imaging near surface heterogeneities by natural migration of backscattered surface waves s u m m a r y we present a migration method that does not require a velocity model to migrate backscattered surface waves to their projected locations on the surface this migration method denoted as natural migration uses recorded green s functions along the surface instead of simulated green s functions the key assumptions are that the scattering bodies are within the depth interrogated by the surface waves and the green s functions are recorded with dense receiver sampling along the free surface this natural migration takes into account all orders of multiples mode conversions and non linear effects of surface waves in the data the natural imaging formulae are derived for both active source and ambient noise data and computer simulations show that natural migration can effectively image near surface heterogeneities with typical ambient noise sources and geophone distributions

interferometric daylight seismic imaging summary claerbout s daylight imaging concept is generalized to a theory of interferometric seismic imaging ii interferometric seismic imaging is defined to be any algorithm that inverts correlated seismic data for the reflectivity or source distribution as examples we show that ii can image reflectivity distributions by migrating ghost reflections in passive seismic data and generalizes the receiver function imaging method used by seismologists interferometric seismic imaging can also migrate free surface multiples in common depth point cdp data and image source distributions from passive seismic data both synthetic and field data examples are used to illustrate the different possibilities of ii the key advantage of ii is that it can image source locations or reflectivity distributions from passive seismic data where the source position or wavelet is unknown in some cases it can mitigate defocusing errors as a result of statics or an incorrect migration velocity the main drawback with ii is that severe migration artefacts can be created by partial focusing of virtual multiples

half year long measurements with a buoy mounted acoustic doppler current profiler in the somali current a self contained upward looking acoustic doppler current profiler adcp mounted in the top float of a subsurface mooring was deployed in the somali current at 2 14 n 45 55 e from september 17 1985 to april 25 1986 the instrument operated at a frequency of 150 khz with a vertical beam angle of 20 vector averaged profiles of horizontal and vertical velocity were recorded every 4 hours using 200 pings per ensemble at a vertical bin length of 8 7 m the mooring was deployed in very rough topography settling in a trough at 337 m depth with the adcp located at 267 m depth data retrieval over the entire recording period was complete with doppler biasing from side lobe reception of vertically traveling rays affecting only the top 20 m below the surface over the 7 month deployment the instrument recorded current profiles encompassing the end of the 1985 summer monsoon and entire winter monsoon and also through the spring transition into the early onset phase of the 1986 summer monsoon significant echo amplitude variations of week to month long duration were observed which were only partially related to horizontal flow variations associated with the monsoons projection of the strong horizontal currents exceeding 150 cm s at times into the vertical component was not observed attesting to fairly exact orientation of the four beams and tilt meters this indicates that the vertical current measurement from adcps can be potentially useful for phenomena with vertical velocities exceeding a few millimeters per second however an analysis of echo amplitude and vertical current variations at the diurnal period suggests that the measured vertical velocity is at least at that period probably dominated by active vertical migration of biological scatterers through the water column

migration of surface seismic data with vsp green s functions seismic migration is the process by which wavefields recorded on or near the surface are mapped back into the subsurface to form an image of the subsurface structure common to all current migration methods is that they rely on a model of the subsurface for computing green s functions these computed green s functions are approximations of the true green s function in the subsurface the more accurate the method used for computing the green s functions and to some extent also the choice of imaging condition the better image we will get most state of the art migration methods will use green s functions that include multiple arrivals and some finite frequency effects allowing them to image fairly complex geologic structures however the computational model in most cases uses only scalar velocity that is smooth on a wavelength scale thus ignoring scattering effects from small scale structure anisotropy and other intrinsic rock effects further whatever the model is it has to be derived from the dat

