on the 1470 year pacing of dansgaard oeschger warm events 1 the oxygen isotope record from the greenland ice sheet project 2 gisp2 ice core was reanalyzed in the frequency and time domains the prominent 1470 year spectral peak which has been associated with the occurrence of dansgaard oeschger interstadial events is solely caused by dansgaard oeschger events 5 6 and 7 this result emphasizes the nonstationary character of the oxygen isotope time series nevertheless a fundamental pacing period of 1470 years seems to control the timing of the onset of the dansgaard oeschger events a trapezoidal time series model is introduced which provides a template for the pacing of the dansgaard oeschger events statistical analysis indicates only a 3 probability that the number of matches between observed and template derived onsets of dansgaard oeschger events between 13 and 46 kyr b p resulted by chance during this interval the spacing of the dansgaard oeschger onsets varied by 20 around the fundamental 1470 year period and multiples thereof the pacing seems unaffected by variations in the strength of north atlantic deep water formation suggesting that the thermohaline circulation was not the primary controlling factor of the pacing period

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

on the role of density and attenuation in three dimensional multiparameter viscoacoustic vti frequency domain fwi an obc case study from the north sea 3 d frequency domain full waveform inversion fwi is applied on north sea wide azimuth ocean bottom cable data at low frequencies 10 hz to jointly update vertical wave speed density and quality factor q in the viscoacoustic vti approximation we assess whether density and q should be viewed as proxy to absorb artefacts resulting from approximate wave physics or are valuable for interpretation in the presence of soft sediments and gas cloud fwi is performed in the frequency domain to account for attenuation easily multiparameter frequency domain fwi is efficiently performed with a few discrete frequencies following a multiscale frequency continuation however grouping a few frequencies during each multiscale step is necessary to mitigate acquisition footprint and match dispersive shallow guided waves q and density absorb a significant part of the acquisition footprint hence cleaning the velocity model from this pollution low q perturbations correlate with low velocity zones associated with soft sediments and gas cloud however the amplitudes of the q perturbations show significant variations when the inversion tuning is modified this dispersion in the q reconstructions is however not passed on the velocity parameter suggesting that cross talks between first order kinematic and second order dynamic parameters are limited the density model shows a good match with a well log at shallow depths moreover the impedance built a posteriori from the fwi velocity and density models shows a well focused image with however local differences with the velocity model near the sea bed where density might have absorbed elastic effects the fwi models are finally assessed against time domain synthetic seismogram modelling performed with the same frequency domain modelling engine used for fwi

double difference elastic waveform inversion with prior information for time lapse monitoring abstractquantitative time lapse seismic monitoring can provide crucial information for enhanced oil recovery geologic carbon storage and enhanced geothermal systems recently developed double difference elastic waveform inversion has the potential to quantitatively monitor reservoirs using seismic reflection data because the approximate location of a reservoir or a target monitoring region is usually known we incorporated this knowledge as prior information into double difference elastic waveform inversion using numerical examples of synthetic time lapse models we found that our new method can quantitatively monitor the changes of elastic properties within reservoirs therefore the double difference elastic waveform inversion with prior information on the location of a monitoring region is a promising tool for quantitatively monitoring reservoir properties changes

continuous seismic surveillance of valhall field since the permanent seismic system was installed over valhall field five years ago 11 surveys have been collected each designed acquired and recorded with a high repeatability and efficiency the time lapse 4d data have been processed using a consistent and fast workflow that generates results within weeks the fast turnaround ensures that information is relevant and has maximum impact on short and long term management decisions

long offset time lapse seismic tested on the valhall lofs data conventional time lapse seismic has been less successful for stiff rock reservoir monitoring such as carbonates this is mainly because of the negligible time lapse changes in the seismic properties therefore we propose to use long offset time lapse seismic as an alternative method to estimate small velocity changes more specifically we monitor the maximum amplitude offset that is beyond critical offset the properties of the maximum amplitude offset are similar to critical offset except that they appear for longer offsets and are frequency dependent increased frequency reduces the gap between this offset and the critical offset we find that the maximum amplitude offset is a function of overburden and reservoir velocity and practically independent of density this method requires a velocity increase across the interface that is to be analyzed this criterion usually is satisfied for stiff rock reservoirs also by long offset acquisition we mean typically 1 to 2 km beyond the critical offset for

