1. Advanced Seismic Imaging GG 6770 Variance Analysis of Seismic Refraction Tomography Data By Travis Crosby

2. Outline Description of field experiment Description of refraction tomography processing and inversion Description of refraction tomography processing and inversion Description of variance analysis Description of variance analysis Results of tomography experiment Results of variance analysis

3. Site Location Map

4. SITE

5. Line Location Map

6. Seismic Data Acquisition Parameters Geophones Geophones - 120 40-Hz vertical component at 5 meter intervals. Source Source - Elastic Weight Drop with 550 pound hammer activated at 20 meter intervals (every 4 th geophone). 31 total shot points per line. Five impacts summed per shot point. Recording Recording - 0.5 ms sample rate, 2000 samples per trace, 1 second listening time.

7. Seismic Refraction Data Common Shot Gather WestEast Created with Vista

8. Seismic Data Processing Parameters - 3188 traveltimes picked (out of 3720 total) - 0.6 meter grid size - One dynamic smoothing schedule 21x11 30 iterations - 3.9 times more knowns (traveltimes) than unknowns (smoothing schedule) - Final residual 3.9 ms Residual vs Iteration

9. Description of Variance Analysis - Velocity data was obtained from 30 inversions where first-arrival time was randomly altered up to ± 4 ms ( ¼ period of average first arrival). - The sample variance of each slowness cell over the 30 samples was determined. - The variance provides an idea of how widely spread the values of each slowness cell are likely to be. - The larger the variance, the more scattered the velocities are. It may be inferred that there is more uncertainty in these parts of the model. - Sample variance: S² = Σ (x i – x ave )² / n-1

10. Tomography Results – Original Data Velocity Profile Ray Density Plot

11. Variance Results Variance Ray Density Plot with color thresholding

12. Variance Results Variance Ray Density Plot without color thresholding