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GeoPhysics > Riley's deck > Flashcards

Riley's deck Flashcards

1
Q

What is geophysics?

A

A subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space.

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2
Q

What does it study/consist of?

A

shape of earth, plate tectonics internal structure, earthquakes and volcano prediction, fluid dynamics, gravity, magnetics, EXPLORATION SEIMOLOGY

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3
Q

What is sea level?

A

Sea level is often referred to as an average level for the surface of one or more of Earth’s oceans from which heights such as elevations may be measured. established by the local Ellipsoid.

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4
Q

What does the surface look like? What are the low and high spots?

A

The geoid does not precisely follow mean sea level, nor does it exactly correspond with the topography of the dry land. It is irregular like the terrestrial surface. It is bumpy. Uneven distribution of the mass of the planet makes it so. If the solid earth had no internal anomalies of density, the geoid would be smooth and almost exactly ellipsoidal. But like the earth itself, the geoid defies such mathematical consistency and departs from true ellipsoidal form by as much as 100 meters in places. Equipotential surface (low spot -100m, high spot 80m)

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5
Q

What two things are considered in the measurement of gravity? Does movement affect “gravity”?

A

earths- mass and rotation

YES (airplanes, with or against rotation)

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6
Q

Must know about ellipsoid !

A

Result of the Earth’s rotation.
pear-shape
Local ellipsoid
Global best fit
Local best fit (North American Datum NAD-83)
UTM 6o zones at equator measurements in m.
Square grid to poles.

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7
Q

What is earths circumference and its radius about the poles and equator.? What is the average radius?

A 
40,000 km circumference 
Difference between the Equator and Poles is approximately 21 km
REqu = 6378.136 km
RPol = 6356.751 km
6371 km average
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8
Q

What is up?

A

vertical from the geoid, plum bob ( weight suspended by string)

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9
Q

What are Geographic coordinates measured in ?

A

Latitude- Degrees-Min-Sec, Decimal degrees

Longitude

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10
Q

what is the UTM?

A

Universal Transvers Mercator

North American Datum (NAD) 27, NAD-83 100m
World Geodetic System (GS84) 
6 degree zones
670 km wide
x, y coordinates in m (plus the zone)
Does not align with geographic direction
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11
Q

What is seismology ?

A

Seismology: Scientific study of the propagation of seismic waves through the earth (often related to earthquakes)

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12
Q

Which one is an instrument used for recording seismic waves? Seismograph or Seismogram.

A

Seismograph!

seismogram is a record of the motion of a passing seismic wave as a function of time.

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13
Q

What are the surface waves ?

A

Rayleigh

love waves

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14
Q

What are the Body waves ?

A

P-waves

S-waves

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15
Q

What are P-waves characteristics?

A 
Primary
Push-Pull  (Compression and dilatation)
Displacement longitudinal
Fastest
Travel in solids and fluids
Most common in exploration 
Longitudinal
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16
Q

What are S-waves characteristics?

A 
Secondary
Shear or transverse
Shake
Displaced perpendicular
Velocity is ½ to the P-wave velocity
double amplitude
Does not propagate through a fluid
transverse
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17
Q

Uses for seismology?

A

inferences of earths interior
find hydrocarbons
find minerals like gold
ANALOGUES- body imaging- Ultrasounds

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18
Q

characteristics of Rayleigh and Love waves?

A

Propagate along the earth’s surface.
Particles travel perpendicular to propagation direction.
highly energetic near the surface
Velocity is much slower than P-wave and S-wave modes
Waves are low frequency
Destructive on land

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19
Q

What mechanism is responsible for earthquakes?

A

Plate is subducting underneath another Plate
and Stick-slip process** along the Zone is the mechanism responsible for earthquakes

**see page 34 lecture 1 for illustration ( essentially plate undergoing subduction sticks and subsequently slips after causing thrust fault)

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20
Q

What is the shadow zone ?

A

Earth’s area from angular distances of 104 to 140 degrees from a given earthquake that does not receive any direct P waves. The shadow zone results from S waves being stopped entirely by the liquid core and P waves being bent (refracted) by the liquid core.

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21
Q

Can waves be converted?

A

Yes.
occurs on interfaces or layer boundaries
every time a wave is reflected it may also be partially converted into a different polarization.

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22
Q

What is a dormant volcano?

A

A dormant volcano is an active volcano that is not erupting, but supposed to erupt again.

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23
Q

How well are we able to predict volcanoes, earthquakes, tsunamis?

A

Volcanoes: quite good

Earthquakes: very poor

Tsunamis: poor

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24
Q

What is a geophone ?

A

Device that converts ground movement (displacement) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure of the earth.

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25
Q

typical sample rate for seismic data ? Using what time source ?

A

0.002 sec. GPS timing

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26
Q

See pages 70-70 L1 for seismic charts

A

.

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27
Q

describe process of processing flow ?

A

Data prep.- (correlation, gain recovery, editing)

Geometric -( CMP sorting, Elevation static, Uphole static, Velocity analysis)

Data Enhancement - ( mute, filters, demultiple, stack, trace balancing)

Imaging -( migration 2D or 3D, time or depth, inversion, Full waveform inversion (FWI))

Interpretation -( Tying well logs )

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28
Q

Why are Reflections curved from source record ?

A

longer raypath

more travel time

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29
Q

source record formula from page 37, L2

A

.

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30
Q

What is CDP?

A

common depth point

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31
Q

What is CMP?

A

common midpoint gathers

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32
Q

How are source gathers and CMP gather similar?

