scattering of vhf radio waves from within the top 700 m of the antarctic ice sheet and its relation to the depositional environment a case study along the syowa mizuho dome fuji traverse

internal layer tracing and age depth accumulation relationships for the northern greenland ice sheet clues to previous ice sheet structure and long term glaciological processes are preserved in the internal layering configuration of the greenland ice sheet information about these internal layers has been retrieved over many parts of the ice sheet with the university of kansas ice penetrating radar we report on the coherence of these layers over very large distances describe a method of tracing these layers along thousands of kilometers of flight line and do so for one flight during the 1999 program for arctic regional climate assessment parca aircraft campaign we determine the ages of these layers based on information at the grip ice core site and extend these ages along the flight line to camp century where they are compared to modeled derived age estimates these ages agree with each other to between 2 and 15 differences that can be substantially reduced with minor changes to the model parameters accumulation rate and shear layer depth finally we are able to derive estimates of accumulation rates along the flight line by fitting the age depth data from layer tracing to a dansgaard johnsen model with a minimization technique providing estimates that match recent accumulation patterns within a few centimeters per year

effects of birefringence within ice sheets on obliquely propagating radio waves in this paper effects of birefringence on radio waves obliquely propagating though polar ice sheets are examined to facilitate interpretations of bistatic and side looking radar data a formalism applicable for arbitrary radar configurations is developed to predict the returned power from within and beneath the ice sheets that have arbitrary alignments of ice crystals ice fabrics we applied this formalism to a range of ice fabrics found in ice cores and assessed the effects of birefringence in terms of ray path configurations ice fabrics and radar frequency predicted frequency dependence of the bed return power replicates prominent features observed at greenland ngrip ice core site results show that birefringence in ice of 1 km or more thickness with strong weak fabric can reduce the power returned from the bed 2 db or more at frequencies higher than 200 mhz 20 mhz as compared to isotropic ice this suggests that quantitative interpretation of the power returned from the bed requires careful assessment of birefringence almost everywhere over the ice sheets application of this formalism also suggests a radar frequency range usable for attenuation measurements possible effects of fabric on synthetic aperture radar processing and a feasibility of remote sensing of ice fabric

instruments and methods geophysical imaging of alpine rock glaciers slope instabilities caused by the disappearance of ice within alpine rock glaciers are an issue of increasing concern design of suitable counter measures requires detailed knowledge of the internal structures of rock glaciers which can be obtained using geophysical methods we examine benefits and limitations of diffusive electromagnetics geoelectrics seismics and ground penetrating radar georadar for determining the depth and lateral variability of the active layer the distributions of ice and water the occurrence of shear horizons and the bedrock topography in particular we highlight new developments in data acquisition and data analysis that allow 2 d or even 3 d structures within rock glaciers to be imaged after describing peculiarities associated with acquiring appropriate geophysical datasets across rock glaciers and emphasizing the importance of state of the art tomographic inversion algorithms we demonstrate the applicability of 2 d imaging techniques using two case studies of rock glaciers in the eastern swiss alps we present joint interpretations of geoelectric seismic and georadar data appropriately constrained by information extracted from boreholes a key conclusion of our study is that the different geophysical images are largely complementary with each image resolving a different suite of subsurface features based on our results we propose a general template for the cost effective and reliable geophysical characterization of mountain permafrost

analysis techniques for coherent airborne radar sounding application to west antarctic ice streams 1 analysis of coherent radar sounding echoes from polar ice sheets can provide information suitable for classifying the subglacial environment echoes from a general interface consist of both specularly reflected and diffusely scattered contributions specular reflection results from smooth uniform interfaces whereas diffuse scattering results from rough nonuniform interfaces and inhomogeneous media this article discusses how these phenomena are important to the acquisition and analysis of coherent radar sounding data reflection results are presented from airborne surveys conducted in 1987 over the downstream portions of whillans ice stream and ice stream c west antarctica additionally reflection and scattering analyses along with new results are presented for repeat profiles flown in 2001 over ice stream c analysis methods include using echo amplitudes to compute reflection coefficients which are used for inferring the dielectric properties of the subglacial material echo phase analysis provides the locations of dominant scattering centers which relate to reflection or scattering from the interface as well as provide interface roughness estimates comparison of low and high resolution imaging obtained from synthetic aperture radar techniques indicates a reflecting and or scattering interface combining the results from these independent analyses provides classification of the subglacial environment classified regions include smooth seawater smooth saturated sediments accreted marine ice rough bottom crevasses mixed conditions with partial subglacial water and dry frozen conditions

glaciers and ice sheets mapping orbiter concept 1 we describe a concept for a spaceborne radar system designed to measure the surface and basal topography of terrestrial ice sheets and to determine the physical properties of the glacier bed our primary objective is to develop this new technology for obtaining spaceborne estimates of the thickness of the polar ice sheets with an ultimate goal of providing essential information to modelers estimating the mass balance of the polar ice sheets and estimating the response of ice sheets to changing climate our new technology concept employs vhf and p band interferometric radars using a novel clutter rejection technique for measuring surface and bottom topographies of polar ice sheets from aircraft and spacecraft our approach will enable us to reduce signal contamination from surface clutter measure the topography of the glacier bed at better than 1 km intervals with an accuracy of 20 m and paint a picture of variations in bed characteristics the technology will also have applications for planetary exploration including studies of the martian ice caps and the icy moons of the outer solar system through the concept developed here we believe that we can image the base and map the three dimensional basal topography beneath an ice sheet at up to 5 km depth

a new ice thickness and bed data set for the greenland ice sheet 1 measurement data reduction and errors ice thickness data collected between 1993 and 1999 using a coherent ice penetrating radar system developed at the university of kansas have been combined with data collected by the technical university of denmark in the 1970s to produce a new ice thickness grid for greenland crossover analysis was used to assess the relative accuracy of the two data sets and they were weighted accordingly and interpolated onto a regular 5 km spacing grid using a kriging interpolation procedure a high resolution land ice mask was used to help constrain the interpolation of the ice thickness data near the ice sheet margins where in the past the relative errors have been largest the ice thickness grid was combined with a new digital elevation model of the ice sheet and surrounding rock outcrops to produce a new bed elevation data set for the whole of greenland the ice thickness grid was compared with the currently available data set differences in the center of the ice sheet where the ice is thickest were of the order of a few percent near the margins however large differences of as much as a factor of 10 were found the total volume of ice contained in the ice sheet was reestimated and found to have a value of 2 93 x 10 6 km 3 the ice thickness grid was used to calculate the spatial pattern of gravitational driving stress over the ice sheet anomalous patterns of stress were found in areas that appeared to be associated with areas of rapid flow

geophysical imaging of alpine rock glaciers slope instabilities caused by the disappearance of ice within alpine rock glaciers are an issue of increasing concern design of suitable counter measures requires detailed knowledge of the internal structures of rock glaciers which can be obtained using geophysical methods we examine benefits and limitations of diffusive electromagnetics geoelectrics seismics and ground penetrating radar georadar for determining the depth and lateral variability of the active layer the distributions of ice and water the occurrence of shear horizons and the bedrock topography in particular we highlight new developments in data acquisition and data analysis that allow 2 d or even 3 d structures within rock glaciers to be imaged after describing peculiarities associated with acquiring appropriate geophysical datasets across rock glaciers and emphasizing the importance of state of the art tomographic inversion algorithms we demonstrate the applicability of 2 d imaging techniques using two case studies of rock glaciers in the eastern swiss alps we present joint interpretations of geoelectric seismic and georadar data appropriately constrained by information extracted from boreholes a key conclusion of our study is that the different geophysical images are largely complementary with each image resolving a different suite of subsurface features based on our results we propose a general template for the cost effective and reliable geophysical characterization of mountain permafrost

