3. SINGLE-SHIP SEISMIC REFRACTION SHOOTING 



M. N. Hill 



I. Introduction 



Shortly after the end of the Second World War it was realized that the diffi- 

 culties of obtaining in Britain two ships for refraction shooting at sea would 

 prohibit the use of the more conventional methods described in Chapter 2. 

 It was, therefore, decided in the Department of Geodesy and Geophysics at 

 Cambridge that a single-ship method must be developed. 



The first system tried was to fire charges suspended beneath buoys. These 

 charges Mere electrically detonated by clocks in the buoys and a preliminary 

 A\arning before firing, and the shot instant, were radio-transmitted to the ship. 

 The receiving hydrophones and their associated amplifiers were connected to 

 the recording equipment in the ship. This method was soon abandoned as 

 laborious and unsafe. At long ranges there would have been danger of the 

 buoys being picked up by unknown ships before the charges had fired. 



The next system tried was first used in shallow water and is that which is 

 still in use, namely the sono-radio buoy system. The first experimental area was 

 in the western approaches to the English Channel and the results were published 

 by Hill and King (1953) and with more observations included by Day et al. 

 (1956). The first deep water experiments were made in 1949 and the results 

 published by Hill in 1952; these experiments were followed by further work in 

 the Atlantic (Hill and Laughton, 1954) and by the w^ork of H.M.S. Challenger 

 (Gaskell et al., 1958). This last paper contains a description of the method of 

 analysis of results. With the method used in all these experiments hydrophones 

 are suspended below buoys which transmit by radio the signals received to the 

 ship from which the charges are fired. With this system (Fig. 1) a number of 

 receiving points can be obtained for each shot; it is usual to lay four buoys at 

 appropriate spacings. Further, the ship is in control of the charge-firing 

 arrangements and the danger is, therefore, greatly reduced from that of the 

 first system. 



The disadvantages of the system compared with the two -ship system are as 

 follows : 



(a) The radio range between the buoys and the ship is limited to about 45 km 

 because of the restricted radio power of the buoys. 



(6) The buoys are not expendable and have to be recovered. This means that, 

 having shot a line, it is necessary to steam back to the buoys, thereby increasing 

 the total steaming distance in comparison with some of the two-ship methods. 

 It further results in weather restrictions; recovery becomes difficult in winds 

 higher than Force 5 to 6. 



(c) Once launched, the buoys are no longer under control and, for example, 

 the amplifier gain settings cannot be altered. Further, it is not possible to 

 quieten the hydrophones by cable streaming methods such as are described in 

 the previous chapter, 



[MS received April, 1962] 39 



