the influence of fracturing process on microseismic propagation in the hydraulic fracturing process the velocity model for microseismic location is usually constructed by well logs seismic data or calibration shots which ignore the influence of the fracturing process in this work we examine its influence by simulating microseismic events and pore pressure variation in the fracturing process besides the microseismicity produced by hydraulic fracture extension we also simulate the microseismicity caused by fluid leakage by means of the critical pore pressure criterion the coates schoenberg method and fracture compliances are then applied to calculate the real time velocity in the fracturing process and 3d ray tracing method is applied to compare the microseismic propagation variation in the different stages of the fracturing process the results of the simulation show that the deviation caused by the fracturing process varies considerably between different receiver locations and the overall deviation increases with the fracturing process finally a new method is constructed to evaluate the travel time deviation in real data

fracture effects in seismic attenuation images reconstructed by waveform tomography we have investigated seismic waveform tomography to characterize fractures in petroleum reservoirs seismic reflection data are used in a frequency domain inversion to reconstruct subsurface attenuation images the images show fracture distributions from which fracture density is estimated fractures smaller than or equal to a half wavelength of seismic act as single scatterers producing images of strong attenuation ellipses and from which fracture density can be estimated when fracture size approaches one wavelength fracture orientation affects the attenuation image horizontal fractures act as individual reflectors and produce strong tomographic attenuation images from which fracture density can be estimated the strength of the attenuation image decreases when the fracture angle relative to horizontal increases vertical fractures produce the weakest attenuation image consequently the accuracy of fracture density measurements decreases with increased fracture angle unless waveform tomography incl

seismic response of fractures by numerical simulation summary fractures occur on a wide range of scales and are important in the study of hydrocarbon reservoirs seismic simulation by finite difference modelling using an equivalent medium method can devise the elastic parameters of a cell intersected by a fracture as those of a medium with an equivalent seismic response numerical experiments confirm that diffractions from the fracture tips are a strong component of the total wavefield however a comparison of boxcar linear angular and elliptic tapering suggests that there is little dependence on shape because the energy involved in a single diffraction is much lower than the incident energy an open fluid filled fracture has stronger effect on the wavefield than the wet and dry multiple crack models because an open fracture would have a stronger dissimilarity to the background rock the density of microcracks within a fracture also has strong effect on the seismic response however the properties of those cracks are not significant to the overall seismic response considering a distribution of a large number of fractures even when the overall density of fractures is held constant longer fractures attenuate seismic energy more than smaller ones for the orientation effect fractures oriented in the direction of propagation seem to affect the wavefield more than those perpendicular because of the incident wave striking the fracture at an angle greater than the critical angle experiments on clustering of the fractures indicate that although clusters which are large compared to the wavelength may attenuate and shield more than for a uniform distribution smaller ones in fact attenuate less because of the healing effect these are important results when trying to characterize the fracture properties including density clustering size and orientation of a fractured reservoir from field seismic data

time scales of pattern evolution from cross spectrum analysis of advanced very high resolution radiometer and coastal zone color scanner imagery we have selected square subareas 110 km on a side from coastal zone color scanner czcs and advanced very high resolution radiometer avhrr images for 1981 in the california current region off northern california for which we could identify sequences of cloud free data over periods of days to weeks we applied a two dimensional fast fourier transform to images after median filtering x y plane removal and cosine tapering we formed autospectra and coherence spectra as functions of a scalar wavenumber coherence estimates between pairs of images were plotted against time separation between images for several wide wavenumber bands to provide a temporal lagged coherence function the temporal rate of loss of correlation decorrelation time scale in surface patterns provides a measure of the rate of pattern change or evolution as a function of spatial dimension we found that patterns evolved or lost correlation approximately twice as rapidly in upwelling jets as in the quieter regions between jets the rapid evolution of pigment patterns lifetime of about 1 week or less for scales of 50 100 km ought to hinder biomass transfer to zooplankton predators compared with phytoplankton patches that persist for longer times we found no significant differences between the statistics of czcs and avhrr images spectral shape or rate of decorrelation in addition in two of the three areas studied the peak correlation between avhrr and czcs images from the same area occurred at zero lag indicating that the patterns evolved simultaneously in the third area maximum coherence between thermal and pigment patterns occurred when pigment images lagged thermal images by 1 2 days mirroring the expected lag of high pigment behind low temperatures and high nutrients in recently upwelled water we conclude that in dynamic areas such as coastal upwelling systems the phytoplankton cells identified by pigment color patterns behave largely as passive scalars at the mesoscale and that growth death and sinking of phytoplankton collectively play at most a marginal role in determining the spectral statistics of the pigment patterns

finite difference modelling of s wave splitting in anisotropic media we have implemented a 3d finite difference scheme to simulate wave propagation in arbitrary anisotropic media the anisotropic media up to orthorhombic symmetry were modelled using a standard staggered grid scheme and beyond monoclinic and triclinic using a rotated staggered grid scheme the rationale of not using rotated staggered grid for all types of anisotropic media is that the rotated staggered grid schemes are more expensive than standard staggered grid schemes for a 1d azimuthally anistropic medium we show a comparison between the seismic data generated by our finite difference code and by the reflectivity algorithm they are in excellent agreement we conducted a study on zero offset shear wave splitting using the finite difference modelling algorithm using the rotated staggered grid scheme our s wave splitting study is mainly focused on fractured media on the scale of seismic wavelenghts small aligned fractures behave as an equivalent anisotropic medium we computed the equivalent elastic properties of the fractures and the background in which the fractures were embedded using low frequency equivalent media theories wave propagation was simulated for both rotationally invariant and corrugated fractures embedded in an isotropic background for one or more than one set of fluid filled and dry fractures s wave splitting was studied for dipping fractures two vertical non orthogonal fractures and corrugated fractures our modelling results confirm that s wave splitting can reveal the fracture infill in the case of dipping fractures s wave splitting has the potential to reveal the angle between the two vertical fractures we also notice that in the case of vertical corrugated fractures s wave splitting is sensitive to the fracture infill

the importance of small scale faulting in regional extension a recurring observation in many studies of extensional basins has been that the amount of extension visible on normal faults for example on seismic reflection profiles is significantly less than the amount of extension indicated by crustal thickness and thermal subsidence1 5 one mechanism suggested to account for this discrepancy is small scale faulting with offsets too small to be resolved seismically6 7 but earthquake studies8 10 indicate that small faults are responsible for only a small fraction of the total seismic moment in an active area scholz and cowie11 have recently attempted to extend this approach to the total strain at the end of a finite deformation interval by combining scaling laws describing the distributions of fault lengths and displacements here we present fault displacement data that directly conflict with scholz and cowie s conclusions and imply that up to 40 of the extension may be missed by summing fault offsets on basin profiles the fault population at the end of a long deformation interval may differ substantially from that responsible for the earthquake population at any one time

ice deformation near sheba 1 the deformation rate of sea ice is a key parameter for determining the evolution of the ice thickness distribution it determines the rate of new ice formation through opening and the rate of ridging through closing and shear an extensive suite of ground based and satellite based measurements of ice motion is used to construct a daily time series of the ice velocity and deformation in the vicinity of the surface heat budget of the arctic ocean sheba ice camp that is suitable for forcing a model of the ice thickness distribution the velocity is interpolated to a square grid that remains centered on the camp has a spacing of 25 km is 400 km on a side and is determined for a 371 day period from 2 october 1997 to 7 october 1998 velocity measurements from buoys advanced very high resolution radiometer avhrr special sensor microwave imager ssmi and radarsat geophysical processing system rgps are merged using optimal interpolation and a kalman filter approach the deformation rate is taken directly from the rgps measurements when available the daily total deformation rate measured on a scale of 100 km near the camp averaged 2 21 d 1 and the standard deviation was 1 78 d 1 the divergence was positive in the early winter and negative through most of the spring and summer there were two major opening closing events one in january and one at the end of july the net divergence over the year was very near zero the vorticity indicated a net rotation of 87 over the year with the winter showing strong anticyclonic turning and the summer showing strong cyclonic turning

nonlinear imaging condition and the effect of source illumination imaging fractures as nonwelded interfaces fluid flow through a fractured reservoir is often controlled by the large fractures seismically imaging these large fractures has the potential to illuminate their hydraulic properties we derived a nonlinear imaging condition considering a medium containing nonwelded interfaces such as fractures for high resolution fracture imaging this was achieved by using the general correlation type representation theorem relating the wavefield between two different states representing different fracture compliances we numerically tested the imaging condition to investigate the effect of the nonlinear term and that of the one sided source illumination assuming a dry fracture we calculated the wavefield from a nonwelded interface we obtained a p wave imaging result from p wave sources in the case of perfect source illumination we found that introducing the nonlinear term in the imaging condition enhances the image because the nonwelded interface is imaged as a thin layer in an otherwise homogeneous medium investigating the effect of one sided illumination by horizontally aligned sources revealed interesting limitations and possibilities the imaging result for a horizontal fracture showed a volumetric distribution of nonzero amplitudes around the polarity change at the fracture that can be misinterpreted as a welded thick layer boundary however when the fracture was not horizontal the imaging result was quite good and was closer to that with a perfect source illumination these led to a new possibility of imaging subvertical fractures from surface seismic measurements or subhorizontal fractures from vertical seismic profiling data assuming that we successfully estimated the perturbed wavefield from the receiver responses

