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Geophysics Flashcards

1
Q

What is Rheology?

A

Study of flow (how things deform: stress to strain)

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2
Q

What are the Earth’s Potential Fields?

A

Gravity field, Magnetic Field

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3
Q

Why do we study the Earth’s Potential Fields?

A

To measure their deviations at the Earth’s surface

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4
Q

What is Seismology?

A

Concerned with wave propagation through Earth

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5
Q

Does seismology change depending on scale?

A

No, Earth scale vs high-res uses the same methods

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6
Q

What changes with scale in Seismology?

A

The Frequency of the wave

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7
Q

What is the Lithosphere?

A

Crust + some Upper Mantle

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8
Q

What is the Moho?

A

Base of the crust

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9
Q

How deep is the Moho?

A

Differing depths of Moho across Earth

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10
Q

Where is the Moho deepest?

A

Deepest under continents

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11
Q

Where is the Moho shallowest?

A

Oceanic crust (uniform thickness)

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12
Q

What does the Moho have to do with seismology?

A

It is a discontinuity at which seismic waves change velocity due to the thickness of the crust.

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13
Q

Who discovered the seismic Moho?

A

Andrija Mohorovic

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14
Q

On continental crust, where is it thickest and thinnest?

A

Thickest under mountains, thinnest under plains

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15
Q

Do seismic waves speed up or slow down when they cross the Moho?

A

Seismic waves speed up as they move from less dense crust to more dense mantle

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16
Q

What increase in speed do seismic waves experience when crossing Moho?

A

6 to 8km/s

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17
Q

What is the crust like around Aberdeen?

A

Thinner as near the sea

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18
Q

What is a Seismic Wave?

A

Energy (elastic strain) that travels away from a source

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19
Q

What is a body wave?

A

Seismic wave that travels through the bulk of a medium

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20
Q

What is a surface wave?

A

Travels along the interface between 2 media with different material properties.

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21
Q

What are P-waves?

A

Body waves (primary, compressional waves)

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22
Q

What are S-waves?

A

Body waves (secondary, shear waves)

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23
Q

Explain deformation of P-waves

A

Alternating compression and dilation
Particle motion is PARALLEL to direction of propagation
Returns to original shape after wave has passed

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24
Q

Explain deformation of S-waves

A

Alternating transverse motion

Particle motion is PERPENDICULAR to direction of propagation

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25
Q

What are the two types of Surface waves?

A

Love Waves and Rayleigh Waves

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26
Q

Explain deformation of Rayleigh waves

A

travel along interface of two media
Amplitude decreases with depth
Looks wavy in a diagram

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27
Q

Explain deformation of Love waves

A

Travels between two layers where top has lower S-wave velocity than deeper layer
transverse particle motion, parallel to interface
Amplitude also decreases with depth

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28
Q

What does velocities of seismic waves depend on?

A

Elastic properties and density of rock which the waves travel through

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29
Q

What is the unit ‘P’

A

Density

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30
Q

What is the unit ‘K’

A

Bulk Modulus

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31
Q

What is the unit ‘μ’

A

Shear Modulus

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32
Q

Do S-waves propagate through fluids?

A

As shear modulus in fluids = 0, velocity of S-waves = 0

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33
Q

Do S-waves travel through the liquid core?

A

No - creates observation of arc 105 degrees from epicentre

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34
Q

Do P-waves travel through the core-mantle boundary?

A

No - P-waves are strongly refracted by boundary

Creates a shadow zone from 105-140 degrees.

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35
Q

General rule for velocity of P-waves?

A

velocity increases with depth

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36
Q

What happens to velocity of P-waves in asthenosphere?

A

Rock is hot enough that is it plastic and therefore waves slow down (low velocity zone)

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37
Q

What happens at the 660km discontinuity?

A

Velocity of P-waves rapidly increase due to increase in pressure between Upper and Lower Mantle

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38
Q

What is Active Seismology?

A

The use of purpose-built sources of seismic waves for investigations into Earth’s Crust and oil/gas exploration. .

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39
Q

What are the units of Length

A

m

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40
Q

What are the units of Mass

A

kg

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41
Q

What are the units of Time

A

s

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42
Q

What are the dimensions of a quantity

A

Refer to the physical entities and their units which are intrinsic to that quantity

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43
Q

What is the principle of dimensional homogeneity?

