Wiener filtering with a seismic underground array at the Sanford Underground Research Facility

A seismic array has been deployed at the Sanford Underground Research Facility in the former Homestake mine, South Dakota, USA, to study the underground seismic environment. This includes exploring the advantages of constructing a third-generation gravitational-wave (GW) detector underground. A major noise source for these detectors would be Newtonian noise (NN), which is induced by fluctuations in the local gravitational field. The hope is that a combination of a low-noise seismic environment and coherent noise subtraction using seismometers in the vicinity of the detector could suppress the NN to below the projected noise floor for future GW detectors. In this paper, certain properties of the NN subtraction problem are studied by applying similar techniques to data of a seismic array. We use Wiener filtering techniques to subtract coherent noise in a seismic array in the frequency band 0.05-1 Hz. This achieves more than an order of magnitude noise cancellation over a majority of this band. The variation in the Wiener-filter coefficients over the course of the day, including how local activities impact the filter, is analyzed. We also study the variation in coefficients over the course of a month, showing the stability of the filter with time. How varying the filter order affects the subtraction performance is also explored. It is shown that optimizing filter order can significantly improve subtraction of seismic noise.