any place and time

pleistocene collapse of the west antarctic ice sheet some glacial sediment samples recovered from beneath the west antarctic ice sheet at ice stream b contain quaternary diatoms and up to 108 atoms of beryllium 10 per gram other samples contain no quaternary diatoms and only background levels of beryllium 10 less than 10 6 atoms per gram the occurrence of young diatoms and high concentrations of beryllium 10 beneath grounded ice indicates that the ross embayment was an open marine environment after a late pleistocene collapse of the marine ice sheet

large scale ice moulding a discussion of genesis and glaciological significance abstract landsat images reveal a previously unsuspected large scale pattern of streamlining within drift which comprises a number of components drumlins and megaflutes form part of the pattern but in addition there are two previously undocumented ice moulded landform elements streamlined lineations of much greater proportions referred to as mega scale glacial lineations and a distinctive cross cutting topology within the grain consideration of the genesis and glaciological significance of such landforms leads to a number of conclusions it is suggested that mega scale glacial lineations were formed under conditions of fast ice flow and their presence may thus record former locations of ice streams or surge events many lineations of varying scales display a pattern that reveals that they were not formed in sub marginal positions extensive sets of lineations must have been formed approximately synchronously thus indicating that lineation generation occurs over a wide range of glaciodynamic conditions from sub marginal positions to interior portions of ice sheets subglacial deformation of tills has been widely invoked to account for rapid glacier motion criteria for identifying such tills are restricted to structures viewed in cross section and it is thus hard to assess how widespread the processes of deformation may have been lineations provide a surface marker of structure that records subsequent deformation from other ice flow events it is argued that the palaeo flow record including cross cutting patterns provides the surface or plan expression of subglacial deformation the degree of modification to pre existing lineations should permit the identification of zones favoured by deformation and thus assist in an assessment of how pervasive deforming bed processes were in influencing ice sheet dynamics

psv wave propagation in a medium with local heterogeneities a hybrid formulation and its application summary hybrid methods which combine so called numerical and analytical methods are a successful tool for computing synthetic seismograms for media with localized heterogeneous regions each of the two methods is applied in that part of the medium in which it can be employed efficiently this paper deals with a combination of a finite difference algorithm applied in the neighbourhood of the heterogeneity and a frequency wavenumber filter for the psv wave equation applied outside this method is an alternative to the use of green s theorem as a wavefield continuation method e g van den berg 1984 in the hybrid method presented here absorption can be incorporated additionally for different models with first and second order discontinuities the hybrid method was tested successfully against the reflectivity method the hybrid method was applied to study the effects of a heterogeneous lower crust on the wavefield this application has been inspired by recent deep seismic reflection profiling which often reveals a highly reflecting lower crust the scattering models have been divided into three groups horizontal vertical and isotropic scatterers the comparison of the reflection responses of these models shows that thin horizontal or small isotropic scatterers yield high reflected energy coherent over a few kilometres in the time window of lower crustal reflections vertical structures characterized by a small correlation length in horizontal direction have less energy in this time window but they can generate a complex moho reflection with a strong moho coda it is also shown that the acoustic approximation is mostly not sufficient to describe the wave propagation in such complex media

efficient 3 d viscoelastic modeling with application to near surface land seismic data three dimensional viscoelastic modeling is implemented by reparameterizing the viscoelastic wave equation for a standard linear solid the formulation uses weighting factors corresponding to relaxation frequencies and composite memory variables this novel 3 d formulation requires less computer memory than the traditional relaxation time formulation because one set of relaxation frequencies can be used for all mechanisms for all parts of a model and only three sets of composite memory variables are needed rather than the seven used in the standard implementation giving a net reduction of 40 in the total required memory computational time is also reduced approximately 25 because of reduced input output i o the algorithm is applied to 3 d modeling of the viscoelastic response of the near surface structure beneath a 3 d reflection survey in the ouachita frontal thrust zone of southeast oklahoma comparison of the 3 d field data with both viscoelastic and elastic response clearly demonstrates the importance of inclusion of viscoelasticity when accurate amplitude fitting is desired observed amplitude and traveltime variations can be explained by shallow velocity and q distributions obtained independently by 3 d tomography

basal mechanics of ice stream b west antarctica 2 undrained plastic bed model based on the results of our studies of the physical conditions beneath ice stream b we formulate a new analytical ice stream model the undrained plastic bed model henceforth the upb model mathematically the upb model is represented by a non linear system of four coupled equations which express the relationships among ice sliding velocity till strength water storage in till and basal melt rate we examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant less than hundreds of years temporal variability is introduced into the upb model only by the direct dependence of till void ratio changes e t on the basal melting rate m r since till strength b e and ice stream velocity u b b change as long as till void ratio varies the first condition for ice stream stability is that of constant till water storage 0 the second condition for ice stream stability arises from the feedback between ice stream velocity till strength and the basal melting rate which depends on shear heating m r u b b this is the weak till condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress b n 1 1 d the salient feature of the upb model is its ability to produce two thermo mechanically controlled equilibrium states one with a strong bed and slow ice velocities ice sheet mode and one with a weak bed and fast ice velocities ice stream mode this bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in west antarctica basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes the upb model demonstrates that ice streams may be prone to thermally triggered instabilities during which small perturbations in the basal thermal energy balance grow leading to generation or elimination of the basal conditions which cause ice streaming

