snow grain size profiles deduced from microwave snow emissivities in antarctica spaceborne microwave radiometers are an attractive tool for observing antarctic climate because their measurements are related to the snow temperature however the conversion from microwave emission to snow temperature is not simple and strongly depends on the emissivity through snow properties this difficulty in predicting the snow property profile for antarctic conditions is the main bottleneck in the retrieval of accurate climate information from microwave radiometers we attempt to explain the vertically polarized emissivity at 19 3 and 37 ghz derived from brightness temperatures acquired by the special sensor microwave imager ssm i and physical temperature from the era 40 re analysis in antarctica the snow emissivities at 19 3 and 37 ghz are nearly equal although a decrease with frequency is expected to explain this we consider various profiles of snow grain size and density and predict their emissivity using a dense medium radiative transfer dmrt model the results show that the emissivities cannot be explained by constant profiles of grain size and density heterogeneous snowpacks need to be considered we first test random variations of snow density and grain radius with depth and then monotonic and continuous variations in the snow grain radius in both cases we show that an overall increase of the snow grain radius with depth is required to match the observed emissivity in antarctica in addition two parameters characterizing the snow grain profiles are retrieved and compared with 1 in situ measurements of grain size at various locations in east antarctica 2 grain size estimated using visible spaceborne radiometers and 3 a semi empirical relationship for grain growth

antarctic snow accumulation mapped using polarization of 4 3 cm wavelength microwave emission different parts of antarctica receive different amounts of snowfall each year in this paper we map the variations of the mean annual snow accumulation across the ice sheet we also quantify the uncertainty in our estimates more objectively than has been possible for earlier maps the new map is produced using observations from satellites and ground based measurements after a logarithmic transformation these are combined using the geostatistical method of continuous part universal kriging to give an estimate of the snow accumulation within each cell of a rectangular grid covering antarctica we also derive spatial averages over the major drainage systems of the ice sheet along with their confidence intervals we obtain a value of 143 4 kg m 2 a 1 for the average rate of snow accumulation upon the grounded ice sheet of antarctica

the influence of the layered character of snow cover on the triggering of slab avalanches one of the principal aims of avalanche warning is to prevent slab avalanches triggered by skiers other than explosives the best practical tool for stability evaluation is the rutschblock test whether the slab may be triggered by a skier depends on various slab characteristics important factors seem to be depth of the weak layer slab hardness and sublayering of the slab the stress distribution induced by a skier is calculated by the finite element method for typical snow cover configurations the additional shear stress is of the same order of magnitude as the shear strength of weak layers besides the critical weak layer prerequisite for a slab avalanche hard layers seem to be important the analysis suggests that a shear failure is most probable at the transition from a hard to a soft layer this corresponds well to observations of slab avalanche profiles the results may help to improve and quantify the analysis of snow profiles

experiments on piping in sandy gravels tests on internally unstable sandy gravels show that a significant proportion of the sand content is washed out by piping at hydraulic gradients far lower than the critical gradient given by a classical theory the probable explanation is that a major part of the overburden load is carried on a framework of gravel particles leaving the sand under relatively small pressures by contrast piping of the sand from stable sandy gravels occurs at approximately the full theoretical gradient the tests broadly confirm kenney s criterion for the internal stability of granular materials des essais realises sur des graves sableuses a instabilite interne importante montrent qu une proportion non negligeable du sable est lessivee pour des gradients hydrauliques bien inferieurs au gradient critique donne par les theories classiques l explication la plus probable est que la majeure partie de la surcharge des couches subjacentes est transmise au reseau des particules graveleuses le sable n etant alors plus soumis qu

numerical simulations of sand migration during gas production in hydrate bearing sands interbedded with thin mud layers at site nghp 02 16 abstract the geomechanical behavior of gas hydrate bearing sands during gas production is complex involving changes in hydrate dependent mechanical characteristics and also redistribution of stresses carried by the hydrate when an interbedded mud layer is present the complex response of the hydrate bearing sediments is more pronounced because hydrate dissociation tends to occur preferentially in the sands adjacent to the hydrate free mud layer which can act as a heat source this leads to non uniform sediment deformation and potentially to shearing deformation along the sand mud layers in addition since the hydrate bearing sand permeability becomes far greater than the mud permeability with hydrate dissociation a greater hydraulic gradient may exist along the interface between the sand mud layers these two behaviors may contribute to sand migration and thus a thorough study on the effect of a thin interbedded mud layer on the extent of sand migration is necessary this paper presents numerical sand migration modeling of sand mud interbedded hydrate bearing sediments found at site nghp 02 16 one of 25 sites drilled as a part of national gas hydrate program expedition 02 nghp 02 in the offshore of india subjected to depressurization for a comparison and better understanding simulation of homogeneous hydrate bearing sediments is also conducted it is found that greater sand migration occurs when the interbedded mud layer is present because of greater shearing deformation and greater pressure gradients at sand mud interface