analytical study of an isotropic viscoplastic sea ice model in idealized configurations abstractanalytic solutions of a mechanical sea ice model are computed in idealized configurations they are then used to study the properties of this model it classically assumes that the ice behaves at large scale as an isotropic viscoplastic medium the plastic regime is characterized by a mohr coulomb yield curve the flow rule corresponds to the one used in granular mediums and depends on a parameter that characterizes the expansion properties of the medium using simple model configurations this study first shows that a sliding of the ice along the coast must be permitted otherwise the model generally has no solution when the plastic regime is active this study then shows that the viscous regime is reached only if the stress remains nearly uniform over a large area for a stress having no particular properties the plastic regime acts everywhere in this case the compressive stress may reach the maximum value allowed by the model close to the coastline the extension of the domain where the c

sequential approach to joint flow seismic inversion for improved characterization of fractured media seismic interpretation of subsurface structures is traditionally performed without any account of flow behavior here we present a methodology for characterizing fractured geologic reservoirs by integrating flow and seismic data the key element of the proposed approach is the identification within the inversion of the intimate relation between fracture compliance and fracture transmissivity which determine the acoustic and flow responses of a fractured reservoir respectively owing to the strong but highly uncertain dependence of fracture transmissivity on fracture compliance the modeled flow response in a fractured reservoir is highly sensitive to the geophysical interpretation by means of synthetic models we show that by incorporating flow data well pressures and tracer breakthrough curves into the inversion workflow we can simultaneously reduce the error in the seismic interpretation and improve predictions of the reservoir flow dynamics while the inversion results are robust with respect to noise in the data for this synthetic example the applicability of the methodology remains to be tested for more complex synthetic models and field cases

extraction of reflected events from sonic log waveforms using the karhunen loeve transform abstractsonic reflection logging a recently developed borehole geophysical scheme is in principle capable of providing a clear view of outside the well bore in this type of acoustic well logging a key technical obstacle is that the reflected wave signal is almost entirely obscured by the directly arriving p s and stoneley wave modes effective extraction of these reflection signals from the full acoustic waveforms is therefore a critical data processing step we have examined the use of the karhunen loeve kl transform combined with a band limiting filter as a technique for the extraction of reflections of interest from a mixture with directly arriving wave modes of much higher amplitude under the assumption that large energy squared amplitude differences exist between each wave component the direct stoneley wave s wave and the p wave are eliminated sequentially by subtracting the most significant principal components after which the remaining signal is seen to be dominated by reflected

relating arctic pack ice stress and deformation under winter conditions 1 together thermodynamic and dynamic processes determine the thickness distribution of the ice cover on polar oceans which governs the exchange of energy between the atmosphere and the ocean key to the dynamic processes is the mechanical behavior of the ice cover during the surface heat budget of the arctic ocean sheba field experiment we deployed sensors to measure the internal ice stress at several locations within a 15 15 km area these measurements are combined with satellite derived ice motion and imagery products the objective is to make a first step toward using these data sources for evaluating sea ice dynamics models by assessing whether the stress signal can be qualitatively linked to the regional scale 10 100 km deformation activity four case studies are presented each with distinguishing characteristics consolidation of the seasonal ice zone against the alaskan coast 5 7 december 1997 advancement of the consolidation zone into the perennial ice pack 11 13 december extreme divergence 14 17 january and consolidation of the pack against wrangel island and the siberian coast 20 23 february the results of this analysis 1 demonstrate that stress measurements are related to the regional deformation behavior of the ice cover 2 confirm that regional scale ice dynamics is primarily a function of coastal geometry and sustained large scale wind direction and magnitude 3 provide continued evidence that the ice pack behaves as a granular hardening plastic and 4 encourage pursuit of efforts to use direct measurements of ice stress and deformation in the formulation and development of sea ice dynamics models

anisotropic internal thermal stress in sea ice from the canadian arctic archipelago results from an ice stress buoy deployed near the center of a multi year floe in the viscount melville sound of the canadian arctic archipelago between 10 october 2010 and 17 august 2011 are presented the position record indicates the landlocked season was approximately 5 months from 18 january to 22 june when the sea ice was fast to melville island and victoria island thermal stresses ranging from 84 to 66 kpa dominate the internal stress record with only a few dynamic stress events 50 kpa recorded before the landlocked season intriguingly the thermal stresses are isotropic before the landlocked ice onset and anisotropic during the landlocked season two possible causes to explain anisotropy in thermal stresses are considered preferred c axis alignment of the ice crystal and land confinement associated with the nearby coastline the orientation of the principal stresses indicates that land confinement is responsible for the anisotropy the stress record also clearly shows the presence of residual compressive stresses at the melt onset suggesting a viscous creep relaxation time constant of several days finally results show an interesting reversal in the sign of the correlation from negative to positive between surface air temperature and thermal stress after the onset of surface melt we attribute this to the onset of water infiltration within sea ice after which colder night temperature leads to refreezing and compressive stresses to the best of the authors knowledge this is the first time that anisotropic thermal stresses have been reported in sea ice

observations of pigment and particle distributions in the western north atlantic from an autonomous float and ocean color satellite profiling floats with optical sensors can provide important complementary data to satellite ocean color determinations by providing information about the vertical structure of ocean waters as well as surface waters obscured by clouds here we demonstrate this ability by pairing satellite ocean color data with records from a profiling float that obtained continuous high quality optical data for 3 yr in the north atlantic ocean good agreement was found between satellite and float data and the relationship between satellite chlorophyll and floatderived particulate backscattering was consistent with previously published data upper ocean biogeochemical dynamics were evidenced in float measurements which displayed strong seasonal patterns associated with phytoplankton blooms and depth and seasonal patterns associated with an increase in pigmentation per particle at low light surface optical variables had shorter decorrelation timescales than did physical variables unlike at low latitudes suggesting that biogeochemical rather than physical processes controlled much of the observed variability after 2 25 yr in the subpolar north atlantic between newfoundland and greenland the float crossed the north atlantic current to warmer waters where it sampled an unusual eddy for 3 months this anticyclonic feature contained elevated particulate material from surface to 1000 m depth and was the only such event in the float s record this eddy was associated with weakly elevated surface pigment and backscattering but depthintegrated backscattering was similar to that previously observed during spring blooms such seldom observed eddies if frequent are likely to make an important contribution to the delivery of particles to depth

seismic modeling of gas chimneys we propose a simple acoustic model explaining the main features of gas chimneys the main elements of the model consist of gas diffusing from a connected fracture network and into the surrounding shale creating an inhomogeneous gas saturation the gas saturation results in an inhomogeneous fluctuating compressional velocity field that distorts seismic waves we model the fracture network by a randomwalk process constrained by maximum fracture length and angleofthefracturewithrespecttothevertical thegassaturation is computed from a simple analytical solution of the diffusion equation and pressure wave velocities are locally obtained assuming that mixing of shale and gas occurs on a scalemuchsmallerthanseismicwavelengths syntheticseismic sections are then computed using the resulting inhomogeneous velocity model and shown to give rise to similar deterioration in data quality as that found in data from real gas chimneys also synthetic common midpointcmpgathers show the same distorted and attenuated traveltime curves as those obtained from a real data set the model shows clearly thatthefeaturesofgaschimneyschangewithgeologicaltime a model parameter in our approach the deterioration of seismicwavesbeingsmallestjustafterthecreationofthegas chimney it seems likely that at least some of the features of gas chimneys can be explained by a simple elastic model in combinationwithgasdiffusionfromafracturenetwork

generalized multiscale finite elements for simulation of elastic wave propagation in fractured media abstractwe applied the generalized multiscale finite element method gmsfem to simulate seismic wave propagation in fractured media fractures are represented explicitly on a fine scale triangular mesh and they are incorporated using the linear slip model the motivation for applying gmsfem is that it can reduce computational costs by using basis functions computed from the fine scale fracture model to simulate propagation on a coarse grid first we apply the method to a simple model that has a uniform distribution of parallel fractures at low frequencies the results could be predicted using a homogeneous effective medium but at higher frequencies gmsfem allows simulation of more complex scattered wavefields generated by the fractures without assuming a specific form of anisotropy a second more complex model has two fracture corridors in addition to a few sparsely distributed fractures simulations compare scattered wavefields for different acquisition geometries the third test case represents

