234 EXPLORATION GEOPHYSICS 



neutralizing current is supplied from a storage battery, through a constant 

 impedance network, to maintain the current at a constant value. The 

 polarity of this neutralizing field is opposite to the ambient field, as indi- 

 cated by the arrows S'N'. 



The recorder is provided with a range switch, with suitable steps, 

 whereby full scale reading may cover values from a few hundred to many 

 thousand gammas, with a readable sensitivity of better than 1 to a few 

 gammas, depending on the scale range. 



The magnetometer may be calibrated by using a Helmholtz coil to set 

 up a magnetic field of specific intensity around the detector unit. Every 

 range scale of the instrument may be calibrated in this way. 



Orientation 



The flux-gate magnetometer must be maintained at a constant average 

 orientation throughout a series of readings, so as not to introduce undesir- 

 able variations by changes in any component of the earth's magnetic field. 

 Gyroscopic orientation alone has been utilized in some instruments, but 

 does not possess the necessary accuracy in orientation. The most successful 

 form of orientation involves some type of servo-control, sometimes work- 

 ing in conjunction with gyroscopic control, whereby the magnetometer is 

 maintained in proper orientation by means of a pair of auxiliary elements 

 similar in type to the magnetometer itself. f Two of such elements mounted 

 at right angles to each other define a plane. When the plane of the two 

 elements is parallel to the magnetic horizon, a zero field is impressed and 

 a zero control signal exists. With the magnetometer mounted normal to 

 the plane of the two control elements, it is oriented parallel to the field and 

 will give an accurate measurement. When the plane is tilted and departs 

 from its correct alignment, the actuating current to the servo-motors is 

 proportional to the sine of the angle of misalignment. The element may be 

 rotated about either axis of the gimbal by one of the servo-motors, each 

 of which is supplied with an actuating current of proper sense to restore 

 that detector element into parallelism with the magnetic horizon. Two 

 amplifiers and their controlling magnetic elements are required. 



The sensitivity of an airborne magnetometer depends on the inherent 

 sensitivity of the measuring device when used on a perfectly stable plat- 

 form and also very much on the magnetic noise level. This arises in part 

 from the lack of perfection in the stability of the platform and from the 

 fixed and variable efifects arising from the permanent and induced mag- 

 netization of the aircraft itself. Under the most favorable conditions of 

 use, the flux-gate type instrument affords a sensitivity of better than one 

 gamma and on the average the data are reliable to one or two gammas. This 

 sensitivity is better than that available with any type of instrument com- 

 monly used for prospecting on the ground. 



t R. D. Wyckoff, "The Gulf airborne magnetometer," Geophysics, Vol. 13, pp. 182-208, 1948. 

 Victor Vacquier, R. F. Simons, and A. W. Hull, "A magnetic airborne detector employing 

 magnetically controlled gyroscopic stabilization," Review of Scientific Instruments, Vol. 18, pp. 

 483-487, 1947. 



