ice streams as the arteries of an ice sheet their mechanics stability and significance ice streams are corridors of fast ice flow ca 0 8 km year within an ice sheet and are responsible for discharging the majority of the ice and sediment within them consequently like the arteries in our body their behaviour and stability is essential to the well being of an ice sheet ice streams may either be constrained by topography topographic ice streams or by areas of slow moving ice pure ice streams the latter show spatial and temporal patterns of variability that may indicate a potential for instability and are therefore of particular interest today pure ice streams are largely restricted to the siple coast of antarctica and these ice streams have been extensively investigated over the last 20 years this paper provides an introduction to this substantial body of research and describes the morphology dynamics and temporal behaviour of these contemporary ice streams before exploring the basal conditions that exist beneath them and the mechanisms that drive the fast flow within them the paper concludes by reviewing the potential of ice streams as unstable elements within ice sheets that may impact on the earth s dynamic system d 2002 elsevier science b v all rights reserved

selective glacial erosion and weathering zones in the coastal mountains of marie byrd land antarctica in the coastal mountains of marie byrd land antarctica there is a juxtaposition of ice moulded landforms at lower altitudes and upstanding summits with weathered surfaces bearing tors this paper uses geomorphological mapping and exposure dating to test two hypotheses commonly used to explain such a landscape contrast either the pattern reflects contrasts in glacial erosion related to the basal thermal regime beneath a former ice sheet or it represents a periglacial trimline marking the upper limit of ice cover during the last glacial maximum lgm cosmogenic nuclide measurements on erratics show that the mountains were covered by ice on several occasions most recently during the lgm similar measurements on bedrock show that fragile landforms on weathered surfaces survived many glaciations nuclide concentrations in bedrock indicate both that subglacial erosion is insignificant and that ice free periods of periglacial weathering are longer at higher elevations we conclude that the pattern of landscape modification reflects both topographically controlled contrasts in the basal thermal regime of overriding ice and the longer exposure to periglacial conditions at higher elevations in addition the combination of nuclide data and the wider pattern of striations shows that the lgm ice cover was thinner than predicted by ice sheet models assuming equilibrium conditions that ice sheet thinning has occurred from 10 400 years ago to the present and that regional flow by overriding ice has been replaced by radial ice flow from local glaciers centred on individual mountain massifs

stochastic reservoir characterization using prestack seismic data reservoir characterization must be based on information from various sources well observations seismic reflection times and seismic amplitude versus offset avo attributes are integrated in this study to predict the distribution of the reservoir variables i e facies and fluid filling the prediction problem is cast in a bayesian setting the a priori model includes spatial coupling through markov random field assumptions and intervariable dependencies through nonlinear relations based on rock physics theory including gassmann s relation the likelihood model relating observations to reservoir variables including lithology facies and pore fluids is based on approximations to zoeppritz equations the model assumptions are summarized in a bayesian network illustrating the dependencies between the reservoir variables the posterior model for the reservoir variables conditioned on the available observations is defined by the a priori and likelihood models this posterior model is not analytically tra

probabilistic petrophysical properties estimation integrating statistical rock physics with seismic inversion a joint estimation of petrophysical properties is proposed that combines statistical rock physics and bayesian seismic inversion because elastic attributes are correlated with petrophysical variables effective porosity clay content and water saturation and this physical link is associated with uncertainties the petrophysical properties estimation from seismic data can be seen as a bayesian inversion problem the purpose of this work was to develop a strategy for estimating the probability distributions of petrophysical parameters and litho fluid classes from seismics estimation of reservoir properties and the associated uncertainty was performed in three steps linearized seismic inversion to estimate the probabilities of elastic parameters probabilistic upscaling to include the scale changes effect and petrophysical inversion to estimate the probabilities of petrophysical variables andlitho fluid classes rock physics equations provide the linkbetween reservoir properties and velocities and lin

hierarchical bayesian lithology fluid prediction a north sea case study seismic 3d amplitude variation with offset avo data from the alvheim field in the north sea are inverted into lithology fluid classes elastic properties and porosity lithology fluid maps over hydrocarbon prospects provide more reliable estimates of gas oil volumes and improve the decision concerning further reservoir assessments the alvheim field is of turbidite origin with complex sand lobe geometry and appears without clear fluid contacts across the field the inversion is phrased in a bayesian setting the likelihood model contains a convolutional linearized seismic model and a rock physics model that capture vertical trends due to increased sand compaction and possible cementation the likelihood model contains several global model parameters that are considered to be stochastic to adapt the model to the field under study and to include model uncertainty in the uncertainty assessments the prior model on the lithology fluid classes is a markov random field that captures local vertical horizonta

