Exam 2 Review Flashcards
(119 cards)
1
Q
What is needed in order to image a layer?
A
A contrast in physical properties between the layers
2
Q
Define Seismic Impedance
A
A measure of how sharp the contrast between two layers is and thus how well we can detect a layer boundary using the reflection technique.
3
Q
What happens to the seismic energy as it encounters a boundary?
A
Some of the energy gets reflected but some of the energy is also transmitted through to the bottom layer.
4
Q
Seismic Impedence Equation I=
A
pv, where p is the denstiy of the layer and v is the velocity of the layer
5
Q
What are the two cases of seismic impedance?
A
1) The density and/or velocity of Layer 2 is large
2) p2v2 with respect to layer 1 is low
6
Q
Describe the first case of impedance
A
The density and/or velocity of Layer 2 is large - specifically the product of density x velocity (p2v2) is large with respect to that of layer 1. Hence the impedance is large and very little seismic energy will transmit through to the bottom layer.
7
Q
What is good about the first case of impedance?
A
It is good for detecting this layer boundary with reflection techniques since most of the energy will get reflected back.
8
Q
What is bad about the first case of impedance?
A
It is very bad for having any chance of detecting lower layers
9
Q
What is the second case of impedance?
A
If p2v2 is low with respect to Layer 1, then we have a weak reflection from the layer boundary which may be difficult to detect.
10
Q
What is the reflection coefficient good for?
A
We can directly quantify how much energy is reflected vs. transmitted.
11
Q
Reflection Coefficient Equation:
A
R= (I2-I1)/(I2+I1) = (p2v2 - p1v1)/(p2v2 + p1v1)
12
Q
What does the reflection coefficient equation only work for?
A
It only works for vertically incident waves! (DRAW DIAGRAM L09-2)
13
Q
Incoming wave energy =
A
reflected energy + transmitted energy
14
Q
Reflection Coefficient = R = (in words)
A
(Amplitude of reflected ray)/(Amplitude of incident ray)
15
Q
Transmission Coefficient = T = (in words)
A
(Amplitude of transmitted ray)/(Amplitude of incident ray)
16
Q
Transmission Coefficient (TC) = T =
A
T = (2*I1)/(I2 + I1) = (2p1v1)/(p2v2 + p1v1)
17
Q
In general, we cannot resolve layering if lambda ______ layer thickness
A
if wavelength is greater than layer thickness, can’t resolve layering
18
Q
What else do you need, aside from a high frequency source, to construct good seismic pulses?
A
a large bandwidth
19
Q
define bandwidth
A
many frequencies
20
Q
Draw diagram for Reflected wave travel times
A
L07-2
21
Q
For reflected arrivals, the angle of reflection is ______ to the angle of the incident ray
A
equal, sigma1=sigma2
22
Q
Total length of ray, 2L= (reflected)
A
sqrt(4h^2+x^2)
23
Q
Travel time, reflected = T =
A
2L/v1 = sqrt(4h^2+x^2)/v1
24
Q
Equation of line T^2, reflected linearized =
A
T^2 = (2h/v1)^2 + (1/v1)^2*x^2
25
Slope of T^2 reflected line =
(1/v1)^2
26
Intercept of T^2 reflected line =
(2h/v1)^2
27
What is x for a vertically incident reflection?
x=0
28
What is t0 the constant for a vertically incident reflection?
t0=2h/v1
29
What shape is this line?
hyperbola, if equation is rewritten this can be seen: T^2/t0^2 - x^2/(v1^2*t0^2) = 1
30
What does the travel time curve asymptotically approach?
A line with the slope 1/v1
31
Why does the travel time curve asymptotically approach this line?
Asymptotically approaches a line with the slope 1/v1, and it does this because increasing the distance between the source and receiver results in not much difference between the direct & reflected wave (draw diagrams that we drew in class review)
32
Draw and label the diagram of travel time curve for rays in a layer over a halfspace
See L07-6
33
Where is the source and receiver for the zero offset experiment?
Source and receiver at the same location
34
What do the seismograms reflect if velocity is constant with each layer?
If velocity is constant with each layer, then time scales linearly with depth, so that the seismograms reflect structure at depth
35
What are some potential challenges with zero-offset experiments?
Noise
time to propagate over layer thickness and back is less than the time length of source
Heterogeneity
Uncertainties in velocity structure
36
Describe the first possible problem with zero offset experiments: (with illustration and solution)
Single records can be noisy: reverberations of energy from near source and near receiver layers can contaminate observations, or, the reflector you wish to image may simply be too weak to generate an observed reflection
(see picture L07-8)
Solution: Use data from non-zero offset geometries and stack (sum up) the data.
37
Describe the second possible problem with zero offset experiments: (with illustration and solution)
The time to propagate over layer thickness and back (t1) is less than the time length of source (ts)
(see picture L07-8)
Solution: Deconvolution: strip the source signal off the seismogram
38
Describe the third possible problem with zero offset experiments: (with illustration and solution)
Heterogeneity may scatter energy to different (non-vertical) directions, biasing results that assume only vertical propagation paths.
