ORIGINAL ARTICLE HVSR technique in near-surface thermal-basin characterization: the example of the Caldiero district (North-East Italy) Laura Agostini • Jacopo Boaga • Antonio Galgaro • Andrea Ninfo Received: 8 August 2014 / Accepted: 23 January 2015 / Published online: 10 February 2015 Ó Springer-Verlag Berlin Heidelberg 2015 Abstract In the present work, non-invasive microtremor survey is applied for the preliminary characterization of the thermal district of Caldiero, in the eastern Po Plain (Italy), a thermal resource known since the Roman times for the presence of warm springs. The work’s aim was to test the suitability of the microtremor passive technique for the pre- liminary characterization of a thermal basin, in terms of resonance properties between the alluvial deposit covers and the rocky bedrock. In particular, the horizontal-to-vertical spectral ratio (HVSR) single-station technique is adopted to estimate the resonance properties of the geothermal basin. More than 100 HVSR single-station measurements are col- lected and analyzed in the area studied. HVSR information is integrated with the findings of the analysis of the data relative to more than 20 deep wells. Such an integration permits the calibration for validation of the geophysical data. The cor- relation between the results from microtremor survey, LiDAR data analysis, and wells information highlighted the main geological structures of the site, which are potentially responsible for the thermal waters arising. HVSR results are then interpreted in terms of regional geological setting, tex- tural, and hydrogeological properties of the superficial de- posits and the underlying rocky bedrock. The results of this method confirm the existence of significant correlations be- tween the resonance behavior of the basin, as determined by the HVSR technique, and the geological assessment. This suggests that the single-station technique could be a very promising tool for a rapid and preliminary non-invasive method for the near-surface general characterization of geothermal basins. Keywords HVSR Á Microtremor analysis Á Thermal basin Á Groundwater resources Introduction This work presents the use of the horizontal-to-vertical spectral ratio single-station microtremor technique (HVSR, Nogoshi and Igarashi 1970) for the preliminary charac- terization of a thermal basin in the Po plain, N-E Italy. HVSR is a technique based on the passive acquisition of natural microtremors. It is known that, in presence of impedance geological contrast in depth, the spectral ratio of the three components of motion furnishes a reliable esti- mation of the resonance properties (Nakamura 1989). The method proves to be an efficient and low-cost preliminary 1D tool for geological characterization (Lermo and Chavez- Garcia 1994), seismic behavior analysis (Castellaro and Mulargia 2009), and thermal-basin geological reconstruc- tion (Galgaro et al. 2014). This non-invasive geophysical technique is applied to the Caldiero thermal district, a thermal resources area located in North-Eastern Italy known since the Roman period (Nicolis 1898 and 1901). More than 100 HVSR single-stations measurements are collected and analyzed in close correlation with the 21 wells distributed all around the district. The microtremor geophysical surveys results are then correlated to the basin characteristics, airborne laser scanners (ALS) remote sensing data, and thermal wells information. The aim of the work is to evaluate the suitability of the microtremor technique for the preliminary characterization of a thermal basin, in terms of resonance properties of the alluvial deposits that cover the rocky bedrock, evaluated by means of the HVSR methods. HVSR results are interpreted in terms of regional geological setting and hydrogeological L. Agostini (&) Á J. Boaga Á A. Galgaro Á A. Ninfo Department of Geosciences, University of Padova, Via G. Gradenigo, 6, 35131 Padua, Italy e-mail: laura.agostini.2@studenti.unipd.it 123 Environ Earth Sci (2015) 74:1199–1210 DOI 10.1007/s12665-015-4109-0