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961.
962.
The Karst Feature Database (KFD) of Minnesota is a relational GIS-based Database Management System (DBMS). Previous karst feature datasets used inconsistent attributes to describe karst features in different areas of Minnesota. Existing metadata were modified and standardized to represent a comprehensive metadata for all the karst features in Minnesota. Microsoft Access 2000 and ArcView 3.2 were used to develop this working database. Existing county and sub-county karst feature datasets have been assembled into the KFD, which is capable of visualizing and analyzing the entire data set. By November 17 2002, 11,682 karst features were stored in the KFD of Minnesota. Data tables are stored in a Microsoft Access 2000 DBMS and linked to corresponding ArcView applications. The current KFD of Minnesota has been moved from a Windows NT server to a Windows 2000 Citrix server accessible to researchers and planners through networked interfaces. 相似文献
963.
964.
965.
The development of subgrain misorientations with strain in dry synthetic NaCl measured using EBSD 总被引:1,自引:0,他引:1
The development of subgrain boundary misorientations with strain in dry, synthetic NaCl polycrystals, deformed at elevated temperature, has been investigated using electron backscattered diffraction (EBSD). At low natural strains, up to 0.5, average misorientations of subgrain boundaries increase with strain and a power law relationship exists between strain and average misorientations. The average misorientations are strongly influenced by grain orientation, suggesting that the misorientation–strain relationship may also be texture dependent in materials with high plastic anisotropy, like NaCl. A slight grain size dependency of the average misorientations was observed. The results indicate that with suitable calibration, average subgrain boundary misorientations may offer a method for estimating the strain accommodated by dislocation creep in NaCl and thus perhaps in other geological materials, although current theories for polycrystalline plasticity imply that misorientations may also depend on stress in some situations. 相似文献
966.
The Holocene Period for the province of West New Britain, Papua New Guinea, is characterised by periodic catastrophic volcanism. The region is mantled in dense wet tropical rainforest, and has been occupied by people since the Pleistocene. Analyses of peat from two nearby sites within a lowland rainforest environment provide us with a macro-level landscape account of the periodic destruction and recovery of the coastal forests during seven periods of volcanic activity in the latter part (2900 yr ago to present) of the Holocene. Radiocarbon dating shows the very close correlation of the peat and tephra layers at both sites, yet the pollen analysis reveals different vegetation communities. These initial results allow us to begin identifying the processes of recovery, and to recognise different ecological pressures placed on vegetation at these neighbouring sites. Evidence of hydrological changes are observed beginning with a marine incursion recorded at Garu Site 3 1360 14C yr B.P. The distinct differences in the vegetation re-establishment and community regeneration rates suggest the greater level of disturbance at Garu Site 1 could be related to the depth of the ashfall, although the proximity of a known human settlement may also be a contributing factor. Of note, palynologically, we found that the fern spore flora is particularly rich and believe it will be useful for ecological interpretation. 相似文献
967.
The conventional interpretation methods of pressuremeter testing effectively approximate pressuremeter membranes as infinitely long. As a result, the effects of the two‐dimensional geometry of pressuremeters are ignored, leading to an overestimation of soil shear strength by pressuremeter testing, as demonstrated in several previous studies. This paper presents results of a numerical study of two‐dimensional geometry effects on self‐boring pressuremeter tests in undrained clay. The results are obtained using critical state soil models with an effective stress formulation. This is in contrast to most (if not all) existing studies on pressuremeter geometry effects, which were based on perfectly plastic soil models (e.g. Yu (Cavity expansion theory and its application to the analysis of pressuremeters. DPhil Thesis, The University of Oxford, 1990), Yeung and Carter (Proc. 3rd Int. Symp. on Pressuremeters, 1990), and Houlsby and Carter (Géotechnique, 1993; 43 (4):567–576)). The present study suggests that the overestimation of soil strength due to the neglect of finite pressuremeter length is significantly affected by the soil model used in the calculations. It is found that for clays with a high overconsolidation ratio (OCR) the strength overestimation predicted using critical state soil models could be considerably smaller than that predicted using perfectly plastic soil models. The main conclusion of this numerical study is that care must be exercised before directly applying any numerically determined pressuremeter geometry correction factors in practice. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
968.
L. G. Tham H. Liu C. A. Tang P. K. K. Lee Y. Tsui 《Rock Mechanics and Rock Engineering》2005,38(1):1-19
Summary To understand the failure mechanism of quasi-brittle materials like rock under tensile stress, observations on the failure process of granite and marble plate specimens under tension are summarized and presented. Micro- and macro-failure properties of rock plates under uniaxial tension have been characterized by using an acoustic emission technique. Acoustic emission signals associated with micro-fractures are captured to locate the sources. An algorithm based on arrival time difference is developed for this purpose. The results reveal clearly the failure processes of rock which include initiation, nucleation and propagation of micro-fractures when the axial stress is close to the peak strength of rock. It is believed that the difference in heterogeneity between granite and marble specimens leads to different fracture shapes and different behaviors of associated acoustic emissions. Numerical simulation of acoustic emissions for two-dimensional tensile test is also carried out. The simulated characteristics are in good agreement with the experimental results. 相似文献
969.
