past rates of accumulation in central west antarctica 1 the spatial variation in ice accumulation is critical to the form and stability of ice sheets in west antarctica knowledge of present ice accumulation rates is restricted to sparse measurements and interpolations across data free regions while information on former ice accumulation is restricted to a small number of ice cores and numerical modeling results here isochronous internal ice sheet layers recorded by airborne radio echo sounding and dated at their stratigraphic position at the byrd ice core site are used to reconstruct the spatial pattern of accumulation across the main west antarctic ice divide for the past 16 000 years mean accumulation calculated for the last 3100 years matches well to present day accumulation rates which is compatible with previous observations on siple dome accumulation between 6400 and 16 000 years ago is calculated to be around half that at present in accordance with recent ice sheet modeling results index terms 1223 geodesy and gravity ocean earth atmosphere interactions 3339 1827 hydrology glaciology 1863 9310 information related to geographic region antarctica 9604 information related to geologic time cenozoic 3349 meteorology and atmospheric dynamics polar meteorology citation siegert m j and a j payne 2004 past rates of accumulation in central west antarctica geophys res lett 31 l12403 doi 10 1029 2004gl020290

tutorial on seismic interferometry part 1 basic principles and applications seismic interferometry involves the crosscorrelation of responses at different receivers to obtain the green s function between these receivers for the simple situation of an impulsive plane wave propagating along the x axis the crosscorrelation of the responses at two receivers along the x axis gives the green s function of the direct wave between these receivers when the source function of the plane wave is a transientas in exploration seismology or a noise signalas in passive seismology then the crosscorrelation gives the green s function convolved with the autocorrelation of the source function direct wave interferometry also holds for 2d and 3d situations assuming the receivers are surrounded by a uniform distribution of sources in this case the main contributions to the retrieved direct wave between the receivers come from sources in fresnel zones around stationary points the main application of direct wave interferometry is the retrieval of seismic surface wave responses from ambient noise and the subsequent tomographic determination of the surfacewave velocity distribution of the subsurface seismic interferometry is not restricted to retrieving direct waves between receivers in a classic paper claerbout shows that the autocorrelation of the transmission response of a layered medium gives the plane wave reflection response of that medium this is essentially 1d reflected wave interferometry similarly the crosscorrelation of the transmission responses observed at two receivers of an arbitrary inhomogeneous medium gives the 3d reflection response of that medium one of the main applications of reflected wave interferometry is retrieving the seismic reflection response from ambient noise and imaging of the reflectors in the subsurface a common aspect of direct and reflected wave interferometry is that virtual sources are created at positions where there are only receivers without requiring knowledge of the subsurface medium parameters or of the positions of the actual sources

point spread functions for interferometric imaging interferometric redatuming is a data driven method to transform seismic responses with sources at one level and receivers at a deeper level into virtual reflection data with both sources and receivers at the deeper level although this method has traditionally been applied by cross correlation accurate redatuming through a heterogeneous overburden requires solving a multidimensional deconvolution problem input data can be obtained either by direct observation for instance in a horizontal borehole by modelling or by a novel iterative scheme that is currently being developed the output of interferometric redatuming can be used for imaging below the redatuming level resulting in a so called interferometric image internal multiples from above the redatuming level are eliminated during this process in the past we introduced point spread functions for interferometric redatuming by cross correlation these point spread functions quantify distortions in the redatumed data caused by internal multiple reflections in the overburden in this paper we define point spread functions for interferometric imaging to quantify these distortions in the image domain these point spread functions are similar to conventional resolution functions for seismic migration but they contain additional information on the internal multiples in the overburden and they are partly data driven we show how these point spread functions can be visualized to diagnose image defocusing and artefacts finally we illustrate how point spread functions can also be defined for interferometric imaging with passive noise sources in the subsurface or with simultaneous source acquisition at the surface

migration of the siple dome ice divide west antarctica the non linearity of the ice flow law or a local accumulation low over an ice divide can cause isochrones internal layers to be shallower under the divide relative to the flanks forming a divide bump in the internal layer pattern this divide signature is analyzed using ice flow models and inverse techniques to detect and quantify motion of the siple dome ice divide west antarctica the principal feature indicating that migration has occurred is a distinct tilt of the axis of the peaks of the warped internal layers beneath the divide the calculated migration rate is 0 05 0 50 m a 1 toward ice stream d and depends slightly on whether the divide bump is caused by the non linearity of ice flow or by a local accumulation low our calculations also suggest a strong south north accumulation gradient of 5 10 x 10 6 a 1 in a narrow zone north of the divide a consequence of divide migration is that pre holocene ice is thickest about 0 5 km south of the present divide position divide motion indicates that non steady processes possibly associated with activity of the bounding ice streams are affecting the geometry of siple dome the migration rate is sufficiently slow that the divide bump is maintained in the internal layer pattern at all observable depths this suggests that major asynchronous changes in the elevation or position of the bounding ice streams are unlikely over at least the past 10 3 10 4 years

