debris bed friction of hard bedded glaciers 1 field measurements of debris bed friction on a smooth rock tablet at the bed of engabreen a hard bedded temperate glacier in northern norway indicated that basal ice containing 10 debris by volume exerted local shear traction of up to 500 kpa the corresponding bulk friction coefficient between the dirty basal ice and the tablet was between 0 05 and 0 08 a model of friction in which nonrotating spherical rock particles are held in frictional contact with the bed by bed normal ice flow can account for these measurements if the power law exponent for ice flowing past large clasts is 1 a small exponent n 2 is likely because stresses in ice are small and flow is transient numerical calculations of the bed normal drag force on a sphere in contact with a flat bed using n 1 show that this force can reach values several hundred times that on a sphere isolated from the bed thus drastically increasing frictional resistance various estimates of basal friction are obtained from this model for example the shear traction at the bed of a glacier sliding at 20 m a 1 with a geothermally induced melt rate of 0 006 m a 1 and an effective pressure of 300 kpa can exceed 100 kpa debris bed friction can therefore be a major component of sliding resistance contradicting the common assumption that debris bed friction is negligible

effects of basal debris on glacier flow glacier movement is resisted partially by debris either within glaciers or under glaciers in water saturated layers in experiments beneath a thick sliding glacier ice containing 2 to 11 debris exerted shear traction of 60 to 200 kilopascals on a smooth rock bed comparable to the total shear traction beneath glaciers and contrary to the usual assumption that debris bed friction is negligible imposed pore water pressure that was 60 to 100 of the normal stress in a subglacial debris layer reduced shear traction on the debris sufficiently to halt its deformation and cause slip of ice over the debris slip resistance was thus less than debris shearing resistance

joint bounded crescentic scars formed by subglacial clast bed contact forces implications for bedrock failure beneath glaciers glaciers and ice sheets are important agents of bedrock erosion yet the precise processes of bedrock failure beneath glacier ice are incompletely known subglacially formed erosional crescentic markings crescentic gouges lunate fractures on bedrock surfaces occur locally in glaciated areas and comprise a conchoidal fracture dipping down ice and a steep fracture that faces up ice here we report morphologically distinct crescentic scars that are closely associated with preexisting joints termed here joint bounded crescentic scars these hitherto unreported features are ca 50 200 mm deep and involve considerably more rock removal than previously described crescentic markings the joint bounded crescentic scars were found on abraded rhyolite surfaces recently exposed 20 years beneath a retreating glacier in iceland as well as on glacially sculpted precambrian gneisses in nw scotland and various precambrian rocks in ontario glaciated during the late pleistocene we suggest a common formation mechanism for these contemporary and relict features whereby a boulder embedded in basal ice produces a continuously migrating clast bed contact force as it is dragged over the hard bedrock bed as the ice embedded boulder approaches a preexisting joint in the bedrock stress concentrations build up in the bed that exceed the intact rock strength resulting in conchoidal fracturing and detachment of a crescentic wedge shaped rock fragment subsequent removal of the rock fragment probably involves further fracturing or crushing comminution under high contact forces formation of joint bounded crescentic scars is favoured by large boulders at the base of the ice high basal melting rates and the presence of preexisting subvertical joints in the bedrock bed we infer that the relative scarcity of crescentic markings in general on deglaciated surfaces shows that fracturing of intact bedrock below ice is difficult but that preexisting weaknesses such as joints greatly facilitate rock failure this implies that models of glacial erosion need to take fracture patterns of bedrock into account

time lapse seismic monitoring some shortcomings in nonuniform processing conceptually repeat or time lapse seismic monitoring is straightforward seismic data sets acquired at different times are calibrated compared and the intersurvey differences analyzed and interpreted in terms of fluid changes or movement

basal flow characteristics of a non linear flow sliding frictionless over strongly undulating bedrock the flow field of a medium sliding without friction over a strongly undulating surface is calculated numerically the results are used to elucidate the basal flow characteristics of glacier flow and they are discussed with reference to known analytical solutions extrusion flow is found to become increasingly pronounced as the value of n where n is a parameter in glen s flow law becomes larger for sinusoidal bedrock undulations a flow separation occurs if the amplitude to wavelength ratio exceeds a critical value of about 0 28 the main flow then sets up a secondary flow circulation within the trough and the ice participating in this circular motion theoretically never leaves it the sliding velocity is calculated numerically as a function of the mean basal shear stress the amplitude to wavelength ratio and the flow parameter n for moderate and high slope fluctuations the sliding velocity is significantly different from what would be expected from results based on the small slope approximation

rheology of ice at the bed of engabreen norway a three dimensional finite element model is used to analyze field data collected as dirty basal ice flowed past an instrumented obstacle at the bed of engabreen a temperate glacier in northern norway the ice is modeled as an incompressible power law fluid with viscosity b 2 1 n 2n d where d is the second invariant of the stretching tensor and b and n are two parameters using measurements obtained in 1996 and 1997 two values of b are obtained one using the measured normal stress difference across the obstacle and the other using the measured bed parallel force over the instrument these two values are not equal probably owing to small frictional forces at the bed unaccounted for in the numerical model hence b ranges between 1 9 10 7 and 3 2 10 7 pa s 1 3 in 1996 and between 2 2 10 7 and 4 1 10 7 pa s 1 3 in 1997 these values are smaller than measured elsewhere for clean glacier or laboratory ice field measurements of water content fabric and texture of the basal ice suggest that unbound water between thin sediment layers and lamellae of clean ice may act as a lubricant and significantly weaken the ice near isotropic fabrics indicate that preferred fabric orientation does not enhance the deformation

anomalous magnetizations in 3 4 b y old barberton mountain land samples the ratio of natural remanence nrm to saturation remanence irms is found to be exceedingly high for a number of basaltic to ultramafic samples from the 3 4 billion year old barberton mountain land greenstone belt south africa although conventional paleointensity methods indicate paleofields of several oersteds demagnetization curves plotted on logarithmic scale clearly show the nrm in these samples to be unlike thermal remanence trm it seems most probable that the observed nrm is a chemical remanence crm related to the greenschist metamorphic event and that the intense magnetizations are perhaps the result of internally generated rather than external fields the probability that the intense magnetization observed in these ancient terrestrial samples is unrelated to the strength of the external field suggests caution in the interpretation of paleointensity results from extraterrestrial materials

a level set method for modeling the evolution of glacier geometry a level set method is proposed for modeling the evolution of a glacier surface subject to a prescribed mass balance this leads to a simple and versatile approach for computing the evolution of glaciers the description of vertical fronts and overriding phenomena presents no difficulties topological changes are handled naturally and steady state solutions can be calculated without integration over time a numerical algorithm is put forth as a means of solving the proposed model of glacier surface evolution it is evaluated by comparing different numerical solutions of the model with analytical and published numerical solutions the level set method appears to be a reliable approach for dealing with different glaciological problems

