conveniently be set up in a motel room or other covered space for record 



analysis . 



Record annotations 



144. Marking reflectors on seismic reflection records can be done 

 directly on the original record, on copies of the record, or on transparent 

 overlays . Permanent markers should not be used on the records or overlays 

 since the initial delineation of reflectors is tentative and may change as the 

 overall pattern is developed and spurious reflectors are identified. Although 

 soft markers such as chalk can be used to mark the original record, the paper 

 can be easily damaged and it is preferable to leave the originals unmarked and 

 delineate the reflectors on a suitable copy or overlays. It is important that 

 all analysis, interpretation, and correlation be accomplished by examination 

 of the original record. Copies and overlays are used only as a base for 

 recording the results of the analysis. 



145. Reflectors are delineated by drawing a line with a suitable marker 

 along the top of the reflector traces. Dashed lines can be used to indicate 

 projections across gaps, and through sections obscured by multiples or other 

 sections where continuity is not certain. Persistent reflectors can be given 

 a letter, number, or code word designation to identify them along each profile 

 and on other records where correlation is established. 



Spurious reflectors 



146. While seismic reflection records show the position and configura- 

 tion of sub-bottom contacts between sediment and rock bodies with contrasting 

 acoustic properties, they also show apparent reflector traces that are spuri- 

 ous and not related to geological factors directly beneath the path of the 

 survey vessel. These false reflectors create two problems: (a) they can be 

 mistaken for true reflectors; and (b) they can obscure valid reflectors on the 

 graphic record. Discrimination between actual and false reflectors is of 

 primary importance in record analysis. False reflectors can usually be iden- 

 tified by certain inherent characteristics that reveal their nature. 



147. The most common type of spurious reflector is known as a "multi- 

 ples." Multiple traces on a record are caused by the signal bouncing between 

 two high- contrast reflectors so that the lower reflector is recorded one or 

 more times in a position below the true reflector elevation. Usually, the 

 most common and prominent multiples are the bottom multiples (Figure 12) . The 

 multiple traces are created when part of the acoustic energy reflected from 



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