QUANTITATIVE ESTIMATION OF SEA-FLOOR FEATURES FROM PHOTOGRAPHS AND THEIR APPLICATION TO NODULE MINING

Methods developed for quantitative estimation of seafloor features fro m seabed photographs and their application for estimation of nodule si zes, coverage, abundance, burial, sediment thickness, extent of rock e xposure, density of benthic organisms, and their lebensspuren have bee n presented. Digitization of the photographs shows variable nodule siz e (< 1 to 10 cm), coverage (< 1 to 75%) and abundance (< 1 to 20 kg/m2 ). Nodule population is inversely proportional to the coverage of the sediment (10-100%) and its thickness (0 to > 10 cm), which causes diff erential burial (0-100%) of nodules. Correlation between nodule parame ters (diameter and coverage) in the photographs and grab recovery is u sed to evolve empirical relationships for estimating nodule abundance in different seabed settings. The rock outcrops (basalts) with a cover age of 6-100% are the sources of nuclei for the nodules, the distribut ion of which is controlled by the local topography. Higher concentrati ons of nodules are observed along the slopes, followed by the crests o f seamounts, and are lowest in the valleys and plains. A population de nsity of 6-7 benthic organisms per 100 m2 belonging to 7 different phy la is observed, with a high frequency of lebensspuren (4-12 traces/m2) in association with nodules. Estimation of these parameters can be us ed as important inputs in the design of the nodule collector, as it wi ll have to encounter a variety of seafloor conditions, such as patchy nodule distribution, rock outcrops, steep slopes, and frequent microto pographic changes, as well as benthic life. The distribution and relat ion of various features with one another can also be used to understan d the possible impact of nodule mining on the seabed. Estimates show t hat for a yield of 3 million tonnes of nodules per year, the volume of sediment disturbed will be between 200 x 10(7) and 500 x 10(7) m3 ove r an area of 300-600 km2 depending upon the average abundance of nodul es. Hence, the nodule collector will have to be a self-propelled syste m, with photographic and acoustic sensors, to enable selective mining and avoid unfavorable areas.