realistic yet simple bottom boundary conditions for glaciers and ice sheets over a nondeformable and impermeable bed the sliding law has to be a relationship between the bottom shear stress b the sliding velocity u and the effective pressure n pi pw where pi is the ice load and pw is the pressure in the interconnected water cavities three horizontal scales are recognized 0 1 m 10 m and 10 km classical theories consider the smallest scale they are of dubious value because temperate ice is neither dry nor totally impermeable the intermediate scale yields sliding laws to be used in glacier modeling melting refreezing processes may then be ignored for modeling large cold ice sheets the sliding law at the largest scale is needed it is suggested that sliding occurs only when a bottom temperate layer exists how to tackle non newtonian rheology is discussed at the intermediate scale a microrelief model consisting of two superimposed bumpy profiles is favored if the smaller one had bumps of equal height b would be double valued but this model is unrealistic the ice avalanche at the allalingletscher in 1965 may be explained otherwise therefore a model with the smaller bumps of unequal height is adopted it yields an asymptotic sliding law at large sliding velocities b n cun1 n with and c as adjustable parameters a third one has to be introduced at low velocities although the correct interpolation function between both extreme cases remains unknown some qualitative results about kinematic waves are obtained

the effect of the subglacial water pressure on the sliding velocity of a glacier in an idealized numerical model in order to interpret observed short term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model in particular the transient stages of growing or shrinking water filled cavities at the ice bedrock interface were analysed it was found that the sliding velocity was larger when cavities were growing than when they had reached the steady state size for a given water pressure the smallest sliding velocities occurred while cavities were shrinking when cavitation is substantial a small drop of water pressure below the steady state value e g by 0 5 bar can temporarily cause backward sliding a limiting water pressure at which sliding becomes unstable is derived the consequences of more realistic assumptions than those of the model are discussed

analyzing three component 3d vertical seismic profiling data a three component 3d vertical seismic profile vsp was acquired over the blackfoot oil field in alberta canada the vsp survey was recorded simultaneously with a surface seismic program the objectives of the vsp were to develop recording logistics data handling and processing procedures and to determine if the 3d vsp volumes could image the glauconitic sand reservoir of the blackfoot field dynamite shots from the surface seismic survey which fell within a 2200 m offset from the recording well were used in the vsp analysis the shots were recorded by a string of three component borehole receivers that was moved seven times resulting in a receiver depth range of 400 to 910 m the borehole data were processed using basic vsp processing techniques that included hodogram analysis wavefield separation using median filters and vsp deconvolution the final p p and p s image volumes were obtained by vsp common depth point and vsp common conversion point stacking the upgoing wavefields followed by f xy de

streamer depth versus vessel and seismic interference noise abstractfor marine seismic surveying it is commonly assumed that the noise level decreases with depth in addition recent advances in broadband seismic have shown that a greater receiver depth is beneficial in preserving low frequency data however in a heavily trafficked ocean noise from other ships including seismic interference is a counteractive process in which the noise actually varies with depth normal modes can be used to explain and predict the ship noise and seismic interference noise level we find that weather noise is dominant below the first mode s cutoff frequency approximately 6 hz ship noise is dominant from that frequency to the upper end of the useful seismic frequency band 80 hz we have used a data set in which the streamer was towed at 8 45 and 60 m depths in three passes over the same area in the north sea the water depth is 135 m on average we observe that the noise level at 45 and 60 m depth is approximately 1 6 times stronger than that at 8 m we find that the air

experimental rock on rock frictional wear application to subglacial abrasion 1 rock on rock wear under known normal loads was measured experimentally using 62 mm diameter rotating discs machined from a range of silicic rock types one granite two metamorphic rocks and three sandstones of porosities varying between 7 and 28 wear debris was removed continuously from the contacting surfaces by water we obtained a wear law of the form dt dx a n m where is contact normal stress mpa dt dx is a dimensionless wear rate meter wear t per meter shear displacement x is porosity and a n and m are empirical constants 10 8 11 8 33 and 4 50 respectively low porosity rocks displayed surface polishing and abraded particle size was substantially smaller than the starting grain size whole grains were plucked from high porosity rocks without any surface polishing so that the wear product particle size remained the same as that of the starting material the empirical wear law describes abrasion beneath a hard bedded temperate glacier provided we can estimate both the in situ normal stress between entrained clasts and the bed and the horizontal velocity of the clasts in the flowing ice normal stress concentration is expected to develop at clast bed contacts from drag against ice flowing toward the bed where pressure melting occurs these experimental data are consistent with wear rates inferred from silt outflow in subglacial streams provided a tenfold stress concentration can occur relative to the ice overburden pressure the rate of production of fine grained wear products under subglacial conditions is substantially less than by tectonic faulting

calving processes and the dynamics of calving glaciers abstract calving of icebergs is an important component of mass loss from the polar ice sheets and glaciers in many parts of the world calving rates can increase dramatically in response to increases in velocity and or retreat of the glacier margin with important implications for sea level change despite their importance calving and related dynamic processes are poorly represented in the current generation of ice sheet models this is largely because understanding the calving problem involves several other long standing problems in glaciology combined with the difficulties and dangers of field data collection in this paper we systematically review different aspects of the calving problem and outline a new framework for representing calving processes in ice sheet models we define a hierarchy of calving processes to distinguish those that exert a fundamental control on the position of the ice margin from more localised processes responsible for individual calving events the first order control on calving is the strain rate arising from spatial variations in velocity particularly sliding speed which determines the location and depth of surface crevasses superimposed on this first order process are second order processes that can further erode the ice margin these include fracture propagation in response to local stress imbalances in the immediate vicinity of the glacier front undercutting of the glacier terminus by melting at or below the waterline and bending at the junction between grounded and buoyant parts of an ice tongue calving of projecting submerged ice feet can be regarded as a third order process because it is paced by first or second order calving above the waterline first order calving can be represented in glacier models using a calving criterion based on crevasse depth which is a function of longitudinal strain rate modelling changes in terminus position and calving rates thus reduces to the problem of determining the ice geometry and velocity distribution realistic solutions to the problem of modelling ice flow therefore depend critically on an appropriate choice of sliding law models that assume that basal velocities are controlled by basal drag can replicate much of the observed behaviour of calving glaciers with grounded termini but an important limitation is that they cannot be used to model floating glacier termini or ice shelves alternative sliding laws that parameterise drag from the glacier margins provide more flexible and robust ways of representing calving in ice sheet models such models can explain a remarkable range of observed phenomena within a simple unifying framework including downglacier increases in velocity and strain rates where basal and or lateral drag diminishes flow acceleration in response to thinning through time the tendency for glaciers to stabilise at pinning points in relatively shallow water or fjord narrowings the constraints on ice shelf stability and the contrasts in calving rates between tidewater and freshwater calving glaciers many unresolved issues remain however including the role played by the removal of backstress in the acceleration of retreating calving glaciers and the controls on melting at and below the waterline

uncertainties in quantitative time lapse seismic analysis most seismic time lapse studies so far have been of a qualitative nature identification of areas with minor or no seismic changes has been used to plan new infill drilling targets increased accuracy in seismic acquisition methods in both conventional streamer surveys and newer methods such as multicomponent sea bed seismic and permanent sensors opens possibilities for the next step quantitative time lapse analysis quantitative methods here mean the estimation of for instance a change in fluid saturation from 20 water to 90 water or the estimation of a pore pressure change of 5 mpa explicit expressions for the uncertainties associated with estimated changes in for instance reservoir pressure and fluid saturation are derived these formulae can be used to compare relative uncertainties between estimated parameters as well as to identify the critical factors in various estimation techniques the importance of accurate rock physics input as well as that of highly repeatable time lapse seismic data is emphasized furthermore uncertainty analysis can be used to find optimal weight factors when the same parameter e g saturation change is estimated by two or three different techniques