winter snow cover on sea ice in the weddell sea measurements of snow thickness temperature salinity density and stratigraphy acquired during the 1992 winter weddell gyre study are presented results indicate that the winter snow cover on sea ice in the weddell sea is extremely variable extreme fluctuations in antarctic synoptic conditions air temperature precipitation humidity and wind speed occur during the austral winter they result in unique modifications and additions to the snow layer during the aging process and act to stabilize an otherwise easily wind redistributed shallow snow cover and develop well packed drift features the latter occur even over relatively undeformed areas of sea ice and have a significant localized effect on the snow thickness distribution significant variability in snow grain size mean 2 73 3 12 mm and density 0 32 0 09 g cm 3 is observed as a result of cyclical switches between high and low temperature gradient metamorphism multiple icy layers indicate multiple thaw freeze events one such event occurred during a 3 day station during which the air temperature rose by 22 c in 12 hours to approximately 0 c this paper also examines mechanisms for flooding of the snow ice interface including snow loading even where the latter is not a factor the layer of snow immediately above the snow ice interface is commonly damp and saline 10 limitations in the data set are discussed and comparisons are drawn with other experiments

δ15n of n2 in air trapped in polar ice a tracer of gas transport in the firn and a possible constraint on ice age gas age differences with respect to gas transport the fim near the surface of an ice sheet can be divided into three zones the uppermost is a convective zone located just below the surface of the ice sheet in which the air is rapidly flushed by convective exchange with the overlying atmosphere below the convective zone there is a diffusive air column in which diffusion is rapid but there is no convection between the bottom of the diffusive air column and the bubble close off region there may be a nondiffusive zone in which diffusion is so slow that negligible gas transport occurs the diffusive air column is characterized by progressive enrichment with depth of 15n in n2 and heavy isotopes of gases in general as predicted using the barometric equation in this paper we present data on the 15n of n2 in recently trapped air samples from 12 ice cores along with numerous downcore samples from byrd vostok and dome c bubble close off depths for these cores calculated from a densification model ranged from 51 to 114 meters below the surface mbs we used these data and the barometric equation to calculate the thickness of the diffusive air column and found that it comprised 46 to 93 of the total firn thickness at our study sites paleo close off depths calculated from the densification model for glacial sections of byrd vostok and dome c are 15 25 m deeper than close off depths today diffusive column heights calculated from 15n varied in a more complex manner the diffusive column height at byrd appears to have decreased from 74 m during the last glacial period to 50 m during the holocene at vostok and dome c the diffusive column height calculated from 15n increase from about 65 m during the last glacial period to about 80 m in the holocene we use records of surface temperature and co2 at byrd and vostok along with their respective chronologies to constrain the ice age gas age difference age throughout the section of the vostok ice core corresponding to the last glacial termination in principle age values calculated from these data can be used to discriminate whether gas in the fim mixes to the bubble close off depth or to a depth equivalent to the diffusive column height in practice however uncertainties in the chronology of byrd and vostok are too great to allow us to distinguish between these two possibilities one can only say that at the time of the last termination age for vostok was between 3 and 10 5 kyr previous estimates fall within this range

comparison of traditional and optical grain size field measurements with snowpack simulations in a taiga snowpack knowledge of snow microstructure is relevant for modelling the physical properties of snow cover and for simulating the propagation of electromagnetic waves in remote sensing applications characterization of the microstructure in field conditions is however a challenging task due to the complex sintered and variable nature of natural snow cover a traditional measure applied as a proxy of snow microstructure which can also be determined in field conditions is the visually estimated snow grain size developing techniques also allow measurement for example of the specific surface area ssa of snow from which the optical equivalent grain size can be derived the physical snow model snowpack simulates evolution of snow parameters from meteorological forcing data in this study we compare an extensive experimental dataset of measurements of traditional grain size and ssa derived optical grain size with snowpack simulations of grain size parameters on average a scaling factor of 1 2 is required to match traditional grain size observations with the corresponding snowpack simulation a scaling factor of 2 1 was required for the optical equivalent grain size standard deviations of scaling factors for the winters of 2011 12 and 2012 13 were 0 36 and 0 42 respectively the largest scaling factor was needed in early winter and under melting conditions

critical hydraulic gradients of internal erosion under complex stress states abstractinternal erosion involves selective loss of fine particles within the matrix of coarse soil particles under seepage flow which affects the hydraulic and mechanical behavior of the soil in this research extensive laboratory internal erosion tests were conducted under complex stress states following three stress paths isotropic drained triaxial compression and triaxial extension stress paths these tests were designed to investigate the initiation and development of internal erosion and the effect of stress state on critical hydraulic gradients the entire erosion process can be divided into four phases stable initiation development and failure accordingly three critical gradients termed as initiation skeleton deformation and failure hydraulic gradients can be defined these critical gradients correspond to the onset of erosion of the fine particles filling the large pores of the skeleton the buckling of the strong force chains formed by the coarse particles and the soil failure re

measurements of mid winter spatial distribution of meltwater saturation under melt freeze conditions crusts may evolve within a snowpack which may favour avalanche initiation by forming a hard bed surface for weakly bonded faceted grains we used a parallel probe saturation profiler ppsp to record the distribution of water contents within the snowpack diurnal effects of melt freeze action on the growth of crusts were monitored with the help of the ppsp device saturation profiles were collected from a partially wet snow cover snow stratigraphy was conducted manually in the morning after overnight freezing to identify the location and the granular compositions of the crusts that had evolved a one to one correspondence between the saturation spikes collected using the ppsp and the actual positions of the crusts was established the ppsp was also used to monitor three dimensional variations in the maximum percolation depths within a south facing snowpack the operation of the ppsp is faster than existing dielectric measurement techniques so it was applied to study the spatial variability of maximum percolation depths on the slopes of different aspects

