extracting time domain green s function estimates from ambient seismic noise 1 it has been demonstrated experimentally and theoretically that an estimate of the green s function between two seismic stations can be obtained from the time derivative of the long time average cross correlation of ambient noise between these two stations this tdgf estimate from just the noise field includes all tensor components of the green s function and these green s function estimates can be used to infer earth structure we have computed cross correlations using 1 to 30 continuous days of ambient noise recorded by over 150 broadband seismic stations located in southern california the data processing yielded thousands of cross correlation pairs for receiver separations from 4 500 km which clearly exhibit coherent broadband propagating dispersive wavetrains across frequency band 0 1 2 hz

an early holocene greenland whale from melville bugt greenland abstract radiocarbon age determination of a greenland whale balaena mysticetus vertebra from melville bugt in northwestern greenland yields an age of 9259 8989 cal yr bp the margin of the greenland ice sheet in melville bugt was situated behind its ad 1950 2000 position in the early holocene at a similar position to that being reached following rapid retreat in recent years such an early deglaciation of areas close to the greenland ice sheet is unusual this probably reflects the unique glaciological setting resulting from the narrow fringe of ice free islands and peninsulas and offshore waters with deep areas that characterize this part of greenland the timing of greenland ice sheet retreat to its present margin varies significantly around greenland

kinematic constraints on glacier contributions to 21st century sea level rise on the basis of climate modeling and analogies with past conditions the potential for multimeter increases in sea level by the end of the 21st century has been proposed we consider glaciological conditions required for large sea level rise to occur by 2100 and conclude that increases in excess of 2 meters are physically untenable we find that a total sea level rise of about 2 meters by 2100 could occur under physically possible glaciological conditions but only if all variables are quickly accelerated to extremely high limits more plausible but still accelerated conditions lead to total sea level rise by 2100 of about 0 8 meter these roughly constrained scenarios provide a most likely starting point for refinements in sea level forecasts that include ice flow dynamics

remote sensing of ocean wave spectra by interferometric synthetic aperture radar the two dimensional power spectra of ocean waves are of great interest not only to oceanographers but also in practical applications such as wave forecasting trans oceanic ship routing and design of coast and offshore installations remote sensing of ocean surface waves can be difficult using conventional synthetic aperture radar sar techniques but waves can be observed clearly by sar in the interferometric configuration insar 1 2 this improvement is due to the ability of insar to provide images of the local surface velocity field in contrast to conventional sar for which the imaging process is related indirectly to the complex modulation of the surface reflectivity by longer waves and currents here we show that insar can be used to obtain wavenumber spectra that are in agreement with power spectra measured in situ this new method thus has considerable potential to provide instantaneous spatial information about the structutre of ocean wave fields

shallow velocity structure of stromboli volcano italy derived from small aperture array measurements of strombolian tremor abstract the properties of the tremor wave field at stromboli are analyzed using data from small aperture arrays of short period seismometers deployed on the north flank of the volcano the seismometers are configured in two semi circular arrays with radii of 60 and 150 m and a linear array with length of 600 m the data are analyzed using a spatiotemporal correlation technique specifically designed for the study of the stationary stochastic wave field of rayleigh and love waves generated by volcanic activity and by scattering sources distributed within the island the correlation coefficients derived as a function of frequency for the three components of motion clearly define the dispersion characteristics for both rayleigh and love waves love and rayleigh waves contribute 70 and 30 respectively of the surface wave power the phase velocities of rayleigh waves range from 1000 m sec at 2 hz to 350 m sec at 9 hz and those for love waves range from 800 to 400 m sec over the same frequency band these velocities are similar to those measured near puu oo on the east rift of kilauea volcano hawaii although the dispersion characteristics of rayleigh waves at stromboli show a stronger dependence on frequency such low velocities are consistent with values expected for densely cracked solidified basalt the dispersion curves are inverted for a velocity model beneath the arrays assuming those dispersions represent the fundamental modes of rayleigh and love waves

beyond the spac method exploiting the wealth of circular array methods for microtremor exploration we explore the wealth of alternative methods for inferring phase velocities of rayleigh waves using vertical component seismograms of microtremors from a circular array of seismic sensors which are formulable along the extension of the popularly used spatial autocorrelation spac method four such methods are illustrated here the centerless circular array cca method the henstridge methods of the zeroth and first orders the h0 and h1 methods respectively and what we tentatively call the fifth v method different methods of phase velocity estimation have different wavelength ranges of good resolution implementation to field data from two sites reveals that the traditional spac method and the h0 method are both capable of producing reasonable estimates of rayleigh wave phase velocities within a relatively narrow range on the short wavelength side whereas the h1 method is valid in a relatively narrow range on the long wavelength side the cca and v methods both remain valid over a very broad range of wavelengths the upper limit extending as far up as several 10s of times the array radius use of a noise compensation technique can further prolong the maximum resolvable wavelength of the cca method we also illustrate the field performance of circle phase methods which give without recourse to the conventional frequency wavenumber analysis estimates for the principal arrival directions of rayleigh waves on the basis of circular array seismograms of microtremors

sea surface velocity vector retrieval using dual beam interferometry first demonstration the dual beam interferometer consists of two interferometric synthetic aperture radars insars one squinted at 20 spl deg forward of broadside and the other 20 spl deg aft to allow measurement of vector surface velocity with only a single aircraft pass estimates of surface velocity vectors in the coastal region during high tidal flow are presented the data were gathered over the barrier islands west of fort myers florida as part of a march 2004 deployment whereas no detailed bathymetry data were available high quality aerial photography appears to be a useful tool in inferring bottom topography and possible current obstructions the retrieved velocity field clearly follows the expected outflow pattern while comparisons with tidal current magnitudes predicted by the u s national ocean service do reveal discrepancies of up to 0 5 m s these differences are most likely due to the contribution of ocean surface waves to the overall insar velocity measurement velocity retrievals for the same area based on the data from different tracks show good consistency the results constitute the first demonstration of vector retrieval of the surface velocity field with a single pass insar system and confirm the robustness of the dual beam interferometry principle

energy density directional spectra of a nearshore wave field measured by interferometric synthetic aperture radar the nearshore wave field within monterey bay california is studied using remote sensing imaging by an interferometric synthetic aperture radar insar and simultaneous ground based measurements it is shown that insar imagery of the ocean surface offers some advantages over conventional synthetic aperture radar because of the direct imaging mechanism of insar quantitative information about the complicated wave field can be obtained the insar derived directional energy density spectra and the corresponding ground based spectra compare well

on the correlation of seismic microtremors 1 we present results of the analysis of microtremor measurements using a small array our analysis is based on the computation of cross correlation functions between stations in both frequency and time domains we obtain similar results in both domains and link those results to the application of aki s spatial autocorrelation method of using a single station pair and to recent studies that have shown that the green s function between two stations can be retrieved from the temporal cross correlation of seismic noise we show that the same simple subsoil structure allows interpretation of our correlation results in time and frequency we observe both love and rayleigh waves however love waves dominate the records in our lower frequency range between 3 6 and 6 hz while rayleigh waves are prevalent in the records in our higher frequency band from 6 to 20 hz frequency domain cross correlation yields better results in the lower frequency range while time domain cross correlation shows very clear results in the higher frequency band thus both analyses are complementary our results show that ambient vibration recorded at the free surface includes different types of waves but that the correlation between any two stations is governed by the more stable propagation mode between them surface waves in the case of a layered medium our results shed some light on the nature of microtremors and on the reasons why the spatial autocorrelation method or the horizontal to vertical spectral ratios are useful in geophysical and site response studies

remote sensing of ocean waves by along track interferometric synthetic aperture radar a new wave retrieval method for the along track interferometric synthetic aperture radar at insar phase image is presented the new algorithm named parametric retrieval algorithm pra uses the full nonlinear mapping relations it differs from previous retrieval algorithms in that it does not require a priori information about the sea state or the wind vector from scatterometer data instead it combines the observed at insar phase spectrum and assumed wind vector to estimate the wind sea spectrum the method has been validated using several c band and x band hh polarized at insar observations collocated with spectral buoy measurements in this paper x band and c band hh polarized at insar phase images of ocean waves are first used to study at insar wave imaging fidelity the resulting phase spectra are quantitatively compared with forward mapped in situ directional wave spectra collocated with the at insar observations subsequently we combine the parametric retrieval algorithm pra with x band and c band hh polarized at insar phase images to retrieve ocean wave spectra the results show that the ocean wavelengths wave directions and significant wave heights estimated from the retrieved ocean wave spectra are in agreement with the buoy measurements

