with over/under source acquisition Mark Egan*, Khadir George El-Kasseh and Nick Moldoveanu, Schlumberger WesternGeco Summary The resolution of marine seismic data is affected by ghost and reverberations occurring in the water layer. A benefit of acquiring ocean-bottom cable data or node data is that the receiver-side ghosts and reverberations can be attenuated by combining pressure data, recorded with hydrophones, with velocity data, recorded with vertical geophones. Resolution of seismic data is also affected by the source ghosts. Acquisition of seismic data with over/under sources allows attenuating the source ghost and in this way to completely deghosting the ocean-bottom recorded data. In this paper, we present the results of an experiment where a 2D oceanbottom cable line was recorded with over/under sources. Introduction Dual-sensor ocean-bottom acquisition has been in use for nearly 20 years and one of the benefits of this method is attenuation of the ghost and water layer reverberations occurring at the receiver side. However, after pressure and vertical velocity sensors are properly combined to attenuate the receiver-side ghosts and reverberations (Barr and Sanders, 1989), the seismic resolution is still affected by the sourceside ghosts and reverberations. One acquisition method that was proposed to address this problem was the vertical source arrays (Moldoveanu, 2000). This method allows separating the upgoing wavefield from the downgoing wavefield and in this way remove the ghost and reverberations that are part of the downgoing wavefield. In Figure 1, we show a seismic event that is followed by a source ghost and a receiver ghost, and the corresponding amplitude spectra. It can be seen that by removing the source and receiver ghosts we can improve not only the high-frequency end but also the low frequencies. justified. Our experiment was motivated by the increased interest in the deep oil and gas reservoirs located on the Shelf. The main objective of the experiment was to increase the low-frequency content of the data and, in this way, the signal-to-noise ratio for the deep seismic events. Because the theory shows that full deghosting of the seismic wavefield can improve the low frequencies, we decided to acquire data with over/under sources. The next section briefly describes the experiment. Over/under source acquisition experiment A Syntron cable was deployed and two 2D lines were acquired with different sources. The acquisition parameters are presented in Table 1. The water depth in the area was between 60 m, and 90 m. Single source Over/under sources Cable length 20000 m 20000 m Receiver interval 50 m 50 m Source volume 6780 in³ 5085 in³ Source depth 11 m 6 m and 11 m Shotpoint interval 37.5 m 37.5 m flip-flop Table 1. Acquisition parameters for the 2D lines We acquired the 2D line with a larger single source array deployed at 11 m because we wanted to enhance the signal-to-noise ratio for the deeper events. The over/under sources were implemented with two source arrays placed in the same vertical plane and at two different depths (Figure 2). The depth of the first array was 6 m and the depth of the second array was 11 m. The inline separation of the two arrays was 37.5 m which was equal with the shotpoint interval. The sources were fired in flip-flop mode, such that two shots were generated at the same x, y location, but, at different depths. This manufactured the desired over/under source geometry. Figure 2. Implementation of over/under sources Figure1. Seismic event with no ghost, with source ghost only and with source and receiver ghost; the corresponding amplitude spectra are in red, blue and green, respectively At the beginning of 2005, we performed an OBC experiment on the Gulf of Mexico Shelf, in the South Timbalier area, an area where there were many obstructions, and an OBC acquisition was fully Data processing sequence Three datasets were processed in order to evaluate the performance of source deghosting, source and receiver deghosting (full deghosting) and to compare the full deghosting with single 6780 in³ array acquisition (Figure 3). 31
Figure 4: OBC full deghosting process with over/under sources Figure 3. The deghosting process applied on each dataset The hydrophone and the geophone components were processed identically with the following processing sequence: Data editing Navigation and seismic data merge De-bubble designature Coherent noise attenuation Receiver gather domain sorting Upgoing wavefield separation (for over/under sources only) Hydrophone and geophone combination Prestack migration velocity analysis Prestack Kirchoff migration Poststack filtering Upgoing wavefield separation for over/under sources Methods to separate the upgoing wavefield from the downgoing wavefield were developed in the last twenty years for vertical seismic profiling (VSP) data and for over/under streamer data (Monk, 1990). The methods used to separate the upgoing from the downgoing wavefield for over/under streamers can also be used for over/under sources, if the wavefield separation algorithms are applied in the receivergather domain. The theoretical basis that allows this application is seismic reciprocity (Ikelle and Amundsen, 2005). Upgoing wavefield separation was performed on over/under sources using the Posthumus method (Posthumus, 1993). Hydrophone and geophone combination Since the first paper on hydrophone