Seismic refraction across the La Bajada Fault

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

Outline • Objectives • Data processing • Results • Conclusions

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

SAGE 2004 Seismic Line 101 457

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

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

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

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

Example 2 Offset (m)

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

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

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

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

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

Seismic refraction across the La Bajada Fault