aspects of hydrocarbon charge of the petroleum system of the yamal peninsula west siberia basin abstract the yamal peninsula is located in the northern portion of the west siberia basin adjacent to the kara sea the sedimentary succession is composed of between 2 5 and 9 km of jurassic through paleogene sediment deposited within lacustrine through deep marine settings although the region is remote and hydrocarbon exploration has been limited a number of discoveries have been made components of the region s petroleum system are however poorly known this study examines aspects of hydrocarbon charge through the integration of new data obtained on core samples from the malygin field and from 10 oils from five fields bovanenkov malygin kharasavey pyasedaysk and new port with published data source rock screening data from the malygin field reveal the presence of organic rich intervals tocs approaching 6 within the cretaceous and jurassic sequences these data also indicate that at the sampling location these intervals are gas prone hydrogen index values less than 300 mg hc g toc within the cretaceous interval the gas prone character appears to be largely depositional in contrast in the jurassic portion of the sequence the hydrogen indices appear to have been reduced as a result of an advanced level of thermal maturity elsewhere on the peninsula where the thermal maturity of the jurassic is less advanced data suggest that the jurassic sequence includes oil prone intervals preliminary review of the oil data suggests that the examined oils belong to a single family i e were derived from a common source and that much of the variation is a result of different alteration and migration histories although the data from this study were incapable of establishing a definitive correlation published information and the lack of an effective cretaceous liquid hydrocarbon source suggests that these oils were derived from the jurassic sequence

thrusting of the central california margin over the edge of the pacific plate during the transform regime wide angle seismic reflection and refraction data mapped geologic structures and marine magnetic anomalies provide evidence that oceanic crust underlies the coastal margin of california from morro bay northward beyond san francisco we interpret the presence of this oceanic slab to be at least partly the result of thrusting of north america over the pacific plate during pliocene to quaternary time well after the principal subduction regime ended likely causes of this thrusting are the change in plate motions at about 3 5 ma and extension in the basin and range province

prestack depth migration of dense wide angle seismic data abstract prestack depth migration of wide angle seismic data represents an extension of traditional imaging with near vertical incidence data because it includes a larger component of the recorded wave field to date however studies that have employed wide angle migration have suffered because only widely spaced data were available and because only very simple synthetic tests were performed although wide angle migration has the potential to increase our ability to image deep crustal structures particularly when closely spaced data are collected a thorough study of this technique has been lacking to address this we present a case study of prestack depth migration of relatively dense synthetic wide angle marine data the objectives are to identify potential benefits and limitations of this approach and answer such fundamental questions as how close the receiver spacing must be for a typical survey to image effectively with wide angle data this will facilitate the design of better seismic experiments our study employs kirchhoff prestack depth migration of variably spaced full wave field synthetic wide angle ocean bottom hydrophone obh data generated using a realistic velocity model based on the passive eastern margin of the united states we show how an increase in obh density improves the migration by increasing the lateral resolution and signal to noise ratio we also investigate the contribution of various offset ranges to the migrated image and show how the wider angle components contribute primarily to the deepest parts of the image with relatively low spatial frequency compared to the near vertical incidence components to investigate how errors in the velocity model affect imaging as a function of offset range we migrate the data using a velocity model derived from refraction and reflection traveltime inversion this example demonstrates the need to obtain an increasingly accurate model as increasingly wider angle data are migrated to effectively image structures in our 200 40 km synthetic velocity model an obh spacing of approximately 2 km is required

reverse time migration from irregular surface by flattening surface topography abstract prestack reverse time migration rtm is a powerful tool to provide high resolution structure images of crust and upper mantle in wide angle seismic experiments as a standard migration algorithm assumes that data are recorded on a flat surface rtm meets severe challenge on the issue of complicated topography where many seismic surveys are carried out up till now there are two popular techniques for handling elevation changes i e elevation static correction or time shift and wave equation datuming numerous studies demonstrate that the elevation static correction is inaccurate for non vertically traveling energy and subsequently may migrate reflectors to incorrect positions wave equation datuming is more accurate but more sophisticated and too computationally demanding for routine use here we present an alternative scheme to deal with topography in prestack rtm that can migrate seismic data acquired in areas with irregular topography to the reflector positions directly without datuming or elevation static corrections in this scheme surface topography is flattened by a transformation of coordinates from cartesian to curvilinear such a transformation is equivalent to map a rectangular grid onto a curved one and is implemented by the introduction of boundary conforming grid then we use an explicit finite difference scheme with second order accuracy to discretize the elastic wave equations and perform the rtm in the curvilinear coordinate system moreover a flux corrected transport fct algorithm adapted from hydrodynamics is incorporated in the rtm implementation which can not only overcome problems of numerical dispersion that arise in finite difference algorithms but also offer the opportunity to use a coarse grid to obtain the same accuracy as conventional finite difference methods on a dense grid the fctrtm method presented here has no limitations on surface irregularity complexity of the subsurface structure or velocity function extensive numerical examples demonstrate the feasibility and effectiveness of our method

