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Geo 3010 > Exam 2 Review Condensed > Flashcards

Exam 2 Review Condensed Flashcards

1
Q

Velocity of p-wave:

A

Vp=sqrt(k + (4/3)u) / p)

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2
Q

Velocity of s-wave:

A

Vs=sqrt(u/p)

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3
Q

Seismic impedance equation:

A

I=pv

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4
Q

reflection coefficient equation:

A

R=I2-I1 / I2+I1

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5
Q

Relationship between wavelength, frequency, wave speed:

A

f=w/2pi=1/T=c/lamda

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6
Q

DRAW AND LABEL A TRAVEL TIME CURVE INCLUDING reflected arrival, t0, t, refracted, direct, and slopes for each line

A

See diagram in notes

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7
Q

Where is the source and receiver for zero offset experiments?

A

Some location

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8
Q

What is the first potential challenge of the zero offset experiment? Draw a diagram and present a solution.

A

1) 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

Solution: use data from non-zero offset geometries and stack (sum up) the data

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9
Q

What is the second potential challenge of the zero offset experiment? Draw a diagram and present a solution.

A

2) The time to propagate over layer thickness and back (t1) is less than the time length of source (ts)

Solution: deconvolution: strip the source signal off the seismogram

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10
Q

What is the third potential challenge of the zero offset experiment? Draw a diagram and present a solution.

A

3) Heterogeneity may scatter energy to different (non-vertical directions, biasing results that assume only vertical propagation paths.

Solution: forward model: do a grid search of all possible sources of scatterers, summing the data for each possibility (migration)

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11
Q

What is the fourth potential challenge of the zero offset experiment? Draw a diagram and present a solution.

A

4) uncertainties in velocity structure make it difficult to establish depth to reflector

Solution: get help from other means, e.g., drill holes and conduct refraction studies

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12
Q

What are zero offset experiments used for?

A

Used to map out structure

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13
Q

What are the four steps of the common midpoint method?

A

1) collect data w/common bounce point location
2) collect t-times, have hyperbolic moveout
3) normal moveout correction: calculate delta(t), move all
4) stack into one signal trace, results in one strong arrival, no noise

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14
Q

What is the point of stacking?

A

to increase signal to noise ration (where SNR is proportional to sqrt(N) and N is the number of signals in stack)

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15
Q

signal to noise ratio =

A

amplitude of signal/amplitude of noise

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16
Q

timing correction t(x) is approximately equal to

A

t0*[1 + 0.5(x/vt0)^2]

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17
Q

Define moveout:

A

The time difference between arrivals at two different distances

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18
Q

Equation for moveout (delta t):

A

delta t = x2^2 - x1^2 / 2v^2t0

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19
Q

define normal moveout:

A

a travel time moveout with respect to the station at a distance x1=0

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20
Q

Equation for normal moveout:

A

delta(t)NMO = x^2/2v^2t0

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21
Q

Why is stacking useful?

A

a) to beat down noise that should add incoherently, if random
b) to enhance coherent energy, such as the reflections of interest, which should add coherently

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22
Q

What needs to be muted out before stacking?

A

A lot of arrivals on the individual seismograms such as surface waves, shallow layer head waves, etc.

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23
Q

If not on flat land for stacking, one must apply…

A

…elevation corrections, otherwise, you are stacking seismograms that have unaccounted for time shifts in them.

24
Q

How do you know which arrivals are due to multiples?

A

Look at 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

Look at NMO correction as a best fit for the primary arrival
-clean stack at primary, secondary looks noisy & broadened.

25
Q

What do you need for seismic impedance and reflection coefficient?

A

Need a physical contrast between layers

26
Q

R = reflection coefficient = (in words)

A

amplitude of reflected wave/amplitude of incident wave

27
Q

The reflection coefficient only works for….

A

…vertically incident waves

28
Q

What is the reflection coefficient good for?

A

We can directly quantify how much energy is reflected vs. transmitted.

29
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

30
Q

First case of impedance:

A

I(layer 2)&raquo_space; I(layer1)

impedance is large, and very little seismic energy will transmit through to the bottom layer

good for detecting layer boundary since most energy will get reflected back

very bad for having any chance of detecting lower layers

31
Q

Second case of impedance:

A

I(layer 2) ~> or ~< I(layer 1)

weak reflection from the layer boundary, which may be difficult to detect

32
Q

Challenges associated with detecting thin layers:

A

We cant resolve structure that is less than the wavelength of the seismic source
If the structure has a thickness h

33
Q

How to see a fault in reflection profiles?

A

Faults seen be offset

34
Q

What is Mb?

A

body wave magnitude

35
Q

What does Mb measure?

A

measures p-wave amp

36
Q

What is the period of Mb?

A

1 s

37
Q

Why is Mb useful?

A

Quickest to measure, based on p-waves, first arriving, rapid hazard response

38
Q

What is ML?

A

local wave magnitude

39
Q

What does ML measure?

A

largest amp on seismogram

40
Q

What is the period of ML?

A

Wood anderson central period, 0.8s

41
Q

Why is ML useful?

A

Easy to calculate, works well in some environments (S. Cal, UT), accurate for local distances, relatively smaller EQ’s

42
Q

What is Ms?

A

Surface wave mag

43
Q

What does Ms measure?

A

Rayleigh amp

44
Q

What is the period of Ms?

A

20 s

45
Q

Why is Ms useful?

A

does a better job estimating magnitude for large mag EQ

46
Q

What is Mw?

A

moment wave mag

47
Q

What does Mw measure?

A

moment of energy over entire trace, related to Mo

48
Q

What is the period of Mw?

A

XXXX

49
Q

Why is Mw useful?

A

Takes time to calculate, most accurate estimate

50
Q

Draw and label each of the beachballs

A

See notes

51
Q

What fault is divergent and where is it?

A

normal, mid ocean ridges

52
Q

What fault is convergent and where is it?

A

reverse, subduction zones

53
Q

What fault is transform and where is it?

A

strike slip, plate boundaries

54
Q

What are the steps of t^2 - x^2 technique?

A 
w/ two points calculate slope
slope=1/v1^2, v1=1/sqrt(m)
t0^2=intercept
t0=2h/v1 = sqrt(intercept)
h=t0v1/2
55
Q

two way travel time for vertical incidence t0=

A

t0=2h/v

56
Q

why does the hyperbola asymptotically approach direct arrival?

A

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)

Geo 3010 (7 decks)

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