along track focusing of airborne radar sounding data from west antarctica for improving basal reflection analysis and layer detection this paper presents focused synthetic aperture radar sar processing of airborne radar sounding data acquired with the high capability radar sounder system at 60 mhz the motivation is to improve basal reflection analysis for water detection and to improve layer detection and tracking the processing and reflection analyses are applied to data from kamb ice stream west antarctica the sar processor correlates the radar data with reference echoes from subsurface point targets the references are 1 d responses limited by the pulse nadir footprint or 2 d responses that include echo tails unfocused sar and incoherent integration are included for comparison echoes are accurately preserved from along track slopes up to about 0 5deg for unfocused sar 3deg for 1 d correlations and 10deg for 2 d correlations the noise clutter levels increase from unfocused sar to 1 d and 2 d correlations but additional gain compensates at the basal interface the basal echo signal to noise ratio improvement is typically about 5 db and up to 10 db for 2 d correlations in rough regions the increased noise degrades the clarity of internal layers in the 2 d correlations but detection of layers with slopes greater than 3deg is improved reflection coefficients are computed for basal water detection and the results are compared for the different processing methods there is a significant increase in the detected water from unfocused sar to 1 d correlations indicating that substantial basal water exists on moderately sloped interfaces very little additional water is detected from the 2 d correlations the results from incoherent integration are close to the focused sar results but the noise clutter levels are much greater

mechanisms of fast flow in jakobshavn isbrae west greenland part iii measurements of ice deformation temperature and cross borehole conductivity in boreholes to the bedrock at a site on the ice sheet adjacent to the jakobshavn ice stream in west greenland ice deformation rates and temperatures have been measured in boreholes to the bedrock at 830 m depth enhanced deformation rates were recorded just below the holocene wisconsin transition at 680 m depth a 31 m layer of temperate ice and the temperature minimum of 22 c at 520 m depth were detected the good agreement of these data with results of a two dimensional thermomechanically coupled flow model implies that the model input is adequate discrepancies between modelled and measured temperature profiles on a flowline at the ice stream centre have been attributed to effects not accounted for by the model we have suggested that the convergent three dimensional flow leads to a vertical extension of the basal ice entering the stream a thick basal layer of temperate and wisconsin ice would explain the fast flow of this ice stream as a test of this hypothesis the new core borehole conductivity cbc method has been used to compare conductivity sequences from the ice stream to those of the adjacent ice sheet the correlation thus inferred suggests that the lowest 270 m of the ice sheet correspond to the lowermost 1700 m of the stream and consequently that the lower part of the ice stream has experienced a very large vertical extension

positive degree day factors for ablation on the greenland ice sheet studied by energy balance modelling ice ablation is related to air temperature by the positive degree day factor variations of the positive degree day factor in west greenland are studied using an energy balance model to simulate ablation under different conditions degree day factors for simulated and measured ice ablation at nordbogletscher and qaman rssup sermia agree well with values around 8 mm d 1 c 1 degree day factors for snow are less than half those for ice energy balance modelling shows that degree day factors vary with summer mean temperature surface albedo and turbulence but there is only evidence of large positive degree day factors at lower temperatures and with low albedo 0 3 the greatest effect of albedo variations 0 3 0 7 is at lower temperatures while variations in turbulence have greater effect at higher temperatures current models may underestimate runoff from the greenland ice sheet by several tenths because they use a degree day factor for melting ice that is too small for the colder parts of the ice sheet i e the upper ablation area and the northerly margin

basal melt at northgrip modeled from borehole ice core and radio echo sounder observations from temperature measurements down through the 3001 m deep borehole at the north greenland icecore project northgrip drill site it is now clear that the ice at the base 3080 m below the surface is at the pressure melting point this is supported by the measurements on the ice core where the annual layer thicknesses show there is bottom melting at the site and upstream from the borehole surface velocity measurements internal radio echo layers borehole and ice core data are used to constrain a time dependent flow model simulating flow along the north northwest trending ice ridge flow line leading to the northgrip site also time dependent melt rates along the flowline are calculated with a heat flow model the results show the geothermal heat flow varies from 50 to 200 mw m 2 along the 100km section of the modeled flowline the melt rate at the northgrip site is 0 75 cm a 1 but the deep ice in the northgrip core originated 50 km upstream and has experienced melt rates as high as 1 1 cm a 1

evidence for divergent plate boundary characteristics and crustal spreading on venus detailed examination of the topography and morphology of western aphrodite terra reveals numerous features that are similar to terrestrial divergent plate boundary characteristics individual domains between fracturezone like discontinuities contain a variety of bilaterally symmetrical topographic elements that suggest that topographic features have been created at rise crests rifted and separated and moved laterally to their present symmetrical positions the topographic and morphologic similarities together with strikingly similar mirror image map patterns on both sides of the rise axis suggest that western aphrodite terra shares the characteristics of oceanic divergent plate boundaries and is the site of crustal spreading on venus topographic profiles are consistent with spreading rates of the order of several centimeters per year

improved estimation of the mass balance of glaciers draining into the amundsen sea sector of west antarctica from the cecs nasa 2002 campaign in november december 2002 a joint airborne experiment by centro de estudios cientificos and nasa flew over the antarctic ice sheet to collect laser altimetry and radio echo sounding data over glaciers flowing into the amundsen sea a p 3 aircraft on loan from the chilean navy made four flights over pine island thwaites pope smith and kohler glaciers with each flight yielding 1 5 2 hours of data the thickness measurements reveal that these glaciers flow into deep troughs which extend far inland implying a high potential for rapid retreat interferometric synthetic aperture radar data insar and satellite altimetry data from the european remote sensing satellites ers 1 2 show rapid grounding line retreat and ice thinning of these glaciers using the new thickness data we have re evaluated glacier fluxes and the present state of mass balance which was previously estimated using ice thicknesses deduced largely from inversion of elevation data assuming hydrostatic equilibrium the revised total ice discharge of 241 5 km 3 a 1 exceeds snow accumulation by 81 17 km 3 a 1 of ice equivalent to a sea level rise of 0 21 0 04 mm a 1 this magnitude of ice loss is too large to be caused by atmospheric forcing and implies dynamic thinning of the glaciers this is confirmed by ice flow acceleration observed with insar we attribute the flow acceleration and ice thinning to enhanced bottom melting of the ice shelves by a warmer ocean which reduces buttressing of the glaciers and in turn accelerates them out of balance

mass balance of the greenland ice sheet at high elevations comparison of ice discharge from higher elevation areas of the entire greenland ice sheet with total snow accumulation gives estimates of ice thickening rates over the past few decades on average the region has been in balance but with thickening of 21 centimeters per year in the southwest and thinning of 30 centimeters per year in the southeast the north of the ice sheet shows less variability with average thickening of 2 centimeters per year in the northeast and thinning of about 5 centimeters per year in the northwest these results agree well with those from repeated altimeter surveys except in the extreme south where we find substantially higher rates of both thickening and thinning

tectonics and evolution of venus the global tectonics of venus differs significantly from that of earth most markedly in that the surface is covered predominately by gently rolling terrain there apparently are no features like ocean rises the gravity is positively correlated with topography at all wavelengths and the few highlands are estimated to be supported or compensated at a depth of approximately 100 kilometers the surface of venus appears to be covered mainly by an ancient crust the high surface temperature making subduction difficult it seems likely that well over 1 billion years ago water was destabilized at the surface and soon after plate tectonics ceased the highlands appear to be actively supported presumably as manifestations of long enduring hot spots

coherent radar ice thickness measurements over the greenland ice sheet we developed two 150 mhz coherent radar depth sounders for ice thickness measurements over the greenland ice sheet we developed one of these using connectorized components and the other using radio frequency integrated circuits rfics both systems are designed to use pulse compression techniques and coherent integration to obtain the high sensitivity required to measure the thickness of more than 4 km of cold ice we used these systems to collect radar data over the interior and margins of the ice sheet and several outlet glaciers we operated both radar systems on the nasa p 3b aircraft equipped with gps receivers radar data are tagged with gps derived location information and are collected in conjunction with laser altimeter measurements we have reduced all data collected since 1993 and derived ice thickness along all flight lines flown in support of program for regional climate assessment parca investigations and the north greenland ice core project radar echograms and derived ice thickness data are placed on a server at the university of kansas http tornado rsl ukans edu greenlanddata htm for easy access by the scientific community we obtained good ice thickness information with an accuracy of 10 m over 90 of the flight lines flown as a part of the parca initiative in this paper we provide a brief description of the system along with samples of data over the interior along the 2000 m contour line in the south and from a few selected outlet glaciers

