Wide-Azimuth Towed Streamer Acquisition
Wide-azimuth towed streamer acquisition (WATS) is a marine seismic acquisition technique that records seismic data over a broader range of source-to-receiver azimuths than conventional narrow-azimuth towed streamer surveys — achieved by using multiple seismic source vessels and multiple receiver streamer vessels (or by performing multiple passes over the same area with different lateral offsets between the source and the receiver spread) to illuminate the subsurface from many different horizontal directions, providing a more complete wavefield sampling that significantly improves seismic imaging quality beneath complex geological structures such as salt bodies, salt flanks, and highly faulted carbonates where conventional narrow-azimuth data fails to provide adequate subsurface illumination; in a conventional narrow-azimuth survey, the source and receiver streamers are towed in the same direction with the receiver cables deployed directly behind the source vessel, recording reflections primarily from the inline direction (along the sail line) with minimal cross-line (perpendicular) azimuth coverage; in WATS surveys, the geometry is designed to distribute recorded traces across a full 0-360 degree azimuth distribution (or a wide sector thereof) by using separate source vessels that fire from positions far to the side of the receiver spread while the receiver streamers tow behind a dedicated streamer vessel, creating wide offsets and high azimuths that illuminate reflectors below complex structures from directions that conventional surveys do not reach; the improved azimuthal sampling from WATS surveys provides better velocity model building for prestack depth migration (critical for accurate depth prediction below salt), better suppression of interbed multiples (which are attenuated more effectively when they have different azimuthal moveout than the primary reflections), and better characterization of azimuthal anisotropy from natural fractures and stress-induced velocity variations that can only be measured when the wavefield is sampled over a full range of azimuths.
Key Takeaways
- Sub-salt imaging in deepwater Gulf of Mexico was the primary driver for developing WATS technology, as conventional surveys failed to adequately illuminate the highly prospective reservoirs beneath Miocene and Jurassic salt bodies — the Gulf of Mexico's most prolific deepwater plays (Miocene sands beneath autochthonous Mesozoic salt, Jurassic Norphlet sands beneath allochthonous mini-basins) are located directly beneath or adjacent to massive salt bodies that strongly deflect and attenuate seismic waves in ways that narrow-azimuth surveys cannot fully compensate for; salt has very high P-wave velocity (approximately 4480 m/s) compared to the surrounding sediments (2000-3000 m/s at equivalent depths), creating strong refraction and reflection at the salt-sediment interface that dramatically alters the geometry of ray paths from any specific source-receiver pair; illuminating a sub-salt target from multiple azimuths (as WATS provides) ensures that some azimuths provide constructive ray paths through the salt geometry even when the conventional inline azimuth creates shadow zones where the salt body deflects all energy away from the target; the improved illumination from WATS surveys in the Gulf of Mexico has been directly credited with enabling discoveries of hundreds of millions of barrels of recoverable oil that were poorly imaged or invisible on conventional surveys.
- Velocity model building for prestack depth migration (PSDM) benefits enormously from wide-azimuth data because different azimuths provide independent constraints on the velocity field — the accuracy of PSDM (which requires an accurate 3D velocity model to correctly position reflections in depth) depends on having velocity variations measured independently in multiple directions, because any single azimuth's moveout velocity is sensitive to both the velocity magnitude and the velocity anisotropy in that direction; with conventional narrow-azimuth data, the velocity model built from moveout analysis in a single azimuth direction cannot separate isotropic velocity heterogeneity from azimuthal anisotropy, leading to velocity models that have systematic errors in structurally complex areas; WATS data provides moveout information from multiple azimuth sectors, enabling tomographic velocity inversion that independently constrains the isotropic velocity background and the azimuthal anisotropy, producing more accurate velocity models that result in better-focused, more accurately positioned sub-salt images; in the deepwater Gulf of Mexico, the improvement in PSDM image quality from WATS velocity models compared to conventional survey velocity models has been demonstrated on multiple large field cases, with significant improvements in both the crispness of salt-sediment boundary definition and the continuity of sub-salt reflections.
- Azimuthal AVO analysis from WATS data enables direct detection of natural fracture orientation and density from surface seismic — in fractured reservoirs (tight carbonates, fractured basement, naturally fractured shales), the elastic wave propagation velocity varies with the direction of wave travel relative to the fracture orientation, creating a seismic anisotropy that manifests as azimuthal variation in reflection amplitude and NMO velocity; conventional narrow-azimuth surveys can detect fracture-induced anisotropy only in the inline azimuth, which provides no information about the fracture azimuth or the magnitude of anisotropy; WATS data with full azimuthal sampling allows computation of azimuthal AVO and azimuthal velocity (AVAZ) analysis that directly measures the direction and strength of the fracture-induced anisotropy; the fast velocity direction (the azimuth with highest seismic velocity) corresponds to the fracture strike direction (fractures are stiffer parallel to their plane than perpendicular to it), providing a map of fracture orientation across the survey area from surface seismic alone; this fracture orientation information is directly used in well placement decisions (horizontal wells should be drilled perpendicular to the fracture strike to maximize the number of fractures intersected by the wellbore) and in hydraulic fracture design (where natural fractures interact with hydraulic fractures to create complex fracture networks).
