108 EXPLORATION GEOPHYSICS 



intensity component in less than one-half hour. Such a storm would be 

 apparent to the operator of a field magnetometer if he were taking readings 

 at the time. A severe storm may show changes back and forth of the 

 amount mentioned at 30 to 45-minute periods with a dying out or decrease 

 in amplitude with time, but with the possibility that the storm will later 

 come back at high intensity. 



Any magnetic field work must quite obviously be suspended during a 

 severe storm. Any stations taken would be unreliable, even though a base 

 magnetometer were used to record daily variation. Fortunately such storms 

 do not occur often; ordinarily not more than one or possibly two take 

 place during the course of a field season. 



The hay type of magnetic storm is a steady swell, a rise or fall in 

 magnetic force, that may last for two to three hours. It may result in 

 some 20 minutes change in declination, although the vertical and horizontal 

 components of magnetic force are most affected. The bay type of storm 

 usually repeats itself at about a 24-hour interval. 



Under ordinary conditions, for field work in the United States it is 

 possible to make arrangements with the nearest magnetic observatory to 

 advise by wire collect of any significant magnetic disturbances. For the 

 Western part of the country the observatory at Tucson, Arizona, is 

 usually the nearest one. As a practical matter the usual base checks, or 

 base instrument for obtaining daily variation, will show if an unusual 

 condition exists. An erratic station, or stations, which do not fit into the 

 contouring may be due to such disturbances. 



MAGNETIC PROSPECTING INSTRUMENTS 



The Compass. — The most familiar magnetic instrument is the com- 

 pass. When pointing to the magnetic north, a compass needle aligns itself 

 with the horizontal component of the earth's magnetic field at the point 

 of observation. This property of magnets is utilized in magnetic instru- 

 ments using magnetic needles ; that is, a magnetic needle suspended and 

 free to rotate will align itself with the direction of the lines of force of 

 the magnetic field. 



A small counterweight is necessary on a compass needle. For work in 

 the northern hemisphere, it is placed on the south side of the pivot. This 

 counterweight balances the needle by introducing a small moment of force 

 due to gravity and causes the needle to swing in a horizontal plane. The 

 resultant center of gravity is therefore shifted to the south side of the 

 pivot. The need for this counterweight brings out the fact that the lines 

 of force of the earth's magnetic field in most areas have an appreciable 

 dip. The needle seeks to align itself with the direction of the earth's 

 magnetic lines of force and would do so in so far as possible, rubbing on 

 the compass case, were the counterweight not used. 



