the influence of fluid sensitive dispersion and attenuation on avo analysis summary analysis of seismic data suggests that hydrocarbon deposits are often associated with higher than usual values of attenuation but this is generally ignored during amplitude versus offset avo analysis the effect can be modelled with equivalent medium theory based on the squirt flow concept but the excess attenuation is associated with strong velocity dispersion consequently when we study reflections from the interface between such an equivalent medium and an elastic overburden we find that the reflection coefficient varies with frequency the impact of this variation depends on the avo behaviour at the interface class i reflections tend to be shifted to higher frequency while class iii reflections have their lower frequencies amplified we calculate synthetic seismograms for typical models using the reflectivity method for materials with frequency dependent velocities and attenuations and find that these effects are predicted to be detectable on stacked data two field data sets show frequency anomalies similar to those predicted by the analysis and we suggest that our modelling provides a plausible explanation of the observations

the influence of fluid type on elastic wave velocity and attenuation in volcanic rocks abstract volcano seismic signatures are the convolution of source and path effects the source signature provides important information about the dynamic rock deformation and fluid resonance processes within the volcano but path effects are the result of the volcanic plumbing the edifice physical conditions and the complex geological structure complicating the inversion of seismic data further is the influence of fluid properties here we use ultrasonic experiments to understand the effect of variable fluid viscosity and bulk modulus on p wave attenuation and velocity for volcanic rocks from white island volcano whakaari new zealand our experimental conditions for high porosity ash tuff and low porosity lava resemble shallow and deep volcano facies respectively p wave velocity vp and quality factor qp increase with the viscosity and bulk modulus of the saturating fluid and decrease with porosity and fracture volume tuffs are highly sensitive to saturating fluid with variations in vp of up to 67 and qp of 383 vp and qp in fractured lavas are slightly less sensitive to fluids 24 and 296 respectively but vp can be used to identify lithology for these tuff vs lavas comparison of the experimental data with rock physics models suggests that squirt flow is the dominant attenuation mechanism for viscosities greater than that of water while scattering losses may be important in gas saturated rocks our results have significant implications for the interpretation of seismic tomography studies and for volcano hazard monitoring based on the characteristics of detected seismicity

numerical analysis of velocity dispersion in multi phase fluid saturated porous rocks seismic waves are subject to velocity dispersion when they propagate in fluid saturated porous media in this work we explore the velocity dispersion behavior of p and sv waves in multi phase fluid saturated porous reservoirs while taking into account the effects of multi phase pore fluids on the effective viscosities that control the wave induced fluid flow the effective viscosities associated with the hydrocarbon saturation of a synthetic sandstone reservoir saturated with different pore fluid mixtures are calculated using the refutas model we then analyze the frequency dependent velocity dispersion variation rate and characteristic frequency for different fluid saturation cases by employing chapman s dynamic equivalent medium theory the results demonstrate that the hydrocarbon proportions and types in multi phase mixed pore fluids significantly affect the magnitude and characteristic frequencies of velocity dispersion features for both the p and s waves the dispersion anomalies of sv waves are in general larger than those of the p waves this indicates that the velocity dispersion anomalies of sv waves are equally sensitive to fluid saturation as the p waves and should not be neglected the velocities at lower frequencies e g 10 and 100 hz within the seismic frequency range show a more remarkable decrease with increasing hydrocarbon proportion than those at higher frequency 1000 hz the numerical examples help to improve the understanding of the frequency dependent avo inversion from seismic reflection data

dispersions and attenuations in a fully saturated sandstone experimental evidence for fluid flows at different scales dispersion and attenuation of elastic waves have been observed in crustal rocks existing theories and experimental measurements evidenced the existence of different energy loss mechanisms in rocks fully saturated by a newtonian fluid one major candidate is fluid flow at different scales separating three regimes the drained undrained and unrelaxed regimes here two elastic transitions between these three regimes are investigated both the cause and the consequence of the drained undrained transition are evidenced because the second transition measured occurs at higher frequency where no global fluid flow occurs it in turn is associated to the undrained unrelaxed transition or squirt flow for both transitions the amount of open microcracks seems to be the major contributor to the magnitudes of attenuation

effects of contact line movement on the dissipation of waves in partially saturated rocks a model is presented to describe the attenuation of seismic waves in a partially saturated rock in particular we include the dissipation due to contact line movement in some cases this can dominate the dissipation mechanisms the model includes both dissipation due to viscosity and contact line movement this additional dissipation can increase the specific dissipation function by an order of magnitude this could account for some of the observed difference between theory and experiment an exact calculation of the dissipation due to contact line movement in an in viscid fluid is also presented

slow waves in fractures filled with viscous fluid stoneley guided waves in a fluid filled fracture generally have larger amplitudes than other waves and therefore their properties need to be incorporated in more realistic models in this study a fracture is modeled as an infinite layer of viscous fluid bounded by two elastic half spaces with identical parameters for small fracture thickness i obtain a simple dispersion equation for wave propagation velocity this velocity is much smaller than the velocity of a fluid wave in a biot type solution in which fracture walls are assumed to be rigid at seismic prospecting frequencies and realistic fracture thicknesses the stoneley guided wave has wavelengths on the order of several meters and an attenuation q factor exceeding 10 which indicates the possibility of resonance excitation in fluid bearing rocks the velocity and attenuation of stoneley guided waves are distinctly different at low frequencies for water and oil the predominant role of fractures in fluid flow at field scales is supported by permeability data showing an increase of several orders of magnitude when compared to values obtained at laboratory scales these data suggest that stoneley guided waves should be taken into account in theories describing seismic wave propagation in fluid saturated rocks

anelasticity and dispersion in dry unconsolidated sands laboratory measurements reveal frame modulus dispersion and creeping behavior in room dry samples of unconsolidated quartzo feldspathic upper miocene turbidites from the wilmington field ca a single standard linear solid model predicts both the dispersion and the creep response similar behavior is observed in ottawa sand samples when mixed with wetted montmorillonite in this case the magnitude of dispersion and creep increase with increasing clay content we suggest that the anelastic behavior of the wilmington sands is due to the presence of micas and a small amount of clay the transition frequency of 2 10 4hz i e between high and low frequency behavior suggests that seismic sonic and ultrasonic measurements of rock moduli are quite similar in the absence of fluids the static frame modulus should be used to model tectonic processes and long term production related changes in oil field environments

holocene climate change and sea level oscillations in the pacific coast of mexico abstract a 2 75 m long core was recovered from la joya lagoon at the southern pacific coast of mexico the base of the core gave an age of ca 5600 a bp the ams 14c dating results indicate a continuously depositional history over the past 5600 years with varied sedimentation rates of 0 022 0 08 cm yr sediment samples were analyzed for pollen diatoms loss on ignition loi magnetic susceptibility and elemental analysis to study the environmental and marine changes ocurred during the holocene in the study site of the pacific coast our data provided indications of the paleoenviromental changes in the study site from the south pacific coast of mexico a complete marine environment from ca 5600 to ca 3200 a bp a transitional lagoon from ca 3200 to 1100 a bp and followed by a shift toward a close lagoon with fresh water during the last 1100 years the pollen data the presence of pollen from cucurbitaceae and secondary taxa related to disturbed vegetation of la joya record show evidence of agricultural activities and human occupation in the study area might exist 5000 a bp

