HR: 0800h
AN: S31A-0267    [Abstracts]
TI: Passive Seismic Reflectivity Imaging with Ocean-Bottom Cable Data
AU: * Hohl, D
EM: Detlef.Hohl@shell.com
AF: Shell International E&P, Inc., Research & Development, 3737 Bellaire Blvd., Houston, TX 77025 United States
AU: Mateeva, A
EM: Albena.Mateeva@shell.com
AF: Shell International E&P, Inc., Research & Development, 3737 Bellaire Blvd., Houston, TX 77025 United States
AB: The idea of imaging the subsurface reflectivity distribution by correlating long traces of seismic ``noise'' (i.e. seismic data recorded without active sources) goes back more than 30 years [1]. To this day, passive seismic reflectivity imaging has not been exploited for business use in the E&P industry. The conditions for successful passive seismic reflection imaging have greatly improved over the past few years, and the prize of cheap continuous sourceless seismic imaging and possibly monitoring is still large. Nearly unlimited quantities of very high quality passive noise data are now available from permanent 4C ocean bottom cable (OBC) installations. In the present contribution, we report our initial results for single-line (2D) OBC data collected in the North Sea and GOM. The OBCs used for the experiment are of length 6-10 km with 4C receivers spaced 50 m apart. They are deployed in both shallow and deep water over large hydrocarbon reservoirs. Passive noise data were recorded for 8-24 h periods, sometimes several times, and months apart. In the analysis presented here only the hydrophone records are used, and the data from all recording periods are used together to produce a single 2D migrated reflectivity section. We observe that environmental noise (e.g. boat and rig activity) play an important role for imaging and usually requires pre-migration seismic processing steps to filter out unwanted signals. At the core of our image generation and processing sequence is the crosscorrelation of noise trace pairs and subsequent prestack time migration [1] with a velocity model established for the active-source OBC data processing. We compute 4 sec of lag time to either side of t=0. After removing unwanted signals (e.g. seafloor interface waves) from these ``virtual shot gathers'' one can clearly detect the linear-moveout direct water wave with velocity 1500 m/s, and a linear interface wave with velocity 2000 m/s. Other ``events'' with moveout are visible, but the gather is contaminated with water-column multiples. These multiples must be attenuated before migration by standard techniques. In the next processing step all (``virtual'') shot-receiver traces are migrated to obtain a subsurface image. We can clearly identify several continuous shallow events on the passive images that correspond to events known from the active source survey images. Our results, based only on a very small fraction of the available data, are thus very encouraging. No coherent events are visible below a time of about 1.5 sec, presumably still due to data paucity. We also observe that the image quality nearest very strong seafloor noise sources (rigs etc.) is very poor due to the large shear motion induced by such activity. Even in the final migrated image using input data that had water column multiples attenuated, multiples are still degrading the image quality, and we cannot distinguish primaries from multiples. It remains to be seen whether standard OBC data processing techniques like PZ summation can be used on multi-component passive data with the same beneficial effect as on active source data. More sophisticated preprocessing in conjunction with larger data volumes (2 or more OBC lines to approximate 3D coverage, and longer recording periods) and ``mining'' for particularly beneficial data are currently being pursued for image quality improvement. [1] For a review see: G. Schuster, J. Yu, J. Sheng and J. Rickett, Geophys. J. Int. 157 (2004), 838, and references therein.
DE: 7290 Computational seismology
SC: Seismology [S]
MN: Fall Meeting 2005