Critical properties of spherically symmetric accretion in a fractal medium     

Nirupam Roy  Arnab K. Ray 《Monthly notices of the Royal Astronomical Society》2007,380(2):733-740

Spherically symmetric transonic accretion of a fractal medium has been studied in both the stationary and the dynamic regimes. The stationary transonic solution is greatly sensitive to infinitesimal deviations in the outer boundary condition, but the flow becomes transonic and stable when its evolution is followed through time. The evolution towards transonicity is more pronounced for a fractal medium than it is for a continuum, and in the former case the static sonic condition is met on relatively larger length scales. The dynamic approach also shows that there is a remarkable closeness between an equation  of motion for a perturbation in the flow, and the metric of an analogue acoustic black hole. The stationary inflow solutions of a fractal medium are as much stable under the influence of linearized perturbations as they are for the fluid continuum.  相似文献   

Plio-Quaternary reactivation of the Neogene margin off NW Algiers, Algeria: The Khayr al Din bank     

Abdelkarim Yelles  Jacques Déverchère  Rabah Bracène  Pierre Strzerzynski  Guillaume Bertrand  Thierry Winter  Pascal Le Roy  Hamou Djellit 《Tectonophysics》2009,475(1):98-116

The Algiers region, northern Algeria, is known to be seismically active, with recurrent large (M>6) earthquakes. Because of the lack of high-resolution bathymetry, the offshore structures remained for a long time poorly known. Thanks to a new marine data base (MARADJA 2003 cruise), the offshore part of the margin is accurately mapped, and new active and recent structures are described. West of the bay of Algiers, the margin enlarges, forming the Khayr al Din bank, interpreted as a tilted block of the passive margin born during the opening of the Algero-Provençal basin. At the slope break, a 80 km-long fault-tip Quaternary fold, namely the Khayr al Din fault, extends at the foot of the margin off NW Algiers and represents the largest active structure of the coastal area, together with the Sahel anticline. We also map for the first time a set of overlapping, en echelon active folds in the upper part of the Khayr al Din bank, located off previously known active structures on land. Most of these faults represent actually a threat for the Algiers region in terms of seismic hazard but also geological hazards, such as tsunamis, as most of them depicts significant dimensions and slip rates. The highest long-term horizontal shortening rate is found on the Khayr al Din fault and is estimated at 0.5 ± 0.1 mm/yr, with a maximal magnitude of 7.3, which provides one of the highest seismogenic potential in the region.A new tectonic framework for the Algiers region is proposed, in which the main south-dipping offshore structure, of opposite vergence relative to most thrusts on land, appears to be nowadays the main driving fault system, as also found further east in the Boumerdès (M 6.8) 2003 rupture zone. The overall apparent pop-up structure of the recent and active faults may result from a progressive migration of the plate limit from the Late Miocene, north-dipping suture zone on land, to the Quaternary, south-dipping main Khayr al Din fault at sea, suggesting a process of subduction inception.  相似文献   

Geochemical signatures of Late Holocene paleo-hydrological changes from Phulera and Pokharan saline playas near the eastern and western margins of the Thar Desert,India     

P.D. Roy  Y.C. Nagar  N. Juyal  W. Smykatz-Kloss  A.K. Singhvi 《Journal of Asian Earth Sciences》2009,34(3):275-286

Stratigraphical, mineralogical, geochemical and optical dating methods were used to reconstruct paleo-hydrological changes in two playas (Phulera, 500 mm/a and Pokharan, 200 mm/a) in near extremum climatic regions of the Thar Desert. Sediment successions in shallow profiles from Phulera and Pokharan contain three and four stratigraphic units, respectively, each with characteristic geochemical properties. These units reflect changes in chemical weathering, detrital input, salinity and provide a measure of the changes in precipitation (i.e. monsoon) through time.Sediments from Pokharan suggest short rainfall events during ca. 6.6–4 ka, relatively stable fresh water (higher and persistent rainfall) regime during 4–2.3 ka, and a hyper saline (low rainfall) condition during 2.3–1.1 ka. Sediments at Phulera, record hyper saline (low rainfall) lacustrine conditions during <2.3 ka to >1.4 ka. Higher abundance of gypsum in Pokharan (2.3–1.1 ka) and proto-dolomite in Phulera (2.3–1.4 ka) are nearly synchronous and reflect enhancement of salinity. Selenite crystals in Pokharan and large desiccation cracks in buried horizons at Phulera reflect desiccation of playas at ca. 2 ka. Both playas progressively became less saline after 1.4 ka. Given the regional nature of this record, these changes are attributed to fluctuation of the monsoon over the Indian sub continent.  相似文献   

Stationarity Scores on Training Images for Multipoint Geostatistics   总被引：2，自引：2，他引：0  

Piotr W. Mirowski  Daniel M. Tetzlaff  Roy C. Davies  David S. McCormick  Nneka Williams  Claude Signer 《Mathematical Geosciences》2009,41(4):447-474

This research introduces a novel method to assess the validity of training images used as an input for Multipoint Geostatistics, alternatively called Multiple Point Simulation (MPS). MPS are a family of spatial statistical interpolation algorithms that are used to generate conditional simulations of property fields such as geological facies. They are able to honor absolute “hard” constraints (e.g., borehole data) as well as “soft” constraints (e.g., probability fields derived from seismic data, and rotation and scale). These algorithms require 2D or 3D training images or analogs whose textures represent a spatial arrangement of geological properties that is presumed to be similar to that of a target volume to be modeled. To use the current generation of MPS algorithms, statistically valid training image are required as input. In this context, “statistical validity” includes a requirement of stationarity, so that one can derive from the training image an average template pattern. This research focuses on a practical method to assess stationarity requirements for MPS algorithms, i.e., that statistical density or probability distribution of the quantity shown on the image does not change spatially, and that the image shows repetitive shapes whose orientation and scale are spatially constant. This method employs image-processing techniques based on measures of stationarity of the category distribution, the directional (or orientation) property field and the scale property field of those images. It was successfully tested on a set of two-dimensional images representing geological features and its predictions were compared to actual realizations of MPS algorithms. An extension of the algorithms to 3D images is also proposed. As MPS algorithms are being used increasingly in hydrocarbon reservoir modeling, the methods described should facilitate screening and selection of the input training images.  相似文献   

