Summary

A method for displaying prestack seismic data that highlights azimuthal anisotropy is shown. Using this method to make observations of azimuthal anisotropy in prestack seismic data makes it clear that azimuthal anisotropy is observed in all types of seismic data and that it is widespread from the shallowest to the deepest seismic reflectors. Therefore, we must account for anisotropy when we are processing and interpreting these data.

Introduction

Visualizing anisotropy in prestack seismic data has been difficult. Cheadle et al (2001) describe a means of displaying prestack offset and azimuth data as a 3D cube. Henceforth, I will refer to this as a COCA (Common- Offset, Common-Azimuth) Cube. Todorovic-Marinic (unpubl. results) came up with a way to view these cubes in 2D that allows a more intuitive understanding of the anisotropic variation in conjunction with the AVO variation in the data. This and more conventional methods are used to demonstrate the large degree of azimuthal anisotropy in both land and marine prestack P-wave seismic data.

Method

The COCA cube extends the common offset stack by adding the third dimension of azimuth (Cheadle et al, 2001). In our implementation, a 3D cube is created where each bin represents a range of shot-receiver offsets and azimuths. For example, the first bin might represent all azimuths from 0-10 degrees and all offsets from 0-100 m. Then all traces that have these parameters are stacked and the resulting trace placed in this bin. The process is repeated for all desired combinations of offsets and azimuths. E.g. an offset binning of 0 - 4000 m by 100 m and an azimuth binning of 0 – 180 degrees by 10 degrees would result in 40 offsets x 18 azimuths = 720 bins. Each binned offset is then represented as an inline and each binned azimuth as a cross-line in a 3D seismic volume, which for this example would have 40 inlines and 18 crosslines, corresponding to offsets 50, 150, …, 3950 m and azimuths 5, 15, …, 175 degrees, respectively. The resulting 3D volume can be visualized using any 3D visualization software, as is shown in Figure 1. We have been using it for quality control of azimuthal processes, such as azimuthal NMO and azimuthal AVO, in much the same way that the common offset stack is used for quality control of conventional NMO and AVO. Here the COCA Cube is used to demonstrate the ubiquitous nature of azimuthal anisotropy and the significant degree of this anisotropy that can be observed in prestack seismic data.

Observations

Using the COCA cube has allowed us to visualize azimuthal anisotropy in a way we were unable to do before. This has demonstrated to us that azimuthal anisotropy occurs throughout the seismic section from top to bottom and in all types of environments. Using the COCA Cube and other tools that allow for visualization of anisotropy in prestack seismic data, I present prestack observations of azimuthal anisotropy gleaned from seismic gathers from various seismic surveys throughout the world.

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