MAZ depth-velocity modelling and imaging with azimuthal anisotropy*

Abstract

Processing of multi and wide-azimuth seismic data faces some new challenges, and one of them is depth-velocity modelling and imaging with azimuthal velocity anisotropy.

Analysis of multi-azimuth data very often reveals noticeable fluctuations in moveout between different acquisition directions.

They can be caused by several factors:

1. real azimuthal interval velocity anisotropy associated with quasi-vertical fractures or present day stress field within the sediments;

2. short-wavelength velocity heterogeneities in the overburden;

3. TTI (or VTI) anisotropy in the overburden; or,

4. random distortions due to noise, multiples, irregularities in the acquisition geometry, etc.

In order to build a velocity model for multi-azimuth pre-stack depth migration (MAZ PSDM) taking into account observed azimuthal anisotropy, we need to recognise, separate and estimate all the effects listed above during iterative depth-velocity modelling.

Analysis of seismic data from a full azimuth 3D seismic land survey revealed the presence of strong spatially variable azimuthal velocity anisotropy that had to be taken into consideration. Using real data examples we discuss major steps in depth processing workflow that took such anisotropy into account: residual moveout estimation in azimuth sectors; separation of different effects causing apparent azimuthal anisotropy (see A–D above); iterative depth-velocity modelling with azimuthal anisotropy; and, subsequent MAZ anisotropic PSDM.

The presented workflow solved problems with azimuthal anisotropy in our multi-azimuth dataset. Some of the lessons learned during this MAZ project are relevant to every standard narrow azimuth seismic survey recorded in complex geological settings.