Nonconventional geophysical surveys for particular problems of civil engineering

Abstract

Geophysical surveys for Civil Engineering deal with shallow targets. So the problem is not in general the depth of penetration but the need of a very high resolution. This is not an easy task as we work exactly in that “weathered layer” that deeper surveys tend to jump out at a l l , because of high frequencies attenuation, high velocity gradients and local heterogeneities. A number of approaches (high resolution shallow reflection techniques, GRM refraction, etc.) are available. Recently we had to deal with particular situations in different fields (tunnel design, landslides in layered rocks, etc.) where these techniques were not applicable. Seismic tomographic techniques were employed but it was necessary to adapt them to the difficult operational conditions. Surface-tohole measurements were performed. A program has been expressly implemented to deal with the very high vertical velocity gradients: it takes into account both curved direct waves and head waves travelling in deeper high velocity layers. The result is similar to a VSP. It was possible to put in evidence low velocity layers as thin as 0.5 m also in complex stratifications in noisy areas. Data were later confirmed with a very good accuracy. The method proved to be fast, reliable and much less costly than the crosshole tomography. Also geoelectrical surveys were tested to study the l ands l ides : the jo in t use (and inversion) of res i s t iv i ty and IP soundings helped to clear some complex situations.