Abstract

Driven, primarily, by the desire to image sub salt targets in the GOM, the industry has made significant improvements in towed streamer acquisition technology, to improve imaging through complex over-burdens. This paper will review a new towed streamer technique which makes a full azimuth seismic measurement with ultra-long offsets (14,000m), which was designed to mitigate imaging limitations of the commonly used WAZ designs in the US GOM. The final design was based on a very intensive modeling study, on real and synthetic models, using a finite difference modeling approach.

Introduction

As the "easy to produce" hydrocarbon reservoirs become more sparse, the industry is exploring in more challenging areas such as deep water, pre-salt, sub basalt etc.. The complexity of these geologies significantly increases the seismic imaging challenges. Conventional marine towed streamer (NAZ) acquires a very limited azimuth range because of the long/thin geometry of the surface receiver spread. While the source generates a wavefield which propogates in all directions, the receiver spread only records a very limited proportion of the reflected wavefield. If the target and over burden geology is relatively benign (low dip and homogeneous) even this limited spread can illuminate the reservoir reasonably consistently. However, if the target and overburden geology is complex, the seismic rays will bend and scatter on their way to and from the reservoir, resulting in inconsistent illumination. Over the last ten years the industry has developed towed marine solutions which generate a more diverse set of ray paths by more fully sampling the azimuth range, which consequently more evenly illuminate target reservoirs in geologically complex areas.

These techniques include multi-azimuth (MAZ), wide-azimuth (WAZ), rich-azimuth (RAZ) and more recently Coil Shooting. MAZ is a technique in which a conventional NAZ survey is acquired multiple times using different primary orientations; Generally, the data is merged post stack either using a weighted or unit summation. WAZ is a technique in which multiple vessels are used to extend the range of azimuths collected and RAZ itechnique in which WAZ geometry surveys are repeated in multiple acquisition directions.

Coil Shooting is a technique in which a marine towed streamer vessel follows a circular pre-plot. This circular line is then repeated in the inline and crossline direction, to build up fold, offset distribution and azimuth distribution. As well as acquiring a dataset very well sampled in azimuth, the technique benefits from some acquisition efficiencies associated with the very sdowntime (line change) between circular lines. This downtime is of the order of minutes as opposed to hours for conventional race-track acquisition.

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