  Infrasonic and seismic waves are supposed to be the main contributors to the
gravity-gradient noise (Newtonian noise) of the third generation subterranean
gravitational-wave detectors. This noise will limit the sensitivity of the
instrument at frequencies below 20 Hz. Investigation of its origin and the
possible methods of mitigation have top priority during the designing period of
the detectors. Therefore long-term site characterizing measurements are needed
at several subterranean sites. However, at some sites, mining activities can
occur. These activities can cause sudden changes (transients) in the measured
signal, and increase the continuous background noise, too. We have developed a
new algorithm based on discrete Haar transform to find these transients in the
infrasound signal. We found that eliminating the transients decreases the
variation of the noise spectra, and hence results a more accurate
characterization of the background noise. We also carried out experiments for
controlling the continuous noise. Machines operating at the mine was turned on
and off systematically in order to see their effect on the noise spectra. These
experiments showed that the main contributor of the continuous noise is the
ventilation system of the mine.
