Multiazimuth Towed Streamer Acquisition

Key Takeaways

• Azimuthal anisotropy detection is the primary geophysical motivation for multiazimuth acquisition in fractured reservoir characterization, because the presence of aligned open fractures in a reservoir creates a rock with different seismic velocity and reflectivity when measured parallel versus perpendicular to the fracture strike: P-wave velocity is higher along the fracture direction (parallel to aligned cracks, which have minimal effect on wave propagation in the direction the wave travels) than perpendicular to the fractures (where the cracks must be opened by the passing wave, slowing propagation); similarly, the amplitude of the seismic reflection from the top of a fractured reservoir varies with azimuth (amplitude-versus-azimuth, or AVAz analysis), with the maximum amplitude in the azimuth direction perpendicular to the fractures and the minimum in the fracture direction; multiazimuth data provides the azimuthally-sampled gathers needed to perform AVAz analysis and derive the fracture strike direction and intensity across the 3D seismic volume; in fractured carbonate reservoirs (such as the Austin Chalk in Texas, the Asmari Formation in the Middle East, and the Midale carbonate in the Weyburn field in Saskatchewan) where natural fracture permeability is the dominant control on well productivity, the fracture orientation maps derived from multiazimuth seismic guide horizontal well placement and completion design.
• Illumination improvement for complex subsurface geometries is the second major benefit of multiazimuth acquisition, particularly beneath salt bodies and beneath steeply dipping structures that are poorly illuminated by single-azimuth surveys: the specular reflection from a steep reflector (such as the flank of a salt body or a steeply dipping fault plane) returns to the surface at a receiver offset and azimuth that depends on the angle of incidence, which for steep reflectors may be far from the inline direction; a survey sailing in only one azimuth receives the steep-reflector return only at the receivers in one specific azimuth range, missing the energy that returns at other azimuths; adding survey passes at additional azimuths brings this energy into the dataset, improving the coverage of steep reflectors in the pre-stack gathers used for depth migration and providing additional constraints for the velocity model building in salt flank areas; the combination of multiazimuth data with full waveform inversion velocity model building and reverse time migration has significantly improved sub-salt imaging quality in the Gulf of Mexico and North Sea compared to earlier single-azimuth surveys in the same areas, enabling the discovery of resources in sub-salt plays that had been geologically present but seismically inaccessible with earlier acquisition strategies.
• Wide-azimuth towed streamer (WATS) acquisition is a related but distinct approach from multiazimuth that uses a much wider cross-line receiver spread than conventional narrow-azimuth surveys, achieved by operating multiple source vessels offset from the recording vessel or by using wide cross-line streamer spreads, to provide a wide range of receiver azimuths from a single sailing direction rather than requiring multiple sail passes: conventional narrow-azimuth surveys (NATS) have a cross-line spread of 300-600 meters (the width of the streamer array), giving source-receiver azimuths that cluster tightly around the in-line direction; WATS surveys achieve cross-line spreads of 6,000-12,000 meters (using offset source vessels or multi-vessel configurations), providing azimuths up to 60-90 degrees from the inline; the combination of WATS and MATS (conducting WATS surveys in multiple azimuth directions) provides the most complete azimuthal sampling possible with towed streamer systems, approaching the full-azimuth coverage achievable with ocean-bottom receiver systems at a significantly lower cost because towed streamer operations are faster than node or cable receiver deployments.
• Processing of multiazimuth data requires specialized workflows that combine the surveys from different azimuth directions into a coherent merged dataset while preserving the azimuthal variation information that distinguishes MATS from single-azimuth processing: the simplest approach (azimuth averaging or isotropic processing) merges all azimuths into a single stack that has better illumination than any single azimuth but discards the azimuthal variation information needed for fracture analysis; anisotropic processing preserves the azimuthal variation by processing each azimuth sector separately through migration and then stacking or inverting the azimuthal gathers for fracture parameters; the cross-azimuth merging of prestack gathers from different sail lines requires careful regularization of the data to a common surface grid (interpolating the irregular shot-receiver midpoint distribution from each sail line to a regular CMP grid) before migration, and care in matching the amplitude, phase, and time-shift between surveys from different azimuths that may have been acquired months apart with different weather, vessel, and water sound velocity conditions; the near-surface static consistency between azimuth passes is particularly challenging in areas of strong near-surface velocity variation (shallow gas pockets, variable water depth along the sail line) where the different azimuths sample the near-surface at different locations with different statics corrections.
• Economic trade-offs between MATS and alternative full-azimuth acquisition methods (ocean-bottom nodes, OBC) determine the survey design in specific areas: MATS is typically less expensive per square kilometer than full-azimuth ocean-bottom methods but more expensive than single-azimuth conventional surveys, because the same area must be sailed two, three, or four times at different headings rather than once; the cost premium for MATS relative to single-azimuth is typically 50-200% (depending on the number of azimuth passes), while the cost premium for ocean-bottom nodes (which provide full azimuth coverage in a single deployment) is typically 400-600% over single-azimuth conventional; in areas where azimuthal anisotropy information is important for field development decisions (fracture orientation mapping, drilling hazard identification, 4D seismic time-lapse monitoring with better repeatability) and where the subsea infrastructure makes OBC impractical, MATS provides the best balance of azimuthal information quality and survey cost; deepwater areas (where OBC deployment cost increases dramatically with water depth) favor MATS as the primary wide-azimuth solution, while shallow-water producing fields (where OBC or seabed node deployment is practical) often choose the higher repeatability and full azimuth coverage of bottom-deployed receivers despite the higher acquisition cost.