circle shooting

Circle shooting in marine seismic acquisition is a specialized survey geometry in which the seismic vessel travels in a circular or spiraling path around a fixed surface point directly above the target structure, acquiring shot records at continuously varying azimuths from 0 to 360 degrees at a constant or incrementally increasing offset radius, generating an azimuthally rich dataset that provides illumination of the subsurface target from all compass directions simultaneously rather than from only the two to four azimuths available in conventional narrow-azimuth or wide-azimuth towed-streamer surveys; circle shooting is used in deepwater petroleum exploration and development when the subsurface target requires full-azimuth illumination to resolve complex structure, image beneath gas clouds or salt bodies, or characterize azimuthal anisotropy caused by aligned fractures or horizontal stress fields in tight carbonate or basement reservoir targets. In Western Canada Sedimentary Basin operations, circle shooting is not commonly employed because WCSB targets are accessible by land seismic and the relatively flat-lying stratigraphy does not require full-azimuth acquisition; however, the technique is relevant to WCSB companies with international portfolios in the Gulf of Mexico, West Africa, and Brazilian pre-salt where circle shooting datasets are routinely acquired. The geometry of a circle shoot places the seismic source and receiver array on the vessel, which orbits the central point (the apex of the subsurface dome or the crest of the salt-related structure) at a nominal offset of 3 to 10 kilometres, recording full-fold seismic data at each azimuth as the vessel completes one or more circuits; the resulting seismic data volume has offset-azimuth sampling that allows azimuthal velocity analysis, anisotropy inversion, and migration using full-azimuth angle gathers that substantially improve subsurface imaging quality in geologically complex settings compared to conventional towed-streamer acquisitions. Circle shooting acquisition is operationally challenging because the vessel must maintain precise circular track while managing streamer feathering from crosscurrent, coordinating with supply vessels, and ensuring consistent shot interval around the full orbit, with data quality most sensitive to position accuracy at the closest offsets (near-offset shadow zone fill) and to streamer depth control at the far-offset turning geometry where the outer streamers experience differential current forces.

• Circle shooting geometry and acquisition parameters for deepwater salt and carbonate targets: A circle shoot acquisition design specifies the target apex coordinates (typically the mapped crest of the subsurface structure), the nominal acquisition radius (3 to 8 km from apex to vessel track, selected to provide the desired range of incidence angles at target depth), the shot interval (12.5 to 25 m along the circular track), and the number of complete circuits (typically 1 to 3 for full-fold datasets). In Gulf of Mexico deepwater circle shoots targeting sub-salt carbonate reservoirs at 6 to 9 km depth, the acquisition radius is set to achieve incidence angles of 30 to 50 degrees at the target, angles sufficient for amplitude-versus-offset (AVO) analysis and anisotropy estimation; the streamer configuration uses dual-source single-vessel shooting with 6 to 12 streamers at 100 m separation providing 600 to 1,200 m crossline aperture per pass. The circle shoot geometry generates CMP gathers with full 360-degree azimuth coverage at each offset class, enabling 5D interpolation (inline, crossline, offset, azimuth, frequency) in pre-processing that fills illumination gaps beneath salt overhangs, gas chimneys, and seafloor topography that create acquisition shadows in conventional straight-line surveys; the azimuthal completeness of circle shoot data is the primary justification for its use despite the 3 to 5 times higher cost per km2 compared to conventional wide-azimuth surveys.
• Azimuthal anisotropy analysis from circle shooting data and its application to fracture characterization: The full-azimuth offset gathers generated by circle shooting enable azimuthal P-wave velocity analysis that detects and quantifies seismic anisotropy caused by aligned vertical fractures (HTI anisotropy) or horizontal stress contrast (orthorhombic anisotropy) in the overburden and reservoir; in circle shoot processing, the azimuthal velocity variation is extracted by sorting CMP gathers into azimuth sectors (typically 8 to 16 sectors of 22.5 to 45 degrees each) and measuring the NMO velocity in each sector, with the azimuthal velocity difference (fast direction minus slow direction, typically 2 to 8 percent in fractured carbonate reservoirs) and fast-azimuth direction providing the fracture orientation and relative intensity. In West African pre-salt carbonate reservoirs targeted by circle shooting, azimuthal anisotropy analysis from circle shoot data identifies the dominant fracture azimuth (correlating with regional stress fields from borehole image logs in nearby wells) and maps the lateral variation in fracture intensity across the structure, directly informing horizontal well landing azimuth and perforation cluster design in the appraisal program. In WCSB tight carbonate and Duvernay shale plays, the equivalent fracture characterization is performed with multi-azimuth land seismic acquired in orthogonal or star-pattern geometries that approximate the azimuthal coverage of circle shoots in a land acquisition framework.
• Processing workflow for circle shooting data: azimuthal gathers, WATS migration, and anisotropy inversion: Circle shooting data processing uses specialized workflows distinct from conventional towed-streamer processing because the non-linear acquisition geometry and full-azimuth sampling require azimuth-preserved processing from demultiple through migration. Key circle shoot processing steps include wide-azimuth towed-streamer (WATS) regularization (resampling the irregularly sampled circle shoot data to a regular 5D grid of inline-crossline-offset-azimuth using minimum-norm or POCS interpolation), azimuthal multiple attenuation (surface-related multiple elimination adapted for non-linear geometry), and wide-azimuth pre-stack depth migration (PSDM) using full-waveform inversion (FWI) velocity models that honour the azimuthal velocity structure of the salt and subsalt overburden. The output of circle shoot PSDM is a full-azimuth angle gather at each surface bin, from which azimuthal AVO attributes (intercept and gradient as functions of azimuth) are extracted for fracture and stress inversion; these attributes guide attribute-calibrated reservoir characterization at the appraisal stage of deepwater carbonate development projects. Computing requirements for circle shoot processing are 10 to 100 times greater than equivalent-aperture conventional surveys due to the 5D data volume and iterative FWI velocity updates; dedicated HPC clusters or cloud processing (AWS, Azure) are the standard platforms for circle shoot PSDM workflows.
• Circle shooting versus wide-azimuth towed-streamer (WATS) and multi-azimuth (MAZ) surveys in deepwater exploration: Circle shooting, WATS, and MAZ surveys are three alternative approaches to achieving azimuthal illumination of complex deepwater targets, each with distinct cost-coverage tradeoffs. MAZ acquires the same target on two to four separate straight-line passes at different vessel headings (typically 0, 45, 90, 135 degrees), combining the resulting azimuth-sector datasets in processing to approximate full-azimuth coverage at 2 to 3 times the cost of a single-azimuth survey; MAZ is the most common approach for deepwater Gulf of Mexico exploration where rectangular lease blocks favour straight-line vessel tracks. WATS uses multi-vessel configurations (typically two source vessels and one or more receiver vessels) with wide crossline separations of 600 to 1,200 m to achieve wide-azimuth coverage in a single-pass straight-line survey; WATS provides good azimuth sampling for structural imaging but less complete azimuth coverage than circle shooting at any given offset. Circle shooting provides the most complete azimuth coverage at the target but requires continuous circular vessel track that is only practical for isolated deepwater targets (single domes, salt diapirs, pinnacle structures) not suited to area surveys; for large-area reconnaissance, WATS or MAZ surveys are more efficient.
• Circle shooting operational challenges: streamer feathering, far-offset geometry, and vessel coordination: The principal operational challenge in circle shooting is maintaining streamer geometry throughout the circular vessel track, particularly at the turning points where crosscurrent forces cause unequal feathering of port and starboard streamer arrays; streamer feathering of 10 to 30 degrees is typical in 0.5 to 1.0 knot crosscurrents on the circular track, with the outermost streamers experiencing the greatest deviation from the planned geometry. Modern circle shoot operations manage feathering with deflectors (paravanes and steerable lead-ins) and real-time streamer positioning using acoustic ranging and GPS buoys at streamer tail ends, with the vessel navigation system continuously adjusting heading to maintain the nominal circular track within 50 to 100 m position tolerance. Source vessel coordination in dual-source circle shoots requires precise synchronization of shot timing between the two independent source arrays firing at alternating intervals to achieve continuous 12.5 m shot point spacing around the circle; dedicated vessel management software tracks both source arrays, streamer positions, and the target apex reference simultaneously, integrating GPS and acoustic positioning to maintain shot geometry quality metrics throughout each orbit.