wetting front advance and freezing of meltwater within a snow cover 1 observations in the canadian arctic in a naturally stratified snow cover the movement of meltwater into dry snow is complicated by the interaction of the wetting front with stratigraphic horizons field observations showed that when the wetting front reached premelt stratigraphic horizons water ponded at the interface and then flow fingers developed and penetrated the lower stratum the flux in these fingers which was increased to about twice that of the surface flux was used to feed water to the impeding horizons where it froze to form ice layers these ice layers were the major source of latent heat released within the snow cover and they were responsible for the warming of the snow and the underlying soil these continuous ice layers grew only at stratigraphic boundaries because of this ice layer growth the wetting front advance was retarded and the arrival of meltwater at the snow cover base was significantly delayed owing to a cold substrate the strong heat flux from the snow into the soil delays the warming of the snow cover and limits runoff after the snow is isothermal at 0 c by the refreezing of soil infiltration and the development of a basal ice layer

recording microscale variations in snowpack layering using near infrared photography deposition of snow from precipitation and wind events creates layering within seasonal snowpacks the thickness and horizontal continuity of layers within seasonal snowpacks can be highly variable due to snow blowing around topography and vegetation and this has important implications for hydrology remote sensing and avalanche forecasting in this paper we present practical field and post processing protocols for recording lateral variations in snow stratigraphy using near infrared nir photography a fuji s9100 digital camera modified to be sensitive to nir wavelengths was mounted on a rail system that allowed for rapid imaging of a 10 m long snow trench excavated on the north side of toolik lake alaska 68 3 n 149 36 w post processing of the images included removal of lens distortion and vignetting a tape measure running along the base of the trench provided known locations control points that permitted scaling and georeferencing snow layer heights estimated from the nir images compared well with manual stratigraphic measurements made at 0 2 m intervals along the trench n 357 r2 0 97 considerably greater stratigraphic detail was captured by the nir images than in the manually recorded profiles nir imaging of snow trenches using the described protocols is an efficient tool for quantifying continuous microscale variations in snow layers and associated properties

glide avalanche response to an extreme rain on snow event snoqualmie pass washington usa rain on snow events trigger immediate and delayed avalanches as liquid water penetrates the snowpack we present results from an extreme rain on snow event that triggered a glide avalanche near snoqualmie pass washington usa snoqualmie pass recorded 463 cm of snowfall from 13 december 2008 to 6 january 2009 this period of snowfall was followed by a strong southwesterly tropical flow that resulted in an extreme rain on snow event sensors at snoqualmie pass recorded 285 mm of precipitation over a 52 hour period flooding slush flows landslides and avalanches resulted from the influx of precipitation snow heights decreased rapidly over the period with settlement rates approaching 80 mm h 1 liquid water infiltrated and flowed through the snowpack within a few hours of the arrival of rain yet many of the major avalanches occurred 12 30 or more hours after the onset of rain and water outflow a glide avalanche occurred 30 hours after the onset of rain and the establishment of drainage through the snowpack increasing glide rates correlate with periods of rapid snow settlement here glide rates approached 670 mm h 1 although glide and settlement rates increased during periods of intense precipitation glide failure occurred 8 hours after peak precipitation and outflow

sand production model in gas hydrate bearing sediments abstract this paper provides a comprehensive analytical formulation that entails the features of sand production in gas hydrate bearing sediments including grain detachment migration sediment deformation and hydrate dissociation the formulation is thermo hydro mechanically coupled such that grain detachment causes stress reduction sediment shear deformation induces grain detachment and grain flow alters multiphase fluid pressure and temperature profiles through a series of analyses the sensitivity of sand production related parameters on resulting solid volume changes is evaluated furthermore the effect of various operational methods on mitigating sand production in hydrate bearing sediments during gas production is numerically investigated it is found that out of the different operational methods investigated lowering depressurization rate was the most effective in reducing sand production for a given gas production

visualizing meltwater flow through snow at the centimetre to metre scale using a snow guillotine movement of liquid water through snowpacks remains one of the least understood aspects of snow hydrology liquid water movement through snowpacks is generally recognized to occur in distinct flow paths rather than as uniform flow through a homogeneous porous medium dye tracer experiments have been used in studies of meltwater flow through snow since the 1930s although dye tracer experiments have provided valuable qualitative information about meltwater pathways quantitative descriptions of their spacing and location are not commonly available because of the difficulty in precisely excavating and measuring pathways here we provide a new proof of concept instrument we term a snow guillotine that provides more quantitative information from dye tracer experiments conducted on melting snowpacks photographs are taken of each cross section over a 1 m distance application of image processing and geostatistical analysis allows collection of high resolution 1 cm3 three dimensional data on meltwater flow through a snowpack the results show preferential flowpaths with the majority of vertical flow occurring in the upper 20 55 cm of the snowpack while fewer preferential flowpaths are apparent below 100 cm the number of vertical flowpaths in the upper half of the snowpack averaged almost 100 per m2 with the highest number of flowpaths reaching almost 300 per m2 layer interfaces were found to significantly increase the volume of dye indicating dominance by lateral flow at these boundaries at each stratigraphic interface the number of individual clusters decreased and it was more likely for a dyed pixel to be part of a large cluster geostatistical analyses showed that there were large increases in correlation lengths and the connectivity function at stratigraphic layers in contrast to low values between layers for example the buried ice layer in experiment a at 169 170 cm showed separation distances of 20 cm in contrast two rows above this layer the separation distance was only 2 cm implementation of the snow guillotine provides the ability to conduct geostatistical analyses on field measurements of meltwater flow while providing three dimensional quantitative data of unprecedented spatial resolution copyright 2010 john wiley sons ltd

