making avo sections more robust1 when large quantities of seismic data are involved it is impossible to examine all gathers by eye for avo anomalies the standard approach is to compute for each amplitude profile at a specific time on each gather the intercept and gradient of a straight line fit to seismic amplitudes these intercepts and gradients are each plotted as a sort of seismic section an intercept section and a gradient section estimation of the intercept and gradient for a straight line fit to each amplitude profile proceeds traditionally via least squares two undesirable features can be hidden from the user by the fitting procedure namely i the effect of outlying or uncharacteristic amplitudes on the intercept and gradient estimates and ii complete breakdown of the straight line model for the amplitudes thus rendering meaningless the intercept and gradient estimates it should be remembered that least squares can always fit any sequence of numbers to any other sequence of numbers checks are needed to show that the result is meaningful it is shown that statistically robust estimation methods can greatly limit the damage done by outlying amplitudes and that a simple test on the model the runs statistic is capable of detecting breakdown of the straight line assumption it is demonstrated using two seismic data sets that these two techniques used in tandem facilitate much better quality control of avo intercept and gradient calculations

avo as an exploration tool gulf of mexico case studies and examples over the past decade amplitude variation with offset avo has evolved from a relatively new technology to increasingly mature applications particularly successful results have been achieved both offshore and onshore in the us gulf coast careful processing detailed petrophysical modeling from complete log suites and avo analyses closely calibrated to measured properties of target reservoir sands have demonstrated effectiveness however routine application of avo has not been without problems and misapplication has led to disappointing results

theory of cbm avo i characteristics of anomaly and why it is so abstractbased on the inverse correlations between methane content and elastic parameters of coalbed methane cbm reservoirs and on geomodulus approximation of zoeppritz equations we study characteristics of cbm amplitude versus offset avo anomaly our research first suggests that cbm avo cannot and needs not cling to poisson s ratio as avo for conventional gas c gas does and that various approximations of zoeppritz equations for c gas avo can be applied to cbm avo though the elastic contrasts at boundaries of cbm reservoirs cannot be said to be small compared to unity we then show that the sweet spots within a brine saturated high and or medium rank cbm reservoir can have only class iv avo anomaly the reason is that all of the three elastic reflectivities i e reflectivity reflectivity and reflectivity at one and the same boundary of a cbm reservoir are always of the same sign and that reflectivity or density reflectivity or both are significant in comparison with reflectivity

forcing of resonant modes on a fringing reef during tropical storm man yi 1 open basin resonant modes have been observed at tidal frequencies on coastal shelves but their excitation on coral reef platforms has only been suggested the topography of and water depth over most fringing reefs correspond to resonant periods that are the order of tens of minutes and fall outside of the energetic part of the wave spectrum during tropical storm man yi low frequency near resonant oscillations dominated the variance of the sea surface elevation at the shoreline of ipan reef guam the excitation of the resonant modes resulted both from a large increase in water level over the reef due to wave setup which increased the resonant frequencies and the occurrence of wave group forcing with time scales commensurate with these increased resonant frequencies

hydrodynamics of spur and groove formations on a coral reef 1 spur and groove sag formations are found on the fore reefs of many coral reefs worldwide although these formations are primarily present in wave dominated environments their effect on wave driven hydrodynamics is not well understood a two dimensional depth averaged phase resolving nonlinear boussinesq model funwavec was used to model hydrodynamics on a simplified sag system the modeling results show that the sag formations together with shoaling waves induce a nearshore lagrangian circulation pattern of counter rotating circulation cells the mechanism driving the modeled flow is an alongshore imbalance between the pressure gradient pg and nonlinear wave nlw terms in the momentum balance variations in model parameters suggest the strongest factors affecting circulation include spur normal waves increased wave height weak alongshore currents increased spur height and decreased bottom drag the modeled circulation is consistent with a simple scaling analysis based on the dynamical balance of nlw pg and bottom stress terms model results indicate that the sag formations efficiently drive circulation cells when the alongshore sag wavelength allows for the effects of diffraction to create alongshore differences in wave height without changing the mean wave angle

groundwater ion content precursors of strong earthquakes in kamchatka russia the kamchatka peninsula located in the far east of russia is a geologically active margin where the pacific plate subducts beneath the north american and eurasia plates this area is characterised by frequent and strong seismic activity magnitudes reaching 8 6 and epicentres are generally distributed offshore along the eastern coast of the peninsula for many years hydrogeochemicals have been sampled with a mean interval of three days to measure the most common ions in the groundwater of five deep wells in the southern area of the kamchatka peninsula in the last decade five earthquakes with m 6 5 have occurred at distances less than 250 km from these wells these earthquakes were powerful enough for them to be considered as potential originators of precursors in order to reveal possible precursors of these earthquakes we analysed the groundwater ion contents the quasi periodic annual variation was filtered out together with other slow trends and then we smoothed out the high frequency fluctuations that arise from errors in a single measurement when examining the data we labelled each signal with an amplitude greater than three times the standard deviation as an irregularity and we made a first attempt at defining an anomaly as an irregularity occurring simultaneously in more than one parameter at each well in a second definition we used the existence of an irregularity occurring simultaneously in each ion at more than one well then on the basis of past results worldwide and the time interval between the earthquakes observed we chose 158 days as the maximum temporal window between a possible anomaly and the subsequent earthquake with the first anomaly definition we identified 6 anomalies with 4 possible successes and 2 failures for the five earthquakes considered capable of producing precursors we obtained precursors in three cases with the second anomaly definition we identified 10 anomalies with 7 possible successes and 3 failures and we obtained precursors in four of the five earthquakes a schematic model explaining aspects of the complex relationship linking earthquakes and ion content anomalies is also presented

