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排序方式: 共有371条查询结果,搜索用时 15 毫秒
61.
Sean M. Crotty 《Urban geography》2013,34(7):993-1017
Day-labor hiring sites are common features of the contemporary North American landscape. These are public and semi-public spaces where mostly male residents congregate daily in hopes of being hired for short-term work. Research on contemporary day-labor markets in the United States to date tends to be policy-oriented, intended to reduce the injustices that are a common part of life as a day laborer. Unfortunately, very little is understood about the spatial organization of day-labor markets. Drawing on more than five years of mixed-methods research in the San Diego Metropolitan Area (SDMA), this paper takes two important steps toward a spatial understanding of day-labor hiring sites. First, it demonstrates that informal hiring sites are established in locations that maximize laborers’ chances of finding employment. Second, it establishes a geo-spatial typology of hiring sites for the SDMA that can be used to better tailor day-labor support efforts and policy to site-level context. 相似文献
62.
An experimental study of a minette from the Milk River area, southern Alberta, Canada 总被引:1,自引:1,他引:0
Buhlmann et al. (Can J Earth Sci 37: 1629–1650, 2000) studied the minettes and xenoliths from the Milk River area of southern Alberta, Canada. Based on previous work, they hypothesized that the minettes were derived from a source containing phlogopite?+?clinopyroxene?±?olivine, at pressures ≥1.7?GPa. To test this hypothesis, liquidus experiments were performed on a primitive minette between 1.33 and 2.21?GPa and between 1,300 and 1,400?°C to constrain the mineralogy of its source region. We found a multiple saturation point along the liquidus at 1.77 GPa and 1,350?°C, where the liquid coexists with orthopyroxene and olivine. Neither phlogopite nor clinopyroxene were found to be liquidus phases, which is inconsistent with Buhlmann et al.’s hypothesis. We suggest that our minette is not primary, but had re-equilibrated with harzburgitic mantle subsequent to formation. In such a scenario, partial melting of a veined source containing mica and clinopyroxene occurred at or near the base of the Wyoming craton (~200?km). Minimal heating or the introduction of hydrous fluids into the source would be required to induce partial melting. Rapid ascent rates, coupled with slow cooling rates, of the “primary minette magma” would preserve the high temperature observed in our experiments. At ~58?km, our “primary minette magma” likely stalled and re-equilibrated with the harzburgite surroundings. 相似文献
63.
Sean A. Shaw M. Jim Hendry Joseph Essilfie-Dughan Tom Kotzer Dirk Wallschläger 《Applied Geochemistry》2011,26(12):2044-2056
The distribution of As, Mo, Ni and Se in the aqueous and solid phases of U mine tailings in the Deilmann Tailings Management Facility (DTMF), located at the Key Lake mine in northern Saskatchewan, Canada, was defined using as-discharged tailings samples collected monthly starting in 1996 and core samples collected from the DTMF from 3 locations on 3 occasions between 2004 and 2009. These data indicated that the DTMF can be divided into two geochemical zones; tailings generated from the Deilmann ore body between 1996 and 2000 and tailings from the McArthur River ore body, mined after July 2000. The Deilmann tailings solids are generally characterized by greater elemental concentrations than those in the McArthur River tailings, particularly for As, Co and Ni. These elevated concentrations are attributed to the abundance of Ni–Co–As–S minerals in this ore. The mean aqueous concentrations of As, Mo, Ni, and Se are 3.7, 23.2, 0.14, and 0.02 mg L−1, respectively, in the Deilmann porewaters and 0.20, 4.16, 0.06, and 0.03 mg L−1, respectively, in the McArthur River porewaters. Similarly, the mean As, Mo, Ni, and Se solid phase concentrations are 5.89 × 103, 69.9, 3.20 × 103, and 17.4 μg g−1, respectively, for the Deilmann tailings and 440, 13.6, 551, and 3.03 μg g−1 in the McArthur River tailings. Statistically, pH remained unchanged from the time of discharge while Eh values have remained oxic with no significant change between the two tailings types over 15 a of residence time. Results from sequential extractions, thermodynamic modeling, and results of complimentary research indicated that ferrihydrite solubility is the dominant control on aqueous As concentrations and also plays a role in controlling dissolved Mo, Ni and Se. 相似文献
64.
