kirchhoff migration using eikonal equation traveltimes the use of ray shooting followed by interpolation of traveltimes onto a regular grid is a popular and robust method for computing diffraction curves for kirchhoff migration an alternative to this method is to compute the traveltimes by directly solving the eikonal equation on a regular grid without computing raypaths solving the eikonal equation on such a grid simplifies the problem of interpolating times onto the migration grid but this method is not well defined at points where two different branches of the traveltime field meet also computational and data storage issues that are relatively unimportant for performance in two dimensions limit the applicability of both schemes in three dimensions a new implementation of a gridded eikonal equation solver has been designed to address these problems a 2 d version of this algorithm is tested by using it to generate traveltimes to migrate the marmousi synthetic data set using the exact velocity model the results are compared with three other images an f x migration a standard for comparison a kirchhoff migration using ray tracing and a kirchhoff migration using traveltimes generated by a commonly used eikonal equation solver the f x migrated image shows the imaging objective more clearly than any of themore kirchhoff migrations and the authors advance a heuristic reason to explain this fact of the kirchhoff migrations the one using ray tracing produces the best image and the other two are of comparable quality less

gravel ripples on the inner scotian shelf abstract sidescan sonar and photo video surveys have revealed the presence of symmetrical gravel bedforms interpreted as wave generated ripples with wavelengths of 1 3 to 3 0 m occurring in patches and narrow ribbons in depths of 15 to 65 m on the inner scotian shelf direct observations and sediment sampling from the pisces iv submersible have been used to ground truth surface based survey dam and to quantify relationships between the bedforms and the processes responsible for their formation the ripples are formed in polymodal sandy gravels with modal sizes ranging from 0 1 to 45 mm evaluation of threshold criteria for coarse sediments together with an 11 year record from a nearby wave rider buoy suggests that existing models for wave entrainment of gravel on rippled surfaces ove estimate the critical velocity by a factor of approximately 2 and that conditions suitable for transport of the coarsest 45 mm size mode sampled in the ripple crests occur on average 43 hr yr analysis of gravel ripples on the inner scotian shelf and of other examples noted in the recent literature extends the range of empirical data for oscillation ripple wavelengths by a factor of 3 and for ripple grain size by a factor of 5 the gravel ripples display a linear scaling to wave orbital diameter similar to that reported for orbital ripples in sand use of such relations for paleoenvironmental interpretation of gravel ripples preserved in the rock record is complicated by the difficulty of selecting appropriate grain size and wave parameters conditions suitable for potential formatio of large scale symmetrical ripples in coarse sediments extend from the outer continental shelf to shallow shore face settings with possible fetch lengths as short as 20 km under intense storms

three dimensional prestack plane wave migration of teleseismic p to s converted phases 1 theory 1 we present the theoretical foundations for a prestack migration technique to image teleseismic p to s converted phases the method builds on teleseismic p wave deconvolution pseudostation stacking neal and pavlis 1999 and on the idea of using a plane wave decomposition for imaging as introduced by treitel et al 1982 deconvolution operators are constructed by pseudostation stacking of the array aligned to the incident p wave arrival times to produce a space variable deconvolution operator the resulting data are then muted to remove the deconvolved direct p wave pulse and pseudostation stacked over a grid of feasible slowness vectors the pseudostation stack interpolates the wave field onto a regular grid along earth s surface producing a series one per slowness vector of uniformly sampled three dimensional data cubes two space variables and time the plane wave components can be propagated downward using a form of approximate ray tracing with a three dimensional earth model this yields a series of distorted cubes topologically equivalent to the original uniformly sampled data cubes these data volumes are summed as a weighted stack with the weights derived from an integration formula for inverse scattering based on the generalized radon transform this allows an image of the subsurface to be constructed on an event by event basis beneath the array we apply this technique to data from the lodore array that was deployed in northwestern colorado the results suggest the presence of a major lithospheric scale discontinuity defined by a south dipping boundary

reservoir fluid substitution effects on seismic profile interpretation a physical modeling experiment p wave seismic surveys are performed on realistic physical models of a reservoir formed of interbedded sand and shale layers and filled with gas water and oil they show characteristic behaviour which can be used to distinguish between fluids with significant implications for the interpretation of time lapse experiments discontinuous reflections originate in gas filled reservoirs with widely varying amplitude compared to oil or water filled reservoirs anomalous changes in seismic amplitude difference attributes for reflections within the reservoir are seen for gas oil substitution oil water substitution results in only weak variations of reservoir elastic properties thus amplitude difference attributes may be unsuitable for monitoring changes in oil water saturation however direct differences between the time traces are strong on time lapse sections strong amplitude difference anomalies do not necessarily correspond to the region where the fluid variations occur this is significant for monitoring oil field development by water injection

lowstand shorefaces transgressive incised shorefaces and forced regressions examples from the viking formation joarcam area alberta abstract in the joarcam beaverhill lake area of alberta there are three long 40 km narrow 5 km sandbodies in the late albian viking formation they are surrounded by offshore open marine mudstones and have previously been interpreted as offshore bars however the sandbodies are both underlain and overlain by erosion surfaces joarcam is the only field with abundant cores and the sandbody shows a sandier upward and coarsening upward succession all three fields have recently been reinterpreted in the literature as lowstand shoreface deposits but the order in which they formed remains controversial our well log cross sections suggest that the lindbrook shoreface was the first to form at a time of maximum lowstand of sea level it was truncated as sea level began to rise and the lindbr ok transgressive surface of erosion tse has been traced into the erosion surface that underlies joarcam joarcam can therefore be interpreted as a shoreface incised during a pause in an overall transgression both the lindbrook and joarcam shorefaces appear to be truncated by the regressive surface of erosion rse that underlies beaverhill lake this suggests that beaverhill lake is the youngest of the shorefaces and implies a second major fall of relative sea level the lowstand shorefaces tend to be thinner because of reduced accommodation space during sea level fall they tend to prograde farther beaverhill lake about 23 km because of abundant sediment supply at lowstand but consist largely of sandstone directly overlying the rse at beaverhill lake the shoreface sediments prograded at least 10 km before any mud in the offshore to shoreface transition was preserved by contrast the transgressive shorefaces tend to be thicker because of accommodation space created during transgression however they tend not to prograde so far joarcam up to 13 km possibly because of reduced supply of sediment to the shoreface during transgression the two proposed episodes of lowering of relative sea level are tentatively correlated with the two separate valley incisions documented at crystal about 90 km to the southwest these incisions represent type 1 sequence boundaries but the boundaries are not continuous regressive surfaces of erosion from crystal to the joarcam beaverhill lake area in both cases the falling stage rses have been eroded by the subsequent tses these tses are the only surfaces preserved between the incised valleys and the lowstand shorefaces

electromagnetic borehole fields in a layered dipping bed environment with invasion electromagnetic modeling of an induction sonde 1 100 khz in a dipping bed environment is a 3 d problem the capability for such an analysis is necessary for interpretation of oil well logs in offshore environments where most holes are deviated 3 d geometrical effects require vector field analysis the method accounts for transverse magnetic mode tm coupling arising from surface charges deposited by eddy currents passing through bed boundaries if borehole and invasion effects are included the only available rigorous analytical methods are finite elements or finite difference techniques these approaches require large scale computing in contrast our method is approximate and is an extension of the geometrical factor theory and born approximation the variational method does not require matrices and is numerically simpler than the more rigorous finite element method the method uses a new electric field vector integral equation developed by chew the formulation accounts for low frequency behavior a

