GNGTS 2017 - 36° Convegno Nazionale

422 GNGTS 2017 S essione 2.2 depth varies from over 500 m in the north to 300-400 m in the south. The archipelago is made up of 22 atolls of circular or elongated shape, the sizes varying from some kilometers to many tens of kilometers, and contains more than 1000 islands which cover a surface of 298 km 2 . Every atoll is formed by a marginal rim surrounding a lagoon commonly less than 50-60 m deep, though some reach depths of more than 80 m. The atoll lagoons are characterized mainly by sandy sediments; a large amount of the sand (more than 50%) derives from mechanical erosion and from bioerosion of the reef. A minor amount is due to benthonic organisms living in the sand with fewer contributions from planktonic foraminifers. Magoodhoo, is a remote small island on the Faafu Atoll (Fig. 1B), at a distance of 134 km from Malé, Republic of Maldives capital, with a population of 683 inhabitants distributed on a surface of less than 0.36 km 2 . Field measurements. The basic goal of single-station ambient vibration measurements is the detection of seismic impedance contrasts in the subsoil, that are responsible for seismic resonance phenomena (e.g., Kramer, 1996). The technique consists of recording and analyzing the seismic ambient noise to retrieve information about subsoil stratigraphy from seismic tremor. In particular, we use HVSR technique, which consists of studying the ratio between the Vertical and Horizontal spectral components of the tremor recorded at a single station. The method, provides a direct estimate of the resonance frequencies of subsoil. This well known capability allows to translate the H/V curves into stratigraphy (Castellaro et al. , 2005). The theoretical bases of this technique are simple only in a 2 layer Fig. 2 - Results of HVSRs using microtremor measurements.