A

The quantity in each term must have the same dimensions (physical units).
Terms can also be dimensionless
A = BD+C (same units)

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44
Q

The displacement (s) of a vehicle moving at a constant acceleration depends on:

A 
Elapsed time (t)
Acceleration (a)
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45
Q

Is displacement a kinematic equation?

A

Yes, it deals with position in time, displacement, velocity or acceleration of something
s=kat

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46
Q

Is displacement vector or tensor quantity?

A

Vector (m)

magnitude, direction, origin

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47
Q

Part of the strain of an object is its change in….

A

length per unit length (elongation)

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48
Q

Is strain a tensor or vector quantity

A

Tensor

magnitude, direction of displacement gradient, direction of plane of action

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49
Q

What are the dimensions of strain

A

strain is dimensionless

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50
Q

Is velocity a tensor or vector quantity?

A

Vector

magnitude, direction, origin

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51
Q

What is velocity?

A

Rate of change of displacment

Change in length/change in time

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52
Q

Units of velocity?

A

m/s or ms’-1

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53
Q

Is acceleration a vector or tensor quantity?

A

Vector

Rate of change (in time) of velocity

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54
Q

Units of acceleration?

A

m/s/s or ms’-2

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55
Q

Is force a vector or tensor quantity?

A

Vector

magnitude, direction, origin

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56
Q

Units of force?

A

Newton (N)

1 N = 1 kgms’-2

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57
Q

Is stress a vector or tensor quantity?

A

Tensor

magnitude, direction of forces, direction of strain action

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58
Q

Units of stress?

A

Pascal (Pa)

1 Pa = 1 Nm’-2

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59
Q

What principle stress do normal faults have?

A

stress 1 - most compressive (vertical)

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60
Q

What principle stress do thrust/reverse faults have?

A

stress 3 - most tensile (vertical)

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61
Q

What principle stress do strike/slip faults have?

A

stress 2 - intermediate (vertical)

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62
Q

What is Lithostatic Stress?

A

Stress due to the weight of overburden rock - vertical principle stress.

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63
Q

What is Tectonic Stress?

A

Stress due to plate forces - horizontal principle stress

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64
Q

What is elasticity?

A

Stress produces instantaneous strain that propagates away from the source.

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65
Q

What is plasticity?

A

Permanent changes of shape (strain in response to stress above/at yield.

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66
Q

What is viscosity?

A

A given shear stress produces a flow (deformation over time) at constant rate (strain rate)

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67
Q

Is stress a dynamic or kinematic quantity?

A

Dynamic

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68
Q

Is strain dynamic or kinematic quantity?

A

Kinematic

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69
Q

what is k’

A

related to bulk modulus (elastic response to normal stress)

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70
Q

what is μ’

A

related to shear stress (elastic response to sheer stress)

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71
Q

What is the bulk modulus k’ also called

A

Young’s modulus (E)

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72
Q

What is the bulk modulus?

A

Ratio of external pressure change to volume change as a proportion of initial volume.

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73
Q

What is the relationship between strain rate and sheer stress in newtonian viscosity?

A

Strain rate is PROPORTIONAL to applied shear stress.

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74
Q

What is the relationship between strain rate and sheer stress in non-newtonian viscosity?

A

Strain rate is NOT LINEARLY PROPORTIONAL to applied shear stress.

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75
Q

Elasticity is the rheological behaviour that applies to…

A

solids

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76
Q

Viscosity is the rheological behaviour that applies to…

A

fluids

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77
Q

Plasticity is the rheological behaviour that applies to…

A

both solids and fluids (irreversible deformation)

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78
Q

What is plasticity?

A

Permanent deformation occurring after a yield stress is reached.

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79
Q

What are the two types of plastic deformation?

A

Ductile or brittle deformation

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80
Q

What are the two types of viscosity?

A

Dynamic Viscosity and Kinematic Viscosity

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81
Q

What is dynamic viscosity?

A

Relationship between stress and strain rate

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82
Q

What is kinematic viscosity?

