A space-time varying graph for modelling places and events in a network     

Ikechukwu Maduako  Monica Wachowicz 《International journal of geographical information science》2019,33(10):1915-1935

Modelling topological relationships between places and events is challenging especially because these relationships are dynamic, and their evolutionary analysis relies on the explanatory power of representing their interactions across different temporal resolutions. In this paper, we introduce the Space-Time Varying Graph (STVG) based on the whole graph approach that combines directed and bipartite subgraphs with a time-tree for representing the complex interaction between places and events across time. We demonstrate how the proposed STVG can be exploited to identify and extract evolutionary patterns of traffic accidents using graph metrics, ad-hoc graph queries and clustering algorithms. The results reveal evolutionary patterns that uncover the places with high incidence of accidents over different time resolutions, reveal the main reasons why the traffic accidents have occurred, and disclose evolving communities of densely connected traffic accidents over time.  相似文献   

Human influence on seagrass habitat fragmentation in NW Mediterranean Sea     

Monica Montefalcone  Valeriano ParraviciniMatteo Vacchi  Giancarlo AlbertelliMarco Ferrari  Carla MorriCarlo Nike Bianchi 《Estuarine, Coastal and Shelf Science》2010

Habitat fragmentation in meadows of Posidonia oceanica, the most important and abundant seagrass in the Mediterranean Sea, was investigated at a region-wide spatial scale using a synthetic ecological index, the Patchiness Index (PI). We tested the hypothesis that human impacts are the major factor responsible for habitat fragmentation in P. oceanica meadows contrasting fragmentation of meadows located in “anthropized” areas with that of meadows located in areas with low anthropization and considered as virtually “natural”. We also related fragmentation of meadow with the morphodynamic state of the submerged beach (i.e. distinctive types of beach produced by the topography, the wave climate and the sediment composition) in order to investigate the influence of one natural component on the seagrass meadow seascape. Results demonstrated that fragmentation in the P. oceanica meadows is strongly influenced by the human component, being lower in natural meadows than in anthropized ones, and that it is little influenced by the morphodynamic state of the coast. The use of landscape approaches to discriminate natural disturbance from human impacts that affect seagrass meadows is thus recommended for the proper management of coastal zones.  相似文献   

Compositional variability and air-sea flux of ethane and propane in the plume of a large, marine seep field near Coal Oil Point, CA   总被引：1，自引：1，他引：0  

Susan Mau  Monica B. Heintz  Franklin S. Kinnaman  David L. Valentine 《Geo-Marine Letters》2010,30(3-4):367-378

Large quantities of methane (C1), ethane (C2), and propane (C3) emanate from shallow marine seeps near Coal Oil Point (COP), California. Concentrations of these gases were analyzed in the surface water down-current of the seep field over a 15-month period. The variable proportions of C1, C2, and C3 analyzed in gas bubbles emitted from 16 distinct seeps in the COP field encompass much of the variability found in the surface waters down-current. However, waters with disproportionate levels of C1 suggest the presence of additional C1 sources. Based on three spatial surveys, covering areas up to 280 km², C2 and C3 air-sea fluxes were estimated to be in the order of 3.7 and 1.4 μmol day^?1 m^?2, respectively. Only 0.6% of C2 and 0.5% of C3 in the dissolved plume originating from the COP seep field are transferred to the atmosphere in the study area, with the fate of the remainder uncertain.  相似文献   

Event-scale pebble mobility observed by RFID tracking in a pre-Alpine stream: a field laboratory     

Vladislav Ivanov  Alessio Radice  Monica Papini  Laura Longoni 《地球表面变化过程与地形》2020,45(3):535-547

