CMP

Common midpoint (CMP) is the fundamental acquisition geometry principle underlying modern multichannel reflection seismic surveys, in which the midpoint on the surface between a seismic source and a receiver is shared by multiple source-receiver pairs with different offsets (source-to-receiver distances), so that each CMP location receives energy reflected from the same subsurface reflection point on a horizontal reflector, allowing the recorded traces from all source-receiver pairs sharing that midpoint to be gathered into a CMP gather for velocity analysis, normal moveout (NMO) correction, and stacking to enhance signal-to-noise ratio and suppress random noise by a factor equal to the square root of the fold (the number of traces in the gather); in Western Canada Sedimentary Basin seismic exploration and reservoir characterization programs, CMP acquisition is the universal standard for 2D seismic lines shot along WCSB exploration corridors across the Alberta Plains, Saskatchewan Williston Basin, and Foothills thrust belt, and for the 3D seismic programs that cover WCSB Cardium, Viking, Montney, and Duvernay development areas with spatial CMP coverage sufficient for reservoir-scale structural and stratigraphic interpretation. The fold of a CMP gather, defined as the number of independent source-receiver pairs whose midpoints coincide at that location, is controlled by the receiver group interval, shot interval, and number of active receiver channels in the recording spread; in standard WCSB 2D seismic programs, fold of 60 to 120 is achieved by deploying receiver arrays at 25 to 50 m intervals over a 3 to 6 km active spread length and shooting at half the receiver interval (12.5 to 25 m shot point spacing), so that each CMP bin accumulates one additional trace for every shot; in WCSB 3D programs, CMP bins are typically 12.5 by 12.5 m to 25 by 25 m in plan view, with fold of 30 to 80 achieved by crossline receiver lines spaced 200 to 400 m apart and inline shot lines spaced 200 to 400 m apart in the orthogonal acquisition geometry standard for WCSB Cardium and Viking development programs. The CMP stacking process assumes that after NMO correction (which flattens the hyperbolic moveout of reflection arrival times across offset), all traces in the gather represent reflections from the same subsurface point and can be summed to increase the signal-to-noise ratio relative to any single-fold trace; this assumption is valid for horizontal or gently dipping reflectors and short offsets, but breaks down in WCSB Foothills thrust structures with steep dips and strong lateral velocity variation where the reflecting point migrates updip from the surface midpoint location and CMP stacking requires dip moveout (DMO) correction before stacking or pre-stack depth migration (PSDM) to correctly position the reflection.

