Determination of bedrock depth in Universitas Indonesia using the seismic refraction method

Abstract

The seismic refraction method is used to determine the exact bedrock depth for placing a foundation pole. The study was conducted in Universitas Indonesia precisely at the Fasilkom Universitas Indonesia complex. The seismic survey configuration consists of 24 geophone channels with a length of 67.5 m, geophone intervals of 2.5 m, and near offset of 10 m. The wave source was generated using a hammer, and the distance between blows was 5 m. The secondary data used was geological data from SPT (Soil Penetration Test) borehole as a reference for comparison of seismic survey results. Seismic refraction data was processed using traditional techniques, namely the Hagedoorn’s Plus-Minus Method and tomographic inversion using Rayfract software. The correlation between the results of the process with the geological data from SPT drill point shows good results. However, the Plus-Minus Haggedorn method results are only able to show one refractor because of the data limitation, in contrast to the inversion method, which was able to show more than one refractor. There are two main refractors at a depth of 6 meters and 12 meters, and the adequate depth obtained only reaches 15 m. The maximum speed obtained is also around 900 m/s. It can be concluded up to a depth of 15 meters, and there is no recommended rock layer for placement of deep foundations for high rise buildings. A seismic survey with a longer seismic line is needed to get an underground picture exceeding 15 meters.