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Unformatted text preview: EAS 44600 Groundwater Hydrology Lecture 4: Porosity and Permeability Dr. Pengfei Zhang Porosity of Earth Materials The porosity of earth materials is defined as the part of rock or soil that is void space, often expressed as a percentage: T v V V n = (4-1) where n is the porosity, V v is the void volume, and V T is the total volume. Porosity has the units of 3 . . . 3 V E R voids L L . To describe porosity, or any other aquifer parameter, it is useful to first define the concept of representative elementary volume (R.E.V.). The R.E.V. is a volume of aquifer over which the porosity, for example, is describable by a single value. This concept is necessary because a volume smaller than the volume that fits the definition of a representative elementary volume will yield different aquifer parameters depending upon where one “places” the R.E.V on the sample. To illustrate, let us take a look at Figure 4-1. Both diagrams represent the same porous media. In the diagram on the left, the squares, which represent a poor choice for a R.E.V., delineate volumes for which very large differences in porosity would be obtained. In the diagram on the right, the R.E.V.’s are sufficiently large that an equivalent porosity is obtained for both of the R.E.V.s. Figure 4-1. Representative elementary volume (R.E.V.). Some typical kinds of porosity associated with various rocks are shown in Figure 4-2. The interstitial porosity of rocks (Figure 4-2 a through d) is referred to as primary porosity , whereas the fracture or solution porosity (Figure 4-2 e and f) is called secondary porosity . 4-1 Figure 4-2. Relation between texture and porosity. (a) Well-sorted sedimentary deposit having high porosity; (b) poorly sorted sedimentary deposit having low porosity; (c) well-sorted sedimentary deposit consisting of pebbles that are themselves porous, so that the deposit as a whole has a very high porosity; (d) well-sorted sedimentary deposit whose porosity has been...
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- Spring '10
- representative, Sediment, High porosity, Pengfei Zhang Porosity