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611.
We present an efficient algorithm using a complex variables formulation for the computation of the gravity effect of 2D polygonal bodies having densities varying both laterally and with depth. The first derivatives of the gravity effect are also provided in order to enable the computation of the Jacobian matrix, which is necessary for linear inverse gravity problems. A geophysical example based on numerical assumptions about the density contrast on a well-studied basin area shows the applicability of the algorithm. 相似文献
612.
613.
Mrcia Cristina Costa de Azevedo Francisco Gerson Araújo Antnio Gomes da Cruz-Filho Andr Luiz Machado Pessanha Mrcio de Araújo Silva Ana Paula Penha Guedes 《Estuarine, Coastal and Shelf Science》2007,75(4):468-480
This study analyzed the factors structuring demersal fish community in a tropical bay in southeastern Brazil. The results were used to quantify the partitioning of ecological variation among the environmental, spatial and temporal components molding the fish community. Three bay zones (inner, middle and outer) were defined according to depth and salinity gradient. Monthly samplings were conducted by bottom trawl tows during daylight hours, between October 1998 and September 1999. In each zone, three replicate samples were taken. Ninety-three fish species from 73 genera and 37 families were recorded in the 108 samples. Two demersal fish assemblages were evidenced, one in the inner and the other in the outer zone. These assemblages were characterized by changes in species composition and relative abundance. Depth, followed by transparency and salinity, influenced spatial pattern of fish assemblages. The largest part of the explained variation occurred as a result of the spatial structure of environmental variables, which means that both species and environmental variables presented similar spatial structure. The spatial effect, not the seasonal, explained the highest part of species variations. The amount of unexplained variation was relatively high (76%), even assuming that part of it is due to nondeterministic fluctuation, which could be due to local effects of unmeasured (biotic and abiotic) controlling variables. Knowing the relative importance of these factors can be of decisive importance when applying casual hypotheses in the framework of some precise ecological theory and should facilitate management, planning, and usage of bay resources. 相似文献
614.
Study of the transfer from the Earth to a halo orbit around the equilibrium pointL
1 总被引:2,自引:0,他引:2
G. Gómez A. Jorba J. Masdemont C. Simó 《Celestial Mechanics and Dynamical Astronomy》1993,56(4):541-562
The purpose of this paper is to study a transfer strategy from the vicinity of the Earth to a halo orbit around the equilibrium pointL
1 of the Earth-Sun system. The study is done in the real solar system (we use the DE-118 JPL ephemeris in the simulations of motion) although some simplified models, such as the restricted three body problem (RTBP) and the bicircular problem, have been also used in order to clarify the geometrical aspects of the problem. The approach used in the paper makes use of the hyperbolic character of the halo orbits under consideration. The invariant stable manifold of these orbits enables the transfer to be achieved with, theoretically, only one manoeuvre: the one of insertion into the stable manifold. For the total v required, the figures obtained are similar to the ones given by the standard procedures of optimization. 相似文献
615.
Carlos José Freire Machado Maria Marlúcia Freitas Santiago Luiz Alberto Ribeiro Mendonça Horst Frischkorn Josué Mendes Filho 《Aquatic Geochemistry》2007,13(2):187-196
Flow modeling and hydrogeochemical inversion calculations were performed on the aquifer systems of the Crato-Juazeiro Graben
in the Araripe Sedimentary Basin of the Cariri Valley, Northeast Brazil, in order to assess aquifer interconnections through
an aquitard of the Araripe Plateau. Steady state hydraulic head numeric calculations with the finite difference method used
Modflow software with hydrologically based boundary conditions and a piezometric surface as initial condition. Hydrogeochemical
inversion with PHREEQC was performed for two water samples representing typical initial and final waters along the flow path.
Flow simulation shows that approximately 20% of all recharge water from the superior aquifer system passes through the Santana
aquitard reaching the middle aquifer system. Quantification of mineral dissolution/precipitation processes, ion exchange and
microbiological redox reactions taking place in the aquifer systems also indicates hydraulic interconnection of the superior
and middle aquifer systems through the Santana aquitard. 相似文献
616.
617.
