discrete element modeling of stress and strain evolution within and outside a depleting reservoir stress changes within and around a depleting petroleum reservoir can lead to reservoir compaction and surface subsidence affect drilling and productivity of oil wells and influence seismic waves used for monitoring of reservoir performance currently modeling efforts are split into more or less coupled geomechanical normally linearly elastic fluid flow and geophysical simulations there is evidence from e g induced seismicity that faults may be triggered or generated as a result of reservoir depletion the numerical technique that most adequately incorporates fracture formation is the dem discrete element method this paper demonstrates the feasibility of the dem here pfc particle flow code to handle this problem using an element size of 20 m 2 d and 3 d simulations have been performed of stress and strain evolution within and around a depleting reservoir within limits of elasticity the simulations largely reproduce analytical predictions the accuracy is however limited by the element size when the elastic limit is exceeded faulting is predicted particularly near the edge of the reservoir simulations have also been performed to study the activation of a pre existing fault near a depleting reservoir

a physical explanation of the cumulative area distribution curve a physical explanation for the behavior of the cumulative area distribution cad based on the tokunaga channel network model is given the cad is divided into three regions the first region for small areas is dependent on hillslope flow accumulation patterns and represents the catchment average of the hillslope flow accumulation in the diffusive erosion dominated areas upstream reaches of the catchment the second region represents that portion of the catchment dominated by fluvial erosion this region is well described by a log log linear power law which results from the scaling properties of the channel network the scale exponent b is highly sensitive to a parameter of the tokunaga stream numbering scheme the exponentb converges to 0 5 for higher order tokunaga networks for parameters consistent with topological random networks small networks have lower values ofb which asymptotic converges tob 0 5 as the catchment scale increase the third region reflects the lowest reaches of the channel network the scale of the catchment and is a boundary effect an explicit analytical solution to the scaling properties in the second region is derived on the basis of the tokunaga network model the cumulative distribution of drainage area is the percent age of the catchment that has an area per unit width draining through a point which is greater than or equal to a given area it is a geomorphologic measure that characterizes part of the catchment hydrology and is a measure commonly used by geo morphologists and hydrologists to characterize the scale invari ant structure of channel networks tarboton et al 1989 ro driguez iturbe et al 1992 inaoka and takayasu 1993 la barbera and roth 1994 this cumulative area distribution cad characterizes the scaling properties of drainage area the cad and its scaling properties are key components in the development of physically based models that link geomorphol ogy and hydrology perera 1997 ibbitt et al 1997 perera 1997 showed that slope of the cad in both the hillslope and channel domains is important for describing the geomorphic impact on subsurface saturation excess runoff generation using a digital elevation model dem of a catchment the drainage path through and the drainage area per unit width of each point in the catchment can be computed for a grided dem the cumulative area a is determined as the number of pixels within the catchment which have drainage area a num ber of pixels draining through greater than or equal to some specified drainage area a

combination of precise leveling and insar data to constrain source parameters of the mw 6 5 26 december 2003 bam earthquake we used new precise leveling data acquired 40 days after the bam earthquake in combination with radar interferometry observations from both ascending and descending orbits to investigate static deformation associated with the 2003 bam earthquake we invert this geodetic data set to gain insight into the fault geometry and slip distribution of the rupture the best fitting dislocation model is a steeply east dipping right lateral strike slip fault that has a size of 11 by 8 km and strikes n2 w we find that such smooth geometry fits available geodetic data better than previously proposed models for this earthquake our distributed slip model indicates a maximum strike slip of 3 m occurring about 3 to 5 km deep the slip magnitude and depth of faulting taper to the north where the fault approaches the bam city inclusion of crustal layering increases the amount of maximum slip inferred at depth by about 4

a note on induced stress changes in hydrocarbon and geothermal reservoirs abstract earthquakes have been induced by oil and gas production where pore pressures have decreased in some cases by several tens of mpa it has previously been suggested that such earthquakes are caused by poroelastic stressing of crust surrounding the reservoir induced earthquakes are also common in geothermal fields such as the geysers where strong correlations between both steam production and condensate injection and earthquake activity have been observed over the last several decades stress measurements within hydrocarbon reservoirs show that the least horizontal stress decreases with declining reservoir pressure as predicted by poroelasticity for circular disk shaped reservoirs isothermal reduction in pore pressure induces a relative horizontal tension within the reservoir production induced stressing may promote frictional sliding on pre existing faults within the reservoir itself normal faulting is promoted if the regional stress is extensional and the biot coefficient is sufficiently large 0 85 for reasonable coefficients of friction on the other hand dilatant fracturing and normal faulting are always promoted in extensional environments near the edge of the reservoir or in regions of high pore pressure gradient it is suggested that such fracturing could enhance fracture permeability in tight rocks adjacent to portions of the reservoir that experience large reductions in pore pressure due to production in regional compressional environments production modestly favors reverse faulting above and below the reservoir the ratio of thermal to poroelastic stress can be quite large in geothermal reservoirs such as the geysers reservoir wide energy balance considerations suggest that the average temperature has declined at the geysers by 6 c during the past 20 years reservoir average stress changes are thus on the order of 2 mpa and are certainly much larger near injection wells and steam producing fractures

