Final Flashcards
Refraction seismology principles
Critical angle
wavefront
Huygens’ wavelet
head waves
Critical angle
If angle > ic then wave is reflected
Huygens’ wavelets
Idea that every point on a wave front is a source of wavelets
Head Waves
Wave the enters a high-velocity medium whose incident and reflected wave is at the critical angle
Time-Distance diagrams (Also draw)
Reflected waves
Refracted waves
Direct waves
First arrivals
Critical refraction usually first arrival
To study waves we mark the fist arrivals
Multiple Layers (refraction)
- 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
Dipping interfaces (Draw this)
Tilting interface does not change value of critical angle, but rotates the ray diagram by the angle of the dip
True vs. apparent dip (Draw this)
Draw this
Hidden layer (draw this)
draw
Reflection seismology principles
sections, limitations, NMO
Seismic Sections
- 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
Velocity determination with NMO (Draw)
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
Multiple Layers (reflection)
Deeper layers need to consider the refractions from the layers above.
Use root mean square velocity
Dix Formula
Calculates the interval velocity of a specific layer
Stacking
in reflection seismology the shot is not repeated but a line of receivers is used
Dipping Reflectors
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.
Diffractions
If reflectors not roughly horizontal, diffraction occurs
Diffraction hyperbolas are generated
Migration (Draw)
Correction for displacement distortion
Bow tie structures
CDP Stacking (Draw)
Common Depth Point
Reflections from CDP are received by successive receivers
Reflector
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
Resolution
Two pulses are hard to distinguish then they are less than half a wavelength apart
Resolution can be improved by using a shorter pulse
Synthetic reflection seismograms
Accounts for all of a trace, not just first arrivals
Earth’s Gravity
9.80m/s^2
Micro-gravity
Location of sub surface cavities, like tombs
Small Scale gravity
Mapping bedrock topography, and mineral exploration
Medium scale gravity
Location of salt domes for oil exploration
Large scale gravity
Estimation of crust thickness
g variation with latitude
Gravity is greater at the poles
@ equator Earth is 21km larger
Centrifugal acceleration reduces g, effect largest at equator, has 0 effect on poles
Sphere anomalies
+ if more dense than surroundings, - if less dense
“Hump shaped”
Sheet anomalies
Shallower = more sharp
More dip = graph offset
Data correction: Latitude
due to Earth’s rotation
Data correction: Eötvös
When measurement taken while moving
Data correction: Topographic
to avoid supposition of mass being buried under a level plain
Topographic Data correction: Free-Air
When measured from the air
g decreases
Topographic Data correction: Bouguer
When balloon stays at same altitude, but Earth rises, like when going from a field to a plateau.
Topographic Data correction: Terrain
When there is a hill on this plateau or there is as valley
Bouguer Anomaly
All corrections added
Residual vs. regional anomaly
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)
Inversion problem with gravity
- bodies with different shapes can produce exactly same anomalies
- alternative models can match the observed anomaly equally well
3*. anomalies depend only on density differences or contrasts
