Seismic Reflection and Refraction

Sonar is Seismic Reflection

Sonar is Seismic Reflection

Artifacts in Seismic Reflection

Migration

Migration

Migration

Migration

Migration

Migration

Salt Dome, Unmigrated

Salt Dome, Migrated

Faults, Unmigrated

Faults, Migrated

Thrust Faults, Unmigrated

Thrust Faults, Migrated

Real Salt Dome

Real Faults

Seismic Reflection, Lake Superior

Interpretation of Deep Seismic Data

Seismic Refraction

Seismic Refraction

Using Travel Time Data

Two-Layer Case

Two-Layer Case
• Snell’s Law:
V
1
/V
2
= sin i/sin r
• If r = 90, sin r = 1, sin i = V
1
/V
2
• s = h/cos i
• x = h tan i
• t = 2s/V
1
+ (d-2x)/V
2
• At crossover, t = d/V
1

Two-Layer Case
• t = 2s/V
1
+ (d-2x)/V d/V
1
2
=
• 2sV
2
+ (d-2x)V
1
• 2sV
2
- 2xV
• s = h/cos i
1
= dV
= d(V
2
2
-V
1
)
• x = h tan i
• 2hV d(V
• 2h(V
2 d(V
2
2
2
/cos i – 2hV
1
-V
1
-V
1
)
– V
1 sin i) =
)cos i tan i =

Two-Layer Case
• 2h(V
2
– V
1 sin i) = d(V
2
-V
1
)cos i
• sin i = V
1
/V
2
• 2h(V
2
– V
1
2 /V
2
) = d(V
2
-V
1
)cos i
• cos i = √ (1- sin 2 i) =
√ (1- V
1
2 /V
2
2 ) =
(1/V
2
) √ (V
2
2 - V
1
2 )

Two-Layer Case
• 2h(V
2
– V
1
2 /V
2
) = d(V
2
-V
1
)(1/V
2
) √ (V
2
2 - V
1
2 )
• 2h(V
2
2 – V
1
2 ) = d(V
2
-V
1
) √ (V
2
2 - V
1
2 )
• 2h(V
2
+ V
1
) = d √ (V
2
2 - V
1
2 )
• h = (d/2) √ (V
2
− V
1
)/(V
2
+ V
1
)

Multiple Layers

Velocity Inversion

Mystery of the Critical Ray Path
• How does the signal “know” when to come back up?
• How does a finite amount of energy travel along a boundary of zero thickness?

A Better Way to Visualize

Wave Fronts

Bow Wave

Bow Wave

Seismic Refraction

Why Are the Graphs Different?

That’s Why

Impossible? No

Seismic Refraction: A Fault

Seismic Refraction: A Fault

Seismic Refraction: Irregular Surface

Seismic Reflection
• Strengths
– Very detailed
– Can resolve complex detail
– Easy to Interpret even raw data
• Weaknesses
– Expensive
– Computer Intensive

Seismic Refraction
• Strengths
– Good for horizontal layers
– Good for sea floor studies
– Can cover long traverses quickly
– Relatively inexpensive
• Weaknesses
– Poor detail
– Raw data not intuitive
– Hard to resolve complex structures
– Ambiguous