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

sequence stratigraphic significance of sedimentary cycles and shell concentrations in the upper jurassic lower cretaceous of kachchh western india abstract upper jurassic and lower cretaceous siliciclastic shallow water sediments of the kachchh basin western india form strongly asymmetric coarsening upward cycles which are interpreted as recording changes in relative sea level deepening shallowing cycles these cycles correspond to depositional sequences in which deposits of the lowstand systems tract are not present the sequence boundary coinciding with the transgressive surface shell concentrations are found in the transgressive lags at the base of the transgressive systems tract tst in the maximum flooding zone mfz and at or close to the top of the highstand systems tract they belong to six assemblages five of them dominated by large bivalves such as seebachia herzogina gryphaea gervillella megacucullaea pisotrigonia and indotrigonia the sixth by the coral amphiastraea three types of shell concentrations can be distinguished that differ from each other in a number of ecological and taphonomic features such as species diversity preservation quality orientation in cross section percentage of disarticulation and degree of biogenic alteration characteristic features of concentrations at the base of the tsts are moderate time averaging sorting a preferred convex up orientation and nearly total disarticulation of shells they are suggestive of an environment in which reworking and local transport were frequent events similar features are shown by concentrations near the tops of the hsts except that there shells were largely concentrated in lenses and in pavements rather than in beds as in the transgressive lags associated sedimentary structures indicate deposition above fair weather wave base in a high energy environment concentrations occurring in the mfz in contrast are autochthonous and highly time averaged having accumulated during times of low rates of sedimentation below storm wave base this is supported by their high preservation quality comparatively high percentage of articulated shells shells of infaunal organisms commonly preserved in life position biogenic alteration being the most important taphonomic agent the dominant elements of these shell concentrations i e seebachia megacuccullaea and indotrigonia in the upper jurassic and pisotrigonia in the lower cretaceous are endemic to the ethiopean faunal province and belong to lineages that rapidly evolved during this time period

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

monthly periodicity in acoustic reflections and vertical motions in the deep ocean 1 a recent 1 5 years long record of acoustic doppler current profiler adcp data from the canary basin north atlantic ocean likely reflects vertical zooplankton migration between 800 and 1400 m this record clearly distinguishes 3 major periodicities of down and upgoing motions to within a precision of 1 400 a daily a seasonal and a monthly cycle largest daily excursions occur during full moon the directly observed hourly mean vertical velocity amplitudes of w 0 025 0 01 m s 1 are too slow for particles from the observational depths to reach the zone of moon and only very weak sun light penetration in half a day it is shown that no physical internal wave geochemical or sinking food mechanism can trigger the daily and monthly cycles which are coupled it is speculated that an entrained biorhythm running precise internal biochemical clocks controls the vertical migration

full wavefield migration of vertical seismic profiling data using all multiples to extend the illumination area vertical seismic profiling data can provide a high resolution reservoir image because the receivers are located close to the reservoir and hence the wavefield encounters less distortion in the overburden compared to surface seismic data however conventional migration of vertical seismic profiling data using only upgoing primary wavefields often suffers from poor illumination imaging artefacts and low image reliability especially at image locations away from the well we propose full wavefield migration to image vertical seismic profiling data using the primaries and all orders of surface multiples and internal multiples the downgoing internal or surface multiples are treated as an additional source of illumination in full wavefield migration we aim to estimate a high resolution reflectivity image of the subsurface with this extended illumination the algorithm is recursive in depth like conventional wavefield extrapolation based migration however it incorporates the non linear transmission and scattering effects at each depth level further full wavefield migration is posed as a constrained least squares inversion problem that could be solved using a conjugate gradient method we propose an iterative full wavefield forward modelling engine as the backbone of this inversion scheme the parameter used in the modelling is subsurface reflectivity full wavefield modelling iteratively incorporates the non linearity of the wavefield due to multiple scattering where every iteration utilizes one higher order of scattered wavefields to estimate the subsurface reflectivity in addition the constrained inversion helps in reducing the extrapolation artefacts and provides a high resolution image of the reservoir in this paper we discuss the concept of full wavefield migration and demonstrate its potential as an imaging tool for vertical seismic profiling data using synthetic examples key words vertical seismic profiling true amplitude migration inversion modelling illumination

twilight vertical migrations of zooplankton in a chilean fjord time series of acoustic backscatter and vertical velocity profiles were obtained at three sites along a chilean fjord with the purpose of determining dominant structures of vertical migrations of the sound scattering layer ancillary data obtained with stratified net samples indicated that the sound scattering layer may have been dominated by euphausiids and decapods therefore distributions of acoustic backscatter anomalies and vertical velocities were attributed to vertical migrations of predominantly these organisms migration patterns were dominated by twilight excursions in which organisms swam toward the water surface at sunset spent 100 m this migration strategy can also be termed semidiel migration as two double excursions were linked to light levels the reasons for this twilight migration remain uncertain but it is possible that the up and down motion around sunset was related to predation avoidance hunger satiation state ontogeny seaward transport evasion or reaction to the environmental shock from the pycnocline or a combination of all or some of them in contrast the sunrise double excursion was probably linked to feeding requirements by organisms that need to spend the day at great depth with no food available this study demonstrated the existence of semidiel patterns throughout the fjord and through prolonged periods in addition identification of this pattern by acoustic backscatter was complemented by direct vertical velocity measurements it is proposed that twilight vertical migration is a common strategy in chilean fjords

