All Flashcards
Sound waves
Simple pressure wave
Series of high pressure - low pressure areas
Travels mainly in gases/liquids.
Seismic waves
Travels in solids
More complicated propagation
Mainly a propagation of an elastic wave.
Rayleigh wave
A type of surface wave that travels along a free interface. Particle motion is elliptical in a plane perpendicular to the interface and retrograde. 90% of speed of S waves.
P-wave
Compressional elastic body wave. Particles oscillate about a fixed point but in the direction of propagation of wave energy. P waves are fastest of seismic waves.
S-wave
An elastic body wave in which particles oscillate about a fixed point but in a direction perpendicular to the direction of propagation of the wave energy. Cannot travel through liquids as they don’t support shear. Half speed of P-waves.
Love wave
A type of surface wave which occurs when the shear body wave velocity in the surface medium is lower than that in the underlying strata. Love waves are characterised by horizontal motion normal to the direction of travel with no vertical motion. (Polarised shear wave that travels slightly faster than a Rayleigh wave).
P wave velocity equation
= {(k+(4/3)u)/p}
{}=sqrt
K= bulk modulus
U=shear modulus
P=densityVelocity of S wave equation
{u/p}
U=shear modulus
P= density
PcP
P reflection from CMB
PKP
Ray through outer core
PKiKP
Reflection from inner core
PKIKP
Ray through inner core
PKKKP
Multiple reflection from inner CMB
SKS
S through mantle as P through outer core.
ScP
S trough mantle reflected P from CMB
Anisotropy
Variation with orientation.
Global seismology
Provides info on global earth structure and argue scale velocity anomalies.
Smaller scale structure is difficult to image particularly away from earthquake source regions.
Controlled source seismology
Allow higher resolution studies
Can provide information away from tectonic regions
Seismic refraction and reflection techniques
Snells law
Sini/sinr=v(1)/v(2)
Sin (I (c))
V1/v2
If velocity decreases with depth…
Critical refraction cannot happen
The slow layer will be invisible in the time distance plot
All results at/below this layer will appear systematically too deep
Reflection coefficient
R= (I2-I1)/(I2+I1)
Travel time equation
2L/V
T(NMO)
X^2/(4Vh)
ReSort into CMP gathers because
Each reflection in a CMP father has bounced from a single point so
The geometry of its different path lengths allow velocity to be measured
Repeat reflections can be summed. STACKING
Disadvantages of reflections methods
Cost and complexity
Time consuming
Sophisticated technology
Needs much computing power
Reflection seismology
Form of echo sounding to detect interfaces below ground (reflectors)
Seismic section vs. Geological section
Time not depth on y axis
Dipped reflectors imaged incorrectly
Multiples
Reflectors may not correspond to lithology
Velocities determines by
Move out of reflection
Factors of earthquake effects
How big is the earthquake
How far away is it
What is the ground like
Richter scale
Measures the amplitude of shaking - on a specific seismometer at a specific distance.
Seismic moment
M(0) = uS
u=shear strength of the faulted rock [Pa=N/m^2]
S=area of rupture
= average displacement.
Moment magnitude scale
M(w) = ((2/3) log(M0) -10.7)
Earthquake magnitude :
1 change in magnitude Unit =
Factor of _ in ground motion amplitude
Factor of _ in energy release
Factor of 10 in ground motion amplitude
Factor of 32 in energy release
Why does amplitude decay with distance
1) attenuation
2) geometric spreading.
types of faults (earthquakes)
Strike slip - horizontal,shear
Normal - horizontal, extension
Thrust - horizontal, compression
Local ground motion during earthquake is controlled by
Magnitude, distance, local amplification, type of faulting, relative direction and orientation of the quake.
Topographic load pushes down the Moho and creates a __________ gravity anomaly above
Positive
Receiver functions
Incident p wave transmitted as P and converted S waves from interface (moho).
These arrive at different times. The difference is sensitive to the local geology near the seismometer only.
The common part of Ray path can be subtracted by deconvolution to leave receiver function for the region below.
Surface wave tomography
Slowest seismic waves also the waves that decrease in amplitude more slowly with distance so often useful for large scale studies.
Isostatically compensated topography has ….free air anomaly
NO
Free air gravity anomalies show non isostatically compensate bathymetry in the _____
OCEAN
Resolution in tomography depends on
Density of Ray paths
Receiver functions (from difference in converted P and S wave arrivals) give structure
In region of seismometer
Surface wave tomography depth sensitivity depends on
Wavelength
What generates magnetic fields
Magnetism
Electric currents
Time varying electric fields
What generates the earths (internal) magnetic field
Permanent or induced magnetism in the crust and upper mantle.
Dynamo action in the liquid outer core.
Induced magnetism
Magnetism in materials is produced by the magnetic moments of the electrons in a material.
Generally these magnetic moments are distributed randomly and do not lead to overall magnetism of the material.
Diamagnetism
Occurs in all materials Applied magnetic field generates anti aligned magnetism Results in very repulsion Commonly referred to as non magnetic Examples include water copper gold.
Paramagnetism
Occurs in materials with unpaired electrons Randomly orientated on their own Aligned by applied magnetic field Results in weak attraction Examples include aluminium titanium
Permanent magnetism
Has unpaired electrons with magnetic moment which align in an applied external magnetic field.
Atomic magnetic moments have a tendency to align with each other.
With the alignment the magnetic moments do not cancel out and we acheive permanent magnetisation of the material
The cutie temperature is the temperature where the material looses it’s ordered magnetic structure and looses its permanent magnetisation.