wave transformation over coral reefs ocean wave attenuation on coral reefs is discussed using data obtained from a preliminary field experiment and from the seasat altimeter marked attenuation of the waves is observed the rate being consistent with existing theories of bottom friction and wave breaking decay in addition there is a significant broadening of the spectrum during propagation across reefs three dimensional effects such as refraction and defraction can also lead to substantial wave height reduction for significant distances adjacent to coral reefs as a result a matrix of such reefs provides significantly more wave attenuation than may initially be expected

tracking the amplitude versus offset avo by using orthogonal polynomials1 inversion for s wave velocities from the amplitude variation with offset of p wave data is far from being a standard routine in the seismic processing sequence however the need for tracking the amplitude versus offset avo occurs in several situations for example in order to estimate the zero offset amplitude to reveal areas with particular avo characteristics or to compress the avo so that it is more easily obtainable at a later stage of the seismic processing furthermore weak reflections can occasionally due to the effect of the angle dependent reflectivity have a polarity shift with offset resulting in a very poor or even vanishing stack response in such cases the reflection event has to be represented by some other property than its mean amplitude or stack value we outline how the avo of seismic data may be extracted and classified by the use of orthogonal polynomials the main advantage of this method compared to a general polynomial fit is that the avo may be classified by a unique spectrum of polynomial coefficients this is in analogy to fourier coefficients where the orthogonal basis is harmonic functions the set of orthogonal polynomials is constructed entirely from the set of offset coordinates and these polyno mials are defined only on the offset window considered compared to a fourier transform this is a major advantage since there is no effect of a limited spatial bandwidth the avo of normal moveout corrected data may be represented by a data gather where the orthogonal polynomial coefficients are given as time traces with each trace revealing a certain avo characteristic for instance the stack is proportional to the zeroth order coefficient the mean gradient is given by the firstorder coefficient while the second order coefficient indicates whether the avo increases and then decreases or vice versa

amplitude versus angle analysis of local angle domain common image gathers with prestack gaussian beam migration of seismic data the analysis of seismic reflection amplitude versus angle amplitude versus offset ava avo is an important tool for investigating the physical properties of subsurface rocks and or reservoirs we propose to generate local angle domain common image gathers adcigs from the surface recorded seismic data for ava analysis via gaussian beam migration gbm using a deconvolution imaging condition the traditional seismic reflection amplitude analysis is often based on the avo response from common middle point cmp gathers however the avo analysis based on the horizontally layered medium assumption can achieve success only in simple geological cases our approach provides an ava response during the migration process that reveals true local physical properties in the image domain even if the image point is at a tilted interface or has a complicated overburden geological structure we test our approach with a two layer velocity model with a tilted interface and with the benchmark marmousi2 model that has a complicated geological structure from these two numerical examples we demonstrate that our method faithfully provides the ava response that can be used to further interpret the rock reservoir physical properties

semi empirical treatment of wave transformation outside and inside the breaker line a semi empirical method is developed which predicts the wave height shoaling up to the break point on laboratory and natural beaches linear shoaling is enhanced with empirical curves for the wave shape obtained from field measurements of wave height through the break point and the surf zone made on an ocean beach in southern australia from 1987 to 1989 wave height shoaling breaking and attenuation are unified in a mixed lagrangian eulerian numerical model of random wave height transformation which with the semi analytical enhancements provides improved predictions of the height transformation of individual waves through the surf zone this is demonstrated for a wide variety of cases from the field and laboratory

reflectivity template a quantitative intercept gradient avo analysis to study gas hydrate resources a case study of iranian deep sea sediments abstract presence of gas hydrate and free gas in iranian part of makran accretionary prism changes the elastic properties of unconsolidated sediments and produces sharp bottom simulating reflectors bsrs which are observed on the 2 d seismic data different methods have been applied to estimate the gas hydrate and free gas saturations in marine sediments based on seismic measurements most of these methods are based on relating the elastic properties to the hydrate and free gas saturations and remotely estimating their concentration in this regard using the effective medium theory emt which was developed for different modes of hydrate distribution is more considered among other rock physics theories the main concern about saturation estimations based on emt is that the velocities of the hydrate bearing sediments primarily depend on how they are distributed within the pore space therefore understanding the modes of hydrate distribution at least cementing or non cementing modes is necessary to decrease the estimation uncertainties the first intention of paper is to investigate amplitude variation versus offset avo analysis of bsr to determine the hydrate distribution modes the results from the probable saturation revealed that if the hydrate cements the sediment grains bsr would show the avo class iv and if hydrate does not cement the sediment grains then bsr would show either the avo class ii or class iii depending on the free gas saturation just beneath the bsr the second intention of paper is to introduce some templates called reflectivity templates rts for quantitative study of hydrate resources these templates are provided based on the emt to quantify the hydrate and free gas near the bsr validation of this approach by synthetic data showed that a reliable quantification could be achieved by intercept gradient rts only if these attributes are determined with a high accuracy and good assumptions are made about the mineralogical composition and porosity of the unconsolidated host sediments the results of this approach applied to a 2 d marine pre stack time migrated seismic line showed that less than 10 of the gas hydrate accumulated near to the bsr in anticlinal ridge type structure of iranian deep sea sediments the free gas saturation near to the bsr by assuming a homogeneous distribution was less than 3 and by assuming patchy distribution was about 3 10