characteristics of the impedance variation in clastic rock reservoirs and lithology interpretation method for the threshold volume abstract an analysis of the petrophysical characteristics of a reservoir demonstrated that the variations in wave velocity were controlled by sedimentary and diagenetic differences resulting in regular variations in the acoustic impedance the sedimentary differences are mainly associated with the grain size sorting and shale content and lead to a slow horizontal change in the wave impedance of the reservoir the diagenetic differences are associated with compaction pressure dissolution and cementation and lead to a rapid vertical change in the wave impedance of the reservoir the classic single threshold method only resolved the vertical variation in the acoustic impedance as a result the threshold volume method is proposed the results of a case study demonstrate that the proposed method yields a more accurate and reliable interpretation of lithology using acoustic impedance than the single threshold method in particular the method effectively solves the uncertainty in the sand interpretation using the plane variation of impedance caused by the deposit

anisotropy of experimentally compressed kaolinite illite quartz mixtures the anisotropy of physical properties is a well known characteristic of many clay bearing rocks this anisotropy has important implications for elastic properties of rocks and must be considered in seismic modeling preferred orientation of clay minerals is an important factor causing anisotropy in clay bearing rocks such as shales and mudstones that are the main cap rocksofoilreservoirs thepreferredorientationofclaysdepends mostly on the amount of clays and the degree of compaction to studytheeffectoftheseparameters wepreparedseveralsamples compressingattwoeffectiveverticalstressesamixtureofclays illite and kaolinite and quartz silt with different clay quartz ratios the preferred orientation of the phases was quantified with rietveld analysis on synchrotron hard x ray images pole figures for kaolinite and illite display a preferred orientation of clay platelets perpendicular to the compaction direction increasing in strength with clay content and compaction pressure quartz particles have a random orientation distribution aggregate elastic properties can be estimated by averaging the singlecrystal properties over the orientation distribution obtained from the diffraction data analysis calculated p wave velocity anisotropy ranges from 0 pure quartz sample to 44 pure clay sample highly compacted but calculated velocities are much higher than measured velocities this is attributed to uncertainties about single crystal elastic properties and oriented micropores and limited grain contacts that are not accounted for in themodel inthiswork wepresentaneffectivemethodtoobtain quantitative data helping to evaluate the role of clay percentage and compaction pressure on the anisotropy of elastic properties ofclay bearingrocks

experimental mechanical compaction of reconstituted shale and mudstone aggregates investigation of petrophysical and acoustic properties of sw barents sea cap rock sequences abstract this study investigates petrophysical and acoustic properties of experimentally compacted reconstituted samples of seal sequences from the southwestern barents sea the aggregates were collected from drill cuttings of mudstone and shale formations of two exploration wells 7220 10 1 salina discovery and 7122 7 3 goliat field the washed and freeze dried samples were characterized for grain size distributions geochemical analyses and mineralogical compositions a total of 25 compaction tests 12 dry and 13 brine saturated were performed with a maximum effective vertical stress of 50 mpa the laboratory measurements demonstrated that petrophysical and acoustic properties of argillaceous sediments can change within a sedimentary basin and even within a given formation the results show that the collected aggregates from goliat field are compacted more compared to salina discovery the maximum and minimum compaction are measured in samples collected from snadd and fuglen formations respectively the final porosity of brine saturated specimens varies between 5 and 22 the ultrasonic velocity measurements depict that samples with the same porosity values can have a broad range of velocity values the resulting compaction trends in this study were compared to published compaction curves for synthetic mixtures of quartz and clay all compaction trends show higher porosity reduction than the silt fraction with 100 quartz comparison of experimental compaction result of each mudstone and shale aggregate with its corresponding acquired well log data helps to delineate the burial history and exhumation in the study area a net exhumation of 950 m and 800 m is estimated at salina and goliat wells respectively the outcomes of this study can provide insights for hydrocarbon prospect discovery in a pre mature sedimentary basin in terms of exploration and production and also for geological co2 storage sites the experimental results may provide information for well log and seismic interpretation basin modeling and seal integrity of investigated horizons

persistently well ventilated intermediate depth ocean through the last deglaciation during the last deglaciation 18 11 thousand years ago existing radiocarbon 14c reconstructions of intermediate waters in the mid to low latitude oceans show widely diverging trends with some broadly tracking the atmosphere and others suggesting extreme depletions these discrepancies cloud our understanding of the deglacial carbon cycle because of the diversity of hypotheses needed to explain these diverging records for example injections of 14c dead geological carbon mixing of extremely isolated waters from the abyssal ocean or changes in sites of deep water ventilation here we present absolutely dated deglacial deep sea coral 14c records of intermediate waters from the galapagos platform close to the largest reported deglacial 14c depletions together with data from the low latitude atlantic our records indicate coherent well equilibrated intermediate water 14c ventilation in both oceans relative to the atmosphere throughout the deglaciation the observed overall trend towards 14c enriched signatures in our records is largely due to enhanced air sea carbon isotope exchange efficiency under increasing atmospheric p rm co 2 these results suggest that the 14c depleted signatures from foraminifera are likely sedimentary rather than water mass features and provide tight 14c constraints for modelling changes in circulation and carbon cycle during the last deglaciation intermediate depth waters in the atlantic and pacific oceans were well equilibrated with the atmosphere through the last deglaciation according to radiocarbon data from deep sea corals

effects of fluid shear resistance and squirt flow on velocity dispersion in rocks abstractlaboratory measurements of rocks saturated with high viscosity fluids such as heavy oil bitumen magma kerogen etc often exhibit considerable seismic velocity dispersion which is usually underestimated by the biot theory over the years grain scale dispersion mechanisms such as squirt local flow and shear relaxation nonzero shear stress in the pore fluid have been more successful in explaining the measured dispersion we developed a new method to quantify the combined high frequency effects of squirt and shear dispersion on the effective moduli of rocks saturated with viscous fluids viscous fluid at high frequencies was idealized as an elastic solid of finite shear modulus hydraulically locked in stiff and soft pores this method entailed performing solid substitution in stiff pores of a dry rock frame which itself was unrelaxed due to solid filled soft pores the unrelaxed frame stiffness solutions required information on the pressure dependency of the rock stiffness and porosity

robust prestack q determination using surface seismic data part 1 method and synthetic examples the accurate determination of seismic attenuation or 1 q is useful for signal enhancement and reservoir characterization to arrive at the necessary accuracy however a number of issues must be addressed in the measurement technique specifically spectral interference from closely spaced reflections is a major concern in addition to the assumptions and errors associated with the raypath geometries of the reference and measured reflections we have developed a robust method for measuring attenuation from prestack surface seismic gathers that helps minimize these issues in our prestack q inversion technique the presence of spectral interference was first reduced by making use of a variable window time frequency transform to minimize the effects of the remaining interference we then made use of an inversion scheme operating simultaneously in the frequency and traveltime difference coordinates a by product of this inversion was a collection of the frequency independent amplitude changes which in the

effective medium modeling of velocity dispersion and attenuation in isotropic rocks abstractcalculation of the velocity of elastic waves that propagate in saturated rocks is complicated by many dispersion and attenuation mechanisms wave induced fluid flow at pore and mesoscopic scales is responsible for substantial dispersion and attenuation effects in addition dispersion and attenuation phenomena in rocks are due to spatial heterogeneities in density and elastic properties whenever the wavelength of a propagating wave is of the same scale as the size of the pores dispersion takes place in the form of multiple scattering we have developed an effective medium model that reproduced the combined effects on acoustic velocities of four wave attenuation mechanisms by replicating results predicted by existing models in saturated isotropic rocks biot s and squirt flow and acoustic scattering from spherical pores and randomly oriented penny shaped cracks first the strengthening of rocks and wave velocity increase with frequency associated with fluid related dispersion was modeled by int

