average seasonal variation of the atlantic equatorial currents from historical ship drifts abstract surface currents in the tropical atlantic were studied using historical ship drift data these are the only available data capable of resolving the long term seasonal fluctuations of currents over a broad geographical region the north equatorial countercurrent was found to extend as a continuous eastward flowing current across the atlantic in the band 5 8 n from july to december the maximum average eastward velocity was 30 cm s 1 during july september east of 20 w the countercurrent continued throughout the year west of 20 w and from january to june the countercurrent disappeared and westward currents were observed average westward velocity reached 20 cm 1 in april and may centered near 40 w the annual cycle in surface velocity 20 cm s 1 amplitude agrees closely with that of geostrophic velocity inferred from variations in thermocline depth across the counter current jets centered near 2 n and 4 s separated by a minimum near 1 s the northern jet has a large 14 cm s 1 semiannual v

reflection and tunneling of ocean waves observed at a submarine canyon 1 ocean surface gravity waves with periods between 20 and 200 s were observed to reflect from a steep walled submarine canyon observations of pressure and velocity on each side of the canyon were decomposed into incident waves arriving from distant sources waves reflected by the canyon and waves transmitted across the canyon the observed reflection is consistent with long wave theory and distinguishes between cases of normal and oblique angles of incidence as much as 60 of the energy of waves approaching the canyon normal to its axis was reflected except for waves twice as long as the canyon width which were transmitted across with no reflection although waves approaching the canyon at oblique angles cannot propagate over the canyon total reflection was observed only at frequencies higher than 20 mhz with lower frequency energy partially transmitted across analogous to the quantum tunneling of a free particle through a classically impenetrable barrier

modification of a linear bar trough system by a standing edge wave abstract detailed fluid motions and bathymetric changes in the lacustrine nearshore zone at wymbolwood beach georgian bay canada were monitored during storms to test theoretical postulates about the importance of low frequency wave motions to nearshore circulation of water and sediments eighteen continuous resistance wave staffs and two electromagnetic current meters were deployed along one shore normal and two shore parallel instrument arrays over a period of two months during one intense storm a mode 3 standing edge wave at 0 035 hz was the predominant low frequency motion prior to the storm the bathymetry was characterized by linear bar trough systems and during the storm these features migrated onshore and became rhythmic in response to the length scales and relatively fixed locations of nodal and antinodal lines of the standing edge wave however a well defined crescentic bar as predicted by the model under the condition of zero net sediment flux divergence did not evolve because the hydrodynamic forcing ceased before an equilibrium condition could be attained nevertheless the location of scour and accretion features demonstrated the tendency towards such a crescentic form the offshore and alongshore irregularities of the post storm bathymetry are not readily explained by alternative hydrodynamic interactions

evidence for thin oceanic crust on the extinct aegir ridge norwegian basin ne atlantic derived from satellite gravity inversion 1 satellite gravity inversion incorporating a lithosphere thermal gravity correction has been used to map crustal thickness and lithosphere thinning factor for the ne atlantic predicted oceanic crustal thicknesses in the norwegian basin are between 4 and 7 km on the extinct aegir ridge increasing to 9 14 km at the margins consistent with volcanic margin continental breakup at the end of the paleocene the observation from gravity inversion and seismic refraction studies of thin oceanic crust produced by the aegir ridge in the oligocene may have implications for the temporal evolution of asthenosphere temperature under the ne atlantic during the tertiary thin oligocene oceanic crust may imply cool normal asthenosphere temperatures during the oligocene in contrast to elevated asthenosphere temperatures in the paleocene and miocene recent as indicated by the formation of volcanic margins and iceland respectively

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

seismic tomographic imaging of the eastern mediterranean mantle implications for terminal stage subduction the uplift of anatolia and the development of the north anatolian fault

wave energy saturation on a natural beach of variable slope time series of flow were measured across the inner surf zone during a storm these data were used to quantify the dependence of wave height transformed from measured flow and velocity on local slope and depth similar to previous studies as incident waves broke and propagated into the surf zone wave energy became saturated and wave height was strongly dependent on depth however the ratio of rms wave height to local depth rms was found not to be constant but to vary between 0 29 and 0 55 rms increased with local slope and was independent of deepwater wave steepness thus the surf zone similarity parameter the ratio of slope to the square root of steepness did not adequately parameterize rms

evolution of surface gravity waves over a submarine canyon 1 the effects of a submarine canyon on the propagation of ocean surface waves are examined with a three dimensional coupled mode model for wave propagation over steep topography whereas the classical geometrical optics approximation predicts an abrupt transition from complete transmission at small incidence angles to no transmission at large angles the full model predicts a more gradual transition with partial reflection transmission that is sensitive to the canyon geometry and controlled by evanescent modes for small incidence angles and relatively short waves model results for large incidence angles are compared with data from directional wave buoys deployed around the rim and over scripps canyon near san diego california during the nearshore canyon experiment ncex wave heights are observed to decay across the canyon by about a factor 5 over a distance shorter than a wavelength however a spectral refraction model predicts an even larger reduction by about a factor 10 because low frequency components cannot cross the canyon in the geometrical optics approximation the coupled mode model yields accurate results over and behind the canyon these results show that although most of the wave energy is refractively trapped on the offshore rim of the canyon a small fraction of the wave energy tunnels across the canyon simplifications of the model that reduce it to the standard and modified mild slope equations also yield good results confirming that evanescent modes and high order bottom slope effects are of minor importance for the energy transformation of waves propagating across depth contours at large oblique angles

