cross season relation of the south china sea precipitation variability between winter and summer the present study reveals cross season connections of rainfall variability in the south china sea scs region between winter and summer rainfall anomalies over northern south china sea in boreal summer tend to be preceded by the same sign rainfall anomalies over southern south china sea in boreal winter denoted as in phase relation and succeeded by opposite sign rainfall anomalies over southern south china sea in the following winter denoted as out of phase relation analysis shows that the in phase relation from winter to summer occurs more often in el nino la nina decaying years and the out of phase relation from summer to winter appears more frequently in el nino la nina developing years in the summer during the el nino la nina decaying years cold warm and warm cold sea surface temperature sst anomalies develop in tropical central north pacific and the north indian ocean respectively forming an east west contrast pattern the in phase relation is associated with the influence of anomalous heating cooling over the equatorial central pacific during the mature phase of el nino la nina events that suppresses enhances precipitation over southern south china sea and the impact of the above east west sst anomaly pattern that reduces increases precipitation over northern south china sea during the following summer the impact of the east west contrast sst anomaly pattern is confirmed by numerical experiments with specified sst anomalies in the el nino la nina developing years regional air sea interactions induce cold warm sst anomalies in the equatorial western north pacific the out of phase relation is associated with a rossby wave type response to anomalous heating cooling over the equatorial central pacific during summer and the combined effect of warm cold sst anomalies in the equatorial central pacific and cold warm sst anomalies in the western north pacific during the mature phase of el nino la nina events

densification of polar snow measurements modeling and implications for altimetry density profiles in the upper 10 14 m of snow have been measured along a 500 km traverse across the greenland ice sheet using a neutron scattering technique repeat measurements over periods ranging from a few days to 5 years allow strain rates to be determined as a function of depth very large strain rates are observed in the surface layer of snow over summer periods in the underlying multiyear snow strain rate decreases with decreasing porosity however once this effect has been removed the effect of increasing overburden pressure is counteracted by increasing strength of the material there are fluctuations in strain rate associated with the annual layering which indicate that winter and summer snow have different strengths based on these observations we derive a new densification equation which includes the effect of snow density and snow type and the effect of temperature described by an arrhenius expression with activation energy of the order of 110 kj mol 1 and an exponential prefactor determined simply by the temperature history of the snow for multiyear snow and meteorological conditions that do not vary from year to year our equation reduces to a form similar to the herron and langway equation for first stage densification using the new equation we calculate the sensitivity of compaction rate to short term fluctuations in temperature and accumulation as 0 11 0 20 m a 1 k 1 and 0 33 0 95 m a 1 meters water equivalent 1 respectively and discuss the consequent uncertainty in satellite measurements of the long term elevation trend in this area of the greenland ice sheet

ultrasonic velocity and attenuation during co2 injection into water saturated porous sandstone measurements using difference seismic tomography we undertook laboratory based seismic measurements with dense sensor array at ultrasonic frequencies during the injection of co2 into a water saturated sandstone specimen the resulting high quality seismic data enabled detailed determination of the relative velocity and attenuation coefficient of the compressional wave using difference seismic tomography which directly inverses time lapse changes in rock properties from time lapse changes in observed data co2 migration and water displacement were clearly mapped using tomographic images of relative velocity and the attenuation coefficient the final and largely stabilised volume fraction of co2 in the pore space of the sample is about 30 40 on average the p velocity fell by 7 5 12 and 14 5 and the attenuation coefficient q 1 increased by factors of 3 3 2 7 and 3 7 as a result of the replacement of water with co2 during the injection of gaseous liquid and supercritical co2 respectively as a function of gas saturation both the velocity and attenuation data are in good agreement with results obtained using the white and dutta ode model for partial saturation indicating that viscous losses due to fluid diffusion are of significant importance for compressional waves travelling at ultrasonic frequencies in porous rocks

wave attenuation in partially saturated porous rocks new observations and interpretations across scales abstract seismic waves propagating in porous rocks saturated with two immiscible fluids can be strongly attenuated predicting saturation effects on seismic responses requires a sound understanding of attenuation and velocity dependencies on the fluid distribution decoding these effects involves interpreting laboratory experiments analyzing well log data and performing numerical simulations despite striking differences among scales flow regimes and frequency bands some aspects of wave attenuation can be explained with a single mechanism wave induced pressure diffusion different facets of wave induced pressure diffusion can be revealed across scales

continuous porosity characterization metric scale intervals in heterogeneous sedimentary rocks using medical ct scanner abstract although computed tomography ct scanning has been regularly applied to core analyses in petroleum geology there is still a need to improve our ways to document porosity and porosity distribution in the entire pore scale spectrum from the tens of nanometer to the meter scale porosity imaging is particularly crucial for complex and heterogeneous rocks such as hydrothermally altered and fractured carbonates the present work proposes an improved method using medical ct to reliably estimate reservoir porosity an in house core flooding setup allowed to analyse several individual core samples scanned simultaneously dry and saturated as well as continuous core sections up to 1 5 m long without any prior knowledge of samples three dimensional alignment and subtraction of the two data sets dry and saturated states results in the generation of 3d porosity matrices the methodology tested on a large set of reference core material shows a strong correlation between conventional gas porosimetry techniques and porosity from ct scan the added value of the porosity measurements by ct scan is first of all the generation of 3d images of pore network allowing to assess spatial attributes of macropores their distribution and connectivity secondly the ct scan method also provides continuous porosity profile at the millimetric scale both developments are crucial for the understanding of reservoir rock properties

mechano chemical interactions in sedimentary rocks in the context of co2 storage weak acid weak effects abstract due to the corrosive nature of dissolved co2 the potential short or long term alteration of rock properties represents a major issue in several sites where natural co2 circulation is observed as well as in reservoirs targeted for storage of anthropogenic co2 to date this has been primarily studied from a transport chemical perspective with laboratory evidence of microstructural modifications together with the consequences for flow properties compared to the transport chemical aspects the mechanical chemical aspects have been less investigated though it is to be expected that mechanical properties e g elastic properties failure parameters and time dependent mechanical behaviour could potentially be affected in a similar manner to hydraulic parameters yet since co2 is a weak acid the ph drop is expected to be moderate with a likely lower limit close to 4 0 the buffering of ph by calcite minerals present in most reservoirs targeted for storage may further limit the ph drop as well as confining it to a localized rock volume around the injection well this leads to the question of the magnitude and time spatial scales of chemically mediated mechanical processes during co2 sequestration the authors propose to address this issue by reviewing recent laboratory based studies restricted to sedimentary rocks namely reservoir rocks carbonate or sandstone intact or fractured caprocks and fault rocks key findings include the following 1 the short term impact on the elastic and inelastic behaviour of intact caprocks remains limited 2 shear strength weakening is likely to be respectively low and low to moderate for shale clay rich and anhydrite rich faults but without modifying slip stability in either case 3 the largest impact is located within carbonate reservoirs but with a broad range of reported responses depending on hydrodynamic conditions closed or open and on dissolution regime uniform or channelling and 4 creep experiments confirm that co2 induced dissolution may enhance long term compaction of carbonate reservoirs but the magnitude of acceleration varying from non significant to 50 times depends to a large extent on site specific conditions grain size ph temperature effective stress state etc which renders any direct extrapolation from laboratory to reservoir scale difficult finally some directions for future research studies are discussed

southeast greenland high accumulation rates derived from firn cores and ground penetrating radar despite containing only 14 of the greenland ice sheet by area the southeastern sector has the highest accumulation rates and hence receives 30 of the total snow accumulation we present accumulation rates obtained during our 2010 arctic circle traverse derived from three 50 m firn cores dated using geochemical analysis we tracked continuous internal reflection horizons between the firn cores using a 400 mhz ground penetrating radar gpr gpr data combined with depth age scales from the firn cores provide accumulation rates along a 70 km transect we followed an elevation gradient from 2350 to 1830m to understand how progressive surface melt may affect the ability to chemically date the firn cores and trace the internal layers with gpr from the firn cores we find a 52 0 43 m w e a 1 increase in average snow accumulation and greater interannual variability at the lower site than the upper site the gpr profiling reveals that accumulation rates are influenced by topographic undulations on the surface with up to 23 variability over 7 km these measurements confirm the presence of high accumulation rates in the southeast as predicted by the calibrated regional climate model polar mm5

