A case study: imaging OBS multiples of South China Sea

Transcription

1 Mar Geophys Res (2012) 33:89 95 DOI /s ORIGINAL RESEARCH PAPER A case study: imaging multiples of South China Sea Xiangchun Wang Changliang Xia Xuewei Liu Received: 14 October 2011 / Accepted: 2 February 2012 / Published online: 21 February 2012 Ó Springer Science+Business Media B.V Abstract The subseafloor structure offshore South China Sea was imaged using first-order water-layer multiples from ocean-bottom seismometer data and the results were compared to conventional imaging using primary reflections. The mirror-imaging method employs a primariesonly reverse time pre-stack depth migration algorithm to image the receiver ghosts. The additional travel path of the multiples through the water layer is accounted for by a simple manipulation of the velocity model and processing datum: the receivers lie not on the sea floor but on a sea surface twice as high as the true water column. Migration results show that the multiple-migrated image provides a much broader illumination of the subsurface than the conventional image using the primaries, especially for the very shallow reflections. The resulting image from mirror imaging has illumination comparable to the vertical incidence surface streamer (single-channel) reflection data. Keywords Pre-stack depth migration Mirror-imaging method Multiples Introduction Surface-towed streamers provide excellent seismic data for exploration, development, and production monitoring. X. Wang (&) X. Liu Key Laboratory of Geo-detection, China University of Geosciences (Beijing), Ministry of Education, Beijing , China wangxcqs@126.com C. Xia Overseas Business Department of Geophysical Research Institute, BGP Inc. of CNPC, Zhuozhou City , Hebei Province, China However, streamers have limitations and this motivates a quest for alternative technologies. When using streamers, obstacles such as production platforms require undershooting and the data lack near offsets and have anomalous azimuth distributions. Obstacles are not the only motivation for ocean-bottom Seismometer () technology. Even in the absence of obstacles, nodes offer a number of significant advantages over conventional surface towed streamers. Their quiet recording location on the sea floor, ability to record both P and S waves, wide azimuth shooting geometries and ability to be deployed under and around underwater obstructions are just a few of these advantages. One alternative to nodes is the use of ocean bottom cables (OBC), where sensors are embedded in cables rather than nodes. OBC operations have lower cost than in shallow water, but are more limited by depth and obstacles such as pipelines and seabed installations. Also, cables have an inherent in-line and cross-line asymmetry which may compromise vector fidelity. Now the main method to image the data is using the primary reflection events (Ray et al. 2005; Zillmer et al. 2005; Pedersen et al. 2010). However, the ray path geometry of having a single recording point on the sea floor means that conventional imaging of near surface reflections and the sea floor is seriously compromised. This fundamental problem, often exacerbated by having sparsely spaced, is no illumination for reflectors whose depth under the sea floor is less than the spacing. Here we exploit this feature of water-layer multiples by treating them as signal. Several authors have used waterlayer multiples in migration of data (Godfrey et al. 1998; Ronen et al. 2005; Pica et al. 2006; Grion et al. 2007; Muijs et al. 2007; Ranjan et al. 2009), either in combination with primaries or separately. We follow the method of

