Mini earthquakes measure rock properties
Recently, Dr. Gary Pavlis of Indiana University, was creating his own mini earthquakes in Lead. Pavlis leads the Active Seismic Source Experiments collaboration. Pavlis's team actively creates the earthquake by dropping a weight on the suface.
The collaboration works in conjunction with the Deep Underground Gravity Laboratory (DUGL), which has an array of seismometers that are listening for seismic activity. Rather than waiting for a seismic event, the collaboration is generating really small earthquakes as a way to measure rock properties.
"This is purely about seismology and seismic wave propagation," Pavlis said.
Using a series of motion sensors pulled behind a trailer, members of the team periodically stopped to drop a weight. The data they collect will test current theoretical models about rock properties and could tell them how seismic waves are scattered on the Earth's surface. Underground a series of free component geophones, or earth phones, measure ground motion in three directions. This is really important, Pavlis said.
Most seismic measurements are taken on the Earth?s surface, which is heavily influenced by weather and erosion. "All kinds of geologic processes attack the rock, especially rock that is fairly old like the Black Hills," Pavlis said. Trees and other plants break down the hard surface, changing the characteristics of the rock by turning it into soil.
"Studying rocks from the surface makes it more difficult to understand real rock properties because you are always filtering through a very complex screen," Pavlis said. Underground it's a controlled environment, allowing them to better characterize the rock. "That's why Sanford Lab is an ideal place to do this experiment."
John Stigall, a network engineer from Indiana University, added, "This allows us to study wave propagation from all directions."
The experiments will help the team better understand how seismic waves propagate in rocks that are a mix of materials with different properties at a wide range of scales. Specifically, the rocks at Sanford Lab are known to be anisotropic, Pavlis said. That means their mechanical properties depend upon direction. Theory says seismic waves in such materials do not behave in a manner commonly taught as the concept of P (primary) and S (secondary or shear) waves. P waves, which are equivalent to sound waves, are not purely longitudinal and S waves split in two and travel at different wave speeds along a fast and slow axis.
To test current theories of anisotropic wave propagation, the team installed arrays of 24 triaxial geophones 4,100 and 4,850 feet underground. They recorded waves created by hammer blows in three perpendicular directions to sort out different wave propagation modes. Additionally, they ran a 1-kilometer seismic reflection profiling experiment on the 2000 Level to record how waves are reflected from Earth's surface and observed underground.
"A key point about Sanford Lab is that we can literally map rock properties at a finer scale and directly relate the results to our measurements."