Common Midpoint Gathers

The CSG data are reorganized so that they are in the form of common midpoint gathers, where any trace in a CMG has the same source-receiver midpoint as any other trace in the CMG.

A CMG collected along the Oquirrh Fault, Utah is shown in Figure 1.9. Here the midpoint x_m and half-offset x_h coordinates in a CMG are related to the source x_s and geophone x_g coordinates in a CSG by

x_m= (x_g+x_s)/2 ;  x_h=(x_g-x_s)/2 . (1.2)

The beauty of the CMG is that each trace contains reflection energy that sampled the same part of the reflector as the other traces in the CMG. As an example, the R3 reflection events along the R3 hyperbola in Figure  all emanated from apex of deepest raypath shown in this figure. This redundancy will be exploited (as discussed later) by stacking these redundant reflections together to increase the S/N ratio of the seismic record. In contrast, the R2 reflections in the CSG in Figure 1.8 emanate from different parts of this reflector and so do not redundantly sample the R2 reflector.

  

Figure 1.9: Similar to previous figure, except 1). traces in CSG's are reorganized into a CMG, and 2). the above data were collected along Oquirrh Fault, Utah. Note, each ray shown above is connected with a source-receiver pair, and all such pairs share the common midpoint location denoted by the filled square box. The thick hyperbolic lines describe the moveout curves for primary reflections.

Figure 1.10 depicts the rays and traces associated with CSG's and CMG's, and the html movie shows how these data were created. Note, all of the common midpoint (CMP) rays in the second to the bottom graph in Figure 1.10 emanate from the same reflection point.

The number of traces in a CMP gather defines the foldof the data. For example, the total number of traces in Figure 1.9 define the fold number. Large fold data means we have redundantly sampled a subsurface reflection point many times, so that after stacking (explained below) we will most likely have a stacked trace with a good S/N ratio. From the html movie, note the fold of a CMG decreases as the midpoint position approaches the end of the recording aperture.

  

Figure 1.10: (Top 2 graphs) CSG's and associated rays. (Bottom 2 graphs) CMG seismograms and rays associated with a common midpoint in the 2-layer model. Check out the html movie to see how the CSG's were generated and reorganized into CMG's.

The next step in order to realize the goal of ideal ZO data is to apply time shifts (i.e., Normal Moveout (NMO) corrections to be discussed in the next section) to the traces in a CMP gather to correct them to the zero-offset reflection time. These corrected traces in the CMG are then added together (i.e., stacked) to produce a stacked trace with a large S/N ratio. Presumably, most of the coherent noise can be eliminated by this stacking process because the time shifts only aligned the reflections with one another so that only the primary reflections coherently added together after stacking.