Nodule distribution characteristics and associated seafloor features: factors for exploitation of deepsea Fe-Mn deposits
Authors: Sharrna Rahul 
Published: October 2008

The heterogenous distribution of nodule sizes, coverages, abundances; their variable burial and association with sediments and rock outcrops on the seafloor have emerged as crucial parameters for consideration of the exploitation potential of the manganese nodules in the Central Indian Ocean.

Nodules between 2 and 4 cm in sizes account for almost 60–70% of those evaluated from photographic and grab sample data, whereas 75% of the nodules by weight are between 2 and 6 cm size. Distribution of nodules with abundances ranging from 1-20 kg/sqm, as well as coverages of 1% to 90% on the seafloor, will be important considerations while demarcating the potential areas for nodule mining.

Mining of areas with higher concentration of nodules along the slopes associated with less sediment would result in enhanced recovery of nodules, as compared to the low nodule concentration areas in the valleys and the plains where thick sediment cover occurs. The mining head will have to encounter substrates, such as hard rock exposures, with or without Fe-Mn encrustations, as well as soft sediment layers in the nodule fields. It may also have to collect buried nodules from under the sediment cover, as well as negotiate the uphill or downhill slopes in the nodule areas.

These requirements suggest that the nodule collector must be a dynamic (active) system with sensors to detect areas of potential nodule concentrations, as well as features which are hazardous (or unfavorable) i for mining operations.

Introduction

Data from seabed photographs (Fig. 1) has been evaluated for distribution of nodules and the parameters associated with them, such as the rock outcrops, fe-mn encrustations, sediments, nodule sizes, relative abundance, and compared with grab data, in the Central Indian Ocean Basin. The objective of this paper is to evaluate the seafloor as well as nodule distribution characteristics in view of their role in the exploitation of the manganese nodules.

Data Acquisition

Data for this study was collected using narrow beam echo sounding for generation of bathymetry maps (Fig. 2), whereas freefall grabs with cameras were used for photography of the seafloor and correlation of nodule data with grab samples. Nodule coverage was calculated from photographs. The nodule coverage at each location was correlated with grab abundance and an empirical formula was worked out, as follows:

(available in full paper)

The number of data points used for this computation are 329 and the coefficient of correlation, r = 0.74. Since, the photographs record only exposed nodules, whereas the grabs can collect buried nodules as well, the relation between abundances from photo and grab data, expressed as relative abundance (RA), which is an indicator for the extent of burial of nodules1 was calculated.

(available in full paper)

Distribution of Nodules and associated features

Nodule Coverage. Of the total (988) photographs evaluated, no nodules were observed in many of them (66.77%), which shows that there are many locations where the nodules are not exposed in the study area. Of the remaining photos, nodule coverage ranges from 1 to 90% (Fig. 3); 72% of which, have very low nodule density (&lt;20% coverage), about 19% have medium density (20–40% coverage) and 9% have high nodule density (above 40% coverage).