simulating migrated and inverted seismic data by filtering a geologic model the simulation of migrated and inverted data is hampered by the high computational cost of generating 3d synthetic data followed by processes of migration and inversion for example simulating the migrated seismic signature of subtle stratigraphic traps demands the expensive exercise of 3d forward modeling followed by 3d migration of the synthetic seismograms this computational cost can be overcome using a strategy for simulating migrated and inverted data by filtering a geologic model with 3d spatial resolution and angle filters respectively a key property of the approach is this the geologic model that describes a target zone is decoupled from the macrovelocity model used to compute the filters the process enables a target orientedapproach by which a geologically detailed earth model describing a reservoir is adjusted without having to recalculate the filters because a spatial resolution filter combines the results of the modeling and migration operators the simulated images can be compared directly to a real migration image we decompose the spatial resolution filter into two parts and show that applying one of those parts produces output directly comparable to 1d inverted real data two dimensional synthetic examples that include seismic uncertainties demonstrate the usefulness of the approach results from a real data example show that horizontal smearing which is not simulated by the 1d convolution model result is essential to understand the seismic expression of the deformation related to sulfate dissolution and karst collapse

least squares reverse time migration of multiples abstractthe theory of least squares reverse time migration of multiples rtmm is presented in this method least squares migration lsm is used to image free surface multiples where the recorded traces are used as the time histories of the virtual sources at the hydrophones and the surface related multiples are the observed data for a single source the entire free surface becomes an extended virtual source where the downgoing free surface multiples more fully illuminate the subsurface compared to the primaries since each recorded trace is treated as the time history of a virtual source knowledge of the source wavelet is not required and the ringy time series for each source is automatically deconvolved if the multiples can be perfectly separated from the primaries numerical tests on synthetic data for the sigsbee2b and marmousi2 models show that least squares reverse time migration of multiples lsrtmm can significantly improve the image quality compared to rtmm or standard reverse time migratio

late orogenic svecofennian deformation in sw finland constrained by pegmatite emplacement ages late stages of the svecofennian orogeny in sw finland were related to a tranpressional stress field and dextral movements along crustal scale shear zones under decreasing temperature and pressure conditions in the kemio area a minimum estimate for the time span of movements along one of these shear zones is obtained from the ages of 1840 to 1830 myr old microcline granite sheets related to early ductile deformation and by dating structurally late rare mineral granite pegmatites related to brittle deformation one pegmatite was emplaced when the rheological conditions in the gabbro changed from ductile to semi ductile and brittle it has u pb ferrotapiolite ages ranging between 1807 0 2 9 myr 2 s and 1803 1 2 9 2 0 myr 2 s another pegmatite emplaced under brittle conditions has an u pb ferrotapiolite age of 1802 9 1 3 myr 2 s these pegmatites were emplaced preferentially in gabbroic rocks that showed brittle to semi ductile deformation at a time when their more felsic host rocks still showed ductile deformation the age range bracketed by the microcline granite sheets and the structurally late pegmatites indicates that ductile deformation related to the transpressional late svecofennian tectonic regime in southwestern finland persisted for at least 30 40 myr

reverse time migration a prospect of seismic imaging methodology abstract reverse time migration rtm is a seismic imaging method to map the subsurface reflectivity using recorded seismic waveforms the practice in exploration seismology has long established a two fold approach of seismic imaging using velocity modeling building to establish the long wavelength reference velocity models and using seismic migration to map the short wavelength reflectivity structures among various seismic migration methods for different situations rtm is the only method that is capable to use all seismic wave types that can be computed numerically being initiated in early 1980 s rtm seeks an image of the subsurface reflectivity as the best match in an image space between the extrapolation of time reversed waveform data and the prediction based on estimated velocity model and source parameters judging the image quality in the same space of forming the images is more advantageous than the approaches of modeling and inversion which seek the solution in the model space but judge its fitness in data space considering that most seismic migration applications today still use primary reflection as the only signal the capability of rtm to use all computable wave types is unique and helpful reducing the imaging artifacts due to mistaking non primary waves as primary reflections hence we refer to those rtm algorithms using only primary reflections as the first generation rtm methods and the rtm algorithms making a full use of primary reflections multiple reflections and other non primary waveform data as the second generation rtm methods this paper reviews the development history of the rtm along with its major challenges current solutions and future perspectives

