SS : The Role of Wide Azimuth in Subsalt Imaging
Authors: Fromyr Eivind 
Published: July 2010

High quality sub-salt imaging has remained a challenge for the seismic industry over the last two decades. Recent modeling and case histories have demonstrated that increased cross-line offset improves the image quality significantly: illumination is enhanced and multiples are better attenuated in complex subsalt environments (Regone, 2006; Fromyr et.al. 2008). In themselves they present new challenges in terms of survey design, acquisition and processing. Another important element of enhanced imaging is related to bandwidth and in particular the amount of low frequency energy we can emit and record. This is closely related to deghosting of source and receiver. It has been demonstrated that the receiver side ghost can be completely eliminatd by dual sensor technology and the source ghost can be effectively reduced by placing sources at different depths (Tengham et.al. 2007; Cambois et.al, 2009).

Introduction

The benefits of wide azimuth acquisition are products of at least three significant consequences of the greater diversity in surface coverage:improved reflector illumination,improved noise reduction, andhigher fold. These benefits are most significant when target reflectors are beneath complicated structural (salt) settings, hence the applicability to deepwater central Gulf of Mexico salt basins. This paper presents results from the Crystal program, acquired from late 2006 through April 2008 in the Keathley Canyon area. The particular wide azimuth arrangement used for this project is patterned after concepts published by Threadgold et al. (2006). Previous WATS surveys have been commissioned to assess and develop particular prospects. The Crystal survey is of much larger exploration scale. It covers an area nearly 14,000 km2, or 580 OCS blocks. Due to the sheer size of the survey, the fold in this exploration WATS design is reduced from that of development surveys.

A detailed 3-D depth velocity model is required to properly image the entire collection of independent traces. This model is built using all traces collected from both tiles - the entire 20-streamer array. The sequence of velocity model building followed rather conventional steps: iterative 3-D reflection tomography for sediment overburden, turning-wave Kirchhoff PSDM for top-salt interpretation, targeted salt-overhang modeling, Kirchhoff and WE PSDM for base-salt geometry interpretation, sub-salt WE PSDM scanning for sediment update.

Results

The WATS acquisition overlaps a portion of a legacy NATS 3D survey. The WATS data are profoundly superior, revealing clear salt boundary delineation and intra-sedimentary structural behavior. These differences can be attributed directly to the wide azimuth data available, which overcome the limitations of a single acquisition direction in complex geological structures. A cascaded approach of 3-D SRME and Radon also contributes to significantly better results.

Preserving seismic bandwidth

Ghosts in marine seismic acquisition are generated by the sea-surface. All up-going waves are reflected back as down-going waves with a reversed polarity, and interfere constructively for some frequencies and destructively for other frequencies. This phenomenon occurs twice: on the source side and on the receiver side.

The affected frequencies depend solely on source and receiver depths. Marine seismic acquisition therefore involves a trade-off between the various frequency ranges. To record high frequencies sources and receivers have to be towed shallow, which strongly attenuates low frequencies. Conversely, a deep tow favors low frequencies at the expense of high frequencies.

The combination of dual-sensor streamer and multi-level source dramatically increases the bandwidth of marine seismic data. In particular, the low frequencies are significantly enhanced, which provides better penetration and improved Q estimation. The dual-sensor streamer effectively removes the receiver ghost, while the multi-level source attenuates the source ghost. The multi-level source is biased towards low frequencies and has an anisotropic radiation pattern.