wave field processing of data from a large aperture seismic experiment in southwestern oklahoma data from a wide aperture survey performed in january 1985 in southwestern oklahoma have been processed using prestack imaging and the images are interpreted to reveal a new picture of the crustal structure in the wichita uplift and the anadarko basin as the data were in recording apertures of approximately 90 km it was necessary to develop new processing concepts and software a complete new imaging system based on common source gathers was conceived implemented tested and applied to these data the heart of the system consists of velocity estimation by slant stacking and acoustic prestack reverse time finite difference migration the migration produces reflectivity images in regions of 50 km 100 km has no dip restrictions and operates in an arbitrarily complicated two dimensional velocity distribution for comparison the data were also processed using standard common midpoint processing but this is a less valid approach for wide aperture data the main ambiguities and limitations in processing and interpretation are a consequence of the small number of shots processing and interpretation of the data provide a new picture of the subsurface structure of southwestern oklahoma the environment is clearly compressive with thrust faulting as the main accommodating mechanism in the upper and central crust beneath a possible detachment zone at 30 km depth the lower crust has responded to the compression by thickening by 50 the moho is interpreted to be 45 km in depth beneath the thrust zone shallowing to 40 km beneath the wichita uplift and the anadarko basin

depth image focusing in traveltime map based wide angle migration wide aperture prestack depth migration requires application of challenging and time consuming velocity analysis and depth focusing collectively referred to here as depth focusing we present an approach to depth focusing using 1 a detailed starting velocity model obtained by a 1 d transformation of the first arrival times followed iteratively by 2 interactive analysis of the common image gathers 3 computation of coherency attributes of the wavefield in the depth domain and 4 2 d traveltime tomography to update the background velocity model we employ two interactive method of migration velocities refinement in the first method similar to the common midpoint velocity spectrum approach residual velocity updates are picked directly from the common image gathers in another method analogous to the common velocity stacks we pick the velocity updates from the areas of maximum coherency in depth sections that are migrated using rescaled traveltime maps both types of migration velocity picks

seismic tomography in boreholes summary synthetic seismic tomographic data sets for cross hole and hole to surface arrays are computed and inverted inversion imaging is performed through an iterative projection solution to a matrix equation the orientation as well as the position of an anomaly relative to the source and recorder arrays affects both the rate of convergence of the solution and the resolution obtained data density and distribution determine the physical validity of the image obtained at convergence borehole tomography is apparently a viable method for imaging inter hole structure

tracing the subducted oceanic crust beneath the central california continental margin results from ocean bottom seismometers deployed during the 1986 pacific gas and electric edge experiment large aperture seismic data were collected on several ocean bottom seismometers obs deployed along a deep crustal seismic profile that was shot across the central california continental margin the line of shots extends from the oceanic crust seaward of the santa lucia escarpment to the california coast near morro bay and crosses the santa lucia basin santa lucia bank santa maria basin and hosgri fault zone the obs data permit us to trace the subducted oceanic crust from seaward of the santa lucia escarpment to beneath the central part of the santa maria basin a complementary study using onshore recordings of the same shots extends the crustal model onshore just seaward of the santa lucia escarpment the oceanic crust is subhorizontal and covered by a thin layer of low velocity sediment the velocity 4 5 km s and gradient 1 20 1 25 km s km of the upper oceanic crust in this region are well determined and agree with earlier determinations of the crustal structure of the eastern pacific beneath the santa lucia escarpment and santa lucia basin the oceanic crust dips approximately 16 to the east it is overlain by material with a velocity that increases from 4 8 to 6 4 km s at a depth of 1 7 5 5 km below the seafloor beneath the santa lucia basin a low velocity zone may be sandwiched between the subducted crust and this shallow high velocity material which we interpret to represent obducted oceanic crustal material beneath the eastern edge of santa lucia basin the dip of the subducted oceanic crust decreases to less than 2 the configuration of the subducted crust in this region is consistent with imbrication of the subducted crust as suggested by others based on common midpoint reflection data beneath the central santa maria basin the top of the subducted oceanic crust is at a depth of about 14 16 km and the moho is at 19 21 km

lithospheric evolution in the wake of the mendocino triple junction structure of the san andreas fault system at 2 ma as the mendocino triple junction mtj moves northwards up the north american margin the tectonic regime changes from subduction to strike slip for the first few million years following triple junction migration the san andreas fault system consists of several strike slip faults distributing deformation over a region 150 km wide this same region is expected to be affected by a slab gap beneath north america created by the northward removal of the subducting gorda plate and into which asthenospheric mantle is thought to rise to crustal depths the onshore and offshore mendocino triple junction seismic experiment mtjse provides a continuous seismic velocity reflectivity cross section across the deforming zone from the pacific ocean basin to the eastern edge of the california coast ranges the accretionary complex rocks that make up most of the crustal thickness are underlain by a 5 10 km thick high velocity 6 4 7 2 km s 1 layer at the base of the crust that extends from the pacific to at least 50 km and probably 90 km east of the san andreas fault the top of the lower crustal layer deepens from 7 km beneath the pacific ocean basin at the west end of the profile to 23 km at the east end by a gentle 5 10 eastward dip punctuated by abrupt offsets at the san andreas and maacama fault zones at each fault the top of the lower crust is offset by up to 4 km down to the east the moho is similarly deformed beneath the faults although by only 2 km such localized deformation of the moho implies that these two strike slip faults penetrate through the entire crust to the upper mantle good agreement between seismic velocity and seismic reflectivity in the vicinity of the faults gives confidence in these results although details of the offset beneath the san andreas fault are better resolved than those under the maacama fault seismic velocities in the upper mantle show only a small change along the profile from 8 1 km s 1 beneath the pacific to about 7 9 km s 1 beneath the coast ranges we infer that upwelling of asthenosphere into the slab gap is limited laterally or a lithospheric lid is present in the slab gap by 2 ma gravity data and crustal density structure show that most of the margin width is in local airy isostasy with the changes in crustal thickness near the strike slip faults corresponding closely to changes in surface topography the crustal blocks defined by the strike slip faults appear to be independently in isostatic equilibrium provided that the mantle beneath the coast ranges has a somewhat lower density than that beneath the pacific plate the densities in the coast range upper mantle are consistent with limited temperature elevation suggesting that the asthenospheric mantle is present beneath the depth of seismic energy penetration from our survey