A

A quantity where no dynamic quantities are involved

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83
Q

Units of dynamic viscosity are..

A

Poise (nm’-2)

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84
Q

What is a Poise the same as?

A

Pa s

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85
Q

What are the units of kinematic viscosity?

A

m’2 s’-1

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86
Q

What are the applications of Seismic methods?

A

Hydrocarbon Exploration
Mineral Exploration
Hi-Res shallow Exploration

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87
Q

What is the purpose of seismic surveys?

A

To measure the time taken for a seismic wave to travel from source (at known location or near the surface) down into the ground where it is refracted and/or reflected back to the surface where it is detected by receivers at known locations.

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88
Q

What is useful about travel-times?

A

Travel-times allow seismic wave paths to be inferred and seismic wave velocities to be calculated.

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89
Q

What is Snell’s Law of refraction?

A

sin’i’/sin’r’ = v1/v2 or. sin∅1/v1 = sin∅2/v2

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90
Q

What is the law of reflection?

A

Angle of Incidence = Angle of Reflection

Largest reflection is for the normal ray (I = 90)

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91
Q

What determines the angles of refraction/reflection?

A

Velocity contrasts between layers

92
Q

What is acoustic impedance?

A

Determines occurrence and amplitude of reflected phases

If acoustic impedance is the same there is no reflection and all the energy is transmitted across interface

93
Q

Formula for acoustic impedance?

A 
z = p x v   
p = density
v = seismic velocity
94
Q

What is the purpose of seismic refraction surveys?

A

Acquire info about subsurface from P-waves or S-waves that are refracted and post-critically reflected when incident upon a boundary with velocity contrast

95
Q

What is the purpose of seismic reflection surveys?

A

Acquire info about subsurface from P-waves or S-waves that are critically reflected on a boundary with an acoustic impedance contrast

96
Q

What is the formula for direct ray?

A 
Td= X/V1
x = offset 
V1 = velocity of boundary 1
97
Q

What line is for the direct ray arrivals?

A

The straight line with slope 1/V1

98
Q

What line is for the refracted arrivals?

A

Straight line with slope 1/V2

99
Q

What is the line for reflected arrivals?

A

The hyperbolic curve

100
Q

What is the critical distance?

A

At the critical distance, the reflected arrival is coincident with the first critically refracted arrival.
Travel times of the two are the same.

Distance between source and first refracted wave recorded.

101
Q

What is the crossover distance?

A

Offset at which the critically refracted waves takes over the direct wave (precedes).

102
Q

What is the outcome of a Seismic survey?

A

Wide-Angle reflection and refraction (WARR)

103
Q

Does the reflected phase ever arrive first?

A

NO - reflected phase is never first to arrive

104
Q

What is done after a seismic reflection survey?

A

Seismic traces are stacked and manipulated to improve quality of arrivals.

105
Q

What is the 1st stage of SR processing?

A

Display all traces from a single airgun shot

106
Q

What is the 2nd stage of SR processing?

A

Delete noisy traces

107
Q

What is the 3rd stage of SR processing?

A

Correct amplitudes for energy loss during long travel times

108
Q

What is the 4th stage of SR processing?

A

Select traces with a common depth point (CDP gather)

109
Q

What is the 5th stage of SR processing?

A

Remove direct wave (mute)

110
Q

What is the 6th stage of SR processing?

A

Correct travel-times for range dependent delays

111
Q

What does stacking do?

A

Improves quality of ‘signal’ which is otherwise embedded in the noise.

112
Q

What is the 7th stage of SR processing?

A

Sum all the traces in a CDP gather (stack)

113
Q

What is the 8th stage of SR processing?

A

Compress signal wavelet (better resolution)

114
Q

What is the 9th stage of SR processing?

A

Display all stack traces

115
Q

What is the 10th/final stage of SR processing?

A

Interpet geology

116
Q

Is it true that the higher the frequency of the seismic source signal then the smaller the wavelength of the seismic pulse and the better the resolution?

117
Q

Is it true the higher the velocity of the propagation medium, then the larger the wavelength of the seismic pulse, the lower the resolution.

