Unit 2: How seismic waves travel through the ground

Basic concepts in refraction seismology. Andy Parsekian, University of Wyoming, aparseki@uwyo.edu


Summary

This unit presents core underlying principles needed to understand refraction seismology concepts including refraction of rays, types of seismic waves, interpreting information about subsurface materials from seismic properties and developing conceptual models of the subsurface environment.

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Learning Goals

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 relevent to urban and environmental geophysics
  • Cognitive: Empower students to use basic physical equations to predict properties of the subsurface

Context for Use

In the previous unit, Introduction to seismic refraction, 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 towards being able to apply the equations for themselves.

Description and Teaching Materials

Part 1: Seismic waves for subsurface exploration & 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 raypaths. This part finishes with a prompt to think 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) PRIVATE FILE 19.4MB Oct20 21)
  • Student handout, how seismic waves travel (Microsoft Word 2007 (.docx) PRIVATE FILE 76kB Oct20 21): 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 & 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 two-layer structure. The concept of material properties is revisited with a prompt to think about both the structure and seismic properties of the subsurface near to the student's home.

Materials provided:

Teaching Notes and Tips

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.

Assessment

  • 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) 74kB Oct12 21).
  • Assessment questions, how seismic waves travel (Microsoft Word 2007 (.docx) PRIVATE FILE 83kB Oct20 21). These questions can be given to students as homework, on quizzes, or on exams.

Solutions for instructors

References and Resources