Engineering Geophysics

Authors: Geraldine C. Anukwu, Mohd H. B. Mohammed, Daniel Obi – Enadhuze Abstract The University of Lagos (UNILAG), Lagos, Nigeria, and Bukit Ledang, Malaysia, are witnessing rapid development in terms of the construction of new buildings. Geologically, while Lagos is a coastal zone characterized bysedimentary rocks, Bukit Ledang has a hilly topography with rocks from the Paleozoic to Quaternary ages. In this study, we present the ambient noise recordings obtained from locations in each study area and analyze them using theNakamura technique of horizontal to vertical spectral ratio (HVSR) to determine the predominant fundamental frequency. The result indicates that the fundamental frequency for Bukit Ledang ranges from 2.32 to 2.56 Hz, while that for UNILAG ranges from 0.85 to 1.29 Hz. The peak amplitude for Bukit Ledang (>2) is higher than that of UNILAG (< 1.5), an indication that the impedance contrast between the overlying layers and seismic bedrock is greater at Bukit Ledang, suggesting a possibility for higher amplification. The depth to the stiff layer estimated for Bukit Ledang is about 20 m, while that of UNILAG is about 30. These estimates correlate well with borehole logs from both areas. This study has shown that the characteristics of the HVSR curve are a function of the geology and, as such, can be utilized to provide good and reliable information about site effect parameters such as the fundamental site frequency. Also, with this technique, design criteria can be defined for new buildings to reduce the probability of soilstructureresonance that can result in collapse in the event of ground motion. Furthermore, this technique is well suited for urban areas and can be readily deployed for microzonation studies and other applications.