Time-lapse Measurements of Shallow Horizontal Wave Velocity over a Compacting Field

Permanent ocean bottom cable (OBC) seismic systems represent a major emerging technology for the acquisition of time-lapse data over producing oil and gas fields. With permanent sensors on the seafloor, we can measure waves that are either not present or typically ignored on conventional (streamer) seismic surveys. Scholte waves and critically refracted compressional (CRC) waves, which propagate horizontally through the shallow subsurface, are two possibilities. These waves carry information on the shear and compressional properties of the shallow sediments and are therefore expected to respond to surface geomechanical changes that are driven by deep hydrocarbon production. Here, using existing active source data from the OBC array in the Valhall field located offshore Norway, we investigate time-lapse monitoring with shallow seismic waves. The large number of 4C surface receiver stations and a dense shot grid make the Valhall data an excellent laboratory for the investigation of time-lapse effects of Scholte and CRC waves. We find that these waves are observable field-wide and that time lapse changes in their velocities are measurable with high accuracy. Applications include reservoir-monitoring, studies of dependence of in situ rock velocity on strain, understanding of OBC noises and 4D shallow hazards.