marine deep seismic reflection profiles off central california a strong reflection horizon at two way travel time of approximately 6 s is observed in a deep seismic profile across the outer continental shelf of central california it is interpreted as the seismic image of subducted oceanic crust emplaced prior to the change from principally convergent to principally transcurrent motion between the pacific and north american plates during the late paleogene the reflector dips landward at a very shallow angle and is at a depth of 14 15 km under santa lucia bank the reflection is not observed or at best is very discontinuous under the inner shelf santa maria basin this suggests that tectonic or other processes have produced significantly different structural styles or compositions on the two sides of the santa lucia bank fault under the outer shelf a prominent apparently deeper later arrival time horizon dips more steeply and diverges from the 6 s reflector the deep horizon is at least partially composed of diffracted energy but is nearly linear after migration possible interpretations are that the horizon indicates crustal imbrication or out of plane diffractions alternatively it is a relict feature imparted to the crust at the now inactive pacific farallon spreading ridge reflective zones at intermediate depths are observed in apparently accreted sediments in parts of the santa lucia and santa maria basins these features could represent technically induced fabrics within the accretionary complex or they could be coherent depositional sequences

velocity analysis by iterative profile migration in conventional seismic processing velocity analysis is performed by using the normal moveout nmo equation which is based on the assumption of flat horizontal reflectors imaging by migration either before or after stack is done normally in a subsequent step using these velocities in this paper velocity analysis and imaging are combined in one step and migration itself is used as a velocity indicator because unlike nmo migration can be formulated for any velocity function migration based velocity analysis methods are capable of handling arbitrary structures i e those with lateral velocity variations in the proposed scheme each shot gather profile is migrated with an initial depth velocity model profile migration is implemented in the x domain but the actual implementation of profile migration is not critical as long as it is not done in a spatial wavenumber domain which would preclude handling of lateral velocity variations after migration with an initial velocity model the

interpretation of si concentrations in lake sediments three case studies the biogenic si concentration in a sediment can be determined as the non detrital si concentration by normalization of the total si concentration with al this procedure is based upon the assumptions that 1 that al exists predominantly in detrital i e minerogenic particles and 2 that biogenic si mainly diatom frustules is the dominating non detrital si phase this paper focuses on the reasons for the variations of the non detrital si concentration in these lake sediments data from three lakes are presented representing three principally different cases regarding the si concentration the processes controlling the concentrations of detrital and non detrital si are discussed

recording marine airgun shots at offsets between 300 and 700 km this paper demonstrates that under favorable conditions by using multichannel recording and subsequent stacking of adjacent records marine airgun shots have been detected at offset distances up

shortening within underplated oceanic crust beneath the central california margin structures within a tectonically underplated layer of early miocene age oceanic crust indicate that the lower crust of offshore central california has been deformed under compression prior to the inception of the san andreas transform system along the continental margin shortening in the lower crust is inferred from the geometry of two structures the santa lucia monocline and the santa maria antiform delineated by an integrated interpretation of multichannel and wide angle seismic reflection data in conjunction with regional gravity data and teleseismic receiver functions the 700 km of multichannel seismic data and 300 km of wide angle seismic data from the 1986 pacific gas and electric edge seismic experiment form the primary data base used to establish the northwesterly trend and extent of the monocline and the wnw trend of the antiform in cross sectional view the santa lucia monocline is a 30 kink in the oceanic crust while the santa maria antiform is characterized by local thickening of the oceanic crust these structures are probably the result of convergence between the monterey and arguello microplates that took place as subduction along the margin waned the multichannel seismic reflection data image two miocene age shelf basins the santa lucia and offshore santa maria basins that formed within a transtensional stress regime the offshore santa maria basin directly overlies the santa maria antiform suggesting that either the upper and lower crustal stress regimes were completely decoupled in miocene time or that the two features formed in different places differential slip between the upper crust and lower crust may have been accommodated along a detachment zone located above oceanic crust convergence within the oceanic crust and apparent differential motion between upper and lower crust are indicative of the processes that can affect the transition from subduction to an oceanic continental transform zone

