The Interpretation of Magnetometer Array Studies

Abstract

Two‐dimensional arrays of three‐component magnetometers have introduced a new geophysical technique useful in the study of the upper mantle and crust, particularly in tectonically active regions. Sources of incident magnetic fields include geomagnetic substorms and the daily variation. Methods and difficulties in interpretation are discussed in the light of the first five years’ use of magnetometer arrays. Maps of Fourier transform coefficients assist qualitative interpretation but differ from other geophysical anomaly maps in combining the source field and the response of the Earth. For quantitative interpretation the normal (source) and anomalous (response) fields must be separated. Methods of separation and their limitations are considered. Numerical modelling of structures to fit normalized anomalous fields for two‐dimensional geometries of conductors and fields is successful in some cases. In others, although the variation field anomaly and the current system are long and narrow, the behaviour is not two‐dimensional because the regions of induction are distant and/or of unknown shape. Such ‘current concentration’ anomalies are common in the crust and can give rise to very large anomalies. In cases where the conductive geometry is complicated and not even crudely two‐dimensional, Fourier transform anomaly maps are not very useful as their features are extremely variable, probably because of interference of the anomalous fields. In such cases transfer functions may prove helpful. Development of three‐dimensional numerical modelling has started but is cost‐limited at the present stage. Copyright © 1973, Wiley Blackwell. All rights reserved