主要區別——孔隙度與滲透率

孔隙度和滲透率都是岩石和土壤的特性。孔隙度和滲透率之間的主要區別在於孔隙度是岩石之間空間的度量，而滲透率是流體在岩石之間流動的容易程度的度量。

什麼是多孔性(porosity)？

孔隙度測量岩石之間的空隙。它被認為是岩石中的空隙（或孔隙）體積與總體積的比值，通常用分數或百分比表示。由於流體可以佔據岩石之間的自由空間，孔隙度也是衡量岩石能容納多少流體的一個指標。

There are several methods to measure the porosity of a sample of soil. One method used to measure porosity is called the Archimedes method. To test porosity using this method, the mass of a sample of soil is first measured when the soil is dry. Let us call this mass . Then, water is added to the sample until the soil sample is saturated and the mass of the soil sample and water,  is measured. Next, the saturated sample is immersed in water, and the “mass” of the submerged sample,  is measured again. (Note that in fact we are measuring weights, however, the factors of  cancel out, so you can treat the mass readings taken from a spring balance as readings of weight).

Then, the mass of water added to the sample is  . If the density of water is  , then the volume of pores in the soil sample is  .

The total volume of soil is the volume of the fluid displaced when the sample is submerged in water. From the Archimedes’ Principle, the total volume is given by  . Then, the porosity  is given by  . Therefore,

另一種測量孔隙度的方法是使用波義耳定律。在這種方法中，氣體被用來填充土壤顆粒之間的孔隙。根據波義耳定律，對於在恆定溫度下質量固定的氣體，氣體的壓力與其體積成反比。為了利用波義耳定律確定孔隙度，使用了兩個相連的腔室。這些腔室的容積是已知的，這些腔室中氣體的壓力可以用壓力計測量。

Initially, the two chambers are separated by closing a valve between them. A container with a known volume, called the core, is filled with the soil sample. Let us call the volume of the core . The core is placed inside one of the chambers and  the air is taken out of this chamber. Gas is still present in the other chamber, and the air pressure inside this chamber is measured. Let us refer to the volume and pressure in this chamber as  and  .

Measuring Porosity using the Boyle’s Law method.

Next, the valve between the two chambers is opened, allowing for air from the first chamber to fill up the second chamber as well. The pressure in the chambers is measured again, and let this pressure be  . Using Boyle’s law, the total volume occupied by the gas, which we will call  , can now be calculated:

The volume  can be given by,

where includes the volume of the second chamber and the tubing between the two chambers. Now, the above expression allows us to calculate the volume of pores  . gives the total volume of the soil sample. The porosity  is then given by,

什麼是滲透(permeability)？

滲透率是測量流體在岩石之間流動的容易程度。具有高滲透性的土壤樣品允許液體容易地流過，而液體更難流過具有低滲透性的土壤。例如，可以用具有高滲透性的材料建造路面，這樣水就不會積聚在路面上的水坑中。

Material forming “permeable pavements”.

Mathematically, permeability is described in Darcy’s law. Darcy’s law is a formula describing the flow rate of fluid through a porous medium. According to this law, if a fluid with a viscosity of  flows through a volume of porous material with a cross-sectional area   for a distance  , then the total volume of fluid discharged per second is given by:

where  is the pressure at the start and  is the pressure of fluid after flowing through length .  The c***tant  in this formula is defined as the permeability of the medium. The unit of  is square metres (m2).

多孔性(porosity)和滲透(permeability)的區別

它測量什麼

孔隙度測量岩石之間或土壤中的空隙量，作為總體積的一部分。

滲透率衡量流體流經物質的容易程度。

單位

孔隙度是體積的比率，所以它沒有單位。

滲透率以面積為單位（在國際單位制中為m2）。

Image Courtesy:

“A permeable paver dem***tration, Austin’s Ferry, Ta**ania, Australia” by JJ Harrison ([email protected]) (Own work) [CC BY-SA 3.0], via Wikimedia Comm***