Understanding coarse sediment transport is crucial for the prediction of sediment migration and the consequent development of fluvial morphologies. In this study, cobble displacements in a pre-Alpine creek have been recorded by means of radio frequency identification (RFID). Pebble monitoring has been systematically performed after each rainfall event with moderate precipitation, in order to exclude the superimposition of sediment displacements induced by triggering factors acting at different times. The analysis of the collected data was carried out through the application of both a principal component analysis and the Buckingham Π theorem. The experimental trends were interpreted considering the ratio of mobile pebbles, the pebbles' displacement and virtual velocity as the dependent variables. These quantities mostly depend on the event peak discharge, with a nonlinear increase of the travelled distance and a growth of up to two orders of magnitude of the virtual velocity (for an approximately 10× increase in peak discharge). An inverse dependency of the virtual velocity on the event duration was also observed. A comparison of the results obtained with those from laboratory investigations of bedload transport mechanics evidenced the differences in parametric trends associated with sediment mobility in the two environments. This contrast brings forward the combination of multiple drivers of sediment mobility, such as local morphology, sediment dimensions and flow unsteadiness, warranting a further in-depth investigation. Representation of results in a dimensionless form is suggested as a good practice to analyse data from case studies characterized by different scales. © 2019 John Wiley & Sons, Ltd.  相似文献   

Seismic depth imaging of iron-oxide deposits and their host rocks in the Ludvika mining area of central Sweden     

Lena Bräunig  Stefan Buske  Alireza Malehmir  Emma Bäckström  Monica Schön  Paul Marsden 《Geophysical Prospecting》2020,68(1):24-43

The development of cost-effective and environmentally acceptable geophysical methods for the exploration of mineral resources is a challenging task. Seismic methods have the potential to delineate the mineral deposits at greater depths with sufficiently high resolution. In hardrock environments, which typically host the majority of metallic mineral deposits, seismic depth-imaging workflows are challenged by steeply dipping structures, strong heterogeneity and the related wavefield scattering in the overburden as well as the often limited signal-to-noise ratio of the acquired data. In this study, we have developed a workflow for imaging a major iron-oxide deposit at its accurate position in depth domain while simultaneously characterizing the near-surface glacial overburden including surrounding structures like crossing faults at high resolution. Our workflow has successfully been showcased on a 2D surface seismic legacy data set from the Ludvika mining area in central Sweden acquired in 2016. We applied focusing prestack depth-imaging techniques to obtain a clear and well-resolved image of the mineralization down to over 1000 m depth. In order to account for the shallow low-velocity layer within the depth-imaging algorithm, we carefully derived a migration velocity model through an integrative approach. This comprised the incorporation of the tomographic near-surface model, the extension of the velocities down to the main reflectors based on borehole information and conventional semblance analysis. In the final step, the evaluation and update of the velocities by investigation of common image gathers for the main target reflectors were used. Although for our data set the reflections from the mineralization show a strong coherency and continuity in the seismic section, reflective structures in a hardrock environment are typically less continuous. In order to image the internal structure of the mineralization and decipher the surrounding structures, we applied the concept of reflection image spectroscopy to the data, which allows the imaging of wavelength-specific characteristics within the reflective body. As a result, conjugate crossing faults around the mineralization can directly be imaged in a low-frequency band while the internal structure was obtained within the high-frequency bands.  相似文献   

Scaling properties of the spatio–temporal distribution of earthquakes: a multifractal approach applied to a Californian catalogue     

C. Godano  P. Tosi  V. Derubeis  & P. Augliera 《Geophysical Journal International》1999,136(1):99-108

We study the spatio–temporal scaling properties of the NCSN California catalogue for the period 1975–1995 (about 137 000 events) using a multifractal approach. The study is based on a new method of declustering earthquake catalogues based on looking for independent events. This technique makes use of the properties of the multifractal distribution of the inter-event times. A comparison test between our method and the Gardner and Knopoff (GK hereafter) approach reveals that our method produces a catalogue of independent events (monofractal Poissonian catalogue), while the GK declustered catalogue remains multifractal. The main difference lies in the assumption that the events can be considered to be correlated at large distances. The spatial properties of the declustered catalogue reveal a distribution of epicentres that is more complex than the distribution for the complete catalogue. We suggest that such results are very similar to the scaling properties of fully developed turbulence.  相似文献   

