The Effect of Varying Damage History in Crystalline Rocks on the P- and S-Wave Velocity under Hydrostatic Confining Pressure     

O. O. Blake  D. R. Faulkner  A. Rietbrock 《Pure and Applied Geophysics》2013,170(4):493-505

Cracks play a very important role in many geotechnical issues and in a number of processes in the Earth’s crust. Elastic waves can be used as a remote sensing tool for determining crack density. The effect of varying crack density in crystalline rock on the P- and S-wave velocity and dynamic elastic properties under confining pressure has been quantified. The evolution of P- and S-wave velocity were monitored as a suite of dry Westerly granite samples were taken to 60, 70, 80 and 90 % of the unconfined uniaxial strength of the sample. The damaged samples were then subjected to hydrostatic confining pressure from 2 MPa to 200 MPa to quantify the effect of varying crack density on the P- and S-wave velocity and elastic properties under confining pressure. The opening and propagation of microcracks predominantly parallel to the loading direction during uniaxial loading caused a 0.5 and 6.3 % decrease in the P- and S-wave velocity, respectively. During hydrostatic loading, microcracks are closed at 130 MPa confining pressure. At lower pressures the amount of crack damage in the samples has a small but measureable effect. We observed a systematic 6 and 4 % reduction in P- and S-wave velocity, respectively, due to an increase in the fracture density at 2 MPa confining pressure. The overall reduction in the P- and S-wave velocity decreased to 2 and 1 %, respectively, at 50 MPa. The elastic wave velocities of samples that have a greater amount of microcrack damage are more sensitive to pressure. Effective medium modelling was used to invert elastic wave velocities and infer crack density evolution. Comparing the crack density results with experimental data on Westerly granite samples shows that the effective medium modelling used gave interpretable and reasonable results. Changes in crack density can be interpreted as closure or opening of cracks and crack growth.  相似文献   

Hypervelocity stars from the Andromeda galaxy     

Blake D. Sherwin  Abraham Loeb  Ryan M. O'Leary 《Monthly notices of the Royal Astronomical Society》2008,386(3):1179-1191

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The kinematics and spatial distribution of stellar populations in E+A galaxies   总被引：1，自引：0，他引：1  

Michael B. Pracy  Harald Kuntschner  Warrick J. Couch  Chris Blake  Kenji Bekki  Frank Briggs 《Monthly notices of the Royal Astronomical Society》2009,396(3):1349-1369

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Arctic mercury depletion and its quantitative link with halogens     

Huiting Mao  Robert W. Talbot  Barkley C. Sive  Su Youn Kim  Donald R. Blake  Andrew J. Weinheimer 《Journal of Atmospheric Chemistry》2010,65(2-3):145-170

Gas phase elemental mercury (Hg°) was measured aboard the NASA DC-8 research aircraft during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign conducted in spring 2008 primarily over the North American Arctic. We examined the vertical distributions of Hg° and ozone (O₃) together with tetrachloroethylene (C₂Cl₄), ethyne (C₂H₂), and alkanes when Hg°- and O₃-depleted air masses were sampled near the surface (<1 km). This study suggests that Hg° and O₃ depletions commonly occur over linear distances of ~20–200 km. Horizontally there was a sharp decreasing gradient of ~100 ppqv in Hg° over <10 km in going from the bay near Ellesmere Island to the frozen open ocean. There was a distinct land-ocean difference in the vertical thickness of the Hg°-depleted layer – being variable but around a few 100 meters over the ocean whereas occurring only very near the surface over land. Data support that atmospheric mercury depletion events are driven by Hg° reactions with halogen atoms. Derived from data collected aboard the DC-8, we present mathematical expressions giving the rates of Hg° and O₃ depletions as a function of the radical concentrations. These relationships can be a useful metric to evaluate models that attempt to reproduce springtime Arctic Hg° and/or O₃ depletion events, and they can also be employed to obtain order-of-magnitude estimates of radical concentrations and the ratio [Br]/[Cl].  相似文献   

10,000 years of climate control over carbon accumulation in an Iberian bog(southwestern Europe)     

Xabier Pontevedra-Pombal  Daniel Castro  Martín Souto  Isabel Fraga  William H.Blake  Maarten Blaauw  José A.López-Sáez  Sebastián Pérez-Díaz  Marcos Valcárcel  Eduardo García-Rodeja 《地学前缘(英文版)》2019,10(4):1521-1533

The northwest region of the Iberian Peninsula is home to a unique ecosystem of bogs,which are particularly sensitive to projected climate cha nge.In this context,the rate of carbon(C)accumulation in Chao de Veiga Mol.an intact raised bog,was analysed.Changes in the accumulation rate over the past10 millennia were determined in a peat core of 847 cm in depth,with a high mean rate of peat growth(11 yr cm^-1,0.09 cm yr^-1).An age-depth model was generated from 22 14C dates and fallout radionuclides.Chronological,stratigraphical and physico-chemical data confirmed the existence of a single cycle of peat formation throughout the Holocene and the formation of ombrotrophic peat 9500 years ago.The total mean C content was 50.2%,and over 10 millennia 583 kg C m^-2 accumulated at a mean rate of 35.3 g C m^-2 yr^-1,with a long-term(apparent)rate of carbon accumulation in the catotelm of59.9 g C m^-2 yr^-1.These values are much higher than reported for other Iberian peatlands and are amongst the highest documented for peatlands in the northern hemisphere.The dynamics of C accumulation and other measured parameters reveals important variations throughout the Holocene.They could be associated with the main climatic events described in the northern hemisphere and are highly consistent with models established for northern latitudes.The Chao de Veiga Mol raised bog is unique and of great potential value for carrying out high resolution palaeoenvironmental studies,especially in relation to regional and Iocal modulations in southern Europe.  相似文献   

