Conversion and comparability of data presentations on seismic background noise

Data on seismic background noise are collected, amongst others, for assessing the suitability of sites for temporary or permanent seismic recordings. Site quality requirements depend on the task of seismic observations and thus on their resolution, dynamic range, bandwidth and frequency range. Till now noise data are collected with a wide range of instruments, both analog and digital, of different bandwidth, resolution and transfer functions. Accordingly, noise appearence in seismic records, amplitude- and frequency-wise, differs and the various kinds of noise spectra derived therefrom vary too. They are not easily comparable amongst each other and with older presentations of noise spectra derived from analog records. Also, when having determined noise power density spectra from digital records it is not so obvious what this means in terms of noise ground motion amplitudes or noise appearance in records of different bandwidth and vice versa. The paper does not aim at serving as a guide to station site selection but rather to present and comment the relationships to be used for the conversion of power and amplitude spectra into different kinematic units and for calculating from the spectral representations of seismic noise the related frequency dependent RMS or average peak amplitudes of different bandwidth and vice versa. For the new global high (NHNM) and low-noise model (NLNM) given by Peterson (1993) in dB of acceleration power density the related velocity and displacement power spectral densities are presented both graphically and tabulated. Examples for the application of the conversion relationships and the effect of bandwidth on noise and signal amplitudes are given. For a selected data set from a site-selection noise survey in NW Iran the suitability of some potential sites is assessed by comparison with the NLNM.