the key role of lower level meridional shear in baroclinic wave life cycles a series of idealized nonlinear life cycle experiments is performed to compare changes in life cycle behavior caused by upper level and near surface meridional shear of the initial zonal wind it is shown that both the eddy kinetic energy and the zonal flow accelerations produced during a cycle of growth and equilibration respond primarily to the meridional shear of the zonal wind near the surface and only weakly to shear near the tropopause near the critical shear for transition from anticyclonic to cyclonic life cycle behavior the zonal flow accelerations are minimized and the eddy persistence is maximized above this critical shear eddy breaking on the poleward side of the jet increases and strong cyclonic zonal wind shears are generated the influence of baroclinic shears is minimized by using dipolar wind anomalies that are zero near the center of a basic baroclinic jet and by taking advantage of the fact that the life cycle response is very sensitive to small changes in the magnitude of initial meridional shear the small baroclinic shears contribute to the differences in the sense that a cyclonic shear decreasing with height is slightly more effective in inducing cyclonic behavior than is a barotropic cyclonic shear with the same surface value upper tropospheric eddy momentum fluxes by linear normal modes are also much more sensitive to lower tropospheric meridional shear than to upper tropospheric meridional shear the primary response of normal modes to lower tropospheric meridional shear is to change the momentum flux at all levels

the structure and nonlinear evolution of synoptic scale cyclones life cycle simulations with a cloud scale model abstract high resolution simulations of idealized baroclinic wave life cycles on both the f plane and the plane are performed the anelastic nonhydrostatic equations are employed in these analyses and motions are assumed dry adiabatic and inviscid apart from a weak horizontal diffusion cyclogenesis is documented in considerable detail and features the formation of an occlusion with spiraling temperature contours that cuts off completely from the main warm air mass late in the life cycle in all cases thereafter the occlusion develops independently of the remaining warm cold frontal cusp and decays by becoming zonally elongated later redevelopment involving reorganization of the occlusion into the more nearly circular shape that obtains at the time of cut off can reoccur on the f plane leading to an oscillatory life cycle features of the simulated synoptic scale vortices that are in common with observed cyclones are the comma shaped vertical velocity pattern the spiraling temperature and vorticity

discrimination between pressure and fluid saturation changes from time lapse seismic data explicit expressions for computing saturation and pressure related changes from time lapse seismic data have been derived and tested on a real time lapse seismic data set necessary input is near and far offset stacks for the baseline seismic survey and the repeat survey the method has been tested successfully in a segment where pressure measurements in two wells verify a pore pressure increase of 5 to 6 mpa between the baseline survey and the monitor survey estimated pressure changes using the proposed relationships fit very well with observations between the baseline and monitor seismic surveys 27 of the estimated recoverable hydrocarbon reserves were produced from this segment the estimated saturation changes also agree well with observed changes apart from some areas in the water zone that are mapped as being exposed to saturation changes which is unlikely saturation changes in other segments close to the original oil water contact and the top reservoir interface are also estimated and conf

baroclinic development within zonally varying flows previous idealized modelling studies have shown the importance of across jet barotropic shear to the resulting evolution of cyclones anticyclones surface based fronts and upper level fronts meanwhile many observational studies of cyclones have shown the importance of along jet variations in the horizontal wind speed i e confluence and diffluence this study investigates the importance of these along jet zonal for zonally oriented jets variations in the horizontal wind speed to the resulting structures and evolutions of baroclinic waves using idealized models of growing baroclinic waves an idealized primitive equation channel model is configured with growing baroclinic perturbations embedded within confluent and diffluent background flows when the baroclinic perturbations are placed in background confluence the lower tropospheric frontal structure and evolution initially resemble the shapiro keyser cyclone model with a zonally oriented cyclone strong warm front and bent back warm front later as the baroclinic wave is amplified in the stronger downstream baroclinicity the warm sector of the cyclone narrows becoming more reminiscent of the norwegian cyclone model the upper level frontal structure develops with a southwest northeast orientation and becomes strongest at the base of the trough where geostrophic cold advection is occurring in contrast when the baroclinic perturbations are placed in background diffluence the lower tropospheric frontal structure and evolution resemble the norwegian cyclone model with a meridionally oriented cyclone strong cold front and occluded front the upper level frontal structure is initially oriented northwest southeast on the western side of the trough before becoming zonally oriented weak geostrophic temperature advection occurs along its length these results are compared to those from previous observational and idealized modelling studies copyright 2007 royal meteorological society