new and improved determinations of velocity of ice streams b and c west antarctica measurements of velocity have been made on and next to ice streams b and c west antarctica the results are more precise than previous work and constitute a 93 increase in the number of values these velocities are used to describe the confluence of flow into the ice streams and the development of fast ice stream flow the onset of fast streaming flow occurs in many separate tributaries that coalesce down glacier into the major ice streams for those inter stream ridges that have been studied the flow is consistent with steady state along ice stream b gradients in longitudinal stress offer little resistance to the ice flow the transition from basal drag control to ice shelf flow is achieved through reduced drag at the glacier base and increased resistance associated with lateral drag velocities in the trunk of ice stream c are nearly zero but those at the up glacial head are similar to those at the head of ice stream b

numerical viscoelastic modelling by the spectral laguerre method seismic wave propagation in a viscoelastic media can be described by a system of integro differential equations the solution of such equations requires special methods when using finite difference techniques in the time domain in the frequency domain the integral terms are represented by complex elastic parameters this paper presents an efficient algorithm for viscoelastic modelling based on the integral laguerre transform for the approximation of temporal derivatives and for the calculation of convolution integrals for the calculation of spatial derivatives it is possible to use various methods finite difference and finite element techniques spectral and pseudo spectral methods we then obtain a system of algebraic equations with a matrix independent of the parameter m i e the degree of the laguerre polynomials in this case only the right hand side of the system has recurrent dependence on the parameter m which is an analogue of the temporal frequency in the frequency domain the obtained system with a large number of right hand sides can be solved using fast methods where the matrix is transformed only once as opposed to the frequency domain approach when the matrix is transformed for each temporal frequency

till beneath ice stream b 2 structure and continuity during the 1984 1985 antarctic field season 8 3 km of high resolution seismic reflection data were collected in order to image a thin till layer beneath ice stream b west antarctica two parallel seismic reflection lines were oriented transverse to ice flow these data show that the till layer varies in thickness but is continuous over almost the entire length of the profiles with an average thickness of 6 5 m the upper surface of the till layer is smooth but the lower boundary is fluted parallel to flow these till filled flutes are as much as 13 m deep and 1000 m across nowhere on the profiles can any feature be discerned to penetrate more than a few meters into the ice from the bed reflection events from lithified sediments of unknown type are observed to be truncated by the till in an angular unconformity

combining field and modelling techniques to assess rockfall dynamics on a protection forest hillslope in the european alps adequate management of a mountain forest that protects downslope areas against impacts of rockfall requires insight into the dynamics of the hillslope environment therefore we applied a combined approach using field and modelling techniques to assess the determining factors for rockfall source areas rockfall tracks and rockfall runout zones on a forested slope in mountainous terrain the first objective of this study was to understand why rockfall occurs in the study area the second objective was to translate the knowledge obtained in the field into a model that simulates rockfall dynamics on a forested slope realistically the third objective was to assess which hillslope characteristics primarily determine the distribution of active rockfall tracks to achieve these objectives we made a geomorphological map of the whole study area and we measured the major discontinuity planes in the bedrock that are exposed in the rockfall source areas furthermore a test site for simulation modelling within the larger study area was defined in which both a forest and a hillslope inventory were carried out the available data and our developed rockfall simulation model allowed us to assess the slope characteristics that mainly determine the distribution of areas affected by rockfall we found that in decreasing order of importance both standing and felled trees the surface roughness and rockfall resistant shrubs primarily determine the distribution of rockfall affected areas simulation tests without a forest cover produced similar rockfall runout zones as fossil rockfall events identified in the field we believe that the combined field and modelling approach is a prerequisite for understanding how forests can protect against rockfall

rheological nonlinearity and flow instability in the deforming bed mechanism of ice stream motion contrary to what has recently been assumed in modeling the proposed deforming bed mechanism for the rapid motion of antarctic ice streams the rheology of water saturated till is probably highly nonlinear according to information from soil mechanics and preliminary experiments on till from the base of ice stream b the equivalent flow law exponent n is probably as high as 100 and the nonlinearities of the shear stress and effective pressure dependences are closely linked the high nonlinearity has important consequences for the deforming bed mechanism a flow system operating by this mechanism can be unstable as a result of feedback from the generation of basal water by shear heating of basal till the short term feedback effect is analyzed for a perturbation in a model ice stream in which the basal meltwater is transported through a distributed system of narrow gap conduits at the ice till interface although the analysis is approximate and some of the system parameters are poorly known the results suggest that the deforming bed mechanism is unstable for n 20 the apparent lack of such an instability in the currently active ice streams implies that their motion is controlled not by the deforming bed mechanism but by some other as yet unidentified mechanism

three dimensional simulation of a magnitude 7 75 earthquake on the san andreas fault simulation of 2 minutes of long period ground motion in the los angeles area with the use of a three dimensional finite difference method on a parallel supercomputer provides an estimate of the seismic hazard from a magnitude 7 75 earthquake along the 170 kilometer section of the san andreas fault between tejon pass and san bernardino maximum ground velocities are predicted to occur near the fault 2 5 meters per second and in the los angeles basin 1 4 meters per second where large amplitude surface waves prolong shaking for more than 60 seconds simulated spectral amplitudes for some regions within the los angeles basin are up to 10 times larger than those at sites outside the basin at similar distances from the san andreas fault