linearized avo and poroelasticity the technique of amplitude variation with offset avo allows geoscientists to extract fluid and lithology information from the analysis of prestack seismic amplitudes various avo parameterizations exist all of which involve the sum of three weighted elastic constant terms in present day avo approaches the weighting terms involve either knowledge of the incidence angle only or knowledge of both the incidence angle and the in situ vp vs ratio we have used the theory of poroelasticity to derive a generalized avo approximation that provides the estimation of fluid rigidity and density parameters we have combined two previously independent avo formulations thus reducing instead of adding to the total number of formulations this new approach requires knowledge of a third parameter to compute the weights the dry rock vp vs ratio we have derived a new equation and applied it to model and real data sets the new formulation has allowed us to estimate fluid properties of the reservoir in a more dire

monsoonally influenced circulation around coral reef islands and seasonal dynamics of reef island shorelines synchronous measurements of reef flat waves and nearshore currents were made around the perimeter of three coral reef islands in the central maldives nearshore current velocities were shown to be tidally modulated in most instances with maximum velocities associated with lower tidal stages peak currents were consistently observed on the windward and lateral flanks of island shorelines 0 3 0 4 ms 1 the relatively low elevation of reef surfaces and medium to low energy incident wave energy hs 0 2 0 3 m accounts for low tidal stage peaks in velocity as this corresponds to the period when wave setup induced current processes are maximised significantly all nearshore currents were unidirectional and alongshore the generation of unidirectional alongshore flow patterns results from island configuration which deflects flow and variations in reef geometry that control spatial variations in wave setup around the reef platform edge and at the island shoreline which can drive alongshore currents the presence of a nearshore moat that channelizes flow in the nearshore and variations in reef geometry where topographic low points act as preferential drainage points results show that changes in monsoon conditions that modulate wind and wave patterns from the west to northeast can force substantial changes in reef platform and nearshore current flows but that the degree of change is dependent on reef platform shape and position in the atoll the circular study island was found to exhibit a near complete reversal in unidirectional flow from northeast flow in the westerly monsoon to southwest flow in the northeast monsoon however the elongate and triangular shaped reef platforms exhibited more localised changes in flow patterns significantly those shorelines that exhibit the greatest changes in shoreline currents correspond to the sections of coast that exhibit the largest morphological adjustments between seasons results provide significant insights for the morphodynamics of reef platform islands in particular the nearshore process regime is dominated by alongshore processes furthermore reef platform shape and the magnitude of change in direction of wave approach to reef platforms provide critical indicators of the morphodynamic behaviour of reef platform islands

compressional velocity structure and poisson s ratio in marine sediments with gas hydrate and free gas by inversion of reflected and refracted seismic data south shetland islands antarctica abstract a bottom simulating reflector bsr on the south shetland margin antarctic peninsula was surveyed within the progetto nazionale di ricerche in antartide pnra high resolution multichannel mcs reflection profiles were acquired and a three component ocean bottom seismograph obs was deployed where the bsr was particularly strong the compressional velocity field associated to the bsr was determined by the inversion of travel times of reflected mcs data and refracted obs data events information on shear wave velocities was extracted from the horizontal components of the obs records the analysis indicates the presence of velocity anomalies both above and below the bsr that are not related to the lithologic characteristics of the sedimentary column but can be associated to gas hydrate and free gas presence in pore space amplitude versus offset avo analysis was performed on common depth point gathers which correspond to the obs position the study determined reflection coefficient trends that provided qualitative information about gas hydrate and free gas abundance a theoretical model for elastic porous media was applied to quantify amounts of gas hydrate and free gas the resulting theoretical velocity curves were compared to interval velocity obtained by seismic inversion a quantitative estimation of free gas and gas hydrates trapped in the sediment pore spaces was made by interpreting velocity anomalies with respect to reference velocity curves normally consolidated marine terrigenous sediments the comparison of the results obtained by these independent methods seismic inversion avo analysis and theoretical model gives consistent values of poisson s ratio and gas hydrate and free gas estimation

circulation in enewetak atoll lagoon1 currents at enewetak atoll marshall islands were measured on the reef margins in the channels and in the lagoon lagoon circulation is dominated by wind driven downwind surface flow and an upwind middepth return flow this wind driven flow has the characteristics of an ekman spiral in an enclosed sea lagoon flushing is accomplished primarily by surf driven water input over the windward eastern reefs and southerly drift out the south channel mean water residence time is 1 month while water entering the northern portion of the atoll takes about 4 months to exit

factors facilitating or limiting the use of avo for coal bed methane there are similarities and differences in employing amplitude variation with offset avo to explore for gas sand reservoirs as opposed to coal bed methane cbm reservoirs the main similarity is that large poisson s ratio contrasts resulting in avo gradient anomalies are expected for both kinds of reservoirs the main difference is that cleating and fracturing raise the poisson s ratio of a coal seam as it improves its reservoir potential for cbm while gas always lowers the poisson s ratio of a sandstone reservoir the top of gas sands usually has a negative avo gradient leading to a class one two or three anomaly depending on the impedance contrast with the overlying caprock on the other hand the top of a cbm reservoir has a positive avo gradient leading to a class four anomaly three environmental factors may limit the usage of avo for cbm reservoirs the smaller contrast in poisson s ratio between a cbm reservoir and its surrounding rock variations in the caprock of a specific cbm reservoir and the fact that cbm is not always free to collect at structurally high points in the reservoir however other factors work in favor of using avo the strikingly high reflection amplitude of coal improves signal noise ratio and hence the reliability of avo measurements the relatively simple characteristics of avo anomalies make them easy to interpret because faults are known to improve the quality of cbm reservoirs faults accompanied by avo anomalies would be especially convincing a 3d avo example offered in this paper shows that avo might be helpful to delineate methane rich sweet spots within coal seams

