Single-Azimuth Towed Streamer Acquisition

Key Takeaways

• Single-azimuth acquisition geometry is defined by the vessel and streamer configuration: a modern SAZ vessel typically tows 8 to 16 streamers of 5 to 8 km length, separated by 75 to 150 meters in the cross-line direction (giving a total across-the-ship spread of 600 to 2,400 meters), with air gun arrays fired from port and starboard of the vessel at fixed intervals (typically every 12.5 or 25 meters of vessel forward progress); the resulting CMP fold (the number of independent source-receiver pairs that contribute to each CMP bin) depends on the number of streamers, the shot interval, the bin size, and the ratio of the shot interval to the trace-to-trace spacing along the streamer; a typical 10-streamer configuration with 75-meter separation and 12.5-meter shot interval provides a CMP fold of approximately 80 to 120 in the in-line direction and 5 in the cross-line direction (the number of streamer positions that overlap for a given CMP bin), with the 5:1 in-line-to-cross-line fold ratio reflecting the narrow-azimuth nature of the data and the limited cross-line aperture relative to the in-line aperture; the narrow cross-line aperture means that reflections can only be recorded from source-receiver pairs within approximately 20 to 45 degrees of the sail-line azimuth, creating blind azimuths where the formation is not illuminated by any source-receiver pair and cannot contribute to the final image.
• The cost and productivity advantage of SAZ over wide-azimuth and full-azimuth acquisition methods is substantial and determines its dominance in exploration markets where the imaging improvement from richer azimuth is not justified by the additional cost: a SAZ survey with a single vessel and 10 streamers can acquire 600 to 1,500 square kilometers per day in favorable conditions (calm sea state, minimal feathering, no pre-plot deviations), while a WAZ survey using 2 to 3 vessels in a coordinated geometry acquires the same area at 30 to 50 percent of the SAZ productivity with the additional vessel cost; an ocean-bottom node survey (full-azimuth) acquires the same area at 1 to 5 percent of SAZ productivity and at 5 to 20 times the cost per square kilometer, justified only in specific applications (active fields with subsea infrastructure, very complex imaging challenges in shallow water, time-lapse monitoring) where the full-azimuth and four-component data provide unique value; for frontier exploration surveys in areas without major imaging challenges (deepwater basins without thick salt, carbonate shelf surveys, passive margin slope appraisal), SAZ typically provides adequate data quality at the lowest cost, making it the appropriate acquisition geometry by default unless a specific technical need for richer azimuth can be demonstrated from the geological setting and prospect assessment.
• Acquisition footprint in SAZ data arises from the regular, repeating pattern of source and receiver positions that creates systematic variations in CMP fold, offset distribution, and azimuthal coverage across the survey area: because each sail line is parallel to the adjacent lines (separated by a fixed cross-line spacing of 75 to 300 meters), the CMP bins along each sail line have consistently high fold while bins between sail lines have lower fold and different offset distributions; this creates a cross-line striping pattern in the fold map and in the seismic amplitudes that is visible as linear noise in the final image after processing; the acquisition footprint is particularly problematic in shallow target imaging where the Fresnel zone (the area of the reflector contributing to a single CMP) is small and the fold variation between high-fold (over the sail line) and low-fold (between sail lines) bins creates amplitude variations that mimic stratigraphic features; in SAZ data over structurally complex areas (salt domes, tilted fault blocks) where the illumination of the target changes rapidly with offset and azimuth, the acquisition footprint can create false structural relief or apparent amplitude anomalies that are artifacts of the uneven illumination rather than real geological features; reducing acquisition footprint requires either using a denser sail-line spacing (smaller cross-line interval, at proportional cost increase) or applying 5D regularization in processing to reduce fold variation without additional acquisition cost.
• SAZ data processing follows the standard marine processing sequence (demultiple including SRME, deconvolution, velocity analysis, NMO correction, DMO or migration, stacking, and post-stack processing) without the additional azimuthal analysis steps required for wide-azimuth data: because all source-receiver pairs in a SAZ CMP gather have approximately the same azimuth, there is no azimuthal information to extract from the gather and the standard isotropic NMO correction and stacking are applied; in anisotropic geological settings (fractured reservoirs, shale with strong intrinsic anisotropy), the SAZ data will show an incorrect NMO velocity if the data is acquired perpendicular to the fast velocity direction of the anisotropic formation, but this bias cannot be identified or corrected from SAZ data alone; pre-stack depth migration (PSDM) using full-waveform inversion (FWI) velocity models can substantially improve the imaging quality of SAZ data over what is achievable with time-domain processing and RMS velocity analysis, but the FWI velocity model derived from SAZ data may itself be biased by the limited azimuthal sampling (FWI requires adequate data coverage in all directions to correctly update the velocity model at each subsurface point, and SAZ data provides asymmetric coverage); the combination of high-quality SAZ data with FWI and PSDM is the current industry standard for most deepwater exploration applications outside salt provinces.
• The transition from SAZ to richer-azimuth acquisition in specific geological contexts represents one of the most significant technical decisions in marine seismic survey design: the survey design team must assess whether the geological setting and specific exploration objectives require the improved azimuthal coverage of WAZ or full-azimuth methods or whether SAZ will provide adequate imaging at lower cost; key factors that favor SAZ include absence of major overburden velocity complexity (no thick salt, no highly variable shallow geology), structural targets where four-way closure can be mapped with SAZ illumination, sediment-dominated basins without significant azimuthal anisotropy, and financial constraints that prevent the additional vessel cost of WAZ; factors that favor richer-azimuth acquisition include presence of thick or overhanging salt bodies that shadow subsalt targets in SAZ illumination, fractured reservoir objectives where AVAZ analysis is planned, complex fold-thrust belt overburden with strong lateral velocity variation, time-lapse monitoring applications where repeatable full-azimuth illumination is required for accurate 4D seismic attribute changes, and mature fields where incremental improvement in imaging quality has high economic value from more accurate well placement; in practice, many operators acquire SAZ as a rapid initial survey to evaluate prospectivity and then acquire WAZ or OBN over the most prospective areas for detailed pre-drill characterization, using the SAZ as a scout survey to guide the more expensive acquisition investment.