Geophysical characterization of a small pre-Alpine catchment

This paper presents an application of surface wave methods to the characterization of a small-scale mountain catchment. The area represents a well-identified hydrological catchment having a limited size and moderate slope. Geologically, the site is characterized by weathered basaltic rocks, with a large percentage of clay sediments, and strong lateral variability. The general aim of the investigation is to obtain structural information about the system to understand its hydrogeological and hydrological behavior, and particularly the identification of a solid bedrock. Electrical methods (surface electric resistivity tomography, and frequency domain electromagnetic methods) are strongly influenced by the presence of clay, and while they evidence differences between different areas of the catchment, these measurements could not highlight any significant bedrock. The high electrical conductivity prevented use of ground penetrating radar as a means of investigating the site structure. The presence of a shallow strong seismic refractor (possibly the water table) caused also problems in the interpretation of classical refraction seismics by obscuring the deeper structure of the site. Ultimately, we resorted to the use of surface wave data as they are (a) sensitive mostly to the shear velocity of the matrix, thus circumventing the problem with the possible shallow water table, (b) these data are collected during the traditional seismic acquisition and carry the largest part of energy. The use of seismic surface waves in this unconventional environment must face the problem that the usual models used for the surface wave interpretation are 1D. Besides, the presence of topography introduces a further, possible deviation from the typical interpretation model. We studied the effect of a moderate topography in the surface wave processing and we used the so-called multi-offset phase analysis to select effective 1D portions where we can confidently use the standard multi-channel analysis of surface wave. Thus, we reconstructed (quasi-)2D shear wave sections by putting side by side the 1D profiles, with no lateral constraint that would be inconsistent with the high heterogeneity of the site. The final shear wave velocity model results in a consistent picture of the subsurface, and locates the bedrock at more than 20. m depth. This result is also confirmed by several independent measurements of horizontal to vertical spectral ratio and is corroborated by borehole data. This result has a major impact on the hydrological model conceptualization.