Optimum sampling interval for evaluating ferromanganese nodule resources in the central Indian Ocean

Abstract

A study to estimate manganese nodule abundance (weight of nodules in kg/m sup(2)) was carried out in a small area of the abyssal plains covering a one-degree square block in the central Indian Basin. Abundance was assessed at various intervals by progressively reducing the grid spacing. Sampling the corners of the 1 degree survey block (approximately 110-km spacing), i.e., four stations with 5-7 free-fall operations (sampling locations) in each case, indicated a nodule abundance of 3.50 kg/m sup(2). By reducing the sampling spacing to four grid units (0.5 degrees survey blocks) and sampling the entire block at eight stations (25 locations), the average abundance of the block was 3.36 kg/m sup(2). Further reduction of the grid to 0.25 degrees survey blocks and sampling in 16 grid units (70 sampling locations) increased the abundance to 4.41 kg/m sup(2). For 64 grid units in the 1 degree block (sampling in 0.125 degrees survey blocks), a substantially higher value was recorded, i.e., 5.31 kg/m sup(2) or about 1.5 times the abundance obtained at a 1 degree spacing. Adding 25 more stations in 0.0625 degrees survey blocks (intervals of sampling locations approximately 500 m) resulted in a negligible change in abundance, the average value of the one-degree block being 5.23 kg/m sup(2). These data demonstrate that, for estimating nodule resources in the region, it is important to adopt a close-grid sampling strategy, so that areas with lower abundance can be relinquished and areas with higher abundance can be confidently identified. To ascertain exact nodule abundance for mine-track selection, it may be sufficient to restrict detailed grid surveys to areas with marked variations in topography and nodule abundance, rather than carrying out such detailed (albeit less cost effective) surveys at a very narrow spacing (0.0265 degrees) over the entire pioneer area