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Determining subsurface geology with seismic refraction tomography survey

Seismic refraction tomography survey is one of the geophysical techniques that is the most popular and commonly used to determine subsurface geology in engineering application. It is fast, reliable, cheaper and cover bigger area in shorter time compared with borehole drilling and other geophysical techniques in providing continuous information on subsurface geology along the lengths of the seismic survey lines. However, the success of seismic refraction tomography survey depends on a few factors such as noise background, top soil features, geology of the site, limitation of the equipment, understanding of the theories and experiences in interpreting the seismic refraction tomography survey data. The method of seismic refraction tomography survey measures the first arrival time of the primary waves through the earth material after seismic signals were generated at several shot points with sledge hammer or explosives. In this paper, the applications of seismic refraction tomography survey in investigating the subsurface geology were discussed in two case studies. The first case study discussed the effect of geophone interval in the limit of depth penetration of seismic refraction tomography survey. Increasing the geophone interval would increase the limit of depth penetration, which helped in mapping the very deep bedrock profile. The second case study showed that the seismic refraction tomography survey results were able to indicate the highly irregular bedrock profiles and complexity of the subsurface geology such as shear zones. It was able to show the extent of the shear zones at the project area, which would have been missed by merely doing borehole drilling. Hydrothermally altered granite and quartz veins were encountered in the boreholes located along the shear zones. Both case studies showed good correlations between boreholes and seismic refraction tomography survey results.