probabilistic lithofacies prediction from prestack seismic data in a heterogeneous carbonate reservoir abstractwe mapped probabilities of lithology and fluid based on seismic and well observations in a heterogeneous carbonate reservoir of the sichuan basin southwest china thus characterizing the reservoir complexity and identifying the potential sweet spot rock physical analysis showed that the presence of a vuggy fracture porosity system combined with the fluids effect complicate the elastic responses of heterogeneous carbonates this also gives physical insight into how the elastic properties of different lithofluid classes can be distinguished with the application of bayesian linearized amplitude variation with offset inversion we found that the posterior distribution of p and s wave velocities were reliably extracted whereas the inverted density tended to show high uncertainty due to a lack of wide angle data in the migrated seismic gathers finally a bayesian approach was implemented to propagate uncertainty from prestack seismic data to lithofluid classes in an integrated framework the gas

joint probabilistic petrophysics seismic inversion based on gaussian mixture and markov chain prior models abstractseismic reservoir characterization focuses on the prediction of reservoir properties based on the available geophysical and petrophysical data the inverse problem generally includes continuous properties such as petrophysical and elastic attributes and discrete properties such as lithology fluid classes we have developed a joint probabilistic inversion methodology for the prediction of petrophysical and elastic properties and lithology fluid classes that combined statistical rock physics and bayesian seismic inversion the elastic attributes depend on continuous petrophysical variables such as porosity and clay content and discrete lithology fluid classes through a nonlinear rock physics relationship together the seismic model relates the elastic attributes such as velocities and density to their seismic response reflectivity traveltime and amplitudes the advantage of our integrated approach is that the inversion method accounts for the uncertainty associated to each step of the mo

stochastic reservoir characterization using prestack seismic data reservoir characterization must be based on information from various sources well observations seismic reflection times and seismic amplitude versus offset avo attributes are integrated in this study to predict the distribution of the reservoir variables i e facies and fluid filling the prediction problem is cast in a bayesian setting the a priori model includes spatial coupling through markov random field assumptions and intervariable dependencies through nonlinear relations based on rock physics theory including gassmann s relation the likelihood model relating observations to reservoir variables including lithology facies and pore fluids is based on approximations to zoeppritz equations the model assumptions are summarized in a bayesian network illustrating the dependencies between the reservoir variables the posterior model for the reservoir variables conditioned on the available observations is defined by the a priori and likelihood models this posterior model is not analytically tra

oceanic crustal thickness versus spreading rate a compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness away from plateaus is 6 km no systematic increase of crustal thickness with spreading rate is observed instead the data show large variations in crustal thickness at slow spreading ridges 3 8 km for half rates 30 mm yr the large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid ocean ridges both data sets support the speculation of a transition from a 3 d structure of crustal accretion at slow ridges to a 2 d accretion pattern at fast ridges

temporal evolution of overlapping spreading centers at 16 20 n on the east pacific rise abstract a compilation of sea beam data on the right stepping overlapping spreading center osc at 16 20 n on the east pacific rise epr and in the surrounding region is presented sea beam data from the rise flanks at 16 20 n reveal three deep basins at least one of which is flanked by a curved ridge whose morphology is similar to that at the offset the widths of these offaxis basins range from 8 to 13 km and allow an estimate to be made of the sizes of the offsets at which they were formed it is proposed that they represent traces on the rise flank which provide a record of the development of the offset and a basis for prediction of its future evolution the repeated pattern of abandoned overlap basins and cuspate ridges supports the notion that oscs are episodic on a time scale of approximately 200 000 years and are cyclic in nature it also reveals an alternation of dominance between the southern and northern segments which have shifted the location of the discontinuity along the axis so as to create zig zag traces of basins on the ridge flanks

probabilistic petrophysical properties estimation integrating statistical rock physics with seismic inversion a joint estimation of petrophysical properties is proposed that combines statistical rock physics and bayesian seismic inversion because elastic attributes are correlated with petrophysical variables effective porosity clay content and water saturation and this physical link is associated with uncertainties the petrophysical properties estimation from seismic data can be seen as a bayesian inversion problem the purpose of this work was to develop a strategy for estimating the probability distributions of petrophysical parameters and litho fluid classes from seismics estimation of reservoir properties and the associated uncertainty was performed in three steps linearized seismic inversion to estimate the probabilities of elastic parameters probabilistic upscaling to include the scale changes effect and petrophysical inversion to estimate the probabilities of petrophysical variables andlitho fluid classes rock physics equations provide the linkbetween reservoir properties and velocities and lin

hierarchical bayesian lithology fluid prediction a north sea case study seismic 3d amplitude variation with offset avo data from the alvheim field in the north sea are inverted into lithology fluid classes elastic properties and porosity lithology fluid maps over hydrocarbon prospects provide more reliable estimates of gas oil volumes and improve the decision concerning further reservoir assessments the alvheim field is of turbidite origin with complex sand lobe geometry and appears without clear fluid contacts across the field the inversion is phrased in a bayesian setting the likelihood model contains a convolutional linearized seismic model and a rock physics model that capture vertical trends due to increased sand compaction and possible cementation the likelihood model contains several global model parameters that are considered to be stochastic to adapt the model to the field under study and to include model uncertainty in the uncertainty assessments the prior model on the lithology fluid classes is a markov random field that captures local vertical horizonta

imaging along strike variations in mechanical properties of the gofar transform fault east pacific rise a large part of global plate motion on mid ocean ridge transform faults rtfs is not accommodated as major earthquakes when large earthquakes do occur they often repeat quasiperiodically we focus here on the high slip rate 14 cm yr gofar transform fault on the equatorial east pacific rise this fault is subdivided into patches that slip during mw 5 5 6 earthquakes every 5 to 6 years these patches are separated by rupture barriers that accommodate slip through swarms of smaller events and or aseismic creep we performed an imaging study to investigate which spatiotemporal variations of the fault zone properties control this segmentation in mechanical behavior and could explain the specific behavior of rtfs at the global scale we adopt a double difference approach in a joint inversion of active air gun shots and microseismicity recorded for 1 year this data set includes the 2008 mw 6 gofar earthquake the along strike p wave velocity structure reveals an abrupt transition between the barrier area characterized by a damaged fault zone of 10 20 reduced vp and a nearly intact fault zone in the asperity area the importance of the strength of the damage zone on the mechanical behavior is supported by the temporal s wave velocity changes which suggest increased damage within the barrier area during the week preceding the mw 6 earthquake our results support the conclusion that extended highly damaged zones are the key factor in limiting the role of major earthquakes to accommodate plate motion along rtfs

a proposed latest triassic to earliest cretaceous microfossil biozonation for the english channel and its adjacent areas abstract three detailed biostratigraphic zonations based on ostracods foraminiferids and dinocysts are proposed for the latest triassic through to the earliest cretaceous of the portland wight basin and its adjacent areas a total of 112 zones zonules 34 ostracod zones subzones 25 foraminiferid zones 53 dinoflagellate zones subzones and zonules are recognized all three zonation schemes are based on samples obtained from both outcrop the dorset coast and boreholes the latter from ditch cuttings with some conventional and sidewall core samples in association with published data and have where possible been calibrated to the standard ammonite zones recognized throughout northwest europe