repeatability issues of 3 d vsp data increased repeatability is recognized as one major issue for improving the time lapse seismic technology as a reservoir management tool a 3 d vertical seismic profiling vsp data set acquired over a period of two days is used to analyze how repeatable a permanent installed geophone array can be and how repeatability changes with inaccuracies in source positioning it is found that for a frequency range between 3 5 and 50 hz the difference root mean square rms level between two recorded traces belonging to two different shots is about 8 this fact shows that there is a potential for acquiring very accurate time lapse seismic data by using a permanently installed downhole geophone array repeatability variation with increasing shot separation distances is analyzed showing a rapid decrease in repeatability as the accuracy of the positioning of the repeat survey decreases horizontal geophone components show approximately the same degree of repeatability compared to the vertical component but horizontal geophone data is slightly more sensitive to positioning errors the results show that repeated 3 d vsp surveys preferably using permanently installed geophone arrays might be an efficient tool for detailed and precise monitoring of fluid and pressure changes within a hydrocarbon reservoir

the importance of pressurized subglacial water in separation and sliding at the glacier bed l effet de l eau sous glaciaire sous pression sur le processus de glissement est examine a partir d un parametre appele indice de decollement du lit qui exprime les contraintes perpendiculaires au lit et la pression d eau les donnees proviennent de trois glaciers de formes differentes une loi puissance du type de weertman avec un exposant voisin de 3 modifiee pour tenir compte de l effet de la pression des eaux sous glaciaires apparait comme etant la meilleure

nrm irm s demagnetization plots of intrusive rocks and the origin of their nrm abstract alternating field demagnetization characteristics of laboratory induced thermal remanence trm and saturation isothermal remanence irm s have been determined for a collection of samples representing the major intrusive rock types these results are compared to nrm irm s curves for the same intrusive specimens and for a smaller number of extrusive and high grade metamorphic rocks samples a number of the intrusive rock specimens which contained primary magnetic mineralogies display nrm irm s curves which are similar in character to their trm irm s counterparts the nrms of these samples are generally unidirectional and resistant to thermal demagnetization suggesting their thermal origin intrusive samples with dissimilar nrm irm s and trm irm s curves are often directionally unstable or contain secondary or altered magnetic mineralogies in contrast to the mixed behavior of the intrusive samples the nrms for a collection of extrusive rock samples are generally trm like whereas the nrms of high grade metamorphic rock samples more often display characteristics which are inconsistent with a simple thermal origin of nrm this study suggests that nrm irm s curves may be useful in evaluating whether the remanent magnetization of crystalline rocks is predominantly of primary or secondary origin

dynamic controls on glacier basal motion inferred from surface ice motion 1 current heuristic laws that relate the motion of glaciers due to sliding along the bed to the subglacial water pressure fail to reproduce variations in sliding speed on timescales of specific hydrologic events such as lake drainage rainfall or surging this may be due to the importance of subglacial cavity evolution and shifts in the glacier stress field both of which are not accounted for in typical sliding laws we use multiple time series of surface motion over a 66 day period at brei amerkurjokull iceland to infer changes in bed separation and longitudinal force budget we observe multiple distinct periods of increased surface motion and uplift corresponding to periods of rainfall and or increased temperatures we find consistent hysteresis and lags between motion and variations in both the bed separation and longitudinal stress gradient that we attribute to the redistribution of normal stresses at the bed during cavity growth increases in the longitudinal stress gradient suggest a downglacier stress transfer during increased basal motion that is consistent with increased drainage system efficiency toward the terminus our results suggest that the transient evolution of the subglacial drainage system and shifts in the glacier stress field are important controls on basal motion

glacial erosion and bedrock properties in nw scotland abrasion and plucking hardness and joint spacing subglacial erosion beneath glaciers occurs predominantly by abrasion and plucking producing distinct erosional forms the controls on the relative importance of abrasion vs plucking are poorly understood on the one hand glacial conditions that favour or suppress cavity formation ice velocity ice thickness and water pressure are thought to favour plucking or abrasion respectively conversely bedrock properties are also known to control landforms but this has rarely been analysed quantitatively in this study we compare landforms and bedrock properties of sandstone and quartzite at the bed of a palaeo ice stream near ullapool in nw scotland the boundary between the rock types is at right angles to the westward palaeo ice flow and palaeoglacial conditions on both rock types were similar we report quantitative parameters for bedrock properties schmidt hammer hardness and joint spacing and use morphometric parameters to analyse the landforms torridon sandstone is soft but thick bedded and with a wide joint spacing erosional bedforms include roche moutonnees with smoothed tops and concave stoss sides whalebacks and elongate p forms indicating a high proportion of abrasion over plucking cambrian quartzite is hard but thin bedded with narrow joint spacing erosional landforms are angular to subangular with abundant plucked lee faces suggesting a high proportion of plucking over abrasion hardness and joint spacing thus exert a strong control on subglacial erosional landforms and the mechanisms that formed them thus glacial conditions ice velocity ice thickness can only be inferred from glacial erosional landforms if the effects of bedrock properties of the substrate are considered

estimation of layer thickness and velocity changes using 4d prestack seismic data in some hydrocarbon reservoirs severe compaction of the reservoir rocks is observed this compaction is caused by production and is often associated with stretching and arching of the overburden rocks time lapse seismic data can be used to monitor these processes since compaction and stretching cause changes in layer thickness as well as seismic velocities it is crucial to develop methods to distinguish between the two effects we introduce a new method based on detailed analysis of time lapse prestack seismic data the equations are derived assuming that the entire model consists of only one single layer with no vertical velocity variations the method incorporates lateral variations in relative velocity changes by utilizing zero offset and offset dependent time shifts to test the method we design a 2d synthetic model that undergoes severe reservoir compaction as well as stretching of the overburden rocks finally we utilize the method to analyze a real 2d prestack time lapse seismic line from t

field test of a permanent in well fiber optic seismic system a deviated borehole was drilled and completed to test systems for deployment of permanent in well three component fiber optic seismic sensors two systems were developed and tested for deployment of these sensors the first system involved using a passive mandrel device to enable coupling of the sensors simply using the weight of the tubing the second system involved use of an active mandrel that incorporates a spring loaded pad device to isolate the sensor from the tubing vibration in addition reference wireline vertical seismic profile data were acquired both in the empty cased borehole and through the tubing at the same location as the fiber optic sensor analysis of the data showed that best results were obtained using the active design tubing related resonances were noted on data acquired using the passive design whereas the active design did a good job of discriminating among these undesirable signals

reservoir monitoring of steam assisted gravity drainage using borehole measurements this paper presents the results from a research project focusing on permanent crosswell geophysical methods for reservoir monitoring during steam assisted gravity drainage a feasibility study indicated detectable differences in seismic and electrical reservoir properties based on expected changes in temperature and fluid saturation during the production of extra heavy oil as a result of this a permanent cross well system was installed at the leismer demonstration area located in the athabasca oil sands region in alberta canada baseline data sets including cross well seismic three dimensional vertical seismic profiling and cross well electrical resistivity tomography have been acquired comparisons between conventional surface seismic and downhole seismic data show an increase in resolution and frequency content as expected steam assisted gravity drainage induced time lapse effects are clearly visible in the 3d vertical seismic profiling and electrical resistivity tomography data sets even after a few months of oil production in general the 3d vertical seismic profiling images show a higher resolution than the surface seismic data in particular when dealing with vertical positioning of the time lapse events the electrical resistivity tomography baseline shows clear separation between zones of high and low electrical resistivity and during 23 months of electrical resistivity tomography measurements the maximum reduction of resistivity is 85 time lapse observations from acoustic and electrical borehole data correspond well and are also supported by temperature measurements in the two observation wells emerging technologies updated models improved flexibility and reduced costs will allow future reservoir monitoring with surface and borehole data in combination or even with borehole data exclusively key words borehole geophysics resistivity seismics time lapse