- WATS acquisition cost is substantially higher than conventional narrow-azimuth surveys but is justified by the improved imaging in complex geological environments — a conventional 3D towed streamer survey uses a single source vessel and 8-12 streamer cables, with acquisition cost typically $5,000-$20,000 per square kilometer of survey area; a WATS survey using multiple source vessels and dedicated streamer vessels may cost $30,000-$80,000 per square kilometer — a factor of 3-6 increase in acquisition cost for the same survey area; this cost premium is justified in complex geological environments where the conventional survey fails to adequately image the target (because the well locations based on conventional data are uncertain enough to justify several failed exploration wells costing $50-100 million each), but is not justified in simpler geological environments where conventional surveys adequately illuminate the target; the decision to commission a WATS survey versus a conventional survey should be preceded by a survey design study (using ray-tracing or full-waveform modelling to simulate the illumination from both acquisition geometries) that quantifies the imaging improvement expected from WATS and compares it to the cost premium, ensuring that the additional acquisition investment is justified by the drilling decision improvement it enables.
- Ocean bottom seismic (OBS) and ocean bottom node (OBN) technology provides an alternative approach to wide-azimuth illumination by placing receivers on the seafloor and acquiring data from surface sources that sail freely across the survey area — unlike towed streamer surveys (where both source and receivers are at the sea surface, limiting the geometry to a narrow azimuth range), OBS/OBN surveys place 4-component receivers (recording 3 components of ground motion plus hydrophone pressure) on the seafloor, allowing sources to sail in any direction above the receivers and record a complete 360-degree azimuth distribution of incident seismic energy; OBN surveys are the most flexible and completest azimuthal sampling method available, but also the most expensive (deploying, repositioning, and retrieving seafloor nodes requires specialized vessels and significant operational time); for the most challenging sub-salt imaging problems in deepwater Gulf of Mexico and ultra-deep Brazilian pre-salt, OBN surveys have been shown to provide significant additional imaging improvement over WATS towed streamer surveys, justifying costs of $50,000-$150,000 per square kilometer for the most critical prospect areas near planned exploration or appraisal wells.
Fast Facts
The first large-scale commercial WATS survey in the deepwater Gulf of Mexico was the Mad Dog WATS survey acquired by BP in 2004, using three source vessels shooting simultaneously into a 14-streamer, 11-km-wide receiver spread to illuminate the sub-salt Paleocene and Miocene reservoirs of the Mad Dog field. The improvement in sub-salt image quality compared to the previous conventional survey was dramatic — fault patterns that were barely visible became sharply defined, and the salt body's irregular base became much more precisely mapped, allowing more accurate depth prediction for the producing reservoir intervals. The Mad Dog WATS survey set the template for what became the standard acquisition approach for deepwater sub-salt exploration in the Gulf of Mexico over the following decade.
What Is Wide-Azimuth Towed Streamer Acquisition?
Wide-azimuth towed streamer acquisition is the seismic industry's answer to the salt problem: when a massive salt body sits above your reservoir and deflects conventional seismic energy away from the target you're trying to image, you illuminate the target from more directions until the geometry gives you ray paths that work. WATS surveys use multiple source vessels and specially configured receiver spreads to record seismic energy from a wide range of horizontal azimuths, ensuring that sub-salt targets are illuminated from angles that conventional surveys can't access. The result is seismic imaging beneath salt that was previously poor quality or effectively blind — opening up hundreds of millions of barrels of recoverable oil to discovery and development in deepwater basins where conventional surveys made the reservoirs look too uncertain to drill. It costs three to six times more than a conventional survey. The discoveries it enabled have returned that investment many times over.
Synonyms and Related Terminology
Wide-azimuth towed streamer acquisition is abbreviated WATS. Related terms include narrow azimuth (the conventional survey geometry that WATS replaces for complex imaging), ocean bottom node (OBN, the alternative full-azimuth acquisition technology), prestack depth migration (the processing technique that uses WATS data for sub-salt imaging), azimuthal anisotropy (the fracture characterization information WATS enables), sub-salt imaging (the primary application that drove WATS development), velocity model (the tomographic product improved by WATS multi-azimuth data), illumination (the ray-path coverage metric that WATS improves), and seismic acquisition (the field operation category that WATS belongs to).
Why Wide-Azimuth Acquisition Changed What Was Discoverable in Deepwater Gulf of Mexico
The deepwater Gulf of Mexico's Miocene sub-salt play — which has yielded fields like Thunder Horse, Atlantis, and Mad Dog with billions of barrels of combined reserves — could not have been discovered at the pace it was without wide-azimuth seismic. Conventional narrow-azimuth surveys produced images beneath the salt that were too poor to justify drilling the deep, expensive wells needed to test these structures with confidence. WATS surveys improved the imaging enough to see the structural geometry clearly, map the reservoir extent credibly, and locate appraisal wells with enough precision to avoid missing the target by drilling into the salt flank or into the wrong fault block. The technology didn't create the oil — the geology did. But WATS made the oil visible enough to drill for it, and visible early enough in the exploration-to-production cycle to make the economics of deepwater development work at reasonable risk levels. In exploration, the quality of the image determines the quality of the decision. WATS made the image good enough to make decisions worth billions of dollars.