f Reservoir definition using triaxial walkaway VSPs

A gas discovery was made by British Gas with an exploration well in block 47/4b of the UK continental shelf. It was important to define the extern of the reservoir particularly to the North West . The structural history of this location is extremely complex, panicularly with respect to the near surface . Ray-path distortion through the complex overburden means conventional surface seismic data is of extremely poor quality, making interpretation difficult . British Gas embarked on an extensive walkaway and offset VSP program on well 47/4b- 5 The success of these surveys to image the Rotliegende azimuthally from the well was dependent on the ability of the processing to compensate for ray - path distortion together with a confident separation of shear and compressional components. The key step involved the extension of parametric inversion developed for VSP to walkaway geometry which resolved the raw triaxial data info its constituent wavefields (Fig 2). This allowed conventional migration with the 3D-lattice geometry to define the extent of the field (Fig 1 ).