model based amplitude versus offset and azimuth inversion for estimating fracture parameters and fluid content abstractthe normal to shear fracture compliance ratio is commonly used as a fluid indicator in the seismic frequency range the fluid indicator lies between the values for isolated fluid filled fractures and dry fractures and it is not easy to discriminate the fluid content assuming that the fracture surfaces are smooth we use 1 n ndry with n and ndry representing the normal fracture weakness of the saturated and dry rock to indicate fluid types and to define a fluid influencing factor the fluid influencing factor is sensitive to the fluid properties the aspect ratio of the fractures and the frequency conventionally the amplitude versus offset and azimuth avoa inversion is formulated in terms of the contrasts of the fracture weaknesses across the interface assuming that the fractures are vertical with the same symmetry axis we consider fractures with arbitrary azimuths and develop a method to estimate fracture parameters from wide azimuth seismic data the proposed avoa inversion algo

seasonal changes in beach morphology along the sheltered coastline of perth western australia abstract seasonal change in beach morphology is traditionally ascribed to a variation in the incident wave energy level with calm conditions in summer resulting in wide beaches with pronounced subaerial berms and energetic conditions in winter causing narrow beaches with nearshore bar morphology the coastline of perth western australia is characterised by a large seasonal variation in the incident wave height and local beaches exhibit a distinct seasonal change in morphology however these morphological changes are better explained by a seasonal reversal in the littoral drift direction than by variations in the incident wave energy conditions in summer when northward sediment transport prevails due to sea breeze activity beaches located south of coastal structures headlands or rocky outcrops become wider due to the accumulation of sediment against the obstacle these beaches will subsequently erode in winter during storms when the longshore sediment transport is toward the south in contrast beaches located north of obstacles will become narrower during summer and wider during winter the usefulness of the dimensionless fall velocity h b w s t where hb is the breaker height ws is the sediment fall velocity and t is the wave period as a predictor of presence absence of bar morphology and beach type was investigated it was found that fluctuates around the threshold of bar formation 1 5 2 over a variety of time scales daily weekly and seasonally these temporal variations in in conjunction with the relatively low wave energy level that characterises the coast negates the development of beach and nearshore morphology that is in equilibrium with the hydrodynamic conditions as a result bar occurrence and beach type can not be readily predicted using along the perth coast

numerical upscaling in 2 d heterogeneous poroelastic rocks anisotropic attenuation and dispersion of seismic waves the presence of stiffness contrasts at scales larger than the typical pore sizes but smaller than the predominant seismic wavelengths can produce seismic attenuation and velocity dispersion in fluid saturated porous rocks this energy dissipation mechanism is caused by wave induced fluid pressure diffusion among the different components of the probed geological formations in many cases heterogeneities have elongated shapes and preferential orientations which implies that the overall response of the medium is anisotropic in this work we propose a numerical upscaling procedure that permits to quantify seismic attenuation and phase velocity considering fluid pressure diffusion effects as well as generic anisotropy at the sample s scale the methodology is based on a set of three relaxation tests performed on a 2d synthetic rock sample representative of the medium of interest it provides a complex valued frequency dependent equivalent stiffness matrix through a least squares procedure we also derive an approach for computing various poroelastic fields associated with the considered sample in response to the propagation of a seismic wave with arbitrary incidence angle using this approach we provide an energy based estimation of seismic attenuation a comprehensive numerical analysis indicates that the methodology is suitable for handling complex media and different levels of overall anisotropy comparisons with the energy based estimations demonstrate that the dynamic equivalent viscoelastic medium assumption made by the numerical upscaling procedure is reasonable even in presence of high levels of overall anisotropy this work also highlights the usefulness of poroelastic fields for the physical interpretation of seismic wave phenomena in strongly heterogeneous and complex media

wave energy gradients across a maldivian atoll implications for island geomorphology exposure to wave energy has been used to account for a range of ecological geological and geomorphic processes in coral reef systems but few attempts have been made to quantify spatial variations in energy at the atoll scale this study presents results of measurements of wave energy on reef platforms across south maalhosmadulu atoll maldives and their implications for island geomorphology the atoll has a perforated rim 37 effective aperture and experiences predictable shifts in monsoon winds from the west 8 months and northeast 4 months results show that wave energy affecting the atoll is considerably greater during the westerly monsoon atoll structure promotes significant changes in wave energy and wave characteristics across the atoll short period 3 8 s monsoon driven wave energy which is significant on windward reefs is dissipated on the peripheral reef network and the density of lagoonal patch reefs limits development of locally generated wind wave energy across the lagoon however longer period swell 8 20 s propagates through the lagoon to leeward reefs a windward to leeward decay in wave energy is evident in the westerly monsoon but not in the northeast monsoon when long period swell from the southwest remains significant on western reefs net energy calculations that account for seasonal changes in wave energy across the atoll identify a steep west east gradient that has geomorphic significance for island building western reefs are dominated by westerly flowing energy that is 4 5 7 times the total energy input elsewhere in the atoll wave energy on central reefs is balanced whereas net energy on eastern reef platforms is dominated by eastward propagating waves this steep energy gradient provides a physical explanation for the presence and distribution of islands on reef platforms across the atoll and provides quantitative support for the theory of gardiner gardiner j s 1903 the fauna and geography of the maldives and laccadives archipelagoes cambridge university press london to explain the prevalence of islands on the eastern atoll reef rim

spatiotemporal variability of typhoon impacts and relaxation intervals on jaluit atoll marshall islands storms are important drivers of geomorphic change within atoll settings causing rapid island erosion while also initiating island building through the generation of vast quantities of reef derived sediment due to the combined effects of storms and sea level rise reef islands are thought to be on an erosional trajectory jaluit atoll in the marshall islands was struck by typhoon ophelia in a d 1958 causing significant geomorphic change to reef islands using aerial photographs as well as recent satellite imagery we track the impacts of the typhoon and the multidecadal recovery of islands ophelia caused a significant reduction in total land area of jaluit atoll from 9 95 km 2 to 9 45 km 2 5 1 with islands on the northeast rim collectively reducing in size from 4 72 km 2 to 4 14 km 2 12 2 between 1976 and 2006 73 of 87 islands increased in size with the total landmass exceeding the pre typhoon area 10 25 km 2 however we observe considerable spatiotemporal variability of impacts and relaxation following the typhoon results indicate that despite significant typhoon driven erosion and a relaxation period coincident with local sea level rise islands have persisted and grown

seismic dispersion and attenuation in saturated porous rocks with aligned fractures of finite thickness theory and numerical simulations part 2 frequency dependent anisotropy abstractnumerous theoretical models have been proposed for computing seismic wave dispersion and attenuation in rocks with aligned fractures due to wave induced fluid flow between the fractures and the embedding background however all these models rely on certain assumptions for example infinitesimal fracture thickness or dilute fracture concentration which rarely hold in real reservoirs and thus limit their applicability to alleviate this issue theoretical models for periodically or randomly spaced planar fractures and penny shaped cracks were recently extended by the authors to the case of finite fracture thickness for p waves propagating perpendicular to the fracture plane theoretical predictions under low and relatively high fracture density were then assessed by comparing with corresponding numerical simulations however the case of arbitrary incidence angles as well as the behaviors of s waves remained unexplored in this work we therefore extended the prediction results to the full stif

