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991.
The major strategy used to prevent the discharge of highly saline groundwater to the River Murray in southeastern Australia is groundwater interception and disposal. The basic design principle assumes that the extraction of groundwater from an aquifer hydraulically connected to the river, using a line of pumps positioned close and roughly parallel to the river, will decrease piezometric heads thereby reducing the discharge of saline groundwater to the river. The paper considers one of these schemes which was designed for the Mildura area on the basis of a hydrogeological investigation. It analyses the effects on piezometric head and groundwater salinity due to the groundwater interception scheme and adjacent irrigation activity over a period of several years from January 1980. It is shown that piezometric heads have decreased significantly in the stretch close to the river. A slight reduction in groundwater salinity is also apparent in this stretch except for an area between the river and a holding basin used for disposal of the saline effluents emanating from the groundwater interception scheme. This general reduction in groundwater salinity is mainly caused by pumping from the groundwater interception scheme and recharge from irrigation. The exception in the trend in groundwater salinity is due to the movement of a highly saline body of groundwater from the holding basin towards the River Murray. Results of this Australian experience should be helpful to the designers of similar salinity mitigation schemes elsewhere. 相似文献
992.
A technique developed by Lighthill for finding the asymptotic solution of an inhomogeneous wave equation with constant coefficients is applied to the study of wave propagation in magneto-atmospheres. The geometry of the wave number surface plays an important role in determining the generation and propagation of various types of magneto-atmospheric waves from a localized forcing region. Examples of these wavenumber surfaces are exhibited for various magnetic field strengths and wave frequencies. The asymptotic far field is tabulated for a time-harmonic, spatially gaussian, localized forcing term. 相似文献
993.
We performed in situ Th-Pb dating of monazite in upper amphibolite facies pelitic schist from the Grouse Creek Mountains in northwest Utah. Sixty-six ages from inclusions in four garnet grains range from 37 to 72 Ma and decrease with radial distance from garnet cores. The age range of 30 matrix monazite grains overlaps and extends to younger ages than inclusions (25-58 Ma). The monazite grains are not intersected by cracks in the garnets, through which dissolution, reprecipitation or Pb loss might occur, and are generally too small (<20 μm) to allow for more than one age determination on any one grain. Processes that might explain inclusion ages that decrease with radial distance from garnet cores include: (1) Pb diffusion in monazite, (2) dissolution and reprecipitation of monazite, and (3) co-crystallization of monazite and garnet. After consideration of these possibilities, it is concluded that the co-crystallization of monazite and garnet is the most plausible, with monazite neoblasts deriving REEs from the breakdown of muscovite. Garnet ages derived by regression of the inclusion ages and assuming a constant rate of volume increase during garnet growth yield model ages with a maximum difference between core and rim of 22 m.y. 相似文献
994.
We present three-dimensional numerical models of the ascent of a plume under a spreading ridge and the concomitant melt generation and crust formation using a mantle viscosity which depends on pressure, temperature, melt content, and, optionally, water content. From the velocity field of these convection models we compute the viscous stress tensor in the mantle. Assuming that melt-rich channels or melt-filled dikes are oriented parallel to the maximum compressional stress, we calculate the orientation of such dikes in the partially molten zone of the plume head and beneath the ridge. For the central part of the plume we find dike orientations parallel to the ridge in shallow parts of the plume head, while they are ridge-perpendicular at greater depth. The boundary between these two regimes is shifted downwards for a water-rich plume which dehydrates upon melting. The two regimes of different dike or melt channel orientations seem to be in good agreement with seismic observations on depth-dependent seismic anisotropy, indicating a preference of our wet model. We find a modest focusing of melt towards the spreading center beneath normal ridge in both models, but also in the fully dehydrated parts of the plume head in the model where water was included; those are regions where upwelling is essentially passive. In contrast, the plume in the water-free model and the only partly dehydrated parts of the plume in the water-bearing model tend to develop a dike orientation which moves melt away from the spreading center; these regions are characterized by active upwelling. The defocusing of melt in actively upwelling plume mantle has radial symmetry about the plume axis, i.e. an outward orientation of the dikes is not only visible in planes perpendicular to the ridge, but also along the ridge. 相似文献
995.