3 d tomographic imaging of near surface seismic velocity and attenuation as a result of the similarity between velocity and attenuation imaging we have implemented both using the same 3 d tomography software with simple variable changes the resulting sets of linear equations are solved by the simultaneous iterative reconstruction technique sirt the algorithm is applied to 3 d estimation of near surface velocity and attenuation distributions from 3 d surface survey field data from the ouachita frontal thrust zone in southeastern oklahoma the images obtained correlate well with the known surface geology resolution analysis by computation of point spread functions indicates highest resolution in the direction parallel to the densest distribution of survey points the receiver lines

extent and basal characteristics of the m clintock channel ice stream abstract the m clintock channel palaeo ice stream drained the northwestern margin victoria island arctic canada of the laurentide ice sheet functioning for 200 years prior to 10 ka bp from its bed imprint it is reconstructed as 720 140 km with a cross sectional area of 100 km2 comparable in size to the hudson strait ice stream it drained an estimated 80 000 km3 of ice enough to significantly affect the shrinking ice sheet and drive the keewatin ice divide south eastwards such discharges likely produced heinrich events and influenced the temperature and salinity of the western arctic ocean ice stream location was controlled by a topographic trough but glaciological regulation rather than properties of the bed controlled margin locations and hence ice stream width the ice stream eroded into pre existing unconsolidated sediments and left a pattern of subglacially produced landforms recording a snapshot view of the bed prior to shutdown frictional shutdown occurred once downcutting through sediments reached bedrock in places high sediment availability appears critical for ice stream functioning with a drift thickness of 50 m required to sustain its 200 year lifespan debris discharge was exceptionally high 73 000 m3 a 1 m 1 this ice stream demonstrates non steady response of ice sheets to climate change and their large impact on mass balance and stability of ice sheets

basal mechanics of ice stream b west antarctica 1 till mechanics data from laboratory geotechnical tests on till recovered from beneath ice stream b west antarctica at the upstream b camp hereinafter the upb till show that failure strength of this till is strongly dependent on effective stress but is practically independent of strain and strain rate these data support use of a coulomb plastic rheology in modeling of ice stream behavior and subglacial till deformation our testing program combined triaxial ring shear and confined uniaxial tests to investigate till strength and compressibility results show that the upb till follows closely coulomb s equation in which shear strength is a linear function of normal effective stress apparent cohesion near zero and internal friction angle equal to 24 till compressibility is best described by a logarithmic function that relates void ratio to normal effective stress in general the behavior of the upb till is consistent with other experimental evidence regarding mechanical behavior of granular materials based on our laboratory results we formulate the compressible coulomb plastic till model in which there are three interrelated primary state variables shear strength void ratio and normal effective stress this model is used in the second part of our study to simulate response of subglacial till to realistic effective stress forcings these simulations demonstrate that the model is capable of reproducing fundamental aspects of subglacial till kinematics 1 occurrence of tilt rate oscillations and negative tilt rates in tiltmeter records and 2 distributed till deformation to depths of 0 1 1 0 m beneath the ice base our laboratory and modeling results substantiate application of the compressible coulomb plastic model in simulations of the motion of ice stream b over its weak till bed

