MIL-01-03

Love waves: A menace to shallow shear wave reflection surveying

Summary

Shallow shear wave reflection profiling, in comparison to compressional wave surveys, requires a greater awareness of and ability to discriminate the many sources and associated forms coherent noise can take on shot gathers and the manifestation of that noise on CMP stacked sections. Shallow shear wave reflection surveying is an attractive alternative to compressional wave reflection surveys in some situations due in large part to the increased resolution potential at a given dominant reflection frequency, the polarized nature of the energy, insensitivity to pore materials, and the relationship of shear wave velocity to rigidity/stiffness of materials. Unlike compressional wave arrivals where reflections are the only coherent hyperbolic events, reflections and surface waves can appear hyperbolic on shear wave shot gathers. This unique difference should eliminate the complete confidence placed in unique interpretations of all hyperbolic events with a zero offset apex as reflections. Without careful attention to true measured velocities (ground truth) and complete removal of all energy arriving within the noise cone, Love waves will likely stack coherently on CMP sections. Interpretations made of stacked shear wave data which do not include sample shot gathers where reflection events are clearly identifiable outside the noise cone should not be considered legitimate and need to be assumed coherent noise.

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