coral reef drag coefficients water depth dependence abstracta major challenge in modeling the circulation over coral reefs is uncertainty in the drag coefficient because existing estimates span two orders of magnitude current and pressure measurements from five coral reefs are used to estimate drag coefficients based on depth average flow assuming a balance between the cross reef pressure gradient and the bottom stress at two sites wind stress is a significant term in the cross reef momentum balance and is included in estimating the drag coefficient for the five coral reef sites and a previous laboratory study estimated drag coefficients increase as the water depth decreases consistent with open channel flow theory for example for a typical coral reef hydrodynamic roughness of 5 cm observational estimates and the theory indicate that the drag coefficient decreases from 0 4 in 20 cm of water to 0 005 in 10 m of water synthesis of results from the new field observations with estimates from previous field and laboratory studies indicate that coral r

depth limited design wave heights in very flat regions abstract engineers working on the flat tops of offshore coral reefs or offshore banks require design criteria for depth limited wave heights over horizontal beds particularly in regions leeward of the bank or reef edge surf zone this paper presents convincing evidence of the fact that in engineering practice both in the laboratory and in the field the value of 0 55 is the largest wave height to water depth ratio realizable for stable shallow water oscillatory waves propagating in water of constant depth horizontal bed this value is much lower than that often used in engineering practice namely a value of about 0 8 which is historically based on the theoretical considerations and laboratory experiments associated with solitary waves this result based on a review of laboratory data and the original results of a major field experiment undertaken on the great barrier reef of australia leads to a significant reduction in design wave forces design crest levels and wave induced scour velocities at the bed

nearshore circulation in a tropical fringing reef system 1 the role of waves tide and wind on the circulation of a fringing reef system was investigated using data collected during a 6 week field experiment in a section of ningaloo reef off western australia the high correlation observed between current velocities and wave height throughout the system revealed the dominant role wave breaking plays in driving the overall reef lagoon circulation whereas the modulation of the currents at tidal frequencies suggested that the wave driven currents responded to tidal variations in the mean water level over the reef the influence of the various forcing mechanisms on the current field was investigated for both high and low frequency bands wave breaking was found to be the dominant forcing mechanism for the low frequency subtidal currents with the subtidal flow pattern consisting of a cross reef flow over the reef alongshore flow in the lagoon and water exiting back to the ocean through the main channel the tides controlled the high frequency current variability via two mechanisms one associated with the ebb flood cycle of the tides and the second associated with tidal modulations of the wave driven currents wind forcing and buoyancy effects were both found to be negligible in driving the circulation and flushing of the system during the observation period flushing time scale estimates varied from as low as 2 h to more than a day for the wide range of observed incident wave heights the results suggest that the circulation of ningaloo reef will be strongly influenced by even a small mean sea level rise

boundary layer turbulence and flow structure over a fringing coral reef measurements of velocity and rates of turbulence were made across a fringing coral reef in the gulf of aqaba red sea to determine the effect that the rough topography has on boundary layer mixing and flow dynamics observations were made at two fore reef sites and a nearby sandy slope the friction velocity u and drag coefficient cd were determined directly from turbulent reynolds stresses measured using acoustic doppler velocimeters values of cd for the coral substrates ranged from 0 009 to 0 015 three to five times greater than over the sandy bottom site the turbulence dissipation rate e was determined by fitting spectra of vertical velocity to the theoretical 5 3 law expected for the inertial subrange of turbulence there was a local balance between production and dissipation of turbulent kinetic energy signifying that we could estimate u from either the mean velocity profile turbulence or dissipation rate of turbulent kinetic energy estimates from all three measures agreed well with mean u uo ranging from 0 10 6 0 03 to 0 12 6 0 03 indicating that existing turbulent boundary layer flow theory can be applied to flows over the rough topography of coral reefs the bottom topography by enhancing both reef scale and local drag and mixing levels allows reef biota to more effectively exchange dissolved and particulate matter with oceanic waters

the application of high resolution 3d seismic data to model the distribution of mechanical and hydrogeological properties of a potential host rock for the deep storage of radioactive waste in france abstract in the context of a deep geological repository of high level radioactive wastes the french national radioactive waste management agency andra has conducted an extensive characterization of the callovo oxfordian argillaceous rock and surrounding formations in the eastern paris basin as part of this project an accurate 3d seismic derived geological model is needed the paper shows the procedure used for building the 3d seismic constrained geological model in depth by combining time to depth conversion of seismic horizons consistent seismic velocity model and elastic impedance in time it also shows how the 3d model is used for mechanical and hydrogeological studies the 3d seismic field data example illustrates the potential of the proposed depth conversion procedure for estimating density and velocity distributions which are consistent with the depth conversion of seismic horizons using the bayesian kriging method the geological model shows good agreement with well log data obtained from a reference well located closest to the 3d seismic survey area modeling of the mechanical parameters such as shear modulus young modulus bulk modulus indicates low variability of parameters confirming the homogeneity of the target formation callovo oxfordian claystone 3d modeling of a permeability index ik seis computed from seismic attributes instantaneous frequency envelope elastic impedance and validated at the reference well shows promising potential for supporting hydrogeological simulation and decision making related to safety issues

