GRAVITATIONAL METHODS 



327 



The area under and immediately adjacent the instrument is leveled to 

 within one-half of a centimeter for a radius of at least one and one-half 

 meters. (Figure 184.) 



Surveying 



The choice of method of survey- 

 ing depends on the topography and on 

 the accuracy of data desired. In areas 

 of rugged topography, surveying may 

 be carried along 16 azimuths and as 

 far as 250 meters from the instru- 

 ment. In many surveys, 8 azimuths 

 and distances of from 70 to 100 me- 

 ters are sufficient. (However, in areas 

 of relatively mild topography, it is 

 common practice not to carry the sur- 

 veying for topographic details beyond 

 30 to 40 meters from the balance.) 



The manner in which the stations 

 are laid out depends on the type of 

 subsurface structures expected in the 

 area under investigation. For average 

 reconnaissance work, the stations are 

 placed from one-half to one-quarter 

 of a mile apart. For detailed work, 

 the station separation may be con- 

 siderably less than one-quarter of a mile. The lines of stations are usually 

 run in a manner to conform as far as possible to the roads and trails 

 present in the area under investigation without altering to a large degree 

 the station net agreed upon before starting the survey. 



Instrument Set-Up 



After arriving at the leveled station site, which should be well marked 



by a surveying crew in advance, 

 the observer drives three wood or 

 steel stakes into the ground so that 

 their tops are in the same hori- 

 zontal plane. The height of the 

 stakes, as projected from the 

 ground, should always be of the 

 same standard value. After the 

 stakes have been leveled carefully 

 with a hand level, the aluminum 

 base plate is laid upon them with 



Fig. 185.— Final set-up of torsion-balance sta- itS north grOOVe pointing UOrth. 



sSSctu're 'SSwind."'"' "' ^""'^''"^ *° ^°^'^ The next step is to set up the hut, 



Fig. 184. — Initial leveling of station site in 

 immediate vicinity of the instrument, a, stadia 

 rod equipped with level bubble for leveling; b, 

 portions of collapsible house for instrument. 



