Earth's field magnetometry

In the last decade a vast expansion has been experienced in the use of observations and measurements of the Earth's magnetic field. The type and quality of the observations required by newly developed geomagnetic research interests, in themselves partly stimulated by the possibilities offered by new types of magnetometers, greatly increased the amount of effort which has been applied to further developing the instruments and devising new ones. The demand arose from two main sources. The first was the evolution, with the advent of satellite experimentation, of magnetospheric physics, and the second derived from the eventual acceptance of reversed magnetization in rocks as being due to reversal of the Earth's field itself. This paved the way to the chronological interpretation of alternating positive and negative anomalies, and stimulated vigorous research into ocean floor spreading and crustal movements. It is not possible in a single review article to describe all the variants of sensors and detection systems which have been developed to meet the very wide range of observations currently required, for example for survey by land, sea and air, measurement of minute magnetization of rock samples in paleo and archeo-magnetism and for rocket, satellite and space probe applications. Recent excellent reviews of the objectives of including magnetometers in spacecraft experimentation, of the instruments used and of the special techniques which have been developed to overcome the operational and interpretational difficulties of that branch of geomagnetism are given by Heppner (1967) and Ness (1970). The subject of aeromagnetic surveying is discussed in depth by Hood and Ward (1969). This review is, therefore, limited to the generalities of the devices currently in use for ground-based magnetic measurements. The operating principles of each detection system are explained in simple physical terms. Any important developments or extensions of basic technique which are of particular significance are discussed, together with their essential characteristics and limitations. Details of electronic detection systems are not described except where appropriate reference cannot be included. The instruments described are in the following main classifications: magnetometers using nuclear and atomic resonance, saturable cores, suspended magnets, induction coils and superconducting devices. In addition, the magnetometer systems which use artificially created fields to allow vector measurements to be made by total field sensors are described. Their operation performance is assessed and the use of bias and backing-off fields are discussed. The use of the proton gyromagnetic ratio as the absolute standard of geomagnetic field measurement is summarized. There follows a discussion of the requirement for routine ground-based geomagnetic data to be supplied to the scientific community in digital form for direct machine read-in, and some speculation about practicable automatically recording observatory systems. This review was completed in April 1972.