modelling the reorientation of sea ice faults as the wind changes direction a discrete element model of sea ice is used to study how a 900 change in wind direction alters the pattern of faults generated through mechanical failure of the ice the sea ice domain is 400 km in size and consists of polygonal floes obtained through a voronoi tessellation initially the floes are frozen together through viscous elastic joints that can break under sufficient compressive tensile and shear deformation a constant wind stress gradient is applied until the initially frozen ice pack is broken into roughly diamond shaped aggregates with crack angles determined by wing crack formation then partial refreezing of the cracks delineating the aggregates is modelled through reduction of their length by a particular fraction the ice pack deformation is neglected and the wind stress is rotated by 90 degrees new cracks form delineating aggregates with a different orientation our results show the new crack orientation depends on the refrozen fraction of the initial faults as this fraction increases the new cracks gradually rotate to the new wind direction reaching 90 degrees for fully refrozen faults such reorientation is determined by a competition between new cracks forming at a preferential angle determined by the wing crack theory and at old cracks oriented at a less favourable angle but having higher stresses due to shorter contacts across the joints

computing ocean surface currents over the coastal california current system using 30 min lag sequential sar images as compared with conventional methods for measuring ocean surface currents spaceborne synthetic aperture radar sar offers cloud penetrating ocean current observation capability at high spatial resolution while some studies have shown the potential of sar for studying ocean surface currents through feature tracking they have only analyzed a few images to demonstrate the basic measurement technique and no concise general technique has been developed this paper shows the application of the maximum cross correlation mcc method to generate ocean surface currents from nearly two years of available sequential spaceborne c band sar imagery from the envisat asar and ers 2 advanced microwave instrument sar sensors over the coastal california current system the data processing strategies are discussed in detail and results are compared with hf radar measured currents one dimensional wavenumber spectra of the sar derived surface currents agree with the k 2 power law as predicted by submesoscale resolution models comparisons with hf radar currents show encouraging results with mcc sar vectors oriented slightly counterclockwise relative to hf radar vectors mcc sar surface currents are found to have larger magnitudes than hf radar currents approx 11 cm s which may be due to the fact that sar penetrates only a few centimeters into the ocean surface whereas hf radar currents are averaged over the top 1 m of the ocean surface the larger part of this magnitude difference is contained in the along shore component which can be attributed to higher hf radar accuracy in the direct radial cross shore measurements as compared with along shore components derived from multiple cross shore radial measurements

the fracture of ice on scales large and small arctic leads and wing cracks from observations and calculations of crack patterns in ice it is suggested that a similar mechanism may account for cracking over a wide range of scales

reservoir fracture characterization from seismic scattered waves the measurements of fracture parameters such as fracture orientation fracture density and fracture compliance in a reservoir is very important for field development and exploration traditional seismic methods for fracture characterization include shear wave birefringence gaiser and dok 2001 dok et al 2001 angerer et al 2002 vetri et al 2003 and amplitude variations with offset and azimuth avoa ruger 1998 shen et al 2002 hall et al 2003 liu et al 2010 lynn et al 2010 these methods are based on the equivalent medium theory with the assumption that fracture dimension and spacing are small relative to the seismic wave length so a fracture zone behaves like an equivalent anisotropic medium but fractures on the order of seismic wave length are also very important for enhanced oil recovery and they are one of the important subsurface scattering sources that generate scattered seismic waves willis et al 2006 developed the scattering index method to extract the fracture scattering characteristics by calculating the transfer funtion of a fracture zone fang et al 2011 proposed a modification of the si method the fracture transfer function ftf method that leads to a more robust fracture characterization in this paper we use both laboratory data and field data to explore the capability of the ftf method

fracture clustering effect on amplitude variation with offset and azimuth analyses abstracttraditional amplitude variation with offset and azimuth avoaz analysis for fracture characterization extracts fracture properties through analysis of reflection avoaz to determine anisotropic parameters e g thomsen s parameters that are then related to fracture properties the validity of this method relies on the basic assumption that a fractured unit can be viewed as an equivalent anisotropic medium as a rule of thumb this assumption is taken to be valid when the fracture spacing is less than 10 under the effective medium assumption diffractions from individual fractures destructively interfere and only specular reflections from boundaries of a fractured layer can be observed in seismic data the effective medium theory has been widely used in fracture characterization and its applicability has been validated through many field applications however through numerical simulations we find that diffractions from fracture clusters can significantly distort the avoaz signatures when a

the echo of a fault or fracture the seismic response of single faults joints or fractures of large planar extent is analyzed these are modeled as nonwelded interfaces in spite of the large range of scale all are assumed to behave according to linear slip theory such a model has been considered theoretically and experimentally before the aim of this paper is to give a physical interpretation to such a linear slip interface to provide simple analytical formulae for the scattering response even when the fracture is embedded in an anisotropic background medium and to relate the properties of this scattering response in the isotropic case to the physical features of the fracture the analysis shows that the reflectivity and transmissivity of the fracture depend on slowness along the fracture and on frequency the frequency dependence arises from the fact that even though the fracture is assumed to be an interface of zero thickness it still has at least two characteristic widths that provide the length scales necessary for scatt

poroelastic modeling of seismic boundary conditions across a fracture permeability of a fracture can affect how the fracture interacts with seismic waves to examine this effect a simple mathematical model that describes the poroelastic nature of wave fracture interaction is useful in this paper a set of boundary conditions is presented which relate wave induced particle velocity or displacement and stress including fluid pressure across a compliant fluid bearing fracture these conditions are derived by modeling a fracture as a thin porous layer with increased compliance and finite permeability assuming a small layer thickness the boundary conditions can be derived by integrating the governing equations of poroelastic wave propagation a finite jump in the stress and velocity across a fracture is expressed as a function of the stress and velocity at the boundaries further simplification for a thin fracture yields a set of characteristic parameters that control the seismic response of single fractures with a wide range of mechanical and hydraulic properties these boundary conditions have potential applications in simplifying numerical models such as finite difference and finite element methods to compute seismic wave scattering off nonplanar e g curved and intersecting fractures

impact of a new anisotropic rheology on simulations of arctic sea ice 1 a new rheology that explicitly accounts for the subcontinuum anisotropy of the sea ice cover is implemented into the los alamos sea ice model this is in contrast to all models of sea ice included in global circulation models that use an isotropic rheology the model contains one new prognostic variable the local structure tensor which quantifies the degree of anisotropy of the sea ice and two parameters that set the time scale of the evolution of this tensor the anisotropic rheology provides a subcontinuum description of the mechanical behavior of sea ice and accounts for a continuum scale stress with large shear to compression ratio and tensile stress component results over the arctic of a stand alone version of the model are presented and anisotropic model sensitivity runs are compared with a reference elasto visco plastic simulation under realistic forcing sea ice quickly becomes highly anisotropic over large length scales as is observed from satellite imagery the influence of the new rheology on the state and dynamics of the sea ice cover is discussed our reference anisotropic run reveals that the new rheology leads to a substantial change of the spatial distribution of ice thickness and ice drift relative to the reference standard visco plastic isotropic run with ice thickness regionally increased by more than 1 m and ice speed reduced by up to 50

spatial scaling of arctic sea ice deformation 1 arctic sea ice deformation arises from spatial gradients in the ice velocity field this deformation occurs across a wide range of spatial scales from meters to thousands of kilometers we analyze 7 years of sea ice deformation data from the radarsat geophysical processor system rgps covering the western arctic ocean we find that the mean deformation rate is related to the spatial scale over which it is measured according to a power law with exponent 0 2 over a scale range from 10 to 1000 km e g deformation rate doubles for a 30 fold reduction in scale both the exponent and the deformation rate have distinct annual cycles the exponent becomes more negative in summer as the ice pack weakens and internal stresses are not as readily transmitted over long distances the deformation rate reaches a minimum in late winter when the ice pack is strongest the deformation also exhibits considerable localization in which the largest deformation rates are confined to smaller and smaller areas as the scale of measurement decreases this supports a model for sea ice based on granular or fracture mechanics the scaling exponent in the power law relationship tends to be larger in magnitude where the concentration of multiyear ice is low consistent with a thinner and weaker ice pack with decreasing multiyear ice in the arctic and a thinning ice pack an increase in the deformation rate has already been documented from buoy data however the net effect of several deformation thickness feedbacks is still uncertain

seismic modelling for geological fractures based on knowledge of a commutative group calculation of the rock stiffness and on some geophysical assumptions the simplest fractured medium may be regarded as a fracture embedded in an isotropic background medium and the fracture interface can be simulated as a linear slip interface that satisfies nonwelded contact boundary conditions the kinematic displacements are discontinuous across the interface whereas the dynamic stresses are continuous across the interface the finite difference method with boundary conditions explicitly imposed is advantageous for modelling wave propagation in fractured discontinuous media that are described by the elastic equation of motion and nonwelded contact boundary conditions in this paper finite difference schemes for horizontally vertically and orthogonally fractured media are derived when the fracture interfaces are aligned with the boundaries of the finite difference grid the new finite difference schemes explicitly have an additional part that is different from the conventional second order finite difference scheme and that directly describes the contributions of the fracture to the wave equation of motion in the fractured medium the numerical seismograms presented to first order show that the new finite difference scheme is accurate and stable and agrees well with the results of previously published finite difference schemes the coates and schoenberg method the results of the new finite difference schemes show how the amplitude of the reflection produced by the fracture varies with the fracture compliances later comparisons with the reflection coefficients indicate that the reflection coefficients of the fracture are frequency dependent whereas the reflection coefficients of the impedance contrast interface are frequency independent in addition the numerical seismograms show that the reflections of the fractured medium are equal to the reflections of the background medium plus the reflections of the fracture in the elastic fractured medium this article is protected by copyright all rights reserved