sensitivity of eddy induced heat transport to diabatic forcing compensation of the poleward eddy heat transport by the heat transport of an eddy induced mean meridional overturning cell is a common feature in many eddy resolving ocean models it has been argued that this is the result of the weak thermal driving of the ocean as the actual air sea coupling is scale dependent it might be questioned whether the approximation of weak thermal driving is relevant for the oceanic eddy field in this paper the role of diabatic forcing in modifying eddy mean flow interaction is investigated emphasis has been placed on the sensitivity of the eddy induced change in heat transport to the sea surface thermal boundary condition experiments have been performed with a multilayer isopycnic primitive equation model of an idealized north atlantic subtropical and subpolar gyre for different values of the air sea coupling solutions with and without transient eddies have been compared the air sea coupling mostly affects the upper ocean thermal and velocity fields a decrease of the coupling timescale pushes the separation point of the midlatitude jet further northward and induces a tight recirculation southwest of the separated jet these effects are enhanced by the eddies in the present model there is compensation of the eddy heat transport for sea surface temperature sst relaxation times longer than 150 days a breakdown of the compensation occurs for sst relaxation times shorter than 50 days the average upper layer depth is 200 m in between is a transition regime for strong thermal driving the eddy induced change in total heat transport is of the same order as the eddy heat transport

suppressing unwanted internal reflections in prestack reverse time migration prestack reverse time migration a wave equation technique using two way propagation correctly handles multiarrivals and enables imaging of overturned reflections however image artifacts occur when backscattered waves cross correlate these artifacts are particularly strong where high velocity contrasts occur a method for removing unwanted internal reflections during propagation of both the source and receiver wavefields is presented this method applies a directional damping term to the wave equation in areas of the velocity model where unwanted reflections occur tests on synthetic data show good suppression of image artifacts

mesoscale stability of an ocean current in the bryan cox model the bryan cox primitive equation model is examined in terms of the stability of an ocean current by intercomparison with the other models taking a focus on different representations finite difference schemes of continuous stratification the bryan cox level model in which density is calculated at the same depth as horizontal velocity versus the other level model with density calculated at the same depth as vertical velocity as well as a layer isopycnal model in which vertical displacement of isopycnals represents density changes stability analyses are carried out for highly idealized cases a basic flow with vertical shear only the two level models are first numerically examined for nongeostrophic cases whose results are then confirmed with analytical solutions of the quasi geostrophic versions the analytical solutions are extended to the multiple level models the instability of the basic current in the bryan cox model is shown to be overestimated with low vertical resolution and sensitive to distribution of vertical shear among levels as the vertical resolution becomes higher the growth rate rapidly approaches that of the continuous model while additional unstable modes appear at high wavenumbers these modes are neutral in the other two models these deficiencies are related to its representation of stratification in the bryan cox model it is suggested that the bryan cox model should be carefully applied to a study of mesoscale processes

comparison of mesoscale meanders and eddies simulated by quasi geostrophic and primitive equation models a case study a comparison is made between meanders and eddies simulated by a two layer quasi geostrophic numerical model and those simulated by a bryan cox type primitive equation model at various values of the rossby number the model geometry and parameter settings are based on a previous study of the norwegian coastal current by ikeda et al 1989 the aim is to identify the dynamical effects of increasing the rossby number and to assess the performance of the quasi gestrophic qg model when applied to meandering currents which are outside its formal range of validity the growth rates of meanders in the primitive equation pe model are particularly sensitive to the vertical resolution used this sensitivity is strongest at short wavelengths and suggests that very fine vertical resolution may be needed in the bryan cox model for accurate simulation of meander growth the qg model simulates the phase speed of meanders in the pe model well but overestimates the growth rate at moderate rossby number by about 25 when the rossby number is 0 69 as the rossby number is increased the asymmetry of the primitive equation vorticity equation comes increasingly into play and at finite meander amplitude this results in small intense cyclones and large weak anticyclones this effect which is absent in the quasi geostrophic model can lead to significant changes in the potential vorticity field and hence by implication in the concentration fields of other tracers the principal trends in the solutions as the rossby number is increased are as follows 1 the disturbance becomes less energetic in a dimensionless sense at all stages of its life cycle 2 the effects of the disturbance become increasingly trapped near the position of the original frontal jet and 3 cross frontal transport particularly offshore transport of coastal water is inhibited as coastal water is wrapped around the intense cyclones our results provide guidance in interpreting the results of ikeda et al and those of other eddy models

