SEISMIC METHODS 841 



for handling the extra cable, storage box for extra seismometers and parts, 

 and floor space directly aft of the cab for tool box, etc. Figure 526 shows a 

 typical survey pick-up truck; one or two are used by each shooting crew. 

 Carriers are provided for the alidade and transit tripods, a steel box for 

 storing the instruments, "U" supports for stadia rods, and storage box for 

 stakes, flag material, etc. 



Fig. 526.— Survey pick-up truck. A, brush guard; B, support for transit 



and alidade tripods; C, "U" supports for stadia rods; D, steel box with lock 



for storage of surveying instruments. (Courtesy of Griffin Tank and Welding 

 Service.) 



Testing of Equipment, — At periodic intervals, as well as when sus- 

 picion is aroused, the following tests should be made to determine the 

 operating characteristics of the equipment: (a) time parallax or phase re- 

 lationship of the recording channels; (b) accuracy of the timing system; 

 and (c) general circuit condition. 



Phase lag between the channels can be detected by placing all of the 

 seismometers at the same location and firing a regular reflection shot. 

 If all channels give the same time of arrival and character of record, the 

 equipment assembly is operating satisfactorily. If differences are recorded, 

 tests should be conducted to determine whether the trouble is due to the 

 location of the seismometers. This can be checked by interchanging the 

 anomalous seismometer with one of the others. If a seismometer is faulty, 

 it should be corrected or replaced. 



The character of the record is influenced considerably by the material 

 at and adjacent the seismometer. If differences in character occur during 

 field operations, the anomalous seismometer should be interchanged with 

 one of the others. If the trouble still exists, the anomalous seismometer 

 should be replaced. Persistence of the trouble will then indicate difficulty 

 in the amplifying, filtering, or galvanometer system of the trace. 



The timing system should be checked for accuracy by comparison with 

 a separate timing fork. The reference timing circuit should be connected 



