tropical atlantic coral oxygen isotopes glacial interglacial sea surface temperatures and climate change abstract we have generated a detailed oxygen isotope time series from the fossil coral reefs from offshore barbados the barbados coral based record is a unique paleoceanographic data set with an equivalent sedimentation rate in excess of 600 cm kyr where not only is the annual signal uniquely preserved but seasonality as well oxygen isotope values during the late glacial and lgm 16 20 14c kyrs 19 24 calendar kyrs are 2 3 heavier than corresponding living specimens and indicate a regional cooling on the order of 4 5 c there is also an isotopic expression of the bolling pre boreal climate oscillation with values reflecting a cooling during the younger dryas seasonality a key diagnostic of the state of the climate system remained the same or slightly less than present and indicates that the observed coolings were a change in the mean state of the western tropical atlantic pan tropic cooling during the last glaciation is best explained by a change in the radiative balance of the tropics variable tropical sea surface temperatures during climate oscillations such as the younger dryas challenge the paradigm that climate change only cascades from variations in north atlantic deep water production

petroleum systems of the adiyaman region southeastern anatolia turkey abstract the petroliferous cretaceous sequence in the adiyaman region has been identified as a continental platform type system several source rocks exist within the sequence including shales mudstones and carbonates of the derdere karababa and karabogaz formations these source rocks contain mostly an oil prone type ii kerogen in the adiyaman region most oil is produced primarily from pores largely secondary in the karababa dolomitic wackestones and derdere dolomites oil is preserved in structural traps formed during the late cretaceous late miocene deformations and in stratigraphic traps prior to re migrating into late structural traps critical processes controlling the hydrocarbon accumulations occurred from the upper cenomanian lower turonian to aquitanian 93 22 ma the mass of petroleum expelled from the source rocks ranges from 4 0 to 10 3 mg hc g rock calculated fractional conversion and expulsion efficiency values of these source rocks have been found to be about 0 60 and 80 respectively the data presented in this study point to apparently low expelled efficiency values in the area since they also indicate that a significant amount of hydrocarbons have been lost due to exposure as the result of uplift or have escaped been expelled from the source rocks i e generated and are preserved somewhere awaiting discovery

island wakes and headland eddies a comparison between remotely sensed data and laboratory experiments island wakes and headland eddies in coastal waters are identified from spaceborne and airborne remotely sensed imagery in the visible wavelengths the imagery were obtained for the bristol and english channels united kingdom which are characterized by high tidal currents and levels of turbidity suspended matter in the surface waters is used as a passive tracer for the flow features scaling parameters e g the reynolds ekman and rossby numbers obtained from the depth averaged equation of motion are presented based upon information on oceanographic conditions at the times of the overpasses the parameters are compared then with data obtained from laboratory model investigations presented by other experimenters in the published literature the remotely sensed data demonstrate that in coastal waters with nonuniform obstacles and bathymetry for a given ekman number the flow regimes occur at a lower rossby number than might be expected from laboratory experiments in the absence of a method for the accurate determination of the horizontal eddy viscosity hence the reynolds number it is concluded that an island wake parameter wolanski et al 1984a should be adopted to describe such features in coastal waters

synthetic pn and sn phases and the frequency dependence of q of oceanic lithosphere the oceanic lithosphere is an extremely efficient waveguide for high frequency seismic energy in particular the propagation of the regional to teleseismic oceanic pn and sn phases is largely controlled by properties of the oceanic plates the shallow velocity gradient in the sub moho lithosphere results in a nearly linear travel time curve for these oceanic phases and an onset velocity near the material velocity of the uppermost mantle the confinement of pn sn to the lithosphere imposes a constraint on the maximum range that a normally refracted wave can be observed the rapid disappearance of sn and the discontinuous drop in pn sn group velocity beyond a critical distance dependent upon the local thickness of the lithosphere are interpreted as a shadowing effect of the low q asthenosphere wave number integration was used to compute complete synthetic seismograms for a model of oceanic lithosphere the results were compared to data collected during the 1983 ngendei seismic experiment in the southwest pacific the pn sn coda is successfully modeled as a sum of leaky organ pipe modes in the sediment layer and oceanic water column while scattering is present to some degree it is not required to explain the long duration and complicated nature of the pn sn wave trains the presence of extremely high frequencies in pn sn phases and the greater efficiency of sn than pn propagation are interpreted in terms of an absorption band rheology a shorter high frequency relaxation time for p waves than for s waves results in a rheology with the property that q q at low frequency while q q at high frequency consistent with the teleseismic pn sn observations the absorption band model is to viewed as only an approximation to the true frequency dependence of q in the oceanic lithosphere for which analytic expressions for the material dispersion have been developed

