1 Department of Geoscience, Science and Technology, Aarhus University2 Dept. of Gescience, University of Padova3 WesternGeco4 Department of Geoscience, Science and Technology, Aarhus University
The Surface Wave Method is a popular tool for geotechnical characterization since it supplies a cost effective testing procedure capable of retrieving the shear wave velocity structure of the near surface. Several acquisition and processing approaches have been developed to infer the Rayleigh wave dispersion curve which is then inverted. Typically, single component vertical and multi channel receivers are used. In most cases the inversion of the dispersion properties is carried out assuming that the experimental dispersion curve corresponds to a single mode, mostly the fundamental Rayleigh mode, unless clear evidence dictates the existence of a more complex response, e,g. in presence of low velocity layers and inversely dispersive sites. A correct identification of the modes is essential to avoid serious misinterpretation of the subsoil model. Here we consider the typical case of higher mode mis-identification known as ‘osculation’ (‘kissing’). In general it is called ‘osculation point’ the point where the energy peak shifts at low frequencies from the fundamental to the first higher mode. This jump occurs, with a continuous smooth transition, around a well-define frequency where the two modes get very close to each other reaching similar Rayleigh velocity. It is known ‘osculation’ happens generally in presence of strong velocity contrasts, typically with a fast bedrock underlying loose sediments. The practical limitations of the acquired data affect the spectral and modal resolution, making it often impossible to identify the presence of more than one mode. In some cases, modes have a common root and cannot be separated at the osculation point. In such cases, mode misidentification can create a large overestimation of the bedrock velocity and a large error on its depth. In this work we examine the subsoil conditions that can generate this unwanted condition, and the common field acquisition procedures that can contribute to producing data having such deceptive Rayleigh dispersion characteristics. We show how this mode mis-identification depends strongly on the usual approach of measuring only the vertical component of ground motion, as the mode osculation is linked to the Rayleigh wave ellipticity polarization, and therefore we conclude that multi-component data, using also horizontal receivers, can help discern the multi-modal nature of surface waves. Finally we introduce a-priori detectors of subsoil conditions, based on passive micro-tremor measurements, that can act as warnings against the presence of mode osculation, and relate these detectors to the frequencies at which dispersion curves can be misidentified. Theoretical results are confirmed by real data acquisition tests.