Imaging beneath gas clouds using 3D prestack Kirchhoff time migration of PS-converted waves—A case study from the North Sea

Abstract

We present a practical approach to imaging structures beneath gas clouds using PS-converted waves. In this approach, we developed methods to build the migration velocity model and to perform 3D prestack Kirchhoff time migration on a PC cluster and improved the processing flow for PS-converted waves. The technique was successfully applied to a 3D/4-C marine data set acquired in August 2001 and made available to us by Kerr-McGee North Sea (UK).This was a North Sea survey with a nominal receiver area of 10.8 km 2 . Two swath data sets were acquired. Each swath data set had an inline geometry using two receiver cables and 22 shot lines. The survey was centered on a domed structure which is obscured by a gas chimney. Faulting is thought to be present beneath the summit of the dome. Because the P-waves are attenuated by gas clouds, the P-wave image of the structure beneath the gas chimney is dimmed, but PS-converted waves can image that structure. In this study, we processed the PS-converted wave data of the 3D/4-C data set from one swath. The objective of the processing was to define the top of the structure at the reservoir target level (3.0 s PP time and 5.5 s PS time). We demonstrated how the PS-converted wave processing was able to clarify structural details.3D prestack Kirchhoff time migration (PKTM) and an appropriate migration velocity model play a crucial role in imaging structures beneath gas clouds using PS-converted waves. Firstly, we present the principle of 3D PKTM and its implications for a PC cluster. Then we demonstrate how to build the migration velocity model. After that we show the results of 2D processing of P-and PS-converted waves for 2D lines at the location of receiver cables, and the results of 3D processing of PS-converted waves for one swath data set. Finally, we present the interpretation results based on the 3D migrated images and our conclusions.Principles of 3D PKTM. 3D PKTM can produce an image at any desired location in a 3D space. Figure 1 shows the relationship among the scatterpoints (image points), shots, and receivers in 3D PKTM. The energy from a trace related to a shot and a receiver must be distributed to all possible scatterpoints in a surface according to the relevant traveltime. In other words, the energy from all shots and receivers is summed at each location to construct an image of the scatterpoints. This is usually implemented as a weighted summation based on the raypaths.In 3D PKTM, the traveltime of a PS-converted wave (t ps ) for anisotropic media is calculated using the double-squareroot (DSR) equation derived by Li et al. (2001): (1) where x p is the horizontal distance from the source to the scatterpoint; x s is the horizontal distance from the scatterpoint to the receiver; t p0 is the vertical traveltime for the Pwave; t s0 is the vertical traveltime for the S-wave; V P is the velocity for the P-wave, and V S is the velocity for the S-wave. η eff and ζ eff are anisotropy parameters for the P-and S-waves, respectively. However, the offset between the source and re...

Reducing the Uncertainty in Parameter Estimation and Data Processing of PS Converted Waves

“…Faulting is thought to be present beneath the summit of the dome. Since the P-waves are attenuated by gas clouds, the P-wave image of the structure beneath the gas chimney is dimmed, but PS converted waves can image that structure (Dai, et al 2007). Here we compare the results obtained using the original approach and the new approach.…”

SS-traveltime parameters from PP and PS reflections

“…Nevertheless, especially in recent years, significant progress has been made, allowing processing and imaging of converted waves to be carried out in a similar way as for nonconverted waves. See, e.g., Dai et al ͑2007͒, Li et al ͑2007͒, and further references therein.…”

A new technique for recognizing fractured zones in P-P~reflection fields, applied to the study of a North Sea oil reservoir

“…when the problem of finding these areas is difficult for methods based on azimuthal analysis (for example, AVO). One of the main problems for Dai et al [2007] and Li et al [2007], was to display the reflectors in a relatively shallow dome-shaped structure placed beneath the "neck" at = 3200 − 3600 ms in Figure 3. Dai et al [2007] used Prestack Kirchoff Time Migration (PKTM) and transformation of PS-waves (Figure 9.…”

“…One of the main problems for Dai et al [2007] and Li et al [2007], was to display the reflectors in a relatively shallow dome-shaped structure placed beneath the "neck" at = 3200 − 3600 ms in Figure 3. Dai et al [2007] used Prestack Kirchoff Time Migration (PKTM) and transformation of PS-waves (Figure 9. As gas clouds "soften" the P-waves, the PKTM technique applied to P-waves did not give an effective result (Figure 10.…”