the ava amplitude versus angle signature of small scale reflector topography routine amplitude versus offset or amplitude versus angle ava analysis is founded on the predicted reflectivity of a planar boundary between isotropic homogeneous half spaces yet many real boundaries of interest to hydrocarbon exploration may violate these assumptions here we evaluate consequences of boundary roughness i e small scale sub resolution topography on the reflecting boundary to find whether sub resolution topography can deceive routine ava analysis and produce misleading hydrocarbon indicators we use noise free synthetic cmp and stacked section seismograms generated with a kirchhoff integral method for offsets up to 4000 m the reflecting boundary was at 2000 m depth below a single overburden layer and comprised isolated mounds or channels sinusoidal cross section 5 40 m high and 100 750 m wide and more complex roughness simulated with seven 1 5 2 km long bathymetric profiles across modern day river beds with dune and bar features up to approximately 20 m or 20 ms twt vertical relief and 10 100 m or more lateral extent flat boundary responses were taken as the reference against which to compare waveforms and amplitudes through semblance analysis ava intercept a and slope b cross plots and inferred poisson s ratios physical properties of the media were based on shales and brine or oil sands from an offshore uk oilfield for the cmp centred on the topographic feature isolated mounds and channels produced correlated excursions of a and b from the flat boundary response by approximately 35 200 simulating the familiar background trend but sometimes oblique to it inferred poisson s ratios were around 0 3 but for some channels often fell as low as 0 2 potentially interpretable as gas sand for complex boundary topographies on a shale brine sand model ava parameters were extracted for 29 cmps 100 m apart along each of seven profiles on a cross plot they spanned the flat boundary response on a linear trend b 0 01 0 02 1 72 0 09 a similar to reported real data and a realistic mudrock trend but with outliers both above and below it that could be interpreted as real weak anomalies poisson s ratios were 0 30 0 01 between the expected values for brine and oil sand this suggests that boundary roughness may contribute to observed trends on cross plots and possibly small but potentially laterally extensive false ava anomalies may also be induced

anisotropic frequency dependent spreading of seismic waves from first arrival vertical seismic profile data analysis abstracta model of first arrival amplitude decay combining geometric spreading scattering and inelastic dissipation is derived from a multioffset 3d vertical seismic profile data set unlike the traditional approaches the model is formulated in terms of path integrals over the rays and without relying on the quality factor q for rocks the inversion reveals variations of geometric attenuation wavefront curvatures and scattering and the effective attenuation parameter with depth both of these properties are also found to be anisotropic scattering and geometric spreading focusing and defocusing significantly affect seismic amplitudes at lower frequencies and shallower depths statistical analysis of model uncertainties quantitatively measures the significance of these results the model correctly predicts the observed frequency dependent first arrival amplitudes at all frequencies this and similar models can be applied to other types of waves and should be useful for true amplitude studi

wave generated flow on coral reefs an analysis for two dimensional horizontal reef tops with steep faces waves breaking on the seaward rim of a coral reef generate a flow of water from the exposed side of the reef to the sheltered side and or to either channels through the reef rim or lower sections of the latter this wave generated flow is driven by the water surface gradient resulting from the wave set up created by the breaking waves this paper reviews previous approaches to modelling wave generated flows across coral reefs and discusses the influence of reef morphology and roughness upon these flows laboratory measurements upon a two dimensional horizontal reef platform with a steep reef face provide the basis for extending a previous theoretical analysis for wave set up on a reef in the absence of a flow gourlay m r 1996b wave set up on coral reefs 2 set up on reefs with various profiles coastal engineering 28 1755 to include the interaction between a unidirectional flow and the wave set up the laboratory model results are then used to demonstrate that there are two basic reef top flow regimes reef top control and reef rim control using open channel flow theory analytical relationships are derived for the reef top current velocity in terms of the offreef wave conditions the reef top water depth and the physical characteristics of the reef top topography the wave set up and wave generated flow relationships are found to predict experimental values with reasonable accuracy in most cases the analytical relationships are used to investigate wave generated flows into a boat harbour channel on heron reef in the southern great barrier reef c 2005 elsevier b v all rights reserved

wave driven circulation of a coastal reef lagoon system abstract the response of the circulation of a coral reef system in kaneohe bay hawaii to incident wave forcing was investigated using field data collected during a 10 month experiment results from the study revealed that wave forcing was the dominant mechanism driving the circulation over much of kaneohe bay as predicted theoretically wave setup generated near the reef crest resulting from wave breaking established a pressure gradient that drove flow over the reef and out of the two reef channels maximum reef setup was found to be roughly proportional to the offshore wave energy flux above a threshold root mean square wave height of 0 7 m at which height setup was negligible on the reef flat the wave driven currents increased approximately linearly with incident wave height however the magnitude of these currents was relatively weak typically 20 cm s 1 because of i the mild fore reef slope of kaneohe bay that reduced setup resulting from a combination of frictional wave damping and its re

