Research paper OpenHVSR: imaging the subsurface 2D/3D elastic properties through multiple HVSR modeling and inversion S. Bignardi n , A. Mantovani, N. Abu Zeid University of Ferrara, Via Ludovico Ariosto 35 - 44121, Ferrara, Italy article info Article history: Received 16 December 2015 Received in revised form 11 April 2016 Accepted 17 May 2016 Available online 19 May 2016 Keywords: HVSR Microtremor Soil amplification Soil response 2-D 3-D abstract OpenHVSR is a computer program developed in the Matlab environment, designed for the simultaneous modeling and inversion of large Horizontal-to-Vertical Spectral Ratio (HVSR or H/V) datasets in order to construct 2D/3D subsurface models (topography included). The program is designed to provide a high level of interactive experience to the user and still to be of intuitive use. It implements several effective and established tools already present in the code ModelHVSR by Herak (2008), and many novel features such as: -confidence evaluation on lateral heterogeneity -evaluation of frequency dependent single parameter impact on the misfit function -relaxation of V p /V s bounds to allow for water table inclusion -a new cost function formulation which include a slope dependent term for fast matching of peaks, which greatly enhances convergence in case of low quality HVSR curves inversion -capability for the user of editing the subsurface model at any time during the inversion and capability to test the changes before acceptance. In what follows, we shall present many features of the program and we shall show its capabilities on both simulated and real data. We aim to supply a powerful tool to the scientific and professional community capable of handling large sets of HSVR curves, to retrieve the most from their microtremor data within a reduced amount of time and allowing the experienced scientist the necessary flexibility to integrate into the model their own geological knowledge of the sites under investigation. This is especially desirable now that microtremor testing has become routinely used. After testing the code over different datasets, both simulated and real, we finally decided to make it available in an open source format. The program is available by contacting the authors. & 2016 Elsevier Ltd. All rights reserved. 1. Introduction Early uses of microtremor for microzonation studies dates back to the 1950s. However, they became increasingly popular in the 90s after the paper of Nakamura (1989), who first introduced the H/V concept (i.e. the ratio between the Fourier spectra of the horizontal and vertical components of the seismic ambient noise). The use of microtremor for the estimation of the local site effects has become increasingly popular especially thanks to its simple approach which only requires the use of a single three-component seismograph and thanks to its applicability which is at present time enhanced by the availability of a wide range of low cost in- struments. It is now well understood that the peaks of a HVSR curve occur at the resonance frequencies of the measurement site and are connected to the acoustic impedance contrasts in the subsurface, so that valuable information about the potential seis- mic amplification at sites where soft sediments resides over bedrock can be achieved by investigating the microtremors. The theoretical basis of the method still remains a matter of discussion. In the early seventies several Japanese scientists (Nogoshi and Igarashi, 1971; Shiono et al., 1979; Kobayashi, 1980) assessed the physical significance of the H/V ratio showing that there is a direct relationship with the ellipticity of Rayleigh waves. Nakamura (1989), on the other hand explained H/V peaks as caused by multiple reflections of vertically incident SH waves. Despite these different explanations, the H/V technique is now widely used for site-specific investigations and microzonation studies (Mucciarelli and Gallipoli, 2001; Scherbaum et al., 2003; Gallipoli et al., 2004a; D’Amico et al., 2008; Albarello et al., 2011). Other applications of the H/V regard the first-order estimation of the geometry of the main seismic reflector and the mapping of the sediments thickness overlying the seismic bedrock (Parolai et al., 2002; Hinzen et al., 2004; D’Amico et al., 2008). Another relevant application of mi- crotremors, regard the identification of the fundamental frequency of buildings and the soil-structure interaction (Mucciarelli and Gallipoli, 2001; Gallipoli et al., 2004b). Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/cageo Computers & Geosciences http://dx.doi.org/10.1016/j.cageo.2016.05.009 0098-3004/& 2016 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: sedysen@gmail.com (S. Bignardi). Computers & Geosciences 93 (2016) 103–113