Integration of Downhole Data Reduction Techniques for Determination of V_S Profiles

The downhole method is one of the most widely used in situ seismic tests to determine shear wave velocity (V_S) profiles for site characterization and site response analysis. Although easy to perform in the field, it has always been a tedious task to accurately interpret the downhole data based on the arrival times of shear waves at the borehole receiver. Commonly used methods are the direct, interval, and Snell’s Raypath methods. Snell’s Raypath method has been known to provide the best results for downhole processing at multilayered subsurface sites as it considers the actual travel path of waves considering the refraction along the wave travel path. The direct method is considered better when the soil layers are uniform with reduced chances of refraction. In this study, V_S profiles are estimated for two boreholes 100 m deep at a deep silty deposit underlain by sandstone rocks. The Snell’s Raypath method was observed to be more suitable for shallow subsurface to capture a greater variation in properties and need for a higher resolution in shallow subsurface. The direct method was found suitable for deeper layers where dense soil and rock layers are located, which show more uniform layering with depth.