Deepwater High-Contrast Volcanics
Not all challenges to reflection tomography exist in the overburden. In this example from west of Shetlands, a reservoir is encased in a high-contrast volcanic interval. The top volcanic generates a recorded refraction, however, the base of the volcanic layer, which is also the top of the reservoir, does not.
Snell’s Law infers that a contrast in velocities from fast to slow at such a boundary, and for the offset range used in this acquisition, make transmission FWI difficult. The contrast in velocities also makes reflection tomography challenging because of a lack of data in midpoint gathers. Therefore, the PGS full wavefield FWI was used. Both transmitted and reflected energy contributed and no conditioning or discrimination of these wavefields was required.
The images of the update from FWI show that the intra-volcanic reservoir is captured by PGS full wavefield FWI and is shown as a slowdown (blue) encased within the faster volcanic layer. The same approach has been used for intra- and pre-salt updates on PGS library data from the Santos Basin.