Delimitation of the outer limits of the continental shelf

The continental shelf as defined under the United Nations Convention on Law of the Sea (UNCLOS) comprises the seabed and subsoil of the submarine areas of a coastal State that extend beyond its territorial sea throughout the natural prolongation of its land territory to the outer edge of the continental margin (comprising the geological shelf, slope and rise), or to a distance of 200 nautical miles from the territorial sea baselines where the outer edge of the continental margin does not extend up to that distance (


The regime prescribed for the outer limits of the continental shelf where it extends beyond 200 M is contained in Part VI (articles 76 to 85) and Annex II of UNCLOS.  The Convention also makes an exception to these criteria for the continental margins of the coastal States in the southern part of the Bay of Bengal (“Statement of Understanding concerning a specific method to be used in establishing the outer edge of the continental shelf” adopted by the Third United Nations Conference on the Law of the Sea on the 29th August 1980). 


A coastal State which intends to establish the outer limits of its continental shelf beyond 200 nautical miles has to submit particulars of such limits to the United Nations Commission on the Limits of the Continental Shelf (CLCS) along with supporting scientific and technical data within a specific timeframe from the date the country ratified the Convention.   The CLCS, which is a 21 member UN body of experts on geology, geophysics and hydrography will consider the data and other material submitted by the coastal State and make recommendations in accordance with article 76 and the Statement of Understanding. 


Considering that India’s continental shelf extends beyond the 200 nautical miles from the territorial sea baselines, the Ministry of Earth Sciences (MoES) undertook a major multi-institutional national programme of collecting, processing, analyzing and documenting the requisite scientific and technical information for delineating the outer limits of the continental shelf in the Arabian Sea and the Bay of Bengal including the western offshore areas of the Andaman-Nicobar Islands. The task was implemented by National Centre for Antarctic and Ocean Research, an autonomous institute of Ministry of Earth Sciences with close participation of Ministry of External Affairs, Naval Hydrographic Office of Ministry of Defence, National Institute of Oceanography and National Geophysical Research Institute of Council of Scientific and Industrial Research, Geological Survey of India, Directorate General of Hydrocarbons, and Oil and Natural Gas Corporation.


In one of the largest ever marine geophysical surveys conducted by India, over 31,000 line km of multichannel seismic reflection, gravity and magnetic data together with bathymetric information were acquired along 42 pre-determined profiles. In addition 100 Ocean Bottom Seismometers (OBS) were deployed with a significant retrieval rate of 92% and high quality wide angle seismic reflection and refraction data were obtained at critical locations. The reflection data was utilised to derive the depth to the basement along the different profiles through a process of iterative modelling of the two-way travel time of the acoustic signals passing through the water and the sediments and sedimentary rocks occurring above the basement.  From this information and the knowledge of the depth to the seabed, the thickness of the sediments above the basement at thousands of discrete points along these profiles was calculated.   Whereas the OBS data was used to constrain the velocity information obtained from the reflection surveys, the magnetic and gravity data were utilised to gather additional evidence on the nature of the basement. 


The entire data acquisition programme comprising of 11 legs was implemented during the year 2002-2004 in which over 40 Indian scientists and officials from 7 national organizations and institutes participated. This was followed by the processing of data, its interpretation with rigorous quality checks and analysis which were carried out concurrently at three national institutions, over a period of over two years. 


The studies carried out by the Indian scientists have provided a wealth of scientific data of the nature of the seabed and subseabed in and off the Indian EEZ.  With two of the world’s thickest accumulations of sediments on the seabed (the “Indus Fan” in the Arabian Sea and the “Bengal Fan” in the Bay of Bengal) derived from the Himalayas and a complex geological setting in the western offshore of Andaman-Nicobar islands, the data interpretation has been a challenging exercise in more ways than one.  While the scientific and technical information gathered would amirably serve its primary purpose of delineating the outer limits of India’s continental shelf beyond 200M, the surveys have also furnished invaluable data that could form the cornerstone of the country’s planned endeavours in the oceanic realm.  Further detailed studies on the data collected are expected to provide the scientific community with answers such long-debated questions as the history and geological evolution of the seas and offshore sedimentary basins around us, the origin and evolution of such enigmatic features as the 85o East Ridge in the Bay of Bengal, the Laxmi and Laccadive Ridges in the Arabian Sea, the Gulf of Mannar, the offshore extent of the Deccan volcanics, the reasons for the association of gravity lows in the Bay of Bengal with structural highs, the development of the fans vis-a-vis the origin and growth of the Himalayas etc.  The studies also open a new vista in the exploration for hydrocarbons in the offshore areas of the extended continental shelf beyond 200M.  In addition, the data gathered is expected to provide specific insights related to such areas as marine ecosystems, unconventional energy, mineral resources, and hazards resulting from extreme events, such as earthquakes and tsunamis.


On the 11th May, 2009, India filed to the CLCS, her first partial submission under the provisions of article 76 for a continental shelf extending beyond 200 nautical miles (M) from the Indian baselines (   A second partial submission for another part of the extended shelf under the provisions of the Statement of Understanding has also been finalised and provided to the MEA for filing before the CLCS.  India already has 12 nautical miles of territorial sea and 200 nautical miles of the Exclusive Economic Zone (EEZ) measured from the baselines.  With the anticipated addition of approximately 1.2 million square km of extended continental shelf from the two submissions to the ~2 million sq. km of EEZ, India’s seabed-subseabed area would become almost equal its land area of 3.274 million sq. km.


Marine Geophysical Database

Considering the huge volume of geoscientfic data collected as a part of the Programme and its intrinsic value, a marine geophysical datacenter has been established at NCAOR.  The Data Centre allows for a web-based geospatial database of the marine geophysical and bathymetric information pertaining to the Indian continental shelf. The metadata functionality of the database describes the primary details in terms of when and how the data was collected, the nature of data, how the data was processed, necessary supporting information that went in to the processing etc.  The database also facilitates customized GIS-based interface for easy retrieval of data from a NAS, queries based on different scientific inputs, and web based input/output interface to facilitate the application to run on internet/ intranet with login authentication. The database format is also flexible enough to allow for both vertical and lateral growth.