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841.
New observations of fracture nucleation are presented from three triaxial compression experiments on intact samples of Westerly
granite, using Acoustic Emission (AE) monitoring. By conducting the tests under different loading conditions, the fracture
process is demonstrated for quasi-static fracture (under AE Feedback load), a slowly developing unstable fracture (loaded
at a `slow' constant strain rate of 2.5 × 10−6 /s) and an unstable fracture that develops near instantaneously (loaded at a `fast' constant strain rate of 5 × 10−5 /s). By recording a continuous ultrasonic waveform during the critical period of fracture, the entire AE catalogue can be
captured and the exact time of fracture defined. Under constant strain loading, three stages are observed: (1) An initial
nucleation or stable growth phase at a rate of ~ 1.3 mm/s, (2) a sudden increase to a constant or slowly accelerating propagation
speed of ~ 18 mm/s, and (3) unstable, accelerating propagation. In the ~ 100 ms before rupture, the high level of AE activity
(as seen on the continuous record) prevented the location of discrete AE events. A lower bound estimate of the average propagation
velocity (using the time-to-rupture and the existing fracture length) suggests values of a few m/s. However from a low gain
acoustic record, we infer that in the final few ms, the fracture propagation speed increased to 175 m/s. These results demonstrate
similarities between fracture nucleation in intact rock and the nucleation of dynamic instabilities in stick slip experiments.
It is suggested that the ability to constrain the size of an evolving fracture provides a crucial tool in further understanding
the controls on fracture nucleation. 相似文献
842.
Arthur Rodgers Hrvoje Tkalcic David McCallen Shawn Larsen Catherine Snelson 《Pure and Applied Geophysics》2006,163(1):55-80
We report site response in Las Vegas Valley (LVV) from historical recordings of Nevada Test Site (NTS) nuclear explosions
and earthquake recordings from permanent and temporary seismic stations. Our data set significantly improves the spatial coverage
of LVV over previous studies, especially in the northern, deeper parts of the basin. Site response at stations in LVV was
measured for frequencies in the range 0.2–5.0 Hz using Standard Spectral Ratios (SSR) and Horizontal-Vertical Spectral Ratios
(HVR). For the SSR measurements we used a reference site (approximately NEHRP B ``rock' classification) located on Frenchman
Mountain outside the basin. Site response at sedimentary sites is variable in LVV with average amplifications approaching
a factor of 10 at some frequencies. We observed peaks in the site response curves at frequencies clustered near 0.6, 1.2 and
2.0 Hz, with some sites showing additional lower amplitude peaks at higher frequencies. The spatial pattern of site response
is strongly correlated with the reported depth to basement for frequencies between 0.2 and 3.0 Hz, although the frequency
of peak amplification does not show a similar correlation. For a few sites where we have geotechnical shear velocities, the
amplification shows a correlation with the average upper 30-meter shear velocities, V30. We performed two-dimensional finite difference simulations and reproduced the observed peak site amplifications at 0.6 and
1.2 Hz with a low velocity near-surface layer with shear velocities 600–750 m/s and a thickness of 100–200 m. These modeling
results indicate that the amplitude and frequencies of site response peaks in LVV are strongly controlled by shallow velocity
structure. 相似文献
843.
David Popp 《Climatic change》2006,77(3-4):311-341
Because of the long-term nature of the climate problem, technological advances are often seen as an important component of any solution. However, when considering the potential for technology to help solve the climate problem, two market failures exist which lead to underinvestment in climate-friendly R&D: environmental externalities and the public goods nature of new knowledge. As a result, government subsidies to climate-friendly R&D projects are often proposed as part of a policy solution. Using the ENTICE model, I analyze the effectiveness of such subsidies, both with and without other climate policies, such as a carbon tax. While R&D subsidies do lead to significant increases in climate-friendly R&D, this R&D has little impact on the climate itself. Subsidies address the problem of knowledge as a public good, but they do not address the environmental externality, and thus offer no additional incentive to adopt new technologies. Moreover, high opportunity costs to R&D limit the potential role that subsidies can play. While R&D subsidies can improve efficiency, policies that directly affect the environmental externality have a much larger impact on both atmospheric temperature and economic welfare. 相似文献
844.
