Cone of rock over Davis Campus mapped

This colorful cone is a three- dimensional map of rock above the Davis Campus. The point of the cone is at the 4850 Level. Note the Open Cut at the top left, on the surface. Rock formations include the Ellison (dark blue), the Northwest (light blue), the rhyolite dike (yellow), amphibolite (lavender), the Flagrock (gold), the Homestake (brown), the Poorman (green) and the Yates Member (hot pink at the bottom).
Credit: 
Graphic by Kathy Hart

Sanford Lab consulting geologist Kathy Hart today is putting the finishing touches on a project that can best be visualized as a three-dimensional map of a very large, inverted cone of rock. (See the graphic.) The tip of the cone rests on the Davis Campus 4,850 feet underground. The circular base of the cone is on the surface of the Black Hills, with the Open Cut on the north and Kirk Gulch on the south.

Hart has entered geological data for all the rock in this cone—graphically and in a database. Sanford Lab Geologist Tom Trancynger compiled data on rock density and geochemistry for the study. It’s a complex project, Hart says, because the rock includes seven major rock formations and various subgroups within those formations. The project was further complicated because much of the rock in the cone is not ore-bearing formations, so Homestake Mining Co. did not take core samples there. Hart and Trancynger, however, were able to find data from a number of other sources, including archived scientific papers.

 Science Liaison Director Jaret Heise says their work will help physicists better predict background noise at experiments on the 4,850-foot level. “Knowing the rock properties and the various formations above the Davis Campus will allow researchers to calculate and simulate the flux of muons showering down through the rock into the area of the Davis Campus,” Heise says.

The muon “flux” is a measure of the flow of subatomic particles called muons, which are heavier cousins of electrons. Muons are created when cosmic ray particles slam into the upper atmosphere, and the muon flux is so intense on the surface of the earth the “noise” would overwhelm sensitive experiments.

Most muons are stopped by nearly a mile of rock, but some do penetrate to the 4850 Level. When they collide with atoms near Davis Campus, Heise said, they can spawn neutrons or gamma radiation that can interfere with sensitive underground experiments. Heise chose a 45-degree cone of rock for the study to capture the particles most likely to interfere with experiments. The data will be helpful to both the Large Underground Xenon (LUX) dark matter detector and the Majorana Demonstrator as well as to possible future experiments.

Heise said the two geologists have created a valuable resource. “The over-arching motivation for this is, you don’t want physicists doing geology,” he joked. No worries. The cone of rock over the Davis Campus has had its portrait painted by professionals.