Overburden of Meteorite Impact Crater at Bukit Bunuh, Lenggong, Perak (Malaysia) using 2-D Resistivity Imaging Dr. M.M. Nordiana Lecturer, Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia, Penang, Malaysia e-mail: [email protected] Dr. Rosli Saad Senior Lecturer Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia, Penang, Malaysia e-mail:[email protected] Dr. Mokhtar Saidin Professor, Centre for Global Archeological Research Malaysia, 11800 Universiti Sains Malaysia, Penang, Malaysia e-mail:[email protected] Noer El Hidayah Ismail Postgraduate Student, Centre for Global Archeological Research Malaysia, 11800 Universiti Sains Malaysia, Penang, Malaysia e-mail:[email protected] Andy A. Bery Postgraduate Student, Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia, Penang, Malaysia e-mail: [email protected] Ragu Ragava Rao Postgraduate Student, Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia, Penang, Malaysia e-mail: [email protected] ABSTRACT 2-D resistivity method is to measure the apparent resistivity of the subsurface at Bukit Bunuh, Lenggong, Perak (Malaysia) along North-South (8 km), West-East (8 km), Northwest- Southeast (6.61 km) and Southwest-Northeast (1.84 km). The survey line was covered using Pole-dipole array with 5 m minimum electrode spacing. The purpose of the 2-D resistivity method was to detect structure and fracture of the shallow subsurface, map hard and compact granite bodies (bedrock) from the overburden material at the meteorite impact area. The results show the first zone with resistivity value of 10-800 ohm-m and thickness 5-60 m was interpreted as alluvium consist of boulders (weathered granite) with resistivity value of >6000 ohm-m. The second zone with resistivity value >20 000 ohm-m was granitic bedrock. Based - 363 - Vol. 19 [2014], Bund. B 364 on the 2-D resistivity imaging results, fractures and faults were identified within the granitic bedrock and other potential areas of poor rock quality. KEYWORDS: 2-D resistivity imaging, Bukit Bunuh, meteorite impact, fracture, overburden INTRODUCTION Bukit Bunuh is situated in Lenggong valley, north of Kota Tampan, Perak state, Malaysia. Generally the Lenggong valley consists of few lithologies which are alluvium, tefra dust and granitic rock. Most of alluvium units are situated along the river area where the quaternary sediment consist of alluvium and tefra dust. The granitic rock is from Jurassic end-Carbonaceous low era being dominated the whole of Lenggong valley and originated from Bintang Range at the west of Lenggong (Mokhtar, 1993). The topography is exceeding 600ft MSL. Sungai Perak is situated at 61ft MSL, flows from north to south and situated at the east of Bukit Bunuh. Bukit Bunuh consists of the older granitic rock. The granite intrusion was occurred throughout Malaysia at Mesozoic era which is 200MY ago (Alexander, 1962). Pebbles dominated by quartz and quartzite are found at the elevation 200-350ft in Lawin, Bukit Jawa, Kampung Telemong and Kota Tampan. Colluviums found at the lower zone (<600ft) are characterized by a lots of fractures and fault zones. The rocks physical characteristics shows that they are fractured and angled fractured include chert, flint and agate. The ground resistivity is related to various geological parameters such as mineral and fluid content, porosity and degree of water saturation in the rock. Variations in electrical resistivity may indicate changes in composition, layer or contaminant levels (Loke, 1994). 2-D RESISTIVITY IMAGING METHOD The resistivity measurements are normally made by injecting current into the ground through two current electrodes, C1 and C2 and measuring the resulting voltage difference at two potential electrodes, P1 and P2 (Figure 1). The resistivity of a soil or rock is dependent on several factors that include amount of interconnected pore water, porosity, amount of total dissolved solid such as salts and mineral composition such as clays. The 2D resistivity method is described by (Zohdya et al, 1974; Sumner, 1976; Reynolds, 1997; and Rubin and Hubbard, 2006). The resistivity method basically measures the resistivity distribution of the subsurface materials. Table 1 shows the resistivity value of some typical rocks and soil materials (Keller and Frischknecht, 1996). Igneous and metamorphic rocks typically have high resistivity values. The resistivity of these rocks is mainly dependent on the degree of fracturing. Since the water table in Vol. 19 [2014], Bund. B 365 Malaysia is generally shallow, the fractures are commonly filled with ground water. The greater the fracturing, the lower is the resistivity value of the rock. I V C1 P1 P2 C2 Equipotential surfaces Current lines Figure 1: Four-point electrode configuration in