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511.
Because of late metamorphic and tectonic overprints, the reconstruction of prograde parts of P–T paths is often difficult. In the SW Variscan French Massif Central, the Thiviers-Payzac Unit (TPU) is the uppermost allochthon emplaced above underlying units. The TPU experienced a Barrovian metamorphism coeval with a top-to-the-NW ductile shearing (D2 event) in Early Carboniferous times (ca. 360–350 Ma). The tectonic setting of the D2 event, compression or synconvergence extension, remains unclear. Using the THERMOCALC software and the model system MnNCKFMASH, the peak P–T conditions are estimated from garnet rims and matrix minerals and the prograde evolution is deduced from garnet core compositions. The combination of these two approaches demonstrates that the TPU experienced pressure and temperature increases before reaching peak conditions at 6.6–9.0 +/− 1.2 kbar and 615–655 +/− 35 °C. This kind of P–T path shows that the regional D2 event corresponds to crustal thickening. 相似文献
512.
Phu Le Vo 《GeoJournal》2007,70(1):75-89
The management of water resources is an unfinished effort of the international community. Rapid urbanization has transcended
the management capacity of governments in developing countries. Since the renovation policy launched in 1986, Ho Chi Minh
City, Vietnam, has experienced the fastest urbanization and industrialization process. This has placed severe constraints
on the use of water resources and management capacity of the local government. The abstraction of groundwater has exceeded
the limiting volume (520,000 m3/day) and the annual drawdown of water table is 2–3 m. In addition, the quality of urban water bodies is increasingly exacerbated
by a huge volume of untreated industrial and domestic wastewater. These are hampering water demand, use and the capacity of
the municipal authority in managing water resources. The purpose of this paper is to analyze the impact of urbanization on
water resources. Current issues and challenges in the management practices of water resources are discussed. It will propose
a new paradigm of water management in Ho Chi Minh City. 相似文献
513.
Arnoldo Valle-Levinson Nandita Sarkar Rosario Sanay Doris Soto Jorge León 《Estuaries and Coasts》2007,30(1):113-126
Underway current velocity profiles were combined with temperature and salinity profiles at fixed stations to describe tidal
and subtidal flow patterns in the middle of the northernmost Chilean fjord, Estuario Reloncaví. This is the first study involving
current velocity measurements in this fjord. Reloncaví fjord is 55 km long, 2 km wide, and on average is 170 m deep. Measurements
concentrated around a marked bend of the coastline, where an 8-km along-fjord transect was sampled during a semidiurnal tidal
cycle in March 2002 and a 2-km cross-fjord transect was occupied, also during a semidiurnal cycle, in May 2004. The fjord
hydrography showed a relatively thin (<5 m deep), continuously stratified, buoyant layer with stratification values >4 kg
m−3 per meter of depth. Below this thin layer, the water was relatively homogeneous. Semidiurnal tidal currents had low amplitudes
(<10 cm s−1) that allowed the persistence of a surface front throughout the tidal cycle. The front oscillated with a period of ca. 2.5
h and showed excursions of 2 km. The front oscillations could have been produced by a lateral seiche that corresponds to the
natural period of oscillation across the fjord. This front could have also caused large (2 h) phase lags in the semidiurnal
tidal currents, from one end of the transect to the other, within the buoyant layer. Tidal phases were relatively uniform
underneath this buoyant layer. Subtidal flows showed a 3-layer pattern consisting of a surface layer (8 m thick, of 5 cm s−1 surface outflow), an intermediate layer (70 m thick, of 3 cm s−1 net inflow), and a bottom layer (below 80 m depth, of 3 cm s−1 net outflow). The surface outflow and, to a certain extent, the inflow layer were related to the buoyant water interacting
with the ambient oceanic water. The inflowing layer and the bottom outflow were attributed to nonlinear effects associated
with a tidal wave that reflects at the fjord's head. The weak subtidal currents followed the morphology of the bend and caused
downwelling on the inside and upwelling on the outside part of the bend. 相似文献
514.
Sediment erosion results from hydrodynamic forcing, represented by the bottom shear stress (BSS), and from the erodability of the sediment, defined by the critical erosion shear stress and the erosion rate. Abundant literature has dealt with the effects of biological components on sediment erodability and concluded that sediment processes are highly sensitive to the biota. However, very few sediment transport models account for these effects. We provide some background on the computation of BSS, and on the classical erosion laws for fine sand and mud, followed by a brief review of biota effects with the aim of quantifying the latter into generic formulations, where applicable. 相似文献
515.
