CDP

Common depth point (CDP), also called common midpoint (CMP) in modern reflection seismic acquisition terminology, is the geometric point on the subsurface reflector that is equidistant from multiple source-receiver pairs in a seismic survey, such that each source-receiver offset pair in a CDP gather reflects energy from the same subsurface point despite originating from different surface positions, enabling the seismic processor to stack (sum) these multiple traces to produce a composite trace with greatly improved signal-to-noise ratio and to apply normal moveout (NMO) velocity analysis to determine the seismic velocity field that is used to convert reflection arrival times to depth for Western Canada Sedimentary Basin structural interpretation, reservoir mapping, and horizontal well trajectory planning. The fundamental value of the CDP method over single-coverage seismic acquisition is redundancy: in a CDP survey with a fold of 60 (60 source-receiver pairs contributing to each CDP gather), random noise averages down by the square root of the fold (factor of 7.7), dramatically improving the ability to detect weak reflections from thin WCSB reservoir intervals such as the Montney siltstone (10 to 80 m thick), Cardium sandstone (1 to 15 m thick), or Duvernay shale (5 to 40 m thick) that are below the single-shot detection threshold in the presence of near-surface noise. The CDP geometry is achieved in 2D seismic acquisition by recording a linear shot-receiver array where each shot activates a spread of receivers, and the midpoint of each source-receiver pair is a CDP location; the CDP spacing equals half the receiver group interval, so a 10 m receiver group interval produces a CDP grid with 5 m spacing along the seismic line. In 3D seismic acquisition (the standard for WCSB horizontal well programs since the mid-1990s), CDP locations are binned into a regular areal grid (typically 12.5 m x 12.5 m or 25 m x 25 m for WCSB Montney and Duvernay programs) that each accumulates multiple source-receiver pairs from different azimuths, providing both the fold required for stacking and the azimuthal coverage required for anisotropy analysis that identifies natural fracture orientations relevant to WCSB hydraulic fracturing design. The CDP stack velocity (the root-mean-square velocity at each CDP location and each two-way reflection time, determined by the best NMO correction) is the primary seismic deliverable from WCSB 3D surveys alongside the stacked amplitude volume, and it is used by WCSB geophysicists and exploration geologists to build the velocity model that converts the seismic time domain into the depth domain for drilling target location, fault mapping, and reservoir thickness estimation in Montney, Duvernay, Viking, Cardium, and Devonian reef play fairways across Alberta and British Columbia. Understanding CDP geometry, the fold concept and its signal-to-noise improvement relationship, the NMO velocity analysis workflow applied to CDP gathers, the 3D CDP bin size selection criteria for WCSB horizontal well imaging requirements, and the depth conversion process that delivers the subsurface maps used for WCSB exploration targeting gives WCSB exploration geophysicists, seismic processors, geological interpreters, and drilling engineers the seismic data acquisition and processing foundation to design surveys, evaluate seismic data quality, and use stacked and migrated CDP volumes for accurate WCSB well location and reservoir characterization.

