Identification of Contact Zone using 2D Imaging Resistivity with EHR technique Postgradute Student M.M. Nordiana Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia, Penang, Malaysia e-mail: [email protected] Senior Lecturer (Dr.) Rosli Saad Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia, Penang, Malaysia e-mail:[email protected] Postgraduate Student Nur Azwin Ismail Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia, Penang, Malaysia e-mail:[email protected] Postgraduate Student Nisa’ Ali Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia, Penang, Malaysia e-mail: [email protected] ABSTRACT This paper illustrates findings of subsurface investigation and 2D imaging resistivity with Enhancing Horizontal Resolution (EHR) technique on a selected site at Batang Merbau, Tanah Merah, Kelantan (Malaysia). The purpose of this study is to characterize interbedded sedimentary with regards to the rock type. Four survey lines were employed at the study area. Results are presented in inversion model resistivity form. The results of the study showed that the study area has a maximum of three main zones, alluvium with resistivity value of 10-800 Ωm, sandstone with resistivity value of 1000-3000 Ωm and granitic bedrock with resistivity value of >3000 Ωm. There is a contact zone between granite and sandstone which produces fractures. Application of 2D imaging resistivity with Enhancing Horizontal Resolution (EHR) technique was successfully used in subsurface investigation. KEYWORDS: Subsurface, 2D imaging resistivity, Enhancing Horizontal Resolution (EHR) technique, contact zone, fracture INTRODUCTION In nature, rocks may be brought into contact through deposition, intrusion, faulting and shearing. The fundamental contacts, unconformities, intrusive contacts that form are normal depositional contacts, unconformities, intrusive contacts, faults and ductile shear zones. The ways in which rock bodies fit together is deduced from geologic mapping, supplemented wherever possible by drilling and geophysical data (Davis, 1984). Geotechnical investigation techniques such as borehole, excavation and geological mapping etc are not only destructive methods, but they are also not cost effective in obtaining this information; - 197 - Vol. 17 [2012], Bund. B 198 furthermore, these techniques usually provides a localized insight into the subsurface conditions. Geophysical techniques on the other hand are non-invasive, cost effective and can give a detail picture of the subsurface geometry with maximum time and cost effectiveness. 2-D resistivity imaging (Griffths and Barker, 1993; Loke and Barker, 1996) is now gaining importance in subsurface investigations. (Bichler and Bobrowsky, 2004; Lapena and Lorenzo, 2003). In this method, high resolution imaging of the electrical properties of the subsurface can be obtained at a reduced cost. The study adopted 2D imaging resistivity with Enhancing Horizontal Resolution (EHR) technique for the investigation. About four survey lines were conducted at Batang Merbau, Tanah Merah, Kelantan (Malaysia) to determine the subsurface resistivity distribution in the area. GENERAL GEOLOGY The Colluvium/Terrace member refers to the sequence of gravel beds, lateritic layers and residual sediments. In Malaysia, this unit is exposed to the south of Rantau Panjang area, southern part of Pasir Mas district, Tanah Merah district and Machang district. These sediments are composed mainly of sand, gravelly sand, clayey sand, gravel, gravel beds and laterite, friable to very firm, abundant mottle, red to reddish brown, poorly sorted and moderate amounts of iron concretions. The gravel beds unit is locally exposed at Ban Ai Su Re. Gravel beds with sand lenses, semi- consolidated, commonly characterize this unit. Class of gravel beds vary from gravel to boulders. They are made up of sandstone, quartz, chert, granite and schist. The sand layers are characteristically reddish brown to red, medium to coarse grained, poor to moderately sorted, moderately cemented and subangular to subrounded. This unit is 15-20 m thick and dips to the south (185°/ 5°-10°, dip direction/ dip angle). Lateritic layers unit is exposed mainly at undulating landforms of Waeng district, Sungai Padi district and western part of Sungai Kolok district. There are mainly characterized by sand, silty sand, gravelly sand and gravel, yellow color. Abundant mottles, strong brown to red and iron concretions are also common. The thickness of this unit varies from 2-6 m. The residual sand unit overlies the Lateritic layers unit with gradational contact. This very thick sequence (>15 m) is underlain by Triassic Kemahang Granite/ Sukhirin granite at the northern part of Sukhirin district. The sequence consists of