A first detection of terrestrial gravity noise in gravitational-wave detectors is a formidable challenge. With the help of environmental sensors, it can in principle be achieved before the noise becomes dominant by estimating correlations between environmental sensors and the detector. The main complication is to disentangle different coupling mechanisms between the environment and the detector. In this paper, we analyze the relations between physical couplings and correlations that involve ground motion and LIGO strain data h(t) recorded during its second science run in 2016 and 2017. We find that all noise correlated with ground motion was more than an order of magnitude lower than dominant low-frequency instrument noise, and the dominant coupling over part of the spectrum between ground and h(t) was residual coupling through the seismic-isolation system. We also present the most accurate gravitational coupling model so far based on a detailed analysis of data from a seismic array. Despite our best efforts, we were not able to unambiguously identify gravitational coupling in the data, but our improved models confirm previous predictions that gravitational coupling might already dominate linear ground-to-h(t) coupling over parts of the low-frequency, gravitational-wave observation band.
Bibliographical noteFunding Information:
We thank R. Schofield for his invaluable comments to this paper. M. P. R. and K. V. were supported by funding from the NSF under Grants No. PHY-1607385, No. PHY1607391, No. PHY-1912380, and No. PHY-1912514. M. W. C., J. D., and S. E. D. are members of the LIGO Laboratory, supported by funding from the U.S. National Science Foundation. LIGO was constructed by the California Institute of Technology and Massachusetts Institute of Technology with funding from the National Science Foundation and operates under cooperative agreement PHY0757058. M. W. C. was supported by NSF Grant No. PHY-1505373 and by the David and Ellen Lee Postdoctoral Fellowship at the California Institute of Technology. B. J. J. S. was supported by the ARC Future Fellowship FT130100329. The authors also gratefully acknowledge the support of the Australian Research Council under the ARC Centre of Excellence for Gravitational Wave Discovery, Grant No. CE170100004 and Linkage Infrastructure, Equipment and Facilities Grant No. LE130100032. The authors acknowledge the use of Matlab and Mathematica for some of the theoretical and numerical analyses.