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THE ACCURACY OF POLYMETALLIC NODULE RESOURCES ESTIMATION IN THE PACIFIC IN THE IOM AREA BASED ON A SAMPLES COLLECTED USING A BOX CORER

J. Mucha

Abstract

One of many factors determining the success of the future exploitation of oceanic polymetallic nodules is the amount of resources and the accuracy of their estimation. The potential exploitation requires accurate estimation of resources classified as measured resources according to the Australian JORC Code (The Joint Ore Reserves Committee Code). The issue of the accuracy of polymetallic nodule resources estimation was analyzed in the area of the Clarion-Clipperton Fracture Zone in the Pacific administered by the InterOceanMetal Joint Organization (IOM), the contractor of the International Seabed Authority (ISA) since 2001. The area of about 4 200 km2 selected for the analysis is one of the most prospective ones due to the abundance of nodules. The analysis was based on samples collected from the ocean floor using a box corer operated from the research ship. The average distance between sampling sites was about 8 km. The assessment of the accuracy of estimates was made for 15 square calculation blocks with a side of 17 km and an area of 300 km2. They roughly correspond to the areas of annual exploitation, assuming that the average abundance of wet nodules is 10 [kg/m2] and the annual production is 3 million tons. The use of geostatistical block kriging enabled to determine relative standard kriging errors as a measure of the accuracy of estimates of nodule resources. To minimize the risk of mining investment, it was assumed that the nodule resources estimation error in the case of calculation blocks corresponding to the areas of annual exploitation must not exceed 5%. The calculated estimation errors range from 10.6% to 26.6% (with a median equal to 13.1%) and are much higher than the assumed threshold value of 5%. The low accuracy of polymetallic nodule resources estimation is due to the small volume of samples collected by box corer, the large distance between sample collection points, and a relatively high regional (with a coefficient of variation close to 40%), and local variability in the abundance of nodules (discontinuous variation) as indicated by the nugget effect in the geostatistical semivariogram. Subsequent simulations have confirmed that achieving the assumed accuracy would require the collection of a large number of samples from the ocean floor, which is unrealistic given the huge labor intensity and high costs. The only rational way to achieve the assumed accuracy is to combine resource estimates based on box corer sampling method with an additional polymetallic nodule resources estimation based on bottom photographs, taken by the research ship or AUV (Autonomous Underwater Vehicle).

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DOI resolver: doi.org/10.5593/sgem2019/1.1/s01.072

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