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991.
Improving vertical GPS precision with a GPS-over-fiber architecture and real-time relative delay calibration 总被引:2,自引:0,他引:2
Daniel Macias-Valadez Rock Santerre Sophie Larochelle René Landry Jr. 《GPS Solutions》2012,16(4):449-462
A limitation of GPS positioning is that the vertical component is generally two to three times less precise than the horizontal components. In a previous work by R. Santerre of Laval University and G. Beutler of University of Bern, it was shown in simulations that it is possible to improve the GPS vertical positioning precision by using a multi-antenna GPS receiver and a precise calibration technique of the relative hardware delay between the antennas and the receiver. However, no actual implementation of the system was done to prove the concept until now. A new multi-antenna, GPS-over-fiber architecture with real-time delay monitoring, designed and implemented to improve the vertical precision is presented. The improvement in vertical precision arises from the elimination of the relative receiver clock error in single difference, between antennas, and the precision real-time calibration of the relative hardware delay. Experiments were conducted with a zero baseline and a short baseline configuration. The results show, as expected by the theory and the simulations, a two to three times improvement in the precision of the vertical component such that it reached the same level of performance as the horizontal components. These promising results will enable the use of this type of configuration in several applications where the same precision in all 3D components is essential and could not be achieved before with standard GPS positioning techniques. 相似文献
992.
993.
Comparison of statistical algorithms for detecting homogeneous river reaches along a longitudinal continuum 总被引:1,自引:0,他引:1
Seven methods designed to delineate homogeneous river segments, belonging to four families, namely — tests of homogeneity, contrast enhancing, spatially constrained classification, and hidden Markov models — are compared, firstly on their principles, then on a case study, and on theoretical templates. These templates contain patterns found in the case study but not considered in the standard assumptions of statistical methods, such as gradients and curvilinear structures. The influence of data resolution, noise and weak satisfaction of the assumptions underlying the methods is investigated. The control of the number of reaches obtained in order to achieve meaningful comparisons is discussed. No method is found that outperforms all the others on all trials. However, the methods with sequential algorithms (keeping at order n + 1 all breakpoints found at order n) fail more often than those running complete optimisation at any order. The Hubert-Kehagias method and Hidden Markov Models are the most successful at identifying subpatterns encapsulated within the templates. Ergodic Hidden Markov Models are, moreover, liable to exhibit transition areas. 相似文献
994.
F. Guillocheau D. Rouby C. Robin C. Helm C. Le Carlier de Veslud J. Braun 《Basin Research》2012,24(1):3-30
The terrigeneous sediment budget of passive margin basins records variations in continental relief triggered by either deformation or climate. Consequently, it becomes a major challenge to determine sediment accumulation histories in a large number of basins found in various geodynamic contexts. In this study, we developed a GIS‐based method to determine the sediment budget at the scale of a whole basin (from the upstream continental onlap to the most distal deepest marine deposits) and the associated uncertainties. The volume of sediments preserved in the basin for each time interval was estimated by interpolation between cross‐sections and then corrected from in situ production and porosity to obtain terrigeneous solid volumes. This approach was validated by applying it to Namibia–South African passive margin basins for which independent data are available. We determined by a statistical approach the variances associated with each parameter of the method: the geometrical extrapolation of the section (8–43%), the uncertainties on seismic velocities for the depth conversion (2–10%), on the absolute ages of stratigraphic horizons (0.2–12%), on the carbonate content (0.2–46%) and on remaining porosities estimation (3–5%). Our estimates of the accumulated volumes were validated by comparison with previous estimates at a lower temporal resolution in the same area. We discussed variations in accumulation rates observed in terms of relief variations triggered by climate and/or deformation. The high accumulation rates determined for the Lower Cretaceous, progressively decreasing to a minimum in the Mid‐Cretaceous, are consistent with the progressive relaxation of a rift‐related relief. The following increase to an Upper Cretaceous maximum is consistent with a major relief reorganization driven either by an uplift and/or a change to more humid climate conditions. The lower accumulation rate in the Cenozoic suggests a relief reorganization of lesser amplitude over that period. 相似文献
995.
