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techniques used are quite different from those used in searching for undersea 

 oil structures where penetration of a thousand meters or more is required. Rarely 

 does this kind of mineral work require sediment penetrations of more than 

 twenty meters. 



MAPPING SHALLOW BEDS 



In trying to design an instrvunent to map shallow beds beneath shallow water 

 there are several problems.. As always in sonic work, there must be a compro- 

 mise between the high frequency required for good definition and the low 

 frequency required for penetration — the possibilities ranging from about 0.5 kc 

 to 5.0 kc. Generally the higher frequencies will penetrate 5 to 10 meters of very 

 fine sand but the lower part of the frequency range is required if gravel and 

 cobbles or thick sediment is likely to be encountered. The wave length of the 

 sound should be at least ten times the diameter of the individual stones. In shallow 

 water it is necessary to have a very short duration pulse ; otherwise the out- 

 going signal will mask the returning reflections.. Moreover, in shallow water 

 there usually are reverberations when the the outgoing signal bounces back and 

 forth between seafloor and sea surface. These surface multiples may obscure 

 direct reflections from the deeper strata. 



It is usually best to tow the sound generating and receiving instruments in 

 a submerged "flsh" well astern of the geophysical ship. This reduces ship noise 

 and surface noise. The use of air-backed downward-looking reflectors in the fish 

 above the instruments greatly reduces the initial surface reverberation and 

 improves the record quality. 



Exi>erience has shown that a frequency of about one kc with a very steep half- 

 wave sound pulse is generally effective and gives acceptable definition. Although 

 the apparent uncertainty at one kc is about one meter, in practice if all the rec- 

 ords are measured against the same datum, the error is much less. 



One can never be certain that sonic methods will be effective in an untried 

 area. On one occasion our geophysicists encountered a very rough coral bottom 

 beneath shallow sand which gave no coherent reflection. Another time, gaseous 

 muds in one sub-bottom layer looked like a faulted hard strata on the record and 

 masked all beds beneath. However, in especially favorable circumstances it 

 has been possible to detect mid-sediment details of such things as old beach facies 

 and layers of minerals or mussel shells. With multiple hydrophone arrays and 

 data-processing techniques, buried cracks and gullies that may contain mineral 

 concentrations can be located and mapped. 



NATURE OF BOTTOM 



There is another means of determining the nature of the bottom. In water 

 depths to 50 meters, geologists trained as divers can descend and directly exam- 

 ine sediments or rocky outcrops. Sometimes this gives them wholly new ideas 

 about geological processes and suggests means of more effectively using instru- 

 ments and sampling techniques. In somewhat deeper water, lights and television 

 cameras can be mounted on a sled and towed slowly along the bottom to give 

 watchers on the ship a continuing picture of the sea floor. 



Although the sonic profiler produces essential information about the sub-sea 

 structure and allows the geologist to identify old beach deposits, buried stream 

 channels, and other natural traps in bedrock, it is necessary to take actual 

 samples of the mineralized materials to determine the presence and depth of 

 minerals or to outline an ore body. Usually several kinds and qualities of samples 

 are required. 



It is relatively easy to obtain rough samples of the bottom material by pump- 

 ing or airlifting samples of the unconsolidated sediment to the surface to deter- 

 mine if any interesting minerals are present. One can obtain somewhat better 

 samples by lowering a weighted cylindrical caisson which penetrates the bottom 

 by using water to wash away the material ahead of the rim. As it descends, the 

 material inside is pumped to the surface and allowed to flow over a riffled trough 

 which retains the heavy minerals. Dimensions of the sample and depth of pene- 

 tration are usually not known well enough so that the samples can be used as a 

 basis for calculating mineral grade and ore body size for investment purposes. 



THE ROCKEATEB 



After much study of the problem of how to obtain accurate samples beneath 

 very rough seas, in 1963 Ocean Science and Engineering. Inc. built the pros- 

 pecting ship Rockeater to sample the diamondiferous gravels off the coast of 

 Southwest Africa. 