potential field data in integrated frontier basin geophysics successes and challenges on australia s continental margin abstract offshore frontier sedimentary basins are characterised by a lack of constraining geological and geophysical data this lack of data is generally the result of deep water 500 m difficult geology volcanics and salt remoteness and harsh met ocean conditions these characteristics present significant challenges to marine surveying which means that frontier basins tend to be underexplored with continuing interest in exploration for energy resources in frontier regions many frontier basins around the world have been the focus of increasingly sophisticated geophysical studies that integrate a range of methodologies including those based on potential field gravity and magnetic data underexplored frontier basins around australia s continental margin have received increased attention during the last decade largely as a result of government funded programs of precompetitive data acquisition and analysis the components of this work that have relied heavily on potential field data include first pass depth to basement estimation using spectral techniques applied to magnetic data enhancement of gravity and magnetic images to aid the identification of basin depocentres and to facilitate onshore offshore geological interpretation of basement structure multi scale edge detection applied to gravity and magnetic data to aid the interpretation of basement structure 3d forward and stochastic inverse modelling of gravity data to guide seismic interpretation of sediment thickness and basement structure and using supercomputers for high resolution regional scale 3d inverse modelling of magnetic and gravity data to constrain the physical properties of the crust despite the additional insight offered by this work efforts to understand frontier basins are not without challenges one of the most fundamental of which is to ensure that non specialists are not misinterpreting data e g wrongly interpreting artefacts arising from specific processing the other main challenge in australian frontier basins arises from a lack of constraints on crustal structure this leads to significant ambiguity when using gravity data to infer sediment thickness or to understand the nature of basement this ambiguity could be vastly reduced through the acquisition of seismic refraction data that focuses on imaging crustal structure further opportunities exist in using alternative methods for automated depth to basement estimation incorporating process oriented rather than static potential field modelling and in applying 3d forward and inverse gravity and magnetic modelling to other australian frontier basins

modeled alongshore circulation and force balances onshore of a submarine canyon alongshore force balances including the role of nonlinear advection in the shoaling and surf zones onshore of a submarine canyon are investigated using a numerical modeling system delft3d swan the model is calibrated with waves and alongshore flows recorded over a period of 1 5 months at 26 sites along the 1 0 2 5 and 5 0 m depth contours spanning about 2 km of coast field observation based estimates of the alongshore pressure and radiation stress gradients are reproduced well by the model model simulations suggest that the alongshore momentum balance is between the sum of the pressure and radiation stress gradients and the sum of the nonlinear advective terms and bottom stress with the remaining terms e g wind stress and turbulent mixing being negligible the simulations also indicate that unexplained residuals in previous field based estimates of the momentum balance may be owing to the neglect of the nonlinear advective terms which are similar in magnitude to the sum of the forcing pressure and radiations stress gradients and to the bottom stress

infragravity waves over a natural barred profile measurements of cross shore flow were made across the surf zone during a storm as a nearshore bar became better developed and migrated offshore measured infragravity band spectra were compared to synthetic spectra calculated numerically over the natural barred profile assuming a white run up spectrum of leaky mode or high mode edge waves as in earlier studies the spectra compared closely however for some frequencies the energy of the measured spectrum exceeded the energy of the synthetic spectrum suggesting that the run up spectrum was not white but had dominating frequencies utilizing cross shore flow data and synthetic spectra from a number of cross shore locations an equivalent run up spectrum was calculated for each day on the first day of the storm the equivalent run up spectrum indicated a dominant wave that had a node in velocity reasonably close to the bar crest later during the storm when the bar had migrated farther offshore there was no evidence for a dominant motion having a velocity node at the bar crest the structure of the equivalent run up spectrum compared well with spectra of direct measurements of run up obtained several hundred meters away we have no clear evidence in support of the theory that infragravity waves might form or force the offshore migration of a bar to confirm this finding longer records obtained synoptically over a developing bar are required the dominant wave observed early in the storm was consistent with symond and bowen s 1984 theoretical prediction of resonant amplification of discrete frequencies over a barred profile

modeling of nonlinear wave propagation over fringing reefs abstract the applicability of existing nonlinear triad spectral models for steep slopes 0 1 0 2 characteristic of reef environments was investigated using both deterministic phase resolving and stochastic phased averaged formulations model performance was tested using laboratory observations of unidirectional wave transformation over steep and smooth bathymetry profiles the models developed for mild slopes were implemented with minimal modifications the inclusion of breaking parametrizations and linear steep slope corrections required by laboratory data the deterministic model produced typically more accurate predictions than the stochastic one but the phase averaged formulation proved fast enough to allow for an inverse modeling search for the optimal breaking parametrization the effects of the additional assumptions of the stochastic approach resulted in a slower than observed evolution of the infragravity band despite the challenge posed by the fast wave evolution and energetic breaking characteristic to the steep reef slopes both formulations performed overall well and should be considered as good provisional candidates for use in numerical investigation of wave current interaction processes on steep reefs

skewness in the nearshore zone a comparison of estimates from marsh mcbirney current meters and colocated pressure sensors many of the field studies that have undertaken an investigation of surf zone dynamics e g nearshore sediment transport study canadian coastal sediment study have relied on marsh mcbirney electromagnetic current meters to measure the velocity field data from these instruments have been used to compute higher order moments of the velocity field which are used as an input in many sediment transport models however recent laboratory measurements by aubrey and trowbridge 1985 have raised questions regarding the suitability of marsh mcbirney current meters for surf zone studies and the accuracy of higher order velocity moments fortunately colocated pressure and velocity measurements were made during field programs in effect this provides a calibration check in the field of the performance of these instruments a cross spectral analysis indicates that colocated marsh mcbirney current meter measurements are highly coherent this is true for measurements from inside and outside the surf zone as a result normalized skewness estimates computed with the aid of the bispectrum from these colocated current meter measurements differ by only a few percent a cross spectral analysis between colocated pressure and velocity measurements show a similar distribution of variance for wind wave frequencies observed differences between normalized skewness estimates from colocated pressure and velocity measurements are shown to be consistent with the theoretical expectation that pressure has a nonlinear dependence on velocity these results suggest that the marsh mcbirney current meter is suitable for the estimation of nondimensional velocity moments even in breaking wave conditions