recognizing unsteadiness in the transport systems of dilute pyroclastic density currents laboratory density currents generated with unsteady source conditions provide insight into the distances and timescales over which unsteadiness at the vent should persist in natural dilute pyroclastic density currents pdcs the laboratory experiments comprise heated 20 m talc particles turbulently suspended in air and introduced as density currents into an 8 5 6 2 6 m air filled chamber the densimetric and thermal richardson froude stokes and settling numbers are all similar to those of natural dilute pdcs although the experiments reynolds numbers are several orders of magnitude less than natural pdcs the experiments are fully turbulent and are thus dynamically similar to some dilute natural pdcs unsteadiness in the experiments is generated by adding two pauses of duration t to the eruption e g a 100 s experiment comprising three 33 s pulses separated by two pauses of t 10 s propagation distance of the leading head is proportional to the square root of time positions of the trailing pulses have similar time dependence but they generally travel slightly faster than and thus catch and merge with the leading current trailing pulses are more easily distinguished from the body of the preceding current when t is large analysis of turbulent structures through space and time shows that unsteadiness at the eruption source can be distinguished from turbulent fluctuations in the currents when t is greater than the integral turbulent timescale of the current body body a statistical measure of the characteristic timescale of unsteadiness within a turbulent flow the distances over which unsteadiness in the transport system persist scale with t body and the ratio of the thermal richardson numbers of the trailing and leading pulses as t body increases from 2 to 4 trailing pulses remain distinct over an increasing fraction of the lead runout distance when subsequent pulses have higher rit than the leading current they coalesce with the leading pulse at more proximal distances but when rit is much lower they remain distinct over distances similar to that of the leading head even when t is similar to body for currents that have depositional timescales comparable to or shorter than body deposits should be expected to preserve a record of unsteadiness over the distances that unsteady pulses persist

past and future grounding line retreat of the west antarctic ice sheet the history of deglaciation of the west antarctic ice sheet wais gives clues about its future southward grounding line migration was dated past three locations in the ross sea embayment results indicate that most recession occurred during the middle to late holocene in the absence of substantial sea level or climate forcing current grounding line retreat may reflect ongoing ice recession that has been under way since the early holocene if so the wais could continue to retreat even in the absence of further external forcing

pyroclastic flows from the 1991 eruption of unzen volcano japan pyroclastic flows from unzen were generated by gravitational collapse of the growing lava dome as soon as the parental lobe failed at the edge of the dome spontaneous shattering of lava occurred and induced a gravity flow of blocks and finer debris the flows had a overhanging tongue like head and cone or rollershaped vortices expanding outward and upward most of the flows traveled from 1 to 3 km but some flows reached more than 4 km burning houses and killing people in the evacuated zone of kita kamikoba on the eastern foot of the volcano the velocities of the flows ranged from 15 to 25 m s on the gentle middle flank observations of the flows and their deposits suggest that they consisted of a dense basal avalanche and an overlying turbulent ash cloud the basal avalanche swept down a topographic low and formed to tongue like lobe having well defined levees it is presumed to have moved as a non newtonian fluid the measured velocities and runout distances of the flows can be matched to a bingham model for the basal avalanche by the addition of turbulent resistance the rheologic model parameters for the 29 may flow are as follows the density is 1300 kg m3 the yield strength is 850 pa the viscosity is 90 pa s and the thickness of the avalanche is 2 m the ash cloud is interpreted as a turbulent mixing layer above the basal avalanche the buoyant portions of the cloud produced ash fall deposits whereas the dense portions moved as a surge separated from the parental avalanche the ash cloud surges formed a wide devastated zone covered by very thin debris the initial velocities of the 3 june surges when they detached from avalanches are determined by the runout distance and the angle of the energy line slope a comparison between the estimated velocities of the 3 june avalanches and the surges indicates that the surges that extended steep slopes along the avalanche path detached directly from the turbulent heads of the avalanches the over running surge that reached kita kamikoba had an estimated velocity higher than that of the avalanche this farther travelled surge is presumed to have been generated by collapse of a rising ash cloud plume

