Quantitative estimation of seafloor features from photographs and their application to nodule mining

Abstract

Methods developed for quantitative estimation of seafloor features from seabed photographs and their application for estimation of nodule sizes, coverage, abundance, burial, sediment thickness, extent of rock exposure, density of benthic organisms, and their lebensspuren have been presented. Digitization of the photographs shows variable nodule size (less than 1 to 10 cm), coverage (less than 1 to 75%) and abundance (less than 1 to 20 kg/m super(2)). Nodule population is inversely proportional to the coverage of the sediment (10-100%) and its thickness (0 to greater than 10 cm), which causes differential burial (0-100%) of nodules. Correlation between nodule parameters (diameter and coverage) in the photographs and grab recovery is used to evolve empirical relationships for estimating nodule abundance in different seabed settings. The rock outcrops (basalts) with a coverae of 6-100% are the sources of nuclei for the nodules, the distribution of which is controllEd. by the local topography. Higher concentrations of nodules are observed along the slopes, followEd. by the crests of seamounts, and are lowest in the valleys and plains. A population density of 6-7 benthic organisms per 100 m super(2) belonging to 7 different phyla is observed, with a high frequency of lebensspuren (4-12 traces/m super(2)) in association with nodules. Estimation of these parameters can be used as important inputs in the design of the nodule collector, as it will have to encounter a variey of seafloor conditions, such as patchy nodule distribution, rock outcrops, steep slopes, and frequent microtopographic changes, as well as benthic life. The distribution and relation of various features with one another can also be used to understand the possible impact of nodule mining on the seabed. Estimates show that for a yield of 3 million tonnes of nodules per year, the volume of sediment disturbed will be between 200 x 10 super(7) and 500 x 10 super(7) m super(3) over an area of 300-600 km super(2), depending upon the average abundance of nodules. Hence, the nodule collector will have to be a self-propelled system, with photographic and acoustic sensors, to enable selective mining and avoid unfavorable areas