# CMP sorting

Series Investigations in Geophysics Öz Yilmaz http://dx.doi.org/10.1190/1.9781560801580 ISBN 978-1-56080-094-1 SEG Online Store

Seismic data acquisition with multifold coverage is done in shot-receiver (s, g) coordinates. Figure 1.5-8a is a schematic depiction of the recording geometry and ray paths associated with a flat reflector. Seismic data processing, on the other hand, conventionally is done in midpoint-offset (y, h) coordinates. The required coordinate transformation is achieved by sorting the data into CMP gathers. Based on the field geometry information, each individual trace is assigned to the midpoint between the shot and receiver locations associated with that trace. Those traces with the same midpoint location are grouped together, making up a CMP gather. Albeit incorrectly, the term common depth point (CDP) and common midpoint (CMP) often are used interchangeably.

Figure 1.5-8b depicts the geometry of a CMP gather and raypaths associated with a flat reflector. Note that CDP gather is equivalent to a CMP gather only when reflectors are horizontal and velocities do not vary horizontally. However, when there are dipping reflectors in the subsurface, these two gathers are not equivalent and only the term CMP gather should be used. Selected CMP gathers obtained from sorting the deconvolved shot gathers (Figure 1.5-7) are shown in Figure 1.5-9.

Figure 1.5-10 shows the superposition of shot-receiver (s, g) and midpoint-offset (y, h) coordinates, and raypath geometries for various gather types. The (y, h) coordinates have been rotated 45 degrees relative to the (s, g) coordinates. The dotted area represents the coverage used in recording the seismic profile along the midpoint axis, Oy. Each dot represents a seismic trace with the time axis perpendicular to the plane of paper. The following gather types are identified in Figure 1.5-10:

1. Common-shot gather (shot record, field record),
3. Common-midpoint gather (CMP gather, CDP gather),
4. Common-offset section (constant-offset section),
5. CMP-stacked section (zero-offset section).

The recording cable length is FG and the line length is AD. The number of dots along the offset axis (cross-section 3) is equal to the CMP fold. The fold tapers off at the ends of the profile (segments AB and CD). Full-fold coverage along the line is at midpoints over segment BC. The diagram in Figure 1.5-10 is known as a stacking chart and is useful when setting up the geometry of a line for preprocessing. If there is a missing shot or a bad receiver, the affected midpoints are identified easily (Exercise 1-15).

For most recording geometries, the fold of coverage nf for CMP stacking is given by

 ${\displaystyle {n_{f}}={\frac {{n_{g}}\Delta g}{2\Delta s}},}$ (12)

where Δg and Δs are the receiver-group and shot intervals, respectively, and ng is the number of recording channels. By using this relationship, the following rules can be established:

1. The fold does not change when alternating traces in each shot record are dropped.
2. The fold is halved when every other shot record is skipped, whether or not alternating traces in each record are dropped.

## References

1. Claerbout, 1976, Claerbout, J. F., 1976, Fundamentals of geophysical data processing: McGraw-Hill Book Co.