crossline

Crossline in three-dimensional seismic acquisition and interpretation refers to a seismic profile oriented perpendicular to the direction of primary data acquisition (the inline direction), forming one axis of the orthogonal two-dimensional grid of common midpoint (CMP) bins that defines the spatial framework of a 3D seismic volume; the inline direction is conventionally the direction along which receiver cables were deployed and along which source points advance during acquisition (typically the direction of dominant structural dip or the dominant geological feature orientation selected to maximize reflector imaging), while the crossline direction is perpendicular to inline, typically oriented along regional strike, and each crossline is identified by a crossline number that increases by one with each CMP bin width in the crossline direction. In Western Canada Sedimentary Basin 3D seismic programs covering Cardium, Viking, Montney, Duvernay, and Devonian carbonate plays, the standard bin grid is square (12.5 by 12.5 m or 25 by 25 m per bin in each direction) so that the inline and crossline bin spacings are equal, providing isotropic spatial sampling in both directions; however, practical land acquisition limitations (road access for vibroseis sources, receiver line deployment along land survey grid lines at 80 chain = 1,609 m spacing in Alberta's Dominion Land Survey) often produce rectangular receiver-line patterns where the crossline direction has wider receiver line spacing (200 to 600 m) than the inline direction (25 to 50 m receiver group interval), creating fold asymmetry between inline and crossline directions that is corrected in processing by trace interpolation in the under-sampled crossline direction. The crossline number increments by 1 for each CMP bin step perpendicular to the acquisition direction; for a WCSB Cardium 3D program with 25 m bin spacing starting at crossline 1 in the southwest corner of the survey, crossline 400 is located 400 times 25 m = 10,000 m north of the starting line; together with the inline number, the crossline number provides the unique identifier for every CMP bin in the 3D volume, and the (inline, crossline) coordinate pair is the standard reference system used in WCSB seismic interpretation workstation software (Petrel, Kingdom, OpendTect) for positioning wells, faults, and horizon picks in the 3D grid. In WCSB 3D seismic interpretation workflows, crossline sections are extracted from the 3D volume perpendicular to the inline direction and displayed as vertical 2D profiles of seismic amplitude versus two-way time and crossline position; interpreters toggle between inline sections (displaying geological features along the principal acquisition direction), crossline sections (displaying features in the perpendicular direction), and time slices (horizontal planes at constant two-way time) to build a complete three-dimensional understanding of WCSB reservoir geometry, structural closure, and stratigraphic architecture.

