Summary. Introduction

Transcription

1 Multivessel coil shooting acquisition with simultaneous sources Nick Moldoveanu¹, Ying Ji², Craig Beasley¹ ¹WesternGeco, ²Schlumberger Cambridge Research Summary Multivessel coil shooting is a towed-streamer acquisition method that provides full-azimuth, long-offsets seismic data, and has been used in the Gulf of Mexico for subsalt exploration since A typical configuration consists of two streamer vessels and two source vessels, each vessel deploying a single source array. In this paper we propose a new design for multivessel coil shooting acquisition that is based on eight source arrays and simultaneous shooting. To prove that this design is geophysically feasible, we performed seismic simulation with 3D finite-difference acoustic modeling and depth imaging using reverse time migration (RTM). Based on the results of this simulation, a field experiment was proposed. A new method for active source separation was introduced and tested on synthetic and real data. Introduction The typical acquisition configuration for multivessel coil shooting consists of two streamer vessels and two source vessels, with a single source array firing sequentially deployed on each vessel. Because there are two recording vessels and four sources, we call this configuration 2 x 4. Each vessel sails along a separate circle and the source interval along each circle is 150 m. Figure 1 shows the shot distribution for a single coil. This pattern of four circles overlaps in x- and y- directions with dx, and dy distances, respectively, covering the entire survey area. Circle radius and distances dx and dy, are design parameters that are defined based on survey objectives. Coil surveys are designed to have a random spatial distribution of sources and receivers (Moldoveanu, 2010). (Moore et al., 2012). For land surveys, different simultaneous source methods have been used since 1983 to improve efficiency of vibroseis acquisition: simultaneous shooting with phase rotation (Garotta, 1983), slip sweep (Rozemond, 1996), high-fidelity vibroseis (Allen et. al, 1998), independent simultaneous sweeping (Howe et. al, 2008). Application of simultaneous shooting for land surveys increased in the last few years, and this has had a significant effect on vibroseis acquisition efficiency and source sampling. Processing of simultaneous source data can be passive or active. Passive separation does not require us to separate the sources and relies on migration to properly position, in space and time, seismic energy corresponding to each individual source. Passive separation can be used when seismic sources are separated by a large distance. One challenge of this approach is the residual noise left in the data due to source interferences. Research is very active in this area and various methods have been proposed to mitigate this problem (Schuster et. al, 2010). Active separation requires separating the sources. Different methods were developed lately for active source separation, the most effective being based on modeling and inversion (More, 2010). One important assumption of these methods is shot dithering, i.e., the seismic sources fire at random times. In the next sections, we will describe the new design and discuss some processing aspects. New design for multivessel coil shooting The new design for multivessel coil shooting is based on eight source arrays, four of them firing simultaneously. This can be implemented without increasing the number of vessels by deploying a dual-source array on each vessel. We call this configuration 2 x 8. The separation between source arrays on the streamer vessel is ½ of the streamer interval, and 4 to 6 times larger for the source arrays on the source vessels (Figure 2). S1, S3, S5, and S7, and S2, S4, S6, and S8 will fire at random times around the fixed firing time of source S1 and source S2, respectively. Figure 1: Source distribution from four source arrays along four circles. The Simultaneous source shooting concept for marine acquisition was introduced in 1997 (Beasley et al., 1997). However, the first commercial 3D towed-streamer survey with simultaneous sources was acquired only in 2011 Figure 2: Multivessel coil shooting with eight source arrays. SEG Las Vegas 2012 Annual Meeting Page 1

