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101.
102.
Miguel Angel de Lucas Pardo Marieke Bakker Thijs van Kessel Francesco Cozzoli Johan Christian Winterwerp 《Ocean Dynamics》2013,63(9-10):1137-1150
The Markermeer is a large and shallow man-made freshwater lake in the Netherlands, characterized by its high turbidity. As part of a study aiming to mitigate this high turbidity, we studied the water–bed exchange processes of the lake’s muddy bed. The upper centimeter’s–decimeter’s of the lake bed sediments mainly consists of soft anoxic mud. Recent measurements have proved the existence of a thin oxic layer on top of this soft anoxic mud. This oxic layer, which is much easier to be eroded than the anoxic mud, is believed to be related with Markermeer’s high-turbidity levels. Our hypothesis is that the thin oxic layer develops from the anoxic mud, enhanced by bioturbation. Actually, we will demonstrate that it is the bioturbated state of the bed that increases its erodability, and not the oxidation state of the sediments. In particular, we will refer to bioturbation caused by meiobenthic fauna. The objective of this study is therefore to determine the influence of the development of the thin oxic layer on the water–bed exchange processes, as well as to establish the role of bioturbation on those processes. This is done by quantifying the erosion rate as a function of bed shear stresses, and at different stages of the development of the oxic layer. Our experiments show that bioturbation increases the rate at which Markermeer sediments are eroded by almost an order of magnitude. The short-term fine sediment dynamics in Markermeer are found to be driven by the complex and highly dynamic interactions between physics, chemistry, and biology. Finally, the long-term fine sediment dynamics are driven by the erosion of the historical deposits in the lake’s bed, which is only possible after bioturbation, and which leads to an increase of the stock of sediments in the lake’s muddy bed. 相似文献
103.
Miguel A. Ortega-Huerta Enrique Martinez-Meyer Stephen L. Egbert Kevin P. Price A. Townsend Peterson 《国际地球制图》2013,28(3):7-20
Abstract An important methodological and analytical requirement for analyzing spatial relationships between regional habitats and species distributions in Mexico is the development of standard methods for mapping the country's land cover/land use formations. This necessarily involves the use of global data such as that produced by the Advanced Very High Resolution Radiometer (AVHRR). We created a nine‐band time‐series composite image from AVHRR Normalized Difference Vegetation Index (NDVI) bi‐weekly data. Each band represented the maximum NDVI for a particular month of either 1992 or 1993. We carried out a supervised classification approach, using the latest comprehensive land cover/vegetation map created by the Mexican National Institute of Geography (INEGI) as reference data. Training areas for 26 land cover/vegetation types were selected and digitized on the computer's screen by overlaying the INEGI vector coverage on the NDVI image. To obtain specific spectral responses for each vegetation type, as determined by its characteristic phenology and geographic location, the statistics of the spectral signatures were subjected to a cluster analysis. A total of 104 classes distributed among the 26 land cover types were used to perform the classification. Elevation data were used to direct classification output for pine‐oak and coastal vegetation types. The overall correspondence value of the classification proposed in this paper was 54%; however, for main vegetation formations correspondence values were higher (60‐80%). In order to obtain refinements in the proposed classification we recommend further analysis of the signature statistics and adding topographic data into the classification algorithm. 相似文献
104.
This study aims to determine and to model the relationship of matric suction versus water content, known as soil?Cwater characteristic curve (SWCC), for a tropical soil profile at the southeastern Brazil. This soil profile consists of a colluvial, lateritic silty clay, with thickness of about 6.5?m. The filter paper and pressure plate techniques were used to determine the SWCC. Specimens were trimmed from the undisturbed soil samples collected along soil profile depth and submitted to drying paths. Impregnated thin-layer plates and a petrographic microscope were used to examine the structure and mineralogical composition of the soil samples. Mercury intrusion porosimetry tests were performed on some soil samples to obtain the frequency histogram of the pores. SWCC with bimodal pore-size distribution were obtained by the filter paper technique and SWCC with unimodal pore-size distribution were obtained by the pressure plate technique. The SWCC showed values of air-entry ranging from 1 to 3?kPa, depending on the technique used, resembling soils with macroporosity, as the lateritic soils. Combining both techniques, the SWCC could be fitted by a model that takes into consideration soils with macro and microporosity. 相似文献
105.
