Seismic refraction across the La Bajada Fault

1. Seismic refraction across the La Bajada Fault Yusliza Mohd Sufian UNIVERSITY OF TEXAS AT AUSTIN SAGE 2004

2. Outline • Objectives • Data processing • Results • Conclusions

3. Objective To find layers deeper than the basalt layer (below 80 m depth) using the wide angle reflection method Why? High velocity basalt flows limits imaging underlying layers or structures using near vertical incidence reflection

4. SAGE 2004 Seismic Line 101 457

5. Data Processing • Deconvolution • Wide angle reflection method • T2 vs. X2 method

6. Deconvolution • to eliminate reverberation and some of surface waves • Example

7. Wide angle reflection • Definition reflection where the angle of incidence is near or greater than the critical angle • Concept by increasing the offset between source and receiver, the energy reflected on a seismic interface increases as compared to the energy transmitted through the interface Relative energy Angle of incidence

9. ID wide angle reflection • Faster than head waves • Example 1

10. Example 2 Offset (m)

11. T^2 vs X^2 • Calculate 1-D velocity model • Parameters time, t (s) and offset, x (m)

12. Results 1-D velocity model at shotpoint 101 Data points for each layer at shotpoint 101

13. Data points for each layer at shotpoint 457 1-D velocity model at shotpoint 457

14. Stratigraphy of the 1-D velocity model Alluvium Basalt rubble Santa Fe Group Santa Fe Group 1-D velocity model at 101 1-D velocity model at 457

15. Conclusions • Wide angle reflection method helps to identify layers beneath the basalt layer • Layer with velocity more than 3 km/s at depth 400 to 500 m depth • No basalt layer in the velocity model because it is a thin layer