geophysical constraints on the dynamics and retreat of the barents sea ice sheet as a paleobenchmark for models of marine ice sheet deglaciation our understanding of processes relating to the retreat of marine based ice sheets such as the west antarctic ice sheet and tidewater terminating glaciers in greenland today is still limited in particular the role of ice stream instabilities and oceanographic dynamics in driving their collapse are poorly constrained beyond observational timescales over numerous glaciations during the quaternary a marine based ice sheet has waxed and waned over the barents sea continental shelf characterized by a number of ice streams that extended to the shelf edge and subsequently collapsed during periods of climate and ocean warming increasing availability of offshore and onshore geophysical data over the last decade has significantly enhanced our knowledge of the pattern and timing of retreat of this barents sea ice sheet bsis particularly so from its late weichselian maximum extent we present a review of existing geophysical constraints that detail the dynamic evolution of the bsis through the last glacial cycle providing numerical modelers and geophysical workers with a benchmark data set with which to tune ice sheet reconstructions and explore ice sheet sensitivities and drivers of dynamic behavior although constraining data are generally spatially sporadic across the barents and kara seas behaviors such as ice sheet thinning major ice divide migration asynchronous and rapid flow switching and ice stream collapses are all evident further investigation into the drivers and mechanisms of such dynamics within this unique paleo analogue is seen as a key priority for advancing our understanding of marine based ice sheet deglaciations both in the deep past and in the short term future

spontaneous generation of pure ice streams via flow instability role of longitudinal shear stresses and subglacial till 1 a significant portion of the ice discharge in ice sheets is drained through ice streams with subglacial sediment till acting as a lubricant the known importance of horizontal friction in shear margins to ice stream dynamics suggests a critical role of longitudinal stresses the effects of subglacial till and longitudinal stresses on the stability of an ice sheet flow are studied by linear stability analysis of an idealized ice till model in two horizontal dimensions a power law viscous constitutive relation is used explicitly including longitudinal shear stresses the till which has compressible viscous rheology affects the ice flow through bottom friction we examine the possibility that pure ice streams develop via a spontaneous instability of ice flow we demonstrate that this model can be made intrinsically unstable for a seemingly relevant range of parameters and that the wavelengths and growth rates that correspond to the most unstable modes are in rough agreement with observed pure ice streams instabilities occur owing to basal friction and meltwater production at the ice till interface the most unstable wavelength arise because of selective dissipation of both short and long perturbation scales longitudinal stress gradients stabilize short transverse wavelengths while nye diffusion stabilizes long transverse wavelengths the selection of an intermediate unstable wavelength occurs however only for certain parameter and perturbation structure choices these results do not change qualitatively for a newtonian ice flow law indicating no significant role to shear thinning although this may very well be due to the restrictive assumptions of the model and analysis

geomorphological significance of lichen colonization in a present snow hollow hoya del cuchillar de las navajas sierra de gredos spain abstract this paper discusses the results of a lichenometrical and geomorphological study of one of the few remaining active snow hollows in the central region of the iberian peninsula the study area located on a glacial shoulder is called hoya del cuchillar de las navajas a protalus rampart occurs at the base of the hollow our studies conducted between 1992 and 1998 were designed to determine the geomorphological characteristics of hoya the mobility of the deposits and the characteristics of the snow cover these data formed the basis for a study of the lichen colonization on the blocks and on the wall surrounding the snow hollow all of the lichen species found were analyzed according to their abundance distribution and the extent of their surface cover measurements of the diameter of the thalli of the species rhizocarpon geographicum were also obtained thalli of this species were found to require a mean snow free growing season of at least 95 days 13 5 weeks per year maximum mean thallus diameters indicate that the protalus rampart was formed during the little ice age and became inactive 130 years ago

modelling the inception of the patagonian icesheet abstract we use a coupled climate icesheet model to simulate the inception and growth of the patagonian icesheet assuming a temperature lowering of 6 c and a uniform pressure gradient driving windfields an icesheet builds up with dimensions and morphology similar to that of the last glacial maximum analysis of the stages of growth reveals the following conclusions different sectors of the 1800 km long icesheet behave differently as a result of latitudinal variations in mass balance the fastest growth occurs in latitudes 45 52 s where the north and south patagonian icefields merge and expand to the coast within 1500 model years the most favourable topography for glacier initiation is the presence of high elevation basins low elevation summits in the mountain chain take a long time to become covered by ice because of the delay in ice spreading along the main mountain axis feedback between the increasing elevation of the icesheet and increasing mass balance on its windward flank enhances the rate of migration of the icesheet towards the precipitation source there is a lee side effect whereby easterly mountains are glaciated early on but are then starved of snow as ice builds up to the windward such locations should hold an empirical record of the early stages of a glacial cycle