rivera ocean seismic experiment rose overview the rivera ocean seismic experiment rose was designed as a combined sea and land seismic program to utilize both explosive and earthquakes to study a number of features of the structure and evolution of a mid ocean ridge a major oceanic fracture zone and the transition region between ocean and continent the primary region selected for the experiment included the rivera fracture zone the crest and eastern flank of the east pacific north of the rivera and adjacent areas of baja california and mainland mexico the experiment included 1 study of the east pacific rise south of the orozco fracture zone primarily using ocean bottom recording and explosive sources 2 a seismicity program at the orozco and 3 a land based program of recording natural events along the coastal region of mexico a considerable amount of useful data was obtained in each of the three subprograms in the marine parts of the experiment we were able to address a variety of problems including structure and evolution of young oceanic crust and mantle structure and dynamics of the east pacific rise seismicity of the orozco fracture zone and partitioning of energy transmission between the ocean volume and the crust lithosphere on land the fortuitousmore occurrence of the petatlan m7 6 earthquake of march 14 1979 permitted the acquisition of an excellent data set of foreshocks and aftershocks of this large event which provide new insight into the filling of a major seismic gap in the region this overview describes the scientific rationale and the design of the experiments along with some general results less

reservoir lithology classification based on seismic inversion results by hidden markov models applying prior geological information abstract hidden markov models hmms have been applied to predict reservoir lithologies using seismic inversion results as inputs this approach takes into account the conditional probabilities between different lithologies i e the vertical transitions in sedimentary sequences these properties are used as prior geological information in order to relate the seismic inversion results to the true well log data hmms need to be trained based on the expectation maximization theory application of the resulting model on a synthetic example from the book cliffs utah usa showed that most lithologies are classified correctly even for some thin layers a comparison with point wise methods in which data samples are treated independently from each other such as k means and fuzzy logic classifiers leads to the conclusion that the spatial correlation in hmms allows better lithological predictions because the prior information accounts for the geological depositional processes a real case study with data from the vienna basin austria is performed in which lithologies in a 3d cube are obtained based on properties from seismic inversions via trained hmms while the vertical sequences are shown to be reasonably well predicted the horizontal continuities are not this indicates that the future research should focus on the lateral geological relationships

a rock physics strategy for quantifying uncertainty in common hydrocarbon indicators we present a strategy for quantifying uncertainties in rock physics interpretations by combining statistical techniques with deterministic rock physics relations derived from the laboratory and theory a simple example combines gassmann s deterministic equation for fluid substitution with statistics inferred from log core and seismic data to detect hydrocarbons from observed seismic velocities the formulation identifies the most likely pore fluid modulus corresponding to each observed seismic attribute and the uncertainty that arises because of natural variability in formation properties in addition to the measurement uncertainties we quantify the measure of information in terms of entropy and show the impact of additional data about s wave velocity on the uncertainty of the hydrocarbon indicator in some cases noisy s data along with noisy p data can convey more information than perfect p data alone while in other cases s data do not reduce the uncertainty we apply the formulation to a well log example for detecting the most likely pore fluid and quantifying the associated uncertainty from observed sonic and density logs the formulation offers a convenient way to implement deterministic fluid substitution equations in the realistic case when natural geologic variations cause the reference porosity and velocity to span a range of values

palaeoecology and stratigraphic distribution of middle jurassic ostracods from the lower inferior oolite of the cotswolds england abstract ostracods extracted from limestones of the lower inferior oolite scissum murchisonae zones are analysed in terms of the sample correlation coefficient r of species pairs and the limestone petrography within each lithofacies faunal composition is demonstrated to vary according to 1 grain size 2 the proportion of lime mud to spar cement in the matrix and 3 allochem composition the nature of these inter relationships indicates that the distribution of ostracods was strongly influenced by 1 size sorting of valves and carapaces prior to burial 2 preferential winnowing and redistribution and 3 bioturbation assemblages apparently unaffected by these processes are considered to represent biocoenoses three palaeocommunities are reconstructed for those carbonate environments where the terrigenous input was low 1 primary weed fauna 2 weed sediment fauna low energy 3 interstitial reef fauna the predominance of one ostracod community over another is correlated with the availability of phytal or non phytal sediment micro habitats in transitional carbonate regimes where terrigenous input was high and or bottom conditions were turbulent a fourth ostracod palaeocommunity is postulated viz terrigenous mud fauna this fauna characterised by abundant thick shelled ornamented cytheraceans is compared with certain late toarcian assemblages of terrigenous clastic association finally the stratigraphic distribution and application of the fauna are evaluated in relation to the species ranges proposed for the jurassic succession in britain and northwestern germany

a two dimensional tomographic study of the clipperton transform fault from the marine refraction data recorded on five instruments during the clipperton area seismic survey to investigate compensation classic experiment in 1994 we construct a compressional velocity model for a 108 km long profile across the clipperton transform we apply a new seismic tomography code that alternates between ray tracing and linearized inversions to find a smooth seismic velocity model that fits the observed refraction travel times the solution to the forward ray tracing problem is a hybrid of the graph or shortest path method and a ray bending method the inversion is performed with least squares penalties on the data misfit and first derivatives of the seismic structure starting with a one dimensional compressional velocity model for oceanic crust the misfit in the normalized travel time residuals is reduced by 96 decreasing the median travel time residual from 110 to 25 ms the compressional velocity structure of the clipperton transform is characterized by anomalously low velocities about 1 0 km s lower than average beneath the median ridge and parallel troughs of the transform domain the low compressional velocities can be explained by an increased porosity due to fracturing of the oceanic crust we found crustal thicknesses of 5 6 5 9 km under the transform fault to produce the best fit of the pmp phase arrivals and pg pn crossovers since the crust is not thin beneath the transform parallel troughs and the velocity anomaly is not confined to the median ridge we find uplift by serpentinite diapirs unlikely as an explanation for the relief of the median ridge a median ridge that is the result of brittle deformation due to compression across the transform domain is however compatible with our results the upper crust is thicker to the north of the transform than to the south which is likely a consequence of the contrast in temperature structure of these two spreading segments

avo classification of lithology and pore fluids constrained by rock physics depth trends elastic properties of rocks are strongly influenced by local geological trends and can change markedly even within a sedimentary basin critical geologic factors that control elastic properties are either related to depositional environment or burial history knowledge about the expected change in seismic response as a function of depositional or compactional trends will increase the ability to predict hydrocarbons especially in areas with little or no well log information in other words by understanding the geologic constraints in an area of exploration one can reduce the range of expected variability in rock properties and hence reduce the uncertainties in seismic reservoir prediction figure 1 depicts this problem where we assume there is well log control only in the shallow interval on the shelf edge before extending the exploration into more deeply buried zones or to more distal deepwater environments it is important to understand the rock physics trends in the area

