Z26 EXPLORATION GEOPHYSICS 



eliminate the necessity for temperature correction under ordinary- 

 conditions. 



It will be noticed that in Figure 183 an extra dot appears at the bottom 

 of the row in the reference dot line. When the instrument is set up and 

 ready for operation in the field, it is oriented in position 3, and the beam 

 released. The lights are then turned on by turning the timing clock. These 

 extra spots are photographed while the beams are swinging, and there- 

 fore they do not produce a dot. The instrument then moves itself around 

 into position 1 (N-S). Forty minutes elapse while the beams quiet down 

 and come to rest before the lights again come on and the four dots for 

 position 1 are photographed. This extra reference line dot serves to orient 

 the plate, being placed in the lower left-hand corner when the plate is read 

 or scaled. 



Field Procedure. — The organization of the torsion balance field 

 parties depends on many factors both technical and economic. In the 

 early days of torsion balance prospecting in America, two-instrument 

 parties having a crew of 6 to 8 men were common. At the present time, 

 a one-instrument party may comprise: 1 observer, 1 surveyor, and 2 

 helpers ; a two-instrument party may comprise : 1 or 2 observers, 1 sur- 

 veyor, and 3 helpers. A large, four-instrument party may include a total 

 of 9 to 14 men. 



Testing of Instrument 



Prior to taking a torsion balance into the field, the instrument should 

 be adjusted, checked and its constants determined. The work can best be 

 done at the laboratory where adequate equipment and shop facilities are 

 available. 



Transportation of Instrument 



The method of transportation to the area under investigation and from 

 station to station depends on many factors : notably, climate, topography, 

 the character of the roads and trails, surface conditions, and to some 

 extent, the number of men and instruments in the party. On marshy 

 ground, boats, canoes or marsh buggies may be used. On dry lands hav- 

 ing relatively good roads and trails, the torsion balance is transported either 

 in a trailer provided with a torsion balance shelter or in a passenger car or 

 light truck equipped for this purpose. 



Choice of Site 



The choice of the instrument site depends both on its preliminary 

 location on the map to suit the general balance net to be used in the area 

 under investigation and on its topographic details. The topographic 

 details must be such that observations made with the instrument will be 

 of value for predicting underground structure and ore bodies. The prop- 

 erties which the site should have in order to minimize the topographic 

 effects on the gradient and curvature values are summarized on p. 332. 