a dynamical model of kara sea land fast ice this paper introduces modifications to the traditional viscous plastic sea ice dynamical model which are necessary to model land fast ice in the kara sea in a realistic manner the most important modifications are an increase in the maximum viscosity from the standard value of max 2 5 108s p to max 1013s p and to use a solver for the momentum equation capable of correctly solving for small ice velocities the limit here is set to 10 4 m s given these modifications a necessary condition for a realistic fast ice simulation is that the yield curve give sufficient uniaxial compressive strength this is consistent with the idea that land fast ice in the kara sea forms primarily via static arching the modified model is tested and tuned using forcing data and observations from 1997 and 1998 the results show that it is possible to model land fast ice using this model with the modifications mentioned above the model performs well in terms of modeled fast ice extent but suffers from unrealistic break ups during the start and end of the fast ice season the main results are that fast ice in the kara sea is supported by arching of the ice the arches footers resting on a chain of islands off shore

finite difference modeling of sh wave propagation in nonwelded contact media many geological structures of interest are known to exhibit fracturing fracturing directly affects seismic wave propagation because depending on its scale fracturing may give rise to scattering and or anisotropy a fracture may be described mathematically as an interface in nonwelded contact i e as a displacement discontinuity this poses a difficulty for finite difference modeling of seismic wave propagation in fractured media because the standard heterogeneous approach assumes welded contact in the past this difficulty has been circumvented by incorporating nonwelded contact into the medium parameters using equivalent medium theory we present an alternate method based on the homogeneous approach to finite differencing whereby nonwelded contact boundary conditions are imposed explicitly for simplicity we develop the method in the sh wave case in the homogeneous approach nonwelded contact boundary conditions are discretized by introducing auxiliary so called fictitious grid points wavefi

an effective data processing flow for the acoustic reflection image logging abstract recent studies have revealed the great potential of acoustic reflection logging in detecting near borehole fractures and vugs the new design of acoustic reflection imaging tool with a closest spacing of 10 6m and a certain degree of phase steering makes it easier to extract the reflection signals from the borehole mode waves for field applications of the tool we had developed the corresponding processing software acoustic reflection imaging in this paper we have further developed an effective data processing flow by employing multi scale slowness time coherence for reflection wave extraction and incorporating reverse time migration for imaging complicated subtle structures with the strong effects of borehole environment applications of the processing flow to synthetic data of acoustic reflection logging in a fractured formation model and interface model with fluid filled borehole generated by 2d finite difference method and to the physical modelling data from a laboratory water tank as well as to the field data from two wells in a western chinese oil field demonstrate the validity and capability of our multi scale slowness time coherence and reverse time migration algorithms

generation of spatially correlated fracture models for seismic simulations summary the critical geometrical parameters that quantify the spatial distribution of natural fractures are the orientation length and position of fractures knowledge of their spatial distribution is important as they control the movement of subsurface fluids and also influence seismic waves propagating in the subsurface however generating realistic models of all of these geometrical parameters to use in forward seismic modelling or inversion applications can become very difficult especially when constraints are available only at a few sparse well locations hence this provides strong motivation for applying seismic data to estimate these quantities in field settings and reliable seismic modelling provides important constraints for interpretation and inversion the discrete fracture network dfn approach has been used frequently to generate models with stochastic distributions of fractures based on sparse well and seismic data however most of these studies lack any constraint from physical models of the behaviour of fractured media in this paper we implement and extend an alternative modelling technique to generate several realizations of a fracture model beginning with theoretical results for the strain energy of a fractured material and propose ways to better incorporate geological field observations the method utilizes an elastic energy function that sums the interactions of all pairs of fractures present in the model the energy for each pair depends on the distance between the two fractures their orientations lengths and some material properties this energy function also serves as an objective function for a simulated annealing sa algorithm used to obtain multiple realizations of correlated fracture networks we improve earlier versions of this technique by incorporating periodic boundary conditions including criteria to limit the maximum range of pair wise calculations and suggesting methods to constrain models to match field data assuming that the host rock is isotropic and homogeneous this method produces orthogonal sets of fractures a pattern that is commonly observed during basin formation or subsidence this shows how the method can be utilized to provide constraints with a physical basis to facilitate the development of fracture models we also outline how the fracture model can be converted into an anisotropic seismic velocity model to compute synthetic seismograms for reservoir characterization applications

time evolution of surface chlorophyll patterns from cross spectrum analysis of satellite color images sequences of coastal zone color scanner czcs images from the offshore region adjacent to vancouver island canada have been analyzed to estimate the time rate of decorrelation of surface phytoplankton chlorophyll pigment patterns in these high latitude high pigment areas czcs derived pigment estimates were lower than those obtained from ship samples by about a factor of 3 their frequency distributions were skewed in opposite directions and subareas of the images often showed a discontinuity in the frequency distribution at a concentration of 1 5 mg m 3 where the algorithm changes czcs bands we selected cloud free subareas that were common to several images separated in time by 1 17 days image pairs were subjected to two dimensional auto spectrum and cross spectrum analysis in an array processor and spectra of squared coherence were formed the squared coherence estimates for several wave bands were plotted against time separation in analogy with a time lagged cross correlation function threshold levels for significant coherence were estimated from many realizations of squared coherence calculated for pairs of synthetic random uncorrelated fields with specified power law behavior 1 5 near the observed range 1 5 2 for wavelengths of 50 150 km significant coherence is lost after 7 10 days and for wavelengths of 25 50 km significant coherence is lost after 5 7 days in both cases offshore regions maintain coherence longer than coastal regions for wavelengths of 12 5 25 km only the offshore regions maintained coherence after 1 day but that was clearly lost after the next time separation of 6 days the implication for the formation of monthly average large scale surface maps to estimate open ocean productivity e g esaias et al 1986 is that all mesoscale patterns 150 km length scale will not be resolved

3d simulation of seismic wave propagation in fractured media using an integral method accommodating irregular geometries fractures play an important role in most carbonate and unconventional reservoirs precise identification of fractures and their associated properties from seismic data have a significant impact on reservoir management and hydrocarbon recovery traditional seismic methods for fracture identification and characterization include shear birefringence and amplitude variations with offset and azimuth these techniques are based on equivalent medium theory assuming that fracture dimensions and spacing are small relative to the seismic wavelength hence the overall population of fractures is equivalent to a homogeneous anisotropic medium we focus on discrete fracture sets with spacing on the order of the seismic wavelength where individual fractures have large areas but small apertures such fracture sets scatter seismic waves and they are important to characterize because they affect fluid flow in the subsurface analyzing scattering induced wave patterns observed in seismograms can provide crucial guidanc

a bayesian framework for fracture characterization from surface seismic data we describe a methodology for quantitatively characterizing the fractured nature of a hydrocarbon or geothermal reservoir from surface seismic data under a bayesian inference framework fractures provide pathways for fluid flow in a reservoir and hence knowledge about a reservoir s fractured nature can be used to enhance production of the reservoir the fracture properties of interest in this study to be inferred are fracture orientation and excess compliance where each of these properties are assumed to vary spatially over a 2d lateral grid which is assumed to represent the top of a reservoir the bayesian framework in which the inference problem is cast has the key benefits of 1 utilization of a prior model that allows geological information to be incorporated 2 providing a straightforward means of incorporating all measurements across the 2d spatial grid into the estimates at each grid point 3 allowing different types of measurements to be combined under a single inference procedure and 4 providing a measure of uncertainty in the estimates the observed data are taken from a 2d array of surface seismic receivers responding to an array of surface sources well understood features from the seismic traces are extracted and treated as the observed data namely the pwave reflection amplitude variation with acquisition azimuth amplitude versus azimuth or avaz data and fracture transfer function ftf data avaz data are known to be more sensitive to fracture properties when the fracture spacing is significantly smaller than the seismic wavelength whereas fracture transfer function data are more sensitive to fracture properties when the fracture spacing is on the order of the seismic wavelength combining these two measurements has the benefit of allowing inferences to be made about fracture properties over a larger range of fracture spacing than otherwise attainable geophysical forward models for the measurements are used to arrive at likelihood models for the data the prior distribution for the hidden fracture variables is obtained by defining a markov random field mrf over the lateral 2d grid where we wish to obtain fracture properties the fracture variables are then inferred by application of loopy belief propagation lbp to yield approximations for the posterior marginal distributions of the fracture properties as well as the maximum a posteriori map and bayes least squares bls estimates of these properties verification of the inference procedure is performed on a synthetic dataset where the estimates obtained are shown to be at or near ground truth for a large range of fracture spacings

