SECT. 1] SINGLE-SHIP SEISMIC REFRACTION SHOOTING 41 



ship's transmitter has relatively great power. The disadvantage of the method 

 lies in the fact that once radio contact is lost from the buoy transmitter the 

 quality of the seismic signals cannot be judged until the buoys are recovered. 

 However, the quality of the recording is improved by eliminating the radio link, 

 and multi-channel recording is possible. 



Apart from single-ship operation, sono-radio buoys can also be used with 

 advantage where two ships are available. The receiving ship lies amidst an 

 array of buoys spaced to conform to any pattern suitable for the particular 

 problem. 



2. Instrumentation 



A. Hydrophones 



In the system at present used by the Cambridge group the sensing head of 

 the hydrophone consists of a barium titanate tube radially polarized and silvered 

 in four longitudinal strips inside and outside. The tubes are 1 in. O.D. and 

 approximately f in. I.D. and 1 in. in length. Each of the longitudinal strips can 

 be considered as a separate hydrophone element and the four are electrically 

 connected in series. By this means the voltage sensitivity is increased by a 

 factor of approximately four times from the value it would have if the tube 

 were fully silvered. 



After the electrical connections have been made the tube is completely 

 potted in epoxy resin to form a cylinder 1| in. in diameter. This cylinder is 

 longer than the barium titanate tube and is machined at one end to take an 

 "O" ring for sealing into the preamplifier tube. At the other end the cylinder 

 has a domed end; the intention being that this dome should prevent eddy 

 formation if the hydi-ophone has water moving past it. The domed end 

 has an axial hole in it : this is loaded with an appropriate amount of lead shot 

 to provide horizontal trim for the complete assembly. A photograph of the 

 sensing head is shown in Fig. 2, 



The preamplifier tube is made from a corrosion resistant aluminium alloy of 

 wall thickness approximately yg ^^- ^^^ of outside diameter equal to that of 

 the sensing head. The length of this tube is selected to make the whole assembly 

 neutrally buoyant. The tube contains a four-stage thermionic valve (DF66) 

 amplifier, a gain control, and the battery supj)ly consisting of one 1^-V cell 

 and one 22i-V battery. The frequency response of the hydrophone and pre- 

 amplifier is flat from 5 c/s to 5000 c/s. 



The coaxial hydrophone cable is 300 ft in length, rubber covered, and ap- 

 proximately ^ in. in outside diameter. For the first 100 ft from the hydrophone 

 the cable is buoyed with tubular floats approximately every 10 ft to make it 

 overall neutrally buoyant. At the 100-ft position a 10 lb weight is attached and 

 thence the cable is connected directly to the buoy. It is thus expected that the 

 hydrophone and 100 ft of cable will stream out horizontally at a depth of 200 

 ft below the buoy (Fig. 3). 



