Combined application of land seismic methods in engineering research

Research subject. The upper part of a geological section (low-velocity layer).Aim. To demonstrate the feasibility of combining two seismic methods (active and passive) for determining the depth of bedrock roof and the presence of structural changes in the soil layer.Materials and methods. Geophysical methods in engineering studies are faced, as a rule, with the task of determining the depth and relief of rock occurrence. This task, although not being the sole one, is of particular importance. Among the variety of geophysical methods, seismic methods are the most informative and mobile. In this article, we compare the results of a combined application of the methods of refracted waves and microseismic sounding in different geological conditions. These are two basically different land seismic methods. The former method is active and has a source of elastic vibrations. The latter method is passive and records natural- and artificial-origin surrounding background microtremor. At each of the sites under study, one profile was selected, on which sections were built along P-waves Vp and S-waves Vs. The microtremor was recorded at several points, along with calculation of the spectral power density of the displacement rate and the spectral ratio curve. Given the rate of shear waves in the loose layer, a conversion formula from the frequency domain (Nakamura transfer characteristic) to depth is used. Thus, at the observational points of microseisms, the depth of boundaries contrasting in acoustic rigidity can be calculated and traced.Results. On the example of different soil conditions, the correlation between velocity cross-sections and Nakamura microseism results is shown.Conclusions. The combination of the classical refracted wave method and microseismic sounding provides a more detailed information on the structural features of the upper part of a geological section.

refracted wave method, engineering surveys, seismic waves, P-waves, S-waves, microtremor, HVSR method, Nakamura method