EVA: GPS-based extended velocity and acceleration determination 总被引:1,自引:0,他引:1
Dagoberto Salazar Manuel Hernandez-Pajares Jose Miguel Juan-Zornoza Jaume Sanz-Subirana Angela Aragon-Angel 《Journal of Geodesy》2011,85(6):329-340
In this work, a new GPS carrier phase-based velocity and acceleration determination method is presented that extends the effective
range of previous techniques. The method is named ‘EVA’, and may find applications in fields such as airborne gravimetry when
rough terrain or water bodies make difficult or impractical to set up nearby GPS reference receivers. The EVA method is similar
to methods such as Kennedy (Precise acceleration determination from carrier phase measurements. In: Proceedings of the 15th
international technical meeting of the satellite division of the Institute of Navigation. ION GPS 2002, Portland pp 962–972,
2002b) since it uses L1 carrier phase observables for velocity and acceleration determination. However, it introduces a wide network
of stations and it is independent of precise clock information because it estimates satellite clock drifts and drift rates
‘on-the-fly’, requiring only orbit data of sufficient quality. Moreover, with EVA the solution rate is only limited by data
rate, and not by the available precise satellite clocks data rate. The results obtained are more robust for long baselines
than the results obtained with the reference Kennedy method. An advantage of being independent of precise clock information
is that, beside IGS Final products, also the Rapid, Ultra-Rapid (observed) and Ultra-Rapid (predicted) products may be used.
Moreover, the EVA technique may also use the undifferenced ionosphere-free carrier phase combination (LC), overcoming baseline
limitations in cases where ionosphere gradients may be an issue and very low biases are required. During the development of
this work, some problems were found in the velocity estimation process of the Kennedy method. The sources of the problems
were identified, and an improved version of the Kennedy method was used for this research work. An experiment was performed
using a light aircraft flying over the Pyrenees, showing that both EVA and the improved Kennedy methods are able to cope with
the dynamics of mountainous flight. A RTK-derived solution was also generated, and when comparing the three methods to a known
zero-velocity reference the results yielded similar performance. The EVA and the improved-Kennedy methods outperformed the
RTK solutions, and the EVA method provided the best results in this experiment. Finally, both the improved version of the
Kennedy method and the EVA method were applied to a network in equatorial South America with baselines of more than 1,770 km,
and during local noon. Under this tough scenario, the EVA method showed a clear advantage for all components of velocity and
acceleration, yielding better and more robust results. 相似文献
106.
The ionosphere: effects,GPS modeling and the benefits for space geodetic techniques 总被引:12,自引:8,他引:4
Manuel Hernández-Pajares J. Miguel Juan Jaume Sanz Àngela Aragón-Àngel Alberto García-Rigo Dagoberto Salazar Miquel Escudero 《Journal of Geodesy》2011,85(12):887-907
The main goal of this paper is to provide a summary of our current knowledge of the ionosphere as it relates to space geodetic
techniques, especially the most informative technology, global navigation satellite systems (GNSS), specifically the fully
deployed and operational global positioning system (GPS). As such, the main relevant modeling points are discussed, and the
corresponding results of ionospheric monitoring are related, which were mostly computed using GPS data and based on the direct
experience of the authors. We address various phenomena such as horizontal and vertical ionospheric morphology in quiet conditions,
traveling ionospheric disturbances, solar flares, ionospheric storms and scintillation. Finally, we also tackle the question
of how improved knowledge of ionospheric conditions, especially in terms of an accurate understanding of the distribution
of free electrons, can improve space geodetic techniques at different levels, such as higher-order ionospheric effects, precise
GNSS navigation, single-antenna GNSS orientation and real-time GNSS meteorology. 相似文献
107.
Pérez-López R Nieto JM López-Cascajosa MJ Díaz-Blanco MJ Sarmiento AM Oliveira V Sánchez-Rodas D 《Marine pollution bulletin》2011,62(2):405-411
This study reports the annual amount of heavy metals discharged by industrial activity into the estuary of the Ría of Huelva (SW Spain). The findings showed that the discharged metals found in highest amounts were Fe (11 t y−1), Zn (3.4 t y−1) and Mo (0.88 t y−1). There were other metals with high pollutant charge, such as Ti (232 kg y−1), As (228 kg y−1), Ni (195 kg y−1), Pb (100 kg y−1), Cr (39 kg y−1) and Cd (33 kg y−1). These results were compared with pollutants transported via the Tinto and Odiel rivers from abandoned mining activities in the Iberian Pyrite Belt (IPB), and it was deduced that the amounts spilled exclusively by industries were less than 1% in relation to the total discharge. Hence, the treatment of residues from the IPB should be the priority goal to improve water quality in the estuary. 相似文献
108.