new model of reef island evolution maldives indian ocean a new model of reef island evolution based on detailed morphostratigraphic analysis and radiometric dating of three islands in south maalhosmadulu atoll maldives is presented islands initially formed on a foundation of lagoonal sediments between 5500 and 4500 yr b p when the reef surface was as much as 2 5 m below modern sea level islands accumulated rapidly during the following 1500 yr effectively reaching their current dimensions by 4000 yr b p since then the high circum island peripheral ridge has been subject to seasonal and longer term shoreline changes while the outer reef has grown upward reducing the energy window and confining the islands this new model has far reaching implications for island stability during a period of global warming and raised sea level which will partially reactivate the energy window although it is not expected to inhibit upward reef growth or compromise island stability

sensitivity of seismic attenuation and phase velocity to intrinsic background anisotropy in fractured porous rocks a numerical study many researchers have analyzed seismic attenuation and velocity dispersion due to wave induced fluid flow wiff related to the presence of fluid saturated fractures embedded in an isotropic porous background most fractured formations do however exhibit some degree of intrinsic elastic and hydraulic anisotropy of the background and the impact of which on the effective seismic properties remains largely unexplored in this work we extend a numerical upscaling procedure to account for the potential intrinsic elastic and hydraulic anisotropy of the background to do this we represent the background of a representative sample of the fractured formation of interest with an anisotropic poroelastic medium and apply a set of relaxation experiments to compute the effective anisotropic seismic properties a comprehensive numerical analysis allows us to observe that for samples containing hydraulically connected fractures the anisotropic behavior of both p and s waves differs significantly from that observed for an isotropic background the anisotropy of the stiffness of the background plays a fundamental role for wiff between the fractures and the background as well as for wiff between connected fractures conversely the anisotropy of the background permeability affects the characteristic frequency the angle dependence and the magnitude of the effects related to wiff between fractures and background in addition different correlations between hydraulic and elastic background anisotropy lead to different degrees of effective seismic anisotropy our results therefore indicate that accounting for the effects of intrinsic background anisotropy on wiff is crucial for a quantitative interpretation of seismic anisotropy measurements in fluid saturated fractured formations

projected atoll shoreline and run up changes in response to sea level rise and varying large wave conditions at wake and midway atolls northwestern hawaiian islands abstract atoll islands are dynamic features that respond to seasonal alterations in wave conditions and sea level it is unclear how shoreline wave run up and erosion patterns along these low elevation islands will respond to projected sea level rise slr and changes in wave climate over the next century hindering communities preparation for the future to elucidate how these processes may respond to climate change extreme boreal winter and summer wave conditions under future sea level rise slr and wave climate scenarios were simulated at two atolls wake and midway using a shallow water hydrodynamic model nearshore wave conditions were used to compute the potential longshore sediment flux along island shorelines via the cerc empirical formula and wave driven erosion was calculated as the divergence of the longshore drift run up and the locations where the run up exceed the berm elevation were also determined slr is projected to predominantly drive future island morphological change and flooding seaward shorelines i e ocean fronted shorelines directly facing incident wave energy were projected to experience greater erosion and flooding with slr and in hypothetical scenarios where changes to deep water wave directions were altered as informed by previous climate change forced pacific wave modeling efforts these changes caused nearshore waves to become more shore normal increasing wave attack along previously protected shorelines with slr leeward shorelines i e an ocean facing shoreline but sheltered from incident wave energy became more accretive on windward islands and marginally more erosive along leeward islands these shorelines became more accretionary and subject to more flooding with nearshore waves becoming more shore normal lagoon shorelines demonstrated the greatest slr driven increase in erosion and run up they exhibited the greatest relative change with increasing wave heights where both erosion and run up magnitudes increased wider reef flat fronted seaward shorelines became more accretive as all oceanographic forcing parameters increased in magnitude and exhibited large run up increases following increasing wave heights island end shorelines became subject to increased flooding erosion at wake and accretion at midway with slr under future conditions windward and leeward islands are projected to become thinner as ocean facing and lagoonal shorelines erode with leeward islands becoming more elongate island shorelines will change dramatically over the next century as slr and altered wave climates drive new erosional regimes it is vital to the sustainability of island communities that the relative magnitudes of these effects are addressed when planning for projected future climates

seismic attenuation and velocity dispersion in heterogeneous partially saturated porous rocks summary using a numerical approach we explore wave induced fluid flow effects in partially saturated porous rocks in which the gas water saturation patterns are governed by mesoscopic heterogeneities associated with the dry frame properties the link between the dry frame properties and the gas saturation is defined by the assumption of capillary pressure equilibrium which in the presence of heterogeneity implies that neighbouring regions can exhibit different levels of saturation to determine the equivalent attenuation and phase velocity of the synthetic rock samples considered in this study we apply a numerical upscaling procedure which permits to take into account mesoscopic heterogeneities associated with the dry frame properties as well as spatially continuous variations of the pore fluid properties the multiscale nature of the fluid saturation is taken into account by locally computing the physical properties of an effective fluid which are then used for the larger scale simulations we consider two sets of numerical experiments to analyse such effects in heterogeneous partially saturated porous media where the saturation field is determined by variations in porosity and clay content respectively in both cases we also evaluate the seismic responses of corresponding binary patchy type saturation patterns our results indicate that significant attenuation and modest velocity dispersion effects take place in this kind of media for both binary patchy type and spatially continuous gas saturation patterns and in particular in the presence of relatively small amounts of gas the numerical experiments also show that the nature of the gas distribution patterns is a critical parameter controlling the seismic responses of these environments since attenuation and velocity dispersion effects are much more significant and occur over a broader saturation range for binary patchy type gas water distributions this analysis therefore suggests that the physical mechanisms governing partial saturation should be accounted for when analysing seismic data in a poroelastic framework in this context heterogeneities associated with the dry frame properties which do not play important roles in wave induced fluid flow processes per se should be taken into account since they may determine the kind of gas distribution pattern taking place in the porous rock

the archaean gneiss complex in e hebei province north china geochemistry and evolution abstract the caozhuang and shuichang gneiss complexes from the western margin of the qianan granite gneiss dome eastern hebei north china were investigated for their isotopic and trace element geochemistry the oldest rocks so far recognized in china occur near caozhuang in the south of the area the caozhuang gneiss complex consists of supracrustal rocks and multiphase orthogneisses the supracrustals comprise a sequence of amphibolites quartzites felsic gneisses marble and bif which are thought to be laid down in a shallow water environment a ca 3 5 ga sm nd isochron has been obtained from the amphibolites and also from four felsic gneisses of the caozhuang supracrustals the orthogneisses from the caozhuang complex appear to have been generated in three main cycles at ca 3 5 ca 2 7 and ca 2 5 ga the earliest huangbaiyu gneiss was broken up and contaminated by the intrusions of granodiorites and late granitic gneisses the granodiorites have yielded a ca 2 7 ga rb sr isochron the relatively high isr 0 7057 2 2 for the rocks suggesting that the granodiorites were formed by anatexis of older continental crust late granitic gneisses have a u pb zircon age of 2 5 ga and they are remarkably similar in appearance and chemical composition to the huangbaiyu gneisses suggesting that the gneiss complex as a whole is a mixture of gneiss types of widely different ages the shuichang gneiss complex contains supracrustal rocks granitic rocks and charnockites the supracrustal rocks include greywackes semi pelites and bif which have been metamorphosed in the granulite facies and occur within a polyphase complex of deformed leucogranites microcline granites and charnockites the age of the supracrustals is not known except that they predate charnockites of 2 65 ga there is a close relationship between the production of the granites and the charnockites it is suggested that differences in mineral and chemical composition between granites and charnockites could result from partial melting processes occurring in the shuichang metasedimentary gneisses

