Seismic radiation from wind turbines: observations and analytical\nmodeling of frequency-dependent amplitude decays

Abstract

Abstract. In this study, we determine spectral characteristics and amplitude decays of wind turbine induced seismic signals in the far field of a wind farm (WF) close to Uettingen/Germany. Average power spectral densities (PSD) are calculated from 10\u2009min time segments extracted from (up to) 6-months of continuous recordings at 19 seismic stations, positioned along an 8\u2009km profile starting from the WF. We identify 7 distinct PSD peaks in the frequency range between 1\u2009Hz and 8\u2009Hz that can be observed to at least 4\u2009km distance; lower-frequency peaks are detectable up to the end of the profile. At distances between 300\u2009m and 4\u2009km the PSD amplitude decay can be described by a power law with exponent b. The measured b-values exhibit a linear frequency dependence and range from b\u2009=\u20090.39 at 1.14\u2009Hz to b\u2009=\u20093.93 at 7.6\u2009Hz. In a second step, the seismic radiation and amplitude decays are modeled using an analytical approach which approximates the surface-wave field. Since we observe temporally varying phase differences between seismograms recorded directly at the base of the individual wind turbines (WTs), source-signal phase information is included in the modeling approach. We show that phase differences between source signals have significant effects on the seismic radiation pattern and amplitude decays. Therefore, we develop a phase-shift-elimination-method to handle the challenge of choosing representative source characteristics as an input for the modeling. To optimize the fitting of modeled and observed amplitude decay curves, we perform a grid search to constrain the two model parameters, i.e., the seismic shear wave velocity and quality factor. The comparison of modeled and observed amplitude decays for the 7 prominent frequencies shows very good agreement and allows to constrain shear velocities and quality factors for a two-layer model of the subsurface. The approach is generalized to predict amplitude decays and radiation pattern for WFs of arbitrary geometry.\n