cenozoic landscape evolution of the convoy range to mackay glacier area transantarctic mountains onshore to offshore synthesis on the basis of geomorphic mapping we reconstruct the landscape evolution of the convoy range to mackay glacier area of the rifted margin of the transantarctic mountains and compare the land record with that obtained from offshore marine sedimentary rocks in the ross sea three landform assemblages reflect 1 fluvial planation and dissection 2 local glaciation under temperate conditions and 3 overriding by the east antarctic ice sheet overall landscape evolution is typical of a passive continental margin contrasts in morphology between the convoy range and the adjacent dry valleys and royal society mountains reflect the varying location of the initial drainage divide in relationship to the rifted coast in a wider synthesis we draw the following conclusions for the 260 km long mcmurdo sector of the transantarctic mountains denudation since rifting at ca 55 ma has removed a wedge 4 7 km thick at the coast declining inland to 1 km most denudation occurred in the eocene from planation and incision of river valleys near the coast a subsequent pulse of denudation most rapid at 34 31 ma and declining until ca 17 ma coincided with further crustal extension and a change from cool temperate to polar climate during the same interval there was a progressive decrease in landscape modification by warm based glaciers and or rivers between 14 8 and 13 6 ma the maximum overriding antarctic ice sheet flowed northeastward across the mountains leaving meltwater features crossing high altitude saddles and areally scoured bedrock near the coast since 13 6 ma the landscape has seen little change under a hyperarid polar climate either by local glaciers or subaerially fragile middle miocene surficial deposits still survive there has been tectonic stability since the middle miocene

the last glacial maximum and deglaciation in southern south america abstract this paper models the extent of the patagonian icesheet during the last glacial maximum lgm and its subsequent deglaciation it constrains a new coupled icesheet climate numerical model with empirical evidence and simulates the icesheet at the lgm and at stages of deglaciation under lgm conditions an icesheet with a modelled volume slightly in excess of 500 000 km3 builds up along the andes there is a marked contrast between the maritime and continental flanks of the modelled icesheet with positive mass balance exceeding 2 m in the west and declining tenfold to the east modelled ice velocities commonly reach 400 m yr 1 in the western fjords the model is most sensitive to variations in temperature and good agreement between modelled ice extent and empirical evidence was achieved by applying a temperature decrease of 6 c relative to present day temperatures with constant wind fields over the model domain assuming a stepped start to deglaciation modelled ice volumes declined sharply contributing 1 2 m to global sea level 80 of it within 2000 years the empirical record suggests that such a stepped warming occurred around 17 500 17 150 cal yr ago

the influence of competition between lichen colonization and erosion on the evolution of soil surfaces in the tabernas badlands se spain and its landscape effects article i nfo badlands often contain a mosaic of soil surface types with contrasting hydrological behaviour which drives their short term geomorphic evolution the tabernas badlands in semiarid se spain show a complex mosaic of bare ground biological soil crusts and plant covered patches and high variability of covers and morphologies previous work has identified the surfaces that act as sources of runoff and sediments and those that act as sinks the pathways of runoff between surfaces and the runoff and erosion at catchment scale however surfaces without vascular plants can be quite dynamic with important effects on geomorphic processes this work aims to generate hypotheses about the dynamics of both bare soil 34 of the area and biological soil crusts 33 and to provide a first estimation of the growth rate of terricolous lichens as increase of coverage and their geomorphological implications the dynamics of ten representative soil surfaces were photographically monitored over 13 years recording the cover and pattern of bare soil and of the two main kinds of biological soil crust brown crust and white crust erosion deposition were measured by erosion pins in unbounded plots and the hydrological and erosional behaviour monitored in bounded plots under natural and simulated rainfall biological colonization and crust growth were studied from 2004 within cleared plots in four crust communities microclimate was continuously monitored in each community after 13 years net erosion was recorded only in previously eroded slopes and divides vegetated sites and those covered by biological soil crust remained more or less invariant or recorded sedimentation in all white crust surfaces whole crust cover increased by 3 on average while macrolichen cover increased by nearly 30 within the driest brown crust macrolichen cover increased by 7 while the whole crust decreased by 3 according to previous work lichen cover particularly of white crust is often greater in north to east facing orientations because of the higher slope stability and the longer duration of moisture however the increase of the whole biological soil crust can depend more on species composition and previous cover and less or not at all on the aspect although certain species do grow faster in particular orientations within the plots cleared in 2004 the greatest increase occurred in the pioneering brown crust which reached an average of 26 cover after one year in spite of it receiving the most insolation significant microclimatic differences exist among the communities mainly in radiation soil moisture and thallus surface temperature terricolous lichen crusts primary colonizers of bare sufficiently stable areas can grow quickly and by increasing soil stability and development and modifying surface hydrology they can influence surface dynamics and ultimately landform dynamics crust growth rate probably decreases as its cover increases and it probably also oscillates with microclimatic oscillations however the high potential growth rate recorded allows detection not only of climatic changes but also short term possibly seasonal periods of stability extensive lichen crusts indicate low erosion rates and particular climatic conditions variations of crust cover could indicate climatic or land use variations

