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81.
The measurement of the signal to noise ratio in continuous seismic profiling systems is carried out by means of a small general purpose digital computer equipped with an analog to digital converter. The computer is programmed to compute and compare the power spectrum of a sample of noise to the power spectrum of a sample of noise plus an echo. Power spectra of some twenty to fifty shots are averaged to yield reliable estimates of the signal to noise ratio as a function of frequency. The observed spectra agree well with spectra calculated from signals measured near to an air-gun. This measurement of signal to noise ratio as a function of frequency provides unambiguous information for the adjustment of the necessary band-pass filter thus improving the operation of a continuous seismic profiler.  相似文献   
82.
Measurements of surface partial pressure of CO2 and water column alkalinity, pHT, nutrients, oxygen, fluorescence and hydrography were carried out, south of the Canary Islands during September 1998. Cyclonic and anticyclonic eddies were alternatively observed from the northwestern area to the central area of the Canary Islands. Nutrient pumping and vertical uplifting of the deep chlorophyll maximum by cyclonic eddies were also ascertained by upward displacement of dissolved inorganic carbon. A model was applied to determine the net inorganic carbon balance in the cyclonic eddy. The fluxes were determined considering both the diffusive and convective contributions from the upward pumping and the corresponding horizontal transport of water outside the area. An increase in the total inorganic carbon concentration in the upper layers inside the eddy field of 133 mmol C m− 2 d− 1 was determined. The upward flux of inorganic carbon decreased the effect of the increased primary production on the carbon dioxide chemistry. The reduced fCO2 inside the cyclonic eddy, 15 μatm lower than that observed in non-affected surface water, was explained by thermodynamic aspects, biological activity, eddy upward pumping and diffusion and air–sea water exchange effects.  相似文献   
83.
The composition of Hippidion diet, and possible changes that could relate to its extinction, were evaluated in the Argentinean-Chilean Central Andes, a Neotropical environment characterized by arid to semiarid conditions (Andean hot and cold deserts). Microhistological analyses were made on feces of Hippidion found at Los Morrillos (31°43′S–68°42′W, 3000 m a.s.l.) and Gruta del Indio (34°35′S, 68°22′W, 660 m a.s.l.). At Gruta del Indio the diet of Hippidion was based mainly on woody species. At Los Morrillos, it was based on herbaceous species.This flexibility in diet composition could be a relative adaptive advantage allowing a longer permanence of this species in comparison to others recorded in the region (such as Megatheriumand Mylodon). Nevertheless, this advantage was not enough to guarantee its survival during the Holocene. Extinction could have been affected by diverse agents, such as growing aridification of the area, increasing competition with other species (mainly Lama guanicoe), and human presence, along with a relatively low population density (as expected from the limited presence of Hippidion at the archaeological and palaeontological sites of South America. At Gruta del Indio significant changes in the diet of Hippidion corresponding to different intervals of the period 31,000–9000 14C BP are not evident. Given this evidence for similar diets for Hippidion throughout the late Quaternary, other factors need to be considered to explain the extinction of this horse.  相似文献   
84.

Background  

The origin of the spatial variability of dissolved As concentrations in shallow aquifers of the Bengal Basin remains poorly understood. To address this, we compare here transects of simultaneously-collected groundwater and aquifer solids perpendicular to the banks of the Hooghly River in Chakdaha, India, and the Old Brahmaputra River in Araihazar, Bangladesh.  相似文献   
85.
A multi-lithology diffusive stratigraphic model is considered, which simulates at large scales in space and time the infill of sedimentary basins governed by the interaction between tectonics displacements, eustatic variations, sediment supply, and sediment transport laws. The model accounts for the mass conservation of each sediment lithology resulting in a mixed parabolic, hyperbolic system of partial differential equations (PDEs) for the lithology concentrations and the sediment thickness. It also takes into account a limit on the rock alteration velocity modeled as a unilaterality constraint. To obtain a robust, fast, and accurate simulation, fully and semi-implicit finite volume discre tization schemes are derived for which the existence of stable solutions is proved. Then, the set of nonlinear equations is solved using a Newton algorithm adapted to the unilaterality constraint, and preconditioning strategies are defined for the solution of the linear system at each Newton iteration. They are based on an algebraic approximate decoupling of the sediment thickness and the concentration variables as well as on a proper preconditioning of each variable. These algorithms are studied and compared in terms of robustness, scalability, and efficiency on two real basin test cases.  相似文献   
86.
The Castellón Plain alluvial aquifer, Spain, is intensively exploited to meet the demand for agricultural irrigation and industrial water supply. The geochemistry of its groundwater shows complex salinization in the northern and southern parts of the aquifer, with significant pollution from human origin in the central portion. Boron content and B isotope geochemistry are useful for distinguishing between various sources of pollution and their relative importance in different parts of this aquifer. Boron concentrations in the groundwater vary between 0.01 and 0.85 mg/L. In the more saline groundwaters, found at the northern and southern ends of the study area, the presence of B is linked to inputs from seawater and water with a calcium-magnesium sulphate facies, which feed the aquifer and clearly influence the chemistry of its waters. Evidence of B adsorption processes in some samples is shown by the low B/Cl ratios and the high values of δ11B. In the central portion of the aquifer, the high B/Cl ratios and the strongly negative δ11B are related to pollution of human origin.  相似文献   
87.