holocene environmental variability in southern greenland inferred from lake sediments abstract sediments from qipisarqo lake provide a continuous holocene paleoenvironmental record from southern greenland following deglaciation and glacio isostatic emergence of the basin from the sea 9100 cal yr b p proxies of lake paleoproductivity including biogenic silica and organic matter increased markedly until 6000 cal yr b p and thereafter remained stable over the ensuing warm three millennia subsequent decreases in these proxies most dramatically between 3000 and 2000 cal yr b p show the lake s responses to initial neoglacial cooling intervals of ameliorated limnological conditions occurred between 1300 and 900 and between 500 and 280 cal yr b p briefly interrupting the decreasing trend in productivity that culminated in the little ice age increased lake productivity during the latter half of the 20th century which reflects the limnological response to circum arctic warming still has not reached peak holocene values the neoglacial climate of the last 2000 yr includes the most rapid high amplitude environmental changes of the past nine millennia the norse thus migrated around the north atlantic ocean region in the most environmentally unstable period since deglaciation lacustrine sediment records provide a context with which to consider future environmental changes in the labrador sea region in particular any future warming will be superposed on a regional climate system that is currently exhibiting highly unstable behavior at submillennial timescales

remote sensing of ocean waves the surface wave process program experiment an airborne synthetic aperture radar sar has been used as an interferometer to obtain direct calibrated measurements of the ocean wind wave directional spectrum flights over the same area from three different directions produced reasonably consistent results the sar derived wave data agree well with simultaneous observations by a conventional wave measuring system and with a novel in situ acoustic doppler system

the recession of the inland ice margin during the holocene climatic optimum in the jakobshavn isfjord area of west greenland recent subsurface mapping of parts of the greenland inland ice margin in the region of jakobshavn isbrae indicates that the fjord system in the period of at least 2700 4700 calendar yr b p was more ice free than at present and that the front of the glacier was at least 15 km behind the present position the 14c datings of subfossils brought to the present ice margin fit with the climatic records from ice cores and confirm the favourable conditions for greenland s first settlers the sarqaq people who arrived in the region about 4000 yr ago to find hunting grounds 10 20 larger than the present

foraminiferal reconstruction of mid to late holocene ocean circulation and climate variability in disko bugt west greenland benthic foraminifera from two cores collected from disko bugt a large marine embayment in west greenland are used to investigate hydrographic changes during the mid to late holocene the records document fluctuations in the west greenland current wgc linked to large scale changes in north atlantic circulation and for one core impacts of local climate changes in particular changes in the cold east greenland current egc and the warm more saline irminger current ic influence the wgc and propagate into disko bugt the cores record subsurface warming associated with the holocene thermal maximum htm from c 6 to 5 ka cal bp gradual cooling of wgc from 5 ka cal bp was interrupted by an abrupt warm interval from 3 9 to 3 5 ka cal bp before cooling became more prominent from 3 5 to 2 ka cal bp this cold period corresponds to the well documented northern hemisphere cooling of the neoglacial and we interpret it as reflecting enhanced flux from the egc and reduced ic flux to the wgc in disko b

shallow structure of deception island antarctica from correlations of ambient seismic noise on a set of dense seismic arrays summary we investigated the shallow velocity structure of deception island volcano antarctica using correlations of ambient seismic noise we selected long records of noise obtained by eight seismic arrays deployed along the inner coast of deception during the period 2003 2005 using these data we calculated average dispersion curves and estimated local 1 d velocity models for the array sites the combination of these profiles allowed us to obtain a comprehensive model of the shallow velocity structure of the island the volcano is composed of relatively soft layers of pyroclastic deposits and sediments extending to a depth of about 400 m with different degrees of compaction two layers with thicknesses of about 100 and 300 m and s wave velocities of around 0 2 0 8 and 0 7 1 1 km s 1 respectively can be differentiated the deeper structure is highly variable in terms of wave velocities and layer depths although the resolving capabilities are reduced for these layers the larger s wave velocities in the range 1 3 2 8 km s 1 indicate that they can be associated with pre caldera structures and products there are substantial differences between the different models which can be spatially related to heterogeneities in the volcano structure the lowest s wave velocities may be related to the alterations produced by hydrothermal activity near the surface on the contrary the largest velocities occur near the caldera border revealing the presence of compact materials at shallow depths sharp lateral variations can also be observed in the northwest of the bay which points to the presence of nw se faults and or strong velocity gradients

determination of elastic properties of shallow sedimentary deposits applying a spatial autocorrelation method abstract the capability of the spatial autocorrelation method for the determination of the elastic properties of shallow sedimentary deposits has been investigated at a test site in which the subsurface structure is known the technique has been shown very innovative and convenient for this kind of study as no artificial sources are needed and also overcomes the problems of soil invasive techniques the method uses the three components of the seismic microtremor recorded simultaneously at different configuration arrays with the possibility to obtain dispersion curves of rayleigh and love waves these curves together with the horizontal to vertical spectral ratio of the seismic microtremors can be used to calculate the s wave velocities of the sediments in order to test the goodness of this procedure a specific experiment was performed with real data the underground model obtained from this methodology is compared with the geotechnical information available our results are in a good agreement with those models obtained previously by s wave refraction experiments and borehole measurements

focal spot imaging based on zero lag cross correlation amplitude fields application to dense array data at the san jacinto fault zone we image the subsurface below a dense seismic array straddling the clark branch of the san jacinto fault zone in southern california the analysis is based on focal spots of surface waves associated with the zero lag amplitudes of noise cross correlations computed between all stations of the dense array local medium properties are inferred from the spatially variable focal spot size and shape based on the first zero crossing of amplitude versus distance distributions the method provides simultaneous estimates of wave speed apparent attenuation and anisotropy without solving a tomographic inverse problem the obtained images of the frequency dependent seismic velocity distributions are consistent with independent estimates from a far field rayleigh wave tomography we observe an anticorrelation between our apparent attenuation coefficient and seismic velocity and a fault parallel alignment of fast propagation directions with greater structural complexity to the southwest of the fault the results imply a complex fault zone structure including a waveguide to the northeast of the fault that is continuous across the observed depth range and a low velocity structure to the southwest associated with a shallow sedimentary basin

early holocene history of the west greenland ice sheet and the gh 8 2 event abstract the margin of the greenland ice sheet retreated rapidly during the first few thousand years of the holocene during this period of relative warmth known as the holocene thermal maximum ice core records identify a significant short lived cooling event at approximately 8 4 8 0 ka cal yr bp the gh 8 2 event associated with a 5 7 c fall in mean annual air temperature over the centre of the ice sheet in this paper we constrain the history of the ice sheet margin in disko bugt west greenland and that of a major ice stream jakobshavns isbrae during the early holocene which incorporates the interval of the gh 8 2 event our work is based on a new relative sea level curve and minimum age estimates for the timing of deglaciation from two field sites combined with a review of previously published research from the study area we identify important differences in the chronology of ice margin recession during the early holocene most noticeably the margin of jakobshavns isbrae retreated well inland of the adjacent ice sheet at this time we conclude that the early holocene fjord stade moraines in disko bugt do not record a uniform ice sheet margin response to the gh 8 2 event rather these moraines are diachronous and formed between c 10 8 ka cal yr bp their age varying as a function of the interplay between topography and ice sheet ice stream dynamics we hypothesise that one cause for the lack of an identifiable response to the gh 8 2 event is because topographic controls dominated ice sheet behaviour at this time in lowland areas any increase in ice sheet mass balance was probably associated with an increase in calving rather than any major advance of a grounded ice sheet margin

late holocene relative sea level rise and the neoglacial history of the greenland ice sheet in west greenland early and mid holocene relative sea level rsl fall was replaced by late holocene rsl rise during the neoglacial after 4 3 cal ka bp thousand calibrated years before present here we present the results of an isolation basin rsl study completed near to the coastal town of sisimiut in central west greenland rsl fell from 14 m above sea level at 5 7 cal ka bp to reach a lowstand of 4 0 m at 2 3 1 2 cal ka bp before rising by an equivalent amount to present differences in the timing and magnitude of the rsl lowstand between this and other sites in west and south greenland record the varied interplay of local and non greenland rsl processes notably the reloading of the earth s crust caused by a neoglacial expansion of the greenland ice sheet gis and the subsidence associated with the collapse of the laurentide ice sheet forebulge this means that the timing of the sea level lowstand cannot be used to infer directly when the gis advanced during the neoglacial the rise in late holocene rsl is contrary to recently reported bedrock uplift in the sisimiut area based on repeat gps surveys this indicates that a belt of peripheral subsidence around the current ice sheet margin was more extensive in the late holocene and that there has been a switch from subsidence to uplift at some point in the last thousand years or so

