The site characterisation of future underground gravitational wave detectors is based on spectral properties of the low frequency seismic noise. The evaluation of the collected long term seismological data in the Mátra Gravitational and Geophysical Laboratory revealed some aspects that are not apparent in short term spectral noise characterisation. In this paper we survey the methodology. In particular, we argue that the spectral properties are best represented by percentiles of the data instead of the mode, because it is noisy, sensitive to the discretization and intrinsic averaging, therefore it is less suitable for a robust characterisation. The suitable cumulative measures are also scrutinized.