the magnitude of a mid holocene sea level highstand in the strait of makassar abstract knowledge on the timing and magnitude of past sea level changes is essential to understand modern and future sea level variability holocene sea level data from literature on the west coast of sulawesi central indonesia suggest that this region experienced two relative sea level highstands over the last 6000 years with magnitudes exceeding two meters however recent datasets from the indo pacific region do not support high magnitude sea level oscillations during the holocene in tectonically stable far field locations here we present a new high precision mid holocene sea level dataset from the spermonde shelf off southwest sulawesi we surveyed 21 fossil microatolls on the reef flats of two coral islands pulau panambungan and pulau barrang lompo and referred their elevations to local mean sea level and to the height of living coral radiometrically calibrated ages from emergent fossil microatolls on pulau panambungan indicate a relative sea level highstand not exceeding 0 5 m above present at ca 5600 cal yr bp the highstand is followed by a relatively rapid sea level fall towards present sea level that was reached at around 4000 cal yr bp fossil microatolls from nearby pulau barrang lompo show the same trend however with a coherent negative vertical offset of about 0 8 m compared to their equivalents on pulau panambungan the largely consistent gradients of both trends 0 14 mm yr 1 the consistent elevation of living microatolls in the spermonde and a number of instructive geomorphic features indicate a localized post formational and probably recent drop of the fossil microatolls on the densely populated island pulau barrang lompo the relative sea level trend inferred from pulau panambungan is well within the range of geophysical predictions based on anice selen ice sheet model which predict a highstand that is significantly lower than those predicted by other gia models for this area although a complete interpretation of the holocene sea level history will require additional high resolution datasets from this and surrounding territories in se asia our results suggest that there was merely a single holocene highstand in central indonesia the magnitude of which was substantially lower than hitherto assumed

imaging fractures and sedimentary fabrics using shear wave splitting measurements made on passive seismic data summary the ability to detect aligned fractures using seismic anisotropy provides a valuable tool for exploiting hydrocarbon reservoirs better perhaps the most direct way of identifying anisotropy is by observing shear wave splitting however the interaction of shear waves with subsurface structure is often complicated although fractures in hydrocarbon reservoirs are usually subvertical shear waves recorded on downhole receivers from microseismic events in or near the reservoir are not likely to have travelled vertically as such interpreting splitting measurements made on such waves is a non trivial problem here we develop an approach to model the effects of subsurface structure on non vertically propagating shear waves rock physics theory is used to model the effects of sedimentary fabrics as well as fractures allowing us to use shear wave splitting measurements to invert for aligned fractures we use synthetic examples to demonstrate how it is possible to assess in advance how well splitting measurements will image structures and how this is highly dependent on the available range of ray coverage finally we demonstrate the inversion technique on a passive seismic data set collected during hydraulic fracture stimulation despite an unfavourable source receiver geometry the strike of an aligned fracture set is accurately identified

frequency dependent seismic anisotropy due to fractures fluid flow versus scattering abstractanisotropy is a useful attribute for the detection and characterization of aligned fracture sets in petroleum reservoirs unfortunately many of the traditional effective medium theories for modeling the seismic properties of fractured rock are insensitive to the size of the constituent fractures for example the same pattern of anisotropy may be produced by a high concentration of small stiff cracks or by a lower concentration of large compliant fractures the distinction between these models is important for assessing permeability anisotropy because fluid flow is dominated by the largest fractures one method to gain further insight is through the analysis of frequency dependent shear wave splitting in microseismic data because fracture compliance is frequency dependent and microseismic data are relatively rich in frequency content we compared two potential mechanisms causing frequency dependent compliance of fractures 1 squirt flow in fractured porous rock and 2 wave scattering over r

the southern oscillation index wave climate and beach rotation short embayed beaches bounded by headlands are a common feature along the southern and central coastline of new south wales australia many of these embayed beaches have experienced severe erosion at their southern end over the last decade previous studies have suggested that this erosion may be the result of an oscillatory medium term phenomenon known as beach rotation the present study was undertaken with the objectives of 1 establishing definitive links between the southern oscillation index soi wave climate and beach rotation and 2 determining the physical processes governing beach rotation data from two similar new south wales beaches were analysed using time series analysis techniques and image processing techniques using argus video images in this study the results indicate that the northern end of this type of beach accretes during el nino phases while the southern end erodes resulting in a net clockwise rotation of the beach the opposite occurs during la nina phases resulting in a net anti clockwise rotation of the beach the beach response at the northern and southern ends lags soi trend shifts by 3 and 17 months respectively waves are predominantly incident from the southeast during both el nino and la nina phases offshore wave height is positively correlated with the soi while offshore wave direction is negatively correlated with the soi on average the number of storms per year doubles from el nino to la nina based on these links between beach width fluctuations soi and incident wave conditions a conceptual model of beach rotation is presented the model describes the combinations of cross shore and longshore sediment transport and hydrodynamic processes that are expected to result in the observed clockwise and anti clockwise beach rotation during el nino and la nina phases respectively

effect of fracture fill on frequency dependent anisotropy of fractured porous rocks in fractured reservoirs seismic wave velocity and amplitude depend on frequency and incidence angle the frequency dependency is believed to be principally caused by the wave induced flow of pore fluid at the mesoscopic scale in recent years two particular phenomena patchy saturation and flow between fractures and pores have been identified as significant mechanisms of wave induced flow however these two phenomena are studied separately recently a unified model was proposed for a porous rock with a set of aligned fractures with pores and fractures filled with two different fluids existing models treat waves propagating perpendicular to the fractures in this paper we extend the model to all propagation angles by assuming that the flow direction is perpendicular to the layering plane and is independent of the loading direction we first consider the limiting cases through poroelastic backus averaging and then we obtain the five complex and frequency dependent stiffnesses of the equivalent ti medium as a function of the frequency the numerical results show that when the bulk modulus of the fracture filling fluid is relatively large the dispersion and attenuation of p waves are mainly caused by fractures and the values decrease as angles increase almost vanishing when the incidence angle is 90 propagation parallel to the fracture plane while the bulk modulus of fluid in fractures is much smaller than that of matrix pores the attenuation due to the partial saturation mechanism makes the fluid flow from pores into fractures which is almost independent of the incidence angle this article is protected by copyright all rights reserved

response of reef island shorelines to seasonal climate oscillations south maalhosmadulu atoll maldives 1 the maldives experience seasonal shifts in monsoon winds from the west northwest to northeast the morphologic response of reef island beaches and shorelines to these predictable shifts in climate was examined on eight islands in south maalhosmadulu atoll based on global positioning system surveys of island shoreline planform in january and june 2002 and february 2003 surveys show that islands exhibit large gross changes 31 120 of beach area in shoreline position between seasons such changes reflect large reversals in sediment flux of 9 23 103 m3 on a biannual basis driven by seasonal reversals in wind and wave conditions annual net change is small 2 15 suggesting islands spatially exhibit a dynamic equilibrium an island oscillation index io is defined that describes the spatial extent of shoreline change around islands island shape ellipticity e is found to be positively correlated with io in the expression io 1 021e 0 275 suggesting island shape is a better indicator of the susceptibility of island shorelines to morphological change than wave energy exposure

frequency dependent anisotropy due to meso scale fractures in the presence of equant porosity fractured rock is often modelled under the assumption of perfect fluid pressure equalization between the fractures and equant porosity this is consistent with laboratory estimates of the characteristic squirt flow frequency however these laboratory measurements are carried out on rock samples which do not contain large fractures we consider coupled fluid motion on two scales the grain scale which controls behaviour in laboratory experiments and the fracture scale our approach reproduces generally accepted results in the low and high frequency limits even under the assumption of a high squirt flow frequency we find that frequency dependent anisotropy can occur in the seismic frequency band when larger fractures are present shear wave splitting becomes dependent on frequency with the size of the fractures playing a controlling role in the relationship strong anisotropic attenuation can occur in the seismic frequency band the magnitude of the frequency dependence is influenced strongly by the extent of equant porosity with these results it becomes possible in principle to distinguish between fracture and microcrack induced anisotropy or more ambitiously to measure a characteristic fracture length from seismic data

