Unit 3: Case studies and limitations of GPR

Sarah Kruse, University of South Florida, skruse@usf.edu
Christine Downs, University of South Florida, cmdowns@usf.edu


Summary

This unit considers some ways to distinguish objects from each other while emphasizing the possibility of misinterpreting data. GPR imaging can be an excellent tool for detecting buried objects. However, as with all geophysical methods, it does have limitations. GPR will show you where something is buried, but not what it is. Targets like tree roots, pipes, shallow graves can all have a very similar signatures in GPR data.

This unit also introduces ground conductivity and its role in wave attenuation. GPR is not just about the targets. The ground characteristics control how the transmitted GPR wave behaves; geology and soil matter! Relatively conductive materials like clay and groundwater cause the GPR wave to quickly lose energy as it travels away from its source.

Used this activity? Share your experiences and modifications

Learning Goals

Students will be able to...

  1. describe wave attenuation in terms of ground properties, both in words and graphically
  2. explain how a tree root might look similar to/different than a grave
  3. recognize a GPR depth slice versus a GPR profile.
  4. describe how GPR data could be misinterpreted
  5. describe environments where GPR will likely not work.

Context for Use

This unit is intended for undergraduate students with no previous exposure to geophysics. There is no math or science prerequisite.

The instructor can expect to spend 1-2 class periods with this material depending on class length and size. Student material is contained in the PowerPoint. The instructor also has the option to use the other examples of GPR in engineering and forensic application for class discussions. Assessment activities may be done in groups or as individuals.

Description and Teaching Materials

The teaching material includes a PowerPoint presentation with links to an external web site and an interactive app that provide additional instructional content. The videos embedded in the PowerPoint are animations of GPR wave propagation in different settings and the resulting radargrams. The interactive app allows students to set 'see' how much the GPR signal attenuates as a function of depth.

Presentation: What are the limitations of GPR
Click to view

Student material includes a worksheet, think-pair-share questions, and synthesis questions. The worksheet is designed to be completed as students following along with the slide presentation or outside of class and referring to the slides. This worksheet may also serve as a source of quiz or test questions if desired. Think-pair-share questions are best implemented during class time. These may also serve as short essay questions to be answered individually.

There is an opportunity in this module to assign a writing exercise. Students may be asked to address the last part of the worksheet in an essay. They can write from the perspective of a GPR expert who was asked why GPR was used at the same site twice, but only successful the first time.

Teaching Notes and Tips

Key concepts in this unit

  1. Multiple profiles are necessary to properly image a 3D feature.
  2. Identify areas/environments in their local region where GPR would likely be successful or unsuccessful.
  3. Explain conductivity and permittivity.
  4. GPR signal is both waves and rays.

Contextualizing the use of GPR to locate unmarked graves

One of the case studies in this module involves the use of GPR to evaluate a site for the presence of unmarked graves. Unmarked grave sites in the U.S. include cemeteries for enslaved people, where slave holders typically made no effort to mark or protect burial sites, and cemeteries on the grounds of "Indian boarding schools" run by U.S. government agencies, where Native American students were buried without their families' knowledge or consent. The module overview page has contextual information about using GPR to locate unmarked graves and incorporating this content into your teaching, and a list of references you can explore to learn more.

Assessment

The Unit 3 handout includes fill-in-the-blank questions to confirm that students read through the PowerPoint and links, and open-ended questions that can be used in a think-pair-share format. The assessment is designed to be completed as students following along with the slide presentation or outside of class and referring to the slides. This worksheet may also serve as a source of quiz or test questions if desired. Think-pair-share questions are best implemented during class time. These may also serve as short essay questions to be answered individually.

Solutions for instructors

References and Resources

Resources

  • Interactive app that illustrates wave attenuation in a user-selected ground material.
  • Geosci.xyz is an open-source educational resource for the geosciences. The site provides a comprehensive explanation of GPR theory that the instructor may refer to as needed to expand on this unit.
  • Suggestions for supporting students (Microsoft Word 2007 (.docx) 16kB Dec6 21)

References

Explore references about African-American slave cemeteries and Native American boarding schools on the module overview page.