1. How Shallow Earth Structure Is Determined
A Classroom Exercise Demonstrating Seismic Refraction Use in the Real World
NSTA, St. Louis, 2007
John Taber, IRIS
Michael Wysession, Washington University, St. Louis

2. Objective(s)
Explore how refracted seismic waves are used to determine earth structure.
Reinforce the concepts of reflected and refraction of waves through a real world example.

4. The deepest drill hole was 12.7 km.
So…how do we know all this?!?!

5. Seismic Refraction The deepest drill hole was 12.7 km.
So…how do we know all this?!?!
Seismic Refraction

10. Identify: Direct wave
Head wave
Refracted wave
Reflected wave

12. Direct Wave

13. Direct Wave
Reflected Wave

14. Direct Wave
Reflected Wave
Refracted Waves

16. Direct wave: Slope is 1/v1
Distance
Direct wave: Slope is 1/v1
Time

17. Direct wave: Slope is 1/v1
Distance
Direct wave: Slope is 1/v1
Time
1. Measure Direct Wave Slope and you get v1 !!!!

18. i1 v1 i2 v2

19. The first head wave is a reflected wave at the critical angle
ic ic

20. ic ic
The Head wave moves along the surface at the SPEED OF THE LOWER LAYER!!!

21. Direct wave: Slope is 1/v1
Distance
Direct wave: Slope is 1/v1
Time
Head wave: Slope is 1/v2

22. Direct wave: Slope is 1/v1
Distance
Direct wave: Slope is 1/v1
Time
Head wave: Slope is 1/v2
2. Measure Head Wave Slope and you get v2 !!!!

23. Direct wave: Slope is 1/v1
Distance xd
Direct wave: Slope is 1/v1
Time
Head wave: Slope is 1/v2
3. Measure the cross-over distance, xd , to get the DEPTH to the boundary !!!!

24. How do you get the depth to the boundary from the cross-over distance, xd ?

25. How do you get the depth to the boundary from the cross-over distance, xd ?
Direct wave: TD(x) = x / v1

26. How do you get the depth to the boundary from the cross-over distance, xd ?
Direct wave: TD(x) = x / v1
Set TD (x) = TH (x)

27. How do you get the depth to the boundary from the cross-over distance, xd ?
Direct wave: TD(x) = x / v1
Set TD (x) = TH (x)
Depth d

28. How do you get the depth to the boundary from the cross-over distance, xd ?
Direct wave: TD(x) = x / v1
Set TD (x) = TH (x)
3. Measure v1 , v2 , and xd , and you can get the Depth to the boundary!!!!
Depth d

29. Example:

30. Step 1. Find the slope of the Direct wave.
Example:
Step 1. Find the slope of the Direct wave.

31. Step 2. Find the slope of the Head wave.
Example:
Step 2. Find the slope of the Head wave.

32. Example: xd
Step 3. Find the slope of the Head wave.

33. xd Step 3. Find the slope of the Head wave.
Example: xd
Step 3. Find the slope of the Head wave.
Step 4. Compute the Depth to the boundary.

34. What about multiple layers?

35. What about dipping layers?

36. What about variable layers?
Mid-Atlantic Ridge off South America
Computer model
Ray trace diagram
100 km

39. Q: How do they know the compositions?

40. A: Laboratory experiments on real rocks!

41. North American Head P-wave Speeds

42. North American Direct P-wave Speeds

43. North American Crustal Thicknesses

44. Global Crustal Thicknesses