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

permeability effects on the seismic response of gas reservoirs summary in this work we analyse the role of permeability on the seismic response of sandstone reservoirs characterized by patchy gas water saturation we do this in the framework of johnson s model which is a generalization of white s seminal model allowing for patches of arbitrary geometry we first assess the seismic attenuation and velocity dispersion characteristics in response to wave induced fluid flow to this end we perform an exhaustive analysis of the sensitivity of attenuation and velocity dispersion of compressional body waves to permeability and explore the roles played by the johnson parameters t and s v which characterize the shape and size of the gas water patches our results indicate that within the typical frequency range of exploration seismic data this sensitivity may indeed be particularly strong for a variety of realistic and relevant scenarios next we extend our analysis to the corresponding effects on surface based reflection seismic data for two pertinent models of typical sandstone reservoirs in the case of softer and more porous formations and in the presence of relatively low levels of gas saturation we observe that the effects of permeability on seismic reflection data are indeed significant these prominent permeability effects prevail for normal incidence and non normal incidence seismic data and for a very wide range of sizes and shapes of the gas water patches for harder and less porous reservoirs the normal incidence seismic responses exhibit little or no sensitivity to permeability but the corresponding non normal incidence responses show a clear dependence on this parameter again especially so for low gas saturations the results of this study therefore suggest that for a range of fairly common and realistic conditions surface based seismic reflection data are indeed remarkably sensitive to the permeability of gas reservoirs and thus have the potential of providing corresponding first order constraints

geomorphological impacts of high latitude storm waves on low latitude reef islands observations of the december 2008 event on nukutoa takuu papua new guinea abstract low latitude reefs and reef islands usually experience relatively benign climatic and hydrodynamic conditions due to their location near to the equator outside of the major storm belts and they typically exhibit geomorphological traits that reflect the prevailing low energy conditions for example algal ridges are poorly developed reef flat boulder zones are modest or lacking rubble banks are rare and reef islands tend to be low and dominated by sand nukutoa is a low lying triangular shaped reef island of 6 ha located on the eastern rim of takuu atoll 4 45 s 157 2 e papua new guinea approximately 300 km northeast of bougainville the approximately 450 residents of takuu all live on nukutoa in december 2008 takuu was struck by several days of very high water levels and waves which washed completely over approximately 50 of nukutoa gps shoreline mapping and topographic surveys of the island were undertaken in the days immediately prior to the event and were repeated immediately after homes and village infrastructure were damaged during this event which eroded around 60 of the shoreline and deposited a sand sheet averaging around 50 mm thick over approximately 13 of the island this event was generated by two distant storms one located 6000 km away near 50 n and affected a wide area of the western pacific oral histories record at least five similar events since the 1940s in this paper we document the geomorphic impacts of the december 2008 event and discuss the possible significance of similar events in the past and in the future

modelling frequency dependent seismic anisotropy in fluid saturated rock with aligned fractures implication of fracture size estimation from anisotropic measurements measurements of seismic anisotropy in fractured rock are used at present to deduce information about the fracture orientation and the spatial distribution of fracture intensity analysis of the data is based upon equivalent medium theories that describe the elastic response of a rock containing cracks or fractures in the long wavelength limit conventional models assume frequency independence and cannot distinguish between microcracks and macrofractures the latter however control the fluid flow in many subsurface reservoirs therefore the fracture size is essential information for reservoir engineers in this study we apply a new equivalent medium theory that models frequency dependent anisotropy and is sensitive to the length scale of fractures the model considers velocity dispersion and attenuation due to a squirt flow mechanism at two different scales the grain scale microcracks and equant matrix porosity and formation scale fractures the theory is first tested and calibrated against published laboratory data then we present the analysis and modelling of frequency dependent shear wave splitting in multicomponent vsp data from a tight gas reservoir we invert for fracture density and fracture size from the frequency dependence of the time delay between split shear waves the derived fracture length matches independent observations from borehole data

spatial and temporal variations in wave characteristics across a reef platform warraber island torres strait australia a field experiment was conducted at warraber island torres strait australia to investigate spatial and temporal variations in wave characteristics and energy across a mesotidal coral reef platform measurements of water depth were obtained using five pressure sensors deployed across a 2 7 km section of reef flat from july 3 5 2001 the reef surface was uneven and consisted of an outer reef flat a central reef flat depression an inner reef ramp a palaeo reef surface and the shoreline water levels decreased landward across the platform with tide ranges at the shoreline being almost 50 lower than at the outer reef flat rising and falling tides were characterised by a bimodal energy distribution with both short period 0 3 s and wind 3 8 s waves present higher water levels were dominated by wind waves the highest waves occurred at high tides associated with nocturnal tidal cycles with hs decreasing from 0 5 to 0 2 m from the outer reef flat to the shoreline wave energy at swell 8 20 s and infragravity 20 s frequencies was negligible across the reef platform although there was evidence of wave groups at higher water levels reef geometry and changes in water level determine the magnitude of wave energy on the reef platform up to 85 95 of incident wave energy is attenuated by the central reef flat depression at high and low tide respectively and strong linear relationships exist between hs and h at all locations both wave height and wave type are strongly depth dependent critical reef rim depths required to produce hs of a given size vary spatially across the reef rim due to variations in reef topography a distinct depth related threshold exists at which short period and wind wave dominance reverses over a 14 day spring neap tidal cycle the time of occurrence of wave action diminishes across the reef platform to the shoreline larger waves hs 0 2 m occur for only 9 of time at the outer reef flat and for less than 0 5 over the remaining reef platform this implies that on mesotidal reef platforms sediment entrainment and transport are severely constrained under normal wave energy conditions and significant change is likely restricted to extreme events

sedimentology and preservation potential of carbonate sand sheets deposited by the december 2004 indian ocean tsunami south baa atoll maldives observations of carbonate sand sheets deposited by the december 2004 indian ocean tsunami across island surfaces in the maldives archipelago provide an opportunity to investigate tsunami flow behaviour across relatively simple coastal forms this paper presents data on tsunami sand sheet deposits for five islands located along an east west transect through the central atoll of south baa sedimentary data include field measurement of sand sheet planform thickness and volume descriptions of sedimentary structure and stratigraphic relationships with underlying surfaces laboratory measurement of grain size as a proxy of settling velocity and observations of post depositional changes to sand sheet characteristics two years following the indian ocean tsunami results show cross atoll trends in volume and form of sand sheets that are interpreted as a proxy of waning and scattering of energy as the wave train interacted with the shallow waters and numerous islands of the atoll on individual islands the basin topography resulted in tsunami overwash but no return flow so that sand sheets are the product of unidirectional flow with evidence in grain size trends for minor acceleration across landward slopes a key finding is the evidence for rapid post depositional changes to the form and extent of tsunami deposits within two years of deposition reworking of the seaward edge of sand sheets has destroyed the thickest part of the deposit and the thinner landward part of the sand sheets has been reworked by bioturbation this study concludes that the preservation potential of these tsunami deposits is low to moderate with the only probable recognizable signature being local variation in the micro topography of island surfaces