inversion migration tomography seismic inversion broadly enough defined is equivalent to doing migration and reflection tomography simultaneously diffraction tomography and inversion work best when sources and receivers surround the region of interest as in medical imaging applications theoretical studies have shown that high vertical wavenumber velocity perturbations are resolved by inverting surface seismic reflection data but the low vertical wavenumbers must be obtained using a separate step such as velocity analysis or reflection tomography i propose that a nonlinear iterative inversion that updates a varying background velocity obtains all wavenumbers that are resolvable separately by migration and tomography the background velocity must contain reflectors to provide data on both upward and downward transmission paths through the earth and hence the low wavenumbers by considering the downward transmission paths to be between surface sources and buried image geophones and the upward transmission paths to be between surfa

analysis of the resolution function in seismic prestack depth imaging we consider the problem of estimating subsurface quantities such as velocity or reflectivity from seismic measurements because of a limited aperture and band limited signals the output from a seismic prestack reconstruction method is a distorted or blurred image this distortion can be computed using the concept of resolution function which is a quantity readily accessible in the fourier space of the model the key parameter is the scattering wavenumber which at a particular image point is defined by the incident and scattered ray directions in a given background model any location in any background model can be considered in general the resolution function will depend on the following four quantities the background velocity model the frequency bandwidth the wavefield type and the acquisition geometry we first establish the resolution function for a general scattering model assuming local reaction we then adapt this result for two well known scattering models born and kirchhoff for each of these approximations the corresponding resolution function is derived and discussed finally by employing a simple synthetic data example we demonstrate the ability of the resolution function to predict the image distortions

prograde metamorphic zoning in amphibolite facies pelitic gneisses from kemio southwest finland the metamorphism of the svecofennides in southwestern finland has been studied in pelitic gneisses from the kemio region three metamorphic zones have been distinguished which are characterized in order of increasing grade by the parageneses muscovite quartz biotite sillimanite k feldspar and cordierite k feldspar temperature and load pressure estimates of metamorphism range from 560 25 c 3 0 5 kb for the upper boundary of the muscovite quartz zone to 670 25 c 4 0 5 kb for the cordierite k feldspar zone at a partial water pressure of 0 4 0 1 ptotal

on the relation between seismic interferometry and the migration resolution function seismic interferometry refers to the process of retrieving new seismic responses by crosscorrelating seismic observations at different receiver locations seismic migration is the process of forming an image of the subsurface by wavefield extrapolation comparing the expressions for backward propagation known from migration literature with the green s function representations for seismic interferometry reveals that these seemingly distinct concepts are mathematically equivalent the frequency domain representation for the resolution function of migration is identical to that for the green s function retrieved by seismic interferometry or its square in the case of double focusing in practice they differ because the involved green s functions in seismic interferometry are all defined in the actual medium whereas in migration one of the green s functions is defined in a background medium

overview and classification of wavefield seismic imaging methods the literature and seismic processing practice overflow with a diversity of wave equation migration methods so what makes wave equation migration a wave equation migration what is the commonality of these methods and how do they differ this article provides a framework for understanding different wave equation migration methods

directional oriented wavefield imaging a new wave based subsurface illumination imaging condition for reverse time migration the key objective of an imaging algorithm is to produce accurate and high resolution images of the subsurface geology however significant wavefield distortions occur due to wave propagation through complex structures and irregular acquisition geometries causing uneven wavefield illumination at the target therefore conventional imaging conditions are unable to correctly compensate for variable illumination effects we propose a generalised wave based imaging condition which incorporates a weighting function based on energy illumination at each subsurface reflection and azimuth angles our proposed imaging kernel named as the directional oriented wavefield imaging compensates for illumination effects produced by possible surface obstructions during acquisition sparse geometries employed in the field and complex velocity models an integral part of the directional oriented wavefield imaging condition is a methodology for applying down going up going wavefield decomposition to both source and receiver extrapolated wavefields this type of wavefield decomposition eliminates low frequency artefacts and scattering noise caused by the two way wave equation and can facilitate the robust estimation for energy fluxes of wavefields required for the seismic illumination analysis then based on the estimation of the respective wavefield propagation vectors and associated directions we evaluate the illumination energy for each subsurface location as a function of image depth point and subsurface azimuth and reflection angles thus the final directionaloriented wavefield imaging kernel is a cross correlation of the decomposed source and receiver wavefields weighted by the illuminated energy estimated at each depth location the application of the directional oriented wavefield imaging condition can be employed during the generation of both depth stacked images and azimuth reflection angle domain common image gathers numerical examples using synthetic and real data demonstrate that the new imaging condition can properly image complex wave paths and produce high fidelity depth sections