surface currents in the caribbean sea as deduced from lagrangian observations the results of a satellite tracked drifting buoy experiment conducted in the caribbean sea from october 1975 through june 1976 are presented the buoy trajectories suggest that the aves rise beata ridge and nicaraguan rise are regions where considerable mesoscale variability occurs in the surface flow the variability is in the form of current deflections small diameter eddies ranging in size from 20 to 100 km large amplitude meanders with amplitudes greater than 150 km and large diameter eddies with diameters greater than 200 km which form on and near these rises and ridges in the central caribbean where the topographic relief is not as dramatic the flow is predominantly zonal intensifications in the surface flow occur off the coast of colombia south of the nicaraguan rise and south of the yucatan strait

mapping climatological seasonal variations of surface currents in the tropical atlantic using ship drifts the seasonal variability of current velocities in the tropical atlantic was studied by grouping ship drift velocity observations into 2 5 boxes and calculating monthly mean velocity values these values were used to calculate and map the annual mean velocity the seasonal variation about the mean the annual and semiannual harmonics and the first two empirical orthogonal functions eofs the seasonal variation is primarily zonal in the equatorial band and in the north equatorial countercurrent necc and primarily alongshore near the coast of south america maxima of seasonal variation of 23 cm s are centered in the necc near 6 n 42 5 w and in the gulf of guinea near 2 n 7 5 w most 80 of the variance in the necc and along the western boundary of the area studied has an annual period most of the variance along the equator in the mid atlantic has a semiannual period over the whole region 49 of the seasonal variance is explained by the annual harmonic and 69 is explained by a combination of the annual and semiannual harmonics the second eof contains 29 of the variance of the data set and shows a simultaneous speeding slowing of the major equatorial currents the south equatorial current sec the north brazil current the necc and the guinea current along their principal axes of variation with a concurrent slowing speeding of the guyana current and the brazil current the pattern of variation resembles a large equatorial gyre centered along 4 n that flows clockwise from may to september and counterclockwise most of the rest of the year the first eof accounts for 35 of the variance and shows a simultaneous speeding slowing of eastward velocity from 5 s to 10 n except in the western sec the western end of the north brazil current and the guinea current the out of phase fluctuations apparent in the western sec near the equator are explained by the slowing and reversal of the south equatorial current near 2 n 40 w between april and june this slowing of the sec is associated with the start up of the necc which accelerates eastward in may june and july and flows eastward across the atlantic from july through december the sec divides seasonally near the eastern tip of brazil where residual alongshore velocities are northward for half the year peaking during may and june and southward for the other half of the year a second division can be seen north of the amazon where the north brazil current continues up the coast into the guyana current during the first half of the year but partially retroflects into the necc during the last half of the year a fast eastward flow in the necc coincides with below average velocities in the guyana current and above average velocities in the western sections of the north brazil current and the sec north of the equator

lagrangian flow observations of the amazon river discharge into the north atlantic nine satellite tracked drifters with near surface drogues were launched in or near the mouth of the amazon river on the north brazil shelf during 1989 1991 at four different stages of river discharge the drifters initially moved along shelf toward the northwest over the north brazil shelf with mean speeds varying from 41 to 128 cm s and a maximum along shelf velocity of 197 cm s many drifters experienced strong acceleration near 2 n between the 5 m and 20 m isobaths caused in part by the sharp reduction in cross shelf cross sectional area near the shore perpendicular cabo norte shoal the drifter trajectories showed strong cross shelf tidal variability in the flow field near the amazon river mouth and the largest low frequency along shelf variability was located over the open shelf just north of cabo norte good visual correlation between drifter along shelf velocity and along shelf wind stress supports the dynamical model of lentz this issue that subtidal fluctuations in near surface along shelf currents in the amazon river plume are strongly wind driven drifters left the north brazil shelf after about 5 to 30 days three drifters moved northwestward parallel to the northeast coast of south america crossed the shelf break near 8 n and moved into deeper north atlantic water where they began looping in anticyclonic eddies often called retroflection eddies or loopers with periods of approximately 11 days as they moved northwestward to the caribbean sea four drifters initially moved along shelf toward the northwest and then offshore between 3 n and 7 n where they followed the retroflection of the north brazil current only one drifter entered the zonal current system that characterizes the equatorial atlantic circulation

measurements of tidal flow around a headland with a shipboard acoustic doppler current profiler a shipboard acoustic doppler current profiler adcp and moored current meters were used to obtain detailed measurements of the spatial structure of the tidal flow around a headland in vineyard sound massachusetts where tidal currents typically range from 50 to 70 cm s 1 eight shipboard surveys were conducted each of which followed one of five trapezoidal tracks in the vicinity of the headland completing 11 12 circuits in the course of the tidal cycle 12 4 hours the measurements from the adcp compare favorably with moored velocity measurements at two locations rms deviations of 3 cm s 1 but the comparison showed more scatter at two other sites rms deviations of 7 cm s 1 the adcp measurements from separate cruises were merged to form a composite spatial representation of the tidal and residual currents providing resolution of the spatial structure of the flow around the headland at scales from several hundred meters to 10 km the semidiurnal tidal flow is relatively uniform in space roughly following the bathymetry around the headland while a residual current field consists of counterrotating eddies on either side of the headland with spatial scales of 5 8 km and velocities as high as 25 cm s 1 the instantaneous current field indicates the formation of transient eddies on the downstream side of the headland during flood and ebb with clear evidence of flow separation near the tip of the headland

