SEISMIC METHODS 845 



already designated by the operator. From his controls in the recording 

 truck, the operator communicates with the shooter by telephone or radio. 

 The shooter repeats the depth and magnitude of charge, reports that 

 personnel, equipment, etc. are safely located, and states his readiness 

 to fire. The final order to fire is given by the operator, who then sets 

 the recording equipment into operation when the shooter signals back.* 

 After the records have been developed, the operator carefully studies them 

 in order to direct the shooter regarding depth and pounds or sticks of 

 explosive to be used in the shots to follow. Upon completion of the shoot- 

 ing, the hole is suitably filled to prevent damage to crops and injury to man 

 and cattle. Sometimes when casing is used, it is pulled later by the drilling 

 crew. When the holes are deep, it is cheaper and more efficient to have a 

 special crew with suitable equipment to transport, pull, and lay down casing 

 at the shot-points. 



The day's records are taken to the party headquarters and usually are 

 computed on the following day. The first step in the interpretation of the 

 records is to mark the shot time period and to determine "weathering" or low 

 velocity layer data. The records are then picked for refractions or reflec- 

 tions. Next, the data are computed and plotted — generally on cross 

 sections from which mapping proceeds. Subsequent field operations are 

 outlined by the supervisor in collaboration with the party chief from studies 

 of the current data. 



A schematic layout of a reflection crew in operation, together with the path of 

 the reflection and the record, is shown in Figure 528. The record indicates the shot- 

 instants and the onsets of the reflections at the three reflecting horizons. For a re- 

 fraction crew, the paths of the refracted rays and the refraction record would be 

 shown ; otherwise the arrangement is much the same except for distances between 

 seismometers and between operator and shot-hole. 



Transportation and Communication 



The mobile equipment required for effective seismic operations is largely 

 dependent on local conditions. Generally, the automotive equipment will 

 comprise : recording or instrument truck, shooting truck, drill truck, water 

 truck for drilling operations, surveyor's car, permit man's car, and party 

 chief's car. The applicability and relative merits of standard tire equipment, 

 dual or tandem conversion wheels, caterpillar treads, or large size marsh 

 tires depend on the region surveyed. Even under favorable field conditions 

 it is desirable, from the viewpoint of good mobility and minimum periods of 

 delay, to equip the heavier trucks with four-wheel drives or dual tires. 

 When field conditions are difficult, it is necessary to equip the heavier 

 trucks with winches, and all explosive-carrying vehicles should be equipped 

 with ground straps or chains. 



* Sometimes the operator is provided with means to fire the shot, and patents 

 have been granted for a device to make the shot-instant coincide with one of the time 

 marks on the record. However, the practice of shooting by the observer is rightly 

 criticised as dangerous for the shooting crew and chance passers-by at the shot-point. 



