This award will support operation of an underground seismometer array in the Homestake mine in South Dakota. The array currently includes eight seismic stations, probing the available depth and vast horizontal extent of the mine, each operating one high-sensitivity broadband seismometer. The award will support a graduate student who will, with supervision by the PI and collaborators, perform the necessary upgrades and maintenance of the array, as well as ensure acquisition of data by the array over the two-year project period. The student will also conduct several data analysis projects, including first studies of the underground seismic noise, studies of the local seismicity, and effects of the surface on the seismic field content and propagation.
The unique data provided by this array will reveal important characteristics of the seismic motion underground such as its amplitude, modal content, and effects of rock non-uniformity. Good understanding of underground seismic motion will be crucial for the development of motion-sensitive experiments in physics, such as gravitational wave detectors or tests of the equivalence principle that underlies Einstein's theory of general relativity. Due to a very strong interest in this data among the geophysics and seismology communities, the data will be made available to these communities via the "Incorporated Research Institutions for Seismology" (IRIS) program. The data will also be made available to high-school students in the Twin Cities area through the "Interactions In Understanding the Universe" (I2U2) program, exposing them to the real data with all its subtleties and imperfections, and to the large computing resources that are used to solve some of the most complicated scientific problems today.
Due to the relatively large seismic noise and large fluctuations in the gravitational field on the surface of the Earth, there is a growing consensus in the gravitational-wave community that the next generation of terrestrial gravitational-wave detectors will be built underground. Such detectors would enable observations of numerous astrophysical and cosmological events and processes, that would otherwise be inaccessible to surface-based detectors. The Deep Underground Gravity Laboratory (DUGL) project is the first attempt to quantify the advantages of the underground environment for purposes of gravitational wave detection. In particular, we have developed a small three-dimensional array of seismometers at the Homestake mine, SD, and we collected data with it during the duration of the project. The underground, closely-packed, three-dimensional configuration of high-sensitivity broadband instruments operating in a seismically quiet environment has resulted in a unique set of measurements of the seismic wave field. In particular, the data has confirmed the expected 10-fold reduction in the seismic noise at 1 Hz at the depth of 4100 feet (as compared to the surface), and has identified a variety of surface-based seismic processes that are significantly suppressed at 2000 feet or 4100 feet depth. We have used a Wiener filter approach to actively suppress the effects of the seismic noise on a detector, demonstrating a 10-fold suppression (or better) at frequencies around 0.1 Hz. This technique could be directly applied to a future underground gravitational-wave detector to suppress seismic noise effects in the gravitational-wave strain data. We have also developed a new technique for measuring the modal content and directionality of the seismic wave field, drawing on the methods used in the gravitational-wave physics. In addition to informing the design and the site-selection for the future gravitational-wave detectors, this technique will enable studies of interest in geophysics that aim to improve understanding of the seismic wave propagation (reflections and scattering processes, modal conversions). This project has therefore started to connect the geophysics and gravitational-wave physics communities, and has facilitated some knowledge transfer between the two communities. Such interactions are expected to only strengthen with time, as new data analysis techniques are developed and applied across the boundaries. The DUGL project has also provided research training for graduate and undergraduate students in the field of gravitational wave physics. The students had opportunities to work with the state-of-the-art instrumentation in the field, to learn different skills (ranging from electronics and data acquisition to various software and operational system platforms), and to contribute to the analysis of the data acquired at Homestake. They also had opportunities to interact and work with experts in different fields, such as geophysics and engineering. Finally, they received the appropriate safety training for working in an underground environment.
