Final

Question 1

Refraction seismology principles

Answer 1

Critical angle
wavefront
Huygens’ wavelet
head waves

Question 2

Critical angle

Answer 2

If angle > ic then wave is reflected

Question 3

Huygens’ wavelets

Answer 3

Idea that every point on a wave front is a source of wavelets

Question 4

Head Waves

Answer 4

Wave the enters a high-velocity medium whose incident and reflected wave is at the critical angle

Question 5

Time-Distance diagrams (Also draw)

Answer 5

Reflected waves
Refracted waves
Direct waves

Question 6

First arrivals

Answer 6

Critical refraction usually first arrival

To study waves we mark the fist arrivals

Question 7

Multiple Layers (refraction)

Answer 7

• There is critical rays for each interface
• critical angle depends upon the velocities above and below it
• Ray paths depend on the thickness and velocities of the layers above

Question 8

Dipping interfaces (Draw this)

Answer 8

Tilting interface does not change value of critical angle, but rotates the ray diagram by the angle of the dip

Question 9

True vs. apparent dip (Draw this)

Answer 9

Draw this

Question 10

Hidden layer (draw this)

Answer 10

draw

Question 11

Reflection seismology principles

Answer 11

sections, limitations, NMO

Question 12

Seismic Sections

Answer 12

• not a true vertical section,
• times cannot be easily converted to depths
• reflectors may not come from directly below the source
• There may be multiple reflections in addition to the primary reflection

Question 13

Velocity determination with NMO (Draw)

Answer 13

NMO = Normal move out
-the effect that the distance between a seismic source and a receiver (the offset) has on the arrival time of a reflection

Question 14

Multiple Layers (reflection)

Answer 14

Deeper layers need to consider the refractions from the layers above.
Use root mean square velocity

Question 15

Dix Formula

Answer 15

Calculates the interval velocity of a specific layer

Question 16

Stacking

Answer 16

in reflection seismology the shot is not repeated but a line of receivers is used

Question 17

Dipping Reflectors

Answer 17

If a reflector is dipping, the apparent dip and position on a seismic section are changed

The reflector appears shallower and with a less steep dip than what really is.

Question 18

Diffractions

Answer 18

If reflectors not roughly horizontal, diffraction occurs

Diffraction hyperbolas are generated

Question 19

Migration (Draw)

Answer 19

Correction for displacement distortion

Bow tie structures

Question 20

CDP Stacking (Draw)

Answer 20

Common Depth Point

Reflections from CDP are received by successive receivers

Question 21

Reflector

Answer 21

Rays reflect when they meet an interference with an abrupt change in seismic velocity
The bigger the contrast between the two sides the stronger the reflection

Question 22

Resolution

Answer 22

Two pulses are hard to distinguish then they are less than half a wavelength apart
Resolution can be improved by using a shorter pulse

Question 23

Synthetic reflection seismograms

Answer 23

Accounts for all of a trace, not just first arrivals

Question 24

Earth’s Gravity

Answer 24

9.80m/s^2

Question 25

Micro-gravity

Answer 25

Location of sub surface cavities, like tombs

Question 26

Small Scale gravity

Answer 26

Mapping bedrock topography, and mineral exploration

Question 27

Medium scale gravity

Answer 27

Location of salt domes for oil exploration

Question 28

Large scale gravity

Answer 28

Estimation of crust thickness

Question 29

g variation with latitude

Answer 29

Gravity is greater at the poles
@ equator Earth is 21km larger
Centrifugal acceleration reduces g, effect largest at equator, has 0 effect on poles

Question 30

Sphere anomalies

Answer 30

+ if more dense than surroundings, - if less dense

“Hump shaped”

Question 31

Sheet anomalies

Answer 31

Shallower = more sharp

More dip = graph offset

Question 32

Data correction: Latitude

Answer 32

due to Earth’s rotation

Question 33

Data correction: Eötvös

Answer 33

When measurement taken while moving

Question 34

Data correction: Topographic

Answer 34

to avoid supposition of mass being buried under a level plain

Question 35

Topographic Data correction: Free-Air

Answer 35

When measured from the air

g decreases

Question 36

Topographic Data correction: Bouguer

Answer 36

When balloon stays at same altitude, but Earth rises, like when going from a field to a plateau.

Question 37

Topographic Data correction: Terrain

Answer 37

When there is a hill on this plateau or there is as valley

Question 38

Bouguer Anomaly

Answer 38

All corrections added

Question 39

Residual vs. regional anomaly

Answer 39

Residual = the anomaly of the rocks in the absence of the dyke has to be estimated This is substracted from the total anomaly to give the residual anomaly

Regional = The anomaly subtracted (due to granite
and others) is the regional anomaly)

Question 40

Inversion problem with gravity

Answer 40

1. bodies with different shapes can produce exactly same anomalies
2. alternative models can match the observed anomaly equally well
   3*. anomalies depend only on density differences or contrasts