Subsurface Methods as Applied in Geophysics 



1049 



intensity between the points. Magnetometers may be read to approximately 

 one or two gammas, depending upon the sensitivity of the instrument. 

 Corrections for temperature fluctuations are generally applied to field 

 observations, as variations in temperature change the instrument sensitiv- 

 ity by varying the moment of the magnetic system. (See fig. 550.) 



Rapidly gaining in popularity for reconnaissance surveys is the first 

 geophysical instrument to be used successfully in a moving aircraft, the 

 saturable-core or flux-gate magnetometer. According to Muffly,-^^ 



. . . this type of magnetometer is basically a rod or strip of ferromagnetic 

 material acting as an open magnetic core for one or more a.c. windings con- 

 nected to exciting and indicating circuits which can measure the magnetization 

 produced in the core by an ambient magnetic field. 



Figure 551. Magnetic vectors over an ore body at Falconbridge, 

 Canada. (After Eve and Keys.) 



The instrument has a sensitivity of about one gamma, is continuously 

 recording, and measures the total magnetic-field intensity. The total-field 

 recording of the saturable-core magnetometer requires changes in the 

 interpretation technique ordinarily applied to field magnetic data of the 

 vertical-component magnetometer. Using modern aircraft such as the DC-3, 

 surveys may be successfully carried out over land, swamp, jungle, or 

 sea in a minimum of time. 



The chief problem confronting the operators of air-borne equipment 

 is the accurate determination of position. War-born, high-frequency radio 

 methods are being used for many of the surveys. The use of the air-borne 

 magnetometer has not only speeded up field operations but has also in- 

 creased the accuracy of the results. By making observations above the 

 earth's surface, interference due to near-surface effects is largely eliminated. 



^^ Muffly, Gary, The Airborne Magnetometer: Geophysics, vol. 11, no. 3, pp. 321-334, July 1946. 