2 90 Mar Geophys Res (2012) 33:89 95 Ronen et al. (2005) and Ranjan et al. (2009) to image the primaries and multiples separately. This method is also discussed in Grion et al. (2007). We utilize downgoing, first-order, receiver-side water-layer multiples from to image the shallow structure beneath the sea floor. Sometimes these events are referred to as receiver ghosts. Our objective is to compare the results from primaries and multiples and to demonstrate how water-column multiples can be utilized to image the shallow subsurface structure. In the following sections, we describe the primaries illumination problem and the solution method followed by imaging the synthetic data using the reverse time pre-stack depth migration program we developed. Then we present imaging results from a real data set of South China Sea. The illumination problem and solution method present one large disadvantage over their streamer counterparts, the illumination problem. The sparse spacing of typical surveys provides poor illumination, especially when considering reflectors with a depth under the sea floor that is less than the spacing (Fig. 1). In addition, if any (the third from the left in Fig. 1) fail, the problem is greatly increased. The resulting P-wave images therefore typically have large gaps in the shallow subsurface with extremely poor ray coverage, ultimately producing unsatisfactory images with little or no fold between receivers. The illumination problem is especially significant when investigating shallow targets such as gas hydrates. The illumination problem is caused because only the conventional upgoing primary reflections are normally imaged (Fig. 2a). However, downgoing multiple reflections bounce from the same reflectors as the primary reflections after reflection from the sea surface. Therefore, when Fig. 1 Poor illumination of sparse. Note the gaps in shallow reflectors coverage. This problem is exacerbated if any components fail imaging multiples the sea surface acts as a mirror reflecting the image of the subsurface structure (Fig. 2b). The mirroring of the water column and moving of the depths to the mirrored sea floor above the shot locations is essentially carried out on the input velocity/depth model used in the migration (Fig. 2c). Modifying the geometry in this way allows an otherwise standard migration program to be used to image the downgoing reflections. The crucial principal that makes this method superior is that the downgoing reflections are better than the upgoing primaries because they offer a wider aperture of illumination for shallow subsurface reflectors (Fig. 3a, b). This effect is most prevalent for the sea floor, which could otherwise not be imaged at all (Fig. 3c). Migration There are several migration methods used in subsurface imaging. Depending on their underlying assumptions, these methods can usually be classified as either Kirchhoff (Schneider 1978; Sun et al. 2000) or wavefield extrapolation (Gazdag 1978; Stolt 1978; Bleistein 1987) methods. The Kirchhoff methods explicitly introduce a high-frequency approximation of the wave-equation. In areas with complex geology where multipathing occurs, Kirchhoff methods may not provide reliable subsurface images (Biondi 2006). In wavefield extrapolation methods, multipathing is handled in a natural way. Here we use the reverse time pre-stack depth migration to image the data. Compared with the other migration methods, reverse time migration is based on the exact wave equation but not its approximate formula and extrapolates wavefield along time axes instead of depth axes. So it has good precision and has no dip restriction or velocity variation restriction. The migration was performed using the Fortran program we developed. In short, we calculate the wavefield of the subsurface from the maximum time to the minimum time. Each grid point under sea floor was thought to be a second source according to the Huygens principle. So the travelling time that the wave travels directly from the to the grid point is the excitation time imaging condition for each grid point (Ding et al. 2008). When we carry out the processing, if the calculated equals to the excitation time of a grid point, then that grid point becomes an image point with its value equals to the value of the wavefield. In order to testify our imaging Fortran program we design a simple velocity model with two layers. The velocity of the first layer is 2,000 m/s and the thickness is 670 m; the velocity of the second layer is 2,200 m/s and the thickness is 200 m (Fig. 4a). There are 400 traces with 5-m trace interval and the shot lies in the middle of the model.

3 Mar Geophys Res (2012) 33: (a) (a) (b) (b) Mirrored (c) Mirrored (c) Mirrored Fig. 2 Raypaths of a upgoing primaries, and b downgoing multiples. The downgoing receiver ghost in b can be treated as an upgoing primary reflected downward from the sea surface. For migration, multiples can be treated as primaries assuming that the data is not recorded on the sea floor but above a layer with twice the thickness of the water column (c) Fig. 3 Illumination of upgoing primaries (a) is narrower than that of the downgoing multiples (b). In particular, the sea floor cannot be imaged with the upgoing primaries but it can be imaged with the downgoing multiples (c)