gabor frame based gaussian packet migration we present a gaussian packet migration method based on gabor frame decomposition and asymptotic propagation of gaussian packets a gaussian packet has both gaussian shaped time frequency localization and space direction localization its evolution can be obtained by ray tracing and dynamic ray tracing in this paper we first briefly review the concept of gaussian packets after discussing how initial parameters affect the shape of a gaussian packet we then propose two gabor frame based gaussian packet decomposition methods that can sparsely and accurately represent seismic data one method is the dreamlet gaussian packet method dreamlets are physical wavelets defined on an observation plane and can represent seismic data efficiently in the local time frequency space wavenumber domain after decomposition dreamlet coefficients can be easily converted to the corresponding gaussian packet coefficients the other method is the gabor frame gaussian beam method in this method a local slant stack which is widely used in gaussian beam migration is combined with the gabor frame decomposition to obtain uniform sampled horizontal slowness for each local frequency based on these decomposition methods we derive a poststack depth migration method through the summation of the backpropagated gaussian packets and the application of the imaging condition to demonstrate the gaussian packet evolution and migration imaging in complex models we show several numerical examples we first use the evolution of a single gaussian packet in media with different complexities to show the accuracy of gaussian packet propagation then we test the point source responses in smoothed varying velocity models to show the accuracy of gaussian packet summation finally using poststack synthetic data sets of a four layer model and the two dimensional seg eage model we demonstrate the validity and accuracy of the migration method compared with the more accurate but more time consuming one way wave equation based migration such as beamlet migration the gaussian packet method proposed in this paper can correctly image the major structures of the complex model especially in subsalt areas with much higher efficiency this shows the application potential of gaussian packet migration in complicated areas

a hybrid wave propagation simulation technique for ocean acoustic problems intermediate range bottom interacting deep ocean acoustic experiments up to 50 km offset in water depths exceeding 1 km using frequencies in the 100 500 hz range encompass a variety of different scales the water column varies smoothly over many wavelengths whereas the seafloor is rough at scales from 1 m to several kilometers we present a general hybrid technique to solve the two dimensional acoustic viscoelastic equations for problems that include these different wave propagation regimes this approach makes realistic large scale computations tractable and assures high accuracy the technique which we call hybrid adaptive regime visco elastic simulation technique harvest consists of three parts a gaussian beam method is used to propagate the source wave field through a vertically varying water column to the scattering region near the seafloor this extrapolated source wave field is inserted into a viscoelastic finite difference grid on which the complex acoustic anelastic interaction of the wave field with the rough seafloor is computed the backscattered wave field collected on hypothetical vertical and horizontal lines is then extrapolated by means of the kirchhoff integral to a receiver array distant from the scattering locale the fidelity of the method is demonstrated by comparison with solutions from a wkbj approximation method and full finite difference simulations

hummocky cross stratification tropical hurricanes and intense winter storms most previous workers have inferred a storm origin for hummocky cross stratification which typically occurs in shallow marine deposits on the modern earth the only storms capable of profoundly affecting shallow marine depositional environments are severe tropical cyclones hurricanes and mid latitude winter wave cyclones intense winter storms this paper examines the palaeogeographic distribution including palaeolatitude and palaeogeographic setting of 107 occurrences of hummocky cross stratification ranging in age from the proterozoic to recent in each of these stratigraphic units both palaeolatitude and palaeogeography are consistent with a direct storm influence associated with the passage of hurricanes or winter storms directly over the site of deposition this palaeogeographic evidence lends support to the inferred storm origin for hummocky cross stratification further the distribution of the structure suggests that most occurrences 73 were generated by tropical hurricanes the remaining 27 being generated by intense mid latitude winter storms the preferential generation of hummocky cross stratification by hurricanes is consistent with 1 the known differences in the nature of the bottom flows generated by the two major storm types and 2 the inferred nature of the flows which form hummocky cross stratification hurricanes couple less effectively with the water column than do intense winter storms due to this ineffective coupling hurricane generated bottom flows tend to be oscillatory or multidirectional dominant with only minor unidirectional components of motion in contrast intense winter storms generally do couple effectively with the water column generating bottom flows which possess a dominant or significant unidirectional component most previous workers have suggested that hummocky cross stratification forms under oscillatory or multidirectional dominant flow thus it is conceptually reasonable that the vast majority of ancient occurrences of hummocky cross stratification were probably hurricane generated as suggested by the aforementioned palaeogeographic distribution the proterozoic palaeozoic neogene and quaternary were times when global climate was similar to that of today the distribution of hummocky cross stratification deposited during these times suggests that both hurricanes and intense winter storms occupied latitudinal belts during these times which were essentially identical to those occupied by their modern counterparts the mesozoic and palaeogene were non glacial times when global climate was much warmer than that of today the distribution of hummocky cross stratification deposited during this interval suggests that hurricanes occurred more frequently at higher latitudes during non glacial times than they do at present the possibility of a broadened hurricane belt during the mesozoic and palaeogene is consistent with climatic considerations a limited number of mesozoic and palaeogene rock units containing hummocky cross stratification were deposited in palaeogeographic settings that preclude a direct hurricane influence these examples were deposited in the middle latitudes suggesting that intense winter storms continued to form hummocky cross stratification in the middle latitudes during these much warmer times some previous workers have suggested that tsunamis may be capable of generating hummocky cross stratification the palaeogeographic distribution of the structure does not support an origin due to tsunamis lacustrine examples of hummocky cross stratification reported herein are the first known non marine occurrences they suggest that storm effects strongly influence the sedimentary record of some lakes

amplitude versus angle analysis of local angle domain common image gathers with prestack gaussian beam migration of seismic data the analysis of seismic reflection amplitude versus angle amplitude versus offset ava avo is an important tool for investigating the physical properties of subsurface rocks and or reservoirs we propose to generate local angle domain common image gathers adcigs from the surface recorded seismic data for ava analysis via gaussian beam migration gbm using a deconvolution imaging condition the traditional seismic reflection amplitude analysis is often based on the avo response from common middle point cmp gathers however the avo analysis based on the horizontally layered medium assumption can achieve success only in simple geological cases our approach provides an ava response during the migration process that reveals true local physical properties in the image domain even if the image point is at a tilted interface or has a complicated overburden geological structure we test our approach with a two layer velocity model with a tilted interface and with the benchmark marmousi2 model that has a complicated geological structure from these two numerical examples we demonstrate that our method faithfully provides the ava response that can be used to further interpret the rock reservoir physical properties

cardium formation u cretaceous at seebe alberta storm transported sandstones and conglomerates in shallow marine depositional environments below fair weather wave base as a result of detailed mapping at seebe alberta we have defined five coarsening upward sequences in the turanian cardium formation the sequences begin with bioturbated mudstones and coarsen int

2 d wavepath migration prestack kirchhoff migration km is computationally intensive for iterative velocity analysis this is partly because each time sample in a trace must be smeared along a quasi ellipsoid in the model as a less costly alternative we use the stationary phase approximation to the km integral so that the time sample is smeared along a small fresnel zone portion of the quasi ellipsoid this is equivalent to smearing the time samples in a trace over a 1 5 d fat ray i e wavepath so we call this wavepath migration wm this compares to standard km which smears the energy in a trace along a 3 d volume of quasi concentric ellipsoids in principle single trace migration with wm has a computational count of o n1 5 compared to km which has a computational count of o n3 where n is the number of grid points along one side of a cubic velocity model our results with poststack data show that wm produces an image that in some places contains fewer migration artifacts and is about as well resolved as the km

