34 SHOR [chap. 2 



method to determine variations in structure over a larger area. The shooting 

 ship maintains constant distance from the receiving ship and varies its azimuth. 



G. Detailed Area Surveys 



Detailed area surveys require continuous profiling. In this case reversed 

 profiles are placed end-to-end either along a straight course or in a series of 

 closed loops. 



5. Miscellaneous Variations 



In addition to the methods described above, a number of variants have been 

 used or are under development for special purposes. One of the most important 

 of these is shooting and recording on the sea floor in deep water. This was 

 attempted with some success at Woods Hole in 1937-40 (Ewing et al., 1946) 

 using a self-contained shooting and recording system that was dropped to the 

 bottom, fired its charges and took its ow^l records, and floated back to the sur- 

 face when it was done. The project was plagued with instrumental problems 

 and was halted by World War 11. The data to be gained from such a system 

 are of great importance, for the layers beneath the ocean on which w-e have 

 the least information are those in the first kilometer below the bottom, refracted 

 arrivals from which are infrequently received. A bottom-shooting and/or 

 recording system would improve the information on these layers. Recent 

 development A\ork on such a system has been in progress at LGO. 



Systems for performing refraction work from a single ship have been used 

 almost from the beginning. LGO and WHOI have used the "'whaleboat" 

 method, in which shots were fired on fixed schedule from a small boat and the 

 firing time recorded against a chronometer mark. Such a boat has been used 

 at SIO using radio communications with the ship. Such operations are practic- 

 able only in areas of calm weather. The most successful single-ship operation 

 is that described in this volume by Hill (Chapter 3), in which sonobuoys have 

 been used for the receiving station. 



A radically different method of operation, adapting the methods used in oil- 

 exploration seismology to deep-sea problems, has been developed by Savit 

 et al. (1962) of the Western Geophysical Company. In this method, the shooting 

 ship stays at a fixed position, and the receiving ship tows a long streamer with 

 24 groups of hydrophones. Shots are fired each time the streamer has been 

 moved ahead its own length. Low-impedance hydrophones without pre- 

 amplifiers are used. The signals are passed through wide-band amplifiers and 

 are tape recorded on a 24-channel ta])e recorder for later playback with filtering 

 on to a variable-area record. This method of operation gives more data points 

 per mile along each profile and permits the use of automatic data-handling 

 systems. The disadvantages are the high initial cost of the equipment and the 

 difficulty of reducing noise in the towed hydrophone system to a level that will 

 permit recording arrivals from the mantle at long range. 



