Summary. Introduction

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1 The contribution of wide-azimuth point-receiver acquisition to the success of full-wave inversion HongYan Li, Denes Vigh, and Jerry Kapoor, WesternGeco Summary Low frequencies are critical for the success of full-wave inversion (FWI). With the point-receiver marine and land acquisition system, low frequencies are significantly improved compared with conventional surveys. For marine data, the lowest frequency band that can be used for FWI is 2-3 Hz. For land data, the lowest frequency band that we can use for FWI is 2-4 Hz. Wide-azimuth (WAZ) acquisition facilitates FWI by highly improved data coverage with a higher signal-to-noise (S/N) ratio, better illumination, and longer offset. FWI was successfully carried out with both marine and land data sets from lowto high-frequency bands. The final FWI velocity models were validated by both Kirchhoff depth migration and reverse-time migration (RTM). Introduction Exploration in complex geological areas requires more accurate imaging and velocity model building techniques. With the cost-effective compute capacity and fast algorithms, RTM has been put into production to bring better imaging, and full wave inversion (FWI) has been used to produce more accurate velocity models (Vigh et al., 2010); thereby, improving imaging. FWI uses a two-way wave equation to produce high-resolution velocity models by minimizing the difference between recorded and predicted seismic data; it relies more on the recorded data rather than interpretation. FWI requires very low frequencies or decent initial velocity models for the convergence because it has the halfwavelength limitation, i.e., the predicted data should be in the half wavelength of the observed data. Traditional tomography generally creates smooth velocity model with spacial resolution ~300 to 1000 m, which requires the lowest frequency to start FWI: 3 Hz or even lower. The lowest frequency of traditional acquisition is 5 or 6 Hz, or even higher. Therefore, the new velocity modeling technique (FWI) requires a new acquisition system to capture more low frequencies. Point-receiver recording (Ongkiehong, 1988; Blacquiere and Ongkiehong, 2000) and the ability to group form to optimum trace sampling delivers new standards in seismic acquisition. The use of good cable and gun geometry design, deeper-tow source and receivers, special low-cut field filters (1.75Hz), and finely spaced point receivers, enabled low-frequency signal / noise separation techniques. Marine surveys (Martin et al., 2000) are able to achieve an average 40% improvement in bandwidth, and the lowest frequency is below 2 Hz (Figure 1). For land acquisition (Baeten et al., 2000) with broad-bandwidth, low-distortion geophone accelerometer (GAC) sensors, the low frequencies can be acquired at 2 Hz. Compared with traditional land surveys, densely spaced point-receiver acquisition facilitates the suppression of high-amplitude noise that often dominates the low frequencies while keeping the low-frequency signals. Moreover, air waves, ground roll, and other coherent noises are easier to remove with closely spaced point-receiver data. After noise attenuation, the point-receiver data are grouped to the optimum spacing according to the target frequency band. WAZ acquisition (Kapoor et al., 2005; Moldoveanu and Egan, 2006) brings wider azimuth and much higher fold data coverage, thereby improving resolution, S/N ratio, and illumination. WAZ data also have longer offsets than narrow-azimuth (NAZ), which contributes to the convergence of FWI. Data description This paper compares point-receiver, wide-azimuth marine data with traditional narrow-azimuth marine data, and point-receiver land data with traditional land survey data to demonstrate the improvement of low frequencies from densely spaced point-receiver acquisition. To show how the lower frequencies impact FWI, we started marine data sediment FWI with the same WAZ pointreceiver data set from the Gulf of Mexico (GoM), but at different starting frequencies. One flow started from a frequency band at 2-3 Hz, and then 2-4, 2-6, and 2-8 Hz; the other flow was only with frequency band at 2-10 Hz. Kirchhoff depth migration was done with the two final FWI models for comparison. For land data, after comparing the spectrum of pointreceiver data with traditional land data, we were able to determine the lowest frequency that can be used for FWI with the two different data sets. A low-cut filter was applied to the point-receiver land data to simulate traditional land data. Refraction FWI (Jaiswal et al., 2009) was carried out from the possible lowest frequency band with point-receiver land data and the simulated conventional land data. Kirchhoff depth migration was carried out with the initial model and the final FWI model for comparison. Results SEG San Antonio 2011 Annual Meeting 2555