The linear dunes of the southern Kalahari dunefield constitute one of the major palaeoenvironmental proxies in the region. The application of optically stimulated luminescence (OSL) dating since the1990s and advancements in the depth of sampling using augering equipment over the past few years have permitted the reconstruction of linear dune accumulation chronostratigraphies for entire dune profiles from base to crest. These methods are applied to four dunes in the Mariental–Stampriet region of the southern Kalahari dunefield, sampled at predominantly 0.5 m intervals. Individual dunes record multiple phases of dune construction, but with only a few phases recorded consistently between two or more of the dunes. Results from the 48 OSL ages produced here extend the aeolian accumulation record for the southern Kalahari dunefield through the last three glacial–interglacial cycles with two ages from the early part of MIS6. A synthesis of all existing luminescence ages for the southern Kalahari reveals that the dunefield has been partially active throughout much of the past 120 ka. There are no clear clusters of ages within OSL age errors. This is in contrast to previous syntheses of ages for this region. In addition, these new data from Mariental–Stampriet dunes show that clusters in grouped dune OSL ages can be spuriously produced as a function of reducing the sampling frequency with depth within the dunes, from 0.5 to 1 mintervals. This has significant implications for previous conclusions regarding discrete phases of aeolian accumulation based on sampling at 1 m intervals and less vertically intensive sampling techniques. The total luminescence data set of 136 ages for the southern Kalahari implies that this dunefield has been close to the threshold of reactivation throughout much of the late Quaternary. 相似文献
996.
Richard Thomas 《Journal of Geographical Systems》2001,3(3):283-301
This paper applies a compartmental epidemic model to estimating the mixing relations that support the transfer of HIV infection
between risk populations within the countries of Western Europe. To this end, a space-time epidemic model with compartments
representing countries with populations specified to be at high (gay men and intravenous drug injectors ever with AIDS) and
low (the remainder who are sexually active) risk is described. This model also allows for contacts between susceptible and
infectious individuals by both local and international travel. This system is calibrated to recorded AIDS incidence and the
best-fit solution provides estimates of variations in the rates of mixing between the compartments together with a reconstruction
of the transmission pathway. This solution indicates that, for all the countries, AIDS incidence among those at low risk is
expected to remain extremely small relative to their total number. A sensitivity analysis of the low risk partner acquisition
rate, however, suggests this endemic state might be fragile within Europe during this century. The discussion examines the
relevance of these mixing relationships for the maintenance of disease control.
Received: 4 May 2000 / Accepted: 9 July 2000 相似文献
997.
Fluid variability in 2 GPa eclogites as an indicator of fluid behavior during subduction 总被引:5,自引:0,他引:5
Jane Selverstone Gerhard Franz Sabine Thomas Stephen Getty 《Contributions to Mineralogy and Petrology》1992,112(2-3):341-357
Fluid activity ratios calculated between millimeter- to centimeter-scale layers in banded mafic eclogites from the Tauern Window, Austria, indicate that variations in a
H
2
O existed between layers during equilibration at P approximately equal to 2GPa and T approximately equal to 625°C, whereas a
CO
2 was nearly constant between the same layers. Model calculations in the system H2O–CO2–NaCl show that these results are consistent with the existence of different saturated saline brines, carbonic fluids, or immiscible pairs of both in different layers. The data cannot be explained by the exisience of water-rich fluids in all layers. The model fluid compositions agree with fluid inclusion compositions from eclogite-stage veins and segregations that contain (1) saline brines (up to 39 equivalent wt. % NaCl) with up to six silicate, oxide, and carbonate daughter phases, and (2) carbonic fluids. The formation of crystalline segregations from fluid-filled pockets or hydrofractures indicates high fluid pressures at 2 GPa; the record of fluid variability in the banded eclogite host rocks, however, implies that fluid transport was limited to local flow along individual layers and that there was no large-scale mixing of fluids during devolatilization at depths of 60–70 km. The lack of evidence for fluid mixing may, in part, reflect variations in wetting behavior of fluids of different composition; nonwetting fluids (water-rich or carbonic) would be confined to intergranular pore spaces and would be essentially immobile, whereas wetting fluids (saline brines) could migrate more easily along an interconnected fluid network. The heterogeneous distribution of chemically distinct fluids may influence chemical transport processes during subduction by affecting mineral-fluid element partitioning and by altering the migration properties of the fluid phase(s) in the downgoing slab. 相似文献
998.
Generalized seismological data functionals 总被引:3,自引:0,他引:3
999.
1000.