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:
- Common-shot gather (shot record, field record),
- Common-receiver gather,
- Common-midpoint gather (CMP gather, CDP gather),
- Common-offset section (constant-offset section),
- 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).
Figure 1.5-8 (a) Seismic data acquisition is done in shot-receiver (s, g) coordinates. The raypaths are associated with a planar horizontal reflector from a shot point (indicated by the solid circles) to several receiver locations (indicated by the triangles). The processing coordinates, midpoint-(half) offset, (y, h) are defined in terms of (s, g): y = (g + s)/2, h = (g − s)/2. The shot axis here points opposite the profiling direction, which is to the left. On a flat reflector, the subsurface is sampled by reflection points which span a length that is equal to half the cable length. (b) Seismic data processing is done in midpoint-offset (y, h) coordinates. The raypaths are associated with a single CMP gather at midpoint location M. A CMP gather is identical to a CDP gather if the depth point were on a horizontally flat reflector and if the medium above were horizontally layered.
Figure 1.5-10 (a) A hypothetical stacking chart (adapted from ). Each dot represents a single trace with the time axis perpendicular to the plane of the page. Shot-geophone (s, g), and midpoint-offset (y, h) coordinates are superimposed with the (y, h) plane rotated 45 degrees with respect to the (s, g) plane. Here, (1) is a common-shot gather, (2) is a common-receiver gather, (3) is a CMP gather, (4) is a common-offset section, and (5) is a CMP-stacked section. The remaining notation is defined in the text. (b) Raypaths associated with (1) a common-shot gather, (2) a common-receiver gather, (3) a CMP gather, and (4) a common-offset section. Solid triangles denote receiver locations and solid circles denote shot locations, x is the effective cable length — the difference between the maximum and minimum offsets, E denotes a midpoint and E’ denotes a depth point on a flat reflector.
For most recording geometries, the fold of coverage nf for CMP stacking is given by
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:
- The fold does not change when alternating traces in each shot record are dropped.
- The fold is halved when every other shot record is skipped, whether or not alternating traces in each record are dropped.
- Claerbout, 1976, Claerbout, J. F., 1976, Fundamentals of geophysical data processing: McGraw-Hill Book Co.
- Velocity analysis
- Normal-moveout correction
- Multiple attenuation
- Dip-moveout correction
- CMP stacking
- Poststack processing
- Residual statics corrections
- Quality control in processing
- Parsimony in processing