Download Cambridge Checkpoints 2014 HSC Physics Chapter 4 Geophysics

Cambridge Checkpoints 2014 HSC Physics
Chapter 4 Geophysics Study Notes
Geophysics
Properties of earth materials
Elasticity:
Density:
Thermal
conductivity:
Thermal
gradient:
Fourier’s
Law:
Magnetic
susceptibility:
Rock
(permanent )
magnetism
Electrical
conductivity
Electrical
resistivity
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–1K–1); significant
is determining the rate of heat transfer
the rate at which temperature changes with distance (units are K m –1)
Governs the amount of heat transferred: Q = kAT/x, where Q is the heat transferred
(J), A is the heat transfer area(m2), 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 (m2); 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/
Geophysical methods
Seismic
Gravitational
Magnetic
Paleomagnetic
Electrical
Electromagnetic
Radiometric
• shock waves (sound) sent through the earth
• path followed is monitored for travel time (gives speed), reflection, refraction and
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
• reveals information about tectonic plate movements and speed
• conductivity of rocks gives information about rock types
• electrical currents can be used to induce polarisation of certain rock types
• ground-penetrating radar can be used to image structures and rock characteristics
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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Geothermal
Chapter 4 Geophysics Study Notes
• 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
Remote sensing: absorption and reflection of radiation
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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)
Remote sensing: applications
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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
Gravitational field at/near/above the Earth’s surface
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experiments by Jean Richer showed that g varied over the Earth’s surface
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Richer (1671) measured g by using a pendulum, using
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T = 2p
l
4p 2 l
or g =
g
T2
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
gR 2
ME
g = G 2 or M E =
G
R
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 E
=
T 2 4p 2
where r is the orbit radius of the satellite (NB not the altitude) , and T is the satellite period
satellites that are not geo-synchronous (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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Chapter 4 Geophysics Study Notes
Gravimeters
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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
Data treatment in gravity surveys
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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
Uses of gravity methods in resource exploration
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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
Earthquake waves
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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
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sinq1 sin q 2
=
v1
v2
3
); they can also totally internally reflect
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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
Geophones and seismometers
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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)
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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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Chapter 4 Geophysics Study Notes
Evidence for the inner structure of the Earth’s core
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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
Seismic methods
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artificial seismic 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
Magnetic field of the Earth
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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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Chapter 4 Geophysics Study Notes
this is recorded in the magnetism of ancient rocks (paleomagnetism)
Plate tectonics
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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
Benefits of geophysical methods
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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
human-made 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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