494 



SEISMIC METHODS 



[Chap. 9 



Firing Chambtr 



The instant of explosion may be computed from the time of arrival of 

 the sownd wave, taking into accomit the velocity of sound, barometric 

 pressure and temperature, wind direction and velocity, and the distance 

 of seismographs from the firing point. AppUcation of this method de- 

 creased when charges were buried at greater depth, but it was later revived 

 for the purpose of determining the distance between firing point and 

 receiver in diflScult terrain. In this procedure a "sound" charge is placed 

 on the ground above the buried main charge and a blastophone is used 

 for recording the sound waves. The use of the light transmitted by the 

 explosion was mentioned by L. Mintrop in one of his patents. The possi- 

 bilities of recording an explosion through its heat radiation have never 

 been investigated. 



In the transmission of the shot instant by wire, a contact in an electrical 

 circuit is made or broken at the instant of the explosion. Double wire or 

 ground return may be used. The indicating devices are electromagnets 



with mirror armatures, telephone receivers 

 or loudspeaker elements with mirrors attached 

 to their diaphragms, oscillographs, string gal- 

 vanometers, and the Uke. These will be dis- 

 cussed in more detail in the article on radio 

 transmission. The transmission circuit (1) 

 may be broken at the instant of firing (a) by 

 wrapping it around the charge, (&) by a series 

 cap, or (c) by a relay in the firing circuit; (2) 

 it may also be closed by a relay, actuated (a) 

 by the firing circuit or (6) by a series cap; 

 or (3) the transmission circuit may be coupled directly to the firing cir- 

 cuit by (a) a resistor or (6) a transformer. 



Wrapping the transmission line around the charge has the disadvantage 

 of placing four lines in the shot hole. A series cap is more convenient for 

 breaking the line. For closing the circuit by a series cap, the firing relay 

 shown in Fig. 9-32 has been used in connection with refraction shot-instant 

 transmission. Two field phones are usually connected to the shot-instant 

 transmission line as shown in Fig. 9-33. The first arrangement is actually 

 a three-line circuit. Communication is not interrupted when the shot- 

 instant circuit is broken; the phone current passes through the indicator, 

 which is avoided in the second arrangement. However, the use of a single 

 line requires switching from communication to shot transmission. 



Various arrangements are applied (particularly in reflection work) to 

 couple the firing line directly to the transmission line. A smaU amount 

 of current, not enough to set off the cap but sufficient to attract the 

 armature of the indicator, may be passed from a battery through the 



snap tprlnj 



''piston 



American Askania Corp. 



Fig. 9-32. Series cap firing 

 relay for wire or radio trans- 

 mission. 