Numerical modeling of environmental flows using DAM: Some preliminary results     

Roy A. Walters  David R. Plew 《Acta Geophysica》2008,56(3):918-934

Development of closures and parameterizations for subgrid scale effects is a significant and longstanding problem in the numerical simulation of environmental flows. The model described herein uses a rigorous approach for developing double-averaged governing equations — first a traditional Reynolds averaging to derive the Reynolds averaged Navier-Stokes equation (RANS), then a volume average to derive a set of double-averaged equations (DANS). An existing finite element flow model is then modified to accommodate these equations. This process gives rise to several new terms that require closures, as well as a new equation for free surface elevation. This paper is directed toward model development and uses several existing closure schemes as test cases.  相似文献   

Geospatial modelling for goral habitat evaluation   总被引：1，自引：0，他引：1  

S. P. S. Kushwaha  S. Munkhtuya  P. S. Roy 《Journal of the Indian Society of Remote Sensing》2000,28(4):293-303

This study attempts to evaluate the habitat suitability of Chilla Sanctuary of Rajaji National Park for goral(Nemorhaedus goral) spread over 25859 sq km area. The IRS-1B false colour imagery, Survey of India topographical maps and ground observations were used to generate the spatial information on the extent of forest cover, waterholes, slope, settlements and the road network in and around the Sanctuary. The geospatial modelling was attempted using Arc/Info geographic information system (GIS). While proximity to waterholes, open forests with intermittent grasslands and steep slopes (30 degree or more) were considered as favourable conditions, the proximity to roads/settlements and flat terrain was taken as unfavourable condition. The results of this study showed that about 14 per cent area of the Sanctuary is highly to moderately suitable for goral. An additional 5 per cent area becomes available to goralif gujjars (tribals living inside sanctuary) are settled outside the sanctuary. Besides highlighting the endangered condition of the habitat for goral, this study demonstrates potential of remote sensing and GIS techniques for wildlife habitat suitability evaluation.  相似文献   

A field experiment on the development of sedimentary structures in a gravel‐bed river     

Hélène Lamarre  André G. Roy 《地球表面变化过程与地形》2008,33(7):1064-1081

  相似文献   

A new concept in Euler deconvolution of isolated gravity anomalies   总被引：2，自引：0，他引：2  

Roy  Agarwal  & Shaw 《Geophysical Prospecting》2000,48(3):559-575

Euler's homogeneity equation has been used to develop a new technique to interpret the gravity anomalies over some simple geometrical sources, namely a finite horizontal line/vertical line, a finite vertical ribbon, a semicircular dome/basin and an isosceles triangle approximating an anticline/syncline. A linear over-determined system of equations has been solved to compute the depth, the horizontal location and the structural index, all treated as free parameters. The concept of a variable structural index provides better depth estimates and helps to identify the source geometry. Nomograms have been prepared to compute an additional model parameter, namely the horizontal/vertical extent of a line, the vertical extent of a ribbon and the radius of a dome/basin. The efficacy of the proposed method has been evaluated using two real field examples.  相似文献   

A Photogravitational Hill Problem and Radiation Effects on Hill Stability of Orbits     

V.V. Markellos  A.E. Roy  M.J. Velgakis  S.S. Kanavos 《Astrophysics and Space Science》2000,271(3):293-301

We introduce a photogravitational version of Hill's problem to include the effect of radiation of the primaries, and discuss its equilibrium points and zero velocity curves. As a first application we use this model to study Hill stability of orbits around the small primary. The estimates obtained for the maximum distance of Hill stable orbits are compared to the predicted maximum sizes of accretion disks in binary stars according to recent theories based on periodic orbits as streamlines of the disks. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Spatial Modeling for Base-Metal Mineral Exploration Through Integration of Geological Data Sets     

Venkataraman  G.  Madhavan  B. Babu  Ratha  D. S.  Antony  Joju P.  Goyal  R. S.  Banglani  S.  Roy  S. Sinha 《Natural Resources Research》2000,9(1):27-42

This study involves the integration of information interpreted from data sets such as LandsatTM, Airborne magnetic, geochemical, geological, and ground-based data of Rajpura—Dariba,Rajasthan, India through GIS with the help of (1) Bayesian statistics based on the weights ofevidence method and (2) a fuzzy logic algorithm to derive spatial models to target potentialbase-metal mineralized areas for future exploration. Of the 24 layers considered, five layers(graphite mica schist (GMS), calc-silicate marble (CALC), NE-SW lineament 0–2000 mcorridor (L4-NESW), Cu 200–250 ppm, and Pb 200–250 ppm) have been identified from theBayesian approach on the basis of contrast. Thus, unique conditions were formed based onthe presence and absence of these five map patterns, which are converted to estimate posteriorprobabilities. The final map, based on the same data used to determine the relationships, showsfour classes of potential zones of sulfide mineralization on the basis of posterior probability.In the fuzzy set approach, membership functions of the layers such as CALC, GMS, NE-SWlineament corridor maps, Pb, and Cu geochemical maps have been integrated to obtain thefinal potential map showing four classes of favorability index.  相似文献