RIASSUNTO
ABSTRACT
This paper reviews the status of mining offshore for sand and gravel on a world-wide basis. It discusses the technology for exploration and evaluation of sea floor mineral targets, as well as mining, transportation, and processing. Large operations in Japan and Europe are described, based upon personal observations of the author. The U.S. situation is outlined and opinions offered as to the outlook for the future.
INTRODUCTION
Sand and gravel are broad terms used to cover almost any ground rock or mineral. Size distribution and performance specifications are more important that mineralogy per se but quartz is the most common constituent. Sand, of different size bands, ranges from 0.06 to 2 mm diameter: gravel from 2 to 8 mm.
Sand and gravel together comprise a low unit value mineral commodity (--$2.55 per metric ton in the U.S. in 1982) that is used in nearly every country in the world for construction aggregate and fill. This commodity is mined from the seafloor in several areas of the world for those same purposes and, in addition, for the restoration of stormeroded beaches.
Commercially important marine sand and gravel occurs as beds and lenses of unconsolidated aggregate, generally Pleistocene or younger, lying at or near the surface of the continental shelf under 20 to 30 meters of water. Glacial deposits are common in Northern latitudes; submerged beach and river deposits are mined elsewhere. Aragonite ""sand"", precipated from seawater, is mined in The Bahamas for construction aggregate as well as for use in the glass industry.
INDUSTRIAL OPERATIONS
Deposits currently mined offshore occur where favorable marine geology, metropolitan (not necessarily coastal) markets, navigable water, and a favorable legal regime occur in juxtaposition. The two largest industries have developed in Japan and Northern Europe. Each is discussed blow.
Japan
Rapid depletion of onshore deposits of construction aggregate has caused Japan to tap sand deposits in coastal waters. In recent years a fleet of about 900 small vessels has recovered 15 to 20% of all aggregate mined in Japan. Most of the mining is for sand for use as fine aggregate, which is costly to grind from crushed rock. Coarse aggregate is supplied increasingly by crushing rock. Annual production is 60 to 70 million tons and is expected to increase. By way of comparison, this rate of mining is double the rate of mining off Northern Europe although the latter operations have received more attention in the Western literature.
Most of the mining occurs off Western Japan, as shown in Figure 1. About two-thirds of the production comes from the Seto Inland Sea; most of the remainder comes from the north coast of Kyushu, off Fukuoka. In general, the seafloor upplies western Japan while eastern Japan is relatively uncrowded and so is able to utilize on land deposits of sand and gravel. Tokyo's huge demand is satisfied from onshore because of its great distance from marine deposits.