a wideband radar for high resolution mapping of near surface internal layers in glacial ice snow accumulation rate is an important parameter in determining the mass balance of polar ice sheets accumulation rate is currently determined by analyzing ice cores and snow pits inadequate sampling of the spatial variations in the ice sheet accumulation has resulted in accumulation rate uncertainties as large as 24 we designed and developed a 600 900 mhz airborne radar system for high resolution mapping of the near surface internal layers for estimating the accumulation rate of polar ice sheets our radar system can provide improved spatial and temporal coverage by mapping a continuous profile of the isochronous layers in the ice sheet during the 2002 field season in greenland we successfully mapped the near surface layers to a depth of 200 m in the dry snow zone 120 m in the percolation zone and 20 m in the melt zone we determined the water equivalent accumulation rate at the nasa u spl i bar 1 site to be 34 9 spl plusmn 5 1 cm year from 1964 to 1992 this is in close agreement with the ice core derived accumulation rate of 34 6 cm year for the same period

crystal orientation fabrics within the antarctic ice sheet revealed by a multipolarization plane and dual frequency radar survey 1 to investigate the viscosity structure of ice sheets induced by crystal orientation fabric cof we carried out a multipolarization plane and dual frequency radar survey in east antarctica radar surveys were done along a 670 km long flow line from dome fuji toward the coast and two transverse lines of 300 km and 20 km length respectively the radar echoes were highly dependent on the polarization plane for ice depths between about 40 and 60 of the ice thickness in the lower reaches of the convergent ice flow sector approaching the outlet glacier when the polarization was perpendicular to the ice flow echoes were about 10 db stronger than when the polarization was parallel to the ice flow this feature was not clear in the upper part of this convergent flow sector farther inland where ice flow is divergent or parallel the radar echo varied by several decibels because of changes of the radar polarization and had maxima in two orientations dual frequency data showed that the cause of the reflections was changes in cof multipolarization data identified anisotropic reflectivities and birefringence as causes of the anisotropic radar echoes in the lower and upper reaches respectively with the aid of ice core based studies on cof we show that ice is composed of stacked layers of single pole and vertical girdle fabrics in the lower reaches in contrast we argue that changes of single pole clustering cause isotropic reflectivities in the upper reaches we also discuss on the development of cof along ice flow and its implication to ice sheet dynamics

present and past climate control on fjord glaciations in greenland implications for ird deposition in the sea calving of icebergs is the dominant ablation mechanism for large outlet glaciers from the greenland ice sheet except in northernmost greenland where bottom melting from floating glaciers dominates this difference is controlled by present climate conditions glacial geological evidence indicates that the transition between the associated types of fjord glaciations moved north south in response to past climate change in cold periods local melt out of debris from the bottom of an increasing number of floating glaciers reduces the potential for iceberg transport of ird thus the marine ird signal of greenland origin is not a simple cold climate signal our findings are discussed in the context of the ongoing debate about the kind of ice transporting ird icebergs or sea ice

regionalized models for the thermal evolution of the earth abstract traditional models for the heat loss in oceanic and continental regions are combined into a regionalized model for the thermal evolution of the earth the need for regionalization is obvious when one considers that the mantle loses 3 to 4 times as much heat per unit area in oceanic regions than in continental areas the present day rate of heat loss together with a geochemical estimate of the concentration of heat producing elements in the earth fixes the response time of the thermally convecting mantle the response time in turn can be used to select the most reasonable representation for mantle convection in terms of the sensitivity of viscosity on temperature and layering versus mantle wide circulation present geochemical estimates of the bulk composition of the earth are most easily reconciled with the observed heat flow if the mantle is layered and its rheology is slightly less temperature dependent than generally assumed the layered system can produce sufficiently high temperatures to explain the high magnesian komatiites of the archean one difficulty with the models is that they predict widespread melting at shallow depth in the early stages of earth history but do not address how such melting affects and alters the heat transfer mechanisms

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

within river nutrient processing in chalk streams the pang and lambourn uk this work examines baseflow nutrient concentrations and loads along two rural chalk streams the pang and lambourn soluble reactive phosphorus srp and boron b concentrations in these streams were heavily influenced by point source inputs and the effects of downstream flow accretion and dilution unlike b which is chemically conservative srp loads were also strongly influenced by in stream processing resulting in uptake of srp particularly immediately downstream of sewage effluent discharges where rates of srp uptake were highest for the upper river pang up to 80 of srp loads were lost within 4 km downstream of compton sewage treatment works stw and on the river lambourn up to 55 of srp loads were lost within 1 6 km downstream of east shefford stw in contrast nitrate no3 concentrations at sites along the pang and lambourn were largely controlled by groundwater inputs and plant uptake during periods of high photosynthetic activity in spring and summer and silicon si by diatom uptake in april may there were net gains in no3 loads along the river reaches as a result of volumetric increases in groundwater discharge and compared with srp the role of in stream processing of no3 appeared low examination of srp exchange by bed sediment and uptake of srp into algal biofilms indicated that biofilms accounted for only a very small percentage of in stream p uptake but that bed sediment srp exchanges had a more important control on baseflow srp concentrations and loads point source p remediation at east shefford stw by removal of p from final effluent p stripping resulted in 70 90 reductions in river water srp loads after introduction of p stripping at east shefford stw bed sediments immediately downstream of the stw switched from being net sinks to net sources of srp our results show that in the immediate aftermath of p stripping bed sediment srp release was responsible for a 30 g p l 1 rise in river water srp along this reach while this increase in srp concentration as a result of bed sediment srp release is potentially ecologically significant it is small in relation to the increase in srp concentrations from effluent prior to p stripping which resulted in increases in srp concentration of up to 500 g p l 1 there was a six month lag between the introduction of p stripping at east shefford stw and bed sediment epc0 recovering to equilibrium levels with the overlying river water and thus negligible srp release recovery of bed sediments to equilibrium levels is likely to have occurred as a result of winnowing and removal of high epc0 sediment and delivery of lower epc0 sediment from upstream under higher more variable flow conditions and greater rates of in channel sediment erosion delivery more rapid recovery of bed sediment epc0 levels following p stripping might be expected

focused synthetic aperture radar processing of ice sounder data collected over the greenland ice sheet the authors developed a synthetic aperture radar sar processing algorithm for airborne spaceborne ice sounding radar systems and applied it to data collected in greenland by using focused sar phase corrected coherent averaging they improved along track resolution by a factor of four and provided a 6 db processing gain over unfocused sar coherent averaging without phase correction based on a point target analysis for a greenland ice sounding data set also they demonstrated that the focused sar processing reduced clutter and enabled them to identify bedrock interface returns buried in clutter using focused sar technique they processed data collected over a key 360 km long portion of the 2000 m contour line of southwest greenland to the best of their knowledge these are the first high quality radar ice thickness measurements over this key location moreover these ice thickness measurements have been used for improving mass balance estimates of the greenland ice sheet

a low frequency airborne gpr system for wide area geophysical surveys the case study of morocco desert abstract the paper deals with development and validation of a vhf band airborne radar designed for subsurface exploration the radar system is a pulsed one and transmits a chirp pulse at the central frequency of 165 mhz with 40 mhz bandwidth a preliminary system validation has been performed by means of flight trials over morocco desert in may 2018 an ad hoc signal processing technique has been developed to produce focused images of the scene under investigation starting from raw and motion uncompensated data in particular the data processing technique is based on a microwave tomographic approach that has the novel feature to compensate the distortion effects produced by the non rectilinear motion of the aircraft based on navigation data the analysis and interpretation of the data is supported by comparison with electromagnetic simulations of the scene

a semiautomated multilayer picking algorithm for ice sheet radar echograms applied to ground based near surface data snow accumulation over an ice sheet is the sole mass input making it a primary measurement for understanding the past present and future mass balance near surface frequency modulated continuous wave fmcw radars image isochronous firn layers recording accumulation histories the semiautomated multilayer picking algorithm sampa was designed and developed to trace annual accumulation layers in polar firn from both airborne and ground based radars the sampa algorithm is based on the radon transform rt computed by blocks and angular orientations over a radar echogram for each echogram s block the rt maps firn segmented layer features into peaks which are picked using amplitude and width threshold parameters of peaks a backward rt is then computed for each corresponding block mapping the peaks back into picked segmented layers the segmented layers are then connected and smoothed to achieve a final layer pick across the echogram once input parameters are trained sampa operates autonomously and can process hundreds of kilometers of radar data picking more than 40 layers sampa final pick results and layer numbering still require a cursory manual adjustment to correct noncontinuous picks which are likely not annual and to correct for inconsistency in layer numbering despite the manual effort to train and check sampa results it is an efficient tool for picking multiple accumulation layers in polar firn reducing time over manual digitizing efforts the trackability of good detected layers is greater than 90