• Crossline orientation selection and structural alignment in WCSB 3D seismic design: The choice of inline versus crossline orientation in WCSB 3D seismic programs is driven by structural geology, receiver access, and the dominant direction of geological features to be imaged: inlines are typically oriented perpendicular to the dominant structural strike (parallel to dip direction) so that structural dip is apparent on inline sections, while crosslines run parallel to strike and display the along-strike continuity of WCSB reservoir sands, reefs, and carbonate buildups. In WCSB Cardium and Viking pool development programs where reservoir sand bodies are oriented northeast-southwest (parallel to Cretaceous paleoshoreline trends), the inline direction is commonly oriented northwest-southeast (perpendicular to the sandstone body long axis) and crosslines are northwest-southeast (parallel to shoreline), so that individual crossline sections capture the along-strike extent of the Viking or Cardium sand fairway that controls reserve distribution. In WCSB Foothills programs where thrust faults and fold axes trend northwest-southeast parallel to the Rocky Mountain front, inlines are oriented northeast-southwest (across structural strike) and crosslines run northwest-southeast (along structural strike), allowing crossline sections to display the lateral continuity of thrust-bounded anticlines for structural trap mapping.
• Crossline fold, sampling, and spatial aliasing in WCSB orthogonal 3D acquisition: Crossline fold (the number of independent source-receiver midpoints in each crossline bin direction) is controlled by the crossline receiver line spacing divided by twice the bin width; for a standard WCSB Cardium 3D program with 25 m bins and 300 m crossline receiver line spacing, the maximum crossline fold is 300 / (2 times 25) = 6 traces per crossline bin, compared to an inline fold of 60 from the receiver group interval and shot point spacing. The imbalance between inline fold (60) and crossline fold (6) in this geometry produces azimuth-dependent fold variation within each bin: near-inline azimuths (source-receiver pairs nearly parallel to inline) have much higher representation in the stack than near-crossline azimuths (source-receiver pairs nearly parallel to crossline), introducing azimuthal anisotropy in the stacked image that can mask or mimic azimuthal amplitude variation from natural fracture systems in WCSB Montney and Duvernay shale plays. Crossline spatial aliasing (temporal frequency folded into the wrong crossline wavenumber) occurs when the crossline sampling interval exceeds half the spatial wavelength at the maximum frequency of interest; for WCSB data with dominant frequency 50 Hz and target interval velocity 3,000 m/s (wavelength 60 m), aliasing-free crossline sampling requires bin widths of 30 m or less, satisfied by the standard 25 m WCSB bin but not by the 50 m bins used in cost-reduced reconnaissance programs.
• Crossline interpolation and regularization in WCSB 3D seismic processing: When field crossline receiver line spacing (200 to 600 m in typical WCSB land programs) is coarser than the nominal bin size (12.5 to 25 m), the CMP bins between receiver lines receive few or zero traces in the crossline direction, creating systematic gaps in the trace distribution that degrade migration performance and introduce crossline-oriented acquisition footprint in the stacked amplitude maps. WCSB 3D processing addresses crossline data gaps through trace interpolation algorithms (anti-leakage Fourier interpolation, minimum weighted norm interpolation, or 5D interpolation including time, inline, crossline, offset, and azimuth) that reconstruct missing traces from the available data; 5D interpolation for WCSB datasets of 50 to 300 km2 is computationally intensive (6 to 24 hours processing time on GPU-accelerated workstations) but significantly improves crossline fold uniformity and reduces acquisition footprint before pre-stack migration. Crossline fold regularization is particularly critical for WCSB AVO analysis and azimuthal amplitude analysis because under-sampled crossline azimuths artificially bias the AVO gradient and azimuthal anisotropy estimates toward the better-sampled inline azimuths.
• Crossline seismic attributes and reservoir characterization in WCSB 3D interpretation workflows: Crossline sections from WCSB 3D seismic volumes are used in reservoir characterization to map lateral reservoir continuity, identify stratigraphic terminations (onlap, truncation, pinch-out), and delineate geological boundaries that are difficult to image on inline sections due to the acquisition geometry; in WCSB Viking incised valley fills and Cardium shelf sand lobes, the crossline sections perpendicular to the valley or shoreline trend display the width and termination geometry of the sand bodies that control well drainage area and recovery factor. Seismic attribute volumes computed from WCSB 3D data (instantaneous amplitude, coherence, curvature, spectral decomposition frequency slices) are displayed on crossline sections and on attribute maps extracted along interpreted horizon surfaces; the coherence attribute in particular highlights discontinuities (faults, karst, fracture swarms) in WCSB Devonian carbonate reservoirs on crossline sections because coherence breaks appear as linear features that are easier to trace on perpendicular crossline sections than on inline sections parallel to the feature trend. In WCSB Montney and Duvernay shale plays, crossline time slices of azimuthal amplitude anisotropy reveal the dominant natural fracture strike direction by showing which crossline azimuth has the higher reflection amplitude (parallel to open fractures), informing horizontal well landing azimuth selection to maximize hydraulic fracture interaction with the natural fracture network.
• Crossline direction and receiver line orientation in WCSB simultaneous source acquisition: Modern WCSB 3D seismic programs increasingly use simultaneous source (SiSo) acquisition in which two or more vibroseis crews shoot independently encoded sweeps from different locations at the same time, with the encoded shot signals separated in processing to reconstruct the individual shot records; in SiSo acquisition, the crossline direction of source deployment (perpendicular to receiver lines) is designed to maximize the inline-crossline separation between simultaneous sources to minimize crosstalk leakage in the separation processing. WCSB SiSo programs with receiver lines oriented north-south (crossline direction east-west) deploy the two vibroseis fleets on alternating east-west receiver lines separated by 200 to 400 m in the crossline direction; this crossline source separation provides enough differential moveout in the crossline direction to allow the SiSo separation algorithm (iterative inversion in the crossline f-k domain) to distinguish the two source records with crosstalk energy below 40 dB of the primary reflection amplitude in WCSB shallow to intermediate depth programs (Cardium and Viking targets at 1,000 to 2,000 m).

Crossline Section Mapping WCSB Viking Incised Valley Width

A WCSB Viking pool development program in east-central Alberta used crossline sections from a 240 km2 3D seismic volume (25 by 25 m bins, 60-fold, inline direction northeast-southwest perpendicular to valley trend) to map the width of a Viking incised valley fill that had been poorly constrained from two 2D seismic lines oriented along the valley axis. The 2D inline sections showed the valley fill as a high-amplitude package but could not define its lateral extent in the crossline direction. Crossline sections through the 3D volume at 25 m crossline spacing revealed a valley fill width of 1,400 to 1,800 m (varying along the 14 km valley length visible in the survey), consistently narrower than the 2,500 m width estimated from the 2D geometry and well spacing. The corrected valley width reduced the volumetric OOIP estimate from 1.8 million m3 to 1.2 million m3, redirecting the infill well program from 12 planned wells to 8 wells optimally placed within the crossline-defined sand body boundaries, avoiding 4 wells that would have been drilled off the sand fairway based on the 2D interpretation.

Related Terms

Inline is the counterpart direction in 3D seismic; inlines run parallel to acquisition and perpendicular to structural strike in WCSB programs, displaying structural dip and reservoir thickness variation. Common midpoint (CMP) bins are the fundamental unit of the 3D seismic grid; each bin is identified by its (inline, crossline) coordinate pair and accumulates all source-receiver midpoints within its boundaries. Seismic attributes such as coherence and curvature displayed on WCSB crossline sections and horizon slices delineate faults, fractures, and stratigraphic boundaries invisible on amplitude sections. Trace interpolation fills crossline sampling gaps in WCSB 3D programs where coarse receiver line spacing leaves bins without traces; 5D interpolation reconstructs missing crossline offset-azimuth combinations before migration. Acquisition footprint in WCSB 3D programs is predominantly crossline-oriented from regular receiver line spacing perpendicular to inline; crossline fold regularization and interpolation reduce footprint before amplitude-based reservoir characterization.