possible impacts of spring sea surface temperature anomalies over south indian ocean on summer rainfall in guangdong guangxi region of china based on observational and reanalysis data for 1979 2015 the possible impacts of spring sea surface temperature anomalies ssta over the south indian ocean on the inter annual variations of summer rainfall in guangdong and guangxi provinces i e the guangdong guangxi area gg were analysed in this study the physical mechanism behind these impacts was explored two geographic regions over 65 e 95 e 35 s 25 s and 90 e 110 e 20 s 5 s were defined as the western pole region and the eastern pole region respectively for the gg summer precipitation pgg related south indian ocean dipole ssta pattern r siodp the difference between springtime sst anomalies averaged over the western pole region and that averaged over the eastern pole region was defined as the r siodp index the correlation between the spring r siodp index and gg summer precipitation can reach up to 0 52 in the spring of positive r siodp anomaly southerly winds over the western pole of the r siodp weaken whereas the southeast trade winds over the eastern pole strengthen by means of the wind evaporation sst feedback mechanism the enhanced southeast trade winds can weaken the evaporation over the western pole of the r siodp and enhance the evaporation over the eastern pole this results in a sustained positive ssta in the western pole of the r siodp and a sustained negative ssta in the eastern pole whereby the distribution of the sstas maintains until summer the sst dipole abnormally enhances the cross equatorial airflow near 105 e which intensifies the anomalous anti cyclonic circulation over south china sea at 850 hpa and simultaneously results in abnormal enhancement of water vapour transport to gg additionally the sst dipole promotes abnormal divergence in the lower troposphere and abnormal convergence in the upper troposphere over the maritime continent mc region moreover the low level convergence in gg is enhanced which results in abnormal enhancement of ascending motion in the gg that is conducive to positive summer rainfall anomaly in this region in this study the spring r siodp index the sst to the east of australia and to the east of southern africa and the north atlantic oscillation nao index were used to construct a statistical prediction model for the inter annual variability of the gg summer rainfall anomaly this model can well predict the accuracy of the inter annual variation of gg summer rainfall

saturation path dependency of p wave velocity and attenuation in sandstone saturated with co2 and brine revealed by simultaneous measurements of waveforms and x ray computed tomography images abstractwe performed a laboratory experiment to measure compressional p wave velocity and its attenuation in low permeability striped sandstone at ultrasonic frequency during drainage co2 injection and imbibition brine injection x ray computed tomography ct images of the rock sample were simultaneously recorded during the injections on the basis of the ct images we calculated the co2 saturation inside a prolate spheroid shaped volume the first fresnel zone that enclosed the wave propagation path the relationship of p wave velocity and that of attenuation to the co2 fraction were then obtained both relationships showed nonuniqueness with remarkable hysteresis in a drainage imbibition cycle to explain the nonunique relationship of p wave velocity to the co2 fraction we investigated the continuous random patchy saturation model by using exponent type autocorrelation the nonunique relationship of velocity to the co2 fraction was explained by the model with a correlation length from an order

automated mapping of local layer slope and tracing of internal layers in radio echograms an automated method is presented for tracing layers in radio echograms the method is designed to work with most radio echo sounding echograms and has been successfully tested with a 180 210 mhz multichannel coherent depth sounder to accurately trace layers first approximate layer positions are calculated by integrating the local layer slope which is inferred by the intensity response to a slanted filter then the positions are refined using an iterative optimization the layers are traced using an active contour model or snake which can be constrained to conserve both echogram features and smooth layers with this technique it is possible to trace internal layers over distances of several hundred kilometers the method was tested between two greenland deep ice cores where the age depth relation is known

indian ocean capacitor effect on indo western pacific climate during the summer following el nino significant climate anomalies persist through the summer june august after el nino dissipates in spring over the equatorial pacific they include the tropical indian ocean tio sea surface temperature sst warming increased tropical tropospheric temperature an anomalous anticyclone over the subtropical northwest pacific and increased mei yu baiu rainfall over east asia the cause of these lingering el nino effects during summer is investigated using observations and an atmospheric general circulation model gcm the results herein indicate that the tio warming acts like a capacitor anchoring atmospheric anomalies over the indo western pacific oceans it causes tropospheric temperature to increase by a moist adiabatic adjustment in deep convection emanating a baroclinic kelvin wave into the pacific in the northwest pacific this equatorial kelvin wave induces northeasterly surface wind anomalies and the resultant divergence in the subtropics triggers suppressed convection and the anomalous anticyclone the gcm results support this kelvin wave induced ekman divergence mechanism in response to a prescribed sst increase over the tio the model simulates the kelvin wave with low pressure on the equator as well as suppressed convection and the anomalous anticyclone over the subtropical northwest pacific an additional experiment further indicates that the north indian ocean warming is most important for the kelvin wave and northwest pacific anticyclone a result corroborated by observations these results have important implications for the predictability of indo western pacific summer climate the spatial distribution and magnitude of the tio warming rather than simply whether there is an el nino in the preceding winter affect summer climate anomalies over the indo western pacific and east asia

airborne radar and ice core observations of annual snow accumulation over thwaites glacier west antarctica confirm the spatiotemporal variability of global and regional atmospheric models 1 we use an airborne radar method verified with ice core accumulation records to determine the spatiotemporal variations of snow accumulation over thwaites glacier west antarctica between 1980 and 2009 we also present a regional evaluation of modeled accumulation in antarctica comparisons between radar derived measurements and model outputs show that three global models capture the interannual variability well r 0 9 but a high resolution regional model racmo2 has better absolute accuracy and captures the observed spatial variability r 0 86 neither the measured nor modeled accumulation records over thwaites glacier show any trend since 1980 although an increase in accumulation may potentially accompany the observed warming in the region the projected trend is too small to detect over the 30 year record

the extreme drought event during winter spring of 2011 in east china combined influences of teleconnection in midhigh latitudes and thermal forcing in maritime continent region abstractthe middle and lower reaches of the yangtze river mlryr in china experienced an extremely severe and persistent drought event from january to may of 2011 using both the observational data and ncep ncar reanalysis features of the drought event and the related circulation anomalies were investigated it is found that the precipitation during the investigated period of 2011 was deficient mostly along the yangtze river the water vapor diverged from mlryr southward into the bay of bengal south china sea and the philippines there were two factors facilitating the drought event one was the quasi stationary rossby wave related teleconnection which propagated eastward at midhigh latitudes from the north atlantic to east asia reinforcing the siberian high and the east asian trough henceforth resulting in the divergence anomalies in mlryr in the lower troposphere this quasi stationary wave train though originating from the north atlantic region was not essentially related to the north atlantic

interdecadal changes in the relationship between southern china winter spring precipitation and enso winter spring precipitation in southern china tends to be higher lower than normal in el nino la nina years during 1953 1973 the relationship between the southern china winter spring precipitation and el nino southern oscillation enso is weakened during 1974 1994 during 1953 1973 above normal southern china rainfall corresponds to warmer sea surface temperature sst in the equatorial central pacific there are two anomalous vertical circulations with ascent over the equatorial central pacific and ascent over southern china and a common branch of descent over the western north pacific that is accompanied by an anomalous lower level anticyclone during 1974 1994 above normal southern china rainfall corresponds to warmer sst in eastern south indian ocean and cooler sst in western south indian ocean two anomalous vertical circulations act to link southern china rainfall and eastern south indian ocean sst anomalies with ascent over eastern south indian ocean and southern china and a common branch of descent over the western north pacific present analysis shows that south indian ocean sst anomalies can contribute to southern china winter spring precipitation variability independently the observed change in the relationship between southern china winter spring rainfall and enso is likely related to the increased sst variability in eastern south indian ocean and the modulation of the pacific decadal oscillation

relationships between convective activity in the maritime continent and precipitation anomalies in southwest china during boreal summer using the outgoing long wave radiation olr data from noaa the era interim reanalysis products from ecmwf and daily observations at 756 stations provided by national meteorological information center of china meteorology administration we have examined the relationships of interannual variations of summer precipitation in southwest china with the convective activity in the maritime continent mc region by employing the singular value decomposition svd method and the regional climate model regcm4 4 the first svd mode here after svd1 indicates that high precipitation anomalies in southwest china correspond to abnormally weak convective activity in northeastern mc and strong convective activity in southwestern mc if the time series of coefficients of svd1 is in positive phase when convective activity are anomalously strong in southern mc the anomalous divergence at 700 hpa and convergence at 200 hpa are observed over the tropical western pacific and south china sea the propagation of wave energy at 700 hpa from western europe and the tropics provide a favorable condition for inducing more precipitation in most of southwest china the atmospheric water vapor transport from northern indochina and the south china sea to southwest china intensifies while the western pacific subtropical high is stronger than normal and extends more westward all these results along with the simulations demonstrate that the summer precipitation in southwest china is significantly affected by the convective activity over the mc region these results above are helpful for our better understanding the role of the mc in regulating the summer climate in southwest china

direct laboratory observation of patchy saturation and its effects on ultrasonic velocities maximizing the recovery of known hydrocarbon reserves is one of the biggest challenges facing the petroleum industry today optimal production strategies require accurate monitoring of production induced changes of reservoir saturation and pressure over the life of the field time lapse seismic technology is increasingly used to map these changes in space and time however until now interpretation of time lapse seismic data has been mostly qualitative in order to allow accurate estimation of the saturation it is necessary to know the quantitative relationship between fluid saturation and seismic characteristics elastic moduli velocity dispersion and attenuation the problem of calculating acoustic properties of rocks saturated with a mixture of two fluids has attracted considerable interest gist 1994 mavko and nolen hoeksema 1994 knight et al 1998 for a comprehensive review of theoretical and experimental studies of the patchy saturation problem see toms et al 2006