crustal thickness beneath libya and the origin of partial melt beneath as sawda volcanic province from receiver function constraints this study investigates crustal thickness and properties within the libyan region results obtained from 15 seismic stations belonging to the libyan center for remote sensing and space science are reported in addition to 3 seismic stations publically available using receiver functions the results show crustal thicknesses ranging from 24 km to 36 km with uncertainties ranging between 0 10 km to 0 90 km more specifically crustal thickness ranges from 32 km to 36 km in the southern portion of the libyan territory then becomes thinner between 24 km and 30 km in the coastal areas of libya and thinnest between 24 km and 28 km in the sirt basin the observed high vp vs value of 1 91 at one station located at the as sawda volcanic province in central libya indicates the presence of either partial melt or an abnormally warm area this finding suggests that magma reservoirs beneath the libyan territory may still be partially molten and active thereby posing significant earthquake and volcanic risks the hypothesis of an active magma source is further demonstrated though the presence of asthenospheric upwelling and extension of the sirt basin this study provides a new calculation of unconsolidated sediment layers by using the arrival time of the p to s converted phases the results show sediments thicknesses of 0 4 km to 3 7 km with the vp vs values ranging from 2 2 to 4 8 the variations in crustal thickness throughout the region are correlated with surface elevation and bouguer gravity anomalies which suggest that they are isostatically compensated

waves currents sediment flux and morphological response in a barred nearshore system abstract a shore normal array of seven bi directional electromagnetic flowmeters and nine surface piercing continuous resistance wave staffs were deployed across a multiple barred nearshore at wendake beach georgian bay canada and monitored for a complete storm cycle time integrated estimates of total itvf and net invf sediment volume flux together with bed elevation changes were determined using depth of activity rods the three bars ranging in height from 0 10 to 0 40 m accreted during the storm 0 03 m and the troughs were scoured 0 05 m sediment reactivation depths reached 0 14 m and 12 of the nearshore control volume was mobilized however the invf value for the storm was less than 1 of the control volume revealing a near balance in sediment volume in the bar system landward migration of the inner crescentic and second sinuous bars occurred in association with an alongshore migration of the bar form itself the outermost straight shore parallel bar remained fixed in location the surf zone was highly dissipative throughout the storm 3 8 10 2 192 10 2 and the wave spectrum was dominated by energy at the incident frequency spectral peaks at frequencies of the first harmonic and at one quarter that of the incident wave were associated with secondary wave generation just prior to breaking and a standing edge wave respectively the former spectral peak was within the 95 confidence band for the spectrum while the latter contributed not more than 10 to the total energy in the surface elevation spectrum even near the shoreline during the storm wave height exceeded 2 m h s and periods reached 5 s t p k orbital velocities exceeded 0 5 m s 1 u rm s and were above the threshold of motion for the medium to fine sands throughout the storm shore parallel flows in excess of 0 4 m s 1 were recorded with maxima in the troughs and minima just landward of the bar crest the rate and direction of sediment flux is best explained by the interaction of antecedent bed slopes with spatial gradients in the mean and asymmetry of the shore normal velocity field these hydrodynamic parameters represent steady flows superimposed on the dominantly oscillatory motion and assumed a characteristic spatial pattern from the storm peak through the decay period increases spatially in the magnitudes of both the mean flows and flow asymmetries cause an increasing net transport potential erosion decreases in these values spatially cause a decreasing net transport potential and thus deposition these transport potentials are increased or decreased through the gravity potential induced by the local bed slope shore parallel flow was important in explaining sediment flux and morphological change where orbital velocities mean flows and flow asymmetries were at a minimum

transformation of long surface and tsunami like waves in the ocean with a variable bathymetry we consider a transformation of long linear waves in an ocean with a variable depth we calculate the transformation coefficients the coefficients of transmission and reflection as the functions of frequency and the total depth drop for three typical models of bottom profile variation i piecewise linear ii piecewise quadratic and iii hyperbolic tangent profiles for all these cases exact solutions are obtained analysed and graphically illustrated this allows us to derive the transformation coefficients in the analytic form for the subsequent comparison with the results of approximate numerical or experimental study we show that the results obtained are in agreement with both the energy flux conservation and lamb s formulae in the limiting case of zero frequency we also study wave transformation on the underwater barriers and trenches of different shapes and compare the results obtained

crustal stretching style variations in the northern margin of the south china sea linking deep seismic profiles with regional scale gravity inversion is a powerful tool to deduce the architecture of rifted margins and their structural evolution here we map upper and lower crustal thicknesses of the northern south china sea scs margin in order to investigate the occurrence of depth dependent crustal extension from the proximal to the distal margin by comparing upper and lower crustal stretching factors we find that the northern margin of the scs is segmented in three parts 1 sedimentary basins where upper crust is stretched more than lower crust 2 distal margin where lower crust is stretched more than upper crust 3 mostly proximal margin regions where the two layers have similar stretching factors our results suggest that sedimentary basins and distal margin prominently feature depth dependent extension however accommodated by different processes while differential thinning within sedimentary basins appears to be governed by lateral pressure variations inducing lower crustal flow we suggest the distal margin to be affected by a combination of mantle flow induced lower crustal shearing and sequential fault activity during crustal hyper extension

