common reflection point, Oil & Gas Glossary

What Is a Common Reflection Point?

Common reflection point (also called CRP or true reflection point) is the location in the subsurface where a seismic wave traveling down from a surface source reflects off a geological interface and travels back up to be recorded by a surface receiver. For flat, horizontal reflectors, the common reflection point coincides with the common midpoint (CMP) located halfway between the source and receiver at the surface. However, when reflectors dip, when lateral velocity variations exist, or when complex structures such as salt flanks redirect ray paths, the reflection point is displaced from the midpoint and can only be correctly positioned through seismic migration — the computational process that moves energy back to its true subsurface location.

Key Takeaways

For a dipping reflector, the CRP shifts updip from the CMP by a distance proportional to the reflector dip angle and the source-receiver offset.
Pre-stack depth migration (PSDM) is the standard method for converting CMP gathers into true CRP gathers in complex geology, with processing costs exceeding $500,000 per survey in some deep-water programs.
Amplitude versus offset (AVO) analysis requires true CRP gathers because incorrect reflection-point positioning contaminates the amplitude-offset relationship used for fluid and lithology prediction.
In sub-salt environments such as the Gulf of Mexico, ray-path bending through the salt body can displace CRPs by hundreds of meters from their apparent CMP positions.
Full-waveform inversion (FWI), adopted widely after 2010, refines the velocity model used to compute CRP positions, improving image focus and AVO reliability simultaneously.

CRP Geometry, Dipping Reflectors, and Migration

The conceptual simplicity of the common reflection point masks significant geometric complexity in real subsurface settings. For a flat reflector, each source-receiver pair at the surface reflects off a point directly below the midpoint between them, and stacking all traces in a CMP gather correctly reinforces the signal from that single subsurface location. This is the foundation of conventional CMP processing. But as soon as a reflector tilts by even a few degrees, the reflection point moves updip from the midpoint — a seismic ray traveling at a finite angle must bounce off a steeper surface that lies updip from where a horizontal surface would intersect the ray path. At high dips such as those encountered on the flanks of salt domes or in thrust belt settings, the displacement between CMP and CRP can reach hundreds of meters, rendering an unmigrated or incorrectly migrated stack misleading for both structural mapping and reservoir characterization.

Migration corrects this displacement by using the recorded wavefield and a model of subsurface velocity to compute where each reflection event actually originated. Time migration, the simpler algorithm, works well in areas of gentle dip and modest lateral velocity variation and can handle most conventional clastic plays adequately. Depth migration — either post-stack or, more powerfully, pre-stack depth migration — directly computes reflector positions in true depth using a detailed three-dimensional velocity model and is required wherever velocity varies significantly in the horizontal direction. The quality of depth migration depends critically on the accuracy of the velocity model, which is why velocity model building through tomographic updating and FWI has become the central challenge of modern seismic processing in complex basins.

Once pre-stack depth migration has been applied, the resulting gathers at each image point are ideally CRP gathers — collections of traces all reflecting from the same subsurface point at different angles of incidence. These angle or offset gathers are the input for pre-stack inversion and AVO analysis. If migration has been accurate, the amplitude on each trace varies with angle only because of changes in acoustic impedance and Poisson's ratio at the reflector — the physical quantities that distinguish gas-bearing sands from brine-filled sands, or carbonates from shales. Any residual displacement of reflection points due to imperfect migration corrupts the gather geometry and introduces spurious amplitude-versus-offset trends that can lead an interpreter to incorrectly predict reservoir fluid content, a costly mistake in exploration and appraisal drilling decisions.

Fast Facts: Common Reflection Point

Abbreviation: CRP (also TRP — true reflection point in some processing workflows)
Equivalence condition: CRP equals CMP only for flat, horizontal reflectors with laterally uniform velocity
Updip displacement formula: CRP offset from CMP = (offset/2) × sin(dip angle)
Migration algorithm for complex geology: pre-stack depth migration (PSDM) using 3D velocity model
Key application: AVO analysis and pre-stack inversion require true CRP gathers for valid fluid prediction
Sub-salt CRP error: can exceed 300 m in deep Gulf of Mexico settings without full-waveform inversion
Velocity model tool: tomographic inversion and FWI refine the model used to compute CRP positions
Industry adoption of PSDM: became routine in deep-water exploration programs during the late 1990s

Seismic Interpreter Tip:

Before running AVO analysis on a prospect, verify with the processing team that a true CRP gather — not an unmigrated CMP gather — was used as input. Ask specifically whether the migration velocity model was updated through tomography or FWI after initial model building. A gather labeled "migrated" in legacy processing reports may have used a smooth, unconverged velocity model that leaves significant residual moveout and CRP displacement, invalidating the AVO gradient calculation.