water saturation effects on elastic wave attenuation in porous rocks with aligned fractures summary elastic wave attenuation anisotropy in porous rocks with aligned fractures is of interest to seismic remote sensing of the earth s structure and to hydrocarbon reservoir characterization in particular we investigated the effect of partial water saturation on attenuation in fractured rocks in the laboratory by conducting ultrasonic pulse echo measurements on synthetic silicacemented sandstones with aligned penny shaped voids fracture density of 0 0298 0 0077 chosen to simulate the effect of natural fractures in the earth according to theoretical models our results show for the first time contrasting variations in the attenuation q 1 o fp and s

location and dynamics of the antarctic polar front from satellite sea surface temperature data the location of the antarctic polar front pf was mapped over a 7 year period 1987 1993 within images of satellite derived sea surface temperature the mean path of the pf is strongly steered by the topographic features of the southern ocean the topography places vorticity constraints on the dynamics of the pf that strongly affect spatial and temporal variability over the deep ocean basins the surface expression of the pf is weakened and the pf meanders over a wide latitudinal range near large topographic features width and temperature change across the front increase and large scale meandering is inhibited elevated mesoscale variability is seen within and downstream of these areas and may be the result of baroclinic instabilities initiated where the pf encounters large topographic features the strong correlations between topography and pf dynamics can be understood in the context of the planetary potential vorticity ppv or f h field mean ppv at the pf varies by more than a factor of 2 along its circumpolar path however at the mesoscale the pf remains within a relatively narrow range of ppv values around the local mean away from large topographic features the pf returns to a preferred ppv value of 25 10 9 m 1 s 1 despite large latitudinal shifts the mean paths of the surface and subsurface expressions of the pf are closely coupled over much of the southern ocean

fluids and frequency dependent seismic velocity of rocks summary compressional and shear velocities of rocks are dependent on frequency and this dispersion may be significant even within the seismic band the amount and position of dispersion will be largely a function of fluid properties distribution and motion in the pores velocities are thus directly coupled to rock permeability and pore compliance propagation often will be in the high frequency regime even at a few tens of hertz static or low frequency models such as gasmann will fail under such conditions in addition biot theory will not correctly model wave propagation in many cases

acoustic and elastic characterization of marine sediments by analysis modeling and inversion of ultrasonic p wave transmission seismograms ultrasonic p wave transmission seismograms recorded on sediment cores have been analyzed to study the acoustic and estimate the elastic properties of marine sediments from different provinces dominated by terrigenous calcareous and diatomaceous sedimentation instantaneous frequencies computed from the transmission seismograms are displayed as gray shaded images to give an acoustic overview of the lithology of each core centimeter scale variations in the ultrasonic waveforms associated with lithological changes are illustrated by wiggle traces in detail cross correlation multiple filter and spectral ratio techniques are applied to derive p wave velocities and attenuation coefficients s wave velocities and attenuation coefficients elastic moduli and permeabilities are calculated by an inversion scheme based on the biot stoll viscoelastic model together with porosity measurements p and s wave scatter diagrams are constructed to characterize different sediment types by their velocity and attenuation porosity relationships they demonstrate that terrigenous calcareous and diatomaceous sediments cover different velocity and attenuation porosity ranges in terrigenous sediments p wave velocities and attenuation coefficients decrease rapidly with increasing porosity whereas s wave velocities and shear moduli are very low calcareous sediments behave similarly at relatively higher porosities foraminifera skeletons in compositions of terrigenous mud and calcareous ooze cause a stiffening of the frame accompanied by higher shear moduli p wave velocities and attenuation coefficients in diatomaceous ooze the contribution of the shear modulus becomes increasingly important and is controlled by the opal content whereas attenuation is very low this leads to the opportunity to predict the opal content from nondestructive p wave velocity measurements at centimeter scale resolution

radiocarbon variability in the western equatorial pacific inferred from a high resolution coral record from nauru island we have generated a high resolution coral delta sup 14 c record spanning the last 50 years to document the seasonal and interannual redistribution of surface waters in the western tropical pacific prebomb 1947 endash 1956 delta sup 14 c values average minus 63 per thousand and have a total range of 30 per thousand values begin to increase in 1957 reaching a maximum of 137 per thousand in mid 1983 large interannual variability of up to 80 per thousand closely follows the el ni tilde n o southern oscillation enso during each enso warm phase delta sup 14 c values begin to increase reflecting the reduction of low sup 14 c water upwelling in the east and the invasion of subtropical water into the western equatorial tropical pacific maximum delta sup 14 c values are in phase or lag the corresponding sea surface temperature maxima in the eastern tropical pacific whereas the rapid return to more negative delta sup 14 c is in phase with eastern pacific enso indices the highest amplitude excursions occur during the 1965 1966 and 1972 1973 events when the sup 14 c contrast is highest between the eastern pacific and subtropics the 1982 1983 el ni tilde n o although a larger enso event has a lower delta sup 14 c amplitude reflecting the penetration of bomb radiocarbon into the equatorial undercurrent and the reduced contrast in delta sup 14 c betweenmore thermocline and subtropical surface waters at that time this coral record demonstrates the potential for using similar radiocarbon time series for documenting variability in pacific shallow circulation over interannual and decadal timescales copyright 1998 american geophysical union less

oceanic radiocarbon separation of the natural and bomb components an improved method has been developed for the separation of the natural and bomb components of the radiocarbon in the ocean the improvement involves the use of a very strong correlation between natural radiocarbon and dissolved silica this method is applied to radiocarbon measurements made on samples collected during the geochemical ocean sections study geosecs transient tracers in the ocean tto and south atlantic ventilation experiment save expeditions on the basis of this new separation we provide not only an estimate of the global inventory of bomb 14c at the time of the geosecs survey but also the distribution of bomb radiocarbon along four thermocline isopycnals in each ocean we also document the evolution of the bomb 14c inventory and penetration along thermocline isopycnals in the north atlantic ocean between the times of the geosecs 1972 1973 and tto 1980 1982 surveys and in the south atlantic ocean between the times of the geosecs 1973 and save 1987 1989 surveys in addition we show that the bomb tritium to bomb 14c ratio expressed in the tritium unit tu 81 units 100 for waters entering the thermocline of the northern hemisphere is about 9 times higher than for those entering the southern hemisphere thermocline this contrast offers long term potential as an indicator of inter hemispheric transport of upper ocean waters

an observational study of fronts and frontal mergers over the continental united states abstract the stormscale operational and research meteorology fronts experiment systems tests storm fest field program was carried out over the central united states to investigate the structure and dynamics of fronts and their attendant precipitation systems adjacent to complex topography and to evaluate new observing networks and analysis techniques for mesoscale research and weather forecasting this study presents the analysis of data taken between 8 and 10 march 1992 which was the most active period of storm fest this three day observing period was characterized by the passage of an extratropical cyclone and related fronts the synoptic scale conditions related to the cyclone and frontal evolutions are described multiple frontal zones i e pacific and arctic cold fronts quasi stationary polar front dryline maritime warm front and associated regions of precipitation are documented and their origins and interactions are explored these features both well defined and subtle were observed wi

surface convergence induced by cold fronts aloft and prefrontal surges abstract the magnitude of surface convergence produced by the movement of cold fronts aloft and prefrontal surges is derived by applying the linear divergence equation to observed surface pressure traces for pacific northwest warm occlusions and from a mesoscale model simulation of a warm occlusion like structure in the central united states convergence values of approximately 10 4 s 1 are found to be generated locally for periods of about 1 h yielding vertical displacements of 10 50 hpa it is hypothesized that such convergences should noticeably enhance condensation rates in the widespread lower stratiform clouds associated with warm occlusions and could be a key mechanism for the triggering of squall lines by cold fronts aloft in cyclones in the central united states