investigation of flow and flow noise around a seismic streamer cable in marine seismic explorations flow noise from the turbulent boundary layer that forms around a streamer cable due to its relative motion through water significantly affects the quality of collected data understanding this noise generation mechanism is valuable for the development of future seismic streamer cables we qualitatively characterize the area of turbulent flow surrounding a seismic streamer cable and relate this characteristic to the statistics of the measured noise signal the main finding is that the boundary layer thickness around a seismic streamer appears to be about 25 cm in an ocean environment this is significantly larger than the thickness of 2 5 to 5 cm that has been reported in the literature from laboratory experiments we attribute this discrepancy to the unsteadiness of the ocean environment estimations of the spatial extent of the recorded boundary layer noise indicate that the optimal hydrophone separation needs to be about 0 5 m for the noise to be uncorrelated the sign

sliding of till over bedrock scratching polishing comminution and kinematic wave theory the observed ratio of removal of material by i scratches gouged by large clasts and ii polishing by debris suggests that the latter is the dominant form of bedrock erosion we examine this with a multi stage breakdown model of glacial debris which includes active erosion by fines these models suggest that a mother clast can through the action of its daughter debris erode several times its own volume of bedrock till sliding over bedrock is likely to be a potent source of scouring by proposing a till bedrock sliding law we investigate the kinematics of sliding till bodies ice flow naturally tends to thin and extend till cover even in the absence of longitudinal gradients in the applied stress thicker till cover has an increased effective pressure at its base a lower sliding velocity and for larger thicknesses a decrease in sediment flux with thickness this implies backward moving kinematic waves and shocks it is suggested that this is related to the blunt upstream faces of drumlins and that drumlinization can be a consequence of debris sliding over bedrock

time lapse tomography in time lapse analysis we have to distinguish the seismic response changes due to oil and gas production at a reservoir over the years from several other causes such as the recording signature and random noise in this paper we focus our attention on the velocity macromodel provided by seismic tomography which is a basic tool for the data regularization its depth or time migration and a possible final subtraction among different vintages we show first that we cannot use just a single velocity model for all data sets because of seasonal variations of the overburden velocity which is mainly due to seawater temperature in marine cases and to the water table depth in land cases however we can exploit the basic assumption of time lapse analysis for constraining reflection refraction tomography i e by imposing the constraint that the layer structure and the local velocities do not change outside the reservoir and in the shallowest part over time we thus get coupled models that are physically consistent with a better spatial coverage and higher information redundancy the new method is illustrated by a marine case history from the north sea

hydraulics of subglacial cavities modele theorique du comportement a l etat stationnaire des cavites remplies d eau et interconnectees sur le lit du glacier

sliding with cavity formation we present a model for the determination of a sliding law in the presence of sub glacial cavitation this law determines the basal stress at a clean ice bedrock interf ace in terms of the velocity and effective pressure the method is based on an exact solut ion of the nye kamb linearly viscous sliding problem with cavities and uses ideas of lliboutry 979 to construct via re norma lization methods an approximate law for gene ral bedrock form we show that for a bedrock whose spectrum has a power law behaviour one obtains a sliding law which gives the basal shear stress proportional to a power of the velocity and to a power of the effective pressure the effect of subgl acial cavitation on the drainage system is exam ined using recent ideas of kamb for sufficiently high velocities drainage through a r6thlisberger tunnel system is unstable and drai nage takes place through the linked system of cavities this leads to a reduction of the effective pressure and by taking account of this one can rewrite the sliding law in terms of st ress and velocity only this sliding law can be multi v alued and it is suggested that this unde rlies the dynamic phenomenon of surges

low frequency seismic deghosting we evaluated a solution to seismic deghosting that deghosts the low frequency components of the seismic pressure data in an approximation that neglected the dependence on wavenumbers the low frequency deghosted pressure field was computed trace by trace as the sum of the pressure field and its scaled temporally integrated and temporally differentiated fields we gave simple numerical examples that demonstrated the concept the method was found to deghost data up to a frequency that is typically half of the second notch frequency on the low frequency side the deghosting method was limited by the signal to noise ratio the lowfrequency deghosting technique can be appropriate to apply to the part of seismic data that have penetrated and reflected beneath complex and attenuating overburdens such as basalt salt and chalk

on the formulation of basal debris drag for the case of sparse debris deux modeles pour la formulation de l abrasion et de l entrainement basal dus a la friction roche roche sont presentes pour le cas de debris epars entraines dans les couches basales d un glacier tempere la relation entre les 2 modeles est donnee pour le cas d un lit sinusoidal

monitoring co2 injection for carbon capture and storage using time lapse 3d vsps carbon capture and storage ccs is the process through which a nearly pure carbon dioxide co2 stream is captured separated from flue gas or other industrial processes compressed transported to an appropriate storage site and injected deep underground into a geological formation where it can be safely stored for long term geologic storage benson 2005 large sedimentary basins such as the illinois michigan and western canadian sedimentary basins are good targets for ccs as they are in close proximity to large co2 emitters and are composed of the appropriate saline formations and overlying nonpermeable formations in 2003 the u s department of energy s national energy technology laboratory doe netl created a nationwide network of federal state and private sector partnerships to determine the most suitable technologies regulations and infrastructure for future ccs in different areas of the north america office of fossil energy 2013

broadband seismic over under sources and their designature deghosting abstractthe source depth influences the frequency band of seismic data due to the source ghost effect it is advantageous to deploy sources deep to enhance the low frequency content of seismic data but for a given source volume the bubble period decreases with the source depth thereby degrading the low frequency content at the same time deep sources reduce the seismic bandwidth deploying sources at shallower depths has the opposite effects a shallow source provides improved high frequency content at the cost of degraded low frequency content due to the ghosting effect whereas the bubble period increases with a lesser source depth thereby slightly improving the low frequency content a solution to the challenge of extending the bandwidth on the low and high frequency side is to deploy over under sources in which sources are towed at two depths we have developed a mathematical ghost model for over under point sources fired in sequential and simultaneous modes and we have found an inverse mode

multisensor streamer recording and its implications for time lapse seismic and repeatability abstract effective time lapse or 4d processing seeks to extract the 4d signature with minimal background noise to help understand reservoir behavior in the production phase and to optimize producer and injector well placement nonrepeatability of source and sensor positions in the presence of overburden heterogeneity limits our ability to repeat seismic data and to deliver 4d signals with low background noise levels to compensate for imperfect receiver repositioning wavefield regularization techniques can enable wavefield matching on a grid common to both data vintages often there is a trade off between reducing the receiver positioning mismatch and crossline interpolation error from wavefield spatial aliasing through the use of towed multisensor marine seismic streamers that record collocated pressure p and acceleration measurements y and z and the application of a matching pursuit interpolation technique a 3d dealiased reconstructed and deghosted pressure wavefield can be obtained on a dens

