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Diamond deposits are particularly difficult to sample because very high values 

 are concentrated in tiny stones which are dispersed through huge quantities of 

 sand and gravel. There is no readily detectable pattern to their distribution 

 although generally the higher values are closer to bedrock. Sometimes a dozen 

 stones wiU collect in a single small rocky depression while those around it will 

 be barren. And of course the usual graduations in value are absent: either there 

 is a diamond in the sample or there is not. Moreover, there can be a big differ- 

 ence in value of diamonds of the same size — and for stones of equal quality, the 

 value increases exponentially with an increase in size. This means that large 

 samples are required from sample holes of precisely known size. Even so, there 

 is a tendency to substantially undervalue (relative to mining results) the 

 deposits. 



Rockeater sampled by drilling large holes — SO cm in diameter — with a heavily 

 weighted bit 8 meters long. Steel claws at the tip loosen the gravel and high- 

 pressure jets churn it into suspension so that an airlift system can bring it to 

 the surface. The ship is held firmly in place by four anchors and is able to drill 

 in waves four meters high. Samples are processed on board in a heavy media 

 plant, and the final concentrate is hand sorted for diamonds. It drills about 30 

 holes averaging about five meters deep in a 12-hour day. In the first year of 

 operation over 4,000 holes were completed and diamond deposits containing 

 millions of carats were outlined. 



The Rockeater system has been operating for three years and is regarded as 

 a very successful solution to that specific problem. However, for other kinds of 

 mineral deposits there are equally satisfactory and less expensive solutions. 



Undersea placer deposits of gold, tin and rutile are somewhat easier to evaluate. 

 Smaller samples (8 to 16 cm diameter sample holes) give adequate data and 

 value-contour charts made from such samples are more meaningful. The equip- 

 ment available for making such holes and recovering samples therefrom has gen- 

 erally been inadequate for coarse gravels or rough water. 



THE PROBLEM OF SAMPLES 



In order to produce results on which an engineer or geologist will stake his 

 reputation, the sampling system must be able to take a continuous sample of 

 precisely known size throughout the unconsolidated section of sediments and 

 gravels to bedrock — preferably a continuous undisturbed core so that layering 

 can be studied and sample values carefully segregated according to depth. There 

 must be no losses from the sample or additions to it ; the material immediately 

 above the bedrock (often the site of the best values) must be retrieved. 



It is not easy to obtain such samples, but several methods have been used in 

 the protected waters of Indonesia, Alaska, and Thailand. For example, the 

 ancient manual Banka placer drill has heen used from rafts. Although samples 

 taken by this method have obvious deficiencies, the uncertainties seem to be 

 counter-balanced by processing losses and poor measurement of dredge volumes 

 so that the results are apparently usable. 



Other sampling in tidelands, bays and estuaries have been done by wash- 

 boring or by jetting a pipe into the bottom and pumping or air-lifting the material 

 to the surface. None of these produce a clean sample of known dimensions and 

 geologists can seldom agree on what the results mean. Another method is to use 

 a pile-driver-like engine to drive a double-walled casing into the bottom, con- 

 tinuously pumping the enclosed material up with what is called "reverse circula- 

 tion." The sample produced is clean and reliable but any layering is destroyed and 

 the device is too cumbersome to use offshore in rough seas. 



Therefore, the Horton Sampler was developed to meet our requirements for 

 inexpensive offshore sampling. It was designed to penetrate 30 meters of the 

 very coarse gold-bearing gravels of Alaska beneath 50 meters of water and has 

 been used to sample tin deposits off Tasmania. The Horton Sampler is a vibrating 

 tool (120,000 impact pounds 100 times a second) in which a 15 cm diameter 

 smooth pipe is driven into the sea floor to take a continiious, undisturbed sample 

 of the entire unconsolidated section. The pipe is in 3.3 meter joints and the ma- 

 terial inside can be retrieved either as a core or it can be pumped out as the 

 pipe descends. It must be mounted on a ship which can be held in position above 

 the hole by multiple anchors. In rough water one assumes that the ship will be 

 continually in motion following an orbit similar to that of the water particles in 

 the passing waves (a circle whose diameter is equal to the wave height) . 



In deeper waters where the material of interest is most likely to be surface 

 nodules containing phosphates or manganese, the customary sampling method 



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