Using Data from the Trans-California Seismic Refraction Profile to Illustrate the Utility of Travel-Time Diagrams

Magali BillenU. C. Davis, Dept. of

GeologyCourse: Exploration

Geophysics

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Exploration Geophysics

• Upper division (juniors/seniors)• Elective course

– No pre-requisites beyond basic math/physics required for geology majors (eeck!!)

• Also serves as elective for engineering students

• Class size varies from 10 - 20 students.

Course Goals• Master basic analysis of several geophysical field methods– Seismic reflection/refraction– Gravity surveying– Resistivity– Electrical methods

• Develop understanding of basic crustal structure & geophysical anomalies– Geometry: size, thickness, depth, shape, magnitude

– Physical origin: density, composition, water

Challenges• Topic is outside my area of research.– Access to interesting/useful data is limited.

• Analysis is formulaic & students get bored or overwhelmed by the multiple steps required.

• Students have highly variable math skills & physics background.

Possible Solution?• Redesign course to focus on a region.

• New, additional goal is to learn something about regional geology & tectonics.– Example: Northern California, from the coast to the Sierras.

• Each technique is taught in the context of learning about some aspect of the subsurface structure for this region.– Example: use seismic refraction data to determine crustal thickness of the Great Valley.

Baby-Steps in Course Re-Design• Step 1: find

appropriate data sets– Search web (seismic data, gravity profiles)

– Search journal articles• Initial analysis were often simple & straightforward

• Step 2: integrate data sets into lectures– Can start by using data as examples.

• Step 3: rewrite homework assignments– apply analysis to regional data sets.

Cost vs. Rewards• Costs: time, lots of time!!• Rewards:

– More interesting for ME!!– More interesting for the students.– More effective at instilling long-term knowledge of when/where methods are useful.

– More effective at developing a deeper knowledge & appreciation of methods & Earth’s structure.

– More exposure to real geologic/tectonic data.

Refraction Seismology Homework

• Data from published paper:– Trans-California Seismic Profile - Pahute Mesa to San Francisco Bay, D. S. Cardier, Anthony Qamar, T. V. Mc Evilly, BSSA, v. 60, p 1829-1846, 1970.

• First homework on refraction seismology.

• Lecture introduced:– Survey geometry.– Seismic wave paths in the sub-surface.

– Travel-time relationship.– Relationships between travel time curve & subsurface structure.

Refraction Seismology Concepts

• Basic data is a travel-time curve (linear).

• Offset vs. arrival time

• Slope is related to layer velocity.– v2 = 1/m2

• Intercept is related to layer thickness.– z = tiv1v2/2(v2

2-v12)1/2

– v1 from slope of direct arrival.

Homework Exercise• Data

– Background explanation about Nevada Test Site as a seismic source.

– Introduction from paper.– Map of stations & source,– Table of station distances & arrival times.

Homework Exercise• Plot Data– Three sets of arrivals: Pn, Pg, P2

– Note haven’t told students what these mean.

• Identify linear arrivals• Determine slopes of

lines and intercepts• Calculate layer velocity

and thickness• Use previous knowledge,

geology books, google to determine what crustal layers these might be.

Results/Student Feed-Back• Positive feedback to having geologic goal.

• Liked learning about NTS seismic sources.• Straight-forward to plot data & get results.

• Some confusion about interpreting data in terms of crustal structure.

• Would like to use this data set again in another assignment... – look at travel-time residuals to get basic info. on non-planar structure.