Field trial measurements of spatial and temporal correlations of electromagnetic fields in a coastal environment
Publish date: 2001-01-01
Report number: FOI-R--0178--SE
Written in: Swedish
An interesting problem when developing a system for magnetic anomaly detection (MAD) of foreign objects concerns the ability of a separate reference sensor to provide sufficient information on the electromagnetic background to make it possible to filter out the signal of the object itself from the noisy raw-data. In this report a study of the spatial and temporal correlations of the geomagnetic field for a typical coastal environment is presented. The surroundings of Djupviken in the southern archipelago of Stockholm were chosen for this purpose. The measurements showed a strong frequency dependence of the correlations, with the higher frequency components less correlated in both time and space. For unfiltered data, where the dc-component had been removed by a subtraction of the mean value of the time series, time lags in the range of 3.6 to 4.7 hours were required for reaching the first zero-crossing of the autocorrelation curve. For data, which have been high-pass filtered with a cut-off frequency of 20 mHz (corresponding to the lowest frequency of the MAD-frequency band), a lag time of about 10 s was obtained instead. In the same way the time average of the spatial correlation decreased by the filtering. For a measurement performed with a distance of 780 m between the magnetometers it decreased from 0.62 to 0.26, and for a magnetometer separation of 520 it decreased from 0.73 to 0.40.