NEEP602 Course Notes (Spring 1996)
Resources from Space
Next: Stage 3 -- Up: Plan of Exploration Previous: Delineation of Areas
Stage 2 - Exploration on Surface to Determine Tonnage and Grade
The first objective of on-surface exploration should be the more accurate delineation of minable regolith in terms of depth of regolith and freedom from blocks too large to be handled by the mining system selected. Drilling and trenching could do this but could involve a program of prohibitive magnitude and cost. Some method of rapid coverage of large areas of regolith is desirable. The only two methods of which I am aware are seismic surveying and radar scanning from a mobile vehicle.
Seismic methods were used by Nakamura and others (1975) to determine depth of regolith at the Apollo 11, 12, 14, 15, and 16 landing sites. They concluded that the lunar surface is covered by a low-velocity layer of material ranging from 3.2 to 12.2 m in depth. At the base of the regolith there was a sharp increase in P-wave velocity from 100 m/sec to 250-400 m/sec. Results of Cooper and others (1974) for the Apollo 14, 16, and 17 sites are similar. The seismic method would yield much information on depth of regolith but would not give continuous profiles necessary to detect blocks.
The second method is downward-looking radar scanning from a mobile vehicle. Use of radar would be favored by the sharp change in velocity at the base of bedrock (and presumably at the top of a block of rock) and by the dryness of the regolith. Radar scanning would give a continuous profile across an area along the line of traverse. Blocks of rock would give bumps on the profile. It would be far faster and far cheaper than seismic surveying and would give information of much better quality.
The work outlined above should provide the basis for firm estimates of tonnage of minable regolith in the portions of Mare Tranquillitatis investigated. However, grade in terms of contents of various gases can only be determined by sampling to whatever depth of mining is planned. Core drilling will be the preferred method of sampling. A choice will have to be made between onsite analysis of cores and transmittal of core to a base site for removal of cores and analysis. The latter would seem to offer advantages in uniform handling of core and recovery of gases. The pattern of drillholes will in general be that used in terrestrial mineral exploration of deposits that are so poorly exposed and internally unpredictable as to offer no guides to drill hole location. In such cases, drillholes on a grid pattern or equally spaced along equidistant parallel lines are generally used. Statistical methods are used in evaluating the results in terms of overall grade. Spacing of holes is ultimately dictated by the nature and scale of variation in grade as indicated in the earlier stages of exploration.
The depth of drillholes will be dictated by the depth of regolith and the depth to which regolith can be excavated with the mining system ultimately adopted. My estimate of helium resources in minable regolith in Mare Tranquillitatis is based on an average mining depth of 3 m, but this is probably a minimum. Drilling to depths of 4 m is probably a realistic goal for present planning.
Trenching is always an alternative to drilling as a means of exploring unconsolidated surficial materials. Trenching could yield either large bulk samples or channel samples from the walls of trenches. Either procedure, however, would be more cumbersome and more time-consuming than drilling.
Next: Stage 3 -- Up: Plan of Exploration Previous: Delineation of Areas
WCSAR-TR-AR3-9301-1
Eugene N. Cameron
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