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521.
The morphology of the small stalkless Saccocoma tenella is unique among crinoids. It is characterized by an extremely light skeleton with dish-like lateral wings on the proximal brachials and peculiar paired vertical processes flanking the food grooves of more distal brachials. The arms are heavily branched. The lateral wings obviously were involved in vertical movement. For the vertical processes a “baffle rail” function for arm curling and “snap swimming” has been postulated, with muscles between the processes. However, there is no evidence that the processes were connected by muscles. For food collection a “pulsating funnel” model in the water column is advocated, with the processes serving to collect plankton during upward movement of the arms. The resulting mouth-up position is supported by the biostratinomy. Saccocoma tenella is considered to have been pelagic, a benthic lifestyle is rejected on ecologic and taphonomic grounds. Adorally-curled arms are considered a reaction to hostile environment before death, not a taphonomic artefact. 相似文献
522.
Andreas S. Stordal Hans A. Karlsen Geir Nævdal Hans J. Skaug Brice Vallès 《Computational Geosciences》2011,15(2):293-305
The nonlinear filtering problem occurs in many scientific areas. Sequential Monte Carlo solutions with the correct asymptotic
behavior such as particle filters exist, but they are computationally too expensive when working with high-dimensional systems.
The ensemble Kalman filter (EnKF) is a more robust method that has shown promising results with a small sample size, but the
samples are not guaranteed to come from the true posterior distribution. By approximating the model error with a Gaussian
distribution, one may represent the posterior distribution as a sum of Gaussian kernels. The resulting Gaussian mixture filter
has the advantage of both a local Kalman type correction and the weighting/resampling step of a particle filter. The Gaussian
mixture approximation relies on a bandwidth parameter which often has to be kept quite large in order to avoid a weight collapse
in high dimensions. As a result, the Kalman correction is too large to capture highly non-Gaussian posterior distributions.
In this paper, we have extended the Gaussian mixture filter (Hoteit et al., Mon Weather Rev 136:317–334, 2008) and also made the connection to particle filters more transparent. In particular, we introduce a tuning parameter for the
importance weights. In the last part of the paper, we have performed a simulation experiment with the Lorenz40 model where
our method has been compared to the EnKF and a full implementation of a particle filter. The results clearly indicate that
the new method has advantages compared to the standard EnKF. 相似文献
523.
A new numerical model was developed to simulate regional sediment transport and shoreline response in the vicinity of tidal inlets based on the one-line theory combined with the reservoir analogy approach for volumetric evolution of inlet shoals. Sand bypassing onshore and sheltering effects on wave action from the inlet bar and shoals were taken into account. The model was applied to unique field data from the south coast of Long Island, United States, including inlet opening and closure. The simulation area extended from Montauk Point to Fire Island Inlet, including Shinnecock and Moriches Inlets. A 20-year long time series of hindcast wave data at three stations along the coast were used as input data to the model. The capacity of the inlet shoals and bars to store sand was estimated based on measured cross-sectional areas of the inlets as well as on comprehensive bathymetric surveys of the areas around the inlet. Several types of sediment sources and sinks were represented, including beach fills, groin systems, jetty blocking, inlet bypassing, and flood shoal and ebb shoal feeding. The model simulations were validated against annual net longshore transport rates reported in the literature, measured shorelines, and recorded sediment volumes in the flood and ebb shoal complexes. Overall, the model simulations were in good agreement with the measured data. 相似文献
524.
Hans Rickman 《Meteoritics & planetary science》2014,49(1):8-20
This review starts with a brief historical overview of the subject, after which some recent papers attempting to improve the understanding of comet injection from the Oort Cloud and the origin of new comets are discussed. Special attention is paid to the importance of nongravitational effects in comet orbit determination, the synergy between stellar encounters and the galactic tides for the injection dynamics, and the role of planetary perturbations. The field is thus shown to be advancing rapidly, and brief comments on possible implications for studying the origin of the cloud are made. 相似文献
525.
H. Cloos 《International Journal of Earth Sciences》1933,24(6):395-396
Ohne Zusammenfassung 相似文献
526.
