Pete’s Lab: Porosity and Permeability


Hello everyone I’m Peter Bower welcome to Pete’s Lab today , we have another one of our short little lectures vignettes if you will on
subjects related to ground fields and groundwater today’s topic is porosity Porosity is the percentage of open space in rock or regolith usually we think of rock as being completely
solid the but in many cases there are fractures or cracks sometimes open spaces and that can provide some porosity Regolith on the other hand is the lose
Material at the surface of the earth mud sand gravel and so forth and it is composed of particles and those particles have
spaces between them which can give you a rather large amount of open space or as we will see porosity there’s some other subjects that we’re going to
touch upon permeability bulk volume
bulk density density of particles volumes of the particles and so forth we’ll make some measurements and we’ll discuss
their significance I have here a pebble quartz and if one were to try to determine the volume
of this pebble most people understand that if we have a graduated cylinder in this case a hundred and ten milliliters
of water that by dropping the pebble into the water the water level will rise and we should be able to determine the volume of the pebble by difference so let me drop this in here and we can see that the water level has gone up and the new water level is a hundred and thirty milliliters meaning that the difference one thirty minus one
ten twenty milliliters is the volume of a pebble milliliters we should also understand is the
same volume as a little cube one centimeter on a side that is a cubic centimeter or CC is exactly the
same volume as a milliliter while most understand that one can determine
the volume of a pebble by volume different by dropping it into a graduated cylinder with some liquid in it it gets a little more confusing when we have more than one particle but it’s really the same principle if we were to dump all these particles in this case a nice pile of half-inch plastic bedes into a liquid surely the volume would rise and what would we have done we would have measured the volume of all of these particles not just one but many here for instance is a beaker of sand consisting
of hundreds and hundreds of thousands of particles we could do the same thing by dropping
sand into a water and watching the water level rise in determining the volume of all particles that’s exactly what we’re going to do okay so let’s begin the experiment before you begin the experiment you should all have a copy of the handout particles porosity bulk density and particle density if you don’t have it pause the video and go to the web site and print it out and then start watching the video again so lets begin the first thing that we’ll want to do is
to determine the weight of this beaker okay the weight of the beaker is one hundred
and seventy one point three grams so if you would please enter that data on you’re sheet in item number two the way of the beaker and the next thing we’re going to do is determine the weight of the beaker
plus the particles again the particles are the plastic that is styrene beads the is one half inch in diameter and we’re going to use a weight to assist us they’re we’re getting close and the wait of the beaker plus the beads is five hundred and thirty nine point eight grams so please enter that on your sheet five thirty nine-point eight by subtracting the weight of the beaker from the weight of the beaker plus the particles we
can find out the weight of the particles and that turns out to be three hundred and sixty
six point seven grams se we’ve determined one thing and that is the weight of the particles the next thing that we could do that
would be Fairly straightforward it would be to determine the bulk volume that is the volume of the
particles plus the spaces between the particles and we can do that by putting the particles into the graduated cylinder try to get them level
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the divisions on this graduated cylinder
are in increments of ten milliliters the volume of the beads is four hundred and forty Milliliters so please enter that on your sheet and now we’re going to determine the volume of the particles I have here some
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water that I put some dye in and I’m going to put this into the graduated cylinder and the volume here is three hundred and ninety milliliters and the next step the dramatic step is to pour the beads into this graduated cylinder exactly
as we did for the single quartz pebble before only were doing it maybe a hundred times over and we can see that the water level has
risen of course and we can also note that the spaces between the particles are completely
filled with liquid here the pour space that is the open space between
the particles is connected and fluids water in this case can move freely between the particles this is an important term called permeability it means
simply the ability of fluids to be transmitted by rock or regolith and clearly the spaces between these particles are connected and
fluid moves easily between them the new volume is six hundred and forty milliliters
so if you enter that on your sheet and then you can see by looking at your sheet that we can now determine
the volume of the particles by subtracting the initial volume of the water from the final that is three ninety from six forty gives us two hundred
and fifty milliliters that’s the volume of the particles now since the bulk volume was the volume of the particles plus the volume of the pour space we can also now determine the amount of open space parting these particles the original bulk volume was four hundred and
forty milliliters the volume of the particles was two hundred and fifty
so subtracting two fifty from four forty gives us one hundred and ninety milliliters which is the volume of the open space or pour space between the particles the rest of this is really some simple calculations we can determine the porosity that’s number nine
on your sheet which is the percentage of open
space and we do that by dividing one ninety the volume of the pour space over the total volume the bulk volume
of four forty and we get forty three percent so forty three percent of the volume of these
particles is open space and conversely the particles take up fifty seven percent of the space we can also now determine the density remember
density is the weight divided by the volume so we could do the bulk density which would be the weight which is we determined right at the beginning three
hundred sixty six point seven grams divide that by the bulk volume of four hundred and forty milliliters and we find out that the bulk density is point eight three grams per milliliter or grams per cubic centimeter that’s particularly useful much more useful is the density of the particles themselves if we
know the density of the particles themselves it may give us some information that would be very useful in determining what the particles are made of in this case we divide three hundred sixty six point seven of
weight of the particles by the volume
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of the particles two hundred fifty milliliters and we find out that the density of these styrene beads is one point four seven grams per cubic centimeter which makes sense the density of water is one gram per cubic centimeter and of course these beads have sunk to the bottom if they were less dense than water they would float and we wouldn’t be able to do the experiment okay so we have now shown you how to measure porosity the percentage of open space in a collection of particles in this case the percentage of open space in
the pile of plastic beads is about forty three percent we weighed the particles we determine the bulk
volume of the particles and pour space we determine the volume of the particles the
density of the particles and so forth all these measurements are a very important for studying groundwater and the flow of fluids through regolith on the surface of the earth we could have done the same experiment
with this with this sand and determined it’s porosity and also the density of the particles permeability to remind you again is the
ability of fluids to flow through rock or regolith in order for that to happen of course there
has to be porosity but the porosity must also be connected so
that the liquid can move from one place to another a good example of a material that has very high porosity but dosen’t allow fluids to move is Styrofoam take a Styrofoam cup and if you look at the walls of the styrofoam cup you’ll see that
it’s made of very tiny bubbles foam made from styrene and those bubbles are not connected % they’re all individually wrapped in plastic and of course we know the Styrofoam cup t does not was allow liquids to pass through it there are some rocks such as pumice that has
similar properties very very light and airy lots of open space but the air space is not connected at all so permeability is the ability of um fluids to move within sand lets say or even through rock where you have fractures and this number of porosity we’ll see later on the very important um piece of information that we need in order to use Darcy’s
law which allows us to calculate the rate at which water flows in sub surface until next time this is Peter Bower in Pete’s Lab

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