42 MEASUREMENT PROCEDURES [Chap. 3 



with miniature detecting units. As depth to geologic bodies is com- 

 mercially a most desirable quantity, wells are used whenever possible. 

 Examples are: seismic determination of average velocities; measurement 

 of resistivity, temperature, and radioactivity in wells ; and observations at 

 different levels of mines. Although attempts have been made to survey 

 from moving supports (automobiles, ships, floats, submarines, and planes) 

 little has been accomplished along this line, since measurements on moving 

 supports introduce a reduction in accuracy. The present trend in geo- 

 j^hysics is more toward accuracy than speed. The use of moving objects 

 and moving detectors plays a part in the military and oceanographic fields 

 of geophysics. Stationary detectors for determining variations in physical 

 effects of stationary objects are used in the recording of magnetic variations, 

 corrosion voltages, traffic vibrations, strain gauging, sound ranging, and in 

 small-scale laboratory investigations where model ore bodies may be used 

 and detectors remain stationary. Table 3 gives a summary of these 

 procedures. 



