Characterization of the sedimentary cover at the Himalayan foothills using active and passive seismic techniques

The characterization of the sediments, down to bedrock, is very important from the seismological point of view in order to study the possible earthquake effects (site effects). Resonance frequency and shear-wave velocity profile are the main features used to estimate the thickness and stiffness of the sedimentary cover. To map these characteristics different geotechnical, geophysical and seismological methods have been developed and applied over a last few years. In this work, different soil investigation methods have been applied around the Himalayan foothills, focusing on three sites with different soil characteristics that span from the Doon valley to the Ganga foreland basin. Active and passive array experiments were carried out: Multichannel Analysis of Surface Waves (active MASW), Passive Remote MASW and f–k technique. A dispersion curve was estimated for every site covering a wider frequency band rather than if only one method would have been used. Moreover, ambient noise measurements were also recorded in order to apply the H/V method and to estimate the resonance frequencies. Combining the information provided from all methods and using the neighbourhood algorithm, the best suitable shear (S) wave velocity profiles were estimated for each area. In this way, soil sediments were characterized by the resonance frequency, the soil thickness and the mean S-wave velocity. It has been demonstrated that the use of different methods give coherent and more robust results than when only one method is applied. This greatly contributes to the credibility of the results.