Spread
In seismic acquisition, a spread (also called a receiver spread or geophone spread) is the geometrical arrangement of the geophone groups (or hydrophone groups in marine surveys) that are active and recording during the detonation of a seismic source shot, defining the set of source-receiver offsets and azimuths that contribute to the seismic record for that shot point, with the spread geometry (the number of active channels, the group interval, the total spread length, the offset range from near to far, and the split-dip, end-on, or off-end configuration relative to the source position) determining the maximum offset available for velocity analysis and AVO studies, the fold of coverage (number of common-midpoint traces contributing to each CMP bin), the angular aperture for migration, and the ability to suppress noise and multiples through array forming and common-midpoint stacking; the spread geometry is one of the most critical design variables in a seismic acquisition program, with spread length and offset distribution directly controlling the resolution of the velocity model (longer offsets provide better velocity discrimination but also more moveout stretch and NMO stretch at far offsets), the fold of coverage (more channels in the spread increase fold, improving signal-to-noise ratio after stack), and the cost of the survey (more active channels require more cable or wireless receivers, larger recording systems, and more source points to maintain constant geometry across the survey area).
Key Takeaways
- Spread configuration (the geometric relationship between the shot point and the receiver array) determines the offset distribution and the symmetry of the resulting CMP gathers: an end-on spread (all receivers on one side of the shot) produces an asymmetric offset distribution (only positive offsets, from near offset at the closest receiver to far offset at the most distant); a split-dip spread (receivers symmetrically distributed on both sides of the shot) produces a symmetric CMP gather with equal numbers of short and long offsets from both sides; a far-field spread (with a long near-trace offset, using a gap between the shot and the first live group) is used when surface wave (ground roll) suppression requires separating the near-offset channels dominated by direct wave and refraction from the reflection records; in marine seismic, the standard configuration is an end-on spread with the streamer towed behind the source vessel, with the source at the near end and the farthest receiver at the tail end of the streamer 3 to 10 km behind; in land 3D surveys, the spread is defined in both the inline and crossline directions, with the number of active receiver lines and the line spacing controlling the crossline offset distribution and minimum azimuth sampling.
- Maximum offset in a seismic spread controls the ability to discriminate velocity from moveout and to perform AVO (amplitude variation with offset) analysis: the velocity resolution of an NMO velocity analysis is proportional to the ratio of maximum offset to the two-way travel time at the target depth (the "offset-to-depth ratio"), with an offset-to-depth ratio of 0.5 to 1.0 typically required to resolve velocity differences of 2 to 5 percent between adjacent layers; for a target at 3,000 m depth with a 2,000 m/s interval velocity (two-way time approximately 3 seconds), a maximum offset of 3,000 to 5,000 m is required for adequate velocity discrimination, which in turn requires a spread length of at least 3 to 5 km; AVO analysis requires that the far-angle reflections (typically 30 to 45 degrees incidence for Class II/III AVO anomalies) are recorded within the spread, which for a 3,000 m target at 2,000 m/s requires offsets of 1,730 to 3,000 m (tan(30 to 45 degrees) times the target depth); beyond the critical offset (where NMO stretch severely distorts the wavelet), far-trace data must be muted before stack, so there is a practical trade-off between including long offsets for AVO and velocity analysis and the degradation of stack quality from stretched far traces.
- Seismic fold (the number of seismic traces contributing to each CMP bin) is determined by the ratio of the active spread length to the station interval, divided by two for a split-dip spread: with a spread of 240 channels at 25-meter group interval (total spread length 5,975 m), split-dip configuration (120 channels each side of the shot), and shot interval equal to the group interval (25 m), the nominal CMP fold is 120/2 = 60 (each CMP bin receives 60 contributing traces from different shot-receiver pairs whose midpoint falls in the bin); actual fold in 3D surveys depends additionally on the number of active receiver lines (fold increases linearly with the number of receiver lines in the crossline direction) and on the receiver line spacing relative to the shot line spacing; the minimum fold required to achieve the target signal-to-noise ratio after stack depends on the noise level in the survey area (fold 30 to 60 for quiet areas, fold 100 to 200 for noisy areas with high cultural noise or surface wave energy) and the stacking velocity contrast between signal and noise (which determines how effectively stack attenuates coherent noise); modern seismic surveys typically acquire 120 to 480 fold or more in high-noise environments, with recent ultra-high-density surveys (simultaneous source acquisition with dense receiver grids) achieving effective fold exceeding 1,000 through simultaneous source separation.
