Geophysics Flashcards
(227 cards)
1
Q
What is Rheology?
A
Study of flow (how things deform: stress to strain)
2
Q
What are the Earth’s Potential Fields?
A
Gravity field, Magnetic Field
3
Q
Why do we study the Earth’s Potential Fields?
A
To measure their deviations at the Earth’s surface
4
Q
What is Seismology?
A
Concerned with wave propagation through Earth
5
Q
Does seismology change depending on scale?
A
No, Earth scale vs high-res uses the same methods
6
Q
What changes with scale in Seismology?
A
The Frequency of the wave
7
Q
What is the Lithosphere?
A
Crust + some Upper Mantle
8
Q
What is the Moho?
A
Base of the crust
9
Q
How deep is the Moho?
A
Differing depths of Moho across Earth
10
Q
Where is the Moho deepest?
A
Deepest under continents
11
Q
Where is the Moho shallowest?
A
Oceanic crust (uniform thickness)
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.
13
Q
Who discovered the seismic Moho?
A
Andrija Mohorovic
14
Q
On continental crust, where is it thickest and thinnest?
A
Thickest under mountains, thinnest under plains
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
16
Q
What increase in speed do seismic waves experience when crossing Moho?
A
6 to 8km/s
17
Q
What is the crust like around Aberdeen?
A
Thinner as near the sea
18
Q
What is a Seismic Wave?
A
Energy (elastic strain) that travels away from a source
19
Q
What is a body wave?
A
Seismic wave that travels through the bulk of a medium
20
Q
What is a surface wave?
A
Travels along the interface between 2 media with different material properties.
21
Q
What are P-waves?
A
Body waves (primary, compressional waves)
22
Q
What are S-waves?
A
Body waves (secondary, shear waves)
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
24
Q
Explain deformation of S-waves
A
Alternating transverse motion
Particle motion is PERPENDICULAR to direction of propagation
25
What are the two types of Surface waves?
Love Waves and Rayleigh Waves
26
Explain deformation of Rayleigh waves
travel along interface of two media
Amplitude decreases with depth
Looks wavy in a diagram
27
Explain deformation of Love waves
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
28
What does velocities of seismic waves depend on?
Elastic properties and density of rock which the waves travel through
29
What is the unit 'P'
Density
30
What is the unit 'K'
Bulk Modulus
31
What is the unit 'μ'
Shear Modulus
32
Do S-waves propagate through fluids?
As shear modulus in fluids = 0, velocity of S-waves = 0
33
Do S-waves travel through the liquid core?
No - creates observation of arc 105 degrees from epicentre
34
Do P-waves travel through the core-mantle boundary?
No - P-waves are strongly refracted by boundary
| Creates a shadow zone from 105-140 degrees.
35
General rule for velocity of P-waves?
velocity increases with depth
36
What happens to velocity of P-waves in asthenosphere?
Rock is hot enough that is it plastic and therefore waves slow down (low velocity zone)
37
What happens at the 660km discontinuity?
Velocity of P-waves rapidly increase due to increase in pressure between Upper and Lower Mantle
38
What is Active Seismology?
The use of purpose-built sources of seismic waves for investigations into Earth's Crust and oil/gas exploration. .
39
What are the units of Length
m
40
What are the units of Mass
kg
41
What are the units of Time
s
42
What are the dimensions of a quantity
Refer to the physical entities and their units which are intrinsic to that quantity
43
What is the principle of dimensional homogeneity?
The quantity in each term must have the same dimensions (physical units).
Terms can also be dimensionless
A = BD+C (same units)
44
The displacement (s) of a vehicle moving at a constant acceleration depends on:
```
Elapsed time (t)
Acceleration (a)
```
45
Is displacement a kinematic equation?
Yes, it deals with position in time, displacement, velocity or acceleration of something
s=kat
46
Is displacement vector or tensor quantity?
Vector (m)
| magnitude, direction, origin
47
Part of the strain of an object is its change in....
length per unit length (elongation)
48
Is strain a tensor or vector quantity
Tensor
| magnitude, direction of displacement gradient, direction of plane of action
49
What are the dimensions of strain
strain is dimensionless
50
Is velocity a tensor or vector quantity?
