GNGTS 2015 - Atti del 34° Convegno Nazionale

140 GNGTS 2015 S essione 3.3 Some conclusions. In spite of the limited adopted equipment (one 4-channel seismograph, one 3C geophone and four vertical-component geophones) and the simple and straightforward field procedures, the proposed acquisition and analysis procedures can provide a significant amount of data to jointly analyze in order to fully constrain an inversion procedure eventually capable of providing a robust subsurface model free from ambiguities and non-uniqueness of the solution. The adopted procedures appear particularly relevant especially in case of surveys to accomplish in urban areas (for instance for seismic-hazard studies), characterized by heavy logistical problems and limited room to deploy and move the sensors. While the determination of the HVSR curve and the active acquisitions are not significantly influenced by the electronic noise produced by the acquisition system, the analyses performed according to the Miniature Array Analysis of Microtremors result instead strongly affected by this sort of noise and the use of high-quality A/D units results then mandatory. Furthermore, an improper geophone coupling (in urban areas planting a geophone can be quite tricky) can determine low-quality data that would then mirror in poor analyses. In fact, because of the mathematics intrinsically involved in the Miniature Array analyses, even relatively-small differences in the amplitudes of the acquired traces can produce major problems and prevent from the determination of fully reliable dispersion curves. Data quality should be then checked already on the field even by simply computing the rms values for all the acquired traces. If the rms amplitude of one trace significantly deviates from the median value, that is the evidence that some problem occurred during the acquisition of that trace. Common causes are represented by a bad coupling, problems with the geophone-cable connections or, finally (worst case), by the fact that the coil of that geophone is not consistent with that of the other geophones (in this case the geophone must be substituted). From the practical point of view is important to underline that while the HVSR and MAAM data necessarily refer to very local conditions, the active data refer instead to the average subsurface conditions between the source and the receiver. In case of significant superficial lateral variations, at the high frequencies (i.e. the shallowest layers) it is then possible a relatively poor match for the three objective functions here considered. For this reasons, HVSR and MAAM data should be considered only in the low-frequency range (incidentally often poorly defined in the active data). Acknowledgements. The present work was partially supported by the Visiting Professor Program of the King Saud University (Riyadh, Saudi Arabia) and the authors are grateful to the Deanship of the Scientific Research of the King Saud University (Riyadh, Saudi Arabia) for the financial support (PRG-1436-06 Research Grant). References Arai H. and Tokimatsu K.; 2004: S-wave velocity profiling by inversion of microtremor H/V spectrum . Bull. Seism. Soc. Am, 94 , 53–63. Arai H. and Tokimatsu K.; 2005: S- ave velocity profiling by joint inversion of microtremor dispersion curve and horizontal-to-vertical (H/V) spectrum . Bull. Seism. Soc. Am., 95 : 1766-1778 Bhattacharya S.N.; 1983: Higher order accuracy in multiple filter technique. Bull. Seismol. Soc. Am., 73 : 1395– 1406. Cho I., Senna S. and Fujiwara H.; 2013: Miniature array analysis of microtremors . Geophysics, 78 , KS13–KS23. Dal Moro G., Moura R.M. and Moustafa S.R.; 2015c: Multi-component Joint Analysis of Surface aves . J. Appl. Geophysics , 119 , 128-138. Dal Moro G., Moustafa S.R. and Al-Arifi N.; 2015b: Efficient acquisition and holistic analysis of Rayleigh waves, Proceedings of the Near-Surface EAGE 2015 (Turin - Italy) Dal Moro G., Keller L. and Poggi V.; 2015a: A Comprehensive Seismic Characterization via Multi-Component Analysis of Active and Passive Data . First Break, 33, 45-53. Dal Moro G.; 2014: Surface ave Analysis for Near Surface Applications . Elsevier, ISBN 9780128007709, 252pp Ohori M., Nobata, A. and Wakamatsu K.; 2002: A comparison of ESAC and FK methods of estimating phase velocity using arbitrarily shaped microtremor analysis . Bull. Seism. Soc. Am., 92 , 2323–2332. Picozzi M. and Albarello D.; 2007: Combining genetic and linearized algorithms for a two-step joint inversion of Rayleigh wave dispersion and H/V spectral ratio curves . Geophys. J. Int., 169 , 189-200.