Jorge H. Faccipieri

Recovering diffractions in CRS stacked sections

The common reflection surface (CRS) method has been proposed as an alternative to the classical common midpoint method to further improve the signal-to-noise ratio, as well as to obtain additional kinematic parameters that are useful for a number of imaging purposes. In the CRS method, reflected events are enhanced by stacking along a generalized hyperbolic moveout, referred to as the CRS moveout. However, during the process of CRS stacking, diffractions are likely to be attenuated or even suppressed. Diffracted events are important since they carry high-resolution information about the subsurface structure. By the use of a modified version of the CRS technique, diffractions can be enhanced in the same way as reflections. This paper proposes a combined approach where the CRS stack is superimposed on the CRS diffraction-enhanced stack. In this way we can recover the diffractions in CRS stacked sections. The potential of the method has been demonstrated using marine seismic data acquired offshore Brazil.

Common-reflection-point time migration

Since the early days of seismic processing, time migration has proven to be a valuable tool for a number of imaging purposes. Main motivations for its widespread use include robustness with respect to velocity errors, as well as fast turnaround and low computation costs. In areas of complex geology, in which it has well-known limitations, time migration can still be of value by providing first images and also attributes, which can be of much help in further, more comprehensive depth migration. Time migration is a very close process to common-midpoint (CMP) stacking and, more recently, to zero-offset commonreflection- surface (CRS) stacking. In fact, Kirchhoff time migration operators can be readily formulated in terms of CRS parameters. In the nineties, several studies have shown advantages in the use of common-reflection-point (CRP) traveltimes to replace conventional CMP traveltimes for a number of stacking and migration purposes. In this paper, we follow that trend and introduce a Kirchhoff-type prestack time migration and velocity analysis algorithm, referred to as CRP time migration. The algorithm is based on a CRP operator together with optimal apertures, both computed with the help of CRS parameters. A field-data example indicates the potential of the proposed technique.

Stretch-free generalized normal moveout correction: Stretch-free GNMO correction

The effective application of normal moveout correction processes mainly depends on four factors: the chosen traveltime approximation, the stretching associated with the given traveltime, crossing events and phase changes, the last two being inherent to the seismic data. In this context, we conduct a quantitative analysis on stretching considering a general traveltime expression depending on half-offset and midpoint coordinates. Through this analysis, we propose a mathematically proven procedure to eliminate stretching, which can be applied to any traveltime approximation. The proposed method is applied to synthetic and real data sets, considering different traveltime approximations and achieved complete elimination of stretching.

A CRS-based Heterogeneity Attribute

An attribute describing heterogeneity is proposed based on the diffracted-wave contribution. In areas of complex geology the amount of diffracted energy will increase relative the specular reflections. By use of the modified CRS technique, a reliable measure of the diffractions can be obtained. Proper scaling with the output from a conventional CRS stack gives a normalized measure of the complexity of the geology. The new attribute has possible use both in seismic texture analysis and as a weight factor employed to combine CRS based reflection- and diffraction-enhanced stacks. The potential of the heterogeneity attribute is demonstrated using seismic data from offshore Brazil.

Improved CMP and CRS Stacking by a Combined Use of Conventional and Automatic Velocity Estimation

Stacking may be considered one of the most important steps in seismic processing providing an approximate zero-offset section with an enhanced signal-to-noise ratio in which the NMO velocity is the key quantity. In this work, we will compare the conventional and automatic approaches to NMO velocity estimation employing both CMP and CRS type of stacking of a 2D real marine data set. Moreover, based on the results obtained we propose a combined approach which has the potential to significantly improve the CRS stacking process.