Innovations for geophysical monitoring of 3D and 4D marine surveys

T ime-lapse seismic surveys are carried out to reveal production changes in the subsurface reservoir. Ensuring optimal repeatability between the different vintage surveys provides a direct way to minimize 4D noise unrelated to reservoir changes. Positioning repeatability is one of the main issues in 4D towed-streamer acquisition. It is primarily optimized by steering the vessel, source and streamers to match the previous acquisition (or baselines). The remaining mismatches in positions must be assessed to ensure the high quality of the data being acquired. We address the 4D repeatability of positioning through a geophysical target-oriented approach, where repeatability is assessed from the impact on seismic illumination. New and complementary repeatability indicators are derived for onboard quality control at gradual discrimination scales, ranging from shots through navigation lines up to the full acquisition. Stability of the airgun source is important for both 3D and 4D surveys. Seismic data must be decoupled from the source signal variability in order to correctly resolve the quantitative features related to subsurface properties. During seismic acquisition, it is therefore important to estimate the far-field source signature and monitor its stability to detect out-of-specification shots. The reconstructed far-field source signature can then be used for further shot-to-shot deconvolution of the seismic data.