In order to test the sensitivity of the transitional phase of the 2006 West African monsoon (WAM) onset to different mechanisms, weather research and forecasting (WRF) model simulations have been carried out addressing the role of the Saharan heat low (SHL) and its sensitivity to the albedo field and to the northern Africa orography, and the role of the sea surface temperature (SST) in the eastern tropical Atlantic and Mediterranean. Lowering albedo over the desert region induces a northward location of the inter-tropical convergence zone (ITCZ), while removing mountains in North Africa reduces rainfall over West Africa. Shifting SST forward by 15?days leads to a northward location of the ITCZ before the WAM onset. However none of these factors modifies the timing of the WAM onset in 2006. The transitional phase of the 2006 WAM onset has been examined in more detail. The enhancement of SHL intensity, combined with the development of the oceanic cold tongue in the Guinea gulf, leads to low-level moisture flux divergence in the ITCZ reducing rainfall and increasing low-level humidity over the Sahel. However, weakening of convection can be clearly attributed to dry-air intrusions in mid-levels, originating from the subtropical westerly jet and associated with Rossby wave pattern over North Africa. Sensitivity tests on the synoptic scale forcing outside of the WRF model domain confirm the dominating role of large-scale dynamics to control the transitional phase of the WAM onset and its timing. However it is shown that the regional factors can modulate this larger scale forcing. 相似文献
996.
The ability of general circulation models to simulate tropical cyclones and their precursors over the North Atlantic main development region 总被引:1,自引:1,他引:0
Anne Sophie Daloz Fabrice Chauvin Kevin Walsh Sally Lavender Deborah Abbs Frank Roux 《Climate Dynamics》2012,39(7-8):1559-1576
The ability of General Circulation Models (GCMs) to generate Tropical Cyclones (TCs) over the North Atlantic Main Development Region (MDR; 10–20°N, 20–80°W; Goldenberg and Shapiro in J Clim 9:1169–1187, 1996) is examined through a subset of ocean–atmosphere coupled simulations from the World Climate Research Programme (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel data set and a high-resolution (0.5°) Sea Surface Temperature (SST)-forced simulation from the Australian Conformal-Cubic Atmospheric Model GCM. The results are compared with National Center for Environmental Prediction (NCEP-2) and European Center for Medium Range Weather Forecasts Re-Analysis (ERA-40) reanalyses over a common period from 1980 to 1998. Important biases in the representation of the TC activity are encountered over the MDR. This study emphasizes the strong link in the GCMs between African Easterly Waves (AEWs) and TC activity in this region. However, the generation of AEWs is not a sufficient condition alone for the models to produce TCs. Precipitation over the Sahel, especially rainfall over the Fouta Djallon highlands (cf. Fig.?1), is playing a role in the generation of TCs over the MDR. The influence of large-scale fields such as SST, vertical wind shear and tropospheric humidity on TC genesis is also examined. The ability of TC genesis indices, such as the Genesis Potential Index and the Convective Yearly Genesis Potential, to represent TC activity over the MDR in simulations at low to high spatial resolutions is analysed. These indices are found to be a reasonable method for comparing cyclogenesis in different models, even though other factors such as AEW activity should also be considered. 相似文献
997.
New experimental determination of Li and B partition coefficients during upper mantle partial melting 总被引:1,自引:1,他引:0
Luisa Ottolini Didier Laporte Nicola Raffone Jean-Luc Devidal Brieuc Le Fèvre 《Contributions to Mineralogy and Petrology》2009,157(3):313-325
Despite the growing interest for Li and B as geochemical tracers, especially for material transfer from subducting slabs to
overlying peridotites, little is known about the behaviour of these two elements during partial melting of mantle sources.
In particular, mineral/melt partition coefficients for B and to a lesser extent Li are still a matter of debate. In this work,
we re-equilibrated a synthetic basalt doped with ~10 ppm B and ~6 ppm Li with an olivine powder from a spinel lherzolite xenolith
at 1 GPa–1,330°C, and we analyzed Li and B in the run products by secondary ion mass spectrometry (SIMS). In our experiment,
B behaved as a highly incompatible element with mineral/melt partition coefficients of the order of 10−2 (D
ol/melt = 0.008 (0.004–0.013); D
opx/melt = 0.024 (0.015–0.033); D
cpx/melt = 0.041 (0.021–0.061)), and Li as a moderately incompatible element (D
ol/melt = 0.427 (0.418–0.436); D
opx/melt = 0.211 (0.167–0.256); D
cpx/melt = 0.246 (0.229–0.264)). Our partition coefficients for Li are in good agreement with previous determinations. In the case
of B, our partition coefficients are equal within error to those reported by Brenan et al. (1998) for all the mineral phases analyzed, but are lower than other coefficients from literature for some of the phases (up to
5 times for cpx). Our measurements complement the data set of Ds for modelling partial melting of the upper mantle and basalt generation, and confirm that, in this context, B is more incompatible
than previously anticipated. 相似文献
998.
