GETSI Teaching Materials >IGUaNA > Teaching Materials > Locating Subsurface Features using Gravity and Magnetics > Unit 3: Gravity and Magnetics Field Data Exercises
GETSI's Earth-focused Modules for Undergraduate Classroom and Field Courses
showLearn More
hideHide
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 »
show Download
hideHide
The instructor material for this module are available for offline viewing below. Downloadable versions of the student materials are available from this location on the student materials pages. Learn more about using the different versions of GETSI materials »

Download a PDF of all web pages for the instructor's materials

Download a zip file that includes all the web pages and downloadable files from the instructor's materials

Unit 3: Gravity and Magnetics Field Data Exercises

Using magnetic and gravity field to make interpretations about the subsurface. Andy Parsekian, University of Wyoming (aparseki@uwyo.edu)

Download a ZIP file of this Unit

Initial Publication Date: June 23, 2023

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 magnetic 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.

Share your modifications and improvements to this activity through the Community Contribution Tool »

Learning Goals

Students will be able to:

  • measure changes in gravitational force by repeating observations within a structure (stairwell experiment)
  • plot and interpret 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)
  • recognize and describe limitations and nonuniqueness (equifinality) in geophysical prospecting map results
  • justify interpretations of a measured dataset by invoking physical properties and principles
  • give examples of applications of gravity and magnetic geophysical prospecting applications in their own neighborhood or region.

Context for Use

This unit is designed to be the capstone exercise for the IGUaNA 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 concept of karst and sinkholes is introduced as a low-density subsurface target 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 Gravity Surveying Field Data Example (PowerPoint 2007 (.pptx) 14.1MB Sep17 24)
  • GeoPark Sinkhole Gravity Lab IGUaNA_gravity_lab.xlsx (Excel 2007 (.xlsx) 47kB Jan19 23) This lab has two workbook 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 worksheet below.
  • GeoPark Sinkhole Gravity Worksheet iguana_maggrav_unit3_gravity_lab.docx (Microsoft Word 2007 (.docx) 1.3MB Sep17 24)
  • Answer keys, solution sets, 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 the Weyandt farm, 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 gravimiter away from the center of mass (Earth) 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, particularly using instrumentation resources available at the 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 IGUaNA teaching materials.

Materials Provided:

Teaching Notes and Tips

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 convert the entire raw dataset into a GoogleEarth KML file and tour the site using GoogleEarth.
  • As a bonus activity, students can attempt to apply Peter's Half Slope methods to estimate depth to the target for Line North 3 in the Excel workbook.

Gravity Stairwell Lab

  • As with any instrument that has a single small screen, most gravimeters are best suited to the smallest group size 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.
  • It is best to divide students into groups of five or fewer, if possible.

Assessment

Lab Reports

  • GeoPark Gravity Lab: The questions at the end of the worksheet document may be assigned as the basis for a lab report write-up that can be graded as individual or team-based assessments.
  • GeoPark Gravity Lab Rubric iguana_maggrav_unit3_labreport_rubric.docx (Microsoft Word 2007 (.docx) 76kB Sep17 24)

Solutions for Instructors

References and Resources

Stairwell Gravity Lab

The stairwell gravity lab was based on this lab written by Dr. Sam Peavy (GSW), inspired by Dr. Ed Robinson (VTech): https://serc.carleton.edu/NAGTWorkshops/geophysics/activities/18907.html

Getting to know your smartphone magnetometer

Although not developed by IGUaNA, this exercise is a good way to do a magnetometer survey without needed specific geophysical instrumentation: https://serc.carleton.edu/NAGTWorkshops/online_field/activities/237566.htm

Already used some of these materials in a course?
Let us know and join the discussion »

Considering using these materials with your students?
Get advice for using GETSI modules in your courses »
Get pointers and learn about how it's working for your peers in their classrooms »

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 »