<p>In recent years, small dense arrays with inter-station distances ~1-km are widely deployed for various geotechnical proposes, including exploring the subsurface geothermal reservoirs, monitoring the hydraulic fracturing, and estimate the local seismic hazards. Those small-dense arrays record several hours or days microtremor datasets and can be used to extract noise H/V ratio and short-period Rayleigh wave dispersion curve. To fulfill those geotechnical purposes, the extracted H/V ratio and Rayleigh wave dispersive curves are used to invert for 1-D layered velocity structures based on traditional ray theory, ignoring the 3-D Rayleigh wave propagating effects. In the present-day, with advances in computational power, 3-D numerical seismic wave propagation in realistic Earth models has become feasible and this has led to efforts to invert full waveforms. However, as H/V ratios from microtremors are produced by multiple random sources close to the surface with complex scattering effects, they are still unrealistic to be simulated by 3-D numerical seismic wave simulations. In this study, we use a strategy to incorporate the theories of H/V ratios from microtremor and the 3-D waveform simulations to invert for shallow sub-surface speed model using a small array. The details of the strategy are that we first use the traditional method to build an initial speed model by jointly inverting the Rayleigh wave dispersion curves and H/V ratios, and then, we further update the initial model with full-wave inversion of the Rayleigh waveforms calculated from ambient noise cross-correlations. The small array used here was composed of 21 broadband seismic stations, aimed to monitor the underground hydraulic fracturing. Therefore, the newly build model has important geotechnical usages of locating stimulated micro-seismic events and diagnose the hydraulic fracturing effects.</p>
Thin superficial layer and lateral heterogeneities in southern Sweden using short-period Rayleigh-wave dispersion
