SS: The Future of Seismic Imaging; Reverse Time Migration and Full Wavefield Inversion - Reverse Time Migration Imaging and Model Estimation

Abstract

The Role of Reverse Time Migration in Imaging and Model Estimation Introduction The key depth imaging technology addressed in this article is Reverse Time Migration (RTM). It will be compared to Kirchhoff, beam and other wave equation migration (WEM) techniques. There will be an emphasis on reducing exploration cycle time and risk. Recent trends such as TTI anisotropy and wide azimuth applications will also be discussed. RTM is most suitable in geologic regimes that exhibit significant amounts of salt. Typically, there are contrasts in velocity and steeply dipping features in this setting. The key conclusion from this article will be that by using RTM throughout the imaging sequence, including model building, the total cycle time of the project can be significantly reduced. Exploration risk is also reduced by RTM because it produces images that are more accurate in terms of structure and amplitudes. Sub-Salt Imaging Challenges The challenges of subsalt imaging are daunting. Salt structures are formed when salt sheets intrude into the higher-density sediments deposited above them. The resulting domes, walls, pillows, ridges and fountains are complex. They can be free-floating or remain attached to the base salt layer as irregular, mushroom-shaped bodies called diapirs. The map in Figure 1 gives an indication of how common these highly complex salt bodies are in exploration. Imaging in salt rich domains is difficult since the seismic waves reflected off the steep flanks of subsurface features travel near horizontal so it is a problem for standard imaging algorithms to image them. Further the top of salt structures is often highly rugose, which causes the scattering of seismic waves into multiple paths. These factors, if not taken into consideration, can yield misleading information about the location and geometry of prospective formations. In the 80's, the Kirchhoff depth migration method was used to image seismic data. From the 1990's, wave equation migration methods were used in combination with Kirchhoff methods to try to fully resolve structures not adequately resolved with Kirchhoff migration. Starting in 2005, reverse time migration became commercially viable in all phases of the imaging sequence.