3,940 research outputs found
Plug-in estimation of level sets in a non-compact setting with applications in multivariate risk theory
This paper deals with the problem of estimating the level sets of an unknown distribution function . A plug-in approach is followed. That is, given a consistent estimator of , we estimate the level sets of by the level sets of . In our setting no compactness property is a priori required for the level sets to estimate. We state consistency results with respect to the Hausdorff distance and the volume of the symmetric difference. Our results are motivated by applications in multivariate risk theory. In this sense we also present simulated and real examples which illustrate our theoretical results.Level sets ; Distribution function ; Plug-in estimation ; Hausdorff distance ; Conditional Tail Expectation
Ătude du comportement physique et hydrogĂ©ochimique d'un dĂ©pĂŽt de rĂ©sidus miniers en pĂąte dans des conditions de surface = Study of the Physical and Hydrogeochemical behaviour of surface paste disposal
Lâindustrie miniĂšre gĂ©nĂšre chaque annĂ©e une importante quantitĂ© de rejets solides que lâon peut classer en deux principales catĂ©gories : (i) les stĂ©riles : fragments de roche de granulomĂ©trie assez grossiĂšre et (ii) les rejets de concentrateur : minerai trĂšs finement broyĂ© dont on a extrait les substances Ă intĂ©rĂȘt Ă©conomique. Une partie des
rejets de concentrateur est retournĂ© sous terre sous forme de remblais miniers (hydraulique ou en pĂąte) et une autre partie est stockĂ©e en surface dans des parcs Ă rĂ©sidus. Ces parcs Ă rĂ©sidus, souvent ceinturĂ©s dâimportantes digues, sont des ouvrages difficiles Ă gĂ©rer et coĂ»teux Ă restaurer; ils sont Ă la base de nombreuses catastrophes (on dĂ©nombre a peu prĂšs deux ruptures de digues de parcs Ă rĂ©sidus par an dans le monde). Par ailleurs, le principal problĂšme environnemental liĂ© Ă lâexploitation miniĂšre et particuliĂšrement Ă la gestion des rejets est le drainage minier acide (DMA). Il se produit quand des minĂ©raux sulfureux sont exposĂ©s Ă lâeau et Ă lâair. Lâoxydation libĂšre alors de lâaciditĂ© dans le milieu favorisant ainsi la mise en solution de certains contaminants comme des mĂ©taux lourds.
Les résidus épaissis en pùte sont obtenus en épaississant des rejets de concentrateur jusqu'à la densité voulue (en général plus de 80% par masse de solide), puis en les
mĂ©langeant avec de lâeau et un liant. AprĂšs le mĂ©lange des divers ingrĂ©dients, on obtient un matĂ©riau ayant la consistance dâune pĂąte et qui peut ĂȘtre transportĂ© par
pompage ou par gravitĂ© Ă lâaide dâun rĂ©seau de tuyauterie. Depuis une vingtaine dâannĂ©es, ils Ă©taient principalement utilisĂ©s pour remblayer les chantiers souterrains
(remblais miniers en pĂąte cimentĂ©s). Mais, pour remplacer les parcs Ă rĂ©sidus « traditionnels » et mieux gĂ©rer le DMA certaines exploitations miniĂšres utilisent cette mĂ©thode pour lâentreposage en surface (dĂ©pĂŽt de pĂąte en surface ou DPS).
Le but de ce travail Ă©tait dâĂ©tudier le comportement environnemental de rĂ©sidus sulfureux stockĂ©s en utilisant la mĂ©thode des DPS. Plus particuliĂšrement nous avons
cherché à améliorer les propriétés de la pùte en ajoutant de faibles quantités de ciment. Pour cela nous avons réalisé des essais en colonnes, nous avons utilisé un
model physique instrumentĂ© et utilisĂ© des outils de modĂ©lisation numĂ©rique. Les rĂ©sultats montrent principalement que lâajout de ciment peut fortement amĂ©liorer les propriĂ©tĂ©s environnementales de rĂ©sidus sulfureux, cependant une mauvaise utilisation du ciment peut avoir lâeffet inverse. Des tests en colonnes (prĂ©parĂ©es avec des rĂ©sidus trĂšs gĂ©nĂ©rateurs dâacide contenant 54% de pyrite) ont montrĂ© que lâintercalation de couches contenant 2% de ciment entre des couches non cimentĂ©es permettait dâamĂ©liorer de maniĂšre significative le comportement environnemental du dĂ©pĂŽt. Lâajout de couches contenant 1% de ciment donnait de plus mauvais rĂ©sultats que la configuration sans ciment.