4 92 Mar Geophys Res (2012) 33:89 95 (a) b Fig. 4 a The velocity model with the first layer s velocity 2,000 m/s and thickness 670 m and the second layer s velocity 2,200 m/s and thickness 200 m. b The modeled synthetic data with the time interval 0.5-ms and 3,000 samples. c The image of the modeled synthetic data, we can see that the image is consistent with the velocity model Depth, km The modeled wavefield is shown as Fig. 4b with 0.5-ms time sample interval and 3,000 samples per trace. We use our program to image the wavefield and achieve the image section (Fig. 4c). We can see that the image is consistent with the velocity model. Case study (b) (c) Depth, km Now we present results of migration from a survey on the South China Sea. The area is known to contain gas hydrates, which are solid, ice-like crystalline substances formed from water molecules containing methane. Methane hydrates are stable under low temperature and high pressure and are detected on seismic data by identifying the characteristic bottom-simulating reflector (BSR), representing the base of the hydrate stability zone. To investigate the shallow sediment structure above the BSR, seismic single-channel (SCS) were collected in this area and data were collected along 30 parallel lines normal to the margin. There were 800 shots in each line and the shot interval was about 25 m. The shot line spacing was kept at 50 m. Six were deployed in water depth of about 1,500 m along the central line perpendicular to the margin. The first order multiples of the data has high signal to noise ratio (Fig. 5). Wavefield separation The ocean-bottom seismometers lie at the boundary separating two different media (acoustic to elastic) and hence a distinction must be made between wavefields just above and below the sea floor (Amundsen and Reitan 1995; Osen et al. 1999; Schalkwijk et al. 1999). The initial source signal that travels directly through the water column gets partially reflected at the water sediment interface and partially transmitted into the seabed. For example, the direct wave and the wave reflected at the sea floor arrive almost at the same time at the receiver just above the sea floor. Therefore, the direct event contains both upgoing and downgoing components just above the sea floor but only downgoing just below. The receiver-side (water-layer) multiples are also both upgoing and downgoing above the sea floor but only downgoing below the sea floor.

5 Mar Geophys Res (2012) 33: Fig. 5 First-order multiples of the data Fig. 6 One migrated image of the upgoing wavefield below the sea floor Similarly, when an upward-propagating signal (reflection event) encounters the water sediment interface, it gets partially transmitted into the water column and partially reflected back into the seabed. Therefore, the reflected signal (primary) is both upgoing and downgoing just below the sea floor but only upgoing just above the sea floor. In our work, we followed the method of Ranjan et al. (2009) to decompose the wavefields. Comparison of migrations Here reverse time pre-stack depth migration was applied to the upgoing primary wavefield just below the sea floor and the first-order multiples just above the sea floor. The input velocity model for the travel time computation was obtained from 2D travel time tomography of the and singlechannel reflection data in the area (Dash 2007). The input velocity model for the mirror migration of multiples was designed such that the receivers were placed effectively at the sea surface (not on the sea floor) of a water column twice as thick as the original water layer. First, the upgoing wavefield below the sea floor was migrated. Illumination is very poor (Fig. 6, we have made the depth to time conversion): the sea floor and the shallow reflectors immediately below the sea floor are not imaged. However, migration of the downgoing wavefield just above the sea floor produces a much better image (Fig. 7, also we have made the depth to time conversion). Lateral illumination is much enhanced and shallow layers are imaged clearly. Also we can see that the mirror-migrated image is comparable to the image obtained from single-channel streamer data (Fig. 8).

6 94 Mar Geophys Res (2012) 33:89 95 Fig. 7 One migrated image of the downgoing wavefield above the sea floor Fig. 8 Single-channel seismic data along the central line Conclusions Although multiples are often discarded as noise, they might contain additional information about the subsurface. When properly imaged, they can provide complementary information on parts of the subsurface not illuminated by primary reflections. Ocean-bottom receiver ghosts can be imaged using existing migration algorithms by allowing different elevation for the receivers. In our study, the image produced from migration of receiver ghosts alone is comparable to the near vertical reflection image obtained from single channel surface-towed streamers. It is much better than the image produced from migration of the upgoing primary wavefield just below the sea floor. The improvement, especially for shallow near-sea floor targets, is mainly because of wider illumination and reduced exposure to shallow inhomogeneous anomalies under the seabed. Although lateral illumination from mirror imaging depends primarily on the extent of the source patch, it is much better in deepwater environments where primaries and first-order, water-layer multiples are clearly distinguishable. For the data presented here, illumination from multiples is particularly wide because the shooting area is significantly larger than the receiver area. Mirror imaging has significant implications for processing and acquisition of data, especially in deepwater. In particular, mirror imaging is well suited to a dense-shot and sparse-receiver geometry because it enables greater tolerance to large intervals between the receivers.

