high archean climatic temperature inferred from oxygen isotope geochemistry of cherts in the 3 5 ga swaziland supergroup south africa new and compiled oxygen isotope data combined with the results of geological and sedimentological studies demonstrate that enclaves of synsedimentary to very early diagenetic cherts are widely preserved in the 3 5 3 2 ga swaziland supergroup barberton greenstone belt south africa the low 1 8 o values of these cherts indicate extremely high ocean temperatures of 55 85 c previously the large depletion in 1 8 o shown by all barberton cherts relative to their phanerozoic counterparts has been attributed to low 1 8 o in archean oceans chert formation during late diagenesis wholesale loss of 1 8 o during alteration and or regional silicification of sediments around hot springs these alternative explanations are not compatible with the new results cherts in the onverwacht group display an isotopic stratigraphy that is inversely repeated in conglomerates in the overlying fig tree and moodies groups demonstrating that the chert 8 0 o values were fixed prior to archean uplift and erosion which started at 3 26 ga the maximum 1 8 o value in barberton cherts 22 is lower than the minimum values 23 in phanerozoic bedded cherts precluding late diagenesis as the explanation of the overall low 1 8 o values regional metamorphic hydrothermal or long term resetting of original 1 8 o values is also precluded by preservation of 1 8 o values across different metamorphic grades and by systematic 1 8 o differences among interbedded chert types stratigraphic units and conglomerate clasts the 7 1 8 o variation of these archean cherts is similar to that of phanerozoic deep sea cherts formed when opal converted to microquartz during burial but the actual archean values are 10 lower marine opal was apparently converted to microquartz during burial to depths of 1 km cherts with 1 8 o 15 reflect conversion during deepest burial or in local areas of enhanced geothermal gradient and or hydrothermal activity cherts with higher 1 8 o values formed during early diagenesis and indicate an extremely hot archean ocean and surface environment

palaeoclimatology evidence for hot early oceans reply robert chaussidon the possibility of a change in 18o values of the oceans has been discussed for the past 30 years1 and the comment by shields and kasting2 does not really bring any new insight into this issue the carbonate 18o curve3 is indistinguishable from that of cherts and shows a huge scatter caused by local geological processes this scatter in 18o up to 20 at a given age far exceeds the sensitivity of the isotope thermometer about 2 per 10 c it therefore prevents any precise additional test of the model and no consensus was ever reached in the literature on this debate from oxygen isotope data alone our approach4 by contrast was to try to find another isotopic proxy 30si that could bring new and independent constraints to test the hypothesis of hot oceans in the early precambrian our interpretation of the 30si values in terms of temperature is fully coherent not only with the 18o of cherts but also with that of marine carbonates and certainly does not overlook these data in addition we indicated4 that our sample set has not the time resolution that would be necessary to look for an effect on silicon isotopes of precambrian global glaciations no chert samples from such periods were analysed

the effects of nonrectangular cross section friction and barotropic fluctuations on the exchange through the strait of gibraltar abstract the inviscid two layer hydraulic model of farmer and armi has two types of solution in the eastern end of the strait of gibraltar for flow that is critical at camarinal sill the solution corresponding to maximal exchange has a fast supercritical flow with a shallow interface whereas the solutions corresponding to submaximal exchange show a much slower subcritical flow with a rather deep interface we extend their model to include physically important effects such as nonrectangular cross section friction and barotropic fluctuations allowing for realistic cross sections reduces the exchange and raises the interface everywhere along the strait realistic interfacial friction has little influence on the exchange but lateral friction at the sloping sides of the cross sections has a significant effect on the flow and brings the two solutions supercritical and subcritical closer together the effect of the earth s rotation is also summarized but dealt with in more detail in a companion paper by bor