fundamental issues in the geology and geophysics of venus a number of important and currently unresolved issues in the global geology and geophysics of venus will be addressable with the radar imaging altimetry and gravity measurements now forthcoming from the magellan mission among these are the global volcanic flux and the rate of formation of new crust the global heat flux and its regional variations the relative importance of localized hot spots and linear centers of crustal spreading to crustal formation and tectonics and the planform of mantle convection on venus and the nature of the interactions among interior convective flow near surface deformation and magmatism

the thermal regime of venus models for the thermal evolution of venus are constructed numerically the mantle is proposed to consist of two independently convecting layers upper and lower mantles the floating crust keeps the temperature of the upper surface of the convecting mantle near 1200 c and the core may solidify at temperatures below the liquidus previously these models were calculated with the usual parameterization of convection based mostly on investigations of constant viscosity convection now this parameterization is modified to take into account new numerical investigations of convection in media with complicated rheology the thermal evolution of venus is assumed to have begun 4 6 billion years b y ago it can be divided into three periods 1 adjustment of the upper mantle to the thermal regime of the lower mantle 0 5 b y 2 transition of the entire mantle to the asymptotic regime 3 4 b y 3 asymptotic regime parameters of the convecting planet in the asymptotic regime do not depend on initial conditions the planet forgets its initial state and are found analytically the present day heat flux is equal to 50 erg cm 2s 1 the results are close to those obtained previously comparison of the present thermal models of venus and earth with the recent studies of melting of the fe fes system suggest that the absence of a dipole magnetic field is closely connected to the absence of solidification of the venusian core at present the position of the iron triple point e liquid makes this conclusion very probable the thermal regime of the venusian crust is also discussed convection in the lower part of the crust is shown to play a role in certain regions with specific crustal composition the prevailing mechanism of heat transfer to the surface is advection by magmas produced by melting the lower layers of the crust

changes in the velocity structure of the greenland ice sheet using satellite radar interferometry observations of greenland we detected widespread glacier acceleration below 66 north between 1996 and 2000 which rapidly expanded to 70 north in 2005 accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year as more glaciers accelerate farther north the contribution of greenland to sea level rise will continue to increase

remote sensing of glacier and ice sheet grounding lines a review abstract the precise positioning and long term monitoring of the grounding line forming the boundary between grounded and floating ice of marine ice sheets and tidewater glaciers is critical for assessing ice sheet glacier stability ice sheet glacier mass balance calculations and numerical ice modelling however mapping of the grounding line is a challenging task since it is a subglacial and transient feature remote sensing techniques do not map the grounding line directly but locate surface features in the grounding zone that serve as proxies for the true grounding line position the large variety of methods products and publications as well as an inconsistent use of terminology additionally complicate the topic we present the first detailed review of existing remote sensing techniques and data for grounding line mapping benefits and limitations of the different techniques and data sources are discussed and illustrated with examples we identify a present tradeoff between accuracy and both spatial coverage and temporal resolution of the derived grounding line products rather new methods like sentinel 1 sar synthetic aperture radar differential range offset tracking drot and pseudo crossover radar altimetry pcra offer the potential for recent measurements and to fill gaps in some areas nevertheless there is a high demand of new spaceborne radar sensors that provide time series of very short repeat pass data to enable regular grounding line monitoring with most accurate differential sar interferometry dinsar on a continental scale

a low cost glacier mapping system an old portable 60 mhz radar has been upgraded with a new digital data processing and acquisition system and a new antenna construction enabling a fast and low cost installation on atwin otter aircraft augmented by a laser altimeter and kinematic global positioning system gps the system has the capability of acquiring accurate data on location and ice surface elevation and adequate quality data on ice thickness the system has been applied successfully in mapping the nioghalvfjerdsfjorden glacier northeast greenland in spite of the difficult conditions with melting water on the glacier surface the measurements from the floating part of the glacier have been evaluated by comparison of radar data with laser altimeter and in situ measurements

geochemical cycling of phosphorus in rivers abstract concentrations of contaminants in fresh waters are controlled not only by point and diffuse sources within the catchment but by a multitude of biogeochemical reactions riverine processes that may occur in the water column during transport as an example the results of mass balance studies using snapshot field sampling techniques for p transport in a large river in northern england r swale are summarised to identify the processes that contribute to internal cycling within the river and their relative size compared with the total export of p the interaction of p with sediments is an important factor controlling concentrations of soluble p in the water column during both low and high river flows with in river storage in some instances amounting to 30 of the load throughput antecedent weather conditions are identified as important in controlling the pool of fine sediment and associated p available for remobilisation during storm pulses examples of p reactions with sediments that have been identified from field studies are briefly reviewed firstly a mass balance approach to elucidate the net interaction of p with river bed sediments is presented and then two specific interactions of p with sediments are discussed a the coprecipitation of phosphate with calcite in lakes during phytoplankton blooms and in benthic algal biofilms on river sediments and b the formation of vivianite in the anoxic zone of bed sediment in eutrophic lakes and rivers phosphate coprecipitation with calcite is an important mechanism controlling p concentrations in some hardwater lakes and has been found to contribute as much as 30 of the dry mass associated with the colonisation and development of algal biofilms on hardwater river beds vivianite has been detected in anoxic river sediments up to 20 by mass receiving treated sewage although little is known about the conditions of formation and how widespread the occurrence of vivianite is in sediments

a low frequency ice penetrating radar system adapted for use from an airplane test results from bering and malaspina glaciers alaska usa ice thickness measurements are needed to calculate fluxes through fast flowing outlet glaciers in greenland alaska patagonia and antarctica however relatively high attenuation of radio waves by dielectric absorption and volume scattering from englacial water hampers detection of the bed through warm deep ice in the past we have had success measuring ice thickness of temperate glaciers using a ground based monopulse radar system operating at low frequencies 2 mhz here we adapt the same system to operate from an airplane test flights over bering glacier alaska usa detected the bed through ice up to 1250 m thick flights across the seward malaspina glacier system alaska resolved the ice thickness of malaspina glacier but strong hyperbolic shaped returns obscured the bed echo through the seward throat it is likely that this clutter in the signal was caused by off nadir returns from chaotic surface crevasses that are ubiquitous in the throat region

greenland ice sheet response to external forcing kinematic wave modeling is used to evaluate possible responses of the greenland ice sheet to changes in its surface mass balance in the approach followed here the reference state is defined based on measured velocity and discharge flux along the central flow line of petermann glacier in the northwest and perturbations on this state are considered the results indicate that significant rates of thickness change can occur immediately after the prescribed change in surface mass balance but adjustments in flow rapidly diminish these rates to a few centimeters per year at most full adjustment of the ice sheet requires times of the order of 1000 years the instability mechanism known as the jakobshavn effect is discussed and based on observational evidence as well as results from prior modeling studies it is concluded that this is an unlikely mechanism for destabilizing major drainage basins of the greenland ice sheet

radar attenuation and temperature within the greenland ice sheet 2015 american geophysical union all rights reserved the flow of ice is temperature dependent but direct measurements of englacial temperature are sparse the dielectric attenuation of radio waves through ice is also temperature dependent and radar sounding of ice sheets is sensitive to this attenuation here we estimate depth averaged radar attenuation rates within the greenland ice sheet from airborne radar sounding data and its associated radiostratigraphy using existing empirical relationships between temperature chemistry and radar attenuation we then infer the depth averaged englacial temperature the dated radiostratigraphy permits a correction for the confounding effect of spatially varying ice chemistry where radar transects intersect boreholes radar inferred temperature is consistently higher than that measured directly we attribute this discrepancy to the poorly recognized frequency dependence of the radar attenuation rate and correct for this effect empirically resulting in a robust relationship between radar inferred and borehole measured depth averaged temperature radar inferred englacial temperature is often lower than modern surface temperature and that of a steady state ice sheet model particularly in southern greenland this pattern suggests that past changes in surface boundary conditions temperature and accumulation rate affect the ice sheet s present temperature structure over a much larger area than previously recognized this radar inferred temperature structure provides a new constraint for thermomechanical models of the greenland ice sheet

in stream phosphate removal by suspended sediments transported from volcanic catchments summary to better understand in stream phosphorus delivery and transformation processes we conducted field measurements and laboratory experiments to study the natural function of phosphate removal by suspended sediments from tributary streamflows of the abukuma river watershed field measurements revealed that phosphate concentrations and fluxes decreased along the abukuma river mainstream due to inflows from a tributary catchment located in a volcanic mountain range in the laboratory experiments water samples collected from tributaries were mixed with those from the abukuma river mainstream to examine the cause and effect of phosphate removal in the mainstream suspended sediments in the tributary samples removed up to about 70 of phosphate in the mainstream sample depending on the amount of sediment added x ray fluorescence spectrometer detected a high ratio of aluminum oxide on the surface of suspended sediments and scanning electron microscopic images showed relatively uniform 0 2 0 5 m spherical particles these findings and the geological condition of the tributary catchments studied led us to conclude that halloysite minerals were likely to comprise the majority of suspended sediments and play a significant role in phosphate adsorption the experimental results were up scaled and verified using field measurements in conclusion natural phosphate removal in the abukuma river can be attributed primarily to high ratios of aluminum oxide minerals transported by volcanic tributaries suggesting a significant hydrological contribution of volcanic catchments to in stream phosphate transport

