GETSI Teaching Materials >Locating Subsurface Features using Gravity and Magnetics > Unit 3: Gravity and Magnetics Field Data Exercises
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This module is part of a growing collection of classroom-tested materials developed by GETSI. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs.
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Unit 3: Gravity and Magnetics Field Data Exercises

Andy Parsekian, University of Wyoming

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Initial Publication Date: June 23, 2023 | Revision: July 15, 2026
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Summary

This activity focuses on practical application of concepts discussed in Units 1 and 2 through the analysis of measured or provided field datasets. Students will follow experimental procedures to measure gravity changes inside a building and/or analyze geophysical prospecting data. Students will gain the ability to interpret real datasets, justify their findings, describe limitations of the measurements, and produce graphics to communicate their data.

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Learning Goals

Students will be able to:

  • Measure changes in gravitational force by repeating observations within a structure (stairwell experiment)
  • Plot andinterpret results of gravity measurements at different elevations
  • Interpret magnetic prospecting data to locate anomalies associated with human infrastructure
  • Interpret gravity prospecting data to locate anomalies related to geologic hazards (i.e., voids, sinkholes)
  • Evaluate  limitations and nonuniqueness (equifinality) in geophysical prospecting map results
  • Justify interpretations of a measured dataset by invoking physical properties and principles
  • Identify examples of applications of gravity and magnetic geophysical prospecting applications in students' own neighborhoods or regions.

Context for Use

This unit is designed to be the capstone exercise for the GETSI module on Gravity and Magnetics. It builds on Unit 1: Locating Buried Objects Using Gravity and Unit 2: Environmental Magnetism. However, it could be used as a standalone unit in an intermediate class focused on geophysics, i.e., where students have obtained some of the background through other course work.

Description and Teaching Materials

Part 1: Avoid Voids—Mapping gravity in karst terrain for hazard assessment

This part addresses the need for geophysical measurement of the subsurface to assess natural hazards in developed areas. The concepts of karst and sinkholes are introduced as a low-density subsurface targets for gravity surveys. An overview of sinkhole processes is presented along with motivation for the importance of using geophysics to locate these features as potential hazards to humans. A dataset is provided from the University of South Florida GeoPark (Tampa) site where a collapsed sinkhole is measured along a linear transect using a gravimeter. The lab leads students through analyzing the dataset based on principles learned in Unit 1 of this module.

Materials Provided:

  • Introduction to the GeoPark Gravity Survey Presentation (PowerPoint 2007 (.pptx) 13.1MB Jul14 26)
  • GeoPark Sinkhole Gravity Lab Worksheet (Microsoft Word 2007 (.docx) 4MB Jul14 26)
  • GeoPark Sinkhole Gravity Data (Excel 2007 (.xlsx) 146kB Jul14 26) This workbook has two sheets that allow instructors to choose between a lab exercise that has the regression calculations for removing the local trend computed (easier), and a second version where the students must enter the formulas themselves to compute the local trend removal regressions. Students will work through this workbook guided by the lab worksheet below.
  • Answer keys and a grading rubric are available for instructors. See the "Assessment" section of this page, below.

Part 2: A magnetic attraction to iron—Detecting "lost" infrastructure

This part addresses the need for magnetic geophysics to locate and map buried infrastructure. A case study is introduced where old oil and gas piping runs that have been "lost" due to limited record-keeping are identified in medium-scale mapping surveys on Weyandt Farm in western Pennsylvania. A dataset is provided where magnetic anomalies are revealed. The lab leads students through analyzing the dataset based on principles learned in Unit 2 of this module.

Materials Provided:

Part 3a: Measure your own gravity data—The stairwell experiment

Why does gravity go down when we go up? This part allows students to explore the Free Air correction by calculating it for themselves. In this classic experiment, students move the gravimeter away from Earth's center of mass by making measurements in a tall stairwell. It can be completed either with the students measuring their own data or by using the provided dataset. The hands-on portion of this lab can be completed almost anywhere since most academic buildings will have some kind of stairway.

Why do we care? Gravity measurements can be a useful way to evaluate hazards and map the subsurface. Several "corrections" are needed to remove parts of the gravity signal that otherwise prevent us from detecting our target in the subsurface. The Free Air Correction is one of these corrections that is essential for accurate interpretation of gravity measurements.

Materials Provided:

Part 3b: Measure your own gravity data—Define your own target

This part will guide you through the steps of collecting your own dataset, if you are able to obtain access to instruments (ex. University of Wyoming Near Surface Geophysics Instrumentation Center). Guidelines are provided for defining a research question, doing background research, and interpreting your results. A review of acquisition and analysis is provided with links to the primary materials on these topics covered elsewhere in the GETSI teaching materials.

Materials Provided:

Teaching Notes and Tips

GeoPark Sinkhole Gravity Lab

  • As a bonus activity, instructors could provide students only the raw data from the gravity survey, and ask them to attempt to correct for drift using the technique introduced in Unit 3 Part 3's stairwell gravity lab. This data is included as a separate sheet in the GeoPark Sinkhole Gravity Data (Excel 2007 (.xlsx) 146kB Jul14 26) spreadsheet.

Weyandt Farm Magnetic Lab

  • As a bonus activity, students could use a more advanced plotting program (Surfer, R, Python, Matlab, etc.) to plot the entire provided dataset as a scatterplot with the color of each point scaled to the magnetic value.
  • As a bonus activity, students can view the entire raw dataset in the KML file and tour the site using Google Earth (or another GIS).

Gravity Stairwell Lab

  • As with any instrument that has a single small screen, most gravimeters are best suited to the smallest group size possible. It is best to divide students into groups of five or fewer, if possible.
  • The leveling exercise is a great hands-on experience and as many students as possible should rotate through it.
  • Be aware that people moving around on the staircase during leveling will make it almost impossible to level.
  • Movement during measurement (i.e., students walking around the stairwell) will reduce the data quality. This can be a teachable moment: some gravimeters (e.g. Scintrex CG5) will plot the measurement in real time. Students can watch this under quiet and noisy conditions.

Assessment

Formative assessments can be conducted via class and small group discussions as students move through each section of the labs. Even if students will be submitting separate lab reports, it is recommended to allow students to discuss their thoughts and findings together or as a class before embarking on the report.

The questions at the end of the lab worksheet documents may be assigned as the basis for lab report write-ups that can be graded as individual or team-based summative assessments. An example lab report rubric and spreadsheet solutions for each lab are provided below that can be used for grading.

References and Resources

Stairwell Gravity Lab - The stairwell gravity lab was based on this lab written by Dr. Sam Peavy (Georiga Southwestern State University), inspired by Dr. Ed Robinson (Virginia Tech): https://serc.carleton.edu/NAGTWorkshops/geophysics/activities/18907.html

Getting to know your smartphone magnetometer - Although not a GETSI product, this exercise is a good way to do a magnetometer survey without needing specific geophysical instrumentation: https://serc.carleton.edu/NAGTWorkshops/online_field/activities/237566.htm

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This module is part of a growing collection of classroom-tested materials developed by GETSI. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs.
Explore the Collection »