2 One important advantage of this design for simultaneous source implementation is the large separation between the sources that fire simultaneously. The minimum distance between two sources firing at approximately the same time is 9 km. We setup a seismic simulation experiment for multivessel coil shooting with the proposed new design. The SEG advanced modeling (SEAM) model was used to generate 3D acoustic isotropic finite-difference data with a frequency bandwidth from 1 Hz to 20 Hz. The data was generated with surface multiples. The acquisition parameters are listed in Table 1. No. of streamer vessels 2 No. of source vessels 2 No. of source arrays for the current design 4 No. of source lines for the current design 4 No. of source arrays for the new design 8 No. of source lines for the current design 8 No. of streamers 10 Streamer length 8000 m Streamer separation 120 m Group interval 25 m Shot interval current design 150 m Shot interval new design 75 m Average coverage fold for 25 m x 25 m 872 for the current design Average coverage fold for 25 m x 25 m for 3568 the new design Table 1: Acquisition parameters used in simulation In Figure 5, we show the unique azimuth coverage for the new design. The maximum expected fold is 36, corresponding to 36 azimuths sectors of 10⁰ each. Three data sets were generated: (1) Current design with four source arrays shooting sequentially (2) New design with four sources shooting simultaneously, without source dithering (3) New design with four sources shooting simultaneously, with source dithering Our test objective was to compare the three data sets. By comparing migration results for data sets 1 and 2, and 1 and 3, we can determine the effect of simultaneous shooting on imaging. From comparison of data sets 2 and 3 we can estimate the effect of shot dithering on imaging. Figure 4: Coverage fold for the new 2x8 design From a geophysical point of view, the main benefit of the new design is improved source sampling: the number of shots is increased four times and the shot points are distributed along eight source lines. Examples of coverage fold for the current design and the new design are shown in Figure 3 and Figure 4 respectively. As the group interval used in modeling was 25 m instead of 12.5 m (the standard group interval used in marine acquisition), the color bar for the fold was adjusted (see white numbers). Figure 5: Unique azimuth coverage for the new 2x8 design An example of a shot gather from the 3 rd data set is shown in Figure 6. This illustrates the complexity of the seismic wavefield that is generated from four sources shooting at approximately the same time. Figure 3: Coverage fold for current 2 x 4 design In Figure 7 we present the RTM image of the current design and in Figure 8 the RTM image of the new design. The images were generated from simulation of five-coil SEG Las Vegas 2012 Annual Meeting Page 2

3 acquisition. The improved signal-to-noise ratio is noticeable for the new design and this is due to better source sampling. The very low level of crosstalk noise can also be noticed. This is explained by the large distance between sources that fire at the same time, and the random distribution of source locations in coil shooting acquisition. demonstrate the benefit of source dithering for reducing the crosstalk noise for passive source separation. In the next section we will introduce a new method for active source separation that requires source dithering. Figure 6: Example of shot gather generated from four sources shooting with dithered times. Figure 8: RTM image obtained from 5 coils acquired with new 2x8 coil acquisition design Processing aspects Processing of data acquired with simultaneous sources can be done with passive or active separation of sources. In each case, the first step in the processing sequence is the attenuation of marine noise. To verify if the onboard noise attenuation processing sequence applied on 2 x 4 coil data can be used for processing of 2 x 8 coil data, we simulated combined shots from the existent 2 x 4 coil data acquired in the Gulf of Mexico, and applied the current processing flow (Moldoveanu, 2011). A simulated simultaneous shot is presented in Figure 9. The data were acquired with single hydrophones and do not have any filter applied. Figure 10 shows the data after noise attenuation was applied. Figure 7: RTM image obtained from five coils acquired with current 2 x 4 coil acquisition design Source dithering is important not only for active source separation, but also for passive separation, because it could reduce the amount of undesired crosstalk noise. We selected the dither times to be a uniform random variable within a certain range, +V milliseconds to V milliseconds, and to allow proper preservation of the very low frequencies (Jiang and Abma, 2010). The comparison of data sets 2 and 3 is not shown in the abstract, but the results SEG Las Vegas 2012 Annual Meeting Page 3