characterization and formation of melt layers in polar snow observations and experiments from west antarctica author posting international glaciological society 2005 this article is posted here by permission of international glaciological society for personal use not for redistribution the definitive version was published in journal of glaciology 51 2005 307 312 doi 10 3189 172756505781829395

the crossover stress anisotropy and the ice flow law at siple dome west antarctica we used observations and modeling of siple dome west antarctica a ridge ice divide to infer the importance of linear deformation mechanisms in ice sheet flow we determined the crossover stress a threshold value of the effective deviatoric stress below which linear flow mechanisms dominate over nonlinear flow mechanisms by combining measurements of ice properties with in situ deformation rate measurements and a finite element ice flow model that accounts for the effects of viscous anisotropy induced by preferred crystal orientation fabric we found that a crossover stress of 0 18 bar produces the best match between predicted and observed deformation rates for siple dome this means that including a linear term in the flow law is necessary but generally the flow is still dominated by the nonlinear glen n 3 term the pattern of flow near the divide at siple dome is also strongly affected by crystal fabric measurements of sonic velocity which is a proxy for vertically oriented crystal fabric suggest that a bed parallel shear band exists several hundred meters above the bed within the ice age ice

near wellbore vsp imaging without overburden in seismic exploration a complex overburden can cause defocused and distorted seismic images of deeper target areas while in surface seismic imaging it is difficult to overcome this problem vertical seismic profile vsp borehole seismic imaging with its well oriented recording geometry has proven able to circumvent the overburden s detrimental impact recently vspnear wellboresaltflankimagingwithout knowledge of the overburden has shown encouraging results in this paper we introduce another method for a more common imaging purpose imaging near wellbore horizons without knowledge of the overburden a key component of ourmethodisareverseray tracingtechniquethatusesactual vspdirect wave traveltime picks and extrapolates through a local velocity model to obtain the direct wave traveltimes in the vicinity of the well the extrapolated traveltimes then serve as references for migration of the vsp reflected data from near wellbore horizons the merits of this method are that all source side effects such as shot position and triggeringtimestaticsareavoided anditcanefficientlyandrobustly imagethenear wellborehorizonsbecauseitdoesnotneedan estimated overburden velocity model prone to errors however the image is limited to a diamond shaped area around the well whose size is proportional to the vsp array a synthetic vsp experiment on the seg eage 2d overthrust modelshowsthatthismethod withalocalevenanestimated 1d velocity model effectively images structural features suchasdipdirectionanddiscontinuitiesofthenear wellbore horizons

local vertical seismic profiling vsp elastic reverse time migration and migration resolution salt flank imaging with transmitted p to s waves to avoid the defocusing effects of propagating waves through salt and overburden with an inaccurate overburden velocity model we introduce a vertical seismic profiling vsp local elastic reverse time migration rtm method for salt flank imaging by transmitted p to s waves this method back projects the transmitted ps waves using a local velocity model around the well until they are in phase with the back projected pp waves at the salt boundaries the merits of this method are that it does not require the complex overburden and salt body velocities and it automatically accounts for source side statics in addition the method accounts for kinematic and dynamic effects including anisotropy absorption and all other unknown rock effects outside of this lo cal subsalt velocity model numerical tests on an elastic salt model and offset 2d vsp data in the gulf of mexico using a finite difference time domain staggered grid rtm scheme partly demonstrate the effectiveness of this method over interferometry

a method for determining ice thickness change at remote locations using gps ice thickness changes at remote locations on ice sheets can be determined by means of precise global positioning system gps surveys with interferometric solutions remote sites are precisely surveyed relative to gps receivers on rock repeat observations of the position of a remote site provide its vertical velocity the difference between this velocity and accumulation rate is an indicator of change in ice sheet thickness allowance must be made for the movement of survey markers due to firn compaction and down slope ice motion to allow for firn compaction very longpoles arc placed to a sufficient depth in the firn that the densification rate can be considered steady this assumption may be tested by measurements with poles set to different depths an analysis of errors in pilot studies indicates that the limit to precision is the determination of accumulation rate

interferometric imaging of a salt flank using walkaway vsp data vsp has long been a tool for investigating the location of salt flanks traditionally a salt proximity survey has been used to estimate the location of salt boundaries relative to a wellbore however this technique depends on accurate knowledge of the shape of the salt and the local sediment velocities in addition this method does not work well with complicated 3d salt structure beyond the traditional salt proximity survey many researchers have developed various methods and strategies for improving the locating and or imaging of salt flanks o brien et al 2002 brandsberg dahl et al 2003 zhao et al 2006 with these methods the quality of the resultant image depends on a prior knowledge of the possibly anisotropic overburden velocity field for very long offsets and also is concerned with multi pathing through salt invalidating a 2d assumption in looking for a simpler approach to this problem a single well imaging concept may be attractive using downhole sources and receivers it is possi