rocks under strain strain induced time lapse time shifts are observed for depleting reservoirs pressure depletion as a result of oil and gas production creates changes in the stress and strain fields of the rock material both inside and outside the reservoir seismic waves propagating through these rocks have different traveltimes before and after depletion because the layer thicknesses change and the seismic velocity is altered by the changes in the stress and strain fields the cumulative change in two way traveltimes with depth is referred to as time lapse time shift

vp vs ratio versus differential stress and rock consolidation a comparison between rock models and time lapse avo data the compressional to shear wave velocity ratio vp vs is an important parameter in seismic amplitude versus offset avo analysis and this parameter plays a key role especially for lithology and fluid prediction methods the p wave velocity is a key parameter in traditional pressure prediction methods because overpressure often results in a velocity reduction however for avo based pore pressure prediction methods one expects that the vp vs ratio also is a key parameter the hertz mindlin geomechanical model predicts a constant vp vs ratio as the differential stress changes in a dry package of identical spheres ultrasonic core measurements show increased vp vs ratios as the differential stress decreases especially for unconsolidated wet sands thus one is likely to assume that the vp vs ratio is dependent on rock consolidation by combining the hertz mindlin model with the gassmann model we show how to obtain a sim ple rock physics framework including both the differential stress and

two paradigms of baroclinic wave life cycle behaviour two idealized baroclinic wave 6 life cycles examined here suggest a framework of opposite extremes a in which to view the behaviour of real synoptic scale disturbances in middle latitudes and b in which to interrelate the synoptic and wave theoretic viewpoints using the saturation propagation saturation sps picture of wave 6 life cycle behaviour the two life cycles dented by lc1 and lc2 are higher resolution versions of the simmons hoskins basic and anomalous cases showing strong and weak late decay of eddy kinetic energy eke they illustrate in varying degrees two extreme types of behaviour here disignated anticyclonic and cyclonic and epitomized by strongly contrasting upper air trough behaviour anticyclonic behaviour dominates the late stages of lc1 and is characterized by backward tilted thinning troughs being advected anticyclonically and equatorward as in the commoner cases of planetary scale mid stratospheric rossby wave breaking cyclonic behaviour dominates lc2 and is characterized by forward tilted broadening troughs wrapping themselves up cyclonically and poleward producing major cut off cyclones in high latitudes these morphologies are visualized by upper air maps of potential temperature on the nominal tropopause defined as a constant potential vorticity surface some atmospheric mid latitude disturbances examined here using the same visualization applied to operational analyses show the same two extreme types of trough behaviour together with intermediate cases the sps picture is re examined using eliassen palm and refractive index cross sections it is shown in particular by reference to a wave activity theorem of haynes that the late stages of lc2 can be looked upon as a remarkably clear and morphologically novel large amplitude counterpart of the nonlinear reflection scenario of rossby wave critical layer theory the late stages of lc1 by constrast look more akin to an nonlinear critical layer absorption scenario lc2 exhibits region of largely undular pv contours adjacent to a nonlinear of irreversiby deformen pv contous in the latter region pv rearrangement and hence absorption of rossby wave activity has largely ceased this accounts for the more persistent ekee in the lc2 case

inertia gravity waves spontaneously generated by jets and fronts part i different baroclinic life cycles abstract the spontaneous generation of inertia gravity waves in idealized life cycles of baroclinic instability is investigated using the weather research and forecasting model two substantially different life cycles of baroclinic instability are obtained by varying the initial zonal jet the wave generation depends strongly on the details of the baroclinic wave s development in the life cycle dominated by cyclonic behavior the most conspicuous gravity waves are excited by the upper level jet and are broadly consistent with previous simulations of o sullivan and dunkerton in the life cycle that is dominated by anticyclonic behavior the most conspicuous gravity waves even in the stratosphere are excited by the surface fronts although the fronts are no stronger than in the cyclonic life cycle the anticyclonic life cycle also reveals waves in the lower stratosphere above the upper level trough of the baroclinic wave these waves have not been previously identified in idealized simulations the sensiti