Cardoso I Pais MP Henriques S da Fonseca LC Cabral HN 《Marine pollution bulletin》2011,62(5):992-1001
The importance of establishing the ecological quality of estuarine systems has been widely acknowledged and led to the development of several fish community-based multimetric indices. Nevertheless, a question rose about the accuracy of these tools when natural disturbance is acting upon the organization of the systems’ communities. Four multimetric indices were used to examine their ability to differentiate the ecological status of five small estuarine systems (southern Portugal), and also to test if they reflected the level of anthropogenic pressures. Fish assemblages from Mira, Odeceixe and Aljezur (in the Southwest coast), Gilão and Bensafrim (in the South coast) estuaries were sampled seasonally for one year, and anthropogenic sources of pressure were identified and quantified. We found that although the applied indices provided information on ecological condition differentiation among systems, they are unable to explain different classes of ecological status in systems with equivalent pressure levels. 相似文献
109.
Costa PM Caeiro S Lobo J Martins M Ferreira AM Caetano M Vale C DelValls TÁ Costa MH 《Marine pollution bulletin》2011,62(1):55-65
Juvenile Senegalese soles were exposed through 28-day laboratory and field (in situ) bioassays to sediments from three sites of the Sado estuary (W Portugal): a reference and two contaminated by metallic and organic contaminants. Fish were surveyed for ten hepatic histopathological alterations divided by four distinct reaction patterns and integrated through the estimation of individual histopathological condition indices. Fish exposed to contaminated sediments sustained more damage, with especial respect to regressive changes like necrosis. However, differences were observed between laboratory- and field-exposed animals, with the latest, for instance, exhibiting more pronounced fatty degeneration and hepatocellular eosinophilic alteration. Also, some lesions in fish exposed to the reference sediment indicate that in both assays unaccounted variables produced experimental background noise, such as hyaline degeneration in laboratory-exposed fish. Still, the field assays yielded results that were found to better reflect the overall levels of contaminants and physico-chemical characteristics of the tested sediments. 相似文献
110.
Mabel Mena Avto Goguitchaichvili Miguel Cervantes Solano Juan Francisco Vilas 《Studia Geophysica et Geodaetica》2011,55(2):279-309
The Early Cretaceous may be considered a key period for understanding the evolution of the Earth’s magnetic field. Some still
unsolved problems are related to the mode of paleosecular variation (PSV) of the Earth’s magnetic field before and during
the Cretaceous Normal Superchron. We report here a detailed rock-magnetic, paleomagnetic and paleointensity investigation
from 28 lava flows (331 standard paleomagnetic cores) collected in the Argentinean part of the Parana Flood Basalts (Formation
Posadas) in order to contribute to the study of PSV during the early Cretaceous and to obtain precise Cretaceous paleomagnetic
pole positions for stable South America. The average paleofield direction is precisely determined from 26 sites, which show
small within-site dispersion and high directional stability. Five sites show evidences for the self-reversal of thermoremanent
magnetization. 23 sites yielded normal polarity magnetization and only 3 are reversely magnetized. Moving windows averages
were used to analyze the sequential variation of virtual geomagnetic pole’s (VGP) axial positions. Interestingly, the axial
average VGP path traces an almost complete cycle around the geographical pole and passes near the location of all previously
published Paraná Magmatic Province poles. Both paleomagnetic poles and average VGP paths are significantly different from
the pole position suggested by fixed hotspot reconstructions, which may be due to true polar wander or the hotspot motion
itself. Only 15 samples from 5 individual basaltic lava flows, yielded acceptable paleointensity estimates. The site mean
paleointensities range from 25.2 ± 2.2 to 44.0 ± 2.2 μT. The virtual dipole moments (VDMs) range from 4.8 to 9.9 × 1022 Am2. This correspond to a mean value of 7.7 ± 2.1 × 1022 Am2 which is 96% of the present day geomagnetic field strength. These intensities agree with the relatively high values already
reported for Early Cretaceous, which are consistent with some inferences from computer simulations previously published. 相似文献