Propagation characteristics of split shear-waves are studied to gain an understanding of the effects of attenuation anisotropy. It is shown that, compared to velocity anisotropy, attenuation anisotropy is a more difficult quantity to measure, being dependent on the attenuation of the faster split shear-wave and the velocity anisotropy. Measuring changes in attenuation anisotropy from repeated shear-wave experiments is also considered, with a view to monitoring EOR processes. Three automatic methods for measuring shear-wave splitting parameters are developed and tested on a synthetic VSP data set contaminated with different amounts of random noise. Standard signal processing techniques are investigated using the synthetic data, to determine whether they distort or improve observations on shear-wave splitting. VSPs from a North Sea gas field and the Geysers geothermal zone, California, are analysed for shear-wave splitting. The North Sea VSP consisted of four wide offset source locations, relying on <i>P</i>-waves being mode-converted to <i>SV</i>-waves at the top of the cap-rock overlying the gas saturated reservoir sands. Fast shear-wave polarizations and time delays are measured from shear-waves in the reservoir region using the three automatic techniques previously developed. Results suggest a predominant crack orientation of N47<SUP>o</SUP>W agreeing with maximum horizontal stress directions found from earthquake focal mechanisms and borehole breakout data. One of the main problems associated with these North Sea VSPs was that the source to borehole azimuths were very nearly parallel to the crack strike, resulting in poor observations of shear-wave splitting.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:642570 |
| Date | January 1990 |
| Creators | Campden, David A. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/13301 |
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