weathered porhyritic biotite-hornblende granite and is characterized by sand, gravelly sand, loose to friable, moderately sorted. The thickness ranges from 5 to approximately 25 m (Malaysian and Thai working group, 2006). Tanah Merah is composed of andesite flow, andesitic tuff and agglomerate but predominantly pyroclastics. It is mostly extrusive in nature, probably contemporaneous with the deposition of the associated sedimentary rocks. The most impressive andesite in the Transect area is located in the Tanah Merah area, to the east of the Temangan Ignimbrite. The volcanic rocks comprise strongly sheared, highly altered andesite, andesitic tuff and agglomerate. Agglomerate is characterized by the abundance of rhyolitic clasts within thin to medium layers (30-40 cm). The colours of these rocks are generally pale brown, pale purple grey to light greenish grey with white spots (1-5 cm in diameter). Volcanic clasts are white or yellowish grey in colour. MacDonald (1967) assigned this rock unit as Carboniferous to Triassic in age. Aw (1990) reported that the main volcanic activities occurred during Permo-Triassic period. Vol. 17 [2012], Bund. B 199 CONTACT BETWEEN GRANITE AND SANDSTONE This is nonconformity. The contact is between an igneous and sedimentary rock formation. Granite should be on the bottom and sandstone on top, if this is not the case then some type of over-turning has taken place. The depositional environment allowed sand to acculate over the top of the granite and later consolidated into a rock. This is called a contact. Granite will change into sandstone when it is eroded into sand and then settle into a pile someplace, and then be exposed to large amounts of pressure to create a sedimentary rock. The arrangement of how the layers of sandstone over massive granite, with an inclined planar contact between the two have at least three possibilities or three hypotheses to evaluate. Hypothesis 1: The granite was intruded as a mass of magma into its present position below the sandstone. sandstone sandstone granite Hypothesis 2: The contact between the sandstone and granite is a fault, a surface along which the two rock masses have moved relative to each other into their present position. sandstone granite sandstone granite Hypothesis 3: The contact between the sandstone and granite is an unconformity, a surface of ancient erosion at which granite was weathered and eroded and then sand was deposited on top of it from the sandstone. This particular kind of unconformity would be nonconformity, a surface of erosion between an overlying sedimentary rock and an underlying unlayered rock. sandstone sandstone granite granite If Hypothesis 1 were correct, we might expect to see that the sandstone had been baked by the heat from the granitic magma, and perhaps recrystallized. If Hypothesis 2 were correct, we might expect to see ground-up or pulverized rock (called "fault gouge") and linear tracks of deformation showing the direction of fault movement (called "slickensides"). If Hypothesis 3 were true, we might expect to see pebbles of granite in the basal sandstone (Noblett, 1986). Ductile shear zones produce contacts entirely different from contacts marking ordinary faults. Unlike ordinary brittle fault surfaces, ductile shear zones commonly do not display any physical break. Instead, differential translation of rock bodies, separated by the shear zone, is achieved entirely by ductile flow. Markers pass through ductile shear zones without necessarily losing their continuity, but the effects of the shearing are reflected in distortion of the markers and in the transformation of the original rocks to sheared rocks like mylonites. Distortion is expressed both in shape and orientation changes. Vol. 17 [2012], Bund. B 200 Ductile shear zone contacts are most commonly formed in igneous or metamorphic environments, where elevated temperature and/ or confining pressure render one or both wall rocks ductile. However, ductile shear zones can also form during soft-sediment deformation of unconsolidated sands and mud. In such environments, in such environments, the ductile response is due to the water-rich nature of the sediments. Geologic mapping has revealed that ductile shear zones are tabular to curved, centimeters to kilometers thick, and marked by penetrative foliations. Shear zones can accommodate either shortening or stretching of crustal rocks. Diagrams drawn by Ramsay (1980) illustrate two of the possibilities (Figure 1). Older rock can be brought up against younger, higher level rock during crustal shortening (Figure 1A); relatively young, high-level rock can be sheared down onto deeper, older rocks during crustal extension (Figure 1B). Properties of ductile shear zone contacts