Aircraft Measurements of Dimethyl Sulfide (DMS) Using a Whole Air Sampling Technique     

I. J. Simpson  J. J. Colman  A. L. Swanson  A. R. Bandy  D. C. Thornton  D. R. Blake  F. S. Rowland 《Journal of Atmospheric Chemistry》2001,39(2):191-213

We present a technique for the measurement of dimethyl sulfide (DMS) from airborne and ground-based platforms, using whole air sampling followed by gas chromatography with mass spectrometer and flame ionization detection. DMS measurements that were obtained during the 1999 NASA Pacific Exploratory Mission-Tropics B showed excellent agreement with independent in-flight DMS measurements, over a wide range of concentrations. The intercomparison supports two key results from this study, first that DMS can be accurately quantified based on ethane and propane per-carbon-response-factors (PCRFs), and second that DMS is stable in water-doped electropolished stainless steel canisters for at least several weeks. In addition, our sampling frequency and duration are flexible and allow detail in the vertical structure of DMS to be well captured. Sampling times as fast as 8 s were achieved and these data are suitable for DMS flux calculations using the mixed-layer gradient technique. Correlations between DMS and other marine tracers can also be readily investigated by this whole air sampling technique, because DMS is analyzed together with more than 50 simultaneously sampled hydrocarbons, halocarbons, and alkyl nitrates. The detection limit of the DMS measurements is 1 part per trillion by volume (pptv), and we conservatively estimate the accuracy to be ±20% or 3 pptv, whichever is larger. The measurement precision (1 ) is 2–4% at high mixing ratios (> 25 pptv), and 1 pptv or 15%, whichever is larger, at low mixing ratios (<10 pptv).  相似文献   

Using cosmogenic beryllium–7 as a tracer in sediment budget investigations     

W. H. Blake  D. E. Walling  & Q. He 《Geografiska Annaler: Series A, Physical Geography》2002,84(2):89-102

Recent advances in the use of the environmental radionuclides caesium–137 and unsupported lead–210 to quantify medium– and longer–term rates of erosion and sediment accumulation have proved of considerable value in catchment sediment budget investigations. However, there remains a need to explore the potential for using other shorter–lived radionuclides to provide evidence of sediment mobilisation, transport and storage over shorter timescales and particularly for individual events. This contribution reports the results of a study aimed at exploring the potential for using beryllium–7 (⁷Be, t _½= 53.3 days) to meet this requirement. The study investigated the use of ⁷Be as a sediment tracer in three key components of the sediment budget, namely, soil erosion and sediment mobilisation from slopes, the transport, storage and remobilisation of fine sediment in river channels and overbank deposition on river floodplains. The results presented clearly demonstrate the potential for using ⁷Be to obtain information on short–term and event–based sediment redistribution rates for use in catchment sediment budget investigations.  相似文献   

Effects of mining-induced seismic events on a deep underground mine     

S. M. Hsiung  W. Blake  A. H. Chowdhury  T. J. Williams 《Pure and Applied Geophysics》1992,139(3-4):741-762

The performance of an underground excavation at the Lucky Friday Mine that was subjected to repetitive episodes of mining-induced seismic activities is assessed. Field measurements indicate that an underground excavation may respond to mining-induced seismicity with step changes in displacement of the rock mass around the excavation or step changes in excavation closures. However, step changes in displacement or closure were also observed without the presence of mine seismic events. This behavior may be explained using the concept of stick-slip on joints or bedding planes within the rock mass. The stick-slip mechanism is believed to be controlled by the state of stresses at a joint or bedding plane. Through this mechanism, joint displacement is accumulated in a progressive fashion leading to a weakening of rock mass around an excavation. As a result, the excavation becomes less stable and therefore is more susceptible to seismic impacts. The concept of using peak particle velocity to assess damage threshold for underground excavations may not be conservative when considering the effect of rock mass fatigue.  相似文献   

The effect of heated effluents from power plants on seagrass (Thalassia) communities quantitatively comparing estuaries in the subtropics to the tropics     

A. Thorhaug  N. Blake  P.B. Schroeder 《Marine pollution bulletin》1978,9(7):181-187

The need for most efficient use of energy is abruptly in conflict with the need to maintain environmental quality in terms of heated effluents released from power plants. This study synthesizes biological results found from four effluent canals from power plants releasing heat on shallow estuaries: tropics, edge of tropics and subtropics. The dominant near-shore community in the Gulf of Mexico and Caribbean is that of Thalassia, rich in animal and plant abundance and biomass. Summer mean temperatures in all three estuaries ranged around 30°C. Results show sustained temperatures of +5°C in summer denude the area of the Thalassia community. Temperatures +1 and +1.5°C were minimally damaging to the studied areas and can be considered rational release temperatures. Insufficient data was available at +2°C to make a statement; +4°C areas showed intense damage to the biological communities at all locations; +3°C effect varied from severe damage in the subtropics to 40% damage in the tropics. Recommended summer heated effluent release for power plants eliminating heat onto Thalassia beds would certainly nerver be greater than +3°C. A trade-off decision must be made by decision-makers as to +3 or +2°C as the upper permissable limit. Winter release of effluent temperature in areas where winter temperatures fall below 25°C could be above +3°C apparently without damage. Tropical and subtropical ecosystems are on the brink of disaster and small increments of change by man's activities can push them beyond tolerance limits. Evidently the marine tropics differ from marine temperate zones in their capacity to assimilate man's activities.  相似文献   

Gradients in physical parameters in zoned felsic magma bodies: Implications for evolution and eruptive withdrawal     

J.A. Wolff  G. Wrner  S. Blake 《Journal of Volcanology and Geothermal Research》1990,43(1-4)

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