Unit 1: Locating Buried Objects Using Gravity

Sarah Kruse, University of South Florida

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

Summary

This in-class/lab/homework activity introduces students to the idea that the gravitational acceleration g varies across Earth's surface and is sensitive to buried features. Students are asked to "read" and run interactive animations of the equation for gravitational acceleration g = GM/r² to understand how measured gravity profiles are sensitive to the mass and depth of buried objects.

Exploration Methods, Environmental Science | College Introductory, College Lower (13-14)

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Learning Goals

Students will be able to:

Describe situations where underground features have missing mass or extra mass that could be measured by a gravity survey
Define each of the variables in the equation for gravitational acceleration
Describe how the mass and depth of the buried feature each influence gravitation acceleration measured at the surface
Verify that the equation for gravitational acceleration matches the results of both numerical calculations and an animation of a gravity profile
Explain why gravity is a potential field
Justify the need for elevation corrections, using examples from data

Context for Use

This unit is designed at a very basic level for introductory college classes in which students may not have thought about detecting buried objects. The tools and activities are designed to:

Provide motivation for the gravity method
Get students to "read" an equation by describing the meaning of the variables with sketches, words, and plugging in numerical values
Check students' understandings of the equation against simulations of gravity profiles

Description and Teaching Materials

This unit consists of a 5-minute gravity video animation, an interactive gravity profile simulator, and a sequence of three worksheets. The unit is designed so that it can be done outside of a classroom but would be more efficient if students worked in class or lab with an instructor available to answer questions in real time. Students simply watch the video animation, and work through the worksheets.

Part 1: What Kinds of Buried Features Can You Detect by Measuring Gravity?

5-minute gravity video animation
Unit 1 Part 1 Student Worksheet (Microsoft Word 2007 (.docx) 5.7MB Jul15 26) This worksheet can be done by hand. Students should have watched the gravity video animation. The worksheet also points students to a YouTube video showing sinkhole subsidence. One question requires an internet link to answer.

Part 2: "Reading" and Visualizing the Equation for Gravitational Acceleration

Unit 1 Part 2 Student Worksheet (Microsoft Word 2007 (.docx) 3.8MB Jul15 26) This worksheet requires calculations on a computer or phone.

Part 3: How Does the Mass and Depth of a Buried Object Affect Gravity Readings over the Object?

Unit 1 Part 3 Student Worksheet (Microsoft Word 2007 (.docx) 6.6MB Jul15 26) This worksheet requires students to run the gravity profile simulator.
Gravity profile simulator

Teaching Notes and Tips

The worksheets can be done on paper or electronically. On paper is easier for the sketch question in Part 2, but harder for showing the simulation results in Part 3. Electronically is harder for the sketch, but the simulation results can be cut and pasted. Part 2 requires calculations that could be done on a phone.

The gravity profile simulator has not been tested on phones.

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Assessment

Formative assessments can be carried out as class conversations after watching the gravity animation video, between worksheets, and as student questions arise. The Unit 1 worksheets assess all the learning goals above and are intended to be the summative assessments for the unit. The worksheets could be done individually or in pairs. Solutions for instructors are available for each worksheet below.