2 With WAZ point-receiver marine data, different lowfrequency filters were applied to a same shot to determine the lowest frequency that can start FWI. Figures 1-(g) show the original shot and the filtered data with frequency bands of 2-10 Hz, 2-8 Hz, 2-6 Hz, 2-5 Hz, 2-4 Hz, and 2-3 Hz, respectively. Acoustic tilted transversely isotropic (TTI) sediment FWI was carried out from 2-3 Hz seismic data with the tomography TTI velocity model. Figure 2 shows a depth slice of 2-3 Hz gradient at a depth ~10000 ft. Different sediment basins and salt flanks can be identified quite clearly on the gradient. After 2-3 Hz, TTI sediment FWI was carried out with frequency bands of 2-4 Hz, 2-6 Hz, and 2-8 Hz. TTI Kirchhoff depth migration was run with the FWI initial model and the FWI final velocity model for comparison (Figure 3). The Kirchhoff gathers became very flat with the FWI model. In the same area, we did some FWI testing with a frequency band at only 2-10 Hz with the same initial velocity model. Kirchhoff migration was run with the initial and final FWI models. The gather flatness does not really change with the FWI velocity model, although Kirchhoff gathers get better focusing than the tomography model in some areas. Figure 4 shows the FWI initial model (4a), FWI velocity model with only 2-10 Hz frequency band (4b), and FWI model with four frequency bands (Figure 4c). With multi-scale FWI (Bunks et al., 1995) from lower to higher frequencies, FWI is able to update velocity from large to small scale to flatten gathers, increase event focusing, and reduce the wavelet print on the velocity model. Conventional NAZ marine seismic data from the same area will be used to compare with WAZ point-receiver data to address the advantage of WAZ point-receiver acquisition for FWI. Point-receiver land acquisition data and traditional land acquisition data were both analyzed to determine the lowest frequency band that could be used for FWI. The original trace spacing of point-receiver land data is m, and they are grouped to 12.5 m or 25 m after some noise attenuation and ground-roll removal. By comparing shot records from point-receiver land acquisition with shots from a conventional land survey and after some data processing, the point-receiver data have more coherent signals with a higher S/N ratio. From the corresponding spectra of the two data sets, the lowest frequency in the point-receiver land data is below 3 Hz, while the lowest frequency in the conventional land data is 7-8 Hz. A 7-Hz low-cut filter was applied to the point-receiver land data to simulate conventional land data. Refraction FWI with the point-receiver data set started from a 2-5 Hz frequency band, and then proceeded to 2-7, 2-9, and 2-11 Hz. Refraction FWI was done with the simulated conventional data set at 7-11 Hz. The initial FWI model is a heavily smoothed refraction tomography model, more like a 1D model. Figure 5 compares the first iteration gradient at 2-5 Hz from point-receiver data with the first iteration gradient at 7-11 Hz from the simulated conventional survey. The polarity is basically opposite with the two gradient files. By comparing the observed vs. predicted data and checking the Kirchhoff gather flatness of the initial velocity model, the gradient from the lower-frequency band will update the velocity in the right direction, while the gradient with the higher-frequency band will not make FWI converge. With refraction FWI, we were able to update up to 1 km in depth with ~4 km maximum offset data. A rapid velocity change at the shallow static zone was able to be inverted. Kirchhoff and RTM migration were implemented with the initial velocity model and the FWI final velocity model to evaluate the velocity update from FWI. The Kirchhoff gathers become much flatter with the FWI velocity model. Both Kirchhoff and RTM images become more coherent and the events are much flatter. Conclusions Point-receiver technology delivers more low frequencies to both marine and land data sets with a higher S/N ratio, which is critical for the success of FWI. Wide-azimuth or full-azimuth acquisition bring better illumination, longer offset, and higher-fold data coverage, which are also very important for FWI. Successful FWI not only depends on compute capacity, fast algorithms, and expertise, but highquality low-frequency data. Acknowledgement We thank WesternGeco acquisition, processing, and research and development teams for developing the techniques that were used in the paper. We also thank WesternGeco management for permission to publish this paper. SEG San Antonio 2011 Annual Meeting 2556

3 (c) (d) (e) (f) (g) Figure 1: One shot from WAZ point-receiver marine data with different frequency bands. original shot; 2-12 Hz; (c) 2-10 Hz; (d) 2-8 Hz; (e) 2-6 Hz; (f) 2-4 Hz, (g) 2-3 Hz. Figure 2: The 2-3 Hz FWI gradient at depth ~10000 ft from WAZ point-receiver marine data in Gulf of Mexico. SEG San Antonio 2011 Annual Meeting 2557

4 (c) Figure 3. TTI tomography velocity model; TTI FWI velocity model after five iterations with only 2-10 Hz frequency band; (c) FWI velocity model after eight iterations with 2-3 Hz, 2-4 Hz, 2-6 Hz, and 2-8 Hz frequency band. The yellow ellipse corresponds to a shale body area that shows clearly on (c). Figure 4: Kirchhoff depth-migrated gathers with TTI FWI initial model and final velocity model after eight iterations. Figure 5: FWI gradient with refraction tomography model at frequency band 7-11 Hz, and 2-5 Hz. SEG San Antonio 2011 Annual Meeting 2558

5 EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2011 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 Baeten, G. J. M., V. Belougne, L. Combee, E. Kragh, A. Laake, J. E. Martin, J. Orban, A. Özbek, and P. L. Vermeer, 2000, Acquisition and processing of point receiver measurements in land seismic: 70th Annual International Meeting, SEG, Expanded Abstracts, Blacquiere, G., and L. Ongkiehong, 2000, Benefits of single sensor recording: Presented at the 62nd Annual International Conference and Exhibition, EAGE. Bunks, C., M. Saleck, S. Zaleski, and G. Chavent, 1995, Multiscale seismic waveform inversion: Geophysics, 60, , doi: / Jaiswal, P., C. A. Zelt, R. Dasgupta, and K. K. Nath, 2009, Seismic imaging of the Naga thrust using multiscale waveform inversion: Geophysics, 74, no. 6, WCC129 WCC140, doi: / Kapoor, J., C. Stork, and M. Egan, 2005, Benefits of low frequencies for subsalt imaging: 75th Annual International Meeting, SEG, Expanded Abstracts, Martin, J., A. Özbek, L. Combee, N. Lunde, S. Bittleston, and E. Kragh, 2000, Acquisition of marine point receiver seismic data with a towed streamer: 70th Annual International Meeting, SEG, Expanded Abstracts, Moldoveneanu, N., and M. S. Egan, 2006, From narrow-azimuth to wide- and rich-azimuth acquisition in the Gulf of Mexico: First Break, 24, Ongkiehong, L., 1988, A changing philosophy in seismic data acquisition: First Break, 6, Vigh, D., B. Starr, J. Kapoor, and H. Li, 2010, 3D full-waveform inversion on a Gulf of Mexico WAZ data set: 80th Annual International Meeting, SEG, Expanded Abstracts, 29, SEG San Antonio 2011 Annual Meeting 2559