Sébastien Castelltort Stefan Nagel Frédéric Mouthereau Andrew Tien-Shun Lin Andreas Wetzel Boris Kaus Sean Willett Shao-Ping Chiang Wei-Yi Chiu 《Journal of Asian Earth Sciences》2011,40(1):52-71
This work presents sedimentological observations and interpretations on three detailed sections of the Pliocene Yutengping/Ailiaochiao formations, deposited in the early stages of collision in Taiwan. Seven facies associations record paleoenvironments of deposition ranging from nearshore to lower offshore with a strong influence of tidal reworking, even in shelfal sub-tidal environments, and a pro-delta setting characterized by mass-flows. The association of shallow facies of the upper offshore to lower shoreface with pro-delta turbidite facies sourced in the orogen to the east suggests a peculiar setting in which turbidite deposition occurred below wave base but on the shelf, in water depths of probably less than 100 m. This adds to the examples of “shallow turbidites” increasingly commonly found in foreland basins and challenges the classical view of a “deep” early underfilled foreland basin. Time series analysis on tidal rhythmites allow us to identify a yearly signal in the form of periodic changes of sand-supply, energy and bioturbation that suggests a marked seasonality possibly affecting precipitation and sediment delivery as well as temperature. The Taiwan foreland basin may also present a potentially high-resolution record in shallow sediments of the early installation of monsoonal circulation patterns in east Asia. We confirm partly the paleogeography during the early stages of collision in Taiwan: the Chinese margin displayed a pronounced non-cylindrical geometry with a large basement promontory to the west in place of the modern Taiwan mountain range. Collision in Taiwan may have happened at once along the whole length of the modern mountain range, instead of progressively from north to south as classically considered. 相似文献
65.
66.
River restoration and bank stabilization programs often use vegetation for improving stream corridor habitat, aesthetic and function. Yet no study has examined the use of managed vegetation plantings to transform a straight, degraded stream corridor into an ecologically functional meandering channel. Experimental data collected using a distorted Froude‐scaled flume analysis show that channel expansion and widening, thalweg meandering and riffle and pool development are possible using discrete plantings of rigid, emergent vegetation, and the magnitudes of these adjustments depend on the shape of the vegetation zone and the density of the vegetation. These experimental results were verified and validated using a recently developed numerical model, and model output was then used to discuss mechanistically how rivers respond to the introduction of in‐stream woody vegetation. Finally, a hybrid method of meander design is proposed herein where managed vegetation plantings are used to trigger or force the desired morphologic response, transforming a straight, degraded reach into a more functional meandering corridor. It is envisioned that such numerical models could become the primary tool for designing future stream restoration programs involving vegetation and assessing the long‐term stability of such activities. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
67.
Design and commissioning of a laminar soil container for use on small shaking tables 总被引:2,自引:0,他引:2
This paper describes the design, fabrication and commissioning of a single axis laminar shear box for use in seismic soil–structure interaction studies. A laminar shear box is a flexible container that can be placed on a shaking table to simulate vertical shear-wave propagation during earthquakes through a soil layer of finite thickness. The laminar shear box described in this paper was designed to overcome the base shear limitations of a small shaking table at The University of Western Ontario. The design details of the box are provided in addition to results of dynamic tests performed to commission the box. A synthetic clay comprising sodium bentonite mixed with diluted glycerin was used as the model soil and 1-G similitude theory was employed to maintain model to prototype similarity. The model soil was compacted into the container in lifts to achieve soil stiffness that increased with depth. A series of shaking table tests and numerical analyses that were performed to study the performance of the laminar box and non-linear seismic behavior of the model clay are described. The results of this study show that the laminar box does not impose significant boundary effects and is able to maintain 1-D soil column behavior. In addition, the dynamic behavior of the model clay during scaled model tests was found to be consistent with the behavior measured during cyclic laboratory tests. 相似文献
68.
Barry L. Roberts Sean A. McKenna 《Stochastic Environmental Research and Risk Assessment (SERRA)》2009,23(2):227-236
The identification and characterization of target areas at former bombing ranges is the first step in investigating these
sites for residual unexploded ordnance. Traditionally, magnetometer surveys along transects are used in identifying areas
with high densities of magnetic anomalies, which are likely former target areas. Combining magnetometer survey data with other
data sources may reduce the level of survey data required for site characterization, increasing characterization efficiency.
Here, several techniques for incorporating secondary information into kriging estimates of magnetic anomaly density are investigated
for a former bombing range located near Pueblo, Colorado. In particular, kriging with external drift, collocated ordinary
cokriging, and simple kriging with local means (SKLM) are used to incorporate information from a secondary variable. The secondary
variable consists of a grid of crater density values derived from a topographic light detection and ranging (LIDAR) analysis.
The craters, which are clearly identifiable in the LIDAR data, were generated through munitions use at the site and are therefore
related to the target locations. The results from this study indicate that the inclusion of the secondary information in the
kriging estimates does benefit target area characterization and provides a means of elucidating target area details from only
limited magnetometer transect data. For the Pueblo site, the use of SKLM with the crater density as a secondary variable and
only limited magnetometer transect data, provided results comparable to those obtained from using much larger magnetometer
transect data sets. 相似文献
69.
70.
Sean P Healey Jock A Blackard Todd A Morgan Dan Loeffler Greg Jones Jon Songster Jason P Brandt Gretchen G Moisen Larry T DeBlander 《Carbon balance and management》2009,4(1):9