beyond the petroleum system geohorizons the first joint aapg amgp association of mexican petroleum geologists hedberg conference on geologic aspects of petroleum systems was held october 2 6 1994 in mexico city mexico this research conference attracted nearly 150 geoscientists from 15 countries 41 papers and 27 posters were presented the opposite response occurred when the petroleum system concept was presented more than 20 yr ago it was largely overlooked in the past decade however interest in this concept as an exploration tool has been growing rapidly the research conference addressed the concept itself its elements and processes the tools and methods used to identify a petroleum system and many case studies we summarize responses by the participants to the following three questions first what is gained from using the petroleum system concept second in what new directions will the petroleum system concept take us in the future third has anything new come out of this research conference

coalbed methane resources of the appalachian basin eastern usa abstract in 2002 the u s geological survey usgs assessed the technically recoverable undiscovered coalbed gas resources in the appalachian basin and black warrior basin assessment provinces as about 15 5 trillion cubic feet although these resources are almost equally divided between the two areas most of the production occurs within relatively small areas within these provinces where local geological and geochemical attributes have resulted in the generation and retention of large amounts of methane within the coal beds and have enhanced the producibility of the gas from the coal in the appalachian basin coalbed methane cbm tests are commonly commercial where the cumulative coal thickness completed in wells is greater than three meters 10 ft the depth of burial of the coal beds is greater than 100 m 350 ft and the coal is in the thermogenic gas window in addition to the ubiquitous cleating within the coal beds commercial production may be enhanced by secondary fracture porosity related to supplemental fracture systems within the coal beds in order to release the methane from microporus coal matrix most wells are dewatered prior to commercial production of gas two total petroleum systems tps were defined by the usgs during the assessment the pottsville coal bed gas tps in alabama and the carboniferous coal bed gas tps in pennsylvania ohio west virginia eastern kentucky virginia tennessee and alabama these were divided into seven assessment units of which three had sufficient data to be assessed production rates are higher in most horizontal wells drilled into relatively thick coal beds than in vertical wells recovery per unit area is greater and potential adverse environmental impact is decreased

low salinity pools at barbados west indies their origin frequency and variability a vertical array of conductivity temperature recorders moored off the west coast of barbados west indies from may 1996 to november 1997 revealed a heterogeneous and variable salinity pattern punctuated by five pools of low salinity water 34 5 practical salinity units psu entering the region a typical pool extended to 30 m depth and lasted 25 days although one pool extended to 47 m and lasted 94 days water samples taken from a pool in may 1997 have radium 228 226 activity ratios of 1 consistent with previous measurements in barbados of water that originated in the amazon river mixing zone the amazon water likely was translated to barbados in rings spawned from the north brazil current analysis of sea height anomaly and residual derived from the topex poseidon satellite supports this conclusion and reveals that contrary to previous studies rings are shed throughout the year mostly during spring the pools of low salinity water and their associated velocities dramatically changed the already variable flow in our study area we believe the complex salinity and flow we observed represented the disorganized remnants of north brazil current rings that were at or near the ends of their lives the changes we observed in the velocity and water structure are interesting in their own right as evidence of the barbados region as a mixing zone and for their influence on recruitment of larval fishes to the reef along the island s west coast

shale gas play risk of the lower cambrian on the yangtze platform south china this paper describes the establishment of shale gas play risk workflow and discusses the major geologic risks for the lower cambrian shale gas play on the yangtze platform the shale gas play risk is assessed by considering the probability of occurrence po of hydrocarbon charge reservoir and retention hcrr of a shale gas play the po of each of these three factors is determined according to geological understanding and data availability the risk of these factors is categorized into high risk po 0 3 relatively high risk 0 3 3 5 and poor hydrocarbon retention are the key geologic risks of the lower cambrian shale gas play uncertainty is inherent in the play risk assessment owing to sparse well data coverage understanding the play risks and uncertainties warrants further study of the impact of extremely high thermal maturities and burial history and how it facilitates decision making for exploration investment

