A 3D Ocean Bottom Cable Repeatability Study

Abstract

IntroduetionThere has been a resurgence of interest lately in using repeated seismie surveys as a reservoir description and production monitoring tooI. Although studies of repeated seismie surveys for mapping steam fronts go back more than 10 years, recent advances have extended time lapse or 4-D seismie to map gas-oil and oil-water contacts with significant economie bene fit (Greaves and Fulp, 1987;Pullin et al., 1987;Dunlop et al., 1991;Johnstad et al., 1995). Recent studies, e.g., Watts, 1995, 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 aresuit, the question arises how best to carry out the data acquisition and processing for a 4-D seismie program so that the data are repeatable to the extent that differences are attributable to movement of the reservoir fluids rather than acquisition and processing.In this paper we present a case study ofthe repeatability ofthe ocean bottom cable (aBC) system based on two colocated 3-D surveys acquired and processed independently.First we compare off-the-shelf 3-D migrated data volumes. Then, beginning with migration, we back up in the processing sequence until we arrive at analogous flows for the two surveys. We found that in this case it was essential to reprocess the data from the prestack stage to account for known differences to achieve the best results. AcquisitionTwo aBC surveys were carried out with a separation of four months over an area that contained no known hydrocarbon production or other subsurface mechanism for changing the seismie response. The survey plan was identical for the two surveys, but for a variety of reasons, such as varying currents and wind, the receiver positions were not identical between the two surveys. Nevertheless, the flexibility of the aBC method allowed changes in receiver positions to be offset by corresponding changes in shot locations yielding fmal fold, offset, and azimuth distributions that were very similar. The same recording instruments were used for the two surveys with one significant difference: while one survey employed a 3 Hz instrument low cut filter, the other used an 8 Hz filter. As we shall see, dealing with such seemingly minor differences in the initial stages of processing is key to insuring repeatability. Off-the-shelf 4-DIn some areas with extensive 3-D coverage, existing 3-D surveys overlap and thus the appealing possibility arises to do 4-D seismie simply by studying the existing different surveys. To test the notion of such raw repeatability, we began our investigation with the existing data which were processed with similar, but independent sequences. Adding to the acquisition differences noted above, veloeities were interpreted independently, reflection staties, though smalI, had been derived from and applied to one data volume but not the other, and, although identical migration algorithms were used, the migration velocity fields and aerial e...

Surface-consistent matching filters for time-lapse processing

“…Because of the success of this study and the potential value seen in applying those lab measurements, seismic data was used to monitor steam injection used to enhance heavy oil recovery (Greaves and Fulp, 1987). By late 1990's, seismic monitoring technology had been reported in many published studies including Johnstad et al (1993);Eastwood et al (1994); Lumley et al (1997);Beasley et al (1997); Ross and Altan (1997). The first SEG Distinguished Instructor Short…”

“…(Beasley et al, 1997;Ross and Altan, 1997;Rennie et al, 1997; Porter-Hirsche and Hirsche, 1998). Other differences can arise from inconsistent processing of time-lapse data due to different processing software or personnel, advances in processing technologies, etc.…”

Repeatability Of 3D Ocean Bottom Cable Seismic Surveys

“…Preliminary results from a case study of repeatability of the ocean bottom cable (aBC) system based on two collocated 3-D surveys acquired and processed independently were presented by Beasley et al 6 • 7 • They showed that "off-the-self' final migrated data volumes have significant differences and that 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 while reprocessing the data.…”