improving avhrr resolution through data cumulation for mapping polar ice sheets abstract data cumulation is a digital image processing technique in which several scenes of the same area taken under similar viewing conditions are combined into a single image of enhanced spatial and radiometric resolution this is effected by the reduction of aliasing in the along scan and along track sampling of scenes and by the reduction of noise content in the combined scene the technique hinges on two key steps accurate image coregistration to a precision significantly better than 1 pixel and cumulation of multiple image data grids derived from the individual images into a single image we report here on the development and initial results of the application of data cumulation using advanced very high resolution radiometer avhrr satellite images of the polar ice sheets the process can improve the real spatial resolution of avhrr data in the along scan direction from 1000 m to 650 m and in the along track direction from 1400 m to 700 m justifying a decrease in pixel size to about 500 m the technique is demonstrated in an image of a portion of the ross ice shelf in antarctica

seismic interferometry using multidimensional deconvolution and crosscorrelation for crosswell seismic reflection data without borehole sources crosswell reflection method is a high resolution seismic imaging method that uses recordings between boreholes the need for downhole sources is a restrictive factor in its application for example to time lapse surveys an alternative is to use surface sources in combination with seismic interferometry seismic interferometry si could retrieve the reflection response at one of the boreholes as if from a source inside the other borehole we investigate the applicability of si for the retrieval of the reflection response between two boreholes using numerically modeled field data we compare two si approaches crosscorrelation cc and multidimensional deconvolution mdd si by mdd is less sensitive to underillumination from the source distribution but requires inversion of the recordings at one of the receiver arrays from all the available sources we find that the inversion problem is ill posed and propose to stabilize it using singular value decomposition the results show that the reflections from deep boundaries are retrieved very well using both the cc and mdd methods furthermore the mdd results exhibit more realistic amplitudes than those from the cc method for downgoing reflections from shallow boundaries we find that the results retrieved from the application of both methods to field data agree well with crosswell seismic reflection data using borehole sources and with the logged p wave velocity

thermodynamics of basal freeze on predicting basal and subglacial signatures of stopped ice streams and interstream ridges we have constructed a numerical model that simulates the response of subglacial sediments to basal freeze on the model is set up to emulate the basal zone of drilling sites in the ross sea sector of thewest antarctic ice sheet we treatbasal freeze on atanice sedimentinterfaceasathermodynamicprocessthatcouplestheflowofwater heat and solutes inunfrozensubglacial sedimentsunderlyinga freezingicebase thecouplingof these flows occurs through the clapeyron equation which specifies the dependence of the basal freezing melting temperature on ice pressure water pressure solute concentration and surface tension effects thermally driven water flow is induced when an ice base becomes supercooled below the pressure melting point because ice water surface tension inhibits ice growth in small pore spaces of fine grained subglacial sediments our model results show that basal freeze on is capable of inducing considerable changes in the basal zone of both ice streams and interstream ridges these changes are associated with specific signatures that compare with borehole observations and geophysical surveys water pres sure levels are reduced and thick layers of debris laden basal ice develop these basal ice layers andunderlying sedimentscontain a distinct isotopic signal the predicted stable iso tope ratios reflect rayleigh type isotopic fractionation whose significance increases with increasing freezing rates supercooling of the ice base induces also measurable changes in the ice temperature profile of the glacier till porosity represents another quantity whose evolution is influenced strongly by basal freeze on in particular measurements of vertical porositydistributionbeneathstoppedicestreamscouldbeusedtoback calculatethetiming of the onsetof basal freezing our modelresults show thatthebasal zoneof ice streams and interstream ridges responds sensitively to changes inbasal melting freezing rates this sen sitivity may allow reconstruction of past conditions beneath ice streams and interstream ridges from measurements made on basal ice samples and subglacial sediment samples our model resultsalso indicate thatmeltwater from fast flowing ice streams may be driven towardsthe freezing icebase of interstream ridges

changes in the west antarctic ice sheet since 1963 from declassified satellite photography comparison of declassified satellite photography taken in 1963 with more recent satellite imagery reveals that large changes have occurred in the region where an active ice stream enters the ross ice shelf ice stream b has widened by 4 kilometers at a rate much faster than suggested by models and has decreased in speed by 50 percent the ice ridge between ice streams b and c has eroded 14 kilometers these changes along with changes in the crevassing around crary ice rise imply that this region s velocity field shifted during this century

