<p>We give an overview of a new toolbox for easy and fast beamforming analysis of three-component ambient seismic noise and discuss examples from different seismic arrays to solve different application challenges. From only a couple of hours of array recordings, the beamformer provides estimates of surface wave dispersion curves, surface wave azimuthal anisotropy, frequency-dependent wavefield composition including surface and body waves, and the direction of arrival for different wave types and frequencies. The beamformer can be used with three-component arrays from the lab to the field scale, provided ambient noise is available in the corresponding frequency range. Compared to standard (single-component) beamforming analysis, our approach integrates all three components recorded at every seismometer. Considering the phase shifts across the components, it identifies wave-specific particle motion and hence discriminates different wave types on account of their polarisation. The new implementation of the beamformer does not use the cross-spectral density matrix of the data explicitly (as done, for example, by the MUSIC algorithm and Capon beamformer), which reduces computation times significantly and makes it feasible to compute beam responses for a full day of data recorded on 100s of stations on a standard laptop PC. The toolbox will be available on github for both MATLAB and Python.</p> <p>In an example from Los Humeros geothermal field (Mexico) we show Rayleigh wave azimuthal anisotropy as a function of frequency, corresponding to varying fast directions as a function of depth. A good agreement between the observed anisotropy and stress data from well logs as well as geological information indicates that fast directions correlate with the orientation of major faults and dykes. Anisotropy analysis thus provides a means to assess fault properties at depth, giving information about potential secondary permeability – a vital parameter in deep geothermal plays. Beamforming analysis of noise recordings in the Groningen area (Netherlands) reveals dominant prograde motion in both fundamental and 1<sup>st</sup> higher mode Rayleigh waves. This behaviour is indicative of a large impedance contrast between the very low shear-velocities in sedimentary basins and the underlying bedrock. The resolution of particle motion as a function of frequency allows us to observe the osculation frequency where fundamental and 1<sup>st</sup> higher mode Rayleigh waves approach each other and both modes change particle motion from prograde to retrograde and vice versa. The osculation frequency can be used to estimate the depth of the major impedance contrast, that is, the depth of the sedimentary basin. While body wave observations must be interpreted with care, considering the resolution capabilities of the array with respect to the expected (larger) wavelengths, the examples show that body waves contribute to the ambient noise wavefield with varying degree as a function of frequency, challenging the assumption of surface wave dominance common in ambient noise studies. Overall, we demonstrate that our beamforming toolbox provides direct information about structural features as well as fundamental a-priori information on wavefield composition and source characteristics, valuable for further ambient noise methods.</p>

<p>The Highland Boundary Fault (HBF) delineates a fundamental division in the topography and surface geology in Scotland, separating 1000-500Ma metamorphic rocks to the north from predominantly ~440-360Ma sedimentary rocks of the Midland Valley to the south. Despite detailed geological mapping of the HBF and surrounding areas, the role(s) of the HBF in the tectonic history of Scotland is contested. On one hand, the HBF may represent a major plate boundary that was active initially as a strike-slip, then reactivated as a high angle thrust fault. On the other hand, some argue that lateral movement on the HBF was limited, and the topographic break seen at the HBF is primarily due to differences in erosion rates. Seismicity on the HBF has been reported in both the instrumental and historical records, including a M4.8 earthquake in Comrie in 1839 and an earthquake swarm in Aberfoyle in 2003. Notably, no seismicity has been observed in northeast Scotland. It may be that there is no seismicity in this region, or that the distribution of seismic instrumentation has been insufficient to detect very small magnitude earthquakes (<M2).</p><p> </p><p>To address these questions, in March-May 2022 we deployed a new network of 10 seismometers in north eastern Scotland as part of the PICTS (Probing Into the Crust Through eastern Scotland) project, which, together with a BGS Seismology permanent station, DRUM, form three transects across the HBF. These instruments form the first dense seismometer deployment in this region and data from them will allow us to place high-resolution constraints on the structure of the crust and uppermost mantle across the HBF, determine crustal thickness in this region, and to investigate if any seismicity is occurring on the eastern portion of the HBF.</p><p> </p><p>Here we present preliminary results from the data recorded on seismometers from the PICTS project, including images of crustal structure from receiver function analysis that show differing crustal structure to the north and south of the HBF.</p><p> </p>

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<p>Faults and fractures are crucial parameters for geothermal systems as they provide secondary permeability allowing fluids to circulate and heat up in the subsurface. In this study, we use an ambient seismic noise technique referred to as the three-component (3C) beamforming to detect and monitor faults and fractures at a geothermal field in Mexico.</p><p>Three-component (3C) beamforming extracts the polarizations, azimuths, and phase velocities of coherent waves as a function of frequency, providing a detailed characterisation of the seismic wavefield. In this study, 3C beamforming of ambient seismic noise is used to determine surface wave velocities as a function of depth and propagation direction. Anisotropic velocities are assumed to relate to the presence of faults giving an indication of the maximum depth of permeability, a vital parameter for fluid circulation and heat flow throughout a geothermal field.</p><p>We perform 3C beamforming on ambient noise data collected at the Los Humeros Geothermal Field (LHGF) in Mexico. The LHGF is situated in a complicated geological area, being part of a volcanic complex with an active tectonic fault system. Although the LHGF has been exploited for geothermal resources for over three decades, the field has yet to be explored at depths greater than 3 km. Thus, it is currently unknown how deep faults and fractures permeate and the LHGF has yet to be exploited to its full capacity.</p><p>3C beamforming was used to determine if the complex surface fracture system permeates deeper than is currently known. Our results show that anisotropy of seismic velocities does not decline significantly with depth, suggesting that faults and fractures, and hence permeability, persist below 3 km. Moreover, estimates of fast and slow directions, with respect to surface wave velocities, indicate the orientation of faults with increasing depth. The North-East and North-West orientation of the fast direction corresponds to the orientation of the Arroyo Grande and Los Humeros faults respectively. Various other orientations of anisotropy align with other major faults within the LHGF at depths permeating to 6 km.</p>