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991.
Andrea M. Snelling Melanie J. Leng Christopher C. Kendrick 《Proceedings of the Geologists' Association. Geologists' Association》2011,122(1):82-91
Here we show the use of graptolite periderm for chemostratigraphic study. Using material from the Aeronian (Silurian) interval from Wales and Scotland as examples, we show that the carbon isotope composition of the periderm (δ13Cgrap.) provides a signal that is locally different but not consistently so from surrounding whole-rock samples (δ13Cwhole-rock). Graptolite periderm δ13C seems not influenced by astogenetic stage of development or gross rhabdosome type and differences between δ13Cgrap. from different metamorphic grades are minimal. Taken as a whole, the Aeronian interval examined shows little overall change, but large variations are seen on the small scale, possibly reflecting very local carbon cycling. For carbon isotope stratigraphy in such rocks, therefore, large-scale bulk sampling will likely reduce inhomogeneities and give more reproducible results. Furthermore, in situations (for instance associated with sea level fluctuations) where terrestrial organic matter has been incorporated into the sediment, then graptolite carbon may more faithfully reflect bulk marine organic matter. 相似文献
992.
This paper presents two comparisons or tests for a Lagrangian model of zooplankton dispersion:numerical schemes and time steps.Firstly,we compared three numerical schemes using idealized circulations.Results show that the precisions of the advanced Adams-Bashfold-Moulton(ABM) method and the Runge-Kutta(RK) method were in the same order and both were much higher than that of the Euler method.Furthermore,the advanced ABM method is more efficient than the RK method in computational memory requirements and time consumption.We therefore chose the advanced ABM method as the Lagrangian particle-tracking algorithm.Secondly,we performed a sensitivity test for time steps,using outputs of the hydrodynamic model,Symphonie.Results show that the time step choices depend on the fluid response time that is related to the spatial resolution of velocity fields.The method introduced by Oliveira et al.in 2002 is suitable for choosing time steps of Lagrangian particle-tracking models,at least when only considering advection. 相似文献
993.
994.
Nieves R. Brisaboa M. Andrea Rodríguez Rodrigo A. Troncoso 《International journal of geographical information science》2013,27(2):280-304
A spatial data set is consistent if it satisfies a set of integrity constraints. Although consistency is a desirable property of databases, enforcing the satisfaction of integrity constraints might not be always feasible. In such cases, the presence of inconsistent data may have a negative effect on the results of data analysis and processing and, in consequence, there is an important need for data-cleaning tools to detect and remove, if possible, inconsistencies in large data sets. This work proposes strategies to support data cleaning of spatial databases with respect to a set of integrity constraints that impose topological relations between spatial objects. The basic idea is to rank the geometries in a spatial data set that should be modified to improve the quality of the data (in terms of consistency). An experimental evaluation validates the proposal and shows that the order in which geometries are modified affects both the overall quality of the database and the final number of geometries to be processed to restore consistency. 相似文献
995.
Piero Guilizzoni Aldo Marchetto Andrea Lami Stefano Gerli Simona Musazzi 《Journal of Paleolimnology》2011,45(4):433-445
We propose a palaeolimnological method for inferring past total phosphorus (TP) concentrations in lake water from spectrophotometrically-measured
sedimentary pigments, particularly total carotenoids (TC). Our approach is based on a highly significant statistical correlation
(P < 0.0001) between pigment concentrations (total carotenoids) in the surface sediment of 28 Italian lakes (subalpine, large,
deep, shallow, volcanic) and TP concentrations measured in these lakes at overturn when the core was collected. A transfer
function was developed from this “training” set, and used to estimate past TP concentrations from pigment concentrations in
sediment cores. The results generally agreed with TP values as measured by long-term water quality monitoring programs. Contrasting
results were obtained by a comparison with diatom-inferred TP. While the diatom model showed a tendency to overestimate TP
values higher than 100 μg l−1, the pigment model correctly estimated TP in lakes when TP was <100 μg l−1, but not when lakes were rich in macrophytes. In fact, lakes with extensive populations of aquatic submersed macrophytes
and epiphytes are outliers in terms of the TC versus TP relationship. The root mean square error of prediction of the pigment
model is lower than those derived from certain diatom—based inference models. The predicted and residual values are not related
to the estimated values and their average is not statistically different from zero. Errors were estimated via a ‘leave-one-out’
re-sampling technique. The proposed method permits rapid and relatively inexpensive determination of reference trophic conditions. 相似文献
996.
Andrea Zanini Peter K. Kitanidis 《Stochastic Environmental Research and Risk Assessment (SERRA)》2009,23(5):565-577
The estimation of field parameters, such as transmissivity, is an important part of groundwater modeling. This work deals
with the quasilinear geostatistical inverse approach to the estimation of the transmissivity fields from hydraulic head measurements.
The standard quasilinear approach is an iterative method consisting of successive linearizations. We examine a synthetic case
to evaluate the basic methodology and some modifications and extensions. The first objective is to evaluate the performance
of the quasilinear approach when applied to strongly heterogeneous (or “high-contrast”) transmissivity fields and, when needed,
to propose improvements that allow the solution of such problems. For large-contrast cases, the standard quasilinear method
often fails to converge. However, by introducing a derivative-free line search as a polishing step after each Gauss–Newton
iteration, we have found that convergence can be practically assured. Another issue is that the quasilinear procedure, which
uses linearization about the best estimate to evaluate estimation variances, may lead to inaccurate estimation of the variance
of the estimated variable. Our numerical results suggest that this may not be a particularly serious problem, though it is
hard to say whether this conclusion will apply to other cases. Nevertheless, since the quasilinear approach is an approximation,
we propose a potentially more accurate but computer-intensive Markov Chain Monte Carlo (MCMC) procedure based on conditional
realizations generated through the quasilinear approach and accepted or rejected according to the Metropolis–Hastings algorithm.