point measurements of mass balance of the greenland ice sheet using precision vertical global positioning system gps surveys rates of ice sheet thickening or thinning in greenland are measured using the coffee can technique this entails computing the difference in the vertical velocity of markers anchored in firn and the long term rate of snow accumulation the velocities are obtained from repeat surveys using the global positioning system gps with corrections for densification and along slope flow this difference is the local mass balance for two sites in western greenland camp century and a site just south of the egig line crawford point results show ice sheet thinning a third west greenland site inland from upernavik is close to balance two sites dye 2 in western greenland and summit are thickening slowly

spatial patterns in mass balance of the siple coast and amundsen sea sectors west antarctica this is the publisher s version also available electronically from http www ingentaconnect com

present day antarctic ice mass changes and crustal motion the peak vertical velocities predicted by three realistic but contrasting present day scenarios of antarctic ice sheet mass balance are found to be of the order of several mm a one scenario predicts local uplift rates in excess of 5 mm a these rates are small compared to the peak antarctic vertical velocities of the ice 3g glacial rebound model which are in excess of 20 mm a if the holocene antarctic deglaciation history portrayed in ice 3g is realistic and if regional upper mantle viscosity is not an order of magnitude below 10 exp 21 pa s then a vast geographical region in west antarctica is uplifting at a rate that could be detected by a future global positioning system gps campaign while present day scenarios predict small vertical crustal velocities their overall continent ocean mass exchange is large enough to account for a substantial portion of the observed secular polar motion omega m bar and time varying zonal gravity field j sub 1

seasonal variations of glaciochemical isotopic and stratigraphic properties in siple dome antarctica surface snow six snow pit records recovered from siple dome west antarctica during 1994 are used to study seasonal variations in chemical major ion and h 202 isotopic deuterium and physical stratigraphic properties during the 1988 94 period comparison of d measurements and satellite derived brightness temperature for the siple dome area suggests that most seasonal sd maxima occur within 4 weeks of each 1 january several other chemical species h 2o2 non sea salt nss so 4 2 methanesulfonic acid and no3 show coeval peaks with sd together providing an accurate method for identifying summer accumulation sea salt derived species generally peak during winter spring but episodic input is noted throughout some years no reliable seasonal signal is identified in species with continental sources nssca 2 nss mg 2 nh 4 or nsscl visible strata such as large depth hoar layers 5 cm are associated with summer accumulation and its metamorphosis but smaller hoar layers and crusts are more difficult to interpret a multi parameter approach is found to provide the most accurate dating of these snow pit records and is used to determine annual layer thicknesses at each site significant spatial accumulation variability exists on an annual basis but mean accumulation in the sampled 10 km 2 grid for the 1988 94 period is fairly uniform

dating the siple dome antarctica ice core by manual and computer interpretation of annual layering the holocene portion of the siple dome antarctica ice core was dated by interpreting the electrical visual and chemical properties of the core the data were interpreted manuallyandwith acomputeralgorithm the algorithm interpretation was adjusted to be consistent with atmospheric methane stratigraphic ties to the gisp2 greenland ice sheet project 2 ice core 10 be stratigraphic ties to the dendrochronology 14 c recordandthedatedvolcanic stratigraphy thealgorithm interpretation ismorecon sistent andbetter quantifiedthanthe tedious and subjective manual interpretation

climate variability in west antarctica derived from annual accumulation rate records from itase firn ice cores thirteen annually resolved accumulation rate records covering the last 200 years from the pine island thwaites and ross drainage systems and the south pole are used to examine climate variability over west antarctica accumulation is controlled spatially by the topography of the ice sheet and temporally by changes in moisture transport and cyclonic activity a comparison of mean accumulation since 1970 at each site to the long term mean indicates an increase in accumulation for sites located in the western sector of the pine island thwaites drainage system accumulation is negatively associated with the southern oscillation index soi for sites near the ice divide and periods of sustained negative soi 1940 42 1991 95 correspond to above mean accumulation at most sites correlations of the accumulation rate records with sea level pressure slp and the soi suggest that accumulation near the ice divide and in the ross drainage system may be associated with the mid latitudes the post 1970 increase in accumulation coupled with strong slp accumulation rate correlations near the coast suggests recent intensification of cyclonic activity in the pine island thwaites drainage system

