THE RESISTIVITY STRUCTURE OF THE EASTERN BANDUNG BASIN BASED ON CSAMT DATA

• Main Author: HARJA (NIM: 30204001), ASEP
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/11881
• Institution: Institut Teknologi Bandung

Bandung basin is an intramontane basin located in West Java, extending from west to east along 35 km and north to south along 15 km distance, with elevation of 660-680 m. The plain in the eastern part is the basin centre with lake deposit as primary sediment filling the basin. Investigation of the subsurface structure and thickness of the basin is the main topic in this research. Beside the deeper structure of the basin, the shallow structure is also very important to be revealed since human activities are concentrated in this part. The latter is supposed to explain phenomenon related to the flood and drought that frequently occur in the area.<p>Controlled-source audio-frequency magnetotellurics (CSAMT) is a highly effective electromagnetic (EM) method to deploy in this area. Its robustness toward electromagnetic noises related to human and industrial activities particularly in the eastern part of the basin is the strong point of this method. It uses a grounded horizontal electric dipole as artificial source of electromagnetic signal that ensures data with a high signal to noise (S/N) ratio. This method is capable to map subsurface resistivity structure with high sensitivity to resistivity contras and deeper penetration.<p>In the first step of this research, a full solution CSAMT modeling was modified to recognize the characteristics of the electromagnetic responses around EM signal source to obtain optimum distances and directions of receivers from the transmitter. The second step is incorporating the full solution 1D CSAMT forward modeling formulation into the inversion scheme. It explicitly integrates the effects of a finite electric dipole source which accommodates all components of the EM fields (near-field, transition-field, and far-field). This inversion scheme was tested using synthetic data for homogenous and layered-earth models. The results show good ability of the scheme in reconstructing the given models. Application of the inversion scheme to the observed CSAMT data in the eastern part of Bandung basin yields a general feature of its subsurface 1D resistivity structure. It is found that low resistivity distribution is widely spread out at depths greater than 250 m.<p>2D inversion scheme was then used to the far-field component of CSAMT data (plane wave assumption) in order to obtain resistivity cross-sections that are more suitable with the basin's structure complexity. The results show that until the depth of more than 2000 m, no high resistivity structure is found. This unlikely indicated the presence of volcanic rocks beneath the area. The subsurface resistivity distribution is dominated by tens of ohm-m, indicating that the basement comprises deep marine sediment. In addition, clay lens are also indicated in the resulting resistivity structure. Based on the inverted resistivity cross-sections, it is found that a low resistivity elongation extends in southeast-northwest direction at about 1000 m depth which may be attributed to the presence of a fault or structure of paleo morphology process.<p>A low resistivity distribution is widely found at shallow depths in the center of the eastern part of Bandung basin. This low resistivity distribution is correlated to the presence of clay. This impermeable layer is responsible for the frequent occurrences of flood and drought in the area. The same result is shown by the DC-Resistivity cross sections in which resistivity value smaller than 10 Om dominates the shallow subsurface. Undulation pattern of sediment is also identified from the resistivity sections at depth more than 1000 m which is correlated to the change of base level attributed to tectonic activity. The undulated layer of sediment and the presence of the fault are also found from gravity data modeling observed along a line that coincides with one of the CSAMT lines.