glaciology of the british isles ice sheet during the last glacial cycle form flow streams and lobes abstract the last ice sheet over the british isles together with other mid latitude pleistocene ice sheets and in contrast to the modern ice sheets of greenland and antarctica had a relatively low profile low summit elevation and extensive elongated lobes at its margin a thermo mechanically coupled numerical ice sheet model driven by a proxy climate has been used to explore the properties that would have permitted these characteristics to develop the approach the key to quantitative palaeoglaciology is to determine the boundary conditions that permit the simulated ice sheet to mimic the evolution of the real ice sheet through the last glacial cycle simulations show how a british ice sheet may have been confluent with a scandinavian ice sheet during some parts of its history and how unforced periodic and asynchronous oscillations could occur in different parts of its margins marginal lobes are a reflection of streaming within the ice sheet such streams can be ephemeral dynamic streams located because of ice sheet properties or fixed streams whose location is determined by bed properties the simulations that best satisfy constraints of extent elevation and relative sea levels are those with major fixed streams that strongly draw down the ice sheet surface in these the core upland areas of the ice sheet were cold based at the last glacial maximum basal streaming velocities were between 500 and 1000 ma 1 compared with surface velocities of 10 50 ma 1 in inter stream zones shear stresses were as low as 15 25 kpa in streams compared with 70 110 kpa in upland areas and 60 84 of the ice flux was delivered to the margin via streams

the impact of migration on avo amplitude variation with offset avo analysis is often limited to areas where multidimensional propagation effects such as reflector dip and diffractions from faults can be ignored migration inversion provides a framework for extending the use of seismic amplitudes to areas where structural or stratigraphic effects are important in this procedure sources and receivers are downward continued into the earth using uncollapsed prestack migration instead of stacking the data as in normal migration the prestack migrated data are used in avo analysis or other inversion techniques to infer local earth properties the prestack migration can take many forms in particular prestack time migration of common angle sections provides a convenient tool for improving the lateral resolution and spatial positioning of avo anomalies in this approach a plane wave decomposition is first applied in the offset direction separating the wavefield into different propagating angles the data are then gathered into common angle sections and migrated one angle at a time the common angle migrations have a simple form and are shown to adequately preserve amplitude as a function of angle normal avo analysis is then applied to the prestack migrated data examples using seismic lines from the gulf of mexico show how migration improves avo analysis in the first set of examples migration is shown to improve imaging of subtle spatial variations in bright spots subsequent avo analysis reveals dim spots associated with dry hole locations that were not resolvable using traditional processing techniques including both conventional avo and poststack migration a second set of examples shows improvements in avo response after migration is used to reduce interference from coherent noise and diffractions a final example shows the impact of migration on the spatial location of dipping avo anomalies in all cases migration improves both the signal to noise ratio and spatial resolution of avo anomalies

lithology and fluid prediction from prestack seismic data using a bayesian model with markov process prior lithology and fluid prediction from prestack seismic data using a bayesian model with markov process prior

bayesian estimation in seismic inversion part i principles1 this paper gives a review of bayesian parameter estimation the bayesian approach is fundamental and applicable to all kinds of inverse problems its basic formulation is probabilistic information from data is combined with a priori information on model parameters the result is called the a posteriori probability density function and it is the solution to the inverse problem in practice an estimate of the parameters is obtained by taking its maximum well known estimation procedures like least squares inversion or l 1 norm inversion result depending on the type of noise and a priori information given due to the a priori information the maximum will be unique and the estimation procedures will be stable except in theory for the most pathological problems which are very unlikely to occur in practice the approach of tarantola and valette can be derived within classical probability theory the bayesian approach allows a full resolution and uncertainty analysis which is discussed in part ii of the paper

