Subsurface evaluation using geophysical method in granite formation

Abstract

More accurate underground information is needed in determining the layers of rock and soil for excavation work. This study was conducted to evaluate the use of seismic refraction method and 2-D electrical resistivity in determining the sub-surface layer of rock and soil. The information from the borehole is also used as a comparison of the accuracy of the information. However, borehole data is only in the drilled location which causes the information obtained is not comprehensive. Incomplete information will result in delays in project implementation when the bedrock layer is found while work on the site is in progress. The need for more accurate sub-surface information can also assist geotechnical engineers in determining surface excavation work, especially in granite formation areas. The study was conducted on three (3) resistivity lines and seven (7) seismic lines at the proposed site of the construction of the Research Complex in Universiti Pertahanan Nasional Malaysia, Sungai Besi. Resistivity data were analyzed using Res2Dinv software and seismic refraction using ReflexW software. The results clearly show through 2-D resistivity of the presence of granite layer at shallow depth with a resistance value of less than 1000 ohm-m and the ripping method for excavation work is appropriate. Through the seismic method shows the thickness of the rippable layer is in the range of less than 15m. Seismic data show a good correlation with borehole data. While the correlation of the resistivity data with the borehole data, there is a slight difference that may be due to the position of the borehole which is not aligned with the seismic line. The conclusion of this study shows that the 2-D resistivity method and seismic refraction is able to provide more accurate sub-surface information especially for the bedrock profile in determining the surface excavation method in addition to the borehole.