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

basal hydraulic system of a west antarctic ice stream constraints from borehole observations pressure and tracer measurements in boreholes drilled to the bottom of ice stream b west antarctica are used to obtain information about the basal water conduit system in which high water pressures are developed these high pressures presumably make possible the rapid movement of the ice stream pressure in the system is indicated by the borehole water level once connection to the conduit system is made on initial connection here also called breakthrough to the basal water system the water level drops in a few minutes to an initial depth in the range 96 117 m below the surface these water levels are near but mostly somewhat deeper than the flotation level of about 100m depth water level at which basal water pressure and ice overburden pressure are equal which is calculated from depth density profiles and is measured in one borehole the conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed the thickness of the gap has to be about 2 mm to account for the water level drop on breakthrough and about 4 mm to fit the results of a salt tracer experiment indicating downstream transport at a speed of 7 5 mm s 1 the above gap conduit model is however ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole and also by the large hole to hole variation in measured basal pressure which if present in a gap conduit system with 2 or 4 mm would result in unacceptably large local water fluxes an alternative model that avoids the e objections called the gap opening model involves opening a gap as injection proceeds starting with a thin film the injection of water under pressure lifts the ice mass a round the borehole creating a gap 3 or 4 mm wide at the ice bed interface evaluated quantitatively the gap opening model accounts for the volume of water that the basal watery system accepts on breakthrough which obviates the gap conduit model in order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits of which the most promising type is the canal conduit proposed theoretically by walder and fowler 1994 flat low conduits incised into the till 0 1 m deep and perhaps 1 m wide with a flat ice roof the basal water pressure data suggest that the canals are spaced 50 300 m apart much closer than r tunnel would be the deepest observed water level 117 m i the most likely to reflect the actual water pressure in the canals corresponding to a basal effective pressure of 1 6 bar in this interpretation the shallower water levels are affected by loss of hydraulic head in the narrow passageway s that connect along the bed from borehole to canal s once a borehole has frozen up and any passageways connecting with canals have become closed a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till given enough time for pressure equilibration this pressure varies considerably with time over the equivalent water level range from 100 to 113 m basal pressure sensors 500 m apart report uncorrelated variations whereas sensors in boreholes 25 m apart report mostly but not entirely well correlated variations of unknown origin in part of the record remarkable anticorrelated variations are interspersed with positively correlated one and there are rare abrupt excursions to extreme water levels as deep as 125m and as shallow as 74 m a diurnal pressure fluctuation intermittently observed may possibly be caused by the ocean tide in the ross sea the lack of any observed variation in ice stream motion when large percentagewise variations in basal effective pressure were occurring according to our data suggests that the observed pressure variations are sufficiently local and so randomly variable from place to place that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed

basal sliding of ice stream b west antarctica a tethered stake apparatus is used to measure basal sliding in a borehole on ice stream b west antarctica about 300 km upstream east from its grounding line near the head of the ross ice shelf a metal stake emplaced at the top of a layer of unfrozen till underlying the ice is connected by a tether line to a metering unit that measures the tether line as it is pulled out from the borehole by the stake as a result of basal sliding the measured sliding motion includes any actual slip across the ice till interface and may include in addition a possible contribution from shear deformation of till within about 3 cm of the interface this 3 cm figure follows from a qualitative model of the movements of the stake in the course of the experiment based on features of the record of apparent sliding alternative but less likely models would increase the figure from 3 cm to 10 cm or 25 cm in any case it is small compared to the seismically inferred till thickness of 9 m measured apparent sliding averages 69 of the total motion of 1 2 m d 1 over 26 days of observation if a 3 5 day period of slow apparent sliding 8 of the total motion is included in the average the occurrence of the slow period raises the possibility that the sliding motion switches back and forth between c 80 and c 8 of the total motion on a time scale of a few days however it is likely that the period of slow apparent sliding represents instead a period when the stake got caught on the ice sole if the slow period is therefore omitted the indicated average basal sliding rate is 83 of the total motion in either case basal sliding predominates as the cause of the rapid ice stream motion in the last 2 days of observation the average apparent sliding rate reached 1 17 m d 1 essentially 100 of the motion of the ice stream if till deformation contributes significantly to the ice stream motion the contribution is concentrated in a shear zone 3 cm to possibly 25 cm thick at the top of the 9 m thick till layer these observations if applicable to the west antarctic ice sheet in general pose complications in modeling the rapid ice streaming motion

subglacial conditions under the last ice sheet in northwest germany ice bed separation and enhanced basal sliding abstract theoretical reconstruction of ice thickness during the weichselian glaciation in a portion of northwest germany yields maximum thicknesses of about 280 m some 50 km into the ice sheet from the margin this corresponds to averaged driving stress at the glacier bed in the order of 17 kpa the increase in ice thickness away from the margin was non uniform caused by undulations in pre existing topography steady state subglacial water pressure heads were high up in the ice sheet reaching the flotation point in places hydraulic lifting of the ice sheet probably occurred causing non deforming bed conditions where transformation of basal shear stresses to soft subglacial sediments was prevented by a water film thin stringers of undeformed sorted sediments occurring in tills are attributed to basal meltwater washing during phases of ice bed separation the basal water film was over 5 mm thick using a quantitative approach coupled with field data we demonstrate that neither internal ice deformation nor bed deformation contributed much to the ice motion instead the rapid movement of the weichselian ice sheet was caused by basal sliding with ploughing the ice sheet was probably characterized by an unstable behaviour