least squares reverse time migration inversion based imaging toward true reflectivity abstract as an inversion based imaging algorithm least squares reverse time migration lsrtm refines seismic images toward true reflectivity earth models it overcomes the shortcomings of conventional migration algorithms via iterative least squares inversion to gradually correct migration errors lsrtm also suppresses side lobes of image wavelets to generate images with high spatial resolution recovered weak events and steep dips are enhanced during iterations with balanced illumination the true amplitude behavior of lsrtm will greatly benefit quantitative seismic interpretations lsrtm can be a practical image domain broadband solution to generate high resolution images with fine layers as well as accurate complex structures

neoarchean deep marine paleotemperature evidence from turbidite successions abstract numerous studies using various proxies have been conducted to ascertain the temperature of the archean ocean if as some of these investigations indicate the archean ocean was relatively hot with a cooling trend through time to the phanerozoic this would have had consequences for sediment erosion transport and deposition substantially higher surface temperatures would cause the viscosity of water an important variable in defining flow hydrodynamics to be lowered in particular less viscous water would have affected the development of current ripples these are dependent on the existence of the laminar sublayer as the sweeps of turbulent eddies impinging on this layer build small mounds that evolve into ripples the laminar sublayer thins at higher temperatures allowing sand grains of smaller sizes to destroy it at modern surface temperatures current ripples cannot form in coarse grained sands and if a hotter ocean existed in the archean it would have inhibited current ripple formation in medium grained sands and possibly even fine grained sands examination of numerous neoarchean deep water turbiditic successions in western superior province led to the realization that ripple cross lamination is rare in all these units as the formation of current ripples is dependent on the flow depositing sediment of the proper grainsize velocities in the appropriate range and existence of a viscous sublayer and the first two prerequisites were met during deposition from these decelerating turbidity currents the existence of the viscous sublayer during the majority of flows must be questioned the occurrence of current ripple cross lamination in well exposed turbidites of the neoarchean mckellar harbour formation was quantified and found to be substantially less than the abundance of ripple cross lamination present in phanerozoic turbidites indicating that the neoarchean deep ocean may have been considerably warmer than the deep ocean of today

palaeoclimatology evidence for hot early oceans arising from f robert m chaussidon 969 972 2006 10 1038 nature05239 robert chaussidon reply the oxygen isotopes in sedimentary cherts siliceous sediments have been used to argue that the precambrian oceans were hot with temperatures of up to 70 c at 3 3 gyr before present1 robert and chaussidon2 measure silicon isotopes in cherts and arrive at a similar conclusion we suggest here that both isotope trends may be caused by variations in seawater isotope composition rather than in ocean temperatures if so then the climate of the early earth may have been temperate as it is today and therefore more consistent with evidence for precambrian glaciations and with constraints inferred from biological evolution

observations of the deepwater flow into the baltic sea it has earlier been hypothesized that the deep water flowing into the baltic sea forms a bottom pool in the arkona sea just inside the entrance sills the flow from the pool farther into the baltic sea was assumed to be baroclinic geostrophic and controlled by the pool stratification this was supported by a realistic statistical description of the deepwater flow into the baltic sea computed using historical vertical density profiles from a hydrographical station in the arkona sea however basin wide synoptic measurements of density and currents were not available for a direct verification of the pool model a hydrographic survey was undertaken in the arkona sea shortly after a major inflow event in the beginning of 1993 the conductivity temperature depth sections reveal a thick bottom pool of deep water separated by a halocline from the surface water of baltic origin approaching the northern coastal boundary from the central parts of the pool the halocline sank by about 20 m before hitting the bottom the ship mounted acoustic doppler current profiler recorded a complicated current field however subtraction of the assumed barotropic part gives a current field quite similar to the baroclinic geostrophic current field computed from the density distribution thus a dense bottom pool including a baroclinic geostrophic boundary current along the northern flank of the pool is quite evident from our measurements

