Geophysics/Tectonics

1. Geophysics/Tectonics GLY 325

2. The Wave Equation Modeled

3. The Wave Equation Modeled

4. The Wave Equation Modeled

5. The Wave Equation Modeled Each of the things labeled is called a phase. Phases, in layman's terms, represent a part of the original source energy that has done something.

6. The Wave Equation Modeled For a two-layer model with a flat interface, the three main P-wave arrivals are: (1) Direct Wave, (2) Refracted Wave (Head Wave), and (3) Reflected Wave.

7. 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.

8. The Wave Equation Modeled The seismologists best friend is the eye, so use it to your advantage!

9. 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.

10. The Wave Equation Modeled You will learn to do this...

11. Refraction Interpretation We’ll now look specifically at refractions and discuss interpretation. First, let’s look at something beyond the two-layer case.

12. Multilayer Model

13. Multilayer Model

14. Multilayer Model

15. Multilayer Model Distance Time

16. Multilayer Model Distance Direct Wave 1st Layer Refraction Time 2nd Refraction 1st Reflection 2nd Reflection

17. Multilayer Model Distance Direct Wave 1st Layer Refraction Time 2nd Refraction

18. Multilayer Model What we use for interpretation are the first arrivals. We do this because the first arrivals are the easiest to find on a seismogram. Distance Time

19. 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 1st Layer Refraction Time 2nd Refraction

20. Multilayer Model All you need is to determine how many layers you think there are, and then calculate the slopes and intercept times for each. Distance Slope = 1/V0 t1 Slope = 1/V1 t2 Slope = 1/V2 Time

23. Refraction Interpretation We’ll no look specifically at refractions and discuss interpretation. First, let’s look at something beyond the two-layer case.

24. Interpreting Refractions This technique of slope and intercept calculations to determine the subsurface velocity structure work well of the layers are homogeneous and flat, with two exceptions: (1) Thin-Layer Problem (2) Low-Velocity Layer

25. Interpreting Refractions (1) Thin-Layer Problem -- if a middle layer is thin, the refraction may not show up as a first arrival. d1 V0 d2 V1 V2 V0 < V1 < V2

26. Interpreting Refractions (1) Thin-Layer Problem -- if a middle layer is thin, the refraction may not show up as a first arrival. Distance Direct Wave Time 2nd Refraction 1st Layer Refraction

27. Interpreting Refractions (1) Thin-Layer Problem -- So you interpret the data as TWO LAYERS. Distance Direct Wave Time 2nd Refraction 1st Layer Refraction

28. Interpreting Refractions (1) Thin-Layer Problem -- So you interpret the data as TWO LAYERS. Distance Direct Wave Your “1st Refraction” Time

29. Interpreting Refractions (1) Thin-Layer Problem -- Then, the thickness of the first layer (d1) will be too large and the depth to the top of the third layer (d1+d2) will be too small. Real Your Model d1 V0 V2

30. Interpreting Refractions (2) Low-Velocity Layer -- If a middle layer is lower in propagation velocity that the layer above it, a head wave (critical refraction) cannot be generated. No Head Wave Here d1 V0 d2 V1 V2 V0 > V1 < V2

31. Interpreting Refractions (1) Low Velocity Layer -- Then, the thickness of the first layer (d1) will be too large. Real Your Model d1 V0 V2

32. Interpreting Refractions This technique of slope and intercept calculations to determine the subsurface velocity structure work well of the layers are homogeneous and flat, with two exceptions: (1) Thin-Layer Problem -- can interpret if you can properly identify the refraction from the thin layer even though it doesn’t show up as a first arrival. (2) Low-Velocity Layer -- NO SOLUTION without other supporting evidence (borehole tie or other geophysical data).