380 



SECONDARY PRESSURE WAVES 



of amplifiers, and need for high sensitivity and good low frequency re- 

 sponse. It has been found that placing gauges too close to the expand- 

 ing gas sphere gave rise to spurious random noise on the records, pre- 

 sumably as a result of disturbance of the gauge and associated gear by 

 the shock wave and later outward flow of water. Satisfactory working 

 distances depend on the size of charge ; a rough estimate from experience 

 is that the gauge should be at least 2 maximum bubble radii from the 

 charge, 60 feet from a 300 pound charge having been found satisfactor3^ 

 A gauge system which is to measure bubble pressures must also 

 withstand the force of the shock wave, as it can hardly be inserted in 

 place during the time between the two events. This obvious fact 

 presents a number of difficulties and limitations. Tourmaline piezo- 



P =0 



Fig. 9.4 



Pressure-time curve for shock wave and bubble pulse from 300 

 pounds of TNT fired at a depth of 50 feet. 



electric gauges, which are the simplest and most rugged, are compara- 

 tively insensitive, and their use introduces the further complication of 

 cable signal. This phenomenon, discussed in section 5.8, is particularly 

 disturbing in bubble pulse measurements. This is because the large 

 stresses, induced by the shock wave as well as by the pulse of interest, 

 develop this spurious charge which cannot leak away rapidly, owing to 

 the necessity for good low frequency response of the electrical sj^stem if 

 the long duration pulses are to be faithfulty recorded. 



The preceding shock wave also occasions difficulties with electrical 

 recording equipment in large charge work, as it often must strike the 

 recording site before the desired gauge signal occurs. Unless the ampli- 

 fiers used are especially protected against microphonics, the recording 

 system introduces serious distortion and may even become inoperative. 

 This is a particularly difficult problem because of the need for amplifiers 

 of high sensitivity and, by ordinary standards, superlative low-frequency 

 response (see sections 5.8, 5.9). 



Perhaps less sophisticated, but equally serious, problems arise in 

 assuring known, satisfactory, and reproducible experimental conditions. 