7 Mar Geophys Res (2012) 33: Acknowledgments This research is funded by the Fundamental Research Funds for the Central Universities of China (2011YYL023), National Basic Research Program of China (973 Program, 2009CB219505) and International Science & Technology Cooperation Program of China (Grant No. 2010DFA21630). References Amundsen L, Reitan A (1995) Extraction of P- and S-waves from the vertical component of the particle velocity at the sea floor. Geophysics 60(1): Biondi BL (2006) 3D Seismic imaging. Investig Geophys Ser 14, SEG Bleistein N (1987) On the imaging of reflectors in the earth. Geophysics 52(7): Dash R (2007) Crustal structure and marine gas hydrate studies near Vancouver Island using seismic tomography. University of Victoria, Dissertation Ding R, Li Z, Sun X, Tong Z (2008) Prestack reverse time depth migration and the imaging condition. Prog Geophys 23(6): Gazdag J (1978) Wave equation migration with the phase-shift method. Geophysics 43(7): Godfrey RJ, Kristiansen P, Armstrong B, Cooper M, Thorogood E (1998) Imaging the Foinaven ghost. In: 68th annual international meeting, SEG, expanded abstracts, vol 17, pp Grion S, Exley R, Manin M, Miao XG, Pica A, Wang Y, Granger PY, Ronen S (2007) Mirror imaging of data. First Break 25(11): Muijs R, Robertsson JOA, Holliger K (2007) Prestack depth migration of primary and surface-related multiple reflections: part I-Imaging. Geophysics 72(2):S59 S69 Osen A, Amundsen L, Reitan A (1999) Removal of water-layer multiples from multicomponent sea-bottom data. Geophysics 64(3): Pedersen Ø, Ursin B, Helgesen HK (2010) One-way wave-equation migration of compressional and converted waves in a VTI medium. Geophysics 75(6):s237 s248 Pica A, Manin M, Granger PY, Marin D, Suaudeau E, David B, Poulain G, Herrmann P (2006) 3D SRME on data using waveform multiple modeling. In: 76th annual international meeting, SEG, expanded abstracts vol 25, pp Ranjan D, Spence G, Hyndman R, Grion S, Wang Y, Ronen S (2009) Wide-area imaging from multiples. Geophysics 74(6):Q41 Q47 Ray A, Noltem B, Herron D (2005) An experimental nodal acquisition from the thunder horse field. Gulf of Mexico. The Leading Edge 24(4): Ronen S, Comeaux L, Miao XG (2005) Imaging downgoing waves from ocean bottom stations. In: 75th annual international meeting, SEG, expanded abstracts vol 24, pp Schalkwijk KM, Wapenaar CPA, Verschuur DJ (1999) Application of two-step decomposition to multicomponent ocean-bottom data: theory and case study. J Seism Explor 8: Schneider WA (1978) Integral formulation for migration in two and three dimensions. Geophysics 43(1):49 76 Stolt RH (1978) Migration by Fourier transform. Geophysics 43(1): Sun Y, Qin F, Checkles S, Leveille JP (2000) 3-D prestack Kirchhoff beam migration for depth imaging. Geophysics 65(5): Zillmer M, Reston T, Leythaeuser T, Flueh ER (2005) Imaging and quantification of gas hydrate and free gas at the Storegga slide offshore Norway. Geophys Res Lett 32:L doi: / 2004GL021535