Sergio Pacca 《Climatic change》2007,84(3-4):281-294
Greenhouse gas (GHG) emissions from hydroelectric dams are often portrayed as nonexistent by the hydropower industry and have
been largely ignored in global comparisons of different sources of electricity. However, the life cycle assessment (LCA) of
any hydroelectric plant shows that GHG emissions occur at different phases of the power plant’s life. This work examines the
role of decommissioning hydroelectric dams in greenhouse gas emissions. Accumulated sediments in reservoirs contain noticeable
levels of carbon, which may be released to the atmosphere upon decommissioning of the dam. The rate of sediment accumulation
and the sediment volume for six of the ten largest United States hydroelectric power plants is surveyed. The amount of sediments
and the respective carbon content at the moment of dam decommissioning (100 years after construction) was estimated. The released
carbon is partitioned into CO2 and CH4 emissions and converted to CO2 equivalent emissions using the global warming potential (GWP) method. The global warming effect (GWE) due to dam decommissioning
is normalized to the total electricity produced over the lifetime of each power plant. The estimated GWE of the power plants
range from 128–380 g of CO2eq./kWh when 11% of the total available sediment organic carbon (SOC) is mineralized and between 35 and 104 g of CO2eq./kWh when 3% of the total SOC is mineralized. Though these values are below emission factors for coal power plants (890 g
of CO2eq./kWh), the amount of greenhouse gases emitted by the sediments upon dam decommissioning is a notable amount that should
not be ignored and must be taken into account when considering construction and relicensing of hydroelectric dams. 相似文献
618.
Andreia Dias Elias Luiz Fernando De Ros Ana Maria Mizusaki Koji Kawashita 《Gondwana Research》2007,11(4):553-567
The isotopic composition of diagenetic carbonates, sulfates, illites and associated diagenetic constituents of the Carboniferous Juruá sandstones, Solimões Basin, largest hydrocarbon reservoirs of northern Brazil, was analyzed in conjunction with petrographic characterization and chemical composition. Eolian Juruá sandstones, deposited in a coastal sabkha of a wide cratonic sag, are the best reservoirs, but diagenetic processes introduced strong and complex heterogeneities, which affect hydrocarbon production. Dolomite and anhydrite cementation, together with compaction, exerts the main control on the quality of the reservoirs. Early dolomite cements precipitated under strong evaporation, while late dolomites are related to thermal decarboxylation. Late anhydrite cements are related to the interbedded evaporites and possibly to the oxidation of dissolved sulfide. Authigenic illite K–Ar ages are related to the voluminous Triassic basic magmatism (around 200 Ma), and to the Jurassic–Cretaceous tectonism (around 150 Ma), which may have also affected the late anhydrite and dolomite–ankerite cementation. This isotopic study revealed the major conditions of the diagenetic processes that strongly affected the Juruá reservoirs, which are essential for the development of models that will optimize the production, as well as decrease the risks in the exploration for new Juruá reservoirs. 相似文献
619.
620.
Luiz Bruner de Miranda Belmiro Mendes de Castro Björn Kjerfve 《Estuaries and Coasts》1998,21(2):204-214
Bertioga Channel is a partially mixed (type 2) tidal estuary on the coastal plain of São Paulo, Brazil. Hourly current and salinity measurements during neap and spring tides in July 1991 yielded information about the physical structure of the system. Peak along-channel velocities varied from 40 cm s?1 to 60 cm s?1 during flood tides and from 70 cm s?1 to 100 cm s?1 during ebb tides. Net vertical velocity profiles indicate that the net current reverses directions at a depth of 2.5–3.0 m in the halocline. Due to appreciable fortnightly tidal modulation, the estuary alternates from being highly stratified (type 2b) during neap tides, with advection and diffusion contributing equally to the net upstream salt flux, to being moderately stratified (type 2a) during spring tides, when 90% of the net upstream salt transport is the result of effective tidal diffusion. Decomposition of the salt flux indicates that the relative contribution to the upstream salt transport by gravitational circulation shear is greater than the oscillatory tidal flux by a factor of 2.6 during neap tides. The oscillatory tidal flux is generated by the correlation of the tidal components of the u-velocity and salinity and is responsible for approximately the same amount of upstream salt transport, during neap and spring tides. However, during spring tides, this oscillatory term is greater than the other salt flux terms by a factor of 1.4. The total salt transport, through a unit width of the section perpendicular to the flow, was within 2% of the sum of the seven major decomposed, advective and dispersive terms. On the assumption that the Bertioga Channel is laterally homogeneous, the results also indicate that the estuary is not in steady state with respect to salt flux. 相似文献