SECT. 1] 



CONTINUOUS REFLECTION PROFILING 



67 



contains a striking example in which 10 multiples were recorded across a small 

 area ; unfortunately we have no clue about the reason for such high reflectivity. 

 In continuous sub-bottom profiling, surface-bottom multiples are often super- 

 imposed on the sub-bottom echoes ; in addition, there may be other multiple 

 reflection sequences involving many different combinations of reflectors. In 

 Fig. 19 not only are there three bottom-surface multiples but a complex 

 pattern of sub-bottom echoes are repeated as well. In Fig. 20 a similar observa- 

 tion was made where the sub-bottom reflectors were so deep that bottom-surface 

 multiples and the sub-bottom-surface-bottom multiples are completely 

 separated. These examples clearly illustrate one of the principal difficulties of 



DEPTH 



IN 

 FATHOMS 

 (C=800 



FM/SEC) 



Sg^SHi?S^3?^«*«Pls*l 



OTTOM REFLECTION 



SUB-BOTTOM 

 REFLECTIONS 



^•? Si^-SECOND BOTTOM 



REFLECTION 



SUB-eOTTOM REFLECTIONS 

 DELAYED BY ONE 

 SURFACE-BOTTOM 

 REFLECTION 



—THIRD BOTTOM 



REFLECTION 



SUB-BOTTOM REFLECTIONS 

 DELAYED BY TWO 

 SURFACE-BOTTOM 

 REFLECTIONS 



100 



1.625 NAUTICAL MILES - 



Fig. 19. Multiple-bottom-surface reflections recorded 10 iniles northeast of Provincetown, 



Massachusetts. (After Knott.) 



interpretation ; painstaking identification of probable multiple echoes is a 

 crucial step in analyzing continuous jDrofiles. 



In addition to providing "false" echoes the multiples may mask deeper 

 reflections. Furthermore, several instances have been observed where strong 

 reflectors seem to have returned so much energy that there was little left to be 

 transmitted on to deeper horizons. McClure et al. (1958) observed such strong 

 scattering in buried shell beds in the Gulf of Mexico (Fig. 7). Similar apparent 

 masking occurred in Narragansett Bay (Fig. 18). Possibly both scattering and 

 masking could be circumvented to some degree by proper choice of pulse 

 shape (i.e. lower frequencies more dominant). 



