328 EXPLORATION GEOPHYSICS 



which is a portable shelter consisting usually of a light wood frame covered 

 inside and outside with thin plywood. The space between the plywood 

 layers and frame is filled with a heat insulating material. (Figure 185.) 

 After the hut has been set up, the torsion balance is placed on the aluminum 

 base plate. The turntable is then placed on the pedestal and clamped in 

 position. Next, the upper part of the balance is placed on the turntable, 

 and the turntable is leveled and oriented. 



After a new photographic plate has been inserted and the electric light 

 connections and the performance of the electrical contact mechanism 

 checked, the instrument is left on the station site to record automatically. 

 (If a visual torsion balance is used, the observer reads the instrument and 

 rotates it into the next position at certain fixed intervals until a satisfac- 

 tory series of readings has been obtained.) 



The instrument is usually kept at each station long enough to have a 

 repeated observation and check reading in one azimuth at least. 



Number of Stations Occupied Per Day 



The number of stations occupied per day depends on several factors, 

 such as type of instrument used and its condition, topography, climate, 

 precision desired in the observations, and the number of men in the crew. 

 Under ordinary conditions, a four-instrument crew can occupy 12 to 14 

 stations a day. The daily average of a two-instrument party may be 6 or 7 

 stations. A one-instrument party consisting of two men generally will 

 occupy 2 or 3 stations per day at most. 



Field Computations and Graphs 



It is common practice to compute the observations pertaining to the 

 evaluation of the gradient and curvature values at the field headquarters, 

 preferably on the same day in which the observations were obtained. The 

 computation and plotting of the results are done under the supervision of 

 the party chief. After the data obtained in a day's work have been com- 

 puted, the graphs and computed data usually are sent to a home office 

 immediately for compilation into the work maps. 



Computing the Torsion-Balance Data. — The following example out- 

 lines the calculations necessary in computing the uncorrected gravity com- 

 ponents. The data from the photographic plate is processed through a form 

 sheet to systematize calculations and minimize errors. 



On page 329 are given readings from the plate for station No. 3 of a survey on the 

 Wild Horse Park structure, Pueblo County, Colorado, April 21, 1930. Torsion Balance 

 No. 529, previously referred to, was used. 



Prime ( ' ) refers to balance I and ( " ) to balance II. The left hand column 

 1, 2, 3, refer to orientation positions. The plate readings for the positions of rest of 

 the balance n' and n" are entered opposite the orientation positions to which they refer. 

 For example, for balance I, orientation position 1, the scale reading of the dot was 

 96.3 ; for position 2, 96.0, etc. 