A Demonstration of the Uncertainty in Projections of UK Climate Change Resulting from Regional Model Formulation 总被引:1,自引:0,他引:1
David P. Rowell 《Climatic change》2006,79(3-4):243-257
Regional climate models (RCMs) are now commonly used to downscale climate change projections provided by global coupled models
to resolutions that can be utilised at national and finer scales. Although this extra tier of complexity adds significant
value, it inevitably contributes a further source of uncertainty, due to the regional modelling uncertainties involved. Here,
an initial attempt is made to estimate the uncertainty that arises from typical variations in RCM formulation, focussing on
changes in UK surface air temperature (SAT) and precipitation projected for the late twenty-first century. Data are provided
by a relatively large suite of RCM and global model integrations with widely varying formulations. It is found that uncertainty
in the formulation of the RCM has a relatively small, but non-negligible, impact on the range of possible outcomes of future
UK seasonal mean climate. This uncertainty is largest in the summer season. It is also similar in magnitude to that of large-scale
internal variations of the coupled climate system, and for SAT, it is less than the uncertainty due to the emissions scenario,
whereas for precipitation it is probably larger. The largest source of uncertainty, for both variables and in all seasons,
is the formulation of the global coupled model. The scale-dependency of uncertainty due to RCM formulation is also explored
by considering its impact on projections of the difference in climate change between the north and south of the UK. Finally,
the implications for the reliability of UK seasonal mean climate change projections are discussed. 相似文献
845.
Evaporation over a Heterogeneous Land Surface: EVA_GRIPS and the LITFASS-2003 Experiment—An Overview
The Evaporation at Grid/Pixel Scale (EVA_GRIPS) project was realised in order to determine the area-averaged evaporation over a heterogeneous land surface at the scale of a grid box of a regional numerical weather prediction or climate model, and at the scale of a pixel of a satellite image. EVA_GRIPS combined surface-based and airborne measurements, satellite data analysis, and numerical modelling activities. A mesoscale field experiment, LITFASS-2003, was carried out in the heterogeneous landscape around the Meteorological Observatory Lindenberg (MOL) of the German Meteorological Service in May and June, 2003. The experiment was embedded in the comprehensive, operational measurement program of the MOL. Experimental determination of surface fluxes on a variety of spatial scales was achieved by employing micrometeorological flux stations, scintillometers, a combination of ground-based remote sensing instruments, and the Helipod, a turbulence probe carried by a helicopter. Surface energy fluxes were also derived from satellite data. Modelling work included the use of different Soil–Vegetation–Atmosphere Transfer schemes, a large-eddy simulation model and three mesoscale atmospheric models. The paper gives an overview on the background of EVA_GRIPS, and on the measurements and meteorological conditions during LITFASS-2003. A few general results are discussed. 相似文献
846.