failure envelope of first year arctic sea ice the role of friction in compressive fracture 1 new experiments have been performed on the brittle compressive failure of columnar grained first year sea ice with the s2 growth texture harvested from the floating cover on the arctic ocean the ice was proportionally loaded biaxially across the columns at 10 c at 1 5 10 2 s 1 i e within the regime of brittle behavior the results when combined with earlier measurements of tensile strength by richter menge and jones 1993 allow the complete brittle failure envelope to be constructed the envelope is symmetric about the loading path r 2 1 1 owing to the isotropic character of the material within the horizontal plane of the ice sheet but is asymmetric with respect to the compressive compressive and tensile tensile quadrants owing to the relative weakness of the material under tension the compressive strength reaches a maximum along the loading path r 0 5 terminal failure occurs through either splitting unconfined loading coulombic shear faulting under lower confinement r 0 2 or spalling under higher confinement 0 2 r 1 an integral component of compressive failure under low confinement is the internal friction coefficient its value under the conditions of the experiments i 0 92 0 09 is deduced from the failure envelope the internal friction coefficient is shown through postterminal failure measurements to be closely similar to the coefficient of friction at the onset of sliding along the coulombic faults granite exhibits similar behavior modeling reveals that the terminal failure stress under low confinement can be reasonably well described in terms of the comb crack mechanism schulson et al 1999 renshaw and schulson 2001 of brittle compressive failure comparison with observations and measurements in the field indicate that the failure processes that operate on the smaller scale are similar to at least some of those that operate on the larger scale implying that the physics of failure may be scale independent

including poroelastic effects in the linear slip theory abstractnumerical simulations of seismic wave propagation in fractured media are often performed in the framework of the linear slip theory lst therein fractures are represented as interfaces and their mechanical properties are characterized through a compliance matrix this theory has been extended to account for energy dissipation due to viscous friction within fluid filled fractures by using complex valued frequency dependent compliances this is however not fully adequate for fractured porous rocks in which wave induced fluid flow wiff between fractures and host rock constitutes a predominant seismic attenuation mechanism in this letter we develop an approach to incorporate wiff effects directly into the lst for a 1d system via a complex valued frequency dependent fracture compliance the methodology is validated for a medium permeated by regularly distributed planar fractures for which an analytical expression for the complex valued normal compliance is determined in the framework of quas

changes in zooplankton habitat behavior and acoustic scattering characteristics across glider resolved fronts in the southern california current system abstract we report cross frontal changes in the characteristics of plankton proxy variables measured by autonomous spray ocean gliders operating within the southern california current system sccs a comparison of conditions across the 154 positive frontal gradients i e where density of the surface layer decreased in the offshore direction identified from six years of continuous measurements showed that waters on the denser side of the fronts typically showed higher chl a fluorescence shallower euphotic zones and higher acoustic backscatter than waters on the less dense side transitions between these regions were relatively abrupt for positive fronts the amplitude of diel vertical migration dvm inferred from a 3 beam 750 khz acoustic doppler profiler increased offshore of fronts and covaried with optical transparency of the water column average interbeam variability in acoustic backscatter also changed across many positive fronts within 3 depth strata 0 150 m 150 400 m and 400 500 m revealing a front related change in the acoustic scattering characteristics of the assemblages the extent of vertical stratification of distinct scattering assemblages was also more pronounced offshore of positive fronts depth stratified zooplankton samples collected by mocness nets corroborated the autonomous measurements showing copepod dominated assemblages and decreased zooplankton body sizes offshore and euphausiid dominated assemblages with larger median body sizes inshore of major frontal features

scattering by hydraulic fractures finite difference modeling and laboratory data reservoir production can be stimulated by creating hydraulic fractures that effectively facilitate the inflow of hydrocarbons into a well considering the effectiveness and safety of the operation it is desirable to monitor the size and location of the fracture in this paper we investigate the possibilities of using seismic waves generated by active sources to characterize the fractures first we must understand the scattering of seismic waves by hydraulic fractures for that purpose we use a finite difference modeling scheme we argue that a mechanically open hydraulic fracture can be represented by a thin fluid filled layer the width or aperture of the fracture is often small compared to the seismic wavelength which forces us to use a very fine grid spacing to define the fracture based on equidistant grids this results in a large number of grid points and hence computationally expensive problems we show that this problem can be overcome by allowing for a variation in grid spacing in the finite

two dimensional finite difference seismic modeling of an open fluid filled fracture comparison of thin layer and linear slip models within the context of seismic wave propagation fractures can be described as thin layers or linear slip interfaces in this paper numerical simulations of elastic wave propagation in a medium with a single fracture represented by these two models are performed by 2d finite difference codes a variable grid isotropic code for the thin layer model and a regular grid anisotropic code for the linear slip model numerical results show excellent agreement between the two models for wavefields away from the fracture the only discrepancy between the two is the presence of a slow wave traveling primarily within the fracture fluid of the thin layer model the comparison of the computational cost shows that modeling of the linear slip model is more efficient than that of the thin layer model this study demonstrates that the linear slip model is an efficient and accurate modeling approach for the remote seismic characterization of fractures

a field investigation of fracture compliance a field measurement of fracture compliance is described the aim was to determine how compliance scales with fracture size and specifically how laboratory measurements of fracture compliance compared with field estimates from sonic and seismic data a test site was constructed consisting of three 40 m vertical boreholes drilled in the floor of a carboniferous limestone quarry detailed knowledge of the rocks in the test area was obtained from core analysis wireline logging and local area fracture mapping seismic cross hole surveys were performed using a sparker source with a dominant frequency of 2000 hz and hydrophone receivers the rocks had a compressional wave velocity anisotropy of 10 which was attributed to the presence of predominantly horizontal partially open fractures estimates of normal fracture compliance within a range from 2 5 10 13 m pa to 3 5 10 12 m pa were obtained from both the cross hole data and the sonic log data this is an order of magnitude greater than values obtained from laboratory experiments which are reported elsewhere

numerical simulation of wave propagation in media with discrete distributions of fractures effects of fracture sizes and spatial distributions summary we model seismic wave propagation in media with discrete distributions of fractures using the pseudospectral method the implementation of fractures with a vanishing width in the 2 d finite difference grids is done using an effective medium theory that is the coates and schoenberg method fractures are treated as highly compliant interfaces inside a solid rock mass for the physical representation of the fractures the concept of linear slip deformation or the displacement discontinuity method is used according to this model the effective compliance of a rock mass with one or several fracture sets can be found as the sum of the compliances of the host background rock and those of all the fractures to first order the background rock and fracture parameters can be related to the effective anisotropic coefficients which govern the influence of anisotropy on various seismic signatures we test the validity of the method and examine the accuracy of the synthetic seismograms by a comparison with theoretical ray traveltimes we present three numerical examples to show the effects of different fracture distributions the first example shows that different spatial distributions of the same fractures produce different wavefield characteristics the second example examines the effects of variation of fracture scale length size compared with the wavelength the final example examines the case of fractures with a power law fractal distribution of sizes and shows how that affects the wavefield propagation in fractured rock we conclude that characterization of fractured rock based on the concept of seismic anisotropy using effective medium theories must be used with caution scale length and the spatial distributions of fractures which are not properly treated in such theories have a strong influence on the characteristics of wave propagation

elastic decohesive constitutive model for sea ice 1 one of the dominant characteristics of arctic ice is the development and persistence of leads leads are critically important for climate modeling since they are a source for new ice and an avenue for heat exchange between the atmosphere and ocean therefore a specific representation for leads in a constitutive model for pack ice is considered essential presented here is an elastic decohesive constitutive model whose purpose is to specifically indicate when a lead is initiated to provide the orientation of the lead to give the mode of failure and to allow for the numerical determination of the width of leads both laboratory and in situ data of sea ice provide the motivation for the basic structure of the model sample paths in stress and strain space are used to illustrate how the model can simulate failure the model can predict multiple failure planes a feature that is necessary to simulate crack branching preexisting planes of weakness such as those formed from previous leads can also be accommodated

modeling the high frequency component of arctic sea ice drift and deformation abstract buoy observations of sea ice drift show that sea ice motion and deformation contain substantial high frequency variability at subdaily timescales however numerical simulations of the sea ice dynamics normally do not include processes on such short timescales instead by applying traditional water drag formulations for the ocean sea ice boundary layer the inertial frequencies tend to be omitted in the spectra of modeled sea ice motion and deformation due to fictitious ocean damping here a dynamic thermodynamic sea ice model is presented that includes inertial embedding of the sea ice into the oceanic boundary layer to reproduce sea ice motion and deformation not only at daily and lower frequencies but also at subdaily frequencies up to 2 cycles per day and higher comparison with buoy data shows that the embedded model much more successfully simulates the observed sea ice motion in regard to the high frequency component the results demonstrate that both inertial embedding of the sea ice oc

