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921.
Experiments [T. Irifune (1994) Nature 370, 131–133; E. Ito et al. (1998) Geophys. Res. Lett. 25, 821–824; A. Kubo, M. Akaogi (2000) Phys. Earth Planet. Int. 121, 85–102] indicate that (Mg,Fe)SiO3 perovskite, commonly believed to be the most abundant mineral in the Earth, is the preferred host phase of Al2O3 in the Earth’s lower mantle. Aiming to better understand the effects of Al2O3 on the thermoelastic properties of the lower mantle, we use atomistic models to examine the chemistry and elasticity of solid solutions within the MgSiO3(perovskite)–Al2O3(corundum)–MgO(periclase) mineral assemblage under conditions pertinent to the lower mantle: low Al cation concentrations, P=25–100 GPa, and T=1000–2000 K. We assess the relative stabilities of two likely substitution mechanisms of Al into MgSiO3 perovskite in terms of reactions involving MgSiO3, MgO, and Al2O3, in a manner similar to the 0 Kelvin calculations of Brodholt [J.P. Brodholt (2000) Nature 407, 620–622] and Yamamoto et al. [T. Yamamoto et al. (2003) Earth Planet. Sci. Lett. 206, 617–625]. We determine the equilibrium composition of the assemblage by examining the chemical potentials of the Al2O3 and MgO components in solid solution with MgSiO3, as functions of concentration. We find that charge coupled substitution dominates at lower mantle pressures and temperatures. Oxygen vacancy-forming substitution accounts for 3–4% of Al substitution at shallow lower mantle conditions, and less than 1% in the deep mantle. For these two pressure regimes, the corresponding adiabatic bulk moduli of aluminous perovskite are 2% and 1% lower than that of pure MgSiO3 perovskite. 相似文献
922.
In single‐streamer acquisition, the use of acoustic transducers to constrain the receiver positions is not possible, thus implying the use of compass birds to gather information on the streamer location. The compasses are, however, sensitive to the declination of the local magnetic field of the earth, and local fluctuations not accounted for can degrade the accuracy of reconstructed positions. In order to correct these small‐scale fluctuations, we propose a simple deterministic method to calculate a spatial correction to apply to the compasses that enhances the positioning accuracy. The local compass declination is calculated after a first reconstruction on the whole survey area. This method was applied with success to navigation data from a 3D survey offshore Japan, and the positioning accuracy was improved to the level of the DGPS accuracy. 相似文献
923.
Soazig Le Bégat Hervé Chauris † Vincent Devaux Sylvain Nguyen Mark Noble 《Geophysical Prospecting》2004,52(5):427-438
The estimation of velocity models is still crucial in seismic reflection imaging as it controls the quality of the depth‐migrated image, which is the basis of geological interpretation. Among the numerous existing methods for velocity determination, tomographic methods are very attractive for their efficiency and ability to retrieve heterogeneities of the medium. We present three tomographic methods in order to estimate heterogeneous velocity models from 2D prestack PP reflection data: a traveltime tomography in the time‐migrated domain, a traveltime and slope tomography in the non‐migrated time domain, and a slope tomography in the depth‐migrated domain. The first method (traveltime tomography in the time domain) is based on continuous picked events, whereas the two slope tomographic methods, one in the time domain and the other in the depth domain, are based on locally coherent events, with no assumptions about reflector geometry or the unknown velocity field. The purpose of this paper is not to describe in detail the theoretical basis and implementation of the methods, but to apply and compare their output using the same marine real data set. Based on the estimated velocity models, the migrated images and the common‐image gathers from the three processing routes, the relative strengths and weaknesses of the methods are discussed. Finally, similarities are indicated and potential alternative approaches are proposed. 相似文献
924.
Defining and Managing Sustainable Yield 总被引:8,自引:0,他引:8
Mark Maimone 《Ground water》2004,42(6):809-814
925.
Measurements of seismic anisotropy in fractured rock are used at present to deduce information about the fracture orientation and the spatial distribution of fracture intensity. Analysis of the data is based upon equivalent-medium theories that describe the elastic response of a rock containing cracks or fractures in the long-wavelength limit. Conventional models assume frequency independence and cannot distinguish between microcracks and macrofractures. The latter, however, control the fluid flow in many subsurface reservoirs. Therefore, the fracture size is essential information for reservoir engineers. In this study we apply a new equivalent-medium theory that models frequency-dependent anisotropy and is sensitive to the length scale of fractures. The model considers velocity dispersion and attenuation due to a squirt-flow mechanism at two different scales: the grain scale (microcracks and equant matrix porosity) and formation-scale fractures. The theory is first tested and calibrated against published laboratory data. Then we present the analysis and modelling of frequency-dependent shear-wave splitting in multicomponent VSP data from a tight gas reservoir. We invert for fracture density and fracture size from the frequency dependence of the time delay between split shear waves. The derived fracture length matches independent observations from borehole data. 相似文献
926.
