Planimetric and volumetric changes of reef islands in response to wave conditions          下载免费PDF全文

Mirella B. Costa  Eduardo C. Macedo  Eduardo Siegle 《地球表面变化过程与地形》2017,42(15):2663-2678

Reef islands are morphologically dynamic features located on atolls and platform reefs that are very sensitive to wave‐induced processes on different timescales. The planform morphological evolution of reef islands is widely described; however, the mechanisms of the volumetric variations in response to wave energy are still poorly documented. To assess their multitemporal vertical and horizontal mobility, we performed a series of synchronous measurements of the volumetric changes and incident wave energies at two reef islands and a shingle bank at the Rocas Atoll in the South Atlantic Ocean. The results show the differences in the magnitudes and locations of the sediment mobility between the reef islands. Whereas one island remained stable on all timescales, with only small volumetric changes concentrated at its extremities, the other island (Farol Island) showed high mobility, especially during the energetic northern swell season. The gross volumetric change reached 10.03 × 10³ m³ (5% of the total island volume) on a daily timescale; however, on a seasonal scale, the gross erosion was compensated by the gross accretion, indicating a cyclical seasonal pattern. Moreover, the observed volumetric changes induced by the waves on both daily and seasonal timescales did not result in large shoreline displacements. However, long‐term oceanward erosion and substantial lagoonward accretion were observed at Farol Island on a decadal scale, resulting in a pronounced change in its planform morphology. This appears to be promoted by at least three sediment transport pathways induced by waves at the atoll, including sediment adjustment between the reef islands. Our results show that reef islands on the same atoll can have very distinct morphological behaviors on daily, seasonal and decadal scales in response to the same boundary conditions. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

Source wavelet correction for practical Marchenko imaging: A sub-salt field-data example from the Gulf of Mexico     

Constantin Mildner  Filippo Broggini  Carlos Alberto da Costa Filho  Johan O.A. Robertsson 《Geophysical Prospecting》2019,67(8):2085-2103

Imaging a target zone below a salt body can be challenging because large velocity contrasts in the overburden between the salt and surrounding sediments generate internal multiples, which interfere with primary reflections from the target level in the imaging process. This can lead to an erroneous interpretation of reflections in the sub-salt area if multiples are misinterpreted as primaries. The Marchenko redatuming method may enable imaging of the sub-salt target area where the effect of the multiply-scattering overburden is removed. This is achieved by creating a redatumed reflection response where virtual sources and receivers are located below the overburden using a macromodel of the velocity field and the surface reflection data. The accuracy of the redatumed data and the associated internal multiple removal, however, depends on the accurate knowledge of the source wavelet of the acquired reflection data. For the first time, we propose a method which can accurately and reliably correct the amplitudes of the reflection response in field data as required by the Marchenko method. Our method operates by iteratively and automatically updating the source function so as to cancel the most artefact energy in the focusing functions, which are also generated by the Marchenko method. We demonstrate the method on a synthetic dataset and successfully apply it to a field dataset acquired in a deep-water salt environment in the Gulf of Mexico. After the successful source wavelet estimation for the field dataset, we create sub-salt target-oriented images with Marchenko redatumed data. Marchenko images using the proposed source wavelet estimation show clear improvements, such as increased continuity of reflectors, compared to surface-based images and to conventional Marchenko images computed without the inverted source wavelet. Our improvements are corroborated by evidence in the literature and our own synthetic results.  相似文献   

Spatial distribution and estimation of rainfall trends and erosivity in the Epitácio Pessoa reservoir catchment,Paraíba,Brazil   总被引：1，自引：0，他引：1  

da Silva  Richarde Marques  Santos  Celso Augusto Guimarães  da Costa Silva  Jorge Flávio Cazé Braga  Silva  Alexandro Medeiros  Brasil Neto  Reginaldo Moura 《Natural Hazards》2020,102(3):829-849

Natural Hazards - The main goals of this study are to better understand the spatial and temporal variabilities in rainfall and to identify rainfall trends and erosivity for the period from 1963 to...  相似文献   

