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Common Reflection Points

Multiple shots and receivers are used in a reflection seismic profiling specifically so that some subsurface points are sampled more than once. Ultimately the goal is to identify all the reflections due to that point on the various seismograms and stack these to get an enhanced signal to noise ratio. The idea is illustrated below left.

The collection of seismic traces that correspond to a particular midpoint is called a Common Midpoint (CMP) gather. In older literature, this collection of traces was referred to as a Common Depth Point (CDP) gather. That is not strictly correct as the diagram below right illustrates.

From Kearey, Philip

A series of six shots and associated receivers that would have reflections from a common point. When the layers are plane and horizontal then this common reflection point lies midway between the source and receiver.

From Kearey, Philip

Common depth point (CDP) reflection profiling. (a) A set of rays from different shots to detectors are reflected off a common point on a horizontal reflector. (b) The common depth point is not achieved in the case of a dipping reflector.

From Kearey, Philip and Micheal Brooks, An Introduction to Geophysical Exploration. 2nd ed. Blackwell Science: 1991.