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Using a colocated ring laser and an STS-2 seismograph, we estimate the ratio of Rayleigh-to-Love waves in the secondary microseism at Wettzell, Germany, for frequencies between 0.13 and 0.30 Hz. Rayleigh wave surface acceleration was derived from the vertical component of STS-2, and Love wave surface acceleration was derived from the ring laser. Surface wave amplitudes are comparable; near the spectral peak about 0.22 Hz, Rayleigh wave amplitudes are about 20% higher than Love wave amplitudes, but outside this range, Love wave amplitudes become higher. In terms of the kinetic energy, Rayleigh wave energy is about 20-35% smaller on average than Love wave energy. The observed secondary microseism at Wettzell thus consists of comparable Rayleigh and Love waves but contributions from Love waves are larger. This is surprising as the only known excitation mechanism for the secondary microseism, described by Longuet-Higgins (1950), is equivalent to a vertical force and should mostly excite Rayleigh waves.

About a decade ago, noise-based monitoring became a key tool in seismology. One of the tools is passive image interferometry (PII), which uses noise correlation functions (NCF) to retrieve seismic velocity variations. Most studies apply PII to vertical components recording oceanic low-frequent ambient noise (< 1 Hz). In this work, PII is applied to high-frequent urban ambient noise (> 1 Hz) on three three-component sensors. With environmental sensors inside the subsurface and in the air, we are able to connect observed velocity variations with environmental parameters. Temperatures below 0 °C correlate well with strong shear wave velocity increases. The temperature sensors inside the ground suggest that a frozen layer of less than 5 cm thickness causes apparent velocity increases above 2  % , depending on the channel pair. The observations indicate that the different velocity variation retrieved from the different channel pairs are due to different surface wave responses inherent in the channel pairs. With dispersion curve modelling in a 1-D medium we can verify that surfaces waves of several tens of metres wavelength experience a velocity increase of several percent due to a centimetres thick frozen layer. Moreover, the model verifies that Love waves show larger velocity increases than Rayleigh waves. The findings of this study provide new insights for monitoring with PII. A few days with temperature below 0 °C can already mask other potential targets (e.g. faults or storage sites). Here, we suggest to use vertical components, which is less sensitive to the frozen layer at the surface. If the target is the seasonal freezing, like in permafrost studies, we suggest to use three-component sensors in order to retrieve the Love wave response. This opens the possibility to study other small-scale processes at the shallow subsurface with surface wave responses. [ABSTRACT FROM AUTHOR]

The observed secondary microseismic wave-field is composed by Rayleigh and Love waves. While the presence and amount of Rayleigh waves can be explained well, the generation of Love waves is still under debate. We investigate both internal multiple scattering of surface waves and interaction with strong seafloor topography as possible mechanisms that could be responsible for the generation of Love waves in the secondary microseism.In a first study, we assume that the secondary microseism source generates mainly vertically polarized waves, and that the Love waves are produced during the propagation of the wavefield. To study this hypothesis, we model the wavefield in a strongly random von Karman type medium with a free surface. To understand more about the relationship between the surface wave types and the parameters of the random medium, we start with a parameter study. We alter the model fluctuation strength, the correlation length and the layering structure of the medium. We find that the fluctuation strength has a major influence on the Love to Rayleigh ratio. We observe that the Love to Rayleigh ratio reaches a maximum value of 0.4 after approximately 4 scattering mean free paths. This value does not explain the observed Love to Rayleigh ratios in the secondary microseism, which lies between 0.6 and 1.2. Moreover, we would need an unrealistically strongly scattering medium to reach this value. Considering scattering properties similar to those encountered in oceanic crust for wavefields around 0.2Hz, we obtain a Love to Rayleigh ratio of 0.1, which significantly below the typically observed ratio of 0.6-1.2.In a second study, we consider the case of a Longuet-Higgins type source in the water column moving over a set of sea mounts with different maximum heights and values for slope steepness. Furthermore, we model the effect of a ocean microseism noise source moving over a rough patch of realistic seafloor topography. We quantify the ratio of Love to Rayleigh wave energy in the fundamental and higher modes around the sea mounts and around the bathymetry structure. For frequency ranges around the secondary microseism, topography contrasts of the order of 500m generate Love-to-Rayleigh ratios in the range commonly observed in the field. [ABSTRACT FROM AUTHOR]