Integrated geophysical approach for imaging sedimentary outcrop at Northwestern Peninsular Malaysia

Abstract

An outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. In recent years, outcrop studies have become very important to understand the geology of the earth. The lack of studies on outcrops depends on the interpretations and observations at the surface of the earth only. Besides that, lack of specific physical value for the type of materials can cause misinterpretation of data. Therefore, 2-D resistivity and ground penetrating radar (GPR) were integrated to characterize the sedimentary outcrop. There are three study areas with the exposed outcrops located at Northwestern Peninsular Malaysia, which are Bukit Chondong, Guar Jentik and Bukit Kukus. The 2-D resistivity survey was performed using SAS4000 Terrameter with Pole-dipole array configuration while for GPR, the frequency of the antenna used is 250 MHz. The uppermost Kubang Pasu Formation at Bukit Chondong consists of repetitive facies of sandstone and mudstone. The mudstone and sandstone exhibit resistivity value of 20 to 130 and 100 to 400 Ωm, respectively whereas GPR shows contrast image in a reflection of the signal strength when the radar wave penetrates through different mediums of mudstone and sandstone. The resistivity value at Guar Jentik shows red mudstone with low resistivity value (10 - 150 Ωm) and sandstone gives higher resistivity (500 - 700 Ωm). The low contrast reflectivity in GPR is interpreted as red mudstone and the high contrast is sandstone. At Bukit Kukus, the chert facies give resistivity value of 1400 to 45000 Ωm while claystone gives 400 to 1000 Ωm, respectively. The chert area is low attenuation, thus permitting high contrast image in the radargram of the GPR and the low contrast is interpreted as claystone. The results show that 2-D resistivity and GPR methods well characterized the geological features of sedimentary outcrops. The inversion model of 2-D resistivity shows that the trend of contouring resistivity value is successfully matched with the outcrops’ geological features. The radargram from the GPR showed compatibility to sedimentary structures in the outcrops. Therefore, both methods will give great potential for further studies on sedimentary outcrop. Key words: 2-D resistivity, ground penetrating radar (GPR), features, Bukit Chondong, Guar Jentik, Bukit Kukus.