MULTIPLE SEPARATION THEORY

Reflection seismograms gathered over media with weak lateral velocity variations will exhibit primary and multiple events with different hyperbolic moveout characteristics. The stepout of primary events at a given time and offset are distinctive from the stepout of water bottom related multiple events for the same time and offset. This is because primary energy arrives from much deeper than multiple energy, and primary energy generally has traveled through higher velocity media than the multiple events. Both of these factors tend to cause the primary wavefield to arrive at the surface at an angle of incidence that is much closer to vertical than does the coincident multiple energy wavefield. On CMP gathers the angle of incidence of an upcoming wavefield manifests itself in the stepout of the event. We use the stepout of events to distinguish primary energy from multiple energy.

In a space parameterized by (t,h,p), where p is the stepout of an event, the moveout trends of the primaries and multiples are separated into distinctive regions. The primary and first water bottom related multiple events will follow a surface in this space. The following general character of primary and multiple events in (t,h,p) space are true if dv/dz >= 0:

• If stepout and time are constant, the offsets of primary events will be greater than the offsets water bottom related multiple events.
  
• If offset and stepout are constant, the time at which primary events occur is earlier than the time of water bottom related multiple events.
  
• If time and offset are constant, the stepout of a primary event is less than the stepout of water bottom related multiple events.

With the prediction of the primary trend and first water bottom related multiple trend a mask can be designed to isolate the multiple energy from the primary energy.