Exploration is crucial for any mining operation, including one on the moon. In this case I use the term exploration as a mining term which includes activities that determine whether or not a mineral or element is present in a prospective mining area. But I think anyone would agree that all places on the moon are not made equal when it comes to mining. Take, for example, that the moon has a surface area a little bit smaller than that of the Continent of Asia. Do you think that I could start a mine anywhere in Asia and find what I am looking for?
There needs to be some way to narrow down the possibilities when selecting the site. A few givens will be obvious immediately, especially if we are looking for water. If water is the target mineral, there are a only few large craters at the northern and southern hemisphere that are shadowed all year long. But, if we are hunting for helium-3 or metals or sulfur, we need to do something to figure out where there is more..say, sulfur, and make a plan to go there. In this case, I want to use sulfur for the remainder of this discussion because it seems to have the most practical use as a cement in a type of lunar concrete that has been proposed.
The Exploration Geologist on Earth has a number of tools in his bag. He usually starts with a series of maps that help him narrow down what likely geologic structures might have concentrated the minerals of interest and whether that particular unit occurs in one spot or extensively. Aerial photographs are often used for this purpose initially. Then as targets are developed, multispectral spectrographic imagery could be used to look for telltale signatures of the presence of a particular mineral or element (depending on the technique). Additionally both types of maps could help him determine if there are helpful outcrops that he could go and sample to verify.
Where would we find such maps of the moon? All the results from the 1994 Clementine mission, which includes UV to visible and Infrared Images are available online for any interested parties. http://www.lpi.usra.edu/lunar/tools/clementine/ Also, the more recent Lunar Reconnaissance Orbiter has a suite of instruments that can detect and map any number of minerals that could be used to narrow down a mining target. http://lunar.gsfc.nasa.gov/index.html The limitations are numerous and could include the specific element of interest (Clementine looks like there are 5 spectral bands available, which is quite limiting) and the pixel resolution of the instrument itself – some of the data is only available on the kilometer scale resolution, much larger than a pit-type mine could be. The LRO has a much better camera that has imaged tracks and equipment left behind from the Apollo missions. This scale and wider are useful for mining applications.
As a starting point, these satellite images of the moon would be used to determine areas with the highest concentrations of the ore of interest. The preliminary results of the mapping could be used to estimate a volume of ore present in the target location if some work was done in advance to calibrate the spectra to a given mineral concentration.
The next step would be to get boots on the ground and begin sampling to verify the results of the spectral mineral map. In this particular case we could get lucky and find that one of the Apollo missions sampled material that happened to be spectrally very similar and use the samples that they collected as a type of verification. Alternatively, the meteorite bombardment that pulverized and mixed the upper layer of the moon, the regolith, could be so well mixed that there is not enough variation to be picky. In this case, any target area with a sufficiently deep and extensive regolith could be adequate. However, the odds are that the people who are doing the mining are going to want to optimize their mining by finding a location with even incrementally better concentrations of sulfur. And the only way to verify that is by getting on the ground and developing a sampling plan.
Sampling is a surprisingly complicated subject. In the easiest case, you have a guy who walks around with a shovel and bags and a map and picks up samples roughly evenly distributed across the target area. However, there is a dark and sullied corner of science that deals with how many samples are statistically required to get a representative understanding of the material present. The statistics are driven by the extent of the ore body, the structural limits of the ore body, the consistency of the sample results, the magnitude of the sample results, the sensitivity of instrument measuring the ore concentration, and any secondary elements that could mask the presence of the ore, among others. Reading through this list it is easy to see that planning for the correct number, spacing, and depth of samples requires knowledge of the ore body that a geologist may not have until after the sampling has started. This puts the geologist in charge of the budget in an odd spot where planning for the adequate amount of data may not be possible without clairvoyance or a good deal with the devil. In this case it is always good to make a facts-based estimate for the number of samples and then double it.
Another kicker is precisely how we sample. There are a couple different methods that immediately come to mind. We have already discussed the idea of taking a shovel and filling a bag. There are also small tools to core out shallow boreholes from soft material such as regolith. And then there are the big boys, using a drilling rig to collect a deep continuous sample of core or chips.
Here is a cartoon of a layer of ore-bearing regolith sitting above a layer of barren regolith. The ore is concentrated on the top by, say, cosmogenic processes – solar wind has been enriching the upper foot of regolith with He3 for the past billion years with little interruption. Each type of sample is shown, a small shovel excavation, a deep shovel excavation, a small borehole and a deep borehole. The benefit to a shovel excavation is that it is quick and easy, but the results can be misleading since it is artificially sampling the upper volume of the layer more than the base and so the results would have a somewhat higher concentration bias. The deep shovel excavation results would dilute the concentration by introducing the barren regolith beneath. The small borehole provides an evenly distributed sample that looks to be ideal for this application. Deep drilling would not be necessary for sampling a shallow ore body. The appropriateness of these sampling methods is largely dependent on the type of deposit that exists and removing bias can be a process that takes time and planning and often, can’t be perfected until after mining has begun.
The results of this sampling plan would then be turned into a map that shows the concentration of the mineral or element present at each sampling location. The maps could be contoured or used in a mine modeling program that extrapolates the results between boreholes to try and better determine the extent of the deposit. Once a sampling plan is completed, the concentration and total extent of the ore is defined, and then the first stages of mining are ready to begin.
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