Reconciling Storegga tsunami sedimentation patterns with modelled wave heights: A discussion from the Shetland Isles field laboratory     

Alastair G. Dawson  Sue Dawson  Stein Bondevik  Pedro J. M. Costa  Jon Hill  Iain Stewart 《Sedimentology》2020,67(3):1344-1353

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Particle precipitation in Brazilian geomagnetic anomaly during magnetic storms     

N.B. Trivedi  D.B. Rai  I.M. Martin  J.M. Da Costa 《Planetary and Space Science》1973,21(10):1699-1704

Particle precipitation in Brazilian geomagnetic anomaly during magnetic storms is investigated using riometer and VLF propagation data. It is found that during large storms the changes in the ionosphere caused by particle precipitation are detectable. There is a good correlation between the behavior of the absorption and the variations of the magnetic field intensity during different phases of a storm. In particular, there seems to be a close relationship between the precipitation of high energy particles and short-period fluctuations of the magnetic field intensity of the order of 5–6 min. During the main phase of the storm, when the field intensity reaches its minimum, the flux of soft electrons also plays a significant role in producing absorption. The nature of precipitation associated with a sudden commencement appears to be more complex; the predominance of low or high energy particle flux may depend on the magnitude of the field increase. The amplitude and phase records of VLF signals also show the effect of the disturbance, but it is difficult to correlate the changes in these records with the features observed on the magnetogram, because only a small part of the propagation path lies in the region of the anomaly. A more detailed analysis of riometer data from different stations and VLF phase and amplitude records for different paths will be helpful in understanding the mechanism of particle precipitation associated with magnetic disturbances. In future experiments it may also be fruitful to look for detectable radiation emitted by the precipitating electrons, for example, Cherenkov and synchrotron radiation.  相似文献   

Active and passive arching stresses outside a deep trapdoor     

Costa  Yuri D. J.  Zornberg  Jorge G. 《Acta Geotechnica》2020,15(11):3211-3227

Acta Geotechnica - The classic trapdoor configuration has been useful to examine the changes in stresses expected on buried structures. However, the primary focus of previous studies has been on...  相似文献   

Coral reefs of the Glorieuses Islands,western Indian Ocean     

MH Schleyer  L Bigot  Y Benayahu 《African Journal of Marine Science》2018,40(3):331-339

The benthic fauna on reefs around the Glorieuses Islands, a small and protected Indian Ocean archipelago northwest of Madagascar, was surveyed in November 2015, focusing particularly on the orders Alcyonacea (soft corals and gorgonians) and Scleractinia (stony corals). The species richness of both groups was rather low for a protected, relatively pristine environment in the region. Though certain soft ‘fugitive’ alcyonacean species were noticeably abundant, other soft corals were rare. Sediment-tolerant faviid corals were abundant among the scleractinians. The sediment around the reefs was fine-grained and white, and appeared to be derived from abundant green macroalgae Halimeda and coralline seaweeds. The abundance of these, in turn, might have been attributable to nutrient enrichment from guano deposited on one of the islands by migratory seabirds, causing the aforementioned anomalies in coral biodiversity and abundance.  相似文献   

Using Mahalanobis Distance to Detect and Remove Outliers in Experimental Covariograms     

Drumond  David Alvarenga  Rolo  Roberto Mentzingen  Costa  João Felipe Coimbra Leite 《Natural Resources Research》2019,28(1):145-152

Experimental variograms are crucial for most geostatistical studies. In kriging, for example, the variography has a direct influence on the interpolation weights. Despite the great importance of variogram estimators in predicting geostatistical features, they are commonly influenced by outliers in the dataset. The effect of some randomly spatially distributed outliers can mask the pattern of the experimental variogram and produce a destructuration effect, implying that the true data spatial continuity cannot be reproduced. In this paper, an algorithm to detect and remove the effect of outliers in experimental variograms using the Mahalanobis distance is proposed. An example of the algorithm’s application is presented, showing that the developed technique is able to satisfactorily detect and remove outliers from a variogram.