(see picture L07-8)
Solution: Forward model: do a grid of scatterers, summing the data for each possibility ("migration")
39
Describe the fourth possible problem with zero offset experiments: (with illustration and solution)
Uncertainties in velocity structure make it difficult to establish depth to reflector (the classic trade off between v and z)
(see picture L07-8)
Solution: Get help from other means, e.g., drill holes and conduct refraction studies.
40
Signal to Noise ratio =
amplitude of signal/amplitude of noise
41
Basic idea of common mid-point stacking
The basic idea is that, if we can choose where we put our recorders and sources of energy, then we can build in some sampling redundancy in our approach, and ultimately stack multiple recordings to improve the signal-to-noise ratio (SNR) of our measurements.
Collect data that shares the same central reflection point off an interface
42
The common midpoint method is based on finding...
all source-receiver combinations that have the same bounce point - or common midpoint
43
Draw a diagram of what the graph first looks like when plotting for the common midpoint method
see last figure L08-2
44
What does the graph trace out for the common midpoint method?
hyperbola
45
What do you do once you have the initial graph for the common midpoint method?
Make a stack for each common midpoint, but first, we need to apply a timing correction for the hyperbolic travel-time moveout with distance.
46
timing correction for common midpoint method:
t(x)~~t0*(1 + 0.5(x/vt0)^2)
47
Define moveout:
the time difference between arrivals at two different distances
48
delta(t)=
t(x2) - t(x1) = (x2^2 -x1^2)/(2v^2t0)
49
Normal moveout (NMO) = (define and equation)
a travel-time moveout with respect to the station at a distance x1=0, =x^2/(2v^2t0)
50
Why is stacking useful?
a) to beat down noise that should add incoherently, if random, and b) to enhance coherent energy, such as the reflections of interest, which should add coherently.
51
What needs to be muted or zeroed out before stacking?
A lot of arrivals on the individual seismograms such as surface waves, shallow layer head waves, etc.
52
What must you apply if you are not on flat land for stacking?
One must apply elevation corrections if you are not on flat land. Otherwise, you are stacking seismograms that have unaccounted for time shifts in them
53
Huygen's Principle:
States that every point in a wave field can be viewed as a secondary source of energy. The coherent coalescing of the secondary expanded wavefronts is the new wavefront.
54
Draw a diagram for Huygen's Principle, with annotations
See L08-11
55
What is the benefit of incorporating Huygen's Principle into formulations?
Gives us a way to deal with complicated elastic media.
56
Define migration:
We can average data at any point by summing adjacent distances along distances along diffraction hyperbolas.
57
With dipping layers, where is the common midpoint?
It doesn't exist
58
How do you know which arrivals are due to multiples?
The key is to look at the intercept times
The intercept time for the 1st multiple will be 2x the intercept time of the primary arrival
The intercept time for the 2nd multiple will be 4x the intercept time of the primary arrival
59
How do you know which arrivals are due to multiples (using NMO correction)?
Another clue comes from applying the NMO correction as a best fit for the primary arrival. The secondary arrivals won't line up perfectly...so what happens then if you stack the arrivals is that you get a nice clean stack on the primary, but the secondary looks noisy and broadened.
60
How do earthquakes happen?
Elastic rebound theory
61
Elastic rebound theory:
As a rock gets stressed it initially undergoes strain elastically, but remember, there is a limit to how much stress a material can undergo before we pass through the elastic regime into plastic deformation. When the rock deforms plastically, when it breaks, it releases the energy we record on seismometers.
62
Where do earthquakes happen? By the elastic rebound theory, two criteria must be met:
1) There must be a movement that will stress material past the elastic limit
2) The material must be able to break by brittle fracture
So it occurs..in the LITHOSPHERE! (usually)
63
The size of the earthquake is directly proportional to how large of an _____ on the fault slips.
area
64
What is the best measure of the size of an earthquake?
Scalar seismic moment (M0)
65
Scalar Seismic Moment, M0 =
M0 = uDA
where
u = shear modulus (=rigidity)
D = fault displacement (=offset)
A = fault area (where offset occurred)
66
ML = local magnitude, who did it come from?
Charles Richter in the 1930s
67
ML equation =
ML = logA(delta) + 2.76log(delta) - 2.48
A is displacement amplitude in 10^-6 m
delta is distance in km, between 10-600 km
Measurements are made on a Wood-Anderson seismograph
68
ML center frequency:
0.8 sec
69
What is measured with ML?
When measuring magnitude based on the local magnitude scale, what is measured is the largest amplitude on the seismic trace (but at a very specific frequency range)
70
Mb: Body Wave Magnitude =
Mb = log(A/T) + Q(h,delta)
A=amplitude
T=period
Q=an empirical function of h,delta
71
What is measured with Mb?
Mb is usually measured from the first few cycles of the P-wave
72
Ms: surface wave magnitude =
Ms=log(A/T) + 1.66log(delta) + 3.3
A=maximum amplitude of the Rayleigh waves on the vertical component of motion.