a theoretical treatment of the sliding of glaciers in the absence of cavitation a model is proposed for the description of glacier sliding which includes the nonlinearity of the flow law for ice the model describes coupled flow problems in the basal ice and a thin water film together with a temperature problem in the underlying bedrock to determine the sliding law relating basal velocity to basal stress the sliding theory should be formulated as a boundary layer to the larger scale bulk ice flow dimensional analysis indicates that the regelative component of ice velocity may be neglected provided roughness is absent at the smallest wavelengths and then the ice flow effectively uncouples from the other problems in this case with the crucial but unrealistic assumption that the flow law for temperate ice is independent of the moisture content there exist complementary variational principles that describe the functional form of the sliding law and give bounds on the magnitude of the 9roughness9 coefficient these principles are valid for nonlinear stress strain rate relations and for non vanishing bedrock corrugation and indicate how the basal velocity is determined by two parameters that together describe the degree of roughness of the bed specific estimates are then given finally the main weakness in the model as a predictor of quantitatively accurate results is pointed out that is that the variation of moisture within the basal layer and the resultant effect on the flow law are neglected a valid description of this phenomenon does not yet appear to be available

time lapse monitoring of the duri steamflood a pilot and case study this article describes a successful 4 d seismic pilot project in duri field indonesia the site of the world s largest steamflood two baseline and six monitor 3 d seismic surveys were recorded over the same steam injection pattern between 1992 and 1995 seismic analysis thermal simulation and acoustic modeling show the engineering relevance of these data and provide new insights into the seismic response of reservoirs during active steamflooding analyses of the data demonstrate that for the duri steamflood the horizontal and vertical distribution of steam can be tracked over time

numerical modelling of subglacial erosion and sediment transport and its application to the north american ice sheets over the last glacial cycle abstract present day sediment distribution offers a potentially strong constraint on past ice sheet evolution glacial system models gsms however cannot address this constraint while lacking appropriate representations of subglacial sediment production and transport incorporating these elements in gsms is also required in order to quantify the impact of a changing sediment cover on glacial cycle dynamics towards these goals we present a subglacial process model hereafter referred to as the sediment model that incorporates mechanisms for sediment production entrainment transport and deposition bedrock erosion is calculated by both hallet s and boulton s abrasion laws separately and by a novel quarrying law parametrized as a function of subglacial cavity extent these process oriented erosion laws are compared against a simple empirical relationship between erosion rate and the work done by basal stress sediment entrainment is represented by philip s law for regelation intrusion and soft bed deformation is included as a subglacial sediment transport mechanism the model is driven by the data calibrated mun 3d gsm and a newly developed subglacial hydrology module the sediment model is applied to the last north american glacial cycle and predicts sediment thickness and cumulative erosion patterns results are obtained in the context of a sensitivity analysis and are compared against the present day distribution of glacigenic sediment and geological estimates of laurentide ice sheet erosion given plausible ranges for the sensitivity parameters chosen a priori based on available literature or on heuristic arguments the calculated erosion depths overlap with the geological estimates of laurentide erosion most of the runs in the sensitivity set produce unrealistically thick and continuous moraines along the eastern southern and western margins of the north american ice complex which suggests that the model overestimates sediment entrainment and thus englacial transport a realistic sediment distribution is only obtained when the entrainment rate is capped at the average basal melting rate which suggests that modelled entrainment and basal melting rates should be of the same order of magnitude

overburden complexity and repeatability of seismic data impacts of positioning errors at the oseberg field north sea repeatability of seismic data plays a crucial role in time lapse seismic analysis there are several factors that can decrease the repeatability such as positioning errors varying tide source variations velocity changes in the water layer marine data and undesired effects of various processing steps in this work the complexity of overburden structure as an inherent parameter that can affect the repeatability is studied a multi azimuth three dimensional vertical seismic profiling data set with 10 000 shots is used to study the relationship between overburden structure and repeatability of seismic data in most repeatability studies two data sets are compared but here a single data set has been used because a significant proportion of the 10 000 shots are so close to each other that a repeatability versus positioning error is possible we find that the repeatability decreases by a factor of approximately 2 under an overburden lens furthermore we find that the x and y components have approximately the same sensitivity to positioning errors as the z component for the same events in this three dimensional vertical seismic profiling experiment this indicates that in an area with complex overburden positioning errors between monitor and base seismic surveys are significantly more critical than outside such an area this study is based on a three dimensional three component vertical seismic profiling data set from a north sea reservoir and care should be taken when extrapolating these observations into a general four dimensional framework

variable source depth acquisition for improved marine broadband seismic data in marine seismic data acquisition varying the source depth along a sail line gives diversity in sequential shot gather frequency spectra undesired alterations of the frequency spectra are created by the source ghost and by air gun bubble oscillations by deliberately varying the source depth along a sail line it is possible to obtain a seismic data set that will have energy more evenly distributed within the main frequency band of the source output this is obtained when data acquired with different source depths are stacked in imaging we formulated a simple inverse problem that seeks to find the optimal distribution of source depths over a sequential series of shots that shape the amplitude spectrum of the final image into a desired shape we assumed that the data are receiver side deghosted that designature could be applied to each shot gather and that the shot gathers could be redatumed to a common datum prior to imaging

seasonal scale abrasion and quarrying patterns from a two dimensional ice flow model coupled to distributed and channelized subglacial drainage abstract field data and numerical modeling show that glaciations have the potential either to enhance relief or to dampen topography we aim to model the effect of the subglacial hydraulic system on spatiotemporal patterns of glacial erosion by abrasion and quarrying on time scales commensurate with drainage system fluctuations e g seasonal to annual we use a numerical model that incorporates a dual morphology subglacial drainage system coupled to a higher order ice flow model and process specific erosion laws the subglacial drainage system allows for a dynamic transition between two morphologies the distributed system characterized by an increase in basal water pressure with discharge and the channelized system which exhibits a decrease in equilibrium water pressure with increasing discharge we apply the model to a simple synthetic glacier geometry drive it with prescribed meltwater input variations and compute sliding and erosion rates over a seasonal cycle when both distributed and channelized systems are included abrasion and sliding maxima migrate 20 up glacier compared to simulations with distributed drainage only power law sliding generally yields to a broader response of abrasion to water pressure changes along the flowline compared to coulomb friction sliding multi day variations in meltwater input elicit a stronger abrasion response than either diurnal or seasonal variations alone for the same total input volume an increase in water input volume leads to increased abrasion we find that ice thickness commensurate with ice sheet outlet glaciers can hinder the up glacier migration of abrasion quarrying patterns computed with a recently published law differ markedly from calculated abrasion patterns with effective pressure being a stronger determinant than sliding speeds of quarrying rates these variations in calculated patterns of instantaneous erosion as a function of hydrology sliding and erosion model formulation as well as model forcing may lead to significant differences in predicted topographic profiles on long time scales