a two-layer model of resistivity, ρ1 and ρ2. I, current; U, voltage; C, current electrode; P, potential electrode (Modified from Said, 2007). Table 1: Resistivity values of common rocks and soil materials (Keller and Frischknecht, 1996). Material Resistivity (ohm-m) Alluvium 10 to 800 Sand 60 to1000 Clay 1 to 100 Groundwater (fresh) 10 to 100 Sandstone 8 - 4 x 103 Shale 20 - 2 x 103 Limestone 50 – 4 x 103 Granite 5000 to 1,000,000 LITERATURE REVIEW A seismic reflection and three-dimensional wide angle tomographic study of a buried, ~200 km diameter, Chicxulub impact crater in Mexico reveals the kinematics of central structural uplift and peak-ring formation during large crater collapse. The seismic data show downward/inward radial collapse of the transient cavity in the outer crater, and upward/outward collapse within the central structurally uplifted region. Peak rings are formed by the interference between these two flow regimes, and involve significant radial transportation of material. Hydrocode modeling Vol. 19 [2014], Bund. B 366 replicates the observed collapse features. Impact-generated melt rocks lie mostly inside the peak ring; the melt appears to be clast-rich and differentiated, with a maximum thickness of 3.5 km at the center (Morgan et al., 2000). Eduardo et al. (2010) presented initial results of subsurface fracturing and deformation study in the Chicxulub crater from seismic reflection data. The analysis is based on the instantaneous seismic attributes envelope amplitude, instantaneous frequency and Q factor, at selected sectors of the crater by looking at petrophysical properties and seismic attenuation. Shock effects with shattering and fracturing of Mesozoic target rocks show a decreasing trend, away from the rim zone. Cretaceous carbonates show less attenuation inside the crater than in exterior sectors. The relative attenuation quality factor, Q is lower in sections outside the crater rim compared to inside the rim, and particularly at depth within the Crataceous sequence. Carbonates in the western sector are characterized by slightly larger attenuation than in the eastern sector, suggesting radial asymmetries in fracturing and deformation within Chicxulub. Located in Northern Alberta, the Steen River structure (~25 km diameter) is the largest known impact crater remnant in the Western Canadian sedimentary basin. Current hydrocarbon production is ~800 barrels of oil per day (bopd) and 30 million cubic feet per day (MMcf/d) gas, mostly from the Keg River and Slave Point Formations, over a fraction of the crater rim. The feature is buried under ~200 m of Creataceous cover and has no apparent surface expression. Thus, Mazur et al. (2002) used geophysical and drilling techniques to explore the structure. All of the known reservoirs are within the rim uplift where structural closures are on the order of 50 m vertical. In an attempt to image such areas, more than 150 seismic reflection lines have been acquired around the rim since the late 1960s. Generally, these lines show coherent reflections up to the rim uplift with little to no seismic coherency interior to the crater’s rim. At least four of the more than 50 wells drilled around the structure have revealed false highs produced by velocity pull-up effects, possibly related to horizontal velocity contrasts close to the Cretaceous unconformity. Refraction seismology techniques are being tried to help characterize these difficult areas that currently present significant exploration challenges. Noh et al. (2010) discussed magnetic gradiometer survey to analyze the subsurface structure correspond to the crater. Gradiometer is one of the components in geophysical magnetic method and it can measure the local disturbances in the earth’s magnetic field cause by the presence of magnetic materials. Therefore, the quantification of sand variation concentration of magnetic grains in rock can be defined. The study site was in Bukit Bunuh, the northern state of Perak, Malaysia. Archeological research shows the evidence of shock metamorphisms (suevite breccias) and crater morphology (Bukit Bunuh rings). There were eight survey lines with a maximum of 100 m station spacing has been conducted usinng 0.9 m magnetic sensor spacing. The data were interpreted both qualitatively and quantitatively. The qualitatively interpretation is based on anomalies signature on the contour maps, and quantitative interpretation is based on data Vol. 19 [2014], Bund. B 367 modeling. Few borehole data and other geophysical methods such as gravity and resistivity surveys have been carried out for monitoring and verification purpose. Thus, by comparing all the result, the preface of the crater structure was confirmed. Donald et al. (2000) described,