contact spectroscopy for determination of stratigraphy of snow optical grain size we present a technique for in situ measurement of the vertical and spatial stratigraphic distribution of snow optical grain size with a coupled contact illumination probe and field spectroradiometer accurate measurements of optical equivalent grain size are critical for modeling radiative properties of snow such as spectral albedo and microwave emission measurements of the spectral reflectance of the snow pit surface are made at 2 cm intervals in the vertical plane under constant illumination and view geometries we invert the integral of the continuum normalization of the ice absorption feature with maximum at 1 03 mm wavelength for optical equivalent grain size using the validated model of nolin and dozier 2000 that has accuracy of 10 50 mm across the grain size range 50 900 mm results are presented for an alpine site in southwest colorado usa across the ablation season and for a greenland ice sheet site at the onset of snowmelt these results suggest that traditional measurements of grain size using a hand lens are nearly accurate only for rounded grains r 2 0 41 rmse 160 mm for polycrystals and faceted grains the hand lens approach is very inaccurate r 2 0 03 and 0 24 rmse 1206 and 1010 mm respectively we demonstrate an order of magnitude improvement in modeling of shortwave spectral albedo and net shortwave flux with contact spectroscopy measurements of grain size stratigraphy over those from a hand lens

extreme anomalous atmospheric circulation in the west antarctic peninsula region in austral spring and summer 2001 02 and its profound impact on sea ice and biota exceptional sea ice conditions occurred in the west antarctic peninsula wap region from september 2001 to february 2002 resulting from a strongly positive atmospheric pressure anomaly in the south atlantic coupled with strong negative anomalies in the bellingshausen amundsen and southwest weddell seas this created a strong and persistent north northwesterly flow of mild and moist air across the wap in situ satellite and ncep ncar reanalysis nnr data are used to examine the profound and complex impact on regional sea ice oceanography and biota extensive sea ice melt leading to an ocean mixed layer freshening and widespread ice surface flooding snow ice formation and phytoplankton growth coincided with extreme ice deformation and dynamic thickening sea ice dynamics were crucial to the development of an unusually early and rapid short retreat season negative ice extent anomaly strong winds with a dominant northerly component created an unusually compact marginal ice zone and a major increase in ice thickness by deformation and over rafting this led to the atypical persistence of highly compact coastal ice through summer ecological effects were both positive and negative the latter including an impact on the growth rate of larval antarctic krill and the largest recorded between season breeding population decrease and lowest reproductive success in a 30 yr adelie penguin demographic time series the unusual sea ice and snow cover conditions also contributed to the formation of a major phytoplankton bloom unexpectedly the initial bloom occurred within compact sea ice and could not be detected in sea viewing wide field of view sensor seawifs ocean color data this analysis demonstrates that sea ice extent alone is an inadequate descriptor of the regional sea ice state conditions from both a climatic and ecological perspective further information is required on thickness and dynamics deformation

winter snow cover variability on east antarctic sea ice copyright 1998 by the american geophysical union analysis of the first detailed data set of snow characteristics collected over east antarctic sea ice in winter confirms that on small scales snow on antarctic sea ice is highly variable in both thickness and properties high amplitude cyclical variability in atmospheric forcing related to the passage of storms is responsible for the high degree of textural heterogeneity observed changes in snow properties were examined over a 3 week period during which a largely icy snow cover formed at near freezing temperatures metamorphosed to snow in which facetted crystals and depth hoar dominated as the air temperature plummeted even on flat ice significant localized thickening of snow occurs in the form of barchan dunes although we observed great variability in snow thickness and properties on local scales overall snow thickness distribution and the complex textural assemblage of snow types are similar from region to region similar observations were made by sturm et al 1998 in west antarctica large scale similarities are also apparent in mean snow density grain size and bulk snow salinity although high variability is again found across individual floes rapid depth hoar formation is a ubiquitous process that greatly affects the density texture grain size and effective thermal conductivity of the snow cover the observed heterogeneity results in varying snow effective thermal conductivities the mean bulk effective thermal conductivity computed from the proportion of observed snow types is 0 164 w m 1 k 1 significantly lower than values typically used in large scale sea ice modeling but similar to that derived by sturm et al 1998 in a near simultaneous experiment in the bellingshausen and ross seas it varies from 0 097 to 0 383 w m 1 k 1 in different snow pits the findings support those of sturm et al 1998 that periodic flooding and subsequent snow ice formation which are also ubiquitous processes effectively diminish the degree to which basal snow processes create inhomogeneities in the snow pack

