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

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

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

Grotzinger et al., 2006

Ray Paths Burger, 1992

Air Ground Roll Reflection Head Direct

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.

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.

Students As Seismic Waves

Laying Out Seismic Cable 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

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

Ground Roll Reflection Head Direct

Direct

Head

Reflection

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

Ground Roll Reflection Head Direct

Head Direct

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?