PS-wave processing in complex land settings: statics correction, wave-mode separation and migration

Saúl Ernesto Guevara

This thesis concerns processing issues with elastic waves in complex land settingscharac-terized by rough topography and geological structures with irregular geometry and proposes some solutions. It is focused on PS-waves (converted from P to S at the reflection point).

The near-surface layer (NSL) is addressed first. An uphole field experiment using explo-sive sources, located at a surface 2D-3C seismic line, allowed detailed local analysis of the NSL S-wave velocity (VS) model. S-waves generated by the sources were identified, from which a local VS model was obtained. Analogous S-waves were also identified on the surface seismic line (which is not usually expected). Tomographic inversion of S-wave refractions from the seismic line enabled the generation of a NSL VS model for the complete line. How-ever, because of the line sampling, it lacked the critical low velocity of the shallowest zone provided by the uphole.

Statics correction for PS waves is focused on next. This is a demanding processing effort for PS waves, due to the S-wave properties in the NSL. A new statics correction method is proposed, based on the cross-correlation of traces from adjacent receivers, assuming that the only delay time is the receiver statics. This method, termed CRGS (for Common Receiver Gather Statics), was first tested and validated on synthetic data, after which it was applied to real data with encouraging results. This new method overcomes shortcomings of other methods currently used since, in addition to being automatic, it requires neither velocity information nor identification of PS reflections.

Topography and wave-mode separation are also addressed. A wave-mode separation method is proposed which considers the free-surface effect for a zone with rough topography and lateral variations in its elastic properties. This method was tested on synthetic data with promising results.

Finally, PreStack Depth Migration (PreSDM) methods are tested for PP and PS-waves in the presence of topography. Two scalar PreSDM methods, Kirchhoff and PSPI (for phase-shift plus interpolation), were implemented. The resulting images on synthetic data with and without wave-mode separation proved their reliability for accurately locating seismic events.