retrograde garnet zoning at low water pressure in metapelitic rocks from kemio sw finland the compositional zoning of almandine spessartine garnet has been studied in pelitic gneisses from the kemio region sw finland garnets with inverse zoning outward increase in mn abound in the low grade musc qz zone garnets with inverse zoning normal zoning outward decrease in mn and with a homogeneous composition occur in the higher grade sill kfs zone the formation of garnet with inverse zoning is explained by continuous retrograde reactions in the model system k2o al2o3 feo mno mgo sio2 h2o at strongly reduced partial water pressures

prism waves in seafloor canyons and their effects on seismic imaging complex seafloor bathymetry can create significant challenges for subsurface imaging and geologic interpretation of seismic exploration and monitoring data steep seafloor canyons that cut through continental shelf areas can produce very strong seismic wavefield distortions neglecting such wavefield complexity can result in inaccurate velocity models significant imaging errors misleading amplitudes and erroneous geologic interpretations we have evaluated the kinematic and dynamic effects of seismic prism waves generated by seafloor canyons prism waves are seismic waves that undergo consecutive reflections at a scattering interface before propagating to the recording sensor array we have demonstrated that strong prism waves can be generated for realistic seafloor canyon geometries and we determined how their adverse effects can contaminate seismic imaging and velocity estimation

finite difference calculation of travel times the travel times of the first arriving seismic waves through any velocity structure can be rapidly computed on a two or three dimensional numerical grid by finite difference extrapolation from point to point wavefronts rather than the traditional rays are tracked head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 03 per cent are found between the results of this technique and raytracing for a complex two dimensional model this scheme is useful for the windowing of finite difference calculations to increase computing speed and promises to aid in earthquake location tomographic inversion and kirchhoff migration in structures that have strong lateral velocity gradients

modeling of marine seismic profiles in the t x and τ p domains in many cases comparison of real data with synthetic seismograms provides additional constraints on the velocity depth profiles obtained with simple inversion techniques obtaining a satisfactory match between the real and computed data usually requires several trials with different models but can be performed rapidly if the theoretical seismograms are themselves easily interpretable i e if the major contributions which make up the synthetic traces can be identified and separated in horizontally stratified media modeling is further simplified and is faster if the simulation techniques are implemented in the domain of the intercept time and the ray parameter p the generalized reflection and transmission matrix method is well suited for these purposes and may be used to generate synthetic seismograms in both the t x and p planes computation of plane wave seismograms is straightforward and merely corresponds to an inverse fourier transform of the overall reflectivity matrix for each ray parameter

reverse time migration migration of stacked or zero offset sections is based on deriving the wave amplitude in space from wave field observations at the surface conventionally this calculation has been carried out through a depth extrapolation we examine the alternative of carrying out the migration through a reverse time extrapolation this approach may offer improvements over existing migration methods especially in cases of steeply dipping structures with strong velocity contrasts this migration method is tested using appropriate synthetic data sets

information on elastic parameters obtained from the amplitudes of reflected waves seismic amplitude variation with offset data contain information on the elastic parameters of geological layers as the general solution of the inverse problem consists of a probability over the space of all possible earth models we look at the probabilities obtained using amplitude variation with offset avo data for different choices of elastic parameters a proper analysis of the information in the data requires a nontrivial definition of the probability defining the state of total ignorance on different elastic parameters seismic velocities lame s parameters etc we conclude that mass density seismic impedance and poisson s ratio constitute the best resolved parameter set when inverting seismic amplitude variation with offset data