a comparison of two spectral wave models in the southern california bight abstract two models a spectral refraction model longuet higgins and a parabolic equation method pem refraction diffraction model kirby are used to simulate the propagation of surface gravity waves across the southern california bight the bight contains numerous offshore islands and shoals and is significantly larger 300 km by 300 km than regions typically studied with these models the effects of complex bathymetry on the transformation of incident wave directional spectra s0 f 0 which are very narrow in both frequency and direction are difficult to model accurately as s0 f 0 becomes broader in both dimensions agreement between the models improves and the spectra predicted at coastal sites become less sensitive to errors in the bathymetry grid to tidal changes in the mean water depth and to uncertainty in s0 f 0 itself the smoothing associated with even relatively narrow 0 01 hz 5 bandwidth s0 f 0 is usually sufficient to bring the model predictions of shallow water energy into at least qualitative agreement however neither model is accurate at highly sheltered sites the importance of diffraction degrades the predictions of the refraction model and a positive bias o 10 of the deep ocean energy in the refraction diffraction model estimates believed to stem from numerical noise kirby may be comparable to the low wave energy the best agreement between the predicted spectra generally occurs at moderately exposed locations in deeper waters within the bight away from shallow water diffractive effects and in the far field of the islands in these cases the differences between the models are small comparable to the errors caused by tidal fluctuations in water depth as waves propagate across the bight the accuracy of predicted energies at these sites is likely to be limited by the uncertainty in specifying s0 f 0

observations of infragravity waves infragravity wave periods of one half to a few minutes energy levels observed for about 1 year in 8 m water depth in the pacific and in 8 and 13 m depths in the atlantic are highly correlated with energy in the swell frequency band 7 to 20 s periods suggesting the infragravity waves were generated locally by the swell the amplification of infragravity wave energy between 13 and 8 m depth separated by 1 km in the cross shore is about 2 indicating that the observed infragravity motions are dominated by free waves not by group forced bound waves which in theory are amplified by an order of magnitude in energy between the two locations however bound waves are more important for the relatively few cases with very energetic swell when the observed amplification between 13 and 8 m depth of infragravity wave energy was sometimes 3 times greater than expected for free waves bispectra are consistent with increased coupling between infragravity waves and groups of swell and sea for high energy incident waves

a field study of wave reflections from an exposed dissipative beach abstract this paper describes a field study of wave reflections from an exposed dissipative beach using only water surface elevation data from five wave height sensors many insights were gained into beach reflections and their detection a technique normally reserved for laboratory use was successfully applied to surf zone data to separate incident and reflected infragravity wave energy spectra the reflected wave energy was found to be significant for all wave periods greater than 20 seconds and often total for wave periods greater than 30 seconds the evidence was heavily weighted towards the observed reflected wave activity being essentially two dimensional with the influence of three dimensional edge waves being minimal other findings relate to the value of wave energy spectra computed from a single surf zone sensor the relevance of single slope approximations of beach profiles and the relationship between incident infragravity wave activity and the longer period swash excursions on a beach face

nearshore water motion and mean flows in a multiple parallel bar system abstract measurements of surface waves and nearshore water motion were made with wave staffs and electromagnetic currentmeters across the straight outer bars of a multiple parallel bar system at wasaga beach georgian bay ontario three sets of experiments were carried out between 1983 and 1985 and this paper examines the characteristics of spatial and temporal variations in incident waves and mean flows incident wave spectra during storms are typical of fetch limited conditions with a prominent peak in the wind wave frequency and a sharp cut off towards lower frequencies there is little evidence of significant energy at infragravity frequencies mean flows perpendicular to the shoreline during storms measured at 0 1 m above the bed are offshore with speeds of 0 04 0 10 m s 1 during moderate storms and 0 25 m s 1 during intense fall storms longshore currents are relatively weak and their direction closely follows changes in the incident wind angle about shore perpendicular there is no evidence of rip cell development and offshore return flow occurs as an undertow uniformly alongshore forming a simple two dimensional net circulation pattern the presence of the undertow very close to the bed is an important control on the direction of suspended sediment transport and thus in maintaining the equilibrium of the nearshore bars

deep structure of the west african continental margin congo zaire angola between 5 s and 8 s from reflection refraction seismics and gravity data summary the deep structure of the west african continental margin between 5 s and 8 s was investigated using vertical reflection and wide angle reflection refraction techniques during the zaiango project a joint programme conducted in 2000 april by ifremer and totalfinaelf to penetrate below the salt layer a non conventional low frequency seismic source was used in the single bubble mode together with ocean bottom instruments hydrophones and seismometers and a 4 5 km long streamer that recorded multichannel seismic reflection mcs the data show that the continental crust thins abruptly over a lateral distance of less than 50 km from 30 km thick below the continental platform based on gravity data to less than 4 km thick below the lower congo basin that formed prior to the aptian salt deposition this subsalt sedimentary basin 180 km wide 4 km thick with velocities varying from 4 7 km s 1 to 5 8 km s 1 at the bottom is located between the foot of the continental slope and the oceanic domain it is underlain by crust of an intermediary or transitional type between continental crust and what can be recognized as oceanic crust in the transitional zone a crustal upper layer is present below the pre salt sedimentary basin 3 to 7 km thick with velocities increasing from 5 8 km s 1 at the top to 6 8 km s 1 at the bottom of the layer this layer appears to thin regularly from 6 7 km thick below the depocentre of the pre salt sedimentary basin to 3 4 km thick below the western termination of the basin below this upper crustal layer an anomalous velocity layer 7 2 to 7 8 km s 1 is documented below the eastern side of the basin where the crustal thinning is at a maximum the origin of this layer is unknown several arguments like rifting duration between 15 ma and 30 ma or the absence of seaward dipping reflectors precludes the hypothesis of underplated mantle material but other hypotheses such as serpentinized material or high grade metamorphic crustal rocks or a mixture of mafic and ultramafic crustal rocks are plausible near the ocean termination of the basin the transitional zone is bounded to the west by a basement ridge that is clearly documented on two profiles 7 11 and 14 having a dense ocean bottom seismometer hydrophone obs obh spacing on these profiles an anomalous velocity layer is present in the westernmost part of the transitional zone below the basement ridge and in the oceanic domain this layer absent on profile 3 may be related either to oceanization and slow seafloor spreading processes or to a consequence of the rifting process