Mark A Clilverd Ellen Clarke Henry Rishbeth Toby D G Clark Thomas Ulich 《Astronomy& Geophysics》2003,44(5):5.20-5.22
Mark A Clilverd, Ellen Clarke, Henry Rishbeth, Toby D G Clark and Thomas Ulich look forward to a little less solar activity in 2100, using direct and proxy records of past solar and geomagnetic activity. 相似文献
927.
The inability to detect the presence of viable Helicobacter pylori bacteria in environmental waters has hindered the public health community in assessing the role water may play in the transmission of this pathogen. This work describes a cultural enrichment method coupled with an H. pylori‐specific PCR to identify these bacteria in water. While far from perfected at the present time, this represents an exciting new approach to studying the significance of water as a transmission mechanism for H. pylori. Evidence is presented that indicates culturable H. pylori bacteria were found using this enrichment/PCR method in a local groundwater source. 相似文献
928.
Hannington Mark D. Kjarsgaard Ingrid M. Galley Alan G. Taylor Bruce 《Mineralium Deposita》2003,38(4):423-442
Mineralium Deposita - The massive sulfide deposits of the Kristineberg area, Sweden, occur within a 2- to 3-km-thick succession of felsic volcaniclastic rocks belonging to the Skellefte Group. The... 相似文献
929.
Brian Fry Donald M. Baltz Mark C. Benfield John W. Fleeger Arian Gace Heather L. Haas Zoraida J. Quiñones-Rivera 《Estuaries and Coasts》2003,26(1):82-97
Brown shrimp (Farfantepenaeus aztecus) are an important commercial aquatic species experiencing loss of inshore marsh nursery habitat in coastal Louisiana. To
study inshore brown shrimp movements and identify aspects of essential habitat important for sustaining brown shrimp populations,
we collected juvenile brown shrimp in April and May 2000, the time of annual maximum brown shrimp abundance, in a small 1-km2 marsh area on the central Louisiana coast. Drop sampling showed average shrimp densities of 1.6–2.4 m−2 in shallow marsh ponds and seining indicated lower densities of 0.5–0.9 m−2 in nearby shallow channel and open bay sites. Smaller shrimp (< 50 mm) fed disproportionately on benthic diatoms and small
harpacticoid copepods, while large shrimp fed more frequently on larger-bodied amphipods and tanaids. We developed novel chemical
approaches to estimate patterns of shrimp residency and movement using carbon and nitrogen stable isotopic determinations.
Resident shrimp had isotopic values similar to average foods and showed consistent isotopic spacings between fast and slow
turnover tissues. Residency was highest (47–55%) in ponds and shallow channel habitats and much less in open bays and deep
channels (4–27%). There was sparse evidence for dietary specialization among individull shrimp. The results support the view
that small 10–20 mm postlarval and juvenile brown shrimp arriving in estuaries from offshore waters continue movement through
sub-optimal habitats (deep channels and open bays), but exhibit much less movement once an optimal habitat (marsh ponds or
shallow channel margins) is reached. This study also indicated that combining estimates of shrimp densities, residency, growth
rate, and mortality allows evaluation of the importance of different habitat types for shrimp production. Shallow ponds that
in many ways resemble fertile aquaculture ponds appear to be hot spots for brown shrimp production, and coastal preservation
and restoration efforts should focus on these areas as important for sustaining shrimp fisheries. 相似文献
930.
Melt droplets formed by the impact of a large meteorite impact event(s) have been found in upper Eocene marine sediments from the Atlantic, Pacific, and Indian oceans. The number of discreet impacts that occurred to form these melt droplets can be gleaned by compositionally analysing the droplets and establishing the number of distinct groups. Previous studies have inferred two, three, or more source impact events, although we believe the statistical techniques used to distinguish the groupings are open to criticism. Multivariate and univariate analysis (including discriminant analysis) of the major-element composition of an increased data set of impact melt ejecta droplets have been performed. The results demonstrate that the uppermost ejecta layer is geochemically distinct from other late Eocene melt ejecta. Our statistical analysis suggests two underlying, purportedly stratigraphically separate ejecta layers, possess minimal differences that cannot be distinguished clearly from one another by discriminant analysis, which adds to the plausibility that they have a common source. Finally, our results reveal apparent disparities between the new major-element data from this study and data compiled from existing sources. 相似文献