Six transmissivity fields with increasing contrast were generated and one thousand conditional realizations were computed
for each studied case. The MCMC procedure proposed in this work gives an overall more accurate picture than the quasilinear
approach but at a considerably higher computational cost. 相似文献
997.
Zelia Dionnet Martin D. Suttle Andrea Longobardo Alessandra Rotundi Luigi Folco Vincenzo Della Corte Andrew King 《Meteoritics & planetary science》2020,55(7):1581-1599
Giant micrometeorites (MMs; 400–2000 µm) are exceedingly rare and scientifically valuable. Three‐dimensional nondestructive characterization by X‐ray computed tomography (X‐CT) provides information on the petrography and thus petrogenesis of MMs and serves as a guide to maximize subsequent multi‐analytical studies on such precious planetary materials. Here, we discuss the results obtained by X‐CT on 22 giant MMs and the classification based on their 3‐D density contrast images. Scoriaceous and unmelted MMs have distinct porosity ranges (10–40 vol% versus 0–25 vol%, respectively). We observe a porosity variation inside scoriaceous MMs, which allows their atmospheric entry flight history to be resolved. For the first time, spinning entry is explicitly demonstrated for four partially melted MMs. Furthermore, we are able to resolve the thermal gradient in a single particle, based on porosity variation (seen as a progressive increase in pore abundance and size with higher peak temperatures). Moreover, we explore parent body alteration through the 3‐D analysis of pores distribution, showing that shock fabrics are either absent or weakly developed in our data set. Finally, owing to the detection of pseudomorphic chondrules, we estimate that the intensively aqueously altered C1 or CI‐like material could represent 18% of the MM flux at this size fraction (400–1000 µm). 相似文献
998.
Late quaternary speleogenesis and landscape evolution in the northern Apennine evaporite areas 下载免费PDF全文
Andrea Columbu Veronica Chiarini Jo De Waele Russell Drysdale Jon Woodhead John Hellstrom Paolo Forti 《地球表面变化过程与地形》2017,42(10):1447-1459
Gypsum beds host the majority of the caves in the north‐eastern flank of the Apennines, in the Emilia Romagna region (Italy). More than six hundred of these caves have been surveyed, including the longest known epigenic gypsum cave systems in the world (Spipola‐Acquafredda, ~11 km). Although this area has been intensively studied from a geological point of view, the age of the caves has never been investigated in detail. The rapid dissolution of gypsum and uplift history of the area have led to the long‐held view that speleogenesis commenced only during the last 130 000 years. Epigenic caves only form when the surface drainage system efficiently conveys water into the underground. In the study area, this was achieved after the dismantling of most of the impervious sediments covering the gypsum and the development of protovalleys and sinkholes. The time necessary for these processes can by constrained by understanding when caves were first formed. The minimum age of karst voids can be indirectly estimated by dating the infilling sediments. U–Th dating of carbonate speleothems growing in gypsum caves has been applied to 20 samples from 14 different caves from the Spipola‐Acquafredda, Monte Tondo‐Re Tiberio, Stella‐Rio Basino, Monte Mauro, and Castelnuovo systems. The results show that: (i) caves have been forming since at least ~600 kyr ago; (ii) the peak of speleogenesis was reached during relatively cold climate stages, when rivers formed terraces at the surface and aggradation caused paragenesis in the stable cave levels; (iii) ~200 000 years were necessary for the dismantling of most of the sediments covering the karstifiable gypsum and the development of a surface mature drainage network. Besides providing a significant contribution to the understanding of evaporite karst evolution in the Apennines, this study refines our knowledge on the timescale of geomorphological processes in a region affected by rapid uplifting. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
999.
Umberto Tomassetti António A. Correia Francesco Graziotti Andrea Penna 《地震工程与结构动力学》2019,48(11):1297-1318
Typical low-rise masonry buildings consist of unreinforced masonry (URM) walls covered with various timber roof configurations generally supported or finished by masonry gables. Post-earthquake observations and experimental outcomes highlighted the large vulnerability of the URM gables to the development of overturning mechanisms, both because of the inertial out-of-plane excitation and the in-plane timber diaphragm deformability. This paper presents the static and dynamic experimental seismic performance of three full-scale roofs tested via quasi-static cyclic and shake table tests. Two of them were tested as part of a whole full scale one-storey and two-storey building. A single-degree-of-freedom (SDOF) numerical model is calibrated against experimental data and proposed for the analysis of this roof typology's dynamic behaviour. Several sets of analyses were conducted to assess the vulnerability of these structural components and to study the effect of the whole building's characteristics (eg, number of storeys and structural stiffness and strength) on the seismic performance of this roof typology. 相似文献
1000.
Influence of the nonlinear behavior of viscous dampers on the seismic demand hazard of building frames 总被引:1,自引:0,他引:1 下载免费PDF全文
This paper analyzes the influence of damper properties on the probabilistic seismic performance of building frames equipped with viscous dampers. In particular, a probabilistic methodology is employed to evaluate the influence of the damper nonlinearity, measured by the damper exponent, on the performance of structural and nonstructural components of building frames, as described by the response hazard curves of the relevant engineering demand parameters. The performance variations due to changes in the damper nonlinearity level are evaluated and highlighted by considering two realistic design scenarios and by comparing the results of a set of cases involving dampers with different exponents designed to provide the same deterministic performance. By this way, it is possible to evaluate the influence of the nonlinear response and of its dispersion on the demand hazard. It is shown that the damper nonlinearity level strongly affects the seismic performance and different trends are observed for the demand parameters of interest. A comparison with code provisions shows that further investigation is necessary to provide more reliable design formulas accounting for the damping nonlinearity level. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献