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Fundamentals Of Geophysics > All > Flashcards

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1
Q

Sound waves

A

Simple pressure wave
Series of high pressure - low pressure areas
Travels mainly in gases/liquids.

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2
Q

Seismic waves

A

Travels in solids
More complicated propagation
Mainly a propagation of an elastic wave.

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3
Q

Rayleigh wave

A

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.

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4
Q

P-wave

A

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.

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5
Q

S-wave

A

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.

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6
Q

Love wave

A

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).

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7
Q

P wave velocity equation

A 
= {(k+(4/3)u)/p}
{}=sqrt
K= bulk modulus 
U=shear modulus 
P=density
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8
Q

Velocity of S wave equation

A

{u/p}
U=shear modulus
P= density

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9
Q

PcP

A

P reflection from CMB

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10
Q

PKP

A

Ray through outer core

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11
Q

PKiKP

A

Reflection from inner core

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12
Q

PKIKP

A

Ray through inner core

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13
Q

PKKKP

A

Multiple reflection from inner CMB

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14
Q

SKS

A

S through mantle as P through outer core.

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15
Q

ScP

A

S trough mantle reflected P from CMB

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16
Q

Anisotropy

A

Variation with orientation.

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17
Q

Global seismology

A

Provides info on global earth structure and argue scale velocity anomalies.
Smaller scale structure is difficult to image particularly away from earthquake source regions.

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18
Q

Controlled source seismology

A

Allow higher resolution studies
Can provide information away from tectonic regions
Seismic refraction and reflection techniques

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19
Q

Snells law

A

Sini/sinr=v(1)/v(2)

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20
Q

Sin (I (c))

A

V1/v2

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21
Q

If velocity decreases with depth…

A

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

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22
Q

Reflection coefficient

A

R= (I2-I1)/(I2+I1)

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23
Q

Travel time equation

A

2L/V

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24
Q

T(NMO)

A

X^2/(4Vh)

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25
Q

ReSort into CMP gathers because

A

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

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26
Q

Disadvantages of reflections methods

A

Cost and complexity
Time consuming
Sophisticated technology
Needs much computing power

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27
Q

Reflection seismology

A

Form of echo sounding to detect interfaces below ground (reflectors)

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28
Q

Seismic section vs. Geological section

A

Time not depth on y axis
Dipped reflectors imaged incorrectly
Multiples
Reflectors may not correspond to lithology

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29
Q

Velocities determines by

A

Move out of reflection

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30
Q

Factors of earthquake effects

A

How big is the earthquake
How far away is it
What is the ground like

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31
Q

Richter scale

A

Measures the amplitude of shaking - on a specific seismometer at a specific distance.

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32
Q

Seismic moment

A

M(0) = uS

u=shear strength of the faulted rock [Pa=N/m^2]
S=area of rupture
= average displacement.

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33
Q

Moment magnitude scale

A

M(w) = ((2/3) log(M0) -10.7)

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34
Q

Earthquake magnitude :
1 change in magnitude Unit =
Factor of _ in ground motion amplitude
Factor of _ in energy release

A

Factor of 10 in ground motion amplitude

Factor of 32 in energy release

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35
Q

Why does amplitude decay with distance

A

1) attenuation

2) geometric spreading.

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36
Q

types of faults (earthquakes)

A

Strike slip - horizontal,shear

Normal - horizontal, extension

Thrust - horizontal, compression

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37
Q

Local ground motion during earthquake is controlled by

A

Magnitude, distance, local amplification, type of faulting, relative direction and orientation of the quake.

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38
Q

Topographic load pushes down the Moho and creates a __________ gravity anomaly above

A

Positive

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39
Q

Receiver functions

A

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.

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40
Q

Surface wave tomography

A

Slowest seismic waves also the waves that decrease in amplitude more slowly with distance so often useful for large scale studies.

41
Q

Isostatically compensated topography has ….free air anomaly

A

NO

42
Q

Free air gravity anomalies show non isostatically compensate bathymetry in the _____

A

OCEAN

43
Q

Resolution in tomography depends on

A

Density of Ray paths

44
Q

Receiver functions (from difference in converted P and S wave arrivals) give structure

A

In region of seismometer

45
Q

Surface wave tomography depth sensitivity depends on

A

Wavelength

46
Q

What generates magnetic fields

A

Magnetism
Electric currents
Time varying electric fields

47
Q

What generates the earths (internal) magnetic field

A

Permanent or induced magnetism in the crust and upper mantle.
Dynamo action in the liquid outer core.

48
Q

Induced magnetism

A

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.

49
Q

Diamagnetism

A 
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.
50
Q

Paramagnetism

A 
Occurs in materials with unpaired electrons
Randomly orientated on their own 
Aligned by applied magnetic field
Results in weak attraction
Examples include aluminium titanium
51
Q

Permanent magnetism

A

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.

