3D High-Resolution Imaging Using Separated Wavefields

Transcription

1 3D High-Reolution Imaging Uing Separated Wavefield S. Lu (Petroleum Geo-Service), N.D. Whitmore (Petroleum Geo- Service), H. LeGleut (Petroleum Geo-Service) & A. Long (Petroleum Geo- Service) SUMMARY Conventional wave equation migration extrapolate upcoming boundary data generated from primary reflection to image the uburface. In marine eimic data proceing, ea urface related multiple are a major challenge. Particularly, when the water bottom i hallow, hort period multiple are difficult to deal with. Acquiition footprint of eimic imaging i another iue for hallow water bottom geology, where the primary reflection do not have the complete illumination coverage to the hallow target, including the water bottom. In thi paper, we review a method of eparated wavefield imaging uing ea urface related multiple. We preent a data proceing work flow of eparated wavefield imaging including up and -going wavefield eparation uing dual-enor data, wave equation migration of wavefield extrapolation and imaging condition, and a pot proceing tep of amplitude balancing. A hallow water example from Aia-Pacific i teted, where the eparated wavefield imaging generate remarkable high reolution 3D image. The eparated wavefield image ha greater area of uburface illumination, which mitigate the trong acquiition footprint. The method and reult can be ueful to reduce well drilling geohazard.

2 Introduction Conventional wave equation migration extrapolate upcoming boundary data generated from primary reflection to image the uburface. The ue of ea urface related multiple in wave equation depth migration ha been dicued by Berkhout and Verchuur (1994) and Guitton (2002). Whitmore et al. (2010) demontrate an approach of uing one-way Wave Equation Migration (WEM) to propagate up and -going wavefield and generate a uburface depth image from ea urface related multiple. Lu et al. (2011) preent the firt 3D cae tudy (uing SEAM ynthetic full azimuth data) of eparated wavefield imaging uing multiple with towed treamer 3D acquiition geometry. Source/receiver ghot event are generated by ea urface reflection. Deghoting of eimic data i a critical procedure to remove the pectral notche and generate high reolution broadband uburface image. Acquiition and proceing method are required to remove the ghot wavefield. A method for receiver deghoting i dual-enor (hydrophone and geophone) towed treamer acquiition, followed by up and -going wavefield eparation (Carlon et al., 2007). Thee eparated wavefield can then be ued for imaging. Cro-line acquiition footprint i a common iue for hallow water towed treamer eimic imaging. Imaging of multiple help to uppre the acquiition footprint. For each hot, imaging of primarie produce le uburface illumination than imaging of multiple [Figure 1A]. To the extent that they are recorded, multiple imaging ue maller reflection opening angle than primarie imaging [Figure 1B], and therefore can generate higher reolution image for the ame offet ditribution. In thi paper, we preent a hallow water cae tudy from Aia-Pacific. Uing up and -going wavefield, we produce a high reolution 3D near urface image from ea urface related multiple. In thi example, the multiple image generate more extenive uburface illumination than the image uing only primarie, and help mitigate the acquiition footprint. The image from multiple can be combined with the image from primarie to create an overall better image. Method Dual-enor towed treamer acquiition record both preure and vertical component of particle velocity wavefield. Broadband up and -going wavefield are eparated by applying a frequency-wavenumber P Z ummation algorithm to dual-enor data. In hot profile wave equation migration, the imaging proce i a combination of wavefield ward extrapolation and imaging condition. The conventional depth migration backward propagate the upcoming data a receiver wavefield and forward extrapolate a ynthetic impule wavelet a ource wavefield. Separated wavefield imaging propagate up and -going wavefield a receiver and ource wavefield. In thi paper, we employ an extrapolation method baed on a Fourier Finite Difference operator [Equation 1]. P i preure wavefield; i temporal frequency; c i reference velocity; v i media velocity; i patial derivative; a and b are finite difference coefficient. x, y 2 v 2 2 P 2, 2 c x y i, 1 (1) 2 x y 2 P z c v c v v v 2 a b 2 x, y Up/ imaging principle i applied to uburface ource and receiver wavefield. Conventional wave equation depth migration ue cro correlation imaging condition [Equation 2]. We ue a table verion of deconvolution imaging condition [Equation 3] (Valenciano and Biondi 2003; Guitton et al., 2007) to uppre the cro talk noie generated from unrelated up and -going wavefield. I(x) I(x) x x P P x, x ; P x, x ; (2) Px, x; Pupx, x; (3) x, x ; P x, x ; ( x) up x, y

