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a hundred or more years ago. Areas adjacent to a hard-rock mining district or 

 to mineralized source rocks subjected to long erosion may be promising. 



The prospector looks for steep stream gradients between the mineralized 

 area and the sea which could have transported the minerals. It would be unlikely 

 to find undersea placers if a wide floodpJain with meandering streams intervened. 



Large waves (at least occasionally) and strong coastal currents do an im- 

 portant job of concentration. The offshore miner does not want to have to sort 

 through allthe material brought down by a river any more than the onshore miner 

 would mine a whole mountain to get a single vein. Luckily, the most valuable 

 minerals have relatively high densities. Thus, when the sediment discharged 

 by the river is stirred up by the waves and the light material is carried away by 

 the currents, there is a natural concentration of heavy minerals adjacent to bed- 

 rock. 



The highest concentration of heavy minerals is likely to be beneath the beach. 

 (Note that a beach is defined as rocky fragmental material that moves when 

 subjected to ordinary wave action. This includes the sands and gravels that lie 

 beneath the surf as well as those visible above water.) However, ancient Pleisto- 

 cene beach lines can be found at several depths below present sea level where they 

 form sort of mineralized wind-rows of sand and gravel. Other undersea features 

 that may have been formed at the same time by wave action are seacliffs, lines 

 of gulleys perpendicular to the shoreline, and wave-cut benches. Much of the 

 geophysical work is directed towards finding and mapping these features. 



NON-GEOLOGICAL PROBLEMS 



An undersea prospecting program is influenced by many other non-geological 

 factors which are of critical importance in deciding what minerals to look for 

 and where to look. Among these are bad weather conditions which are not as 

 serious a bar to prospecting as they are to long term mining operations. Occa- 

 sional violent storms, high winds, or a short ice-free season will increase risks 

 and costs to such an extent that the grade of ore required for profitable mining 

 will be several times that of warm calm seas. This greatly alters the prospector's 

 cutoff grade and affects his methods. 



Another consideration is the matter of obtaining clear title to the mineral 

 lands to be prospected. This has not been much of a problem to date because 

 most placer deposits are in nearshore areas subject to the control of the adjacent 

 country. But there are unclaimed oceanic banks and vast areas of deep water 

 where chemical precipitate deposits might be mined and where title is uncertain. 



The undersea mining prospector needs specialized equipment that is precisely 

 suited to his unusual task. He uses geophysical equipment to determine the shape, 

 structure, and mineral constituents of the sea floor. Of these, the configuration 

 and contour of the bottom is the easiest to discover. Recording echo sounders 

 of a wide range of frequencies (from 10 to 300 kc) are inexpensive, portable, 

 and easily available. Since the higher frequencies are attenuated by seawater, 

 work beyond the edge of the continental shelf usually is done with 10 or 12 

 kc instruments. Often an experienced marine geologist can determine the posi- 

 tion of old shorelines, areas of exposed bedrock, seacliffs, and other important 

 features simply by using the hydrographic charts made from an echo sounder 

 record. 



In some circumstances a recording magnetometer towed behind the ship can 

 be useful — the proton-precession variety is the simplest and most convenient 

 form. It is used to map gross magnetic anomalies such as the abrupt change 

 between granite and limestone. Contact metamorphic deposits or tin deposits 

 associated with the granite may be found in this manner. Presumably it can 

 also be used to determined directly the extent of magnetic mineral deposits 

 (magnetite or ilmenite) — either for their own sake or because they are associ- 

 ated with more valuable materials. 



A scintillometer properly encased, can be dragged along the sea floor to deter- 

 mine whether or not radioactive minerals are present in situ or as sand (mon- 

 azite, for example). 



But the mainstay of mining geophysics is the continuous sonic profiler. In effect 

 it is a high-powered low-frequency echo sounder capable of penetrating the 

 unconsolidated material of the sea floor and reflecting from the bedrock beneath. 

 The purpose is to determine the depth and shape of the bedrock, and the fine 

 structure within the unconsolidated sediment. Occasionally the near-surface 

 bedding structure in the bedrock itself is useful, especially if its dip is suflicient 

 to influence the movement of the mineral bearing unconsolidated material. The 



