On utility of space-borne vector magnetic measurements in crustal studies

Abstract

The MAGSAT data over India and contiguous regions for quiet periods (K_p≤3) have been analysed to isolate components of crustal origin. The main field contribution was removed with MGST (4/81) 13th degree and order spherical harmonic expansion. The external current contributions were estimated using the H- and Z-components directly and assuming the potential to have P₁ (cos θ) dependence. A quadratic polynomial for each satellite pass had to be removed to make residuals from adjacent passes agree with each other. These residuals over 2° × 2° blocks were averaged to prepare the anomaly maps. The anomaly maps exhibit better resolution than Langel et al. Features associated with the Indian Ocean geoid low clearly appear in the results. A test on internal consistency of reduction techniques was made by synthesizing B- map through the projection of three vector maps and comparing the same with the map obtained from direct measurements. It is noticed that near the dip equator the features of Z- map are not reflected in the B- map. We conclude that in those regions where the main field direction is such that one or two of its components X, Y or Z become zero or very small, the crustal anomaly in that direction will not influence the B- anomaly for it would have no projection in the direction of the main field.