H. Cloos 《International Journal of Earth Sciences》1943,33(7-8):481-484
Ohne Zusammenfassung 相似文献
527.
Hans Züllig 《Journal of Paleolimnology》1989,2(1):23-40
Cultural eutrophication of lakes occurring over the last 100 years is well known. Less well known is the eutrophication of lakes in earlier, late Quaternary time due to human and other causes. The recent and earlier trophic changes are documented in the sedimentary record by several groups of parameters. Among the most revealing of these are the diverse carotenoid pigments that originate from phytoplankton, photosynthetic bacteria, and other biota. The interpretation of the carotenoids in ancient sediments is facilitated by the study of carotenoids in recent sediments from lakes with relevant limnological and historical information. I support these contentions with evidence from several Swiss lakes, with emphasis on the late Quaternary development of Pfaffikersee and Soppensee.This is the eighth of a series of papers to be published by this journal that was presented in the paleolimnology sessions organized by R. B. Davis and H. Löffler for the XIIth Congress of the International Union for Quaternary Research (INQUA), which took place in Ottawa, Canada in August 1987. Drs. Davis and Löffler are serving as guest editors of this series. 相似文献
528.
Monte Carlo simulations can either be viewed as a numerical method for solving evolutionary equations or as a way for statistically modelling the outcome of chaotic dynamical systems. Thus they are well suited for treating many aspects of cometary dynamics. We present a critical review of past applications of Monte Carlo simulations for both long- and short-period comets indicating the strength and weaknesses of the methods with suggestions for future applications. 相似文献
529.
The use of mud motors and other tools to accomplish forward motion of the bit in extended reach and horizontal wells allows avoiding large amounts of torque caused by rotation of the whole drill string. The forward motion of the drill string, however, is resisted by excessive amount of friction. In the presence of large compressive axial loads, the drill pipe or coiled tubing tends to buckle into a helix in horizontal boreholes. This causes additional frictional drag resisting the transmission of axial load (resulting from surface slack‐off force) to the bit. As the magnitude of the frictional drag increases, a buckled pipe may become ‘locked‐up’ making it almost impossible to drill further. In case of packers, the frictional drag may inhibit the transmission of set‐up load to the packer. A prior knowledge of the magnitude of frictional drag for a given axial load and radial clearance can help avoid lock‐up conditions and costly failure of the tubular. In this study a neural network model, for the prediction of frictional drag and axial load transmission in horizontal wellbores, is presented. Several neural network architectures were designed and tested to obtain the most accurate prediction. After cross‐validation of the Back Propagation Neural Network (BPNN) algorithm, a two‐hidden layer model was chosen for simultaneous prediction of frictional drag and axial load transmission. A comparison of results obtained from BPNN and General Regression Neural Network (GRNN) algorithms is also presented. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
530.
A new experimental system composed of two devices is presented for studying the hydraulic conductivity of rock joints and other interfaces in relation with their mechanical loading. This system allows for investigations on the anisotropy of the hydraulic conductivity, as well as for its heterogeneity. The first device controls the injection of a fluid, regulated in pressure or discharge near the centre of the sample of rock joint, which produces a quasi‐radial flow within the joint. The output flow is measured in real time using a sectorized peripheral membrane for acquisition of data about the directionality or anisotropy of the flow. The second device allows for the measurement of the evolution of the morphology and of the void map of the rock joints during their mechanical loading (compression or shearing). It is a laser beam moved by an (x, y) frame. This frame is installed within the shear box at several stages of the loading, after temporarily moving one rock wall away from the other one, but without unmounting them from the shear box. After the presentation of the experimental system, and of its performances, we give some examples of test results of replicas of rock joints, loaded in compression and shearing. First, these results show that the change in global transmissivity of the samples is strongly correlated with the normal relative displacement of the rock walls. Several statistical analyses of the change in the void map, taking into account the damage of the asperities of the joint, are presented and compared with the results of the directional output flows. Today further studies involving a FEM as well as a finite volume modelling of the flow within the joints, based on the measured void map are in progress for a more precise comparison to the experimental results. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献