118
Q

What is migration

A

fixes the position of seismic reflectors in real life on a graph

119
Q

Name one thing migration can remove

120
Q

Name the sources of seismic waves used in active seismology on land

A

Weight drops
Guns
Explosives
Vibroseis

121
Q

Name the sources of seismic waves used in the ocean

A

Sparkers
Boomers
airguns

122
Q

Name the receivers of seismic waves used in the ocean

A

Ocean Bottom Cables

Hydrophones

123
Q

Name the receivers of seismic waves used on land

124
Q

What is gravity surveying?

A

Measuring variations in Earth’s gravitational field caused by differences in density of subsurface rocks.

125
Q

What is magnetic surveying?

A

Measuring variation in Earth’s magnetic field caused by differences in magnetic properties of rocks in subsurface

126
Q

What are variations measured in?

A

mGal (10’-3 Gal)

127
Q

What is the geomagnetic field described in terms of?

A

Declination
Inclination
Total Magnetic Force Vector

128
Q

What are the magnetic units?

A

Tesla (Nm/A)

Normally measured in nT.

129
Q

Where is flux density greatest?

A

Flux density greater at poles than equator.

Decreases with increasing distance from centre to earth.

130
Q

Order rocks in order of magnetic susceptibility

A

Basic Ig Highest
Acid Ig
Metamorphic
Sedimentary Lowest

131
Q

Do magnetised rocks have remnant magnetisation?

A

No, magnetised rocks have their own induced magnetisation.

Magnetic minerals within a rock may have remnant and induced magnetisation.

132
Q

Is the direction and magnitude of remnant and induced magnetisation always the same?

A

No, the magnitude and direction of either magnetisation can be different.

133
Q

What is the intensity of induced magnetisation based on?

A

Based on magnetic susceptibility

134
Q

What dictates the amplitude and shape of an observed magnetic anomaly?

A

The magnitude and direction of the resultant magnetisation.

135
Q

What gives remnant magnetisation?

A
  1. Cooling of igneous rocks
  2. Sedimentary rocks with grains which aligned themselves during sedimentation
  3. Metamorphosed rocks that have crystallising minerals which align with earth’s field
136
Q

What gives induced magnetisation?

A
  1. certain materials produce own magnetisation depending on field they are put in
  2. Intensity of magnetisation is proportional to field strength of inducing field according to susceptibility of materials.
137
Q

What are the large scale applications of gravity surveying?

A

Estimation of crustal thickness

low density crust over higher density mantle.

138
Q

What are the medium scale applications of gravity surveying?

A

Hydrocarbon industry

location of salt domes

139
Q

What are the small scale applications of gravity surveying?

A 
Mapping bedrock topography
Mineral exploration (massive density compared to host rock)
140
Q

What are the micro scale applications of gravity surveying?

A

Finding caves/tombs

Cavities

141
Q

How do we measure gravity at sea?

A

Lower meter to seafloor

Onboard a ship

142
Q

How do we measure gravity in air?

A

Helicopter

Plane

143
Q

What is the gravity anomaly?

A

We are interested in the difference between a theoretical value and a measurement at a base somewhere. This difference is the gravity anomaly.

144
Q

Formula for gravity anomaly?

A

∆g = Gobs - Gtheor or// ∆g = Gobs - Gbase

145
Q

Before it can be interpreted, the anomaly data must be corrected. How do we do this?

A

Using gravity data reduction

∆g = Gobs + ΣGcorr - Gtheor

146
Q

Why do we correct the gravity anomaly?

A
  1. Drift Correction
  2. Tidal Correction
  3. Latitude Correction
  4. Free-air Correction
  5. Bouguer Correction
  6. Terrain Correction
147
Q

What is drift correction?

A

Instrument drift corrected by linear interpolation

148
Q

What is latitude correction?

A

gravity is less at equators than poles

149
Q

What is tidal correction?

A

variations of gravity due to changing positions of sun and moon easily calculated

150
Q

What is free-air correction?

A

Gravity decreases with distance from centre to earth. Measurements adjusted for elevation differences compared to constant sea level.

151
Q

What is the bouguer correction?

A

accounts for gravitational effects of rocks between the observation elevation and datum elevation

152
Q

What is the terrain correction?

