Comparison of the Aero and marine magnetic data over Peninsular India.

Rigorous reprocessing of the degree sheet aero magnetic maps over peninsular India up to 17° N acquired from the Geological Survey of India was undertaken. These degree sheet maps include surveys that have been carried out by different agencies at different epochs and altitude and have not been corrected for the main field. The data grid spacing has a crucial effect on the nature of the map prepared. In order to utilise the aeromagnetic data for studying the deep crust, we have manually redigitised the degree sheet maps at 6' interval and corrected for the main field using IGRF (International Geomagnetic Reference Field) model corresponding to the epoch and altitude of the data collection. Within the Cuddapah region the aeromagnetic data were not available with us; we have therefore merged our ground magnetic data in this region. In a similar manner the published marine magnetic anomaly data were re-digitised. To be able to prepare a composite magnetic anomaly map the aeromagnetic data were downward continued to the mean sea level. The two data sets are handled independently, as the errors in the two are of different nature. We do not merge the data sets, but just place them adjacent to each other to study the trends of the anomalies. The colour shaded aero cum marine magnetic anomaly map over Peninsula India, thus prepared brings out several features that are presented. From the analysis of the aeromagnetic data, we find that the source rocks of the aeromagnetic anomalies are the host province of Charnokites in the Southern Granulite Terrain (SGT) and the intrusives/iron ore bodies in the Dharwar belt (Harikumar et al, 2000). The marine magnetic data is also analysed in a similar manner, to be able to locate the position of the magnetic sources. We find that the SGT continues to the offshore between Madras and south of Karaikal. North of Madras the trends change in the offshore region. Results of the combined aero and marine magnetic sources are presented.