late svecofennian magmatism and tectonism aland southwest finland abstract the proterozoic late svecofennian group iii granites of sederholm ava seglinge mosshaga and lemland form discrete complexes distributed along a ne sw line across the aland archipelago in southwestern finland they were emplaced 1840 1800 ma after cessation of the main svecofennian orogeny during the subsequent long period of continuing mantle crust interaction and repeated crustal adjustment by shearing which culminated in the major rapakivi granite event current investigations on the island of seglinge suggest that these complexes were rapidly emplaced at high crustal levels over a short time interval a composite pipe of hybrid rocks of intermediate composition was emplaced and with the surrounding country rocks was subsequently disrupted and invaded by porphyritic porphyroclastic granite to form breccia screens and ring dykes field relationships and textures suggest that the granite was emplaced largely as gas borne solids the petrography and mineralogy of the hybrid rock suite is such as could have been produced by contact anatexis assimilation where hot basic melts were impounded beneath older deeper porphyritic granite sheets the granite phase at seglinge may be a sample of the roof of such a source complex mobilised by volatile discharge when the surface was breached compositional change in transit would be minimal the compositional similarity of the seglinge granite and the rapakavi granites is so close as to suggest some genetic kinship the group iii complexes may represent local premature tappings from the developing rapakivi source region the composition indicated for the granites of the source region of the hybrid suites and granites of the group iii complexes differs markedly from that of the early svecofennian granites of the region it is however closely similar to granite sheets described from sw sweden and greenland which have been attributed to crustal depletion associated with basic intrusion similar lower crust mantle processes may have operated during the long spasmodically eventful period which followed the major svecofennian orogenic activity in sw finland

subsolidus migmatization in high grade meta tuffs kurkijarvi southwest finland the phenomenon of migmatization was studied in precambrian metavolcanic gneisses of calc alkaline chemistry outcropping along a prograde amphibolite granulite facies transition in the west uusimaa complex of sw finland this paper discusses one of the studied gneiss levels a garnet bearing qtz plag ksp bio gneiss which was observed to transsect the metamorphic isograd pattern at almost right angle the gneiss was studied for structures whole rock chemistry major trace and ree mineral content microtextures plagioclase anorthite content and fluid inclusions data concerning the latter four subjects are presented migmatization proved to 1 have occurred parallel to compositional banding of the rocks 2 have produced identical leucosome melanosome mesosome mineral parageneses 3 have initiated feldspar garnet poikiloblasthesis and occasionally biotite porphyroblasthesis in leucosome and biotite garnet poikiloblasthesis in melanosome 4 have caused entrapment of unstrained quartz blebs carrying isolated primary two phase pure h2o fluid inclusions of unique filling degree range in the above mentioned feldspar and garnet poikiloblasts 5 have occurred post d1 pre d2 synchronous to amphibolitefacies metamorphism in the subsolidus regime 6 have been affected by d2 in the way of localized mylonitization of the melanosome and quartz migration exudation from adjacent mesosome into leucosome and 7 have had some control by the biotite content of the original compositionally banded rock initial leucosome formation appears to have been controlled by the pre leucosome biotite content the recalculated modal biotite content of the leucosome melanosome combination conspicuously is in the range of 5 20 vol of biotite final extent of the leucosome shows on its turn a marked correlation with mesosome modal biotite content because leucosomes occur carrying a recalculated modal biotite content equalling adjacent mesosome biotite content a second factor is held responsible for the onset of migmatization in the buried and sheared rock deficient water balance migmatization initiated at pt conditions fit for feldspar recrystallization and almandine formation was induced during prograde metamorphism to cancel an established zonation in water pressure or water content parallel to compositional banding zones of low ph2o or wt h2o thereby were converted into leucosomes while zones of higher ph2o or wt h2o remained unaffected and became mesosome that xh2o did not vary at the onset of migmatization is recorded in the isolated pure h2o fluid inclusions contained in the quartz blebs enclosed in the studied leucosome and melanosome poikiloblasts restore of water balance either by internally controlled factors or externally introduced ones halted migmatization and its obliteration of compositional banding

amplitude preserving reverse time migration from reflectivity to velocity and impedance inversion conventional methods of prestack depth imaging aim at producing a structural image that delineates the interfaces of the geologic variations or the reflectivity of the earth however it is the underlying impedance and velocity changes that generate this reflectivity that are of more interest for characterizing the reservoir indeed the need to generate a better product for geologic interpretation leads to the subsequent application of traditional seismic inversion techniques to the reflectivity sections that come from typical depth imaging processes the drawback here is that these seismic inversion techniques use additional information e g from well logs or velocity models to fill the low frequencies missing in traditional seismic data due to the free surface ghost in marine acquisition we found that with the help of broadband acquisition and processing techniques the bandwidth gap between the depth imaging world and seismic inversion world is reducing we outlined a theory that shows how angle domain common image gathers produced by an amplitude preserving reverse time migration can estimate impedance and velocity perturbations the near angle stacked image provides the impedance perturbation estimate whereas the far angle image can be used to estimate the velocity perturbation in the context of marine acquisition and exploration our method can together with a ghost compensation technique be a useful tool for seismic inversion and it is also adaptable to a full waveform inversion framework we developed synthetic and real data examples to test that the method is reliable and provides additional information for interpreting geologic structures and rock properties