microscale oxygen isotope variations in 1 9 ga gunflint cherts assessments of diagenesis effects and implications for oceanic paleotemperature reconstructions abstract variations in the oxygen isotope composition 18 o of five cherts from the 1 9 ga gunflint iron formation canada were studied at the micrometer scale by ion microprobe to try to better understand the processes that control 18 o values in cherts and to improve seawater paleotemperature reconstructions gunflint cherts show clearly different 18 o values for different types of silica with for instance a difference of 15 between detrital quartz and microquartz microquartz in the five samples is characterized by large intra sample variations in 18 o values 18 o of quartz varies from 4 6 to 6 6 at the 20 m scale and from 12 to 14 at 2 m scale isotopic profiles in microquartz adjacent to hydrothermal quartz veins demonstrate that microquartz more than 200 m away from the veins has preserved its original 18 o value at the micrometer spatial resolution of the ion probe data reveal that microquartz has preserved a considerable 18 o heterogeneity that must be regarded as a signature inherited from its diagenetic history modelling of the 18 o variations produced during the diagenetic transformation of sedimentary amorphous silica precursors into microquartz allows us to calculate seawater temperature t sw at which the amorphous silica precipitated and diagenesis temperature t diagenesis at which microquartz formed that reproduce the 18 o distributions mean range and shape measured at micrometer scale in microquartz the two critical parameters in this modelling are the 18 o value and the mass fraction of the diagenetic fluid under these assumptions the most likely ranges for t sw and t diagenesis are from 37 to 52 c and from 130 to 170 c respectively

hydraulic control of sill flow with bottom friction abstract the hydraulics of strait and sill flow with friction is examined using a reduced gravity model it is shown that friction moves the critical or control point from the sill to a location downstream if the strait has constant width the control point lies where the bottom slope is the negative of the drag coefficient cd if cd exceeds the bottom slope everywhere the flow cannot be controlled in the classical sense that energy and flow force are minimized friction also decreases the minimum obstacle height required to establish hydraulically controlled flow in the classical laboratory towing experiment also friction greatly encourages the establishment of stationary hydraulic jumps in the lee of the sill and under certain conditions gives rise to stationary jumps on the upstream face of the obstacle some consequences of these results for deep ocean overflows are given using the iceland faroe overflow as an example

topographic control of oceanic flows in deep passages and straits saddle points between neighboring deep ocean basins are the sites of unidirectional flow from one basin to the next depending on the source of bottom water flow in these sites appears to be topographically controlled so the interface between the bottom water and the water above adjusts itself to permit bottom water flow from the basin that contains a source of bottom water into the next examples in the atlantic include flow in the romanche fracture zone the vema channel the ceara abyssal plain the anegada jungfern passage and the discovery gap but there are many more theoretical predictions of volume flux using a method that requires only conductivity temperature depth data archives and detailed knowledge of bathymetry near the saddle point are compared with volume flux estimates using current meters and or geostrophic estimates for seven cases the ratio of prediction to volume flux estimate ranges from 1 0 to 2 7 some ocean straits that separate adjacent seas are also found to critically control bidirectional flows between basins theory of the influence of rotation on such critical flows is reviewed predictions of volume flux in eight cases are compared with ocean estimates of volume flux from traditional methods

why the oxygen isotopic composition of sea water changes with time the oxygen isotopic composition of oxygen in minerals and oxygen in water this sea water is determined by interactions with equilibrium fractionation is a strong function of fresh silicate rocks interactions at high temperature varying for many minerals from about

the effect of rotation on the surface inflow through the strait of gibraltar abstract gull s theory for rotating hydraulics is applied as a reduced gravity model to the surface inflow through the strait of gibraltar into the mediterranean as it is shown that rotation is mostly important east of tarifa narrows where only the upper layer is active details of the flow depend on the volume flux q and on the potential vorticity of the inflow we examine two extreme cases for this in both examples the model predicts that supercritical inflow separates from the north share about half way between tarifa and gibraltar so that at gibraltar the flow is separated the comparison of i conditions for criticality ii separation widths and iii interface depths and velocities in midstrait between this 2d model and the id models of farmer and armi and of bormans and garrett suggests that the use of a simple 1d model is appropriate for the strait of gibraltar for both subcritical and separated supercritical flows however for the latter the prediction of the point of separation and o

