Common Shot Point Gathers

The traces recorded by a single shot are grouped together to form a common shot gather or CSG, as shown on the LHS of Figure 1.8. Note that the R2 reflection times increase with an increase of source-receiver offset, and these reflections are readily identified for source-receiver offsets greater than 60 feet. Near the zero-offset locations these reflections are buried by near-source noise.

  

Figure 1.8: Common shot gather on left and skeletonized version depicting arrival traveltimes vs X for the direct (D), Refracted (Ref1) and Reflected (R2) arrivals. The apparent velocities dx/dt of these arrivals can be measured from the x-t graph. CSG data collected along W. Valley Fault, Utah,

To cover a greater subsurface area with reflection events, the shot and geophone locations are translated (or rolled along) by the same distance and the CSG experiment is repeated to give another shot gather. These experiments are repeated along a line until sufficient subsurface coverage has been achieved. Play the html movie to see how CSG's are collected and the correspondence between traces and raypaths. It is usually necessary to have a long recording line so that 1). the noise near the source is avoided, 2). the surface wave amplitude has diminished or the slow surface waves arrive much later than the reflections, 3). the moveout trajectory of the reflection events is sufficiently distinguished from coherent noise trajectories so that processing (i.e., stacking) removes the noise, and 4). there is better spatial resolution in the seismic image. The seismic camera is similar to an optical camera, where wider apertures result in better image resolution.

In Figure 1.8, the non-zero offset reflections arrive later than the ideal zero-offset reflection. Therefore, to realize the goal of obtaining an ideal ZO section we must apply time shifts to the non-zero offset reflections to correct them to zero-offset time. To do this we must reorganize the data into a common midpoint  gather (CMG) as explained in the next section.