Unit 2: How Seismic Waves Travel Through the Ground

×

A seismic survey overlooking Reno, NV.

Provenance: J. Louie, UNR
Reuse: If you wish to use this item outside this site in ways that exceed fair use (see http://fairuse.stanford.edu/) you must seek permission from its creator.

Unit 2 Learning Outcomes

Students will be able to:

• describe wave refraction underground
• calculate seismic wave travel time in a layered subsurface
• interpret earth materials based on seismic properties
• conceptualize the subsurface environment by sketching and interpreting diagrams

Unit 2 Teaching Objectives

• Affective: Motivate students' interest in seismic imaging with examples relevant to urban and environmental geophysics
• Cognitive: Empower students to use basic physical equations to predict properties of the subsurface

In the previous unit, Introducing Shallow Seismology, students were introduced to the concept of using physical measurements to make images of the Earth. Unit 2 is designed to facilitate their first practical experience with calculating earth parameters from physical equations. Basic information on seismic waves is presented to support lectures and activities designed to lead students toward being able to apply the equations for themselves.

Part 1: Seismic waves for subsurface exploration and conceptual models of the subsurface environment

This part introduces the idea of seismic (acoustic) waves in the subsurface. Content describes compressional waves used in refraction experiments and provides diagrams of ray paths. This part finishes with a prompt for thinking about what is underground and how that is related to basic conceptualizations of the subsurface structure and materials.

Materials provided:

• Introduction to shallow seismic refraction (PowerPoint 2007 (.pptx) 19.4MB Sep2 24)
• Student handout, how seismic waves travel (Microsoft Word 2007 (.docx) 78kB Sep8 24): This handout is intended for students to follow along during the lecture to make sure they are getting the important information from it. It covers materials from both PowerPoint presentations.
• An answer key for the questions in the handout is available for instructors. See the "Assessment" section of this page, below.

Part 2: Refraction and related equations for seismology and interpreting material properties from geophysics

This part describes how velocities of seismic waves are calculated in refraction experiments, and the basic equations that govern velocity and travel time. Snell's law is introduced in the context of a 2-layer refraction problem, with an Excel-based practical exercise. Travel-time/offset plots are described and context is given for how information can be taken off of these plots and used to calculate a subsurface structure. An Excel-based practical exercise is provided for calculating a 2-layer structure. The concept of material properties is revisited with a prompt for thinking about both the structure and seismic properties of the subsurface near to the student's home.

Materials provided:

• Refracted wave paths and Snell's Law (PowerPoint 2007 (.pptx) 18.1MB Sep2 24)
• Student handout, how seismic waves travel (Microsoft Word 2007 (.docx) 78kB Sep8 24): This handout is the same handout provided in Part 1. It is intended for students to follow along during the lecture to make sure they are getting the important information from it. It covers materials from both PowerPoint presentations.
• Snell's Law exercise (Excel 2007 (.xlsx) 597kB Sep18 24)
  • Snell's Law, worked example (Microsoft Word 2007 (.docx) 34kB Sep18 24) - worked example can be shared with students to help them check their work
  • Snell's Law exercise text from XLS (Microsoft Word 2007 (.docx) 17kB Sep15 24): Text from Snell's Law exercise spreadsheet. This document provides step-by-step instructions for students to complete the Snell's Law exercise. These instructions are also embedded in the spreadsheet.
• 2-layer seismic refraction exercise (Excel 2007 (.xlsx) 2.3MB Sep18 24)
  • 2-layer seismic refraction exercise, worked example (Microsoft Word 2007 (.docx) 877kB Sep18 24) - worked example can be shared with students to help them check their work
  • 2-layer seismic refraction exercise text from XLS (Microsoft Word 2007 (.docx) 21kB Sep15 24): Text from 2-layer seismic refraction exercise spreadsheet. This document provides step-by-step instructions for students to complete the 2-layer seismic refraction exercise. These instructions are also embedded in the spreadsheet.
• Answer keys, solution sets, and a grading rubric are available for instructors. See the "Assessment" section of this page, below.

Instructors may provide a review of Microsoft Excel (and analogous free software) to students before Part 2.

These exercises can be completed using a team-based learning (TBL) or a jigsaw approach:

• For exercise 1 (Snell's Law), the students could be divided up into small groups (~3 students per group), with each team calculating the velocity of the lower layer for a range of refraction angles. After calculations, all groups could add their results to a large graph for the whole class on the whiteboard.
• For exercise 2 (2-layer seismic refraction), the students could be divided up into small groups (3–5 students per group) to complete the exercise. Each group could then transfer their travel-time offset plot and calculations to a section of the whiteboard, followed by a class discussion of results.
• If the students struggle with the exercises, start by providing the "Worked Examples" that break down the work into more explicit steps and do not use Excel. If the students are struggling with the Excel implementation, consider providing the key or "solution" Excel workbook sheets in part or in whole so they can see how to enter the formulas. Alternatively, show the solution sheets to the whole class, and then have the students go back and reproduce the results themselves.

• The Excel-based exercises provided in the Teaching Materials section of this page may be completed by students for a grade, using this rubric for Excel-based exercises (Microsoft Word 2007 (.docx) 75kB Sep8 24).
• Assessment questions, how seismic waves travel (Microsoft Word 2007 (.docx) 77kB Sep15 24). These questions can be given to students as homework, on quizzes, or on exams.

Solutions for instructors

• Answer key, student handout, how seismic waves travel -- private instructor-only file
  Hide
  Answer key, student handout, how seismic waves travel

  
  This file is only accessible to verified educators. If you are a teacher or faculty member and would like access to this file please enter your email address to be verified as belonging to an educator.

  Email Adress Submit

• Answer key, assessment questions, how seismic waves travel -- private instructor-only file
  Hide
  Answer key, assessment questions, how seismic waves travel

  
  This file is only accessible to verified educators. If you are a teacher or faculty member and would like access to this file please enter your email address to be verified as belonging to an educator.

  Email Adress Submit

• Answer key, Snell's Law exercise -- private instructor-only file
  Hide
  Answer key, Snell's Law exercise

  
  This file is only accessible to verified educators. If you are a teacher or faculty member and would like access to this file please enter your email address to be verified as belonging to an educator.

  Email Adress Submit

• Answer key, 2-layer seismic refraction exercise -- private instructor-only file
  Hide
  Answer key, 2-layer seismic refraction exercise

  
  This file is only accessible to verified educators. If you are a teacher or faculty member and would like access to this file please enter your email address to be verified as belonging to an educator.

  Email Adress Submit

• Alternative introductory materials and exercises related to seismic refraction:
  • Introduction to Seismic Refraction activity
  • Seismic Refraction activity
• Advanced practical exercises using MATLAB:
  • Seismic Refraction in Matlab activity
• A hands-on lab exercise for exploring refraction in light waves rather than sound waves:
  • Bending Light activity