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

source rock quality determination from oil biomarkers i a new geochemical technique biomarker analyses of the aspen shale mowry shale equivalent skully s gap wyoming suggest that vital source rock properties e g kerogen hydrogen to carbon h c ratio rock eval hydrogen index hi and maceral composition may be indirectly yet quantitatively predicted from the biomarker parameters measured in oils a strong correlation exists between the homohopane index of aspen source rock extracts and both the h c ratio and hi of the associated kerogen r2 0 93 and 0 83 respectively the percentages of c30 and c28 4 desmethyl steranes relative to total extracted 4 desmethyl steranes correlate with the percentage of marine derived amorphous type ii organic matter in the kerogen r2 0 80 and 0 73 respecti ely and with kerogen h c ratio and hi r2 0 89 and 0 88 respectively for c30 steranes r2 0 89 and 0 83 respectively for c28 steranes the ratio of 4 agr methyl 24 ethylcholestanes to 3s methyl 24 ethylcholestanes in the extracts also correlates with kerogen h c ratio and hi r2 0 93 and 0 92 respectively in contrast the percentage of c29 steranes in the extracts correlates with the percentage of higher plant derived vitrinite in the kerogen r2 0 69 and shows a negative correlation with h c ratio and hi r2 0 81 and 0 66 respectively these relationships were used to determine indirectly the source rock kerogen properties of several mowry derived oils the correlations etween biomarkers and source rock properties observed in the aspen shale probably apply to many deltaic and nearshore marine source rocks that were deposited after the evolution of vascular plants therefore prediction of source rock properties from oils in vertically drained basins could prove extremely useful to explorationists seeking to identify the most oil prone areas in regions such as the u s gulf coast the north sea the beaufort sea and the mahakam and niger deltas an example from the beaufort sea is given in this same issue mccaffrey et al 1994

comparison of eddies off the north brazilian and somali coasts abstract repeated temperature sections using xbts from ships of opportunity along the sea lane off the brazilian coast indicate that a region of mesoscale occurs in which the eddies are particularly well developed in northern summer and fall these are located between approximately 3 10 n are on the order of 200 to 400 km across and may extend to 300 to 400 m in depth relatively fresh surface water apparently from the region of the amazon outflow is often observed in the eddy region similar eddies both in size and latitude are found off the somali coast these are associated with the turning offshore of all or part of the strong northward flowing somali current it is speculated that a somewhat similar circulation may also occur in northern summer in the pacific ocean

the characteristics and significance of conventional and unconventional sinian silurian gas systems in the sichuan basin central china abstract oil and gas discoveries within proterozoic cambrian strata and the majority of shale gas production occurs in relatively stable tectonic regions and characterised by moderate thermal maturity r o o 2 5 the system comprises two components namely a conventional sinian cambrian dolomite hosted gas resource and an unconventional cambrian silurian shale gas deposit both represent trillion cubic meter accumulations the migration accumulation type sinian cambrian gas system is a result of numerous processes including 1 paleo high trapping within a relatively stable tectonic setting 2 appropriate source materials located in a paleo rift trough 3 large scale dolomite grainstone reservoirs located near the paleo high 4 adequate gas produced from in situ crude oil cracking and 5 gypsum salt and shale cap rock preservation the self contained source reservoir cambrian silurian shale gas system is localized by deepwater organic rich shelf facies characterized by high silicon and calcium contents the recent discovery of these two massive gas fields in old and over matured sedimentary strata has effectively extended the upper maturity limit of r o over 3 0 which may indicate the efficacy of broadening current natural gas exploration areas thus potentially increasing global natural gas resources significantly furthermore the discovery may spark interest in natural gas exploration in widely distributed old strata thus encouraging co development of conventional and unconventional petroleum accumulations

an overview of pre devonian petroleum systems unique characteristics and elevated risks abstract the radiation of land plants at the onset of the devonian resulted in a change in the character of available biomass which in turn introduced changes to the nature of petroleum systems this study examines the nature of pre devonian petroleum systems prior to this significant evolutionary event conventional hydrocarbon resources have been associated with pre devonian petroleum systems across the globe including the neoproterozoic infracambrian huqf supergroup of the oman basin the riphean vendian sequences of east siberia the cambrian of the murzuq basin the ordovician of the tarim basin and the silurian qusaiba of saudi arabia although individual accumulations can be quite significant e g hassi messaoud has ultimate recoverable reserves of 10 billion barrels the relative importance of pre devonian derived oils is thought to be limited compared to the global conventional resource base with growing interest in unconventional resources the relative importance of these systems is expected to increase as resource plays such as the ordovician utica shale of the appalachian basin the cambrian of china and possibly the silurian of central and eastern europe develop the advanced ages of these systems result in some unique properties and amplify risks that exist in younger petroleum systems differences in the nature of the biomass contributing to pre devonian source rocks may give rise to oils that display unique geochemical characteristics for example there are some precambrian oils where c 29 steranes dominate even though land plants were absent there are also ordovician source rocks dominated by gloeocapsamorpha prisca a primitive prokaryote which yield oils containing limited amounts of c 20 components and nearly lack pristane and phytane the potential for unconventional reservoirs in pre devonian systems may also be highly dependent upon the age of the system as biological evolution influences the availability and nature of biogenic silica an important factor controlling brittleness and fracability literature has shown that biogenic silica from different sources displays varying degrees of resistance to diagenesis which allows the formation of a silica network and influences brittleness for example radiolaria present during the cambrian are more resistant to diagenesis than diatoms which did not evolve until the jurassic risks associated with preservation of hydrocarbons are also amplified in pre devonian petroleum systems many such systems have been exposed to significant thermal stresses resulting in the cracking of oil and wet gas others have had complex tectonic histories potentially resulting in the breaching of seals or changes in pvt conditions that can result in gas loss gas loss may also occur through diffusion from these older reservoirs as a result of long residence times