true amplitude prestack depth migration most current true amplitude migrations correct only for geometric spreading we present a new prestack depth migration method that uses the framework of reverse time migration to compensate for geometric spreading intrinsic q losses and transmission losses geometric spreading is implicitly compensated by full two way wave propagation intrinsic q losses are handled by including a q dependent term in the wave equation transmission losses are compensated based on an estimation of angle dependent reflectivity using a two pass recursive reverse time prestack migration the image condition used is the ratio of receiver source wavefield amplitudes two dimensional tests using synthetic data for a dipping layer model and a salt model show that loss compensating prestack depth migration can produce reliable angle dependent reflection coefficients at the target the reflection coefficient curves are fitted to give least squares estimates of the velocity ratio at the target the main new result is a procedure

increasing illumination and sensitivity of reverse time migration with internal multiples reverse time migration is a two way time domain finite frequency technique that accurately handles the propagation of complex scattered waves and produces a bandlimited representation of the subsurface structure that is conventionally assumed to be linear in the contrasts in model parameters because of this underlying linear singlescattering assumption most implementations of this method do not satisfy the energy conservation principle and do not optimally use illumination and model sensitivity of multiply scattered waves migrating multiply scattered waves requires preserving the non linear relation between the image and perturbation of model parameters i modify the extrapolation of source and receiver wavefields to more accurately handle multiply scattered waves i extend the concept of the imaging condition in order to map into the subsurface structurally coherent seismic events that correspond to the interaction of both singly and multiply scattered waves this results in an imaging process referred to here as non linear reverse time migration it includes a strategy that analyses separated contributions of singly and multiply scattered waves to a final non linear image the goal is to provide a tool suitable for seismic interpretation and potentially migration velocity analysis that benefits from increased illumination and sensitivity from multiply scattered seismic waves it is noteworthy that this method can migrate internal multiples a clear advantage for imaging challenging complex subsurface features e g in salt and basalt environments the results of synthetic seismic imaging experiments including a subsalt imaging example illustrate the technique

true amplitude angle domain common image gathers from one way wave equation migrations true amplitude wave equation migration provides a quality migrated image of the earth s interior in addition the amplitude of the output provides an estimate of the angular dependent reflection coefficient similar to the output of kirchhoff inversion recently true amplitude wave equation migration for common shot data has been proposed to generate amplitude reliable shot domain common image gathers in heterogeneous media we present a method to directly produce angle domain common image gathers from both common shot and shot receiver wave equation migration generating true amplitude shot domain common image gathers requires a deconvolution type imaging condition using the ratio of the upgoing and downgoing wavefield each downward projected to the image point producing true amplitude angle domain common image gathers requires instead the product of the upgoing wavefield and the complexconjugate of the downgoing wavefield in the imaging condition since multiplication is a more stable comput

biotite breakdown and the formation of gahnite in metapelitic rocks from kemio southwest finland the formation of ferroan gahnite by breakdown of zinc bearing biotite has been studied in metapelitic gneisses from the kemio region in southwestern finland spinel compositions range from herc50 gahn40spin10 to herc15gahn82spin3 biotite contains up to 0 24 wt zno individual spinel grains show a gradual compositional zoning owing to increasing zinc contents towards the grain margins

comparison of kirchhoff and reverse time migration methods with applications to prestack depth imaging of complex structures the performance of two popular migration methods the kirchhoff integral and reverse time migrations is evaluated through applications to imaging complex structures using prestack shot records the migration results from the marmousi model data demonstrate that reverse time migration is more accurate than kirchhoff migration in imaging the steeply dipping faults however the improved accuracy of reverse time migration requires higher computational costs in the application to the alberta foothills data where a good estimate of the velocity model is available however both the kirchhoff and the reverse time migration methods produce almost identical results this implies that in the real world of exploration seismology it will be relatively difficult to identify which method performs better because we never know the exact answer of the subsurface