3d wavepath migration 3d kirchhoff migration km smears a trace s time sample along a quasi ellipsoid in the model space this is a costly and sometimes noisy process as reflection energy is smeared far away from the actual reflector position introducing far field migration artefacts as a reduced form of 3d km 3d wavepath migration wm smears a picked reflection arrival to a small fresnel zone portion centred about the specular reflection point leading to fewer migration artefacts and reduced computation time both the traveltime and the angle of incidence are required by wm for locating the specular reflection point our results with 3d prestack synthetic data show that wm generates fewer migration artefacts and can sometimes define complex structure better than km our results with 3d prestack field data show that wm can mostly suppress migration artefacts and can sometimes resolve reflection interfaces better than km the cpu comparison shows that for both the synthetic and field data examples wm can be more than an order of magnitude faster than km the limitation with 3d wm is that the angle of incidence calculation is sensitive to the recording geometry and the signal to noise s n ratio which can lead to blurred images

local vertical seismic profiling vsp elastic reverse time migration and migration resolution salt flank imaging with transmitted p to s waves to avoid the defocusing effects of propagating waves through salt and overburden with an inaccurate overburden velocity model we introduce a vertical seismic profiling vsp local elastic reverse time migration rtm method for salt flank imaging by transmitted p to s waves this method back projects the transmitted ps waves using a local velocity model around the well until they are in phase with the back projected pp waves at the salt boundaries the merits of this method are that it does not require the complex overburden and salt body velocities and it automatically accounts for source side statics in addition the method accounts for kinematic and dynamic effects including anisotropy absorption and all other unknown rock effects outside of this lo cal subsalt velocity model numerical tests on an elastic salt model and offset 2d vsp data in the gulf of mexico using a finite difference time domain staggered grid rtm scheme partly demonstrate the effectiveness of this method over interferometry

kinematic artifacts in prestack depth migration strong refraction of waves in the migration velocity model introduces kinematic artifacts coherent events not corresponding to actual reflectors into the image volumes produced by prestack depth migration applied to individual data bins because individual bins are migrated independently the migration has no access to the bin component of slowness this loss of slowness information permits events to migrate along multiple incident reflected ray pairs thus introducing spurious coherent events into the image volume this pathology occurs for all common binning strategies including common source common offset and common scattering angle since the artifacts move out with bin parameter their effect on the final stacked image is minimal provided that the migration velocity model is kinematically correct however common image gathers may exhibit energetic primary events with substantial residual moveout even with the kinematically accurate migration velocity model

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

palaeogeographic reconstruction using composite maps with case studies from three continents abstract palaeoenvironmental reconstruction is based partly on the evaluation of stratigraphic lithofacies and biofacies maps which generally show broad similarities but also local discrepancies that can be difficult to reconcile integrating these maps has the advantage that corresponding features are accentuated whereas contrasting details are eliminated or reduced in prominence as all the different basemaps contribute equally towards the final product the composite map gives a more reliable and accurate representation of the palaeogeography than any of its components this is illustrated by case studies of shallow marine fluvial and lacustrine environments from australia south africa and chile

ensialic spreading subsidence in the mesozoic and palaeogene andes of central chile an oceanic plate subducting eastwards below a continental margin and resulting in crustal shortening can be inferred to have been active in central chile during the palaeozoic however the geological record for the mesozoic and palaeogene shows evidence of a persistent tensional regime and absence of crustal shortening although lateral asymmetry inherent to a subducting model is shown by this record a bilateral symmetry is also well substantiated a coupled action of plate subduction and spreading subsidence the latter taking place within the margin of a continental plate is suggested to explain these features

deposition and interpretation of long narrow sandbodies underlain by a basinwide erosion surface cardium formation cretaceous western interior seaway alberta canada abstract sand bodies of the cardium alloformation burnstick allomember occur in three parallel linear trends in the garrington caroline crossfield and lochend oilfields they are extremely long up to 100 km straight and narrow up to 4 km and are encased in marine mudstones suggesting their former interpretation as offshore bars our study of 189 cores and over 800 resistivity well logs shows that below the burnstick the hornbeck allomember consists entirely of dark mudstones deposited in quiet water the hornbeck burnstick contact where seen in cores is extremely sharp in cross sections resistivity well log markers are truncated by the lower surface of the burnstick and the sand bodies rest in asymmetrical one sided elongate scours with erosional relief of up to 7 m above he scoured surface there is a basal lag of mudstone intraclasts and chert pebbles overlain by a sandier upward succession of mudstones sandstones and conglomerates the sandstones are extensively burrowed but show some cross stratification the burnstick sandier upward succession is overlain by black mudstones of the raven river allomember the morphology of the scoured surface as defined in isopach maps does not indicate its formation in an open marine setting the long straight asymmetrical scours are parallel to regional strike and do not suggest subaerial erosion instead we suggest that a major relative lowering of sea level first exposed the hornbeck mudstones to subaerial erosion subsequently marine transgression of this surface removed evidence of its subaerial exposure pauses in transgression stillstands resulted in incised shoreface profiles the asymmetrical scours and at this time rivers supplied coarse sediment to the shoreface this sediment was reworked alongshore to form what are now the very long straight narrow sand bodies continued transgression resulted in the burial of the shoreface sand bodies by black mudstone basal part of the raven river allomember incised shoreface environments provide an alternative to the offshore bar settings suggested for other long linear sand bodies

pulsation tectonics as the control of long term stratigraphic cycles a new theory of pulsation tectonics involving the cyclic eruption of thermal plumes from the d layer in the lowermost mantle explains the long term cycles of marine stratigraphy heating of the mantle changes conditions at the core mantle boundary that result in the quiescence of the earth s magnetic field the rising plumes after a phase lag later induce faster plate spreading and widespread intraplate volcanism subsequently colder rock returning to the lower mantle reestablishes the d thermal boundary layer and sets up conditions for more turbulent core convection and frequent magnetic reversals meanwhile lower mantle plumes cease erupting and the plates spread more slowly the cycles of fast and slow plate spreading and quiet and reversing magnetic field have a duration of 60 100 m y global changes in spreading rate are one of the main causes of large scale 100 300 m eustatic sea level changes and variations in output of volcanic gases consequential changes in albedo global temperature humidity forestation atmospheric and oceanic co2 oceanic acidity and oxygenation lead to variations in the preservation of organic carbon in the oceans and epicontinental seas and in the level of the calcite compensation depth during rising sea levels there is an increase in organic carbon preservation and relatively little calcite is preserved with falling sea levels less organic carbon and more calcite are preserved in the deep sea the long term cycles in carbon and calcite bearing sediments correspond generally to the pulsations of grabau 1934 the long term rhythms which nearly equal the lengths of geologic time periods

a pliocene mega tsunami deposit and associated features in the ranquil formation southern chile abstract an exceptionally large tsunami affected the coastline of southern chile during the pliocene its backflow eroded coarse beach and coastal dune sediments and redistributed them over the continental shelf and slope sandstone dykes and sills injected from the base of the resulting hyperconcentrated flow into underlying cohesive muds assisted in plucking up large blocks of the latter and incorporating them into the flow locally the rip up intraclasts were fragmented further by smaller scale injections to form a distinct breccia of angular to rounded mudstone clasts within a medium to coarse sandstone matrix sandstone sills in places mimic normal sedimentary beds complete with structures resembling inverse gradation planar laminae as well as ripple and trough cross lamination these were probably formed by internal sediment flow and shear stress as the semi liquefied sand was forcefully injected into cracks in borehole cores such sills can easily be misinterpreted as normal sedimentary beds which can have important implications for hydrocarbon exploration

diffraction imaging by uniform asymptotic theory and double exponential fitting seismic diffracted waves carry valuable information for identifying geological discontinuities unfortunately the diffraction energy is generally too weak and standard seismic processing is biased to imaging reflection in this paper we present a dynamic diffraction imaging method with the aim of enhancing diffraction and increasing the signal to noise ratio the correlation between diffraction amplitudes and their traveltimes generally exists in two forms with one form based on the kirchhoff integral formulation and the other on the uniform asymptotic theory however the former will encounter singularities at geometrical shadow boundaries and the latter requires the computation of a fresnel integral therefore neither of these methods is appropriate for practical applications noting the special form of the fresnel integral we propose a least squares fitting method based on double exponential functions to study the amplitude function of diffracted waves the simple form of the fitting function has no singularities and can accelerate the calculation of diffraction amplitude weakening coefficients by considering both the fitting weakening function and the polarity reversal property of the diffracted waves we modify the conventional kirchhoff imaging conditions and formulate a diffraction imaging formula the mechanism of the proposed diffraction imaging procedure is based on the edge diffractor instead of the idealized point diffractor the polarity reversal property can eliminate the background of strong reflection and enhance the diffraction by samephase summation moreover the fitting weakening function of diffraction amplitudes behaves like an inherent window to optimize the diffraction imaging aperture by its decaying trend synthetic and field data examples reveal that the proposed diffraction imaging method can meet the requirement of high resolution imaging with the edge diffraction fully reinforced and the strong reflection mostly eliminated