global positioning system measurements of deformations associated with the 1987 superstition hills earthquake evidence for conjugate faulting large station displacements observed from imperial valley global positioning system gps campaigns are attributed to the november 24 1987 superstition hills earthquake sequence thirty sites from a 42 station gps network established in 1986 have been reoccupied during 1988 and or 1990 displacements at three sites within 3 km of the surface rupture approach 0 5 m eight additional stations within 20 km of the seismic zone are displaced at least 10 cm this is the first occurrence of a large earthquake ms 6 6 within a preexisting gps network best fitting uniform slip models of rectangular dislocations in an elastic half space indicate 130 8 cm right lateral displacement along the northwest trending superstition hills fault and 30 10 cm left lateral displacement along the conjugate northeast trending elmore ranch fault the geodetic moments are 9 4 1025 dyn cm and 2 3 1025 dyn cm for the superstition hills and elmore ranch faults respectively consistent with teleseismic source parameters the data also suggest that postseismic slip along the superstition hills fault is concentrated at shallow depths distributed slip solutions using singular value decomposition indicate near uniform displacement along the elmore ranch fault and concentrated slip to the northwest and southeast along the superstition hills fault a significant component of nonseismic secular displacement is observed across the imperial valley which is attributed to interseismic plate boundary deformation

contrast in crustal structure across the clipperton transform fault from travel time tomography a three dimensional 3 d seismic refraction study of the clipperton transform fault northern east pacific rise reveals anomalously low compressional velocities from the seafloor to the moho we attribute this low velocity anomaly to intensive brittle deformation caused by transpression across this active strike slip plate boundary the seismic velocity structure south of the clipperton transform appears unaffected by these tectonic forces but to the north seismic velocities are reduced over 10 km outside the zone of sheared seafloor this contrast in seismic velocity structure corresponds well with the differences in mid ocean ridge morphology across the clipperton transform we conclude that the amount of fracturing of the upper crust which largely controls seismic velocity variations is strongly dependent on the shallow temperature structure at the ridge axis intermittent supply of magma to the shallow crust north of the clipperton transform allows seawater to penetrate deeper and the cooler crust is brittle to a greater depth than south of the transform where a steady state magma lens is known to exist the crustal thickness averages 5 7 km only slightly thinner than normal for oceanic crust and variations in moho depth in excess of 0 3 km are not required by our data the absence of large crustal thickness variations and the general similarity in seismic structure imply that a steady state magma lens is not required to form normal east pacific rise type crust perhaps a significant portion of the lower crust is accreted in situ from a patchwork of short lived gabbro sills or from ductile flow from a basal magma chamber as has been postulated in some recent ophiolite studies

seismic inversion combining rock physics and multiple point geostatistics a novel inversion technique combines rock physics and multiple point geostatistics the technique is based on the formulation of the inverse problem as an inference problem and incorporates multiple point geostatistics and conditional rock physics to characterize previously known geologic information the proposed implementation combines elements of sampling from conditional probabilities and elements of optimization the technique provides multiple solutions all consistent with the expected geology well log data seismic data and the local rock physics transformations a pattern based algorithm was selected as the multiple point geostatistics component rock physics principles are incorporated at the beginning of the process defining the links between reservoir properties e g lithology saturation and physical quantities e g compressibility density making it possible to predict situations not sampled by log data results for seismic lithofacies inversion on a synthetic test and a real data application demonstrate the validity and applicability of the proposed inversion technique

structure of the earth oceanic crust and uppermost mantle the past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere the application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere borehole seismic instrumentation has become more widely employed ocean bottom seismographs while generally decreasing in number have become significantly more reliable and useful and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes the techniques available for the analysis of seismic data have become more sophisticated waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models and travel time data are regularly inverted directly for structure trial and error modeling has become increasingly unimportant marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere this has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

lithology and fluid prediction from prestack seismic data using a bayesian model with markov process prior lithology and fluid prediction from prestack seismic data using a bayesian model with markov process prior

trace based and geostatistical inversion of 3 d seismic data for thin sand delineation an application in san jorge basin argentina argentina s san jorge basin which straddles the southern provinces of chubut and santa cruz in the heart of patagonia has been producing hydrocarbons since 1907 the region currently accounts for 32 of the country s production the existence and early geologic evolution of the basin is due to the same rift process responsible for the opening of the atlantic ocean in early jurassic times extensive direct faulting and local erosion accompanied the rift evolution thereby facilitating the accumulation of chiefly nonmarine terrigenous sediments well into the early cretaceous at this point andean tectonism became a major source of pyroclastic concentration in the sedimentary column it was also responsible for pervasive batholitic intrusions clastic deposition in the hydrocarbon producing zone is characterized by thick shale laminations of lacustrine and floodplain origin interspersed with much thinner and laterally sparse sand bodies the hydrocarbon reservoirs

palaeoecology and palaeobiogeography of the middle jurassic ostracods of southern tunisia abstract the results of a palaeoecological biostratigraphical and palaeobiogeographical analysis of jurassic ostracods from southern tunisia are presented the shallow to marginal marine deposits of bajocian bathonian krachoua and techout formations and callovian oxfordian age tataouine formation which accumulated under predominantly warm humid climatic conditions yielded diverse ostracod faunas from limnic brackish and marine environments which have been analysed statistically a qualitative cluster analysis yielded fourteen associations and two assemblages which are characterised by different percentages of marine brackish and freshwater species the distribution of the associations in the middle jurassic reflects temporal and spatial salinity gradients which are interpreted to reflect climatic variations humid arid and global eustatic sea level changes in the callovian the ostracods and macrofaunas probably also reflect minor relative sea level changes in accordance with other biostratigraphical data the ostracods support a bajocian bathonian age for the upper krachoua and the lower techout formations and a callovian age for the lower tataouine formation the palaeobiogeographical relationships suggest faunal exchange between southern tunisia and europe southern france sardinia england in the bajocian bathonian and an isolation of the south tunisian ostracod faunas from the european faunas in the callovian probably due to the deepening of the central western tethys predominantly in the bajocian bathonian but also in the callovian brackish conditions resulted in an endemicity of the ostracod faunas as a consequence of the eustatic sea level rise in the callovian species from the eastern north gondwana shelf israel jordan egypt saudi arabia respectively the ethiopian province invaded southern tunisia palaeobiogeographically the ostracods from southern tunisia are intermediate between the ostracods from the arabic northeast african region and northwest africa morocco