4 Figure 9: Simulated shot record from four simulatenous shooting sources; the data were acquired with single sensor and are unfiltered. Discussions and conclusions The main geophysical benefit of the proposed 2x8 design for multivessel coil shooting with simultaneous sources could be the improvement in signal-to-noise ratio due to better source sampling and increased source density. This could be particularly important for areas where the reservoir is covered by a complex overburden and a small amount of energy could reach the reservoir. This new design may allow also us to improve acquisition efficiency by increasing the dx and dy parameters related to coil move-up. Figure 10:Simulated simultaneous shot record after onboard noise attenuation was applied. The result of noise attenuation confirms that standard onboard processing developed for coil data can be applied. We also performed full RTM imaging of the simulated data and compared the results with RTM imaging of the original data. The comparison shows a small amount of artifacts due to the crosstalk noise. As the aim of simultaneous source acquisition is to improve seismic data quality, we consider that an active separation is required. A new algorithm was developed using frequency-diverse filtering (Ji et. al, 2012). The algorithm combines the array response at different frequencies to supress spatial aliasing and convert the data separation problem into an l 1 or l 0 optimization problem. The algorithm works in the common-offset or common-receiver domain. To determine how this algorithm works when a strong seismic event interferes with a weeker sesimic event, we generated a simple synthetic with two aliased events generated by two sources: S1 (Figure 11a) firing regularly, and S2 (Figure 11b) firing with dithering. The interval between traces was 75 m and the maximum frequency was 45 Hz, so the events are aliased. The superposition of the events generated by S1 and S2 is shown in Figure 11c, and the events after source separation are shown in Figures 11d and 11e. The separation error was estimated and it was smaller than 40 db (1%), proving the accuracy of the new algorithm. Processing of the synthetic dataset no. 3 using this new method is ongoing. Amplitudes will be controled, before and after source separation, to be sure that the separation algorithm preserves the amplitudes. After source separation was applied, the processing flow is the same as for sequential shooting data: multiple attenuation, velocity model building and imaging. Figure 11: Seismic events generated by simultaneous shooting of sources S1 and S2; before separation (Figure 11c) and after source separation (Figures 11d and 11e). Figures 11a and 11b show the original events. We demonstrated with synthetic modeling that this new design for multivesel coil shooting is feasible, and based on this, a field experiment was planned. As the volume of data acquired with 2 x 8 coil acquisition will be 10 times larger than the volume of data acquired with 2 x 4 wide-azimuth acquisition, processing with passive separation of sources could be very atractive. The developments in compressive sampling theory for application to seismic acquisition and processing, as well as progress made in imaging algorithms, particularly imaging with multiples and least-square migration, will allow us to process simultaneous source data without active source separation. Acknowledgements We aknowledge WesternGeco and Schlumberger Cambridge Research for permission to present the paper. SEG Las Vegas 2012 Annual Meeting Page 4

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6 EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2012 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 Allen, K. P., M. L. Johnson, and J. S. May, 1998, High-fidelity vibratory seismic (HFVS) for acquiring seismic data: 68th Annual International Meeting, SEG, Expanded Abstracts, Beasley, C., R. Chambers, and Z. Jiang, 1998, A new look at simultaneous sources: 68th Annual International Meeting, SEG, Expanded Abstracts, Howe, D., M. Foster, T. Allen, B. Taylor, and I. Kack, 2008, Independent simultaneous sweeping A method to increase the productivity of land seismic Crewes: 78th Annual International Meeting, SEG, Expanded Abstracts, Ji, Y., E. Kragh, and P. Cristie, 2012, A new simultaneous source separation algorithm using frequencydiverse filtering: Presented at the 82 nd Annual International Meeting, SEG. Jiang, Z., and A. Ray, 2010, An analysis of simultaneous imaging of simultaneous source data: 80th Annual International Meeting, SEG, Expanded Abstracts, Moldoveanu, N., 2010, Random sampling: A new strategy for marine acquisition: 80th Annual International Meeting, SEG, Expanded Abstracts, Moldoveanu, N., 2011, Attenuation of high-energy marine towed-streamer: 81st Annual International Meeting, SEG, Expanded Abstracts, Moore, I., 2010, Simultaneous sources: processing and applications: 72nd Annual International Conference and Exhibition, EAGE, Extended Abstracts, B001. Moore, I., D. Monk, L. Hansen, and C. Beasley, 2012, Simultaneous sources: The inaugural full-field, marine seismic case history from Australia: Presented at the 22 nd Meeting of the ASEG. Schuster, G. T, W. Dai, G. Zhan, and C. Boonyasiriwat, 2010, Theory of multisource crosstalk reduction by phase-encoded statics: 80th Annual International Meeting, SEG, Expanded Abstracts, SEG Las Vegas 2012 Annual Meeting Page 6