observations of near surface currents and temperature in the central and western tropical atlantic ocean eleven satellite tracked drifting buoys were deployed in the central south atlantic ocean during two austral summer and two austral winter cruises between 7 s and 11 s and 23 w and 31 w during austral winter net buoy drift was to the west surface geostrophic flow was to the east between 7 s and 9 s it is proposed that strong southeast trade winds can induce directly driven surface flows to the west that are more intense than the eastward geostrophic flows associated with the south equatorial countercurrent secc a sustained period of eastward drift within the secc was observed during austral summer when the trades are weaker the trajectories indicate surface waters north of 8 s have a mean northward meridional component and those south of 8 s a southward component the buoys which drifted north became entrained into the north brazilian coastal current nbcc and those that drifted south into the brazil current one buoy left the nbcc at about 5 n to drift northeastward in the north equatorial countercurrent necc this trajectory and historical ship drift reports suggest that the necc may extend only to 35 w to 40 w during boreal winter temperature data obtained as the buoys drifted westward and northward suggest that increases in upper layer heat content can be attributed to heat fluxes through the sea surface

morphodynamics of a bar trough surf zone abstract a field study was made of the distinguishing morphodynamic processes operating in a surf zone which perennially exhibits accentuated bar trough topography the longshore bar trough and rhytmic bar and beach states as described by wright and short 1984 characteristic features of the morphology include a shallow bar with a steep shoreward face a deep trough and a steep beach face this morphology which is favored by moderate breaker heights and small tidal ranges strongly controls the coupled suite of hydrodynamic processes in contrast to fully dissipative surf zones the bar trough surf zone is not at all saturated and oscillations at incident wave frequency remain dominant from the break point to the subaerial beach the degree of incident wave groupiness does not change appreciably across the surf zone infragravity standing waves which in dissipative surf zones dominate the inshore energy remain energetically secondary and occur at higher frequencies in the bar trough surf zone analyses of the field data combined with numerical simulations of leaky mode and edge wave nodal antinodal positions over observed surf zone profiles indicate that the frequencies which prevail are favored by the resonant condition of antinodes over the bar and nodes in the trough standing waves which would have nodes over the bar are suppressed sediment resuspension in the surf zone appears to be largely attributable to the incident waves which are the main source of bed shear stress in addition the extra near bottom eddy viscosity provided by the reformed non breaking waves traversing the trough significantly affects the vertical velocity profile of the longshore current whereas the bar is highly mobile in terms of onshore offshore migration rates the beach face and inner regions of the trough are remarkably stable over time

gravity inversion mapping of crustal thickness and lithosphere thinning for the eastern mediterranean the distribution of oceanic and continental crust in the eastern mediterranean region is not well understood but has major implications for tectonic evolution of this region and its petroleum systems in particular the location of the continent ocean boundary cob the ocean continent transition oct structure and crustal thickness within the basin regions is a topic of much debate while seismology especially refraction seismology is an ideal method for locally determining crustal thickness it is limited to 2d as 3d mapping of crustal thickness is not practical or affordable over large areas however a recent development in 3d crustal thickness mapping uses gravity anomaly inversion we illustrate the application of this technique using the example of the eastern mediterranean figure 1 the new 3d gravity inversion technique incorporating a lithosphere thermal gravity anomaly correction is used to map moho depth crustal basement thickness and continental lithosphere thinning we then use thi

on time delays in the nearshore zone between onshore and longshore currents at incident wave frequencies observations of horizontal velocities just outside the surf zone on two natural beaches show unexpected time lags between the onshore offshore currents and the longshore currents at incident wave frequencies these time lags typically around 20 s appear to have been first noticed by katoh 1981 in data from a beach in japan a number of possible hypotheses to explain these time lags are examined of these propagation of secondary waves formed by reflection or diffraction at a point alongshore from the measurement site seems most able to explain the observations this suggests a significance for reflected diffracted waves some distance from breakwaters or headlands which has not previously been recognized in field studies however a number of questions remain that will only be resolved by further field data particularly from beaches far removed from end effects

standing infragravity waves over an alongshore irregular rocky bathymetry rocky reef coastlines typically feature highly variable and often abrupt cross shore and alongshore changes in bathymetry the effects of this irregular rocky bathymetry on the dynamics of infragravity waves are largely unknown most models of infragravity wave dynamics have been developed and validated on smooth alongshore uniform bathymetries which may break down over these highly variable bathymetries a 2 week field experiment was conducted on a rocky reef fringed beach to investigate how the variable bathymetry affects the spatial and temporal variability of infragravity waves the height of short sea swell waves decreased over the shallow reef due to breaking whereas the height of infragravity waves increased toward the shoreline both during a storm event hm0 2 3 m and under moderate wave conditions hm0 1 0 1 8 m the infragravity waves formed a persistent cross shore standing wave pattern along the entire shoreline despite the irregular bathymetry in addition the alongshore components of infragravity waves refracted by the presence of the nearshore reef were observed to propagate in opposite directions up and down the coast resulting in a local alongshore standing wave pattern thus the presence of highly variable nearshore bathymetry which commonly occurs along rocky reef coastlines may produce both cross shore and alongshore standing wave patterns

field observations of surf beat 1 progressive edge waves nineteen biaxial electromagnetic current meters have been used to determine the longshore and on offshore structure of currents at surf beat periods 1 4 min the sensors formed two linear arrays a long shore array within the surf zone and an on offshore array stretching from the shoreline to well beyond the breaker line analysis of the longshore current components yields a clear picture of progressive low mode edge waves with frequency wave number dispersion relations which are in remarkably good agreement with predictions some separation of edge wave modes is found with mode zero energy dominating in the frequency band 0 006 and 0 011 hz and mode one between 0 015 and 0 025 hz on offshore currents present a rather different picture which while not inconsistent with the longshore currents suggests that other sources of energy are also important to the on offshore currents these include standing edge waves probably formed by reflections at nearby scripps canyon and motions which are nonresonantly forced by incoming wave groups