seismic velocity constraints on the material properties that control earthquake behavior at the quebrada discovery gofar transform faults east pacific rise 1 mid ocean ridge transform faults rtfs vary strongly along strike in their ability to generate large earthquakes this general observation suggests that local variations in material properties along rtfs exert a primary control on earthquake rupture dynamics we explore these relationships by examining the seismic structure of two rtfs that have distinctly different seismic coupling we determine the seismic velocity structure at the gofar and quebrada faults on the east pacific rise epr using p wave traveltime tomography with data from two active source wide angle refraction lines crossing the faults we image low velocity zones lvzs at both faults where p wave velocities are reduced by as much as 0 5 1 0 km s 10 20 within a several kilometer wide region at the gofar fault the lvz extends through the entire crust into the seismogenic zone we rule out widespread serpentinization as an explanation for the low velocities owing to the lack of a corresponding signal in the locally measured gravity field the reduced velocities can be explained if the plate boundary region is composed of fault material with enhanced fluid filled porosity 1 5 8 local seismic observations indicate that the high porosity region lies within a 10 km long portion of the fault that fails in large swarms of microearthquakes and acts as a barrier to the propagation of large m 6 0 earthquakes tomographic images of fault structure combined with observed earthquake behavior suggest that epr transform segments capable of generating large earthquakes have relatively intact gabbro within the seismogenic zone whereas segments that slip aseismically or via earthquake swarms are composed of highly fractured 2 km wide damage zones that extend throughout the crust

bayesian estimation in seismic inversion part i principles1 this paper gives a review of bayesian parameter estimation the bayesian approach is fundamental and applicable to all kinds of inverse problems its basic formulation is probabilistic information from data is combined with a priori information on model parameters the result is called the a posteriori probability density function and it is the solution to the inverse problem in practice an estimate of the parameters is obtained by taking its maximum well known estimation procedures like least squares inversion or l 1 norm inversion result depending on the type of noise and a priori information given due to the a priori information the maximum will be unique and the estimation procedures will be stable except in theory for the most pathological problems which are very unlikely to occur in practice the approach of tarantola and valette can be derived within classical probability theory the bayesian approach allows a full resolution and uncertainty analysis which is discussed in part ii of the paper

improved resolution in bayesian lithology fluid inversion from prestack seismic data and well observations part 2 real case study we have performed lithology fluid inversion based on prestack seismic data and well observations from a gas reservoir offshore norway the prior profile markov random field model captures horizontal continuity and vertical sequencing of the lithology fluid variables the prior model is also locally adjusted for spatially varying lithology fluid proportions the likelihood model is inferred from basic seismic theory and observations in wells an approximate posterior model is defined which can be simulated from by an extremely computer efficient algorithm the lithology fluid inversion results are compared to manual interpretations and evaluated by cross validation in one well moreover inversions based on simplified prior models are developed for comparative reasons both lithology fluid realizations and predictions look geologically reasonable the results seem to reflect general reservoir experience and information provided by the prestack seismic data and well observations the lithology fluid propor

seismicity and crustal structure in the orozco fracture zone project rose phase ii a total of 301 earthquakes were recorded in the vicinity of the orozco fracture zone by seven texas ocean bottom seismograph stations during the 2 week period of the rivera ocean seismic experiment rose phase ii using data from the entire rose array hypocenters of 50 earthquakes were determined these revealed two distinct zones of seismic activity within the fracture zone in addition to these earthquake families many very small events were detected by a station located very close to the spreading center of the east pacific rise the magnitudes of these earthquakes defined by their duration times were so small that most of them were recorded only at this station station 14 in continual or swarmlike occurrences the slope of the frequency magnitude distribution of these events is significantly larger than those of other earthquake groups detected during the experiment i e they appear to have an unusually high b value these results suggest that this new population of earthquakes may be associated with volcanic activity or stress release within highly fractured crustal material refraction studies in the fracture zone reveal the presence of a rather high velocity crustal layer 6 9 7 0 km s beneath the experiment zone the moho velocity and the crustal thickness are estimated at 7 8 km s and 6 2 km respectively

seismic inversion for reservoir properties combining statistical rock physics and geostatistics a review there are various approaches for quantitative estimation of reservoir properties from seismic inversion a general bayesian formulation for the inverse problem can be implemented in two different work flows in the sequential approach first seismic data are inverted deterministically or stochastically into elastic properties then rock physics models transform those elastic properties to the reservoir property of interest the joint or simultaneous work flow accounts for the elastic parameters and the reservoir properties often in a bayesian formulation guaranteeing consistency between the elastic and reservoir properties rock physics plays the important role of linking elastic parameters such as impedances and velocities to reservoir properties of interest such as lithologies porosity and pore fluids geostatistical methods help add constraints of spatial correlation conditioning to different kinds of data and incorporating subseismic scales of heterogeneities

petrophysical seismic inversion conditioned to well log data methods and application to a gas reservoir hydrocarbon reservoirs are characterized by seismic welllog and petrophysical information which is dissimilar in spatial distribution scale and relationship to reservoir properties we combine this diverse information in a unified inverse problem formulation using a multiproperty multiscale model linking properties statistically by petrophysical relationships and conditioningthemtowell logdata twoapproacheshelpus 1markov chainmontecarlosampling whichgeneratesmanyreservoir realizationsforestimatingmediumpropertiesandposteriormarginalprobabilities and2optimizationwithaleast squaresiterativetechniquetoobtainthemostprobablemodelconfiguration our petrophysical model applied to near vertical anglestacked seismicdataandwell logdatafromagasreservoir includesadeterministic component based on a combination of wyllie and wood relationships calibrated with the well log data and a random component based on the statistical characterization of the deviations of well log data from the petrophysical transform at the petrophysical level the effects of porosity and saturation on acoustic impedance are coupled conditioning the inversion to well log data helps resolve this ambiguity the combination of well logs petrophysics and seismic inversion builds on the correspondingstrengthsofeachtypeofinformation jointlyimproving 1 cross resolution of reservoir properties 2 vertical resolution of property fields 3 compliance to the smooth trend of property fields and 4 agreement with well log data at well positions

artificial neural network assisted prediction of dissolution spatial distribution in the volcanic weathered crust a case study from chepaizi bulge of junggar basin northwestern china abstract the reservoir rocks in the volcanic weathered crust are characterized by plentiful dissolution pores caverns and fractures however as an essential control factor of the reservoir distribution volcanic dissolution is often ignored in quantitative studies in this paper we propose a complete artificial intelligence workflow to model and predict the volcanic dissolution distribution by integrating the image logging semi quantitative analysis with seismic prediction a synthetic reservoir property named dcv dissolution comprehensive values is generated as logging curves from xrmi resistivity pseudo pictures to indicate the intensity of dissolution these curves are then used as the target for the seismic based prediction in an artificial intelligence framework a multi layer artificial neural network ann model is constructed in order to map the seismic attributes into the dcv the methodology is demonstrated on a real case study from the junggar basin in northwestern china the maps and profiles from the prediction represent the dissolution values in the volcanic weathered crust the predictions are consistent with the geological volcanic dissolution model provided by geological knowledge it is concluded that the spatial dissolution distribution in volcanic weathered crust can be reliably predicted by this integrated method