possible evidence for episodic epeiric marine and fluvial sedimentation and implications for palaeoclimatic conditions c 2 3 1 8 ga kaapvaal craton south africa abstract the debate on palaeoproterozoic atmospheric composition climate and the timing and nature of the postulated great oxidation event possibly at c 2 3 ga remain matters of debate within precambrian geology this paper contributes to the debate through examination of physical sedimentary structures preserved within two separate groups of sedimentary deposits from the central and northern portions of the kaapvaal craton south africa within the c 2 3 2 1 ga pretoria group study of epeiric marine sandstones and mudrocks magaliesberg and silverton formations c 2 1 ga suggests that fluvial input was predominant over low energy reworking by waves winds and tides this together with extensive evidence for microbial mat growth within the littoral sandstones of the magaliesberg formation points to strongly episodic sedimentation by both marine and fluvial processes within two younger basins belonging to the c 2 05 1 8 ga waterberg group anomalously high palaeoslopes calculated according to standard palaeohydrological methods combined with direct field evidence for sediment gravity flows and sheetflood deposition within an overall braided fluvial setting again suggest episodic sedimentation strongly episodic sedimentation can most easily be reconciled with a greenhouse palaeoclimate at least for that part of kaapvaal at that time taken together these data sets point to the postulate that the generally accepted change from a palaeoproterozoic greenhouse through a great oxidation event c 2 3 ga to less extreme climatic conditions may have been gradational and even diachronous across the earth s cratons at that time

origin and mechanisms of k si metasomatism of ca 3 4 3 3ga volcaniclastic deposits and implications for archean seawater evolution examples from cherts of kittys gap pilbara craton australia and msauli barberton greenstone belt south africa abstract the effects of k si metasomatism during the formation of early archean replacement cherts have been quantified in this study by the investigation of two well known stratigraphic sections the msauli chert mc barberton greenstone belt south africa and the kittys gap chert kgc pilbara craton western australia the kgcs have a dacitic precursor similar to duffer formation dacites pilbara craton while the mcs are derived from al depleted komatiites similar to those from the weltevreden formation barberton greenstone belt mass balance calculations reveal that the volcaniclastic deposits had initial porosities of up to 85 vol for the kgc and of 65 vol for the mc secondary porosities 27 vol mc 8 vol kgc produced during k metasomatism are proportional to the dissolution of fe ca mg rich glass and precursor minerals komatiites have a higher chemical exchange potential than dacites each gram releasing 1 2 mmol fe2 2 8 mmol mg2 1 4 mmol ca2 and 1 1 mmol na to seawater together with 4 4 mmol o2 k metasomatism of 1 g of komatiite further implies an uptake of 0 67 mmol of k and 2 7 mmol of h the highest silica uptake is achieved for the kgc 82 mmol g of precursor this silica enrichment most likely operated in the water column and at the sediment water interface by sorption mechanisms on the surface of detrital particles and particulate organic matter as a result of seawater silica saturation acidic conditions ph 5 5 6 5 and hot temperatures 70 c favored the formation of k rich phyllosilicates by interaction with seawater during the early diagenetic alteration of the volcaniclastic particles the widespread occurrence of k si metasomatism in volcanic and sedimentary rocks can be regarded as a general alteration process of the early archean seafloor with a major influence on seawater composition the highly k selective metasomatism confirms previous studies suggesting that the archean ocean was acidic and probably in equilibrium with a co2 rich atmosphere