three dimensional velocity structure of the northern hikurangi margin raukumara new zealand implications for the growth of continental crust by subduction erosion and tectonic underplating traveltimes between shots from nine marine seismic reflection lines and nine onshore recorders were used to construct a 3 d p wave velocity model of the northern hikurangi subduction margin new zealand from north to south between raukumara basin and raukumara peninsula the moho of the overriding plate increases in depth from 17 to similar to 35 km low seismic p wave velocities of 3 5 5 0 km s are localized within a similar to 10 km thick prism in the lower crust of the overriding plate immediately updip of the intersection between the subduction thrust and moho and beneath the topographic crest of east cape ridge and the raukumara range southward this region of low seismic velocities and surface uplift increases in distance from the trench as the thickness of the crust in the overriding plate increases we interpret this low velocity volume to be underplated sedimentary rocks and crustal materials that were tectonically eroded by subduction beneath the trench slope the buoyancy and low strength of these subducted materials are proposed to assist the escape from a subduction channel near the base of the crust and drive local rock uplift in the middle crust our observations of very low velocity suggest high fluid filled porosities of 12 18 and the implied buoyancy anomaly may enhance underplating at greater depths the process is driven by the contrast between upper crustal quartz feldspar mineralogy and the denser diabase or olivine rich lithologies of the lower crust and mantle we estimate a rate of lower crustal underplating at the northern hikurangi margin of 20 7 km 3 ma 1 km 1 since 22 ma we suggest that underplating provides an efficient means of accreting subducted sediment and tectonically eroded material to the lower crust and that the flux of forearc crustal rocks into the mantle at subduction zones may be systematically overestimated

wide angle reflectivity across the torngat orogen ne canada data from a lithoprobe refraction wide angle reflection seismic transect across the torngat orogen in ne canada were depth migrated to produce a record section similar to near vertical incidence seismic data densely spaced airgun shots and a station spacing between 9 and 25 km were able to produce a low frequency image of the lower crustal reflectivity for most of the transect over the western end of the profile a series of east dipping lower crustal reflectors continue down to the moho which is clearly defined at a depth of 36 to 38 km the lower crustal reflectors can be correlated laterally for up to 80 km this reflection fabric is interpreted as evidence for thick skinned deformation caused by convergence of the nain and superior provinces over the eastern end of the line reflectors in the nain province are less continuous and may indicate steeply dipping faults which have not been modified during formation of the orogen

migration of wide aperture onshore offshore seismic data central california seismic images of late stage subduction we conducted a high shot and receiver density onshore offshore seismic experiment in 1986 across the central california transform margin as part of the pacific gas and electric edge deep crustal geophysical survey the data set consists of land recordings of 406 marine air gun shots fired along a 70 km track across the santa maria basin the seismic vessel s track was collinear with 154 land seismic recorders deployed over 50 km across the sur obispo terrane and part of the salinian block the source receiver offsets range from 2 5 km to 120 km the unique recording geometry and high shot and receiver density permit imaging the deeper crustal structure of the california transform margin but pose serious challenges for conventional seismic imaging methods consequently we developed a unified approach to wide angle imaging which incorporates a tomography technique for obtaining initial migration velocity models and a prestack kirchhoff depth migration with depth focusing of the main reflectors for velocity model refinement and imaging this approach retains the velocity and dip information in the data and allows user control over the imaging process we applied this processing to subsets of the onshore offshore data to obtain an upper crustal velocity model and a 32 km long wide angle depth migrated image of the lower crust in the coastal region under the santa maria basin and the sur obispo terrane the migration image shows a part of an underplated oceanic crustal layer interpreted as the monterey plate meltzer and levander 1991 miller et al 1992 which dips coastward at depths of 18 to 28 km the plate appears to be kinked or imbricated beneath the coastline the image correlates well with existing wide angle seismic interpretations based on ray tracing walter and sharpless 1987 putzig 1988 howie et al 1993 the migrated lower crustal image has more structural information than is apparent in conventional ray based interpretations along the same crossing and adjacent profiles the crustal image demonstrates that automated imaging techniques applied to wide angle data can provide images of crustal structure comparable in fidelity to conventionally processed and migrated near vertical incidence data

quantitative imaging of complex structures from dense wide aperture seismic data by multiscale traveltime and waveform inversions a case study an integrated multiscale seismic imaging flow is applied to dense onshore wideaperture seismic data recorded in a complex geological setting thrust belt an initial p wave velocity macromodel is first developed by first arrival traveltime tomography this model is used as an initial guess for subsequent full waveform tomography which leads to greatly improved spatial resolution of the p wave velocity model however the application of full waveform tomography to the high frequency part of the source bandwidth is difficult due to the non linearity of this kind of method moreover it is computationally expensive at high frequencies since a finitedifference method is used to model the wave propagation hence full waveform tomography was complemented by asymptotic prestack depth migration to process the full source bandwidth and develop a sharp image of the short wavelengths the final traveltime tomography model and two smoothed versions of the final full waveform tomography model were used as a macromodel for the prestack depth migration in this study wide aperture multifold seismic data are used after specific preprocessing of the data 16 frequency components ranging from 5 4 hz to 20 hz were inverted in cascade by the full waveform tomography algorithm the full waveform tomography successfully imaged sw dipping structures previously identified as highresistivity bodies the relevance of the full waveform tomography models is demonstrated locally by comparison with a coincident vertical seismic profiling vsp log available on the profile the prestack depth migrated images inferred from the traveltime and the smoothed full waveform tomography macromodels are shown to be on the whole consistent with the final full waveform tomography model a more detailed analysis based on common image gather computations and local comparison with the vsp log revealed that the most accurate migrated sections are those obtained from the full waveform tomography macromodels a resolution analysis suggests that the asymptotic prestack depth migration successfully migrated the wideaperture components of the data allowing medium wavelengths in addition to the short wavelengths of the structure to be imaged

