496 SEISMIC METHODS [Chap. 9 



In Fig. 9-34a the phones are in parallel with the secondary of this trans- 

 former; in h and c the phones (or the speaker input transformers) are in 

 series with it. In all cases the regular seismograph galvanometer is used 

 for the time break, and the output of the shot-transmission transformer 

 is either in series (a and h) or in parallel (c) with the amplifier output 

 transformer. Arrangement c differs from 6 by the use of microphones and 

 speakers, which makes for more convenient operation in a recording 

 truck. 



Wire transmission of the shot instant is applied in short-range refraction 

 and in most reflection work. In all long-range refraction work and for 

 reflection shooting in areas where distance, type of country (swamps), 

 or topography make laying of lines impracticable, radio is used. Trans- 

 mitters vary from about 40 to 200 meters^^* in wave length, and from 

 0.2 to 50 watts in power, which must be great enough to overcome static 

 and to actuate the type of indicator used. Their range may extend 

 to 150 miles or more. Fig. 9-35 shows a portable transmitting and re- 

 ceiving unit. The transfer of the shot instant to the transmitter is gener- 

 ally accomplished by making or breaking the plate circuit. For breaking 

 it, the B battery lead is shot apart ; for connecting it, the firing relay shown 

 in Fig. 9-32 is used. Often the transmitter sends some sort of a signal 

 (produced by buzzer or tuning fork) which is either turned off or on by the 

 shot. On the receiving end the signals are picked up as sound and 

 recorded. 



A great variety of devices are available for recording. In long-range 

 radio time-signal transmission (deep refraction and pendulum surveys) 

 relays are sometimes applied in connection with less sensitive and, there- 

 fore, more rugged indicating devices. These may be mechanical relays 

 or gas-filled tubes (grid-glow tube or thyratron). In the majority of cases 

 relays are avoided, and indicators of special design or oscillograph galva- 

 nometers are applied. 



A simple recording device may be made from a phone receiver by at- 

 taching a mirror to the diaphragm and placing the receiver behind a lens 

 of the same focal length as that used in the (mechanical) seismograph. 

 The receiver is then set up on a stand next to the seismograph. A more 

 effective recorder is made from a magnetic speaker by removing the 

 diaphragm and coupling the driving pin to a light spring fitted with a 

 mirror, or to a mirror suspended on a short platinum-iridium torsion wire. 

 In both cases suitable damping should be used. In refraction work the 

 Askania mirror device has been widely applied (see Fig. 7-24). 



All kinds of oscillographs, vibration galvanometers, and string gal- 



"* In this country, the following frequencies have been allotted by the F. C. C. 

 to geophysical work: 1602, 1628, 1652, 1676, and 1700 kilocycles. 