flow resistance and sediment transport by concentrated overland flow in a grassland valley abstract flow resistance and sediment transport data are needed from well vegetated humid environments to evaluate surface wash erosion and channel incision by overland flow in humid environments runoff in valley floors can reach depths of several centimetres but erosion is often limited by dense grass cover intense grazing reduces grass cover but the impacts of this on sediment transport processes are poorly understood we conducted flume experiments in a grassed valley of coastal california to investigate flow resistance and sediment yield under natural conditions and with progressive clipping of grass cover flow resistance has a laminar like relationship with reynolds number but we attribute this to very low velocity beneath submerged stems and not to the state of flow the sediment transport relations provide support for the concept of a threshold shear stress below which erosion is effectively prevented by surface resistance shear stress partitioning suggests that on a densely grassed surface over 90 of flow resistance is exerted on plant stems this effectively prevents sediment transport at boundary shear stresses as high as 1000 1800 dyn cm2 complete clipping of the grass cover reduces the critical shear stress for sediment transport to 11 38 of that under natural conditions continued surface wash erosion and channel initiation are prevented however by strong soil cohesion provided by a dense root mat even with reduction of root density boundary shear stresses of at least 250 430 dyn cm2 are required for channel incision

variability of precipitation along cold fronts in idealized baroclinic waves abstractprecipitation patterns along cold fronts can exhibit a variety of morphologies including narrow cold frontal rainbands and core and gap structures a three dimensional primitive equation model is used to investigate alongfront variability of precipitation in an idealized baroclinic wave along the poleward part of the cold front a narrow line of precipitation develops along the equatorward part of the cold front precipitation cores and gaps form the difference between the two evolutions is due to differences in the orientation of vertical shear near the front in the lower troposphere at the poleward end the along frontal shear is dominant and the front is in near thermal wind balance while at the equatorward end the cross frontal shear is almost as large at the poleward end the thermal structure remains erect with the front well defined up to the midtroposphere hence updrafts remain erect and precipitation falls in a continuous line along the front at the equatorward end the cores form

incorporating diagnosed intercept parameters and the graupel category within the arps cloud analysis system for the initialization of double moment microphysics testing with a squall line over south china abstractin this study a new set of reflectivity equations are introduced into the advanced regional prediction system arps cloud analysis system this set of equations incorporates double moment microphysics information in the analysis by adopting a set of diagnostic relationships between the intercept parameters and the corresponding mass mixing ratios a reflectivity and temperature based graupel classification scheme is also implemented according to a hydrometeor identification hid diagram a squall line that occurred on 23 april 2007 over southern china containing a pronounced trailing stratiform precipitation region is used as a test case to evaluate the impacts of the enhanced cloud analysis scheme the results show that using the enhanced cloud analysis scheme is able to better capture the characteristics of the squall line in the forecast the predicted squall line exhibits a wider stratiform region and a more clearly defined transition zone between the leading convection and the trailing str

the role of passive microseismic monitoring in the instrumented oil field with the current industry trend toward instrumented oil fields and smart well completions the permanent deployment of geophones or other acoustic sensors to complement standard engineering gauges is being promoted as a way to map reservoir dynamics the biggest push is from the time lapse seismic practitioners although the deployment of permanent seismic instrumentation is also potentially an ideal route to monitor passive seismicity

the linkage between the warm and the cold conveyor belts in an idealized extratropical cyclone abstractthis study continues the investigation of airstreams in idealized moist baroclinic waves and addresses the formation of the cold conveyor belt ccb its linkage to the warm conveyor belt wcb and their impact on the development of a midlatitude cyclone the ccb is identified as a coherent bundle of trajectories characterized by weak ascent and a strong increase of potential vorticity pv along the flow in contrast to the wcb defined as the trajectories with maximum ascent the authors illuminate the role of the two conveyor belts in the formation of two strong pv anomalies that form in the upper wcb negative pv anomaly and lower troposphere ccb positive pv anomaly respectively and thereby establish a link between these airstreams and relevant aspects of the dynamics of extratropical cyclones the ccb moves close to the surface along the colder side of the bent back front and experiences a pv increase as it passes below a region of maximum latent heat release at midtropospheric level

predicting the topographic limits to a gully network using a digital terrain model and process thresholds a digital terrain model is used with process thresholds to predict the extent of a stable gully network in a 5 km2 catchment of the southeastern highlands of australia the model developed by dietnch et al 1992 1993 predicts the topographic controls on channel networks and interprets these in terms of a critical shear stress for channel incision c applied by saturation overland flow we adapt the model slightly to compare the shear stress applied by hortonian overland flow to that applied by saturation overland flow the limits to gully erosion in the catchment are controlled strongly by a topographic threshold that has an inverse relationship between upslope catchment area and local gradient the topographic threshold for channel incision is reproduced using a simple model of hortonian overland flow and a c appropriate for incision into a degraded grass surface c 245 dyn cm2 this is consistent with historical evidence for the timing of gully erosion the study confirms a strong topographic control on the extent of the channel network in a catchment significantly different from the western north america catchments where the topographic threshold was first demonstrated despite its simplicity the model for incision by overland flow appears capable of distinguishing the hydrological processes responsible for channel incision when these are reflected in the relationship between channel network and landscape morphology the model requires relatively simple inputs suggesting it may be useful for mapping gully erosion hazard in actively eroding catchments