Vector
| magnitude, direction, origin
51
What is velocity?
Rate of change of displacment
| Change in length/change in time
52
Units of velocity?
m/s or ms'-1
53
Is acceleration a vector or tensor quantity?
Vector
| Rate of change (in time) of velocity
54
Units of acceleration?
m/s/s or ms'-2
55
Is force a vector or tensor quantity?
Vector
| magnitude, direction, origin
56
Units of force?
Newton (N)
| 1 N = 1 kgms'-2
57
Is stress a vector or tensor quantity?
Tensor
| magnitude, direction of forces, direction of strain action
58
Units of stress?
Pascal (Pa)
| 1 Pa = 1 Nm'-2
59
What principle stress do normal faults have?
stress 1 - most compressive (vertical)
60
What principle stress do thrust/reverse faults have?
stress 3 - most tensile (vertical)
61
What principle stress do strike/slip faults have?
stress 2 - intermediate (vertical)
62
What is Lithostatic Stress?
Stress due to the weight of overburden rock - vertical principle stress.
63
What is Tectonic Stress?
Stress due to plate forces - horizontal principle stress
64
What is elasticity?
Stress produces instantaneous strain that propagates away from the source.
65
What is plasticity?
Permanent changes of shape (strain in response to stress above/at yield.
66
What is viscosity?
A given shear stress produces a flow (deformation over time) at constant rate (strain rate)
67
Is stress a dynamic or kinematic quantity?
Dynamic
68
Is strain dynamic or kinematic quantity?
Kinematic
69
what is k'
related to bulk modulus (elastic response to normal stress)
70
what is μ'
related to shear stress (elastic response to sheer stress)
71
What is the bulk modulus k' also called
Young's modulus (E)
72
What is the bulk modulus?
Ratio of external pressure change to volume change as a proportion of initial volume.
73
What is the relationship between strain rate and sheer stress in newtonian viscosity?
Strain rate is PROPORTIONAL to applied shear stress.
74
What is the relationship between strain rate and sheer stress in non-newtonian viscosity?
Strain rate is NOT LINEARLY PROPORTIONAL to applied shear stress.
75
Elasticity is the rheological behaviour that applies to...
solids
76
Viscosity is the rheological behaviour that applies to...
fluids
77
Plasticity is the rheological behaviour that applies to...
both solids and fluids (irreversible deformation)
78
What is plasticity?
Permanent deformation occurring after a yield stress is reached.
79
What are the two types of plastic deformation?
Ductile or brittle deformation
80
What are the two types of viscosity?
Dynamic Viscosity and Kinematic Viscosity
81
What is dynamic viscosity?
Relationship between stress and strain rate
82
What is kinematic viscosity?
A quantity where no dynamic quantities are involved
83
Units of dynamic viscosity are..
Poise (nm'-2)
84
What is a Poise the same as?
Pa s
85
What are the units of kinematic viscosity?
m'2 s'-1
86
What are the applications of Seismic methods?
Hydrocarbon Exploration
Mineral Exploration
Hi-Res shallow Exploration
87
What is the purpose of seismic surveys?
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.
88
What is useful about travel-times?
Travel-times allow seismic wave paths to be inferred and seismic wave velocities to be calculated.
89
What is Snell's Law of refraction?
sin'i'/sin'r' = v1/v2 or. sin∅1/v1 = sin∅2/v2
90
What is the law of reflection?
Angle of Incidence = Angle of Reflection
| Largest reflection is for the normal ray (I = 90)
91
What determines the angles of refraction/reflection?
Velocity contrasts between layers
92
What is acoustic impedance?
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
Formula for acoustic impedance?
```
z = p x v
p = density
v = seismic velocity
```
94
What is the purpose of seismic refraction surveys?
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
What is the purpose of seismic reflection surveys?
Acquire info about subsurface from P-waves or S-waves that are critically reflected on a boundary with an acoustic impedance contrast
96
What is the formula for direct ray?
