Sea‐bed diffractions and their impact on 4D seismic data

ABSTRACTA sea‐bed reflection is used to estimate changes in water layer velocities between time‐lapse seismic surveys. Such corrections are crucial in order to obtain high‐quality 4D seismic data. This might be a challenge in areas where rough sea‐bed topography creates sea‐bed diffractions that interfere with the sea‐bed reflection, as in several of the northern fields in the Norwegian Sea. These diffractions and diffracted multiples are difficult to attenuate during data processing and become a source of noise in time‐lapse data.In this work we study how cross‐correlation analysis of time‐lapse seismic data at a sea‐bed reflection may be perturbed by the presence of diffractions. The sea‐bed topography from the Norne field is used in 2D finite difference modelling to explain some of the observed variation in a water layer time‐shift in field data. We find that a rough sea‐bed and the diffracted energy it induces cause residual noise on the 4D data. The variation to the water layer time‐shift increases with sea‐bed complexity and is amplified by interaction with other sources of non‐repeatability like water column variations, mis‐positioning and strength of the ice scours creating the diffracted energy. Time‐shift variations with mis‐positioning and velocity changes between the surveys seem to best explain the observed variation in the time‐shift for time‐lapse seismic field data from Norne.