Summary

A 3D synthetic wide-azimuth towed-streamer (WATS) dataset is migrated with different migration techniques. These migration techniques are suited for imaging complex overburden. First, a wave-equation method based on a oneway propagator is used. Second, a two-way method that utilizes one-way propagators for the wavefield extrapolation downward and upward is tested. Finally, a method based on the solution of the two-way acoustic wave equation, also known as Reverse Time Migration (RTM) is selected. We compare the migration results in 2D and 3D and show that the best results are obtained when more information is incorporated in the imaging process, e.g., turning and/or prismatic waves. In practice, the selection of a migration algorithm is based on computational and geophysical considerations. For instance, the complexity of the subsurface tells us if turning waves are needed or not. Our ability to estimate an accurate velocity model helps us to decide which method will produce the best results. Finally, computing resources could present challenges when large datasets need to be migrated, especially for advanced imaging techniques such as RTM.

Introduction

Obtaining accurate images of the subsurface when complex velocity structures are present remains a challenging undertaking. This challenge must be tackled from the acquisition side as well as from the processing side. For instance, wide azimuth geometries show promise by attenuating undesirable artefacts caused by multiples in the migration, and by overcoming illumination issues. On the imaging side, the processing flow should be such that the primary reflections are kept intact and imaged properly by our migration algorithms. To better understand the effects of acquisition and imaging for complex overburden, 2D and 3D synthetic WATS data were generated on the MareNostrum supercomputer in Barcelona (Kaelin et al., 2007). The modelling was conducted as a collaborative effort between 3DGeo Inc, Repsol YPF, and the Barcelona Supercomputing Center (BSC) as part of the Kaleidoscope project. The modelled data were then migrated with advanced imaging techniques suited for handling complex wavefield propagations. We decided to focus on three migration techniques. All of them use shot profiles for input data. The first migration technique utilizes a 1-way propagator for the wavefield propagation (SPM 1-way). This propagator, which incorporates a Fourier Finite-Difference operator with optimized coefficients, is accurate for steep dips. The second migration technique is a two-pass migration, where the down-going wavefield is saved and used as a source for the up-going wavefield (SPM 2-way). This technique has the ability to migrate overturning waves. Last, we use a 2- way propagator based on the acoustic wave equation. This last technique is also known as Reverse Time Migration (RTM). These techniques have their pros and cons, which we analyze and compare in the following sections. This abstract starts with a short description of the synthetic WATS data. Then we present briefly each migration method used for imaging.

Modelling results

For the modelling effort, 4,047 wide-azimuth shots were created. The shots are separated by 536 m in each direction (X and Y).

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