  When monitoring geophysical parameters, data from segments that are
contaminated by noise may have to be abandoned. This is the case, for example,
in the geoelectrical field measurements at some sites in Japan, where high
noise -due mainly to leakage currents from DC driven trains- prevails almost
during 70\% of the 24 hour operational time. We show that even in such a case,
the identification of seismic electric signals (SES), which are long-range
correlated signals, may be possible, if the remaining noise free data are
analyzed in natural time along with detrended fluctuation analysis (DFA).