seismic anisotropy of fractured rock a simple method for including the effects of geologically realistic fractures on the seismic propagation through fractured rocks can be obtained by writing the effective compliance tensor of the fractured rock as the sum of the compliance tensor of the unfractured background rock and the compliance tensors for each set of parallel fractures or aligned fractures the compliance tensor of each fracture set is derivable from a second rank fracture compliance tensor for a rotationally symmetric set of fractures the fracture compliance tensor depends on only two fracture compliances one controlling fracture compliance normal the other tangential to the plane of the fractures the stiffness tensor which is more useful in the consideration of elastic wave propagation through rocks can then be obtained by inversion the components of the excess fracture compliance tensor represent the maximum amount of information that can be obtained from seismic data if the background rock is isotropic and the normal a

including poroelastic effects in the linear slip theory abstractnumerical simulations of seismic wave propagation in fractured media are often performed in the framework of the linear slip theory lst therein fractures are represented as interfaces and their mechanical properties are characterized through a compliance matrix this theory has been extended to account for energy dissipation due to viscous friction within fluid filled fractures by using complex valued frequency dependent compliances this is however not fully adequate for fractured porous rocks in which wave induced fluid flow wiff between fractures and host rock constitutes a predominant seismic attenuation mechanism in this letter we develop an approach to incorporate wiff effects directly into the lst for a 1d system via a complex valued frequency dependent fracture compliance the methodology is validated for a medium permeated by regularly distributed planar fractures for which an analytical expression for the complex valued normal compliance is determined in the framework of quas

seismoacoustic signatures of fracture connectivity wave induced fluid flow wiff between fractures and the embedding matrix as well as within connected fractures tends to produce significant seismic attenuation and velocity dispersion while wiff between fractures and matrix is well understood the corresponding effects related to fracture connectivity and the characteristics of the energy dissipation due to flow within fractures are largely unexplored in this work we use oscillatory relaxation simulations based on the quasi static poroelastic equations to study these phenomena we first consider synthetic rock samples containing connected and unconnected fractures and compute the corresponding attenuation and phase velocity we also determine the relative fluid displacement and pressure fields in order to gain insight into the physical processes involved in the two manifestations of wiff in fractured media to quantify the contributions of the two wiff mechanisms to the total seismic attenuation we compute the spatial distribution of the local energy dissipation finally we perform an exhaustive sensitivity analysis to study the role played by different characteristics of fracture networks on the seismic signatures we show that in the presence of connected fractures both p wave attenuation and phase velocity are sensitive to some key characteristics of the probed medium notably to the lengths permeabilities and intersection angles of the fractures as well as to the overall degree of connectivity of the fracture network this in turn indicates that a deeper understanding of these two manifestations of wiff in fractured media may eventually allow for the extraction of some of these properties from seismic data

physical modelling of reef island topographic response to rising sea levels abstract low lying coral reef islands are considered extremely vulnerable to the impacts of climate change particularly sea level rise and increasing storm magnitude inundation and erosion are expected to destabilise reef islands and render them uninhabitable within decades however such assertions are founded upon the assumption that reef islands are geomorphologically inert landforms whereas recent planform and cross sectional analysis of reef island morphodynamics demonstrates that many islands are highly dynamic landforms changing in size shape and position on the reef platform this paper describes the first physical modelling experiments to model the whole of island morphodynamic response of reef islands to changes in sea level and wave regime a wave basin is utilised to construct a 1 50 scale three dimensional model of fatato island tuvalu and document the whole of island response across a broad range of wave and water level conditions whole of island results corroborate flume studies and provide new insights into the modes and styles of island change spit rotation as well as lagoonward island recession and vertical oceanward crest accretion are identified as the core island responses to increasing water level and wave conditions however results show that the rate and magnitude of physical adjustment is strongly dependent on the rate and magnitude of sea level rise and wave conditions results challenge existing models of future island susceptibility to wave driven flooding demonstrating that washover processes can provide a mechanism to build and potentially maintain island freeboard above sea level these insights highlight an urgent need to incorporate island morphodynamics into flood risk models in order to produce accurate assessments of future wave driven flood risks and better resolve island vulnerability

formation and adjustment of typhoon impacted reef islands interpreted from remote imagery nadikdik atoll marshall islands abstract in 1905 a devastating typhoon hit nadikdik atoll 5 54 n and 172 09 e in the southern marshall islands evidence suggests that large sections of reef islands on nadikdik were overwashed and destroyed comparison of aerial photographs taken in 1945 and modern satellite imagery provides a unique record of the geomorphic adjustment of islands after the typhoon between 1945 and 2010 the vegetated area of islands on nadikdik grew from 0 74 to 0 90 km2 observed changes to nadikdik reef islands manifested through a range of styles and were largely accretionary of note the formation of a new island was tracked from an embryonic deposit to a fully vegetated and stable island over a 61 year period similarly a number of previously discrete islands have agglomerated and formed a single larger island these changes were rapid and indicate that reef island formation can occur quickly evidence suggests that despite the typhoon occurring over a century ago the geomorphic adjustment of islands is still on going

building biogenic beachrock visualizing microbially mediated carbonate cement precipitation using xfm and a strontium tracer the fate of reef islands is a topic of ongoing debate in the face of climate change induced sea level rise and increased cyclone intensity increased erosion and changes to the supply of reef derived sediment may put sand reef cays at risk of dramatic morphological changes these changes may have negative implications for the existence of reef cay environments which host vital sea turtle and bird rookery habitats beachrock consolidated carbonate beach sediment in the intertidal zone forms naturally on many tropical beaches and reduces the erosion rates of these beaches when compared to unconsolidated sand in spite of the critical role beachrock plays in stabilizing some reef cay shores the method of beachrock formation is still incompletely understood in this investigation beachrock was synthesized using beach sand and beachrock samples from heron island great barrier reef australia in aquarium experiments in which natural beachrock formation conditions were simulated beachrock was produced in two aquaria wherein the water chemistry was influenced by microorganisms derived from the natural beachrock inoculum whereas no cementation occurred in an aquarium that lacked a microbial inoculum and was controlled only by physicochemical evapoconcentration the new cements in the synthesized beachrock were analyzed using synchrotron based x ray fluorescence microscopy and were identified using a sr tracer added to the experimental seawater the resulting precipitates cement sand grains together and contain abundant microfossils of the microorganisms on whose exopolymer they nucleated demonstrating the fundamental role microbes play in beachrock formation these results are of interest because beachrock could be utilized as a natural coastline stabilization strategy on sand reef cays and in turn protect the unique habitats reef islands support