the marginal shear stress of ice stream b west antarctica to ascertain whether the velocity of ice stream b west antarctica may be controlled by the stress in its marginal shear zones the snake and the dragon we undertook a determination of the marginal shear stress in the dragon near camp up b by using ice itself as a stress meter the observed marginal shear strain rate of 0 14 a 1 is used to calculate the marginal shear stress from the flow law of ice determined by creep tests on ice cores from a depth of 300 m in the dragon obtained by using a hot water ice coring drill the test specimen orientation relative to the stress axes in the tests is chosen on the basis of c axis fabrics so that the test applies horizontal shear across vertical planes parallel to the margin the resulting marginal shear stress is 2 2 0 3 10 5 pa this implies that 63 100 of the ice stream s support against gravitational loading comes from the margins and only 37 0 from the base so that the margins play an important role in controlling the ice stream motion the marginal shear stress value is twice that given by the ice stream model of echelmeyer and others 1994 and the corresponding strain rate enhancement factors differ greatly e 1 2 vs 10 12 5 this large discrepancy could be explained by recrystallization of the ice during or shortly after coring estimates of the expected recrystallization time scale bracket the 1 h time scale of coring and leave the likelihood of recrystallization uncertain however the observed two maximum fabric type is not what is expected for annealing recrystallization from the sharp single maximum fabric that would be expected in situ at the high shear strains involved 20 experimental data from wilson 1982 suggest that if the core did recrystallize the prior fabric was a two maximum fabric not substantially different from the observed one which implies that the measured flow law and derived marginal shear stress are applicable to the in situ situation these issues need to be resolved by further work to obtain a more definitive observational assessment of the marginal shear stress

numerical modeling of subglacial sediment deformation implications for the behavior of the lake michigan lobe laurentide ice sheet we apply a numerical model of the late wisconsin circa 20 000 years b p lake michigan lobe lml laurentide ice sheet to investigate how fine grained subglacial sediment might influence lobe behavior particularly rapid millennial scale marginal oscillations observed in the geologic record over the canadian shield we assume a rigid bed hard bedded basal boundary condition in areas overlain by fine grained sediment soft bedded the base of the ice is coupled to a deformable sediment layer using a rate dependent stress strain law geotechnical tests of clay rich till deposited by the lml provide control for sediment rheologic parameters simulated cross sectional profiles are consistent with reconstructions from geologic evidence time dependent simulations suggest that a soft bedded lobe could have reached steady state in about 20 000 years or less in contrast to 50 000 to 60 000 years for an otherwise identical hard bedded lobe a soft bedded lobe with sediment viscosity at the experimentally determined value is about twice as responsive to millennial scale shifts in accumulation or ablation as a nonsliding hard bedded lobe but in both cases the response is slower than that indicated by the geologic record results suggest that while strong millennial scale changes in accumulation and ablation can produce responses in hard bedded or soft bedded ice that are consistent with the geologic record changes in subglacial sediment viscosity even relatively modest changes whether independent or in conjunction with climate change might more readily account for millennial and submillennial scale fluctuations of the lobe margin these observations do not exclude a role for sliding but they do provide some perspective from which to evaluate relative contributions of the various processes that influence lobe behavior

a ploughing model for the origin of weak tills beneath ice streams a qualitative treatment abstract glaciological studies of west antarctic ice streams have shown that weak sub ice stream tills provide the basal lubrication that makes fast ice streaming possible under low driving stresses given the significant current interest in time dependent ice stream behavior there is a clear need for a conceptual model of weak sub ice stream tills that treats in a simple but physically correct way the coupling between evolution of till properties and ice stream dynamics as a possible alternative to the previous viscous bed model we propose a ploughing model that is consistent with the experimentally determined coulomb plastic rheology of sub ice stream till in the ploughing model the till is a several meters thick layer of sedimentary material that is disturbed and transported by ploughing that occurs during sliding of a bumpy ice base the thickness of the till layer is determined in the ploughing model by the amplitude of the largest roughness elements ice keels there is no direct proof for the existence of ice bumps and ice keels beneath the modern west antarctic ice streams but bedforms e g megalineations and bundle structures left behind by pleistocene ice streams strongly support our assumption that an ice stream base is irregular generation of new till material occurs when ice keels protrude through the existing till layer and erode the top of the sub till preglacial sediments based on a single tethered stake measurement of engelhardt and kamb j glaciol 44 1998 223 made at the upb camp ice stream b west antarctica fig 1 we estimate that the till flux due to sliding with ploughing is there 3 yr 1 per meter width to balance the estimated till flux in the upb area substrata erosion by ice keels would have to take place at a high but not unreasonable non dimensional rate of 4 assuming 1 contact area in the case of the west antarctic ice streams erosion of sub till materials by ice keels may be particularly fast and unimpeded because these ice streams are overriding unlithified preglacial tertiary sediments the most significant implication of the proposed ploughing model is that it permits treating basal resistance to ice motion as being velocity independent plastic till rheology while allowing subglacial transport of till as in the viscous bed model models of ice streams with a plastic bed exhibit a greater potential for unstable behavior than models of ice streams with viscous beds

deep sea floor evidence of past ice streams off the antarctic peninsula the past existence of a giant ice stream off the northern end of the antarctic peninsula is suggested by a convex upward elongated sediment body now at a mean water depth of 1000 m because of its morphological characteristics i e a set of parallel to subparallel ridges and grooves to 100 km long with an overall width of 25 km we call this depositional body a bundle structure we hypothesize that bundle structures form by accumulation of basal deformation till under their parent ice streams from its location size and overall characteristics the bundle structure described here constitutes the best preserved largest deepest and relatively low latitude evidence of giant ice streams that flowed offshore antarctica during glacial maxima bundle structures reveal the very dynamic behavior of ice caps in the northern antarctic peninsula during the last glacial maximum with catchment areas draining rapidly under marine based critical subglacial conditions bundle structures represent a new megascale streamlined glacial landform

