Acquisition of Data Sets for Gravity Inversion Laboratories

Sarah Titus (Geology) and Bill Titus (Physics/Astronomy), Carleton College

Summary

We collected gravity data across two canyon-spanning bridges in the southwest, USA. These data were analyzed and modeled by students in Geophysics.

Learning Goals

The three specific learning goals of this project are to improve (1) students' understanding of uncertainty in data and models, and (2) their ability to visualize subsurface geologic structures, while simultaneously providing (3) an authentic research experience using expert-level modeling and interpretation techniques. The first goal is part of the broader push in science education to develop students' quantitative literacy skills. The second reflects students' difficulties with component spatial skills, which may include imagining subsurface geologic structures. The third reflects the importance of authentic research experiences at the undergraduate level, which has been cited as a major reason that female undergraduates at Carleton College continue in the sciences (Wilson, 2006).

Wilson, R., 2006, A hothouse for female scientists: At Carleton, working closely with professors leads women to careers in science: The Chronicle of Higher Education, v. LII, p. A13-A15.

Context for Use

We decided to give the Rio Grande Gorge Bridge dataset as a take-home exam in Geophysics. We did not provide students with much information about the problem set-up. (For instance, we never mentioned that it was a bridge.) This was because we wanted to see what kinds of other geometries and scenarios they might develop during the modeling process.

Description and Teaching Materials

The activity consists of a gravity dataset that has been corrected for instrument drift and Earth tides. Students perform all other gravity corrections (latitude, free-air, bouguer, and regional bouguer) as well as the tie to an absolute gravity station. Modeling was done using GravMag, the software that comes with Applied Geophysics textbook by Burger, Sheehan, and Jones.

Teaching Notes and Tips

As a take-home exam, this activity took students about 6-8 hours. One nice aspect is that even when students made mistakes in the gravity correction process, they always ended up with the right shape and usually the right size for the gravity anomaly. This meant that they could still model the data and get realistic geometries (which made it much easier from a grading perspective).

Assessment

The first goal and third goals will be assessed using student performance on the lab exercises and a survey about their attitudes towards their experience in this course.

The second goal will be assessed using pre- and post-course surveys of visualization skills, which Sarah has been using in her course since coming to Carleton. We will also examine student understanding using formative assessments called skill puzzles. These paper and pencil activities were developed by Sarah Titus and Eric Horsman (2009) and are loosely based on concept sketches (Johnson and Reynolds, 2005) that emphasize student- and faculty-drawn diagrams that typically include both visual and verbal components. Instead of using text to describe processes and name particular features within the drawing as is done in concept sketches, skill puzzles are designed to give students practice with particular spatial component skills, such as visual penetrative ability. The verbal portions are usually in the form of questions, which provide some insight into student thought processes. These verbal portions are especially useful when students are unable to draw the correct answer.

Titus, S.J., and Horsman, E., 2009, Characterizing and improving spatial visualization skills: special volume on Research on Thinking and Learning in the Geosciences: Journal of Geoscience Education, v. 57, p. 242-254.

Johnson, J. K., and Reynolds, S. J., 2005, Concept sketches - Using student- and instructor-generated annotated sketches for learning, teaching, and assessment in geology courses: Journal of Geoscience Education, v. 53, p. 85-95).

References and Resources

Please contact either of us if you wish to use these materials for a class.

The textbook reference: Burger, H.R., Sheehan, A.F., and Jones, C.H., 2006, Introduction to Applied Geophysics: Exploring the Shallow Subsurface, W.W. Norton and Company, 600 pp.