lateral velocity variations in the orozco transform fault inferred from observed incident angles and azimuths of p waves summary body waves from earthquake and explosive sources which were recorded by a three component ocean bottom seismometer obs in the orozco transform fault show pronounced systematic deflections of incident angles and azimuths from those expected for a laterally homogeneous velocity structure after considering several phenomena which could affect the observations we conclude that the observed pattern of deflections in part reflects regional lateral velocity variations possible models explaining the observations are examined by tracing rays through three dimensional velocity structures a model in which the moho is elevated beneath a north south trending topographic high to the west of the obs can explain observations from sources to the north west and south and is compatible with seismic refraction results in the region east of the obs a low velocity region simulating a magma chamber results in very steep incident angles and large azimuthal deflections which qualitatively match those observed from earthquakes in the central transform region although these data alone do not provide irrefutable evidence for a magma chamber that the corrections to fault plane solutions and hypocentres implied by this model can explain major inconsistencies in the source parameters calculated relative to a laterally homogeneous velocity model supports its validity

fast probabilistic petrophysical mapping of reservoirs from 3d seismic data abstracta fast probabilistic inversion method for 3d petrophysical property prediction from inverted prestack seismic data has been developed and tested on a real data set the inversion objective is to estimate the joint probability density function pdf of model vectors consisting of porosity clay content and water saturation components at each point in the reservoir from data vectors with compressional and shear wave impedance components that are obtained from the inversion of seismic data the proposed inversion method is based on mixture density network mdn which is trained by a given set of training samples and provides an estimate of the joint posterior pdf s of the model parameters for any given data point this method is much more time and memory efficient than conventional nonlinear inversion methods the training data set is constructed using nonlinear petrophysical forward relations and includes different sources of uncertainty in the inverse problem such as variations in effective pr

impact of rock physics depth trends and markov random fields on hierarchical bayesian lithology fluid prediction early assessments of petroleum reservoirs are usually based on seismic data and observations in a small number of wells decision making concerning the reservoir will be improved if these data can be integrated and converted into a lithology fluid map of the reservoir we analyze lithology fluid prediction in a bayesian setting based on prestack seismic data and well observations the likelihood model contains a convolved linearized zoeppritz relation and rock physics models with depth trends caused by compaction and cementation well observations are assumed to be exact the likelihood model contains several global parameters such as depth trend wavelets and error parameters the inference of these is an integral part of the study the prior model is based on a profile markov random field parameterized to capture different continuity directions for lithologies and fluids the posterior model captures prediction and model parameter uncertainty and is assessed by markov chain monte carlo simulation base

axial intrusion zone beneath the median valley of the mid atlantic ridge at 37 n detected by explosion seismology summary four seismic refraction lines three of which had shots every 250 m were shot across along and parallel to the median valley of the mid atlantic ridge at 37 n a method has been developed for calculating the effect on the travel times of the rough sea floor relief beneath the profiles and has been used to correct all the travel times for this effect most arrivals were from a main refractor of apparent velocity 5 4 to 6 3 km s 1 only beyond 35 km were faster arrivals observed from an 8 09 36 km s 1 refractor the main refractor corresponds in depth at least approximately to the top of layer 3 of the ocean basins but its velocity is significantly less than normal for layer 3 perhaps due to dip a study of time residuals along two profiles across the median valley indicates the presence of a 2 to 3 km wide low velocity zone about 3 2 km s 1 beneath the median valley floor this zone extends over the upper 2 5 km of the crust and is believed to represent a zone of intrusion through which magma passes on its way to the sea floor

oceanic transform earthquakes with unusual mechanisms or locations relation to fault geometry and state of stress in the adjacent lithosphere on oceanic transforms most earthquakes are expected to occur on the principal transform displacement zone ptdz and to have strike slip mechanisms consistent with transform parallel motion we conducted a search for transform earthquakes departing from this pattern on the basis of source mechanisms and locations taken from the harvard centroid moment tensor catalogue and the bulletin of the international seismological centre respectively events with unusual mechanisms occur on several transforms we have determined the source mechanisms and centroid depths of 10 such earthquakes on the st paul s marathon owen heezen tharp menard and rivera transforms from inversions of long period body waveforms relative locations of earthquakes along these transforms have been determined with a multiple event relocation technique much of the anomalous earthquake activity on oceanic transforms is associated with complexities in the geometry of the ptdz or the presence of large structural features that may influence slip on the fault reverse faulting earthquakes occur at a compressional bend in the owen transform in the area of mount error and at the st paul s transform near st peter s and st paul s rocks a normal faulting earthquake on the heezen transform is located at the edge of a pull apart basin marking an extensional offset of the fault normal faulting earthquakes along the tharp menard and rivera transforms may also be related to extensional offsets some events with unusual mechanisms occur outside of the transform fault zone however and do not appear to be related to fault zone geometry for instance earthquakes with mechanisms indicating reverse faulting on ridge parallel fault planes are located near the ridge transform intersections of the st paul s and the marathon transforms possible additional contributors to the occurrence of anomalous earthquakes include recent changes in plate motion differential lithospheric cooling and the development of a zone of weakness along the fault zone but we do not find strong evidence to confirm the influence of these processes

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

bayesian lithology fluid prediction and simulation on the basis of a markov chain prior model a technique for lithology fluid lf prediction and simulation from prestack seismic data is developed in a bayesian framework the objective is to determine the lf classes along 1d profiles through a reservoir target zone a stationary markov chain prior model is used to model vertical continuity of lf classes along the profile the likelihood relates the lf classes to the elastic properties and to the seismic data and it introduces vertical correlation because the seismic data are band limited an approximation of the likelihood model provides an approximate posterior model that is a markov chain the approximate posterior can be assessed by an exact and efficient recursive algorithm the lf inversion approach is evaluated on a synthetic 1d profile that is inspired by a north sea sandstone reservoir with a realistic wavelet colored noise model and a s n ratio of three in the seismic data the results are reliable the lf classes and the interfaces between zones are largely correct the prediction unce

geostatistical inversion of prestack seismic data for the joint estimation of facies and impedances using stochastic sampling from gaussian mixture posterior distributions abstractwe have developed a seismic inversion method for the joint estimation of facies and elastic properties from prestack seismic data based on a geostatistical approach the objectives of our inversion methodology are to sample from the posterior distribution of seismic properties and to simultaneously classify the lithology conditioned by seismic data the inversion algorithm is a sequential gaussian mixture inversion based on bayesian linearized amplitude variation with offset inverse theory and sequential geostatistical simulations the stochastic approach to the inversion allows generating multiple elastic models that match the seismic data to mathematically represent the multimodal behavior of elastic properties due to their variations within different lithologies we adopt a gaussian mixture distribution for the prior model of the elastic properties and we use the prior probability of the facies as weights of the gaussian components of the mixture the solution of the inverse problem is achieve