Clustering properties of Etna seismicity during 1981–1991     

M.S. Barbano  V. De Rubeis  P. Tosi  S. Vinciguerra 《Journal of Seismology》2000,4(2):191-196

Space and time clustering properties ofseismic activity, affecting Etna Volcano (Italy)during 1981–1991, are investigated by fractaldimension analysis. Very interesting volcanic andseismic activity occurred within this time interval.Temporal evolution of the time fractal dimension D _t calculated on a moving window, revealscorrelation with the eruptive processes at differenttime scales confirming results obtained for a differenttime span (De Rubeis et al., 1997). Spatial fractaldimension D _s shows to be negativelycorrelated with the time fractal dimension D _t, suggesting a peculiar dynamic patternassociated with volcanic processes.  相似文献   

Effects of grazing by feral horses,clipping, trampling,and burning on a Georgia salt marsh     

Monica Goigel Turner 《Estuaries and Coasts》1987,10(1):54-60

Responses ofSpartina alterniflora marsh to combinations of feral horse grazing, clipping, simulated trampling, and a late winter burn were studied on Cumberland Island National Seashore, Georgia. Replicated 200-m² plots were established and sampled bimonthly from July 1983 to November 1984. Clipping and trampling each reduced peak aboveground biomass by 20% in 1983 and 50% (clipping) and 55% (trampling) in 1984. A March burn reduced peak aboveground biomass by 35% in 1984. Trampling and burning earch reduced net aboveground primary production (NAPP) by 35%, but clipping did not reduce NAPP. Standing stocks of live rhizomes were correlated with aboveground biomass and were reduced with experimental treatments. Abundance of the periwinkle snail (Littorina irrorata) was also reduced. Horse grazing had a substantial impact on standing stocks and NAPP ofSpartina, but grazing was not uniform throughout the marsh. Moderately grazed plots had NAPP reduced by 25% compared to ungrazed plots. Heavily grazed plots had extremely low NAPP, and abovegroundSpartina never exceeded 40 g m^?2 dry mass compared to 360 g m^?2 within exclosures.  相似文献   

Extreme wave conditions in the tyrrhenian sea     

L. Cavaleri  P.L. De Filippi  G.F. Grancini  G.L. Iovenitti  R. Tosi 《Ocean Engineering》1986,13(2)

A hindcast study of extreme wave conditions in the Tyrrhenian Sea is described. The paper covers the different steps of the work, including the identification and the statistics of the weather patterns in the area, the choice of the storms to be hindcasted, the wind and wave models, a check of the best extremal distributions for the estimate of the extreme values probability, and the estimate of the accuracy of the final results. Detailed examples of the results are given, followed by a discussion of their reliability.  相似文献   

Assessment of uncertainty associated with the estimation of well catchments by moment equations     

Monica Riva  Alberto Guadagnini  Michela De Simoni 《Advances in water resources》2006

Non-local stochastic moment equations are used successfully to analyze groundwater flow in randomly heterogeneous media. Here we present a moment equations-based approach to quantify the uncertainty associated with the estimation of well catchments. Our approach is based on the development of a complete second order formalism which allows obtaining the first statistical moments of the trajectories of conservative solute particles advected in a generally non-uniform groundwater flow. Approximate equations of moments of particles’ trajectories are then derived on the basis of a second order expansion in terms of the standard deviation of the aquifer log hydraulic conductivity. Analytical expressions are then obtained for the predictors of locations of mean stagnation points, together with their associated uncertainties. We implement our approach on heterogeneous media in bounded two-dimensional domains, with and without including the effect of conditioning on hydraulic conductivity information. The impact of domain size, boundary conditions, heterogeneity and non-stationarity of hydraulic conductivity on the prediction of a well catchment is explored. The results are compared against Monte Carlo simulations and semi-analytical solutions available in the literature. The methodology is applicable to both infinite and bounded domains and is free of distributional assumptions (and so applies to both Gaussian and non-Gaussian log hydraulic conductivity fields) and formally includes the effect of conditioning on available information.  相似文献