```
Td= X/V1
x = offset
V1 = velocity of boundary 1
```
97
What line is for the direct ray arrivals?
The straight line with slope 1/V1
98
What line is for the refracted arrivals?
Straight line with slope 1/V2
99
What is the line for reflected arrivals?
The hyperbolic curve
100
What is the critical distance?
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
What is the crossover distance?
Offset at which the critically refracted waves takes over the direct wave (precedes).
102
What is the outcome of a Seismic survey?
Wide-Angle reflection and refraction (WARR)
103
Does the reflected phase ever arrive first?
NO - reflected phase is never first to arrive
104
What is done after a seismic reflection survey?
Seismic traces are stacked and manipulated to improve quality of arrivals.
105
What is the 1st stage of SR processing?
Display all traces from a single airgun shot
106
What is the 2nd stage of SR processing?
Delete noisy traces
107
What is the 3rd stage of SR processing?
Correct amplitudes for energy loss during long travel times
108
What is the 4th stage of SR processing?
Select traces with a common depth point (CDP gather)
109
What is the 5th stage of SR processing?
Remove direct wave (mute)
110
What is the 6th stage of SR processing?
Correct travel-times for range dependent delays
111
What does stacking do?
Improves quality of 'signal' which is otherwise embedded in the noise.
112
What is the 7th stage of SR processing?
Sum all the traces in a CDP gather (stack)
113
What is the 8th stage of SR processing?
Compress signal wavelet (better resolution)
114
What is the 9th stage of SR processing?
Display all stack traces
115
What is the 10th/final stage of SR processing?
Interpet geology
116
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?
yes
117
Is it true the higher the velocity of the propagation medium, then the larger the wavelength of the seismic pulse, the lower the resolution.
yes
118
What is migration
fixes the position of seismic reflectors in real life on a graph
119
Name one thing migration can remove
Bowties
120
Name the sources of seismic waves used in active seismology on land
Weight drops
Guns
Explosives
Vibroseis
121
Name the sources of seismic waves used in the ocean
Sparkers
Boomers
airguns
122
Name the receivers of seismic waves used in the ocean
Ocean Bottom Cables
| Hydrophones
123
Name the receivers of seismic waves used on land
Geophones
124
What is gravity surveying?
Measuring variations in Earth's gravitational field caused by differences in density of subsurface rocks.
125
What is magnetic surveying?
Measuring variation in Earth's magnetic field caused by differences in magnetic properties of rocks in subsurface
126
What are variations measured in?
mGal (10'-3 Gal)
127
What is the geomagnetic field described in terms of?
Declination
Inclination
Total Magnetic Force Vector
128
What are the magnetic units?
Tesla (Nm/A)
| Normally measured in nT.
129
Where is flux density greatest?
Flux density greater at poles than equator.
| Decreases with increasing distance from centre to earth.
130
Order rocks in order of magnetic susceptibility
Basic Ig Highest
Acid Ig
Metamorphic
Sedimentary Lowest
131
Do magnetised rocks have remnant magnetisation?
No, magnetised rocks have their own induced magnetisation.
Magnetic minerals within a rock may have remnant and induced magnetisation.
132
Is the direction and magnitude of remnant and induced magnetisation always the same?
No, the magnitude and direction of either magnetisation can be different.
133
What is the intensity of induced magnetisation based on?
Based on magnetic susceptibility
134
What dictates the amplitude and shape of an observed magnetic anomaly?
The magnitude and direction of the resultant magnetisation.
135
What gives remnant magnetisation?
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
What gives induced magnetisation?
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
What are the large scale applications of gravity surveying?
Estimation of crustal thickness
| low density crust over higher density mantle.
138
What are the medium scale applications of gravity surveying?
Hydrocarbon industry
| location of salt domes
139
What are the small scale applications of gravity surveying?
```
Mapping bedrock topography
Mineral exploration (massive density compared to host rock)
```
140
What are the micro scale applications of gravity surveying?
Finding caves/tombs
| Cavities
141
How do we measure gravity at sea?
Lower meter to seafloor
| Onboard a ship
142
How do we measure gravity in air?
