TECHNIQUES AND RESULTS
In early stages of exploration, it was decided to demarcate an area of at least 30, 000 sq.km and to estimate the abundance, grade and reserves of nodule.
During the first phase regional reconaissance sampling on 100 km spacing was carried out, followed by 50 km grid sampling, thus progressively narrowing down the area for detailed exploration. Regional surveys were carried out at a widely spaced but regular grid pattern to identify the most promising areas of the ocean bed. This includes collection of nodule samples to know their abundance, regional geochemistry and also the bathymetric profiles. The sampling device was chosen according to the requirements. Free fall grab samplers were the most commonly used to obtain nodules samples in addition to photo boomerang grabs, dredge samplers and corers. Bathymetric profiles were obtained by a precision depth recorder and the nature of the sea bed by a side scan sonar. The data were studied through geostatistical methods mainly by trend surface analyses, probability tests and variogram analyses to demarcrate the 'Prime Areas'
Detailed surveys based on the results obtained from regional surveys were concentrated only in the 'Prime Areas' and the number of stations were increased at the reduced grid spacing. At every station two sets of data a re collected. i.e wet aboundance of nodules, which is the average of seven sampling devices, including underwater photographs, and grade of nodules .
The surveys to identify the candidate site formed the final phase of exploration. These were carried out at a close grid spacing to detail the information acquired in previous surveys. The spacing of surveys was fixed by extrapolation/interpolation of results after statistically analysing the data of previous surveys.
The main data base was created at the NIO and the steps involved are schematically presented in the Fig 41. Data base generation fig 42 and data management further enchances resourse evaluation techniques. This necessitated the involvement of Engineers India Limited and Metallurgical and Engineering counsultants.In addition to these two agencies, NGRI and the Operational Research Unit (CSIR) were associated till the time the revised application was filed with the Preparatory Commission.
Bathymetry is an important factor governing the distribution of polymetallic nodules. Bathymetry of the Indian Ocean was carried out by 3.5 and 12 KHz echosounders. A total of 388.00 line Km of bathymetry has been carried out so far. With the help of these bathymetric records 100m bathymetric contour interval maps were prepared at MECON, Ranchi.
Magnetic measurements were carried out by a proton precision magnetometer and 250,000 1 km of magnetic data from Central Indian Ocean have been collected. Magnetic anomaly maps were prepared at MECON, by the NIO scientists and the data were utilised for crustal structure interpretation.
Extensice dredging has been carried out by different types of indigenous dredges and a total of 51 tonnes bulk samples were collected during different cruises. These bulk samples were sent to the different matalurgical laboratories for developing metal extraction processes. About 2.5 tonnes of nodules were seived on board to find out their size distribution. Nearly 75-80 % of the nodules are in the range of 2-6 cm and confining the use of nodules of these sizes for metallurgical purposes is expected to give higher metal yield.
Sampling was carried out by using Preussag freefall grabs. Initially sampling was carried out at 100 km interval. Later this interval was reduced to 50 km, and 12.5 km. Alongwith the boomerang grabs, photoboomerang grabs were also used for underwater photographs to know the distribution of nodules.( fig 30). A Benthos underwater towed camera was also used for continous photography. From the underwater photographs, nodule converage , size and other associated biological features on the sea floor were studied. To date 7,000 boomerang and photoboomerang operations were carried out.
R & D EFFORTS IN EXTRACTION METALLURGY OF POLYMETALLIC NODULES
BACKGROUND:
R & D efforts for extraction of metals from the nodules was started by leading research centres in the country as early as 1983-84. Regional Research laboratory, National Metallurgical laboratory, Hindustan Zinc Ltd., Hindustan Copper Ltd. took the lead in initiating such efforts. A wide variety of process were studied by these centres which may broadly been classified as: