Geodesy, Gravity, Magnetics, Thermal Geophysics, Paleomagnetism and Rock Mechanics

  Since the inception of CSIR-National Geophysical Research Institute (NGRI) the Gravity, Magnetic and Paleomagnetic studies group has been actively involved in research of geodynamic processes as well as exploration of natural resources. Theoretical developments have concentrated on use of spectral decay to determine the depth to the causative sources located at different depths, removal of large wave length isostatic regional anomalies based on zero Free-air anomaly, regional-residual separation is developed based on finite element method, designed long digital filter operators using Hankel transform, algorithms for simultaneous computation of gravity and magnetic anomalies due to 2D and 3D arbitrary shaped bodies bounded by polygons, modified centroid method for estimation of Curie depth from magnetic/aeromagnetic data.

The new Gravity Map of India (2006) is  a flagship product of this Group, prepared in conjunction with several other organisations (GSI, ONGC, OIL) nation-wide.  Data from 51,356 gravity stations at 3 arc interval was included with implemetation of detailed terrain corrections to the gravity stations, and adopting new theoretical gravity formula based on the Geodetic Reference system 1980 (GRS80), and the International Gravity Standardisation Net 1971 (IGSN71) datum.

On the basis of the qualitative study of the Bouguer anomaly along the Narmada-Son Lineament, Qureshy (1964) concluded that the gravity high indicates a horst type structure in which the crustal upliftment was associated with the movement due to incorporation of material from the upper mantle into the crust. This contention is further corroborated by the under-compensation observed in the Airy’s isostatic anomaly map (NGRI, 1975) where the Satpura range is characterized by the positive isostatic anomaly. Recently, a thick accreted igneous layer at the base of the crust is delineated underneath the Deccan Volcanic Province and Rajmahal Traps. The isostatic calculations lead to the inference that the moderate topographic mass in the Southern Granulite Terrane (SGT) is overcompensated, and the mass of about 1.0 km of eroded surface material is yet to be accounted for. The low Te / low rigidity negates the possible presence of requisite mechanical strength that could resist the rebound to erosion-induced crustal unloading. The recent 2D and 3D lithospheric density modelling in SGT and Eastern Indian shields have delineated the Lithosphere-Asthenosphere Boundary (LAB) at a depth of about 130-140 km. The thinned lithosphere provides compelling evidence of lithospheric modification in the above two regions due to prolonged thermal perturbation and/or stretching during the Gondwana breakup.

Extensive G-M studies have been undertaken by the group to delineate the basement configuration over a part of Upper Assam and Arunachal Pradesh, Mizoram and in the eastern part of the Ganga Basin in Bihar. Similar G-M study has been undertaken over the Infra cambrian petroleum system of the Bikaner–Nagaur Basin, for shallow gas exploration in Krishna-Godavari Basin, South Rewa Basin, etc. Under the integrated geophysical exploration programme about 15,000 gravity data in Saurashtra, 3500 gravity data in Kutch, and over 10000 gravity data in Deccan Syneclise in Central India were collected by the group at an interval of ~2 km. Integrated modeling delineated the 3D configuration of the subtrappean Mesozoic sediments present at a depth range of 500 to 3000 meters followed by the basement. The study also delineates a number of high-density intrusive bodies in the basement at an average depth of about 4-7 km that best fits the residual gravity anomaly of the study area. The Group has also carried out extensive G-M study for manganese in central India.

Study of temporal changes in gravity related to changes in mass balance of the Earth has led to research in deformation and seismogenesis on the one hand and hydrological changes on the other.

Paleomagnetism studies in CSIR-NGRI have started way back in 1960’s where old classical instruments like Astatic Magnetometer was designed and operated for measuring the natural remnant magnetization in rocks to the present day sensitive spinner magnetometers. The Group has taken a lead in deciphering the continental drift theory by studying various rock units in India pertaining to different age groups in Geological Time Scale, including igneous, sedimentary and volcanics by determining the Pole Positions and reconstruction of continents in the past. Enormous work was done on dykes from Dharwar craton, Southern Granulite Terrains, Lonar lake, Deccan Traps, Rajmahal and Sylhet Traps, Sedimentary Basins (Siwalik, Cauvery, Kachchh, Vindhyan), Shillong Plateau in NE India, Koyna Borehole drilling Programme and Kimberlites).