repeatability analysis of land time lapse seismic data co2crc otway pilot project case study time lapse seismics is the methodology of choice for remotely monitoring changes in oil gas reservoir depletion reservoir stimulation or co 2 sequestration due to good sensitivity and resolving power at depths up to several kilometres this method is now routinely applied offshore however the use of time lapse methodology onshore is relatively rare the main reason for this is the relatively high cost of commercial seismic acquisition on land a widespread belief of a relatively poor repeatability of land seismic data prevents rapid growth in the number of land time lapse surveys considering that co 2 sequestration on land is becoming a necessity there is a great need to evaluate the feasibility of time lapse seismics for monitoring therefore an understanding of the factors influencing repeatability of land seismics and evaluating limitations of the method is crucially important for its application in many co 2 sequestration projects we analyse several repeated 2d and 3d surveys acquired within the otway co2 sequestration pilot project operated by the cooperative research centre for greenhouse technologies co2crc in australia in order to determine the principal limitations of land time lapse seismic repeatability and investigate the influence of the main factors affecting it our findings are that the intrinsic signal to noise ratio s n signal to coherent and background noise levels and the normalized root meansquare nrms difference are controlled by the source strength and source type however the post stack s n ratio and corresponding nrms residuals are controlled mainly by the data fold for very high fold data the source strength and source type are less critical

cavities and the effective pressure between abrading clasts and the bedrock the mechanical configuration of cavities associated with clasts is discussed and how this affects the water pressure underneath the clast the range of permissible water pressure is determined by flow parallel to the bed rather than by flow towards the bed the actual pressure is controlled by hydraulic connections to areas away from the clast the degree of cavitation provides an additional degree of freedom which adjusts to ensure that horizontal and vertical forces respect the coulomb friction condition

sliding of ice past an obstacle at engabreen norway at engabreen norway an instrumented panel containing a decimetric obstacle was mounted flush with the bed surface beneath 210 m of ice simultaneous measurements of normal and shear stresses ice velocity and temperature were obtained as dirty basal ice flowed past the obstacle our measurements were broadly consistent with ice thickness flow conditions and bedrock topography near the site of the experiment ice speed 0 45 m above the bed was about 130 mm d 1 much less than the surface velocity of 800 mm d 1 average normal stress on the panel was 1 0 1 6 mpa smaller than the expected ice overburden pressure normal stress was larger and temperature was lower on the stoss side than on the lee side in accord with flow dynamics and equilibrium thermodynamics annual differences in normal stresses were correlated with changes in sliding speed and ice flow direction these temporal variations may have been caused by changes in ice rheology associated with changes in sediment concentration water content or both temperature and normal stress were generally correlated except when clasts presumably collided with the panel temperature gradients in the obstacle indicated that regelation was negligible consistent with the obstacle size melt rate was about 10 of the sliding speed despite high sliding speed no significant ice bed separation was observed in the lee of the obstacle frictional forces between sediment particles in the ice and the panel estimated from hallet s 1981 model indicated that friction accounted for about 5 of the measured bed parallel force this value is uncertain as friction theories are largely untested some of these findings agree with sliding theories others do not

laboratory simulations of glacial abrasion comparison with theory glacial abrasion was simulated in experiments in which a small artificial glacier bed was pushed beneath a fixed ice block under pressure the experiments provide a means of testing theoretical models of abrasion particularly those factors that govern the magnitude of stress concentrations beneath abrading rock fragments in preliminary experiments vertical ice flow around a sphere mounted on the bed was studied in subsequent experiments marble tablets were pushed beneath granitic rock fragments frozen into the base of the ice block unlike previous abrasion experiments the sliding velocity was realistic 25 mm d 1 and ice near the bed was at the pressure melting temperature resultant striations closely resemble those observed on glaciated bedrock as predicted by hallet 1979 the component of the ice velocity towards the bed strongly influenced stresses beneath fragments and classical regelation and creep theory provided an approximate estimate of the downward drag force on fragments half of the rock fragments rotated significantly accounting for 10 50 of their motion relative to the bed and influencing abrasion rates and the shear stress supported along the ice bed interface striation patterns indirectly suggest that fragment rotations were inhibited by increases in ice pressure which presumably increased the drag on roughness elements on fragment surfaces this may have resulted from a reduction in the thickness of the water film around fragments facilitated by leakage of water from the bed

accelerated subglacial erosion in response to stick slip motion subglacial stick slip motion speeds erosion by hydrofracturing and in other ways as determined from analysis of the growing body of field data microearthquake monitoring commonly detects subglacial earthquakes likely mostly from stick slip motion of debris laden ice over bedrock source parameters show that many quakes cause enough motion to greatly lower water pressure in cavities on the lee sides of bedrock steps we calculate that the resulting expansion of higher pressure water in nearby cracks promotes hydrofracturing with even relatively small cracks growing unstably under thick glaciers and all cracks growing faster than for aseismic behavior this mechanism also helps generate the step like topography favoring block plucking this stick slip glacier erosion hypothesis suggests that the erosion rate will increase with ice thickness as well as basal shear stress ice flow velocity and water supply

processing and quality control strategies for consistent time lapse seismic attributes a case history on an internal blowout using vintage data abstractmany vintage reflection seismic surveys exist that have nonrepeated acquisition geometries or final stacked migrated sections are obtained using different or nonconsistent processing flows this may lead to derived time lapse attributes that are not internally consistent or even conflicting for instance we have focused on a subsurface gas blowout that occurred in 1989 in the norwegian sector of the north sea the 2d site survey data were acquired in 1988 and 1990 and the 3d data were acquired in 1991 and 2005 the various sets of legacy data are plagued by poor repeatability among data acquisitions application of different processing strategies missing prestack data and the presence of multiples all of these factors severely complicate even qualitative interpretation of the gas migration associated with the underground blowout careful time lapse processing may provide useful information even from such challenging legacy data by first computing numerous attributes including instantaneous am

the sliding velocity over a sinusoidal bed at high water pressure under idealized conditions when pressurized water has access to all low pressure areas at the glacier bed a sliding instability exists at a critical pressure p c well below the overburden pressure p 0 the critical pressure is given by p c p 0 l 2 a where l is the wavelength and a is the amplitude of a sinusoidal bedrock and is the basal shear stress when the subglacial water pressure p w approaches this critical value the area of ice bed contact l becomes very small and the pressure on the contact area becomes very large this pressur is calculated from a force balance and the corresponding rate of compression is obtained using glen s flow law for ice on the assumption that compression in the vicinity of the contact area occurs over a distance of the order of the size of this area l a deformational velocity is estimated the resultant slidin velocity sho the expected instability at the critical water pressure the dependency on other parameters such as wavelength l and roughness a l w as found to be the same as for sliding w ithout bed separation

sea bed diffractions and their impact on 4d seismic data a sea bed reflection is used to estimate changes in water layer velocities between timelapse seismic surveys such corrections are crucial in order to obtain high quality 4d seismic data this might be a challenge in areas where rough sea bed topography creates sea bed diffractions that interfere with the sea bed reflection as in several of the northern fields in the norwegian sea these diffractions and diffracted multiples are difficult to attenuate during data processing and become a source of noise in time lapse data in this work we study how cross correlation analysis of time lapse seismic data at a sea bed reflection may be perturbed by the presence of diffractions the seabed topography from the norne field is used in 2d finite difference modelling to explain some of the observed variation in a water layer time shift in field data we find that a rough sea bed and the diffracted energy it induces cause residual noise on the 4d data the variation to the water layer time shift increases with sea bed complexity and is amplified by interaction with other sources of non repeatability like water column variations mis positioning and strength of the ice scours creating the diffracted energy time shift variations with mis positioning and velocity changes between the surveys seem to best explain the observed variation in the time shift for time lapse seismic field data from norne

