'Electric hammer' reveals rock stress

South Dakota School of Mines geologist Bill Roggenthen and graduate student Jason Van Beek spent much of Friday on the 4850 Level, whacking rocks with a sledgehammer.

The sledgehammer was acting as a ?seismic source,? but Roggenthen calls this particular device ?the electric hammer.? It?s not powered by electricity. Rather, a cable duct-taped to the handle connects the hammer to a seismograph, which records the rock vibrations caused by each whack.

As Roggenthen swung the hammer, Van Beek monitored the seismograph. Mounted on a rock wall nearby, a set of three ?geophones? measured to a fraction of a millisecond the time the sound waves arrived from the hammer. Tiny differences in ?seismic velocity??the speed of sound through rock?will provide data about the characteristics of an important chunk of underground real estate at the Sanford Lab.

Roggenthen and Van Beek were working near an area dubbed the ?Big X,? so named because it?s the intersection of four tunnels. The seismograph and the geophones were installed in the Chiller Cutout, a new excavation at the end of a tunnel called the Exhaust Drift. Roggenthen wielded his electric hammer around a corner, at 18 locations on the wall of a tunnel called the Vent Drift, which connects the Ross and Yates shafts.

The Exhaust Drift and the Vent Drift converge to a point at the Big X, which is near the Yates Shaft. In other words, the rock between the drifts forms an ever narrowing pillar. By hammering on one side of the pillar and recording vibrations on the other, Roggenthen and Van Beek can measure tiny differences in seismic velocities. ?If we can understand, within this triangle, how the seismic velocities vary, we can get an idea of what is happening to the in situ stresses in that rock,? Roggenthen said.

Roggenthen and Van Beek have been collecting data for more than a month as part of the Transparent Earth project, which also has established a three-dimensional seismic array at the Sanford Lab. Their ?electric hammer? findings will be useful to engineers designing underground cavities, to scientists conducting experiments underground and to a broader community of researchers studying seismic velocities in the earth?s crust.

Interesting factoid: Sound travels through rock about 20 times faster than through air.