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Exploration and Environmental Geophysics

Exploration and Environmental Geophysics. Ray Paths Exercise Jeffrey A. Nunn Louisiana State University. Gravity Acquisition Gravity/Magnetics Interpretation Ray Paths Refraction Acquisition Landmark/Refraction Interpretation Reflection Acquisition Reflection Processing.

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Exploration and Environmental Geophysics

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

  2. 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 Laboratory Exercises

  3. Context • Audience: • Graduate/Undergraduate; Some PE students • 20 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. http://www.pi.ac.ae/pge/gallery/geophysics_trip_Mar07.htm Laying Out Seismic Cable http://www.seismo.unr.edu/ftp/pub/louie/class/453/album00/reflection.jpg

  11. Distance Versus Time for Various Students

  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?

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