Geophysical Investigation to Determine the Bedrock Profile at Universiti Pertahanan Nasional Malaysia (UPNM)

Geophysical investigation was carried out at the proposed laboratory construction site at Bukit Besi, Universiti Pertahanan Nasional Malaysia (UPNM) upon the request by the Jabatan Kerjaraya Malaysia (JKR). The objective of the investigation is to delineate granite bedrock topography which was found fluctuate through drilling causing the engineers and the architects have to design a new proposal for the building plan. This may increased the costing of the overall project. The original proposal building plan suggested the foundation of the building is at 20 m below surface. However, data from drilling shows the bedrock was found at 23 m at BH3 and 12 m depth at BH4. In order to reach the level required from the building proposal plan, at least 10 m of rock have to be removed from the site, in addition to the 10 m thickness of top soil. Two geophysical methods were proposed; Seismic Refraction and Resistivity Imaging. The result from seismic survey is shown in a two dimensional profile, plotting the depth of seismic velocity (m/s) traveling in the media against the horizontal distance (m). The velocity of seismic wave travelled in the subsurface can be divided into three ranges; <1500 m/s is a soil layer 1500 - 2000 m/s is a semi weathered and >2000 m/s is a hard layer. Additionally, it can be assume the depth of the fresh granite ranging from 0 m to 23 m from ground surface. On the other hand, the resistivity survey gives the information on resistivity of the subsurface materials and also plotted in a 2D profile. The resistivities value ranges between 50 - 10000 Ωm and can be divided into three classes; <1000 Ωm, 1000 - 3000 Ωm and >3000 Ωm. The thickness of the overburden layer ranges between 0 - 20 m, as represented by a low resistivity value, which also appear at deeper layer. This expected due to the present of fractured within the granite body which is filled by clay mineral or water. As a conclusion, the investigation found the granite body's topography is in an irregular form. Correlation with the drilling data shows an agreement with the results from both seismic refraction and resistivity surveys. (author)