Presents a review of fluxgate magnetometry based on a survey of the literature and on the work of the author and colleagues. The theory is outlined and an evaluation of the merit of the different types of sensors is made based on the sensor geometry. The design of a second-harmonic feedback magnetometer is discussed with respect to sensor configuration, frequency response, noise, offset, temperature dependence and long-term stability. Examples of the parameters are compiled from the literature and some new results on the long-term stability are presented.

https://iopscience.iop.org/article/10.1088/0022-3735/12/4/001/pdf

The fluxgate magnetometer

A review of various types of fluxgate sensors is given and they are divided into two major groups, parallel-and orthogonal-gated fluxgates. A theory for parallel and for orthogonal gating is presented and the gating characteristics for the two cases are calculated from measured B-H curves. The theory is verified by a close fit to the experimentally derived curves and finally a general expression for the output voltage from the two types of fluxgates is presented.

The fluxgate mechanism, part I: The gating curves of parallel and orthogonal fluxgates

The output voltages from various fluxgate probe types are found to have the same general form, and an analytic model has been developed with which the output may be calculated. In the saturation region where the output pulse is developed, the differential permeability can be closely approximated by an exponential, dB/dH = k_{1}H^{K_{2}} , where k 1 and k 2 are on the order of 100 and -2.0, respectively, for cores of Supermalloy or Permalloy 80. Using this model, the effect of the nature of the excitation function on output pulse amplitude and width and the effect of the squareness of the core characteristic can be clearly demonstrated, and good agreement with experimental results has been obtained.

An analytic model for the fluxgate magnetometer

The airborne magnetometer AN/ASQ-3A has been modified and auxiliary equipment has been developed to meet requirements for constructing isomagnetic contours of the total magnetic field over extensive areas for use in magnetic prospecting. The principal features of the instrument system are: (a) a self-oriented magnetometer which utilizes the output of a system of saturable inductors to provide a d-c response of low noise level and drift rate; (b) instruments for aiding the pilot to follow predetermined lines of a grid or network covering the area to be explored; and (c) an interlocked group of recorders for automatically correlating a continuous magnetometer record with plane position and altitude.

The system described has been used extensively by the United States Geological Survey, the United States Naval Petroleum Reserves, and the United States Naval Ordnance Laboratory in aerial magnetic exploration under a wide variety of conditions. Similar systems are also being used by a few commercial companies.

Airborne equipment for geomagnetic measurements

Longitudinal magnetostriction of the core material is found to be the principal source of offset and noise in present state-of-the-art flux gate magnetometers. Magnetostrictive offset is identified with even harmonics of the steady-state induced EMF resulting from the periodic elongation (or contraction) of the core material. Magnetostrictive noise is identified with random fluctuations of the magnetostrictive offset caused by frictional forces exceeding the magnetostrictive stress when the core material is near zero elongation. An analytical model and experimental results of magnetostrictive offset and noise are presented. It is expected that flux gate magnetometers with zero magnetostriction of the core material will yield a significant increase in signal-to-noise ratio with corresponding decreases in offset and magnetomotive drive power.

Magnetostrictive offset and noise in flux gate magnetometers

Air-borne magnetometers developed during the war for the detection of submerged enemy submarines now have become important tools for aerial geophysical exploration. The instruments described utilize the second harmonic outputs produced by the magnetic field in saturated core inductors for field measurement and for control of servo stabilizing systems acting around two axes to maintain the measuring element in alignment with the earth's field. The field to be measured is compared continuously with that produced in the measuring inductor, winding by an accurately controlled direct current. The sensitive element usually is towed in a bombshaped bird sufficiently removed from the airplane to escape the effects of its magnetic field. A continuous record is produced which, when keyed with position records obtained by photography or shoran, may be used in the preparation of maps of total magnetic field intensity.

Air-Borne Magnetometers for Search and Survey

Developed under the impetus of the submarine menace of World War II, the airborne magnetometer has found many peacetime uses. Navy airplanes equipped with magnetometers for exploration of Antarctica were used in the recent United States Navy expedition. An expedition now is studying the Aleutian Alaskan volcanos and the Aleutian submarine trench. From there it will proceed to Hawaii and Bikini.

http://ieeexplore.ieee.org/iel5/6413714/6443600/06443620.pdf

A. J. Tickner

Papers

Air-Borne Magnetometers for Search and Survey

Air-borne magnetometers