Deep Underground Science and Engineering Lab - Topography Shielding Part II

Meters of water equivalent, m.w.e. is the unit of shielding used to describe how much muon flux can be filtered by placing an underground lab at a certain depth. I did a calculation looking at the mwe reported for the DUSEL. DUSEL's main cavern will be placed at approximately 8,000 feet below ground surface and the mwe is reported as 7,200. With a little unit conversion mathemagic, the specific gravity that the scientists used was 2.95. The density of water is 1 gram per cubic centimeter and specific gravity is the factor of times more dense than water. So, the scientists that estimated the mwe for Homestake used some kind of logic that assumed the rock above the lab was 2.95 times the density of water. 

This is interesting for a few reasons. 1) The 2.95 is close to the average density of continental material, the kind of thing that a physicist would look up in a book. 2) The Homestake formation and the surrounding units are highly metamorphosed greenschists, quartzites, and iron formations. The specific gravity of these units, in place, is about 2.85 on the low end for relatively light quartzite up to 5 or 6 for heavy iron formation. The estimate for specific gravity may be low by a bit under a factor of 2. 3) The 8,000 foot depth is based on a mine coordinate system that has a zero elevation somewhere near the top of the open pit (the zero point was mined out when the Homestake company opened up the pit in the 1980s). Anyone who has been to Lead knows that those hills are as much as thousand feet high. The actual depth from the collar of the shaft is much different than the depth from the Kirk trailhead to the 8000 foot level. And 4) the mining method was vertical stope retreat where large blocks of material were mined away and then refilled with a sand slurry. This fill material could be a relatively large fraction of the material between the lab and the ground surface. All of these factors are sure to introduce uncertainty in the back of the envelope mwe calculation. 

Instead, I would suggest using the block model that Barrick trusted to the state to estimate mwe. The computer model includes all the geologic formations, known rock density from drill core, and accurate 3d locations of the drifts and ground surface. These parameters could be used to estimate the mwe for any location that was sampled with diamond drill holes or mined during Homestake's operational period. The alternative would be to characterize the muon flux by accessing the opened caverns and directly measuring the flux. A final option would be to measure the gravity at each level and use the results to determine the effective density of the ground between levels - this method would provide a better understanding of the backfill and dewatering which could change the density after geologic modelling was completed by Homestake. 

I think tomorrow I want to look at the permutations of the best and worst case scenario for specific gravity of the ground above the DUSEL caverns.