1. Exploration and Environmental Geophysics
Ray Paths Exercise
Jeffrey A. Nunn
Louisiana State University

2. Laboratory Exercises Gravity Acquisition
Gravity/Magnetics Interpretation
Ray Paths
Refraction Acquisition
Landmark/Refraction Interpretation
Reflection Acquisition
Reflection Processing
Reflection Interpretation
Resistivity Survey
Resistivity Interpretation
Well Logs (BakerHughes)
GPR Acquisition
GPR Interpretation

3. Context Audience: Skills Graduate/Undergraduate; Some PE students
Weekly Laboratory
Skills
Reflection
Refraction
Direct Wave
Ground Roll

4. Grotzinger et al., 2006

5. Ray Paths
Burger, 1992

6. Air
Ground
Roll
Reflection
Head
Direct

7. Goals
Students gain a strong visual image of how different waves travel different paths at different speeds which causes them to arrive at a geophone at different times.
Interpret arrival data in terms of refraction, reflection, direct wave, and ground roll.

8. Instructions
For the next two exercises, we will break up into groups of four.
Each member of the group will represent one of four waves leaving the source: direct wave, ground roll, reflected wave, and head wave.
All four "waves" will leave the source at the same time and travel at a particular speed and path as directed by the instructor.
ALL students will record the arrival time of each "wave" at each geophone until all 12 geophones have been used.

9. Students As Seismic Waves

10. Laying Out Seismic Cable
Laying Out Seismic Cable

11. Distance Versus Time for Various Students
Geophone
Distance, m
Direct
Reflected
Head
Ground Roll 1 3 10 6 2 5 11 9 13 17 4 12 15 18 22 16 20 29 19 36 7 21 44 8 24 25 48 27 28 53 30 61 33 34 35 66 37 39 74

12. Ground
Roll
Reflection
Head
Direct

13. Direct

14. Head

15. Reflection

16. Things I Learned the Hard Way (Everybody has to “March” at the same cadence)

17. Ground
Roll
Reflection
Head
Direct

18. Head
Direct

19. Laboratory Report
For each geophone, list the order in which the four waves arrive. In addition, sketch the ray paths from the source to geophones 4, 8 and 12
Plot arrival time versus distance for each "wave". Do any of the time versus distance curves fit a straight line? Do any of them not fit a straight line? Explain why they do or don't fit a straight line.
How would the arrival time versus distance curves be different if the distance to the horizontal/parallel surface was greater? How would the arrival time versus distance curves be different if the velocity of the "waves" in the upper medium was slower?
Without any additional information, is it possible to determine that the subsurface layer is dipping? If so, how?