Full-Azimuth Towed-Streamer Acquisition: Coil Shooting, Subsalt Imaging, and Single-Vessel Marine Seismic
Full-azimuth towed-streamer acquisition is a marine seismic technique in which a single vessel records reflection data across the complete 360-degree range of source-to-receiver azimuths by towing its streamers along a continuous circular, or coiled, path rather than the straight parallel lines of conventional acquisition. The method, commercially known as coil shooting, was introduced by WesternGeco around 2008 to solve a specific imaging problem: below thick, complex salt bodies, seismic energy is refracted and scattered so severely that data recorded from only one direction, as in a narrow-azimuth survey, leaves large shadow zones and unreliable structure. Illuminating a subsurface point from every compass direction fills those gaps, sharpens the salt-flank and subsalt image, and supplies the azimuthal information needed to characterize fracture-induced anisotropy and stress orientation. Traditional wide-azimuth (WAZ) surveys achieve a range of azimuths too, but they typically require three or four vessels shooting in coordinated straight parallel lines, an expensive and logistically heavy operation. Coil shooting delivers a comparable or wider azimuth distribution with one vessel following a pre-plotted circle roughly 12 to 25 km in diameter, repeated and stepped across the survey area to build up the required fold, offset, and azimuth coverage. A major operational advantage is the near-elimination of line-change downtime: because the vessel simply continues around the circle, the turn between adjacent coils takes minutes rather than the several hours a conventional streamer vessel needs to swing its 8 to 12 km spread around at the end of a straight line. The streamers, which trail behind the vessel on a curved path, are positioned with acoustic networks and steered with birds to maintain feathering control through the turn. Full-azimuth coil data are processed with anisotropic prestack depth migration to exploit the azimuthal richness. While the Western Canadian Sedimentary Basin is onshore and does not use towed-streamer methods, the same technique is directly relevant to Canadian offshore exploration on the East Coast, where operators image beneath the salt and complex overburden of the Flemish Pass and Orphan Basin under regulation by the Canada-Newfoundland and Labrador Offshore Petroleum Board.
Key Takeaways
- Single vessel, circular path: Full-azimuth acquisition tows streamers along continuous circles roughly 12 to 25 km in diameter, so one vessel records reflections across the full 360-degree azimuth range. This contrasts with narrow-azimuth surveys, which sample essentially one direction, and with multi-vessel wide-azimuth surveys, which need three or four ships shooting straight parallel lines to build azimuth coverage.
- Subsalt imaging is the driver: Thick, complex salt refracts and scatters seismic energy, leaving shadow zones that single-direction data cannot image. Illuminating each subsurface point from every azimuth fills those gaps, sharpens salt flanks and subsalt structure, and is the central reason the technique was developed for salt-province exploration.
- Line-change efficiency: A conventional streamer vessel needs several hours to turn its 8 to 12 km spread at the end of a straight line. In coil shooting the vessel simply continues around the circle, so the transition between adjacent coils takes minutes. This near-continuous recording is a major cost and schedule advantage over race-track geometries.
- Azimuthal anisotropy information: Recording every azimuth captures how seismic velocity varies with direction, which reveals aligned fracture sets and horizontal stress orientation. This azimuthal anisotropy analysis supports fractured-reservoir characterization and geomechanical interpretation that single-azimuth data cannot deliver.
- Anisotropic depth processing: Full-azimuth coil data are migrated with anisotropic prestack depth migration to honour the directional velocity field and the curved acquisition geometry. Accurate streamer positioning through the turn, tracked by acoustic networks and controlled by steering birds, is essential for the geometry to bin correctly.
Coil Geometry and Fold Build-Up
In coil shooting the vessel follows a circular pre-plot, and that circle is repeated and stepped in both the inline and crossline directions to accumulate the fold, offset distribution, and azimuth distribution a modern depth image requires. Because each circle contributes reflections from a rotating range of source-receiver directions, adjacent and overlapping coils progressively fill the azimuth-offset space at every subsurface bin. Survey designers model the coil diameter, spacing, and overlap against the target depth and salt geometry, trading survey duration against azimuthal completeness. Larger circles capture longer offsets useful for deep and subsalt targets, while tighter coil spacing raises fold. The result is a data volume with far richer directional sampling per bin than a straight-line narrow-azimuth survey of comparable vessel time.
Full-Azimuth Versus Wide-Azimuth and Narrow-Azimuth
Narrow-azimuth acquisition, the historic default, samples a single narrow band of azimuths and images poorly beneath salt. Wide-azimuth surveys broaden that band using multiple source and receiver vessels in straight lines, at high cost. Full-azimuth coil shooting reaches the widest azimuth range of the three with a single vessel, and modeling studies indicate the circular geometry can deliver a better distribution of azimuths and offsets than parallel wide-azimuth layouts. The trade-off is a more complex processing flow and careful streamer steering through continuous turns, but for salt-province and structurally complex offshore basins the imaging uplift generally justifies the added processing effort.
Fast Facts
The first commercial coil-shooting survey was acquired in the Gulf of Mexico in 2008, and the appeal of the idea comes down to arithmetic on downtime. A conventional wide-azimuth survey using four vessels loses hours at every line turn across all ships, while a single coil vessel spends its turn-time productively curving into the next circle. Over a multi-month deep-water program, cutting line-change downtime from hours to minutes can recover weeks of vessel time, and offshore seismic vessels can cost well over CAD 250,000 per day to operate, making the efficiency gain financially decisive.
Related Terms
Full-azimuth acquisition is a form of marine seismic surveying, the offshore method of imaging the subsurface with towed sources and hydrophone streamers. Its data are collected on a streamer, the long cable of hydrophones trailed behind the vessel, and each cable is subdivided into a group, the set of receivers feeding one recording channel. The technique's central purpose, resolving structure obscured by salt, ties it to salt dome imaging, where complex velocity contrasts demand illumination from every azimuth.
Real-World Offshore Scenario: Flemish Pass Subsalt Program
An operator exploring a deep-water block in the Flemish Pass off Newfoundland faced a prospect partly shadowed by a salt canopy, where an earlier narrow-azimuth survey had left the subsalt reservoir target ambiguous and could not de-risk a proposed exploration well estimated near CAD 180 million. Rather than mobilize a four-vessel wide-azimuth spread, the operator commissioned a single-vessel full-azimuth coil survey over the structure, recording continuous circles across the salt body under CNLOPB authorization.
Anisotropic prestack depth migration of the full-azimuth data resolved the salt flank and imaged a coherent subsalt closure that the prior survey had smeared, tightening the volumetric range and repositioning the planned well location. The improved image reduced dry-hole risk enough to sanction the well, illustrating how azimuthal completeness can change a multi-hundred-million-dollar drilling decision.