So-called classical microseisms in a frequency range of about f ≈ 0.15 Hz were analyzed by linear least-squares inversion of (cross-) spectra of the components of motion observed in a three-station array on the Island of Sylt (North Sea). As a result of initial data inspection, the analysis was based on a model composed of Rayleigh and Love waves which are incident from one directional sector. The variable model parameters used in the procedure of fitting the model to the data were: (1) directional spread of the incoming “wave beam”, (2) “transfer coefficients”, standing for modal spectral densities relative to the transfer functions of the local subsurface structure, and (3) shear-wave velocities of the sedimentary sequence extending to 3.7 km depth.
For the two applications presented, the optimal models are statistically consistent representations of the observations. Concerning the sedimentary shear-wave velocities, realistic estimates were obtained for the upper (Cenozoic, Cretaceous, and Triassic) layers, whereas the velocity of the bottom layer was not uniquely resolved.
Concerning transfer coefficients and directional spread, they are well suited for delivering quantitative evidence about the spectrum of the source(s).