obliquely incident irregular waves in surf and swash zones a time dependent numerical model is developed for predicting the free surface elevation and depth averaged cross shore and alongshore velocities in the swash and surf zones under obliquely incident irregular waves the assumption of small incident angles is made to simplify the finite amplitude shallow water equations and allow the computation of the cross shore fluid motion using the existing one dimensional model for normally incident waves the developed numerical model is compared with available field data the time dependent model is shown to be capable of predicting the cross shore variations of the root mean square wave height and longshore current in the surf zone the numerical model predicts large cross shore and alongshore velocities near the shoreline the causes of these large velocities are examined but cannot be ascertained for lack of velocity data near the shoreline

waves long waves and nearshore morphology abstract recent field measurements on beaches of different slopes have established that wave motion at periods substantially longer than the incident waves dominates the velocity field close to the shore analysis of a number of extensive data sets shows that much of this long wave motion is in the form of progessive edge waves though forced wave motion standing edge waves and free waves propagating away from the shore may also contribute to the energy theoretically the drift velocities in bottom boundary layers due to edge waves show spatial patterns of convergence and divergence which may move sediment to form either regular crescentic or cuspate features when only one edge wave mode dominates or a bewildering array of bars bumps and holes when several phase locked modes exist together convincing field demonstration of the link between nearshore topography and edge waves only exists for the special case of small scale beach cusps on steep beaches formed by edge waves at the subharmonic twice the period of the incident waves at longer periods the link is proving more difficult to establish due to the longer time scales of topographic changes the interaction between pre existing topography and the water motion and the observation of broad banded edge wave motion which is not readily linked to topography with a well defined scale these ideas are however central to the study of nearshore processes as most of the plausible alternate hypotheses do not seem to lead to quantitative predictions clearly further theoretical and observational work is essential

swan predictions of waves observed in shallow water onshore of complex bathymetry abstract swan model predictions initialized with directional wave buoy observations in 550 m water depth offshore of a steep submarine canyon are compared with wave observations in 5 0 2 5 and 1 0 m water depths although the model assumptions include small bottom slopes the alongshore variations of the nearshore wave field caused by refraction over the steep canyon are predicted well over the 50 days of observations for example in 2 5 m water depth the observed and predicted wave heights vary by up to a factor of 4 over about 1000 m alongshore and wave directions vary by up to about 10 sometimes changing from south to north of shore normal root mean square errors of the predicted wave heights mean directions periods and radiation stresses less than 0 13 m 5 1 s and 0 05 m 3 s 2 respectively are similar near and far from the canyon squared correlations between the observed and predicted wave heights usually are greater than 0 8 in all water depths however the correlations for mean directions and radiation stresses decrease with decreasing water depth as waves refract and become normally incident although mean wave properties observed in shallow water are predicted accurately nonlinear energy transfers from near resonant triads are not modeled well and the observed and predicted wave energy spectra can differ significantly at frequencies greater than the spectral peak especially for narrow band swell

two ship deep seismic soundings in the basins of the eastern mediterranean sea pasiphae cruise summary new results from a two ship refraction and oblique reflection deep seismic cruise are presented to discuss the nature of the crust in the ionian sirte and herodotus abyssal plains these expanding spread profiles were processed and analysed in both the x t and p domains arrival times of reflected and refracted branches are matched by ray tracing in both domains in spite of a shallow evaporitic sequence messinian evaporites deposited on top of a thick sedimentary pile responsible for velocity inversions on many profiles we obtain excellent velocity control down to moho depth pmp and pn 8 4 8 6 km s 1 arrivals are observed the three basins have a relatively thin crust 8 to 11 km overlain by a thick sedimentary cover up to 10km in the herodotus abyssal plain the moho boundary and main crustal units identified in the basins can be followed beneath the calabrian prism to the west and beneath the mediterranean ridge to the east the crustal structure is of oceanic type for both the ionian and sirte basins where typical oceanic layer 2 and 3 are recognized the thin crust of the herodotus basin may be interpreted either as oceanic or thinned continental crust about 10 km thick the top of the crust of the herodotus basin is much deeper therefore the herodotus basin is probably significantly older than the ionian basin triassic versus early cretaceous in age

crustal configuration in the northern levant basin based on seismic interpretation and numerical modeling abstract the interpretation of five 2d pstm seismic reflection sections 14 s twt covering the northern levant basin revealed a total of 10 horizons among which one is interpreted as an interface that may represent the moho the interpretation of seismic packages and their bounding surfaces as well as the seismic facies analysis were constrained by published 2d seismic interpretations of the northern lebanese offshore a total of nine seismic packages are identified in the basin with ages varying from the mid jurassic to the quaternary the filling of the basin is made up of thick cenozoic and mesozoic strata deposited above rifted triassic early jurassic interval the sediments are deposited in deep water mixed settings resulting from high stand systems various types of carbonate platforms and low stand systems siliciclastic and carbonate deep water turbidite complexes carbonate and siliciclastic systems are sealed by 1 1 5 km of evaporites and underlie plio quaternary hemipelagic and pelagic sediments intercalated by turbiditic sheets the time horizons were converted into depth using two methods the first one is based on stacking velocities and the second one on velocities resulting from refraction data 2d crustal modeling was achieved by integrating free air gravity anomaly geoid heights and topography data on the five interpreted pstm seismic lines the models representing five sections across the northern levant basin show a progressively attenuated crystalline crust in an ew direction away from the basin s eastern margin the crystalline crust is best interpreted as a strongly thinned continental crust under the levant basin represented by two distinct components an upper and a lower continental crust the moho appears to be situated between 20 and 23 km in the central and southern lebanese offshore estimated surface heatflow in the basin is around 40 mw m2 which is lower than reported values for the onshore and the margin these differences in heatflow values between the offshore the margin and the onshore have an important impact on hydrocarbon maturation and assessment of potential petroleum systems

