Trans-California Seismic Refraction Profile Analysis
U. C. Davis, Department of Geology
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Homework assignment focused on identifying seismic arrivals and
determining layer thickness and velocity using real-world data from a
Junior/senior level,elective course in the theory and analysis of geophysical field methods.
Designed for a geophysics course.
Skills and concepts that students must have mastered
Students need to have been introduced to travel-time diagrams, including the
form of travel-time curves for reflected, refracted and direct waves. Although
the students are encouraged to use MATLAB for the plotting and analysis,
the assignment can be done on graph paper.
How the activity is situated in the course
One in a series of homework assignments on seismic reflection and refraction
Content/concepts goals for this activity
Interpreting seismic travel-time data for crustal-scale velocity and layer
structure. Recognizing the relationship between parameters in the travel-time diagram
and in the subsurface structure.
Higher order thinking skills goals for this activity
Synthesis of understanding of different travel-time relationships.
Interpretation of results in terms of earth structure.
Other skills goals for this activity
Application of previously derived equations to real data.
It is expected that students will have to seek out the information
needed to answer the interpretive question on the web or in books from
lower division geology courses.
Description of the activity/assignment
Homework assignment asks the students to analyze data from a refraction profile
across California. The data is presented in a table together with the abstract
and map figure scanned from a 1970s article in which the data was published and
analyzed. The assignment requires the student to plot the data, determine
line-fits to the three arrivals, identify the arrivals as the direct,
reflected or refracted arrival and then to determine the layer velocities. They are
asked to sketch a cross section of the profile with the layer properties and to illustrate
representative travel time paths. In addition, they are asked to consider the velocities and layer
thickness and determine what boundary in the continental crust the refracted
waves come from. Finally they are asked to compare their results with the results
presented in the abstract for the journal article and discuss any differences/similarities.
Determining whether students have met the goals
Students have met the goals of the assignment if they correctly identify the
arrivals, determine the layer velocities and the layer thickness. For the
interpretation questions, the students need to demonstrate that they understand
which ray paths in the surface related to which parts of the travel-time curve and
they need to explain why they have made the conclusions based on the results
of their analysis.
More information about assessment tools and techniques.
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The data set included in the homework assignment is from the article
Cardier D. S., A. Qamar, and T. V. McEvilly, Trans-California Seismic
Profile-Pahute Mesa to San Francisco Bay, Bulletin of the Seismological
Society of America, v. 60, no. 6, pp 1829-1846, 1970.