General joint conditional simulations using a fast fourier transform method     

Allan Gutjahr  Bryan Bullard  Sean Hatch 《Mathematical Geology》1997,29(3):361-389

A procedure for generating joint statistically homogeneous random fields is examined. The method is based on the spectral representation theorem. It handles large fields easily and is both rapid and flexible. Algorithm development and examples are presented. The procedure is adapted further to include the possibility of generating fields that are jointly conditioned on data from two related fields.  相似文献   

Chemical denudation rates in Kärkevagge, Swedish Lapland     

Sean W. Campbell  John C. Dixon  Colin E. Thorn  & Robert G. Darmody 《Geografiska Annaler: Series A, Physical Geography》2002,84(3-4):179-185

This study examines the spatial and temporal variability of chemical denudation rates in Kärkevagge, northern Sweden. The chemical flux rates within the valley are strongly influenced by the local geology. Chemical denudation rates determined for the study period are more than double those previously reported in the literature for this valley. Rates of greater than 46t km⁻² a⁻¹ were measured at the valley mouth over the course of the melt season. This difference is likely due to differences in measurement technique compared to that used by past researchers. This rate is also much higher than for other arctic and alpine watersheds. Chemical denudation in Kärkevagge is comparable to larger temperate rivers. The rapid chemical denudation in Kärkevagge is likely due to sulfide weathering creating acid solutions.  相似文献   

Fleet communication to abate fisheries bycatch     

Eric L. Gilman  Paul Dalzell  Sean Martin 《Marine Policy》2006

Fleet communication systems report near real-time observations of bycatch hotspots to enable a fishery to operate as a coordinated “One Fleet” to substantially reduce fleet-wide capture of protected bycatch species. This benefits the bycatch species per se, reduces waste, and can provide economic benefits to industry by reducing risk of exceeding bycatch thresholds and causing future declines in target species catch levels. We describe case studies of fleet communication programs of the US North Atlantic longline swordfish fishery, US North Pacific and Alaska trawl fisheries, and US Alaska demersal longline fisheries, and identify alternative fleet communication program designs to reduce fisheries bycatch. Evidence supports the inference that these three fleet communication programs substantially reduced fisheries bycatch and provided economic benefits that greatly outweighed operational costs. Fleet communication may be appropriate in fisheries where there are strong economic incentives to reduce bycatch, interactions with bycatch species are rare events, adequate onboard observer coverage exists, and for large fleets, vessels are represented by a fishery association.  相似文献   

On the use of an externally deployed geophone package on an Ocean Bottom Seismometer     

Jeremy D. Duschenes  Timothy W. Barash  Paul J. Mattaboni  Sean C. Solomon 《Marine Geophysical Researches》1981,4(4):437-450

We describe a recent modification to the MIT Ocean Bottom Seismometer by which the geophones are housed in a separate package that is deployed on the sea floor about 1 m distance from the main unit several hours after the OBS reaches the ocean bottom. Records from deep-sea experiments and shallow-water tests show two improvements over records from geophones housed in the main instrument package. Signals recorded by the external geophones have a much better signal-to-noise ratio because tape recorder noise and instrument vibrations generated by water currents are effectively eliminated. As a result, the overall frequency response of the sensors to ground motion has a demonstrably smoother spectral shape. The second improvement is that the cross coupling of horizontal instrument motion to vertical ground motion is apparently greatly reduced because of the simpler design for the sensor package.  相似文献   

Prediction of magnetite segregation in dense medium cyclone using computational fluid dynamics technique     