measurements of air sea gas exchange at high wind speeds in the southern ocean implications for global parameterizations velocity is proposed which is consistent with previou s 3 he sf 6 dual tracer result sf rom the coastal and open ocean obtained at lower wind speeds this suggests that factors controlling air sea gas exchange in this region are similar to those in other parts of the world ocean and that the parameterization presented here should be applicable to the global ocean citation ho d t c s law m j smith p schlosser m harvey and p hill 2006 measurements of airsea gas exchange at high wind speeds in the southern ocean implications for global parameterizations geophys res lett 33 l16611 doi 10 1029 2006gl026817

prefrontal wind shift lines in the plains of the united states abstract in the central united states cold fronts are sometimes preceded by a surface wind shift by several hours these wind shifts are examined to determine their origins and characteristics a climatology and a case study of an autumn prefrontal wind shift are presented data collected by the recently implemented oklahoma mesonet and a network of wind profilers permit finescale analyses of the autumn prefrontal wind shift it is found that prefrontal wind shifts are associated with both lee troughs and drylines the climatology reveals that in the central united states fronts are most frequently up to 60 of them preceded by wind shifts just east of the rockies where lee troughs and drylines originate this paper focuses on a case study of a prefrontal wind shift associated with a lee trough that forms when a front approaches the lee trough the prefrontal wind shift associated with the lee trough forms in the lee of the mountains but propagates away from the mountains when downslope flow decreas

deep tropospheric gravity waves created by leeside cold fronts abstract the water vapor channel of the geostationary operational environmental satellite goes 8 reveals narrow 30 km wide elongated 500 1000 km bands that propagate southward on the eastern side of the rocky mountains from colorado to texas two events in which surface and wind profiler observations show that these bands are associated with leeside cold fronts are documented in detail and several other cases are summarized the wind profilers observe vertical motions exceeding 1 m s 1 in narrow plumes at the leading edge of the fronts in broader zones in the upper troposphere and in the lower stratosphere these cause vertical displacements of up to 1 km and are responsible for the signature in water vapor images the bands occur when the rocky mountains block either arctic leeside cold fronts coming from the north or northeast or pacific cold fronts coming from the northwest the blocking changes the frontal orientation and disrupts geostrophic thermal wind balance near the terrain modified fron

estimation of velocity and attenuation coefficient maps from crosswell seismic data using a classic traveltime tomography and an attenuation tomography based on the centroid frequency shift method velocity and attenuation coefficient tomograms are computed from two crosswell seismic data because of the limited frequency band of the data and the difficulties of picking the first transmitted events velocity and attenuation coefficients are assumed independent of frequency during the processing meaning that the attenuation linearly increases with frequency this reduces the possibility of using a theoretical model to interpret the results moreover attenuation results include both scattering and viscous attenuation nevertheless the attenuation coefficient tomograms confirm some of the features seen in the velocity tomograms the sand zones show a higher attenuation although the shale zones and the seals of the reservoirs show a lower attenuation with a kramers kronig relation for the dispersion formula based on frequency independent attenuation coefficient the smooth part of the m

variation in dynamic elastic shear modulus of sandstone upon fluid saturation and substitution experimental acoustic measurements on sandstone rocks at both sonic and ultrasonic frequencies show that fluid saturation can cause a noticeable change in both the dynamic bulk and shear elastic moduli of sandstones we observed that the change in dynamic shear modulus upon fluid saturation is highly dependent on the type of saturant its viscosity rock microstructure and applied pressures frequency dispersion has some influence on dynamic elastic moduli too but its effect is limited to the ultrasonic frequency ranges and above we propose that viscous coupling reduction in free surface energy and to a limited extent frequency dispersion due to both local and global flow are the main mechanisms responsible for the change in dynamic shear elastic modulus upon fluid saturation and substitution and we quantify influences

a family of frontal cyclones over the western atlantic ocean part i a 60 h simulation despite marked improvements in the predictability of rapidly deepening extratropical cyclones many operational models still have great difficulties in predicting frontal cyclogenesis that often begins as a mesoscale vortex embedded in a large scale parent cyclone system in this paper a 60 h simulation and analysis of a family of frontal cyclones that were generated over the western atlantic ocean during 13 15 march 1992 are performed using the pennsylvania state university national center for atmospheric research mesoscale model with a fine mesh grid size of 30 km although it is initialized with conventional observations the model reproduces well the genesis track and intensity of the frontal cyclones their associated thermal structure and precipitation pattern as well as their surface circulations as verified against the canadian meteorological centre analysis and other available observations it is shown that each frontal cyclone is initiated successively to the southwest of its predecessor in the cold sector first appearing as a pressure trough superposed on a baroclinically unstable basic state in the lowest 150 300 hpa then it derives kinetic energy from the low level available potential energy as it moves over an underlying warm ocean surface with weak static stability toward a leading large scale frontal zone and deepens rapidly by release of latent heat occurring in its own circulations one of the frontal cyclones originating in the cold air mass deepens 44 hpa in 42 h and overwhelms the parent cyclone after passing over the warm gulf stream water into the leading frontal zone these cyclones have diameters ranging from 500 to 1100 km as denoted by the last closed isobar and are spaced 1000 1400 km apart between their circulation centers during the mature stage they begin to establish their own cold warm frontal circulations once their first closed isobars appear thus distorting the leading large scale frontal structures and altering the distribution and type convective versus stratiform of precipitation it is found that the frontal cyclones accelerate and experience their central pressure drops as they move from high to low pressure regions toward the parent cyclone center and then they decelerate and fill as they travel away from the parent cyclone their spatial and temporal scales vertical structures as well as deepening mechanisms are shown to differ significantly from those typical extratropical cyclones as previously studied

pathways of anthropogenic carbon subduction in the global ocean the oceanic uptake of anthropogenic carbon is tightly coupled to carbon subduction i e the physical carbon transfer from the well ventilated surface ocean to its interior despite their importance pathways of anthropogenic carbon subduction are poorly understood here we use an ocean carbon cycle model to quantify the mechanisms controlling this subduction over the last decade 90 of the oceanic anthropogenic carbon is subducted at the base of the seasonally varying mixed layer vertical diffusion is the primary mechanism of this subduction contributing 65 of total subduction despite very low local fluxes in contrast advection drives the spatial patterns of subduction with high positive and negative local fluxes our results suggest that vertical diffusion could have a leading role in anthropogenic carbon subduction which highlights the need for an accurate estimate of vertical diffusion intensity in the upper ocean to further constrain estimates of the future evolution of carbon uptake

relating p wave attenuation to permeability to relate p wave attenuation to permeability we examine a three dimensional 3 d theoretical model of a cylindrical pore filled with viscous fluid and embedded in an infinite isotropic elastic medium we calculate both attenuation and permeability as functions of the direction of wave propagation attenuation estimates are based on the squirt flow mechanism permeability is calculated using the kozeny carman relation we find that in the case when a plane p wave propagates perpendicular to the pore orientation q 90 1 attenuation is always higher than when a wave propagates parallel to this orientation q 0 1 the ratio of these two attenuation values q 90 1 q 0 1 increases with an increasing pore radius and decreasing frequency and saturation by changing permeability varying the radius of the pore we find that the permeability attenuation relation is characterized by a peak that shifts toward lower permeabilities as frequency decreases therefore the attenuation of a low frequency wave d