a wave equation migration method for receiver function imaging 1 theory 1 a wave equation based poststack depth migration method is proposed to image the earth s internal structure using teleseismic receiver functions by utilizing a frequency wave number domain one way phase screen propagator for wave field extrapolation in the migration scheme common conversion point ccp stacked receiver functions are backward propagated to construct a subsurface structural image the phase screen propagator migration method takes into account the effects of diffraction scattering and travel time alternation caused by lateral heterogeneities and therefore it is particularly useful for imaging complex structures and deep discontinuities overlain by strong shallow anomalies synthetic experiments demonstrate the validity of the migration method for a variety of laterally heterogeneous models the migrated images show considerable improvement over the ccp images in recovering the model features influences of several factors on the image quality of the poststack migration are further investigated including interstation spacing noise level of the data velocity model used in migration and earthquake distribution incident direction of source fields on the basis of the sampling theorem and previous statistic results we discuss the relation of spatial resolution and signal to noise ratio of the migrated image with the frequency of the data surface station spacing and number of receiver functions used in stacking we show that both ccp stacking and poststack migration of receiver functions need to be designed in a target oriented way for reliable and efficient imaging our results also suggest that careful consideration of earthquake source distribution is necessary in designing seismic experiments aimed at imaging steeply dipping structures

forced regressions in a sequence stratigraphic framework concepts examples and exploration significance sequence stratigraphic concepts suggest that stratal geometries develop and are largely controlled by changes in relative sea level on the shelf lowstand deposits which form during falls and subsequent stillstands of relative sea level can be recognized by the presence of an unconformity at the base the isolated and basinward position relative to the previous shoreline and the abrupt seaward translation of shallow water and shoreline facies into the basin across an unconformity surface this seaward translation of facies and shoreline regression in response to relative sea level lowering is termed a forced regression a forced regression is independent of variations of sediment flux and is in contrast with normal regressions that occur in response to excess sediment flux relative to space available on the shelf i e accommodation forced regressions commonly are associated with a zone of sedimentary bypass subaerial exposure and possible fluvial erosion between the newly formed and preceding shorelines certain shelf sands previously interpreted as offshore or mid shelf sand bodies thus can be reinterpreted as stranded lowstand shorelines associated with forced regressions this alternative interpretation has economic significance insofar as it suggests different subsurface correlations and reservoir geometries with the potential for development of new play types and enhanced recovery in o der fields examples of forced regression can be observed at a variety of scales and ages several such examples include the modern east coulee fan delta and the lower cretaceous viking formation in alberta canada the quaternary rhone delta and the quaternary hudson valley system

gaussian beam migration just as synthetic seismic data can be created by expressing the wave field radiating from a seismic source as a set of gaussian beams recorded data can be downward continued by expressing the recorded wave field as a set of gaussian beams emerging at the earth s surface in both cases the gaussian beam description of the seismic wave propagation can be advantageous when there are lateral variations in the seismic velocities gaussian beam downward continuation enables wave equation calculation of seismic propagation while it retains the interpretive raypath description of this propagation this paper describes a zero offset depth migration method that employs gaussian beam downward continuation of the recorded wave field the gaussian beam migration method has advantages for imaging complex structures like finite difference migration it is especially compatible with lateral variations in velocity but gaussian beam migration can image steeply dipping reflectors and will not produce unwanted reflectio

paraxial ray kirchhoff migration a rapid nonrecursive prestack kirchhoff migration is implemented for 2 d or 2 5 d media by computing the green s functions both traveltimes and amplitudes in variable velocity media with the paraxial ray method since the paraxial ray method allows the green s functions to be determined at points which do not lie on the ray two point ray tracing is not required the green s functions between a source or receiver location and a dense grid of thousands of image points can be estimated to a desired accuracy by shooting a sufficiently dense fan of rays for a given grid of image points the paraxial ray method reduces computation time by one order of magnitude compared with interpolation schemes the method is illustrated using synthetic data generated by acoustic ray tracing application to vsp data collected in a borehole adjacent to a reef in michigan produces an image that clearly shows the location of the reef

imaging complex geologic structure with single arrival kirchhoff prestack depth migration we compare various forms of single arrival kirchhoff prestack depth migration to a full waveform finite difference migration image using synthetic seismic data generated from the structurally complex 2 d marmousi velocity model first arrival traveltime kirchhoff migration produces severe artifacts and image contamination in regions of the depth model where significant reflection energy propagates as late or multiple arrivals in the total reflection wavefield kirchhoff migrations using maximum energy arrival traveltime trajectories significantly improve the image in the complex zone of the marmousi model but are not as coherent as the finite difference migration image by carefully incorporating continuous phase estimates with the associated maximum energy arrival traveltimes we obtain single arrival kirchhoff images that are similar in quality to the finite difference migration image furthermore maximum energy green s function traveltime and phase values calculated within the seismic frequency ban

coarse grained storm beds of the upper cretaceous chungo member wapiabi formation southern alberta canada the upper cretaceous chungo member of the wapiabi formation at mount yamnuska alberta is composed of three cycles two coarsening upward progradational successions overlain by a fining upward transgressive succession the basal 3 m of the transgressive sequence is made up of interbedded sandstone and conglomerate with minor shale that was deposited in a nearshore shallow marine setting discrete coarse grained storm beds in this interval are characterized by 1 a sharp based conglomeratic division with local sole marks and onshore imbricate intra and extraformational clasts 2 a cross stratified conglomeratic division with cross strata dipping predominantly onshore and capped by symmetrical coarse grained ripples 3 a horizontal to sub horizontally laminated sandstone division containing pebble horizons near the contact with underlying conglomerate and commonly capped by symmetrical ripples and 4 a capping shale division deposition of the coarse grained storm beds took place in two stages 1 transport from a beach by offshore directed storm generated combined flows followed by 2 reworking and selective sorting of bed material by asymmetrical oscillatory currents generated by shoaling swell waves propagating onshore stage 2 involved the onshore transport of selected coarse grain sizes which deposited as the reworked portions of coarse grained storms beds divisions 2 and above concurrent with offshore transport of fine sediment that may be deposited as the top portions of hcs storm beds

depositional setting and early diagenesis of the dinosaur eggshell bearing aren fm at bastus late campanian south central pyrenees the late cretaceous aren fm exposed north of bastus in the tremp basin south central pyrenees preserves an excellent record of dinosaur eggs laid in a marine littoral setting different from other cases reported in literature at the bastus site the preferential nesting ground was original beach sand the coastal deposits of aren fm can be grouped into four facies assemblages representing respectively shoreface beachface beach ridge plain and backbarrier lagoon environments shoreface deposits include fine to coarsegrained hybrid arenites and subordinate quartz dominated conglomerates with ripple structures of wave and wave current origin beachface deposits are mainly storm beach conglomerates but parallel laminated foreshore arenites locally occur backbarrier lagoon deposits comprise of washover sandy conglomerates that grade laterally into sandy lime mudstones biomicrites and marls beach ridge sediment wherein the bulk of dinosaur eggs and eggshell debris occurs predominantly is a reddish hybrid arenite that has undergone a complex early diagenetic evolution including marine and meteoric cementation followed by soil development the reddish arenites overlie wave dominated shoreface deposits and in places pass laterally into lagoonal deposits they originally formed shore ridges that became stabilized during progradational episodes by pedogenesis beach ridge sensu otvos e g 2000 beach ridges definitions and significance geomorphology 32 83 108 which also affected the dinosaur eggs the eggshell bearing beach ridge arenites are typically preserved at the top of parasequences forming the systems tracts of a third order sequence thick packages of this facies resulted from aggradation of barrier beach ridge deposits whose preservation below surfaces of transgressive erosion was favoured by incipient lithification