3d seismic characterization of fractures in a dipping layer using the double beam method the characterization of natural and induced fractures in terms of fracture orientation fracture spacing or density and fracture compliance is critical in reservoir development given the multiscale nature of the fracture distribution the commonly used effective anisotropy assumption may not be valid the recently proposed double beam method to characterize the fractured reservoir has the potential to invert for the spatially dependent fracture network information for a horizontal reservoir layer however the inverted results can be biased if the fractured reservoir layer is dipping as a result it is essential to estimate and include the dip angle information of reservoir layers in applying the double beam method we used forward modeling to demonstrate the bias and proposed a new method to correct for the error caused by the reservoir layer dip angle for a dipping layer our new method can correctly invert for the fracture parameters including the fracture orientation and fracture spacing

effect of sea ice rheology in numerical investigations of climate 1 plastic sea ice rheologies that employ teardrop and parabolic lens yield curves and allow varying biaxial tensile stresses have been developed these rheologies together with the previously developed ellipse and mohr coulomb ellipse rheologies are implemented in a thickness and enthalpy distribution sea ice model to examine the rheological effect in numerical investigations of arctic climate the teardrop lens and ellipse rheologies obey a normal flow rule and result in a two peak shear stress distribution the first peak is at the zero shear stress the second is near 16 000 n m 1 for the ellipse and two lens rheologies and near 30 000 n m 1 for the two teardrop rheologies the location of the second peak depends on the fatness of the yield curve and the amount of biaxial tensile stress allowed in contrast the mohr coulomb ellipse rheology based on coulombic friction failure does not tend to create the second peak the incorporation of biaxial tensile stress tends to increase ice thickness in most of the arctic a fatter yield curve tends to increase the frequency of large shear stresses an increased frequency of large shear stresses in conjunction with the inclusion of biaxial tensile stress tends to reduce ice speed and ice export to enhance ice ridging in the arctic interior and to reduce ice ridging in the coastal areas which has a significant impact on arctic spatial ice mass distribution and the total ice budget the teardrop rheologies reduce spatial bias of modeled ice draft against submarine observations more than others by changing ice motion deformation and thickness the choice of plastic rheology also considerably affects the simulated surface energy exchanges particularly in the arctic marginal seas

seismic characterization of fractured reservoirs by focusing gaussian beams abstractnaturally fractured reservoirs occur worldwide and they account for the bulk of global oil production the most important impact of fractures is their influence on fluid flow to maximize oil production the characterization of a fractured reservoir at the scale of an oil field is very important for fluid transport the critical parameters are connectivity and transmittivity plus orientation these can be related to fracture spacing compliance and orientation which are the critical seismic parameters of rock physics models we discovered a new seismic technique that can invert for the spatially dependent fracture orientation spacing and compliance using surface seismic data unlike most seismic methods that rely on using singly scattered diffracted waves whose signal to noise ratios are usually very low we found that waves multiply scattered by fractures can be energetic the direction information of the fracture multiply scattered waves contains fracture orientation and spacing informati

sea ice kinematics and surface properties from radarsat synthetic aperture radar during the sheba drift 1 satellite data are important for providing the large scale context of the surface heat budget of the arctic ocean sheba station and for characterizing the spatial variability of the sea ice in its vicinity the canadian radarsat satellite collected 195 synthetic aperture radar sar images of the sheba site over the course of the 1 year drift the radarsat geophysical processor system rgps used these images to compute the spatial pattern of ice motion within 100 km of the sheba station by tracking features in sequential images from the ice motion data the divergence and shear of the pack ice are estimated the divergence is large from november to january followed by a gradual convergence from february through july the character of the ice motion changes at the end of july from piecewise rigid motion to free drift the ice motion reverts to its winterlike character in late september thus the kinematic summer runs from late july to late september the radar backscatter also goes through seasonal transitions capturing the abrupt onset of melt 29 may and freeze up 15 august the concentration of multiyear ice is about 94 in the fall and its backscatter signature remains stable through spring multiyear and first year ice cannot be distinguished during the summer melt season when the mean backscatter is negatively correlated with the surface air temperature the thermodynamic summer runs from late may to mid august index terms 4207 oceanography general arctic and antarctic oceanography 4275 oceanography general remote sensing and electromagnetic processes 0689 4540 oceanography physical ice mechanics and air sea ice exchange processes keywords sea ice synthetic aperture radar sar sheba

compressive shear faults within arctic sea ice fracture on scales large and small 1 we examined fracture features within arctic sea ice as revealed by landsat 7 and radarsat imagery by an aerial survey and through observations in the laboratory of specimens loaded to brittle failure under biaxial compression regardless of scale which spans the range from kilometers to millimeters the features look alike they consist mainly of narrow lineaments that traverse the field of view plus wing like and comb like secondary cracks the lineaments occasionally intersect and generally exhibit either right lateral or left lateral relative movement like strike slip faults within earth s crust we term them brittle compressive shear faults they form through the linking of en echelon arrays of deformation induced secondary cracks from an application of wing crack and comb crack mechanics we estimated the maximum compressive stress near the onset of faulting and found that our estimates compare favorably with in situ measurements by earlier investigators of ice sheet failure stresses to account for the brittle behavior of the sea ice cover we applied a recent model of the ductile to brittle transition in which the key idea is the competition between stress buildup and stress relaxation at stress concentrators in identifying the nature of sea ice fracture features we advance the view that failure occurs on many scales through highly localized as opposed to uniformly distributed deformation via the operation of scale independent mechanisms

sub daily sea ice motion and deformation from radarsat observations 1 we find a persistent level of oscillatory sea ice motion and deformation superimposed on the large scale wind driven field in may 2002 spring and february 2003 mid winter in the high arctic over a region centered at 85 n 135 w at this latitude the radarsat wide swath sar coverage provides 4 5 sequential observations every day for ice motion retrieval with a sampling interval at the orbital period of 101 minutes periodic correlations in ice motion and deformation can be seen in length scales from 10 km and above and suggest a 12 hr oscillation that is more likely associated with inertial rather than tidal frequencies divergence convergence of 0 1 0 2 peak to peak or rates of 10 7 s is seen in both datasets with the mid winter dataset having smaller values these observations are remarkable in that short period ice motion is previously believed to be inhibited by the strength of the ice pack in the high arctic during winter new ice production due to the recurrent openings and closings at these temporal scales if ubiquitous could be significant within the winter pack

finite difference modeling of faults and fractures for the purposes of seismic propagation a slip fault may be regarded as a surface across which the displacement caused by a seismic wave is discontinuous while the stress traction remains continuous the simplest assumption is that this slip and the stress traction are linearly related such a linear slip interface condition is easily modeled when the fault is parallel to the finite difference grid but is more difficult to do for arbitrary nonplanar fault surfaces to handle such situations we introduce equivalent medium theory to model material behavior in the cells of the finite difference grid intersected by the fault virtually identical results were obtained from modeling the fault by 1 an explicit slip interface condition fault parallel to the grid and 2 using the equivalent medium theory when the finite difference grid was rotated relative to the fault and receiver array no additional computation time is needed except for the preprocessing required to find the relevant cells and their associated moduli the formulation is sufficiently general to include faults in and between arbitrary anisotropic materials with slip properties that vary as a function of position

results from a three dimensional nested biological physical model of the california current system and comparisons with statistics from satellite imagery 1 a three dimensional model of the california current system ccs from 35 n to 48 n extending offshore to 134 w is coupled with a four component trophic model the model reproduces many conspicuous characteristics in the ccs including complex filamentary mesoscale surface features seen in the pigment and temperature from satellite imagery wind driven coastal upwelling at appropriate spatial and temporal scales and the close correlation between prominent features seen in pigment and those in temperature observed by satellites abbott and zion 1985 statistical estimates of the characteristic spatial scales of variability as calculated from the coupled nested model agree with those previously estimated from satellite images for both surface temperature and pigment denman and abbott 1988 1994 model estimates of the characteristic temporal scales of variability from decorrelation times agree with those previously estimated from satellite images typical model decorrelation times lie between 2 and 4 days in agreement with calculations from earlier sequences of coastal zone color scanner czcs and advanced very high resolution radiometer avhrr satellite images denman and abbott 1988 1994

tracking oceanic currents by singularity analysis of microwave sea surface temperature images in the recent years the power of employing singularity analysis of scalar variables to perform pattern recognition in complex images has been evidenced this approach is particularly useful when the image under study corresponds to a scalar variable submitted to a turbulent flow because in that case the arrangement of singularity manifolds corresponds to the multifractal hierarchy from the underlying turbulent flow in this paper we will show how this intimate connection between image processing and physics when applied to microwave sea surface temperature images allows to uncover global circulation patterns in the ocean at a daily basis with the resolution of 1 4 of degree using this technique details not evidenced in the non processed image are revealed the emerging patterns provide a description of the global oceanic currents much richer than the usual global circulation scheme in particular instabilities eddy generation and filamentation are distinctly identified a pattern extraction of this kind is useful in order to measure and track mesoscale oceanic phenomena opening the way to many operational and reanalysis applications