3 d migration of cross spread data resolution and amplitude aspects ideally full prestack migration involves a 4 d integration along the source and receiver coordinates obviously for cross spread data typical for land acquisition this 4 d integration cannot be effectuated an analysis is presented of the resolution and amplitude behavior of full prestack migration applied to cross spread data moreover a proposal is made to modify operators that partly compensate for the diminishing effects of the cross spread aquisition geometry on amplitudes and resolution

viscosity dependent internal variability in a model of the north pacific abstract a 2 resolution isopycnal model of the north pacific ocean is shown to produce anomalies that propagate around the subtropical gyre on the decadal time scale that do not appear in a 1 resolution version of the same model a principal oscillation pattern pop analysis of the isopycnal interface anomaly is performed to examine the dynamics responsible for the anomaly generation the pops show a coherent oscillation around the entire subtropical gyre with two centers of action one in the central mode water cmw region the other in the subtropical countercurrent stcc lead lag covariances between the subduction rate in the cmw and the layer thickness along the oscillation path indicate that anomalous subduction events are not the driving mechanism for the oscillation a linearized quasigeostrophic mode analysis shows that the anomalies are generated by flow instability in the region of the stcc the instability disappears in the 1 model because of changes in the horizontal viscosity which is

offshore imaging with complex overburden understanding gather complexity and resulting attribute accuracy through synthetics the objectives of this study are to determine the effect of complex overburden in offshore continental settings on the seismic wavefield and on the resultant seismic images and attributes seismic waves exhibit complex wave paths when propagating through heterogeneous geologic structures a key step in understanding the consequences for imaging and more importantly amplitude versus angle behavior is to model this complexity in a known earth velocity model the earth model used represented by the seismic velocities should exhibit similar behavior to the real data but still remain as simple as possible this is also the case in velocity model building however the inability to include subtle geologic features leads to inaccuracy and uncertainty in the velocity model understanding how this inaccuracy manifests itself is vital in any predictive exercise using seismic attributes to improve this understanding several synthetic data sets were simulated using full wavefield synthetics it is evident from

migration by extrapolation of time dependent boundary values migration of an observed zero offset wavefield can be performed as the solution of a boundary value problem in which the data are extrapolated backward in time this concept is implemented through a finite difference solution of the two dimensional acoustic wave equation all depths are imaged simultaneously at time 0 the imaging condition and all dips right up to vertical are correctly migrated numerical examples illustrate this technique in both constant and variable velocity media

ring genesis and the related heat transport part ii a model comparison abstract various ocean circulation models have been compared with respect to their performance in the genesis of rings and the subsequent heat transport emphasis has been placed on the role of the spurious diapycnal fluxes of heat and momentum in cartesian models arising when the horizontal dissipation mixes through sloping isopycnals quasigeostrophic isopycnal coordinate and cartesian primitive equation models in a two layer periodic channel domain have been used to simulate the process of eddy detachment from an eastward flowing jet this jet is modeled after the gulf stream east of cape hatteras on this jet a small sinusoidal disturbance is super imposed which through the release of available potential energy grows until it ultimately has developed into ringlike eddies simulations with the cartesian primitive equation model appear to suffer from spurious diapycnal mixing of both heat and momentum this retards the process of rossby wave breaking and prolongs the growth of the meander thus ca

evolution of ptygmatic folds in migmatites from the type area s finland abstract it is proposed that ptygmatic folds in migmatites form during leucosome melanosome differentiation volume loss by depletion of quartz and feldspar takes place in the interlimb domains of folds leaving a residual melanosome buckling leucosomes increase in volume due to this process and are protected from substantial thinning concurrent with tectonic segregation albeit at a lower rate the leucosome expands by replacing the melanosome this process is induced by fluid activity at the leucosome boundary as suggested by the occurrence of secondary offshoot veinlets segregation and growth causes the most typical aspect of ptygmatic folds preservation of an overall buckle shape during the stage when the folds become isoclinal and devoid of interlimb matrix

3d angle gathers from reverse time migration reverse time migration rtm based on directly solving the two way wave equation provides a natural way to deal with large lateral velocity variations and imposes no dip limitations on the seismic images it has become a standard migration tool for subsalt imaging in the gulf of mexico in addition the adoption of wide azimuth waz data acquisition provides better subsurface illumination and more azimuthal information which significantly aids imaging beneath complex overburdens because waz data are richer in the propagation directions of the waves recorded rtm and waz acquisition go hand in hand