effect of sub core scale heterogeneities on acoustic and electrical properties of a reservoir rock a co2 flooding experiment of brine saturated sandstone in a computed tomography scanner the effect of sub core scale heterogeneity on fluid distribution pattern and the electrical and acoustic properties of a typical reservoir rock was studied by performing drainage and imbibition flooding tests with co2 and brine in a laboratory moderately layered rothbach sandstone was used as a test specimen two core samples were drilled one perpendicular and the other parallel to the layering to allow injection of fluids along and normal to the bedding plane during the test 3d images of fluid distribution and saturation levels were mapped by an industrial x ray ct scanner together with simultaneous measurement of electrical resistivity ultrasonic velocities as well as amplitudes the results showed how the layering and the flooding direction influenced the fluid distribution pattern and the saturation level of the fluids for a given fluid saturation level the measured changes in the acoustic and electrical parameters were affected by both the fluid distribution pattern and the layering orientation relative to the measurement direction the p wave amplitude and the electrical resistivity were more sensitive to small changes in the fluid distribution patterns than the pwave velocity the change in amplitude was the most affected by the orientation of the layering and the resulting fluid distribution patterns in some instances the change due to the fluid distribution pattern was higher than the variation caused by the change in co2 saturation as a result the gassmann relation based on uniform or patchy saturation pattern was not suitable to predict the p wave velocity variation overall the results demonstrate the importance of core imaging to improve our understanding of fluid distribution patterns and the associated effects on measured rock physics properties

revised estimates of recent mass loss rates for penny ice cap baffin island based on 2005 2014 elevation changes modified for firn densification repeat airborne or satellite measurements of surface elevation over ice caps are often used tocalculate glacier wide surface mass changes over time however these measurements typically do notacco

consistent patterns of antarctic ice sheet interannual variations from envisat radar altimetry and grace satellite gravimetry summary interannual variations of the antarctic ice sheet due to surface mass balance smb fluctuations are important for mass balance estimates and interpretations to date these variations are primarily assessed by global or regional atmospheric modelling satellite altimetry and satellite gravimetry over the ice sheet provide complementary observations of the related volume and mass effects respectively yet so far the interannual signal contents of these observations have not been extensively studied we compare and jointly interpret envisat radar altimetry ra and grace satellite gravimetry results relying on ra products from the along track repeat satellite ra approach and on the grace 10 d solutions by cnes grgs ra results and grace results are expressed in terms of variations of ice sheet thickness z t and ice equivalent thickness zice t respectively in view of the different errors and limitations of both techniques and of differences between z t and zice t expected due to firn related processes our principal approach is a comparison of qualitative patterns in space and time to adjust the spatial resolution of both data sets we describe the spatial filtering inherent to the regularization of the cnes grgs grace solutions and apply this filtering to the envisat ra height changes in a consistent fashion after correction for glacial isostatic adjustment the spatial patterns of linear trends seen by envisat ra and grace over the period 2002 october to 2009 august agree well not only for the extreme ice losses in the west antarctic amundsen sea sector but also for an alternating sequence of gains and losses along the east antarctic coast our main focus is on interannual signals which we represent by the low pass filtered non linear non seasonal components of the z t and zice t time series these components should reflect interannual smb variations apart from effects of changes in ice flow we find an agreement between the interannual variation patterns from envisat ra and grace with temporal correlation coefficients typically in the order of 0 8 this qualitative agreement prevailing even in the theoretical absence of a simple proportionality between z t and zice t indicates that both observational signals primarily originate in common geophysical variations combining both geodetic data sets aids their interpretation and promises to be valuable for the reduction of smb uncertainties in a case study based on the analysis of both grace and envisat ra we identify a prominent interannual feature in west antarctica in 2005 september october as an event of excess snow accumulation with a mass effect of 82 31 gt

a laboratory study of the dependence of elastic wave velocities on pore scale fluid distribution laboratory data have been collected during a continuous imbibition drainage experiment that show a clear dependence of elastic wave velocities on the details of the pore scale distribution of water and air in a sandstone compressional wave velocity vp was measured at a frequency of 1 mhz shear wave velocity vs was measured at a frequency of 600 khz during the experiment vp showed little variation with the level of water saturation sw during increasing sw through imbibition until sw 0 80 at which point vp increased dramatically when sw was decreased pronounced saturation induced hysteresis was observed in the region 03 0 4 as a simple model we consider the imbibition process as producing a partially saturated state in all pores i e all pores contain both air and water the drainage process in contrast favors the existence of either air filled or water filled pores as elastic wave velocities are very sensitive to the saturation state in the smaller crack like pores these variations in fluid distribution cause related variations in velocities

fluid distribution effect on sonic attenuation in partially saturated limestones extensional and torsional wave attenuation measurements are obtained at a sonic frequency around 1 khz on partially saturated limestones using large resonant bars 1 m long to study the influence of the fluid distribution we use two different saturation methods drying and depressurization when water saturation sw is higher than 70 the extensional wave attenuation is found to depend on whether the resonant bar is jacketed this can be interpreted as the biot gardner white effect the experimental results obtained on jacketed samples show that during a drying experiment extensional wave attenuation is influenced strongly by the fluid content when sw is between approximately 60 and 100 this sensitivity to fluid saturation vanishes when saturation is obtained through depressurization using a computer assisted tomographic ct scan we found that during depressurization the fluid distribution is homogeneous at the millimetric scale at all saturations in contrast during drying heterogeneous s

a reconciled estimate of ice sheet mass balance we combined an ensemble of satellite altimetry interferometry and gravimetry data sets using common geographical regions time intervals and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of earth s polar ice sheets we find that there is good agreement between different satellite methods especially in greenland and west antarctica and that combining satellite data sets leads to greater certainty between 1992 and 2011 the ice sheets of greenland east antarctica west antarctica and the antarctic peninsula changed in mass by 142 49 14 43 65 26 and 20 14 gigatonnes year 1 respectively since 1992 the polar ice sheets have contributed on average 0 59 0 20 millimeter year 1 to the rate of global sea level rise

glacial isostatic adjustment over antarctica from combined icesat and grace satellite data abstract the glacial history of antarctica during the most recent milankovitch cycles is poorly constrained relative to the northern hemisphere as a consequence the contribution of mass changes in the antarctic ice sheet to global sea level change and the prediction of its future evolution remain uncertain the process of glacial isostatic adjustment gia represents the ongoing response of the solid earth to the late pleistocene deglaciation and therefore provides information about antarctic glacial history moreover insufficient knowledge of gia hampers the determination of present day changes in the antarctic mass balance through satellite gravity measurements previous studies have laid the theoretical foundation for distinguishing between signals of ongoing gia and contemporary ice mass change through the combination of satellite gravimetry and satellite altimetry this distinction is made possible by the fact the gia induced changes involving relatively dense rock will produce a different combination of topography and gravity change than those produced by variations in ice or firn thickness due to the lower density of these materials however no conclusive results have been produced to date here we show that by combining laser altimetry and gravity data from the icesat and grace satellite missions over the period march 2003 march 2008 the gia contribution can indeed be isolated the inferred gia signal over the antarctic continent which represents the first result derived from direct observations by satellite techniques strongly supports late pleistocene ice models derived from glacio geologic studies the gia impact on grace derived estimates of mass balance is found to be 100 67 gt yr

seismic attenuation due to wave induced flow analytical expressions for three p wave attenuation mechanisms in sedimentary rocks are given a unified theoretical framework two of the models concern wave induced flow due to heterogeneity in the elastic moduli at mesoscopic scales scales greater than grain sizes but smaller than wavelengths in the first model the heterogeneity is due to lithological variations e g mixtures of sands and clays with a single fluid saturating all the pores in the second model a single uniform lithology is saturated in mesoscopic patches by two immiscible fluids e g air and water in the third model the heterogeneity is at microscopic grain scales broken grain contacts and or micro cracks in the grains and the associated fluid response corresponds to squirt flow the model of squirt flow derived here reduces to proper limits as any of the fluid bulk modulus crack porosity and or frequency is reduced to zero it is shown that squirt flow is incapable of explaining the measured level of loss 10 sup 2 q sup 1 10 sup 1 within the seismic band of frequencies 1 to 10 sup 4 hz however either of the two mesoscopic scale models easily produce enough attenuation to explain the field data

elevation changes in antarctica mainly determined by accumulation variability antarctic ice sheet elevation changes which are used to estimate changes in the mass of the interior regions are caused by variations in the depth of the firn layer we quantified the effects of temperature and accumulation variability on firn layer thickness by simulating the 1980 2004 antarctic firn depth variability for most of antarctica the magnitudes of firn depth changes were comparable to those of observed ice sheet elevation changes the current satellite observational period 15 years is too short to neglect these fluctuations in firn depth when computing recent ice sheet mass changes the amount of surface lowering in the amundsen sea embayment revealed by satellite radar altimetry 1995 2003 was increased by including firn depth fluctuations while a large area of the east antarctic ice sheet slowly grew as a result of increased accumulation