two petroleum systems charge the ya13 1 gas field in yinggehai and qiongdongnan basins south china sea restoring burial history correlating maturities and biomarkers of gas condensate source rocks measuring fluid inclusions and stable carbon isotope data show that two petroleum systems charge the ya13 1 gas field in yinggehai and qiongdongnan basins in the south china sea the first source came from the yanan depression in qiongdongnan basin at a depth of 4450 m with the critical moment of 5 8 ma the second source came from the yinggehai basin at a depth of 4700 m with the critical moment of 2 0 ma the ya13 1 drape anticline trap formed at 10 5 ma which was earlier than gas migration and accumulation the meishan formation with 7 10 mpa 1015 1450 psi expelling pressure and 49 66 mpa 7105 9570 psi overpressure combined effectively seals ya13 1 gases the reservoir of lingshui formation sands has average values of 14 9 porosity and 213 10 3 m2 permeability because the weathering of the feldspathic sands at the end of the oligocene the preserving of k feldspar and quartz overgrowths and dissolution of carbonate cements by organic acid enhanced the reservoir quality consequently trap formation coincided with gas generation migration and accumulation

beam stack imaging using a small aperture array we seek to gain a fuller understanding of seismic coda generation in the continental crust by identifying secondary scattering sources illuminated by a distant primary source we have developed a migration technique to scan seismic coda recorded by a small aperture seismic array for phases generated locally by scattering from large heterogeneities or topographic undulations we use a widely distributed suite of seismic events to illuminate the local crustal volume from different directions and produce an image of the local crust stable apparent secondary seismic sources are observed and interpreted as scatterers excited by the primary events

eddies in the north brazil current retroflection region observed by geosat altimetry mesoscale fluctuations in the western tropical atlantic are analyzed in geosat altimetry sea surface height ssh and geostrophic velocity anomalies to investigate the role of eddies in the north brazil current nbc retroflection zone the detachment of anticyclonic eddies from the nbc retroflection is observed during november through january when the nbc retroflection into the north equatorial countercurrent necc weakens and finally breaks down these eddies are traced over more than 2 months between 50 and 60 w on their way toward the caribbean at average speeds of 15 cm s 1 in one case an apparent merger of two anticyclonic eddies occurs one detached from the retroflection zone and one detached from the necc cyclonic eddies are also observed but are generally less persistent mesoscale ssh variance just west of the retroflection increases by a factor of 2 from early summer to winter mainly because of the anticyclonic eddies interhemispheric water mass transfer associated with the eddy flux out of the nbc retroflection may amount to an average transport of 3 sv

physical property characteristics of gas hydrate bearing reservoir and associated seal sediments collected during nghp 02 in the krishna godavari basin in the offshore of india abstract india s national gas hydrate program expedition 02 nghp 02 was conducted to better understand geologic controls on gas hydrate occurrence and morphology targeting coarse grained sediments along the lower continental slope offshore eastern india this study combines seismic logging while drilling data and a petroleum system approach to provide a regional geologic and lithologic context for 1 gas hydrate morphology and distribution and 2 effects of fine grained sediments clays and other grains smaller than 63 m on gas production in nghp 02 area b in the krishna godavari basin area b seismic data show a buried anticline syncline structure with strong reflectors r1 and r2 that delineate two of the five lithologic units unit i shallowest ii r1 iii iv r2 and v below the bas of gas hydrate stability throughout area b gas hydrate morphology depends on its placement within these units specifically core and grain scale measurements indicate fines content exerts a primary control on the gas hydrate distribution and morphology units i ii and iii are generally fine grained on the anticline crest these units host primarily grain displacing gas hydrate veins linked to pore occupying gas hydrate in thin localized coarser grained deposits diatoms in unit iii increase porosity with depth reaching 70 where it contacts unit iv the gas hydrate reservoir associated with reflector r2 the unit iii lithology and porosity allow fluid and dissolved phase methane to escape unit iv and form gas hydrate in the fine grained overburden within unit iv fine grained layers are interbedded with coarser grained gas hydrate reservoir sands and the fines content even in the sands is high enough to impact hydraulic and mechanical properties during gas production fluid motion during gas production can mobilize fines which can then clog pore throats limiting production rates pore water freshening during gas hydrate dissociation can increase fines mobilization particularly given the smectite identified in the fine grained interbeds