seismic migration problems and solutions historically seismic migration has been the practice science technology and craft of collapsing diffraction events on unmigrated records to points thereby moving migrating reflection events to their proper locations creating a true image of structures within the earth over the years the scope of migration has broadened what began as a structural imaging tool is evolving into a tool for velocity estimation and attribute analysis making detailed use of the amplitude and phase information in the migrated image with its expanded scope migration has moved from the final step of the seismic acquisition and processing flow to a more central one with links to both the processes preceding and following it in this paper we describe the mechanics of migration the algorithms as well as some of the problems related to it such as algorithmic accuracy and efficiency and velocity estimation we also describe its relationship with other processes such as seismic modeling our approach is tutorial we avoid presenting the finest details of either the migration algorithms themselves or the problems to which migration is applied rather we focus on presenting the problems themselves in the hope that most geophysicists will be able to gain an appreciation of where this imaging method fits in the larger problem of searching for hydrocarbons

implementation of elastic reverse time migration using wavefield separation in the frequency domain summary considerable effort has been devoted to the migration of multicomponent data in elastic media with wavefield separation techniques being the most successful most of this work has been carried out in the time domain in this paper we formulate a multicomponent migration technique in the frequency domain reverse time migration can be viewed as the zero lag cross correlation between virtual source and back propagated wavefields cross correlating the helmholtz decomposed wavefields rather than directly correlating the vector displacement fields results in sharper more interpretable images contaminated by fewer crosstalk artefacts the end products are separate p and s wave and if desired ps and sp migration images we test our migration algorithm on synthetic seismic data generated using the seg eage salt dome overthrust and marmousi 2 models we correctly image the location and shape of the target zone for oil exploration using these data sets furthermore we demonstrate that our new migration technique provides good images even when the initial velocity model is only approximate

storm and tide dominated shorelines in cretaceous moosebar lower gates interval outcrop equivalents of deep basin gas trap in western canada the moosebar and gates formations crop out in the deformed foothills belt south of fort st john british columbia they permit examination of the depositional environments of sandstones and conglomerates that are gas bearing in the equivalent wilrich falher interval of the deep basin of alberta and british columbia the moosebar gates interval consists of several upward coarsening and marine to nonmarine sequences moosebar bioturbated shales pass upward into turbidites and offshore storm deposits flow directions indicate a north dipping paleoslope a second coarsening upward sequence at the top of the moosebar and base of the gates terminates in a lenticular fluvial conglomerate up to 30 m thick which is overlain by coals carbonaceous mudstones and sandstones above the carbonaceous zone a major lateral facies change is observed fluvial conglomerates in the south grade into beach conglomerates northward the upper part of the beach conglomerate consists of exposed three dimensional storm berms at least 500 m long up to 1 5 m high with wavelengths of 12 to 24 m the berms trend east west seaward of the conglomerate beach are herringbone cross bedded tidal sandstones with a marine trace fauna graded sharp based conglomerates also are present probably washed out to sea by storms farther north at the same stratigraphic level is a series of partly overlapping conglomerate bars up to 5 m thick and 40 to 50 m across the tops of these bars are covered with storm formed symmetric conglomeratic dunes details of the orientation and geometry of these conglomerate bodies along with their internal structure and pebble fabrics suggest that four major environments fluvial beach offshore conglomerate bars and offshore graded storm conglomerates should be identifiable and predictable in the subsurface

wave formed coarse grained ripples and their relationship to hummocky cross stratification abstract wave formed coarse grained ripples cgr form in water depths ranging from 3 to 160 m on transgressive surfaces in upper and lower shoreface sediments and in finer grained offshore transition deposits cgr have crest spacings of 0 25 3 0 m with amplitudes of 5 35 cm internal stratification is rarely preserved but where present crossbeds dip en echelon away from the ripple crest and individual laminae feather out away from the crest waves that form cgr commonly have wavelengths on the order of 2 5 m with periods of 8 to 14 sec these are not large storm events especially on a geological scale cgr commonly occur in close association laterally and vertically with hummocky cross stratification it is postulated that if sediment is medium to very coarse grained sand or pebbly sand then two dimensional cgr will be formed from oscillatory or oscillatory dominant flow if however sediment is very fine to fine grained sand then three dimensional hummocky cross stratification will be formed the wave conditions under which cgr form are similar to those which would form hummocky cross stratification it is only the grain size that is different reported and calculated wave conditions for hummocky cross stratification are almost identical to those for cgr modern cgr crests trend parallel to the local shoreline and bathymetry as such ancient ripple crests are a useful tool for basin reconstruction

slowness driven gaussian beam prestack depth migration for low fold seismic data subsurface images based on low fold seismic reflection data or data with geometry acquisition limitations such as obtained from ocean bottom seismography obs are often corrupted by migration swing artifacts incorporating prestack instantaneous slowness information into the imaging condition can significantly reduce these migration swing artifacts and improve image quality especially for areas with poor illumination we combine the horizontal surface slowness information of observed seismic data with gaussian beam depth migration to implement a new slowness driven gaussian beam prestack depth migration whereby fresnel weighting is combined naturally with beam summation the prestack instantaneous slowness information is extracted from the original obs or shot gathers using local slant stacks and is combined with a local semblance analysis during migration we propagate the seismic energy downward knowing its instantaneous slowness information at each image location the beam summation is localized in a resolution dependent fresnel zone the instantaneous slowness information controls the beam summation synthetic and real data examples confirm that slowness driven gaussian beam migration can suppress most noise from inadequate stacking and give a clearer migration result

is there evidence for geostrophic currents preserved in the sedimentary record of inner to middle shelf deposits abstract unidirectional oscillatory and combined flow paleocurrent data from many ancient wave dominated coastlines indicate that sediment transport was directed offshore at angles of 70 to 90 from the inferred local paleoshoreline solemarks and parting lineations from turbidite like storm beds and solemarks from hummocky beds indicate that flows transporting sediment offshore were orthogonal to the local shoreline the shore normal orientations of solemarks parting lineation and asymmetric ripples on hummocky cross stratification and their orthogonal relationship with respect to wave ripple crest orientation indicate that the dominant direction of wave approach at the sediment water interface was also orthogonal to the shoreline the formation of hummocky cross stratification by an al ng shore flow component superimposed on oscillatory wave motion is not supported by paleocurrent data rather much of the evidence from the ancient record suggests that hummocky cross stratification forms under high energy oscillatory flow with a weak shore normal combined flow component the orientation of wave ripple crests from the lower shoreface to offshore transitional zone of wave dominated coasts can be used to approximate the local paleoshoreline trend the paleocurrent data from the ancient examples suggest a strong predominance of offshore directed sediment transport as opposed to the nearly shore parallel orientation of currents observed in modern geostrophic flows

finite element analysis for model parameters distributed on a hierarchy of geometric simplices abstracta resurgence of interest in the problem of electrical or electromagnetic scattering from thin conductors either on a line or in a plane has been motivated in recent years by time lapse fracture monitoring in the near surface enhanced geothermal reservoirs and unconventional hydrocarbon plays however finite element modeling of small electrical features in large computational domains results in a disproportionately large number of elements concentrated in a volumetrically insignificant fraction of the mesh and it focuses computational resources away from areas of interest elsewhere such as receiver locations we have developed a novel hierarchical electrical model is proposed for unstructured tetrahedral finite element meshes in which the usual volume based conductivity on tetrahedra is augmented by facet and edge based conductivity on the infinitesimally thin regions between elements doing so allows a slender borehole casing of arbitrary shape to be coarsely approximated by set of connec