remote sensing of southern ocean sea surface temperature implications for marine biophysical models nineteen years of advanced very high resolution radiometer multi channel sea surface temperature avhrr mcsst data were used to calculate monthly averages of sea surface temperatures ssts for a large region of the southern ocean centred on macquarie island between october and february the mcsst data were a reliable source of ssts north of 60 s but their quality i e spatial and temporal density was degraded severely at higher latitudes between april and august the interpolated mcsst data were found to be unreliable in the study area even at latitudes as low as 45 s a specific problem has been identified to the west of tasmania in the winter interpolated climatology which is likely an artefact of interpolation of sparse data the poor coverage of valid mcssts during this time limits the data s application for many uses although the limitations of the mcsst data set are well recognised the possibilities for ecological applications of these data remain largely unexplored and unquantified the potential exists to maximize the information available from this and other similar data sets by determining the appropriate spatial and temporal scales at which they are best applied to biophysical models

a rate and state friction law for saline ice sea ice friction models are necessary to predict the nature of interactions between sea ice floes these interactions are of interest on a range of scales for example to predict loads on engineering structures in icy waters or to understand the basin scale motion of sea ice many models use amonton s friction law due to its simplicity more advanced models allow for hydrodynamic lubrication and refreezing of asperities however modeling these processes leads to greatly increased complexity in this paper we propose by analogy with rock physics that a rate and state dependent friction law allows us to incorporate memory and thus the effects of lubrication and bonding into ice friction models without a great increase in complexity we support this proposal with experimental data on both the laboratory 0 1 m and ice tank 1 m scale these experiments show that the effects of static contact under normal load can be incorporated into a friction model we find the parameters for a first order rate and state model to be a 0 310 b 0 382 and m0 0 872 such a model then allows us to make predictions about the nature of memory effects in moving ice ice contacts

spectral element modelling of fault plane reflections arising from fluid pressure distributions the presence of fault plane reflections in seismic images besides indicating the locations of faults offers a possible source of information on the properties of these poorly understood zones to better understand the physical mechanism giving rise to fault plane reflections in compacting sedimentary basins we numerically model the full elastic wavefield via the spectral element method sem for several different fault models using well log data from the south eugene island field offshore louisiana we derive empirical relationships between the elastic parameters e g p wave velocity and density and the effective stress along both normal compaction and unloading paths these empirical relationships guide the numerical modelling and allow the investigation of how differences in fluid pressure modify the elastic wavefield we choose to simulate the elastic wave equation via sem since irregular model geometries can be accommodated and slip boundary conditions at an interface such as a fault or fracture are implemented naturally the method we employ for including a slip interface retains the desirable qualities of sem in that it is explicit in time and therefore does not require the inversion of a large matrix we perform a complete numerical study by forward modelling seismic shot gathers over a faulted earth model using sem followed by seismic processing of the simulated data with this procedure we construct post stack time migrated images of the kind that are routinely interpreted in the seismic exploration industry we dip filter the seismic images to highlight the fault plane reflections prior to making amplitude maps along the fault plane with these amplitude maps we compare the reflectivity from the different fault models to diagnose which physical mechanism contributes most to observed fault reflectivity to lend physical meaning to the properties of a locally weak fault zone characterized as a slip interface we propose an equivalent layer model under the assumption of weak scattering this allows us to use the empirical relationships between density velocity and effective stress from the south eugene island field to relate a slip interface to an amount of excess pore pressure in a fault zone

chlorophyll dispersal by eddy eddy interactions in the gulf of mexico 1 a lagrangian analysis of the transport and dispersal of plumes observed in satellite derived ocean color images was conducted using a data assimilating model of the gulf of mexico the interaction between pervasive cyclonic and anticyclonic eddies in the gulf generated advective paths that connect remote shelf regions these paths aligned remarkably well with the plume events recorded with the chlorophyll a ocean color product from seawifs two such events were studied in one event material was transported in a thin strip between the northern wall of the loop current and an adjacent cyclone connecting the eastern campheche shelf off the yucatan peninsula and south florida shelves the other event began off the louisiana shelf break as a small plume traced by chlorophyll and then developed into a long and thin feature which meandered to the shelf break off the northern yucatan peninsula moving between a large anticyclone and several adjacent cyclones these results indicate that inter eddy advection plays a crucial role in developing the ocean color patterns observed in the satellite ocean color data

covariation of mesoscale ocean color and sea surface temperature patterns in the sargasso sea during the lifetime of the coastal zone color scanner there were 21 instances in which both satellite derived ocean color and sea surface temperature are simultaneously available over large areas of the sargasso sea these images reveal close correspondence between mesoscale structures observed in temperature and pigment fields in general higher lower pigment biomass occurs in mesoscale features consisting of cold warm temperature anomalies this relationship is consistent with the idea that upward displacement of isopycnals at the base of the euphotic zone by mesoscale eddies is an important mechanism of nutrient supply in the region

three new boundary conditions for the seismic response analysis of geomechanics problems using the numerical manifold method abstract the numerical manifold method nmm provides a unified approach to address continuum discontinuum problems in geotechnical engineering owing to the dynamic nature of its governing equations the nmm should be suitable for modelling dynamic problems such as those associated to earthquakes however due to the current limitations in far field boundary conditions implemented in the original nmm formulation nmm has not been used to carry out seismic response analysis in the present study three new boundary conditions have been developed to extend the capability of the nmm to conduct seismic response analysis 1 the classical viscous boundary condition which allows for the absorption of the seismic wave energy at the boundaries based on the viscous boundaries and the seismic motion input method is also proposed 2 the free field boundary condition which captures the free field motion and absorption of the reflected waves at the sides of the model the algorithms to generate free field mesh and its coupling calculations with the main mesh are also presented 3 the static dynamic unified boundary which models the transition from fixed boundary condition in static state to free field boundary condition in seismic state thus ensuring the accuracy and consistency of the numerical simulation finally five numerical examples are shown to validate the proposed methods the numerical results indicate that the improved nmm can be successfully adopted for seismic response analysis

the fate of upwelled waters in the great whirl august 1995 abstract the great whirl is a large anticyclonic gyre that develops off the northern somali coast during the southwest monsoon in august 1995 the noaa ship malcolm baldrige surveyed the seaward edge of the upwelling zone associated with this gyre the fate of recently upwelled water was followed by mapping surface property distributions along a cool surface feature that extended seaward along the northern edge of the great whirl surface properties t s and chlorophyll a surface velocity adcp and xbt and ctd casts were interpreted in relation to the trajectories of three instrumented surface drifters deployed in the feature cool surface waters correspond in space to the shoaling of the upper thermocline and offshore advection from the coast surface chlorophyll a concentrations decreased from 2 to 3 g l 1 in the upwelling zone to 0 5 1 5 g l 1 in the surface feature and contiguous waters maximum surface velocities in the great whirl were 250 cm s 1 with velocities 100 cm s 1 along the northern perimeter of the gyre decorrelation time scales for u and v velocity components and chlorophyll a fluorescence from the drifters were on the order of 4 to 7 days these times are comparable to those over which the drifters were ejected from the great whirl into the socotra gyre decorrelation times for sea surface temperature were somewhat longer 10 days all three platforms passed between the somali coast and socotra within a week of their deployment and then traveled east into the northern arabian sea

an integrated multi azimuth vsp study for fracture characterization in the vicinity of a well we present the analysis of a multi azimuth vertical seismic profiling data set that has been acquired in a tight gas field with the objective of characterizing fracture distributions using seismic anisotropy we investigate different measurements of anisotropy which are shear wave splitting p wave traveltime anisotropy and azimuthal amplitude variation with offset we find that for our field case shear wave splitting is the most robust measure of azimuthal anisotropy which is clearly observed over two distinct intervals in the target we compare the results of the vertical seismic profiling analysis with other borehole data from the same well cross dipole sonic and formation microlmager data from the reservoir section suggest that no open fractures intersect the well or are present within half a metre of the borehole wall furthermore a detailed dispersion analysis of the sonic scanner data provides no indication of stress induced seismic anisotropy along the logged borehole section we therefore explain the azimuthal anisotropy measured in the vertical seismic profiling data with a model that contains discrete fracture corridors which do not intersect the well itself but lie within the vertical seismic profiling investigation radius we show that such a model can reproduce some basic characteristics of azimuthal anisotropy observed in the vertical seismic profiling data the model is also consistent with well test data that suggest the presence of a fracture corridor away from the well with this study we demonstrate the necessity of integrating different data types that investigate different scales of rock volume and can provide complementary information for understanding the characteristics of fracture networks in the subsurface