and geophone combination was published by Barr and Sanders, several methods have been developed for the attenuation of the ghosts and receiver-side reverberations. Most of these methods require estimating a scalar to multiply the geophone signal prior to the summation with the hydrophone signal. The method used in this study is a non-linear method that does not require estimating any scalar, being a trace-by-trace, and sample-by-sample, data-driven process (Moldoveanu, 1997). Figure 4 describes the full deghosting process that consists of source deghosting for hydrophone and geophone, followed by receiver deghosting. Data analysis and results To evaluate the performance of the full OBC deghosting process we examined firstly the source deghosting step and then the receiver deghosting step. The over source data were processed separately and used as a reference for source deghosting. The following processing results were generated for this analysis: from receiver gathers for the over source from receiver gathers for the under source from receiver gathers after source deghosting after receiver deghosting Prestack migrated data for over/under sources Prestack migrated data for the over source Prestack migrated data for the conventional OBC acquisition with a single 6780 in³ source array The abstract included only some of the results we generated and these will be discussed next. Figure 5 shows the spectra calculated from a hydrophone receiver gather: spectrum of the over source (red), spectrum of the under source (blue) and spectrum after the source degosting was performed (green). The source ghost notch for the over source is at 125 Hz and for the under source at 68 Hz. After the source deghosting was performed, the spectrum has no source notches and the low-frequency content was enhanced. Apparently the higher frequencies were attenuated after wavefield separation but this was only due to the fact that high-frequency noise was present in the data, and this noise was attenuated by the over/under source deghosting process. Figure 6 shows the spectra calculated from a geophone receiver gather. Similar observations can be made for the geophone spectra as were made for the hydrophone spectra. The comparison of the spectra derived from the geophone and hydrophone after over/under source deghosting vs. spectra derived after hydrophone and geophone combination ( receiver deghosting ) is shown in Figure 7. It can be seen that the source deghosting followed by receiver deghosting ( full 32
deghosting process) enhanced the very low frequencies, as well as the higher frequencies, and removed the receiver and source notches. section show a better resolution, improved fault mapping, and an improved signal-to-noise ratio. The full deghosted data for the deep section (Figure 10 and 11) show better low frequency content and an improved signal-to-noise ratio. 1 s Figure 5. Amplitude spectra derived from a hydrophone receiver gather: over source (red), under source (blue) and over-under sources (green) 2 s Figure 8. Prestack migration after receiver deghosting (shallow section) 1 s Figure 6. Amplitude spectra derived from a geophone receiver gather: over source (red), under source (blue) and over-under sources (green) 2 s Figure 7. Amplitude spectra of the geophone after source deghosting (red), amplitude spectra of the hydrophone after source degosting (blue), and hydrophone and geophone combined or full deghosted OBC data (green) The prestack Kirchoff migration was performed for receiver deghosted data ( over source) and for source and receiver deghosted data, and the results are presented in Figure 8 and Figure 9 for a shallow section and Figure 10 and Figure 11 for a deeper section. The full deghosted data for the shallow Figure 9. Prestack migration after source and receiver deghosting (shallow section) Discussions and conclusions The results of this test confirmed that source deghosting can be implemented with over/under source acquisition and in this way full deghosting of OBC data can be performed. OBC full deghosting could be beneficial to improve the signal-to-noise ratio for the deep seismic events. With the current flip-flop implementation of the over/under sources the source sampling is decreased. This issue could be addressed if simultaneous source shooting is considered. Acknowledgements We thank WesternGeco for the permission to present this paper. 33
Main Menu Figure 10. Prestack migration after receiver deghosting (deep section) Figure 11. Prestack migration after source and receiver deghosting (deep section) 34
EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2007 SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web. REFERENCES Bar, F. J., and J. I. Sanders, 1989, Dual-sensor summation of noisy ocean-bottom data: 66th Annual International Meeting, SEG, Expanded Abstracts, 653 656 Ikelle, L., and L. Amundsen, 2005, Introduction to petroleum seismology: Investigations in Geophysics, 12. Moldoveanu, N., 1997, Method for attenuation of reverberations using a pressure-velocity bottom cable: U.S. patent, 5,621,700., 2000, Vertical source array in marine seismic exploration: 70th Annual International Meeting, SEG, Expanded Abstracts, 53 56. Monk, D., 1990, Wavefield separation of twin streamer data: First Break, 8, 96 104. Posthumus, B., 1993, Deghosting of twin streamer configuration: Geophysical Prospecting, 41, 267 286. 35