Common reflection point is also referred to as:

True reflection point (TRP) — processing term emphasizing that migration has correctly repositioned the energy versus an apparent or nominal midpoint location.
Image point — used in migration literature to describe the subsurface location where migrated energy is placed; equivalent to CRP after successful migration.
Reflection point gather — the collection of seismic traces associated with a single subsurface reflection point after pre-stack migration; the ideal input for AVO and inversion.
Depth image point — used specifically in pre-stack depth migration workflows to distinguish the depth-domain image location from the time-domain CMP location.

Frequently Asked Questions About Common Reflection Points

Why does AVO analysis specifically require CRP gathers rather than CMP gathers?

AVO analysis measures how reflection amplitude changes with the angle of incidence at a reflector interface, using the Zoeppritz equations or their linearized approximations to infer Poisson's ratio, shear impedance, and fluid content. This calculation is only valid if all traces in the gather actually reflect from the same point on the same interface at different angles. In a CMP gather over a dipping or structurally complex reflector, different traces reflect from different subsurface points at different depths, mixing amplitude information from geologically distinct locations. The resulting amplitude-offset trend is geologically meaningless. Migration that correctly collapses all energy to a true CRP is therefore a prerequisite, not an option, for defensible fluid prediction from seismic data.

How does lateral velocity variation cause CRP displacement?

When seismic velocity changes horizontally — for example, as a ray passes from a slow shale into a fast carbonate body — Snell's law causes the ray to bend at the velocity interface, redirecting its path in the subsurface. The reflection point of the bent ray is no longer where a straight-ray approximation (the assumption underlying CMP processing) would predict. In salt basins, the very high velocity of halite (approximately 14,700 ft/s versus 5,000 to 8,000 ft/s for surrounding sediments) bends rays dramatically both on the way down and on the way back up, displacing CRPs by amounts that can make a flat-lying sub-salt reservoir appear to dip steeply on an unmigrated section. Accurate 3D velocity model building that explicitly represents the salt geometry is the only remedy for this problem.

What is the practical difference between time migration and depth migration for CRP accuracy?

Time migration assumes that lateral velocity variations are mild enough that reflector positioning can be adequately solved in two-pass, time-domain computations. It is faster, less expensive, and sufficient for simple, gently dipping structures in areas of uniform velocity. Depth migration solves the wave equation in full three-dimensional depth space using an explicit velocity model and correctly handles all ray-bending effects at velocity interfaces. The CRP positions output by depth migration are physically correct in depth provided the velocity model is accurate; time migration only approximates CRP positions and can produce significant structural distortion in complex areas. The deciding criterion is geologic: wherever the overburden contains bodies that create strong lateral velocity contrasts — salt, carbonates interbedded with shales, overpressured zones — depth migration is required for defensible CRP accuracy.

Why Common Reflection Points Matter in Oil and Gas

Accurate knowledge of where seismic energy actually reflects in the subsurface is the foundation of every structural interpretation and reservoir characterization workflow in the exploration and development cycle. Incorrectly positioned reflection points cause false structural closures to appear on seismic maps, lead drilling locations to be offset from the actual crest of a trap, and produce erroneous AVO anomalies that generate false confidence in hydrocarbon presence. In deepwater basins where individual exploration wells cost $100 million or more, the investment in high-quality pre-stack depth migration that correctly positions CRPs is marginal compared to the cost of a dry hole drilled on a phantom structure. For development programs, accurate CRP positioning enables reliable time-to-depth conversion, correct fault geometry, and valid pre-stack inversion for reservoir property mapping — all critical inputs to the well spacing and completion design decisions that determine field economics.

Common Reflection Point (CRP): Seismic Migration and Subsurface Imaging