A study of about 850 different sized nodules from 234 sites in the Central Indian Basin (CIB) showed a clear inverse relationship between size and grade of nodules. Among the different sized nodules, only the small (<2 cm) and medium (2-4 em) sized fractions have higher grade (Cu+Ni+Co = 2.6 and 2.5 %, respectively) and highest weight percent· ages at every dredge haul. It is probably recommended that in CIB only small nodules will have to be processed after screening by size. In order to minimize ecological and environmental problems, it is emphasized that the nodules should be screened on sea bottom during mining, which may be possible by future technological modification of mining devises.
Deep-sea polymetallic nodule deposits in the Indian Ocean have been of economic interest. Although occurrence of polymetallic nodules (PMN) were known in 8 different basins from the Indian Ocean, Siddiquie et al., (1978, 1984), Frazer and Wilson (1980), and Cronan and Moorby (1981) showed that only the Central Indian Basin (CIB) nodules have paramarginal grade that can be exploited on commercial scale. The National Institute of Oceanography (NIO), Dona Paula, Goa, carried out extensive geological and geophysical surveys in cm using Institutional (RV Gaveshani, ORV Sagar Kanya) and chartered vessels (MV Farnella, MV Skandi Surveyor, GA Reay, DSV Nand Rachit and AA Sidorenko) from 1982 to understand the formation processes, and distribution of nodule extension deposits. Various convential sampling devices used for spot seabed sampling include Preussag free fall (FFB) and photo-boomerang (PFB) grabs; Vanveen and Petersson grabs; gravity, boomerang and spade corers, whereas bulk quantity of nodules and crusts were obtained by using indigenously designed bucket dredges with nylon net. The gravity, magnetics and multibeam sonar (Hydrosweep) baseline data were simultaneously collected.