structure and evolution of winter cyclones in the central united states and their effects on the distribution of precipitation part ii arctic fronts abstract the structure and evolution of a shallow but intense cold front commonly referred to as an arctic front and its associated precipitation features that passed through the central united states from 0000 utc 9 march to 0000 utc 10 march 1992 are studied with the aid of observations and outputs from a numerical simulation using the pennsylvania state university national center for atmospheric research mesoscale model mm4 located above the arctic front was a region of midtropospheric frontogenetical confluence that was attended by a thermally direct vertical circulation a large banded precipitation feature for the most part located behind the arctic front was produced by ice crystals from upper level clouds formed by the frontogenetical confluence falling into low level stratocumulus associated with the arctic front the arctic front at the surface separated a region where the precipitation reaching the ground was solid from an adjacent region where the precipitation was liquid a westward m

frequency dependent velocity analysis and offset dependent low frequency amplitude anomalies from hydrocarbon bearing reservoirs in the southern north sea norwegian sector abstractour aim was to identify some of the characteristics of low frequency anomalies specifically we have looked in 3d broadband data from the north sea for any offset dependence in these anomalies and any frequency related change in normal moveout nmo velocity that could influence stacking power over different frequencies after high resolution spectral decomposition two types of low frequency anomaly have been identified associated with hydrocarbon bearing reservoirs 1 at the reservoir top and 2 below the reservoir with a time delay of approximately 100 200 ms both types of anomalies indicate offset dependence on the near offset stacks they are relatively strong but they tend to be absent on the far offset stacks in addition horizon velocity analysis which was performed along the horizons picked at the tops of reservoir and nonreservoir intervals has revealed frequency dependent nmo velocity for nonreservoir events we found no significant difference between the nmo velocities fo

azimuthal variation in p wave signatures due to fluid flow the favored dominant mechanism for attenuation in the upper crust at seismic frequencies is intracrack fluid flow in cracked media the azimuthal attenuation of the p wave amplitudes arising from such flow is predicted to be quite substantial the consequence of this variation in azimuth is a modification in the amplitude behavior of the base event from a cracked reservoir due to transmission through the attenuative layer indeed the effect is of sufficient strength to exacerbate diminish or reverse variations that arise solely due to the reflectivity coefficient thus although this attenuation is always greatest perpendicular to the crack strike the direction of the dimming or brightening of the base reservoir event will depend upon the exact attenuation law and the crack properties the combination of these factors contributes to a wave behavior that can provide a more adequate discrimination between conditions of brine and oil fill than an interpretation assuming the reflection coefficient alone

equivalent medium representation of fractured rock a similarity may be found between various approaches for determining the effects of parallel fractures or aligned cracks on seismic wave propagation at wavelengths that are long compared with the scale length of the cracks fractures can be modeled using an empirical linear slip condition however natural fracture surfaces can also be simulated directly as planar distributions of small isolated areas of slip cracks model 1 or conversely as planar distributions of imperfect interfacial contacts model 2 an alternative is plane surfaces separated by thin continuous layers of viscous fluid or a soft material model 3 we present analytic expressions for the fracture compliances for these three models and using these analytic results compute the effective compliances and stiffnesses of the fractured material as a result it is possible to relate the measured compliances or stiffnesses directly to the statistics of the microstructural details of a fracture given appropriate a priori information on the fracture surfaces the results for model 1 are equivalent to a volume distribution of cracks as studied by hudson 1980 1981 for small crack density the results for model 2 are basically the same as those given by white 1983 for a packing of spheres and finally the results for model 3 are in agreement with those given by backus 1962 for combinations of two constituent layers these results can be extended to the case of nonaligned fractures and to allow for fluid flow between cracks and into a porous matrix rock finally it is shown that the ratio of the normal to shear fracture compliance is a good indicator of the properties of the fracture infill

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

velocity anisotropy and attenuation of shale in under and overpressured conditions the seismic velocity and attenuation of fully saturated shales were measured for the first time under overpressured conditions using the ultrasonic reflection technique shale cores from naturally overpressured horizons in the north sea were tested in the laboratory at confining and pore pressures relevant to in situ conditions a single frequency tone burst pulse wave was used to determine the seismic wave velocities and quality factors of the shale samples with errors less than 0 3 and 0 1 db cm respectively at a frequency of 0 75 mhz sample length changes with varying confining and pore pressure were measured and the pore pressure equilibration time was monitored for each sample the anisotropy of the seismic attributes vp vs qp and qs was determined over a range of differential pressures from 5 to 60 mpa with respect to the predominant foliation the ultrasonic velocity data followed a transversely isotropic pattern depending on the direction of wave propagation with respect to the laminations the poisson s ratio was found to rise by 5 as the shale material progressed from a normally pressured to an overpressured state the quality factor q characteristics were interpreted in terms of pore geometry and connectivity as well as the directional permeability of the transversely isotropic shale material the results were converted to bulk and shear loss modulus defects and a positive bulk loss was observed for waves propagating perpendicular to the lamination plane even above differential pressures of 20 mpa this indicates different levels of biot flow and squirt flow attenuation mechanisms acting within the shale structure depending on the wave propagation and vibration directions

compressional wave attenuation in oceanic basalts to understand better the seismic attenuation in the upper volcanic regions of the oceanic crust compressional wave attenuations of oceanic basalts have been measured as a function of confining pressure using an ultrasonic pulse echo spectral ratio technique capable of measuring attenuations to pressures of 500 mpa seven basalts five from deep sea drilling project cores and two from dredge samples have wide ranges of densities porosities and alterations making possible an analysis of the parameters influencing basalt attenuation attenuation increases with the volume of secondary minerals present and with increasing porosity thus vesicularity and compositional changes associated with basalt alteration will produce variations in attenuation with the application of hydrostatic pressure cracks close thereby reducing attenuations this pressure dependence should be manifested in oceanic layer 2 by decreasing attenuation with depth an inverse relationship between velocity and attenuation is observed at high hydrostatic pressures water saturation increases attenuation at pressures below 200 mpa and enhances the sensitivity of attenuation to pressure thus making the state of saturation important in the 40 to 100 mpa range generally found in layer 2 these results provide a framework for interpreting marine attenuation data

abrupt shift in subsurface temperatures in the tropical pacific associated with changes in el nino radiocarbon 14c content of surface waters inferred from a coral record from the galapagos islands increased abruptly during the upwelling season july through september after the el nino event of 1976 sea surface temperatures ssts associated with the upwelling season also shifted after 1976 the synchroneity of the shift in both 14c and sst implies that the vertical thermal structure of the eastern tropical pacific changed in 1976 this change may be responsible for the increase in frequency and intensity of el nino events since 1976

the structure and evolution of a continental winter cyclone part i frontal structure and the occlusion process the frontal structure and occlusion process in a cyclone of moderate intensity that affected the central united states in january 1995 is examined the deep warm frontal zone associated with this cyclone had a lateral extension to the southwest of the sea level pressure minimum that although characterized by cold air advection near the surface had many of the characteristics of a warm front aloft in fact this feature had a structure similar to the so called bent back fronts previously documented only in association with explosively deepening maritime cyclones the development of a warm occluded structure was investigated with the aid of a numerical simulation of the event performed using the university of wisconsin nonhydrostatic modeling system the development of the warm occluded structure was asynchronous in the vertical occurring first at midtropospheric levels and later near the surface in contrast to the classical occlusion process near the surface the warm occluded front was formed as the warm front was overtaken by the frontogenetically inactive portion of the historical coldfrontal zone at midtropospheric levels the warm occluded structure formed as a result of the cold frontal zone approaching and subsequently ascending the warm frontal zone in accord with a component of the classical occlusion mechanism the observed asynchronous evolution of the occluded structure is proposed to result from the vertical variation in vortex strength associated with the upper level potential vorticity pv anomaly that controls the cyclogenesis it is suggested that the occlusion process begins aloft where the associated vortex strength is greatest and gradually penetrates downward toward the surface during the cyclone life cycle additionally a characteristic treble clef shape to the upper level pv anomaly is shown to be a sufficient condition for asserting the presence of a warm occluded structure in the underlying troposphere