rapid sea level rise soon from west antarctic ice sheet collapse there has been recent concern over whether shrinkage of ice sheets in antarctica caused by climate change could lead to rapid global sea level rise in his perspective bentley discusses the processes at work in ice sheet behavior and concludes that in view of the evidence for ice sheet stability catastrophic changes are unlikely in the next century or two

deformation of till beneath ice stream b west antarctica the behaviour and possible instability of the west antarctic ice sheet depend fundamentally on the dynamics of the large ice streams which drain it model calculations show that most ice stream velocity arises at the bed1 2 and radar sounding has shown the bed to be wet3 but the basal boundary condition is not well understood seismic evidence from the upstream b camp upb on the siple coast of west antarctica4 shows that the ice stream there rests on a layer of unconsolidated sediment averaging 5 or 6 m thick in which the water pressure is only 50 kpa less than the overburden pressure because this thin layer occurs well inland beneath an active ice sheet and rests on a surface showing flutes4 characteristic of glacial erosion5 we presume that it is glacial till we propose here that deformation within the till is the primary mechanism by which the ice stream moves and we discuss implications of this hypothesis

till beneath ice stream b 1 properties derived from seismic travel times seismic experiments conducted on ice stream b part of the marine ice sheet of west antarctica show a meters thick layer immediately beneath the 1000 m thick ice a seismic experiment consisting of wide angle reflection profiling along a line parallel to ice stream flow was conducted to determine the properties of this layer inversion of seismic travel times yields a compressional wave speed of less than 1700 m s 1 and a shear wave speed less than 160 m s 1 for the layer these very low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure based on wave speeds in other saturated unconsolidated sediments we believe that a porosity substantially greater than 0 32 probably around 0 4 and an excess of overburden pressure over pore pressure of only 50 kpa 0 5 bar characterize the layer at this location

drumlinization and drumlin forming instabilities viscous till mechanisms glacially induced flow naturally tends to thin and extended till cover through shock formation even in the absence of longitudinal gradients in the applied stress thicker till cover has an increased effective pressure at its surface and base a lower sliding velocity or deformation rate and above a critical thickness a decrease in wave velocity with thickness leading to reverse facing shocks moving downstream for sliding and for some theologies of internal deformation a decrease in sediment flux with thickness occurs implying backward moving kinematic waves and reverse facing reverse moving shock downstream facing shocks are also formed which move upstream if the till is sliding and downstream if the till is deforming internally eventually shocks coalesce leaving an upstream lacing shock for sliding and a downstream facing shock for internal deformation it is observed that some drumlins have downstream blunt ends only fairly realistic three dimensional drumlin shapes can be produced from symmetric sediment bodies and barchan shapes can be produced from linear forms perpendicular to the ice sheet flow the fact that viscous theories produce drumlinoid forms suggests that on this scale till behaves viscously and the the lower length scale for drumlins represents the plastic viscous transition scale

sedimentation on the ross sea continental shelf antarctica abstract glacial marine sedimentation has prevailed throughout most of the early glacial history of the ross sea region however since late miocene to early pliocene time the west antarctic ice sheet has advanced onto the continental shelf perhaps several times glacial troughs of the western ross sea may also have formed at this time these glacial troughs border outlet glaciers of the east antarctic ice sheet a widespread lodgement till facies was deposited during this glacial advance holocene retreat of this expanded ice sheet left no conspicuous meltwater record so decoupling of the ice sheet from the seafloor rather than rapid melting is implied modern glacial sediment is far less prevalent than biogenic sediment and reworked glacial sediment except in coastal areas adjacent to small ice shelves and ice tongues little or no debris is being transported beyond the calving line of the ross ice shelf siliceous mud and ooze occurs in the western ross sea where surface circulation is most vigorous and warm deep waters up well this siliceous sediment along with terrigenous silt and clay blankets the seafloor below approximately 300 m water depth most of the ross sea continental shelf is deeper than this surface sediments on shallow banks and coastal platforms have clearly been reworked by marine currents the most vigorous bottom current activity occurs in those areas where geostrophic currents impinge onto the shelf debris flows and turbidity currents are also important in eroding and redistributing glacial sediments on the shelf terrigenous silt and clay are more abundant in surface sediment than basal till the source of this silt and clay is problematic the most plausible sources being onshore transport of suspended sediment by impinging currents and subglacial meltwater input