n2o and ch4 variations during the last glacial epoch insight into global processes 1 greenhouse gas measurements along polar ice cores provide important insight into the former composition of the atmosphere its natural variations and the responses to fast climatic changes in the past we present high resolution nitrous oxide n2o and methane ch4 records measured along two ice cores from central greenland covering part of marine isotope stages 3 and 4 in the last glacial epoch the n2o data confirm the hypothesis that n2o shows variations in phase to fast climatic changes observed in the northern hemisphere the so called dansgaard oeschger d o events variations exist not only for events with a long duration 1500 years and more but also for the shorter ones the comparison with ch4 unveils interesting differences between the response of ch4 and n2o to d o events while the average amplitudes of ch4 oscillations associated with d o events are similar to those of the northern hemisphere summer insolation the magnitude of the n2o concentration change instead correlates with the duration of the d o events the records give further insight into the timing of concentration changes at the beginning of d o events they show that for long lasting events the n2o concentration starts to increase before both the sharp increase in the ch4 concentration and the temperature reconstructed for greenland

time lapse seismic imaging using regularized full waveform inversion with a prior model which strategy full waveform inversion is an appealing technique for time lapse imaging especially when prior model information is included into the inversion workflow once the baseline reconstruction is achieved several strategies can be used to assess the physical parameter changes such as parallel difference two separate inversions of baseline and monitor data sets sequential difference inversion of the monitor data set starting from the recovered baseline model and double difference inversion of the difference data starting from the recovered baseline model strategies using syntheticmarmousi data sets we investigate which strategy should be adopted to obtain more robust and more accurate time lapse velocity changes in noise free and noisy environments this synthetic application demonstrates that the double difference strategy provides the more robust time lapse result in addition we propose a target oriented time lapse imaging using regularized full waveform inversion including a prior model and model weighting if the prior information exists on the location of expected variations this scheme applies strong prior model constraints outside of the expected areas of timelapse changes and relatively less prior constraints in the time lapse target zones in application of this process to the marmousi model data set the local resolution analysis performed with spike tests shows that the target oriented inversion prevents the occurrence of artefacts outside the target areas which could contaminate and compromise the reconstruction of the effective time lapse changes especially when using the sequential difference strategy in a strongly noisy case the target oriented prior model weighting ensures the same behaviour for both time lapse strategies the double difference and the sequential difference strategies and leads to a more robust reconstruction of the weak time lapse changes the double difference strategy can deliver more accurate time lapse variation since it can focus to invert the difference data however the double difference strategy requires a preprocessing step on data sets such as time lapse binning to have a similar source receiver location between two surveys while the sequential difference needs less this requirement if we have prior information about the area of changes the target oriented sequential difference strategy can be an alternative and can provide the same robust result as the doubledifference strategy