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

dead delta s former productivity two trillion shells at the mouth of the colorado river the diversion of the colorado river by dams and irrigation projects started in the 1930s triggered the collapse of the colorado delta ecosystem paleontological ecological geochronological stable isotope field and satellite image data provide estimates of the delta s benthic productivity during the 1 k y directly preceding the artificial shutdown of the river at least 2 1012 shells of bivalve mollusks make up the current beaches and islands of the delta the 125 individual valves dated using 14c calibrated amino acid racemization indicate that these shells range in age from a d 950 to 1950 seasonal intrashell cycles in 180 values indicate that average sized bivalves lived at least 3 yr the most conservative calculation based on these numbers indicates that during the time of natural river flow an average standing population of 6 109 bivalve mollusks population density 50 m2 thrived on the delta in contrast the present abundance of shelly benthic macroinvertebrates is 94 lower 3 m2 in 1999 2000 the dramatic decrease in abundance testifies to the severe loss of benthic productivity resulting from diversion of the river s flow and the inadequacy of its partial resumption 1981 to present an integration of paleontological records with geomorphological geochemical and geochronological data can provide quantitative insights into human impact on coastal ecosystems

vertical velocity and vertical heat flux observed within loop current eddies in the central gulf of mexico abstract sixteen months of observations from a surface to bottom mooring in the central gulf of mexico show that acoustic doppler current profilers adcps are useful for directly measuring the vertical velocity within mesoscale anticyclonic eddies such as those shed from the loop current and combining simultaneous temperature measurements vertical heat flux can also be estimated as a covariance of both variables there is evidence of significant and coherent signals of vertical velocity 2 3 mm s 1 and vertical heat temperature transport 10 3 c m s 1 during the presence of three anticyclones a simple analysis shows downward flow near the eddies centers above 350 m and essentially upward flow in the peripheries but below 700 m depth the pattern is indeed the opposite however further study is necessary to determine the eddies interior structures the observations also suggest the existence of a vertical convergence of heat somewhere around 600 m depth and estimations of adiabatic heat flux

patterns in bioclastic accumulation through the phanerozoic changes in input or in destruction evolutionary changes in the ecology and diversity of organisms that produce and destroy calcareous skeletons suggest that bioclastic concentrations themselves might have changed in nature through the phanerozoic empirical data from marine siliciclastic records of ordovician silurian jurassic and neogene ages indicate a significant increase in the thickness of densely packed bioclastic concentrations over geologic time from a primarily thin bedded brachiopod dominated record in the ordovician silurian to a mollusk dominated record with many more and thicker shell beds in the neogene jurassic shell beds vary in thickness with the paleozoic or modern affinities of the chief constituents suggesting along with other evidence that the phanerozoic increase was determined neither by diagenesis nor by a shift in taphonomic conditions on the sea floor but rather by the evolution of bioclast producers namely groups with 1 more durable low organic skeletons 2 greater ecological success in high energy habitats and 3 on the basis of indirect evidence higher rates of carbonate production these results suggest that 1 reproductive and metabolic output has increased in benthic communities over time and 2 the scale of time averaging in benthic assemblages has increased owing to greater hard part durability of modern groups

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

reduced time migration of transmitted ps waves we develop the novel theory of transmitted ps migration and show that ps transmitted arrivals in a gulf of mexico vertical seismic profile vsp data set can be migrated to accurately image a salt sheet even though the receiver array is below the transmitting boundary we also show that migrating transmitted arrivals is effective in illuminating the base of an orebody invisible to pp reflections in general interfaces that bisect wavepath propagation i e the source and receiver are on opposite sides of the interface and therefore invisible to pp reflections can be imaged by migration of ps transmitted waves these results suggest that migration of ps transmitted waves opens new opportunities in imaging nearly vertical impedance boundaries that are typically invisible to conventional reflection imaging of crosswell and vsp data we also present a new interferometric method denoted as reduced time migration which uses the arrival time difference between the direct p wave and subsequent events to incre