M. Narasimha  Mathew Brennan  P.N. Holtham 《International Journal of Mineral Processing》2007

Multi-phase simulations of turbulent driven flow in a dense medium cyclone with magnetite medium have been conducted in Fluent, using the Algebraic Slip Mixture model to model the dispersed phases and the air-core, and both the Large Eddy Simulation turbulence model (LES) and Reynolds Stress Models (RSM) for turbulence closure. The predicted air-core shape and diameter were found to be close to the experimental results measured by gamma ray tomography. It is possible to use the LES turbulence model with ASM multi-phase model to predict the air/slurry interface accurately. Multi-phase simulations (air/water/medium) show appropriate medium segregation effects but over-predict the level of segregation compared to that measured by gamma ray tomography near the wall. This is believed to be because of unaccounted back-mixing of the dispersed phase due to turbulence in the basic Algebraic Slip Mixture model. The predictions of accurate axial segregation of magnetite medium are investigated using the LES turbulence model in conjunction with the multi-phase mixture model and viscosity corrections according to the feed particle loading factor. At higher feed densities the agreement between the Dunglison [Dunglison, M.E., 1999, A general model of the dense medium cylone, PhD thesis, JKMRC, University of Queensland] correlations and experimental measurements and the CFD is reasonably good, but the overflow density is lower than the model predictions. It is believed that the excessive underflow volumetric flow rates are responsible for under prediction of the overflow density. The effect of size distribution of the magnetite has been fully studied. As expected, the ultra-fine magnetite sizes (i.e. 2 and 7 microns) are distributed uniformly throughout the cyclone. As the size of magnetite increases, more segregation of magnetite occurs close to the wall.  相似文献   

Mobilising generosity, framing geopolitics: Narrating crisis in the homeland through diasporic media     

Sean Carter   《Geoforum》2007,38(6):1102-1112

This paper provides an account of the humanitarian interventions enacted by the Croatian–American diaspora during the secessionist conflicts in Yugoslavia during the 1990s. Whilst undeniably an act of generosity towards ‘distant strangers’, actions such as these also represent a much more complex reality – they are an outcome of a complex set of relations and processes, in which the ethical choices of individuals become bound up with nationalist ideologies, geopolitical questions and, crucially, knowledge and understanding of distant events. In particular, this paper considers the ways in which generosity is mobilised through the framing of Balkan geopolitics through diasporic media. In so doing, it becomes possible to deepen the dialogue between work on geography and ethics on the one hand, and critical geopolitics on the other. In particular, the paper argues that due attention needs to be paid to the ways in which such ‘networks of concern’ are constructed in a variety of banal and mundane ways.  相似文献   

Liquefaction remediation by vertical drains with varying penetration depths     

A.J. Brennan  S.P.G. Madabhushi 《Soil Dynamics and Earthquake Engineering》2006,26(5):469-475

Vertical drains have been used as remediation against earthquake-induced soil liquefaction for many years. These are seen to begin fluid dissipation from deeper deposits first. Drains are not necessarily installed to the full depth of the liquefiable layer. To determine the effect of this on the efficiency of drain systems, centrifuge test results are presented. It is seen that not installing all drains through the full liquefiable depth significantly retards their performance, due to the dominance of vertical dissipation. It will be shown that a standard design chart may over-predict an improvement in drain performance.  相似文献   

Isotopic exchange of carbon-bound hydrogen over geologic timescales     

Alex L. Sessions  Sean P. Sylva  John M. Hayes 《Geochimica et cosmochimica acta》2004,68(7):1545-1559