fluids in the lower crust following mendocino triple junction migration active basaltic intrusion geodynamic and plate tectonic models for the mendocino triple junction a fault fault trench triple junction in northwestern california predict a slab free zone south of the triple junction in which asthenospheric mantle upwells to the base of the crust a variety of geological and geophysical data support this model although fine scale 20 km details of the lithospheric structure have been unknown previously seismic investigations in the onshore transform regime south of the mendocino triple junction region reveal very strong short offset reflections from the lower crust and at the crust mantle boundary beneath the entire width of the coast range particularly near lake pillsbury california seismic analysis suggests that these reflections are from discrete zones of fluid the reflector geometry implies that the source of the fluid is within the upper mantle in this tectonic context it is likely that the fluids are largely partial melt segregated from asthenospheric mantle upwelling into the slab free zone the tectonic setting and the location of lake pillsbury relative to the estimated position of the southern edge of the gorda slab and the clear lake volcanic field suggest that volcanism may initiate in this region within the next 400 k y

slowness weighted diffraction stack for migrating wide angle seismic data in laterally varying media wide angle prestack depth migration is an important tool for studying the nature of reflecting boundaries in the earth s crust the slowness weighted diffraction stack swds method has been used to incorporate both two way traveltime constraints and slowness information in the migration for this purpose traveltimes and apparent slownesses of reflected arrivals must be calculated in the image space earlier applications of swds required a 1d or gently varying seismic velocity structure to obtain these quantities by ray tracing in the image space i show that the apparent slownesses can also be derived directly from one way traveltime maps using fermat s principle the sdws is applied to an existing onshore offshore wide angle data set and the example shows that the method can be used to image detailed reflectivity structure at great depths

wavelet correlation filter for wide angle seismic data a new filtering technique for single fold wide angle reflection refraction seismic data is presented the technique is based on the wavelet decomposition of a set of adjacent traces followed by coherence analysis the filtering procedure consists of three steps in the first a wavelet decomposition of traces into different detail levels is performed in the second the coherence attributes for each level are evaluated by calculating cross correlation functions of detail portions contained in a space time moving window finally the filtered traces are obtained as a weighted reconstruction of the trace details each weight is obtained from the coherence attributes distribution estimated in a proper interval a sequence of tests is then conducted in order to select possible optimum or unsuitable wavelet bases the efficiency of the filter proposed was assessed by calculating some properly designed parameters in order to compare it with other standard de noising techniques the proposed method produced a clear signal enhancement in high density wide angle seismic data thus proving that it is a useful processing tool for a reliable correlation of seismic phases

joint inversion of multichannel seismic reflection and wide angle seismic data improved imaging and refined velocity model of the crustal structure of the north ecuador south colombia convergent margin 1 improving seismic imaging of the crust is essential for understanding the structural factors controlling subduction zones processes we developed a processing work flow based on the combined analysis of multichannel seismic reflection mcs and wide angle wa reflection refraction data to derive both shallow and deep velocities suitable for prestack depth migration and to construct a blocky velocity model integrating all identifiable seismic phases contained in mcs and wa data we apply this strategy to the study of the north ecuador sw colombia subduction margin to improve the imaging and geostructural interpretation of a splay fault and surrounding outer and inner margin wedges results show improvements over tomographic inversion of wa data only such as 1 sediment velocity variation across the trench and margin slope that correlates with lateral lithologic changes tectonic compaction and effect of mass wasting processes 2 a two layer velocity structure of the inner wedge basement that is consistent with the crust of an oceanic plateau 3 a complex velocity structure of the outer wedge basement that consists of a deep high velocity 5 0 5 5 km s 1 core and a low velocity zone 3 8 5 0 km s 1 associated with the major splay fault 4 a 1 3 km thick low velocity 3 5 4 0 km s 1 subduction channel that extends beneath the margin outer wedge both the splay fault and subduction channel are expected to direct fluid flows and 5 downdip velocity increase 5 6 km s 1 in the subducting oceanic crust associated with a low 7 8 km s 1 upper mantle velocity possibly reflecting changes in rock nature or properties

deep crustal reflection profiling offshore southern central california deep crustal reflection profiling offshore southern central california shows that the offshore santa maria basin and adjacent shelf and slope are underlain by oceanic crust which underthrusts the margin on common midpoint cmp sections the top of the oceanic crust appears as a strong coherent reflection occurring at 6 s two way time twt the reflection can be traced continuously more than 50 km from the deep ocean basin eastward beneath the continental shelf depth conversion places the top of oceanic crust at 6 16 km depth with a dip of 8 toward the coast beneath the santa lucia high the crustal reflection breaks up into several east dipping segments above the oceanic crust an accretionary wedge increases in thickness from 3 km at the base of the slope to over 12 km beneath the santa lucia high the neogene sedimentary section above the accretionary prism exhibits relatively little deformation implying that most of the accretion occurred during pre neogene convergence between the north american plate and oceanic plates located to the west the offshore santa maria basin exhibits at least two periods of neogene deformation an early extensional phase accompanied by strike slip faulting occurred in the lower to middle miocene followed by compression and basin inversion in the upper miocene lower pliocene the development of structures in the basin began in the northwest and is progressively younger to the southeast faults identified in the offshore sedimentary column offset acoustic basement but are not imaged in the franciscan basement material below 2 3 s twt and do not extend to or offset the deep reflections we suggest that the oceanic crust served as a detachment surface above which the shallow deformation occurred neogene sediments in the offshore basins show only 1 3 km of shortening this implies that shortening due to neogene oblique plate motion between the pacific and north american plates extends east of the offshore santa maria basin and is distributed across the entire transform margin