assessing the sensitivity of the north atlantic ocean circulation to freshwater perturbation in various glacial climate states a striking characteristic of glacial climate in the north atlantic region is the recurrence of abrupt shifts between cold stadials and mild interstadials these shifts have been associated with abrupt changes in atlantic meridional overturning circulation amoc mode possibly in response to glacial meltwater perturbations however it is poorly understood why they were more clearly expressed during marine isotope stage 3 mis3 60 27 ka bp than during termination 1 t1 18 10 ka bp and especially around the last glacial maximum lgm 23 19 ka bp one clue may reside in varying climate forcings making mis3 and t1 generally milder than lgm to investigate this idea we evaluate in a climate model how ice sheet size atmospheric greenhouse gas concentration and orbital insolation changes between 56 ka bp 56k 21k and 12 5k affect the glacial amoc response to additional freshwater forcing we have performed three ensemble simulations with the earth system model loveclim using those forcings we find that the amoc mode in the mild glacial climate type 56k and 12 5k with deep convection in the labrador sea and the nordic seas is more sensitive to a constant 0 15 sv freshwater forcing than in the cold type 21k with deep convection mainly south of greenland and iceland the initial amoc weakening in response to freshwater forcing is larger in the mild type due to an early shutdown of labrador sea deep convection which is completely absent in the 21k simulation this causes a larger fraction of the freshwater anomaly to remain at surface in the mild type compared to the cold type after 200 years a weak amoc is established in both climate types as further freshening is compensated by an anomalous salt advection from the sub tropical north atlantic however the slightly fresher sea surface in the mild type facilitates further weakening of the amoc which occurs when a surface buoyancy threshold 0 6 kg m 3 surface density anomaly to the 56k reference state is stochastically crossed in the nordic seas while described details are model specific our results imply that a more northern location of deep convection sites during milder glacial times may have amplified frequency and amplitude of abrupt climate shifts

on the footprint of anisotropy on isotropic full waveform inversion the valhall case study summary the validity of isotropic approximation to perform acoustic full waveform inversion fwi of real wide aperture anisotropic data can be questioned due to the intrinsic kinematic inconsis tency between short and large aperture components of the data this inconsistency is mainly related to the differences between the vertical and horizontal velocities in vertical transverse isotropic vti media the footprint of vti anisotropy on 2 d acoustic isotropic fwi is illus trated on a hydrophone data set of an ocean bottom cable that was collected over the valhall field in the north sea multiscale fwi is implemented in the frequency domain by hierar chical inversions of increasing frequencies and decreasing aperture angles the fwi models are appraised by local comparison with well information seismic modelling reverse time migration rtm and source wavelet estimation a smooth initial vti model parameterized by the vertical velocity v0 and the thomsen parameters andwere previously developed by anisotropic reflection traveltime tomography the normal moveout vnmo v0 1 2 and horizontal vh v0 1 2 velocity models were inferred from the anisotropic models to perform isotropic fwi the vnmo models allows for an accurate match of short spread reflection traveltimes whereas the vh model after updating by first arrival traveltime tomog raphy fatt allows for an accurate match of first arrival traveltimes ray tracing in the velocity models shows that the first 1 5 km of the medium are sampled by both diving waves and reflections whereas the deeper structure at the reservoir level is mainly controlled by short spread reflections starting from the initial anisotropic model and keeping fixed and models anisotropic fwi allows us to build a vertical velocity model that matches reasonably well the well log velocities isotropic fwi is performed using either the nmo model or the fatt model as initial model in both cases horizontal velocities are mainly reconstructed in the first 1 5 km of the medium this suggests that the wide aperture components of the data have a dominant control on the velocity estimation at these depths these high velocities in the upper structure lead to low values of velocity in the underlying gas layers either equal or lower than vertical velocities of the well log and or a vertical stretching of the structure at the reservoir level below the gas this bias in the gas velocities and the mispositioning in depth of the deep reflectors also shown in the rtm images are required to match the deep reflections in the isotropic approximation and highlight the footprint of anisotropy in the isotropic fwi of long offset data despite the significant differences between the anisotropic and isotropic fwi models each of these models produce a nearly equivalent match of the data which highlights the ill posedness of acoustic anisotropic fwi hence we conclude with the importance of considering anisotropy in fwi of wide aperture data to avoid bias in the velocity reconstruc tions and mispositioning in depth of reflectors designing a suitable parameterization of the vti acoustic fwi is a central issue to manage the ill posedness of the fwi