coastal deformation and great subduction earthquakes isla santa maria chile 37 s isla santa maria at the active margin of south central chile is the result of earthquake related uplift and deformation in the forearc since at least late pleistocene time field mapping dating of key depositional horizons and analysis of seismic refl ection profi les reveal ongoing deformation in this sector of the chilean forearc the 30 km 2 island is located 12 km above the interplate seismogenic zone and 75 km landward of the trench it is situated near the southern termination of the concepcion earthquake rupture segment where charles darwin measured 3 m of coseismic uplift during a m 8 megathrust earthquake in 1835 permanent postearthquake deformation from this earthquake and an earlier event in 1751 is registered by emerged landward tilted abrasion surfaces uplift at 2 m k y and tilting at 0 025 k y of the island have been fairly constant throughout the late quaternary and have resulted in emergence of the island above sea level 31 k y ago the island is composed of a late pleistocene upper tilted surface with two asymmetric tilt domains and holocene lowlands characterized by uplifted and tilted strandlines industry offshore seismic refl ection profi les covering an area of 1800 km 2 and crustal seismicity reveal active reversefault cored anticlines surrounding isla santa maria the principal fault apparently roots in the plate interface thrust these reverse faults in the upper plate result from inversion of late cretaceous to early pliocene normal faults and rift structure of the arauco forearc basin positive inversion of these inherited structures started between 3 6 and 2 5 ma and resulted in continuous shortening rates of 0 8 mm yr the seismic refl ection profi les show that the asymmetric tilt domains and progressive syntectonic sedimentation are linked to the position of the island in the forelimbs of two converging anticlines whereas their backlimbs have been removed by cliff retreat the 2 m uplift contour of the 1835 earthquake is parallel to the strike of active faults and antiforms in the arauco concepcion region the close relation among the asymmetric uplift and tilt of the island modern deformation patterns and reverse faults rooted in the plate interface suggests that slip on the plate interface thrust infl uences localizes and segments surface deformation during large interplate earthquakes furthermore the link between positive inversion of pre existing structures uplift and tilt patterns in the forearc emphasizes the importance of inherited structural fabrics in guiding plate boundary deformation

seismic structure and tectonics of the southern arauco basin south central chile 38 s abstract the arauco basin is located on the continental shelf near the chilean subduction zone 38 s main deformational stages that affected this setting show a good correlation with andean constructional phases we studied the kinematic evolution of the arauco basin using high resolution seismic reflection data across the southern arauco forearc basin three structural domains are identified 1 inversion 2 extension and 3 accretion in addition seven syn kinematic sequences have been recognized overlying the late carboniferous to triassic basement and their relationships with four traditional tectonostratigraphic sequences of the arauco basin s1 late cretaceous syn extension s2 eocene syn extension s3 and s4 eocene syn inversion s5 s6 miocene syn inversion with middle extensional structures and s7 pliocene quaternary post inversion and syn compression shortening began in the southern arauco basin coeval with a major readjustment of the plate convergence rate 34 ma that is represented by inversion structures and kinematic syn inversion sequences a marked erosional unconformity 34 23 ma represents a subsequent event of erosion uplift during the oligocene a contractional miocene deformation phase reinforced the inversion tectonic and generated miocene extensional structures since the pliocene the rapid exhumation of the nahuelbuta range allows the emergence of the arauco basin within the basin pliocene and quaternary sequences were affected by contractional structures including the inversion of the miocene extensional faults the contraction along the arauco basin can be related to the shift to accretionary conditions of the margin and the oblique collision of the mocha fracture zone 3 6 ma at 38 s the mechanics of regional subsidence which affected the basin during the miocene cannot be attributed to major scale extensional structures 1 0 km

common shot fresnel beam migration based on wave field approximation in effective vicinity under complex topographic conditions gaussian beam migration is a versatile imaging method for geologically complex land areas which overcomes the limitation of kirchhoff migration in imaging multiple arrivals and has no steep dip limits of one way wave equation migration however its imaging accuracy depends on the geometry of gaussian beam that is determined by the initial parameter of dynamic ray tracing as a result its applications in exploration areaswith strong variations in topography and near surface velocity are limited combined with the concept of fresnel zone and the theory of wave field approximation in effective vicinity we present a more robust common shot fresnel beam imaging method for complex topographic land areas in this paper compared with the conventional gaussian beam migration for irregular topography our method improves the beam geometry by limiting its effective half width with fresnel zone radius moreover through a quadratic travel time correction and an amplitude correction that is based on the wave field approximation in effective vicinity it gives an accurate method for plane wave decomposition at complex topography which produces good imaging results in both shallow and deep zones trials of two typical models and its application in field data demonstrated the validity and robustness of our method

bay sedimentation as controlled by regional crustal behaviour local tectonics and eustatic sea level changes coquimbo formation miocene pliocene bay of tongoy central chile abstract the north facing bay of tongoy in central chile is flanked by topographic highs in the west and east during the miocene and pliocene the bay extended inland at least 30 km farther south than a present it was filled with muds sands coquinas and gravel during a series of transgressions and regressions related to regional and local tectonic movements combined with global sea level variations 87sr 86sr and microfossil dating indicates transgressions between 11 9 11 2 ma 10 1 9 5 ma 9 0 7 3 ma 6 3 5 3 ma 4 3 2 2 ma and 1 7 1 4 ma the regional tectonic behaviour of the crust shows general uplifting from 10 5 ma to 6 9 ma associated with subduction of the juan fernandez ridge jfr beneath this part of the continent subsidence followed between 6 9 and 2 1 ma in the wake of the southeastward migrating jfr the subsequent subduction of an oceanic plateau similar to the jfr caused rapid uplift that led to the final emergence of the bay above sea level the puerto aldea normal fault along the western limit of the study area was reactivated during the regional uplift and subsidence events with reverse faulting occurring during the latter phase sporadic fault reactivation probably triggered the rapid changes in water depth reflected in the recorded vertical succession of facies

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

delineating complex spatiotemporal distribution of earthquake aftershocks an improved source scanning algorithm summary conventional earthquake location methods depend critically on the correct identification of seismic phases and their arrival times from seismograms accurate phase picking is particularly difficult for aftershocks that occur closely in time and space mostly because of the ambiguity of correlating the same phase at different stations in this study we introduce an improved source scanning algorithm issa for the purpose of delineating the complex distribution of aftershocks without time consuming and labour intensive phase picking procedures the improvements include the application of a ground motion analyser to separate p and s waves the automatic adjustment of time windows for brightness calculation based on the scanning resolution and a modified brightness function to combine constraints from multiple phases synthetic experiments simulating a challenging scenario are conducted to demonstrate the robustness of the issa the method is applied to a field data set selected from the ocean bottom seismograph records of an offshore aftershock sequence southwest of taiwan although visual inspection of the seismograms is ambiguous our issa analysis clearly delineates two events that can best explain the observed waveform pattern

shannon sandstone in hartzog draw heldt draw fields cretaceous wyoming usa reinterpreted as lowstand shoreface deposits abstract the lower campanian shannon sandstone at hartzog draw heldt draw powder river basin wyoming is interpreted in the literature as a shelf ridge complex deposited 160 km from the contemporaneous shoreline this interpretation poses difficult problems and does not discuss the significance of bounding discontinuities preserved in the sandbodies cross sections show that the shannon sandstone is present at several stratigraphic horizons and is bounded by regionally extensive discontinuities the presence of these bounding discontinuities suggests that fluctuations in relative sea level were important in controlling geometry of sandbodies on the shannon shelf in the hartzog draw heldt draw area of wyoming the base of the shannon is a planar surface with respect to a lower bentonite marker and is characterized by the abrupt appearance of bioturbated sandstone on top of black basinal mudstone siderite pebbles 1 cm in diameter are common at this contact in hartzog draw the bioturbated sandstone interval has an average thickness of 10 m and is separated from an overlying unit of cross bedded fine to medium sandstone up to 20 m thick by a ubiquitous 50 cm thick horizon of mud clasts in a cross bedded highly glauconitic medium sandstone these three units are laterally continuous in hartzog draw except where removed by erosion the top of the shannon is marked by an abrupt change from the cross bedded sandstone or bioturbated medium sandstone to laminated black mudstone mud clasts siderite pebbles chert grains and glauconite mark this abrupt contact suggesting erosional transgression this surface can be traced westward where in many areas it is a mud on mud contact marked by a lag of shell debris this transgressive surface which terminates shannon deposition is characterized by at least one regionally extensive onlapping marker in the hartzog draw heldt draw area the presence of depositional discontinuities in the shannon sandstone is suggested by 1 the sharp basal contact 2 the lateral continuity of individual facies 3 the abrupt vertical facies changes and the association of mud clasts siderite pebbles and glauconite accompanying these abrupt juxtapositions of facies 4 the stratigraphic variability and 5 the termination of shannon deposition by erosive transgression marked by onlapping markers these depositional discontinuities are the bounding discontinuities these bounding discontinuities have asymmetrical one sided scour morphologies with steeper landward margins that flatten basinward these geometries and the linearity of the overlying sandstone and the troughs defined by these scoured surfaces suggest incised shoref ce profiles truncated by erosive transgression thus the shannon sandstone in hartzog draw heldt draw is reinterpreted as a lowstand shoreface deposit