an analysis of frontogenesis in numerical simulations of baroclinic waves abstract using a primitive equation pe model we revisit two canonical flows that were previously studied using a semigeostrophic equation sg model in a previous paper the authors showed that the pe and the sg models can have significantly different versions of the large scale dynamics here they report on the implications of this difference for frontogenesis the program for the study of frontogenesis developed by b j hoskins and collaborators is followed to show how in the pe version of the canonical cases the surface warm front develops before the cold front and why the upper level front is a long nearly continuous feature going from ridge to trough the frontogenesis experienced by an air parcel is computed following the parcel to illustrate better the mechanisms involved as the present calculations are carried out longer than most previous ones the relation of the upper frontogenesis to the formation of the upper level cutoff cyclone is also examined trajectory and three dimensional g

a case study of cyclogenesis using a model hierarchy abstract it is universally agreed that cyclogenesis in midlatitudes occurs through baroclinic conversion of the potential energy available from an initial state the mechanical process by which that conversion takes place is a perennial subject of discussion at least as far back as the 1950s it was recognized that in any practical forecast problem the initial condition is influential observational research continues to confirm the prevalence of tropopause level perturbations preceding surface cyclogenesis the observations also suggest that the growing disturbances have time varying vertical structures relating these observations to the classical linear theory of baroclinic instability is not immediately obvious since in the latter the precise form of the initial condition is not important and the theory predicts cyclogenesis with a fixed in time vertical structure these differences between theory and observations are but a few of the many that have been recognized and treated in modified theorie

effects of surface drag on fronts within numerically simulated baroclinic waves abstract a comparative analysis of simulations of baroclinic waves with and without surface drag is presented with particular reference to surface features as in recent studies the present simulations show that compared to simulations with no drag those with surface drag are less inclined to develop a secluded warm sector and that drag weakens the warm front while the cold front remains strong the authors demonstrate that analogous effects occur when ekman pumping is used in nonlinear quasigeostrophic numerical simulations of unstable baroclinic waves in a channel however since the quasigeostrophic model produces symmetric highs and lows in the unstable baroclinic wave the cold and warm fronts are therefore also symmetric and hence equally affected by the ekman pumping the different effect that friction has on the warm front with respect to the cold front in the primitive equation simulations is fundamentally related to the tendency for the lows to be strong and narrow and the highs weak and br

quantitative estimation of compaction and velocity changes using 4d impedance and traveltime changes in some hydrocarbon reservoirs severe compaction of the reservoir rocks is observed this compaction is caused by production and it is often associated with changes in the overburden time lapse or 4d seismic data are used to monitor this compaction process since the compaction causes changes in both layer thickness and seismic velocities it is crucial to distinguish between the two effects two new seismic methods for monitoring compacting reservoirs are introduced one based on measured seismic prestack traveltime changes and the other based on poststack traveltime and amplitude changes in contrast to earlier methods these methods do not require additional empirical relationships such as for instance a velocity porosity relationship the uncertainties in estimates for compaction and velocity change are expressed in terms of errors in the traveltime and amplitude measurements these errors are directly related to the quality and repeatability of time lapse seismic data for a reservoir at 3000 m depth wit h9mo fcompaction and assuming a 4d timeshift error of 0 5 ms at near offset and 2 ms at far offset we find relative uncertainty in the compaction estimate of approximately 50 60 using traveltime information only

pore pressure detection sensitivities tested with time lapse seismic data in an exploration context pore pressure prediction from seismic data relies on the fact that seismic velocities depend on pore pressure conventional velocity analysis is a tool that may form the basis for obtaining interval velocities for this purpose however velocity analysis is inaccurate and in this paper we focus on the possibilities and limitations of using velocity analysis for pore pressure prediction a time lapse seismic data set from a segment that has undergone a pore pressure increase of 5 to 7 mpa between the two surveys is analyzed for velocity changes using detailed velocity analysis a synthetic time lapse survey is used to test the sensitivity of the velocity analysis with respect to noise the analysis shows that the pore pressure increase cannot be detected by conventional velocity analysis because the uncertainty is much greater than the expected velocity change for a reservoir of the given thickness and burial depth finally by applying amplitude variation with offset avo analysis to the same data we demonstrate that seismic amplitude analysis may yield more precise information about velocity changes than velocity analysis

