A 3D-4C ocean bottom seismic data set, originally shot over the Valhall field (North Sea) to image through a gas cloud, was used to investigate the detection of azi-muthal anisotropy through the shear-wave splitting phenomenon. The data were processed in converted wave mode (C-waves), assuming that the conversion occurs at the reflection point and involves downgoing P-waves and upgoing S-waves. In this preliminary study two key horizons were used, the early-Miocene at about 2.8 seconds and the top chalk at about 5.5 seconds. The detection technique involves azimuth dependent analysis of radial-to-transverse energy ratios. Preliminary results show that the technique is capable of measuring azimuthal anisotropy effects despite a cross-spread acquisition geometry, where azimuth and offset are poorly sampled. The results show that the principal directions of anisotropy for both formations are close and coincide with the direction of the main fault system. In addition, due to the significant magnitude of the shear-wave splitting measured at the top chalk level, the converted-wave data show vastly improved resolution when oriented in fast and slow modes.
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Anisotropy 2000: Fractures, Converted Waves, and Case Studies
“This volume contains 25 papers that represent most of the best work in seismic anisotropy in 1998 and 1999. Fracture characterizations and processing of converted waves are the two main topics covered in this volume. They are addressed from both theoretical and practical viewpoints. Also included are papers describing the historical roots of seismic anisotropy.”