Cambridge Checkpoints 2014 HSC Physics Chapter 4 Geophysics Study Notes
Elasticity:
Density:
Thermal conductivity: tendency to return to original shape and size when deforming forces are removed; deforming forces can be compressive, tensile or shearing; stress is the ratio of the deforming force to the area of application (

= F / A ); strain is the ratio of the deformation to the original dimension (

= x / l) ; elasticity measured by the elastic modulus ; two kinds
– bulk modulus and shear modulus; these are equal to the ratio of the applied stress to the resulting strain defined as mass per unit volume (units are kg m
–3
); density affects other parameters e.g. the speed of sound and earthquake waves in earth materials the rate of heat flow through one unit of distance spread over one unit area in one unit of time caused by one unit of temperature difference (units are J m
–1
s
–1
K
–1
); significant is determining the rate of heat transfer the rate at which temperature changes with distance (units are K m
–1
) Thermal gradient:
Fourier’s
Law:
Magnetic susceptibility:
Rock
(permanent ) magnetism
Electrical conductivity
Governs the amount of heat transferred: Q = kA

T/x , where Q is the heat transferred
(J), A is the heat transfer area(m 2 ),

T is the temperature difference, k is the thermal conductivity of the material (J m
–1
s
–1
K
–1
) and x is the thickness of the material. determines magnetic response of materials to a magnetizing field; defined as the intensity of magnetisation ( I ) when placed in a magnetic field ( H ) of unit intensity. The key formula is I = kH , where k is the magnetic susceptibility. Generally, them more ferromagnetic materials in the sample, the greater the value of k .
Many rocks display permanent magnetism (provided the temperature is below the Curie point – a bit above 800 K); this often reflects the direction of the Earth’s magnetic field as the rock cooled below the Curie temperature; important in paleomagnetism symbol

= I / EA ; where I is the current (A), E the electrical field (V m
–1
), and A the cross section area (m 2 ); the greater this quantity the more easily current can flow; different rock types have different values of conductivity (or resistivity (see below)
Symbol

; defined as the inverse of

; hence

= 1/

Electrical resistivity
Seismic • shock waves (sound) sent through the earth
• path followed is monitored for travel time (gives speed), reflection, refraction and
Gravitational
Magnetic absorption
• waves travel faster in more rigid and/or less dense rocks
• waves are reflected at rock type boundaries; refracted when the density/rigidity changes continuously
• hence able to infer information about structure, density and shape
• changes in rock density are reflected in small changes in g above the ground
• hence sensitive measurements of g can reveal dense ore deposits, structural changes and boundaries and salt domes
• very early experiments with pendulums showed the density of the core rocks to be less than the underlying mantle
• accurate measurement of local magnetic field variations gives information about the magnetic susceptibility of local rock types, and hence local rock types and minerals
• presence of rock (permanent) magnetism reveals presence of ferromagnetic minerals
Paleomagnetic
• reveals information about tectonic plate movements and speed
Electrical • conductivity of rocks gives information about rock types
• electrical currents can be used to induce polarisation of certain rock types
Electromagnetic
• ground-penetrating radar can be used to image structures and rock characteristics
Radiometric below the surface
• careful measurements of radioactive isotope emissions can identify the isotopes
(and hence minerals) involved
• isotopic abundances can also be measured
• consequently, known isotopic half-lives can determine age of rocks
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Cambridge Checkpoints 2014 HSC Physics Chapter 4 Geophysics Study Notes
Geothermal
• thermal properties of rocks provide information on rock types, structures and fluids
• can be measured by measurements of heat flow
• can reveal volcanism (e.g. rifts, mid-ocean ridges) and tectonic (plate) structures
 passive remote sensing uses natural radiation (e.g. sunlight); the spectrum of reflected radiation is measured
 different surface types reflect different parts of the spectrum (e.g. visible, infrared, ultraviolet) differently
 electromagnetic radiation is emitted from terrestrial surfaces naturally and human-made
 stereographic photography allows topographical feature mapping
 sensors include radiometers, photometers, charge-coupled devices, film photography
 active remote sensing emits radiation (e.g. RADAR, LiDAR); can measure time delay data, and precise measurements of distance and altitude data

LiDAR includes use of LASER light

Doppler RADAR allows velocity measurements (e.g. moving meteorological systems)
 remote sensing allows measurements in inaccessible/dangerous/fragile areas
 remote sensing possible from aeroplane, ocean buoys, ships, helicopter, balloon, kite and satellite
 satellite observation also allows observation of atmospheric systems (e.g. El Niño )
 aerial/satellite measurements provide time-series data on climate, vegetation, pollution, sea temperatures and weather systems.
 different vegetation types reflect/absorb various bands of the electromagnetic spectrum differently
 polluted air/water reflect/absorb various bands of the electromagnetic spectrum differently
 surface mineralization also affects reflectance/absorbance of various bands of the electromagnetic
 spectrum; observable from aerial/satellite spectrometers aerial surveys with sensitive radiation detectors can locate surface and near-surface radioactive minerals
(gamma radiation); for example uranium-bearing minerals
 iron-bearing ores can be detected by aerial mapping of magnetic anomalies
 experiments by Jean Richer showed that g varied over the Earth’s surface