simulated geomechanical responses to marine methane hydrate recovery using horizontal wells in the shenhu area south china sea abstract the recovery of natural gas hydrate ngh from marine sediments faces challenging due to not only the gas productivity itself but also the possible geohazards such as seafloor subsidence submarine landslide wellbore instability and possible sand production the coupled thermal hydrodynamic mechanical thm processes during ngh recovery are generally complex and numerical simulation tools are needed to assess related geomechanical responses we have extended the biot consolidation model in our previous simulator tough2biot and incorporated into the existing tough hydrate code resulting in a thm simulator for the ngh recovery the thm simulator is used to assess the geomechanical responses to gas recovery from an unconfined hydrate bearing sediment hbs in the shenhu area south china sea we investigated depressurization using constant bottom hole pressure through a horizontal well results show that methane production quickly reaches a stabilized state and the water rate increases linearly the drawdown of pore pressure around the well controls the increase in effective stress subsidence becomes significant after depressurization due to the quickly propagation of pore pressure depressurization in early stage could contribute to more than half of the total subsidence the decreasing production pressure leads to an increase in the methane production rate but deterioration in subsidence a decrease in intrinsic permeability of overlying and underlying layer is undesirable due to its decrease in methane production rate and the worse of subsidence a balance between gas productivity and related geomechanical response must be achieved the methods and preliminary results presented in this study could help us to understand the geomechanical behaviors during ngh recovery and to design trial production schemes under similar conditions

snow crystal growth with varying surface temperatures and radiation penetration the temperature field is derived for a sinusoidally varying surface temperature with varying solar radiation penetration the growth rates of snow crystals are calculated to explain the rapidly growing layers of faceted crystals i e depth hoar that form just below the surface at high altitudes and in polar snow the solutions also show that a layer of wet snow can exist just below the surface even on days when the surface temperature remains sub freezing

observations of snowpack ripening in the sierra nevada california u s a the transition from a dry sub freezing snow cover to isothermal snow freely conducting water was observed near timberline in the sierra nevada california u s a although there were few major stratigraphic boundaries in the snowpacks observed minor textural discontinuities were sufficient to both route water laterally downslope and later create ice layers at these strata during the first few days of snowmelt downslope movement of water parallel to the strata allowed water to bypass lower layers of the snow cover on slopes and converge in microtopographic depressions on level terrain creating isolated drains through the snowpack in addition to these drains flow fingers associated with holes in the ice sheets delivered water to the base of the snow several days prior to the thorough wetting of the snowpack formation of a series of ice lenses just below the snow surface every night released much more latent heat than did ice layer development within the hulk of the snowpack or at its base

determination of timing and location of water movement and ice layer formation by temperature measurements in sub freezing snow measurements of temperature in snow along a vertical profile during the onset of spring melting are used to calculate spatial and temporal temperature gradients and terms of the conduction equation with an internal energy production term are calculated heat transfer information is combined with stratigraphic observations made during melting and allow detailed determination of the timing and location of heterogeneous water movement and of refreezing internal energy production is interpreted as latent heat of refreezing of percolated meltwater times and locations of flow and refreezing of meltwater are calculated and compared to stratigraphic observations of layering and changes in density and liquid water content sequences of melt piping layering and refreezing seen in calculations and in stratigraphy demonstrate retarded flow at fine to coarse transitions flow along such transitions and refreezing at the transitions to form ice layers downslope flow is also observed in the absence of an impermeable horizon to redirect flow from the vertical

model for dry snow metamorphism by interparticle vapor flux kinetic growth metamorphism lowers the mean number of bonds between the ice particles in snow during the beginning state of snow metamorphism at a large mean temperature gradient significant particle growth occurs only at preferred growth sites three dimensional approximations of the sink source geometry are introduced to model particle growth and shrink rates the calculations show that enhanced growth occurs at very small and at large sink source distances the consideration of growth rate enhancement by differences in radii of curvature between sink source particles accounts for metamorphism at very low mean temperature gradients final particle size distributions and the reduction of the particle number density by metamorphism are calculated and may be compared with measured distributions

critical state soil constitutive model for methane hydrate soil 1 this paper presents a new constitutive model that simulates the mechanical behavior of methane hydrate bearing soil based on the concept of critical state soil mechanics referred to as the methane hydrate critical state mhcs model methane hydrate bearing soil is under certain geological conditions known to exhibit greater stiffness strength and dilatancy which are often observed in dense soils and also in bonded soils such as cemented soil and unsaturated soil those soils tend to show greater resistance to compressive deformation but the tendency disappears when the soil is excessively compressed or the bonds are destroyed due to shearing the proposed model represents these features by introducing five extra model parameters to the conventional critical state model it is found that for an accurate prediction of ground settlement volumetric yielding plays an important role when hydrate soil undergoes a significant change in effective stresses and hydrate saturation which are expected during depressurization for methane gas recovery

the layered character of snow covers snow studies have generally ignored the layered nature of snow covers having achieved a good understanding of the properties and processes in homogeneous snow snow scientists should develop more insight into the evolution of the layers and their effects on overall snow response many of the outstanding problems in snow studies can only be solved by dealing with snow as a layered medium the various mechanisms by which layers are generated their effects and some of the outstanding research problems are described here

