Week 8: Groundwater, Non-renewable Energy, Geophysical Exploration

Question 1

Groundwater consists of ___% of the world’s fresh water supply.

Answer 1

24%

Question 2

What is the “water table”?

Answer 2

The “surface” where the groundwater is at zero pressure.

Question 3

Define zone of “aeration”, zone of “saturation”, and “porosity”.

Answer 3

Zone of aeration: the zone above the water table in which water cannot be pumped by wells (not deep enough).

Zone of saturation: open spaces in sediment and rock that are completely filled with water (groundwater)

Porosity: determines the storage capacity for water.

Question 4

Differentiate between primary and secondary porosity.

Answer 4

Primary: water stored between pebble-like sand grains.

secondary: water stored in the fractures of rock and sediment.

Question 5

What are “aquitards” and “aquifers”?

Answer 5

Aquitard: impermeable layers that prevent water movement (ex. clay)

Aquifer: permeable rock strata/sediment that transmits water freely (sands, gravels)

Question 6

What are the causes of karst openings, sinkholes and caverns?

Answer 6

Groundwater weathering and erosion (ex. stalactites on cave ceilings).

Question 7

What is the driving force for the movement of groundwater?

Answer 7

Gravity.

Question 8

What two major factors influence the storage and movement of groundwater?

Answer 8

Porosity (percentage of total volume of rock that consists of pore spaces).

Permeability (ability of a material to transmit a fluid).

Question 9

What is the equation for the “Hydraulic head”, the height of the water table from sea level?

Answer 9

Total hydraulic head (h) = Elevation head (z) + Pressure head (w)

Elevation head = height from sea level to the tip of the measurement (drill)

Pressure head = height of the tip of the measurement to the top of the water table.

Question 10

What is Darcy’s Law?

Answer 10

Hydraulic gradient (i) = the “slope” of the water table; total hydraulic head / distance

v = k (h/L) = k (i)

Where v is the speed (m/s), k is the coefficient of permeability, h is the hydraulic head and L is the distance between the two drill-points.

greater difference in h = faster groundwater flow.

greater difference in height of water table = faster groundwater flow.

greater coefficient of permeability = faster groundwater flow.

Question 11

Define “artesian”.

Answer 11

Artesian: any groundwater under pressure in a confined aquifer that rises above the level of the aquifer.

Question 12

What is a consequence of over-pumping groundwater using wells?

Answer 12

Drawdown (lowering) of the water table.

The water available to recharge the aquifer is much less than that withdrawn (may cause drought).

The ground sinks when water is pumped faster than natural recharge can replace.

Question 13

What are some ways groundwater may be contaminated?

Answer 13

• Highway salt
• Fertilizers
• Organics and Bacteria
• Pesticides
• Chemical and industrial materials
• Leachate from landfills
• Heavy metals from mining
• Acid drainage
• Biological contamination (leaking septic tank in permeable rock)

Question 14

What are two modes of contaminant migration by groundwater?

Answer 14

1. Advection (contaminants “go with the flow”, move according to Darcy’s Law)
2. Diffusion (movement under concentration gradient)

f = -D(dC/dx)

f = mass transfer per area per unit time
C = concentration
D = diffusion coefficient

Question 15

What are LNAPLs?

Answer 15

Light Non-Aqueous Phase Liquid: do not dissolve, but have some soluble components. Lighter than water and floats to top of water table.

Question 16

What are DNAPLs?

Answer 16

Dense Non-Aqueous Phase Liquids: do not dissolve, but have some soluble components. Heavier than water and sink until they reach the aquitard.

Question 17

What are some solutions to groundwater contamination?

Answer 17

• Natural attenuation (solution to pollution is dillution!)
• Excavation
• Pump and treat
• Chemical flushing
• Injection steam flushing
• Vapour extraction

Question 18

Which period does almost all the world’s coal supply come from?

Answer 18

Carboniferous period.

Question 19

How does petroleum form?

Answer 19

Gradual “cooking” of simple aquatic organisms concentrated in source rocks (may produce biogenic gas, methane.)

Question 20

Where may we find petroleum and other geologically-sourced oils?

Answer 20

Structural traps: oil is less dense than water, so it may rise to the top of domes caused by 3D foliation (ex. Zagros Mountains in Iran provided easily-accessible oil) > porous, cap rock is impermeable to oil & gas

Question 21

Name the types of petroleum traps.

Answer 21

Stratigraphic traps (pinch-out, unconformities, reefs).

Structural traps (anticlines, fault traps, salt domes).

Question 22

Describe “seismic exploration”.

Answer 22

• Analyze geometry of rock [for petroleum traps]
• Ships send shockwaves which bounce off and come back (echolocation)
• Trucks thump repeatedly at low frequency to go deep into ground

Question 23

List two examples of unconventional fossil fuel deposits.

Answer 23

1. Heavy oil sands (tar sands, contain sediment, water and bitumen)
2. Oil shale (not shale but fine-grained limestones that contain enormous amounts of kerogen).
3. Shale gas (methane trapped in pores of shale or other fine sedimentary rock

Question 24

What is “fraccing”, and what are the consequences associated with it?

Answer 24

Aquitard rock is split open with high pressure water blasted through a tube with lacerations. Aquitard then becomes aquifer.

• permanently fractures shale rock
• groundwater may become contaminated
• don’t have a whole lot of information on how fraccing can impact surrounding geological environment

Question 25

List four geophysical properties of rocks and minerals and the Geophysics used to identify them.

Answer 25

1. Density (gravity, seismic methods)
2. Conductivity/resistivity (electrical methods, ex. radar)
3. Magnetics (electrical, magnetic)
4. Radioactivity (natural; downhole logging methods)

Question 26

What is Gravity Surveying used for?

Answer 26

• Measure variations in Earth’s gravitational pull related to density
• Detect large features (intermountain basin fills, buried glacial valleys)
• Ice Mass Changes (continental ice sheets like Greenland and Antarctica)

Question 27

What is Seismic Exploration used for?

Answer 27

• Measure time taken for waves to travel through material
  (v = higher in metallic, lower in sediments)
• Petroleum exploration

Question 28

What are the two types of seismic waves? Which ones are fastest?

Answer 28

1. Body waves (P and S)
2. Surface waves

P waves have the fastest travel time and can travel through liquids and solids. S waves are slower and cannot travel through liquid.

Question 29

What are reflection seismics used for, and how do they work?

Answer 29

• Used extensively for petroleum exploration
• Observes the travel times of reflected P-waves
• Boat sends shockwaves which bounce back from oil surfaces to the hydrophones dragged behind the boat
• Vibrosis Trucks bounce at low frequency so waves do not scatter quickly and have time to reach desired surface

Question 30

What is Ground Penetrating Radar used for, and how does it work?

Answer 30

• To find geological structures with varying electrical properties
• Electromagnetic E is transmitted through soil and rock (backscattered from boundaries of strata w/ contrast in elec. properties)

Question 31

What is resistivity surveying used for, and how does it work?

Answer 31

• Two electrodes pinned to the ground, sending current through
• Measures elevation and resistivity of certain geological bodies

Question 32

What is electromagnetic surveying used for, and how does it work?

Answer 32

• The strength of the SECONDARY magnetic field induced by the PRIMARY magnetic field is directly proportional to the CONDUCTIVITY of a material.
• Can be used for mineral exploration (ex. diamond)