  In April 2020, KAGRA conducted its first science observation in combination
with the GEO~600 detector (O3GK) for two weeks. According to the noise budget
estimation, suspension control noise in the low frequency band and acoustic
noise in the middle frequency band are identified as the dominant contribution.
In this study, we show that such noise can be reduced in offline data analysis
by utilizing a method called Independent Component Analysis (ICA). Here the ICA
model is extended from the one studied in iKAGRA data analysis by incorporating
frequency dependence while linearity and stationarity of the couplings are
still assumed. By using optimal witness sensors, those two dominant
contributions are mitigated in the real observational data. We also analyze the
stability of the transfer functions for whole two weeks data in order to
investigate how the current subtraction method can be practically used in
gravitational wave search.
