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21.
Barbara B. Prézelin Max M. Tilzer Oscar Schofield Clivia Haese 《Aquatic Sciences - Research Across Boundaries》1991,53(2-3):136-186
This tutorial was designed for nonbiologists requiring an introduction to the nature and general timescales of phytoplankton responses to physical forcing in aquatic environments. As such, an effort was made to highlight biological markers which might assist in identifying, measuring and/or validating physical processes controlling the variability in the distribution, abundance, composition and activity of phytoplankton communities. Given the recent advances in environmental optics and remote sensing capabilities, a special emphasis was placed on the nature and utility of phytoplankton optical properties in current bio-optical modelling efforts to predict temporal and spatial variability in phytoplankton productivity and growth. 相似文献
22.
Ephemeris errors of GPS satellites 总被引:2,自引:0,他引:2
Oscar L. Colombo 《Journal of Geodesy》1986,60(1):64-84
Ephemeris errors are supposed to be a major factor limiting the usefulness ofGPS in high precision geodesy. Considerations of orbital mechanics suggest that, regardless of their complexity, the uncertainties
in the solar radiation pressure model, the gravity field model, and the estimated initial state, may have simple effects on
the ephemeris. This possibility has been tested by fitting linear combinations of simple functions—chosen on theoretical grounds—to
simulated errors of three-day ephemerides. With a set of five functions for the across-track component, six for the radial,
and seven for the along-track, it has been possible to fit the position errors to better than 1% of theirr.m.s values, in all the caces studied. The simulations included —besides solar radiation pressure errors—gravity field model and
initial state uncertainties, as well as an unknown constant force along the axis of the solar panels. The solar radiation
force was calculated taking into account the shape, orientation, and physical properties (reflectivity and specularity) of
the main parts of the spacecraft, under various conditions of illumination (continuous sunlight, eclipses, etc.). 相似文献
23.
Compilations of historical observations, archaeomagnetic data from ancient fireplaces and palaemagetic results from short cores of sediment from lakes in southeastern Australia, particularly Lake Keilambete, provide a detailed record of the geomagnetic secular variation during the last 3000 years. The independent sets of data are in good agreement if the radiocarbon time scale for the lacustrine record is about 450 years too old. The error is attributed to systematic incorporation of ancient carbon into the lake floor sediments, mainly through erosion of sediment on the crater walls at times of low water level. A significant lag between deposition and the acquisition of stable magnetic remanence is ruled out. Inclination has been abnormally steep during the last 500 years but remained fairly close to the axial dipole field value prior to that. During the last 1000 years the predominant sense of looping of the magnetic vector corresponds to westward drift of the nondipole field. Secular variations on a time scale of ~ 100 years can be resolved by the lacustrine record. 相似文献
24.
The two-dimensional distribution of 26Al across a slice of the Estacado meteorite was measured using non-destructive γ-γ coincidence counting. The maximum activity was found to be 60 ± 2 dpm kg?1 and the variation across the slice rather small. The intensity and distribution are in moderate agreement with existing theoretical models if the pre-atmospheric shape of the meteorite had been a sphere of radius 0.35 ± 0.05 m, but it was more likely to have been slightly non-spherical. 相似文献
25.
Charles E. Barton 《Surveys in Geophysics》1983,5(4):335-368
This review is intended to provide an introduction for the nonspecialist to concepts and techniques which are useful for analysing palaeomagnetic time series. Emphasis is placed on analysis in the frequency domain, particularly the periodogram and maximum entropy methods. The review consists of two parts dealing with techniques and applications respectively. 相似文献
26.
Oscar Escolero Luis E. Marin Eloisa Domínguez-Mariani Sandra Torres-Onofre 《Environmental Geology》2007,51(5):719-723
A hydraulic analysis of the interface between freshwater–saltwater behavior was done in the Merida Yucatan zone, two machines
that constantly register the groundwater levels were installed, and three electric conductivity logs were taken from wells.
When comparing the measured results with the ones obtained using theoretical equations developed to calculate the freshwater–saltwater
interface position, it was proved that in some cases these equations can be applied, and in others not. Two effects that rule
the behavior of karst aquifers in extraordinary conditions were found. 相似文献
27.
Velocity images from stacking depth-slowness seismic wavefields 总被引:1,自引:0,他引:1
Geraint D. Jones Penny J. Barton Satish C. Singh 《Geophysical Journal International》2007,168(2):583-592
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30.
Garrett?B.?Hazelton Gary?AxenEmail author Oscar?Lovera 《Contributions to Mineralogy and Petrology》2003,145(1):1-14
Sized aggregates of glasses (47–84 wt% SiO2) were fused from igneous-derived cohesive fault rock and igneous rock, and step-heated from ~400 to >1,200 °C to obtain their 39Ar diffusion properties (average E=33,400 cal mol?1; D o=4.63×10?3 cm2 s?1). At T<~1,000 °C, glasses containing <~69 wt% SiO2 and abundant network-forming cations (Ca, Fe, Mg) reveal moderate to strong non-linear increases in D and E, reflecting structural modifications as the solid transitions to melt. Extrapolation of these Arrhenius properties down to typical geologic T-t conditions could result in a 1.5 log10 unit underestimation in the diffusion rate of Ar in similar materials. Numerical simulations based upon the diffusion results caution that some common geologic glasses will likely yield 40Ar/39Ar cooling ages rather than formation ages. However, if cooling rates are sufficiently high, ambient temperatures are sufficiently low (e.g., <65–175 °C), and coarse particles (e.g., radius (r) >~1 mm) are analyzed, glasses with compositions similar to ours may preserve their formation ages. 相似文献