Imaging Subsurface Structure of an Urban Area Based on Diffuse-Field Theory Concept Using Seismic Ambient Noise
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Pure and Applied Geophysics
Imaging Subsurface Structure of an Urban Area Based on Diffuse-Field Theory Concept Using Seismic Ambient Noise NILUTPAL BORA,1 RAJIB BISWAS,2 Abstract—Single station ambient noise measurement in campaign mode has of late gained a huge popularity among geoscientists. Herein, we present results of ambient vibration analysis, executed in a highly populated urban area covering 47 survey points. The resonance frequency estimates range from 0.5 to 3 Hz, as found from H/V. Taking H/V curve as input for retrieving subsurface information, we deploy diffuse field assumption (DFA) theory. The obtained shear wave velocity from the inversion of H/V curve through DFA approach provides evidence of the complex nature of the subsurface geological structures. Identifying six characteristic 2D cross-sections of the entire area, we attain prevalence of a lowvelocity intermediate layer with velocity ranging from 128–192 m/s. On the contrary, a relatively high-velocity layer is also obtained (279–471 m/s) which can be treated as sedimentary deposits (may be for some sites as basin basement). The attained results, when extended to a 3D shear wave profile, tally excellently with estimated frequency distribution and its corresponding links with depth wise strata, accompanied by a topographical profile of the surveyed locations. All findings are comprehensively analyzed and interpreted as a proof of concept of implementation of DFA approach towards retrieving subsurface information. Keywords: Site effects, seismic noise, spectral ratio techniques, shear-wave velocity profile, earthquake data analysis, soil characterization.
1. Introduction Inferring seismic response from ambient noise has become a widely established technique which renders quick acquisition as well as cost-effectiveness. Numerous literatures are available which directly deals with effectiveness of quantification of ambient vibrations for delineating local seismic response (e.g. Nakamura 1989; SESAME 2004; D’Amico et al.
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Department of Civil Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, India. 2 Geophysical Lab, Department of Physics, Tezpur University, Tezpur, Assam 784028, India. E-mail: [email protected] 3 Institute of Geosciences, Friedrich-Schiller University Jena, Jena 07749, Germany.
and PETER MALISCHEWSKY3 2008; Albarello and Lunedei 2011; Gallipoli et al. 2011 ; Paolucci et al. 2015; Farrugia et al. 2016). Local site settings play a major role in enhancing seismic damage; although, there are contributions from proximity of source location as well as path effect. In general terms, local site effects may be well-defined as amendment of features of receiving seismic wavefield, arising due to specific characteristics of site geology. There are two ways to quantify site effects; firstly, by using seismic array arrangement (multi-station) (Marano´ et al. 2017) and secondly, by simply using a single-station deployment (e.g., Mucciarelli 1998; Parolai et al. 2005; Picozzi et al. 2005; Foti et al. 2011). Related to
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