the two scale radar wave probe and sar imagery of the ocean a two scale formulation of oceanic backscatter of microwave radiation which has previously been applied to the dual frequency scatterometer is applied here to the two scale radar wave probe and sar imagery of the ocean a two scale radar wave probe is a microwave system which coherently detects signals scattered from a very small patch of the ocean surface it is shown that a sar image may be described as a convolution of the am part of the output of a two scale wave probe with the power spectrum of the fm part of the output thus sar images are not faithful reproductions of cross section variations over the surface distortions occur as a result of variable surface velocities examples of this distortion are derived from two scale wave probe data taken in the gulf of mexico application of a focusing correction does not remove the distortion from the simulated image of a nearly sinusoidal azimuth traveling wave large v r0 ratios minimize the distortion however

shear wave imaging from traffic noise using seismic interferometry by cross coherence we apply the cross coherence method to the seismic interferometry of traffic noise which originates from roads and railways to retrieve both body waves and surface waves our preferred algorithm in the presence of highly variable and strong additive random noise uses cross coherence which uses normalization by the spectral amplitude of each of the traces rather than crosscorrelation or deconvolution this normalization suppresses the influence ofadditive noise and overcomes problems resulting from amplitudevariations among input traces by using only the phase information and ignoring amplitude information the method effectively removes the source signature from the extracted response and yields a stable structural reconstruction even in the presence of strong noise this algorithm is particularly effective where the relative amplitude among the original traces is highly variable from trace to trace we use the extracted reflected shear waves from the traffic noise data to construct a stackedandmigratedimage andweusetheextractedsurfacewaves love waves to estimate the shear velocity as a function of depth this profile agrees well with the interval velocity obtained from the normal moveout of the reflected shear waves constructed by seismic interferometry these results are useful in a wide range of situations applicable to both geophysics and civil engineering

holocene records of effective precipitation in west greenland holocene changes in effective precipitation precipitation evaporation p e were reconstructed for the sondre stromfjord region of southwest greenland using the sediment records of two neighbourin

salt marshes as archives of recent relative sea level change in west greenland abstract temperate latitude salt marshes are a proven environment from which high precision 0 10 0 20 m relative sea level rsl records can be developed over recent decades and centuries such records provide an important link between millennial histories of rsl change and instrumental records the high latitude salt marshes in greenland have not previously been explored as potential archives of recent rsl change here we develop four diatom based transfer functions using contemporary diatom data collected from three salt marshes located 40 km south of the coastal town of sisimiut west greenland our preferred model has a good fit between observed and predicted elevations r 2 0 94 and a root mean square error prediction of 0 19 m we apply the four models to a short sediment profile collected from one of the marshes that formed between c 600 cal year bp and the present three of the four models predict the same trend in which rsl rose from 0 55 0 19 m mean tide level mtl to 0 05 0 19 m mtl between c 600 and 400 cal year bp at a rate of c 2 7 mm year 1 after 400 cal year bp rsl slowed and remained stable until the present day the results of this study demonstrate that greenland salt marshes are potentially valuable archives of data for developing quantitative estimates of rsl change during the last few centuries thereby bridging the gap between existing millennial scale approaches and more recent direct observations of ice sheet behaviour and associated vertical land motions

estimating ocean coherence time using dual baseline interferometric synthetic aperture radar a new technique for measuring the coherence time of the ocean surface at radar wavelengths has been developed and tested this technique requires an interferometric synthetic aperture radar system with at least two unique baselines along the direction of platform motion the coherence time of the surface may be presented as a high resolution coherence time map this technique was tested using the jpl airsar along track interferometer measurements of the ocean coherence at the l band were made at high spatial resolution under a variety of conditions a new operating technique that results in the required two baselines is described some parametric analysis is performed pertaining to the design of such a system a representative interferometric data set acquired at the strait of messina in 1991 is presented to illustrate the technique the interferometric phase data reveal and measure tidal currents flowing through the strait while the coherence time image shows considerable structure not visible in the backscatter or phase image coherence times in and around the strait were measured to be of the order of 0 1 s at the l band which is in agreement with previous estimates and measurements

characterization of shallow geology by high frequency seismic noise tomography summary to study the applicability of the passive seismic interferometry technique to near surface geological studies seismic noise recordings from a small scale 2 d array of seismic stations were performed in the test site of nauen germany rayleigh wave green s functions were estimated for different frequencies a tomographic inversion of the traveltimes estimated for each frequency from the green s functions is then performed allowing the laterally varying 3 d surface wave velocity structure below the array to be retrieved at engineering geotechnical scales furthermore a 2 d s wave velocity cross section is obtained by combining 1 d velocity structures derived from the inversion of the dispersion curves extracted at several points along a profile where other geophysical analyses were performed it is shown that the cross section from passive seismic interferometry provides a clear image of the local structural heterogeneities that are in excellent agreement with georadar and geoelectrical results such findings indicate that the interferometry analysis of seismic noise is potentially of great interest for deriving the shallow 3 d velocity structure in urban areas

interferometric ground roll removal attenuation of scattered surface waves in single sensor data land seismic data are contaminated by surface waves or ground roll these surface waves are a form of source generated noise and can be strongly scattered by near surface heterogeneities the resulting scattered ground roll can be particularly difficult to separate from the desired reflection data especially when this scattered ground roll propagates in the crossline direction we have used seismic interferometry to estimate scattered surface waves recorded during an exploration seismic survey between pairs of receiver locations where sources and receivers coincide these interreceiver surface wave estimates were adaptively subtracted from the data this predictive subtraction process can successfully attenuate scattered surface waves while preserving the valuable reflected arrivals forming a new method of scattered ground roll attenuation we refer to this as interferometric ground roll removal

holocene boreal molluscs in greenland palaeoceanographic implications abstract boreal molluscs now extinct in greenland waters occur as subfossils in west greenland between lat 65 30 and 68 30 n a number of new localities for these warmth demanding molluscs are described and the list of species now comprises the gastropod emarqinula fissura and the bivalvalves heteranomia squamula modiolus arctica islandica panopea norvegica and zirphaea crispata during their time the molluscs formed an isolated northern outpost located in the area which as a result of the complex oceanic circulation today has greenland s highest ocean surface temperatures the occurrences are 14c dated to the interval 8400 4900 yr b p and summer surface temperatures were 1 3 c higher than now for its beginning this period coincides with similar marine optimal periods in east greenland and on the baffin island shelf whereas it took 1000 years before the beginning of warmer than present temperatures on land on the basis of this evidence it is concluded that the rise of sea surface temperatures was afforded by a decrease in inflow of cold polar water and or increase in inflow of warm atlantic water into davis strait rather than climatic change

theory for synthetic aperture radar imaging of the ocean surface with application to the tower ocean wave and radar dependence experiment on focus resolution and wave height spectra a one dimensional model is developed to simulate azimuthal synthetic aperture radar sar imaging of the ocean surface the model developed here is general and can admit both the distributed surface and the velocity bunching approaches computer simulations are performed to test the validity of this model the simulations show that the time dependent modulation patterns due to the radar cross section variation and the velocity bunching effects give optimum focusing around half the phase velocity of the long wave it is shown that the focus dependence is due to an incoherent integration process and not due to motion induced phase errors it is argued that the radar reflectivity is correlated over distances small in comparison with the sar resolution in the simulations the radar reflectivity is assumed to be spatially decorrelated also given are the relative importance of the various effects that degrade the azimuthal resolution it is shown that the limited radar temporal coherence due to the velocity spread of short waves degrades the azimuthal resolution of the sar this coherence time depends on sea state and radar wavelength it is shown that the orbital acceleration effects are small in comparison with the smearing due to the velocity spread it is found that the degraded azimuthal resolution of the radar minimizes the nonlinear effects of velocity bunching the simulations show that in the tower ocean wave and radar dependence experiment toward sar imaging at l band is reasonably linear

implications of increased greenland surface melt under global warming scenarios ice sheet simulations abstract the greenland ice sheet is likely to make a faster contribution to sea level rise in a warming world than previously believed based on numerical modelling using a parameterization of recent results showing surface meltwater lubrication of ice flow zwally et al science 297 557 2002 218 documented correlation between increased ice velocity and increased surface melt as parameterized by positive degree days they argued that surface water is piped directly to the bed with little delay causing increased basal water pressures and basal sliding velocities an effect not included in recent greenland ice sheet models known to the authors using the pennsylvania state university university of chicago thermomechanical flowline model numerous simulations were conducted to test a wide range of parameter space linking surface melt with a new sliding law based on the zwally et al data under three different global warming scenarios 2 co2 4 co2 and 8 co2 comparisons to reconstructions generated with a traditional sliding parameterization illustrate an enhanced sensitivity of the ice sheet to surface warming resulting in higher ablation rates additional thinning and retreat of the margin and a reduction in ice volume leading to an increased contribution to global sea level rise