mapping the megathrust beneath the northern gulf of alaska using wide angle seismic data in the northern gulf of alaska and prince william sound we have used wide angle seismic reflection refraction profiles earthquake studies and laboratory measurements of physical properties to determine the geometry of the prince william and yakutat terranes the aleutian megathrust and the subducting pacific plate in this complex region the yakutat terrane is underthrust beneath the prince william terrane and both terranes are interpreted to be underlain by the pacific plate wide angle seismic reflection refraction profiles recorded along five seismic lines are used to unravel this terrane geometry modeled velocities in the upper crust of the prince william terrane to 18 km depth agree closely with laboratory velocity measurements of orca group phyllites and quartzofeldspathic graywackes the chief components of the prince william terrane to hydrostatic pressures as high as 600 mpa 6 kbar a landward dipping reflector at depths of 16 24 km is interpreted as the base of the prince william terrane this reflector corresponds to the top of the wadati benioff zone seismicity and is interpreted as the megathrust immediately beneath the megathrust is a 4 km thick 6 9 km s refractor which we infer to be the source of a prominent magnetic anomaly and which is interpreted by us and previous workers to be gabbro in eocene age oceanic crust of the underthrust yakutat terrane wide angle seismic data magnetic anomaly data and tectonic reconstructions indicate that the yakutat terrane has been underthrust beneath the prince william terrane for at least a few hundred kilometers wide angle seismic data are consistent with a 9 to 10 landward dip of the subducting pacific plate beneath the outer shelf and slope distinctly different from the inferred average 3 to 4 dip of the overlying 6 9 km s refractor and the wadati benioff seismic zone beneath the inner shelf our preferred interpretation of the geophysical data is that one composite plate composed of the pacific plate of a fairly uniform thickness and the yakutat plate of varying thickness is subducting beneath southern alaska

migration of noisy crustal seismic data a slowness weighted diffraction stack algorithm is used to migrate crustal seismic reflection data slowness information is extracted from the data using a slant stack beam forming process the main purpose of beam forming is to improve the signal to noise ratio over a given range of slowness values localized slant stacks are used to design a coherency weighted slowness filter in the time distance domain through migration the subsurface structure and the observed seismic data are tied via slownessselective diffraction curves a combination of nonlinear slowness filtering with migration gives excellent noise rejection and leads to an improved image in the depth domain the method which is particularly effective for migrating moderate to steep dipping events with low signal to noise ratio is applied to various poststack and prestack synthetic data examples and to a deep crustal data set collected on vancouver island as part of the canadian lithoprobe project

cdp mapping to obtain the fine structure of the crust and upper mantle from seismic sounding data an example for the southeastern china abstract we present an efficient and general approach of common depth point cdp mapping to image the fine structure of the crust and upper mantle we use velocity model and reflector geometries which are the results of conventional interpretation as a reference model to construct our cdp mapping of crust and upper mantle after three steps the calculation of cdp points and reflecting traveltimes the determination of cdp mapping trajectories and the implementation of cdp imaging reflection images were created by cdp mapping technique in which each time element of a seismic trace beginning with the direct arrival is mapped to reflection point our test of the method presented in this paper shows that the algorithm can quickly accurately image the subsurfaces of the given model the result of cdp mapping in the southeastern china shows that the depth of moho is about 34 km there are numerous reflection interfaces within the upper crust and the moho is a transition zone with a thickness of 2 3 km the cdp mapping approach presented here can easily be extended to three dimensions and complex anisotropic media the method can also be used to develop seismic attribute images of the crust and upper mantle

wave field processing of data from the 1986 program for array seismic studies of the continental lithosphere ouachita experiment in may 1986 a wide aperture seismic field experiment was conducted in southwestern arkansas and northwestern louisiana this experiment was sponsored by the incorporated research institutions for seismology as part of the program for array seismic studies of the continental lithosphere the survey line runs from the south edge of the ouachita mountains out into the gulf coastal plain the experiment involved 21 shot points and two major deployments each of 400 recorders with an effective aperture of 200 km the goal of the experiment was to attempt to provide constraints on the diverse development sequences that have been proposed to explain the structure of the area the data were processed using a previously developed prestack common source acoustic finite difference migration to produce images of reflectivity in a slice 200 km long and 40 km deep this migration algorithm has no dip restrictions and uses a two dimensional variable velocity distribution for wave extrapolation interpretation of the results of the prestack processing together with the results of complementary two dimensional ray trace modeling not shown here of the data provides an enhanced picture of the structure of the paleozoic continental margin that is consistent with the observations in most previous geophysical and geological studies the main physical and temporal features of the model are crustal thinning during precambrian rifting a continent ocean boundary sedimentary wedge and carbonate bank thrusting of deepwater sediments during the ouachita orogeny and triassic rifting