• CDP fold and signal-to-noise ratio in WCSB seismic survey design: The fold of a CDP gather is the number of independent source-receiver pairs that contribute a reflection from the same subsurface point; signal stacks coherently (amplitude increases proportionally to fold) while random noise stacks incoherently (amplitude increases as the square root of fold), so the signal-to-noise ratio improvement equals the square root of the fold. WCSB 3D seismic surveys for Montney and Duvernay horizontal well programs specify minimum fold of 60 to 120 per bin, which provides signal-to-noise improvement factors of 7.7 to 11 over single-coverage data; this improvement is necessary to image the thin (5 to 40 m), low-acoustic-impedance-contrast Montney and Duvernay reflectors against the ambient noise from agricultural surface sources, coal mine blasting, and pipeline compression stations in northeast British Columbia and west-central Alberta acquisition environments.
• NMO correction and velocity analysis on WCSB CDP gathers: Normal moveout (NMO) is the increase in two-way travel time (TWT) of a reflection from a flat reflector as the source-receiver offset increases, described by the hyperbolic NMO equation: TWT(offset) = sqrt(TWT(zero-offset)^2 + offset^2 / Vrms^2), where Vrms is the root-mean-square seismic velocity to the reflector. The seismic processor applies NMO correction to each CDP gather by testing a series of Vrms values and selecting the velocity that best flattens the hyperbolic moveout curve (aligning all offset traces so they can be stacked constructively); the resulting velocity function versus TWT at each CDP is the interval velocity model used for WCSB depth conversion. WCSB interval velocities from NMO analysis range from 1,800 to 2,200 m/s in shallow Cretaceous sections to 4,500 to 5,500 m/s in Devonian carbonates, with velocity inversions in overpressured zones (Montney, Duvernay) providing a pore pressure indicator used in WCSB pre-drill pressure prediction.
• 3D CDP bin size selection for WCSB horizontal well reservoir imaging: The CDP bin size in a 3D seismic survey determines the spatial resolution of the subsurface image and must be matched to the structural wavelength and reservoir thickness of the WCSB target. The Nyquist spatial sampling theorem requires a bin size no larger than half the minimum seismic wavelength to be imaged; for a dominant seismic frequency of 50 Hz at a Montney interval velocity of 4,000 m/s, the dominant wavelength is 80 m and the minimum bin size for alias-free imaging is 40 m, but standard WCSB Montney programs use 12.5 to 25 m bins to image the thin reservoir intervals and sub-seismic faults that affect hydraulic fracture placement. Smaller bin sizes require proportionally denser shot and receiver spacing, increasing acquisition cost; the economic optimum for WCSB Montney surveys is typically 12.5 m x 12.5 m bins at $1,200 to $2,500 per km2 acquisition cost.
• CDP azimuthal anisotropy analysis for WCSB fracture characterization: In WCSB Montney and Duvernay multi-azimuth 3D surveys, CDP gathers are sorted by source-receiver azimuth into azimuthal sectors (typically 6 to 8 sectors at 30 to 45 degree increments) and the NMO velocity is analyzed separately in each azimuthal sector. Azimuthal NMO velocity variation (elliptical anisotropy) indicates the presence of aligned fractures or stress-induced anisotropy: the fast velocity direction (minimum NMO moveout) corresponds to the fracture strike direction or maximum horizontal stress direction, with velocity anisotropy of 2 to 8% indicating significant fracture alignment. WCSB operators use azimuthal velocity anisotropy maps from CDP analysis to orient horizontal wells perpendicular to the maximum horizontal stress direction and to design hydraulic fracture stage spacing that avoids communication between natural fracture sets.
• Pre-stack depth migration of CDP gathers for WCSB Foothills structural imaging: In the WCSB Foothills thrust belt where steeply dipping formations and velocity contrasts across thrust faults violate the flat-layer NMO assumption, pre-stack depth migration (PSDM) of CDP gathers replaces the conventional NMO-stack workflow by migrating each CDP trace individually to its true subsurface position before summation, using a full velocity model derived from tomographic inversion of the CDP gather residuals. WCSB Foothills 3D PSDM surveys resolve the structural geometry of thrust-bounded traps in the Turner Valley, Waterton, and Jumping Pound areas that conventional NMO-stack images render as multiples and diffraction noise, providing the accurate fault geometry and reservoir depth maps needed to design directional wells into thrust sheet reservoirs that may be displaced 1 to 5 km horizontally from their surface projection.

CDP Velocity Analysis Revealing Overpressure Anomaly in a WCSB Montney Exploration Well

A WCSB exploration company evaluating a Montney prospect in the Gold Creek area of Alberta used interval velocity analysis of CDP gathers from a 2022 3D seismic survey to predict pore pressure before drilling the first exploration well. The Vrms velocity at the Montney Two-way-travel-time horizon showed a velocity reversal of 8% (Vrms dropping from 3,850 m/s at Doig level to 3,540 m/s at Montney level) compared to offset well sonic log compaction trends that predicted 4,050 m/s Vrms at Montney depth. Applying the Eaton pore pressure prediction method with a normal compaction Vrms of 4,050 m/s and observed Vrms of 3,540 m/s gave a predicted pore pressure gradient of 1.62 g/cc EMW, significantly above the regional normal gradient of 1.15 g/cc. The exploration well was drilled with an 11.8 ppg (1.41 g/cc) mud weight to Doig depth and increased to 13.5 ppg (1.62 g/cc) before entering the Montney; the well encountered pore pressure of 1.58 g/cc EMW at Montney top, confirming the CDP velocity-based overpressure prediction within 2.5% and avoiding the well control event that would have occurred if the pre-drill overpressure had not been identified from the CDP velocity analysis.

Related Terms

Seismic reflection is the wave propagation phenomenon that CDP surveys are designed to record, with reflected energy returning from acoustic impedance contrasts at formation boundaries in the WCSB subsurface; the CDP geometry ensures that multiple independent recordings of each reflection are available for noise cancellation through stacking and velocity determination through NMO analysis. Normal moveout is the offset-dependent travel time increase on CDP gathers that encodes the seismic velocity of each formation layer; NMO velocity analysis on WCSB CDP gathers produces the interval velocity model used for depth conversion, pore pressure prediction, and anisotropy characterization that together define the pre-drill geological model for WCSB horizontal well planning. Seismic stack is the summed trace produced by adding all NMO-corrected CDP gather traces at a common subsurface point, producing the stacked seismic volume that WCSB interpreters use for structural and stratigraphic mapping of Montney, Duvernay, Viking, and Cardium reservoir intervals. Depth conversion transforms the WCSB seismic time domain to the depth domain using the velocity field derived from CDP NMO analysis, with the accuracy of the depth conversion directly affecting the TVD error in the prognosed formation tops that determine WCSB horizontal well landing depths and reservoir entry points. Pre-stack depth migration is the advanced seismic processing technique applied to WCSB Foothills CDP gathers where thrust fault complexity and velocity anisotropy invalidate conventional NMO-stack imaging, migrating each CDP trace individually to its true subsurface position using a full-field velocity model before summation to produce accurate structural images of WCSB Foothills thrust belt reservoir traps.