3 In equation 2-3, I i the uburface image; i a damping parameter to make the deconvolution imaging condition table; tand for moothing in the image pace in the x, y direction. x, y Image are generated hot by hot and are tacked to form a compoite image after migration. The uburface power pectra are generated baed on a boundary data patial amplitude map. The tacked image i pot proceed uing reidual cable footprint removal and amplitude balancing. Example A field data example i teted with a 585km 2 area extracted from a full 3D dual-enor towed treamer urvey over the Tenggol Arch area in offhore Peninula Malayia. The dual-enor urvey i acquired uing 12 cable with 4050m cable length and 75m cable pacing. The acquiition hot pacing i 18.75m and receiver pacing i 12.5m. Data i dealiaed and reampled before migration to allow for migration of 120Hz. Migration are produced for frequencie of both 80Hz and 120Hz. Up and -going wavefield eparation i applied in the hot domain to the urface data. The data conit of ignificant hort period and long period multiple, which are from the hallow water bottom and other major impedance change. When imaging the multiple, the eparated wavefield have no demultiple applied, which mean everal order of ea urface related multiple are ued. Deconvolution imaging condition i applied to attenuate both the cro talk and noie from different order of multiple. The up-coming wavefield after ignature deconvolution, τ-p deconvolution and SRME (Verchuur, 1991) demultiple i ued to produce the image from primarie with a cro correlation imaging condition. Figure 2-4 how image of up to 80Hz migration. Figure 2 compare the inline image from primarie and from multiple in depth (0-3km). Inline image from one hot (colour region) are on top of the 3D full tack image. The comparion how that in the inline direction the multiple image ha more extenive illumination than primarie from one hot, which i explained by the chematic diagram in Figure 1A. Figure 3 compare the cro-line image in two way travel time (0-1600m). Image of multiple mitigate the trong ail line acquiition footprint in thi direction. Figure 4 how the depth lice at 105m of the tet region (25km by 23.4km). The four plot are: (A) image from primarie that i contaminated by very trong ail-line acquiition footprint; (B) raw tack of image from multiple that mitigate the acquiition footprint from adjacent ail-line (the image till ha ome reidual amplitude footprint, primarily due to the lack of fold equalization of the input data); (C) uburface power pectra that are generated from the urface data ued to etimate imaged fold; (D) high reolution final image from multiple after amplitude balancing, which i largely free of ail-line acquiition footprint. Concluion In thi paper, we review and preent a work flow of eparated wavefield imaging uing ea urface related multiple. Up and -going wavefield eparation i required to prepare boundary data a input of wave equation migration; deconvolution imaging condition i ued to upre the cro talk and multiple noie. The eparated wavefield are ued to produce image of both multiple and primarie, where the multiple are intrumental in helping to uppre the acquiition footprint in the image. In the hallow water example from Aia-Pacific, the 3D eparated wavefield image ha remarkable high reolution in the near urface, with the acquiition footprint eentially removed. The method and reult could be very ueful to reduce well drilling geohazard. Acknowledgement We thank Lundin Malayia BV and PETRONAS Carigali Sdn. Bhd. (Lundin Malayia BV partner in PM307) for permiion to ue and publih the data. We thank PGS for permiion to publih the methodology and the reult.

4 Figure 1 (A) Schematic diagram for uburface reflection of primarie (olid line) and ea urface related multiple (dahed line). In wave equation migration, red line are ued a receiver wavefield; blue line are ued a ource wavefield. Imaging from multiple (dahed circle) ha greater extent of illumination than imaging from primarie (olid circle). (B) To image the ame reflector (olid circle) by a ingle hot, primarie imaging (olid line) ue a larger reflection opening angle than multiple imaging (dahed line); therefore multiple produce a higher reolution image than primarie. Figure 2 Inline image from primarie (left) and from multiple (right) in depth from 0 to 3km. Image from one hot (colour region) on top of the 3D full tack image (grey) how that imaging of multiple ha greater extent of uburface illumination than imaging of primarie. Star indicate the ource location; triangle diplay the receive geometry. Figure 3 Cro-line image from primarie (left) and from multiple (right) in two way travel time from 0 to 1600m. Imaging of multiple mitigate the acquiition footprint in thi direction and generate a very high reolution image including detail information of the water bottom reflection. 75th EAGE Conference & Exhibition incorporating SPE EUROPEC 2013 London, UK, June 2013

5 Figure 4 Depth lice at 105m of the tet region (25km by 23.4km): (A) the image from primarie how trong acquiition footprint; (B) raw tack of image from multiple till ha ome reidual amplitude footprint; (C) uburface power pectra are generated from the urface data and conitent with reidual amplitude footprint; (D) high reolution final image from multiple after amplitude balancing i largely free of ail-line acquiition footprint. Reference 1. Berkhout, A. J., and D. J. Verchuur, 1994, Multiple technology: Part 2, migration of multiple reflection: 64th Annual International Meeting, SEG Expanded Abtract, 23, Carlon, D., A. Long, W. Söllner, H. Tabti, R. Tenghamn, and N. Lunde, 2007, Increaed reolution and penetration from a towed dual-enor treamer: Firt Break, 25, Guitton, A., 2002, Shot-profile migration of multiple reflection: 72nd Annual International Meeting: SEG Expanded Abtract, Guitton, A., A. Valenciano, D. Bevc, and J. Claerbout, 2007, Smoothing image condition for hot-profile migration: Geophyic, 72, no. 3, S149 S Lu, S., N.D. Whitmore, A.A. Valenciano, and N. Chemingui, 2011, Imaging of Primarie and Multiple with 3D SEAM Synthetic: 81t Annual International Meeting, SEG Expanded Abtract, Verchuur, D.J., 1991, Surface-related multiple elimination, an inverion approach: Ph.D. Diertation, Delft Univ. of Technology. 7. Valenciano, A., and B. Biondi, 2003, 2D deconvolution imaging condition for hot profile migration: 73rd Annual International Meeting, SEG Expanded Abtract. 8. Whitmore, N. D., A. Valenciano, W Söllner., S. Lu, Imaging of primarie and multiple uing a dualenor towed treamer: 80th Annual International Meeting, SEG Expanded Abtract, 29,