geologically constrained seismic imaging workflow we present the geologically constrained workflow for velocity model building as a case study from offshore brazil the workflow involves basin reconstruction gravity modeling and seismic interpretation in addition to standard prestack depth migration psdm model building tools building a salt model based on seismic evidence can be highly nonunique in a geologically constrained seismic processing workflow the main aim is to use geologic understanding with geophysical models and datasets to improve an input velocity realization for the psdm loop thereby improving image quality all of these methods are inherently uncertain and a final model is based on a range of subjective choices thus a final result that agrees with all sciences still can be completely wrong however an understanding of these choices enables a unique way of testing and constraining the number of antimodels velocity models that fit the observations but are different from the final result this can reduce time spent and uncertainty in geologic evaluation

maximum energy traveltimes calculated in the seismic frequency band prestack kirchhoff migration using first arrival traveltimes has been shown to produce poor images in areas of complex structure to avoid this problem i propose a new method for calculating traveltimes that estimates the traveltime of the maximum energy arrival rather than the first arrival this method estimates a traveltime that is valid in the seismic frequency band not the usual high frequency approximation instead of solving the eikonal equation for the traveltime i solve the helmholtz equation to estimate the wavefield for a few frequencies i then perform a parametric fit to the wavefield to estimate a traveltime amplitude and phase the images created by using these parameters in a kirchhoff imaging algorithm are comparable in quality to those created using full wavefield finite difference shot profile migration

paralic parasequences associated with eocene sea level oscillations in an active margin setting trihueco formation of the arauco basin chile abstract the eocene trihueco formation is one of the best exposed successions of the arauco basin in chile it represents a period of marine regression and transgression of second order duration during which barrier island complexes developed on a muddy shelf the strata are arranged in classical shoaling upward parasequences of shoreface and beach facies capped by coal bearing back barrier lagoon deposits these fourth order cycles are superimposed upon third order cycles which caused landward and seaward shifts of the coastal facies belts the final punctuated rise in sea level is represented by shelf mudrocks with transgressive incised shoreface sandstones relative sea level oscillations as revealed in the stratigraphy of the trihueco formation show a reasonable correlation with published eocene eustatic curves

sedimentologic development of a late oligocene miocene forearc embayment valdivia basin complex southern chile abstract deposition of tertiary sediments in the southeastern part of the valdivia basin commenced during the late oligocene with the rapid incision of rivers draining the palaeo coastal range alluvial fans developing along the steep valley flanks joined gravelly braided streams transporting bedload to the coast tectonic and eustatic stability lasting into the early miocene subsequently allowed lateral erosion of the valley flanks and the development of extensive fluvial and estuarine peat swamps this stable period was interrupted briefly by a volcanic eruption at 23 5 ma shortly after 23 ma rapid uplift caused by an acceleration in plate convergence resulted in renewed erosion of the landscape as reflected in the deposition of coarse river gravels basin subsidence and marine transgression proceeding up the river valleys subsequently formed still deep embayments occasionally disturbed by debris flows and turbidity currents originating on the steep wooded valley flanks at the upper limits of the inlets bayhead deltas tidal flats and beaches existed in some areas fan deltas prograded into the embayments as reflected in the interfingering relationship between conglomerates and marine mudrocks in the deeper parts of the embayments the frequency of debris flows and turbidity currents increased markedly during periods of relative sea level lowstand the landscape was eventually inundated by continued subsidence and marine transgression which lasted until a possibly eustatic sea level fall in the early tortonian

ray equation migration of wide angle reflections from southern alaska a kirchhoff style prestack depth migration algorithm is implemented for wide angle reflections in two dimensional variable velocity acoustic media extrapolation of both source and recorder wavefields is performed by ray tracing the imaging condition at each point in the image space is the total source to recorder travel time since the migration algorithm is based on ray tracing migration can be performed for any medium through which rays can be traced there is no inherent limitation on the size of the region to be imaged regions up to 3 6 103 km2 are utilized in this study the algorithm is illustrated by migration of two synthetic and three real data sets from the u s geological survey s trans alaska crustal transect program images are produced of inferred subduction assemblages in southern alaska emphasis in this paper is on obtaining reflector positions and shapes rather than detailed structural interpretations

inversion of forearc basins in south central chile caused by rapid glacial age trench fill this study examines the response of a forearc to the increase in sediment flux to the trench caused by the onset of glacial denudation in the patagonian andes we investigated shelf coastal basins in south central chile which generally comprise eocene early miocene nearshore facies overlain by late miocene early pliocene deep water siltstones and by late pliocene quaternary nearshore deposits seismic profiles and coastal exposures reveal eocene early pliocene extension followed by ongoing late pliocene compression evidenced from growth strata adjacent to seismically active reverse faults the onset of major global cooling ca 6 ma triggered glacial denudation in the uplifted high andes exhumed material transported along the steep and humid andean western slope increased trench sedimentation rates and caused continuous accretion and subduction of terrigenous material we interpret forearc basin inversion as a response to a decrease in slope and basal friction of the wedge caused by frontal accretion and subduction of water rich material respectively in order to reach a critical taper this process lifted the shelf 1 5 km during the middle pliocene the juan fernandez ridge and chile rise confined 2 km trench fill between 45 and 34 s limiting accretion and basin inversion glacial age trench fill and the steady decrease in plate convergence rate shifted this segment of the margin from erosive to accretionary during the pliocene

topography and significance of a basinwide sequence bounding erosion surface in the cretaceous cardium formation alberta canada abstract the cardium formation turonian coniacian upper cretaceous of the western interior seaway alberta contains six marine sandier upward successions bounded by seven basin wide erosion surfaces in the subsurface erosion surface e5 is associated with major oil and gas fields and is penetrated by thousands of wells a topographic map of e5 based on 6126 well logs and over 1100 measured cores shows a series of subparallel elongate highs steps and lows which trend northwest southeast parallel to tectonic trends and to the actively rising cordillera to the southwest the steps are about 10 km apart and the relative relief step to adjacent low is up to 25 m the steps are asymmetrical with a steeper face to the northeast the e5 erosion surface cuts into offshore marine hummock cross stratified sandstones and interbedded mudstones and it records a lowering of relative sea level our interpretation suggests that erosion was in neither a fully subaqueous nor a fully subaerial environment instead it involved relative sea level lowering and subaerial erosion followed by major modification of the landscape during transgressive shoreface erosion at times of relative stillstand during overall transgression gravel was supplied to the shoreface and was reworked a short distance alongshore by waves these conglomerates now form important oil and gas reservoirs the asymmetry of the steps and lows indicates that the originally flat sea floor was tilted northeastward immediately before erosion giving a gradient of about 1 1000 during transgression shorefaces were incised into the tilted surface to form a stepped morphology also during transgression the sea floor was gradually restored to its pre tilting position each of the six marine sandier upward sequences along with the erosion surface at the top occupies an average of 470 000 years and we estimate that transgression took at least 100 000 to 200 000 years