three dimensional shallow structure from high frequency ambient noise tomography new results for the mygdonia basin euroseistest area northern greece we examine the use of ambient noise cross correlation tomography for shallow site characterization using a modified two step approach initially we extract rayleigh wave traveltimes from correlation traces of vertical component seismic recordings from a local network installed in mygdonia basin northern greece the obtained rayleigh wave traveltimes show significant spatial variability as well as distance and frequency dependence due to the 3 d structure of the area dispersion and anelastic attenuation effects the traveltime data sets are inverted through a surface wave tomography approach to determine group velocity maps for each frequency the proposed tomographic inversion involves the use of approximate fresnel volumes and interfrequency smoothing constraints to stabilize the results in the last step we determine a final 3 d velocity model using a node based monte carlo 1 d dispersion curve inversion the reliability of the final 3 d velocity model is examined by spatial and depth resolution analysis as well as by inversion for different model parameterizations the obtained results are in very good agreement with previous findings from seismic and other geophysical methods the new 3 d vs model provides additional structural constraints for the shallow sediments and bedrock structure of the northern mygdonia basin up to the depth of 200 250 m present work results suggest that the migration of ambient tomography techniques from large scales tens or hundreds of km to local scales few hundred meters is possible but cannot be used as a black box technique for 3 d modeling and detailed geotechnical site characterization

an observation of the directional wave spectrum evolution from shoreline to fully developed abstract the surface contour radar scr is a 36 ghz computer controlled airborne system which produces ocean directional wave spectra with much higher angular resolution than pitch and roll buoys scr observations of the evolution of the fetch limited directional wave spectrum are presented which indicate the existence of a fully developed sea state the jonswap wave growth model for wave energy and frequency was in best agreement with the scr measurements the model of donelan et al correctly predicted the propagation direction of waves in the asymmetrical fetch situation nearshore the donelan et al parameterization is generalized to permit other growth algorithms to predict the correct direction of propagation in asymmetrical fetch situations

fluctuations of local glaciers in greenland during latest pleistocene and holocene time abstract this paper is the first to summarize research on fluctuations of local glaciers in greenland e g ice caps and mountain glaciers independent of the greenland ice sheet during latest pleistocene and holocene time in contrast to the extensive data available for fluctuations of the greenland ice sheet surprisingly little data exist to constrain local glacier extents much of the available research was conducted prior to wide spread use of ams radiocarbon dating and the advent of surface exposure and luminescence dating although there is a paucity of data generally similar patterns of local glacier fluctuations are observed in all regions of greenland and likely reflect changes in paleoclimate which must have influenced at least the margins of the inland ice absolute age data for late glacial and early holocene advances of local glaciers are reported from only two locations disko island and the scoresby sund region subsequent to late glacial or early holocene time most local glaciers were smaller than at present or may have disappeared completely during the holocene thermal maximum in general local glacier advances that occurred during historical time 1200 1940 ad are the most extensive since late glacial or early holocene time historical documents and more recent aerial photographs provide useful information about local glacier fluctuations during the last 100 yrs in all but one area north greenland local glaciers are currently receding from historical extents

shallow structure of mt vesuvius volcano italy from seismic array analysis data from a portable dense seismic array deployed on mt vesuvius in may 1994 during a 2d seismic tomography experiment are analyzed in the present paper the array consisted of two groups of short period geophones 4 5 hz natural frequency formed by 16 and 25 vertical components plus two horizontal components distributed along an arc like shape along the summit crater stacks of later arrivals interpreted as reflected phases provide a significant constraint to a boundary layer located in the depth range 1 5 2 2 km beneath the summit crater with average velocity v 1 8 2 2 km s interpreted as the top of the limestone basement the correlation methods applied on microtremor records allowed to infer the shallow velocity structure up to 400 m beneath the crater rim

progressive increase in ice loss from greenland 1 laser altimeter measurements over greenland show increasing thickening rates above 2000 m reflecting increasing snowfall in a warming climate but near coastal thinning rates have increased substantially since the mid 1990s and net mass loss more than doubled from an average of 4 50 gt yr 1 between 1993 4 and 1998 9 to 57 105 gt yr 1 between 1998 9 and 2004 this increasing trend is very similar to findings from independent mass budget studies but differs widely from ers radar altimeter results this may result from limitations associated with the large ers footprint over sloping and undulating surfaces that typify fast narrow glaciers where thinning is most pronounced

on the impact of an offshore bathymetric anomaly on surf zone rip channels 1 we use a nonlinear morphodynamic model to demonstrate that the presence of a single persistent offshore bathymetric anomaly strongly affects the formation nonlinear evolution and saturation of surf zone rip channels in the case of an offshore bump or trough and waves with oblique incidence a rip channel shoreward of the anomaly is enforced by the more seaward alongshore variability in depth the degree of rip channel enforcement is controlled by the strength of the rotational nature of surf zone rip current circulations which is in turn driven by differential broken wave energy dissipation induced by wave refraction across the offshore bathymetric anomaly the alongshore location of this forced rip channel is more stable with increasing offshore anomaly amplitude decreasing offshore wave obliquity and decreasing bathymetric anomaly distance to the shore simulations show that rip channel behavior downdrift and updrift of the offshore perturbation are different in our numerical experiments downdrift rip channels have systematically larger alongshore scales smaller alongshore migration rates and more erosive megacusps than those updrift rip channels therefore self organize into patterns of different alongshore scales and migration rates as a result of an alongshore perturbation in the wave forcing enforced by wave refraction across an offshore bathymetric anomaly these simulations are qualitatively corroborated by video observations of sandbar behavior during a down state sequence at a site with a persistent offshore trough

analysis of radar sea return for breaking wave investigation 1 low grazing angle backscattering data collected by a coherent dual polarized radar installed on a fixed tower in the ocean are analyzed to investigate the properties of sea spikes attributable to wave breaking the distribution of breaking wave speed is narrow banded with an average speed between 2 0 and 2 6 m s in mixed seas with wind speeds between 7 and 14 5 m s the corresponding breaking wavelength is between 2 5 and 4 3 m the length or velocity scale of wave breaking is not proportional to the length or velocity scale of the dominant wave this observation reflects the localized nature of the breaking process and may have significant implications on quantifying various breaking properties such as the energy dissipation or area of turnover by breaking waves the fraction of sea spike coverage generally increases with wind speed but the trend of increase is modified by the intensity and relative direction of background swell parameterizations of sea spike coverage needs to take into consideration both wind and wave factors similarities and differences between sea spikes and whitecaps are discussed in particular while both quantities show a similar power law dependence on wind speed the fraction of sea spike coverage is considerably higher than that of whitecap coverage this result reflects the prevalence of steep features that produce quasi specular facets and short scale waves bounded to intermediate waves during breaking these quasi specular facets and bound waves contribute significantly to enhancing the radar sea return but may not entrain air to produce whitecap signature

seismic hazard and ground motion characterization at the itoiz dam northern spain this paper presents a new hazard consistent ground motion characterization of the itoiz dam site located in northern spain firstly we propose a methodology with different approximation levels to the expected ground motion at the dam site secondly we apply this methodology taking into account the particular characteristics of the site and of the dam hazard calculations were performed following the probabilistic seismic hazard assessment method using a logic tree which accounts for different seismic source zonings and different ground motion attenuation relationships the study was done in terms of peak ground acceleration and several spectral accelerations of periods coinciding with the fundamental vibration periods of the dam in order to estimate these ground motions we consider two different dam conditions when the dam is empty t 0 1 s and when it is filled with water to its maximum capacity t 0 22 s additionally seismic hazard analysis is done for two return periods 975 years related to the project earthquake and 4 975 years identified with an extreme event soil conditions were also taken into account at the site of the dam through the proposed methodology we deal with different forms of characterizing ground motion at the study site in a first step we obtain the uniform hazard response spectra for the two return periods in a second step a disaggregation analysis is done in order to obtain the controlling earthquakes that can affect the dam subsequently we characterize the ground motion at the dam site in terms of specific response spectra for target motions defined by the expected values sa t of t 0 1 and 0 22 s for the return periods of 975 and 4 975 years respectively finally synthetic acceleration time histories for earthquake events matching the controlling parameters are generated using the discrete wave number method and subsequently analyzed because of the short relative distances between the controlling earthquakes and the dam site we considered finite sources in these computations we conclude that directivity effects should be taken into account as an important variable in this kind of studies for ground motion characteristics