correlations between the in situ acoustic properties and geotechnical parameters of sediments in the yellow sea china knowledge about the marine sediment acoustic properties is a key to understanding wave propagation in sediments and is very important for military oceanography and ocean engineering we developed a hydraulic drived self contained in situ sediment acoustic measurement system and measured for the first time the in situ acoustic properties of sediments on 78 stations in the yellow sea china by employing this system the relationships between the in situ measured acoustic properties and the onboard or laboratory determined geotechnical parameters were analyzed porosity was found to be the dominant factor in reducing velocity in a quadratic fashion velocity showed an increment with bulk density and a decrement with mean grain size and clay content both with a nonlinear dependence acoustic attenuation showed a bell shaped correlation with porosity and mean grain size but reduced with clay content of the sediments the attenuation results indicate that intergrain friction rather than viscous interactions between pore fluid and solid grains is the dominant loss mechanism in our marine sediments the relationships established would be used to predict the geotechnical parameters from in situ measured acoustic properties and vice versa as well as being an indicator of the seafloor processes potential gas bubbles hazard and gas hydrates resources or other suitable targets of acoustic surveys

changing controls on oceanic radiocarbon new insights on shallow to deep ocean exchange and anthropogenic co2 uptake 1 the injection of radiocarbon 14c into the atmosphere by nuclear weapons testing in the 1950s and 1960s has provided a powerful tracer to investigate ocean physical and chemical processes while the oceanic uptake of bomb derived 14c was primarily controlled by air sea exchange in the early decades after the bomb spike we demonstrate that changes in oceanic 14c are now primarily controlled by shallow to deep ocean exchange i e the same mechanism that governs anthropogenic co2 uptake this is a result of accumulated bomb 14c uptake that has rapidly decreased the air sea gradient of 14c c 14c and shifted the main reservoir of bomb 14c from the atmosphere to the upper ocean the air sea 14c gradient reduced further by fossil fuel dilution is now weaker than before weapons testing in most regions oceanic 14c and particularly its temporal change can now be used to study the oceanic uptake of anthropogenic co2 we examine observed changes in oceanic 14c between the woce save 1988 1995 and the clivar 2001 2007 eras and simulations with two ocean general circulation models the community climate system model ccsm and the estimating the circulation and climate of the ocean model ecco observed oceanic 14c and its changes between the 1980s 90s and 2000s indicate that shallow to deep exchange is too efficient in ecco and too sluggish in ccsm these findings suggest that mean global oceanic uptake of anthropogenic co2 between 1990 and 2007 is bounded by the ecco based estimate of 2 3 pg c yr 1 and the ccsm based estimate of 1 7 pg c yr 1

uptake of excess co2 by an outcrop diffusion model of the ocean a carbon cycle model is presented in which direct ventilation of intermediate and deep ocean waters in high latitudes is taken into account the 1 dimensional ocean model is an extension of a box diffusion model including a deep sea outcrop at the surface if both are calibrated in a consistent way the outcrop diffusion ocean takes up more excess co2 than the box diffusion ocean because the outcropping deep water is essentially virgin as to fossil co2 two calibration methods are compared using the distribution either of natural or of bomb produced 14c the latter leads to a higher oceanic uptake of excess co2 than the former and to a better agreement with the observed atmospheric increase long term model responses are also discussed

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

separating natural and bomb produced radiocarbon in the ocean the potential alkalinity method alkalinity method provides an estimate of surface ocean prebomb d 14 c concentrations predictions with the technique appear to be unbiased and have uncertainties which are less than previous techniques index terms 4860 oceanography biological and chemical radioactivity and radioisotopes 4894 oceanography biological and chemical instruments and techniques

the effect of fracture density and stress state on the static and dynamic bulk moduli of westerly granite elastic properties are key parameters during the deformation of rocks they can be measured statically or dynamically but the two measurements are often different in this study the static and dynamic bulk moduli kstatic and kdynamic were measured at varying effective stress for dry and fluid saturated westerly granite with controlled fracture densities under isotropic and differential stress states isotropic fracturing of different densities was induced in samples by thermal treatment to 250 450 650 and 850 c results show that fluid saturation does not greatly affect static moduli but increases dynamic moduli under isotropic loading high fracture density and or low effective pressure results in a low kstatic kdynamic ratio for dry conditions kstatic kdynamic approaches 1 at low fracture densities when the effective pressure is high consistent with previous studies stress induced anisotropy exists under differential stress state that greatly affects kstatic compared to kdynamic as a result the kstatic kdynamic ratio is higher than that for the isotropic stress state and approaches 1 with increasing axial loading the effect of stress induced anisotropy increases with increasing fracture density a key omission in previous studies comparing static and dynamic properties is that anisotropy has not been considered the standard methods for measuring static elastic properties such as poisson s ratio young s and shear modulus involve subjecting the sample to a differential stress state that promotes anisotropy our results show that stress induced anisotropy resulting from differential stress state is a major contributor to the difference between static and dynamic elasticity and is dominant with high fracture density

a new laboratory apparatus for the measurement of seismic dispersion under deviatoric stress conditions abstract a better understanding of seismic dispersion and attenuation of acoustic waves in rocks is important for quantitative interpretation of seismic data as well as for relating seismic data sonic log data and ultrasonic laboratory data in the present work a new laboratory setup is described allowing for combined measurements of quasistatic deformations of rocks under triaxial stress ultrasonic velocities and dynamic elastic stiffness young s modulus and poisson s ratio at seismic frequencies the setup has been used mainly for the study of shales for such rocks it is crucial that the saturation of the samples is preserved which requires fast sample mounting the design of our setup together with a technique that was developed for rapid mounting of strain gauges onto the sample and subsequent sealing of the sample allows for sample preservation which is of particular importance for shales the performance of the new experimental setup and sample mounting procedure is demonstrated with test materials aluminium and polyetheretherketone and two different shale types mancos shale and pierre shale furthermore experimental results are presented that demonstrate the capability of measuring the impact of saturation stress and stress path on seismic dispersion for the tests with mancos shale and pierre shale large dispersion up to 50 in young s modulus normal to bedding was observed increased water saturation of mancos shale results in strong softening of the rock at seismic frequencies whereas hardening is observed at ultrasonic frequencies due to an increase in dispersion counteracting the rock softening the poisson s ratio of mancos shale strongly increases with the level of saturation but appears to be nearly frequency independent we have found that the different types of shale exhibit different stress sensitivities during hydrostatic loading and that the stress sensitivity is different at seismic and ultrasonic frequencies