Geological and geochemical study has been carried out to investigate arsenic contamination in groundwater in Nawalparasi, the western Terai district of Nepal. The work carried out includes analyses of core sediments, provenance study by rare earth elements analyses, 14C dating, and water analyses. Results showed that distribution of the major and trace elements are not homogeneous in different grain size sediments. Geochemical characteristics and sediment assemblages of the arsenic contaminated (Nawalparasi) and uncontaminated (Bhairahawa) area have been compared. Geochemical compositions of sediments from both the areas are similar; however, water chemistry and sedimentary facies vary significantly. Extraction test of sediment samples showed significant leaching of arsenic and iron. Chemical reduction and contribution from organic matter could be a plausible explanation for the arsenic release in groundwater from the Terai sediments. 相似文献
970.
The Paleoproterozoic Ruttan Cu–Zn volcanogenic massive-sulfide (VMS) deposit is a large, relatively low grade, bimodal-siliciclastic
type deposit in the Rusty Lake volcanic belt of northern Manitoba. The deposit contained over 82.8 million tonnes of massive
sulfide, of which 55.7 million tonnes were mined from 1973 to 2002. The deposit consists of a series of moderately to steeply
dipping, south-facing lenses that extend along strike at the surface for 1.1 km and to a depth of 1.0 km. These lenses occur
within a steeply dipping, bimodal volcanic, volcaniclastic and siliciclastic sequence. In the immediate mine area, transitional
calc-alkalic to high-silica (tholeiitic), felsic, and intermediate volcanic/volcaniclastic rocks of the Mine Sequence are
host to, and intercalated with, the massive-sulfide lenses. Transitional tholeiitic to calc-alkalic basalt and andesite are
present in the footwall sequence, approximately 500 m down-section from the ore horizon. The overlying rocks are predominantly
fine-grained volcaniclastics and siliciclastics, but include polyfragmental agglomerate that contains mafic bombs and scoriaceous
felsic fragments. Syn-depositional felsic and mafic dikes, sills, and apophyses are ubiquitous throughout the Mine Sequence,
including the ore lenses, indicating continued, near-vent magmatism, and volcanism during ore formation. Fabrics in altered
hostrocks have consistent, down-plunge stretching lineations to the SSE that suggest the deposit has been elongated by a factor
of ~1.2–1.5; otherwise, the deposit is remarkably undeformed. Syn- and post-depositional faults in the mine area have relatively
minor displacements up to tens of meters. Proximal (within 200 m) footwall rocks exhibit moderate to strong chloritization,
characterized by the upper greenschist to lower amphibolite facies assemblages that include cordierite–almandine–andalusite–sillimanite–biotite ± staurolite ± anthophyllite ± talc,
and local silicification. The proximal hanging wall rocks are characterized by sericite ± gahnite alteration, which is restricted
to within approximately 75 m of the uppermost lenses. Additional gangue minerals are anhydrite and carbonate minerals (siderite,
dolomite, ankerite, and calcite), as well as chlorite, sericite, biotite, talc, and quartz. Carbonate (excluding siderite),
potassium feldspar, silicification and epidotization are common distal alteration zones in the footwall to the Mine Sequence
several kilometers to the northeast. There are three principal groups of massive sulfide lenses; the East lenses, the West
lenses, and the Western Anomaly lenses to the far west. In general, Cu is relatively enriched at the stratigraphic base and
in the center of the deposit, whereas Zn is enriched upsection and at the outer margins. Some of the Zn-rich ore exhibits
primary mineralogical layering. Parts of the West and Western Anomaly lenses show two layers with Cu-rich bases and Zn-rich
tops. The massive sulfide is typically 10–40-m thick; one area along the margin of the main lenses is over 130-m thick and
may represent deposition adjacent to a syn-depositional fault. The main sulfide phases are pyrite, pyrrhotite, chalcopyrite,
sphalerite, and galena, with tetrahedrite as the most abundant trace phase. Gahnite is ubiquitous in the chlorite-rich assemblages
adjacent to the ore lenses. The average base, precious and trace metal contents estimated from Cu and Zn concentrates, and
from millhead grades and recoveries. Metals easily transported as chloride and bisulfide complexes in hydrothermal fluids
including: Pb, Ag, In, Cu, Cd, Au, and Zn are enriched by 1.5–2.5 orders of magnitude in comparison to the bulk continental
crust. Other elements such as Sn, Mo, and As are at near-crustal concentrations, whereas Mn, Ga, and Co are significantly
depleted in comparison to the crust. Calculated metal concentrations in the average hydrothermal fluid based on the average
metal contents are comparable to, or higher than those measured at sediment covered ridge hydrothermal systems, which precipitate
much of their metal budget in the subsurface. Average rare earth element contents for the sulfide are light rare earth element
enriched (LaN/YbN=22) and range from 0.45 to 0.02x chondritic values, with a moderate negative Eu anomaly (Eu*=0.51). Metal and trace element
contents in the Ruttan exhalite horizon, and in proximal (within 1–2 km) exhalites along strike from the 0.6 million tonne
Dar-2 Cu–Zn deposit 12 km south of Ruttan, have positive Eu anomalies, whereas negative Eu anomalies are present at distance.
The positive Eu anomalies reflect high temperature paleoseafloor hydrothermal venting and precipitation of Eu2+-enriched clays and possibly carbonates, and indicate proximity to base-metal deposits. Silver and lead are also enriched
in the exhalites near the deposits, whereas Mn is enriched at ~1–3 km along strike, but not consistently.
Editorial handling: B. Gemmel
An erratum to this article is available at . 相似文献