Helicopter
| Plane
143
What is the gravity anomaly?
We are interested in the difference between a theoretical value and a measurement at a base somewhere. This difference is the gravity anomaly.
144
Formula for gravity anomaly?
∆g = Gobs - Gtheor or// ∆g = Gobs - Gbase
145
Before it can be interpreted, the anomaly data must be corrected. How do we do this?
Using gravity data reduction
| ∆g = Gobs + ΣGcorr - Gtheor
146
Why do we correct the gravity anomaly?
1. Drift Correction
2. Tidal Correction
3. Latitude Correction
4. Free-air Correction
5. Bouguer Correction
6. Terrain Correction
147
What is drift correction?
Instrument drift corrected by linear interpolation
148
What is latitude correction?
gravity is less at equators than poles
149
What is tidal correction?
variations of gravity due to changing positions of sun and moon easily calculated
150
What is free-air correction?
Gravity decreases with distance from centre to earth. Measurements adjusted for elevation differences compared to constant sea level.
151
What is the bouguer correction?
accounts for gravitational effects of rocks between the observation elevation and datum elevation
152
What is the terrain correction?
correction for divergence of actual topography survey
153
Why is the Bouguer Anomaly important?
Variations in the bouguer anomaly reflects the lateral variations in density
154
What does a positive bouguer anomaly mean?
High density feature in low density medium
155
What does a negative bouguer anomaly mean?
Low density feature in high density medium
156
Applications of geomagnetic surveying?
Mapping of geological features and structures
| Ore bodies, igneous dykes, fault boundaries
157
What is the benefit of geomagnetic surveys?
Cheap and easy to use in field
158
What instruments are used in magnetic surveys?
Magnetometers
| 3 types
159
What do magnetometers measure?
Horizontal and vertical components of the geomagnetic field or total field F
160
What is the magnetic anomaly?
It is determined by subtracting the theoretical value from the observed value.
Like the gravity anomaly, it is what we are interested in
161
Formula for magnetic anomaly?
∆F= Fobs - Ftheor
162
What are the magnetic corrections?
Diurnal Corrections
| Geomagnetic Correction
163
What is the diurnal correction
daily variations in Earth's magnetic field
164
What is the geomagnetic correction
magnetic equivalent of latitude correction in gravity.
165
What helps yield info about the geometry and depth of an anomaly?
Shape and amplitude of gravity anomaly data
166
What is the problem with gravity anomaly data?
Different bodies can give similar anomalies
| Sedimentary basin and granite pluton
167
What else is useful to look at with gravity anomaly data?
2nd derivatives of vertical gradients can help indicate difference between bodies.
168
Geological sources that change gravity are....
Variations in rock density
Faulting of sedimentary layers
Other lithological contacts
169
What do residual fields show?
Local anomalies
| such as those associated with young basic intrusions
170
What is the bouguer formula
BAtot = BAreg + BAres
171
What do regional anomaly waves look like?
Long-wavelength
172
What do residual anomaly waves look like?
Shorter-wavelength
173
What do long wavelength regional trends show?
Deep-seated crustal features
174
What do short wavelength anomalies arise from?
Shallower geological features
175
How do you find the residual anomaly?
BAtot - BAreg
176
What is seismic acquisition?
Generation and recording of data
177
What is seismic processing?
Alteration of seismic data to:
suppress noise
enhance signal
migrate seismic events
178
What is seismic interpretation?
analysis of data to generate models
179
What is a seismic trace?
A record of reflected arrival info received at each receiver
record of amplitude vs time
180
How much does seismic data cost?
2-10x as much as processing
181
Importance of good data?
No amount of processing can undo poorly collected data
182
Objective of a good seismic trace
maximise recording of primary reflections
| Minimise recording noise
183
What are the types of seismic reflection gather?
Common source
Common receiver
Common offset
Common Mid-point
184
What is CMP?
Common midpoint is the halfway point between source and receiver at the surface
185
What is a CMP gather?
Traces with same CMP are put together.
186
What is the usefulness of move-out corrections and stacking?
They result in redundancies of data that improves the signal-noise ratio.