seismic inversion for reservoir properties combining statistical rock physics and geostatistics a review there are various approaches for quantitative estimation of reservoir properties from seismic inversion a general bayesian formulation for the inverse problem can be implemented in two different work flows in the sequential approach first seismic data are inverted deterministically or stochastically into elastic properties then rock physics models transform those elastic properties to the reservoir property of interest the joint or simultaneous work flow accounts for the elastic parameters and the reservoir properties often in a bayesian formulation guaranteeing consistency between the elastic and reservoir properties rock physics plays the important role of linking elastic parameters such as impedances and velocities to reservoir properties of interest such as lithologies porosity and pore fluids geostatistical methods help add constraints of spatial correlation conditioning to different kinds of data and incorporating subseismic scales of heterogeneities

impact of rock physics depth trends and markov random fields on hierarchical bayesian lithology fluid prediction early assessments of petroleum reservoirs are usually based on seismic data and observations in a small number of wells decision making concerning the reservoir will be improved if these data can be integrated and converted into a lithology fluid map of the reservoir we analyze lithology fluid prediction in a bayesian setting based on prestack seismic data and well observations the likelihood model contains a convolved linearized zoeppritz relation and rock physics models with depth trends caused by compaction and cementation well observations are assumed to be exact the likelihood model contains several global parameters such as depth trend wavelets and error parameters the inference of these is an integral part of the study the prior model is based on a profile markov random field parameterized to capture different continuity directions for lithologies and fluids the posterior model captures prediction and model parameter uncertainty and is assessed by markov chain monte carlo simulation base

last glacial maximum climate in new zealand inferred from a modelled southern alps icefield abstract we present a simulation of the new zealand southern alps icefield at the last glacial maximum lgm c 30 000 20 000 calendar years ago ka in an attempt to constrain the climate of that period we use a 500 m resolution ice sheet model parameterised using empirical glaciological climatological and geological data specific to the model domain to simulate the entire southern alps icefield we find that an lgm cooling of at least 6 6 5 c is necessary to bring about valley glaciers that extend beyond the mountains however climate topography thresholds related to the elevation and hypsometry of individual catchments control the gradient of the rate of glacier expansion in the domain and in order to remain within geologically reconstructed lgm limits we find that the lgm cooling was most likely associated with a precipitation regime up to 25 drier than today wetter than present scenarios give rise to equilibrium line depressions and ice extents that are incompatible with empirical evidence these results perhaps indicate that either the westerly air masses affecting new zealand during the lgm were drier than today or that they were weaker or zonally displaced with respect to present

quantifying the uncertainty in an avo interpretation lithologic interpretations of amplitude variation with offset avo information are ambiguous both because different lithologies occupy overlapping ranges of elastic properties and because angle dependent reflection coefficients estimated from seismic data are uncertain this paper presents a method for quantifying and combining these two components of uncertainty to get a full characterization of the uncertainty associated with an avo based lithologic interpretation the result of this approach is a compilation of all the lithologies that are consistent with the observed avo behavior along with a probability of occurrence for each lithology a 2 d line from the north sea illustrates how the method might be applied in practice for any data set the interaction between the geologic and measurement components of uncertainty may significantly affect the overall uncertainty in a lithologic interpretation

bayesian lithology fluid prediction and simulation on the basis of a markov chain prior model a technique for lithology fluid lf prediction and simulation from prestack seismic data is developed in a bayesian framework the objective is to determine the lf classes along 1d profiles through a reservoir target zone a stationary markov chain prior model is used to model vertical continuity of lf classes along the profile the likelihood relates the lf classes to the elastic properties and to the seismic data and it introduces vertical correlation because the seismic data are band limited an approximation of the likelihood model provides an approximate posterior model that is a markov chain the approximate posterior can be assessed by an exact and efficient recursive algorithm the lf inversion approach is evaluated on a synthetic 1d profile that is inspired by a north sea sandstone reservoir with a realistic wavelet colored noise model and a s n ratio of three in the seismic data the results are reliable the lf classes and the interfaces between zones are largely correct the prediction unce