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

the combined influences of variable thermal conductivity temperature and pressure dependent viscosity and core mantle coupling on thermal evolution abstract most convection studies of thermal history have not considered explicitly the thermal interaction between the mantle flow and the core we have investigated the influences of variable thermal conductivity and variable viscosity temperature and pressure dependent on the boundary layer and thermal characteristics of the d layer and the evolution of the thermo mechanical profiles of horizontally averaged viscosity and thermal conductivity viscosity contrast due to temperature dependence of up to 30 000 has been considered our results show clearly that variable thermal conductivity though small in magnitude as compared to variations in the viscosity does exert a significant delaying influence on mantle cooling thereby keeping the urey ratio low reducing the growth of the bottom thermal boundary layer and changing the viscosity profiles over time a higher temperature at the core mantle boundary increases the overall time dependent behavior of the thermal boundary layers enhanced radiative conductivity results in faster cooling opposite to the effect of the phonon conductivity component and a superadiabatic temperature gradient in the deep lower mantle finally the initial value of the core mantle boundary temperature can be inferred to wield a strong influence on the subsequent mantle thermal evolution in this model with both variable thermal conductivity and viscosity we may conjecture that other rheological and conductivity complexities such as grain size dependence of mantle properties would also have an impact on the current state of the mantle resulting from the primordial thermal condition

two frequency radar experiments for sounding glacier ice and mapping the topography of the glacier bed we performed airborne experiments using 150 and 450 mhz radars to measure ice thickness on the greenland ice sheet our objectives were to investigate to what degree surface clutter obscures the basal echo when airborne measurements are made at different elevations and at different frequencies we also explored interferometric techniques for processing the data to form swath measurements of ice thickness we found that surface clutter was minimal for either frequency when operated at low aircraft elevations 500 m above the ice sheet surface or over benign regions of the ice sheet because signal to clutter ratios were favorable we found that we could retrieve the swath measurements of ice thickness at both frequencies using an interferometric technique at high elevation surface clutter degraded the 150 mhz signal but the nadir ice thickness was still retrievable the basal return in high elevation 450 mhz data was detectable only after additional beam steering techniques were applied to the data to reduce the surface clutter signal results suggest that interferometric cross track ice thickness measurements can be successfully made given a sufficient number of antenna elements driven at either 150 or 450 mhz and flown at both high and low elevations over the interior ice sheet

hot spot heat transfer its application to venus and implications to venus and earth gross similarities between earth and venus suggest that both planets might be expected to lose heat by the same mechanism available data do not resolve plate tectonics on venus however and other heat loss mechanisms such as hot spot heat loss have been suggested using a model which gives a relationship among surface elevation lithospheric thickness and heat flux we test the hot spot heat loss mechanism for venus and determine that it can easily explain the predicted heat loss of the planet with a modest number of hot spots of the order of 35 approximately 93 of the mapped topography of venus can be explained solely on the basis of lithospheric thickness variations additional compensation is required for topography above a radius of 6053 km and this can be effected by incorporating a variable thickness crust into the model if crust is assumed to be generated on the crests of the hot spots probably by processes associated with volcanism the model is consistent with almost 99 of the mapped venus topography the model is also basically consistent with available gravity data and interpretations which indicate compensated topography and great depths of compensation 100 1000 km for the mid latitudes of the planet the remaining approximately 1 of the topography not explained by hot spot crustal generation is thought to be compensated at a shallower depth primarily by variations in crustal thickness which are not directly related to hot spot volcanism the models relating surface elevation to lithospheric thickness and heat flow are grossly consistent with either a plate tectonic or hot spot heat loss mechanism for venus the range of lithospheric thicknesses implied by the range of elevations on venus implies that if plate tectonics is the dominant heat loss mechanism on venus the mean plate velocities are slower than on earth slow spreading ridges should be resolvable by the available venus topographic data but are not apparent comparison and analysis of the hypsometric curves for earth and venus suggest basic differences between the curves the skewed terrestrial ocean floor curve being consistent with plate tectonics but the relatively symmetrical venusian curve indicates that either plate tectonics differs on venus from the terrestrial analogue or that plate tectonics does not dominate the topographic expression of venus tectonism the venusian curve is consistent with a hot spot heat loss mechanism a hot spot crustal genesis mechanism is proposed which would probably result in crust very similar to the granite greenstone terranes of many terrestrial archaean cratons we speculate that hot spot heat loss may have preceded plate tectonics as the dominant terrestrial heat loss mechanism

ice flow of humboldt petermann and ryder gletscher northern greenland radar interferometry ice penetrating radar profiles and an elevation model are used to determine the velocity fields rates of ice discharge approximate states of balance and catchment area for three large outlet glaciers in northeast greenland discharge through flux gates is calculated for humboldt and petermann gletscher which are found to be in balance at the level that the accumulation is known a large difference between the measured and estimated fluxes for ryder gletscher may be a reflection of unsteady flow behavior for this glacier the patterns ofice flow for the three glaciers considered are each unique showing that the nature of ice discharge varies substantially from basin to basin controlled by bed conditions and the presence of subglacial troughs and obstructions

high geothermal heat flow basal melt and the origin of rapid ice flow in central greenland age depth relations from internal layering reveal a large region of rapid basal melting in greenland melt is localized at the onset of rapid ice flow in the large ice stream that drains north off the summit dome and other areas in the northeast quadrant of the ice sheet locally high melt rates indicate geothermal fluxes 15 to 30 times continental background the southern limit of melt coincides with magnetic anomalies and topography that suggest a volcanic origin

ice sheet bed mapping with airborne sar tomography we develop and then demonstrate a 3 d tomographic ice sounding method applied to very high frequency vhf radar data that produces swath measurements of ice sheet surface topography ice thickness and radar reflectivity of both the surface and bed of the ice sheet first we formulate the ice sheet imaging problem as a problem of estimating signal arrival angles and illustrate how the method resolves ambiguous echoes arriving simultaneously from the left and right sides of the aircraft as well as from the surface and base of the ice sheet we then discuss why we chose the time domain subaperture method for 2 d image formation for ice sounding we apply the tomographic technique to the data that we collected in may 2006 and again in july 2008 from a multiple phase center vhf radar system we present 3 d images of the upper and lower surfaces of the ice sheet and compare the estimated surface topography with ice cloud and land elevation satellite altimeter nadir track measurements and the measured swath ice thickness with independent nadir depth sounder tracks we achieved a 5 m surface topography accuracy and a 14 m ice thickness accuracy

understanding groundwater surface water and hyporheic zone biogeochemical processes in a chalk catchment using fluorescence properties of dissolved and colloidal organic matter understanding groundwater surface water gw sw interaction in chalk catchments is complicated by the degree of geological heterogeneity at this study site in southern england united kingdom alluvial deposits in the riparian zone can be considered as a patchwork of varying grades and types with an equally varied lateral connectivity some display good connection with the river system and others good connection with the groundwater system and by definition poorer connectivity with the surface water by coupling tangential flow fractionation tff with fluorescence analysis we were able to characterise the organic matter in the river and hyporheic zone there is a significant proportion of particulate and colloidal fluorescent organic matter fom within the river system and at depth within the gravels beneath the river channel at depth in the hyporheic zone the surface water inputs are dampened by mixing with deeper groundwater fom the shallow 0 0 5 m below river bed hyporheic zone is highly dynamic as a result of changing surface water inputs from upstream processes labile c in the form of protein like fom appears to be attenuated preferentially compared to fulvic like fluorescence in the hyporheic zone compared to the adjacent gravel and sand deposits these preliminary findings have important implications for understanding nutrient and trace element mobility and attenuation within the groundwater surface water and hyporheic zone of permeable chalk catchments fluorescence analysis of dissolved organic matter has been shown to be a useful environmental tracer that can be used in conjunction with other methods to understand gw sw processes within a permeable chalk catchment

