G4 key idea 2 Flashcards
(15 cards)
porosity
the amount (volume) of pore space in a rock or sediment, usually expressed as a percentage of total rock volume
factors effecting porosity
degree of sorting
amount of diagenesis - a loose unconsolidated rock has a much higher porosity than a rock that has undergone compaction and cementation
grain shape and packing (square/ rhombic)
permeability
the ability of a rock or sediment to transmit fluids such as water, oil or gas
can be expressed as a rate of flow of the fluid through the rock or sediment
factors effecting permeability
porosity
temperature - effects the viscosity of the fluid
viscosity of oil decreases with increasing temp
secondary permeability - presence of fractures such as joints or faults
water table
the water table is held level at which water sits within the ground
rocks above, are unsaturated and have air and water in their pore spaces
rocks below are saturated and only contain water in their pore spaces
shape of the water table generally follows the surface topography
with less relief, intersecting the surface at lakes and rivers
level of water changes depending on the season and level of rainfall
aquifers
aquifers
unconfined - open to the atmosphere, under atmospheric pressure and is recharges by rainwater directly above
confined - overlain by impermeable rocks and the groundwater held within it is under hydrostatic pressure
can only be replenished if it has recharge zones open to the atmosphere
groundwater controls
aquiclude - an impermeable rock that does not transmit water
aquitard - rock which has very low permeability and only allows the transmission of water at very low rates
piezometric surface - an imaginary surface to which groundwater rises under hydrostatic pressure, to produce springs
cones of depression - occur when there is a lowering of the water table in the vicinity of a well
usually due to abstraction of water
sources of pollution
problems with coal and metal-ore waste which contain sulphides
once exposed to oxygen and water, these create acid waters with metals in solution
oil and gas extraction and refining have resulted in contamination by organic pollutants
agriculture contributes to organic waste and hazardous chemicals (pesticides)
and the disposal of domestic and commercial waste requires detailed consideration of underlying geology
residence time
average length of time water spends in an aquifer before it exits (springs, wells, river)
short residence time means there’s less chemical reaction with the rocks but may still contain surface pollutants (pesticides and bacteria)
long resistance time means there’s higher concentrations of dissolved ions
aquifer filtration
process where soil and rock layers filter suspended particles and pollutants from infiltrating water as it moves downward into the aquifer
geophysical surveying
gravity
gravimeter, a suspended object hovered over ground to measure gravitational pull
looks for anomalies (differing densities in rock)
plots this data on a map, allowing us to visualise differing rock types (folds, faults, salt domes)
however, requires integration with other data to confirm
seismic reflections
artificial waves are generated on land (explosions or air guns at sea)
waves are reflected at layer boundaries within sedimentary sequences
location is accurately pinpointed using GPS
travel time of the waves back to the surface tells us its depth
we can then plot a seismic profile showing subsurface layering
however, high cost and environmental impact, won’t work with complex terrains
geochemical prospecting
water sampling
either surface water or groundwater can be collected and tested
background (normal) metal content of most natural waters is only a few ppb so an increase in this would indicate the presence of metallic mineral deposits
also allows us to test safety of drinking water
however, seasonal variations must be taken into account (dilution)
vegetation
metal elements can be taken up by plants through their roots
samples of leaves, twigs and seeds are collected from the chosen plant to check for elevated mineral content
can see mineral deposits deeply buried
however, also positives can occur due to natural surface enrichments and pollution
effectiveness of geochemical surveys
gives quant data
non-invasive (taking samples)
cheaper than borehole drilling
may give inaccurate results due to pollution
time consuming
does not enable large area to be explored
microfossils
collect rock samples or drill cuttings from different locations
then extract the microfossils
by crushing or dissolving to release them
why are microfossils used for correlation
microfossils can survive the drilling process
small so likely to be found in drill core
fossils can be identified by observing through a microscope
if the fossils are the same then the rocks are the same age
microfossils are abundant