wave driven flow over shallow reefs long term 1 month observations of waves and currents over a natural reef are presented which show a strong correlation between offshore rms incident wave height and cross reef currents at subtidal frequencies the energy spectrum of the cross reef currents shows a significant peak at twice the semidiurnal tidal frequency while the spectrum of sea surface elevation over the reef flat shows no corresponding peak furthermore experimental results reported by gourlay 1993 show setup over the reef occurs in the absence of a beach and the cross reef transport decreases with an increase in the sea surface slope across the reef flat due to an increase in setup at the top of the reef face analytic solutions for flow forced by wave breaking over an idealized reef explain the above features of cross reef flows in the absence of a beach through the surf zone on the reef face the cross reef gradient in the radiation stress due to wave breaking is partitioned between balancing an offshore pressure gradient associated with setup over the reef and forcing a mean flow across the reef over the reef flat where the depth is constant there is no forcing due to wave breaking and the flow is driven by a pressure gradient which results from the setup through the surf zone the magnitude of the setup through the surf zone is such that the transport across the reef flat matches the transport through the surf zone which is forced by the gradient in the radiation stress solutions are presented for general reef geometry defined by the reef width and slope of the seaward reef face and incident wave forcing defined by the depth at the breakpoint and the depth of water over the reef as the depth over the reef goes to zero the solutions converge to the plane beach solutions described by longuet higgins and stewart 1964 wave setup is maximized and the cross reef transport is zero in other cases the relative magnitudes of the setup and the cross reef transport depend on the geometry of the reef and the incident wave forcing

wave breaking hydrodynamics within coral reef systems and the effect of changing relative sea level this paper considers the hydrodynamics of wave driven flow across a coral reef and the resultant flushing of its lagoon current depth coefficients are introduced that describe the fractional change of across reef and lagoonal current with change in the relative local sea level i e the depth of water over the reef flat the coefficients are derived from an analytical hydrodynamic model and compared with data from two reefs the first is ningaloo reef in northwest australia which is a typical barrier reef with a narrow coastal lagoon and the second is kaneohe bay in oahu hawaii where the reef is both unusually deep and wide the current depth coefficients are shown to be sensitive to the form of the frictional law on the reef flat and can be deduced from measurements of the astronomical tidal currents in the lagoon both quadratic and linear friction laws are considered for the reef flat and good agreement obtained with a linear law based on the high frequency motion due to surface gravity waves

wave set up on a natural beach wave set up the superelevation of mean water level owing to the presence of breaking incident waves was measured at the shoreline of a natural beach offshore pressure sensors monitored incident wave conditions the set up of the shoreline was found to be about 0 17hs where hs is the significant wave height in deep water

integrated geological and geophysical interpretation case study and lame rock parameter extractions using avo analysis on the blackfoot 3c 3d seismic data southern alberta canada blackfoot field southeast of calgary alberta canada has produced oil and gas from a glauconitic compound incised valley system the glauconitic compound incised valley has three cycles of incision and valley fill lower lithic and upper incised valleys the upper and lower incised valleys are the main reservoirs the geophysical interpretation of compressional pp seismic data resulted in the definition of the compound valley extent and in the mapping of the upper and lower incised valleys a stratigraphic well log template was built using the most significant lithological information and well logs to integrate both geological and geophysical interpretations the well log cross sections and corresponding depth converted seismic were superimposed furthermore a detailed geological facies interpretation of the upper and lower incised valleys was undertaken and incorporated a good correlation was found between the interpreted geological facies and the seismic data response information about the natu

avo as a fluid indicator a physical modeling study information on time lapse changes in seismic amplitude variation with offset avo from a reservoir can be used to optimize production we designed a scaled physical model experiment to study the avo response of mixtures of brine oil and carbon dioxide at pressures of 0 1 03 and 2 07 mpa the small changes in density and velocity for each fluid because of increasing pressure were not detectable and were assumed to lie within the error of the experiment however avo analysis was able to detect changes in the elastic properties between fluids that contained oil and those that did not when the avo response was plotted in the avo intercept gradient domain fluids containing oil were clearly separated from fluids not containing oil this was observed in the avo response from both the top and base of the fluids in the physical model we then compared the measured avo response with the theoretical avo response given by the zoeppritz equations the measured and theoretical avo intercept responses for the t

quantitative seismic pre stack analysis of potential gas hydrate resources in the makran accretionary prism offshore iran abstract gas hydrates are classified as a major unconventional resource and assumed to be a future potential energy source high pressures low temperature stability conditions of the hydrates restrict their occurrence to the permafrost and deep sea regions from the continental slop to the abyssal in these regions where the well information is most often absent or sparse the amplitude variation with angle ava is a common method of hydrate free gas assessment in this method various linear reflection coefficient equations are fitted to a series of data points to compute some pre stack attributes these attributes have the advantage of being an interface property rather than a layer property which makes them an easy to use characterization tool in lack of well data in recent years rock physics templates nomograms have been presented to estimate the hydrate free gas saturations solely based on these interface properties ava inversion approach was applied to the seismic data in the makran accretionary prism offshore iran to estimate the bottom simulating reflector bsr vicinity hydrate and gas saturations this inversion process included four pre stack attributes of the intercept gradient normal incidence ni and poisson reflectivity pr and was limited to the bsr estimated bsr vicinity saturations were ranged from 7 8 5 for hydrate and 5 6 5 for free gas at three representative locations accuracy of each attribute was analyzed by a synthetic modeling in this modeling the effective medium theory emt was considered to calculate the physical properties of hydrate and gas bearing sediments lithological properties and the acquisition geometry were set to the corresponding values of the field data results of the synthetic ava inversion showed that if the hydrate saturation does not exceed 30 the error of the estimation for pre stack attributes would be in acceptable range the expected estimation errors for actual attributes were derived from this synthetic modeling

