Khác biệt giữa bản sửa đổi của “Địa chất thủy văn”
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==Địa chất thủy văn và các lĩnh vực khác==
[[File:Лазоревый грот. Неаполь.jpg|thumb|300px|Painting of [[Ivan Aivazovsky]] ([[1841]]).]]
Hydrogeology, as stated above, is a branch of the earth sciences dealing with the flow of water through aquifers and other shallow [[porous medium|porous media]] (typically less than 450 m or 1,500 ft below the land surface.) The very shallow flow of water in the subsurface (the upper 3 m or 10 ft) is pertinent to the fields of [[soil science]], [[agriculture]] and [[civil engineering]], as well as to hydrogeology. The general flow of [[fluid]]s (water, [[hydrocarbons]], [[Geothermal (geology)|geothermal]] fluids, etc.) in deeper formations is also a concern of geologists, [[geophysics|geophysicists]] and [[petroleum geology|petroleum geologists]]. Groundwater is a slow-moving, [[viscosity|viscous]] fluid (with a [[Reynolds number]] less than unity); many of the empirically derived laws of groundwater flow can be alternately derived in [[fluid mechanics]] from the special case of [[Stokes flow]] (viscosity and [[pressure]] terms, but no inertial term).
The [[math]]ematical relationships used to describe the flow of water through porous media are the [[diffusion equation|diffusion]] and [[Laplace equation|Laplace]] equations, which have applications in many diverse fields. Steady groundwater flow (Laplace equation) has been simulated using [[electrical]], [[Elasticity (physics)|elastic]] and [[heat conduction]] analogies. Transient groundwater flow is analogous to the diffusion of [[heat]] in a solid, therefore some solutions to hydrological problems have been adapted from [[heat transfer]] literature.
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Traditionally, the movement of groundwater has been studied separately from surface water, [[climatology]], and even the chemical and [[microbiology|microbiological]] aspects of hydrogeology (the processes are uncoupled). As the field of hydrogeology matures, the strong interactions between groundwater, [[river|surface water]], [[geochemistry|water chemistry]], soil moisture and even [[climate]] are becoming more clear.
For example: Aquifer [[drawdown]] or [[overdrafting]] and the pumping of [[fossil water]] may be a contributing factor to sea-level rise.<ref>{{
==Định nghĩa và tính chất vật liệu==
Dòng 72:
==== Thủy động lực phân tán ====
Hydrodynamic dispersivity (α<sub>L</sub>, α<sub>T</sub>) is an empirical factor which quantifies how much contaminants stray away from the path of the groundwater which is carrying it. Some of the contaminants will be "behind" or "ahead" the mean groundwater, giving rise to a longitudinal dispersivity (α<sub>L</sub>), and some will be "to the sides of" the pure advective groundwater flow, leading to a transverse dispersivity (α<sub>T</sub>). Dispersion in groundwater arises because each water "particle", passing beyond a soil particle, must choose where to go, whether left or right or up or down, so that the water "particles" (and their solute) are gradually spread in all directions around the mean path. This is the "microscopic" mechanism, on the scale of soil particles. More important, on long distances, can be the macroscopic inhomogeneities of the aquifer, which can have regions of larger or smaller permeability, so that some water can find a preferential path in one direction, some other in a different direction, so that the contaminant can be spread in a completely irregular way, like in a (three-dimensional) delta of a river.
Dispersivity is actually a factor which represents our ''lack of information'' about the system we are simulating. There are many small details about the aquifer which are being averaged when using a [[macroscopic]] approach (e.g., tiny beds of gravel and clay in sand aquifers), they manifest themselves as an ''apparent'' dispersivity. Because of this, α is often claimed to be dependent on the length scale of the problem — the dispersivity found for transport through 1 m³ of aquifer is different from that for transport through 1 cm³ of the same aquifer material.<ref>http://www.cof.orst.edu/cof/fe/watershd/fe537/labs_2007/gelhar_etal_reviewfieldScaleDispersion_WRR1992.pdf</ref>
==== Khuyết tán ====
Diffusion is a fundamental physical phenomenon, which [[Einstein]] characterized as [[Brownian motion]], that describes the random thermal movement of molecules and small particles in gases and liquids. It is an important phenomenon for small distances (it is essential for the achievement of thermodynamic equilibria), but, as the time necessary to cover a distance by diffusion is proportional to the square of the distance itself, it is ineffective for spreading a solute over macroscopic distances. The diffusion coefficient, D, is typically quite small, and its effect can often be considered negligible (unless groundwater flow velocities are extremely low, as they are in clay aquitards).
It is important not to confuse diffusion with dispersion, as the former is a physical phenomenon and the latter is an empirical factor which is cast into a similar form as diffusion, because we already know how to solve that problem.
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Gridded Methods like [[finite difference]] and [[finite element]] methods solve the groundwater flow equation by breaking the problem area (domain) into many small elements (squares, rectangles, triangles, blocks, [[tetrahedron|tetrahedra]], etc.) and solving the flow equation for each element (all material properties are assumed constant or possibly linearly variable within an element), then linking together all the elements using [[conservation of mass]] across the boundaries between the elements (similar to the [[divergence theorem]]). This results in a system which overall approximates the groundwater flow equation, but exactly matches the boundary conditions (the head or flux is specified in the elements which intersect the boundaries).
[[Finite differences]] are a way of representing continuous [[differential operators]] using discrete intervals (''Δx'' and ''Δt''), and the finite difference methods are based on these (they are derived from a [[Taylor series]]). For example the first-order time derivative is often approximated using the following forward finite difference, where the subscripts indicate a discrete time location,
: <math>\frac{\partial h}{\partial t} = h'(t_i) \approx \frac{h_i - h_{i-1}}{\Delta t}.</math>
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====Áp dựng mô hình phần tử hữu hạn====
Finite Element programs are more flexible in design (triangular elements vs. the block elements most finite difference models use) and there are some programs available ([http://water.usgs.gov/software/sutra.html SUTRA], a 2D or 3D density-dependent flow model by the USGS; [[HYDRUS (software)|Hydrus]], a commercial unsaturated flow model; [[FEFLOW]], a commercial modelling environment for subsurface flow, solute and heat transport processes; OpenGeoSys, a scientific open-source project for thermo-hydro-mechanical-chemical (THMC) processes in porous and fractured media;<ref>{{
====Áp dụng mô hình khối lượng hữu hạn====
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*[http://www.igrac.net/ IGRAC International Groundwater Resources Assessment Centre]
*[http://www.agc.army.mil/ US Army Geospatial Center] — For information on OCONUS surface water and groundwater.
*[http://hydrooffice.org/Software/ListOfTools.aspx HydroOffice.org] list of free/commercial tools for Hydrogeology
[[Thể loại:Địa chất học]]
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