coil shooting acquisition

Coil shooting acquisition (also called circular shooting or continuous-azimuth marine seismic) is a marine seismic acquisition method in which the vessel sails overlapping circular paths of 3 to 8 km radius rather than conventional straight parallel lines, continuously rotating the source-receiver azimuth through a full 360 degrees as the vessel completes each coil, providing wide-azimuth or full-azimuth illumination of the subsurface in a single survey pass without requiring multiple orthogonal vessel runs; because each coil circle generates source-receiver midpoints with azimuths spanning the full compass rose, a single coil survey delivers the azimuthal coverage equivalent to three or more conventional narrow-azimuth passes while reducing total vessel time by 30 to 50 percent. Coil shooting was first commercially deployed in the Gulf of Mexico in 2008 by CGG in collaboration with Shell, targeting subsalt reservoirs beneath complex salt canopy geometries that conventional narrow-azimuth (NAZ) surveys illuminated poorly because the salt flanks and base-salt reflectors were only imaged from one angular direction; subsalt imaging quality improved dramatically with wide-azimuth coil illumination because multiple azimuths collectively sample the salt geometry from all lateral directions, providing the angular diversity that full-waveform inversion and reverse-time migration algorithms require to accurately reconstruct the velocity field above and within the salt. Coil shooting has since been applied in the North Sea chalk and Base Cretaceous Unconformity plays, offshore Brazil pre-salt Santos and Campos basins, the Barents Sea, and Canadian offshore areas including Nova Scotia Sable Island basin surveys, where the method's ability to provide uniform azimuth coverage over structurally complex targets is valuable for seismic anisotropy analysis and fracture characterization that narrow-azimuth data cannot deliver; in the Canadian offshore context, coil survey results inform WCSB operators' understanding of azimuthal velocity anisotropy and fracture orientation analysis methods that are also applied to land 3D data over Montney and Duvernay unconventional plays, though land coil shooting analogs use randomized or wide-azimuth orthogonal geometries rather than literal circular vessel tracks.

• Coil shooting geometry, vessel track design, and streamer management in marine surveys: The coil radius is the primary geometric design parameter: a 5 km radius coil with 6 km streamers provides offsets from near-zero to approximately 11 km with full azimuth sampling at midpoints equal to the coil radius, while smaller 3 km radii give higher spatial density but limit maximum offset and larger 8 km radii provide ultra-long offsets for subsalt imaging at the cost of increased vessel turning time. Operationally, coil shooting imposes severe streamer management challenges because turning the vessel in a circle causes the streamers to feather laterally outward due to centrifugal force: at a 5 km radius turn at 4.5 to 5.0 knots, a 6 km streamer array spreads 200 to 400 m from the nominal straight-line position, requiring real-time positioning via acoustic ranging and GPS tail buoys at every 400 m in-cable interval to track the actual midpoint distribution for accurate 5D regularization. Most commercial coil surveys use 10 to 16 streamers spaced 100 m apart, with streamer entanglement during turns as a key operational risk managed by precise vessel speed control (4.5 to 5.0 knots maximum during coil arcs) and by staggering the starboard and port streamer lengths so that outer streamers trail shorter and avoid crossing inner ones during the sharpest parts of the turn.
• 5D regularization and processing of coil seismic data for prestack depth migration: Coil surveys produce inherently irregular data geometry: source-receiver midpoints are continuously distributed across the survey area at all azimuths, but the density of midpoints in any given azimuth-offset bin varies with the coil pitch and vessel track spacing, producing an irregular 5D data tensor in the dimensions of inline position, crossline position, offset, azimuth, and frequency. Standard CMP stacking and prestack migration algorithms assume regular input geometry, so coil data must be regularized onto a uniform 5D grid before migration using Fourier-based or rank-reduction interpolation methods (5D MWNI, Cadzow, or tensor decomposition approaches) that simultaneously fill missing azimuth-offset bins and reconstruct aliased high-frequency spatial content from the dense low-frequency sampling provided by the circular acquisition geometry. 5D regularization adds 15 to 25 percent to total coil processing cost but is non-negotiable: poorly regularized coil data produces migration artifacts and false amplitude anomalies in the subsalt or deep target intervals that cannot be distinguished from genuine geological features without a regularization quality control step comparing the regularized midpoint distribution to the ideal uniform 5D grid.
• Azimuthal velocity analysis and fracture characterization from coil shooting surveys: Because coil surveys continuously sample all azimuths, azimuthal variations in seismic velocity (azimuthal anisotropy) can be measured as a function of offset and azimuth using amplitude versus azimuth (AVAZ) or azimuthal NMO velocity analysis; in fractured reservoirs, seismic waves traveling parallel to the fracture strike travel faster than those traveling perpendicular, producing a characteristic cos(2-theta) azimuthal velocity variation whose fast-axis direction indicates the fracture strike and whose anisotropy magnitude is proportional to fracture intensity. Coil surveys in the Gulf of Mexico Austin Chalk and North Sea Ekofisk chalk plays have used azimuthal velocity anisotropy maps derived from coil data to identify zones of elevated fracture intensity (greater than 5 percent anisotropy) that correspond to historical production sweet spots confirmed by well data, demonstrating that coil-derived fracture maps can guide horizontal well placement in naturally fractured reservoirs; the same azimuthal anisotropy analysis applied to land wide-azimuth data in WCSB Montney and Duvernay programs uses circular shooting geometry analogs (wide-azimuth orthogonal land 3D designs with azimuths spanning 0 to 180 degrees) that provide comparable, though not identical, fracture characterization capability to true marine coil surveys.
• Coil shooting versus ocean-bottom node acquisition for subsalt imaging in deepwater programs: Both coil shooting and ocean-bottom node (OBN) acquisition provide wide-azimuth illumination for subsalt targets but through fundamentally different approaches with distinct economic and image-quality profiles: coil shooting uses a moving source vessel and towed streamers in circular patterns, achieving large-area coverage (200 to 1,000 km2 per vessel-month) at moderate cost but limited by streamer noise, finite maximum cable length (typically 6 to 10 km), and the inability to access ultra-long offsets (greater than 12 km) needed for full-waveform inversion velocity model building below salt bodies thicker than 2 km. OBN acquisition deploys receivers permanently on the seabed with a separate source vessel sailing arbitrary patterns including near-circular coil geometries, enabling ultra-long offsets (10 to 20 km), full-azimuth illumination, and near-zero streamer noise at the receiver (ocean bottom is below the wave-induced noise floor), producing image quality improvements of 20 to 40 percent in subsalt fold and illumination compared to coil shooting for the same target, but at 3 to 5 times the cost per km2. Industry practice has evolved toward using coil shooting for regional exploration surveys (where the economics favor wide-area coverage) and OBN for high-value appraisal and development surveys over specific subsalt fields where image quality improvements directly translate to reduced drilling risk on wells costing $50 million to $200 million each.
• Coil shooting operational efficiency, acquisition footprint, and survey design for maximum subsurface coverage: A key operational advantage of coil shooting over conventional NAZ acquisition is the elimination of sail-line turns between parallel lines, which in conventional acquisition consume 20 to 35 percent of total vessel time as the ship decelerates, turns, accelerates, and stabilizes before the next recording line; coil shooting eliminates this non-productive time by maintaining continuous curved motion at recording speed throughout the survey, reducing cost per km2 by 25 to 40 percent compared to equivalent-azimuth multi-azimuth (MAZ) surveys that achieve similar coverage through multiple NAZ passes. The coil pitch (distance between adjacent coil circle centers in the crossline direction) must be designed to ensure adequate bin fold across all azimuth sectors: a 300 to 500 m pitch between 5 km radius coils provides adequate overlap for surveys with 10 to 16 streamers at 100 m spacing, while a 600 m pitch produces under-sampled azimuth sectors in the crossline direction that require more aggressive 5D interpolation to fill and may introduce interpolation artifacts near the limit of the reconstruction aperture in zones of complex geology such as salt flanks and sub-salt mini-basin boundaries.