non hydrostatic effects on mountain wave breaking in directional shear flows mountain waves excited by narrow 3d orography are investigated using idealized numerical simulations of atmospheric flows with directional wind shear the stability of these waves is compared with the stability of hydrostatic mountain waves the focus is on understanding how wave breaking is modified via gravity wave critical level interaction when non hydrostatic dispersive effects arise the influence of non hydrostatic effects on wave breaking appears to be a function of the intensity of the background shear increasing the stability of the flow inhibiting wave breaking for weak wind shear but decreasing it instead enhancing wave breaking for stronger wind shear

theory of metamorphism of dry snow the growth of ice particles in dry seasonal snow is caused by vapor diffusion among particles due to temperature gradients imposed on the snow cover the diffusion is calculated by using the potential field solutions for electrostatically charged particles the stereography of snow is represented by using a log normal distribution function for a geometrical enhancement factor defined here reasonable crystal growth rates and supersaturations are found the transition between the growth of highly faceted crystals and the growth of highly rounded crystals is determined by the critical supersaturation for the onset of dislocation aided growth thermal convection and the continuous movement of vapor around the particles due to the imposed temperature gradient are accommodated in the theory although both have little effect relative to the interparticle diffusion the growth of a layer of faceted crystals just below semi permeable crusts is explained by showing that crusts can cause the local supersaturation to exceed the critical value faceted crystals are shown to grow most rapidly in the lower warmer portions of the snow cover because of temperature effects on growth rate

instruments and methods recording microscale variations in snowpack layering using near infrared photography deposition of snow from precipitation and wind events creates layering within seasonal snowpacks the thickness and horizontal continuity of layers within seasonal snowpacks can be highly variable due to snow blowing around topography and vegetation and this has important implications for hydrology remote sensing and avalanche forecasting in this paper we present practical field and post processing protocols for recording lateral variations in snow stratigraphy using near infrared nir photography a fuji s9100 digital camera modified to be sensitive to nir wavelengths was mounted on a rail system that allowed for rapid imaging of a 10 m long snow trench excavated on the north side of toolik lake alaska 68838 0 n 149836 0 w post processing of the images included removal of lens distortion and vignetting a tape measure running along the base of the trench provided known locations control points that permitted scaling and georeferencing snow layer heights estimated from the nir images compared well with manual stratigraphic measurements made at 0 2 m intervals along the trench n 357 r 2 0 97 considerably greater stratigraphic detail was captured by the nir images than in the manually recorded profiles nir imaging of snow trenches using the described protocols is an efficient tool for quantifying continuous microscale variations in snow layers and associated properties

snow grain size measurements in antarctica grain size is an important but not well known characteristics of snow at the surface of antarctica in the past grain size has been reported using various methods the reliability reproducibility and inter comparability of which is not warranted in this paper we present and recommend depending on available logistical support 3 techniques of snow grain sampling and or imaging in the field as well as an ariginal digital image processing method which we have proved to provide reproducible and intercomparable measures of a snow grain size parameter the mean convex radius results from more than 500 samples and 3000 images of snow grains presented which yield a still spatially limited yet unprecendtedly wide picture of near surface snow grain size distribution from field work in antarctica in particular except at sites affected by a very particular meteorology surface grains in the interior of the ice sheet are uniformly small o 1 to 0 2 mm the climate related increase of grain size with depth through metamorphism is as expected not spatially uniform our antarctic snow grain size database will continue to grow as field investigators bring new samples images and measures of snow grain size

wetting front advance and freezing of meltwater within a snow cover 2 a simulation model a simulation model is developed which incorporates the effect of flow fingers at the leading edge of the wetting front ice layer growth within and at the base of the snow cover and meltwater infiltration into the underlying frozen soils the model results demonstrated that ice layers grow rapidly over a 24 to 36 hour period due to the conduction of heat into the snow and underlying soil sufficient water is frozen as ice layers to slow the finger wetting front and to account for the rapid snow and soil warming wetting front advance and ice layer growth were found to be sensitive to snow temperature thermal conductivity volume of flow in the fingers and irreducible water saturation simulation of a warm snow cover showed that with reduced ice layer growth the flow fingers were able to transmit rapidly water to the snow cover base before the entire snow cover was wet in the arctic situation the soil was sufficiently cold to ensure basal ice growth throughout the melt period basal ice growth was found to be sensitive to soil infiltration length of the growing period and the snowmelt rate as a result of the negative soil heat flux all meltwater is not available for runoff until both the liquid and thermal requirements of the snow and soil have been satisfied

measuring specific surface area of snow by near infrared photography the specific surface area ssa is considered an essential microstructural parameter for the characterization of snow photography in the near infrared nir spectrum is sensitive to the ssa we calculated the snow reflectance from calibrated nir images of snow pit walls and measured the ssa of samples obtained at the same locations this new method is used to map the snow stratigraphy the correlation between reflectance and ssa was found to be 90 calibrated nir photography allows quantitative determination of ssa and its spatial variation in a snow profile in two dimensions within an uncertainty of 15 in an image covering 0 5 1 0 m 2 even layers of 1 mm thickness can be documented and measured spatial maps of ssa are an important tool in initializing and validating physical and chemical models of the snowpack