dynamic recrystallization and texture development in ice as revealed by the study of deep ice cores in antarctica and greenland the preferred c axis orientation of ice from polar ice sheets develops essentially as a result of intracrystalline slip but dynamic recrystallization appears to alter the kinetics of the development of deformation textures and is at high temperature at the origin of recrystallization textures the purpose of this work is to obtain a better understanding of recrystallization processes that occur in polar ice sheets and to clarify the relationship between dynamic recrystallization and textures the study was based on two deep ice cores from greenland and antarctica the greenland ice core project grip and vostok ice cores the structure along the grip core displays normal grain growth in the first 100 m of the ice sheet and rotation recrystallization and migration recrystallization near the bottom only grain growth and rotation recrystallization appear to occur in the vostok ice core the transition between these recrystallization regimes was studied estimating for interglacial ice the evolution with depth of the dislocation density this calculation has shown the efficiency of grain boundary migration for the absorption of dislocations at vostok the highest value of the dislocation density is found at a depth of about 1000 m and the continuous decrease in the dislocation density below this depth is related to the increase of the grain boundary migration rate it is shown that the driving force required to initiate migration recrystallization is not reached in interglacial ice at vostok the observed textures were compared with those predicted by the self consistent approach recrystallization textures are interpreted by assuming that the less stressed grains i e the best oriented for basal slip are favored by the size advantage of subgrains the recrystallization textures are compared with those of other materials

some interesting seismic noise the seismic data used in geophysical interpretation is normally restricted to common depth point stacked seismic sections the quality of the seismic section is dependent upon the design of the recording system and the skill of the seismic processor the system vessel source and cable for marine work is designed to minimize known sources of noise or at least record them so that they can be later attenuated a major part of the processor s function is to recognize and attenuate this noise he may also be called upon for suggestions to improve the recording system

glacial abrasion and sliding their dependence on the debris concentration in basal ice a model of bedrock abrasion by a temperate glacier is developed that takes into account the significant impeding effect of scattered rock fragments at the ice rock interface on glacier sliding an increase in the debris concentration above a relatively low value can reduce the abrasion rate by decreasing both the flux of rock fragments and the effective forces which press fragments against the bed abrasion of a simple wavy bed is shown to be most rapid for a relatively low debris concentration above which the abrasion varies inversely with the debris content a similar concentration dependence of abrasion is expected for beds of arbitrary geometry but low roughness sliding glaciers will tend to abrade until the basal ice is nearly stagnant at which point rocks of all sizes start to lodge against the bed

the effects of entrained debris on the basal sliding stability of a glacier 1 new laboratory experiments exploring likely subglacial conditions reveal controls on the transition between stable sliding and stick slip motion of debris laden ice over rock with implications for glacier behavior friction between a rock substrate and clasts in ice generates heat which melts nearby ice to produce lubricating water an increase in sliding speed or an increase in entrained debris raises heat generation and thus meltwater production unstable sliding is favored by low initial lubrication followed by rapid meltwater production in response to a velocity increase low initial lubrication can result from cold or drained conditions whereas rapid increase in meltwater generation results from strong frictional heating caused by high sliding velocity or high debris loads strengthening of the interface healing during stick intervals between slip events occurs primarily through meltwater refreezing when healing and unstable sliding are taken together the experiments reported here suggest that stick slip behavior is common from motion of debris laden glacier ice over bedrock

morphology of glacial striae implications for abrasion of glacier beds and fault surfaces glacial striae recently exposed on carbonate bedrock adjacent to saskatchewan glacier alberta provide information about the mechanics of abrasion beneath temperate glaciers and along fault surfaces the striae consist of plowed grooves that vary smoothly in width and depth but are located where similar variations in normal stress between the bed and subglacial rock fragments were unlikely finite element calculations of stresses in bedrock beneath abrading fragments suggest that under constant normal loading plowing elements with steep leading edges will plow progressively deeper into the bed with sliding whereas those with more gently inclined leading edges will climb out of their grooves results of simple experiments in which limestone slabs were pushed beneath fixed plowing elements indicate a similar dependence on contact geometry this dependence together with rotation of fragments during glacier sliding provides a plausible origin for the three characteristic shapes of striae in the area shapes of striae indicate that abrading fragments commonly rotate affecting depths of plowing and the shear stress supported by abrading debris an important refinement in models of glacial abrasion would be the incorporation of fragment rotations into estimates of abrasion rates and debris influenced glacier sliding velocities some striae become progressively deeper and wider down glacier and then end abruptly similar to some fault striae that have been linked to stick slip phenomena they may form as a result of tensile fracture at depth in front of plowing asperities that is not necessarily dependent on stick slip motion other minor structures along faults that are used as shear sense indicators may result from the same process

fluid pressure discrimination in anisotropic reservoir rocks a sensitivity study we investigate how seismic anisotropy influences our ability to distinguish between various production related effects from time lapse seismic data based on rock physics models and ultrasonic core measurements we estimate variations in pp and ps reflectivity at the top reservoir interface for fluid saturation and pore pressure changes the tested scenarios include isotropic shale weak anisotropic shale and highly anisotropic shale layers overlaying either an isotropic reservoir sand layer or a weak anisotropic sand layer we find that for transverse isotropic media with a vertical symmetry axis tiv the effect of weak anisotropy in the cap rock does not lead to significant errors in for instance the simultaneous determination of pore pressure and fluid saturation changes on the other hand changes in seismic anisotropy within the reservoir rock caused by for instance increased fracturing might be detectable from time lapse seismic data a new method using exact expressions for pp and ps reflectivity including tiv anisotropy is used to determine pressure and saturation changes over production time this method is assumed to be more accurate than previous methods

glacier surge mechanism 1982 1983 surge of variegated glacier alaska the hundredfold speedup in glacier motion in a surge of the kind the kind that took place in variegated glacier in 1982 1983 is caused by the buildup of high water pressure in the basal passageway system which is made possible by a fundamental and pervasive change in the geometry and water transport characteristics of this system the behavior of the glacier in surge has many remarkable features which can provide clues to a detailed theory of the surging process the surge mechanism is akin to a proposed mechanism of overthrust faulting

evidence for an early archean geomagnetic field a paleomagnetic study of the komati formation barberton greenstone belt south africa 3 5 ga old peridotitic komatiites of the komati formation barberton greenstone belt south africa have a characteristic nrm natural remanent magnetization that is stable during alternating field and thermal demagnetization the thermal nature of this remanence is demonstrated by successful thellier type paleo intensity determinations on some samples in the temperature range 350 to 575 c the characteristic nrm is an almost total thermal overprint acquired during unroofing and cooling following pervasive greenschist metamorphism at 3470 20 ma the mean direction of magnetization after cleaning is d 326 i 81 k 60 95 8 n 7 sites with a paleopole at 11 s 21 e p m 15 the paleopole falls in southern africa ancestral southern africa was therefore in polar latitudes around 3 5 ga the same position it occupied in the late archean and early proterozoic paleofield intensities are about one half present day intensities for polar latitudes a vigorous earth s magnetic field was already in existence in the early archean implying that the core had a suitable size composition and convective flow to support dynamo action