73
What is Ms scale most appropriate for and why?
Surface waves are not efficiently generated for deep focus earthquakes. For this reason, the Ms scale is most appropriate for shallow focus events.
74
Mw: energy (or momement) magnitude =
Mw=(2/3)logM0-10.73
75
Who invented the Mw scale and what problem does it circument?
Hiroo Kanamori devised a scheme to circumvent the problem of saturation.
76
What is saturation?
There is an upper size limit to the magnitude that can be measured
77
What is the unit issues with Mw equation?
M0 equals the scalar seismic moment measured in dyne*cm,
78
10^5 dyne =
1 N
79
10^7 dyne*cm =
1 N*m
80
How can the total seismic energy, Es, be determined?
The total seismic energy, Es, can be determined from integrating the energy that comprises the total seismogram.
log(Es) = 5.8 + 2.4(mb)
=11.8 + 1.5Ms
81
What does a 1 unit increase in magnitude result in energy increase?
A 1 unit increase in magnitude results in roughly a 32x increase in energy release.
82
How can we tell an earthquake from an explosion?
Most discriminants are based on ratios of P and S wave energy. Both explosions and earthquakes generate P and S waves, however, the relative amount differs according to the type of source.
83
What has more S wave energy? Earthquakes or explosions?
Earthquakes
84
First assumption of plate tectonics:
1) The generation of new plate material occurs by sea-floor spreading. That is, new oceanic lithosphere is generated along the active mid-ocean ridges.
85
Second assumption of plate tectonics:
2) The new ocean lithosphere, once created, forms part of a rigid plate; this plate may or may not include continental material.
86
Third assumption of plate tectonics:
3) The earth's surface area remains constant; therefore sea-floor spreading must be balanced by consumption of plate elsewhere.
87
Fourth assumption of plate tectonics:
The lithospheric plates are capable of transmitting stresses over great horizontal distances without buckling. i.e., the relative motion between plates is taken up only along plate boundaries.
88
Where do earthquakes mostly occur?
Along plate boundaries
89
What are the three types of plate boundaries?
Divergent, Convergent, Transform
90
Define Divergent:
Divergent (constructive) - Here the plates are moving away from each other and new plate material is added to the lithosphere.
91
Where are divergent plate boundaries?
Mid Ocean Ridges
92
Define Convergent:
Convergent (destructive) - These are at subduction zones, where one of the colliding plates descends into the mantle and is destroyed.
93
Where are convergent plate boundaries?
Subduction zones
94
Define Transform:
Transform (conservative) - Here lithosphere is neither created or destroyed. Contact between two plates that slide horizontally past one another, commonly connecting two mid ocean ridges.
95
```
Up = (compression/tension) = (+/-)
Down = (compression/tension) = (+/-)
```
```
Up = compression (+)
Down = tension (-)
```
96
Why is the lower hemisphere used for the beach ball models?
The lower hemisphere is used because it shows the polarity of seismic wave exiting the earthquake source downwards, since these are the seismic rays that ultimately bend back up to the surface that we record at our distant seismic stations
97
Draw the three main types of beach balls and what they represent as well as the +/- in each region
See figure L11-10 bottom of page
98
So what happens at mid ocean ridges?
Earthquakes along MOR's are relatively few compared with those at transform faults and subduction zones, however, the ones we record are shallow and mostly show normal faults.
99
Most of the faults at slow spreading centers appear to dip towards the ______
ridge axis
100
For fast spreading centers, they dip towards _____
half towards ridge axis, other half dip away from axis.
101
What are the fault plane solutions at convergent boundaries dominated by?
Thrust faulting
102
Velocity of P wave=
Vp = sqrt ((k + (4/3)u)/p)
103
Velocity of S wave=
Vs=sqrt(u/p)
104
Seismic Impedance=
I=pv
105
Relation between wavelength, frequency, and wave speed:
f=w/2pi = 1/T = c/lamda
106
Two wave travel time for vertical incidence
t0=2h/v
107
Draw a travel time curve with direct, reflected, refracted waves. Label (with slopes) How are each lines identified?
See diagram from test review notes
108
Challenges associated with detecting thin layers:
We cant resolve structure that is less than the wavelength of the seismic source
If the structure has a thickness h
109
What does Mb measure?
p-wave amplitude
110
What is the period of Mb?
1 sec
111
What does ML measure?
largest amp on seismogram
112
What is the period of MK?
0.8s
113
What does Ms measure?
rayleigh amp
114
What is the period of Ms?
20 s
115
What does Mw measure?
moment of energy over entire ... trace?
116
What is the benefit of Mb?
quickest to measure, based on p-waves, first arriving, rapid hazard response
117
What is the benefit of ML?
easy to calculate, works well in some environments (S.cal Utah), accurate for local distances, relatively smaller earthquakes
118
What is the benefit of Ms?
Does a better job of estimating magnitude for large magnitude earthquakes
119
What is the benefit of Mw?
takes time to calculate, most accurate estimate