gravity lineaments of the cocos plate evidence for a thermal contraction crack origin lineaments in the gravity field with wavelengths of 100 200 km affect the south central pacific because they align with absolute plate motion it has been proposed that they reflect small scale convection cells beneath the lithosphere that become elongated by basal shear alternatively it was suggested that they reflect channelized flow of low viscosity material following the base of the lithosphere toward the east pacific rise or that they result from lithospheric scale thermal contraction cracks here we report about previously undetected gravity lineaments across the cocos plate similarly to the south central pacific lineaments the cocos lineaments affect a plate that is anomalously shallow with seamounts aligning mostly within their troughs however the cocos lineaments strike markedly oblique to absolute plate motion and follow instead trajectories that are perpendicular to seafloor isochrons a characteristic best explained by the thermal contraction crack model the presence of steep scarps at the base of seamounts and the seismic imaging of faults striking perpendicular to isochrons further support this interpretation assuming that the slow subsidence rates of the south central pacific and cocos plates reflect a warmer upper mantle we propose that the associated thinner elastic plates favor the formation of thermal contraction cracks a thinner elastic plate may also explain the pattern of ridge propagation in both areas at large ridge offsets with a history of steady migration the propagating segments have been those cutting into the shallower flanks consistent with the concept that a warmer thinner plate is more easily cracked

bayesian lithology and fluid prediction from seismic prestack data a fast bayesian inversion method for 3d lithology and fluid prediction from prestack seismic data and a corresponding feasibility analysis were developed and tested on a real data set the objective of the inversion is to find the probabilities for different lithology fluid classes from seismic data and geologic knowledge the method combines stochastic rock physics relations between the elastic parameters and the different lithology fluid classes with the results from a fast bayesian seismic simultaneous inversion from seismic data to elastic parameters a method for feasibility analysis predicts the expected modification of the prior probabilities to posterior probabilities for the different lithology fluid classes the feasibility analysis can be carried out before the seismic data are analyzed both the feasibility method and the seismic lithology fluid probability inversion were applied to a prospect offshore norway the analysis improves the probability for gas sand from 0 1 to about 0 2 0 4 with s

a monte carlo approach to the joint estimation of reservoir and elastic parameters from seismic amplitudes inversion of seismic data and quantification of reservoir properties such as porosity lithology or fluid saturation are commonly executed in two consecutive steps a geophysical inversion to estimate the elastic parameters and a petrophysical inversion to estimate the reservoir properties we combine within an integrated formulation the geophysical and petrophysical components of the problem to estimate the elastic and reservoir properties jointly we solve the inverse problem following a monte carlo sampling approach which allows us to quantify the uncertainties of the reservoir estimates accounting for the combination of geophysical data uncertainties the deviations of the elastic properties from the calibrated petrophysical transform and the nonlinearity of the geophysical and petrophysical relations we implement this method for the inference of the total porosity and the acoustic impedance in a reservoir area combining petrophysical and seismic information in our formulation the porosity and impedance are related with a statistical model based on the wyllie transform calibrated to well log data we simulate the seismic data using a convolutional model and evaluate the geophysical likelihood of the joint porosity impedance models applying the monte carlo sampling method we generate a large number of realizations that jointly explain the seismic observations and honor the petrophysical information this approach allows the calculation of marginal probabilities of the model parameters including medium porosity impedance and seismic source wavelet we show a synthetic validation of the technique and apply the method to data from an eastern venezuelan hydrocarbon reservoir satisfactorily predicting the medium stratification and adequate correlation between the seismic inversion and well log estimates for total porosity and acoustic impedance

velocity structure from forward modeling of the eastern ridge transform intersection area of the clipperton fracture zone east pacific rise in the spring of 1994 we undertook an extensive geophysical study of the clipperton fracture zone fz on the fast spreading east pacific rise the clipperton area seismic study to investigate compensation experiment classic included surveys to examine the deep structures associated with the fracture zone and adjacent northern ridge segment in this paper we report the results from five seismic profiles acquired over the eastern ridge transform intersection rti including profiles over the rti high the northern ridge segment and the eastern transform region the travel time data for crustal phases moho reflections and mantle phases were modeled using two dimensional ray tracing seismic profiles reveal that the crust is similar in thickness north and south of the clipperton fz despite differences in axial topography that have previously been interpreted in terms of differences in magma supply when compared to older crust the northern ridge axis is characterized by lower seismic velocities and higher attenuation in our model a low velocity zone exists beneath the ridge axis probably associated with a zone of partial melt and or very high temperatures within the transform zone we find that the southeastern trough is underlain by nearly normal crustal structure the crust is slightly thinner than the adjacent aseismic extension but not enough to compensate for the depths of the trough toward the rti the trough is replaced by an intersection high which appears underlain by a thickened crust and a thicker upper crustal section both characteristics indicate that the intersection high is a volcanic feature produced by excess volcanism at the intersection the volcanism acts to fill in the transform trough creating the thicker crust that extends under the eastern aseismic extension of the transform our results show that the northern ridge segment often identified as magma starved displays the crustal thickness and apparent signal attenuation characteristic of a plentiful but perhaps episodic magma supply

bayesian wavelet estimation from seismic and well data a bayesian method for wavelet estimation from seismic and well data is developed the method works both on stacked data and on prestack data in form of angle gathers the seismic forward model is based on the convolutional model where the reflectivity is calculated from the well logs possible misties between the seismic traveltimes and the time axis of the well logs errors in the log measurements and seismic noise are included in the model the estimated wavelets are given as probability density functions such that uncertainties of the wavelets are an integral part of the solution the solution is not analytically obtainable and is therefore computed by markov chain monte carlo simulation an example from sleipner field shows that the estimated wavelet has higher amplitude compared to wavelet estimation where well log errors are neglected and the uncertainty of the estimated wavelet is lower

stochastic inversion of facies from seismic data based on sequential simulations and probability perturbation method abstractwe presented a new methodology for seismic reservoir characterization that combined advanced geostatistical methods with traditional geophysical models to provide fine scale reservoir models of facies and reservoir properties such as porosity and net to gross the methodology we proposed was a stochastic inversion where we simultaneously obtained earth models of facies rock properties and elastic attributes it is based on an iterative process where we generated a set of models of reservoir properties by using sequential simulations calculated the corresponding elastic attributes through rock physics relations computed synthetic seismograms and finally compared these synthetic results with the real seismic amplitudes the optimization is a stochastic technique the probability perturbation method that perturbs the probability distribution of the initial realization and allows obtaining a facies model consistent with all available data through a relatively small number of iterations the pr

