Accuracy analysis of shear wave velocity profile by semicircular masw and seismic downhole tests

Abstract

The multichannel analysis of surface waves (MASW) test is often used to estimate the shear wave velocity (Vs) profile. Geophone’s layouts and the dispersion-inversion analysis type affect the accuracy of estimating the Vs profiles from active MASW tests. Geophone’s responses in a semicircular layout are expected to provide different wave attenuations in the active MASW test. This indicates the challenge in obtaining the correct Vs profile. Before estimating the Vs profile, this study analyses the geophone’s response in a semicircular layout using the Fourier amplitude spectrum (FAS) and dispersion image. The FAS attenuation characteristics of sensors in the semicircular MASW layout can figure out the effective frequency range of the fundamental mode dispersion curves. The active MASW tests were conducted at a field site, where geophones were placed at 1m spacing on the perimeter of the semicircular layout around each borehole. The wavefield from 12 geophones was captured by applying an impulse at an offset distance of 6m from each end of the semicircular array. The dispersion images were generated in the forward and reverse shots of the semicircular sensor layout. The fundamental mode dispersion curves were picked, and Vs profiles were estimated using the teaching-learning-based optimization algorithm. The borehole was located at the centre of the semicircular MASW layout, and Downhole seismic tests were performed at this location to collect the shear waveform at each 1m interval along the borehole depth to obtain the Vs profiles. The accuracy of Vs from the semicircular MASW test was reported up to 8m depth by comparing it with the Downhole Seismic test result.