52
Q

Curie temperature

A

The temperature where the material loses it’s ordered magnetic structure and loses its permanent magnetism

53
Q

Why is curie temp important for Geophysics

A

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.

54
Q

Gauss’ Law

A

Every source of magnetic field must have a corresponding sink

55
Q

Dipole

A

Simplest magnetic field has 2 poles with opposite polarity

56
Q

Describing magnetic field structure requires knowing 3 pieces of info

A

Vector components
Declination, horizontal intensity, vertical intensity
Declination, inclination, total field intensity

57
Q

1 Gal

A

1 cm s-2

Gravity unit

58
Q

Variation of gals Pole to Equator

A

5200mGals

59
Q

Variation with Height

A

1mGal per 5m

60
Q

Variation due to geology

A

MicroGal to 100s of milligals

61
Q

Variation due to Earth shape

A

1000s of mGals

62
Q

International Gravity Formula

A

Accounts for variations of gravity with distance form equator.
2 effects: rotation of earth and oblateness of earth caused by rotation.

63
Q

What influences the value of g

A 
Rock density
Latitude
Tides
Height
Topography
64
Q

Correction to measured gravity

A 
Tides
Instrument drift
Latitude
Free air correction
Bouguer Correction
Terrain Correction
65
Q

Full Bouguer anomaly =

A

(observed gravity + FAC -

- BSC - TC) -g(0)

66
Q

What does a full Bouguer Anomaly represent?

A

density contrasts relative to background (~2.67gm/cc)

geoid - ellipsoid differences (long-wavelength)

67
Q

Geoid

A

equipotential surface close to mean sea level

equipotential surface = approximates mean sea level extrapolated into continents

68
Q

Gravitational potential formula

A

= -GM/r

69
Q

Applications of static gravity field

A

Geodynamics
Crustal geology
Ocean currents

70
Q

Measurements of changing gravity field are sensitive to

A 
Hydrology
Changes in ice mass
Isostatic adjustment
Earthquakes
Volcanoes
71
Q

Surface wave tomography

A

Slowest seismic waves but also the waves that decrease in amplitude more slowly with distance so often useful for large scale studies.

72
Q

Isostatically compensated topography has roughly __ free air anomaly

A

NO

73
Q

Free air gravity anomalies show

A

Non isostatically compensated bathymetry in the ocean

74
Q

Resolution in tomography depends on

A

Density of Ray paths

75
Q

Receiver functions from difference in converted p and S wave arrivals give

A

Structure in region of seismometer

76
Q

Surface wave tomography depth sensitivity depends on

A

WAVELENGTH

77
Q

What generates magnetic fields

A

Magnetism
Electric currents
Time varying electric fields

78
Q

What generates the earths internal magnetic field

A

Permanent or induced magnetism in the crust and upper mantle

Dynamo action in the liquid outer core.

79
Q

Induced magnetism

A

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

80
Q

Diamagnetism occurs in all materials:

A

Applied magnetic field generates anti aligned magnetism
Results in very weak repulsion
Commonly referred to as a non magnetic
Examples include water copper gold

81
Q

Paramagnetism occurs in

A 
Materials with unpaired electrons 
Randomly oriented on their own 
Aligned by applied magnetic field 
Results in weak attraction 
Examples include aluminium titanium
82
Q

Permanent magnetism

A

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

83
Q

Cutie temperature

A

Is the temperature where the material loses it’s ordered magnetic structure and loses it’s permanent magnetisation.

84
Q

Why is cutie temp important for Geophysics

A

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.

85
Q

Gauss’ Law

A

Every source of magnetic field must have a corresponding sink

86
Q

Dipole

A

Simplest magnetic field with 2 opposite poles.

87
Q

Describing magnetic field :

A 
Vector components (north east down)
Declination, horizontal intensity, vertical intensity 
Declination, inclination, total field intensity.
88
Q

Units of magnetic field intensity

A

Tesla = kg/As^2
nT
Old unit is the gamma
Ranges between 25000-65000

89
Q

Ground based geomagnetic observatories

A

First network begun in 1836 by Gauss

Expanded during 1957

90
Q

Swarm constellation

A

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.

91
Q

Time variations of earths field

A 
Strength
Structure 
Pole locations
Sign of polarity
Reversal frequency 
Dipole to multiple ratio
92
Q

Secular variation

A

Changes over time scales of a year or more mostly reflect changes in the earths interior particularly the iron rich core

93
Q

Geodynamo

A

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.

94
Q

Reversals of the magnetic field

A

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

95
Q

Reversals of magnetic field

A

Decay of magnetic field strength
deviation from dipole to multipole at surface
Swap of magnetic north and south poles

96
Q

Paleomagnetism : seafloor spreading

A

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.

97
Q

Geomagnetic storms can

A

Disrupt radio communication
Interfere with GPS
damage satellites
Knock out power grids

98
Q

Solar wind interaction with the earths magnetic field

A

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

Fundamentals Of Geophysics (1 decks)

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