NATURE OF PROJECT: GEOPHYSICAL SURVEY
CLIENT: BERJAYA HILLS RESORT BERHAD
DURATION: 2 MONTHS
LOCATION: BUKIT TINGGI, BENTONG, PAHANG
INTRODUCTION
Geomapping Technology Sdn. Bhd. (GMTSB) was appointed by Berjaya Hills Resort Berhad (BHRB) to carry out geophysical survey on the study area. The purpose of this survey works is to identify the subsurface profile of 2D image and correlated to the slope failure by measuring the depth of existing boulders, presence of bedrock and detecting water table below the surface. This information will be used by engineers for slope mitigation. The fieldwork was carried out between 13𝑡ℎ January to 19𝑡ℎ January 2020 to complete all the survey lines of the area.
METHODOLOGY
Eight (8) lines of 2D electrical resistivity tomography (ERT) and induced polarization (IP) geophysical surveys were conducted at slope failure location no. 2 (road junction Golf Club), Berjaya Hills Resort known as CS 1, CS 2, CS 3, CS 4, CS 5, CS 6, CS 7 with 120 m length and CS 8 with 30.0 m length. Schlumberger array was utilized with 3.0 m electrode spacing for survey lines with 120 m length and 0.75 m electrode spacing with 30 m survey length. Testing equipment for characterization of subsurface electrical resistivity in this project is known as ABEM Terrameter LUND Imaging System, consisting of the Terrameter LS, electrode cables, cable joints, cable jumpers and electrodes. The procedure of testing was carried out according to the recommendations of the ABEM INSTRUCTION MANUAL TERRAMETER LS. After the surveys were conducted, raw data from Terrameter LS was transferred to the software program called RES2DINV software for process and analysis.
RESULT AND DISCUSSION
Interpretation of 2D image of resistivity and induced polarization surveys were based on the geology of the study area. Geologically, Bukit Tinggi area is located at the southwest of Pahang and covered by three main granitic body considered as part of Main Range Granite Batholith. There are Bukit Tinggi granite, Kuala Lumpur granite and Genting Sempah microgranite. The area is bounded by Bukit Tinggi Fault Zone and Kuala Lumpur Fault Zone with NW-SE trending.
Result of resistivity survey presents the subsurface profile (Figure 1) consist perched water, weathered granite Grade V with high water content, weathered granite Grade IV and existing of hard material (granite boulders). High resistivity value shows the in-situ granite boulders presence at the upper part to middle part of slope (B to C). The existence of low resistivity value at the centre (C) of survey lines were correlated to the result of induced polarization survey to interpret the water table and high–water content zone.
Figure 1: Subsurface profile of resistivity survey for line CS 3 processed by RES2DINV Software
Schematic diagram of interpretation geological cross section was produced based on 2D image of resistivity and induced polarization surveys at the study area as shown in Figure 2. The water table were marked from upper part of slope through the fracture of hard material to the middle part of slope. There are presence of water flowing at the center of slope (Figure 3) shows the study area has high water table boundary which control by geology structure in the subsurface.
Figure 2: Interpretation geological cross section based on line CS 3 at the study area
Figure 3: Water flowing along the middle of slope
Eight line of interpreted geological cross section were improvised into 3D model as illustrated in Figure 4. Generally, all the line gave almost the same subsurface result. From the Point B to Point C, there are existence of in situ granite boulders with high resistivity value (more than 1000 Ωm) which undergo weathering process of their parent rock (granite). This granite boulders maybe had some fracture throughout the weathering process which act as water pathway from the upper to the middle part of slope. At middle part of slope, there are water flowing to the surface indicates the water table of this area were ended at this area. This water flowing was absorbed back to the area from Point C to Point A due to low resistivity and chargeability values and causing the area to become weak and trigger to slide.
Figure 4: 3D model interpretation of geological cross section based on eight (8) line surveys at Berjaya Hills, Pahang Darul Makmur
