SECT. 1] SINGLE-SHIP SEISMIC REFRACTION SHOOTING 45 



F. The Ship's Radio Receivers 



There are two receiving aerials in the ship, one mounted on the forecastle and 

 the other astern. These aerials are commercial dipole directional television 

 aerials; one has its maximum response ahead and the other astern thus enabling 

 good recejation to be obtained both on the way out and on the way back to the 

 buoys. Relatively broad band aerial amplifiers are installed in the ship near the 

 aerials and the high-level r.f. signals are thence fed to the receivers in the 

 laboratory. Each receiver is tuned to a particular buoy. 



The audio output from each receiver is filtered to provide one band from 

 5-500 c/s and a second from 5-50 c/s. 



Each receiver has a tuning indicator and a radio frequency signal level 

 indicator. 



G. The Recorder 



The output from the receivers is recorded on an ultraviolet-light 12-channel 

 galvanometer recorder. This type of recording allows inspection of the record 

 before processing. Time marks are produced by a phonic motor driven from a 

 crystal-controlled frequency source. 



The methods of analysis of the results are as described in Chapter 1 . 



3. Operation 



The number of buoys normally used is four. These, in deep-water areas, are 

 spaced along the line of shots at intervals of about 1 mile. The first shot is fired 

 about 4 miles from the nearest buoy and thereafter at fixed intervals of time, 

 usually between 4 and 8 an hour. The ship's speed is approximately 8 knots. 

 Between 10 and 20 shots will be fired for a full profile. 



The explosive used is "Geophex", which does not require watertight con- 

 tainers or primers when initiated with aluminium detonators. Slow-burning 

 fuse, specially covered in order to stop penetration of sea-water, allows the 

 charge to sink and the ship to get clear before detonation. A length of fuse 

 giving a burning time of 80 sec is standard; the charge depth is then approxi- 

 mately 300 ft. 



The charge size is varied from about 5 lb for the close shots to a maximum of 

 300 lb for the longest range shots. 



It is usual to keep the observation roughly plotted as the information comes 

 in; thus, on the passage back to the buoys, any gaps can be filled in. 



To avoid unnecessary searching for the buoys on returning to them, sound- 

 ranging shots of 2 1 lb weight are fired when the range is estimated as being 

 about 3 miles from the nearest buoy. In good visibility the buoys can be seen 

 at about 1 mile distance. 



If the opportunity exists, shots are fired from both ends of the line of buoys. 



In shallow-water work the technique is unchanged apart from the spacing 

 between shots and buoys. 