synthetic seismogram modelling of the oceanic pn phase the seismic oceanic pn phase is characterized by low spatial attenuation a high frequency wavetrain of extremely long duration and an onset velocity near 8 0 km s 1 in an effort to explain these properties models have been proposed for the mode of propagation of this phase which include energy entrapment within a low velocity channel1 2 propagation within an extremely low attenuation lithosphere waveguide3 4 and forward scattering from local heterogeneity within the oceanic crust5 6 through synthetic seismogram modelling we have generated realistic oceanic pn phases for an oceanic lithosph re model that did not contain any of these intrinsic properties the synthetic pn phase can be explained rather simply as a set of refractions from the lower lithosphre with subsequent reverberations within the oceanic water column and sediment layer many of the spectral properties associated with layer reverberation observed in the synthetic pn phase are revealed in data collected in the south west pacific complicated oceanic lithosphere models are not required to explain the gross characteristics of pn propagation

investigation of kuroshio induced cold core eddy trains in the lee of the izu islands using high resolution satellite images and numerical simulations abstract a series of synthetic aperture radar sar images acquired during the summer of 2006 revealed island wakes in the lee of the izu islands south of japan the wakes were formed of not only meandering disturbances but also a series of eddies with diameters on the order of those of the islands the kuroshio flowed near these islands in the summer of 2006 indicating that the island wakes were induced by a kuroshio island interaction satellite sea surface temperature sst and chlorophyll a chl a images observed during the summer of 2006 revealed low sst and high chl a wakes some of which included low sst eddy trains it is thus inferred that the phenomenon entailed upwelling and mixing processes and biological productivity known as the island mass effect high spatial resolution sst derived by combining a landsat infrared channel and avhrr sst clearly revealed a well defined sst front associated with the island wakes and a 1 km scale low sst wake pattern a numerical simulation was performed to investigate the formation mechanism the simulation qualitatively reproduced the cold eddy pattern with eddy driven mixing developing a mixed layer down to 200 m causing low sst island wakes the shedding frequency and spacing of the model produced eddies were roughly close to those of the karman vortex theory suggesting that karman type cold eddy trains are commonly formed behind the islands when the kuroshio strong flow impinges on them

crust mantle boundary reflectors in nevada from ambient seismic noise autocorrelations summary the depth of reflector layers in the earth s crust is usually estimated using controlled sources or earthquake signals ambient seismic noise however can also be used for this purpose we develop and apply a new method based on continuous waveform analysis to estimate the two way p wave reflection component of the green s function beneath each station the green s functions are retrieved from continuous record autocorrelation stacks at broad band sensor locations within the usarray earthscope transportable array in the western great basin and the sierra nevada in a region with complex crustal and upper mantle structure in this paper we show evidence of a reflector at the crust mantle boundary moho discontinuity derived for the first time from ambient noise autocorrelations using short period 1 s data our results compare well with earthquake and controlled source investigations and with tomography findings in the region moho depth is difficult to resolve seismically because of the lack of favourable spatial distribution of source and receiver geometries in contrast our method can be applied at any desired sensor spacing to estimate earth reflector depth beneath surface located sensors providing unprecedented resolution

eddy formation in unsteady flows unsteadiness of flow at m2 s2 tidal frequencies is demonstrated to be a dominant factor forming eddies in natural shallow water and continental shelf environments numerical hydrodynamic modelling and field measurements show that during the later stages of the half tidal cycle in a low friction environment currents in the lee of an obstruction accelerate with supporting sea gradients opposite to the adjacent inertial free stream which is still decelerating these opposed currents appear as a phase eddy and ideally can endure for 3 hours in m2 s2 reversing currents case studies are cited and numerical hydrodynamic modeling is applied to examine the formation of these circulation patterns

water mass and coastal current variability near barbados west indies moored current meter observations complimented by conductivity temperature depth ctd profiles and shipboard acoustic doppler current profiler measurements have revealed two distinctly different coastal circulation patterns south of the island of barbados west indies in the springs of 1990 and 1991 in the first year an abrupt increase in speed and change in direction of the currents south of the island was found to be associated with a large anticyclonic horizontally sheared circulation and a marked variation in temperature salinity characteristics the shift in currents was also accompanied by the incursion of a shallow pool of low salinity water a strong internal semidiurnal tidal signal and a change in the location of larval fish assemblages west of the island rapid variations in current speed and direction were also observed in 1991 though less persistent than in the previous year in this year the island was at one time completely surrounded by a large shallow pool of low salinity water a comparison was made of the 1990 hydrographic observations with ctd data from the national oceanographic data center archive for the region 0 20 n 40 60 w this suggests that the variation in hydrographic characteristics was due to the intrusion of a water mass of north brazil current origin which displaced the ambient north atlantic subtropical underwater around the island several recent studies have demonstrated that mesoscale anticyclonic eddies formed near the retroflection region of the north brazil current can be advected toward the eastern caribbean before being dissipated it is concluded that the most likely cause of the observed high energy event in 1990 was the passage of such an eddy past barbados

