232 EXPLORATION GEOPHYSICS 



the measurements are made at lower elevations to give more emphasis to 

 the local features, v^hich are superimposed on the deeper-lying regional 

 effects. Work of this type usually is done from automobile, motorship, or 

 helicopter. Each type of survey fulfills a need and serves a useful purpose 

 in the exploration picture. 



The stationary-type magnetometer was one of the first instruments used 

 in exploration geophysics. It has remained unchanged in basic principle 

 for nearly two decades. Various research projects have been devoted to 

 the development of an improved mobile-type magnetometer, and many 

 instruments have been proposed. The most successful is the "flux-gate" 

 type which operates due to the sensitivity of saturated core devices to super- 

 posed or ambient magnetic fields. These magnetic sensitive devices have no 

 moving parts and hence are inherently independent of the disturbances 

 caused by acceleration. They are therefore ideal for mobile operation. The 

 development of the high permeability core materials has made possible the 

 high sensitivity of the flux-gate type instruments. For use on the ground, it 

 has few advantages over the conventional magnetometer. 



The magnetization curve of many ferromagnetic materials is non-linear 

 (Figure 72), and coils having such iron cores change their effective alter- 

 nating current reactance when a unidirectional magnetic field is impressed 

 on the field set up by an alternating current flowing in the coil. This change 

 in effective reactance also results in a distortion of the alternating current 

 wave form. For ordinary iron core materials, the unidirectional field must 

 be quite intense, but for certain high-permeability alloys, the earth's field 

 is of suf^cient intensity (.6 gauss) to induce a magnetization which is an 

 appreciable part of the total saturation. A coil with such a core will be 

 sensitive to small changes in field intensity and becomes the heart of a flux- 

 gate magnetometer by operating the core in the saturation region at the 

 sharp knee in the B-H curve. 



Schematically,! as may be seen by referring to Figure 115, the sensing 

 element has a magnetic core that comprises essentially two thin strips of 

 Mumetal, a high-permeability alloy, wound with identical windings through 

 which an alternating sine wave exciting current is passed. This exciting 

 current is indicated at A and A'. The two strips of Mumetal are 

 joined together at each end to make a closed magnetic circuit. The two 

 windings are connected so as to make the alternating fields additive around 

 the closed core circuit. However, an external ambient field applied along 

 the axis of the core aids the exciting field in one strip and opposes the 

 field in the other strip. During the period that the ambient and the exciting 

 fields are additive, the total flux is increased and the saturation point is 



t James R. Balsley, Jr., "The airborne magnetometer", U. S. Department of the Interior, 

 Geophysical Investigations, Preliminary Report 3, 1946. 



Gary Muffly, "The airborne magnetometer", Geophysics, Vol. 11, pp. 321-334, 1946. 



E. P. Felch, W. J. Means, T. Slonczewski, L. G. Parratt, L. H. Rumbaugh, and A. J. Tickner, 

 "Airborne magnetometers", Electrical Engineering, Vol. 66, pp. 680-685, 1947. 



John E. Quaile, "Airborne submarine detector equipment", Military Engineer, Vol. 39, No. 

 258, pp. 166-167, April, 1947. 



