Characterization of rock vibrations at the Homestake mine - Our first steps at Homestake

By: Jan Harms, Angelo Sajeva, Riccardo Desalvo and Vuk Mandic 

Our experiments at Homestake are determined by two main goals:

1. We want to characterize seismic noise and rock density fluctuations in an underground environment.

2. We want to assess the quality of the Homestake mine as a candidate for a third generation underground gravitational-wave detector.

As shown in the schematic section of the Homestake mine, we set up stations at various levels (blue dots do not indicate the actual locations) and investigate how much seismic noise decreases with increasing depth and measure variations of rock vibrations at different locations (i.e. coherence between different stations). In principle, these two kinds of measurement are sufficient to qualify the mine as a good or bad candidate for an underground antenna.

In the future, we want to add optical instruments to the array of mechanical seismometers. These instruments are needed to distinguish between the two fundamental types of seismic waves, shear waves and pressure waves, since standard seismometers are always sensitive to both types of waves. One of these instruments will be the optical bar which measures the pressure waves. Another one, the shear pendulum will also be sensitive to shear waves. Presumably, optical instruments will allow to make more sensitive measurements of seismic noise.

Another question which needs to be answered is whether the seismic waves come from all directions with equal strength (i.e. if the wave field is isotropic). If waves coming from certain directions contribute more energy to the seismic noise than other directions, the wave field is called anisotropic. To measure this, one would have to increase the number of seismic stations, align them into a 3D grid and measure coherence between their data.

The authors gladly acknowledge the support of the National Science Foundation for their effort, and SUSEL and the SDSTA for hosting them.