Near-surface Shear Velocity Structure Estimation using Ground-roll in Moran Area, Central Upper Assam Basin, India

Abstract

The frequency dependent characteristics of Rayleigh-wave, which is considered to be noise during body-wave prospecting, can be utilized to infer near surface elastic properties. To generate the near surface VS profiles, initially spectral analysis of surface waves (SASW) was used, which has now been replaced by more sophisticated multi-channel analysis of surface waves (MASW). In the present study, MASW for impulsive source is applied to the seismic (impulsive source) datasets acquired in the Moran area of the central part of upper Assam basin to estimate the near-surface S-wave velocity (Vs) structure with the objective of its application to static correction during S-wave reflection data processing, and geotechnical and environmental studies in the area. Multichannel surface waves are extracted from conventional seismic record. The multichannel analysis includes construction of dispersion curves from observed Rayleigh-waves, and inversion of VS profile from calculated dispersion curves. The near surface 1D VS profiles, derived from the extracted ground roll vary from 200 m/sec (near the surface) to about 450 m/sec at 40m depth, and provides a good estimation of vertical shear velocity structure in the area. The 2D near surface shear velocity structure (VS) is generated by horizontal interpolation of individual vertical profiles. The results are in agreement with the available three component low velocity layer (3C-LVL) data.