the effect of snow on vehicle generated seismic signatures vehicle generated seismograms recorded under summer and winter conditions at fort devens ma are analyzed and compared the appearance and frequency content of the recorded ground motion change dramatically from summer to winter when a 0 7 m thick snow layer is present the snow layer causes a 10 db reduction in the amplitude of the acoustically coupled signal relative to the signal generated where the vehicle contacted the ground a magnitude squared coherence analysis and a simple wiener prediction model of the acoustic transmission near the observation point reinforce this conclusion

surface and melt pond evolution on landfast first year sea ice in the canadian arctic archipelago the evolution of landfast sea ice melt pond coverage surface topography and mass balance was studied in the canadian arctic during may june 2011 and 2012 using a terrestrial laser scanner snow and sea ice sampling and surface meteorological characterization initial melt pond formation was not limited to low lying areas rather ponds formed at almost all premelt elevations the subsequent evolution of melt pond coverage varied considerably between the 2 years owing to four principle temporally variable factors first the range in premelt topographic relief was 0 5 m greater in 2011 rougher surface than in 2012 smoother surface such that a seasonal maximum pond coverage of 60 and maximum hydraulic head of 204 mm were reached in 2011 versus 78 and 138 mm in 2012 a change in the meltwater balance production minus drainage caused the ponds to spread or recede over an area that was almost 90 larger in 2012 than in 2011 second modification of the premelt topography was observed during mid june due to preferential melting under certain drainage channels some of the lowest lying premelt areas were subsequently elevated above these deepening channels and unexpectedly became drained later in the season third ice interior temperatures remained 1 2 c colder later into june in 2012 than in 2011 even though the ice was 0 35 m thinner at melt onset thereby delaying permeability increases in the ice that would allow vertical meltwater drainage to the ocean finally surface melt was estimated to account for approximately 62 of the net radiative flux to the sea ice cover during the melt season

texture and strength changes of buried surface hoar layers with implications for dry snow slab avalanche release buried layers of surface hoar are the failure plane for many slab avalanches including fatal human triggered avalanches in various mountain regions these layers may persist as weak layers in the snow cover for weeks or months it is therefore essential for operational avalanche forecasters to monitor the evolution of persistent weak layers such as buried surface hoar traditional grain shape observations of isolated grains with a magnifier and crystal screen do not show bonding that is decisive for strength in this study we used in situ microphotography and observations of texture to complement strength measurements from shear frame tests buried layers of surface hoar consist of crystals most of which extend from the layer below to the layer above and may exhibit a columnar or truss like structure observations and measurements show that texture and crystal size change little over periods of up to several months during which the snowpack remains dry under these conditions layer thickness decreases while density and strength increase based on field measurements we argue that the increase in strength is primarily due to penetration of the surface hoar crystals into the adjacent layers especially at the bottom of the buried surface hoar layer where bonding is critical the weak bonding at the bottom implies that shear failure occurs at the lower interface rather than within the weak layer on slopes we find that surface hoar crystals that were initially surface normal are tilted downslope faster than predicted by published shear strain rates for settled snow indicating that shear strain is concentrated in these layers the characteristic texture of buried surface hoar columnar or truss like permits collapsing at the time of fracture the gravitational energy released by the displacement of the slab may contribute to the extensive fracture propagation associated with buried surface hoar layers

spatio temporal influence of tundra snow properties on ku band 17 2 ghz backscatter during the 2010 11 boreal winter a distributed set of backscatter measurements was collected using a ground based ku band 17 2ghz scatterometer system at 26 open tundra sites a standard snow sampling procedure was completed after each scan to evaluate local variability in snow layering depth density and water equivalent swe within the scatterometer field of view the shallow depths and large basal depth hoar encountered presented an opportunity to evaluate backscatter under a set of previously untested conditions strong ku band response was found with increasing snow depth and snow water equivalent swe in particular co polarized vertical backscatter increased by 0 82db for every 1cm increase in swe r 2 0 62 while the result indicated strong potential for ku band retrieval of shallow snow properties it did not characterize the influence of sub scan variability an enhanced snow sampling procedure was introduced to generate detailed characterizations of stratigraphy within the scatterometer field of view using near infrared photography along the length of a 5m trench changes in snow properties along the trench were used to discuss variations in the collocated backscatter response a pair of contrasting observation sites was used to highlight uncertainties in backscatter response related to short length scale spatial variability in the observed tundra environment

formation of refrozen snowpack layers and their role in slab avalanche release 1 in a variety of snow climates numerous slab avalanches release over crusts consisting of refrozen snow slab avalanches sometimes release in weak layers of faceted crystals that developed while underlying wet layers froze into crusts often within a day weak layers of faceted crystals can also develop when less permeable and more conductive crusts alter the temperature and vapor pressure gradients these processes create interfaces where differences in grain radii can contribute to weak bonding in western canada layers of faceted crystals facet layers on crusts are most common in early and late winter when thaws and rain are more frequent also thin facet layers occur in spring when surface melting by solar radiation is common shear strength tests on facet layers show an initial strength loss during faceting followed by a slow strength increase the spatial distribution of poorly bonded crusts can be interpreted from the interaction of terrain and meteorology that caused the antecedent wet layer