late quaternary glacial phases in the iberian peninsula abstract the only glaciers existing today in the iberian peninsula are small features located in the pyrenees though their number and extension has undergone significant changes over the late quaternary the wide range of glacial landforms and deposits distributed across different iberian ranges suggests the occurrence of several past periods with larger glacial systems the objective of this research is to summarize the current knowledge on the spatial and temporal patterns of glacial activity in the iberian mountains during the late quaternary to this purpose the chronological framework was divided in six periods glaciations prior to the last glacial cycle middle pleistocene last glacial cycle late pleistocene termination 1 holocene little ice age lia and present day the data were geographically divided considering the mountain systems where glacial evidence exists pyrenees cantabrian range nw ranges central range iberian range and sierra nevada during quaternary cold stages ice accumulated in the head valleys of these mountain ranges and glaciers flowed down valleys in all cases glaciers remained confined within the mountain systems and did not reach the surrounding lowlands depending on the combination of temperatures and moisture conditions more or less ice was stored in some ranges there is evidence of middle pleistocene glaciations one potentially correlating with marine isotope stage mis 12 and another correlating with mis 6 with glaciation dated to ca 130 170 ka however most of the glacial records correspond to the last glacial cycle and subsequent termination the maximum glacial expansion of this last pleistocene glaciation stage occurred well before the global last glacial maximum lgm between 30 and 60 ka in the cantabrian mountains and pyrenees at ca 30 ka in sierra nevada and nw ranges and almost synchronously to the lgm in the central range and iberian range a massive glacial retreat occurred in all ranges at 19 20 ka but the long term deglaciation process was interrupted by cold intervals such as the oldest and younger dryas which favoured glacial expansion in the highest mountains temperature increase recorded during the holocene conditioned the melting of glaciers which only reappeared in the highest massifs during the coldest periods such as the lia however post lia warming led to glacier disappearance in the cantabrian mountains sierra nevada and most massifs of the pyrenees together with an accelerated shrinkage of the small glaciers still existing in this range at elevations near 3000 m

maximum glacial advance and deglaciation of the pinar valley sierra de gredos central spain and its significance in the mediterranean context abstract pinar valley is located on the north side of galana peak 40 15 21 n 5 18 00 w 2564 m a s l sierra de gredos in the center of the iberian peninsula surface exposure ages based on the in situ accumulation of 36 cl were obtained from six moraine boulders and from three bedrock outcrops to investigate the timing of both the last local glacial maximum and the deglaciation the oldest moraines which probably overrode older glacial deposits are coeval to the global last glacial maximum subsequently the pinar glacier underwent minor pulsations close to its maximum position followed by general glacier retreat after 17 16 ka small cirque glaciers probably remained during the terminal pleistocene but they had completely disappeared by 10 ka

european ice sheet modelling initiative eismint model intercomparison experiments with first order mechanics 1 the european ice sheet modelling initiative eismint intercomparison experiments with thermomechanical coupling are repeated with an ice sheet model that applies the first order approximation for computing the flow field the experiments impose radially symmetric boundary conditions most of the previous results have shown the loss of implied radial symmetry i e the formation of distinct regularly spaced spokes of cold ice in the simulated basal temperatures a similar result is also observed in the presented first order model results the computed velocity components scatter widely along the marginal grid points this indicates that the spokes may be triggered by the poor representation of the margin with a regular grid where the steep gradients in the surface enhance the numerical errors additionally the generally applied second order discretization scheme tends to decouple even and odd numbered grid points thus leading to wavy solutions with a wavelength of two grid cells these patterns strongly suggest that the loss in radial symmetry is a numerical artifact

bayesian lithology and fluid prediction from seismic prestack data a fast bayesian inversion method for 3d lithology and fluid prediction from prestack seismic data and a corresponding feasibility analysis were developed and tested on a real data set the objective of the inversion is to find the probabilities for different lithology fluid classes from seismic data and geologic knowledge the method combines stochastic rock physics relations between the elastic parameters and the different lithology fluid classes with the results from a fast bayesian seismic simultaneous inversion from seismic data to elastic parameters a method for feasibility analysis predicts the expected modification of the prior probabilities to posterior probabilities for the different lithology fluid classes the feasibility analysis can be carried out before the seismic data are analyzed both the feasibility method and the seismic lithology fluid probability inversion were applied to a prospect offshore norway the analysis improves the probability for gas sand from 0 1 to about 0 2 0 4 with s

last glacial maximum and deglaciation of sierra de gredos central iberian peninsula abstract the study area is located in the sierra de gredos a portion of the sistema central range of central spain specifically in the gredos gorge on the north side of pico almanzor 2596 masl where glacial forms are present down to an altitude of 1410 m 10 km from the headwall this paper presents cosmogenic 36 cl surface exposure dates of four closely spaced lateral moraine ridges within the area of maximum advance of the ancient glacier 8 km from the headwall and two glacially polished bedrock thresholds 3 and 5 km from the headwall the dates are overall coherent and indicate a maximum advance at 26 24 ka subsequently the glacier front stabilized around its maximum position for 3 ka with small scale fluctuations resulting in small closely spaced moraines ridges glacier recession began after 21 ka accelerating sharply by 16 ka the glacier had probably disappeared from the gredos gorge by 15 ka there are no traces of older or more recent moraines than those dated in this study these results are in agreement with the well known late pleistocene climatic evolution of the northern atlantic and with the glacial chronology of the alps and other mediterranean mountains however the gredos gorge has no geomorphic evidence of the younger dryas cold event probably because of the low altitude and southern character of these mountains