an overview of laboratory apparatuses to measure seismic attenuation in reservoir rocks intrinsic wave attenuation at seismic frequencies is strongly dependent on rock permeability fluid properties and saturation however in order to use attenuation as an attribute to extract information on rock fluid properties from seismic data experimental studies on attenuation are necessary for a better understanding of physical mechanisms that are dominant at those frequencies an appropriate laboratory methodology to measure attenuation at seismic frequencies is the forced oscillation method but technical challenges kept this technique from being widely used there is a need for the standardization of devices employing this method and a comparison of existing setups is a step towards it here we summarize the apparatuses based on the forced oscillation method that were built in the last 30 years and were used to measure frequency dependent attenuation in fluid saturated and or dry reservoir rocks under small strains 10 8 10 5 we list and discuss important technical aspects to be taken into account when working with these devices or in the course of designing a new one we also present a summary of the attenuation measurements in reservoir rock samples performed with these apparatuses so far

impact of model resolution on simulated wind drifting snow and surface mass balance in terre adelie east antarctica this paper presents the impact of model resolution on the simulated wind speed drifting snow climate and surface mass balance smb of terre ad elie and its surroundings east antarctica we compare regional climate model simulations at 27 and 5 5 km resolution for the year 2009 the wind speed maxima in terre ad elie and the narrow glacial valleys of victoria land are better represented at 5 5 km resolution because the topography is better resolved drifting snow sublimation is 100mma 1 in regions with high wind speeds our results indicate a strong feedback between topography wind gradients and drifting snow erosion as a result smb shows much more local spatial variability at 5 5 km resolution that is controlled by drifting snow erosion whereas the large scale smb gradient is largely determined by precipitation drifting snow processes lead to ablation in the narrow glacial valleys of victoria land the integrated smb equals 86gt although wind climate drifting snow processes and smb variability are better represented at 5 5 km the area integrated smb is not significantly different between the simulations at 27 and 5 5km a horizontal resolution of 27 km is sufficient to realistically simulate ice sheet wide smb

influence of persistent wind scour on the surface mass balance of antarctica in the antarctic interior assessments of surface mass balance may overestimate accumulation because high winds remove some of the annual snowfall geophysical observations reveal localized zones of persistent wind scour where little or no snow accumulates that are predicted to occur across approximately 5 of the antarctic surface accurate quantification of surface snow accumulation over antarctica is a key constraint for estimates of the antarctic mass balance as well as climatic interpretations of ice core records1 2 over antarctica near surface winds accelerate down relatively steep surface slopes eroding and sublimating the snow this wind scour results in numerous localized regions 200 km2 with reduced surface accumulation3 4 5 6 7 estimates of antarctic surface mass balance rely on sparse point measurements or coarse atmospheric models that do not capture these local processes and overestimate the net mass input in wind scour zones3 here we combine airborne radar observations of unconformable stratigraphic layers with lidar derived surface roughness measurements to identify extensive wind scour zones over dome a in the interior of east antarctica the scour zones are persistent because they are controlled by bedrock topography on the basis of our dome a observations we develop an empirical model to predict wind scour zones across the antarctic continent and find that these zones are predominantly located in east antarctica we estimate that 2 7 6 6 of the surface area of antarctica has persistent negative net accumulation due to wind scour which suggests that across the continent the snow mass input is overestimated by 11 36 5 gt yr 1 in present surface mass balance calculations

effects of partial liquid gas saturation on extensional wave attenuation in berea sandstone extensional wave attenuation measurements on berea sandstone were made during increasing imbibition and decreasing drainage brine saturations measurements on samples with both open pore and closed pore surfaces were made using the resonant bar technique the frequency dependence was examined using the forced deformation method the attenuation was found to be dependent on saturation history as well as degree of saturation and boundary flow conditions the sample with open pore surface had a larger attenuation which peaked at greater brine saturations than the sample with closed pore surface during drainage the attenuation reached a maximum at about 90 brine saturation as opposed to about 97 brine saturation during imbibition the variation of the size and number of air pockets within the rock can account for this discrepancy the magnitude of the attenuation peak value decreases substantially with decreasing frequency to the extent that no attenuation peak with saturation was apparent at seismic frequencies say below 100 hz

experimental evidence for the biot gardner theory seismic wave attenuation has been a subject of interest during the last 40 years because it may be of use in interpreting seismic data from this attenuation parameter more detailed information about the lithology of the subsurface may be deduced if we understand the absorption mechanisms by which dissipation of seismic energy is governed we are therefore studying in the laboratory the effects of different parameters such as porosity permeability pore fluid and saturation state on the absorption of seismic waves in porous rocks over a wide spectrum ranging from seismic to ultrasonic frequencies burkhardt et al 1986

numerical modeling and laboratory measurements of seismic attenuation in partially saturated rock to better understand the effects of fluid saturation on seismic attenuation we combined numerical modeling in poroelastic media and laboratory measurements of seismic attenuation in partially saturated berea sandstone samples although in laboratory experiments many physical mechanisms for seismic attenuation take place simultaneously with numerical modeling we separately studied the effect of a single physical mechanism wave induced fluid flow on the mesoscopic scale using the finite element method we solved biot s equations of consolidation by performing a quasistatic creep test on a 3d poroelastic model this model represents a partially saturated rock sample we obtained the stress strain relation from which we calculated frequency dependent attenuation in the laboratory we measured attenuation in extensional mode for dry and partially water saturated berea sandstone samples in the frequency range from 0 1 to 100 hz all the measurements were performed at room pressure and temperature conditions from numerical simulations we found that attenuation varies significantly with fluid distribution within the model in addition to binary distributions we used spatially continuous distributions of fluid saturation for the numerical models such continuous saturation distribution was implemented using properties of an effective single phase fluid by taking into account the matrix anelasticity we found that wave induced fluid flow on the mesoscopic scale due to a continuous distribution of fluid saturation can reproduce seismic attenuation data measured in a partially saturated sample the matrix anelasticity was the attenuation measured in the room condition dry sample

characterisation and multifaceted anisotropy assessment of corvio sandstone for geological co2 storage studies we present a comprehensive characterisation of the physical mineralogical geomechanical geophysical and hydrodynamic properties of corvio sandstone this information together with a detailed assessment of anisotropy is needed to establish corvio sandstone as a useful laboratory rock testing standard for well constrained studies of thermo hydro mechanical chemical coupled phenomena associated with co2 storage practices and for geological reservoir studies in general more than 200 core plugs of corvio sandstone 38 1 and 50 mm diameters 2 1 length to diameter ratio were used in this characterisation study with a rock porosity of 21 7 1 2 dry density 2036 32 kg m 3 and unconfined compressive and tensile strengths of 41 3 28 and 2 3 0 14 mpa respectively geomechanical tests show that the rock behaves elastically between 10 and 18 mpa under unconfined conditions with associated young s modulus and poisson s ratio of 11 8 2 8 gpa and 0 34 0 01 gpa respectively permeability abruptly decreases with confining pressure up to 10 mpa and then stabilises at 1 md ultrasonic p and s wave velocities vary from about 2 8 3 8 km s 1 and 1 5 2 4 km s 1 respectively over confining and differential pressures between 0 1 and 35 mpa allowing derivation of associated dynamic elastic moduli anisotropy was investigated using oriented core plugs for electrical resistivity elastic wave velocity and attenuation permeability and tracer injection tests corvio sandstone shows weak transverse isotropy symmetry axis normal to bedding of 10 for velocity and 20 for attenuation

velocity saturation relation in partially saturated rocks modelling the effect of injection rate changes the relationship between p wave velocity and fluid saturation in a porous medium is of importance for reservoir rock characterization forced imbibition experiments in the laboratory reveal rather complicated velocity saturation relations including rollover like patterns induced by injection rate changes poroelasticity theory based patchy saturation models using a constant fluid patch size are not able to describe these velocity saturation relations therefore we incorporate a saturation dependent patch size function into two models for patchy saturation this recipe allows us to model observed velocity saturation relations obtained for different and variable injection rates the results reveal an increase in patch size with fluid saturation and show a reduction in the patch size for decreasing injection rate this indicates that there can exist a distinct relation between patch size and injection rate we assess the relative importance of capillarity on velocity saturation relations and find that capillarity stiffening impairs the impact of patch size changes capillarity stiffening appears to be a plausible explanation when a decrease in the injection rate is expected to boost the importance of capillarity