Richer (1671) measured g by using a pendulum, using
T
=
2 p l or g
=
4 p
2 l g T
2
 where T is the period of the pendulum and l is the length

Richer found that as he approached equatorial latitudes the value of g increased
 reason: Earth is an oblate spheroid, not spherical (‘fatter’ at equator)
 rotation of Earth also reduces g at equator (centrifugal force)
 local variations also caused by high density mineral ore bodies in vicinity of measurements or neighboring
mountains
 mass of Earth can be calculated (assuming uniform density and spherical shape), from g
=
G
M
R 2
E or
M
E
= gR
2
G
 where G is the universal constant of gravitation and R is the radius of Earth
 mass of Earth can also be calculated from satellite data, from: r
3
=
GM p
2
E
T
2
4
 where r is the orbit radius of the satellite (NB not the altitude) , and T is the satellite period
 satellites that are not geosynchronous (e.g. communication satellites) travel above different paths of the Earth’s surface
 as the value of g changes their centripetal acceleration and path will change slightly
 this enables g to be measured
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Cambridge Checkpoints 2014 HSC Physics Chapter 4 Geophysics Study Notes
 simple gravimeters use spring-mass systems; when g increases the extension of the spring increases
 more sophisticated versions are evacuated, with very sensitive springs
 such instruments need to be perfectly level so that the weight of the mass is directed vertically downwards
 gravity surveys are usually used for detection of high or low density material under the surface
 these changes g
 called gravity anomalies
(changes from the ‘expected’ value, indicating ); often measured in mgal
 the unit is a tribute to Galileo)

1 mgal is equal to 1

10 -5 m s
–2
(or 0.01 N kg
–1
)
 most measurements are not absolute, but with reference to a datum station (reference point)
 high density ore bodies will increase g; low density ones will decrease g
 g values are also affected by latitude (Earth is not spherical), and altitude ( g reduces with altitude)

to be sure the g anomalies mean ore/oil/water under the ground, necessary to remove these other effects:
- latitude correction (shape of Earth)
- free air correction (altitude)
- Bouguer correction (where altitude is due to extra mass underneath you)

g(corrected) = g(measured) + free air correction – Bouguer correction – latitude correction

typical values of these corrections (from the 2006 BOS examination):
- free air: 0.3 mgal per metre of altitude above sea level
- Bouguer: 0.1 mgal per metre above the datum station (reference point)
- Latitude: 0.8 mgal per metre above the datum station
 g anomalies > zero can mean(shallow) high density material

examples are chromite (chromium ore), hematite (iron ore) and barite (barium ore)
 ganomalies < zerocan mean(shallow) low density material
 examples are halite (rock salt), weathered kimberlite (diamond bearing), diatomaceous earth (used in cat litter, insecticide and dynamite), underground lakes and oil reservoirs

P waves (primary waves)are longitudinal compression waves (as are sound waves)

S waves (secondary waves) are shear waves and are transverse waves

P waves travel faster than S waves and arrive first
 both kinds are referred to as body waves (they travel through the body of the Earth)

S waves cannot pass through liquids and gases (these have no resistance to shear)

P waves can travel through solids, liquids and gases
 the ‘shake’ direction of P waves is parallel to their direction; S waves shake perpendicular to their direction
 both kinds of wave refract when the speed of the waves changes with changing rock/material; both follow
Snell’s Law at a boundary (in the form sin q
1
= sin
); they can also totally internally reflect v
1 v
2 q
2
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Cambridge Checkpoints 2014 HSC Physics Chapter 4 Geophysics Study Notes
 both wave types can reflect at these boundaries (same laws as for sound and light)
 refraction of S waves tend to be greater than that of P waves (lower speed)