A

correction for divergence of actual topography survey

153
Q

Why is the Bouguer Anomaly important?

A

Variations in the bouguer anomaly reflects the lateral variations in density

154
Q

What does a positive bouguer anomaly mean?

A

High density feature in low density medium

155
Q

What does a negative bouguer anomaly mean?

A

Low density feature in high density medium

156
Q

Applications of geomagnetic surveying?

A

Mapping of geological features and structures

Ore bodies, igneous dykes, fault boundaries

157
Q

What is the benefit of geomagnetic surveys?

A

Cheap and easy to use in field

158
Q

What instruments are used in magnetic surveys?

A

Magnetometers

3 types

159
Q

What do magnetometers measure?

A

Horizontal and vertical components of the geomagnetic field or total field F

160
Q

What is the magnetic anomaly?

A

It is determined by subtracting the theoretical value from the observed value.
Like the gravity anomaly, it is what we are interested in

161
Q

Formula for magnetic anomaly?

A

∆F= Fobs - Ftheor

162
Q

What are the magnetic corrections?

A

Diurnal Corrections

Geomagnetic Correction

163
Q

What is the diurnal correction

A

daily variations in Earth’s magnetic field

164
Q

What is the geomagnetic correction

A

magnetic equivalent of latitude correction in gravity.

165
Q

What helps yield info about the geometry and depth of an anomaly?

A

Shape and amplitude of gravity anomaly data

166
Q

What is the problem with gravity anomaly data?

A

Different bodies can give similar anomalies

Sedimentary basin and granite pluton

167
Q

What else is useful to look at with gravity anomaly data?

A

2nd derivatives of vertical gradients can help indicate difference between bodies.

168
Q

Geological sources that change gravity are….

A

Variations in rock density
Faulting of sedimentary layers
Other lithological contacts

169
Q

What do residual fields show?

A

Local anomalies

such as those associated with young basic intrusions

170
Q

What is the bouguer formula

A

BAtot = BAreg + BAres

171
Q

What do regional anomaly waves look like?

A

Long-wavelength

172
Q

What do residual anomaly waves look like?

A

Shorter-wavelength

173
Q

What do long wavelength regional trends show?

A

Deep-seated crustal features

174
Q

What do short wavelength anomalies arise from?

A

Shallower geological features

175
Q

How do you find the residual anomaly?

A

BAtot - BAreg

176
Q

What is seismic acquisition?

A

Generation and recording of data

177
Q

What is seismic processing?

A

Alteration of seismic data to:
suppress noise
enhance signal
migrate seismic events

178
Q

What is seismic interpretation?

A

analysis of data to generate models

179
Q

What is a seismic trace?

A

A record of reflected arrival info received at each receiver

record of amplitude vs time

180
Q

How much does seismic data cost?

A

2-10x as much as processing

181
Q

Importance of good data?

A

No amount of processing can undo poorly collected data

182
Q

Objective of a good seismic trace

A

maximise recording of primary reflections

Minimise recording noise

183
Q

What are the types of seismic reflection gather?

A

Common source
Common receiver
Common offset
Common Mid-point

184
Q

What is CMP?

A

Common midpoint is the halfway point between source and receiver at the surface

185
Q

What is a CMP gather?

A

Traces with same CMP are put together.

186
Q

What is the usefulness of move-out corrections and stacking?

A

They result in redundancies of data that improves the signal-noise ratio.

187
Q

What is a seismic source?

A

A device that provides energy for acquisition of seismic data.

188
Q

Ideal characteristics of a seismic source?

A 
Strength
Bandwidth
Signal-noise
Consistency
Environmental Impact
189
Q

What are the two types of receivers for seismic acquisition on land and in the sea?

A

Geophones: velocity
Hydrophones: pressure

190
Q

What happens from acquisition to processing?

A

Raw shot records are combined

End result is a seismic stack

191
Q

What is NMO?

A

Process of aligning at the same time the same primary event from different offsets

A stack is what is created after NMO.

192
Q

What is a bright spot?

A

Results from increase in acoustic impedance contrast when hydrocarbons cause an increase in the reflection coefficient

193
Q

What is a dim spot?

A

A local low amplitude anomaly. Opposite of a bright spot.