an experimental study of acoustic responses on the injection of supercritical co2 into sandstones from the otway basin quantitative knowledge of the acoustic response of rock from an injection site on supercritical co2 scco2 saturation is crucial for understanding the feasibility of time lapse seismic monitoring of co2 plume migration a suite of shaley sandstones from the injection interval of the crc 2 well otway basin australia is tested to reveal the effects of supercritical co2 injection on acoustic responses co2 is first injected into dry samples flushed out with brine and then injected again into brine saturated samples such a suite of experiments allows us to obtain acoustic velocities of the samples for a wide range of co2 brine saturations from 0 to 100 on injection of scco2 into brine saturated samples the rocks exhibit a decrease of compressional velocities by about 7 with the increase of co2 saturation from 0 to a maximum of about 50 anisotropy of the shaley sandstones from the otway basin must be taken into account as the difference in the velocities normal and parallel to bedding is comparable with the perturbation due to co2 injection and the samples of different orientations exhibit transition from gassmann hill to gassmann wood bound at different scco2 saturations changes of the dry samples before and after the co2 injection if any are not traceable by acoustic methods

snow megadunes in antarctica sedimentary structure and genesis 1 megadune fields occupy large areas in the interior of the east antarctic ice sheet and are the result of unusual snow accumulation and redistribution processes they therefore are important to surface mass balance and ice core interpretation field observations gps gpr and surface measurements have provided a detailed description of megadune sedimentation and morphology over a 70 km2 area located 200 km east of dome c a combination of remote sensing analysis using landsat and satellite radar altimetry and field measurements indicate that slope in the prevailing wind direction spwd and climatic conditions play a crucial role in megadune genesis the megadune areas tend to be characterized by slightly steeper regional slope and the presence of highly persistent katabatic winds the megadunes represent 2 to 4 m amplitude waves of 2 to 5 km wavelength formed by variable net accumulation ranging between 25 leeward faces to 120 windward faces of the accumulation in adjacent nonmegadune areas leeward faces are characterized by glazed sastrugi free surfaces and extensive depth hoar formation windward faces are covered by large rough sastrugi up to 1 5 m in height

subtropical sst dipole events in the southern indian ocean sea surface temperature sst anomalies in the subtropical southern indian ocean show interannual dipole events that are seasonally phase locked to the austral summer a positive phase of the event is characterized by cold sst anomalies in the eastern part i e off australia and warm sst anomalies in the southwestern part south of madagascar such an event is found to produce above normal rainfall over many regions in south central africa the cooling of sst in the eastern part is mainly caused by the enhanced evaporation this is associated with stronger winds along the eastern edge of the subtropical high which is strengthened and shifted slightly to the south during the event on the other hand relative decrease in the seasonal latent heat loss due to reduced evaporation dominates the warming in the southwestern part evolution of such subtropical dipole events shows quite a contrast to that of the tropical dipole events discovered recently in the indian ocean

microstructure dependent densification of polar firn derived from x ray microtomography the densification of dry polar snow and firn results in a continuous increase of density with depth accompanied by significant density fluctuations within seasonal layers density measurements of high spatial resolution reveal a persistent minimum of density fluctuations in the vicinity of the snow firn transition 0 55 0 65 g cm 3 in firn core records in this study we give an explanation for the fluctuation minimum by applying a new method of x ray microtomography to obtain three dimensional 3 d structural data of a greenland firn core at 13 different depths between 10 and 78 m a set of 16 samples of 40 cm total length for each depth interval was measured a reconstructed firn segment of 40 cm covers 1 2 years of snow accumulation using digital image analysis techniques different structural parameters are estimated including 3 d pore and particle sizes and specific surface areas it is shown that the densification rates of snow and firn layers consisting of coarse particles are much higher than those consisting of fine particles within the same depth interval this causes a density crossing of fine and coarse grained layers with a minimum of density variations at the crossover point this crossing over implies that formerly dense layers in the seasonal density signal are not of the same origin as dense layers in the deeper part of the firn column and that the seasonal density signal will totally change shape with depth it is speculated that in coarse and fine grained firn the dominant mechanism of densification acts over different regimes of density

instruments and methods in situ measurements of firn compaction profiles using borehole optical stratigraphy we have developed a technique in which we use a borehole video camera and post processing software to make a record of the optical brightness as a function of depth in polar firn we call this method borehole optical stratigraphy to measure firn compaction we note the positions of optical features on the borehole wall detected by an initial baseline log we track the displacements of these features in subsequent logs the result provides a measurement of the relative vertical motion and thus compaction of the firn over the survey period we have successfully used this system at summit greenland to measure the depth distribution of firn column shortening experienced in a borehole over three 1 year periods the uppermost 30 m of the firn at summit is compacting as predicted by a simple steady state model implying that the firn density profile at summit is at or close to steady state over the past 70 years

simultaneous core sample measurements of elastic properties and resistivity at reservoir conditions employing a modified triaxial cell a feasibility study adaptations of existing triaxial cells for ultrasonic p and s wave measurements are well documented this paper proposes further modification of such a cell so that also resistivity measurements can be carried out simultaneously at reservoir conditions by employing the top cap and the pedestal of the cell as electrodes axial resistivity measurements are now feasible in order to minimize the polarization effect of this two electrode arrangement careful analyses have been carried out to optimize the choice of electrode coating and measurement frequency band radial resistivity measurements are also included in the system by introducing a strap electrode system in a reservoir under production changes in both saturations temperature if steam injection and stresses can take place therefore the modified triaxial system should be able to measure the integrated effects on the acoustic parameters and electric responses caused by variations in each of these parameters the feasibility of the system to obtain such reliable information is demonstrated employing a small selection of core samples in the future such combined measurements on reservoir core samples can be used to link both seismic and electromagnetic observations to the actual earth model and constrain both modelling and inversion

pressure and fluid effect on frequency dependent elastic moduli in fully saturated tight sandstone we developed a system to explore the effects of pressure and fluid viscosity on the dispersion and attenuation of fully saturated tight sandstones especially at seismic frequencies calibration of the new system revealed that the system can operate reliably at frequencies of 2 200 106 hz tight sandstone with a crack pore microstructure was tested under nitrogen gas dry brine and glycerin saturation a frequency dependent effect was not found for the dry case however apparent dispersion and attenuation for the undrained unrelaxed transition was clearly observed for sample under brine or glycerin saturation the magnitude of which was largely suppressed by increasing effective pressure the measurement results illustrated that increasing the fluid viscosity or the effective pressure will shift the dispersion curve to the lower frequency range a simple squirt flow model with dual porosity scheme was used to compare with the measurement results although the estimated values deviated slightly from the data the trend fitted the saturated data relatively well especially at low effective pressures therefore considering the crack pore microstructure of the tight sandstone dispersion and attenuation are induced predominantly by the squirt flow stiffening effect from cracks to pores

the unusual summer of 1994 in east asia iod teleconnections 1 an extremely hot and dry summer of 1994 was reported in east asian countries using observational data we have demonstrated that the indian ocean dipole iod is at least one possible cause of the abnormal east asian summer climate an anomalous cyclonic circulation over the western pacific and the southern china weakened the monsoonal northward flow in the lower troposphere an anomalous anticyclonic circulation with the equivalent barotropic structure around japan korea and the northeastern part of china caused the hot and dry summer of 1994 this accumulation of the lower potential vorticity in the far east is related to the wave activity from the mediterranean sahara region the monsoon desert mechanism connects a rossby wave source with the iod induced diabatic heating around the bay of bengal another rossby wave train pattern was generated in the upper troposphere and propagates northeastward from the southern china both the rossby wave patterns influenced the circulation changes over east asia

in situ measurements of antarctic snow compaction compared with predictions of models 1 we describe in situ measurements of the compaction of antarctic snow at three different sites in antarctica the rate of compaction was measured hourly over various depth intervals for up to two years these measurements show that compaction at each of the sites occurs through slow viscous deformation of the snowpack with no significant contribution from sudden collapse of weak layers the measured rates of compaction at the coldest site exhibit a strong seasonality consistent with a temperature dependent sintering mechanism having activation energy of 70 kj mol 1 at the two warmer sites activation energies of 80 and 120 kj mol 1 provide slightly better agreement with the observations published models of snow compaction underestimate the temperature sensitivity a good match to our observations is provided by a semi empirical model based on rate equations for lattice diffusion nabarro herring creep of material around pores combined with normal grain growth this model also provides a theoretical basis for a widely used empirical model of snow compaction the rate coefficient for lattice diffusion inferred from our measurements is considerably higher than published values however and other creep mechanisms cannot be ruled out