correlation migration using gaussian beams of scattered teleseismic body waves correlation migration for structural imaging using gaussian beams is described for the inversion of passively recorded teleseismic waves gaussian beam migration is based on an overcomplete frame based representation of the seismic wave field and uses localized slant stack windows of the data paraxial gaussian beams are then utilized for the backpropagation of the seismic waves into the me dium the method can provide stable imaging of seismic data in smoothly varying background media where caustics and triplicated arrivals exist we develop gaussian beam migration for structural imaging which allows for incident teleseismic waves from beneath the structure using directly scattered or surface reflected phases the method is applied to synthetic data computed for a collisional zone model and the inversions show that the gaussian beam migration can image structure using passive teleseismic data the method is next applied to synthetic data that have been con volved with an observed pulse from the 1993 cascadia experiment the autocorre lation is computed and the autocorrelated data are migrated by using gausian beams for direct p to s scattered waves and surface reflected waves although the migra tions of the autocorrelation data with the source pulse included have more noise than the migrations of the impulsive source data the subduction zone structure can still be clearly identified in the migrated results without removal of the source pulses

segmentation of megathrust rupture zones from fore arc deformation patterns over hundreds to millions of years arauco peninsula chile 1 this work explores the control of fore arc structure on segmentation of megathrust earthquake ruptures using coastal geomorphic markers the arauco nahuelbuta region at the south central chile margin constitutes an anomalous fore arc sector in terms of topography geology and exhumation located within the overlap between the concepcion and valdivia megathrust segments this boundary however is only based on 500 years of historical records we integrate deformed marine terraces dated by cosmogenic nuclides syntectonic sediments published fission track data seismic reflection profiles and microseismicity to analyze this earthquake boundary over 10 2 10 6 years rapid exhumation of nahuelbuta s dome like core started at 4 1 2 ma coeval with inversion of the adjacent arauco basin resulting in emergence of the arauco peninsula here similarities between topography spatiotemporal trends in fission track ages pliocene pleistocene growth strata and folded marine terraces suggest that margin parallel shortening has dominated since pliocene time this shortening likely results from translation of a fore arc sliver or microplate decoupled from south america by an intra arc strike slip fault microplate collision against a buttress leads to localized uplift at arauco accrued by deep seated reverse faults as well as incipient oroclinal bending the extent of the valdivia segment which ruptured last in 1960 with an mw 9 5 event equals the inferred microplate we propose that mechanical homogeneity of the fore arc microplate delimits the valdivia segment and that a marked discontinuity in the continental basement at arauco acts as an inhomogeneous barrier controlling nucleation and propagation of 1960 type ruptures as microplate related deformation occurs since the pliocene we propose that this earthquake boundary and the extent of the valdivia segment are spatially stable seismotectonic features at million year scale

three dimensional topographic and bathymetric effects on magnetotelluric responses in jeju island korea summary magnetotelluric mt surveys have been conducted to investigate the volcanic structure in jeju island in the south sea of korea the island is oval in shape 31 km wide and 73 km long with a peak elevation of 1950 m at mt halla located in the centre of the island mt data were obtained in a wide range of frequencies in a mid mountain region and may be affected by distortion of electric fields due to topographic and bathymetric effects the purpose of this paper is to investigate the topographic and bathymetric effects on mt data for a four layered model having the topographic and bathymetric undulation of jeju island using an edge finite element method the model effectively reproduces a general feature of mt sounding curves observed on the island the topographic effect appears only at high frequencies whereas the sea effect causes a split of apparent resistivity curves in different polarizations at low frequencies since the island has a gentle slope of less than 10 except in the close vicinity of mt halla it has a similar effect of a change in thickness in the top layer in contrast the sea effect can be observed as quasi 2 d rather than 3 d along the major axis of the island and generates a parallel shift of the log apparent resistivity curve at low frequencies

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

theory of induction sonde in dipping beds the analysis necessary for the computation of the response of an induction sonde in dipping beds is developed the boundary value problem solved is for a bed of uniform thickness surrounded by two infinitely thick shoulder beds the source is a magnetic dipole oriented in an arbitrary direction and located in any of the three media the resulting electromagnetic fields in all three media are obtained from these fields induction logs are calculated for a focused induction sonde 6ff40 passing through a dipping bed under the assumptions of negligible borehole effect and no invasion the analysis is also used to obtain correction charts for dipping beds

offset domain pseudoscreen prestack depth migration the double square root equation for laterally varying media in midpoint offset coordinates provides a convenient framework for developing efficient 3 d prestack wave equation depth migrations with screen propagators offset domain pseudoscreen prestack depth migration downward continues the source and receiver wavefields simultaneously in midpoint offset coordinates wavefield extrapolation is performed with a wavenumber domain phase shift in a constant background medium followed by a phase correction in the space domain that accommodates smooth lateral velocity variations an extra wide angle compensation term is also applied to enhance steep dips in the presence of strong velocity contrasts the algorithm is implemented using fast fourier transforms and tri diagonal matrix solvers resulting in a computationally efficient implementation combined with the common azimuth approximation 3 d pseudoscreen migration provides a fast wavefield extrapolation for 3 d marine streamer data migration of the 2 d marmousi model shows that offset domain pseudoscreen migration provides a significant improvement over first arrival kirchhoff migration for steeply dipping events in strong contrast heterogeneous media for the 3 d seg eage c3 narrow angle synthetic dataset image quality from offset domain pseudoscreen migration is comparable to shot record finite difference migration results but with computation times more than 100 times faster for full aperture imaging of the same data volume

topographic corrections to slowness measurements in parsimonious migration of land data parsimonious migration requires that incident and emergent angles be measured via apparent slownesses from the seismic data being migrated when slownesses are measured from land data it is the apparent slowness along the topography not the horizontal slowness that is being measured thus errors are introduced into the incident and emergent angle estimates which are defined via horizontal slownesses these errors can be corrected using the local topographic dips a 2d field data example shows that after correction a migrated image has significantly improved coherency

3 d finite element analysis of induction logging in a dipping formation mark electromagnetic induction emi by a magnetic dipole located above a dipping interface is of relevance to the petroleum well logging industry the problem is fully three dimensional 3 d when formulated as above but reduces to an analytically tractable one dimensional 1 d problem when cast as a small tilted coil above a horizontal interface the two problems are related lay a simple coordinate rotation an examination of the induced eddy currents and the electric charge accumulation at the interface help to explain the inductive and polarization effects commonly observed in induction logs from dipping geological formations the equivalence between the 1 d and 3 d formulations of the problem enables the validation of a previously published finite element solver for 3 d controlled source emi

redatuming through a salt canopy and target oriented salt flank imaging we describe a new shortcut strategy for imaging the sediments and salt edge around a salt flank through an overburden salt canopy we tested its performance and capabilities on 2d synthetic acoustic seismic data from a gulf of mexico style model we first redatumed surface shots using seismic interferometry from a walkaway vertical seismic profile survey as if the source and receiver pairs had been located in the borehole at the positions of the receivers this process creates effective downhole shot gathers by completely moving surface shots through the salt canopy without any knowledge of overburden velocity structure after redatuming we can apply multiple passes of prestack migration from the reference datum of the bore hole in our example first pass migration using only a simple vertical velocity gradient model reveals the outline of the salt edge a second pass of reverse time prestack depth migration using full two way wave equation was performed with an updated velocity model that consisted of the velocity gradient and salt dome the second pass migration brings out dipping sediments abutting the salt flank because these reflectors were illuminated by energy that bounced off the salt flank forming prismatic reflections in this target oriented strategy the computationally fast redatuming process eliminates the need for the traditional complex process of velocity estimation model building and iterative depth migration to remove effects of the salt canopy and surrounding overburden this might allow this strategy to be used in the field in near real time