mass balance and accumulation rate across siple dome west antarctica snow accumulation rates and rates of ice thickness change mass balance are studied at several sites on siple dome west antarctica accumulation rates are derived from analyses of grossbeta radioactivity in shallow firn cores located along a 60 km transect spanning both flanks and the crest of the dome there is a north south gradient in snow accumulation rate across the dome that is consistent with earlier radar mappingof internalstratigraphy orographicprocessesprobablycontrolthisdistribution mass balance is inferred from the difference between global positioning system gps derived vertical velocities and snow accumulation rates for sites close to the firn core locations results indicatethatthere is virtually no netthicknesschangeat fourof thefive sites the exception is at the northernmost site where a small amount of thinning is detected that appearstobe inconsistentwith other studies a possiblecause ofthisanom alousthinning is recent retreatof the grounding line of ice stream d

inversion for salt flank geometry using transmitted p and s wave travel times accurate delineation of the salt flank is important for oil and gas exploration in areas with salt intrusion we describe the application of deformable layer tomography dlt to invert for the geometry of salt flank using travel times of transmitted p and s waves from surface sources to downhole receivers the dlt allows us to take advantage of a common situation that the salt velocity is known and generally invariant but we need to determine the variable geometry of salt flank we demonstrate our new method using a physical model experiment mimicking the setup of a walkaway vertical seismic profiling vsp survey we first use picked p wave arrivals to invert for the salt flank geometry the sinusoid shape of the salt flank is estimated fairly by the dlt due to the uneven p wave raypath coverage as an improvement we further incorporate the s wave arrivals in the dlt the picking of s wave arrivals is assisted with modeled s wave arrivals based on the p wave dlt model the dlt solution using both p and s wave arrivals delineates the salt flank geometry more accurately than that using p wave arrivals alone the salt flank delineation using dlt can complement with the conventional salt proximity and migration methods it could also serve as constraints or the initial model for the full waveform inversion of salt geometry

ultra high resolution snapshots of three multi decadal periods in an antarctic ice core we offer the first sub seasonal view of glacial age archives from the siple dome a sdma ice core using the ultra high resolution capabilities of a newly developed laser ablation inductively coupled plasma mass spectrometry la icp ms 121 m sampling resolution system capable of conducting multi element glaciochemical analysis our ultra high resolution data demonstrates that 1 the sdma ice core record can be annually dated based on seasonality in chemical inputs at a depth not previously possible using previous glaciochemical sampling methods 2 winter accumulation at the sd site was greater than summer accumulation during the three late glacial periods selected 15 3 17 3 21 4 ka ago in this study and 3 resulting annual layer thicknesses results show greater variability than the current sd ice core depth age model brook and others 2005 possibly due to depositional effects such as wind scouring and or decadal variability in snow accumulation that is not captured by the resolution of the current depth age model

evidence for late pleistocene thinning of siple dome west antarctica 1 we use a two dimensional full stress thermomechanical flow band model to investigate late pleistocene and holocene histories of ice thickness and ice flow at siple dome in west antarctica measurements obtained from an ice core and borehole near the ice divide and measurements of radar detected internal stratigraphy are used as targets for modeling models that account for the climate history accumulation rate and temperature alone yield a poor match to observations histories of ice dynamics ice thickness ice divide location and ice flow enhancement also need to be included our favored model includes 350 m of thinning at siple dome between 15 000 and 14 000 years ago onset of divide flow 3000 years ago ongoing divide migration starting 2500 years ago and evolution of ice fabric over the past 30 000 years the timing of thinning is earlier than that inferred from geologic evidence from the margins but is coincident with a 20 m rise in global sea level about 14 500 years ago meltwater pulse 1a this opens the possibility that thinning was a result of increased activity of the ice streams in the late pleistocene forced by meltwater pulse 1a the contribution to meltwater pulse 1a as a result of this thinning was likely small

sensitivity of the divide position at siple dome west antarctica to boundary forcing a linearized perturbation about a two dimensional vialov nye icesheet profile is used to investigate the sensitivity of the divide position at siple dome west antarctica to small changes in the accumulation pattern and in the elevation of its lateral boundaries at the margins of ice streams c and d relaxation time scales for the icesheet surface and divide position are derived from the perturbation theory for siple dome these time scales are short 450 800 years for surface adjustment and 200 350 years for divide position adjustment these short time scales indicate that siple dome responds quickly to forcing at its boundaries therefore the recent migration of the siple dome divide determined from previous work is probably a response to an ongoing sustained forcing rather than a response to a long past climate event such as the transition from the last glacial maximum to the holocene based on our analysis the inferred rate of migration of the siple dome divide could be attained be