generation of seiches by cold fronts over the southern north sea 1 seiches affecting the port of rotterdam are generated in the southern north sea their generation is investigated with observations and numerical simulations a wavelet analysis of the observations both at sea and in the harbor shows that low frequency energy 0 1 2 0 mhz does indeed occur at sea prior to each seiche event in rotterdam an analysis of 6 years of weather charts indicates that all 51 seiche events in this period with amplitude exceeding 0 25 m coincided with the passage of a low pressure weather system some of these low pressure systems included a sharp cold front classical or ana whereas others included a more diffuse cold front split or kata numerical simulations with a hydrodynamic model driven by meteorological observations reproduced the seiches for situations with the sharp cold fronts correctly the seiches that were simulated for situations with the gradual cold fronts do not agree with the observations which is ascribed to the inadequate atmospheric forcing of the hydrodynamical model

modification of edge waves by barred beach topography a numerical model to calculate edge wave profiles and wavelengths for arbitrary bottom profiles and given frequency was developed utilizing the model wave equation of berkhoff 1972 following smith and sprinks 1975 surface profiles and longshore wave numbers were found for fixed wave periods using an eigenvalue approach the model was verified by calculating profiles and dispersion for a plane beach of mild slope and comparing results with the analytic solution of eckart 1951 and ursell 1952 the model was then applied to the case of a beach with multiple offshore bars with bar positions determined by the antinodal positions of a standing incident wave of a set frequency calculation of surface profiles for a range of frequencies using a fixed beach indicated that antinodes of the edgewave profiles tend to be trapped by the bars suggesting the possibility that bars affect the long wave climate in such a way as to lead to self maintenance over extended periods no effect of the bars on the dispersion relationship was found and it was concluded that for the particular beach profile used the edge waves respond to the mean beach slope over the offshore length of the profile

refraction and reflection of infragravity waves near submarine canyons 1 the propagation of infragravity waves ocean surface waves with periods from 20 to 200 s over complex inner shelf water depths from about 3 to 50 m bathymetry is investigated with field observations from the southern california coast a wave ray path based model is used to describe radiation from adjacent beaches refraction over slopes smooth changes in bathymetry and partial reflection from submarine canyons sharp changes in bathymetry in both the field observations and the model simulations the importance of the canyons depends on the directional spectrum of the infragravity wave field radiating from the shoreline and on the distance from the canyons averaged over the wide range of conditions observed a refraction only model has reduced skill near the abrupt bathymetry whereas a combined refraction and reflection model accurately describes the distribution of infragravity wave energy on the inner shelf including the localized effects of steep walled submarine canyons

physical linkages between an offshore canyon and surf zone morphologic change the causes of surf zone morphologic changes observed along a sandy beach onshore of a submarine canyon were investigated using field observations and a numerical model delft3d swan numerically simulated morphologic changes using four different sediment transport formulae reproduce the temporal and spatial patterns of net cross shore integrated between 0 and 6 5 m water depths accretion and erosion observed in a 300 m alongshore region a few hundred meters from the canyon head the observations and simulations indicate that the accretion or erosion results from converging or diverging alongshore currents driven primarily by breaking waves and alongshore pressure gradients the location of convergence or divergence depends on the direction of the offshore waves that refract over the canyon suggesting that bathymetric features on the inner shelf can have first order effects on short term nearshore morphologic change this article is protected by copyright all rights reserved

wave driven setup and alongshore flows observed onshore of a submarine canyon 1 the effect of alongshore variations in the incident wavefield on wave driven setup and on alongshore flows in the surfzone is investigated using observations collected onshore of a submarine canyon wave heights and radiation stresses at the outer edge of the surfzone water depth 2 5 m varied by up to a factor of 4 and 16 respectively over a 450 m alongshore distance resulting in setup variations as large as 0 1 m along the shoreline water depth 0 3 m even with this strong alongshore variability wave driven setup was dominated by the cross shore gradient of the wave radiation stress and setup observed in the surfzone is predicted well by a one dimensional cross shore momentum balance both cross shore radiation stress gradients and alongshore setup gradients contributed to the alongshore flows observed in the inner surfzone when alongshore gradients in offshore wave heights were large and a simplified alongshore momentum balance suggests that the large o 1 kg s2 m observed setup induced pressure gradients can drive strong o 1 m s alongshore currents

edge waves on the sydney coast pressure and current oscillations at periods of 40 s to 17 min observed during storm conditions at two locations separated by 560 m in the alongshore direction in the coastal ocean near sydney australia indicate the existence of infragravity waves having amplitudes of 20 cm and velocities of 10 cm s 1 the observed infragravity waves appear to be locally forced by the wind wave envelope through radiation stress yet the observed alongshore phase differences of the infragravity waves are consistent with those predicted from free edge wave theory for low mode edge waves travelling northward and the relationship of pressure to velocity at each location is also consistent with free edge wave theory as a function of time the infragravity wave spectral energy grows and decays in step with the longer period wind waves suggesting a continuous transfer of energy the infragravity waves appear to contain energy in both directly forced and freely propagating edge wave oscillations the edge waves may be generated either by radiation stress as outlined above by a resonant triad mechanism or by a combination of the two

