A case study for 3D land data from Mexico shows that high-resolution image and attribute volumes from CRS time processing directly add to a fast and accurate depth model building and imaging.
A superior initial depth model that reproduces the subsurface dip without require continuous horizons is provided by CRS tomographic inversion. In alternative model building approaches, however, the lack of horizon continuity in low-fold areas is the major draw-back for layer-based model building, whereas the structural dip is not honoured by vertical Dix inversion of stacking velocities. The smooth CRS tomography model is well suited for an initial PostSDM of the CRS stack, which exhibits an excellent subsurface resolution in comparison to prestack time migration.
The first phase of CRS-based depth processing comprising CRS Tomography and CRS PostSDM, thus provides an increase of subsurface resolution from time processing results at little additional effort. The second phase gains additional accuracy by the costly steps of model update, and PreSDM. In order to improve the image in areas with salt intrusions, the salt bodies are interpreted in the CRS PostSDM result, and inserted into the CRS tomography model. Further runs of PreSDM and model updates complete this depth imaging phase.
Seismic depth processing is the decisive step to reconstruct the structural geometry in the subsurface. Depending on the desired accuracy, both, the depth model building, and the depth imaging can be very time-consuming and costly steps. In order to increase the depth resolution and signal quality especially in data of varying fold or quality, the CRS technique can be integrated at all stages of the general depth imaging procedure.
In this case study, a depth processing is based on the initial CRS time processing of 3D seismic land data from Mexico. At the surface, acquisition of these data had to deal with several inhabited areas, that caused strong variations of fold, and of data quality. In the subsurface, the Tertiary and Mesozoic sediments are disturbed by strong salt tectonics in part of the survey. The low fold areas, and the complicated subsurface represented the main challenges for depth processing.
The CRS time processing provides high-resolution volumes of both, the CRS image, and the CRS stacking parameter or attributes, for subsequent depth processing. The high information contents of the densely sampled CRS attributes may well be used for a fast initial reconstruction of the velocity-depth model by CRS tomography. Especially with respect to the varying data quality and the complicated subsurface, this approach has significant advantages in comparison to two other fast methods of layer-based model building, or of Dix inversion of stacking velocities, respectively.
With the CRS tomography model, a first straightforward depth imaging can be preformed by poststack depth migration (PostSDM) of the CRS stack. The result exhibits an excellent structural resolution in comparison to conventional prestack time migration (PreSTM) . This first cycle of depth processing thus provides a fast and effective option to obtain preliminary depth sections that improve resolution beyond time processing.