Download Porosity - Geology Rocks

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The specification
states that you need
to be able to:
 Define and explain
the following terms:
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Porosity
Permeability
hydrostatic pressure
hydraulic gradient
Aquifers
water table.
Porosity and Permeability
 What is meant by the terms porosity and
permeability?
Porosity: Amount of pore space between grains:
Porosity = Volume of pore space x 100%
Total volume of rock
Porosity and Permeability 2
Factors influencing porosity:
 Grain shape
 Packing
 Amount of cement and smaller grains
between grains.
 Sedimentary rocks have a greater
porosity as a rule than igneous and
metamorphic rocks that are made of
interlocking crystals.
 Some examples:
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Limestone
Sandstone
Shale
Clay
Sand
Gravel
Granite
Basalt
10%
18%
18%
45%
35%
25%
1%
1%
Permeability:
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Definition: The ability of a liquid to
pass through a rock.
Permeable = water can pass
through, varies from high to low.
Impermeable = Cannot pass
through.
The larger the pore spaces the more
permeable the rock will be
especially if the pore spaces are well
connected.
A moderately porous rock may not
necessarily be permeable, as the
pore spaces may not be connected.
E. g. shale has a porosity of 18%
similar to sandstone but clay
minerals hold onto the water and so
it is impermeable.
Post Depositional Changes:
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1.
2.
3.
4.
These can affect porosity and
permeability:
Faulting (increase)
Jointing (increase)
Cementation (decrease)
Metamorphism will crystallise the
rock and reduce the porosity.
Hydrostatic Pressure
Normal water pressure exerted at any point in a water
body at rest.”
 This increases with depth.
Hydraulic Gradient
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Forms when areas of low and
high hydrostatic pressure are
connected e.g. within an
artesian well.
The water in the high-pressure
zone will flow towards the lowpressure zone (at the surface).
In artesian systems the well is
dug into the aquifer below the
water table.
This creates a strong hydraulic
gradient and water keeps
flowing.
This will become clear when
we have covered artesian
basins.
Aquifers
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The water table tends to follow the
surface topography.
Water Table: Surface of the
saturated groundwater zone.
If it happens to occupy a position
above land then water seeps out
forming lakes, marshes that fill until
they reach the water table level.
Where a steep surface and the water
table meet then springs will occur.
This is particularly common in
limestone, which are porous and
can dissolve in water forming cave
systems.
Springs will occur where the
permeable aquifer meets an
impermeable rock below.
Aquifers 2
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There are different types of
aquifer:
Confined aquifers: This is
an aquifer between 2
impermeable rocks.
The 2 sandwiching rocks
are called aquicludes.
Unconfined aquifers: Not
covered by impermeable
rock.
Water table level may vary
depending on the season.
Aquifers 3
Perched aquifers: Smaller
aquifers that hold amounts of
water higher than the
regional water table e.g.
valley sides.
 These can often lead to the
formation of springs.
Water can be extracted from
these aquifers in a number of
ways:
WELLS
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These are dug or drilled
down into an unconfined
aquifer and must extend
below the water table.
The water is either
pumped out mechanically
or drawn out by hand.
The main problem is that
over time if used
excessively output may
exceed the main meteoric
input and water table
level can drop locally.
ARTESIAN WELLS
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A well is dug or drilled into
a confined aquifer.
 This time no pumping is
needed as water in the
aquifer is under
considerable pressure.
 The well acts like a
pressure release valve and
water flows out under it’s
own pressure.
 E. g. the Great Australian
Artesian basin in
Queensland.
Artesian Wells 2
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In artesian systems the well is
dug into the aquifer below the
water table.
This creates a strong
hydraulic gradient and water
keeps flowing.
When the aquifer is folded
producing a broad synform
then an artesian basin is
formed e.g. the Australian
basin covers 1.5 million km2.
Water is picked up in the
mountains in the east and the
water slowly moves through
the rock west towards an arid
region, where wells up to 1.5
km deep.