wave set up on coral reefs 1 set up and wave generated flow on an idealised two dimensional horizontal reef wave set up may be significant in determining water levels on coral reefs particularly in microtidal environments and hence is an important factor for the design of reef top structures and for the stability of reef top islands laboratory experiments have been made on a two dimensional model of an idealised horizontal reef under two different conditions corresponding to a fringing reef or closed lagoon situation and a platform reef or open lagoon situation both wave set up on the reef top and the wave generated flow across the reef were measured and related to wave and tide level conditions all other factors being the same wave set up is greatest at low tide levels whereas wave generated flow is greater at higher tide levels the magnitude of the set up on a platform reef with a wave generated flow is less than on a fringing reef without any net flow by an amount equal to the velocity head of the flow across the reef dimensionless parameters ghrt o and q gho3 are found to be functions of relative submergence parameters hrho or r hr ho for values of ghr hr ho 1 waves break on the reef top and radiation stress theory can be used to calculate set up for ghr hr ho 0 7 waves break on the reef face and set up is determined by broadcrested weir control at the reef edge the symbols are defined as follows g is gravitational acceleration hr is still water depth over horizontal reef top ho is offreef wave height equivalent deep water value q is discharge per unit length of reef edge t is wave period and ghr is maximum wave set up on reef top

how lime sand islands in the south china sea have responded to global warming over the last 30 years evidence from satellite remote sensing images abstract as global warming threatens their existence there are widespread concerns regarding the persistence of lime sand islands and the future of mid ocean atoll nations to investigate how climate change has affected lime sand islands changes in vegetation and other characteristics of such islands in the south china sea scs were investigated from 1989 to 2019 using 67 satellite images first boundaries of the lime sand islands and their vegetation were extracted using an active contour extraction procedure called the gradient vector flow snake model afterwards the spatial extents were estimated by enclosing the extracted boundaries and the digital shoreline analysis system was used to calculate beach widths finally area growth rates and speeds were used to evaluate areal changes in the lime sand islands and their respective vegetation based on the estimated area growth rates area growth speeds and beach widths the lime sand islands in scs eroded over the past three decades whereas their vegetation expanded further analysis suggested that direct inundation caused by sea level rise might not be clearly identified from the satellite images however other climate change related factors were most likely responsible for the observed island erosions these factors included higher wave energy stronger typhoon intensity and destructiveness and accelerated coral reef degradation in addition the observed expansion of vegetation on the lime sand islands was likely due to the increase in precipitation in a warming world the results show that 1 the lime sand islands will continue to erode whereas vegetation will continue to expand 2 as vegetation growth is significantly inhibited by salt water when it is adjacent to the ocean vegetation areas on lime sand islands may start to continuously decline overall this study is the first to quantitatively examine changes in scs lime sand islands due to global warming

detection of gas in sandstone reservoirs using avo analysis a 3 d seismic case history using the geostack technique the geostack technique is a method of analyzing seismic amplitude variation with offset avo information one of the outputs of the analysis is a set of direct hydrocarbon indicator traces called fluid factor traces the fluid factor trace is designed to be low amplitude for all reflectors in a clastic sedimentary sequence except for rocks that lie off the mudrock line the mudrock line is the line on a crossplot of p wave velocity against s wave velocity on which water saturated sandstones shales and siltstones lie some of the rock types that lie off the mudrock line are gas saturated sandstones carbonates and igneous rocks in the absence of carbonates and igneous rocks high amplitude reflections on fluid factor traces would be expected to represent gas saturated sandstones of course this relationship does not apply exactly in nature and the extent to which the mudrock line model applies varies from area to area however it is a useful model in many basins of the world including the one

estimating and interpreting velocity uncertainty in migrated images and avo attributes estimating a suitable velocity field for use in prestack depth migration is inherently uncertain because of limitations on the available data and estimation techniques this uncertainty affects both the migrated depth of structures and their amplitudes in the inverted images these effects can be estimated by performing multiple migrations with a set of velocity fields and colocating features in the migrated images this lets us examine the imaging procedure s sensitivity to changes in the velocity field so we can assess both structural and amplitude uncertainties in migrated images these two types of uncertainties affect interpretation in different ways for instance with structural uncertainty interpretation we consider the change in migrated location of structures when deciding on drilling locations optimizing well trajectories or computing uncertainty in volumetric calculations with amplitude uncertainty or amplitude versus offset avo uncertainty interpretation we consider 1 uncertainty in c

bottom stress estimates and their prediction on the northern california continental shelf during code 1 the importance of wave current interaction abstract high quality near bottom boundary layer measurements obtained at a midshelf location 90 m water depth in the code region off northern california are described bottom tripod velocity measurements and supporting data obtained during typical spring and early summer conditions june 1981 during code 1 are analyzed to obtain bath velocity profiles and mean bottom stress and bottom roughness estimates during the time period described the mean near bottom 0 997 approximately 30 percent of the time effects induced by unsteadiness from internal waves result in some degradation of the profiles 0 96 r 0 997 the rest of the time mean stress profiles indicate the logarithmic layer is approximately a constant stress layer the near bottom flow field is composed of mean currents and oscillatory currents due to well typing mean u values estimated from measurements greater than 30 cm above the bottom have magnitudes of 0 5 1 0 cm s 1 mean stress valu

distribution of melt along the east pacific rise from 9 30 to 10 n from an amplitude variation with angle of incidence ava technique author posting the author s 2015 this article is posted here by permission of the royal astronomical society for personal use not for redistribution the definitive version was published in geophysical journal international 203 2015 1 21 doi 10 1093 gji ggv251