elastic wavefield interferometry using p wave source vsps at the wamsutter field wyoming in this study elastic wavefield interferometry was used to recover p and s waves from the 3d p wave vibrator vsp data at wamsutter field in wyoming s wave velocity and birefringence is of particular interest for the geophysical objectives of lithology discrimination and fracture characterization in naturally fractured tight gas sand reservoirs because we rely on deconvolution interferometry for retrieving interreceiver p and s waves in the subsurface the output fields are suitable for high resolution local reservoir characterization in 1d media where the borehole is nearly vertical data at the stationary phase point is not conducive to conventional interferometry strong tube wave noise generated by physical sources near the borehole interfere with s wave splitting analyses also converted p to s wave ps wave polarity reversals occur at zero offset and cancel their recovery we developed methods to eliminate tube wave noise by removing physical sources at the stationary phase point and pertur

a high precision method for measurement of paleoatmospheric co2 in small polar ice samples we describe a high precision method now in use in our laboratory for measuring the co 2 mixing ratio of ancient air trapped in polar ice cores occluded air in ice samples weighing 8 15 g is liberated by crushing with steel pins at 35 c and trapped at 263 c in a cryogenic cold trap co 2 in the extracted air is analyzed using gas chromatography replicate measurements for several samples of high quality ice from the siple dome and taylor dome antarctic ice cores have pooled standard deviations of 0 9 ppm this high precision technique is directly applicable to high temporal resolution studies for detection of small co 2 variations for example co 2 variations of a few parts per million on millennial to decadal scales

a 12 000 year record of explosive volcanism in the siple dome ice core west antarctica 1 air mass trajectories in the southern hemisphere provide a mechanism for transport to and deposition of volcanic products on the antarctic ice sheet from local volcanoes and from tropical and subtropical volcanic centers this study extends the detailed record of antarctic south american and equatorial volcanism over the last 12 000 years using continuous glaciochemical series developed from the siple dome a sdma ice core west antarctica the largest volcanic sulfate spike 280 g l occurs at 5881 b c e other large signals with unknown sources are observed around 325 b c e 270 g l and 2818 b c e 191 g l ages of several large equatorial or southern hemisphere volcanic eruptions are synchronous with many sulfate peaks detected in the sdma volcanic ice chemistry record the microprobe fingerprinting of glass shards in the sdma core points to the following antarctic volcanic centers as sources of tephra found in the sdma core balenny island pleiades mount berlin mount takahe and mount melbourne as well as mount hudson and possibly mount burney volcanoes of south america identified volcanic sources provide an insight into the poorly resolved transport history of volcanic products from source volcanoes to the west antarctic ice sheet

extracting earth s elastic wave response from noise measurements recent research has shown that noise can be turned from a nuisance into a useful seismic source in seismology and other fields in science and engineering the estimation of the system response from noise measurements has proven to be a powerful technique to convey the essence of the method we first treat the simplest case of a homogeneous medium to show how noise measurements can be used to estimate waves that propagate between sensors we provide an overview of physics research dating back more than 100 years showing that random field fluctuations contain information about the system response this principle has found extensive use in surface wave seismology but can also be applied to the estimation of body waves because noise provides continuous illumination of the subsurface the extracted response is ideally suited for time lapse monitoring we present examples of time lapse monitoring as applied to the softening of soil after the 2011 tohoku oki earthquake the detection of a precursor to a landslide and temporal changes in the lunar soil

vertical strain measurements in firn at siple dome antarctica we measured vertical strain in the firn at siple dome antarctica using two systems both of which measure relative displacements over time of metal markers placed in an air filled borehole one system uses a metal detecting tuned coil and the other uses a video camera to locate the markers we compare the merits of the two systems we combine steady state calculations and a measured density profile to estimate the true vertical velocity profile this allows us to calculate a depth age scale for the firn at siple dome our steady state depth age scale has ages 10 15 younger at any given depth when compared to depth age scales derived by layer counting in a core 40 m away the age of a visible ash layer at 97 m in the core is 665 30 years in agreement with a similar analysis conducted at taylor dome antarctica where the same ash is also seen providing an additional dated tie point between the two cores

