  Correlated magnetic noise from Schumann resonances threatens to contaminate
the observation of a stochastic gravitational-wave background in
interferometric detectors. In previous work, we reported on the first effort to
eliminate global correlated noise from the Schumann resonances using Wiener
filtering, demonstrating as much as a factor of two reduction in the coherence
between magnetometers on different continents. In this work, we present results
from dedicated magnetometer measurements at the Virgo and KAGRA sites, which
are the first results for subtraction using data from gravitational-wave
detector sites. We compare these measurements to a growing network of permanent
magnetometer stations, including at the LIGO sites. We show how dedicated
measurements can reduce coherence to a level consistent with uncorrelated
noise. We also show the effect of mutual magnetometer attraction, arguing that
magnetometers should be placed at least one meter from one another.
