GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 3.2 615 MULTI-MODE MULTI-OFFSET PHASE ANALYSIS OF SURFACE WAVE DATA (MMMOPA) I. Barone 1 , G. Cassiani 1 , C. Strobbia 2 1 Università degli Studi di Padova, dipartimento di Geoscienze, Padova, Italy 2 RealTimeSeismic, Pau, France Introduction. Seismic surface waves have started to be analysed in the late 1950’s, with the purpose of retrieving useful information about the subsoil (Jones, 1958; Jones, 1962; Ballard, 1964). In the 1980’s spectral methods for surface waves analysis were introduced and applied in the context of geotechnical studies (Nazarian and Stokoe II, 1984), but it is only in the late 1990’s that the surface wave methods gained popularity, thanks to the development of multichannel techniques (Park et al. , 1999; Xia et al. , 1999). Both active and passive techniques have been developed. Among the active, the Multi-Offset Phase Analysis (MOPA) (Strobbia and Foti, 2006) is a fairly recent technique for evaluating seismic surface wave dispersion and estimating the presence of lateral variations. Despite its robustness, MOPA has a limitation: it is based on the assumption of one predominant mode, usually the fundamental mode, in the wave propagation. If this condition is not satisfied, MOPA can still be used if the different modes are well separated: a preliminary processing step is required, which consists in isolating each mode, for example by filtering the data in f-k domain. Otherwise, when the different modes are close to each other and/or lateral variations are quite important, isolating modes prior to the analysis might be not applicable. We present here a new approach, which can be considered as an extension of MOPA to the two-modes case, with possible extensions to more modes. The Multi-Mode MOPA(MMMOPA) is based on the concurrent analysis of both amplitude and phase, and is capable of extracting dispersion curves for at least two different interfering modes. Fig. 1: From top to bottom: amplitude, unwrapped phase and unwrapped phase after LMO correction for the interference between two sinusoidal signals, a and b, with frequency f = 6 Hz, amplitudes A a = 10 and A b = 5 and apparent propagation velocities c a = 100 m/s and c b = 160 m/s. Velocity used for LMO is v LMO = 100 m/s.