a record of atmospheric co2 during the last 40 000 years from the siple dome antarctica ice core 1 we have measured the co2 concentration of air occluded during the last 40 000 years in the deep siple dome a hereafter siple dome ice core antarctica the general trend of co2 concentration from siple dome ice follows the temperature inferred from the isotopic composition of the ice and is mostly in agreement with other antarctic ice core co2 records co2 rose initially at 17 5 kyr b p thousand years before 1950 decreased slowly during the antarctic cold reversal rose during the younger dryas fell to a local minimum at around 8 kyr b p and rose continuously since then the co2 concentration never reached steady state during the holocene as also found in the taylor dome and epica dome c hereafter dome c records during the last glacial termination a lag of co2 versus siple dome isotopic temperature is probable the siple dome co2 concentrations during the last glacial termination and in the holocene are at certain times greater than in other antarctic ice cores by up to 20 ppm mol co2 mol air while in situ production of co2 is one possible cause of the sporadic elevated levels the mechanism leading to the enrichment is not yet clear

changes in the configuration of ice stream flow from the west antarctic ice sheet surface based ice penetrating radar profiles on the northeast flank of siple dome support the hypothesis that a curvilinear scar first observed in advanced very high resolution radiometer satellite imagery represents the margin of a formerly active ice stream the scar defines the southwestern boundary of an ice stream flowing from ice stream c to ice stream d close to where it enters the ross ice shelf our studies show that the scar coincides with a trough and upward step in surface topography approximately 5 km across underlain by a zone of disturbed internal stratigraphy revealed by the radar burial depth of the disturbed zone enables us to calculate the time of shutdown as occurring prior to approximately 1 3 ka the configuration of the ice streams draining the west antarctic ice sheet into the ross ice shelf evidently changes with time and attempts to predict the evolution of the ice sheet must incorporate this observation

emplacement of the taupo ignimbrite by a dilute turbulent flow an ignimbrite is a pumice rich deposit that records the passage of a ground hugging ash flow a pyroclastic flow generated by the collapse of a volcanic eruption column1 geologists study such deposits to reconstruct the parent eruptions and to predict the consequences of future eruptions the 1 800 yr old taupo ignimbrite of new zealand has been interpreted to represent en masse emplacement by an avalanche like flow with a volumetric solids concentration in excess of 30 ref 2 the evidence for this is equivocal however and here we propose the alternative view that the deposit was emplaced by a relatively dilute and turbulent density current3 we present an isothermal hydraulic model the results of which taken together with existing observations suggest that the total flow rate of solids and gas was about 40km3s 1 for around 15 minutes this intense flux resulted in a flow which had a near vent solids concentration of 0 3 by volume was about 1 km thick and travelled outward from the vent with a typical speed of 200 ms 1 in view of the good agreement between predicted and observed radial trends in the taupo deposit we suggest that the origin of other ignimbrites with similar characteristics should be reconsidered

redatuming through a salt canopy and target oriented salt flank imaging we describe a new shortcut strategy for imaging the sediments and salt edge around a salt flank through an overburden salt canopy we tested its performance and capabilities on 2d synthetic acoustic seismic data from a gulf of mexico style model we first redatumed surface shots using seismic interferometry from a walkaway vertical seismic profile survey as if the source and receiver pairs had been located in the borehole at the positions of the receivers this process creates effective downhole shot gathers by completely moving surface shots through the salt canopy without any knowledge of overburden velocity structure after redatuming we can apply multiple passes of prestack migration from the reference datum of the bore hole in our example first pass migration using only a simple vertical velocity gradient model reveals the outline of the salt edge a second pass of reverse time prestack depth migration using full two way wave equation was performed with an updated velocity model that consisted of the velocity gradient and salt dome the second pass migration brings out dipping sediments abutting the salt flank because these reflectors were illuminated by energy that bounced off the salt flank forming prismatic reflections in this target oriented strategy the computationally fast redatuming process eliminates the need for the traditional complex process of velocity estimation model building and iterative depth migration to remove effects of the salt canopy and surrounding overburden this might allow this strategy to be used in the field in near real time

mitigation of defocusing by statics and near surface velocity errors by interferometric least squares migration with a reference datum abstractimaging seismic data with an erroneous migration velocity can lead to defocused migration images to mitigate this problem we first choose a reference reflector whose topography is well known from the well logs for example reflections from this reference layer are correlated with the traces associated with reflections from deeper interfaces to get crosscorrelograms interferometric least squares migration ilsm is then used to get the migration image that maximizes the crosscorrelation between the observed and the predicted crosscorrelograms deeper reference reflectors are used to image deeper parts of the subsurface with a greater accuracy results on synthetic and field data show that defocusing caused by velocity errors is largely suppressed by ilsm we have also determined that ilsm can be used for 4d surveys in which environmental conditions and acquisition parameters are significantly different from one survey to the next the limitations of ilsm are that it requires prior knowledge of