Delineating structural features and deciphering radiometric concentration zones along the west coast of India using aerogeophysical data

摘要

A regional multi-sensor airborne geophysical survey was carried out with 2.5 km line spacing and flight altitude of 150 m above ground level (AGL) over an area of 36,900 km2 employing magnetic and spectrometric sensors in Hosdurg-Vengurla area of western offshore of India. The objective of the airborne geophysical survey was to delineate the continuity of magnetic lineaments and structural features in the continental shelf region and to identify the spectrometric elemental concentration zone along the west coast. The area is occupied by rocks of Peninsular Gneissic Complex (PGC) intruded by basic dykes and granites. The southeastern part of the area is occupied by granulites represented by charnockites and khondalites. In the northern part, the PGC is overlain by metabasalts / schistose rocks and phyllites, variegated phyllites, argillite and metagreywackes belonging to the Neoarchean Bababudan Group of Dharwar Supergroup of the Western Dharwar Craton. The results of aeromagnetic surveys have brought out clearly the contacts between the different lithological units. The granulite group of rocks exposed in the southeastern part is well-delineated in the form of high-intensity magnetic anomalies trending NNW-SSE around Hosdurg, Kasargod, Manjeswar and Srinivasa Nagar, which extend further west in the offshore region up to 15 km. The aeromagnetic anomaly map shows the extension of the magnetic lineaments that are interpreted from high-altitude data in to the offshore region. Prominent NNW-SSE- to NW-SE-trending magnetic discontinuities were found to be associated with west coast fault system, besides E-W- to ENE-WSW- and NE-SW-trending discontinuities. In addition, two large prominent and circular-shaped anomalies of high magnetic intensity, disposed towards west of St Mary's Island and west of Gokarna are likely to be associated with basic intrusive bodies within PGC. The spectral analysis of magnetic data reveals that the magnetic interface over the land area is deeper than that in the offshore region. The estimated depth of magnetic interface over offshore is relatively shallow and is in good agreement with free-air gravity data. The ternary image of spectrometric data (K, Th and U) shows the distribution and relative abundance of elements and correlates well with the geology of the area. The spectrometric maps (K, U and Th) helped in delineating the element concentration zones. Moderate concentrations of thorium are found along the coast near Hosdurg and Mangalore and few moderate concentration zones of uranium are observed over granulites.