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111.
Sediment origin and budget in Sepetiba Bay (Brazil) - an approach based on multielemental analysis 总被引:1,自引:0,他引:1
Sediment origin and transport were determined in a shallow 447 km2 coastal lagoon in Brazil, using the distribution of major elements in bottom and suspended sediments. Applying multivariate
analysis, the sum of normalized concentrations of Ti, Fe, and Al in sediments was selected to trace the terrigenous influence
in bottom sediments, whereas Ca, Si, and K were used as indicators of marine influence. Sepetiba Bay bottom sediments are
dominated by up to 80% terrigenous inputs. The open bay sediments are enriched in P, Mg, K, and Ca because of the intensive
water column primary production. The inner bay forms an independent circulation cell with a predominance of terrigenous sediments.
The results suggest a long residence time for particles in the bay, demonstrated by the magnitude of resuspension flux compared
to the total sediment input to the bay.
Received: 29 May 1996 · Accepted: 17 December 1996 相似文献
112.
In this paper we study the infiltration of DNAPL in a porous medium containing a single low-permeable lens. Our aim is to
determine whether or not DNAPL infiltrates into the lens. A key role is played by the capillary pressure: DNAPL cannot infiltrate
into the lens unless the capillary pressure exceeds the entry pressure of the lens. In the model this is reflected by an interface
condition, the extended capillary pressure condition. To derive analytical approximations we first consider a steady-state
DNAPL plume in a homogeneous medium. This results in an estimate of the DNAPL plume width as a function of depth, and an asymptotic
solution for small saturations. Assuming that the extent of the lens is much larger than the width of the unperturbed DNAPL
plume in the homogeneous medium, we derive an explicit criterion for DNAPL infiltration into the lens in terms of a critical
inflow rate. A numerical algorithm is presented in which the extended capillary pressure condition is incorporated. The numerical
and analytical results show good qualitative agreement.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
113.
114.
A general correspondence principle is presented that relates any time-domain electromagnetic diffusion field to an electromagnetic wavefield in a 'corresponding' configuration. The principle applies to arbitrarily inhomogeneous and anisotropic media and arbitrary transmitters and receivers. For the correspondence between the two types of electromagnetic fields to hold, the electric conductivity in the diffusive case and the permittivity in the wavefield case should have the same spatial variation, while the permeability distributions in space in the two cases are to be identical. Essential steps in the derivation of the correspondence principle are the use of the time Laplace transformation of causal signals, taken at real, positive values of the transform parameter, the Schouten-Van der Pol theorem in the theory of the Laplace transformation, and the reliance upon Lerch's theorem of the uniqueness of the interrelation between causal field quantities and their time-Laplace-transform representations at real, positive values of the transform parameter. Correspondence is then established between the tensorial Green's functions in the two cases, where the Green's functions are the point-receiver responses (either electric or magnetic field) to point-transmitter excitations (either electric- or magnetic-current source).
Through the correspondence principle, all transient electromagnetic wavefields (where losses are neglected) have as a counterpart a transient diffusive electromagnetic field (where the electric displacement current is neglected). The interrelation yields the tool to compare quantitatively the potentialities of the two types of fields in transient electromagnetic geophysical prospecting.
Finally, a general medium-parameter scaling law for time-domain electromagnetic wavefields is presented. 相似文献
Through the correspondence principle, all transient electromagnetic wavefields (where losses are neglected) have as a counterpart a transient diffusive electromagnetic field (where the electric displacement current is neglected). The interrelation yields the tool to compare quantitatively the potentialities of the two types of fields in transient electromagnetic geophysical prospecting.
Finally, a general medium-parameter scaling law for time-domain electromagnetic wavefields is presented. 相似文献
115.