Béatrice Luais Christian Le Carlier de Veslud Yves Géraud 《Geochimica et cosmochimica acta》2009,73(10):2961-2977
We have carried out a comparative Rb-Sr, Sm-Nd and Lu-Hf isotopic study of a progressively deformed hercynian leucogranite from the French Massif Central, belonging to the La Marche ductile shear zone, in order to investigate the respective perturbation of these geochronometers with fluid induced deformation. The one-meter wide outcrop presents a strongly deformed and mylonitized zone at the center, and an asymmetric deformation pattern with a higher deformation gradient on the northern side of the zone. Ten samples have been carefully collected every 10 cm North and South away from the strongest deformed mylonitic zone. They have been analyzed for a complete major, trace element data set, oxygen isotopes, Rb-Sr, Sm-Nd and Lu-Hf isotopic systematics.We show that most of major and trace elements except SiO2, alkaline elements (K2O, Rb), and some metal transition elements (Cu), are progressively depleted with increasing deformation. This depletion includes REE + Y, but also HFS elements (Ti, Hf, Zr, Nb) which are commonly considered as immobile elements during upper level processes. Variations in elemental ratios with deformation, e.g. decrease in LREE/MREE- HREE, Nd/Hf, Th/Sr, increase in Rb/Sr, U/Th and constant Sr/Nd, lead to propose the following order of element mobility: U ? Th > Sr = Nd ? Hf + HREE. We conclude in agreement with previous tectonic and metallogenic studies that trace element patterns across the shear zone result from circulation of oxidizing F-rich hydrothermal fluids associated with deformation. A temperature of the fluid of 470-480 °C can be deduced from the δ18O equilibrium between quartz-muscovite pairs.Elemental fractionation induces perturbation of the Rb-Sr geochronometer. The well-defined 87Rb/86Sr-87Sr/86Sr correlation gives an apparent age of 294 ± 19 Ma, slightly younger than the 323 ± 4 Ma age of leucogranites in this area. This apparent age is interpreted as dating event of intense deformation and fluid circulation associated with mass transfer, and exhumation of the ductile crust shortly after the leucogranite emplacement. Sm-Nd and Lu-Hf isochron-type diagrams do not define any correlation, because of the low fractionated Sm/Nd and Lu/Hf ratios. Isotopic data demonstrate that only the Lu-Hf geochronometer system is not affected by fluid circulation and gives reliable TDM age (1.29 ± 0.03 Ga) and εHf signatures. By contrast, the Sm-Nd geochronometer system gives erroneous old TDM ages of 2.84-4 Ga. There is no positive εNd-εHf correlation, because of decreasing εNd values with deformation at constant εHf values. However, εNd-εHf values remain in the broad εNd-εHf terrestrial array, which strongly indicates that fluid-induced fractionation can contribute to the width of the terrestrial array. The strong εHf negative values of the leucogranite are similar to metasedimentary granulitic xenoliths from the French Massif Central and confirm the generation of the leucogranite by several episodes of reworking of the lower crust. 相似文献
999.
1000.
Managing environmental problems in Cuban karstic aquifers 总被引:1,自引:0,他引:1
The study area is located along the Dead Sea Rift, the climate is considered arid in its southern margin near the Dead Sea,
which is the lowest water reservoir found on the globe (412 m BSL), to semiarid in its northern part. During the last few
decades, the water resources became depleted limiting the natural development of the agricultural settlements, which are the
most common type of communities in the region. Previous studies suggested that a large amount of freshwater is lost as the
result of salinization processes, which occur when fresh groundwater from the mountain aquifers, flow into the saline clastic
Neogene aquifer complex. In order to comprehend this complex system, a detailed outlining of the regional hydrogeological
system is essential. Since there are no boreholes, which penetrate the aquiferous rock sequences within the Rift, it was necessary
to interpolate a large variety of data from several fields of geosciences. The methods applied included geological mapping,
geophysical modeling based on interpretation of seismic profiles and geochemical modeling based on chemical and isotopic analysis
of runoff, sediments and groundwater. The combined modeling based on results from the different types of analyses implied
to several conclusions relevant to the regional water management policy: (1) groundwater becomes saline as it flows from the
margins of the Rift to its center. Therefore, it is recommended to exploit it along the foothills of the rift escarpment.
(2) Geophysical modeling indicated that the foothills and the Karstic mountain aquifer extend into the subsurface of the valley
and can be farther exploited (up to 15 mm3 per annum) by relatively shallow wells. (3) Several mechanisms of groundwater salinization were deciphered: (a) the dense
vertical faulting systems act as potential conduits for saline water, which flow-up from deep-seated sources and penetrate
into the fresh aquifers. (b) Fresh groundwater in the clastic aquifer complex is rare, furthermore, two evaporates bodies
were encountered (Auja and Zaharat el Qurein), also acting as sources for fresh water salinization. (c) Although the quantity
of runoff recharge to the Jordan Valley aquifer complex is negligible, the increase in its salt-content (TDS) turns this negligible
freshwater recharge to a significant contributor of salts. 相似文献