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EXPLORATION GEOPHYSICS 



Applications and Economic Value. — In addition to engineering information, the 

 film provides a visual check on the geological outcrops in connection with studies of 

 surface geology and also shows the need for terrain corrections in connection with 

 gravity surveys. 



One instrument man with two rodmen, each with a car, can do the work which 

 normally requires two transit men, each with rodman and car. Not only is the volume 

 of work increased, but often the accuracy is improved. The camera has some operating 

 advantages over the surveying instrument. For example, heat wave effects cannot be 

 detected on the film, partly because of the short exposure time as compared with the 

 low frequency of these effects, and partly because the film possesses a certain selective 

 sensitivity to certain portions of the visible spectrum. 



A further increase in speed is effected by the virtual elimination of re-runs in the 

 field, since the film can be reviewed in the office at any time without the costly pro- 

 cedure of sending men, cars and equipment back to the field when a mis-tie occurs in 

 a traverse. 



A few days training is required to learn the operation of the field and office instru- 

 ments. An elementary knowledge of photography is helpful, but is not a prerequisite. 

 The simplicity of operation enables junior engineering personnel to handle the field 

 work and permits checking and supervision of each step by senior personnel from a 

 central office location. 



Fig. 241. — (l)Gravimeter on tripod in truck, with (2) extension 

 leg resting on ground, (3) battery box for thermostat and motor 

 battery. (L. M. and F. W. Mott-Smith, The Petroleum Engineer.) 



Field Procedure. — The heavier semi-portable gravimeters are 

 usually transported in a light closed panel truck. While making a measure- 

 ment the instrument must rest on a firm base. For this purpose it is sup- 

 ported on a sturdy tripod arrangement inside the truck. When a measure- 

 ment is to be made, the extension support is lowered to the ground through 

 a hole in the floor of the truck. Figure 241 shows the instrument on such 

 a tripod ready for a measurement. This method is advantageous in that 

 the meter, and the operator, may be protected from inclement weather. 



The lighter instruments may be transported in a jeep or light field 