Coil Shooting Enabling Subsalt Discovery in the Gulf of Mexico

A Gulf of Mexico operator acquired a 650 km2 coil survey over a Lower Tertiary Wilcox subsalt prospect at 7,500 m depth beneath a 2,200 m thick salt canopy. The coil survey used 12 streamers at 100 m spacing, 5 km coil radius, and 8 km streamer length, providing offsets to 13 km with full azimuth coverage. Prestack depth migration of the regularized coil data using a full-waveform inversion velocity model resolved the base-salt reflection with 85 percent lateral continuity compared to 40 percent on a previous conventional NAZ survey over the same area. A prospective Wilcox fan closure of 35 km2 was identified beneath the salt at 7,500 m that was absent from the NAZ image. The exploration well drilled on the closure penetrated 180 m of net pay in the Wilcox, confirming the coil-imaged structure. Operator-estimated reserves of 280 million barrels were attributed to the coil survey's ability to image through the salt body that had made the prospect undrillable on NAZ data.

Related Terms

Wide-azimuth seismic is the broader category; coil shooting is one method of achieving wide-azimuth illumination in marine surveys alongside multi-azimuth (MAZ) surveys and ocean-bottom node acquisition; azimuthal coverage is the primary quality criterion distinguishing these methods. Prestack depth migration (PSDM) is the primary imaging algorithm applied to coil data after 5D regularization; full-waveform inversion velocity model building from coil ultra-long-offset data improves PSDM image quality for subsalt targets in GoM and Brazil pre-salt programs. Marine seismic acquisition encompasses towed-streamer methods including both conventional NAZ and coil shooting; streamer feathering during coil turns requires real-time acoustic positioning to track actual midpoint distribution for 5D regularization. Seismic anisotropy measurement from coil data azimuthal velocity analysis identifies fracture strike and intensity in naturally fractured carbonates and tight reservoirs; the same principle is applied to land wide-azimuth 3D surveys in WCSB Montney and Duvernay programs. Ocean-bottom node (OBN) acquisition is the higher-cost, higher-quality alternative to coil shooting for subsalt imaging; OBN provides ultra-long offsets and zero streamer noise at 3-5 times the cost per km2, used for appraisal and development over confirmed subsalt fields.