aspects of the baroclinic boundary layer abstract the ekman taylor boundary layer model is solved for the case of a linear variation of the geosptophic wind with height the two layer model couples a monin obukhov similarity layer to an ekman layer with a vertically constant eddy diffusivity the presence of the thermal wind contributes both an along isotherm and a cross isotherm component to the boundary layer flow the along isotherm flow is supergeostrophic and results from the net downward transport of geostrophic momentum by the eddies the cross isotherm flow is toward the warm air and results from the coriolis deflection of the geostrophic momentum rich air aloft that has been mixed downward the effect of the baroclinity i e the thermal wind shear on the wind field is conveniently summarized geometrically the model predicts that the surface vorticity increases in regions of cyclonic thermal vorticity i e the vorticity of the thermal wind however anticyclonic thermal vorticity produces convergence of the low level warmward flow

numerical simulations of maritime frontogenesis abstract a hydrostatic primitive equation model initialized in a highly baroclinically unstable state was used to simulate maritime cyclogenesis and frontogenesis in order to identify boundary layer physical processes important in maritime frontogenesis several different simulations were performed in an effort to isolate impacts due solely to the boundary layer moist processes were not included an adiabatic and inviscid simulation provided the control for these experiments two different boundary layer parameterizations were used a k theory parameterization featuring richardson number dependent eddy diffusivity and a second order closure scheme with prognostic equations for the turbulence quantities results indicated that strong warm and cold fronts formed in the adiabatic and inviscid case but that the vertical motion fields were weak in the k theory simulation the results were somewhat more realistic with stronger vertical motion in both the k theory and second order closure simulations the b

relating 4d seismics to reservoir geomechanical changes using a discrete element approach a modified discrete element method is briefly introduced and used for modelling reservoir geomechanical response during fluid injection and depletion the modified approach works as a continuum method until some local failure is initiated after which it behaves like a discrete element method on a polygonal lattice the method is advantageous for modelling fracture developments in rocks it is applied here to synthetic models of two reservoirs taken from the north sea gullfaks and elginfranklin for gullfaks two cases of water injection were modelled one with low horizontal effective stress and the other with low vertical effective stress vertical fractures are developed in the first case whereas horizontal fractures are developed in the second case this would not have been seen using traditional methods based on 4d seismics data for the gullfaks field one may envision that horizontal fractures could have been formed the elgin franklin synthetic model is used to study various scenarios of changing stress field around the depleting reservoir based on 4d seismics data from this field one may see changes that could be interpreted in terms of possible fault reactivation

seismic time lapse effects and stress changes examples from a compacting reservoir compaction of unconsolidated reservoir rock represents a major drive mechanism in many oil and gas reservoirs significant volumes of produced hydrocarbons may be credited to this effect however compaction might also result in surface subsidence a dynamic overburden and local permeability losses in the reservoir

channel head location and characteristics using digital elevation models the drainage network is the conduit through which much surface water and sediment are routed within a catchment in a catchment the position of where hillslopes begin and channels end has long been considered the position of transition between diffusive processes upslope and the more incisive fluvial processes downslope consequently understanding channel head location is an important issue in understanding catchment hydrology and geomorphology this study examines channel head position and characteristics in a catchment in arnhem land northern territory australia in this study the position of channel heads was mapped within the catchment and plotted on a reliable digital elevation model of the catchment it was found that the majority of channel heads have relatively small source areas and that graphical catchment descriptors such as the area slope relationship and cumulative area distribution can provide reliable measures of the field position of the heads of first order streams and the transition from hillslope to channel the area slope relationship and cumulative area distribution are also shown to be good tools for determining digital elevation model grid size which can capture hillslope detail and the transition from hillslope to channel copyright 2005 john wiley sons ltd

use of digital elevation model data for the derivation of the geomorphological instantaneous unit hydrograph the use of digital elevation models dems allows the automatic derivation of channel networks and the quantitative description of the geomorphic characteristics of basins a common method of channel network extraction from dem data is based on the specification of a threshold area at that is the minimum support area required to drain to a point for a channel to form usually an arbitrary constant threshold area value is chosen for channel network extraction in this study the effects of threshold area selection both on the morphometric and scaling properties such as drainage density total channel length horton laws and fractal dimension of a channel network and the associated hydrological response function are analysed the response is obtained following the geomorphological instantaneous unit hydrograph theory two different probabilistic models are used they both relate the characteristic response function of the basin to its dem data derived networks one is derived assuming a strahler stream ordering system and the other is obtained by averaging a flow equation with respect to the network structure described by the width function applications are shown for three mountainous basins in the italian alps a sensitivity analysis is performed to study the influence of the variability of morphometric properties with respect to at on the hydrological response obtained it is shown that the model based on the width function is able to reduce the effects of this variability on the simulated response 1997 by john wiley sons ltd

