Post on 11-Jan-2016
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ESS 135 Lecture 13
Reflection Seismology
Migration
Unmigrated: Note diffraction hyperbolas‘bow ties’
Migrated section
Refractions and reflections from the sea floor (Sheriffand Geldart, Vol1)Sonobuoy refractionprofile Baffin Bay
Velocity Stacking1. Apply normal moveout using various velocities to seismogramsand sum amplitudes to form a stacked Seismogram.
2. Power is sum of amplitudes squared.
3. Plot power as a function of velocity (and dip)The velocity that gives maximum power is the RMS velocity.
Power
Velocity0V
Velocity stacking from Kearey and Brooks
x
h
Apparent Dip
'
x
AhRelationship between Apparent ' and True dips
Let dip =
sin
' atan( / )
asin( / )
'/ atan( / ) / asin( / )
Apparent A
A
Apparent A
A
A A
h h vt
h x
h x
h x
h x h x
Ah
Note:Unmigrated has diffraction Hyperbolas (draped downwards)
Reflection from a pointgives rise to a diffraction orreflection hyperbola
Diffraction Migration (basis of tomography)
Place hyperbola on reflection seismogramand sum along it placing the energy at thepeak Move and repeat. Note at greater depththe hyperbolas have less moveout (are flatter)
Migration collapses diffraction hyperbolas toa point, puts reflectors in their correct positionsand reveals changes clearly such as anticlines, synclines and faults.
Migration drape hyperbola over diffraction hyperbolas
Common depth point shooting stacksall receiver seismograms for which shotsAnd receivers a re equally spaced aboutA common depth point.
At the middle of a survey with n geophoneswith a shot at each geophone the foldIs the number of traces stacked. n/2
7.14 ship moves shots are fired dx/2 wheredx is separation between geophones.
Reflection Refraction Coefficients Normal Incidence
AT
ArAi
2 2 1 1
2 2 1 1
1 1
2 2 1 1
i i
2
where is reflected amplitude
is the transmitted amplitude
is incident amplitude
, and v are densities and velocities of layer 1 and 2
r
i
T
i
r
T
i
A v vR
A v v
A vT
A v v
A
A
A
At surface R is negative
Synthetic Seismograms
Sea Level through Time
Global Change Sea Level 100 m Rise and Fall
Why might it change
1. Ice caps2. Tectonics (more ridges)
Sea Level Rise Melt Icecaps
Hallam et al. (1983) and Exxon eustatic (global) sea level reconstructions for the Phanerozoic eon.
Top lap on lap bottom lap
Sediment is deposited whereflow slackens pace at shelfedge
REFLECTION
2 2
2
21 0
2( )2
2
xt hv
xt
v t
Moveout
1
02
xv
t t
Multiple layers-velocity
2 2 21 1 2 2 3 2
1 2 3
...
...rms
v v vv
1 one way travel time 2 2rmsB B rmsT T
layerB T
v t v tv
t t
Thickness
2B T
layer
t th v
Migration
2 2
2 2
2
Locus of reflection points
x ht
v
R x h
Reflector Properties
2 2 1 1
2 2 1 1
1 1
2 2 1 1
2
v vR
v v
vt
v v
CDP stack Vibroseis Sequence stratigraphy Lithostratigraphic and Chonostratigraphic units