rapid soft bed sliding of the puget glacial lobe the puget lobe of the late pleis inferred to be lodgment till based on the tocene cordilleran ice sheet advanced into presence of oriented stones oriented stria western washington over a thick sequence of tions on stones overconsolidation and unlithified sediments the basal drag due to deformation features sliding over this surface was calculated by 1 this paper investigates the rapid motion of idealizing the sediment bed as a rigid planar the puget lobe over its bed of deformable sedi surface scattered with roughness elements cor ment studies of large vanished glaciers com responding to individual sedimentary particles plement basal motion studies of modern glaci and 2 taking the minimum reconstructed slid ers in which the glacier geometry and velocity ing velocity to be 500 m yr 1 the calculated are well constrained but the characteristics of basal drag minimized by choices of parameters the bed can only be inferred from observations is 400 kpa much higher than the reconstructed over very limited areas kamb et al 1985 gravitational driving stress of 40 kpa indi seismic measurements blankenship et al 1986 cating that a rigid bed and low water pressure or modelling lingle and brown 1987 obser are not consistent with the glacierfs rapid vations on glacier beds recently exposed by motion high subglacial water pressure aver glacial retreat can provide information about aging 90 of the ice overburden 10 mpa is conditions during a somewhat greater time span inferred from overconsolidation of subglacial but are restricted to the marginal environ clays the occurrence and deformation of ment in addition inferences about the bed of water deposited sediments within the till and modern glaciers are limited by the short dura the requirement to drain large quantities of tion and seasonality of the observations a water from the interface suggest that water notable benefit of studying the beds of pieis pressure reached 99 of the ice overburden tocene glaciers is that they record basal ploughing of till by ice entrained clasts per conditions for intervals spanning entire glaci vasive shearing and finally ice bed separa ations and accessible far from the glacier tion become possible as water pressure margins increases at the low inferred effective nor after a brief summary of data available from mal stress the basal drag is reduced from the previous reconstructions of the puget lobe the rigid bed case by water layers which decouple basal conditions of the lobe are explored along the smallest particles from the glacier sole two complementary lines we first present cal and ploughing which reduces the resistance culations of basal drag due to glacier sliding offered by the largest particles the limited for different models of till bed roughness that shear strain observed in much of the till indicate that rapid sliding requires very low implies pervasive shearing did not contribute effective stresses and extensive ice bed significantly to ice motion basal motion was decoupling or ploughing second we summarize confined to the ice bed interface or to dis geological and geotechnical data that indicate tinct faults within the substrate

wave propagation effects on amplitude variation with offset measurements a modeling study wave propagation effects can significantly affect amplitude variation with offset avo measurements these effects include spreading losses transmission losses interbed multiples surface multiple reflections p sv mode converted waves and inelastic attenuation examination of prestack elastic synthetic seismograms suggests that spreading losses and the transmission losses plus compressional interbed multiples are manifest mainly as a time and offset effect on the primary reflections the surface related multiples and the p sv mode converted waves interfere with prestack amplitudes inducing distortions in the avo pattern such distortions cause large variances in avo model fitting prestack viscoelastic synthetic seismograms also suggest that inelastic attenuation further complicates the avo response because of the offset and time variant amplitude decay effects and the phase change due to dispersion together all these effects severely alter avo behavior and result in serious errors in avo parameter

influence of subglacial geology on the position of a west antarctic ice stream from seismic observations ice streams drain much of the interior west antarctic ice sheet and buffer the main ice reservoir from oceanic influences1 2 the slow flowing interior feeds the floating ross ice shelf with ice via fast flowing ice streams3 that are believed to modulate sea level change through their control of inland ice storage understanding ice stream behaviour and predicting the response to climate change4 requires a better knowledge of the subglacial geology5 6 it is known that a thawed ice bed and high pressure basal water are necessary but not sufficient conditions to cause ice streaming7 8 moreover it has been hypothesized that a soft sedimentary bed is also required because of its intrinsic low frictional resistance to flow9 and owing to its high erodibility so as to generate till that can deform and lubricate ice motion10 11 or to bury rough features and smooth the bed for sliding here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill the ice stream is within the basin and the ice outside the basin is slow flowing the basin fill presents an order of magnitude lower frictional resistance to ice flow than the subglacial material outside the basin we conclude that the ice stream position is dependent on subglacial geology

changes in the configuration of ice stream flow from the west antarctic ice sheet surface based ice penetrating radar profiles on the northeast flank of siple dome support the hypothesis that a curvilinear scar first observed in advanced very high resolution radiometer satellite imagery represents the margin of a formerly active ice stream the scar defines the southwestern boundary of an ice stream flowing from ice stream c to ice stream d close to where it enters the ross ice shelf our studies show that the scar coincides with a trough and upward step in surface topography approximately 5 km across underlain by a zone of disturbed internal stratigraphy revealed by the radar burial depth of the disturbed zone enables us to calculate the time of shutdown as occurring prior to approximately 1 3 ka the configuration of the ice streams draining the west antarctic ice sheet into the ross ice shelf evidently changes with time and attempts to predict the evolution of the ice sheet must incorporate this observation

