210 EXPLORATION GEOPHYSICS 



The magnetic observations are made with the regular horizontal and ver- 

 tical component magnetometers. The latter are mounted on a rigid 

 platform above the movable tank. The "depth" of the magnetic body is 

 simulated by varying the height of the magnetometers above the tank. 

 Any desired shape and configuration of the body may be simulated by 

 proper shaping of the sand-iron mixture. 



The iron filings are preferably of the coarse type obtained from milling 

 operations. Those employed by the author were obtained by milling a 

 stack of laminated iron sheets across grain. (The iron sheets came from 

 the core of an old power transformer.) The milling was done on a bias to 

 give short cuttings. A mixture of sand and 10 to 30 per cent of filings, 

 by weight, gives an easily handled material with sufficient magnetic per- 

 meability to allow sharp readings. 



The movable tank arrangement with stationary magnetometer involves 

 considerably more preparatory work than the use of a stationary tank or 

 earth pit with a movable magnetometer. However, more accurate and rapid 

 work may be done with the movable tank arrangement, because the mag- 

 netometer may be adjusted at the beginning of the experiment and not 

 disturbed thereafter. Thus, the errors of misorientation, leveling, and other 

 similar instrumental errors are avoided. The magnetometer is undamped 

 at the beginning of the experiment and readings are made thereafter 

 without disturbing the instrument, A second magnetometer is employed for 

 mapping the diurnal variations. 



GEOMAGNETIC ANOMALIES 



The outer crust of the earth comprises a magnetically heterogeneous 

 assemblage of rocks which exists in the earth's magnetic field. Certain 

 rocks exhibit the properties of magnets (induced or permanent) and super- 

 impose their own magnetic fields on that of the earth. The superimposed 

 fields are termed major, continental, regional, and local anomalies, depend- 

 ing on the scale of the geologic irregularities which produce them. 



The major and continental magnetic anomalies may be defined as varia- 

 tions of the earth's magnetic field associated with major and continental 

 irregularities (inhomogeneities of composition and/or structural distor- 

 tions). Continental anomalies are negative in Europe and positive in 

 North and South America. Major anomalies may show pronounced 

 trends, such as a positive major trend paralleling the Rockies and the 

 Andes and a negative major trend paralleling the Alpine chains. f 



The regional anomalies cover smaller areas, one of the best known 

 being in the Province of Kursk, Russia. Regional anomalies indicate the 

 tectonics and the stratigraphic and the petrographic character of the upper 

 portion of the rock zone. A basin that has strongly magnetic beds, such as 



t W. p. Jenny, Abstract of paper delivered before the American Association of Petroleum 

 Geologists, Oil and Gas Journal, April 11, 1940. 