local migration with extrapolated vsp green s functions we have developed a novel vertical seismic profile vsp imaging method that requires only a local velocity model around the well this method is denoted as a local migration where the model based forward propagation and backward propagation operators are computed using a local velocity model between the well and the target body the velocity model for the complex overburden and salt body is not needed and the source side statics are automatically accounted for in addition kinematic and dynamic effects including anisotropy absorption and other unknown undefined rock effects outside of this local velocity model are mostly accounted for numerical tests on an acoustic 2d sigsbee vsp data set an elastic salt model and offset 2d vsp data in the gulf of mexico partly validate the effectiveness of this method by accurately imaging the sediments next to the vertical well

acoustic backscatter observations with implications for seasonal and vertical migrations of zooplankton and nekton in the amundsen shelf antarctica abstract high temporal resolution profiles of acoustic backscatter were collected in the dotson trough on the amundsen shelf in the antarctica using a bottom moored upward looking acoustic doppler current profiler adcp this data set was used to examine the impact of seasonal variations in surface solar radiation ssr sea ice concentration sic and circumpolar deep water cdw thickness on acoustic backscatter in the lower water column 250 540 m depth a recorded high acoustic backscatter 75 to 70 db at depth 400 m from april to november compared to the rest of the year 90 to 80 db suggests that zooplankton and nekton migrated towards the bottom during winter the depth of maximum mean volume backscattering strength showed a significant correlation with ssr sic and cdw thickness a daily cycle of vertical migration was also recorded this varied with changing surface ice conditions when sea ice cover was low the acoustic backscatter descended at sunrise and ascended at sunset when sea ice cover was high the daily migration was not pronounced and the layer of high acoustic backscatter remained near the bottom this is the first study of seasonal and vertical migration of zooplankton and nekton that has been conducted on the amundsen sea shelf one of the world s most productive areas the findings provide implications to understand the behavior of zooplankton and nekton below the euphotic zone in the southern ocean

salt flank delineation by interferometric imaging of transmitted p to s waves we describe how vertical seismic profile vsp interferometric imaging of transmitted p to s ps waves can be used to delineate the flanks of salt bodies unlike traditional migration methods interferometric ps imaging does not require the migration velocity model of the salt and or upper sediments in order to image the salt flank synthetic elastic examples show that ps interferometric imaging can clearly delineate the upper and lower boundaries of a realistic salt body model results also show that ps interferometric imaging is noticeably more accurate than conventional migration methods in the presence of static shifts and or migration velocity errors however the illumination area of the ps transmitted waves is limited by the width of the shot and geophone aperture which means wide shot offsets and deeper receiver wells are needed for comprehensive salt flank imaging interferometric imaging results for vsp data from the gulf of mexico demonstrate its superiority over the traditional migration method we also discuss other arrivals that can be used for interferometric imaging of salt flanks for comparison reduced time migration results are presented which are similar in quality to those obtained for interferometric imaging we conclude that ps interferometric imaging of vsp data provides the geophysicist with a new tool by which salt flanks can be viewed from both above and below vsp geophone locations

preservation of autochthonous shell beds by positive feedback between increased hardpart input rates and increased sedimentation rates abstract the preservation of nonrapidly buried autochthonous shell concentrations with noncementing epifaunal animals in life position presents a taphonomic dilemma if in fact an increase in shelliness is driven by a decrease in sedimentation rate a 150 cm thick densely packed shell bed with brachiopods from the lower jurassic of morocco shows lower levels of postmortem alteration than shell poor beds indicating that its formation is primarily governed by variations in hardpart input rates varying dominance and size structure of the main shell producer brachiopod zeilleria rehmanni indicate that its increased population density was the main trigger in the shell bed formation thinner and less common microbial crusts in the shell bed than in shell poor beds indicate that higher shelliness is not due to lack of sediment on the basis of actualistic data from modern mussel and oyster shell beds the suspension feeding of a high density population leads to high biodeposition rates through production of

mitigation of defocusing by statics and near surface velocity errors by interferometric least squares migration with a reference datum abstractimaging seismic data with an erroneous migration velocity can lead to defocused migration images to mitigate this problem we first choose a reference reflector whose topography is well known from the well logs for example reflections from this reference layer are correlated with the traces associated with reflections from deeper interfaces to get crosscorrelograms interferometric least squares migration ilsm is then used to get the migration image that maximizes the crosscorrelation between the observed and the predicted crosscorrelograms deeper reference reflectors are used to image deeper parts of the subsurface with a greater accuracy results on synthetic and field data show that defocusing caused by velocity errors is largely suppressed by ilsm we have also determined that ilsm can be used for 4d surveys in which environmental conditions and acquisition parameters are significantly different from one survey to the next the limitations of ilsm are that it requires prior knowledge of