elastic wave propagation in media with parallel fractures and aligned cracks a model of parallel slip interfaces simulates the behaviour of a fracture system composed of large closely spaced aligned joints the model admits any fracture system anisotropy triclinic the most general monoclinic orthorhombic or transversely isotropic and this is specified by the form of the 3 3 fracture system compliance matrix the fracture system may be embedded in an anisotropic elastic background with no restrictions on the type of anisotropy to compute the long wavelength equivalent moduli of the fractured medium requires at most the inversion of two 3 3 matrices when the fractures are assumed on average to have rotational symmetry transversely isotropic fracture system behaviour and the background is assumed isotropic the resulting equivalent medium is transversely isotropic and the effect of the additional compliance of the fracture system may be specified by two parameters in addition to the two isotropic parameters of the isotropic background dilute systems of flat aligned microcracks in an isotropic background yield an equivalent medium of the same form as that of the isotropic medium with large joints i e there are two additional parameters due to the presence of the microcracks which play roles in the stress strain relations of the equivalent medium identical to those played by the parameters due to the presence of large joints thus knowledge of the total of four parameters describing the anisotropy of such a fractured medium tells nothing of the size or concentration of the aligned fractures but does contain information as to the overall excess compliance due to the fracture system and its orientation as the aligned microcracks which were assumed to be ellipsoidal with very small aspect ratio are allowed to become non fiat i e have a growing aspect ratio the moduli of the equivalent medium begin to diverge from the standard form of the moduli for flat cracks the divergence is faster for higher crack densities but only becomes significant for microcracks of aspect ratios approaching 0 3

seismic wave attenuation and dispersion resulting from wave induced flow in porous rocks a review one major cause of elastic wave attenuation in heterogeneous porous media is wave induced flow of the pore fluid between heterogeneities of various scales it is believed that for frequencies below 1 khz the most important cause is the wave induced flow between mesoscopic inhomogeneities which are large compared with the typical individual pore size but small compared to the wavelength various laboratory experiments in some natural porous materials provide evidence for the presence of centimeter scale mesoscopic heterogeneities laboratory and field measurements of seismic attenuation in fluid saturated rocks provide indications of the role of the wave induced flow signatures of wave induced flow include the frequency and saturation dependence of p wave attenuation and its associated velocity dispersion frequency dependent shear wave splitting and attenuation anisotropy during the last four decades numerous models for attenuation and velocity dispersion from wave induced flow have been developed with varying degrees of rigor and complexity these models can be categorized roughly into three groups according to their underlying theoretical framework the first group of models is based on biot s theory of poroelasticity the second group is based on elastodynamic theory where local fluid flow is incorporated through an additional hydrodynamic equation another group of models is derived using the theory of viscoelasticity though all models predict attenuation and velocity dispersion typical for a relaxation process there exist differences that can be related to the type of disorder periodic random space dimension and to the way the local flow is incorporated the differences manifest themselves in different asymptotic scaling laws for attenuation and in different expressions for characteristic frequencies in recent years some theoretical models of wave induced fluid flow have been validated numerically using finite difference finite element and reflectivity algorithms applied to biot s equations of poroelasticity application of theoretical models to real seismic data requires further studies using broadband laboratory and field measurements of attenuation and dispersion for different rocks as well as development of more robust methods for estimating dissipation attributes from field data

the impact of temperate reefs on 34years of shoreline and vegetation line stability at yanchep southwestern australia and implications for coastal setback coastal setbacks are used to protect populations and infrastructure and involve determining baselines and historic trends in shoreline movements assessing these movements can be difficult on coasts with especially complex geomorphology such as due to rock and coral reefs reefs are often assumed to provide coastal protection and stability however little is known about the spatial variability of beach dynamics on beaches with reefs over inter annual and decadal time scales spatial variability in inter annual vegetation line and shoreline stability was assessed on three adjacent beaches at yanchep southwestern australia using 34 years 1974 2008 of aerial photographs over 96 of the study area beach width narrowed over the 34 years this trend was statistically significant on 83 of the middle beach 43 of the northern beach and only 29 of the southern beach the maximum mean annual change in beach width was 1 7 m yr 1 at the south end of the southern beach the first mode of the empirical orthogonal function eof explained 45 of the variability alongshore and correlated significantly p 0 05 with the 90th percentile of annual wave height and annual mean sea level beach rotation and reef controlled currents resulted in cellular beach morphodynamics and vegetation line and shoreline movements varied greatly alongshore on one occasion the beach widened by more than 50 m inter annually while other areas eroded by 50 m the southern beach that was fronted by the most continuous and highest supratidal section of reef was the most temporally variable even the vegetation line here moved by up to 20 m inter annually so cannot be assumed to be a stable baseline for setback the impact of reefs on inter annual shoreline and vegetation line stability should be considered in future guidelines for determining coastal setback

fluid pressure diffusion effects on the excess compliance matrix of porous rocks containing aligned fractures the presence of sets of open fractures is common in most reservoirs and they exert important controls on the reservoir permeability as fractures act as preferential pathways for fluid flow therefore the correct characterization of fracture sets in fluid saturated rocks is of great practical importance in this context the inversion of fracture characteristics from seismic data is promising since their signatures are sensitive to a wide range of pertinent fracture parameters such as density orientation and fluid infill the most commonly used inversion schemes are based on the classical linear slip theory lst in which the effects of the fractures are represented by a real valued diagonal excess compliance matrix to account for the effects of wave induced fluid pressure diffusion fpd between fractures and their embedding background several authors have shown that this matrix should be complex valued and frequency dependent however these approaches neglect the effects of fpd on the coupling between orthogonal deformations of the rock with this motivation we considered a fracture model based on a sequence of alternating poroelastic layers of finite thickness representing the background and the fractures and derived analytical expressions for the corresponding excess compliance matrix we evaluated this matrix for a wide range of background parameters to quantify the magnitude of its coefficients not accounted for by the classical lst and to determine how they are affected by fpd we estimated the relative errors in the computation of anisotropic seismic velocity and attenuation associated with the lst approach our analysis showed that in some cases considering the simplified excess compliance matrix may lead to an incorrect representation of the anisotropic response of the probed fractured rock

equivalent medium representation of fractured rock a similarity may be found between various approaches for determining the effects of parallel fractures or aligned cracks on seismic wave propagation at wavelengths that are long compared with the scale length of the cracks fractures can be modeled using an empirical linear slip condition however natural fracture surfaces can also be simulated directly as planar distributions of small isolated areas of slip cracks model 1 or conversely as planar distributions of imperfect interfacial contacts model 2 an alternative is plane surfaces separated by thin continuous layers of viscous fluid or a soft material model 3 we present analytic expressions for the fracture compliances for these three models and using these analytic results compute the effective compliances and stiffnesses of the fractured material as a result it is possible to relate the measured compliances or stiffnesses directly to the statistics of the microstructural details of a fracture given appropriate a priori information on the fracture surfaces the results for model 1 are equivalent to a volume distribution of cracks as studied by hudson 1980 1981 for small crack density the results for model 2 are basically the same as those given by white 1983 for a packing of spheres and finally the results for model 3 are in agreement with those given by backus 1962 for combinations of two constituent layers these results can be extended to the case of nonaligned fractures and to allow for fluid flow between cracks and into a porous matrix rock finally it is shown that the ratio of the normal to shear fracture compliance is a good indicator of the properties of the fracture infill