- Moving spreads (rolling spreads) in land seismic acquisition advance the receiver array along the survey line as the shot moves forward, maintaining a consistent offset range and fold while progressively covering the survey area: in a standard single-line 2D survey, the receiver cable is advanced one group interval forward after each shot (roll-along acquisition), so the near-trace offset and far-trace offset remain approximately constant as the shot progresses along the line; in a 3D land survey with multiple receiver lines and multiple shot lines, the spread rolls in both inline and crossline directions as the acquisition crew advances the survey grid, requiring careful management of which receiver lines are active (plug-in points) and which are being picked up and redeployed ahead of the shot pattern; the transition between active and inactive receiver groups (the "live" spread boundary) creates edge effects at the margins of the survey where fold drops from nominal to zero, requiring extension of the receiver deployment beyond the planned image area to maintain full fold within the target coverage zone; the concept of "nominal fold" (the fold in the full-offset interior of the survey) versus "migration apron" (the extra receiver deployment needed to allow migration to use all offsets at the image boundary) is fundamental to survey design and cost estimation.
- Marine seismic spread design for 3D acquisition uses multiple streamers (2 to 18 or more streamers in wide-azimuth and full-azimuth surveys) deployed in parallel from a source vessel or from multiple vessels, covering a wide swath of receiver area per sail line: a standard narrow-azimuth 3D marine survey uses 8 to 12 streamers at 100-meter crossline spacing, each 6 to 10 km long with 25-meter group interval, providing a swath width of 700 to 1,100 m of imaged data per sail line and a nominal fold of 80 to 120 from a single source; wide-azimuth surveys (WAZ) use additional source vessels or a combination of active and passive source-receiver geometries to sample a wider range of offset azimuths, improving imaging of azimuthally anisotropic targets (fractured reservoirs, overthrust structures with strong lateral velocity gradients) that are poorly imaged by narrow-azimuth data; full-azimuth surveys (using reciprocal shooting, circular shooting, or node-based receivers on the seafloor) sample the complete 360-degree azimuth range and maximum offset distribution, providing the input for azimuth-sectored velocity analysis and azimuthal AVO that can map fracture orientation and density in tight reservoirs without drilling; the economic premium for WAZ and full-azimuth marine surveys ($50 to $200 per km^2 above standard narrow-azimuth cost) is justified only when the imaging problem is demonstrably azimuth-dependent.
Fast Facts
The earliest seismic refraction surveys in the 1920s and 1930s used very simple spreads: a single shot point with a line of seismographs recording the refracted arrival, with the spread length designed to sample the first arrival from the target refractor; the transition to reflection seismic in the 1930s (pioneered by the Geophysical Research Corporation and later by Geophysics Inc.) required more complex spread designs that could isolate the reflection arrival against the much stronger direct wave and refraction, leading to the development of multichannel recording systems with 6, 12, and eventually 24 channels by the 1940s; the modern era of 96, 240, and 1,000+ channel land acquisition systems (enabled by digital recording, wireless receiver nodes, and high-capacity recording trucks) was driven by the recognition in the 1980s that fold and offset range were the primary determinants of seismic image quality, and that the cost of receiver deployment (cable, geophones, and recorder units) was declining fast enough relative to drilling costs that high-fold, long-spread surveys were economically justified for most exploration and development programs. The introduction of simultaneous source acquisition (blended seismic, vibroseis slip-sweep) in the 2010s effectively decoupled fold from acquisition time by allowing multiple sources to fire simultaneously and separating the blended records in processing, making previously impractical fold levels (500 to 2,000+) achievable on production-rate timescales.
What Is a Seismic Spread?
A seismic spread is the geometric arrangement of active receiver groups (geophones on land, hydrophones in streamers at sea) that record the seismic wavefield from a single shot. The spread configuration (end-on, split-dip, or off-end), length, and number of channels determine the offset range available for velocity analysis and AVO studies, the fold of CMP coverage after stacking, and the azimuth sampling in 3D surveys. Spread design balances the subsurface imaging requirements (target depth, velocity resolution, fold needed for noise attenuation) against the acquisition cost (more channels, longer spreads, and wider swaths all increase cost).
Synonyms and Related Terminology
A seismic spread is also called a receiver spread, geophone spread, shot spread, or receiver array. In marine surveys, the spread is the streamer configuration. Related terms include common midpoint (CMP, the subsurface reflection point equidistant between a source-receiver pair; seismic traces from many different source-receiver pairs whose midpoint falls in the same CMP bin are stacked after NMO correction to increase fold and signal-to-noise ratio; the spread geometry determines how many traces contribute to each CMP bin), fold (seismic fold, the number of independent seismic traces that contribute to a single CMP stack; determined by the ratio of spread length to shot interval for 2D surveys; higher fold improves signal-to-noise ratio after stacking but requires more acquisition channels and shots), offset (the distance between the seismic source and a receiver in the spread; controls the angle of incidence at the reflector and the moveout of the reflection on the CMP gather; the range of offsets in the spread determines the effectiveness of NMO velocity analysis and AVO analysis), amplitude variation with offset (AVO, the change in seismic reflection amplitude as a function of source-receiver offset (angle of incidence); used to detect fluid content, lithology changes, and fractures; requires long-spread data with far offsets reaching 30 to 45 degrees incidence at the target), and streamer (a marine seismic receiver cable containing hydrophone groups at regular intervals, towed behind a survey vessel to record the reflected seismic wavefield; multiple parallel streamers form the marine equivalent of the land geophone spread).