Processing and quality control of flux data during LITFASS-2003 总被引:1,自引:0,他引:1
Matthias Mauder Claudia Liebethal Mathias Göckede Jens-Peter Leps Frank Beyrich Thomas Foken 《Boundary-Layer Meteorology》2006,121(1):67-88
Different aspects of the quality assurance and quality control (QA/QC) of micrometeorological measurements were combined to create a comprehensive algorithm which was then applied to experimental data from LITFASS-2003 (Lindenberg Inhomogeneous Terrain—Fluxes between Atmosphere and Surface: a long term Study). Eddy-covariance measurements of the latent heat flux were the main focus of the QA/QC efforts. The results of a turbulence sensor intercomparison experiment showed deviations between the different eddy-covariance systems on the order of 15%, or less than 30 W m−2, for the latent heat flux and 5%, or less than 10 W m−2, for the sensible heat flux. In order to avoid uncertainties due to the post-processing of turbulence data, a comprehensive software package was used for the analysis of experimental data from LITFASS-2003, including all necessary procedures for corrections and quality control. An overview of the quality test results shows that for most of the days more than 80% of the available latent heat flux data are of high quality so long as there are no instrumental problems. The representativeness of a flux value for the target land-use type was analysed using a stochastic footprint model. Different methods to calculate soil heat fluxes at the surface are discussed and a sensitivity analysis is conducted to select the most robust method for LITFASS-2003. The lack of energy balance closure, which was found for LITFASS-2003, can probably be attributed to the presence of low-frequency flux contributions that cannot be resolved with an averaging time of 30 min. Though the QA/QC system has been developed for the requirements of LITFASS-2003, it can also be applied to other experiments dealing with similar objectives. 相似文献
847.
Xiang-Yu Kou David K. Ferguson Jing-Xian Xu Yu-Fei Wang Cheng-Sen Li 《Climatic change》2006,77(3-4):431-448
The Eryuan palynoflora from the Late Pliocene of western Yunnan, China is described in this paper, and is compared with two contemporary palynofloras from Yangyi and Longling. The palynological data of the three locations are analyzed to reconstruct the climatic parameters of these areas at that time by using the Coexistence Approach. The Late Pliocene climatic parameters of Eryuan are estimated, i.e., the mean annual temperature ranged from 13.3 to 18.6∘C, the mean temperature of the warmest month from 24.6 to 27.5∘C, the mean temperature of the coldest month from 1.9 to 12.1∘C, the difference in temperatures of coldest and warmest month from 14.2 to 16.6∘C, the mean annual precipitation from 619.9 to 1484.3 mm, the mean maximum monthly precipitation from 143.8 to 245.6 mm, and the mean minimum monthly precipitation from 12.7 to 16.4 mm. Both paleovegetation and paleoclimate of the three localities are compared with the modern data at each location. The present study suggests a MAT decrease accompanied by a doubling of the MAP in the Longling area between the Late Pliocene and the present. This seems to be related to the uplift of Gaoligong Mountain in Longling which is now part of the eastern portion (Western Yunnan) of the Tibetan Plateau. 相似文献
848.
Michael A. Wulder Trisalyn A. Nelson Chris Derksen David Seemann 《Climatic change》2007,82(1-2):113-130
Twenty-four winter seasons (1978–2002) of mean February snow water equivalent (SWE) values were analyzed in an exploration of the spatial pattern of temporal variability in snow cover across the non-mountainous interior of Canada. The SWE data were derived from space-borne passive microwave brightness temperatures processed with a land cover-sensitive suite of algorithms. Spatial patterns in the frequency and amount of variability were investigated on an annual basis through comparisons with average trends over all 24 years. Changes in temporal variability through time were also investigated by comparing three eight year time periods to general trends. Analyses were synthesized at the ecozone scale in order to link results both to potential land cover influences on algorithm performance and climatological variability in SWE. Prairie and northern ecozones were typically found to be the most variable in terms of SWE magnitude. Analyses indicate that non-treed land cover classes are generally more variable than treed classes. The results also indicate that extreme weather events appear to be occurring with increasing consistency in the Prairie and Arctic regions. Discerning climatologically significant variability in the time series, compared to algorithm-related issues can be a challenge, but in an era of eroding surface observing networks the passive microwave time series represents an important resource for monitoring and detecting trends and variability in terrestrial snow cover. 相似文献
849.