effects of digital elevation model resolution on derived stream network positions 1 this study examines the effect of digital elevation model dem resolution on the positional accuracy of derived hydrologic networks and quantitatively confirms that dem resolution should be greater than the average hillslope length when used for hydrologic modeling seven hundred kilometers of mapped stream networks are compared to stream networks derived from 17 dems with resolutions ranging from 30 m to 3 km comparison between predicted and mapped streams reveals that accuracy of predicted stream locations decays quickly beyond a dem resolution of 180 m a new application of gyasi agyei et al s 1995 dem resolution suitability test based on average slope and vertical resolution indicates an average hillslope length of between 150 and 180 m tarboton et al s 1991 method of determining average hillslope length based on slope and accumulation areas reconfirms the length to be 150 m and verifies the link between network accuracy and hillslope scale two algorithms are used to derive stream networks the d algorithm tarboton 1997 which allows for flow dispersion and the d8 algorithm o callaghan and mark 1984 which does not a comparison between the d8 and d algorithms shows that modeling flow dispersion is not necessary in steep terrain as both algorithms performed equally well however the d algorithm is found to be less susceptible to modeling erroneous hillside flow convergence making it preferable to the d8 algorithm when used by tarboton et al s 1991 method of determining the average hillslope lengths finally field observation indicates that predicted first and second order streams tend to exist when predicted however mapped streams have a higher positional accuracy

production induced stresses from time lapse time shifts a geomechanics case study from franklin and elgin fields franklin and elgin fields were discovered in 1986 and 1991 respectively within the u k north sea central graben blocks 22 30 and 29 5 the producing reservoirs are contained in the jurassic fulmar shallow marine and pentland fluvial formations at depths of 5100 5600 m subsea the fields presented significant development challenges both in terms of seismic imaging complexity and being in a state of exceptionally high pressure high temperature hpht addressing these challenges meant that production could not commence until 2001

injection induced stress and fracture orientation changes examination of the field evidence suggests that induced vertical fractures in the cohesionless shallow oilsand reservoirs of northeastern alberta are unlikely to remain vertical when large volumes of hot fluids are injected into them numerical models are assembled taking into account the appropriate material properties model geometries and boundary conditions in order to simulate as closely as possible the stress response of a reservoir to induced fracturing the results suggest that generation of horizontal fractures is extremely likely and that thermal input enhances this probability overstress above overburden may be maintained by an elastic skin effect on the fracture boundary continued displacement and pore pressure permeation can overcome this overstress resulting in the cyclic stress buildup breakthrough pattern reported in the literature the inadequacy of static geomechanical models no fracture orientation change is pointed out correct simulation requires correct process comprehension

a physical explanation for an observed area slope elevation relationship for catchments with declining relief a general relationship between the contributing area slope and mean elevation of catchments with relief declining after a tectonic uplift event is presented this relationship is based on the continuity equation for runoff and erosion processes in the catchment the key hypothesis underlying this relationship is that as a catchment declines the nondimensionalized catchment approaches a constant form this hypothesis is verified for computer simulated catchments the area slope elevation relationship covers several cases catchments declining toward a peneplain catchments declining from a high slope dynamic equilibrium resulting from a high rate of tectonic uplift to a low slope one resulting from a lower rate of tectonic uplift and catchments declining from an elevated initial condition as for example in the decline of a mine spoil heap a previously published relationship between slope and area for catchments in dynamic equilibrium and based on runoff and erosion physics is shown to be a special case of this general relationship the new area slope elevation relationship is compared with data from simulated catchments and a field catchment it is thus shown that for declining catchments the area slope elevation relationship is a good predictor of catchment form for catchments with declining relief it is argued that the slope area elevation relationship is sufficient with the planiform drainage pattern to completely define the elevation properties of the catchment such as for instance the hypsometric curve

precipitation banding in idealized baroclinic waves abstractmoist idealized baroclinic wave simulations show the development of precipitation bands from a zonally uniform initial midlatitude jet for a frictionless lower boundary and with no latent heat release or surface heat and moisture fluxes warm advection is strong and a bent back warm front forms although a narrow vertical velocity maximum forms within the area of synoptic scale ascent near the triple point only a wide warm frontal band forms as surface roughness length increases between simulations to that of an ocean then a land surface warm advection is reduced and the cold front becomes stronger relative to the warm front a separate narrow rainband forms along the cold front which is more intense and farther removed from the wide warm frontal band when roughness length is greater in the simulation with roughness length appropriate to the ocean after the narrow band decays the precipitation becomes oriented along the warm conveyor belt in the warm sector when latent heat release is in