187
What is a seismic source?
A device that provides energy for acquisition of seismic data.
188
Ideal characteristics of a seismic source?
```
Strength
Bandwidth
Signal-noise
Consistency
Environmental Impact
```
189
What are the two types of receivers for seismic acquisition on land and in the sea?
Geophones: velocity
Hydrophones: pressure
190
What happens from acquisition to processing?
Raw shot records are combined
| End result is a seismic stack
191
What is NMO?
Process of aligning at the same time the same primary event from different offsets
A stack is what is created after NMO.
192
What is a bright spot?
Results from increase in acoustic impedance contrast when hydrocarbons cause an increase in the reflection coefficient
193
What is a dim spot?
A local low amplitude anomaly. Opposite of a bright spot.
194
What are some fluids in Geology?
Water, Magma, Ice, Gases
195
What is a solid?
If a material after some deformation, resits further deformation.
196
What is a fluid?
A material that deforms indefinitely (ie it flows)
197
What is viscosity of a fluid?
Thickness, stickiness of a fluid.
198
What is the formula for strain rate?
Strain rate is velocity per unit length so,
Stress = n x e'
n= linear coefficient of viscosity e' = strain rate
199
What does viscosity in gases arise from?
Molecular diffusion
200
Does pressure affect viscosity in gases?
No, visocosity of gases is usually independent of pressure.
201
Relationship between temperature and viscosity in gases?
As temperature increases, so does the viscosity in gas.
202
What does viscosity in liquids depend on?
Additional forces between molecules
203
Is viscosity in liquids independent of pressure?
Yes, unless very high P
204
What is the relationship between temperature and viscosity in liquids?
Viscosity in liquids tends to fall with an increase in temperature.
205
What is the difference between viscosity of surface seawater at the poles and equator?
Surface seawater viscosity is about half that at the equator than the poles.
206
What is the viscosity of molten glass?
10¹ - 10³
207
What is the viscosity of basalt lava?
10¹ - 10³
208
What is the viscosity of andesite lava?
10⁵ - 10⁶
209
What is the viscosity of rhyolite lava?
10¹⁰ - 10¹¹
210
What are the controls on magma viscosity?
SI02 content
Dissolved H20, F + Cl
C02
Proportion of crystals
211
How does SIO2 affect magma viscosity?
More SI02 = more viscous
| Less SI02 = less viscous
212
Who was the Navier-Stokes Equation named after?
Claude-Louis Navier
| George-Gabriel Stokes
213
What is the Navier-Stokes Equation?
Describes the motion of fluid substances.
214
What is the Navier-Stokes Equation used for?
Model weather
Ocean Currents
Water flow in a pipe
Air flow around a wing
215
What is Bernoulli's Principle?
The reduction of fluid pressure when fluid velocity increases.
216
What is laminar flow?
Sometimes called streamline flow
| Fluid flows in parallel layers, no disruption between layers
217
What is turbulent flow?
Chaotic motions about the mean flow
rapid variations of pressure and velocity
opposite of laminar flow
218
What does the transition from laminar to turbulent flow depend on?
```
Reynolds Number (Re)
Re = v x L/V
```
v = velocity L=length travelled V= kinematic viscosity
219
What are conditions for transition between laminar and turbulent flow?
Size of object gradually increasing
Viscosity of fluid is decreasing
Density of fluid increasing
220
What its Stokes Law?
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
What is a jet?
continuous source of momentum (product of mass+velocity) dominates buoyancy effect.
222
What is a plume?
continuous source of buoyancy that dominates over momentum
223
What is the difference between a jet and a plume?
Related to the buoyancy of a system vs its momentum
| Quantified by its Richardson number (Ri)
224
What is the Richardson Number?
ratio of potential energy to kinetic energy
Ri = gh/u²
```
g = acceleration due to gravity. u = speed of system
h = height of system
```
225
What does it mean when Ri > 1
Buoyancy is dominant in driving the fluid flow
226
What does it mean when Ri < 1
Buoyancy unimportant, flow already has momentum
227
What are the principles of plate tectonics?
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.