spatial orientation and distribution of reservoir fractures from scattered seismic energy wepresentthedetailsofanewmethodfordeterminingthereflection and scattering characteristics of seismic energy from subsurface fractured formations the method is based upon observations we have made from 3d finite difference modeling of the reflected and scattered seismic energy over discrete systems of vertical fractures regularly spaced discrete vertical fracture corridors impart a coda signature which is a ringing tail of scatteredenergy toanyseismicwaveswhicharetransmittedthrough or reflected off of them this signature varies in amplitude and coherence as a function of several parameters including 1 the difference in angle between the orientation of the fractures and the acquisition direction 2 the fracture spacing 3 the wavelength of the illuminating seismic energy and 4 the compliance or stiffness of the fractures this coda energy is most coherent when the acquisition direction is parallel to the strike of thefractures ithasthelargestamplitudewhentheseismicwavelengths are tuned to the fracture spacing and when the fractures have low stiffness our method uses surface seismic reflection tracestoderiveatransferfunctionthatquantifiesthechangeinan apparent source wavelet before and after propagating through a fracturedinterval thetransferfunctionforanintervalwithnoor low amounts of scattering will be more spikelike and temporally compact the transfer function for an interval with high scattering will ring and be less temporally compact when a 3d survey is acquired with a full range of azimuths the variation in the derived transfer functions allows us to identify subsurface areas with high fracturing and to determine the strike of those fractures wecalibratedthemethodwithmodeldataandthenapplied ittotheemiliofieldwithafracturedreservoir themethodyielded results which agree with known field measurements and previously published fracture orientations derived from ps anisotropy

field measurements of normal and shear fracture compliance abstract field estimates of the normal and shear compliance of water saturated fractures were obtained from a seismic experiment carried out on a wave cut platform of upper caithness flagstone on the north coast of scotland the rocks are cut by two orthogonal sets of vertical fractures vertical geophones were glued to the rock surface and seismic waves were generated by striking the surface with a sledge hammer first arrival traveltimes were obtained for source receiver distances up to 30 m differences of compressional velocity with direction across the platform were interpreted as resulting from the compliance of the fractures intersected by the transmitted waves the average normal compliance for fractures at this site is 4 0 10 12 1 0 10 12 m pa there is much less certainty about the precise value of shear compliance however the normal shear compliance ratio is determined to be 0 1

modeling the seismic response of individual hydraulic fracturing stages observed in a time lapse distributed acoustic sensing vertical seismic profiling surveyseismic response of fracturing stages

3d numerical simulation of elastic wave propagation in discrete fracture network rocks fractures play an important role in controlling rock block stability and the hydraulic properties of fractured rock formations understanding elastic wave propagation in fractured media can result in significant advances for the geophysical prediction of fracture parameters from seismic data however most natural fracture characteristics such as fracture length aperture angle and location are random therefore fracture models must be built discretely and follow some stochastic principles we construct stochastic models of fractured rock samples using a random fracture network rather than a single fracture three dimensional 3d wave field computation is a computationally complex problem here the 3d fourth order in space second order in time displacement stress staggered grid finite difference scheme is used for accurate simulations our numerical examples demonstrate the effects of varying fracture number aperture and length distribution of the fracture network on the seismic response the wave field scattering caused by the contrast between fractures and background media is one of the key features and the resulting scattering is more obvious for s waves than for p waves such an approach can be applied to any fracture network model that provides a link between fracture parameters and seismic attributes

sea ice rheology from in situ satellite and laboratory observations fracture and friction on the basis of an analysis of in situ ice stresses and of satellite derived ice strain rates as well as of a comparison between field and laboratory behaviour we describe an alternative viewpoint for modelling sea ice deformation during winter we propose that fracture and frictional sliding govern inelastic deformation over all spatial and temporal scales even under small stresses consequently winter and or perennial sea ice does not behave as a viscous material even at large scales the normal flow rule is not obeyed as observed during laboratory tests on sea ice samples harvested from the field and stresses are highly intermittent and poorly correlated spatially

fractured reservoirs an analysis of coupled elastodynamic and permeability changes from pore pressure variation equivalent medium theories can describe the elastic compliance and fluid permeability tensors of a layer containing closely spaced parallel fractures embedded in an isotropic background we propose a relationship between effective stress background or lithostatic stress minus pore pressure and both permeability and elastic constants this relationship uses an exponential decay function that captures the expected asymptotic behavior i e low effective stress gives high elastic compliance and high fluid permeability while high effective stress gives low elastic compliance and low fluid permeability the exponential decay constants are estimated for physically realistic conditions with relationships coupling pore pressure to permeability and elastic constants we are able to couple hydromechanical and elastodynamic modeling codes a specific coupled simulation is demonstrated where fluid injection in a fractured reservoir causes spatially and temporally varying changes in pore pressure permeability and elastic constants these elastic constants are used in a 3d finite difference code to demonstrate time lapse seismic monitoring with different acquisition geometries changes in amplitude and traveltime are seen in surface seismic p to s reflections as a function of offset and azimuth as well as in vertical seismic profile p to s reflections and in crosswell converted s waves these observed changes in the seismic response demonstrate seismic monitoring of fluid injection in the fractured reservoir

frequency dependent seismic anisotropy in fractured rock to examine the effects of multiple aligned fractures in rock we have developed a two dimensional elastic finite difference code for fractured media fractures are incorporated into the model explicitly as displacement discontinuity boundary conditions the wavefield is computed using a 4th order staggered grid scheme simulations were performed for a broadband explosion point source center frequency 374 hz located at the center of the model the model consisted of 90 horizontal fractures spaced approximately 1 8 of a wavelength apart the normal and shear fracture stiffnesses were selected such that the normal incidence transmission coeficient is 0 6 the simulations show strong scattering attenuation of the p wave in the vertical direction to the fractures and channeling of guided waves in the horizontal direction ii to the fractures the same code was also used to model wave propagation in an anisotropic medium with equivalent effective moduli for the 90 fracture system significant differences between the amplitudes velocities and frequency content of the waves in the explicit and equivalent medium fracture models were observed these differences result from frequency dependent time delays and filtering across each fracture and channeling along fractures that are not included in the zero frequency effective medium description these effects are especially interesting because they illustrate that the dynamic properties of fractured rock include significant amplitude anisotropy that may prove useful in the characterization of fractured rock

arctic sea ice as a granular plastic an important consideration in understanding sea ice mechanics is the integration of observed sea ice behavior on a floe neighborhood scale 1 10 km into ice dynamics on a regional scale o 50 km we investigate sea ice kinematics from october 1993 through april 1994 using relative motions from 13 drifting buoys with global positioning system navigation in a 20 km array centered on the sea ice mechanics initiative ice camp and we compare these motions to synthetic aperture radar sar derived ice velocities over a 100 by 500 km region in the beaufort sea there is excellent correspondence between the deformation of the buoy array and that from the sar inferred ice dynamics from analysis of the two major northerly wind convergence events of the winter are consistent with a granular hardening plastic conceptual model for beaufort sea ice under continued northerly winds the ice from the alaskan shore to the camp failed in shear and convergence in a progressive manner away from the coast the continuum scale o 10 km is an order of magnitude larger than the grain i e floe size o 1 km the ice motion often forms aggregates of 20 200 km separated by narrow 10 km shear zones similar to granular materials at moderate forcing i e wind stress multiplied by fetch the ice appears to fail along slip lines that occur at an acute angle to each other and to the direction of the wind forcing characteristic of a plastic material at critical state with longer fetch the ice appears to fail in compression perpendicular to the wind direction sea ice appeared to harden on a regional scale after the first event during the second northerly wind event there was a sea ice breakout toward the west apparently due to a lack of lateral confining stress our observations suggest that the ice floes advect through relatively stationary stress fields created by the wind forcing and coastal boundaries for example while the sar and advanced very high resolution radiometer images indicated the presence of the shear feature at the same geographic location for nearly a week buoys would show shearing only for several days as they transited across the region of shear there is a high correspondence between the major internal ice deformation events and persistent weather patterns on a 3 to 5 day temporal scale this implies that sar data collection and analysis for regional sea ice dynamics should be consistent with the wind forcing and have a sampling of less than 3 days

on modeling the anisotropic failure and flow of flawed sea ice the failure and flow of sea ice on scales small and large is characterized by the propagation of oriented leads and cracks in this paper a theory for the dynamical treatment of anisotropic oriented flaws in sea ice is developed and used to examine the interaction of these oriented flaws under idealized stress forcing the essential idea of the theory is to take one or more oriented weak leads imbedded in thick ice a constitutive law for both the thin and thick ice is taken to be similar to laboratory observations which are consistent with mohr coulomb like failure application of normal continuum mechanics boundary conditions leads to anisotropic flow and failure characteristics of this anisotropic composite under both near and far field forcing the local failure characteristics show that there is a preferred orientation for failure with the intersection between leads increasing as confinement increases for spatially separated interacting flaws the theory predicts a preference for the weakening of flaws in a narrow range of angles of 10 20 relative to the principal far field stress imbedding isolated flaws leads to simulated damage directions consistent with local failure characteristics in the case where the ice is considered to have available flaws in all directions fracture propagation is found to proceed by picking out oriented flaws alternating in direction along large scale damage strikes with individual flaw alignments of 20 relative to far field principal stresses the mechanisms responsible for these results and how this anisotropic sea ice rheology might be implemented in current dynamic thermodynamic sea ice models are discussed