Ray Paths

Jeffrey A. Nunn
Louisiana State University
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Students work in groups to "walk" through the ray paths of the direct wave, ground roll, reflected wave, and refracted wave. This exercise gives them a visual image of the ray paths as well as the ability to process distance versus time data.

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Undergraduate/Graduate course in geophysics
Designed for a geophysics course

Skills and concepts that students must have mastered

Reflection, refraction, direct wave, and ground roll

How the activity is situated in the course

First in a sequence of exercises in which students gather seismic data using a 24 geophone system


Content/concepts goals for this activity

Interpreting arrival data in terms of refraction, reflection, direct wave, and ground roll. 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.

Higher order thinking skills goals for this activity

This is a simple exercise but I am always amazed at how many students have difficulty understanding the different ray paths that seismic waves travel. If they don't understand ray paths, then they cannot really learn anything else in seismology.

Other skills goals for this activity

this is a group exercise. The quality of the "data" depends critically on each member of the group doing their job properly.

Description of the activity/assignment

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. 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.
Uses online and/or real-time data
Has minimal/no quantitative component

Determining whether students have met the goals

Students have to draw the ray paths and then discuss how well their data fit a straight line or not in an exercise report. They are also asked to compare results from a horizontal and a dipping layer interface.

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