• CMP gather velocity analysis and NMO correction in WCSB 2D and 3D seismic processing: Velocity analysis in WCSB seismic processing uses the offset-dependent moveout of reflections in the CMP gather to estimate the stacking velocity at each reflector: for a horizontal reflector at two-way time T0, the reflection from a source-receiver pair at offset X arrives at time T = sqrt(T0 squared plus X squared divided by V squared), where V is the root-mean-square (RMS) velocity to the reflector; semblance analysis computes the normalized coherence of the CMP gather as a function of T0 and velocity, and the velocity producing maximum coherence at each T0 is the stacking velocity. In WCSB Plains seismic programs, velocity analysis is performed every 500 m to 2 km along a 2D line (every 10 to 40 CMP bins) where lateral velocity variation from Quaternary glacial sediments and WCSB Cretaceous shale sequences is gradual; in WCSB Foothills programs, velocity analysis must be performed every 100 to 200 m because the allochthonous thrust sheets create abrupt lateral velocity contrasts between high-velocity Paleozoic carbonates and low-velocity Mesozoic clastics that cannot be interpolated over longer distances without introducing NMO correction errors that degrade stack quality.
• CMP fold design and acquisition parameter optimization for WCSB 3D seismic programs: CMP fold in WCSB 3D seismic programs is designed to meet the noise attenuation requirement for the target reflectors: the minimum fold needed to achieve a given signal-to-noise ratio improvement equals the square of the ratio of desired SNR to single-fold SNR, so a WCSB program requiring 6 dB improvement over single-fold needs a fold of 4, while 12 dB improvement requires fold of 16, and practical WCSB Cardium and Viking 3D programs use fold of 40 to 80 to achieve adequate noise suppression for thin-bed reservoir mapping. Bin size (the CMP bin dimensions, which equal half the receiver line spacing by half the shot line spacing in orthogonal acquisition) controls the spatial resolution of the 3D seismic volume: a 12.5 by 12.5 m bin resolves lateral features above approximately 12.5 m horizontal scale (two bins minimum for aliasing-free imaging), suitable for resolving WCSB Cardium reservoir fairway edges and Viking sand body geometries at 1,200 to 2,000 m depth. Larger WCSB 3D programs covering 100 to 400 km2 of a Cardium or Montney development area achieve economies of scale in acquisition cost by using longer receiver lines (12 to 18 km) and wider crossline spacing (400 to 600 m), at the expense of reduced fold in the outer portions of the acquisition patch (near-edge fold tapering) that limits the usable seismic volume to an interior area excluding the 20 to 30 percent of area near acquisition boundaries where fold does not meet target.
• Pre-stack CMP analysis for AVO and amplitude versus offset studies in WCSB gas plays: The offset variation of reflection amplitude within a CMP gather carries information about the elastic contrast between the reflector and the surrounding formation that is not preserved after CMP stacking; amplitude versus offset (AVO) analysis retains the pre-stack CMP gather and examines how the reflection amplitude changes from near-offset to far-offset traces to identify gas sands, fluid contacts, and lithological variations in WCSB exploration targets. In WCSB Cretaceous gas sands (Medicine Hat, Horseshoe Canyon, Viking, Cardium), AVO Class II and Class III anomalies (amplitude increasing with offset on the far traces) indicate low-impedance gas sands above shale baselines, and pre-stack CMP gathers from WCSB 2D and 3D programs are routinely analyzed for AVO before drilling to assess whether amplitude anomalies on the stacked section reflect gas saturation or lithological variation alone. AVO analysis requires careful pre-stack processing of the CMP gathers: geometric spreading correction, surface-consistent amplitude balancing, and multiple suppression must be applied before AVO gradient estimation to prevent processing artifacts from masking or mimicking AVO signatures in the WCSB exploration target intervals.
• CMP binning and irregular geometry challenges in WCSB land seismic acquisition: Land 3D seismic programs in WCSB development areas must adapt the ideal orthogonal acquisition geometry to surface obstacles (roads, pipelines, wellheads, farmsteads, lakes, and muskeg) that prevent regular source and receiver placement, producing irregular CMP bin fold and offset-azimuth distribution that degrades the uniformity of the stacked volume. In WCSB Peace River Plains and WCSB Foothills programs, road density limits shot access to existing road allowances (spacing 1.6 km in Alberta's survey grid), forcing oblique or non-orthogonal acquisition geometries with skewed CMP bin illumination; regularization processing (trace interpolation and bin regularization using Fourier or anti-leakage methods) is applied after CMP binning to restore uniform fold and offset-azimuth distribution before velocity analysis and stacking. The WCSB practice of simultaneous source acquisition (vibroseis crews using encoded sweeps to shoot simultaneously from multiple source points and separate the signals in processing) increases acquisition efficiency by a factor of 2 to 4, but requires careful CMP bin crosstalk suppression to prevent energy from one simultaneous source from contaminating the CMP gathers intended for the other source position.
• CMP versus common reflection point and depth imaging in WCSB Foothills and Duvernay programs: The CMP designation is strictly accurate only for horizontal reflectors where the reflection point on the subsurface interface lies directly below the surface midpoint; for dipping reflectors in WCSB Foothills thrust structures and for refracted ray paths in WCSB formations with strong vertical velocity gradients, the actual reflection point migrates updip from the surface CMP location and the gather is more accurately termed a common reflection point (CRP) gather after accounting for ray-path bending. WCSB Foothills seismic processing uses pre-stack depth migration (PSDM) rather than conventional CMP NMO-stack to handle the combination of steep dip, strong lateral velocity contrasts between thrust sheets, and long offsets needed for deep Paleozoic target imaging below the Lewis thrust; PSDM collapses diffraction energy and migrates dipping reflectors to their true subsurface positions using a three-dimensional velocity model derived from tomographic inversion of the CMP gather moveout, producing a common image point (CIP) gather in depth rather than a CMP gather in time. The transition from CMP time-domain processing (adequate for WCSB Plains programs) to PSDM depth imaging (required for WCSB Foothills and some Montney programs with strong overburden velocity heterogeneity) approximately doubles the processing cost and increases the interpreter skill requirement for building and iteratively updating the subsurface velocity model.

CMP 3D Acquisition Improving WCSB Viking Reservoir Definition

A WCSB Viking operator in central Alberta acquired a 180 km2 3D seismic program over a producing pool to improve reservoir definition before a 12-well infill drilling program. Acquisition parameters: 12.5 by 12.5 m CMP bins, receiver line spacing 300 m, shot line spacing 300 m, target fold 60, maximum offset 3,000 m. Processing included surface-consistent deconvolution, NMO with velocity analysis every 250 m, DMO, and post-stack time migration. The 3D Viking horizon structure map at 1,180 m depth revealed two previously unmapped 40-hectare closures in the eastern pool area that were not visible on the prior 2D seismic grid (6 km line spacing). AVO analysis of the pre-stack CMP gathers across both closures showed Class III anomalies consistent with gas-charged Viking sand; two wells drilled on these closures confirmed producible gas at 25,000 and 31,000 m3/d per well. The 3D CMP coverage also identified two previously producing areas with no AVO anomaly, which were removed from the infill drilling schedule, saving two dry holes estimated at $2.1 million each.

Related Terms

Seismic reflection surveys in WCSB exploration use CMP geometry to accumulate multiple offset traces at each surface midpoint, enabling NMO correction and stacking that improves the signal-to-noise ratio for detecting WCSB Cardium, Viking, and Montney reflectors. Normal moveout (NMO) correction flattens the hyperbolic offset-dependent arrival time of reflections in the CMP gather before stacking; stacking velocity estimated from CMP semblance analysis is the primary input. Seismic processing of WCSB CMP data includes velocity analysis, NMO correction, DMO for dipping reflectors, stacking, and post-stack or pre-stack migration to produce the final interpreted seismic volume. Amplitude versus offset (AVO) analysis uses pre-stack CMP gathers from WCSB 2D and 3D surveys to identify gas sands through offset-dependent amplitude variation before drilling. Pre-stack depth migration (PSDM) replaces conventional CMP NMO-stack in WCSB Foothills and structurally complex Montney programs where dipping reflectors and lateral velocity contrasts require depth-domain imaging rather than time-domain CMP stacking.