deep crustal structure and tectonics in the offshore southern santa maria basin california reprocessed deep penetration seismic reflection profiles across the offshore southern santa maria basin suggest a complex history of tectonic deformation the top of subducted oceanic crust imaged at the north end of edge line ru 10 is disrupted and eventually disappears to the south farther south a strong lower crustal reflection ramps up and may reflect a possible north dipping low angle detachment or a change in crustal geometry associated with the subducted morro fracture zone above a time transgressive unconformity miocene and younger sediments onlap to the south and water bottom deepens suggesting a major regional tilt reversal in post miocene time below the unconformity basement is displaced by largely northwest striking normal faults that we believe form tilted crustal blocks it is possible that cretaceous and paleogene sections are preserved in resulting half grabens some of these normal faults including splays off the outer santa lucia bank fault system were reactivated as minor reverse faults in post miocene time there is no evidence that requires any post 17 5 ma tectonostratigraphic terrane boundary through this area rather much of the structure of the southern santa maria basin appears to be continuous with the rotated western transverse ranges and northern channel islands

seismic evidence for a lower crustal detachment beneath san francisco bay california results from the san francisco bay area seismic imaging experiment basix reveal the presence of a prominent lower crustal reflector at a depth of 15 kilometers beneath san francisco and san pablo bays velocity analyses indicate that this reflector marks the base of franciscan assemblage rocks and the top of a mafic lower crust because this compositional contrast would imply a strong rheological contrast this interface may correspond to a lower crustal detachment surface if so it may represent a subhorizontal segment of the north america and pacific plate boundary proposed by earlier thermo mechanical and geological models

migration moveout analysis and depth focusing prestack depth migration still suffers from the problems associated with building appropriate velocity models the two main after migration before stack velocity analysis techniques currently used depth focusing and residual moveout correction have found good use in many applications but have also shown their limitations in the case of very complex structures to address this issue we have extended the residual moveout analysis technique to the general case of heterogeneous velocity fields and steep dips while keeping the algorithm robust enough to be of practical use on real data our method is not based on analytic expressions for the moveouts and requires no a priori knowledge of the model but instead uses geometrical ray tracing in heterogeneous media layer stripping migration and local wavefront analysis to compute residual velocity corrections these corrections are back projected into the velocity model along raypaths in a way that is similar to tomographic reconstruction while this approa

integrated crustal structure across the south central california margin santa lucia escarpment to the san andreas fault the continental margin of south central california has undergone different styles of deformation over the past 30 m y as the north america pacific plate boundary has evolved from subduction to transform tectonics results presented here identify strike slip and compressional structures in the lower crust that probably developed during this period of tectonic transition an integrated model of crustal structure along deep seismic line pg e 3 crosses the entire continental margin west of the san andreas fault near the latitude of san luis obispo a compressional wave velocity model was developed using coincident marine multi channel seismic and onshore offshore wide angle reflection refraction seismic data and was tested independently with gravity modeling a high velocity layer 6 8 7 0 km s about 6 km thick in the lower crust that extends east possibly as far as the san andreas fault is interpreted as tectonically underplated oceanic crust the upper and middle crust of the patton and sur obispo franciscan terranes extends to depths of 14 to 20 km the velocity and density modeling indicate lower average velocities and densities for the patton terrane supporting earlier interpretations that it is a distinct terrane within the franciscan complex abrupt crustal thickening associated with broken and downdropped oceanic crust beneath the santa lucia escarpment probably results from active transform faulting that occurred along the continental slope from about 22 to 16 ma following the subduction of the pacific arguello ridge an imbrication within the oceanic crust centered beneath the coast is evidence for about 15 km of shortening within the oceanic crust convergence between the monterey and arguello microplates during the final stages of farallon subduction about 24 to 16 ma probably caused the imbrication

structure of archean crust and passive margin of southwest greenland from seismic wide angle data to investigate the structure and composition of the archean crust beneath the shelf of southwest greenland parallel to the passive margin we conducted an onshore offshore seismic wide angle experiment in 1989 one of our goals was to obtain a velocity depth profile along the thinned continental or transitional crust to place constraints on the magmatic rifting history in the northern labrador sea seismic recording instruments at four locations along the coast recorded energy from an air gun array up to 400 km distance due to the dense shot spacing of 100 to 150 m we were able to identify continuous reflected and refracted phases from the upper to lower crust results from a one dimensional extremal inversion indicated a high velocity zone hvz with p wave velocities of more than 7 2 km s at the bottom of the transitional crust combined two dimensional travel time forward modeling and inversion produced a velocity depth model that contains midcrustal and lower crustal discontinuities between 12 and 25 km depth and a moho dipping northward at 30 to 42 km depth an average crustal velocity of about 6 6 km s indicates a high portion of mafic components in the crust the lowermost crust consists of a hvz with a maximum thickness of 8 km and temperature corrected velocities of 7 5 to 7 85 km s the hvz may be explained by an underplated igneous complex often observed under passive margins worldwide metamorphism to garnet pyroxene granulite during postrift isobaric cooling of the 30 to 40 km deep thinned continental crust could explain the velocities that are higher than on other margins a large volume of magmatic material must have been added to the crust but did not produce extrusions magmatic underplating of the thinned continental or transitional crust of sw greenland is likely associated with a possible hot spot magmatism in the davis strait and baffin bay region magmatism associated with continental rifting was found to reach farther south along the northern and central part of the archean block than previously thought

