>,. I ■ ■ :<; ,■ - :' , I <_'. =■; « 



Figure 12. Section of seismic reflection record showing 

 bottom multiple. Note mimicking of bottom topography 

 and series of reflectors dipping to the left. 

 (Meisburger and Field 1975) 



the bottom is re-reflected at the high-contrast air-water interface at the 

 surface, returned again to the bottom and back to the surface where it is 

 picked up by the receivers and recorded in a position below the true bottom 

 reflector corresponding to the two-way travel time. Because of the geometry 

 involved, the apparent depth of the reflector is equal to the water depth at 

 that location. The signal can bounce repeatedly and two or more multiples can 

 appear on the record spaced at intervals equivalent to the water depth, becom- 

 ing progressively weaker as the energy is attenuated. In addition to bottom 

 multiples, there is an almost endless variety of multiples that can be created 

 by signal bouncing between any combination of reflectors. However, random 

 multiples are usually not as prominent and persistent as bottom multiples and, 

 in general, are a lesser problem. 



148. Where multiples of any type occur, they can often be identified 

 because they repeat the configuration of the source reflector and cut across 

 other reflectors. Crossing of true reflector surfaces is rare in nature and 

 crossed reflectors usually indicate that one is spurious. The more difficult 

 problems occur where the bottom, or other source reflector, is even and 



60 