dating of the narssarssuaq stade in southern greenland following the holocene thermal maximum the greenland ice sheet began to expand and many local glaciers formed the maximum extent of most outlet glaciers from the inland ice was reached around 100 to 200 years ago however some exceptions have been recorded in southern greenland the neoglacial narssarssuaq stade marks an older re advance that exceeded the historical advance the dating of the narssarssuaq stade has been much debated here we present new radiocarbon dates from a sediment core collected from a small lake within the moraines formed during this stade a minimum age for the narssarssuaq stade of around 1200 yr bp is suggested from the dates

ocean surface features and currents measured with synthetic aperture radar interferometry and hf radar this paper describes the results of the first quantitative comparison between high resolution ocean surface current fields extracted from interferometric synthetic aperture radar insar measurements and those from a high frequency hf ocean surface current radar oscr system data from each of these radar systems along with supporting measurements from shipboard and buoy mounted sensors were collected during the high resolution remote sensing experiment high res on june 20 1993 on the continental shelf off the coast of cape hatteras north carolina both components of the surface current were obtained from the insar system at roughly 10 m resolution from two orthogonal flight legs over the region separated in time by about 30 min the oscr system measured two dimensional surface current vectors at about 1 km resolution over this same region while the usns bartlett was collecting hydrographic samples and near surface current measurements two dimensional wave spectra as well as meteorological and additional current measurements were collected at two buoys within the experimental area we discuss in the paper two techniques for eliminating the effect of surface wave motion on the insar current estimates one method relies on some knowledge of the local wind and wave field and the use of a microwave scattering model the other method makes use of a few in situ current measurements spaced at different range locations across the insar image using either of these techniques we find that the agreement between the insar and oscr current estimates is generally very good furthermore the insar current and magnitude imagery show the presence of undulating surface features where abrupt changes in the current speed and direction occurred the ship surveys indicate that these features were caused by the collision of water masses of different density we show for the first time in this paper a high resolution area extensive vector surface current map derived from the insar of the two dimensional flow in the vicinity of these features our results demonstrate convincingly that high resolution oceanic surface current vectors can be derived from insar current measurements and that these measurements may be very beneficial for detailed studies of the dynamics of small scale surface features in regions of strong current divergences or shears

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

characteristics of seismic waves composing hawaiian volcanic tremor and gas piston events observed by a near source array a correlation method specifically designed for describing the characteristics of a complex wave field is applied to volcanic tremor and gas piston events recorded by a semicircular array of geos instruments set at the foot of the puu oo crater on the east rift of kilauea volcano hawaii the spatial patterns of correlation coefficients obtained as functions of frequency for the three components of motion over the entire array are similar for gas piston events and tremor and clearly depict dispersive waves propagating across the array from the direction of puu oo the wave fields are composed of comparable amounts of rayleigh and love waves propagating with similar and extremely slow phase velocities ranging from 700 m s at 2 hz to 300 m s at 8 hz the highly cracked solidified lava flow on which the array was deployed and subjacent structure of alternating lava and ash layers formed during repeated eruptions of puu oo since 1983 appear to be responsible for the low velocities observed the results from puu oo stand in sharp contrast to those obtained in an experiment conducted in 1976 on the partially solidified lava lake of kilauea iki rayleigh waves were not observed in kilauea iki but well developed trains of love waves were seen to propagate there with velocities twice as high as those observed near puu oo these differences in the propagation characteristics of surface waves at the two sites may be attributed to the presence of a soft horizontal layer of molten rock in kilauea iki which may have lowered the phase velocity of rayleigh waves more drastically than that of love waves resulting in severe scattering of the rayleigh wave mode on the other hand the thin superficial pahoehoe flow under our array at puu oo may have favored the development of vertical columnar joints more extensively at this location than at kilauea iki which may have reduced the shear moduli controlling the love wave mode the average phase velocities in the frequency band from 2 to 5 hz found at puu oo are roughly similar to those determined for tremor at klyuchevskoy volcano kamchatka but the frequency dependence appears much stronger at puu oo

using proglacial threshold lakes to constrain fluctuations of the jakobshavn isbrae ice margin western greenland during the holocene abstract the future response of the greenland ice sheet gis and its potential contribution to sea level rise are uncertain rapid changes of greenland s outlet glaciers over the past decade have made it difficult to extrapolate ice sheet change into the future this significant short term variability highlights the need for longer term geologic e g holocene records of ice margin fluctuations however a major challenge with reconstructing the gis during the holocene stems from it having been smaller than it is at present thus traditional glacial geologic approaches are not suitable we use radiocarbon dated sediment sequences from seven proglacial threshold lakes spanning 50 km of the western gis margin near jakobshavn isbrae to constrain the timing of early holocene deglaciation the duration that this sector of the western gis was smaller than its present configuration and the timing of its advance during neoglaciation our reconstructions suggest deglaciation 7300 cal yr bp minimum ice extent 6000 5000 cal yr bp and smaller than present ice configuration until at least 2300 cal yr bp for the ice margin south of jakobshavn isbrae and until 400 cal yr bp for the ice margin north of jakobshavn isbrae one relatively large proglacial lake that became briefly ice free during the middle holocene lies in a catchment that likely extends 10s of km inland beneath the gis suggesting significant middle holocene retreat of this portion of the gis the overall pattern of ice sheet change is inconsistent with existing ice sheet model reconstructions for this region but is consistent with numerous paleoclimate proxy and relative sea level data these continuous lacustrine records corroborate but provide closer age control than existing non continuous records of radiocarbon dated reworked bivalves from historical moraines in the region reconstructing ice margin change from proglacial threshold lakes is one of few approaches with the potential to constrain smaller than present ice sheet extent

the correlation of microtremors empirical limits and relations between results in frequency and time domains summary the use of the correlation of microtremor records is on its way to develop into a common tool to estimate local shear wave velocity structure for this reason the establishment of the conditions for the correct use of this method and its limitations when applied to real data is becoming increasingly important in addition to the use of frequency domain spatial correlation technique the spatial autocorrelation spac method the use of time domain correlation to obtain the green s function of the medium is rapidly gaining presence we explore the use of microtremor correlation techniques in the time domain to determine local velocity structure and compare with previous results obtained with the same data using spac our data come from three experiments carried out in parkway and wainuiomata valleys in new zealand using broad band portable stations interstation distances range from 5 m to 2 1 km and our results are useful in the frequency band from 0 1 to 7 hz frequency domain correlation requires an isotropic microtremor field a condition that need not be satisfied in the time domain two station correlations provide useful results due to the temporal stationarity and isotropy in average of the microtremor wavefield this manifests itself in the symmetry of the temporal correlation functions with respect to zero time our results show that the local velocity structure and the interstation distance are the key factors conditioning the frequency range where surface wave dispersion can be correctly measured either in frequency or time domains we confirm that when the interstation distance becomes much larger than the dominant wavelengths only the correlation in time domain is useful all our results indicate that the signal obtained in the correlation of vertical component microtremors is due to the fundamental mode of rayleigh waves which appears as the most stable propagation mode without any indication of body waves

interferometric radar measurement of ocean surface currents we describe a new method of measuring surface currents using an interferometric synthetic aperture radar an airborne implementation has been tested over the san francisco bay near the time of maximum tidal flow resulting in a map of the east west component of the current only the line of sight component of velocity is measured by this technique where the signal to noise ratio was strongest statistical fluctuations of less than 4 cm s 1 were observed for ocean patches of 60 60 m

a new method to determine phase velocities of rayleigh waves from microseisms we have developed a new method to determine phase velocities from the vertical component of microseisms recorded with an array of seismic sensors spaced around the circumference of a circle we calculate two different time histories by taking the average of the seismograms with differing sets of weights for the sensor stations the spectral ratio of these two time histories contains no information on the arrival directions or on the amplitudes of the incoming waves but depends solely on the phase velocities of the arriving modes theoretical considerations indicate that the effects of directional aliasing caused by the use of a finite number of sensors in the field implementation of our method are small in most situations except for short wavelengths the presence of incoherent noise limits the efficacy of our method for long wavelengths in field tests using arrays of three seismic sensors we obtained appropriate estimates of phase velocities in the wavelength range from 5r to 30r where r the array radi

