Near Surface Modeling and Effect on 4D Repeatability In The Presence Of Shallow Gas Anomalies

Time Lapsed 4D seismic surveys are now considered to be an invaluable and essential tool in order to capture, image and analyze valid changes in seismic signal induced by reservoir depletion, or by injection of water or CO2 into the reservoir. 4D monitoring has been proven to be very successful for offshore reservoirs (Haugvaldstad). However, in ultra-shallow water situations, the valid time lapse signal may be obscured with unrepeatable seismic noise, attributable to variations in near surface heterogeneity due to shallow channels with partial gas-saturation, acquisition geometries (e.g. undershoots), strong multiple contamination and ambient noise. These residual unrepeated noises represent the change at the reservoir level and will impact on the quality of the time lapse signal. Currently the best strategy to tackle near surface heterogeneity and other repeatable noises (e.g. multiples) is to repeat source and receiver location as accurately as possible during acquisition. Whilst it is desirable to acquire well repeated surveys, it is not always possible due to limits imposed by operational safety, weather, timing and other factors. Therefore, it is important to create a reliable near surface model and compensate residual differences through robust statistical methods.