Curie temperature
The temperature where the material loses it’s ordered magnetic structure and loses its permanent magnetism
Why is curie temp important for Geophysics
For common Earth materials the curie temp is reached at depth between 10-50km
Interior of earth is paramagnetic and cannot contribute to the earths magnetic field through permanent magnetism
Cooling magma through the curie point allows freezing of the ambient magnetic orientation into the magma
By studying igneous rocks we can study magnetic history of the planet.
Gauss’ Law
Every source of magnetic field must have a corresponding sink
Dipole
Simplest magnetic field has 2 poles with opposite polarity
Describing magnetic field structure requires knowing 3 pieces of info
Vector components
Declination, horizontal intensity, vertical intensity
Declination, inclination, total field intensity
1 Gal
1 cm s-2
Gravity unit
Variation of gals Pole to Equator
5200mGals
Variation with Height
1mGal per 5m
Variation due to geology
MicroGal to 100s of milligals
Variation due to Earth shape
1000s of mGals
International Gravity Formula
Accounts for variations of gravity with distance form equator.
2 effects: rotation of earth and oblateness of earth caused by rotation.
What influences the value of g
Rock density Latitude Tides Height Topography
Correction to measured gravity
Tides Instrument drift Latitude Free air correction Bouguer Correction Terrain Correction
Full Bouguer anomaly =
(observed gravity + FAC -
- BSC - TC) -g(0)
What does a full Bouguer Anomaly represent?
density contrasts relative to background (~2.67gm/cc)
geoid - ellipsoid differences (long-wavelength)
Geoid
equipotential surface close to mean sea level
equipotential surface = approximates mean sea level extrapolated into continents
Gravitational potential formula
= -GM/r
Applications of static gravity field
Geodynamics
Crustal geology
Ocean currents
Measurements of changing gravity field are sensitive to
Hydrology Changes in ice mass Isostatic adjustment Earthquakes Volcanoes
Surface wave tomography
Slowest seismic waves but also the waves that decrease in amplitude more slowly with distance so often useful for large scale studies.
Isostatically compensated topography has roughly __ free air anomaly
NO
Free air gravity anomalies show
Non isostatically compensated bathymetry in the ocean
Resolution in tomography depends on
Density of Ray paths
Receiver functions from difference in converted p and S wave arrivals give
Structure in region of seismometer
Surface wave tomography depth sensitivity depends on
WAVELENGTH
What generates magnetic fields
Magnetism
Electric currents
Time varying electric fields
What generates the earths internal magnetic field
Permanent or induced magnetism in the crust and upper mantle
Dynamo action in the liquid outer core.
Induced magnetism
Magnetism in materials is produced by the magnetic moments of the electrons in a Material.
Generally these magnetic moments are distributed randomly and do not lead to overall magnetism of the material
Diamagnetism occurs in all materials:
Applied magnetic field generates anti aligned magnetism
Results in very weak repulsion
Commonly referred to as a non magnetic
Examples include water copper gold
Paramagnetism occurs in
Materials with unpaired electrons Randomly oriented on their own Aligned by applied magnetic field Results in weak attraction Examples include aluminium titanium
Permanent magnetism
A ferromagnetic material has unpaired electrons with magnetic moment which align in an applied magnetic field
But the atomic magnetic moments have a tendency to align with each other even without an external magnetic field
With the alignment the magnetic moments do not cancel out and we achieve permanent magnetisation of the material
Cutie temperature
Is the temperature where the material loses it’s ordered magnetic structure and loses it’s permanent magnetisation.
Why is cutie temp important for Geophysics
For common Earth materials the cutie temp is reached at a depth of 10-50 km
The interior of the earth is paramagnetic and cannot contribute to the earths magnetic field through permanent magnetism
Cooking magna through the cutie point allows freezing of the ambient magnetic orientation into the magna
By studying igneous rocks we can study the magnetic history of the planet.
Gauss’ Law
Every source of magnetic field must have a corresponding sink
Dipole
Simplest magnetic field with 2 opposite poles.
Describing magnetic field :
Vector components (north east down) Declination, horizontal intensity, vertical intensity Declination, inclination, total field intensity.
Units of magnetic field intensity
Tesla = kg/As^2
nT
Old unit is the gamma
Ranges between 25000-65000
Ground based geomagnetic observatories
First network begun in 1836 by Gauss
Expanded during 1957
Swarm constellation
Earth core dynamics, geodynamo processes and core mantle interaction.
Lithospheric magnetisation and it’s geological interpretation
3D electrical conductivity of the mantle
Currents flowing in the magnetosphere and ionosphere.
Time variations of earths field
Strength Structure Pole locations Sign of polarity Reversal frequency Dipole to multiple ratio
Secular variation
Changes over time scales of a year or more mostly reflect changes in the earths interior particularly the iron rich core
Geodynamo
Time varying geomagnetic field has existed for at least three billion years
Continually generated no permanent magnetisation in core
Convection of liquid iron of outer core generates electric currents and hence a magnetic field
Powered by cooling of the core.
Reversals of the magnetic field
Magnetic field reversed polarity at random intervals between 0.1 to 50 million years
A period with one polarity is a chron - a super chron is a chron lasting at least 10 million years
Last one was 0.78 million years ago
Transition lasts a few centuries to millennia
Reversals of magnetic field
Decay of magnetic field strength
deviation from dipole to multipole at surface
Swap of magnetic north and south poles
Paleomagnetism : seafloor spreading
As magna cooks through cutie temperature, magnetic minerals align with the orientation of the earths ambient magnetic field
Progressive spreading creating new crust at ocean ridge results in symmetric magnetic stripes.
Geomagnetic storms can
Disrupt radio communication
Interfere with GPS
damage satellites
Knock out power grids
Solar wind interaction with the earths magnetic field
Trapped particles bounce from pole to pole along magnetic field lines
In polar regions where the inclination is high particles can penetrate the atmosphere
Ionisation of oxygen and nitrogen molecules