After syntheses of partially molten diopside-forsterite polycrystalline aggregates doped with various solutes, we analyzed the equilibrium segregation of Ni, Mn, Sr, Al, Yb, Y, Nd, La, and Ti at interfaces between diopside/diopside, diopside/forsterite and, forsterite/forsterite grains based on STEM/EDX (scanning transmission electron microscopy/energy dispersive X-ray spectrometry) to examine the effects of ionic size, valence state, co-segregation, and interface type on interface chemistry. We derive relationships between two quantities describing interface segregation and X-ray intensities acquired both from areas that include an interface and from areas that do not. These segregation quantities are (i) interface excess density and (ii) interface enrichment factor, which rely on Gibbsian thermodynamics and the Langmuir-McLean segregation model, respectively. Interface excess densities, which vary from −0.5 to 10 atoms/nm2, indicate that the level of interface excess density depends on solutes and sample assemblage. Interface enrichment factors, which range from almost 1 to 130, reveal that the ionic size of the solutes affects their segregation via production of misfit lattice strain due to the difference between the size of a solute ion and that of the ideal strain-free lattice site. The ionic sizes of Yb and Y are almost identical to the size of the strain-free site; however, their segregation is significant indicating that a difference in valence state between the host elements (i.e., Ca and Mg) and the solutes also drives segregation. In contrast to other solutes, segregation characteristics of Al differ from these simple segregation rules. Segregation quantities do not change with interface type, indicating that the number of sites available for segregants and the driving force for segregation are similar among type of interfaces. We compare the element partitioning between diopside-melt and diopside-interfaces within the same sample assemblages. These two partition coefficients coincide if we approximate the number of segregation sites at interfaces as equivalent to 2 mono-atomic layers. Examination of the energetics in crystal-melt partitioning reveals that the interface segregation energy is essentially equal to the solute solution energy in a crystal. 相似文献
850.
The effects of soil type and chemical treatment on nickel speciation in refinery enriched soils: A multi-technique investigation 总被引:1,自引:0,他引:1
Aerial deposition of Ni from a refinery in Port Colborne, Ontario, Canada has resulted in the enrichment of 29 km2 of land with Ni concentrations exceeding the Canadian Ministry of the Environment’s remedial action level of 200 mg kg−1. Several studies on these soils have shown that making the soils calcareous was effective at reducing chemically extractable Ni, as well as alleviating Ni phytotoxicity symptoms in vegetable crops grown in the vicinity of the refinery. Conversely, dolomitic limestone additions resulted in increased uptake of Ni in the Ni hyperaccumulator Alyssum murale ‘Kotodesh’, a plant whose use was proposed as a remediation strategy for this area. In this paper we use multiple techniques to directly assess the role soil type and lime treatments play in altering the speciation of Ni in the Welland loam and Quarry muck soils around the refinery and relate these findings to Ni mobility and bioavailability.Stirred-flow dissolution experiments using pH 4 HNO3 showed that Ni release from the limed Quarry muck and Welland loam soils was reduced (∼0.10%) relative to the unlimed soils (∼2.0%). Electron microprobe analysis (EMPA) identified approximately spherical NiO and Ni metal particles, which are associated with no other metals, and range from 5 to 50 μm in diameter. Synchrotron micro-X-ray absorption fine structure and X-ray fluorescence spectroscopies showed that Ni and Al layered double hydroxide (Ni-Al LDH) phases were present in both the limed and unlimed mineral soils, with a tendency towards more stable (e.g., aged-LDH and phyllosilicate) Ni species in the limed soil, possibly aided by the solubilization of Si with increasing pH. In the muck soils, Ni-organic complexes (namely fulvic acid) dominated the speciation in both limed and unlimed soils.The results reported herein show that both soil type and treatment have a pronounced effect on the speciation of Ni in the soils surrounding the Port Colborne refinery. We provide the first evidence that Ni-Al LDH phases can form in anthropogenically enriched mineral field soils at circumneutral pH, and can lead to a reduction in Ni mobility. In the organic soils Ni is strongly complexed by soil organic matter; a property enhanced with liming. Interestingly, increased accumulation of Ni by A. murale grown in the limed muck and loam soils indicates that the plant may be capable of removing Ni from those fractions typically considered unavailable to most plants. 相似文献