adaptive least squares rtm with applications to subsalt imaging least squares reverse time migration lsrtm refines the seismic image toward true reflectivity by inversion its iterative nature and modeling capability enable the use of synthetic data to guide the preconditioning of input data when the velocity contains errors dynamic warping can be used to shift the input data and force the traveltime to be consistent with the imperfect migration velocity a crosscorrelation based confidence level is introduced to control the quality of dynamic warping for field data the confidence level also is used as an inverse weighting to adaptively precondition the data residual the adaptive preconditioning automatically balances data fitting in the shallow and deep and speeds up convergence in subsalt both synthetic and field data experiments based in the gulf of mexico show that the adaptive lsrtm can improve the image quality in subsalt effectively and efficiently within only a few iterations the adaptive lsrtm suppresses the salt halo artifacts and increases the signal to noise ratio in poorly illuminated areas it also improves the termination of sediments against salt boundaries and enhances subsalt image coherency compared with conventional rtm the adaptive lsrtm image is more favorable to geologic interpretation

topographic control of rotating deep water flow through the combination of a sill and a horizontal constriction many subsurface passages connecting the deep parts of the oceans have a topography such that the location of the most pronounced horizontal constriction does not coincide with that of the sill when applying rotating hydraulic theory to describe the deep water flow through channels of this type a problematic situation emerges since the standard analysis assumes a well defined controlling section the present study demonstrates how the associated critical flow problem arising from the hydraulic model may be resolved on the basis of functional theoretical analysis the methodology is applied to the deep water flow into the baltic proper through the bornholm channel where multiple critical cross sections were found these as well as the associated critical transport predictions are discussed and it is found that only one of them serves de facto as a control with unidirectional flow

a model for late archean chemical weathering and world average river water abstract interpretations of the geologic record of late archean near surface environments depend very strongly on an understanding of weathering and resultant riverine transport to the oceans the late archean atmosphere is widely recognized to be anoxic p o 2 g 10 5 to 10 13 bars p h 2 g 10 3 to 10 5 bars detrital siderite feco3 pyrite fes2 and uraninite uo2 in late archean sedimentary rocks also suggest anoxic conditions however whether the observed detrital minerals could have been thermodynamically stable during weathering and riverine transport under such an atmosphere remains untested similarly interpretations of fluctuations recorded by trace metals and isotopes are hampered by a lack of knowledge of the chemical linkages between the atmosphere weathering riverine transport and the mineralogical record in this study we used theoretical reaction path models to simulate the chemistry involved in rainwater and weathering processes under present day and hypothetical archean atmospheric boundary conditions we included new estimates of the thermodynamic properties of fe ii smectites as well as smectite and calcite solid solutions simulation of present day weathering of basalt calcite by world average rainwater produced hematite kaolinite na mg saponite and chalcedony after 10 4 moles of reactant minerals kg 1 h2o were destroyed combination of the resultant water chemistry with results for granitic weathering produced a water composition comparable to present day world average river water warw in contrast under late archean atmospheric conditions p co 2 g 10 1 5 and p h 2 g 10 5 0 bars weathering of olivine basalt calcite to the same degree of reaction produced kaolinite chalcedony and na fe ii rich saponite late archean weathering of tonalite trondhjemite granodiorite ttg formed fe ii rich beidellite and chalcedony combining the waters from olivine basalt and ttg weathering resulted in a model for late archean warw with the composition na ca2 fe2 mg2 cl hco 3 sio2 aq the ph of the water was 6 3 and it is much richer in hco 3 and in mg fe relative to ca na compared to present day warw at higher ph2 g e g 10 4 0 bars organic acid anions could be metastable our results are consistent with the thermodynamic stability of fe ii clays pyrite uraninite and under some conditions siderite during weathering and riverine transport overall our results provide a basis for assessing the formation of organic hazes and the mobility of trace elements and nutrients due to fluctuations of the late archean atmosphere

alternating si and fe deposition caused by temperature fluctuations in precambrian oceans precambrian banded iron formations contain thin alternating layers of iron and silica rich minerals microbial culture experiments show that these alternations could be caused by changes in the relative dominance of microbial iron oxidation and abiotic silica precipitation driven by fluctuating ocean temperature

