Rich-Azimuth Towed Streamer Acquisition

Key Takeaways

• Rich-azimuth acquisition geometries are achieved through several complementary approaches that increase the angular diversity of source-receiver paths contributing to each subsurface CMP bin: multiazimuth (MAZ) acquisition uses multiple parallel survey passes over the same area with different sail-line orientations (typically 3 to 8 orientations separated by 30 to 60 degrees), with the data from all passes merged into a single dataset that samples CMP bins from the full range of orientations; wide-azimuth (WAZ) acquisition uses a split configuration with the source vessel sailing one set of lines while independent streamer vessels (gun boats) tow sources in offset positions to create source-receiver offsets in the cross-line direction (perpendicular to the streamer direction) that are not achievable with a single vessel, dramatically expanding the azimuthal coverage from the approximately 20 to 30 degree cone of NAZ to 90 to 120 degrees in the cross-line direction; coil shooting (a proprietary WesternGeco technique using multiple vessels sailing overlapping circular paths) provides the widest azimuthal coverage achievable with towed streamers by generating source-receiver pairs at all azimuths as the vessel continuously circles; each approach involves operational complexity, vessel management challenges, and increased cost compared to standard NAZ acquisition, justified in areas where the subsalt or complex-geology imaging benefit from improved azimuthal coverage exceeds the incremental cost of the richer-azimuth geometry.
• Subsalt illumination improvement is the primary commercial driver for rich-azimuth acquisition in deepwater Gulf of Mexico, Brazilian presalt, and West African salt provinces: the salt body (which has a significantly different seismic velocity, typically 4,480 m/s, compared to the surrounding sediment velocity of 1,800 to 3,500 m/s) refracts seismic rays away from the subsalt target zone for inline source-receiver geometries, creating shadow zones where reflections from the subsalt target cannot be recorded with NAZ acquisition; crossline shot-receiver pairs (which approach the salt body from a different angle) penetrate through parts of the salt flank that would deflect inline energy away from the target, illuminating subsalt CMP bins from azimuths that are blind spots for NAZ; the 3D wave propagation effects that make crossline illumination valuable for subsalt targets cannot be replicated by any processing technique applied to NAZ data -- the physical energy simply does not reach the target with NAZ geometry, regardless of the sophistication of the migration or velocity analysis applied in processing; WesternGeco's introduction of WAZ acquisition in the deepwater Gulf of Mexico (Atlantis and Thunder Horse area in 2005 to 2007) was a watershed event in subsalt exploration, with the WAZ images showing significantly improved subsalt definition compared to NAZ images from the same area acquired only a few years earlier and triggering a rapid industry adoption of rich-azimuth geometries for deepwater exploration.
• Azimuthal anisotropy analysis enabled by rich-azimuth data provides direct information about natural fracture orientation and intensity in the subsurface: in a vertically fractured reservoir (HTI anisotropy, where the symmetry axis is horizontal in the direction perpendicular to the fracture planes), seismic waves traveling parallel to the fracture planes travel faster than waves traveling perpendicular to them, and the reflection amplitude varies with azimuth (azimuthal AVO, or AVAZ) in a systematic pattern controlled by the fracture normal vector and the fracture compliance; with NAZ data (sampled at only one azimuth), the AVAZ signal is invisible; with rich-azimuth data sampled at multiple azimuths across each CMP bin, the azimuthal velocity variation and amplitude variation can be extracted by fitting the Fourier decomposition of azimuthal attributes (the ellipse of velocity variation, the amplitude azimuthal gradient) to reveal the fast and slow directions and the magnitude of anisotropy; in fractured carbonate reservoirs (Austin Chalk, Bone Spring, Monterey, Middle East Cretaceous carbonates) and in unconventional shale plays where the natural fracture network modifies hydraulic fracture propagation, AVAZ analysis from rich-azimuth data provides pre-drill fracture characterization that cannot be obtained from any other remote sensing measurement at the scale of a seismic survey.
• Processing of rich-azimuth data requires azimuth-preserving workflows that maintain the azimuthal diversity of the input data through all processing steps to the final azimuthal stack or azimuth-differentiated output: conventional processing workflows designed for NAZ data (which average all offsets and azimuths during NMO correction and CMP stacking) would destroy the azimuthal information in rich-azimuth data; azimuth-preserving processing maintains separate azimuth sectors (typically 30 to 60 degree bins) through demultiple, deconvolution, velocity analysis, and migration, stacking each azimuth sector separately to produce an azimuthal stack cube; azimuthal velocity analysis (computing the NMO velocity variation with azimuth from the moveout of CMP gathers sorted by azimuth and offset) requires sufficient data density in each azimuth-offset bin to fit a stable velocity, which is only achievable with the bin fold available from rich-azimuth acquisition; 5D interpolation (regularizing the 5D data space of offset, azimuth, and surface position) is commonly applied to rich-azimuth data before processing to fill gaps in the sampling and ensure that each azimuth-offset bin has adequate fold for stable velocity estimation and pre-stack migration; the full processing of a rich-azimuth 3D survey (from demultiple through anisotropic pre-stack depth migration with azimuthal velocity analysis) typically requires 2 to 5 times the computational resources and elapsed time of comparable NAZ processing, reflecting the additional data volume and azimuthal analysis steps.
• The economics of rich-azimuth acquisition must be weighed against its imaging benefit for each specific geological context: WAZ acquisition in deepwater Gulf of Mexico typically costs 1.5 to 3 times the cost of equivalent NAZ acquisition for the same target area, reflecting the use of multiple vessels and the longer acquisition time required to achieve the wider-azimuth coverage; coil shooting costs are typically higher, at 2 to 4 times NAZ cost; the economic justification requires that the improved imaging from rich-azimuth data leads to better exploration decisions (avoiding dry holes from misidentified subsalt structures or from AVO anomalies that are actually multiple contamination in NAZ data) or better development decisions (placing appraisal wells more accurately, characterizing fracture networks that control production performance) that have economic value exceeding the incremental acquisition cost; in mature areas with existing NAZ data, the decision to acquire new rich-azimuth data (at full cost) versus purchasing and merging existing multiazimuth NAZ surveys from different operators (at lower cost but with potential data compatibility issues) requires careful analysis of the achievable azimuthal coverage from the available NAZ vintages and the incremental imaging improvement expected from full WAZ or coil acquisition; industry experience in the deepwater Gulf of Mexico suggests that the subsalt imaging improvement from WAZ over NAZ is large enough to justify the cost in most areas with significant subsalt prospectivity, but the fracture characterization benefit of AVAZ analysis requires specific geological context (fractured reservoirs with proven production response to fracture intensity) to be economically compelling.