Shallow and Deep Time-lapse Effects on Valhall LoFS Converted Wave Data

The Valhall LoFS OBC system provides an excellent opportunity for 4D studies, both for P-wave and converted wave data. We have observed a very good correlation between the 4D effects in Scholte wave velocity, converted wave arrival times and shear wave splitting in the shallow overburden. These effects are indicative of shallow overburden compaction caused by production induced reservoir compaction. The strong shear velocity increase in the shallow overburden causes 4D converted wave time shifts that are up to 15ms large, as well as large changes in Scholte wave velocity, measured from the vertical geophone. Additionally we observe a reduction in shear wave splitting due to the shallow compaction. These strong shallow 4D effects obscure the top reservoir converted wave 4D effects, but by application of a receiver consistent 4D correction we are able to produce top reservoir converted wave 4D time-shifts that are consistent with the P-wave 4D signal.