flow properties in rotating stratified hydraulics abstract this paper discusses three distinct features of rotating stratified hydraulics using a reduced gravity configuration first a new upstream condition is derived corresponding to a wide almost motionless basin and this is applied to flow across a rectangular sill and compared with the case of a zero potential vorticity upstream condition for this geometry it is shown that unidirectional flow permits more water to pass through the sill than bidirectional flow second the general problem is considered of flow from any upstream configuration that passes through sills that vary slowly in depth cross sill and so are effectively many deformation radii wide only two flow configurations permit any realistic amount of flux across the sill either the fluid occupies a narrow region within the sill with a small flux or the fluid occupies a wide region with sluggish geostrophic flow except for at boundary layers at each side in the latter case hydraulic control is not likely to occur the zero

imaging of offset vsp data with an elastic iterative migration scheme vsp data are usually acquired in order to obtain high resolution images of complex structures in reservoirs and near boreholes the authors present an elastic iterative migration scheme which has few limitations regarding the complexity of the geology and where the macromodel for both p and s wave velocities is automatically improved and updated at each iteration they avoid wavefield separation up down and p s and the simplifying assumptions of small dips underlying most such methods the migration scheme is based on elastic inversion theory the wavefield extrapolation is based on a high order coarse grid finite difference solution to the elastic two way wave equation at each iteration the macromodel is updated using a gradient method in which the gradient is computed by correlation of forward modelled fields with back propagated residual fields the first iteration of the migration scheme is equivalent to elastic reverse time migration with an imaging condition similar to claerbout s principle both p and s wave reflections contribute to the images in numerical examples with both synthetic and real offset vsp data they find that increasing the number of iterations improves the image quality images based on both p and s wave energy give more near well information and higher spatial resolution than images on only acoustic energy more in the real data example the authors show that the iterative migration scheme can image a relatively complex geological structure a fault and a small graben intersecting the well can be identified less

ground penetrating radar for high resolution mapping of soil and rock stratigraphy ground penetrating radar is a technique which offers a new way of viewing shallow soil and rock conditions the need to better understanding overburden conditions for activities such as geochemical sampling geotechnical investigations and placer exploration as well as the factors controlling groundwater flow has generated an increasing demand for techniques which can image the subsurface with higher resolution than previously possible the areas of application for ground penetrating radar are diverse the method has been used successfully to map ice thickness water depth in lakes bedrock depth soil stratigraphy and water table depth it is also used to delineate rock fabric detect voids and identify karst features the effective application of the radar for the high resolution definition of soil stratigraphy and fractures in bedrock is highlighted the basic principles and practices involved in acquiring high quality radar data in the field are illustrated by selected case histories one example demonstrates how radar has been used to map the bedrock and delineate soil horizons to a depth of more than 20 m two case histories show how radar has been used to map fractures and changes of rock type to 40 m range from inside a mine another case history demonstrates how radar has also been used to detect and map the extent of groundwater contamination the corroboration of the radar results by borehole investigations demonstrates the power and utility of the high resolution radar method as an aid for interpolation and extrapolation of the information obtained with conventional coring programmes with the advent of new instrumentation and field procedures the routine application of the radar method is becoming economically viable and the method will see expanded use in the future

maximal two layer exchange over a sill and through the combination of a sill and contraction with barotropic flow the analysis of two layer exchange flow through contractions with a barotropic component treated by armi farmer 1986 is extended to include exchange flows over sills and through a combination of a sill and contraction it is shown that exchange over a sill is fundamentally different from exchange through a contraction control at the sill crest acts primarily through the deeper layer into which the sill projects and only indirectly controls the surface layer this asymmetry in the control results in asymmetrical flows the interface depth above the crest is not one half the total depth as assumed in other studies by analogy with flow through contractions but is somewhat deeper the maximal exchange rate is less than for flow through a contraction of equal depth when both a sill and a contraction are present the contraction influences control at the sill crest only if it lies between the sill and the source of denser water the response to barotropic flow is also asymmetrical the transition to single layer flow occurs at much lower speeds for a barotropic component in one direction than the other results of the analysis are applied to exchange flow through the strait of gibraltar which includes both a sill and a contraction it is shown that maximal exchange conditions apply throughout part of the tidal cycle and observations illustrate several of the analytical predictions for barotropic flows including the formation of fronts single layer flow submaximal exchange and reverse flow

