SEISMIC METHODS 843 



minutes later. The instant of the shot was recorded at the point of obser- 

 vation by an electric signal which was transmitted from the shot-point by 

 means of a wire. 



Later, radios were introduced, each operator having a receiver and the 

 shooter a transmitter. The shooter sent a message just before firing the 

 shot, and the instant of explosion (shot-time) was transmitted by radio. 

 Still later, the operator and the shooter used two-way radio communication 

 to make sure that all instruments were in working condition before the heavy 

 charge was fired. 



The quantity of dynamite in the shot depended upon the distance; often 

 charges of several thousand pounds were used. Originally, the shots were 

 placed on the surface of the ground, or, after the first shot, in the crater 

 made by preceding shots. Later the dynamite was put in holes as deep as 

 200 feet. A photograph of an early refraction shot is shown in Figure 527. 

 (The resulting crater was over 20 feet deep.) 



Modern Refraction and Reflection Operations. — From an opera- 

 tional viewpoint, refraction field procedure differs from reflection field 

 procedure in numerous respects : viz., the refracted "first arrivals" and 

 a few correctable subsequent wave trains are of primary interest, to obtain 

 time-distance curves; the size of charge (amount of dynamite) is usually 

 much greater; the seismometer spread length and the distance of the 

 spread from the shot-point are greater ; usually the range of depth investi- 

 gated is more restricted than in reflection work ; and the investigations are 

 essentially lateral in their scope, as compared to vertical for the reflection 

 work. As the work proceeds, the configuration of shot-point and seis- 

 mometer spread is moved forward in increments along a given profile line. 



However, the refraction and reflection field operations and equipment 

 have several similarities. Thus the exploration crew and the general 

 division of duties of the crew members are similar; the field trucks used in 

 the two types of prospecting are similar, etc. Hence, in the general descrip- 

 tion given here, refraction and reflection operations will be distinguished 

 specifically only in the discussion of types of spread and in the methods of 

 improving detection of reflections. 



The Exploration Crew 



A typical seismic party comprises a party chief, assistant party chief, 

 permit man, surveyors, a recording truck staff, shooters, drillers, and com- 

 puters. The general division of duties is given in Table 24. 



The party chief* is responsible for all details of the work and arranges 

 the schedule so that the work is done in the most efficient manner. After 

 the party chief and the supervisor to whom he is responsible have selected 

 the areas to be explored, the permit man secures permission from the 



* In most organizations the party chief is in charge of geophysical interpretation 

 and prospect mapping as well as field operations. In other organizations these two 

 general functions are divided between two persons. 