tsunami as agents of geomorphic change in mid ocean reef islands low lying atoll islands appear highly vulnerable to the effects of climate change and extreme natural events potentially disastrous effects of future sea level rise have been inferred in many studies and the actual impacts of tropical storms on island destruction and formation have been well documented in contrast the role of tsunami in the geomorphic development of atoll islands has not been investigated the sumatran earthquake of 26 december 2004 generated a tsunami that reached the maldives 2500 km away with waves up to 2 5 m high observations on the geomorphic changes resulting from the tsunami are detailed here based on pre and post tsunami profile measurements of island beach and reef topography and gps surveys of the planform shape of islands and beaches of 11 uninhabited islands in south maalhosmadulu atoll maldives erosional and depositional impacts were observed on all islands and these have been quantified in general the changes were of a minor nature with a maximum reduction in island area of 9 and average of 3 75 rather the tsunami accentuated predictable seasonal oscillations in shoreline change including localised erosion reflected in fresh scarps and seepage gullies depositional features in the form of sand sheets and sand lobes emplaced on the vegetated island surfaces provide clear evidence that the tsunami waves washed over parts of all the islands both erosional scarps and overwash deposits were concentrated at the tsunami exposed eastern sides of the islands impacts on leeward shores were primarily accretionary in the form of spit and cuspate foreland extension whereas the nature and magnitude of intra and inter island impacts was variable an east to west decline in aggregate effects was noted detailed consideration of the morphodynamic interaction between the tsunami waves and island morphology show that this cross atoll gradient resulted not just from the reduction in tsunami energy as it passed through the atoll but also from variations in elevation of the encircling island ridge and the quantity and distribution of sediment in the antecedent beach a conceptual model identifying the sequence of changes to individual islands supports the observational data and the pattern of geomorphic changes resulting from the tsunami this model leads to consideration of the longer term impacts of the tsunami on the future stability of islands four scenarios are presented each of which has a different island beach sediment budget and different relaxation time to achieve dynamic equilibrium

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

storms and shoreline retreat in the southern gulf of st lawrence abstract storms play a major role in shoreline recession on transgressive coasts in the southern gulf of st lawrence gsl southeastern canada long term relative sea level rise off the north shore of prince edward island has averaged 0 3 m century over the past 6000 years 0 2 m century over 2000 years this has driven long term coastal retreat at mean rates 0 5 m a but the variance and details of coastal profile response remain poorly understood despite extensive sandy shores sediment supply is limited and sand is transferred landward into multidecadal to century scale storage in coastal dunes barrier washover deposits and flood tidal delta sinks charlottetown tide gauge records show mean relative sea level rise of 3 2 mm a 0 32 m century since 1911 a further rise of 0 7 0 4 m is projected over the next 100 years when differenced from tidal predictions the water level data provide a 90 year record of storm surge occurrence combined with wind wave hindcast and sea ice data this provides a catalogue of potentially significant coastal storms we also document coastal impacts from three recent storms of great severity in january and october 2000 and november 2001 digital photogrammetry 1935 1990 and shore zone surveys 1989 2001 show large spatial and temporal variance in coastal recession rates weakly correlated with the storm record in part because of wave suppression or coastal protection by sea ice large storms cause rapid erosion from which recovery depends in part on local sand supply but barrier volume may be conserved by washover deposition barrier shores with dunes show high longshore and interdecadal variance with extensive multidecadal healing of former inlet and overwash gaps this reflects recovery from an episode of widespread overwash prior to 1935 possibly initiated by intense storms or groups of storms in the latter half of the 19th century with evidence from the storms of 2000 2001 this points to the importance of storm clustering on scales of weeks to years in determining erosion vulnerability as well as the need for a long term large scale perspective in assessing coastal stability the expected acceleration in relative sea level rise together with projections of increasing storm intensity and greatly diminished winter ice cover in the southern gsl implies a significant increase in coastal erosion hazards in future

seismic dispersion and attenuation in saturated porous rocks with aligned fractures of finite thickness theory and numerical simulations part 1 p wave perpendicular to the fracture plane when a seismic wave travels through a fluid saturated porous reservoir containing aligned fractures it induces oscillatory fluid flow between the fractures and the embedding background medium although there are numerous theoretical models for quantifying the associated seismic attenuation and velocity dispersion they rely on certain assumptions such as infinitesimal fracture thickness and dilute fracture concentration which usually do not hold in real reservoirs the objective of this work is to overcome some of these limitations and therefore improve the applicability of the available theoretical models to do so we extend existing models to the finite fracture thickness case for p waves propagating perpendicular to the fracture plane using the so called branching function approach we consider three types of fractures namely periodically and randomly spaced planar fractures as well as penny shaped cracks the extended unified model is then tested by comparing with corresponding numerical sim

qvoa analysis p wave attenuation anisotropy for fracture characterization this paper investigates q anisotropy for characterizing fractured reservoirs specifically the variation of the seismic quality factor q versus offset and azimuth qvoa w e derive an analytical expression for p wave attenuation in a transversely isotropic medium with horizontal symmetry axis hti and provide a method qvoa for estimating fracture direction from azimuthally varying q in pp wave reflection data the qvoa formula is similar to ruger s approximation for pp wave reflection coefficients the theoretical basis for amplitude variation with angle offset avoa analysis the technique for qvoa analysis is similar to azimuthal avo analysis we introduce two new seismic attributes q versus offset qvo gradient and intercept qvo gradient inversion not only indicates fracture orientation but also characterizes q anisotropy we relate the q anisotropy parameter q to fractured medium parameters and invert the qvo gradient to estimate q the attenuation parameter q and thomsen style anisotropy parameter v are found to be interdependent the attenuation anisotropy magnitude strongly depends on the host rock s vs vp parameter whereas the dependence on fracture parameters is weak this complicates the qvo gradient inversion for the fracture parameters this result is independent of the attenuation mechanism to illustrate the qvoa method in synthetic data we use hudson s first order effective medium model of a dissipative fractured reservoir with fluid flow between aligned cracks and random pores as a possible mechanism for p wave attenuation

the leaking bucket of a maldives atoll implications for the understanding of carbonate platform drowning abstract seismic and multibeam data as well as sediment samples were acquired in the south male atoll in the maldives archipelago in 2011 to unravel the stratigraphy and facies of the lagoonal deposits multichannel seismic lines show that the sedimentary succession locally reaches a maximum thickness of 15 20 m above an unconformity interpreted as the emersion surface which developed during the last glacial sea level lowstand such depocenters are located in current protected areas flanking the reef rim of the atoll or in infillings of karst dolinas much of the 50 m deep sea floor in the lagoon interior is current swept and has no or very minor sediment cover erosive current moats line drowned patch reefs whereas other areas are characterized by nondeposition karst sink holes blue holes and karst valleys occur throughout the lagoon from its rim to its center lagoonal sediments are mostly carbonate rubble and coarse grained carbonate sands with frequent large benthic foraminifers halimeda flakes red algal nodules mollusks bioclasts and intraclasts some of them glauconitic as well as very minor ooids finer grained deposits locally are deposited in current protected areas behind elongated faros i e small atolls which are part of the rim of south male atoll the south male atoll is a current flushed atoll where water and sediment export with the open sea is facilitated by the multiple passes dissecting the atoll rim with an elevated reef rim and tower like reefs in the atoll interior it is an example of a leaky bucket atoll which shares characteristics of incipiently drowned carbonate banks or drowning sequences as known from the geological record

frequency dependent anisotropy of porous rocks with aligned fractures naturally fractured reservoirs are becoming increasingly important for oil and gas exploration in many areas of the world because fractures may control the permeability of a reservoir it is important to be able to find and characterize fractured zones in fractured reservoirs the wave induced fluid flow between pores and fractures can cause significant dispersion and attenuation of seismic waves for waves propagating normal to the fractures this effect has been quantified in earlier studies here we extend normal incidence results to oblique incidence using known expressions for the stiffness tensors in the low and high frequency limits this allows us to quantify frequency dependent anisotropy due to the wave induced flow between pores and fractures and gives a simple recipe for computing phase velocities and attenuation factors of quasi p and sv waves as functions of frequency and angle these frequency and angle dependencies are concisely expressed through dimensionless velocity anisotropy and attenuation anisotropy parameters it is found that although at low frequencies the medium is close to elliptical which is to be expected as a dry medium containing a distribution of penny shaped cracks is known to be close to elliptical at high frequencies the coupling between p wave and sv wave results in anisotropy due to the non vanishing excess tangential compliance