ice stream related patterns of ice flow in the interior of northeast greenland ice flow in the interior of the ne quadrant of the greenland ice sheet is focused on the large ice stream draining the north side of the summit dome the rapid ice flow in the stream is apparent in the surface features in the stream and at the margins in the broad scale topography that drives ice flow and in satellite derived ice motion information the patterns of ice flow in the upper half of the stream are remarkable for their level of organization simple geometry and effects on local surface topography the stream begins less than 100 km from the ice divide as a current 15 km wide and then broadens symmetrically downstream by the addition of ice from the sides to a width of more than 60 km elevation data and visible band imagery show that the stream has marginal troughs tens of meters deep in its upper reach which are coincident with regions of high shear strain rate the topography of the margins and undulating surface of the stream is generated by the ice flow the surface undulations in the stream are fixed in location and shape over the 5 year period from 1994 to 1999 the enhanced flow presents a challenge for researchers trying to understand the history of ice discharge from a significant area in the interior of the ice sheet

thickening of the western part of the greenland ice sheet nasa s program for arctic regional climate assessment parca includes measurements of ice velocity and ice thickness along the 2000 m elevation contour line in the western part of the ice sheet here we use these measurements together with published estimates of snow accumulation rates to infer the mass balance or rate of thickening thinning of the ice sheet catchment area inland from the velocity traverse within the accuracy to which we know snow accumulation rates the entire area is in balance but localized regions inland from upernavik isstrom and jakobshavn isbrae both appear to be thickening by about 10 cm a 1

greenland ice sheet high elevation balance and peripheral thinning aircraft laser altimeter surveys over northern greenland in 1994 and 1999 have been coupled with previously reported data from southern greenland to analyze the recent mass balance of the greenland ice sheet above 2000 meters elevation the ice sheet is in balance on average but has some regions of local thickening or thinning thinning predominates at lower elevations with rates exceeding 1 meter per year close to the coast interpolation of our results between flight lines indicates a net loss of about 51 cubic kilometers of ice per year from the entire ice sheet sufficient to raise sea level by 0 13 millimeter per year approximately 7 of the observed rise

direction of arrival estimation for radar ice sounding surface clutter suppression ice sounding radars are able to measure ice sheets by profiling their glaciological features from the surface to the bedrock the current airborne and in particular future space based systems are suffering from off nadir surface clutter which can mask the depth signal of interest the most recent surface clutter suppression techniques are based on multi phase center systems combined with sophisticated coherent postprocessing the performance of the techniques can be improved by accurate direction of arrival doa estimates of the surface clutter this paper deals with data driven doa estimation for surface clutter signals which includes a formulation of the mathematical foundation of spatial aliasing doa estimation is applied to data acquired with the p band polarimetric airborne radar ice sounder at the jutulstraumen glacier antarctica the effects of spatial aliasing related to a large phase center spacing are analyzed and an unwrapping procedure is presented and applied to the data finally doa estimation of full scene data is analyzed and used to show an along track and incidence off nadir angle dependent variation of the effective scattering center of the surface return which is caused by a varying penetration depth

using internal layers from the greenland ice sheet identified from radio echo sounding data with numerical models spatially extensive internal layers have been traced in airborne radio echo sounding res data collected over greenland during the late 1990s by linking internal layers within individual flight lines at crossover points it is possible to identify spatiallycontinuouslayersthatareinterpretedas isochronoussurfaces severalof thesur vey lines pass over the grip core site and this allows us to use the published gripage depth relationship to accurately date these surfaces two layers with ages of 3891 and 6956years bp have been traced over a large part of north greenland accurately dated and spatially continuous isochrones are valuable for both assimilation within and verifi cation of numerical models forexample comparison of isochronous surfaces from a nu merical simulation with those layers observed in res data can be used to inform the choice of parameters e g rheology and climate history usedto force a numerical model to demonstrate the potential of the res data two layers for north greenlandwere used to determine palaeo accumulation rates the inversion from layer depthto accumulation rate requires a three dimensional velocity field this velocity field is constructed by com biningatwo dimensionalbalance velocity fieldwithanassumedverticalstructureforthe horizontal velocity the isochronous layer derived accumulation rates were compared with the bales and others 2001 rates a larger east west gradient was found across the central ice divide for the derived accumulation rate suggesting a trend in the holocene accumulation rates for this region the layers were also compared with isochronous sur faces derived from simulations of a three dimensional thermodynamic ice sheet model using the isochronous layer derived accumulation rates to force the model improved the matchbetween modelled and observed layers

radiostratigraphy and age structure of the greenland ice sheet 2015 the authors several decades of ice penetrating radar surveys of the greenland and antarctic ice sheets have observed numerous widespread internal reflections analysis of this radiostratigraphy has produced valuable insights into ice sheet dynamics and motivates additional mapping of these reflections here we present a comprehensive deep radiostratigraphy of the greenland ice sheet from airborne deep ice penetrating radar data collected over greenland by the university of kansas between 1993 and 2013 to map this radiostratigraphy efficiently we developed new techniques for predicting reflection slope from the phase recorded by coherent radars when integrated along track these slope fields predict the radiostratigraphy and simplify semiautomatic reflection tracing core intersecting reflections were dated using synchronized depth age relationships for six deep ice cores additional reflections were dated by matching reflections between transects and by extending reflection inferred depth age relationships using the local effective vertical strain rate the oldest reflections dating to the eemian period are found mostly in the northern part of the ice sheet within the onset regions of several fast flowing outlet glaciers and ice streams reflections typically do not conform to the bed topography disrupted radiostratigraphy is also observed in a region north of the northeast greenland ice stream that is not presently flowing rapidly dated reflections are used to generate a gridded age volume for most of the ice sheet and also to determine the depths of key climate transitions that were not observed directly this radiostratigraphy provides a new constraint on the dynamics and history of the greenland ice sheet

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

ice sheet bed 3 d tomography information on bed topography and basal conditions is essential to developing the next generation ice sheet models needed to generate a more accurate estimate of ice sheet contribution to sea level rise synthetic aperture radar sar images of the ice bed can be analyzed to obtain information on bed topography and basal conditions we developed a wideband sar which was used during july 2005 to perform measurements over a series of tracks between the gisp2 and grip cores near summit camp greenland the wideband sar included an eight element receive antenna array with multiple phase centers we applied the multiple signal classification music algorithm which estimates direction of arrival signals to single pass multichannel data collected as part of this experiment to obtain fine resolution bed topography this information is used for producing fine resolution estimates of bed topography over a large swath of 1600 m with a 25 m posting and a relative accuracy of approximately 10 m the algorithm derived estimate of ice thickness is within 10 m of the grip ice core length data collected on two parallel tracks separated by 500 m and a perpendicular track are compared and found to have difference standard deviations of 9 1 and 10 3 m for the parallel and perpendicular tracks respectively

interannual variations of snow accumulation on the greenland ice sheet 1985 1996 new observations versus model predictions newly measured time series of net water equivalent accumulation from ice cores are reported for 11 sites located near the 2000 m contour of the greenland ice sheet many of these sites are located in regions where accumulation has not been previously measured we compared these new time series of annual accumulation with previously reported modeled precipitation covering the same time range because ice core accumulation records include both regional and local accumulation components we used adjacent ice core records at six multiple core sites to estimate the local component and extrapolated these results to the five single core sites we investigated the impact of this short scale spatial variability in accumulation on statistical comparisons between the observed accumulation and model precipitation although sealers were required to match the 12 year mean modeled precipitation to the mean observed accumulation a high degree of correspondence between observed interannual accumulation and scaled modeled precipitation was found at 7 of 11 locations the need for scaling and the low correspondence between model simulations and observations at some sites clearly indicate that model improvements are needed although very encouraging a more spatially distributed array of ice core accumulation measurements spanning the same time period as the precipitation modeling is necessary for continued model improvement and validation in agreement with earlier studies the large temporal variability in snow accumulation predicted by modeling and confirmed by ice cores indicates that the ice sheet elevation varies by tens of centimeters from year to year simply because of changing accumulation

