Frequency-dependent multi-offset phase analysis of surface waves: an example of high-resolution characterization of a riparian aquifer

Multi-offset phase analysis (MOPA) of seismic surface waves is an established technique for the extraction of dispersion curves with high spatial resolution, and consequently for the investigation of the subsurface in terms of shear wave velocity distribution. However, field applications are rarely documented in the published literature. In this paper we discuss an implementation of the multi-offset phase analysis consisting of the estimation of the Rayleigh wave velocity by means of a moving window with a frequency-dependent length. This allows maximizing the lateral resolution at high frequencies, while warranting stability at the lower frequencies. In this way, we can retrieve the shallow lateral variability with high accuracy and, and the same time, obtain a robust surface wave velocity measurement at depth. In this paper we apply this methodology to a dataset collected for hydrogeophysical purposes, and compare the inversion results with those obtained by using refraction seismics and electrical resistivity tomography. The surface wave results are in good agreement with those provided by the other methods and demonstrate a superior capability in retrieving both lateral and vertical velocity variations, including inversions. Our results are further corroborated by the lithological information from a borehole drilled on the acquisition line. The availability of MOPA data also allows disentangling a fairly complex interpretation of the other geophysical results.