STIFFNESS PROFILING OF GEOTECHNICAL SITES WITH A SOFT
ENTRAPPED LAYER BY BEAMFORMING TECHNIQUE
Norfarah Nadia Ismail*1, Sung-Ho Joh†1, and Raja Hassanul Musa Raja Ahmad1
1
Department of Civil and Environmental Engineering, Chung-Ang University, Korea
ABSTRACT
Surface-wave methods are beneficial due to their intrusive features and have been successful in
determining subsurface profiles of normally dispersive geotechnical sites. However, when a soft
layer is entrapped between stiffer layers, surface-wave data are complicated by multiple modes of
surface waves and multiple reflections of body waves. Therefore, surface-wave velocities are poorly
evaluated due to the predicament in interpreting surface-wave data. In this paper, a refined version
of the conventional beamforming technique was proposed to make a reliable stiffness profiling even
for the geotechnical sites with a soft entrapped layer. The refined version is capable of a more
practical determination of surface-wave velocities and also a more rational inversion analysis of
surface-wave dispersion relations. Practicality in determination of surface-wave velocities was
implemented by an automated determination of surface-wave velocities and a seamless recovery of
spatially-aliased wave numbers. Also rationality in inversion analysis was attained using apparent
surface-wave velocities based on the locations of source and receivers. Validity of the proposed
scheme was verified using synthetic seismograms generated from a layered system with a soft
entrapped layer. And feasibility and reliability of the proposed method was proved by field
applications at a highway compaction site. Lastly, the results were compared with the ones from
downhole tests, which show a good agreement between two shear-wave velocity profiles.
Keywords: velocity inversion, apparent velocities, inversion, beamforming technique.
*
Corresponding author: Email: shjoh@cau.ac.kr
†
Presenter: Email: norfarahnadia@gmail.com
ITC2012
(Oct 30- Nov 21, 2012, Kuala Lumpur, Malaysia)
1