Ce travail a aussi permis de mettre en Ă©vidence lâimpact important des diffĂ©rentes discontinuitĂ©s (fractures de dessiccation, interfaces entre les couches et fracturations secondaires liĂ©e Ă lâattaque sulfatique). Ces diffĂ©rentes discontinuitĂ©s peuvent crĂ©er un rĂ©seau dâoxydation et dâinfiltration prĂ©fĂ©rentiel favorisant la formation du DMA
Azimuthal anisotropy of Rayleigh-wave phase velocities in the east-central United States
We explore the Rayleigh-wave phase velocity structure of the east-central US in a broad period range (10-200 s). Using a recent implementation of the two-stations method, we first measure interstation dispersion curves of Rayleigh-wave phase velocities along 60 paths. We then invert our collection of dispersion curves for isotropic and azimuthally anisotropic (2Κ and 4Κ) phase-velocity maps. The inversion is performed by a damped, smoothed LSQR, and the output model is parametrized on a triangular grid of knots with a 140 km grid spacing. Using the isotropic component of the phase velocity maps to constrain regional variations in shear velocity and Moho-depth, we observe that over the upper-middle crust depth range (z 1 per cent), and the azimuth of the fast-propagation direction is uniform over the entire region and equal to 54°. Our results suggest that azimuthal anisotropy beneath the east-central US is vertically distributed in three distinct layers, with a different geodynamic origin for each of the
Influence of intensive agriculture and geological heterogeneity on the recharge of an arid aquifer system (SaqâRam, Arabian Peninsula) inferred from GRACE data
This study assesses the detailed water budget of the SaqâRam Aquifer System (520â000âkm2) over the 2002â2019 period using satellite gravity data from the Gravity Recovery And Climate Experiment (GRACE). The three existing GRACE solutions were tested for their local compatibility to compute groundwater storage (GWS) variations in combination with the three soil moisture datasets available from the land surface models (LSMs) of the Global Land Data Assimilation System (GLDAS). Accounting for groundwater pumping, artificial recharge, and natural discharge uniformly distributed over the SaqâRam domain, the GRACE-derived mass balance calculation for water yields a long-term estimate of the domain-averaged natural recharge of (2.4±1.4)âmmâyrâ1, corresponding to (4.4±2.6)â% of the annual average rainfall (AAR). Beyond the regional-scale approach proposed here, spatial heterogeneities regarding the groundwater recharge were identified. The first source of heterogeneity is of anthropogenic origin: chiefly induced by irrigation excess over irrigated surfaces (about 1â% of the domain), artificial recharge corresponds to half of the total recharge of the aquifer. The second source of recharge heterogeneity identified here is natural: volcanic lava deposits (called harrats on the Arabian Peninsula) which cover 8â% of the SaqâRam aquifer domain but contribute to more than 50â% of the natural recharge. Hence, in addition to this application on the Arabian Peninsula, this study strongly indicates a major control of geological context on arid aquifer recharge, which has been poorly discussed hitherto. Due to large lag times of the diffuse recharge mechanism, the annual analysis using this GRACEâGLDAS approach in arid domains should be limited to areas where focused recharge is the main mechanism, while long-term analysis is valid regardless of the recharge mechanism. Moreover, it appears that about 15 years of GRACE records are required to obtain a relevant long-term recharge estimate.</p
Attitude Determination from Single-Antenna Carrier-Phase Measurements
A model of carrier phase measurement (as carried out by a satellite
navigation receiver) is formulated based on electromagnetic theory. The model
shows that the phase of the open-circuit voltage induced in the receiver
antenna with respect to a local oscillator (in the receiver) depends on the
relative orientation of the receiving and transmitting antennas. The model
shows that using a {\it single} receiving antenna, and making carrier phase
measurements to seven satellites, the 3-axis attitude of a user platform (in
addition to its position and time) can be computed relative to an initial
point. This measurement model can also be used to create high-fidelity
satellite signal simulators that take into account the effect of platform
rotation as well as translation.Comment: 12 pages, and one figure. Published in J. Appl. Phys. vol. 91, No. 7,
April 1, 200
âBridgingâ Engineering & Art: An Outreach Approach For Middle And High School Students
In this paper describes a novel outreach approach to high school and middle school students to familiarize them with engineering functions and methods. In this approach students participated in a seven-day summer research camp and learned many engineering skills and tools such as CAD solid modeling, finite element analysis, rapid prototyping, mechanical tests, team working, and communication skills under a project-based bridge design research project. The project choice was intended to integrate principles of artistic design, creative use of public spaces, as well as principles of engineering design. End of the program survey results showed a good understanding about the engineering skills and functions and a high degree of satisfaction among participants.
Reef response to sea-level and environmental changes during the last deglaciation: Integrated Ocean Drilling Program Expedition 310, Tahiti Sea Level
The last deglaciation is characterized by a rapid sea-level rise and coeval abrupt environmental changes. The Barbados coral reef record suggests that this period has been punctuated by two brief intervals of accelerated melting (meltwater pulses, MWP), occurring at 14.08-13.61 ka and 11.4-11.1 ka (calendar years before present), that are superimposed on a smooth and continuous rise of sea level. Although their timing, magnitude, and even existence have been debated, those catastrophic sea-level rises are thought to have induced distinct reef drowning events. The reef response to sea-level and environmental changes during the last deglacial sea-level rise at Tahiti is reconstructed based on a chronological, sedimentological, and paleobiological study of cores drilled through the relict reef features on the modern forereef slopes during the Integrated Ocean Drilling Program Expedition 310, complemented by results on previous cores drilled through the Papeete reef. Reefs accreted continuously between 16 and 10 ka, mostly through aggradational processes, at growth rates averaging 10 mm yr-1. No cessation of reef growth, even temporary, has been evidenced during this period at Tahiti. Changes in the composition of coralgal assemblages coincide with abrupt variations in reef growth rates and characterize the response of the upward-growing reef pile to nonmonotonous sea-level rise and coeval environmental changes. The sea-level jump during MWP 1A, 16 ± 2 m of magnitude in ~350 yr, induced the retrogradation of shallow-water coral assemblages, gradual deepening, and incipient reef drowning. The Tahiti reef record does not support the occurrence of an abrupt reef drowning event coinciding with a sea-level pulse of ~15 m, and implies an apparent rise of 40 mm yr-1 during the time interval corresponding to MWP 1B at Barbados. © 2012 Geological Society of America
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