Geoelectrical Resistivity Imaging and Refraction Seismic Investigations at Sg.Udang, Melaka

Problem statement: A Reconnaissance geophysical survey of an area near Sg.Udang, Melaka was conducted using geoelectrical resistivity and seismic refraction methods. The main objective of this study is to determine the depth of bedrock in the study area. The resistivity imaging measurement employing Wenner electrode configuration was carried out using an ABEM SAS 1000 terrameter and electrode selector system ES464. Electrodes were arranged in a straight line with constant spacing and connected to a multicore cable. The refraction seismic was conducted using 24 channel ABEM Mark6 signal enhancement seismograph with 5 m geophone spacing. The resistivity layer is associated with the residual soil with thickness of about 0.5-3 m. The high velocity layer is an average depth of about 9.4 m. The intermediate velocity zone is associated with weathered schist with thickness of about 2.5 m. The low velocity zone is with thickness of about 6 m. The thickness of residual soil obtained by seismic refraction survey appears to agree very well with the borehole data. Resistivity and the seismic refraction data have been successfully used to determine the thickness of residual soil layer and depth of bedrock. Approach: In this survey, electrodes were arranged in a straight line with constant spacing and connected to a multicore cable. The refraction seismic was conducted using 24 channel ABEM Mark6 signal enhancement seismograph with 5 m geophone spacing. The seismic data have been interpreted using SeisOpt@2D which automatically produced 2-D seismic velocity sections of the subsurface. Results: The resistivity results showed that the subsurface layers are associated with variable resistivity (296-2600Ω. m). The resistivity layer is associated with the residual soil with thickness of about 0.5-3 m. The interpreted 2-D seismic sections showed three different velocity layers. The high velocity layer (1600-2000 m sec^-1) is interpreted to be associated with bedrock at an average depth of about 9.4 m. The intermediate velocity zone (1000-1600 m sec^-1) is associated with weathered schist with thickness of about 2.5 m. The low velocity zone (450-900 m sec^-1) corresponds to clayey silt of residual soil with thickness of about 6 m. Borehole data indicate that the depth of bedrock is about 10 m which appears to be in good agreement with that of seismic results. Conclusion: Interpretation of the resistivity and the seismic refraction data have been successfully used to determine the thickness of residual soil layer and depth of bedrock in the study area. The thickness of residual soil obtained by seismic refraction survey appears to agree very well with the borehole data.