a new model evaluating holocene sediment dynamics insights from a mixed carbonate siliciclastic lagoon bora bora society islands french polynesia south pacific abstract mixed carbonate siliciclastic lagoons of barrier reefs provide great potential as sedimentary archives focusing on paleoenvironmental and paleoclimatic changes as well as on event deposition sediment sources include lagoonal carbonate production the marginal reef and the volcanic hinterland mixed carbonate siliciclastic continent attached coastal lagoons have been intensively studied however their isolated oceanic counterparts have been widely disregarded here we present a new model of holocene sediment dynamics in the barrier reef lagoon of bora bora based on sedimentological paleontological geochronological and geochemical data the lagoonal succession started with a pleistocene soil representing the lowstand systems tract as the rising holocene sea inundated the carbonate platform peat accumulated locally 10 650 9400 years bp mixed carbonate siliciclastic sedimentation started ca 8700 5500 years bp and represents the transgressive systems tract during that time sediments were characterized by relatively coarse grain size and contained high amounts of terrestrial material from the volcanic hinterland as well as carbonate sediments mainly produced within the lagoon siliciclastic content decreases throughout the holocene after the rising sea had reached its modern level sand aprons formed between reef crest and lagoon creating transport pathways for reef derived material leading to carbonate dominated sedimentation ca 6000 3000 years bp during the highstand systems tract however mainly fine material was transported and accumulated in the lagoon while coarser grains were retained on the prograding sand apron from ca 4500 500 years bp significant variations in grain size total organic carbon as indicator for primary productivity ca and cl element intensities as qualitative indicators for carbonate availability and lagoonal salinity are seen such patterns could indicate event re deposition and correlate with contemporaneous event deposits found in the lagoon of nearby tahaa which are supposed to be induced by elevated cyclone activity correspondingly enhanced erosion and run off from the volcanic hinterland as well as lower lagoonal salinity would be associated with intense rainfall during repeated cyclone landfall increased amounts of coarse grained sediment from marginal reef areas would be transported into the lagoon however ti ca and fe ca ratios as proxies for terrigenous sediment delivery have incessantly declined since the mid holocene also benthic foraminiferal faunas do not validate reef to lagoon transport of sediment alternatively the apparent onset of higher hydrodynamic energy conditions can be explained by more permanent southeast trade winds and higher than present sea level which are supposed for the mid late holocene in the south pacific sustained winds would have flushed higher amounts of open ocean water into the lagoon enhancing primary productivity and the amount of pelagic organisms within the lagoon while lowering lagoonal salinity we propose the shift towards coarser grained sedimentation patterns during the mid late holocene to reflect sediment load shedding of sand aprons due to oversteepening of slopes at sand apron lagoon edges during times of stronger trades and higher than present sea level of the highstand systems tract which led to redeposition of sediment even within the lagoon center modern conditions including a sea level fall to modern level were reached ca 1000 years bp and lagoonal infill has been determined to a large part by fine grained carbonate dominated sediments produced within the lagoon and derived from the marignal reef infill of lagoonal accommodation space via sand aprons is estimated to be up to six times higher than infill by lagoonal background sedimentation and emphasizes the importance of the progradation of sand aprons contrary to the commonly supposed assumption that coarse grained sediment layers within fine grained lagoonal successions represent overwash events induced by storms or periods of higher storm activity we postulate a new model of long term lagoonal sediment dynamics including sea level climatic change and geomorphological variation of the barrier reef lagoon

transmission of seismic waves across single natural fractures fractures and other nonwelded contacts are important mechanical and hydrological features of rock masses their effects on seismic wave propagation can be modeled as a boundary condition in the seismic wave equation seismic stress is continuous across such a boundary but seismic particle displacement and seismic particle velocity are not the complete solutions for seismic wave reflection conversion and transmission across a displacement and velocity discontinuity between two half spaces with different densities and elastic properties are derived for all angles of the incident wave the ratio between the seismic stress across this boundary and the seismic particle displacement and velocity are described by a specific stiffness and a specific viscosity respectively a displacement discontinuity results in frequency dependent reflection and transmission coefficients and a frequency dependent group time delay the velocity discontinuity results in frequency independent coefficients and zero delay results of laboratory experiments on compressional and shear wave transmission across three different natural fractures in a quartz monzonite are described measurements were made at different effective stresses under dry and saturated conditions at room temperature it is shown that the effect of these fractures on the spectral amplitudes for compressional and shear pulses transmitted across these fractures are described well by a displacement discontinuity for compressional pulses under dry and saturated conditions and by a combined displacement and velocity discontinuity for shear wave pulses under dry and saturated conditions values of specific stiffness and specific viscosity vary between fractures and increase with increasing effective stress as does the static specific stiffness of these fractures changes in the spectral amplitudes of transmitted pulses are also analyzed in terms of attenuation using the seismic quality factor q which is found to be a function of frequency

short term changes in the morphodynamic states of beaches and surf zones an empirical predictive model a time series of 6 5 years of daily observations of beach states and wave characteristics from southeastern australia was analyzed with the aim of determining the degree to which time varying beach state can be explained and predicted the six commonly occurring beach states dissipative longshore bar trough rhythmic bar and beach transverse bar and rip low tide terrace and reflective discussed earlier were related by means of discrete discriminant analyses to the parameter hb wst where hb is breaker height ws is the fall velocity of the beach sand and t is peak breaker period using a total of 1545 cases two values were used in the analyses the immediate value occurring on the day the particular state was observed and a weighted mean value expressing recently antecedent conditions the immediate value made a negligible contribution to explaining day to day beach state observations however the antecedent conditions showed a strong relationship and provide a successful means of prediction by examining cases where the time derivatives of both state and were near zero it was possible to define the equilibrium conditions associated with each state directions of change erosional or accretionary are predicted in terms of departures of from the equilibrium value appropriate to the beach state prevailing at the time change begins

the geomorphology development and temporal dynamics of tepuka island funafuti atoll tuvalu abstract the evolution sedimentary structure and morphodynamics of tepuka island a leeward vegetated sand cay on funafuti atoll tuvalu was examined based on morphostratigraphic analysis and radiometrically dated island sediments tepuka island formed rapidly on the leeward reef platform between 1100 and 500 cal years bp and presents the first evidence of island formation at current sea level postdating the holocene highstand the island is predominantly composed of foraminifera coralline algae and coral sands derived from the surrounding reef flat morphostratigraphic analysis suggests that the island formed as a central core and prograded rapidly through incremental accumulation results indicate that the vegetated core of tepuka has been stable since formation but the island margins have been morphologically dynamic over the past century the island has reduced in net area by approximately 23 substantial planform changes have been documented which have occurred through differential erosion of the shoreline in two phases lagoon shoreline erosion from 1897 to 1984 and ocean shoreline coupled with extension of the terminal ends of islands from 1984 to 2011 collectively these changes have caused a lengthening and clockwise rotation of the island on the reef platform the investigation highlights significant differences in the geomorphology sediment composition timeframe of formation and post formation island dynamics compared with the storm deposited and comparatively stable motu on the exposed eastern atoll rim