Circle Shooting Resolving Sub-Salt Carbonate Reservoir in Gulf of Mexico Deepwater

A Gulf of Mexico deepwater exploration prospect targeting a Cretaceous carbonate buildup beneath a salt overhang at 7.2 km depth had failed to image the reservoir crest using two previous narrow-azimuth towed-streamer surveys (2004, 2012), with the salt overhang creating an illumination shadow covering 60 percent of the structural closure. A single-vessel circle shoot was acquired in 2019 using a 4.5 km radius orbit with 18 streamers at 100 m separation, completing 2 full circuits at 25 m shot spacing; 5D WATS interpolation and full-waveform inversion velocity updating resolved the salt base geometry and illuminated the carbonate crest through the overhang shadow. The resulting full-azimuth PSDM image showed 280 ms of high-amplitude carbonate reservoir beneath the salt, azimuthal anisotropy analysis indicated a northeast-southwest fracture fabric at 4 percent anisotropy intensity, and AVO inversion confirmed Class II oil signature. The first appraisal well encountered 42 m of net carbonate pay at 98 percent oil saturation, confirming the circle shoot imaging success where two prior conventional surveys had failed to define the structure.

Related Terms

Marine seismic acquisition encompasses all ocean-based seismic methods; circle shooting is the specialized full-azimuth geometry for isolated complex targets where conventional towed-streamer surveys fail to illuminate the subsurface adequately. Azimuthal anisotropy is the primary geophysical target of circle shoot data analysis; full-azimuth P-wave velocity variation of 2-8 percent in fractured carbonates is resolved from azimuth-sector NMO velocity analysis in circle shoot gathers. Wide-azimuth towed-streamer (WATS) is the multi-vessel straight-line alternative to circle shooting; WATS provides better area coverage efficiency while circle shooting provides more complete azimuth sampling for isolated targets. Full-waveform inversion (FWI) velocity model building is essential for circle shoot PSDM imaging quality; dense azimuthal offset sampling improves FWI convergence through complex salt and subsalt velocity structures. Pre-stack depth migration (PSDM) applied to circle shoot full-azimuth gathers resolves sub-salt carbonate geometry that conventional time migration cannot image through complex overburden velocity variations.