Chap. 2] 



METHODS OF GEOPHYSICAL EXPLORATION 



17 



Magnetic instruments. Most widely used in magnetic prospecting are 

 the Schmidt magnetometers. In the Schmidt vertical intensity magnetom- 

 eter (see Fi^. 2-7 and 2-8), a magnetic system is suspended on a knife- 

 edge at right angles to the magnetic meridian; its center of gravity is so 

 arranged that the system is approximately horizontal in the area under 

 test. Deflections from this position are measured with a telescope and 

 scale arrangement, expressed in scale divisions, and are then multiplied 

 by a scale value to give relative vertical intensities. 



In the Schmidt horizontal magnetometer, 

 a magnetic system is suspended in the mag- 

 netic meridian and its center of gravity is 

 so adjusted that the system stands approxi- 

 mately vertical in the area under survey 

 and is deflected by the horizontal force. The 

 methods of taking the readings and applying 

 corrections are the same as for the verti- 

 cal magnetometers, except that for large 

 anomalies of vertical intensity a correction 

 for vertical intensity variations is required. 



In the Hotchkiss superdip, a magnetized 

 needle is suspended on a horizontal pivot 

 and provided with a counter arm so that 

 both the position and the sensitivity of the 

 needle may be controlled. The system may 

 be used at right angles to the direction of 

 the inclination so that it will then measure 

 variations in total intensity. 



The instruments described above furnish 

 the high degree of accuracy required in oil 

 exploration. In mining exploration, how- 

 ever, simpler devices are often quite satis- 

 factory. The earliest instrument of this kind 

 is the Swedish mining compass in which a 

 magnetic needle is suspended on a jewel and 

 a stirrup so that it can rotate about a horizontal and vertical axis. Another 

 early instrument is the dial compass which is a combination of a compass 

 and sun dial. Extensive use has been made of the dipneedle, which is a 

 magnetic needle capable of rotation about a horizontal axis and is essen- 

 tially a vertical-intensity instrument. 



Corrections. The following corrections are required in magnetic ex- 

 ploration: (1) correction for temperature of instrument, arising from the 

 fact that the magnets used for comparison with the earth's magnetic field 

 lose their strength with an increase in temperature; (2) a "base" correction 



American Askania Corp. 



Fig. 2-7. Schmidt-Askania 

 magnetometer. 



