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Reflection Seismology - Introduction to Geophysics - Lecture Slides, Slides of Geology

In the course of introduction to geophysics, The key points are:Reflection Seismology, Image Subsurface, Detailed Imaging, Horizontal Layering, Seismology, Depth Sounding, Reflection, Ship, Pulses, Partially Reflected

Typology: Slides

2012/2013

Uploaded on 04/29/2013

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Seismic Reflection is the most important tool we
have to image subsurface structure.
Provides detailed imaging of approximately
horizontal layering in the earth
Reflection seismology is echo or depth sounding
Reflection Seismology
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Download Reflection Seismology - Introduction to Geophysics - Lecture Slides and more Slides Geology in PDF only on Docsity!

  • Seismic Reflection is the most important tool we

have to image subsurface structure.

  • Provides detailed imaging of approximately

horizontal layering in the earth

  • Reflection seismology is echo or depth sounding

Reflection Seismology

  • A ship is moving and fires air-

guns about every 10 s.

  • The pulses travel downward

and are partially reflected back

up from the reflectors

  • Waves are then recorded by

seismic receivers

Reflection Seismology

  • Yet, there are reasons that the

section is NOT true.

  • Vertical scale is not depth, but

time

  • Actual time is two-way travel-

time (down and back).

  • Can be converted to depth,

but must know velocity

structure.

Seismic Section

  • If layering is not horizontal,

reflections will not come from

directly below the source.

  • Problem solved using a

migration algorithm.

Seismic Section

  • The time to travel to a receiver a distance x away from the source is

Normal Moveout (NMO)

x: src—rec offset (m) T 0 : vertical travel-time ∆t(x): moveout wrt offset hi : ith^ layer thickness (m) V: layer velocity S: source coordinate R: receiver coordinate A: reflection point (mid- point of ray for flat layers).

Normal Moveout Derivation 2 2 0 0 2 2 2 2

2 2

2 2 2 2 2 0 2 0 0

2 (^0 ) 0

2 / ( / 2) 2 / / 2 (^2) ( / 2) 2 (1 ( / 2 )

(^2 1) ( / 2 ) 2 (1 1 ( / 2 ) ) 2 : 1 1 / 2 (^2) ( 2 2

( )

1 / 8 ) (^8 ) 2 4 2

t h v L h x t h v h t v t h x h x h v v t h^ x h h x h v v binomial approximation x t h^ x h h^ hx^ t x v v vh vh t t x t x x v

t t vt v t

ε ε

= = + = = = + = +

= + ≈ + = + ≈ + ≈ + = + = +

≈ + ≈ + ∆ ∆ =

h

v

S M R 0 x/2 x

L^ L

Normal moveout derivation

t: travel-time L: right triangle hypotenuse v: velocity of layer (km/s) t 0 : two-way time x: source-receiver offset h: layer thickness (km)

  • What velocity should be used for two layers?
  • Answer: Root Mean Square (RMS) velocity
  • τi : one wave interval time (top to bottom time)

Multiple Layers

2 2 1 1 2 2 2, 1 2

rms

v v

v

τ τ τ τ

2,

2 0,2 t 2

rms

x

v

t

Can solve for v 2

Can iterate this procedure for deeper layers Docsity.com

  • Another way to calculate velocity of any layer
  • B index: layer bottom
  • T index: layer top
  • v (^) layer : layer velocity
  • t (^) B : 2-way tt to layer bottom
  • t (^) T : 2-way tt to layer top
  • v (^) rms,B RMS velocity to bottom
  • v (^) rms,T RMS velocity to top

Multiple Layers: Dix formula

2 2 rms B , B rms T , T layer B T

v t v t v t t

=

  • Finding layer velocities and stacking done simultaneously
  • Try range of v (^) rms
  • NMO correct the times of the seismograms
  • If the v (^) rms is correct

strong reflections are found

  • If v (^) rms is incorrect

Reflections are not found.

Stacking

Dipping Reflectors

  • If a reflector is dipping, its apparent position and dip are wrong in an unmigrated section.
  • Wave raypaths are least-time paths and hence reflect from up dip points.
  • Unmigrated reflector is shallower and with less dip.
  • Travel-time hyperbola offset: h (thickness), alpha (dip)

∆ x= 2h sin α

Curved Reflectors: Anticline

  • Anticline has simpler response wrt Syncline.
  • An anticline seismic image is broadened
  • At edges of anticline two arrivals exist.

Migration

  • Correcting for the position and shape of the reflector is called migration
  • Complicated and requires large amounts of computer time
  • Be aware of possible distortions in un-migrated sections

Faulted Reflectors

  • Migration removes diffraction effects and reveals features more clearly

Multiple Reflections

  • The positions of multiples can be anticipated from the position of the primary reflectors
  • However, sometimes it is difficult to recognize a primary reflector that comes in with the same TWT as the multiples
  • Can be distinguished
    • Moveout for the primary is less than for the multiple, so it stacks using a higher velocity