the impact of common offset migration on porosity estimation by avo inversion the impact of prestack time migration on porosity estimation has been tested on a 2 d seismic line from the valhall hod area in the north sea porosity is estimated in the cretaceous chalk section in a two step procedure first p wave and s wave velocity and density are estimated by amplitude variation with offset avo inversion these parameters are then linked to porosity through a petrophysical rock data base based on core plug analysis the porosity is estimated both from unmigrated and prestack migrated seismic data for the migrated data set a standard prestack kirchhoff time migration is used followed by simple angle and amplitude corrections compared to modern high cost true amplitude migration methods this approach is faster and more practical the test line is structurally fairly simple with a maximum dip of 5 but the results differ significantly depending on whether migration is applied prior to the inversion the maximum difference in estimated porosity is of the order of 10 about

bayesian wavelet estimation from seismic and well data a bayesian method for wavelet estimation from seismic and well data is developed the method works both on stacked data and on prestack data in form of angle gathers the seismic forward model is based on the convolutional model where the reflectivity is calculated from the well logs possible misties between the seismic traveltimes and the time axis of the well logs errors in the log measurements and seismic noise are included in the model the estimated wavelets are given as probability density functions such that uncertainties of the wavelets are an integral part of the solution the solution is not analytically obtainable and is therefore computed by markov chain monte carlo simulation an example from sleipner field shows that the estimated wavelet has higher amplitude compared to wavelet estimation where well log errors are neglected and the uncertainty of the estimated wavelet is lower

joint avo inversion wavelet estimation and noise level estimation using a spatially coupled hierarchical bayesian model the main objective of the avo inversion is to obtain posterior distributions for p wave velocity s wave velocity and density from specified prior distributions seismic data and well log data the inversion problem also involves estimation of a seismic wavelet and the seismic noise level the noise model is represented by a zero mean gaussian distribution specified by a covariance matrix a method for joint avo inversion wavelet estimation and estimation of the noise level is developed in a bayesian framework the stochastic model includes uncertainty of both the elastic parameters the wavelet and the seismic and well log data the posterior distribution is explored by markov chain monte carlo simulation using the gibbs sampler algorithm the inversion algorithm has been tested on a seismic line from the heidrun field with two wells located on the line the use of a coloured seismic noise model resulted in about 10 lower uncertainties for the p wave velocity s wave velocity and density compared with a white noise model the uncertainty of the estimated wavelet is low in the heidrun example the effect of including uncertainty of the wavelet and the noise level was marginal with respect to the avo inversion results

spatial and temporal variability of periglaciation of the iberian peninsula abstract active periglacial processes are currently marginal in the iberian peninsula spatially limited to the highest mountain ranges however a wide variety of periglacial deposits and landforms are distributed in low and mid altitude environments which shows evidence of past periods of enhanced periglacial activity the purpose of this paper is to summarize the present knowledge of past periglacial activity in the iberian peninsula the chronological framework takes four main stages into account the last glaciation deglaciation holocene and present day processes this study focuses on the highest massifs pyrenees cantabrian range nw ranges central range iberian range sierra nevada as well as other lower elevation environments namely the central iberian meseta during the last glaciation the periglacial belt extended to much lower altitudes than today reaching current sea level in the nw corner of the iberian peninsula a wide range of geomorphological landforms and sedimentary records is indicative of very active periglacial processes during that phase in some cases related to permafrost conditions i e block streams rock glaciers most of the inactive landforms and deposits in low and mid elevations in iberia are also related to this phase the massive deglaciation of the iberian massifs was caused by a gradual increase in temperatures the deglaciation phase was only interrupted by a short period with colder conditions the younger dryas that reactivated periglacial processes in the formerly glaciated cirques of the highest lands specifically with the widespread development of rock glaciers during the holocene periglacial processes have been only active in the highest ranges shifting in altitude according to temperature regimes and moisture conditions the little ice age saw the reactivation of periglacial activity in lower elevations than today currently periglacial processes are only active in elevations exceeding 2500 m in the southern ranges and above 2000 2200 m in the northern massifs higher in sierra nevada in the south of iberian peninsula