surface wave tomography from ambient seismic noise of accelerograph networks in southern korea 1 an efficient and robust cross correlation technique for estimation of surface wave green s function between two locations is applied to a set of ambient noise data recorded by accelerograph networks in the southern korea we have estimated 1843 group velocities of rayleigh waves in the period band of 2 4 seconds from correlation pairs of 91 accelerograph stations relatively dense and uniform coverage of the pairs provides us with high resolution tomographic image of surface wave group velocities in the region the resulting group velocity distribution correlates well with the characteristic features of geological map of the region displaying low velocities for the sedimentary basins and regions of volcanic and metasedimentary rocks and high velocities for the regions where granitoid basement rocks outcrop the short period information from the accelerograph network can enhance resolution of the subsurface structures at depths within a few kilometers which may be predominantly responsible for simulation results of strong ground motions

mass balance of the greenland ice sheet at dye 3 the mass balance of the greenland ice sheet a t dye 3 is estimated on the basis of 0 bserva tions of ice thickness accumulation rate surface velocities and surface strain rates the calculations indicate a rate of increase of surface elevation of 3 cm year with 95 confidence limits of 3 cm year and 9 cm year previous estimates of the mass balance of the greenland ice sheet by the same method reported large imbalances these are most probably due to lack of precise data and the use of quantities measured at the surface as representative of depth averaged quantities the most reliable observations indicate that the interior regions of the greenland ice sheet are at present thickening at a rate of a few centimetres per year a contributing cause for this may be the slow thinning of a bottom layer of relatively soft wisconsin ice resume bilan de masse de l lndlandsis groenlandais a dye 3 le bilan de masse est estime it partir des mesures d epaisseur taux d accumulation vitesses et vitesses de deformation superficielles le calcul don ne un epalss lssement de 3 cm an 1 avec un intervalle de confiance au seuil de 95 allant de 3 cm an 1 it 9 cm an i les estimations anterieures du bilan de l lndlandsis obtenues par la meme methode sont examinees on en deduit que les valeurs de bilan tres eloignees de l equilibre sont certainement dues au manque de donnees preclses et ou a i utilisation de donnees superficielles comme valeurs moyennes sur une

last interglacial arctic warmth confirms polar amplification of climate change the warmest millennia of at least the past 250 000 years occurred during the last interglaciation when global ice volumes were similar to or smaller than today and systematic variations in earth s orbital parameters aligned to produce a strong positive summer insolation anomaly throughout the northern hemisphere the average insolation during the key summer months m j j was ca 11 above present across the northern hemisphere between 130 000 and 127 000 years ago with a slightly greater anomaly 13 over the arctic greater summer insolation early penultimate deglaciation and intensification of the north atlantic drift combined to reduce arctic ocean sea ice allow expansion of boreal forest to the arctic ocean shore across vast regions reduce permafrost and melt almost all glaciers in the northern hemisphere insolation amplified by key boundary condition feedbacks collectively produced last interglacial summer temperature anomalies 4 5 c above present over most arctic lands significantly above the average northern hemisphere anomaly the last interglaciation demonstrates the strength of positive feedbacks on arctic warming and provides a potentially conservative analogue for anticipated future greenhouse warming

a comparison of radiated energy from diamond impregnated coring and reverse circulation percussion drilling methods in hard rock environments abstractwe compared and analyzed hard rock drilling elastic energy emission between diamond impregnated coring and reverse circulation rc percussion drilling methods from an experiment at brukunga south australia the two drilling mechanisms generated very different seismic wavefields this comparison emphasized their energy radiation differences and signal characteristics from the field data by investigating the raw data energy frequency analysis and crosscorrelation tests the seismic records from percussive rc drilling provided a strong indication that the drill bit energy could be suitable for drill bit seismic imaging purposes the energy radiation from the percussive rc bit could also provide high resolution images with borehole seismic acquisition in contrast at comparable drilling conditions the coring drilling using a diamond impregnated coring bit was quiet the radiated acoustic energy from this drilling mechanism was difficult to detect by a surface receiver array and there was no co

extracting coherent wave fronts from acoustic ambient noise in the ocean a method to obtain coherent acoustic wave fronts by measuring the space time correlation function of ocean noise between two hydrophones is experimentally demonstrated though the sources of ocean noise are uncorrelated the time averaged noise correlation function exhibits deterministic waveguide arrival structure embedded in the time domain green s function a theoretical approach is derived for both volume and surface noise sources shipping noise is also investigated and simulated results are presented in deep or shallow water configurations the data of opportunity used to demonstrate the extraction of wave fronts from ocean noise were taken from the synchronized vertical receive arrays used in the frame of the north pacific laboratory npal during time intervals when no source was transmitting

early holocene palaeoceanography and deglacial chronology of disko bugt west greenland two new cores from disko bugt a large marine embayment in west greenland are used to assess the deglacial chronology and palaeoceanography of the area during the early holocene deglaciation of the main part of disko bugt is directly dated for the first time to a minimum of ca 10 2 ka cal bp by this time the jakobshavn isbrae ice stream had retreated to the eastern margins of disko bugt initial deglaciation took place before the relatively warm west greenland current wgc was initiated a strengthening in the wgc is identified from ca 9 2 ka cal bp accompanied by retreat of the ice stream to a shallow bank immediately west of the present day jakobshavn isfjord while in this position the calving and melting ice stream produced large volumes of sediment and meltwater the large volumes of meltwater prevented the relatively warm waters of the wgc reaching the eastern margins of disko bugt the ice stream remained grounded on this bank for over a thousand years until ca 7 9 ka cal bp when reduced sedimentation rates and an increase in boreal fauna point to its retreat inside the present day fjord system

airborne measurements of the wavenumber spectra of ocean surface waves part i spectral slope and dimensionless spectral coefficient an airborne scanning lidar system acquires 3d spatial topography of ocean surface waves from the spatial data wavenumber spectra are computed directly the spectral analyses of two distinctively different wave fields are presented the first one is a quasi steady wave field under active wind generation and the second one is a decaying wave field following a slackening of the wind field subtle differences in different representations of the one dimensional spectrum omnidirectional marginal and traverse are illustrated the spectral properties in terms of the dimensionless spectral coefficient and spectral slope in the equilibrium range are investigated using the wavenumber spectra directly computed from the 3d topography of the ocean surface the results are in excellent agreement with existing data the rapid data acquisition afforded by an airborne system provides an enhanced capability for studying the spatial variation of a wave field with minimal temporal changes in the environmental forcing conditions the data of the 3d surface topography are also ideal for the quantitative investigation of the directional properties of a random wave field

an experimental investigation of wave measurements using a dual beam interferometer gulf stream as a surface wave guide 1 a dual beam interferometric synthetic aperture radar measures remotely two radial components of the ocean surface current from a single flight pass combining two passes over the same area all three orthogonal components of the surface velocity can be retrieved an experiment is conducted near the gulf stream gs boundary a sharp change of the surface velocity of about 1 m s over a 500 m lateral distance is measured the wind and wave condition is dominated by a 14 s swell system and low wind velocity the wave variance inside gs is about twice the wave variance outside the gs in the present data set the difference in the wave variance is considerably higher than that can be expected from wave current interaction an ocean current system with strong shears such as the gs is a wave guide and can trap waves with the right combinations of wavelengths and propagation directions numerical calculations suggest that the wave properties of the data set may satisfy the conditions of wave trapping by the gs the standing wave pattern on the gs side of the sharp velocity front indicative of the long swell bouncing off the current front also offers support for the wave guide hypothesis in this respect the gulf stream can be considered the nature s hydraulic breakwater that can attenuate about 50 of the incident wave energy generated by storms its role in protecting the u s coastlines in the atlantic ocean cannot be overstated

theory of synthetic aperture radar ocean imaging a marsen view this paper reviews basic synthetic aperture radar sar theory of ocean wave imaging mechanisms using both known work and recent experimental and theoretical results from the marine remote sensing marsen experiment several viewpoints that have contributed to the field are drawn together in a general analysis of the backscatter statistics of a moving sea surface a common focus for different scattering models is provided by the mean image impulse response function which is shown to be identical to the spatially varying frequency variance spectrum of the local complex reflectivity coefficient from the analysis has emerged a more complete view of the sar imaging phenomenon than has been previously available a new generalized imaging model is proposed