the lac des iles palladium deposit ontario canada part ii halogen variations in apatite analysis of apatite from the mine block intrusion mbi of the lac des iles igneous complex shows two pronounced trends in the halogens apatite from relatively fresh norite and melanorites from the pd sulfide zone contain up to 57 mol chlorapatite endmember with significant hydroxyapatite component in contrast in altered rock amphibolite and greenschist assemblages the chlorapatite component is typically less than 10 mol with wide variation in the f and oh endmember components the latter trend is attributed to cl loss to degassing and alteration whereas the former is attributed to cl enrichment in the ore bearing rocks it is suggested that the relatively h2o rich and intermediate cl content of the early igneous fluids degassed from the deeper levels of the mbi can explain the high pd pt and pd ir ratios of the deposit a model is presented in which disseminated pd rich sulfides are initially introduced by a high temperature magmatic fluid that also influenced crystallization to produce the gross modal variations of the igneous host rock this high temperature mineralization event was subsequently modified by the influx of late igneous and country fluids at amphibolite to greenschist conditions

cross matching with interpreted warping of 3d streamer and 3d ocean bottom cable data at valhall for time lapse assessment legacy streamer data and newer 3d ocean bottom cable data are cross matched and analysed for time lapse analysis of geomechanical changes due to production in the valhall field the issues relating to time lapse analysis using two such distinctly different data sets are addressed to provide an optimal cross matching workflow that includes 3d warping additionally an assessment of the differences between the imaging using single azimuth streamer and multi azimuth ocean bottom cable data is provided the 3d warping utilized in the cross matching procedure is sensitive to acquisition and processing differences but is also found to provide valuable insight into the geometrical changes that occur in the subsurface due to production as such this work also provides a demonstration of the use of high resolution 3d interpreted warping to resolve the 3d heterogeneity of the compaction and subsidence this is an important tool for valhall and possibly other fields where compaction and subsidence and monitoring thereof are key factors in the reservoir management since the predominant observed production induced changes are compaction of the soft high porosity chalk reservoir due to pore pressure reduction and the resultant overburden subsidence such reservoir compaction could have significant implications for production by changing permeabilities and production rates furthermore the subsidence effects could impact upon subsea installations and well bore stability geomechanical studies that have previously been used to model such subsidence and compaction are only constrained by observed surface displacements and measured reservoir pressure changes with the geological overburden being largely neglected the approaches suggested herein provide the potential for monitoring and assessment in three dimensions including the probable heterogeneity and shearing that is needed for full understanding of reservoir compaction and the resultant effects on the overburden to for example mitigate well bore failures

time lapse seismic reservoir monitoring time lapse seismic reservoir monitoring has advanced rapidly over the past decade there are currently about 75 active projects worldwide and more than 100 cumulative projects in the past decade or so the present total annual expenditures on 4 d seismic projects are on the order of 50 100 million us this currently represents a much smaller market than 3 d seismic but the use of 4 d seismic has grown exponentially over the past decade and is expected to continue to do so

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

integration of the life of field seismic data with the reservoir model at the valhall field the frequent time lapse observations from the life of field seismic system across the valhall field provide a wealth of information the responses from the production and injection wells can be observed through time shift and amplitude changes these observations can be compared to modelled synthetic seismic responses from a reservoir simulation model of the valhall field the observed differences between the observations and the modelling are used to update and improve the history match of the reservoir model the uncertainty of the resulting model is reduced and a more confident prediction of future reservoir performance is provided a workflow is presented to convert the reservoir model to a synthetic seismic response and compare the results to the observed time lapse responses for any time range and area of interest correlation based match quality factors are calculated to quantify the visual differences this match quality factor allows us to quantitatively compare alternative reservoir models to help identify the parameters that best match the seismic observations three different case studies are shown where this workflow has helped to reduce the uncertainty range associated with specific reservoir parameters by updating various reservoir model parameters we have been able to improve the match to the observations and thereby improve the overall reservoir model predictability the examples show positive results in a range of different reservoir modelling issues which indicates the flexibility of this workflow and the ability to have an impact in most reservoir modelling challenges