magmatism versus serpentinization crustal structure along the 13 n segment at the mid atlantic ridge a region of oceanic core complexes occs exists at 13 n on the mid atlantic ridge that is regarded as a type site this site includes two occs at 13 20 n and 13 30 n thought to be in the active and dying stages of evolution and two together called the ashadze complex centred at 13 05 n that are considered to be relict here we describe the results of s wave seismic modelling along an 200 km long 2 d transect traversing south to north through both the mercurius and marathon fracture zones the southern outside corner of the 13 n segment the occs the ridge axis deviation in trend centred at 13 35 n and the youngest oceanic crust of the eastern ridge flank to the north our inversion model and the corresponding vp vs ratio show that the majority of the crust beneath the 13 30 n occ comprises metamorphosed lithologies that have been exhumed to the shallowest subseabed level while basaltic lithologies underlie the 13 20 n occ the transition between these contrasting crustal structures occurs over a distance of 1 9 and equivalent poisson s ratio of 0 3 indicates exhumed and or metamorphosed lithologies beneath the bathymetric depression between them and within the crust beneath the southern occ between the northern and southern flanks of the marathon fracture zone and northern flank of mercurius fracture zone the lower crust has a relatively low vp vs ratio suggesting that the deformation associated with marathon fracture zone which facilitates fluid ingress extends laterally within the lower crust marathon fracture zone itself is underlain by a broad zone of low s wave velocity 2 0 km s 1 up to 20 km wide from the seabed to at least the mid crust that is mirrored in a high vp vs ratio and lower density particularly deeper than 1 km below seabed within its bathymetric footprint volcanic domains are highlighted by a low vp vs ratio of 1 6 and equivalent poisson s ratio of 0 15 our combined seismic and density models favour the localized model of occ evolution they also show a considerable ridge parallel variability in the amount and distribution of magmatic versus metamorphosed crust our results suggest that the current focus of magmatism lies to the north of the 13 20 n occ where the magmatic accretion type seabed morphology observed is mirrored in the pattern of microseismicity suggesting that its inward facing median valley wall fault may link to the 13 20 n occ detachment surface magmatism and active faulting behind to the west the footwall breakaway of the 13 30 n occ and the microseismicity concentrated in a band along its southern flank suggest a readjustment of ridge geometry along axis is underway as part of this a transform offset is forming that will ultimately accommodate the 13 30 n occ in its inside corner on the eastern flank of the ridge axis to the north

joint avo inversion wavelet estimation and noise level estimation using a spatially coupled hierarchical bayesian model the main objective of the avo inversion is to obtain posterior distributions for p wave velocity s wave velocity and density from specified prior distributions seismic data and well log data the inversion problem also involves estimation of a seismic wavelet and the seismic noise level the noise model is represented by a zero mean gaussian distribution specified by a covariance matrix a method for joint avo inversion wavelet estimation and estimation of the noise level is developed in a bayesian framework the stochastic model includes uncertainty of both the elastic parameters the wavelet and the seismic and well log data the posterior distribution is explored by markov chain monte carlo simulation using the gibbs sampler algorithm the inversion algorithm has been tested on a seismic line from the heidrun field with two wells located on the line the use of a coloured seismic noise model resulted in about 10 lower uncertainties for the p wave velocity s wave velocity and density compared with a white noise model the uncertainty of the estimated wavelet is low in the heidrun example the effect of including uncertainty of the wavelet and the noise level was marginal with respect to the avo inversion results

crustal structure in the orozco transform zone seismic refraction data from lines of small shots and from individual large shots recorded during project rose are used to determine the crust and upper mantle velocity structure in the orozco transform zone the objective of this study is to provide constraints on the origin of a deep north south trending trough flanked by two shallow ridges these topographic features are incompatible with the present plate tectonic framework as defined by the regional spreading direction approximately east west and did not display any seismic activity during project rose travel times and amplitudes from reversed profile along one of the ridges reveal an anomalous crustal structure a velocity increase approximating a gradient of 0 5 s 1 extending through most of the crust and decreasing to 0 1 s 1 at the base and a total crustal thickness of about 5 4 km in contrast three profiles from topographically normal oceanic crust indicate a crustal structure typical of oceanic crust throughout the oceans a 1 5 km thick layer in which velocity increases from about 3 km s 1 to 6 7 km s 1 overlying a 4 5 km thick layer with a velocity of about 6 8 km s 1 the upper mantle velocity beneath both structures determined from travel times from large shots at ranges of 30 100 km is about 8 1 km s 1 these results are interpreted to indicate that the trough and flanking ridges represent an extinct spreading center that developed an axial valley as the spreading center progressively shifted to the present axis of the east pacific rise

distribution and palaeoecology of middle jurassic foraminifera from the lower inferior oolite of the cotswolds abstract foraminifera extracted from limestones of the lower inferior oolite scissum murchisonae zones are analysed in terms of the sample correlation coefficient r of species pairs and the limestone petrography in each limestone sample microfaunal composition is related to 1 grain size 2 the proportion of lime mud to spar cement in the matrix 3 allochem composition the nature of these inter relationships provides a basis for determining assemblages most likely to represent a biocoenosis three palaeocommunities are reconstructed 1 primary weed fauna 2 weed sediment fauna 3 interstitial reef fauna each community was likely to have been strongly influenced by the availability of phytal or non phytal habitats in common with recent carbonate foraminifera a morphogenetic study of lenticulina muensteri roemer demonstrates that variation was generated within numerous geometrically distinct growth series where uncoiling could proceed systematically for a given coil diameter variants were subject to postmortem size geometry sorting the affinity and evolutionary development of lower inferior oolite foraminifera are evaluated by comparison with assemblages described from other locations and stratigraphic levels in nw europe the appearance of similar faunas at the lower middle jurassic boundary is correlated with the simultaneous development of shallow marine environments which favoured species capable of adapting to both phytal and non phytal micro habitats

comparisons of gravity anomalies at pseudofaults fracture zones and nontransform discontinuities from fast to slow spreading areas published mechanisms for rift tip propagation at spreading centers include extensional deformation and an initial period of slow spreading we investigate whether the gravity signal and inferred crustal structure at pseudofaults formed in medium to superfast spreading environments resemble the gravity signal at fracture zones or nontransform discontinuities formed in slow spreading environments we find that altimetry based gravity anomalies on the mathematician bauer easter juan fernandez and northern chile ridge pseudofaults located in 75 150 mm yr full rate seafloor spreading environments are similar in amplitude and form to atlantic fracture zones with 20 30 mm yr spreading rates a 5 15 mgal positive mantle bouguer anomaly is observed on the pseudofault bounding the eastern juan fernandez microplate comparable to those at some similar age offset nontransform discontinuities in slow spreading environments our results suggest that the deeps associated with active propagating rift tips result from both a dynamic mantle component and anomalous crust the latter of which remains frozen at pseudofaults we predict that any pseudofaults with age offsets more than 1 m y and not coincident with hotspot volcanism will be associated with thin and possibly unusually dense crust even in superfast seafloor spreading environments

seismic crustal structure and morphotectonic features associated with the chain fracture zone and their role in the evolution of the equatorial atlantic region oceanic transform faults and fracture zones fzs represent major bathymetric features that keep the records of past and present strike slip motion along conservative plate boundaries although they play an important role in ridge segmentation and evolution of the lithosphere their structural characteristics and their variation in space and time are poorly understood to address some of the unknowns we conducted interdisciplinary geophysical studies in the equatorial atlantic ocean the region where some of the most prominent transform discontinuities have been developing here we present the results of the data analysis in the vicinity of the chain fz on the south american plate the crustal structure across the chain fz at the contact between 10 and 24 ma oceanic lithosphere is sampled along seismic reflection and refraction profiles we observe that the crustal thickness within and across the chain fz ranges from 4 6 5 9 km which compares with the observations reported for slow slipping transform discontinuities globally we attribute this presence of close to normal oceanic crustal thickness within fzs to the mechanism of lateral dike propagation previously considered to be valid only in fast slipping environments furthermore the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower order mid atlantic ridge segmentation around the equator