kirchhoff modeling inversion for reflectivity and subsurface illumination because of its computational efficiency prestack kirchhoff depth migration is currently one of the most popular algorithms used in 2 d and 3 d subsurface depth imaging nevertheless kirchhoff algorithms in their typical implementation produce less than ideal results in complex terranes where multipathing from the surface to a given image point may occur and beneath fast carbonates salt or volcanics through which ray theoretical energy cannot penetrate to illuminate underlying slower velocity sediments to evaluate the likely effectiveness of a proposed seismic acquisition program we could perform a forward modeling study but this can be expensive we show how kirchhoff modeling can be defined as the mathematical transpose of kirchhoff migration the resulting kirchhoff modeling algorithm has the same low computational cost as kirchhoff migration and unlike expensive full acoustic or elastic wave equation methods only models the events that kirchhoff migration can image kirchhoff modeling is also

least squares migration of incomplete reflection data a least squares migration algorithm is presented that reduces the migration artifacts i e recording footprint noise arising from incomplete data instead of migrating data with the adjoint of the forward modeling operator the normal equations are inverted by using a preconditioned linear conjugate gradient scheme that employs regularization the modeling operator is constructed from an asymptotic acoustic integral equation and its adjoint is the kirchhoff migration operator we tested the performance of the least squares migration on synthetic and field data in the cases of limited recording aperture coarse sampling and acquisition gaps in the data numerical results show that the least squares migrated sections are typically more focused than are the corresponding kirchhoff migrated sections and their reflectivity frequency distributions are closer to those of the true model frequency distribution regularization helps attenuate migration artifacts and provides a sharper better frequency distribution of estimated reflectivity the least squares migrated sections can be used to predict the missing data traces and interpolate and extrapolate them according to the governing modeling equations several field data examples are presented a ground penetrating radar data example demonstrates the suppression of the recording footprint noise due to a limited aperture a large gap and an undersampled receiver line in addition better fault resolution was achieved after applying least squares migration to a poststack marine data set and a reverse vertical seismic profiling example shows that the recording footprint noise due to a coarse receiver interval can be suppressed by least squares migration

the flow of atlantic water through the strait of gibraltar abstract we describe and analyze observations of the water exchange through the strait of gibraltar the primary observations were taken in april 1986 and included data from moored recording instruments at four locations in the strait together with an intensive survey by ship these measurements included extensive ctd profiling acoustic doppler current profiling numerous profiles from expendable instruments and imaging using a high frequency echo sounder the analysis focuses on the internal hydraulics of the strait and in particular the presence of hydraulic controls and their influence on the exchange our observations during april 1986 show that the maximal exchange condition in which a subcritical flow is bounded by supercritical flow at both ends of the strait did occur although with various subtleties not explicitly incorporated in our previous theoretical developments the atlantic water moving east along the surface encounters a control at tarifa narrows in the eastern part of the strait the mediterranean water passes through controls both at camarinal sill and at spartel sill further west at our westernmost mooring at spartel the outflowing mediterranean water is continuously supercritical with little tidal variability the control at camarinal sill is periodically lost due to tidal action and reappears on a reverse flow during a falling spring tide tangier basin bounded by camarinal and spartel sills acts as an internal reservoir for the outflowing mediterranean water the interface rising and falling through each tidal cycle during spring tides the tangier basin interface rises high enough to flood the control at camarinl sill and produces a reverse flow of the lower layer when control is lost at camarinal sill a travelling bore is released this bore may modify the location of the control acting on the surface layer in tarifa narrows the atlantic water enters the alboran sea as a jet which may have a well defined northern boundary where it separates from the european coast this boundary may be identified by the presence of a slick although the internal jump west of camarinal sill is almost certainly an area of intense mixing frictional effects do not appear to be dominant in the subcritical portion of the flow