Gary C. Aldunate

Modelling, Stack and Imaging: Marine Seismics

This paper summarizes practical aspects of seismic modelling of a marine geological ambient of petroleum exploration of passive sedimentary basins along the Brazilian coast. The results of a consistent attention to stack and imaging demonstrate the basics and the potential of the combined SU/WIT processing systems. The central attention is grounded on the data driven CRS (Common Reflection Surface) stack, as we also look at to establish a workflow for seismic reevaluation of sedimentary basins. The CRS stack is atractive because it is a velocity independent method, and is founded on recovered wave front attributes. Using these attributes, a post-stack Kirchhoff type time migration is automatically carried out.

Método híbrido de migração pré-empilhamento em profundidade no domínio da freqüência em duas etapas com interpolação

This paper shows the application of Phase Shift Plus Interpolation Split-Step (PSPI-SS) wavefield extrapolation operator coupled with the excitation-time imaging condition to perform prestack seismic depth migration of common shot profiles. The migration methods which are characterized by the use of different propagators for the source and receiver wavefields are called hybrid migration methods. Normally, the source wavefield is determined by the computation of the traveltimes from the solution of the eikonal equation. Here the traveltimes were calculated based on a finite difference solution of the eikonal equation (TGRID), a simple geometrical approach (FAST) and also by the paraxial ray tracing method (PARAX). For the extrapolation of the receiver data we applied the PSPI Split-Step operator, that is derived from the conventional Split-Step wave equation method using multiple references velocity. As the source traveltime computation is based on the ray assintotic theory the velocity field needs to be smoothed and the migration results obtained with the Kirchhof migration method, that is a very fast migration procedure, was used to choose the best level of smoothing of the velocity field. In this work the synthetic Marmousi data and a real dataset from Gulf of Mexico were used to test the migration methods proposed here. The migration results with the hybrid methods shown better images in quality then the kirchhoffs results and comparable to those obtained with the full PSPI-SS migration method, but a low computation cost.

Migração pré-empilhamento em profundidade no domínio da freqüência de seções de ondas planas

The prestack migration of common-shot section based on the wave equation has produced high quality seismic images. However, for a 3-D dataset, this process involves a huge number of computational operations and its high cost impedes its daily use in data processing. Methods that can reduce the number of migrations, as plane wave migration, requested for a complete seismic experiment, are very attractive, mainly, in the 3-D seismic data processing. Plane wave migration can also produce high quality images that are comparable to those of shot migration with much less computational cost. The prestack plane wave migration implemented in this work is applied along constant ray parameter sections, obtained through the decomposition of the data arranged in common receiver gathers taking by the Slant-Stack transform. Then these sections are extrapolated in depth using the Split-Step extrapolation operator. The images in depth are obtained by summing all frequencies of interest from all migrated constant ray parameter sections. The source plane wave method was applied in two synthetic seismic datasets and the results achieved with the new migration method simulating a plane wave source has produced good quality subsurface images, comparable with those from common-shot gathers migration with a lower computational cost.

Migração sísmica 2-D pré-empilhamento em profundidade com operadores de extrapolação "split-step"

Three 2D prestack depth migration techniques using split-step extrapolation operators were developed and tested on seismic data sorted into common shot gathers. In the first method, which we name simultaneous split-step migration (SS-S), the migration procedure is carried out simultaneously for the sources and receivers. The recorded receiver data are depropagated in depth and the source wavefield is downward propagated using the split-step operators for both. The final depth section is achieved by summing all the frequencies of interest after the correlation of the propagated and depropagated wavefields, for each depth level and by the sum of all migrated shot gathers. To decrease the computational time of the SS-S method, we can calculate the source wavefields through a finite difference solution of the eikonal equation. This second method we call the hybrid split-step migration method (SS-H). In the third migration method, we combine the SS-S with the PSPI method. In this case the wavefields are depropagated using split-step operators for different velocities and then interpolated as in the PSPI method. We called this method PSPI-SS. The choice of the split-step operator for migration is mainly due to its easy implementation, high accuracy and robustness even in situations with very strong lateral velocity variation. The results we present in this work were obtained using the Marmousi data and also the SEG-EAGE salt model, which present very high geologic complexity. The results obtained with the three differentes methods were compared and all show satisfactory images.