large scale reconstruction of accumulation rates in northern greenland from radar data internal layers in polar ice sheets have been mapped in many areas using information from radio echo sounding surveys the layers contain imprints of past accumulation patterns and ice flow and can potentially provide estimates of past accumulation rates in this study average accumulation rates for the past 14 7 ka in north and central greenland are recovered from the mapped depth of one pronounced isochrone using a simple ice flow model accounting for both vertical and horizontal flow combined with formal inversion techniques using this method upstream variations in the accumulation and flow are taken into account when inferring the accumulation rates from the deep layer this resulted in larger spatial variations in the inferred accumulation rates compared to approaches using a one dimensional ice flow model the largest differences were found in areas of significant horizontal flow the inferred accumulation rates agree with holocene estimates from ice cores in the interior of the ice sheet across the central ice divide the inferred accumulation pattern shows a pronounced gradient in accumulation rates similar to the present day pattern suggesting that the large scale pattern in the interior of central northern greenland has been consistent throughout the past 14 7 ka

improvement of radar ice thickness measurements of greenland outlet glaciers using sar processing extensive aircraft based radar ice thickness measurements over the interior and outlet glacier regions of the greenland ice sheet have been obtained by the university of kansas since1993 with the latest airborne surveys conducted in may 2001 the radar has evolved during this period to a highly versatile system capable of charac terizing ice thickness over a wide variety of ice sheet conditions before 1997 the digital system was limited only capable of storing incoherent data or coherent data with a very large number of presumed signals at a low pulse repetition frequency in1998 the radar was upgraded with modern components allowing coherent datato be stored with a small number of presumed returns for1024 range cellsat a highpulse repetition frequency the new data on icethickness of greenlandoutlet glaciers arearchivedand made availableto the scientific community in the form of radar echograms and derived ice thickness at http tornado rsl ukans edu greenlanddata htm the u s national snow and ice data center nsidc also provides a link to these data and nsidc will eventually serve as the permanent archive of these data improvements in radar sensitivity in outlet glacier regions have been achieved by collecting coherent radar data and applying various signal processing techniques deep outlet glacier channels that were previously unre solvedwith incoherentdata can nowbemappedusingacoherent signal signalcondition ing and synthetic aperture radar sar processing

tracing the depth of the holocene ice in north greenland from radio echo sounding data radio echo sounding surveys over the greenland ice sheet show clear extensive internal layering and comparisons with age depth scales from deep ice cores allow for dating of the layering along the ice divide we present one of the first attempts to extend the dated layers beyond the ice core drill sites by locating the depth of the bolling allerod transition in 400 flight lines using an automated fitting method results show that the transition is located in the upper one third of the ice column in the central part of north greenland while the transition lowers towards the margin this pattern mirrors the present surface accumulation and also indicates that a substantial amount of pre holocene ice must be present in central north greenland

assessing the utility of acoustic communication for wireless sensors deployed beneath ice sheets the environments underneath ice sheets are of high scientific interest wireless sensors offer the prospect of sustained distributed remote sensing in the subglacial environment typically wireless sensor networks use radio frequency rf electromagnetic communications but these are highly attenuated in wet environments in such environments acoustic communications may be more power efficient here we review the literature on acoustic and rf attenuation through ice and other relevant media and present the results of new experiments on acoustic attenuation in glacial ice link budgets for communications from a range of subglacial environments show that acoustic communications are a viable strategy for transmission through water and ice where rf is too highly attenuated to be detected acoustic communication at 30 khz is predicted to be possible through 1 km of glacial ice using a 1 w transmitter such a strategy may be appropriate for shallow ice stream environments around the antarctic and greenland ice sheet margins

stagnant lid convection on venus the effect of strongly temperature dependent viscosity on convection in the interior of venus is studied systematically with the help of finite element numerical models for viscosity contrasts satisfying experimental constraints on the rheology of rocks venus is likely to be in the regime of stagnant lid convection this regime is characterized by the formation of a slowly creeping very viscous lid on top of the mantle venusian lithosphere and is in agreement with the tectonic style observed on venus stagnant lid convection explains large geoid to topography ratios on venus by the thermal thinning of a thick lithosphere the thickness of the lithosphere can be as large as 400 550 km for beta regio and 200 400 km on average geoid and topography data and experimental data on the rheology of rocks provide constraints on the viscosity of the mantle 10 20 10 21 pa s the convective stresses in the interior 0 2 0 5 mpa the stresses in the lid 100 200 mpa the velocity in the interior 0 5 3 cm yr 1 and the heat flux beneath the lithosphere 8 16 mw m 2 parameterized convection calculations of thermal history of venus are difficult to reconcile with a thick present day lithosphere however a sufficiently thick lithosphere can be formed if a convective regime with mobile plates was replaced by stagnant lid convection around 0 5 b y ago one of the possible explanations for the cessation of venusian plate tectonics is that during the evolution of venus stresses in the lid dropped below the yield strength of the lithosphere this model predicts a drastic drop in the heat flux thickening of the lithosphere and suppression of melting and is consistent with the hypothesis of cessation of resurfacing on venus around 0 5 b y ago

a low power consumption radar system for measuring ice thickness and snow firn accumulation in antarctica in order to measure total ice thickness and surface snow accumulation in antarctica we have designed and built a surveying system comprising two types of radar this system is aimed at having low power consumption low weight volume and low construction cost the system has a pulse compression radar to measure ice thickness and a frequency modulated continuous wave fm cw radar designed to measure hundreds of meters of surface snow firn layers with high resolution the pulse compression radar operates at 155 mhz 20 mhz of bandwidth and the fm cw radar operates from 550 to 900 mhz the system was tested in december 2010 at union glacier 79846 0 s 83824 0 w west antarctica during an oversnow campaign where union and other nearby glaciers schanz schneider and balish were covered through 82 km of track ice thickness of 1540 m and snow firn thickness of 120 m were detected in the area the collected data allowed the subglacial topography internal ice structure isochronous and the snow ice boundary layer to be detected here we describe radar electronics their main features and some of the results obtained during the first test campaign further improvements will focus on the adaptation of the system to be implemented on board airplane platforms

radar mapping of isunnguata sermia greenland ice thickness estimates using advanced nadir sounding and tomographic radar processing techniques are compared and combined in a study of isunnguata sermia glacier greenland using an ensemble of operation icebridge flight lines spaced at 500m intervals and running approximately along the flow direction we find there is a statistically excellent comparison between subglacial terrains derived from two dimensional tomography and gridded nadir sounding analysis shows that tomographic data better capture short wavelength 1 2 km patterns in basal terrain but interpolated nadir sounding data yield more spatially extensive and continuous coverage across the glacier especially in deep subglacial troughs using derived surface and basal topography maps we find that driving stress and measured and modeled surface velocity comparisons indicate that basal sliding is an important component of the glacier motion but is also only weakly coupled to the detailed bed topography save for the deepest troughs as might be expected for this land terminating relatively slow moving glacier the subglacial and proglacial topography is similar suggesting the erosional processes acting on the modern glacier bed once helped sculpt the now exposed land

airborne radar sounder for temperate ice initial results from patagonia we describe the development of a low frequency airborne radar specifically designed for the sounding of temperate ice the system operates at a central frequency of 1 mhz and consists of an impulse transmitter with an output voltage up to 5000 v and a digital receiver with a maximum gain of 80 db the radar was deployed on board a casa 212 aircraft which also carries a laser altimeter an inertial navigation system a digital camera and a gps receiver a description of the radar system is provided as well as preliminary results obtained at glaciar tyndall campo de hielo sur southern patagonia icefield where an ice depth of 670 m was reached

radar images of the bed of the greenland ice sheet 1 in this paper we apply radar tomography methods to very high frequency airborne synthetic aperture radar data to measure the ice thickness field and to construct three dimensional basal image maps of a 5 20 km study area located along the southern flank of the jakobshavn glacier greenland unlike ice radar measurements typically made at nadir our approach uses radar echo phase and amplitude measured across an antenna array to determine the propagation angle and signal strength of pixel elements distributed on each side of the aircraft flight path that information combined with knowledge of aircraft position and the assumed dielectric properties of the glacier can be used to measure ice thickness and radar reflectivity across a 3 km wide swath combining ice thickness and surface topography data we estimate basal topography and basal drag we conclude that the glacier is sliding over the bed we use the three dimensional image maps of the bed to inspect the modern subglacial geomorphology and find for the first time beneath the greenland ice sheet assemblages of long ridge groove landforms that are oriented in the direction of the ice flow spatial dimensions 10 to 30 m depths 150 to 500 m spacing and lengths of 10 km or more and correlation with the current ice flow direction suggest that these are glacial erosional features similar to mega grooves observed on deglaciated terrain