deep structure and structural inversion along the central california continental margin from edge seismic profile ru 3 deep penetration seismic reflection profile ru 3 crosses the central california continental margin revealing a subducted oceanic plate a modified accretionary prism and complex structures of the overlying sedimentary basins this structural framework was established by subduction processes during paleogene and earlier time and subsequently was modified by neogene transform motion combined with apparent components of extension and compression subducted rocks are indicated by deep gently dipping reflectors that extend beneath the continental margin for at least 38 km at a depth of about 15 km we interpret the subducted crust as either a part of the pacific plate or more likely a subducted fragment derived from the farallon plate a set of more steeply dipping deep events may indicate faulting within the subducted plate or its boundary with a no slab zone the overlying largely nonreflective layer of accreted material rapidly reaches 10 km in thickness landward of the paleotrench and increases to 15 km in thickness near the coast the santa lucia basin landward of the steep continental slope originated as a slope basin during paleogene subduction the lower strata of this basin were deposited onto and partially incorporated into the accretionary complex the offshore santa maria basin exhibits a variety of compressional structures that formed in the last 3 5 m y and whose locations correspond to an earlier framework of extensional faults structural inversion has occurred in miocene depocenters adjacent to the santa lucia bank fault and at the queenie structure miocene and lower pliocene strata also thicken toward the hosgri fault zone where subsequent compression is characterized by low angle thrusts and folding

image of the moho across the continent ocean transition u s east coast strong wide angle reflections from the moho were recorded by ocean bottom seismic instruments during the 1988 carolina trough multichannel seismic experiment in an area where the moho is difficult to detect with vertical incidence seismic data prestack depth migration of these reflections has enabled the construction of a seismic image of the moho across the continent ocean transition of a sedimented passive margin the moho rises across the margin at a slope of 10 12 from a depth of about 33 km beneath the continental shelf to 20 km beneath the outer rise this zone of crustal thinning defines a distinct 60 70 km wide continent ocean transition zone we interpret the moho in the carolina trough as a jurassic feature formed by magmatic intrusion and underplating during the rifting of pangea

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

effective sub zechstein salt imaging using low frequency seismics results of the grundy 2003 experiment across the variscan front in the polish basin abstract the extent of the variscan deformation front is one of the key problems of the regional geology of the central european permian basin system particularly in its polish part conventional reflection seismics usually fails to produce a satisfactory image of the pre permian strata due to the shielding effect of zechstein upper permian evaporites thus we used a novel seismic acquisition technique to study the base of the permian complex and its variscan basement in the grundy 2003 experiment we combined wide angle reflection refraction measurements with the near vertical reflection seismics by the use of the constant geophone array with dense 100 m receiver spacing occupying 50 km long profile 3d design of the experiment covering 50 10 km area helped in eliminating the effect of out of plane propagations and local inhomogeneities an effective integration of traveltime tomography cdp reflection processing and prestack depth migration of wide angle reflections applied to our data allowed us to present the model in which we deduced the contact zone of the variscan overthrust structure variscan front with its molasse filled foredeep the latter might be a gas generation zone which is of a great importance for hydrocarbons prospecting in this area

crustal structure of the offshore southern santa maria basin and transverse ranges southern california from deep seismic reflection data seismic reflection data from the 1986 edge pacific gas and electric experiment together with refraction and gravity data are used to construct an integrated north south geophysical cross section along the offshore santa maria basin and across the offshore extension of the transverse ranges thrust faults offsetting acoustic basement and folding miocene sediments root in a midcrustal detachment at 12 13 km depth the detachment coincides with an increase in reflectivity seen on the seismic reflection section and the maximum depth of earthquakes a high velocity oceanic crustal layer at the base of the crust 17 22 km and moho 25 km is offset by a low angle thrust fault with an apparent dip 10 to the north offshore point arguello gravity models indicate crustal thickening and a change in the depth to moho from 25 to 30 km in this region however the contribution of elastic flexure from loading of the transverse ranges is considered to be minor in this part of the borderland the cross section constrains neogene shortening to less than 3 based on faulting and folding of the sedimentary section shortening of the upper crust is balanced by a combination of ductile flow and subduction of the oceanic crust beneath the midcrustal detachment

deep seismic imaging of the eastern nankai trough japan from multifold ocean bottom seismometer data by combined travel time tomography and prestack depth migration 1 the easternmost segment of the nankai trough remained unruptured during the 1944 and 1946 earthquakes that affected the rest of the trench it is therefore a zone of seismic gap that also undergoes a peculiar tectonic regime due to its proximity with the izu collision zone we investigate this area with densely sampled active seismic data one hundred ocean bottom seismometers spaced only 1 km apart were deployed along a line perpendicular to the trough a two step tomographic approach designed for this type of acquisition is applied first a long wavelength velocity model is built by a linearized inversion of first arrival travel times its resolution is assessed and its reliability is confirmed through a semiglobal approach in which a large range of starting models is tested second this tomographic model is used to exploit late reflected arrivals with a prestack depth migration the result of which is validated by classically modeling refraction and reflection travel times in the derived structural model thus both velocities and discontinuities are mapped in our model with a minimum of data interpretation and a priori knowledge our results bring new insights on the deep crustal zones of the margin and its evolution the existence of presently inactivated major thrusts is very clearly evidenced in the backstop a general timing for its evolution and deformation is proposed farther seaward the presence of a subducted ridge beneath the accretionary wedge is confirmed its compressive structure and origin are demonstrated it exhibits a thickened lower crust which nature is discussed besides strong analogies between this paleo zenisu subducting ridge and the zenisu ridge observed farther south are also emphasized bringing evidences for a steady state crustal deformation pattern