Crosswell Waveform Tomography in Fine-layered Sediments - Meeting the Challenges of Anisotropy

During June, 2004, BP carried out a crosswell seismic survey in the Rocky Mountains. The crosswell data were acquired in finely layered clastic sediments in the Mesozoic section. The reservoir sands are known to vary laterally in quality, and the crosswell survey was conducted in order to provide a high resolution, sub-seismic image of the target. High-resolution velocity images were obtained using Waveform Tomography. This was particularly challenging due to low signal-to-noise ratios, high intrinsic attenuation, and significant, variable, an-elliptic anisotropy with a vertical transverse isotropy (TIV) symmetry. The final Waveform Tomography results show we can resolve layers down to the order of 4 m in thickness, and that we can accurately track these layers across the target zone. A subtle change in dip in the geology is revealed below a known unconformity. The variability of the anisotropy played a critical role, and the development of fully anisotropic Waveform Tomography is a logical next step.