the correction for the travel time effects of seafloor topography in the interpretation of marine seismic data problems associated with the application of topographic corrections to deep ocean marine seismic data are described synthetic travel time data are used to show that corrections for the delay time introduced by the water column are inferior to corrections for the true water path water path corrections place the shots at the ray entry points on the seafloor require no assumptions about the structure under investigation and maintain the correct distribution of data with range the water path correction can be most accurately applied during the stacking operation that transposes the observed data into intercept time slowness space there is then no requirement to make determinations of slowness and for experiments involving seafloor receiver and surface shots the data may be transposed to the seafloor with complete objectivity water path corrected stacks from different locations may then be compared in the sure knowledge that any observed differences are due to real structural changes and not to topographic correction errors or to invalid assumptions of lateral homogeneity

upper crustal velocity structure in the rose area of the east pacific rise we describe a method to constrain the seismic velocity structure of the upper 500 800 m of young oceanic crust conventional interpretations of seismic refraction profiles provide little or no information about this shallowmost crust the method described here requires the assumption of a linear velocity gradient and utilizes measured values of range travel time and slowness of the observed refracted waves results are reported from several refraction profiles that sample crust between 0 and 4 m y in age on the flanks of the east pacific rise at 12 n these data give an average gradient in the uppermost 800 m of the crust of 3 5 s 1 and a velocity at the seafloor of 2 5 km s ray path and travel time modeling show that the refracted waves sampling the uppermost part of the crust are effectively masked by reflections from the seafloor

earthquakes in the orozco transform zone seismicity source mechanisms and tectonics as part of the rivera ocean seismic experiment a network of ocean bottom seismometers and hydrophones was deployed in order to determine the seismic characteristics of the orozco transform fault in the central eastern pacific we present hypocentral locations and source mechanisms for 70 earthquakes recorded by this network all epicenters are within the transform region of the orozco fracture zone and clearly delineate the active plate boundary about half of the epicenters define a narrow line of activity parallel to the spreading direction and situated along a deep topographic trough that forms the northern boundary of the transform zone region 1 most focal depths for these events are very shallow within 4 km of the seafloor several well determined focal depths however are as great as 7 km no shallowing of seismic activity is observed as the rise transform intersection is approached to the contrary the deepest events are within 10 km of the intersection first motion polarities for most of the earthquakes in region 1 are compatible with right lateral strike slip faulting along a nearly vertical plane striking parallel to the spreading direction another zone of activity is observed in the central part of the transform region 2 the apparent horizontal and vertical distribution of activity in this region is more scattered than in the first and the first motion radiation patterns of these events do not appear to be compatible with any known fault mechanism pronounced lateral variations in crustal velocity structure are indicated for the transform region from refraction data and measurements of wave propagation directions the effect of this lateral heterogeneity on hypocenters and fault plane solutions is evaluated by tracing rays through a three dimensional velocity grid while findings for events in region 1 are not significantly affected in region 2 epicentral mislocations of up to 10 km and azimuthal deflections of up to 45 may result from assuming a laterally homogeneous velocity structure when corrected for the effects of lateral heterogeneity the epicenters and fault plane solutions for earthquakes in region 2 are compatible with predominantly normal faulting along a topographic trough trending nw se the focal depths however are poorly constrained these results suggest an en echelon spreading center or leaky transform regime in the central transform region

regional scale integration of multiresolution hydrological and geophysical data using a two step bayesian sequential simulation approach significant progress has been made with regard to the quantitative integration of geophysical and hydrological data at the local scale for the purpose of improving predictions of groundwater flow and solute transport however extending corresponding approaches to the regional scale still represents one of the major challenges in the domain of hydrogeophysics to address this problem we have developed a regional scale data integration methodology based on a two step bayesian sequential simulation approach our objective is to generate high resolution stochastic realizations of the regional scale hydraulic conductivity field in the common case where there exist spatially exhaustive but poorly resolved measurements of a related geophysical parameter as well as highly resolved but spatially sparse collocated measurements of this geophysical parameter and the hydraulic conductivity to integrate this multi scale multi parameter database we first link the low and high resolution geophysical data via a stochastic downscaling procedure this is followed by relating the downscaled geophysical data to the high resolution hydraulic conductivity distribution after outlining the general methodology of the approach we demonstrate its application to a realistic synthetic example where we consider as data high resolution measurements of the hydraulic and electrical conductivities at a small number of borehole locations as well as spatially exhaustive low resolution estimates of the electrical conductivity obtained from surface based electrical resistivity tomography the different stochastic realizations of the hydraulic conductivity field obtained using our procedure are validated by comparing their solute transport behaviour with that of the underlying true hydraulic conductivity field we find that even in the presence of strong subsurface heterogeneity our proposed procedure allows for the generation of faithful representations of the regional scale hydraulic conductivity structure and reliable predictions of solute transport over long regional scale distances

improved resolution in bayesian lithology fluid inversion from prestack seismic data and well observations part 1 methodology the focus of our study is lithology fluid inversion with spatial coupling from prestack seismic amplitude variation with offset avo data and well observations the inversion is defined in a bayesian setting where the complete solution is the posterior model the prior model for the lithology fluid lf characteristics is defined as a profile markov random field model with lateral continuity each vertical profile is further given as an inhomogeneous markov chain model upward through the reservoir the likelihood model is defined by profile and it relates the lf characteristics to the seismic data via a set of elastic material parameters and a convolution model the likelihood model is approximated the resulting approximate posterior model is explored using an efficient block gibbs simulation algorithm the inversion approach is evaluated on a synthetic realistic 2d reservoir seismic avo data and well observations are integrated in a consistent manner to obtain predictions of the lf characteristics wi

stochastic inversion by matching to large numbers of pseudo wells abstractwe have developed a 1d stochastic algorithm for estimating reservoir properties based on matching large numbers of pseudo wells to seismic angle stacks the pseudo wells are part deterministic and part stochastic 1d stratigraphic profiles with consistent elastic and reservoir properties pseudo wells are sampled from a prior distribution defined by the geological interpretation a rock physics model and a model for the vertical statistics that provides close control of the lithofacies proportions a new set of pseudo wells typically 103 104 tied to the local stratigraphy is constructed for each seismic trace synthetics derived from the pseudo wells using extended elastic impedance are matched to either one or two seismic angle stacks and the best matches are selected and averaged to provide a joint estimate of reservoir properties and impedances and the associated uncertainties the algorithm has been tested on a number of data sets and validated by blind well ties the algorithm is 1d with