The increasing popularity of compound-specific hydrogen isotope (D/H) analyses for investigating sedimentary organic matter raises numerous questions about the exchange of carbon-bound hydrogen over geologic timescales. Important questions include the rates of isotopic exchange, methods for diagnosing exchange in ancient samples, and the isotopic consequences of that exchange. This article provides a review of relevant literature data along with new data from several pilot studies to investigate such issues. Published experimental estimates of exchange rates between organic hydrogen and water indicate that at warm temperatures (50-100°C) exchange likely occurs on timescales of 10⁴ to 10⁸ yr. Incubation experiments using organic compounds and D-enriched water, combined with compound-specific D/H analyses, provide a new and highly sensitive method for measuring exchange at low temperatures. Comparison of δD values for isoprenoid and n-alkyl carbon skeletons in sedimentary organic matter provides no evidence for exchange in young (<1 Ma), cool sediments, but strong evidence for exchange in ancient (>350 Ma) rocks. Specific rates of exchange are probably influenced by the nature and abundance of organic matter, pore-water chemistry, the presence of catalytic mineral surfaces, and perhaps even enzymatic activity.Estimates of equilibrium fractionation factors between organic H and water indicate that typical lipids will be depleted in D relative to water by ∼75 to 140‰ at equilibrium (30°C). Thus large differences in δD between organic molecules and water cannot be unambiguously interpreted as evidence against hydrogen exchange. A better approach may be to use changes in stereochemistry as a proxy for hydrogen exchange. For example, estimated rates of H exchange in pristane are similar to predicted rates for stereochemical inversion in steranes and hopanes. The isotopic consequences of this exchange remain in question. Incubations of cholestene with D₂O indicate that the number of D atoms incorporated during structural rearrangements can be far less than the number of C-H bonds that are broken. Sample calculations indicate that, for steranes in immature sediments, the D/H ratio imparted by biosynthesis may be largely preserved in spite of significant structural changes.  相似文献   

Seasonal growth stimulation of sub-temperate estuarine phytoplankton to nitrogen and phosphorus: An outdoor microcosm experiment     

David A. Flemer  Robert J. Livingston  Sean E. McGlynn 《Estuaries and Coasts》1998,21(1):145-159

A study of nutrient limitation of phytoplankton biomass production with emphasis on nitrate-nitrogen (NO₃ ^?) and ortho-phosphate-phosphorus (PO₄ ^3?) was conducted in Perdido Bay, Alabama-Florida. The experimental design employed 18-1 outdoor microcosms operated in a static renewal mode. Phytoplankton growth responses (i.e., growth stimulation) measured as chlorophyll a (chl a) fell into three principal categories: primary P stimulation occurred mostly during the cooler months at the upper bay (tidal brackish) and mid bay (lower mesohaline) stations; a total of 12 out of 36 experiments; primary N stimulation occurred mostly during the warmer months primarily at the mid-bay station and infrequently at the upper and lower bay stations (upper mesohaline); a total of 7 out of 36 experiments; and N+P costimulation occurred primarily during the warmer months in the upper bay and mid bay and during both warmer and cooler months of the lower bay; a total of 17 out of 36 experiments. Primary P stimulation was generally associated with high ratios of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphate (DIP) (ratio range: 18 to 288). Conversely, primary N stimulation was associated with decreasing DIN:DIP ratios (range 8–46). Redfield ratios of particulate organic N (PON) to particulate organic P (POP) often indicated N limitation (i.e., values often less than 10). PON:chl a ratios often indicated N sufficiency, but three occasions were noted where PON:POP and PON:chl a ratios were not congruent. It is difficult to reconcile the inorganic and organic N and P ratios with the relatively low DIP and DIN concentrations. The phytoplankton assemblage appeared not to be strongly nutrient-limited but, given a nutrient increase, responded differentially to N and P, both seasonally and along the longitudinal salinity gradient. Grazing pressure in concert with nutrient limitation was advanced as an hypothesis to explain N+P co-limitation.  相似文献   

The karstlands of Trinidad and Tobago, their land use and conservation   总被引：2，自引：0，他引：2  

Michael J Day  M Sean Chenoweth 《The Geographical journal》2004,170(3):256-266

The karst landscapes of Trinidad and Tobago, although restricted in area, present a previously undocumented array of tropical karst landforms, including karren, caves, springs, valley systems, and a range of dolines or sinkholes, including an area of polygonal cockpit karst. The karstlands are located primarily in the Northern and Central Ranges of Trinidad and in the southwestern portion of Tobago, and they have developed on carbonate rocks ranging in age from Jurassic to Quaternary. Associated with them is a range of vegetation types and faunal communities, many of which have been much altered by human land management, including forestry and agriculture. Quarrying has destroyed much of the karst in the western part of Trinidad, and urban development and tourism have taken their toll too, especially in the Lowlands of western Tobago. Little of the karst is incorporated within existing protected areas.  相似文献