production potential and stability of hydrate bearing sediments at the site gmgs3 w19 in the south china sea a preliminary feasibility study abstract according to the preliminary geological data of gas hydrate bearing sediments ghbs at site gmgs3 w19 in the third chinese expedition to drill gas hydrates in 2015 a production model using three different recovery pressures was established to assess the production feasibility from both production potential and geomechanical response the simulation results show that for this special class 1 deposit it is a little hard for gas production rate to reach the commercial extraction rate because the degree of hydrate dissociation is limited due to the low reservoir permeability and the permeable burdens however the free gas accumulating in the lower part of the ghbs can significantly increase gas to water ratio it also generates many secondary hydrates in the ghbs at the same time decreasing the well pressure can be beneficial to gas recovery but the recovery increase is not obvious in term of geomechanical response of the reservoir during the gas recovery the permeable burdens are conducive to reduction of the sediment deformation though they don t facilitate the gas recovery rate in addition significant stress concentration is observed in the upper and lower edges of ghbs around the borehole during depressurization because of high pressure gradient and the greater the well pressure drop the more obvious the phenomenon yield failures and sand production easily take place in the edges therefore in order to achieve the purpose of safe efficient and long term gas production a balance between the production pressure and reservoir stability should be reached at the hydrate site the production pressure difference and sand production must be carefully controlled and the high stress concentration zones need strengthening or sand control treatment during gas production besides the sensitivity analyses show that the hydrate saturation heterogeneity can affect the production potential and geomechanical response to some extent especially the water extraction rate and the effective stress distribution and evolution increasing ghbs and its underlying free gas formation permeabilities can enhance the gas production potential but it probably introduces geomechanical risks to gas recovery operations

the permeability of a melting snow cover data from snow lysimeters in california and vermont are used to find the saturated permeability of a melting snow cover in the range of 10 40 10 10 m2 depending on snow density the unsaturated permeability increases as about the third power of liquid saturation the gravity flow theory is shown to be an accurate representation of meltwater drainage from snow covers in two diverse areas even though the snow covers are treated as homogeneous units the variation of saturated permeability with snow density occurs about as predicted by shimizu s formula for dry snow although ice layers decrease the permeability somewhat

field measurements of sintering after fracture of snowpack weak layers 1 this research documents two cases where field workers unintentionally fractured a snowpack weak layer but no avalanche released measurements from before and after the fractures provide unique data sets on the temporal change of snow stability shear strength decreased immediately after fracture on both slopes subsequent strengthening occurred in both cases though the rates differed presumably due to the characteristics of the weak layers our results have two important implications first they suggest the sub critical weak layer fractures assumed as a prerequisite in some snow slab avalanche release models are transient features and future modeling efforts must take this into account second they provide insights into interpreting snow stability tests and assessing the stability of slopes with fractured weak layers

experimental parametric study of suffusion and backward erosion within hydraulic earth structures dikes levees or dams internal seepage flows can generate the entrainment of the soil grains grain transportation affects both particle size distributions and porosity and changes the mechanical and hydraulic characteristics of the earth s structure the occurrence of failures in new earth structures due to internal erosion demonstrates the urgency of improving our knowledge of these phenomena of erosion with this intention a new experimental device has been developed that can apply hydraulic stresses to reconstituted consolidated cohesive soils without cracks in order to characterize the erosion evolution processes that might be present a parametric study was conducted to examine the influence of three critical parameters on clay and sand erosion mechanisms when the hydraulic gradient was low it was concluded that the erosion of the structure s clay fraction was due to suffusion when the hydraulic gradient increased it was concluded that the sand fraction erosion initiation was due to backward erosion the extent of the erosion was dependent on the clay content the study underlines the complexity of confinement stress effects on both erosion phenomena

geomechanical issues in the exploitation of natural gas hydrate abstract as a clean fossil fuel with great reserves natural gas hydrate ngh is widely regarded as an important future alternative energy source ngh is widely distributed in onshore tundra and shallow sedimentary layers in the deep sea these sedimentary layers typically exit shallow burial depth poor diagenesis and low strength characteristics moreover the decomposition of ngh can also greatly reduce reservoir strength therefore ngh development can easily causes many geomechanical problems including reservoir instability sand production and seabed landslides etc which may further trigger a series of environmental disasters such as tsunamis natural gas leakage and the acceleration of global warming this study mainly reviews the research progress regarding geomechanical issues in ngh development including mechanical properties of ngh bearing sediments borehole stability hydraulic fracturing sand production reservoir settlement and seabed landslides in addition to previous research achievements regarding geomechanical problems in ngh exploitation the limitations and challenges are also discussed and several questions and insightful suggestions are put forward for future research from our point of view

inversion of snow parameters from passive microwave remote sensing measurements by a neural network trained with a multiple scattering model the inversion of snow parameters from passive microwave remote sensing measurements is performed with a neural network trained with a dense media multiple scattering model the input output pairs generated by the scattering model are used to train the neural network simultaneous inversion of three parameters mean grain size of ice particles in snow snow density and snow temperature from five brightness temperatures is reported it is shown that the neural network gives good results for simulated data the absolute percentage errors for mean grain size of ice particles and snow density are less than 10 and the absolute error for snow temperature is less than 3 k the neural network with the trained weighting coefficients of the three parameter model is also used to invert ssmi data taken over the antarctic region