modelling landscape evolution under ice sheets we use an ice sheet model glimmer with an erosion component to examine the evolution of landscapes under ice sheets over long time scales in hypothetical situations glimmer is a fully coupled thermomechanical 3 dimensional ice sheet model with a sliding component the model thus has the ability to predict ice thickness the distribution of areas at pressure melting point and basal velocities the erosion rate is assumed to be a simple linear function of the basal velocity and therefore erosion only occurs where the bed is not frozen and basal sliding is possible in addition sliding velocities are assumed to be related to basal water production thus giving a mechanism for smooth temporal and spatial transitions between eroding and non eroding portions of the bed the eroding glacial landscape system is modelled for long term persistent glaciation assuming a variety of initial landscapes results indicate that without the inclusion of basal water production in the calculation of sliding velocities any thermal feedbacks are exaggerated because of steep horizontal basal temperature gradients additionally bed morphology exerts a greater influence under low basal slip conditions because ice cannot respond readily to thermal instabilities we suggest that erosion and valley overdeepening may aid stabilisation of the thermal regime of an ice sheet and that it is possible to generate glacial signals in previously fluvial systems after only 100 kyr of glaciation at conservative erosion rates

improved resolution in bayesian lithology fluid inversion from prestack seismic data and well observations part 1 methodology the focus of our study is lithology fluid inversion with spatial coupling from prestack seismic amplitude variation with offset avo data and well observations the inversion is defined in a bayesian setting where the complete solution is the posterior model the prior model for the lithology fluid lf characteristics is defined as a profile markov random field model with lateral continuity each vertical profile is further given as an inhomogeneous markov chain model upward through the reservoir the likelihood model is defined by profile and it relates the lf characteristics to the seismic data via a set of elastic material parameters and a convolution model the likelihood model is approximated the resulting approximate posterior model is explored using an efficient block gibbs simulation algorithm the inversion approach is evaluated on a synthetic realistic 2d reservoir seismic avo data and well observations are integrated in a consistent manner to obtain predictions of the lf characteristics wi

ice flow pattern and extent of the last scandinavian ice sheet southeast of the baltic sea a review of scientific literature geological maps and available previous digital data on ice marginal positions glacial landforms and sediment distribution are utilised to reconstruct ice streams and lobes and the maximum lgm and subsequent recessional ice marginal positions of the scandinavian ice sheet sis southeast of the baltic sea this paper presents preliminary results of the ongoing research that aims to build a geographic information system gis based model on the extent and timing of the last sis in the area between its maximum extent and the baltic sea digitized subglacial bedforms ice marginal and other glacial landforms and features from published sources are compared and validated against digital elevation model dem this has allowed specifying revising and questioning the location of the lgm and to interpret and correlate the post lgm ice streams with marginal positions morphological evidence demonstrates that large ice streams of between 100 and 300 km lengths were in operation ca 2 3 ka after the lgm and formed the middle and north lithuanian end moraines ice streams from the onega and white sea basins had high lateral slopes and clearly channelled flow while the ice streams that drained the ice sheet through the ladoga ilmen depression and in the eastern baltic usually had a fan shaped flow pattern and morphologically unclear lateral slopes in elevated areas and highlands that are located between ice streams a crossing of lineated bedforms is common one set of the crossing lineations on zemaitia eastern kursa vidzeme haanja latgale and sudoma highlands that are 200 350 km inside of the lgm margin has northwest southeast orientation that in general conforms to the direction of ice flow during the lgm and baltija phases it is expected that the presented compilation will stimulate discussions and scrutiny of earlier published data and generate ideas for future investigations

what controls the location of ice streams ice streams influence ice sheet mass balance and stability but key aspects of their behaviour remain poorly understood this paper reviews and discusses one very important aspect what controls their location in an ice sheet seven potential controls on ice stream location are identified from the literature topographic focusing topographic steps macro scale bed roughness calving margins subglacial geology geothermal heat flux and subglacial meltwater routing for each control the theoretical basis for its link to rapid ice flow is introduced followed by discussion of the evidence of its influence on the location of both contemporary and palaeo ice streams based on this new synthesis topographic focusing subglacial geology meltwater routing and calving margins appear to be most commonly associated with fast ice flow it is clear that rather than a single control however there exist a number of potential controls of varying influence we propose a hierarchy of factors with those occurring at the top of the hierarchy exerting a stronger influence on ice stream location and where present beneath an ice sheet are very likely to be associated with fast flow those factors occurring lower down the hierarchy are less commonly associated with ice streaming but appear to be influential in the absence of more common controls in such a hierarchy topographic focusing in the presence of a calving margin is the primary control in the absence of this ice streams will preferentially occur in areas with favourable subglacial meltwater routing and subglacial geology in the absence of these bed roughness geothermal heat flux and topographic steps may promote ice streaming significantly the primary controls on a given ice stream location are likely to influence its spatial and temporal dynamics ice streams governed by the presence of meltwater routing and or calving processes might exhibit more variable behaviour because these controls can vary over relatively short time scales compared to controls that vary over longer time scales e g geothermal heat flux subglacial roughness geology and topography identifying the controls on ice stream location is therefore of paramount importance when understanding ice stream longevity and their past and future activity