velocity model building by waveform inversion of early arrivals and reflections a 2d case study with gas cloud effects joint full waveform inversion jfwi combines reflection waveform inversion rwi and early arrival waveform inversion ewi to build a large scale velocity model of the subsurface from long offset data the misfit function of jfwi requires an explicit separation between the short spread reflections and early arrivals the feasibility of which is illustrated with a real data case study jfwi is alternated with a waveform inversion migration of short spread reflections to provide a short scale impedance model this model is needed for building the sensitivity kernel of rwi along the two way reflection paths the large scale velocity macromodel built by jfwi can be used as the initial model for classical fwi to enrich the high wavenumber content of the subsurface model we present an application of this workflow to a real 2d obc profile across a gas cloud in the north sea to review its main promises and pitfalls viscoacoustic vti seismic modeling allows us to account for attenuation and anisotropy effects

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

multiparameter full waveform inversion of multicomponent ocean bottom cable data from the valhall field part 1 imaging compressional wave speed density and attenuation multiparameter full waveform inversion fwi is a challenging quantitative seismic imaging method for lithological characterization and reservoir monitoring the difficulties in multiparameter fwi arise from the variable influence of the different parameter classes on the phase and amplitude of the data and the trade off between these in this framework choosing a suitable parametrization of the subsurface and designing the suitable fwi workflow are two key methodological issues in non linear waveform inversion we assess frequency domain visco acoustic fwi to reconstruct the compressive velocity vp the density or the impedance ip and the quality factor qp from the hydrophone component using a synthetic data set that is representative of the valhall oil field in the north sea we first assess which of the vp and vp ip parametrizations provides the most reliable fwi results when dealing with wide aperture data contrary to widely accepted ideas we show that the vp parametrization allows a better reconstruction of both the vp and ip parameters first because it favours the broad band reconstruction of the dominant vp parameter and secondly because the trade off effects between velocity and density at short to intermediate scattering angles can be removed by multiplication to build an impedance model this allows for the matching of the reflection amplitudes while the broad band velocity model accurately describes the kinematic attributes of both the diving waves and reflections then we assess different inversion strategies to recover the quality factor qp in addition to parameters vp and a difficulty related to attenuation estimation arises because on the one hand the values of qp are on average one order of magnitude smaller than those of vp and and on the other hands model perturbations relative to the starting models can be much higher for qp than for vp and during fwi in this framework we show that an empirical tuning of the fwi regularization which is adapted to each parameter class is a key issue to correctly account for the attenuation in the inversion we promote a hierarchical approach where the dominant parameter vp is reconstructed first from the full data set i e without any data preconditioning to build a velocity model as kinematically accurate as possible before performing the joint update of the three parameter classes during a second step this hierarchical imaging of compressive wave speed density and attenuation is applied to a real wide aperture ocean bottom cable data set from the valhall oil field several geological features such as accumulation of gas below barriers of claystone and soft quaternary sediment are interpreted in the fwi models of density and attenuation the models of vp and qp that have been developed by visco acoustic fwi of the hydrophone data can be used as initial models to perform visco elastic fwi of the geophone data for the joint update of the compressive and shear wave speeds

time lapse full waveform inversion with ocean bottom cable data application on valhall field abstractknowledge of changes in reservoir properties resulting from extracting hydrocarbons or injecting fluid is critical to future production planning full waveform inversion fwi of time lapse seismic data provides a quantitative approach to characterize the changes by taking the difference of the inverted baseline and monitor models the baseline and monitor data sets can be inverted either independently or jointly time lapse seismic data collected by ocean bottom cables obcs in the valhall field in the north sea are suitable for such time lapse fwi practice because the acquisitions are of a long offset and the surveys are well repeated we have applied independent and joint fwi schemes to two time lapse valhall obc data sets which were acquired 28 months apart the joint fwi scheme is double difference waveform inversion ddwi which inverts differenced data the monitor survey subtracted by the baseline survey for model changes we have found that ddwi gave a cleaner and more easily interpr

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