A

Horizontal data they appear the same.
same velocity at scatter point
same reflection point (CDP)

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33
Q

do you want to sum data in same CMP gather?

A

yes.

cant sum because of moveout

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34
Q

how do you find V ?

A

get T0, T, and h - solve for V

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35
Q

very near surface - Do you use the first arrival times

A

Yes

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36
Q

How to learn sub-surface structure ?

A

Estimate using surface geology
Ground penetrating radar
First breaks

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37
Q

What is First breaks analysis ?

A

source record
refraction analyses ( Layered medium*)
diff slopes represent diff layers and diff velocities

38
Q

First breaks result from ?

A

refraction

39
Q

pg 49 L2 question

A

look

40
Q

what is snells law?

A

formula used to describe the relationship between the angles of incidence and refraction, referring to waves passing through a boundary between two different isotropic media, such as water, glass, or air.

41
Q

Refraction ray paths and wave fronts- Is there energy injected beyond the critical angle ?

A

NO

42
Q

Refraction ray paths and wave fronts- Does energy propagate layer to layer.

A

Energy in second layer propagates upward into the first layer.

43
Q

What do you use the intercept time for ?

A

get thickness of the first layer

44
Q

How to correct surface travel time ?

A

travel time used to correct all the times on a seismic trace

45
Q

know about velocities

A 
Assumed linear raypaths
can be complex
CMP analysis estimates a hyperbolic approximations
May require raytracing
Velocities more complex
-RMS velocities
-Average velocities
-Interval velocities
46
Q

Greatest discovery in geophysics?

A

RMS velocity

47
Q

RMS velocities used for ?

A

In layered media.

Ray-times to surface use one velocity VRMS.

Used billions of times each day.

48
Q

Shape of RMS velocity ?

A

bell shaped
can be approx. by a hyperbola
Complex rays in depth == straight rays in time**

49
Q

data in recording is called ?

A

Mega data

50
Q

Deconvolution ?

A

math algorithm, remove convolution

51
Q

what is tomography ?

A

Start with a blocky velocity model
Shoot ray paths through current model
Least squares

52
Q

drum head?

A

movement stationary on a circular boundary

53
Q

modes of propagation?

A

mo

54
Q

Which waves pass through polarization?

A

waves vibrating perpendicular to the highway

55
Q

mode of oscillation for Tacoma Narrows Bridge? longitudinal or transverse

A

longitudinal or transverse

56
Q

What is NMO correction

A

Normal moveout correction, hyperbolic moveout

57
Q

Location of reflection energy.- What is migration?

A

Removal of hyperbolic energy
corrects miss positioning
Collapses diffractions back to their sources

58
Q

Mass of big shot on the large refraction project

A

5000kg shot

59
Q

What is PNE’s

A

peaceful nuclear explosions

60
Q

How many PNE’s did Russia’s project have ?

A

19

61
Q

What is greens function?

A

Impulse response

62
Q

Where does diffracted energy come from?

A

mike makes it

63
Q

Kirchhoff window migration?

A

data migrated to any or a part of a window

64
Q

Modeling characteristics

A

Modeling

  • forward process
  • Sum energy on a semicircle
  • spread energy on a diffraction
  • impulse response
65
Q

Migration characteristics

A 
Reverse process*
Sum energy on a diffraction
Spread energy on a semicircle
Impulse response
Transpose process*
66
Q

What is inversion ?

A

process of transforming seismic reflection data into a quantitative rock-property description of a reservoir

67
Q

Diffraction matrix 4D (dimensions) memory size ?

A

Small 2D data (20x20)
-4x108 Words

Real data  (1000X2000)
-4x1012

Can be up to 5D
Huge memory
1016 bytes

68
Q

True or false migration matrix is transpose

A

true

69
Q

r= D^-1 Possible ?

A

nope

70
Q

Formula for modelling

A

Dr = s

71
Q

how many reverse processes are approximated by the transpose processes?

A

many

more stable and less costly

72
Q

Why is inversion non ideal?

A

Very expensive and time consuming.

“Absurd” memory requirements.

73
Q

impulse response is what shape?

A

semi-circle

74
Q

Strain is ?

A

Distortion

75
Q

How many components of stress are there?

A

9

76
Q

3D shear strain. How many components from top surface?

A

2

77
Q

How many components of the spring constant, modulus

A

81

78
Q

How many components are independent?

A

21

79
Q

What does the symmetrical reduction give you ?

A

reduce 9 components to 6

80
Q

The two parameters for isotropic media?

A

lambda and viscosity

81
Q

Know properties

A

Bulk modulus: Volume stress/strain (K) fluid
Shear modulus: Rigidity, angular
Young’s modulus- Longitudinal
Lame or Lambda- fluid incompressibility
Poisson’s ratio- lateral strain/ longitudinal strain (rod)

82
Q

effective stress=

A

total stress - pore pressure

83
Q

what is a gradient ?

A

Forces not equal and opposite

84
Q

What are the signal processing aspects ?

A

Sinusoids
convolution
Fourier transform

85
Q

convolution formula

A

Wr= s

86
Q

deconvolution formula

A

r = (w^-1)

87
Q

How do you get the inverse of the wavelet?

A

z transform
least squares
Fourier transform

88
Q

Fourier transform also known as ?

A

spectral whitening

89
Q

what do you compute with Fourier analysis?

A

Compute the amplitudes of the sinusoids.

N = 2^M, 2^10 = 1024 (FFT)

90
Q

what is FFT

A

fast fourier transform

91
Q

what is correlation?

A

detection

similar to convulsion

GeoPhysics (9 decks)

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