wave attenuation on an offshore coral reef the wave climate along the northeastern tropical coastline of australia is controlled by the great barrier reef gbr however the processes by which the gbr attenuates and transforms waves are little understood as the first part of an on going study of the interaction between waves and coral reefs a field experiment was conducted to study the processes that occur as waves break on an offshore reef and proceed across the reef flat into the lagoon eighteen wave water level and current measuring instruments were deployed and data for a wide range of tide and wave conditions were collected preliminary results for wave attenuation are presented results for wave attenuation across the reef show that wave heights on the reef flat and in its lagoon are controlled by the depth of water over the windward reef flat as the waves travel across the reef flat the ratio of significant wave height to water depth reduces to a value of 0 40 and the ratio of maximum wave height to water depth reduces to a value of 0 6 to 0 8 in the deeper water in the middle of the reef lagoon both the ratios of significant wave height to the depth over the reef flat and maximum wave height to the depth over the reef flat remain in the above ranges however at the mid lagoon position these results are less general as wave heights inside a lagoon are also dependant on wind speed direction and fetch length inside the lagoon

wave propagation effects on amplitude variation with offset measurements a modeling study wave propagation effects can significantly affect amplitude variation with offset avo measurements these effects include spreading losses transmission losses interbed multiples surface multiple reflections p sv mode converted waves and inelastic attenuation examination of prestack elastic synthetic seismograms suggests that spreading losses and the transmission losses plus compressional interbed multiples are manifest mainly as a time and offset effect on the primary reflections the surface related multiples and the p sv mode converted waves interfere with prestack amplitudes inducing distortions in the avo pattern such distortions cause large variances in avo model fitting prestack viscoelastic synthetic seismograms also suggest that inelastic attenuation further complicates the avo response because of the offset and time variant amplitude decay effects and the phase change due to dispersion together all these effects severely alter avo behavior and result in serious errors in avo parameter

the effect of submarine groundwater discharge on nutrient and salinity regimes in a coastal lagoon off perth western australia abstract submarine groundwater discharge sgd into a coastal lagoon off perth western australia contains nitrate and silicate in concentrations two orders of magnitude higher than those of the receiving waters this discharge delivers enough nitrate to replace that dissolved in the lagoon water mass about every eight days and enough silicate to replace the lagoon silicate in about 48 days the delivery rate of nitrate nitrogen by sgd is equal to about 48 of that required for observed growth rates of lagoon macrophytes surface salinity is lower close to the shore as a result of sgd during calm conditions a salinity front was observed in the lagoon with a nearshore pool of nutrient enriched water floating above the more saline ocean water

random breaking waves field verification of a wave by wave algorithm for engineering application abstract a simple numerical algorithm is utilized to test the wave by wave approach to modeling random wave transformation caused by shoaling breaking and reforming across nearshore profiles of arbitrary shape histograms of wave height and associated statistics predicted by the model show favorable comparison to measurements from the nine tests of the duck 85 field experiment with a root mean square error less than 10 for most wave statistics although suitable for use in many engineering applications the model does overpredict the number of waves that reform as they pass over a trough formation which is one facet that warrants improvement using the model predictions as a point of reference the qualitative effects of 1 an offshore wind 2 wave reflection and 3 surf beat are identified and discussed

variability of depth limited waves in coral reef surf zones wave breaking and transformation on coral reef flats is an important process protecting tropical coastlines and regulating the energy regimes of coral reefs however the high hydraulic roughness shallow water and steep bathymetries of coral reefs may confound common surf zone assumptions such as a depth limited and saturated surf zone with a constant wave height to water depth ratio here we examine wave transformation across a coral reef flat during three separate swell events on both a time averaged and a wave by wave basis we use the relationship between significant wave height and water depth to examine the change in surf saturation across the reef flat and compare the measured wave height decay to results of modelled wave energy dissipation in the surf zone our results show that was not cross reef constant and varied according to location on the reef flat and local water depth on average was greatest at the outer reef flat near the reef crest and progressively reduced towards the inner reef flat near the reef lagoon this was most pronounced in shallow water with large values 0 85 at the outer reef flat and small values 0 1 at the inner reef flat this indicates that there is an increase in wave energy dissipation in shallow water most likely due to increased breaker and bed frictional dissipation the measured wave energy dissipation across the entire reef flat could on average be modelled accurately however this required location specific calibration of the free parameters the wave friction factor f and and further suggests that there is no value for either parameter that is universally applicable to coral reef flats despite model calibration inaccuracies were still observed primarily at the outer reef flat these inaccuracies reflected the observed cross reef variation of on the reef flat and potentially the limitations of random wave breaker dissipation models in complex surf zones our results have implications for the use of wave energy dissipation models in predicting breaker dissipation and subsequent benthic community change on coral reef flats and suggest that careful consideration of the free parameters in such models such as f and is required

the seismic response to overpressure a modelling study based on laboratory well and seismic data we investigate the seismic detectability of an overpressured reservoir in the north sea by computing synthetic seismograms for different pore pressure conditions the modelling procedure requires the construction of a geological model from seismic well and laboratory data seismic inversion and avo techniques are used to obtain the p wave velocity with higher reliability than conventional velocity analysis from laboratory experiments we obtain the wave velocities of the reservoir units versus confining and pore pressures laboratory experiments yield an estimate of the relationship between wave velocities and effective pressure under in situ conditions these measurements provide the basis for calibrating the pressure model overpressures are caused by different mechanisms we do not consider processes such as gas generation and diagenesis which imply changes in phase composition but focus on the effects of pure pore pressure variations the results indicate that changes in pore pressure can be detected with seismic methods under circumstances such as those of moderately deep north sea reservoirs