Seismic Refraction Analysis of California Wash and Astor Pass

1. Seismic Refraction Analysis of California Wash and Astor Pass Stephen Hein Mason Kreidler

2. Overview • Project Objectives • Field methods • Analyses • Interpretations • Discussions

3. Project Objectives • California Wash • Analyze the fault running through California Wash, to evaluate seismic hazards for Las Vegas • Astor Pass • Examine regions that were not previously drilled, to see if more promising geophysical prospects exist

4. Equipment: All lines • 7 kg sledgehammer • Seismic source • 100 Hz Geophones • Seismic vibration monitor • Seismic cable with 48 channel • Data transfer • Bison Galileo-21 • Data logger

5. Analysis: All lines • First arrival picks using Viewmat • Better images through Band-Pass filters, TEGain, • Simple calculations using a time-distance plot in excel • V1 and V2 velocities: • Refractor depth: • Dip angle • Estimated low and high velocities • Possible third layers

6. Analysis: All lines • Dr. Pullammanappallil created P-wave seismic velocity sections from our first-arrival pick times using SeisOpt® @2D ( 2011 Optim) • Cross Section with velocities • Refractor depths • Layer velocities

7. Field Methods: California Wash Line 1 • Seismic cable oriented East-West with a total length of 100 meters • 2 meter spacing between each takeout • 6 geophones per takeout oriented parallel to the seismic cable • Sledgehammer hits used to propagate waves into the ground • 10 hammer hits at each source point • A total of 74 source points were taken • 14 to the East of the first geophone at a spacing of 4 meters • 48 at each channel along the line • 12 to the West of the last geophone at a spacing of 4 meters • Total length of hammer hits: 200 meters

8. Layout of seismic cable and geophones (Louie, 2011) Producing waves using a sledgehammer and layout of geophones (Louie, 2011

9. Results: California Wash Line 1 • V1: 666-1104.7 m/s with an average of 885.4 m/s • V2: 1301.2 m/s • Refractor depth between 2.4-15 m • Dip angle of 1.3 degrees

10. Optim'sSeisOpt®@2DTMResults: California Wash Line 1

11. SeisOpt® Interpretation: California Wash Line1 • P-Wave velocity was 600-900 m/s, consistent with simple calculations average of 880 m/s • V2 of ~ 1400 m/s consistent with simple calculations • Estimated refractor depth of 1-10 m, also consistent • Very low eastward dip in first refractor

12. Field Methods: California Wash Line 2 • Seismic cable oriented East-West with a total length of 48 meters • 1 meter spacing between each takeout • 6 geophones per takeout oriented perpendicular to the seismic cable • Sledgehammer hits used to propagate waves into the ground • 10 hammer hits at each source point • A total of 48 source points were taken • 48 at each channel along the line • Total length of hammer hits: 48 meters

13. Example of geophone arrangement and wave propagation at line 2 (Louie, 2011)

14. Results: California Wash Line 2 • V1: 216.7-326.1 m/s with an average of 271.4 m/s • V2: 1274.5 m/s • Refractor depth between 0.69-0.90 m • Dip angle of 0.24 degrees

15. Optim's SeisOpt®@2DTMResults: California Wash Line 2

16. SeisOpt® Interpretation: California Wash Line2 • P-Wave velocity was 200-400 m/s, consistent with simple calculations average of 270 m/s • V2 of ~ 1200 m/s consistent with simple calculations • Estimated refractor depth of 1-2 m, slightly higher than estimated in simple calculations • Minimal evidence of any dip in the first refractor

17. Interpretation of California Wash Lines 1 and 2 • Conformation of good results by overlaying CW2 onto CW1

18. Discussion: California Wash Lines 1 and 2 • Results from Line 1 give a good indication of the dip of the fault running through the wash • Line 2 shows evidence of the amount of slip that occurred that last time on this fault, by the amount of recent, slower velocity, sediment on top of older, higher velocity alluvium

19. Field Methods: Astor Pass • Seismic cable oriented East-West with a total length of 144 meters • 3 meter spacing between each takeout • 6 geophones per takeout oriented parallel to the seismic cable • Sledgehammer hits used to propagate waves into the ground • 10 hammer hits at each source point • A total of 60 source points were taken • 48 at each channel along the line • 12 to the West of the last geophone, at a spacing of 6 meters • Total length of hammer hits: 220 meters

20. Layout of seismic cables and geophones at Astor Pass

21. Results: Astor Pass • V1 of 275-400 m/s with an average of 337.5 m/s • V2 of 1090 m/s • Refractor depth between 15-30 m • Dip angle of 1-5 degrees

22. Optim'sSeisOpt®@2DTMResults: Astor Pass

23. SeisOpt® Interpretation: Astor Pass • P-Wave velocity was 300-400 m/s, consistent with simple calculations of 275-400 m/s • V2 of ~ 1000 m/s consistent with simple calculations of 1090 • Estimated refractor depth of 12-16 m, also consistent with previous calculations • No dip in first refractor

24. Basic Interpretation

25. Discussion: Astor Pass • Results from SeisOpt®did not agree with the hypothesis of seeing faster velocities near the middle of the plot where tufalayers were thought to exist. • These results actually lean towards a hypothesis that most if not all of the tufais already on the surface.

26. Possible Sources of Errors • Wind creating noise on the seismic line. • People walking around on the seismic line while recording data. • First arrival picks may have been off slightly; however, refractor depth and data was consistent throughout the plot, suggesting picks were valid.

27. Conclusion of Astor Pass • Refraction Microtremor results agreed with our Refraction results of not seeing faster velocities near the tufa mounds.