Applying the Seismic Refraction Tomography for Site Characterization

Seismic refraction method is a geophysical method that has been developed for shallow subsurface investigation. It provides 2-dimensional profiles including depth and distance that simplified the characterization of relatively large volumes of the subsurface. Interpretation of seismic refraction data using seismic tomography involves continuous velocity gradient across a subsurface which is more effective for site characterization compared to conventional seismic refraction. Three parallel seismic refraction survey lines were conducted at Kaki Bukit, Perlis with the aim of characterizing the subsurface of the area. 2m geophone spacing was used with total length of 46m. The separation between lines is 20m. Weight drop of 20 kg and steel plate were used as seismic source. A total of 20 shotpoints were performed for survey line L1, 23 shotpoints for survey line L2 and 22 shotpoints for survey line L3. The high quality seismic data obtained were then processed using SeisOptPicker and SeisOpt2D software to produce a seismic tomography section for each survey line. Results indicate that the study area is said to have 4 main layers with velocity increase fairly with depth. The first layer with velocity 300-500 m/s predominantly consists of top soil and form overburden. Second layer with velocity 500-800 m/s is suggested to be a highly weathered limestone. The third layer represents by highly fractured limestone with velocity 800-1500m/s. Limestone bedrock represent by the fourth layer with velocity > 2000m/s. Competent limestone bedrock is identified at survey line L3 with velocity > 3000m/s.