mechanisms for lithospheric heat transport on venus implications for tectonic style and volcanism the tectonic and volcanic characteristics of the surface of venus are poorly known but these characteristics must be closely related to the mechanism by which venus rids itself of internal heat on the other solid planets and satellites of the solar system lithospheric heat transport is dominated by one of three mechanisms 1 plate recycling 2 lithospheric conduction and 3 hot spot volcanism we evaluate each mechanism as a candidate for the dominant mode of lithospheric heat transfer on venus and we explore the implications of each mechanism for the interpretation of venus surface features despite claims made to the contrary in the literature plate recycling on venus cannot be excluded on the basis of either theoretical arguments or present observations on topography and radar backscatter landforms resulting from plate convergence and divergence on venus would differ substantially from those on the earth because of the high surface temperature and the absence of oceans on venus the lack of free or hydrated water in subducted material the possibility that subduction would more commonly be accompanied by lithospheric delamination and the rapid spreading rates that would be required if plate recycling removes a significant fraction of the internal heat if plate recycling occurs on venus the rolling plains and lowlands provinces would be approximate analogs to terrestrial ocean basins in terms of age igneous rock type and formative process highlands on venus would be roughly analogous to terrestrial continents the hypothesis that lithospheric conduction dominates shallow heat transfer on venus leads to the prediction that the lithosphere is thin if venus has a global heat loss per mass equal to that for earth then temperatures marking the base of the thermal lithosphere on earth would be reached on venus at an average depth of about 40 km unless the mantle convective planform can maintain lithospheric regions of persistently low heat flow or unless the present atmospheric greenhouse on venus is geologically recent then such a lithospheric thickness leads to the conclusion that the topographic features contributing to the 13 km of relief on the planet must be geologically young the hypothesis that hot spot volcanism dominates lithospheric heat transfer on venus leads to the prediction that the surface must be covered with numerous active volcanic sources in particular if a typical venus hot spot has a volcanic flux equal to the average flux for the hawaiian hot spot for the last 40 m y then 104 such hot spots are necessary to remove the venus internal heat by volcanism such a number would produce enough volcanic material to resurface the entire planet to a depth of 1 km every 2 m y few areas of the planet would escape resurfacing for geologically long periods of time we find that none of the mechanisms for lithospheric heat transfer on venus can be excluded as unimportant at present it is likely that as on earth a combination of mechanisms operates on venus the strongest conclusion to emerge from this evaluation is that most of the major topographic features and probably many of the surface geological units on venus are young by comparison with the surfaces of the smaller terrestrial planets

substantial thinning of a major east greenland outlet glacier aircraft laser altimeter surveys in 1993 and 1998 over kangerdlugssuaq glacier in east greenland reveal thinning over the 5 year interim of several meters for all surveyed areas within 70 km of the seaward ice front rising to 50 meters in the final 5 km such rapid thinning is best explained by increased discharge velocities and associated creep thinning most probably caused by enhanced lubrication of the glacier bed the calving ice front over the past decade has occupied approximately the same location as in 1966 velocity estimates for 1995 96 are about the same as those for 1966 and 1988 but significantly less than for 1999 suggesting that major thinning began after 1995

advanced multifrequency radar instrumentation for polar research this paper presents a radar sensor package specifically developed for wide coverage sounding and imaging of polar ice sheets from a variety of aircraft our instruments address the need for a reliable remote sensing solution well suited for extensive surveys at low and high altitudes and capable of making measurements with fine spatial and temporal resolution the sensor package that we are presenting consists of four primary instruments and ancillary systems with all the associated antennas integrated into the aircraft to maintain aerodynamic performance the instruments operate simultaneously over different frequency bands within the 160 mhz 18 ghz range the sensor package has allowed us to sound the most challenging areas of the polar ice sheets ice sheet margins and outlet glaciers to map near surface internal layers with fine resolution and to detect the snow air and snow ice interfaces of snow cover over sea ice to generate estimates of snow thickness in this paper we provide a succinct description of each radar and associated antenna structures and present sample results to document their performance we also give a brief overview of our field measurement programs and demonstrate the unique capability of the sensor package to perform multifrequency coincidental measurements from a single airborne platform finally we illustrate the relevance of using multispectral radar data as a tool to characterize the entire ice column and to reveal important subglacial features

radio wave depolarization and scattering within ice sheets a matrix based model to link radar and ice core measurements and its application crystal orientation fabric cof has a large influence on ice sheet flow earlier radar studies have shown that cof based birefringence occurs within ice sheets radio wave scattering in polar ice results from changing physical properties of permittivity and conductivity that arise from differing values of density acidity and cof we present an improved mathematical model that can handle all these phenomena together we use this matrix based model to study the two way propagation of depolarized radio waves that scatter at both isotropic and anisotropic boundaries based on numerical simulations we demonstrate how cof affects the radar signals in terms of radar polarization and frequency we then compare the simulated features with vhf radar data obtained at two contrasting inland sites in east antarctica where cof is known from ice core studies these two sites are dome fuji located near a dome summit and mizuho located in a converging ice flow region data at dome fuji are dominated by typical features resulting from birefringence in contrast both birefringence and anisotropic scattering affect the radar data at mizuho we argue that radar methods can be used to determine principal axes and strength of birefringence in the ice sheets

precipitation over greenland and its relation to the north atlantic oscillation the equation method based on an equivalent isobaric geopotential height in coordinates has been used to retrieve the precipitation over greenland this approach is designed to accurately represent the topographic effects of the greenland ice sheet on atmospheric motion and precipitation the 11 year mean precipitation from 1985 to 1996 over all of greenland is 376 mm yr 1 which is close to the long term mean precipitation of 346 mm yr 1 estimated from glaciological data the precipitation over all of greenland shows that the largest value in 1986 is 472 mm yr 1 and the smallest value in 1995 is 309 mm yr 1 the major interannual variability of the atmospheric circulation in the north atlantic can be represented by the variation of the north atlantic oscillation nao index which is most pronounced during winter it is found that if the nao index increases the total precipitation over greenland decreases and vice versa the correlation coefficient between these two series for 1985 1995 is 0 75 the mean precipitation over southern greenland where the majority of precipitation falls is more closely related to the nao index in winter and their correlation coefficient is 0 80 this relationship can be understood from the composite maps of sea level pressure and greenland precipitation for the high and low index months during months of high nao index values the icelandic low is strong during months of low nao index values the monthly mean low is located to the southwest of greenland over the labrador sea precipitation amounts over the southeast coast of greenland are about 100 mm larger during the low nao index months than the high nao index months precipitation over all of greenland during the low nao index months is higher there are significant downward trends in annual precipitation from 1985 1995 for all of greenland and its southern and central west coastal regions amounting to about 3 per year

parameterized thermal convection in a layered region and the thermal history of the earth parameterized convective schemes for constant viscosity fluids are used to investigate the influence of layering on the relationship between the rate of heat loss from and heat supply to a number of convecting layers of different viscosity and thicknesses simple analytic expressions are obtained which accurately predict the behavior to a step decrease in the heat supply and show that layering considerably increases the response time the same arguments are applied to a planet consisting of spherical shells they show that the response of the earth s heat loss to the decay of radioactive elements depends strongly on whether or not mantle convection extends from the core to the surface if it does so the surface heat flux is only about 50 greater than the heat generation rate however if upper and lower mantle convection occurs in two separate shells as seismological and geochemical arguments suggest the response time is increased the present surface heat flux is then principally controlled by the thermal structure of the earth at the time it was formed the best estimate of the present ratio of the heat loss to heat generation is about two and this value is consistent with the behavior of an earth model consisting of a two layered mantle overlying a core

greenland ice sheet thickness changes measured by laser altimetry precise airborne laser altimetry surveys at locations on the greenland ice sheet that had been accurately surveyed in 1980 and 1981 reveal a thickening in western greenland of up to two meters between 1980 and 1993 we cannot yet state whether this represents a long term trend or the cumulative effects of interannual variability of snow accumulation rates nevertheless the information presented here provides an indication of ice thickness changes across southern greenland in unprecedented detail laser altimetery surveys have now been made over all the major ice sheet drainage basins and will be repeated at regular intervals to provide detailed estimates of ice thickening thinning rates over the entire ice sheet