S waves tend to have a longer range
 speed of P is affected by the materials bulk modulus , shear modulus and density ; speed of S waves depends on shear modulus and density
 typical P wave speeds are 1 to 14 km s
–1
; typical S wave speeds are 1 to 8 km s
–1
 since these speeds tend to change gradually, typical paths within the Earth are curved; see below
 both geophones and seismometers record the acceleration (due to shaking) of the Earth’s surface
 one part of each device is attached to the Earth’s surface and the other a heavy inertial mass that is free to move
 need a pair of each device
– one to record horizontal motion; the other to record vertical motion
 the seismometer shown records shaking on a paper record; the geophone shown produces a voltage output seismometer (horizontal measurement) geophone (vertical measurement)
 as the seismometer table shakes horizontally, the heavy spherical mass lags behind (due to inertia) and the pen records the shaking on the moving paper strip
 as the geophone table shakes vertically, the heavy permanent magnet shakes, causing an emf in the
 coil surrounding it; this emf is amplified and recorded
 seismometers tend to be more sensitive than geophones
 both geophones and seismometers can be used for mineral exploration, to measure time of travel of reflected waves from explosions from measured distances
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Cambridge Checkpoints 2014 HSC Physics Chapter 4 Geophysics Study Notes
 current scientific picture of the interior of the Earth is: solid inner core, liquid outer core, warm and soft inner mantle, cool and brittle outer mantle, and a cold solid crust.
 evidence for this structure based on the paths of S
– and P–waves; the inability of S–waves to travel through the outer core indicates their sold nature
 evidence for the solid inner core is primarily from weak reflections from the outer core/inner core boundary 
 following earthquakes
 evidence also from slower P-waves in the inner core; hypothesis that they convert to S-waves and then back to
P-waves
 artificial seism ic energy is generated by an explosion or a ground ‘shake’ or ‘thump’
 this energy generates P- and S-waves which travel outwards and refract and reflect off rock/material structures
 these waves are detected by geophones or seismometers spaced at known positions
 time of travel to the detectors allows calculation and imaging of rock/material structures
 these calculations are quite complex
 these techniques are preferred in the search for oil and gas; they are inexpensive, fairly accurate and quick
 they do not involve drilling or excavating
 similar to that of a very large bar magnet
 the magnetic north pole is near but not at the geographic north pole
 the magnetic south pole is near but not at the geographic south pole
 the magnetic north pole is the same kind of pole as the south pole on an ordinary bar magnet
 the magnetic south pole is the same kind of pole as the north pole on an ordinary bar magnet
 in most places the Earth’s field has a substantial vertical component (except at the equator)
 the angle between the field direction and the local horizontal is called the dip angle
 at the magnetic poles the dip angle is 90

 the position of the magnetic poles varies with time; this is evident over short time periods (e.g. a few years); this is due to electric currents in the molten outer core that change with time
 there is also evidence of complete reversals of the direction of the Earth’s field
 the most recent occurred around 780 000 years ago
 the causes of these reversals is not understood
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Cambridge Checkpoints 2014 HSC Physics Chapter 4 Geophysics Study Notes
 this is recorded in the magnetism of ancient rocks (paleomagnetism)
 the lithosphere comprises the crust and the upper mantle; it is fairly rigid, compared to the lower mantle, which deforms viscously and through plastic deformation
 the lithosphere is broken up into tectonic plates ; there are seven or eight major plates and minor plates
 the plates meet at plate boundaries ; these are of various kinds:
- transform; the plates grind/slide past each other (e.g. the San Andreas fault)
- divergent; the plates slide apart from each other (e.g. the Great Rift Valley, the Mid-Atlantic ridge)
- convergent: the plates move towards each other, either subducting (sliding under, for example deep marine trenches) or collisional (e.g. the formation of the Himalayas)
 evidence for plate tectonics includes:
- if plates are divergent (e.g. the Mid-Atlantic Ridge), then the sea floor near this ought to be geologically recent, because it has been formed of material rising from the lower mantle; this has been confirmed; paleomagnetism also confirms this (e.g. magnetic stripes both sides of the mid-ocean ridge)
- modern GPS technology can now measure the rate of seafloor movement; some plates diverge at around 7 cm per year
- transform boundaries are associated with shallow earthquakes due to movement between the plates
- fossil comparisons along the edges of continents that were joined in the past (according to plate movement) make sense due to similarities
 the mechanism for the movement of the plates has been suggested as due to convection currents in the lower mantle
 there was initial reluctance to accept the mobility of tectonic plates, largely because a lack of clarity as to a possible mechanism (as recently as the middle of the twentieth century); the evidence detailed above changed scientific opinion
 discovery of large mineral deposits, previously hidden under many layers of sediments
 seismic and gravity methods (for example) are able to penetrate these covering layers
 these techniques are cost and time efficient
 they are non-invasive and hence protect the environment
 geophysical techniques assist archaeologists in locating the best dig locations
 seismic monitoring enables better understanding of earthquake-prone areas, including location of places to avoid large settlements of humanity
 seismic monitoring has enabled distant monitoring of nuclear testing (and other large explosions), including location and blast size
 volcanic explosions can be predicted (using seismic techniques) with enough accuracy to enable evacuation from danger areas
 humanmade explosions can be distinguished from earthquakes because of a different seismic ‘signature’
 remote sensing techniques enable monitoring of dangerous areas, including spreading wildfires
 networks of seismometers can help predict tsunamis
 satellite/aerial monitoring of large weather systems have improved weather forecasting significantly
 satellite/aerial monitoring of reflected radiation is used to monitor destruction of forest/grasslands/other vegetation types on a large scale
 remote sensing has allowed creation of accurate topographical maps

GPS technologies have allowed a host of improvements in (e.g. navigation, tracking of migratory species, measurements of tectonic plate movements, location of lost people in remote locations, etc)
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