anelastic earth structure from the coherency of the ambient seismic field 1 cross correlation of the ambient seismic field is now routinely used to measure seismic wave travel times however relatively little attention has been paid to other information that could be extracted from these signals in this paper we demonstrate the relationship between the spatial coherency of the ambient field and the elastodynamic green s function in both time and frequency domains through measurement of the frequency domain coherency as a function of distance we sequentially recover phase velocities and attenuation coefficients from these measurements we generate 1 d shear wave velocity and attenuation models for southern california the ambient field measurements of attenuation and the exceptional path coverage that results from the many possible interstation measurements allow us to extend q estimates over a range of frequencies that has previously been difficult to analyze using earthquake data measurements from paths that cross major sedimentary basins show both lower wave speeds and lower quality factors than other paths as expected our results indicate that there is a wealth of information available in the spatial coherency of the ambient seismic field

directional wave spectra measured with the surface contour radar abstract the surface contour radar is a 36 ghz computer controlled airborne radar which generates a false color coded elevation map of the sea surface below the aircraft in real time and can routinely produce ocean directional wave spectra with post flight data processing which has much higher angular resolution than pitch and roll buoys when compared with waveriders and the xerb and edeco pitch and roll buoys there is good agreement among the nondirectional spectra there is also good agreement among the angles associated with a1 b1 and a2 b2 fourier coefficients of the spreading function for xerb endeco and the surface contour radar there are indications that the pitch and roll buoys in this study may have calibration problems with the magnitudes of the fourier coefficients of the spreading function and that the radar system determines the fourier coefficients with significantly less noise and bias the high spatial resolution and rapid mapping capability over extensive areas make the surface

wind wave effects on estuarine turbulence a comparison of observations and second moment closure predictions abstractobservations of turbulent kinetic energy dissipation and turbulent stress were collected in the middle reaches of chesapeake bay and were used to assess second moment closure predictions

large fluctuations in speed on greenland s jakobshavn isbrae glacier it is important to understand recent changes in the velocity of greenland glaciers because the mass balance of the greenland ice sheet is partly determined by the flow rates of these outlets jakobshavn isbrae is greenland s largest outlet glacier draining about 6 5 per cent of the ice sheet area and it has been surveyed repeatedly since 1991 ref 2 here we use remote sensing data to measure the velocity of jakobshavn isbrae between 1992 and 2003 we detect large variability of the velocity over time including a slowing down from 6 700 m yr 1 in 1985 to 5 700 m yr 1 in 1992 and a subsequent speeding up to 9 400 m yr 1 by 2000 and 12 600 m yr 1 in 2003 these changes are consistent with earlier evidence for thickening of the glacier in the early 1990s and rapid thinning thereafter our observations indicate that fast flowing glaciers can significantly alter ice discharge at sub decadal timescales with at least a potential to respond rapidly to a changing climate

shear wave velocity structure around teide volcano results using microtremors with the spac method and implications for interpretation of geodetic results deformation analysis and simulation of volcanic edifices require the construction of models of elastic properties of those structures in this paper we present an analysis of microtremor measurements recorded during the performance tests of two temporary seismic arrays installed in the eastern portion of the teide caldera in 1994 we take advantage of recent developments of the spac method and use spatial cross correlation computations to estimate phase velocity dispersion of rayleigh waves at the location of the arrays we show that the extension of the standard spac method is valid in the case of our data justifying its use and supporting the generalization of the spac method to single station pairs the phase velocity dispersion curve obtained was inverted to recover the shear wave profile at the site of the arrays our results indicate that the subsoil structure of the caldera is laterally homogeneous at the scale of a few km about the location of the arrays we obtained about 315 m of volcanic sediments overlying rocks with a shear wave velocity of 2 km s these results are robust and are a starting point to further modelling of deformation permanent or transient at this volcanic edifice which can be useful in the interpretation of different observed fields in fact the computation of deformations and gravity changes due to possible volcanic intrusions in two models one considering the volcanic sediments and the other without considering them provided different results in the near field

remote sensing of ocean currents a method of remotely measuring near surface ocean currents with a synthetic aperture radar sar is described the apparatus consists of a single sar transmitter and two receiving antennas the phase difference between sar image scenes obtained from the antennas forms an interferogram that is directly proportional to the surface current the first field test of this technique against conventional measurements gives estimates of mean currents accurate to order 20 percent that is root mean square errors of 5 to 10 centimeters per second in mean flows of 27 to 56 centimeters per second if the full potential of the method could be realized with spacecraft then it might be possible to routinely monitor the surface currents of the world s oceans

using diffuse field theory to interpret the h v spectral ratio from earthquake records in cibeles seismic station mexico city abstract it has been recently demonstrated that averaging the autocorrelations of fields produced by various almost vertical incoming elastic body plane waves upon a layered system approximately leads to the imaginary part of the corresponding 1d green s functions for deep sources located underneath the receiver kawase et al 2011 thus the ensemble of these waves from deep earthquakes recorded in a station located in the epicentral zone is interpreted as a diffuse field in this short note we extend the study to consider earthquakes recorded in a station located at epicentral distances of up to hundreds of kilometers we consider the horizontal to vertical spectral ratio hvsr of the averaged p s and coda waves and full earthquake records at the cibeles station mexico city accelerometric network and compare these with the results obtained with the corresponding hvsr for the 1d kawase et al 2011 and the 3d sanchez sesma rodriguez et al 2011 diffuse fields models using the signals of 90 earthquakes recorded at cibeles we find that the experimental results have distinctive features compatible with the 3d signature of a diffuse field we interpret this result as a consequence of the multiple paths that seismic waves undergo from the subducting slab to the mexico city valley and to the multiple scattering in a complex tectonic environment our study strongly suggests that we can use strong motion records from earthquakes and apply similar techniques to the ones used to analyze the ambient seismic field online material earthquake catalog

calibrating a glaciological model of the greenland ice sheet from the last glacial maximum to present day using field observations of relative sea level and ice extent we constrain a three dimensional thermomechanical model of greenland ice sheet gris evolution from the last glacial maximum lgm 21 ka bp to the present day using primarily observations of relative sea level rsl as well as field data on past ice extent our new model huy2 fits a majority of the observations and is characterised by a number of key features i the ice sheet had an excess volume relative to present of 4 1 m ice equivalent sea level at the lgm which increased to reach a maximum value of 4 6 m at 16 5 ka bp ii retreat from the continental shelf was not continuous around the entire margin as there was a younger dryas readvance in some areas the final episode of marine retreat was rapid and relatively late c 12 ka bp leaving the ice sheet land based by 10 ka bp iii in response to the holocene thermal maximum htm the ice margin retreated behind its present day position by up to 80 km in the southwest 20 km in the south and 80 km in a small area of the northeast as a result of this retreat the modelled ice sheet reaches a minimum extent between 5 and 4 ka bp which corresponds to a deficit volume relative to present of 0 17 m ice equivalent sea level our results suggest that remaining discrepancies between the model and the observations are likely associated with non greenland ice load differences between modelled and observed present day ice elevation around the margin lateral variations in earth structure and or the pattern of ice margin retreat

mass balance of the greenland ice sheet from 1958 to 2007 we combine estimates of the surface mass balance smb of the greenland ice sheet for years 1958 to 2007 with measurements of the temporal variability in ice discharge d to deduce the total ice sheet mass balance during that time period we find a robust correlation r2 0 83 between anomalies in smb and in d which we use to reconstruct a continuous series of total ice sheet mass balance we find that the ice sheet was losing 110 70 gt yr in the 1960s 30 50 gt yr or near balance in the 1970s 1980s and 97 47 gt yr in 1996 increasing rapidly to 267 38 gt yr in 2007 multi year variations in ice discharge themselves related to variations in smb cause 60 20 more variation in total mass balance smb and therefore dominate the ice sheet mass budget copyright 2008 by the american geophysical union

azimuth falloff effects in two antenna sar measurements of ocean wave spectra estimates of ocean wave spectra obtained from conventional synthetic aperture radar sar images suffer from the azimuth falloff effect which limits the range of azimuth wavenumbers that can be observed by such systems this effect was earlier predicted to be modified by the use of a second receive antenna which is displaced from the first in the along track direction the authors present experimental evidence which is consistent with these theoretical predictions and suggests that the azimuth falloff limitations inherent in conventional sar systems can be overcome by the use of two or more appropriately spaced antennas

