Week 9

Wed Oct 21^st, 2026 | LEC

• Discussion of the basic refraction survey: direct, critically refracted, and head waves. Shot offset. Critical distance. Crossover distance. Change in slope as indicator critically refracted wave has overtaken direct wave.

  • You should be able to make such a drawing.

• Analysis of our field data from last week. Use of Front End Express to pick first-break times, export the time, use a spreadsheet to plot the arrival times vs. distance.

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  • Calculate slopes → calculate velocities.

    • Convert velocities to potential geologic materials.

  • Obtain $t_i$$t_{i}$ from best-fit line for the head wave arrivals.

  • Calculate Layer 1 thickness $h_i=\frac{t_i}{2}\cdot \frac{V_2\cdot V_1}{\sqrt{V_2^2-V_1^2}}$$h_{i}=\frac{t_{i}}{2} \cdot \frac{V_2 \cdot V_1}{\sqrt{V_2^2-V_1^2}}$

• Most importantly - the geologic interpretation. Use your preliminary geologic model, and updates. What did we come up with in class?

Thur Oct 22^nd, 2026 | LAB

• Field exercise rained out - will do next week.

• Reviewed the basic seismic refraction drawing

• Concepts of and formulas for critical distance, crossover distance, and 1st layer thickness.

  • These formulas are in the I2AG reading - see the posted readings on the Inroduction to Seismic Refraction page.

  • Importantly, these formulas generalize to multiple layers. The formulas are not beautiful, but they are understandable.

• Non-ideal subsurfaces - see the non-ideal subsurfaces page.

• Dog Leg FWD and REV seismic refraction data from 2022:

  • Field Notes for both FWD and REV shots.

  • FWD .sgy file

  • REV .sgy file

  In out quick analysis in class, we examined the data set as if it contained only one critically refracted / head wave. This may not be true if we look at the travel-time curve plot more carefully!

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• "COWPOOP2" seismic refraction data from the Dog Leg in 2025

  • Field Notes

  • FWD .sgy file. The location is similar to the 2022 data above, but the profile is about 40 meters longer.