Transcript 0501RefracInterp

Geophysics/Tectonics
GLY 325
The Wave Equation Modeled
Even for models with no noise, identifying phases
may be challenging. However, you can train your
eye/brain to learn how to do this.
The Wave Equation Modeled
You will learn to do this...
The Wave Equation Modeled
So, at any interface, some energy is reflected (at the
angle of incidence) and some is refracted (according
to Snell’s Law). Let’s look at a simple model and just
watch what happens to the P-wave energy...
Refraction Interpretation
We’ll no look specifically at refractions and discuss
interpretation. First, let’s look at something beyond
the two-layer case.
Multilayer Model
Distance
Direct Wave
Time
1st Layer Refraction
2nd Refraction
1st Reflection
2nd Reflection
Multilayer Model
Prior to the first arrivals, only background noise
interferes. After the first arrivals, background noise
plus many other phases are recorded.
Distance
Direct Wave
Time
1st Layer Refraction
2nd Refraction
Case Studies
Why is determining depths to interfaces and seismic
propagation velocities important?
(1) Depth to the Moho (crust/mantle boundary) often
relates to tectonic history:
• Thin crust at rift zones and mid-ocean ridges
• Thick crust at mountain ranges
•Thin to thick transition at continental margins
(2) Velocity of upper mantle (lower lithosphere)
suggests asthenospheric upwelling:
• Low seismic velocity means hot asthenosphere
is shallow and partial melting occurs (continental
rifts & mid-ocean ridges)
• Higher seismic velocity means no
asthenospheric upwelling.
Case Studies
Global Crustal Thickness (km)
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Global Geologic Province
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