are microtremors useful in site response evaluation we have reviewed the applicability of microtremor measurements to evaluate site response of soft soils to this end we evaluated three different techniques commonly used to estimate site effects from microtremor measurements interpretation of fourier amplitude spectra computation of spectral ratios relative to a firm reference station and finally computation of spectral ratios of horizontal components relative to the vertical component of ground motion nakamura s technique these techniques are applied to microtremor records obtained in three cities in mexico mexico city oaxaca and acapulco these cities differ in their local geological conditions and in their seismotectonic environment in order to evaluate the results obtained from microtremor measurements we compare them with standard spectral ratios of the intense s wave part of weak or strong motion records obtained at the same sites our results showed that microtremor measurements can be used to estimate the dominant period of a site with very acceptable reliability in the range 0 3 to 5 hz the best results were obtained with nakamura s technique which also gives a rough estimate of amplification of seismic waves when the local geology is relatively simple simple numerical simulations indicate that the underlying assumptions of nakamura s technique are consistent with the propagation of rayleigh waves these simple numerical simulations also explain why different researchers have been able to successfully characterize 1d site effects using microtremor records regardless of whether they consider microtremors to consist of surface or body waves our results strongly suggest that the technique by nakamura effectively compensates for source effects in microtremor measurements which eliminates a major limitation to their application in earthquake engineering

improved ocean surface velocity precision using multi channel sar this paper investigates new approaches to estimating the motion of the dynamic ocean surface using a multi channel synthetic aperture radar msar with m phase centers arranged in an along track configuration the objective of this paper is to determine the processing methods that produce the finest velocity resolution an issue that arises due to the finite coherence time of radar backscatter produced by the sea surface the investigation is carried out both theoretically using synthesized data produced from a modeled msar covariance matrix as well as experimentally using images collected with a 16 channel system three processing methods are considered linear regression along with a multi baseline phase progression estimation of the velocity centroid and coherent averaging of the shortest baseline interferograms both the theoretical and experimental results indicate that simple averaging of the shortest baseline interferograms often produces the best velocity precision

three dimensional shear wave velocity imaging by ambient seismic noise tomography summary 3 d shear wave velocity images are of particular interest for engineering seismology to obtain information about the local subsoil structure we present a one step inversion procedure based on the computation of high frequency correlation functions between stations of a small scale array deployed for recording ambient seismic noise the calculation of rayleigh wave phase velocities is based on the frequency domain spatial autocorrelation technique constitutively a tomographic inversion of the traveltimes estimated for each frequency is performed allowing the laterally varying 3 d surface wave velocity structure below the array to be retrieved we test our technique by using simulations of seismic noise for a simple realistic site and by using real world recordings from a small scale array performed at the nauen test site germany the results imply that the cross sections from passive seismic interferometry provide a clear image of the local structural heterogeneities and the shear wave velocities are satisfactorily reproduced the velocity structure is also found to be in good agreement with the results of geoelectrical measurements indicating the potential of the method to be easily applied for deriving the shallow 3 d velocity structure in urban areas and for monitoring purposes

development of crescentic bars for a periodically perturbed initial bathymetry 1 the development of crescentic bed patterns starting from an undisturbed beach i e one only perturbed so as to simulate background noise and thereby initiate morphological development is compared with that starting from a beach where monochromatic and in some cases bichromatic bed patterns pre exist using a fully non linear model morfo55 a wide range of different lengthscales and amplitudes is investigated the findings suggest that the pre existence of crescentic bed forms can influence the subsequent morphological development of this beach significantly but that this development is related to the undisturbed beach development whether a pre existing bed form remains under certain forcing conditions depends on the position of the pre existing lengthscale along the undisturbed linear growth rate curve if the pre existing mode shows significant linear growth in the undisturbed scenario this pre existing lengthscale remains otherwise the pre existing mode will be replaced by a bed form with a lengthscale closer to the undisturbed fastest growing mode a pre existing lengthscale that is significantly longer than the undisturbed fastest growing mode will be replaced by a higher harmonic of the original lengthscale for shorter pre existing lengthscales this will be the linear fastest growing mode an increased amplitude of the pre existing bed form accelerates the development toward a new stable situation for small initial amplitudes the initial response of the system to pre existing bed forms could theoretically be described using linearized equations alone the migration rates of both the pre existing and newly arising modes correspond to the migration rate of these modes in the undisturbed scenario

mapping surface currents and waves with interferometric synthetic aperture radar in coastal waters observations of wave breaking in swell dominant conditions airborne and spaceborne interferometric synthetic aperture radars insars produce surface velocity measurements at very high spatial resolutions over a large area the data allow construction of the velocity strain field for highlighting ocean surface processes such as wave breaking and rip currents also coherence between signals from two interferometric channels is a descriptor of the correlation condition of the surface roughness that scatters back the radar signals and it is an indication of the ocean surface turbulence wave breaking is a major turbulence source causing surface roughness decorrelation thus the coherence parameter serves as an independent means for detecting wave breaking the results of breaking detection using roughness decorrelation and critical local acceleration are comparable in this paper the breaking fraction in swell dominant mixed seas along a cross shore transect is compared with several steepness parameters characterizing different length scales of surface waves the highest correlation coefficient from 0 90 to 0 99 is between the breaking fraction and windsea mean square slope contributed primarily by short waves this result reinforces the previous field observations showing that the length scales of breaking waves are much shorter than the energetic components near the spectral peak although dominant waves and the associated wave group modulation are important in triggering the breaking process the large spatial coverage of airborne or spaceborne operation further offers the opportunity to investigate evolution of the surface wave spectrum in high spatial subkilometer resolution this capability is very useful for monitoring the coastal wave and current environment

mass changes of the greenland and antarctic ice sheets and shelves and contributions to sea level rise 1992 2002 changes in ice mass are estimated from elevation changes derived from 10 5 years greenland and 9 years antarctica of satellite radar altimetry data from the european remote sensing satellites ers 1 and 2 for the first time the dh dt values are adjusted for changes in surface elevation resulting from temperature driven variations in the rate of firn compaction the greenland ice sheet is thinning at the margins 42 2g t a 1 below the equilibrium line altitude ela and growing inland 53 2g t a 1 above the ela with a small overall mass gain 11 3g t a 1 0 03 mm a 1 sle sea level equivalent the ice sheet in west antarctica wa is losing mass 47 4g t a 1 and the ice sheet in east antarctica ea shows a small mass gain 16 11 gt a 1 for a combined net change of 31 12 gt a 1 0 08 mm a 1 sle the contribution of the three ice sheets to sea level is 0 05 0 03 mm a 1 t he antarctic ice shelves show corresponding mass changes of 95 11 gt a 1 in wa and 142 10 gt a 1 in ea thinning at the margins of the greenland ice sheet and growth at higher elevations is an expected response to increasing temperatures and precipitation in a warming climate the marked thinnings in the pine island and thwaites glacier basins of wa and the totten glacier basin in ea are probably ice dynamic responses to long term climate change and perhaps past removal of their adjacent ice shelves the ice growth in the southern antarctic peninsula and parts of ea may be due to increasing precipitation during the last century

holocene climate inferred from glacier extent lake sediment and tree rings at goat lake kenai mountains alaska usa lake sediment glacier extent and tree rings were used to reconstruct holocene climate changes from goat lake at 550 m asl in the kenai mountains south central alaska radiocarbon dated sediment cores taken at 55 m water depth show glacial lacustrine conditions until about 9500 cal yr bp followed by organic rich sedimentation with an overall increasing trend in organic matter and biogenic silica content leading up to the little ice age lia through most of the holocene the northern outlet of the harding icefield remained below the drainage divide that currently separates it from goat lake a sharp transition from gyttja to inorganic mud about ad 1660 signifies the reappearance of glacier meltwater into goat lake during the lia marking the maximum holocene postglacial extent meltwater continued to discharge into the lake until about ad 1900 a 207 yr tree ring series from 25 mountain hemlocks growing in the goat lake watershed correlates with other regional tree ring series that indicate an average summer temperature reduction of about 1 c during the 19th century compared with the early mid 20th century cirque glaciers around goat lake reached their maximum lia extent in the late 19th century assuming that glacier equilibrium line altitudes ela are controlled solely by summer temperature then the cooling of 1 c combined with the local environmental lapse rate would indicate an ela lowering of 170 m in contrast reconstructed elas of 12 cirque glaciers near goat lake average only 34 18 m lower during the lia the restricted ela lowering can be explained by a reduction in accumulation season precipitation caused by a weakening of the aleutian low pressure system during the late lia copyright 2008 john wiley sons ltd

glaciers dominate eustatic sea level rise in the 21st century ice loss to the sea currently accounts for virtually all of the sea level rise that is not attributable to ocean warming and about 60 of the ice loss is from glaciers and ice caps rather than from the two ice sheets the contribution of these smaller glaciers has accelerated over the past decade in part due to marked thinning and retreat of marine terminating glaciers associated with a dynamic instability that is generally not considered in mass balance and climate modeling this acceleration of glacier melt may cause 0 1 to 0 25 meter of additional sea level rise by 2100