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