SECT. 1] REFRACTION AND REFLECTION TECHNIQUES AND PROCEDURE 33 



arrivals from each layer is not normally obtained. As used extensively by LGO 

 and WHOI, one ship lies to on station while the second shoots a profile, usually 

 about 40 miles long on deep-sea lines where arrivals from the mantle are being 

 sought. As soon as the final shot of the profile is fired, the shooting ship stops 

 and launches hydrophones ; the receiving ship gets underway and shoots on 

 the same course up to the new receiving position. Ship drift on station normally 

 amounts to several miles and is rarely known in amount until after the fact ; 

 profiles are, therefore, rarely truly reversed, forming instead a parallelogram. 

 It can be assumed if the shooting lines are parallel that the velocities obtained 

 will be true, and any mis-match in the reverse times is normally blamed on 

 drift. On very long profiles in complex areas lateral changes in velocity may be 

 pronounced in the shallow layers so that the velocities and thicknesses obtained 

 are at best averages ; in such cases, where time allows, resort is sometimes 

 made to compound profiling. 



D. Overlapping Profiles 



Overlapping profiles are essentially incomplete reversed profiles so arranged 

 that refraction arrivals from the deepest layer are received at both ends of the 

 fine from shots with the same geographic position. This permits determination 

 of mantle velocity from plots of arrival-time differences to eliminate effects of 

 variation in thickness of the shallower layers. If carried out to full length, 

 these would be very long reversed profiles ; for a given amount of time and 

 explosives, reversed control is sacrificed to get a better velocity determination 

 on the mantle. 



E. Compound Profiles 



Compound profiling involves combinations of the foregoing; in one form, 

 used at SIO on expedition MUKLUK, it involved shooting reversed profiles on 

 the ends of which were attached short unreversed lines to get a more positive 

 check on the sediment and second-layer velocities. Each station then consisted 

 of four shooting lines, two of which were reversed. A more elaborate version 

 of this was the combined LGO-WHOI-Hudson Labs.-Texas A & M MOHOLE 

 expedition of 1959, in which four ships participated, with three receiving along 

 a straight line at any given time and the fourth ship shooting from a point 

 short of the first ship to a point beyond the third. At the conclusion of each 

 run the shooting ship stopped and became a receiver, while the first receiving 

 ship in line picked up its hydrophones and became a shooting ship. This 

 system can give reversals on all layers ; it has as its only drawback the expense 

 and difficulty of co-ordinating a large number of ships and personnel. 



F. Fan Profiles 



In fan profiles the shooting line never passes the receiving position. This is 

 normally done in conjunction with a profile of another type and is a correlation 



