Exploring the Origin of Indian Ocean Geoid Low (IOGL)

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The scientific research program entitled “Exploring the Origin of Indian Ocean Geoid Low” is of significantinterest to the scientific community due to its intriguing characteristic. The scientific proposal to explore the largestgeoid low anomaly was sanctionedas a part of the 12th Five Year plan. NCAOR, Goa has been nodal agency on behalf of MoES, to plan and implement the long term scientific activities for exploring the origin of Indian Ocean Geoid Low and variouspossible causative mechanisms.

The Indian Ocean Geoid Low (IOGL) centred near to south of Sri-Lanka is the most fascinating and largest geoid low that spans a vast areal extent south of the Indian subcontinent. It appears as a very long wavelength feature (> 15,000 km) covering the entire Indian Ocean and dominated by a significant low of -106 m south of Sri Lanka.

 


Figure 1. Schematic diagram showing geoid, ellipsoid and topography.



Figure 2. Geoid shape of Earth (http://www.asu.cas.cz/~bezdek/vyzkum/rotating_3d_globe/)

 

A number of causal mechanisms have been proposed for the global geoid highs and lows. Positive long wavelength geoid anomalies are generally concentrated over subduction zones and hotspots.  Geoid highs over active subduction zones are inferred as resulting from a dominating positive mass anomaly of cold, upper mantle slabs compared with a low from dynamic topography.  Association of positive geoid anomalies with hot spots and associated density anomalies, going down to Core-Mantle Boundary (CMB) have also been explained and modeled. In contrast however, there is no general consensus regarding the source of long wavelength geoid lows. Analysis of the global geoid lows indicates that they are correlated with locations of Mesozoic subduction.  The geoid lows have also been associated with various mechanisms like undulations in Core-Mantle boundary & its interaction with mantle dynamics, Moho depth variation, density void and heterogeneity in lower mantle, etc. Temporal variation of geoid may also be related to variation of water volume due to melting of solid water bodies in the globe.Geoid lows have also been shown to be underlain by high seismic velocities near the base of the mantle. However, in the same region, the upper part of the mantle is marked by anomalously low seismic velocities.

One such geoid low, the largest one, is centered around the south of India.  Several suggestions have been made to explain this geoidal low. Some researchers think that it is caused due to the depression in the Core-Mantle boundary, while others propose density heterogeneities in the upper mantle. It has also been proposed that it is produced due relict of earlier subduction. All the studies are, however, in agreement that it is a deep-seated earth structure.

 

Against the above backdrop, NCAOR has planned a major initiative to study and understand this deep-seated, globally debated earth structure.  A two-fold approach is proposed to adopted: as the first step, a detailed marine geophysical study of the area along a series of transects across the low, from Chagos-Laccadive Ridge in the West through the Southern part of India and Sri Lanka, and across the Bay of Bengal to the western offshore region of Andaman-Nicobar islands is proposed to be carried out.   Concurrently   it is planned to deploy passive ocean bottom seismometers for continuous measurement of seismological data across the IOGL region.

 

 Figure 3. Gun Deck

 

 

 

 

Figure 4.OBS unit used during wide-angle seismic data acquisition.