three dimensional gravity inversion for moho depth at rifted continental margins incorporating a lithosphere thermal gravity anomaly correction summary this paper describes a method for determining moho depth lithosphere thinning factor 1 1 and the location of the ocean continent transition at rifted continental margins using 3 d gravity inversion which includes a correction for the large negative lithosphere thermal gravity anomaly within continental margin lithosphere the lateral density changes caused by the elevated geotherm in thinned continental margin and adjacent ocean basin lithosphere produce a significant lithosphere thermal gravity anomaly which may be in excess of 100 mgal and for which a correction must be made in order to determine moho depth accurately from gravity inversion we describe a method of iteratively calculating the lithosphere thermal gravity anomaly using a lithosphere thermal model to give the present day temperature field from which we calculate the lithosphere thermal density and gravity anomalies for continental margin lithosphere the lithosphere thermal perturbation is calculated from the lithosphere thinning factor 1 1 obtained from crustal thinning determined by gravity inversion and breakup age for thermal re equilibration time for oceanic lithosphere the lithosphere thermal model used to predict the lithosphere thermal gravity anomaly may be conditioned using ocean isochrons from plate reconstruction models to provide the age and location of oceanic lithosphere a correction is made for crustal melt addition due to decompression melting during continental breakup and seafloor spreading we investigate the sensitivity of the lithosphere thermal gravity anomaly and the predicted moho depth from gravity inversion at continental rifted margins to the methods used to calculate and condition the lithosphere thermal model using both synthetic models and examples from the north atlantic

crustal structure and post rift evolution of the levant basin abstract petroleum exploration activity in the deep basins of the eastern mediterranean has been rejuvenated over the last decade with the discoveries of the giant tamar and leviathan gas fields made by noble energy and its partners and more recently with the discovery of the zohr field by eni with exploration efforts pursuing deeper and older leads a first principles quantitative study of the region seems timely while it is generally agreed that the levant basin formed along the northern passive margin of gondwanaland during the early mesozoic its complex post rift evolution has not been documented in detail located in a unique junction of three active tectonic plates and at the distal end of the nile one of the largest rivers on earth the levant basin is characterized by a thick sedimentary section more than half of which has accumulated since the oligocene in the last 15 of its existence this study implements a quantitative basin modelling workflow that assesses the three dimensional crustal geometry of the levant basin and its post rift evolution by integrating 3d gravity inversion and 3d flexural backstripping techniques the focus of the gravity inversion which includes corrections for the lithosphere thermal gravity anomaly resulting from lithosphere thinning and for the magmatic addition from decompression melting has been to predict both the 3d crustal structure and the magnitude of total lithospheric stretching for the levant basin the 3d flexural backstripping workflow using the outputs of the gravity inversion invokes three major processes in its calculations accompanying the layer by layer removal of the stratigraphy i 3d flexural isostatic unloading ii decompaction in response to the removal of overburden and iii reverse thermal subsidence modelling controlled by the stretching factor derived from the gravity inversion backstripping has produced a series of maps and cross sections depicting the evolution of post rift palaeobathymetry in the levant basin the integration of these techniques allows us to assess the evolution of the basin and the various sedimentary and tectonic events that have occurred since basin initiation although the calculated stretching factors indicate that while the lithosphere underlying the levant basin has been very highly stretched and thinned the stretching did not progress to the formation of new oceanic crust predicted palaeobathymetry portrays a thermally subsiding basin reaching maximum water depths at the onset of the cenozoic this deep basin provided ample amount of accommodation space for the large volumes of terrigenous clastics sourced from the contemporaneous uplift and exhumation of the basin margins and delivered in large part at least since the oligocene by the continental drainage of the nile these sediments have been filling and shallowing the basin ever since the basin evolution described here is consistent with available geophysical and geological data and has important implications for petroleum systems modelling within the basin

atlantic equatorial sections during july 1983 two cross equatorial sections of current temperature and salinity were made in the atlantic during july 1983 the first along 24 w extended from 10 n to 10 s and the second along 28 w from 3 s to 10 n measurements extended from the surface to 300 m in the case of currents and to 800 m for temperature and salinity on the large scale these measurements agree well with historical data on a smaller scale the closely spaced stations across the equator reveal a complex structure in the equatorial currents

bars bumps and holes models for the generation of complex beach topography in shallow water any two waves of the same frequency are shown to produce complex patterns of drift velocity above the sea bed if the longshore components hi h of the wave numbers of the two waves are different these steady flow patterns exhibit a longshore periodicity of wave number hi h irrespective of whether the waves propagate in the same direction say hi he positive or in opposite directions he negative the interaction of two edge wave modes is examined in detail the drift velocities are calculated and a simple sediment transport model is used to predict the beach topography that would be in equilibrium with these flow patterns as expected crescentic sand bars are produced by the special case of standing edge waves hi h intriguingly for all other cases a combination of complex transverse bars plus meandering or straight offshore bars result patterns that are surprisingly reminiscent of many published descriptions of complex rhythmic topography the extension of the model to three or more waves produces topography that appears to be very irregular although the pattern should repeat over sufficiently long distances along the beach even with only three waves these distances may be very large compared to the scale of the bars

low frequency meandering of the atlantic north equatorial countercurrent four 19 month time series of indirect measurements of dynamic height were obtained in the tropical atlantic along 28 w at 0 3 6 and 9 n with three inverted echo sounders and one current meter mooring the series were analyzed to study the time latitude variability of the north equatorial countercurrent necc the eastward flow associated with the necc was present at 28 w from 3 to 9 n during most of the observed period except in march april 1983 and april may 1984 periods that coincided with the onset of the wind at the equator the amplitude of the necc s annual cycle was maximum at 6 n and was larger in 1983 than in 1984 the analysis of the time latitude variability of differences in dynamic height shows a long period meridional shift of the necc the core of the current attains its northernmost location during august september in both years and its southernmost location during march april in 1983 and march in 1984 the location of the core is directly related to the position of the intertropical convergence zone from the time series of dynamic height obtained from the indirect measurements geostrophic velocities and transports were estimated and compared with direct observations of currents and values obtained from hydrographic casts