mega scale glacial lineations and cross cutting ice flow landforms landsat images reveal a previously unsuspected large scale pattern of streamlining within drift that is assumed to reflect former phases of ice flow such a glacial grain can be regarded as a landform assemblage comprised of a number of components drumlins and megaflutes form part of the pattern but in addition there are two previously undocumented ice moulded landform elements streamlined lineations of much greater proportions referred to as mega scale glacial lineations and a distinctive cross cutting topology within the grain the ice moulded landform assemblage is described and illustrated with reference to examples from canada possible modes of genesis of such landforms are discussed and their glaciological implications outlined the discovery of this pattern indicates the pervasive nature of subglacial deformation of sediment and demands a radical re interpretation of ice sheet dynamics

kinematic indicators in fault gouge tectonic analog for soft bedded ice sheets abstract polished slip planes resulting from basin and range detachment faulting of mid tertiary volcaniclastics are well exposed at lake pleasant arizona u s a slip planes show erosional features formed by abrasion below moving sheets of gouge coarse grained diamict during repeated faulting and shearing tectonically generated forms are identical to those found in glaciated terrains slip planes show rat tail ridges and grooves ridge in groove structures nail head striations and wear tracks a prominent morphological element consists of flute ridges up to 30 cm wide and at least 7 m long formed of gouge preserved in the lee of boulders projecting as obstacles above slip surfaces crescentic scours around the stoss side of such boulders record enhanced abrasion by streams of gouge debris clasts are striated and show shapes comparable to flat iron clasts produced by glacial abrasion kinematic indicators at lake pleasant are directly comparable in form to flutes and drumlins in glaciated terrains the data presented here not support a recent hypothesis arguing that glacial striations flutes and drumlins are the product of turbulent subglacial meltwater instead diamict produced along fault planes gouge by low grade shearing is directly comparable to diamict facies deformation till formed by pervasive shear at the base of soft bedded ice sheets both facies are tectonically generated cataclastites produced in low grade shear zones and display genetically related but differently scaled streamlined bedforms

abrupt climatic change related to unstable ice sheet dynamics toward a new paradigm ice streams are fast currents of ice that drain some 90 of present day ice sheets and probably drained a similar percent of ice from former ice sheets ice streams may have life cycles akin to but much longer than surge cycles of certain mountain glaciers the floating tongues of marine ice streams coalesce to form floating ice shelves that buttress marine ice sheets and the grounded lobes of terrestrial ice streams stabilize terrestrial ice sheets ice shelves and ice lobes form during the life cycles of ice streams and disintegrate rapidly to initiate new life cycles successive life cycles continue until the ice sheet collapses a new paradigm is postulated in which abrupt climatic change is driven by iceberg outbursts associated with life cycles of marine ice streams and dust storms associated with life cycles of terrestrial ice streams for pleistocene northern hemisphere ice sheets these mechanisms regulated north atlantic surface temperatures and salinity and perhaps planetary albedo and atmospheric co2 and thereby drove climatic change which tended to be abrupt because ice stream life cycles began and ended abruptly ice streams proliferated along the margins of northern hemisphere ice sheets after they advanced beyond the crystalline shields of north america and eurasia and spread over softer sedimentary rocks overlain by thick ice cemented sediments that progressively thawed especially along river valleys and marine troughs an increasing fraction of these ice sheets was then subjected to the unstable life cycles of ice streams as seen in the pronounced spikiness of the late glacial oxygen isotope stratigraphy in the greenland ice core at dye 3 life cycles of greenland ice streams during the holocene seem to be linked to episodes of regional climatic cooling most recently the little ice age when superimposed on the present milankovitch hemicycle of reduced insolation over the north atlantic this could abruptly initiate a new worldwide glaciation cycle in the aftermath of present day co2 greenhouse warming proliferation of antarctic ice stream life cycles during the period of greenhouse warming may also trigger global climatic change by forcing a reorganization of circumpolar thermohaline circulation in the southern ocean

flow mechanism of glaciers on soft beds subhourly measurements of bed deformation bed shear strength subglacial water pressure and surface speed at storglaciaren a glacier in northern sweden showed that the shear strain rates of the bed decrease during periods of high water pressure and surface speed high water pressures appear to be accompanied by a reduction in the coupling of ice with the bed that is sufficient to reduce or eliminate shearing the instability of large ice masses may result from similar decoupling rather than from pervasive bed deformation as has been commonly thought

scattering effect on shallow gas obscured zone imaging in bohai pl19 3 area seismic imaging is challenging in the bohai bay pl19 3 area offshore china bohai bay field is seismically obscured making well penetrations the only reliable source of data for subsurface interpretation with the help of turning ray tomography we are able to obtain a reliable near surface velocity model approximately down to 700 m below sea level using the first arrivals picked from streamer data depth migration using velocities estimated from turning ray tomography has improved shallow structures and fault definition however reservoir level structures from 800 to 1500 m are still poorly imaged a viscoelastic modeling study with assigned variable q and shallow velocity profiles with and without shallow gas induced scatterers demonstrates that scattering is the primary controlling phenomenon causing imaging difficulty within the obscured zone due to scattering imaging tests at the target level were unsuccessful even with precise knowledge of velocity