Application of Refraction Microtremor (ReMi) technique for determination of 1-D shear wave velocity in a landslide area

The application of the Refraction Microtremor (ReMi) method on slopes affected by or prone to landsliding is complicated by the presence of lateral lithological heterogeneities and irregular topography, which may hinder the extension of the geophone array to the minimum lengths (100-200 m) usually adopted in standard applications of this technique. We focus on deriving one-dimensional shear-wave velocity (Vs) vertical profiles from the analysis of microtremor recordings carried out in the municipality of Caramanico Terme (central Italy) where the seismic response has been monitored with a local accelerometer network since 2002. The stability of the ReMi data acquisitions and the reliability of the results in irregular landslide terrain were tested by using ReMi campaigns in three different periods and different acquisition parameters (seismograph channel number, geophone frequency and spacing). We also investigated the possible presence of directional variations in soil properties by carrying out noise recordings along L-shaped arrays. The influence of changing environmental conditions and of different acquisition parameters was tested by comparing the data obtained from different campaigns, using the same acquisition parameters, with the data from simultaneous acquisitions using different parameters. The tests showed that stable results can be obtained under different acquisition conditions provided that i) the ratio between the coherent and incoherent part of ambient noise is sufficiently high and ii) spatial aliasing does not contaminate the signal in the p (slowness)-f (frequency) matrix near the picking area: the latter condition can be satisfied by selecting geophone frequency and spacing appropriate for the site characteristics and for the investigation purpose. The differences in Vs measured in two orthogonal directions did not exceed 10-20 % and their analysis suggests that these directional variations are most likely due to anisotropy in noise source distribution rather than in material properties. The Rayleigh wave velocity dispersion curves obtained from microseimic noise recordings were then inverted with the DINVER software package to derive vertical distribution of the Vs. We reconstructed vertical profiles of Vs through the joint inversion of fundamental and higher modes, constrained by borehole information. The results from a site of a recent deep-seated slope failure showed that both colluvium few tens of meters thick, and the underlying mudstone have lower velocities than those of the same formations present in the surrounding area not involved in mass movements. This suggests that the mudstone has been affected by slope deformations at this site. (C) 2010 Elsevier B.V. All rights reserved.