seismic wave attenuation and dispersion resulting from wave induced flow in porous rocks a review one major cause of elastic wave attenuation in heterogeneous porous media is wave induced flow of the pore fluid between heterogeneities of various scales it is believed that for frequencies below 1 khz the most important cause is the wave induced flow between mesoscopic inhomogeneities which are large compared with the typical individual pore size but small compared to the wavelength various laboratory experiments in some natural porous materials provide evidence for the presence of centimeter scale mesoscopic heterogeneities laboratory and field measurements of seismic attenuation in fluid saturated rocks provide indications of the role of the wave induced flow signatures of wave induced flow include the frequency and saturation dependence of p wave attenuation and its associated velocity dispersion frequency dependent shear wave splitting and attenuation anisotropy during the last four decades numerous models for attenuation and velocity dispersion from wave induced flow have been developed with varying degrees of rigor and complexity these models can be categorized roughly into three groups according to their underlying theoretical framework the first group of models is based on biot s theory of poroelasticity the second group is based on elastodynamic theory where local fluid flow is incorporated through an additional hydrodynamic equation another group of models is derived using the theory of viscoelasticity though all models predict attenuation and velocity dispersion typical for a relaxation process there exist differences that can be related to the type of disorder periodic random space dimension and to the way the local flow is incorporated the differences manifest themselves in different asymptotic scaling laws for attenuation and in different expressions for characteristic frequencies in recent years some theoretical models of wave induced fluid flow have been validated numerically using finite difference finite element and reflectivity algorithms applied to biot s equations of poroelasticity application of theoretical models to real seismic data requires further studies using broadband laboratory and field measurements of attenuation and dispersion for different rocks as well as development of more robust methods for estimating dissipation attributes from field data

an interdecadal change in southern china summer rainfall around 1992 93 the present study documents a pronounced interdecadal change in summer rainfall over southern china around 1992 93 and explores the plausible reasons for this change the summer rainfall is persistently below normal during 1980 92 and above normal during 1993 2002 coherent changes in atmospheric circulation are identified over east asia and the south china sea scs western north pacific wnp the increase in rainfall is accompanied by an increase in lower level convergence midtropospheric ascent and upperlevel divergence over southern china the changes in lower level winds feature two anomalous anticyclones one over the scs subtropical wnp and the other over north china mongolia the outflows from the two anomalous anticyclones converge over southern china leading to anomalous moisture convergence enhanced ascent and increased rainfall the development of the northern anticyclone is related to an increase in the tibetan plateau snow cover in the preceding winter spring that leads to a contrast in temperature change between the plateau and the surrounding regions the relatively small temperature change over the plateau coupled with increases in temperature to the west and the east leads to an increase in surface pressure extending northward from the plateau the development of the southern anticyclone is related to an increase in sea surface temperature in the equatorial indian ocean that enhances lower level convergence and ascent the accompanying upper level divergent flows from the tropical indian ocean to the scs wnp lead to the development of anomalous descent and lower level anomalous anticyclone over the scs wnp

measurements of the elastic and anelastic properties of sandstone flooded with supercritical co2 the aim of this study was to investigate the effects of supercritical co2 scco2 injection on the elastic and anelastic properties of sandstone at seismic and ultrasonic frequencies we present the results of the low frequency and ultrasonic experiments conducted on water saturated sandstone donnybrook western australia flooded with scco2 the sandstone was cut in the direction perpendicular to a formation bedding plane and tested in a hoek triaxial pressure cell during the experiments with scco2 the low frequency and ultrasonic systems and the pump dispensing scco2 were held at a temperature of 42 c the elastic parameters obtained for the sandstone with scco2 at seismic 0 1 hz 100 hz and ultrasonic 0 5 mhz frequencies are very close to those for the dry rock the extensional attenuation was also measured at seismic frequencies for the dry water saturated and scco2 injected sandstones the applicability of gassmann s fluid substitution theory to obtained results was also tested during the experiments

using the temporal variability of satellite radar altimetric observations to map surface properties of the antarctic ice sheet the problem of measuring surface height and snowpack characteristics from satellite radar altimeter echoes is investigated in this paper we perform an analysis of the ers1 altimeter dataset acquired during a 3 day repeat orbit the analysis reveals that there are temporal variations in shapes of the radar altimeter echo and that these variations are linked to meteorological phenomena the time and space scales over which these variations apply are a few to tens of days and a few hundred kilometres respectively this phenomenon if not accounted for can create error in the height measurement a numerical echo model is used to recover snowpack characteristics by taking advantage of the temporal variations of the radar echoes a map of penetration depth of the radar waves in the ku band over the antarctic continent is obtained and suggests that grain size produces the dominant effect on radar extinction in the snowpack at this frequency finally a procedure is proposed to correct the height measurement within the context of ice sheet mass balance survey

a new seismic wave attenuation module to experimentally measure low frequency attenuation in extensional mode abstract a seismic wave attenuation module is developed to experimentally measure the attenuation in extensional mode qe 1 and the young s modulus of copper jacketed 60 mm long and 25 4 mm in diameter samples in a gas medium paterson rig the new module is suitable for natural rock samples and was tested under confining pressure up to 50 mpa and room temperature the module is designed to operate at a strain 10 6 for which rocks behave linearly to calculate attenuation both the applied force and the bulk shortening of the sample are measured employing linear variable differential transformers this technique allows measuring samples with a high degree of heterogeneity attenuation at low seismic frequencies 10 2 102 hz is obtained for rocks at dry and various saturation conditions we present a series of measurements on berea sandstone in a room dry condition and saturated with different fluids water and glycerine solutions with viscosities of 10 cp and 22 cp respectively at a confining pressure of 10 mpa and with a pore pressure of 1 mpa the accuracy of the attenuation data expressed as a phase shift is 0 0019 rad

laboratory measurements of seismic attenuation in sandstone strain versus fluid saturation effects abstractin the past few decades great attention has been focused on uncovering the physics of seismic wave attenuation in fluid saturated rocks however the relationship among many variables affecting attenuation is still not completely clear for instance although the role of strain in enhancing friction dissipation is relatively well known for dry rocks it remains unclear how and how much it affects attenuation in fluid saturated rocks we experimentally measured attenuation in the extensional mode in berea sandstone at strains between 7 8 10 7 and 1 9 10 5 and at frequencies in the seismic bandwidth 1 100 hz these strains were similar to those typically observed in seismic exploration 10 6 we also measured the transient fluid pressure caused when a stepwise stress was applied resulting in such strains for the studied strain range our results indicated that 1 the overall attenuation in dry berea sandstone increased linearly with strain 2 the frequency dependent component of attenuati

effect of brine co2 fracture flow on velocity and electrical resistivity of naturally fractured tight sandstones abstractfracture networks inside geologic co2 storage reservoirs can serve as the primary fluid flow conduit particularly in low permeability formations although some experiments focus on the geophysical properties of brine and co2 saturated rocks during matrix flow geophysical monitoring of fracture flow when co2 displaces brine inside the fracture seems to be overlooked we have conducted laboratory geophysical monitoring of fluid flow in a naturally fractured tight sandstone during brine and liquid co2 injection for the experiment the low porosity low permeability naturally fractured core sample from the triassic de geerdalen formation was acquired from the longyearbyen co2 storage pilot at svalbard norway stress dependence hysteresis and the influence of fluid rock interactions on fracture permeability were investigated the results suggest that in addition to stress level and pore pressure mobility and fluid type can affect fracture permeability during loading and unloading cycles moreo

seismic attenuation effects of pore fluids and frictional sliding seismic wave attenuation in rocks was studied experimentally with particular attention focused on frictional sliding and fluid flow mechanisms sandstone bars were resonated at frequencies from 500 to 9000 hz and the effects of confining pressure pore pressure degree of saturation strain amplitude and frequency were studied observed changes in attenuation and velocity with strain amplitude are interpreted as evidence for frictional sliding at grain contacts since this amplitude dependence disappears at strains and confining pressures typical of seismic wave propagation in the earth we infer that frictional sliding is not a significant source of seismic attenuation in situ partial water saturation significantly increases the attenuation of both compressional p and shear s waves relative to that in dry rock resulting in greater p wave than s wave attenuation complete saturation maximizes s wave attenuation but causes a reduction in p wave attenuation these effects can be interpreted in term

definition of antarctic oscillation index following walker s work about his thmous three oscillations published during the 1920 30s many papers were written about atmospheric oscillations a iburth atmospheric oscillation in the middle and high southern latitudes was lbund and nanted the grotarctic oscillation ao a refiers to a large scale alternation of atmospheric mass between the mid latitudes and high latitudes surthce pressure in order to understand the spatial structure of sea level pressure variation in detail empirical orthogonal fitnction analysis is applied an objective index of the antarctic oscillation index aoi is defined as the diffierence of zonal mean sea level pressure between 40os and 65os the aoi has the potential tbr clarifying climate regimes in the southern hemisphere similar to how the na and the npo has been used in the northern hemisphere were there other oscillation s besides nao npo and s especially in the southern hemisphere it had been supposed that there might be other atmospheric oscillation s in the high southern latitudes although as early as in the first quarter of this century walker 1928 had stated that just as in the north atlantic there is a pressure opposition between the azores and iceland there is an opposition between the high pressure belt across chile and the argentine on the one hand and the low pressure area of weddell sea and the bellingshausen sea on the other the scarcity of data in the southern hemisphere hindered the search tbr new oscillation s during the last two decades more comprehensive data over the southern hemisphere became available and more analyses were carried out see kidson 1975 rogers and van loon 1982 mo and l 7 ite 1985 2 4o 1986 connolley 1997 gong and wang 1998 a tburth atmospheric oscillation in the middle and high southern latitudes was tbund and nanted as the antarctic oscillation a wang 1992 gong and wang 1998 the term ao relrs to a large scale alternation of atmospheric mass between the mid latitude sur hce pressure

