PROJECTS COMPLETED AND THEIR FINDINGS

       a) Hydrocarbon exploration

 

The Mesozoic sediments throughout the world are known to be potential source rocks for hydrocarbon. These sediments are hidden underneath the Deccan trap cover with varying thickness. A systematic scientific study to locate the subtrappean Mesozoic sediments and structures that could act as hydrocarbon entrapment zones was proposed by NGRI using the integrated approach of seismic, gravity, magneto telluric and deep resistivity investigations. With the collaboration of ONGC and OIDB, the region between 20°-23°N latitudes and 68’-78°E longitudes covering the Saurashtra ,Kutch and the Narmada-Tapti block was taken up for exploration. The Gravity group was involved in acquisition, processing and interpretation of gravity data at about 30,000 stations in the above region at 2km grid interval. In addition to the above, the group also has taken up projects in Upper Assam and Arunachal Pradesh, Eastern Ganga basin and Amriti structure in Rewa area of Madhya Pradesh. The following are the details:

 

       i) Gravity Studies for Hydrocarbon Exploration Saurashtra, India.

 

  NGRI recorded gravity data at 10,000 stations at 2 km grid interval in the Western Saurashtra. Combing this data with an almost equal amount of data in eastern Saurashtra combined gravity anomaly maps were prepared. The shift modeling of the data with the constraints from other geophysical methods resulted in delineating of Mesozoic sediments of 2-3 km thick in Jamnagar and Dwarka basin beneath the Deccan traps cover.

 

(ii) Gravity Magnetic Surveys over Amriti Feature in Rewa District, Madhya Pradesh

 

 The gravity and magnetic studies around Amriti feature revealed a NE-SW regional component. A semi-circular gravity low is observed over Amriti Structure.

 

 

(iii) Hydrocarbon exploration Kutch on land, India.

The gravity data acquired at about 4000 stations was processed and modeled along eleven profiles. The results suggest five layer sequence of 5-6 km thickness over lying gneisic basement in the southern portion near Mundra while it reduces to 2-3 km towards northern part of Kutch Main land.

 

(iv) Exploration of Sub-trappean Mesozoic Basins in the Narmada-Tapti region of Deccan synclise.

   Gravity data acquired at 14000 stations at 2 km interval is processed to prepare gravity anomaly maps of the region between the latitude…. longitude. The BA map depicts gravity high between Narmada and Tapti which may be attributed to volcanic intrusives/ shallow basement. The large wave length lows in the northern land eastern part are due to isostatic compensation whereas small wave length gravity lows around Shirpur are due to low density sediments which are vital for oil exploration. Based on these studies, parametric wells were recommended for drilling by DGH, New Delhi.

 

v) Processing and Interpretation of Marine Geophysical data (Gravity, Magnetic and Bathymetric) from five offshore blocks of Western Indian continental Margin.

The offshore Gravity, Magnetic and bathymetry of five off-shore blocks are processed. Modelling along l31 profiles was carried out to delineate the basement configuration. The modeling results constrained from seismic data indicate block faulting in the basement.

 

(vi) Gravity and Magnetic Studies to delineate basement structures in parts of Upper Assam and Arunachal Pradesh.

 

The logistically difficult regions of Upper Assam and Arunachal Pradesh were covered by detailed gravity and magnetic studies. These studies delineated basement highs and lows in the study region. Prominent among them is Manabhum anticline which is a favorable location for oil exploration.

 

  (b) GEODYNAMIC STUDIES

Right from the inception of NGRI, the gravity group has been carrying out gravity measurements throughout the country mainly for geodynamic studies.

 

Scientists of this group played a major role in establishing the Gravity Base Network and reducing it to the IGSN 1971. The large amount of gravity data acquired across different traverses in Central india, Southern Granulite Terrain and Himalayan regions helped in understanding the geodynamic processes of the earth. A few of them are mentioned here briefly.

 

(i) Modelling the tectonic Evolution of Southern Granulite Belt of the Indian shield using integrated geophysical studies.

 

Closely spaced 3000 new gravity and magnetic measurements along Kuppam-Palani geotransect reveal the complex crustal fabric of region. Bipolar gravity and magnetic anomalies along Kuppam-Bhavani section of the transect reveals ancient continental collosion along the transition zone. The modeling reveals a four layer crustal configuration with Moho varying between 41 km to 46. Late Archean continental collision along the transition zone and subsequent crust-mantle interaction beneath the Palghat gap are suggested as the twin mechanism covering the present day crustal evolution of SGT.

 

(ii) Modelling of Gravity Magnetic profiles from Nagaur-Jhalawar across Aravali–Delhi fold belts.

 

This profile in Western India covers Delhi and Aravali Super group of rocks and parts of sedimentary sequence on either side of the travali mountain ranges. The modeling results show large crustal thickness of 42-43 km under Aravali and high density (3.1 g/cm³) lower crust which varies in depth from 20-30 km which may represent underplating due to magmatism that may be even responsible for the uplift of Amaravali region.

 

    (iii) Gravity- Magnetic profile of Seoni-Rajnandgaon.

 

This profile covers part of Bhandara craton in Central India and follows a deep seismic sounding profile recorded earlier. Both free air and Bouguer Anomaly profiles show a good correlation with Satpura high ranges coinciding with gravity “highs” indicating lack of isostatic compensation. The large weave length gravity “highs” and lows towards the north and south of Central India futrure indicate deep seated high and low density bodies respectively.

 

     (iv) Satellite gravity Project

Satellite gravity studies were taken up with the collaboration of both national (Space Application Centre, Ahmedabad) and international institutions (GEOMAR, Kiel, Germany). Under the former project, the,area between 6-25^o N latitude and 66-105⁰ E longitudes was studied with reference to the regional gravity field. The regional density model helped to revealthe continent–ocean interaction across the Indian Peninsula. More detailed studies were carried out across Laxmi Ridge and Basin and Andaman Subduction Zone. Under the Indo-German Project, The regional elevation and Free air gravity data over India and adjoining oceans was compiled to make an overall assessment of the most striking features which is vital for further quantitative studies. Three dimensional forward modeling and application of process oriented models was used to explain the nature of prominent anomalies of the gravity field.

 

      (v) Establishment of Gravity Observatory at Maitri, Antarctica

 

Dr. V.M. Tiwari and Mr. M.B.S.Vyaghreswar Rao constructed gravity observatory and carried out Absolute Gravity measurement using FG-5 at Maitri station in Antarctica during 2003-04. First Absolute Gravity value was recorded during December 2003 to March 2004. They also carried out total magnetic field measurements also along a traverse in the east Antarctic land.