Using Data from the Trans- California Seismic Refraction Profile to Illustrate the Utility of Travel-Time Diagrams Magali Billen U. C. Davis, Dept. of Geology Course: Exploration Geophysics

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 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 , 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. – v 2 = 1/m 2 Intercept is related to layer thickness. – z = t i v 1 v 2 /2(v 2 2 -v 1 2 ) 1/2 –v 1 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: P n, P g, P 2 – 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.