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Nursery use patterns of commercially important marine fish species in estuarine systems along the Portuguese coast     

R.P. Vasconcelos  P. Reis-Santos  A. Maia  V. Fonseca  S. França  N. Wouters  M.J. Costa  H.N. Cabral 《Estuarine, Coastal and Shelf Science》2010

Analysing the estuarine use patterns of juveniles of marine migrant fish species is vital for identifying important sites for juveniles as well as the basic environmental features that characterize these sites for different species. This is a key aspect towards understanding nursery function. Various estuarine systems along the Portuguese coast (Minho, Douro, Ria de Aveiro, Mondego, Tejo, Sado, Mira, Ria Formosa and Guadiana) were sampled during Spring and Summer 2005 and 2006. Juveniles of commercially important marine fish species Solea solea, Solea senegalensis, Platichthys flesus, Diplodus vulgaris and Dicentrarchus labrax, predominantly 0-group individuals, were amongst the most abundant species and had distinct patterns of estuarine use as well as conspicuous associations with several environmental features. Juvenile occurrence and density varied amongst estuaries and sites within them, and differed with species. Sites with consistently high juvenile densities were identified as important juvenile sites (i.e. putative nursery grounds). Through generalized linear models (GLM), intra-estuarine variation in occurrence and density of each of the individual species was largely explained by environmental variables (temperature; salinity; depth; percentage of mud in the sediment; presence of seagrass; importance of intertidal areas; relative distance to estuary mouth; macrozoobenthos densities; and latitude). Decisive environmental factors defining important sites for juveniles varied depending on the system as a result of different environmental gradients, though there were common dominant features for each species regardless of the estuary considered. Analysed environmental variables in the GLM also accounted for inter-estuarine variation in species' occurrence and density. In several estuaries, the identified important juvenile sites were used by many of these species simultaneously and may be of increased value to both management and conservation. Overall, the variability in site features amongst estuaries highlighted the tolerance of these species to different available environmental conditions and provided fundamental information for future spatially explicit modelling of their distribution. This should ultimately enable the prediction of species response to habitat alterations.  相似文献   

The role of fault reactivation in the development of tropical montane lakes     

Pedro Walfir M. Souza-Filho  Roberto Vizeu L. Pinheiro  Francisco R. Costa  José Tasso F. Guimarães  Prafulla K. Sahoo  Marcio S. Silva  Cleverson G. Silva 《地球表面变化过程与地形》2020,45(14):3732-3746

This work details the role of fault reactivation in the development of tropical montane lakes by using basin morpho-structural analysis and seismostratigraphic studies. The upland lakes are severely faulted sinkholes, whose faults penetrate the Quaternary sedimentary units. Four main stages are related to the lake formation: (i) an Early Proterozoic tectonic deformation of the rocks along the southern border of the Carajás Structure, where the lake is placed; (ii) differential erosion by – and building of – the formation of the South Carajás Hill; (iii) Fe-rich crust formation by weathering and gravitational collapse faults following the E–W plateau border and the start of Violão Lake formation during the Pliocene–Pleistocene; and (iv) episodic fault-fracture reactivation by gravitational collapse causing pulses of subsidence in the lake and outlining its faulted borders. Dissolution of the lateritic crust and erosion by runoff drainage under wet climate conditions were coeval with fault activities, which allowed the deposition of relatively thick clastic deposits organized in three main seismostratigraphic units associated with major lake-level fluctuations. Initial fault reactivation under low-level water started lacustrine basin development with deposition of prograding fan deltas related to the main drainage. A second fault reactivation by gravitational collapse increased the lake accommodation space and resulted in the deposition of fine-grained sediments from dilute interflows or overflows until 36 000 cal year BP. At about 31 000 cal year BP, rapid decreases in the lake water level under redox conditions at the sediment/water interface allowed widespread siderite formation. A third gravitational collapse episode was responsible for the increase in the lake area and depth and the returning of clastic/organic deposition up to the present. This tropical montane lake can be seen as a representative example for understanding the formation of other upland lakes controlled by fault reactivation. © 2020 John Wiley & Sons, Ltd.  相似文献