temporal and spatial variations in bomb produced radiocarbon along beagle2003 lines revisits of whp p06 a10 and i03 i04 in the southern hemisphere oceans abstract radiocarbon 14 c in dissolved inorganic carbon was measured during revisit cruises along world ocean circulation experiment hydrographic programme whp lines a10 in the south atlantic i03 i04 in the indian and p06 in the south pacific oceans from august 2003 to january 2004 during the blue earth global expedition 2003 beagle2003 zonal means of the water column inventory of bomb produced 14 c in 2003 2004 in the south atlantic indian and south pacific oceans were about 180 128 and 159 10 12 atoms m 2 respectively the smallest zonal inventory along the i03 line among the three lines was primarily due to a sampling bias because the i03 line in the indian ocean was along 20 s which is more equatorward than the other two lines along approximately 30 s in the south atlantic and south pacific oceans the i03 line in the indian ocean had the smallest zonal inventory of bomb produced 14 c and the largest of bomb produced 137 cs suggesting that the distribution of bomb 14 c in the indian ocean is determined primarily by the thermocline ventilation within the indian ocean the history of bomb 14 c over time suggests that the bomb 14 c inventory in the southern subtropical regions increased steadily up to the early 1990s the rate of increase then slowed between the early 1990s and 2003 2004 because of a decrease in the bomb 14 c influx from the atmosphere to the surface ocean the highest bomb 14 c inventory among the southern subtropical regions was in the subtropical indian ocean however the contribution of the indonesian throughflow from the north pacific and indian oceans to this large inventory in the indian ocean is not clear the 14 c data along the beagle2003 lines in 2003 2004 were compared with those obtained during whp in the 1990s and during the south atlantic ventilation experiment in the late 1980s the zonal averages of the decadal changes in 14 c revealed that bomb 14 c continued to increase between the late 1980s 1990s and 2003 2004 in the southern subtropical regions of the south atlantic and south pacific oceans in the indian ocean an increase in the natural 14 c as a result of circulation changes also contributed to the apparent decadal increase of the bomb 14 c all of our results suggest that the distribution of bomb 14 c in the southern hemisphere is governed primarily by thermocline ventilation processes within the southern hemisphere

effects of pore pressure on compressional wave attenuation in a young oceanic basalt laboratory data are reported for ultrasonic compressional wave attenuation p as a function of pore pressure in a juan de fuca ridge dredge basalt four experiments have been made to determine the relationships between attenuation and quality factor qp and confining and pore pressures in the shallow ocean crust attenuation was measured at 1 a constant differential pressure of 40 mpa 2 confining pressures to 120 mpa and atmospheric pore pressures 3 a constant confining pressure of 50 mpa while varying pore pressures 4 a constant confining pressure of 100 mpa while varying pore pressures for atmospheric pore pressures p ranges from 1 64 db cm to 7 08 db cm qp 24 to 10 in addition attenuation increases systematically with increasing pore pressure and decreasing differential pressure confining pressure pore pressure results from dsdp and odp downhole packer experiments suggest that the hydrostatic pore pressure regime may best approximate in situ conditions for young oceanic crust hydrostatic pore pressures 5000 meters depth reduce qp as much as 35 from normal atmospheric pressure conditions therefore pore pressures generated in the upper oceanic crust may be responsible in part for the observed low seismic qp within layer 2a qp measurements at elevated pore pressures agree well with seismic qp data

a cracked porous medium elastic wave theory and its application to interpreting acoustic data from tight formations rocks in the earth s crust usually contain pores and cracks typical examples include tight sandstone and shale rocks that have low porosity but contain abundant microcracks by extending the classic biot s poroelastic wave theory to include the effects of cracks we obtain an elastic wave theory for porous rocks containing cracks adding crack density and aspect ratio as two important parameters to the original theory because the flat or narrow shaped cracks can easily deform under acoustic wave excitation the acoustic property of a cracked porous rock is quite different for different saturation conditions the predicted fluid sensitivity is used to interpret acoustic velocity log data from tight sand and shale gas formations in both scenarios the new theory correctly predicts the trend of velocity variation with gas saturation the results confirm that the presence of cracks in tight rocks can give rise to significant hydrocarbon signature in the acoustic measurement data allowing for identifying hydrocarbons from the data

laboratory monitoring of p waves in partially saturated sand energy dissipation is observed on seismic data when a wave propagates through a porous medium involving different frequency regimes depending on the nature of rock and fluid types we focus here on the role of partial fluid saturation in unconsolidated porous media looking in particular at p wave phase velocity and attenuation the study consists in running an experiment in a sand filled tank partially saturated with water seismic propagation in the tank is generated in the khz range by hitting a steel ball on a granite plate seismic data are recorded by buried accelerometers and injecting or extracting water controls the partial saturation several imbibition drainage cycles were performed between the water and gas residual saturations a continuous wavelet transform applied on seismic records allowed us to extract the direct p wave at each receiver we observe an hysteresis in phase velocities and inverse quality factors between imbibition and drainage phase velocities and inverse quality factors are then jointly inverted to get a final poro visco elastic model of the partially saturated sand that satisfactorily reproduces the data the model formulation consists in generalizing the biot theory to effective properties of the fluid and medium permeability and bulk modulus in order to properly explain the phase velocity variation as a function of the saturation the strong level of attenuation measured experimentally is further explained by an anelastic effect due to grain to grain sliding adding to biot s losses this study shows that fluid distribution at microscopic scale has strong influence on the attenuation of direct p waves at macroscopic scale and confirms that seismic prospection may be a powerful tool for the characterization of transport phenomena in porous media

relationships among compressional wave attenuation porosity clay content and permeability in sandstones anelastic attenuation is the process by which rocks convert compressional waves into heat and thereby modify the amplitude and phase of the waves understanding the causes of compressional wave attenuation is important in the acquisition processing and interpretation of high resolution seismic data and in deducing the physical properties of rocks from seismic data we have measured the attenuation coefficients of compressional waves in 42 sandstones at a confining pressure of 40 mpa equivalent to a depth of burial of about 1 5 km in a frequency range from 0 5 to 1 5 mhz the compressional wave measurements were made using a pulse echo method in which the sample 5 cm diameter 1 8 cm to 3 5 cm long was sandwiched between perspex lucite buffer rods inside the high pressure rig the attenuation of the sample was estimated from the logarithmic spectral ratio of the signals corrected for beam spreading reflected from the top and base of the sample the results show that for these samples compressio

low frequency seismic properties of thermally cracked and argon saturated granite torsional forced oscillation techniques have been used to measure the shear modulus and strain energy dissipation on cylindrical specimens of a fine grained granite delegate aplite the specimens were subjected to thermal cycling and associated microcracking under varying conditions of confining pressure pc and argon pore fluid pressure pf within the low frequency saturated isobaric regime complementary transient flow studies of in situ permeability and volumetric measurements of connected crack porosity allowed the modulus measurements to be interpreted in terms of the density and interconnectivity of the thermally generated cracks the modulus measurements indicate that newly generated thermal cracks are closed by a differential pressure pc pf which ranges from 120 to 160 mpa for temperatures of 300 600 c this suggests crack aspect ratios on the order of 10 3 the covariation of in situ permeability k and thermal crack density e that we infer from the modulus deficit is consistent with percol

estimation of soil permeability using an acoustic technique a new method of estimating the permeability of soils using an acoustic technique is proposed in this technical note biot s coupled theory of mixtures from the mid 1950s which addresses the relationship between the permeability of saturated soils and the characteristic frequency of elastic waves is used the characteristic frequency is determined from the attenuation versus frequency curves obtained by acoustic sweep tests this study presents the experimental technique for measuring the characteristic frequency of soils and calculation procedure for estimating the permeability of sandy soils this technical note holds discussion for silty or clayey soils

