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Interpreting seismic data in the presence of azimuthal anisotropy; or azimuthal anisotropy in the presence of the seismic interpretation

AXIS Geophysics, Denver, Colorado

Corresponding author: williams@axisgeo.com

The first 20% of the full text of this article appears below.

Measuring P-wave azimuthal anisotropy has been in the recent past an elusive task; therefore, the interpreter ignored this attribute of the seismic data and left the subject to the research and technology group geophysicists. Conversely, the interpreting geophysicist knew that, when measured, the anisotropy could yield important reservoir properties related to fractures and stress fields. However, little did we suspect that P-wave azimuthal anisotropy both in velocity and AVO would change our perception of data interpretation as is happening now.

Two very important changes recently occurred in P-wave anisotropy research. First, very accurate methods of measuring both azimuthally varying velocity and AVO have been developed (see the paper by Jenner in this issue). Second, the very large anisotropy values in the literature were found to be fairly normal and ubiquitous for basins that are not under primary burial. (See the suggested readings for P-wave velocity anisotropy magnitudes.) Therefore, understanding the anisotropy becomes a prerequisite to both obtaining the correct imaging from processing and determining the effects of anisotropy on conventional seismic interpretation. An interpreter will also find that there is no escaping an anisotropic world even when not working with true 3D data. The truth is, the earth does not care which tools we use, it still knows that it is anisotropic.

True 3D data, wide azimuth, are data shot with a complete sampling of source-receiver azimuths with the additional requirement that offsets are fully populated in each azimuth class. These are the data that are appropriate for very accurate measurements of azimuthal velocity and azimuthal AVO. However, most data are not acquired in this way especially in the marine environment. Therefore, little can be done to directly view the anisotropy of the earth with these data. One might then say no problem exists and conclude that there is no . . . [Full Text of this Article]