Full Waveform Modelling for Converted Waves Seismic Reflections in Mountainous and Marine Environment

Abstract

The application of seismic waves allows us to achieve adequate results by compressional wave (P-wave) surveys alone. However, in the presence of gas P-wave transmission disrupts and obscures underlying targets. Many reservoirs don’t present sufficient impedance contrast to the overburden and not reflect P-wave strongly to produce an impedance image. High impedance rock such as basalt or hard volcanic rocks are difficult to image with P-wave. To overcome these challenges shear-wave (S-wave) or converted wave (P-S) surveys are usedfor last 20 years by making the use of down going P waves converting to upcoming S waves at the mode conversion boundaries.The processing of converted waves requires studying asymmetric reflection at the conversion point, difference in geometries and conditions of source and receiver, and the partitioning of energy into orthogonally polarized components. Interpretation of P-S sections incorporates the identification of P-S waves, full waveform modelling, correlation with P-wave sections and depth migration.
The objectives of this study is to model P-S wave reflections in onshore and offshore environment and to examine the major differences in processing of P and P-S wave surveys together with the identifying converted mode reflections by P-wave sources in anisotropic media. To achieve these, realistic mountainous and marine environment models have been developed and synthetic seismograms are generated by full waveform modelling technique. First a mountain foothill model was studied. A Kirchhoff-based technique that includes anisotropic velocities is used for depth migration of P-S waves. The results from depth imaging show that P-S section help in distinguishing amplitude associated with hydrocarbons from those caused by localized stratigraphic changes. Marine model shows a good correlation with identified converted waves. In addition, the full waveform elastic modellingproves useful in finding an appropriate balance between capturing high-quality P-wave data as well as P-S data challenges in a survey.

Key Words: Converted-waves (P-S); P-S Wave; Kirchhoff migration; Depth migration; Gas clouds; Shale diapers