dynamic poroelasticity a unified model with the squirt and the biot mechanisms the velocities and attenuation of seismic and acoustic waves in rocks with fluids are affected by the two most important modes of fluid solid interaction 1 the biot mechanism where the fluid is forced to participate in the solid s motion by viscous friction and inertial coupling and 2 the squirt flow mechanism where the fluid is squeezed out of thin pores deformed by a passing wave traditionally both modes have been modeled separately with the biot mechanism treated in a macroscopic framework and the squirt flow examined at the individual pore level we offer a model which treats both mechanisms as coupled processes and relates p velocity and attenuation to macroscopic parameters the biot poroelastic constants porosity permeability fluid com pressibility and viscosity and a newly introduced microscale parameter a characteristic squirt flow length the latter is referred to as a fundamental rock property that can be determined experimentally we show that the squirt flow mechanism dominates the biot mechanism and is responsible for measured large velocity dispersion and attenuation values the model directly relates p velocity and attenuation to measurable rock and fluid properties therefore it allows one to realistically interpret velocity dispersion and or attenuation in terms of fluid properties changes e g viscosity during thermal enhanced oil recovery eor or to link seismic measurements to reservoir properties as an example of the latter transformation we relate permeability to attenuation and achieve good qualitative correlation with experimental data

effect of fluids and frequencies on poisson s ratio of sandstone samples abstractpoisson s ratio is an important parameter when interpreting measured geophysical and seismic data for an isotropic medium it directly relates to the ratio of p and s wave velocities we have measured as a function of pressure and frequency in fluid saturated sandstones the method of measuring was first tested as a function of pressure and frequency using standard samples the phase shift between radial and axial strains was also measured for all standard samples such as the linear viscoelastic plexiglas the data indicated that tan correlated with and related to a dissipation on then and tan were measured as a function of pressure and frequency for two dry and fluid saturated fontainebleau sandstone samples under dry conditions no frequency dependence and very small pressure dependence were observed unusual behaviors were observed under fluid saturated conditions in particular of one sample indicated a frequency dependent bell shaped dispersion under water and gl

eastern tropical north pacific coral radiocarbon reveals north pacific gyre oscillation npgo variability abstract fluctuations in oceanic circulation and upwelling associated with the north pacific gyre oscillation npgo are the largest source of salinity and nutrient concentration variability across the pacific basin recent observations suggest npgo like variability is intensifying but longer pre instrumental records are required to improve our understanding of npgo amplitude and phase change here using measurements of coral skeletal chemistry from san benedicto island in the eastern tropical north pacific etnp we assess this region s suitability for reconstructing npgo behavior we find that coral geochemical proxy measurements of etnp salinity and dissolved inorganic carbon radiocarbon 14c content reflect npgo driven gyre circulation and regional coastal upwelling these results provide the basis for reconstructing npgo related ocean conditions hundreds of years prior to the modern observational record

a feasibility study of co2 geologic sequestration integrating reservoir simulation rock physics theory and seismic modeling abstractco2 capture and sequestration is a promising approach to reduce carbon emission and mitigate the greenhouse effect we have developed a methodology combining reservoir simulation rock physics theory and seismic modeling to simulate the co2 sequestration and monitoring process based on an idealized geologic model of the sleipner field first we simulated a constant rate co2 injection into the idealized geologic model to study the basics of the two phase flow involved in co2 sequestration the main features of the co2 plume evolution and pressure build up are captured in the simulation results in any co2 sequestration project an important part is monitoring co2 distribution using seismic methods the seismic response of the injected co2 is controlled by its effect on elastic properties of the reservoir rock we built a rock physics model to assess the effect of co2 on wave properties for unconsolidated sand a sensitivity study found that co2 saturation and effective pressure can strongly aff

bomb radiocarbon invasion into the northwestern north pacific invasion of the bomb radiocarbon was investigated in the northwestern north pacific including the subarctic oyashio region the subtropical kuroshio region and the subarctic subtropical mixing transition areas after the geosecs pacific expedition in 1973 the bomb radiocarbon decreased in the oyashio region and increased in the kuroshio region the former was caused by rapid ventilation of the oyashio water the latter was owing to accumulation of the bomb radiocarbon in the intermediate water of the kuroshio region the reverse trends in the temporal variation of radiocarbon between the oyashio and the kuroshio regions lead to virtually small temporal change of the bomb radiocarbon in the transition area where the two waters mix the temporal changes of the bomb radiocarbon suggest that the north pacific intermediate water is formed in the northwestern north pacific

persistently declining oxygen levels in the interior waters of the eastern subarctic pacific abstract fifty years of measurements at ocean station papa osp 50 n 145 w show trends in the interior waters of the subarctic pacific that are both impacted by short term few years to bi decadal atmospheric or ocean circulation oscillations and by persistent climate trends between 1956 and 2006 waters below the ocean mixed layer to a depth of at least 1000 m have been warming and losing oxygen on density surfaces found in the depth range 100 400 m 26 3 27 0 the ocean is warming at 0 005 0 012 c y 1 whereas oxygen is declining at 0 39 0 70 mol kg 1 y 1 or at an integrated rate of 123 mmol m 2 y 1 decrease of 22 over 50 years during this time the hypoxic boundary defined as 60 mol o 2 kg 1 has shoaled from 400 to 300 m in the alaska gyre the 26 2 isopycnal occasionally ventilates whereas at osp 26 0 has not been seen at the ocean surface since 1971 as the upper ocean continues to stratify to interpret the 50 year record at osp the isopycnal transport of oxygenated waters within the interior of the subarctic pacific is assessed by using a slightly modified no parameter broecker w 1974 no a conservative water mass tracer earth and planetary science letters 23 100 107 the highest nitrate oxygen signature in interior waters of the north pacific is found in the bering sea gyre western subarctic gyre and east kamchatka current region as a consequence of winter mixing to the 26 6 isopycnal by mixing with low no waters found in the subtropics and okhotsk sea this signature is diluted as waters flow eastward across the pacific evidence of low no waters flowing north from california is seen along the coasts of british columbia and se alaska oxygen in the subsurface waters of the alaskan gyre was supplied 60 by subarctic and 40 by subtropical waters during woce surveys whereas such estimates are shown to periodically vary by 20 at osp other features discernable in the osp data include periods of increased ventilation of deeper isopycnals on an 18 year cycle and strong short term few month variability caused by passing mesoscale eddies the potential impacts of declining oxygen on coastal ecosystems are discussed

anthropogenic co2 in the southern ocean distribution and inventory at the indian atlantic boundary world ocean circulation experiment line i6 1 the southern ocean where various water masses are formed mode intermediate deep and bottom waters has a high potential to absorb anthropogenic co2 cant however most data based and model estimates indicate low cant inventories south of 50 s in order to investigate this paradox the distribution of cant is estimated between south africa and antarctica world ocean circulation experiment woce line i6 based on a back calculation technique previously used in the north atlantic kortzinger et al 1998 and adapted here for application in the southern ocean at midlatitudes 30 50 s formation and spreading of mode and intermediate waters results in a deep penetration of cant down to 2000 m south of 50 s significant concentrations of cant were estimated in circumpolar deep water 10 mol kg and antarctic bottom water aabw 20 25 mol kg higher concentrations are detected along the continental slope in aabw presumably formed in prydz bay compared to aabw of weddell sea origin the distribution of cant obtained north of 50 s compares well with previous data based and model estimates but large disagreements are found in the south however although transient tracers are not used in the back calculation technique employed here the distribution of cant is remarkably well correlated with cfcs we reevaluated the column inventories of cant for the southern ocean and found higher values at high latitudes 70 90 mol m2 compared to the subtropical subantarctic region 40 80 mol m2 these results support the idea that deep and bottom water formation in the southern ocean is a key process in the natural sequestration of anthropogenic co2