  Newtonian gravitational noise from seismic fields is predicted to be a
limiting noise source at low frequency for second generation gravitational-wave
detectors. Mitigation of this noise will be achieved by Wiener filtering using
arrays of seismometers deployed in the vicinity of all test masses. In this
work, we present optimized configurations of seismometer arrays using a variety
of simplified models of the seismic field based on seismic observations at LIGO
Hanford. The model that best fits the seismic measurements leads to noise
reduction limited predominantly by seismometer self-noise. A first simplified
design of seismic arrays for Newtonian-noise cancellation at the LIGO sites is
presented, which suggests that it will be sufficient to monitor surface
displacement inside the buildings.