characteristics of subglacial drainage systems deduced from load cell measurements rock tunnels beneath engabreen northern norway permit access to the ice bedrock interface beneath a 210 m thick glacier eight load cells have been installed in the bedrock of the glacier sole along a 22 m transect with some interruptions the load cells have been logged at 15 min intervals since december 1992 here we analyse the records until 2003 load cell signals measure stresses acting normal to the bedrock and usually log the pressure of the thin water film between the basal ice and the bed occasionally there are distinct pressure events characterized by short lived hours local minima often followed by a maximum before decaying to background load cell pressure the amplitudes of these pressure events are of the order of 0 01 1 mpa and depend on the placement of the sensor and the state of the subglacial drainage system we identify winter and summer pressure regimes the winter regime is characterized by few pressure events of large pressure amplitude influencing all load cells a lag of 0 6 days is observed between surface forcing rain and pressure events during winter the summer regime typically has periods of daily pressure events of low amplitude no delay is seen between surface forcing and pressure events during the summer regime in summer the onset of a pressure event is correlated with a local maximum of the derivative of the subglacial discharge record whereas no such relation is found during the winter regime the transition from winter to summer may june is easily detectable and is strongly correlated with a rapid increase in subglacial discharge and the transition to a dominating r channel system the autumn transition is less clearly defined but has usually occurred by the beginning of november stress bridging an increase in bed pressure at the edge of low pressure channels is recorded during the summer regime water pressures at the bed are connected or unconnected to the drainage system pressure increases in the connected system beyond local normal stress values lead to an uplift of the connected system and a pressure drop in the unconnected system the occurrence of pressure events is determined by the capacity of the drainage system uplift is controlled by local normal stress values and not mean ice overburden pressure

is it optimal to tow air guns shallow to enhance low frequencies abstractin marine seismic acquisition it used to be commonly accepted that it is optimal to tow the source deep to enhance the low frequency content in the seismic data however mayne and quay found in 1971 that the low frequency response of air guns actually improves as the source depth decreases we evaluated a simple ad hoc theory that demonstrates that two effects are counteracting each other the free surface effect favors deep towed sources whereas the bubble time period increasing with decreasing source depth favors shallower tow depths from a fjord test we found that combining several source depths in an air gun array might flatten and improve the low frequency part of the source spectrum the experiment confirms that various source depths result in local characteristic maxima in the low frequency spectrum corresponding to the bubble time period of the air gun

glacial erosion status and outlook glacier erosion rates range across orders of magnitude and much of this variation cannot be attributed to basal sliding rates subglacial till acts as lubricating fault gouge or sawdust and must be removed for rapid subglacial bedrock erosion such erosion occurs especially where and when moulin fed streams access the bed and are unconstrained by supercooling or other processes streams also may directly erode bedrock likely with strong time evolution erosion is primarily by quarrying aided by strong fluctuations in the water system driven by variable surface melt and by subglacial earthquakes debris bed friction significantly affects abrasion quarrying and general glacier flow frost heave drives cirque headwall erosion as winter cold air enters bergschrunds creating temperature gradients to drive water flow along premelted films to growing ice lenses that fracture rock and the glacier removes the resulting blocks recent subglacial bedrock erosion and sediment flux are in many cases much higher than long term averages over glacial cycles evolution of glacial valley form feeds back strongly on erosion and deposition most of this is poorly quantified with parts open to argument glacial erosion and interactions are important to tectonic and volcanic processes as well as climate and biogeochemical fluxes motivating vigorous research

when 4d seismic is not applicable alternative monitoring scenarios for the arab d reservoir in the ghawar field abstract ghawar the largest oilfield in the world produces oil from the upper jurassic arab d carbonate reservoir the high rigidity of the limestone dolomite reservoir rock matrix and the small contrast between the elastic properties of the pore fluids i e oil and water are responsible for the weak 4d seismic effect due to oil production a feasibility study was recently completed to quantify the 4d seismic response of reservoir saturation changes as brine replaced oil the study consisted of analysing reservoir rock physics petro acoustic data and seismic modelling a seismic model of flow simulation using fluid substitution concluded that time lapse surface seismic or conventional 4d seismic is unlikely to detect the floodfront within the repeatability of surface seismic measurements thus an alternative approach to 4d seismic for reservoir fluid monitoring is proposed permanent seismic sensors could be installed in a borehole and on the surface for passive monitoring of microseismic activity from reservoir pore pressure perturbations reservoir production and injection operations create these pressure or stress perturbations reservoir heterogeneities affecting the fluid flow could be mapped by recording the distribution of epicentre locations of these microseisms or small earthquakes the permanent borehole sensors could also record repeated offset vertical seismic profiling surveys using a surface source at a fixed location to ensure repeatability the repeated vertical seismic profiling could image the change in reservoir properties with production

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

topography climate and ice masses a review topography acts as a filter between regional climate and the consequent response of a glacier or ice sheet it influences the mass and energy inputs and modifies the ice dynamics the evolution of the ice sheet surface and over longer time scales of the bed topography modulates the climatic forcing until we have a better understanding of these topographic linkages the use of geophysical signals from former ice masses to determine palaeoclimates must be regarded as fundamentally flawed

avalanching glacier instabilities review on processes and early warning perspectives avalanching glacier instabilities are gravity driven rupture phenomena that might cause major disasters especially when they are at the origin of a chain of processes reliably forecasting such events combined with a timely evacuation of endangered inhabited areas often constitute the most efficient action recently considerable efforts in monitoring analyzing and modeling such phenomena have led to significant advances in destabilization process understanding improving early warning perspectives the purpose of this paper is to review the recent progress in this domain three different types of instabilities can be identified depending on the thermal properties of the ice bed interface if cold 1 the maturation of the rupture is associated with a typical time evolution of surface velocities and passive seismic activity a prediction of the final break off is possible using these precursory signs for the two other types water plays a key role in the development of the instability if the ice bed interface is partly temperate 2 the presence of meltwater may reduce the basal resistance which promotes the instability no clear and easily detectable precursory signs are known in this case and the only way to infer any potential instability is to monitor the temporal evolution of the thermal regime the last type of instability 3 concerns steep temperate glacier tongues switching for several days weeks during the melting season into a so called active phase followed in rare cases by a major break off event although the prediction of such events is still far from being achievable critical conditions promoting the final instability can be identified

combined measurements of subglacial water pressure and surface velocity of findelengletscher switzerland conclusions about drainage system and sliding mechanism interpretation des mesures de pression dans les forages et des vitesses superficielles la comparaison avec les modeles suggere que l on a neglige le frottement du aux debris dans les modeles et qu il n existe jamais de chenaux cylindriques rectilignes

antarctic ice streams a review an ice stream is a part of an inland ice sheet that flows rapidly through the surrounding ice the distinction between an ice sheet and an outlet glacier is clear in principle but muddy in practice many antarctic glaciers are intermediate in character the ross ice streams which flow through the west antarctic inland ice into the ross ice shelf are distinct in character differing even from other ice streams in the marine ice sheet of west antarctica their surface elevation profiles are low their bed slopes are low and smooth and their driving stresses diminish monotonically downglacier in transverse profile they are broader in relation to ice thickness and exhibit shallower subglacial troughs than other ice streams many models for the fast sliding of glaciers have been applied to the ross ice streams most have included in some form a reduction in basal drag resulting from a lesser effective than glaciostatic pressure at the bed most models are only semi physical at best so need to be tuned by fitting to assumed parameters at some place or places in the real ice sheet the recent discovery of a very small effective pressure beneath one ice stream consequently has led to some gross errors in the velocities predicted by the models the difficulty may be resolved if it is true as recent experiments suggest that ice stream b and by extrapolation other ross ice streams as well slide on a deforming bed that absorbs most or all of the differential motion between the ice and the bedrock