horizontal or nearly so, and true sub-bottom reflection surfaces are likewise 

 even and near horizontal. In such cases, discrimination of multiples is dif- 

 ficult at best, and in shallow water, the multiples may completely obscure any 

 sub-bottom reflectors. 



149. Other common spurious signals that can be recorded are direct 

 arrival, side echoes, and point sources. The direct arrival occurs when the 

 acoustic source and receivers are towed separately, and the arrival of the 

 acoustic signal is recorded directly from source to receiver as distinct from 

 reflected signals. Usually the separation of source and receivers is less 

 than the water depth, and the direct arrival appears in the water column as a 

 strong signal, generally parallel to the zero time line, but may become irreg- 

 ular where the source -receiver spacing changes from time to time (as when boat 

 turns are made) . Side echoes occur where reflections are received from irreg- 

 ular features that are not directly beneath the trackline, but are close 

 enough to produce one or more short reflectors that cut across sub -bottom 

 reflectors at an angle. Point source reflections are a type of side echo pro- 

 duced by a distinct object such as a boulder or rock outcrop that appears on 

 the record as a hyperbola with the object being beneath the apex and the sides 

 cutting across the sub-bottom reflectors. In most cases, the direct arrival, 

 side echoes, and point source signals are readily identifiable and do not 

 unduly complicate record analysis. 



Record analysis 



150. The main purpose of record analysis is to develop a tentative 

 stratigraphic framework of the exploration area that shows the thickness and 

 areal extent of the various sub -bottom rock and sediment bodies. When corre- 

 lated with core data, the lithologic character of these bodies can be estab- 

 lished. Those areas, potentially suitable as a borrow source, are identified, 

 and the thickness, extent, and lithology of overburden material are deter- 

 mined. To do this, the reflectors on each profile line are delineated and 

 correlated with adjacent lines to construct the three-dimensional framework. 



151. An initial assumption is made that all valid traces on the profile 

 lines are of some stratigraphic significance. As core data are acquired, a 

 more complete and reliable interpretation can be made and the initial assump- 

 tion of significance tested. Experience from past operations where cores or 

 borings were also obtained, indicates that, in most cases, reflectors have 



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