Calculating the two-dimensional magnetotelluric Jacobian in finite elements using reciprocity 总被引:6,自引:0,他引:6
Patricia Pastana de Lugão Philip E. Wannamaker 《Geophysical Journal International》1996,127(3):806-810
To speed up the calculation of the field Jacobian for 2-D magnetoteliuric inversion using finite elements, the principle of electromagnetic reciprocity is applied. The governing relationship for the Jacobian of the field along strike is obtained by differentiating the Helmholtz equation with respect to the resistivity of each region in the finite-element mesh. The result is a similar Helmholtz equation for the Jacobian, with new sources distributed over all nodes within the parameter medium. However, according to the principle of electromagnetic reciprocity, the roles of sources and receivers are interchangeable. Utilizing reciprocity, the field values obtained from the original forward problem and for new unit sources imposed at the receivers are then utilized in the calculation of the Jacobian by simple multiplication and summation with finite-element terms at each rectangle in the mesh. For the auxiliary (across-strike) fields, the Jacobian terms are obtained by solving source vectors loaded with parabola coefficients used in the approximation to Maxwell's equations. Jacobian terms for the apparent resistivity ( p a ), the impedance phase (φ) and the vertical magnetic field ( K zy ) are then calculated utilizing the parallel- and auxiliary-field Jacobians. Comparison of Jacobian values obtained from reciprocity calculations and by differencing two forward solutions show that the reciprocity method is accurate and can be used to decrease the number of calculations required to obtain sensitivities by one to two orders of magnitude. 相似文献
116.
Richard A Shakesby David J Boakes Celeste de OA Coelho AJ Bento Gonçalves Rory PD Walsh 《Applied geography (Sevenoaks, England)》1996,16(4):337-355
In newly burnt and unburnt pine and eucalyptus forest in Portugal, overland flow and soil losses were monitored to assess the impacts of the following post-fire treatments: application of different quantities of logging litter; rip-ploughing compared with minimum tillage prior to planting eucalyptus seedlings; and clearance of pine needles and vegetation. Eucalyptus logging litter reduced soil losses by up to 95 per cent. The impact of pine logging litter was equivocal, but removal of pine needles increased soil losses elevenfold. Implications for soil longevity, soil quality and land management strategy are discussed. 相似文献
117.
Frédérick Delay Hélène Housset-Resche Gilles Porel Ghislain de Marsily 《Mathematical Geology》1996,28(1):45-71
A new method for solving the transport equation based on the management of a large numbe of particles in a discretized 2-D domain is presented. The method uses numerical variables to represent the number of particles in a given mesh and is more complex than the 1-D problem. The first part of the paper focuses on the specific management of particles in a 2-D problem. The method also would be valid for three dimensions as long as the medium can be modeled similar to a layered system. As the particles are no longer tracked individually, the algorithm is fast and does not depend on the number of particles present. The numerical tests show that the method is nearly numerical dispersion free and permits accurate calculations even for simulations of low-concentration transport. Because each mesh is considered as a closed system between two successive time steps, it is easy to add adsorption phenomenon without any problem of numerical stability. The model is tested under conditions that are extremely demanding for its operating mode and gives a good fit to analytical solutions. The conditions in which it can be used to best advantage are discussed. 相似文献
118.
119.
During the last decade, exploration and mining of modern-Tertiary heavy mineral beach and raised beach sands along the west
coast of South Africa has developed into a major industry. High resolution radiometric techniques have demonstrated their
use as a quantitative indicator of total heavy mineral concentration (THM) and also have the ability to discriminate between
sediments derived from different provenance terrains. Results indicate that it is possible to calculate the total heavy mineral
concentration from high resolution radiometric measurements, but the ability to quantify the concentration of individual mineral
fractions, such as ilmenite, requires further refinement of the method. Radiometric characteristics of the light mineral fraction
made it possible to distinguish between sediments from mixed marine-aeolian palaeoplacers and active present beach placers.
The effects of in situ alteration of the ilmenite fraction in the palaeoplacers were also reflected by subtle contrasts in
the radiometric character of the deposits. Furthermore, the uniform radiometric character of the sample population indicates
a common provenance for the heavy mineral suite and supports previous results which indicate the metamorphites of the mid-Proterozoic
Namaqua Province as the primary source terrain. The high resolution radiometric techniques discussed offer a broad spectrum
of applications in sedimentology and have the potential to aid heavy mineral exploration. With further refinement it can possibly
be used quantitatively in grade control during mining and beneficiation of heavy mineral deposits.
Received: 19 July 1996 / Accepted: 7 January 1997 相似文献
120.
Geologie en Mijnbouw - 相似文献