The region known as Pampa Plain, in Argentina, is a vast area characterized by slopes of less than 0.05%. The surface sediment is silty sand, mainly Aeolian, referred to as Pampean Loess, which is a phreatic aquifer unit of utmost importance for the water supply of the region. On account of the slight gradient, hydrogeological analyses using only hydraulic measurements are difficult to perform, often leading to confusing results. Thus, the study presented relies on hydrochemical modeling and isotopic determinations as well. The study area comprises three catchments in the inter-mountainous area corresponding to El Moro, Tamangueyú and Seco creeks, and covering 2,570 km2 in the province of Buenos Aires. Measurements of piezometric levels and samples of groundwater, surface water and rainwater were carried out between January and March 2005. Major ion results were analyzed by means of hydrochemical graphs and hydrogeochemical modelling using NETPATH. The resulting data show that it is possible to identify local changes in recharge, flow direction and stream/groundwater relationship by using hydrochemical and isotopic information, which may become a useful and more precise tool for the study of particularly flat landscapes.  相似文献   
88.
An updated analysis of geothermal data from the highland area of eastern Brazil has been carried out and the characteristics of regional variations in geothermal gradients and heat flow examined. The database employed includes results of geothermal measurements at 45 localities. The results indicate that the Salvador craton and the adjacent metamorphic fold belts northeastern parts of the study area are characterized by geothermal gradients in the range of 6–17°C/km. The estimated heat flow values fall in the range of 28–53 mW/m2, with low values in the cratonic area relative to the fold belts. On the other hand, the São Francisco craton and the intracratonic São Francisco sedimentary basin in the southwestern parts are characterized by relatively higher gradient values, in the range of 14–42°C/km, with the corresponding heat flow values falling in the range of 36–89 mW/m2. Maps of regional variations indicate that high heat flow anomaly in the São Francisco craton is limited to areas of sedimentary cover, to the west of the Espinhaço mountain belt. Crustal thermal models have been developed to examine the implications of the observed intracratonic variations in heat flow. The thermal models take into consideration variation of thermal conductivity with temperature as well as change of radiogenic heat generation with depth. Vertical distributions of seismic velocities were used in obtaining estimates of radiogenic heat production in crustal layers. Crustal temperatures are calculated based on a procedure that makes simultaneous use of the Kirchoff and Generalized Integral Transforms, providing thereby analytical solutions in 2D and 3D geometry. The results point to temperature variations of up to 300°C at the Moho depth, between the northern Salvador and southern São Francisco cratons. There are indications that differences in rheological properties, related to thermal field, are responsible for the contrasting styles of deformation patterns in the adjacent metamorphic fold belts.  相似文献   
89.
During the development of the Variscan orogeny, large amounts of granitic melt were produced, giving rise to the intrusion of granitoids at different structural levels. Despite numerous studies, ages available from previous work on the Cévennes granites remain largely imprecise. In order to better constrain the age and emplacement mode of these granites, we have combined U–Pb dating on monazites and zircons and 40Ar/39Ar dating on biotites with petrological observations, major element chemical analysis and SEM zircon imaging on five samples from the Aigoual–St Guiral–Liron and Mont Lozère granitic massifs. The results revealed that granitic intrusions and cooling in Southern Cévennes occurred in a short time span at ∼306 Ma after the main episode of regional metamorphism. Petrological and chemical data suggest that they result from a mixing between mantle-derived basic magmas (lamprophyres) and lower crust acid magmas. At a regional scale the production of these melts occurred at the end of crustal thickening induced by nappe stacking, at the same time as the late anatectic events recorded further north in the Velay dome and the granulite facies metamorphism recorded in metasedimentary granulite enclaves brought up by Tertiary volcanoes of the Velay area (Bournac).  相似文献   
90.
A. Jiménez 《Solar physics》1994,152(1):319-319
Since 1984, simultaneous observations of irradiance and velocity solar acoustic modes, have been carried out by several authors in order to measure the phase difference between irradiance and velocity modes. Following the earliest observations with stratospheric balloon (Frolich and van Der Raay, 1984), a two ground-based stations (Tenerife and Baja California) were established (Jimenez et al, 1990) obtaining coherence results in the frequency range from 2.5 mHz to 4.3 mHz. These phase differences between irradiance and velocity solar acoustic modes are interpreted in terms of the non-adiabatic behaviour of the solar atmosphere. In 1988 the IPHIR (Frolich et al, 1988) instrument flown on the PHOBOS-2 mission to Mars and measured the solar irradiance during 150 consecutive days. The best velocity observations obtained in Tenerife for this period were compared with IPHIR data to compute the phase differences (Schrijver et al, 1991). The final conclusion is that good agreement is attained between space quadsi-space and ground observations which yield a phase diffenrece of about -125 degrees in the frequency range 2.5 mHz to 4.2 mHz, with a slight increase suggested by the data running up to 4.6 mHz.  相似文献   
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