monitoring overburden layer changes and fault movements from time lapse seismic data on the valhall field summary a method developed by roste et al which discriminates between layer thickness and velocity changes is tested on pre stack time lapse seismic ocean bottom cable obc data from the valhall field a key parameter in this discrimination process is the dilation factor which is the relative velocity change divided by the relative thickness change within a given layer the high quality and good repeatability of the obc data enables us to estimate with a reasonable accuracy for values between 0 and 5 for values below 5 complementary information like for instance geomechanics is required for the top reservoir horizon we estimate a maximum subsidence of 0 50 m 0 29 m and a corresponding velocity decrease for the sequence from the seabed to the top reservoir of 2 0 m s 1 0 40 m s 1 time lapse distortion patterns with characteristic time shift versus offset signatures are observed the positions and vertical extents of the distortion zones are determined from ray path considerations and modelling the distortion zones correlate with buried faults indicating that a time lapse distortion zone might be produced by a localized slip in a fault zone we present an extended method which allows for vertical in addition to lateral variations in the relative thickness and velocity changes this method can be viewed as a simplified version of time lapse tomography but involving fewer unknown parameters giving more stability to the estimated changes in thickness and velocity using this technique we are able to estimate for positions with localized time lapse distortions

slope area relationships and sediment dynamics in two alpine streams the paper focuses on the channel network of alpine basins attempting to interpret channel reaches in terms of response to erosion and deposition processes two basins in the dolomites north eastern italy were considered the rio cordon 5 km2 and the upper boite river 163 km2 the channel network was extracted from raster type digital elevation models dem using a slope area threshold criterion a contribution area index cai which combines drainage area a and local slope s cai a0 5s was used to identify channel heads the channel network extracted from the dem was then analysed to recognize cells showing a value of cai lower than the threshold adopted for channel initiation contiguous cells with low values of cai define channel reaches with low transport efficiency field surveys carried out for some selected cases showed a good agreement between prevailing sediment deposition predicted by the analysis of the channel network and observed channel morphological features sediment sources mapped in two study basins were also analysed in relation to the location of channels with high potential for sediment deposition this made it possible to classify the potential role of different types of sediment sources with regard to basin sediment yield topographic characteristics of the channel network expressed by cai were compared with a classification of channel morphology in the rio cordon it was found that cells with low values of cai frequently occur in the riffle pool reaches whereas the percentage significantly decreases in step pool and bedrock channels copyright 2003 john wiley sons ltd

airborne radar observations of a cold front during fastex1 abstract a detailed analysis using airborne doppler radars of an oceanic cold front associated with precipitation core and gap regions is presented the precipitation cores appear to form as a result of the combined effects of horizontal shearing instability and the advection of hydrometeors by the core relative winds in contrast to previous schematic models it is shown that a strong surface discontinuity does not exist along the entire length of the precipitation core the southern section of the core can be accompanied by an abrupt discontinuity and lighter precipitation while the northern section can be associated with more slowly changing variables but heavier precipitation the peak updrafts at the leading edge of the front appeared to be primarily driven by frictional convergence and the acceleration of the vertical vorticity in the boundary layer the overall motion of the cold front was not well predicted using density current theory even though the kinematic structure of the front resembled cla

numerical simulation of baroclinic waves with a parameterized boundary layer a dry three dimensional baroclinic life cycle model is used to investigate the role of turbulent fluxes of heat and momentum within the boundary layer on midlatitude cyclones simulations are performed of life cycles for two basic states with and without turbulent fluxes the different basic states produce cyclones with contrasting frontal and mesoscale flow structures the analysis focuses on the generation of potential vorticity pv in the boundary layer and its subsequent transport into the free troposphere the dynamic mechanism through which friction mitigates a barotropic vortex is that of ekman pumping this has often been assumed to also be the dominant mechanism for baroclinic developments the pv framework highlights an additional baroclinic mechanism positive pv is generated baroclinically due to friction to the northeast of a surface low and is transported out of the boundary layer by a cyclonic conveyor belt flow the result is an anomaly of increased static stability in the lower troposphere which restricts the growth of the baroclinic wave the reduced coupling between lower and upper levels can be sufficient to change the character of the upper level evolution of the mature wave the basic features of the baroclinic damping mechanism are robust for different frontal structures with and without turbulent heat fluxes and for the range of surface roughness found over the oceans

efficient global optimization of reservoir geomechanical parameters based on synthetic aperture radar derived ground displacements abstractwhen large volumes of fluids are removed from or injected into underground formations for e g hydrocarbon and water production co2 storage gas storage and geothermal energy exploitation monitoring of surface deformations coupled to numerical modeling improves our understanding of reservoir behavior the ability to accurately simulate surface displacements however is often impaired by limited information on reservoir geometry waterdrive strength and fluid geomechanical parameters characterizing the geologic formations of interest we have investigated the ability of efficient global optimization ego to reduce the parameter uncertainties usually affecting geomechanical modeling ego is used to identify the parameter set that minimizes the difference in land displacements obtained from synthetic aperture radar sar derived measurements and 3d geomechanical modeling we have tested the approach on the tengiz giant oil field kazakhstan where large uncertainties affect our knowledge of g