Common Depth Point: The Foundation of Multi-Fold Seismic Acquisition

What Is a Common Depth Point?

Common depth point (also called CDP, common midpoint, or CMP) is the subsurface reflection point shared by multiple seismic source-receiver pairs that have different offsets but the same surface midpoint location. In a CDP gather, all traces recorded from shots and receivers equidistant from the same surface midpoint are assembled together, enabling velocity analysis and normal moveout (NMO) correction before stacking the traces to produce a single high-signal-to-noise seismic trace at that midpoint location. The technique, introduced by W. Harry Mayne in 1962, transformed seismic exploration by allowing weak reflections to be enhanced through redundant sampling of the same subsurface point from many source-receiver geometries.

How Common Depth Point Geometry Works

The geometry of CDP acquisition begins with the observation that any seismic source fired at a distance X from a receiver will share its subsurface reflection point with a different source-receiver pair whose midpoint is at the same surface location but whose offset is different. For a flat horizontal reflector, this midpoint is directly above the reflection point at a depth equal to the two-way travel time multiplied by half the average velocity. By collecting all these traces into a gather sorted by common midpoint rather than by common shot, the geophysicist assembles a set of traces that collectively sample the same portion of the reflector from near offset (source and receiver close together, nearly vertical ray paths) to far offset (widely separated source and receiver, steeply inclined ray paths).

The practical benefit of this geometry is the ability to perform velocity analysis and NMO correction on the CDP gather before stacking. Because travel time increases with offset following a hyperbolic moveout curve, the correct stacking velocity can be determined by testing which velocity value flattens the reflection events across the gather. Once NMO-corrected, all traces in the gather have their reflections aligned at the same two-way time, and summing (stacking) them constructively reinforces signal while destructively interfering with random noise. The improvement in signal-to-noise ratio is proportional to the square root of the number of traces stacked, which is the CDP fold. A 60-fold gather produces approximately 7.7 times better S/N than a single trace, transforming borderline-quality data into interpretable seismic sections.

The transition from the CDP label to CMP reflected a more precise understanding of the geometry in the presence of dipping reflectors. When a reflector dips, the actual reflection point for a given source-receiver pair no longer lies directly below the surface midpoint; it migrates in the updip direction. The amount of migration depends on reflector dip and the offset of the source-receiver pair, meaning that traces in a so-called CDP gather from a dipping bed are actually sampling slightly different subsurface points. This smearing effect reduces the effectiveness of stacking and introduces a dip-dependent error in velocity analysis. Migration, performed after stacking or before stacking in pre-stack depth migration workflows, repositions the reflections to their true subsurface locations and restores the full resolution of the acquisition geometry.

Common Depth Point Synonyms and Related Terminology

Common depth point is also referred to as:

• CMP (common midpoint) — the technically preferred modern term, used interchangeably with CDP in most processing workflows; more accurate for dipping reflectors.
• Common reflection point (CRP) — used in pre-stack depth migration to describe the actual subsurface point where energy reflects, accounting for dip and lateral velocity variation.
• CDP gather — the collection of all seismic traces sharing a common midpoint, assembled for velocity analysis and NMO correction before stacking.
• CDP fold — the count of independent traces contributing to each midpoint bin; a direct measure of subsurface coverage density.

Related terms: normal moveout, seismic reflection, velocity analysis, migration, stacking

Frequently Asked Questions About Common Depth Points

What is the difference between CDP fold and spatial resolution in seismic surveys?

CDP fold and spatial resolution are independent attributes of seismic acquisition geometry that are often confused. Fold governs signal-to-noise ratio: more fold means more redundant traces stacked, reducing random noise relative to coherent reflections. Spatial resolution governs the minimum size of subsurface feature that can be detected or delineated, and is controlled primarily by CDP bin size (smaller bins resolve finer features) and the dominant frequency of the seismic wavelet (higher frequency = shorter wavelength = finer resolution). It is possible to acquire a high-fold, low-resolution survey (large bins, many traces per bin) or a low-fold, high-resolution survey (small bins, few traces per bin). Optimal survey design balances both parameters against the imaging objectives and available budget.

Why do marine surveys sometimes have lower effective fold than onshore surveys despite more receivers?

Marine streamer surveys use long cables (3 to 12 km) towed behind the vessel, which sounds like it should produce very high fold. However, ocean currents cause the streamers to feather laterally away from the planned sail line, displacing the midpoints from their nominal positions. When midpoints scatter outside their target bins, the effective fold in each bin drops and the regular geometry needed for optimal stacking is disturbed. Wide-azimuth marine acquisition and ocean-bottom cable surveys partially address this by using multiple vessels or fixed receivers, but the feathering problem means that nominal fold from vessel geometry often overstates actual fold in the processed data, particularly in areas with strong currents such as the North Sea or offshore West Africa.

How does the CDP concept apply to 3D seismic surveys?

In 3D seismic acquisition, the CDP concept is extended to two horizontal dimensions so that midpoints are sorted into rectangular bins defined by inline and crossline coordinates rather than a single line of midpoints. Each bin collects traces from shots and receivers in a range of azimuths and offsets, enabling the survey to image structures in three dimensions. The azimuthal diversity of traces within each bin also allows azimuthal anisotropy analysis for fracture characterization, where the variation in seismic amplitude or velocity with azimuth reveals the orientation and intensity of natural fracture systems in the reservoir. Full-azimuth and wide-azimuth 3D surveys designed to maximize azimuthal sampling have become standard in fractured carbonate and unconventional reservoir development.

Why Common Depth Points Matter in Oil and Gas

The common depth point method is the single most important innovation in reflection seismology, enabling the oil and gas industry to image hydrocarbon traps beneath thousands of meters of complex overburden with the signal-to-noise quality required for economic decision-making. Before CDP stacking, single-fold recording could not distinguish weak reflections from subsurface reservoirs from ambient noise, limiting exploration to simple, shallow structures. Multi-fold CDP acquisition transformed seismic from a qualitative reconnaissance tool into a quantitative reservoir characterization technology, making possible the discovery of giant deepwater fields, sub-salt prospects in the Gulf of Mexico, and complex thrust-belt traps in the Andes and Zagros that would have been invisible to earlier methods. Every modern 2D and 3D seismic survey relies on CDP geometry as the fundamental organizing principle of data acquisition, processing, and interpretation.