Repeatability Of 3D Ocean Bottom Cable Seismic Surveys
Authors: Beasley Craig J. , Chambers Ron E. , Workman Ricky L. , Craft Kenneth L. , Meister Laurent J. 
Published: October 2008

Abstract
The repeatability of seismic data acquisition and processing has become an important question in determining the role of conventional land, marine, and transition zone systems in seismic monitoring of reservoirs. In this paper we examine a case study involving two repeated 3-D seismic surveysacquired using an ocean bottom cable (OBC) system. Although not originally acquired as part of a monitoring project, the survey design and preplanned source and receiver patterns were identical over a significant area. Even thoughthere were slight differences in the source and receiver positions, the final fold, offset and azimuth distributions were very similar. We also found, not surprisingly, that since the two surveys had some differences in acquisition and processing, "off-the-shelf' final migrated data volumes showed significant differences, which could only be slightly reduced by the use of matching filters. However, by careful reprocessing of the prestack data with a sequence designed to maximize repeatability of the signal, the differences were reduced dramatically. The areal distribution of the dual-sensorsummation scalars for the base and monitoring surveys were found to be similar. We also found that small phase differences can give large amplitude differences and that it isessential to account for known differences such as instrument responses and noise characteristics at the start of prestack reprocessing. The level of repeatability required for a givenmonitoring project depends on the specifics of the reservoir and the monitoring goals; however, in this case, repeatability achieved between the two data volumes, after careful prestack reprocessing, indicates that OBC data would be suitable foranalysis of reservoir fluid movement.

Introduction
There has been a resurgence of interest lately in using repeated seismic surveys as a reservoir description and production monitoring tool. Although studies of repeated seismic surveysfor mapping steam fronts go back more than 10 years, recent advances have extended time lapse or 4-D seismic to map gasoil and oil-water contacts with significant economic benefit1,2,3,4. Recent studies by Watts et al.5, have made fortunate use of legacy 3-D data not originally acquired for a 4-D study and found that the varying quality of the different vintages of 3-D data was a factor limiting the resolution of their studies. As a result, the question arises how best to carry out the data acquisition and processing for a 4-D seismicprogram so that the data are repeatable to the extent that differences are attributable to movement of the reservoir fluidsrather than acquisition and processing or other factors.
Preliminary results from a case study of repeatability of the ocean bottom cable (OBC) system based on two collocated 3-D surveys acquired and processed independently werepresented by Beasley et al.6 They showed that "off-the-self' final migrated data volumes have significant differences andthat careful reprocessing of the prestack data to account for known differences was necessary to improve differencing results. This paper will summarize previous results and discuss observations and conclusions derived whilereprocessing the data.