194
Q

What are some fluids in Geology?

A

Water, Magma, Ice, Gases

195
Q

What is a solid?

A

If a material after some deformation, resits further deformation.

196
Q

What is a fluid?

A

A material that deforms indefinitely (ie it flows)

197
Q

What is viscosity of a fluid?

A

Thickness, stickiness of a fluid.

198
Q

What is the formula for strain rate?

A

Strain rate is velocity per unit length so,

Stress = n x e’

n= linear coefficient of viscosity e’ = strain rate

199
Q

What does viscosity in gases arise from?

A

Molecular diffusion

200
Q

Does pressure affect viscosity in gases?

A

No, visocosity of gases is usually independent of pressure.

201
Q

Relationship between temperature and viscosity in gases?

A

As temperature increases, so does the viscosity in gas.

202
Q

What does viscosity in liquids depend on?

A

Additional forces between molecules

203
Q

Is viscosity in liquids independent of pressure?

A

Yes, unless very high P

204
Q

What is the relationship between temperature and viscosity in liquids?

A

Viscosity in liquids tends to fall with an increase in temperature.

205
Q

What is the difference between viscosity of surface seawater at the poles and equator?

A

Surface seawater viscosity is about half that at the equator than the poles.

206
Q

What is the viscosity of molten glass?

A

10¹ - 10³

207
Q

What is the viscosity of basalt lava?

A

10¹ - 10³

208
Q

What is the viscosity of andesite lava?

A

10⁵ - 10⁶

209
Q

What is the viscosity of rhyolite lava?

A

10¹⁰ - 10¹¹

210
Q

What are the controls on magma viscosity?

A

SI02 content
Dissolved H20, F + Cl
C02
Proportion of crystals

211
Q

How does SIO2 affect magma viscosity?

A

More SI02 = more viscous

Less SI02 = less viscous

212
Q

Who was the Navier-Stokes Equation named after?

A

Claude-Louis Navier

George-Gabriel Stokes

213
Q

What is the Navier-Stokes Equation?

A

Describes the motion of fluid substances.

214
Q

What is the Navier-Stokes Equation used for?

A

Model weather
Ocean Currents
Water flow in a pipe
Air flow around a wing

215
Q

What is Bernoulli’s Principle?

A

The reduction of fluid pressure when fluid velocity increases.

216
Q

What is laminar flow?

A

Sometimes called streamline flow

Fluid flows in parallel layers, no disruption between layers

217
Q

What is turbulent flow?

A

Chaotic motions about the mean flow
rapid variations of pressure and velocity
opposite of laminar flow

218
Q

What does the transition from laminar to turbulent flow depend on?

A 
Reynolds Number (Re)
Re = v x L/V

v = velocity L=length travelled V= kinematic viscosity

219
Q

What are conditions for transition between laminar and turbulent flow?

A

Size of object gradually increasing
Viscosity of fluid is decreasing
Density of fluid increasing

220
Q

What its Stokes Law?

A

Concerned with settling velocity
u= D² x y’ / 18η

u = settling velocity D = grain size η= viscosity y’ = (ps - pw)g. ps= sediment pw = water

221
Q

What is a jet?

A

continuous source of momentum (product of mass+velocity) dominates buoyancy effect.

222
Q

What is a plume?

A

continuous source of buoyancy that dominates over momentum

223
Q

What is the difference between a jet and a plume?

A

Related to the buoyancy of a system vs its momentum

Quantified by its Richardson number (Ri)

224
Q

What is the Richardson Number?

A

ratio of potential energy to kinetic energy
Ri = gh/u²

g = acceleration due to gravity.     u = speed of system
h = height of system
225
Q

What does it mean when Ri > 1

A

Buoyancy is dominant in driving the fluid flow

226
Q

What does it mean when Ri < 1

A

Buoyancy unimportant, flow already has momentum

227
Q

What are the principles of plate tectonics?

A

Lithosphere broken up into rigid plates (continental or oceanic) which are in relative motion.

Plate boundaries are either mid-ocean ridges, subduction zones or transform faults

Deformation due to interactions occurs at boundaries

Lithosphere is somewhat elastic and capable of transmitting stress.

Geophysics (1 decks)

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