A portable erosion device (ERIS) was deployed in November 2000 and May 2001 to investigate the spatio-temporal variation of fine-grained sediment erosion over a transect on the intertidal mudflats of Marennes-Oléron Bay, western France. The objective was to relate changes in bed erodability over diurnal emersion periods to bioturbation, rapid changes in sediment characteristics, and microphytobenthos biomass. 相似文献
516.
Kaja Pietsch Paweł Marzec Marcin Kobylarski Tomasz Danek Andrzej Leśniak Artur Tatarata Edward Gruszczyk 《Acta Geophysica》2007,55(2):191-208
The thin-layer build of the Carpathian Foredeep Miocene formations and large petrophysical parameter variation cause seismic
images of gas-saturated zones to be ambiguous, and the location of prospection wells on the basis of anomalous seismic record
is risky. A method that assists reservoir interpretation of standard recorded seismic profiles (P waves) can be a converted wave recording (PS waves).
This paper presents the results of application of a multicomponent seismic survey for the reservoir interpretation over the
Chałupki Dębniańskie gas deposit, carried out for the first time in Poland by Geofizyka Kraków Ltd. for the Polish Oil and
Gas Company. Seismic modeling was applied as the basic research tool, using the SeisMod program based on the finite-difference
solution of the acoustic wave equation and equations of motion. Seismogeological models for P waves were developed using Acoustic Logs; S-wave model (records only from part of the well) was developed on the basis of theoretical curves calculated by means of the
Estymacja program calibrated with average S-velocities, calculated by correlation of recorded P and PS wavefields with 1D modeling.
The conformity between theoretical and recorded wavefields makes it possible to apply the criteria established on the basis
of modeling for reservoir interpretation. Direct hydrocarbon indicators (bright spots, phase change, time sag) unambiguously
identify gas-prone layers within the ChD-2 prospect. A partial range of the indicators observed in the SW part of the studied
profile (bright spot that covers a single, anticlinally bent seismic horizon) points to saturation of the horizon. The proposed
location is confirmed by criteria determined for converted waves (continuous seismic horizons with constant, high amplitude)
despite poorer agreement between theoretical and recorded wavefields. 相似文献
517.
A simple theoretical model of atmospheric radiative equilibrium is solved analytically to help understand the energetics
of maintaining Earth's tropical and subtropical climate. The model climate is constrained by energy balance between shortwave
(SW) and longwave (LW) radiative fluxes. Given a complete set of SW and LW optical properties in each atmospheric layer, the
model yields a unique equilibrium-temperature profile. In contrast, if the atmospheric temperature profile and SW properties
are prescribed, the model yields essentially two distinct LW transmissivity profiles. This bimodality is due to a nonlinear
competition between the ascending and descending energy fluxes, as well as to their local conversion to sensible heat in the
atmosphere. Idealized slab models that are often used to describe the greenhouse effect are shown to be a special case of
our model when this nonlinearity is suppressed. In this special case, only one solution for LW transmissivity is possible.
Our model's bimodality in LW transmissivity for given SW fluxes and temperature profile may help explain certain features
of Earth's climate: at low latitudes the temperature profiles are fairly homogeneous, while the humidity profiles exhibit
a bimodal distribution; one mode is associated with regions of moist-and-ascending, the other with dry-and-subsiding air.
The model's analytical results show good agreement with the European Centre for Medium-Range Weather Forecasts' reanalysis
data. Sensitivity analysis of the temperature profile with respect to LW transmissivity changes leads to an assessment of
the low-latitude climate's sensitivity to the “runaway greenhouse” effect.
Received: 7 December 1999 / Accepted: 19 February 2001 相似文献
518.
Summary. We show that most of the abnormal variations of the transient magnetic field have deflection and canalization of telluric currents as main sources. In order to do so, we develop a formalism which is valid in a limited frequency range which satisfies the following hypotheses: (a) the thickness of the laterally inhomogeneous upper layer is small with respect to the penetration depth of the transient field; (b) self-induction of the heterogeneities of conductivity can be neglected. Theoretical results and observations performed at mid-latitudes are in good agreement.
The main result we establish is that deviation and canalization of the current responsible for the transient abnormal field do not affect their temporal variations. The study of temporal variations of the abnormal field thus provides us with information about the structure of the crust in the region where the deviated and canalized currents are induced. 相似文献
The main result we establish is that deviation and canalization of the current responsible for the transient abnormal field do not affect their temporal variations. The study of temporal variations of the abnormal field thus provides us with information about the structure of the crust in the region where the deviated and canalized currents are induced. 相似文献
519.
520.