coastal ocean circulation near barbados west indies spring 1990 and 1991 results are presented of two oceanographic surveys of the coastal waters surrounding barbados west indies during april and may of 1990 and 1991 in both years observations of the hydrographic structure were made with a dense grid of conductivity temperature depth stations over the upper 500 m of the water column acoustic doppler current profiler adcp measurements of currents in the upper 250 m were made on radial transects from the island during three short surveys in each year one fixed mooring was deployed south of the island for the duration of each cruise results from both years showed the circulation to be energetic highly variable over short timescales and sometimes flowing contrary to historical means surface adcp and geostrophic flow patterns during 1990 suggested that the barbados ridge north of the island played an important role in steering the circulation north of the island the sudden reversal of the current south of the island between legs 1 and 2 of 1990 coupled with the presence of low salinity surface water suggested we may have witnessed an encounter between the island and the northern edge of a thin less than 35 m mesoscale pool of brazilian current retroflexion region origin on the other hand in 1991 several eddies both cyclonic and anticyclonic were observed near the island suggesting the possibility of shedding of von karman type eddies from the island

island wakes in shallow waters field laboratory and numerical results suggest that the free shear layer behind flow separation points and the internal circulation generated by rotation in the eddy are important processes in island wakes in shallow coastal waters

surface and near surface effects on seismic waves theory and borehole seismometer results a simple plane wave model is adequate to explain many surface versus borehole seismometer data sets using such a model we present a series of examples which demonstrate the effects of the free surface near surface velocity gradients and low impedance surface layers on the amplitudes of upcoming body waves in some cases these amplitudes are predictable from simple free surface and impedance contrast expressions however in other cases these expressions are an unreliable guide to the complete response and the full plane wave calculation must be performed large surface amplifications are possible even without focusing due to lateral heterogeneities or nonlinear effects both surface and borehole seismometer site responses are almost always frequency dependent ocean bottom versus borehole seismic data from the 1983 ngendei seismic experiment in the southwest pacific are consistent with both a simple plane wave model and a more complete synthetic seismogram calculation the borehole seismic response to upcoming p waves is reduced at high frequencies because of interference between the upgoing p wave and downgoing p and sv waves reflected from the sediment basement interface however because of much lower borehole noise levels the borehole seismometer enjoys a p wave signal to noise advantage of 3 to 7 db over nearby ocean bottom instruments

the time frequency characteristics of quarry blasts and calibration explosions recorded in kazakhstan ussr summary in this paper we consider two fundamentally different processes that can be responsible for the organization of energy in seismic coda into discrete time independent frequency bands one process involves the resonance of energy in low velocity horizons and the other requires the interaction of time offset wavefields produced by subevents within multiple event mine explosions ripple fired quarry blasts we examined data collected by high frequency seismometers in kazakhstan ussr and observed regular time independent spectral modulations in coda resulting from events strongly suspected to be soviet quarry blasts but not in the coda from single event calibration explosions detonated at similar ranges we conclude these modulations are a source effect and due to ripple firing this modulation is independent of the source receiver azimuth and we infer that the spatial array of subshots in each event must be small we demonstrate that simple linear superposition theory can be used to reproduce effectively the spectral modulation observed in real quarry blasts on the basis of these observations we attempt to discriminate between the two types of events using a spectral pattern based algorithm that seeks time independent features we consider the detrimental effect that resonant energy in low velocity horizons can have on the successful application of our algorithm

on the north brazilian eddy field abstract the region of large eddies occurring off the coast of brazil between approximately 3 n 10 n is revealed in some detail by repeated temperature observations on the along shore sea lane by ships of opportunity augmented by one airborne xbt expendable bathythermograph survey the general pattern of the circulation implied by the temperature structure is similar to that indicated by past hydrographic measurements and drifter tracks in general a large anticyclonic eddy to the north 7 n 10 n as well as a southern eddy like feature 3 n 6 n appear to be permanent patterns of the western north equatorial circulation this pattern somewhat resembles that found in the somali basin during the southwest monsoon

high frequency seismic attenuation of oceanic p and s waves in the western pacific analyses of oceanic p and s data from an earthquake in the kuril islands which occurred along the trend of a 1500 km long ocean bottom hydrophone array deployment near wake island have yielded constraints on high frequency seismic attenuation in the western pacific spatial attenuation and q were determined for group velocities between 8 2 and 3 0 km s and for frequencies between 2 5 and 22 5 hz there is greater attenuation of oceanic p than oceanic s at all frequencies suggesting either the presence of significant bulk attenuation in the lithosphere or that oceanic p propagates as a leaky mode the apparent attenuation of oceanic p and s is distance dependent with lower attenuation beyond 30 the apparent q of oceanic p increases with frequency as q 0 7 from about 300 at 2 5 hz to 1500 at 17 5 hz the apparent q of oceanic s increases with frequency as q 1 1 from about 400 at 2 5 hz to 3000 at 22 5 hz