antarctic firn compaction rates from repeat track airborne radar data ii firn model evaluation the thickness and density of the antarctic firn layer vary considerably in time and space thereby contributing to ice sheet volume and mass changes distinguishing between these mass and volume changes is important for ice sheet mass balance studies evolution of firn layer depth and density is often modeled because direct measurements are scarce here we directly compare modeled firn compaction rates with observed rates obtained from repeat track airborne radar data over a 2 year interval 2009 11 in west antarctica spatially the observed compaction rates exhibit significant variability but when averaged to scales comparable to the model resolution 20 50 km the measurements and model results qualitatively agree a colder and drier period preceding the 2009 survey led to lower compaction rates during the 2009 10 interval when compared to 2010 11 which is partly captured by the firn model spatially higher compaction rates are observed and modeled in warmer regions with higher accumulation

seismic attenuation in partially saturated berea sandstone submitted to a range of confining pressures using the forced oscillation method we measure the extensional mode attenuation and young s modulus of a berea sandstone sample at seismic frequencies 0 5 50 hz for varying levels of water saturation 0 100 and confining pressures 2 25 mpa attenuation is negligible for dry conditions and saturation levels 80 for saturation levels between 91 and 100 attenuation is significant and frequency dependent in the form of distinct bell shaped curves having their maxima between 1 and 20 hz increasing saturation causes an increase of the overall attenuation magnitude and a shift of its peak to lower frequencies on the other hand increasing the confining pressure causes a reduction in the attenuation magnitude and a shift of its peak to higher frequencies for saturation levels above 98 the fluid pressure increases with increasing confining pressure when the fluid pressure is high enough to ensure full water saturation of the sample attenuation becomes negligible a second series of comparable experiments reproduces these results satisfactorily based on a qualitative analysis of the data the frequency dependent attenuation meets the theoretical predictions of mesoscopic wave induced fluid flow wiff in response to a heterogeneous water distribution in the pore space so called patchy saturation these results show that mesoscopic wiff can be an important source of seismic attenuation at reservoir conditions

a laboratory study of low frequency wave dispersion and attenuation in water saturated sandstones abstract laboratory measurements of elastic and anelastic parameters of dry and water saturated sandstones with low 0 7 and 1 1 md and high 425 md permeability have been conducted at seismic frequencies 0 1 to 120 hz the experiments were performed with a laboratory apparatus that measures the stress strain relationship in the linear regime the extensional attenuation in water saturated low permeability sandstones exhibits prominent peaks in the seismic band accompanied by considerable dispersion variations in attenuation and dispersion in the high permeability sandstone are below the measurement error as is the moduli dispersion in all the sandstones in dry condition the experiments demonstrate that for low permeability rocks seismic frequencies do not necessarily correspond to the low frequency limit relaxed pore fluid pressures of acoustic wave dispersion

antarctic firn compaction rates from repeat track airborne radar data i methods while measurements of ice sheet surface elevation change are increasingly used to assess mass change the processes that control the elevation fluctuations not related to ice flow dynamics e g firn compaction and accumulation remain difficult to measure here we use radar data from the thwaites glacier west antarctica catchment to measure the rate of thickness change between horizons of constant age over different time intervals 2009 10 2010 11 and 2009 11 the average compaction rate to approximately 25m depth is 0 33ma exp 1 with largest compaction rates near the surface our measurements indicate that the accumulation rate controls much of the spatio temporal variations in the compaction rate while the role of temperature is unclear due to a lack of measurements based on a semi empirical steady state densification model we find that surveying older firn horizons minimizes the potential bias resulting from the variable depth of the constant age horizon our results suggest that the spatiotemporal variations in the firn compaction rate are an important consideration when converting surface elevation change to ice mass change compaction rates varied by up to 0 12ma exp 1 over distances less than 6km and were on average greater than 20 larger during the 2010 11 interval than during 2009 10

improved understanding of velocity saturation relationships using 4d computer tomography acoustic measurements a recently developed laboratory method allows for simultaneous imaging of fluid distribution and measurements of acoustic wave velocities during flooding experiments using a specially developed acoustic sample holder that combines high pressure capacity with good transparency for x rays it becomes possible to investigate relationships between velocity and fluid saturation at reservoir stress levels high resolution 3d images can be constructed from thin slices of cross sectional computer tomography scans ct scans covering the entire rock core volume and from imaging the distribution of fluid at different saturation levels the x ray imaging clearly adds a new dimension to rock physics measurements it can be used in the explanation of variations in measured velocities from core scale heterogeneities computer tomography gives a detailed visualization of density regimes in reservoir rocks within a core this allows an examination of the interior of core samples revealing inhomogeneities porosity and fluid distribution this mapping will not only lead to an explanation of acoustic velocity measurements it may also contribute to an increased understanding of the fluid flow process and gas liquid mixing mechanisms in rock immiscible and miscible flow in core plugs can be mapped simultaneously with acoustic measurements the effects of core heterogeneity and experimentally introduced effects can be separated to clarify the validity of measured velocity relationships

antarctic ice sheet loss driven by basal melting of ice shelves accurate prediction of global sea level rise requires that we understand the cause of recent widespread and intensifying glacier acceleration along antarctic ice sheet coastal margins atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves potentially limiting their ability to buttress the flow of grounded tributary glaciers indeed recent ice shelf collapse led to retreat and acceleration of several glaciers on the antarctic peninsula but the extent and magnitude of ice shelf thickness change the underlying causes of such change and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum antarctic pattern of ice shelf thinning through increased basal melt we deduce that this increased melt is the primary control of antarctic ice sheet loss through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow the highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of antarctic ice shelves through ocean upwelling in the amundsen and bellingshausen seas and atmospheric warming on the antarctic peninsula this implies that climate forcing through changing winds influences antarctic ice sheet mass balance and hence global sea level on annual to decadal timescales

velocity and attenuation in partially saturated rocks poroelastic numerical experiments we use a poroelastic modelling algorithm to compute numerical experiments on wave propagation in a rock sample with partial saturation using realistic fluid distribution patterns from tomography scans frequencies are in the range 10 to 500 khz the rock is a homogeneous isotropic sandstone partially filled with gas and water which are defined by their characteristic values of viscosity compressibility and density we assume no mixing and that the two different pore fills occupy different macroscopic regions the von karman self similar correlation function is used employing different fractal parameters to model uniform and patchy fluid distributions respectively where effective saturation is varied in steps from full gas to full water saturation without resorting to additional matrix fluid interaction mechanisms we are able to reproduce the main features of the variation in wave velocity and attenuation with effective saturation and frequency as those of published laboratory experiments furthermore the behaviour of the attenuation peaks versus water saturation and frequency is similar to that of white s model the conversion of primary p wave energy into dissipating slow waves at the heterogeneities is shown to be the main mechanism for attenuating the primary wavefield fluid gas patches are shown to affect attenuation more than equivalent patches in the permeability or solid grain properties

measurements of seismic attenuation and transient fluid pressure in partially saturated berea sandstone evidence of fluid flow on the mesoscopic scale s u m m a r y a novel laboratory technique is proposed to investigate wave induced fluid flow on the mesoscopic scale as a mechanism for seismic attenuation in partially saturated rocks this technique combines measurements of seismic attenuation in the frequency range from 1 to 100 hz with measurements of transient fluid pressure as a response of a step stress applied on top of the sample we used a berea sandstone sample partially saturated with water the laboratory results suggest that wave induced fluid flow on the mesoscopic scale is dominant in partially saturated samples a 3 d numerical model representing the sample was used to verify the experimental results biot s equations of consolidation were solved with the finite element method wave induced fluid flow on the mesoscopic scale was the only attenuation mechanism accounted for in the numerical solution the numerically calculated transient fluid pressure reproduced the laboratory data moreover the numerically calculated attenuation superposed to the frequency independent matrix anelasticity reproduced the attenuation measured in the laboratory in the partially saturated sample this experimental numerical fit demonstrates that wave induced fluid flow on the mesoscopic scale and matrix anelasticity are the dominant mechanisms for seismic attenuation in partially saturated berea sandstone

summer rainfall variability in low latitude highlands of china and subtropical indian ocean dipole abstractbased on reanalysis and observational datasets this study proposes a reasonable mechanism for summer rainfall variations over the low latitude highlands llh of china in which a subtropical indian ocean dipole siod like pattern is the key external thermal forcing in summers with a positive siod like pattern sea surface temperature sst anomalies may lead to lower tropospheric divergence over the tropical indian ocean and convergence over the subtropical southwestern indian ocean and arabian sea the convergence over the arabian sea can induce easterly anomalies of the divergent wind component off the eastern coast of the bay of bengal bob while the divergence over the tropical indian ocean can change the interhemispheric vertical circulation and produce a descending motion over the same area and cyclonic anomalies in the rotational wind component over the indian peninsula the combined effect of the divergent and rotational wind anomalies and enhanced interhemispheric vertical circulati

