Pre-stack depth migration for improved imaging under seafloor canyons: 2D case study of Browse Basin, Australia

Abstract

In the Browse Basin, as in many areas of the world, complex seafloor topography can cause problems with seismic imaging. This is related to complex ray paths, and sharp lateral changes in velocity. This paper compares ways in which 2D Kirchhoff imaging can be improved below seafloor canyons, using both time and depth domain processing.In the time domain, to improve on standard pre-stack time migration (PSTM) we apply removable seafloor static time shifts in order to reduce the push down effect under seafloor canyons before migration. This allows for better event continuity in the seismic imaging. However this approach does not fully solve the problem, still giving sub-optimal imaging, leaving amplitude shadows and structural distortion. Only depth domain processing with a migration algorithm that honours the paths of the seismic energy as well as a detailed velocity model can provide improved imaging under these seafloor canyons, and give confidence in the structural components of the exploration targets in this area. We therefore performed depth velocity model building followed by pre-stack depth migration (PSDM), the result of which provided a step change improvement in the imaging, and provided new insights into the area.

Seismic prism waves generated by seafloor canyons and their effects on subsurface imaging

“…Our migration of the data shows that the artifacts produced by these prism waves can appear to be geological. This is most evident in Figure 5, in this case prism waves produce low frequency energy both above and below the real canyon floor (at 500 ms and 830 ms), this is consistent with the effect seen in data in Debenham and Westlake (2013) Figure 4a. With some more complex layering these artifacts could easily be misinterpreted to be real structure.…”

“…This is a significant problem for the oil and gas industry as many targets lie beneath or proximal to these continental shelf breaks. Bisley and MacNeill (2008), Debenham and Westlake (2013) demonstrated that such seafloor canyons present particular challenges on the North-West shelf (NWS) in Western Australia. Neglecting these wavefield complexities can result in inaccurate velocity models, significant imaging errors, misleading amplitudes and uncertain interpretations.…”

“…While these techniques improve normal move-out (NMO) analysis around complex structures, they are based on a single reflection assumption and do not account for multiple scattering events such as prism waves. More recently Debenham and Westlake (2013) demonstrated improvements that resulted from careful velocity model building and Kirchhoff pre-stack depth migration (KPSDM); however this method is also based on a single reflection assumption and does not account for multiple-scatterers such as prism waves. Several imaging techniques are being developed that aim to use multiply-scattered waves as well as primary reflections (Cavalca and Lailly, 2007;Malcolm et al, 2009;Zuberi and Alkhalifah, 2014).…”

Velocity model-building challenges and solutions for seabed- and paleo-canyons: A case study in Campos Basin, Brazil

“…Deeks and Lumley (2015) pointed out that multiple paths of prism waves are usually generated by the canyons, even at short offsets. Prism wave energy becomes stronger in narrow and deep canyons and creates shadow events in stacked images and CDP gathers (Deeks and Lumley, 2015;Debenham and Westlake, 2014). Confidence in picking proper RMO and geological events is low in the presence of prism waves.…”