590 



EXPLORATION GEOPHYSICS 



employed for shallow work of about 1000- foot depth is illustrated in Figure 

 368. This magnetometer is of the suspension type with a torsion head at the 

 top of the instrument for orientation. The initial and final readings (with 

 energizing current ofif, on and off) are made by means of scale and telescope. 

 The instrument has a sensitivity of about 2 gammas per scale division. 



Magnetometer (Flux Type). — An improved, more sensitive type of 

 magnetometer is illustrated in Figure 369. This equipment comprises essen- 

 tially a Permalloy-core flux coil connected to a sensitive flux meter. A 



A, 



r 



iTTBWIim 



Fig. 369. — Magnetometer, flux type. A, flux coil; A', alignment 

 sight; B, potentiometer and batteries for galvanometer lamp; C, 

 flux meter; D, light shield and cover for scale and control panel. 



potentiometer and a reversing switch bring the light beam to the proper 

 scale position, and also compensate for thermo-electric and other spurious 

 potentials. The instrument illustrated has a higher sensitivity of 0.61 gammas 

 per scale division, and a lower sensitivity of 1.93 gammas per scale division. 

 The lower sensitivity is obtained by reading the first reflection of the light 

 beam from the galvanometer, while the higher sensitivity is obtained from 

 the multiple reflected beam. 



Field Procedure 



In practice, the field procedure comprises the following steps: (1) After initial 



adjustment of the instrument, the magnetometer operator records the scale reading of 



the instrument. (2) The direct current power then is applied to the ground and the 



operator records the new reading of the instrument. (3) The power flow is interrupted 