resolution of prominent crustal scatterers near the noress small aperture array summary our goal is to identify prominent scatterers near seismic arrays so that the secondary phases they generate can be anticipated and recognized as being of local not teleseismic origin toward this end we have developed a technique to scan seismic coda recorded by arrays for phases generated locally by scattering of teleseismic energy from large topographical and crustal heterogeneities in this paper we test the technique by using the noress small aperture array in southern norway to image free surface secondary r sources excited by incoming teleseismic p waves in essence our technique suppresses the teleseismic energy and migrates the residual array records to image coherent local sources to assess the azimuthal and radial resolution that the noress array provides we have analysed point source synthetics a preliminary analysis of recorded data provides an initial view of the scatterers in the vicinity of noress and identifies limitations of imaging using the full coda of individual events single events can be used to image scatterers but more robust images with higher azimuthal resolution can be obtained by using a broadly distributed suite of events radial resolution of scatterers can be enhanced by deconvolution a composite image generated by simultaneously considering a suite of 31 deconvolved teleseisms indicates that two secondary r sources are present at or near the free surface near noress one 27 km to the south west of the array appears to be related to dramatic topographic relief at the south west edge of lake mjosa another roughly 10 km to the east of the array is judged by a bootstrapresampling analysis to be only marginally significant this apparent source does not coincide with obvious topography and is likely buried although the time delay that separates the scattered and parent phases depends somewhat on the angle of approach of the incoming waves in general these secondary sources are contributing wave trains to the seismic coda that begin 4 and 11 s respectively after the primary onset

deep cross equatorial flow in the atlantic measured with sofar floats neutrally buoyant sofar floats at nominal depths of 800 1800 and 3300 m were tracked for 21 months in the vicinity of tropical boundary currents in the atlantic near 6 n and at several sites near 11 n as well as along the equator trajectories at 1800 m show a swift 50 cm s narrow 100 km wide southward flowing deep western boundary current dwbc extending from 7 n to the equator the average transport per unit depth in the dwbc was estimated to be 13 8 103 m2 s coupling this value with mean velocities measured in the dwbc by current meters gave a volume transport of 15 106 m3 s between depths of 900 m and 2800 m approximately 6 106 m3 s recirculated northward between the dwbc and the mid atlantic ridge leaving 9 106 m3 s as cross equatorial transport no obvious dwbc nor swift equatorial current was observed by the 3300 m floats a low mean velocity at this depth lay between f 11 and higher velocity cores above and below the 1800 m trajectories also suggest that at times february march 1989 the north atlantic deep water in the dwbc turned eastward and flowed along the equator and at other times august september 1990 the dwbc crossed the equator and continued southward the velocity near the equator calculated by grouping floats in a box along the equator was eastward at 4 1 cm s from february 1989 to february 1990 and westward at 4 6 cm s from march 1990 to november 1990 thus the amount of cross equatorial flow in the dwbc appeared to be linked to low frequency variability of the structure of the equatorial current system floats in antarctic intermediate water at 800 m revealed a northwestward western boundary current although flow patterns were complicated three floats that significantly contributed to the northwestward flow looped in anticyclonic eddies that translated up the coast at 8 cm s six 800 m floats drifted eastward along the equator between 5 s and 6 n at a mean velocity of 11 cm s one reached 5 w in the gulf of guinea suggesting that the equatorial currents at this depth extended at least 35 40 along the equator three of these floats reversed direction near the end of the tracking period

island wakes in shallow coastal waters rattray island northeast australia is 1 5 km long 300 m wide and lies in well mixed water approximately 25 m deep its long axis is inclined at about 60 into the direction of the dominant semidiurnal tidal current the length of the wake in the lee of the island as documented by aerial photographs and satellite imagery appears to equal that of the wake behind a flat plate in a two dimensional flow at a reynolds number of about 10 however current metering drogues measurements and temperature mapping indicate internal wake velocities much greater than would be consistent with such a simple low reynolds number model further estimates of the turbulent eddy coefficient suggest an effective reynolds number more in the vicinity of 103 to reconcile these observational differences and to explain the observed upwelling in the core of the wake an ekman pumping model is proposed it is postulated that the ekman benthic boundary layer driven by rotation in the wake allows the vertical vorticity introduced into the water at the tip of the island at the point of separation to be negated by the vorticity of opposite sign introduced at the bottom further it is shown that a large fraction of the kinetic energy of the upstream flow facing the island is used to drive the wake eddies leading to the conclusions that the trapping of water in the lee of islands greatly increases head losses on continental shelves with numerous islands coral reefs and rock outcrops