Subsurface Imaging of Complex Structures by Reflection Seismic Data

Conventional imaging algorithms based on the Kirchhoff integral have difficulty producing interpretable images when irregularly shaped bodies, such as salt domes or basalt flows, with fast acoustic propagation velocities are present in the overburden. In these cases, imaging methods based on wavefield continuation of the recorded data produce clearer images than Kirchhoff methods. We present the general concepts behind some of the new wavefield-continuation methods that are capable of imaging 3-D prestack data at an acceptable computational cost. We note however, that the most challenging aspect of seismic imaging below complex overburden is not the focusing of the data, but the estimation of the background velocity function. We therefore introduce the concept of the Angle-Domain Common Image Gather (ADCIG), which provides the essential methodology for updating the velocity function from the migrated images. The ADCIG concept is not restricted to one type of migration method, and can be readily adapted to all the known methods for wavefield-continuation migration. A real data example from the deep waters of the Gulf of Mexico demonstrates the tight link between migration and velocity estimation. When the velocity function is updated by iterative wavefield-continuation migration, the imaging results substantially improve with respect to the image obtained using the starting velocity that was initially estimated using conventional Kirchhoff techniques.