geomechanical and geochemical effects on sandstones caused by the reaction with supercritical co2 an experimental approach to in situ conditions in deep geological reservoirs the geochemical and geomechanical behaviour of reservoir rocks from deep saline aquifers during the injection and geological storage of co2 is studied in laboratory experiments a combination of geochemical and geomechanical studies was carried out on various sandstones from the north german basin after the mineralogical geochemical and petrophysical characterization a set of sandstone samples was exposed to supercritical sc co2 and brine for 2 4 weeks in an autoclave system one sample was mineralogically and geochemically characterised and then loaded in a triaxial cell under in situ pressure and temperature conditions to study the changes of the geomechanical rock properties after treatment in the autoclaves geochemical alterations mainly in the carbonate but also in the sheet silicate cements as well as in single minerals of the sandstones were observed affecting the rocks granular structure in addition to partial solution effects during the geochemical experiments small grains of secondary carbonate and other mineral precipitations were observed within the pore space of the treated sandstones results of additional geomechanical experiments with untreated samples show that the rock strength is influenced by the saturation degree the confining pressure the pore fluid pressure and temperature the exposure to pure scco2 in the autoclave system induces reduced strength parameters modified elastic deformation behaviour and changes of the effective porosity in comparison to untreated sandstone samples experimental results show that the volume of pore fluid fluxing into the pore space of the sandstones clearly depends on the saturation level of the sample

distinct patterns of sea surface temperature anomaly in the south indian ocean during austral autumn a neural network based cluster technique called self organizing map som determines 12 distinguishable patterns derived from the sea surface temperature sst anomaly fields in the south indian ocean for 1901 2010 in austral autumn march may two out of twelve patterns patterns 1 and 12 are characterized by a dipole pattern with opposite sst anomalies in the southwest indian ocean and to its northeastern side which is similar to the so called indian ocean subtropical dipole iosd and they are named as traditional iosd t iosd in the present work patterns 2 and 11 show a similar dipole pattern of a northeast southwest orientation which can be significantly distinguished from t iosd due to their distinct location of action centers amplitudes and evolution processes these two patterns were called new iosd n iosd for both positive and negative positive phase of t iosd the anomalous dipole pattern is first observed near madagascar in previous spring and it migrates eastward and forms the typical pattern of t iosd in early summer differently for positive phase of n iosd significant sst anomalies appear to the west of australia continent at first in late spring and a well organized dipole pattern could not be observed until late summer approximately one or two months later than the formation time of t iosd the major asymmetry of n iosd characterizes one month later of the formation time for the negative phase of n iosd compared with its positive phase the possible mechanisms responsible for different evolution process of t iosd and n iosd events were examined the results show that latent heat flux not surprisingly plays an essential role in the formation of both t iosd and n iosd however the contribution of meridional advection by anomalous oceanic current to the n iosd event is much more important than that to the t iosd

potential influence of the november december southern hemisphere annular mode on the east asian winter precipitation a new mechanism as the leading mode of the global atmospheric mass inter annual variability the southern hemisphere annular mode sam may exert potential influences to the northern hemisphere nh climate but the related physical mechanism is not yet clear in this study it is found that the november december nd sam exhibits a significant inverse relationship with the winter precipitation over east asia particularly southern china observational and numerical evidences show that anomalous nd sam is usually associated with a south atlantic pacific dipole sea surface temperature anomaly ssta which persists into ensuring winter the dipole ssta can modulate the variability of the inter tropical convergence zone itcz in pacific subsequently a distinguished atmospheric tele connection pattern is induced and prevails over the nh mid latitude region as a response to the anomalous itcz large areas of high pressure anomalies are triggered at upper troposphere over east asia and centered over southern china which favors less precipitation over east asia particularly southern china and vice versa through such a physical mechanism the notable influence of the nd sam can sustain through the following season and impact on the nh winter climate

laboratory based seismic attenuation in fontainebleau sandstone evidence of squirt flow at seismic frequencies 1 100 hz we studied attenuation in the laboratory using the forced oscillation method we adopted the longitudinal mode of oscillation which yields the young s modulus and the corresponding attenuation here defined as the inverse quality factor a fontainebleau sandstone with a porosity of 8 and a permeability of 12 md was saturated with different fluids and investigated at the confining pressures of 5 10 and 15 mpa at all the measured confining pressures while attenuation was zero for the dry sample for partial and full water saturation it gradually increased from nearly zero to 0 02 with increasing frequency the sample was then fully saturated with glycerin water mixtures of varying viscosities up to that of glycerin 8 92 485 and 1414 cp at the confining pressure of 5 mpa a bell shaped attenuation curve peaking at 6 hz with a magnitude of 0 11 was observed when the sample was fully saturated with glycerin 1414 cp a decrease in viscosity of the saturating fluid shifted the attenuation curve to higher frequencies and an increase in confining pressure caused a decrease in the overall magnitude of attenuation the data obtained for glycerin were compared to a simple squirt flow model with sufficient agreement implying that squirt flow is the dominant mechanism responsible for the observed attenuation

laboratory measurements of a complete set of poroelastic moduli for berea sandstone and indiana limestone measurements have been completed for eight different poroelastic moduli of water saturated berea sandstone and indiana limestone as a function of confining pressure and pore pressure the poroelastic moduli for indiana limestone are generally consistent to 10 which was verified by a formal inversion procedure for independent moduli from the eight measurements for indiana limestone best fit values were drained bulk modulus 21 2 gpa the undrained bulk modulus 31 7 gpa drained poisson s ratio 0 26 undrained poisson s ratio 0 33 and pore pressure buildup coefficient 0 47 at 20 35 mpa effective stress the poroelastic moduli for berea sandstone are generally consistent to 20 the greater inconsistency is most likely caused by the nonlinear variation of the moduli at different strains for berea sandstone best fit values were drained bulk modulus 6 6 gpa undrained bulk modulus 15 8 gpa drained poisson s ratio 0 17 undrained poisson s ratio 0 34 and pore pressure buildup coefficient 0 75 at 10 mpa effective stress

integrated geophysical and hydromechanical assessment for co2 storage shallow low permeable reservoir sandstones geological reservoirs can be structurally complex and can respond to co2 injection both geochemically and geomechanically hence predicting reservoir formation behaviour in response to co2 injection and assessing the resulting hazards are important prerequisites for safe geological co2 storage this requires a detailed study of thermal hydro mechanical chemical coupled phenomena that can be triggered in the reservoir formation most readily achieved through laboratory simulations of co2 injection into typical reservoir formations here we present the first results from a new experimental apparatus of a steady state drainage flooding test conducted through a synthetic sandstone sample simulating real co2 storage reservoir conditions in a shallow 1 km low permeability 1md 26 porosity sandstone formation the injected pore fluid comprised brine with co2 saturation increasing in steps of 20 brine co2 partial flow rates up to 100 co2 flow at each pore fluid stage an unload loading cycle of effective pressure was imposed to study the response of the rock under different geomechanical scenarios the monitoring included axial strains and relative permeability in a continuous mode hydromechanical assessment and related geophysical signatures ultrasonic p wave and s wave velocities vp and vs and attenuations qp 1 and qs 1 respectively and electrical resistivity on average the results showed vp and vs dropped 7 and 4 respectively during the test whereas qp 1 increased 55 and qs 1 decreased by 25 from the electrical resistivity data we estimated a maximum co2 saturation of 0 5 whereas relative permeability curves were adjusted for both fluids comparing the experimental results to theoretical predictions we found that gassmann s equations explain vp at high and very low co2 saturations whereas bulk modulus yields results consistent with white and dutta ode model predictions this is interpreted as a heterogeneous distribution of the two pore fluid phases corroborated by electrical resistivity tomography images the integration of laboratory geophysical and hydromechanical observations on representative shallow low permeable sandstone reservoir allowed us to distinguish between pure geomechanical responses and those associated with the pore fluid distribution this is a key aspect in understanding co2 injection effects in deep geological reservoirs associated with carbon capture and storage practices

fluid mobility and frequency dependent seismic velocity direct measurements the influence of fluid mobility on seismic velocity dispersion is directly observed in laboratory measurements from seismic to ultrasonic frequencies a forceddeformation system is used in conjunction with pulse transmission to obtain elastic properties at seismic strain amplitude 10 7 from 5 hz to 800 khz varying fluid types and saturations document the influence of pore fluids the ratio of rock permeability to fluid viscosity defines mobility which largely controls pore fluid motion and pore pressure in a porous medium high fluid mobility permits pore pressure equilibrium either between pores or between heterogeneous regions resulting in a low frequency domain where gassmann s equations are valid in contrast low fluid mobility can produce strong dispersion even within the seismic band here the low frequency assumption fails since most rocks in the general sedimentary section have very low permeability and fluid mobility shales siltstones tight limestones etc most rocks are not in the lowfrequency domain even at seismic frequencies only those rocks with high permeability porous sands and carbonates will remain in the low frequency domain in the seismic or sonic band