DEVELOPMENT OF INTEGRATED INSTRUMENTATION FOR JOINT DATA ACQUISITION SYSTEM OF DC-RESISTIVITY METHOD AND REFRACTION SEISMIC METHOD FOR SUBSURFACE INVESTIGATION

• Main Author: Imaduddin, Ihsan
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/55234
• Institution: Institut Teknologi Bandung

One way to overcome ambiguity or non-uniqueness of solutions in geophysical measurements is a joint survey, which is to conduct a survey using more than one geophysical method. Two geophysical methods that are widely used in joint surveys, especially for shallow subsurface investigations are the direct current resistivity (DCR) method and the refractive seismic (SR) method. Where the DCR method identifies the subsurface based on the ability to conduct electric current, while the SR method is based on the ability to propagate mechanical waves. There are three aspects in the joint survey are acquisition, inversion, and interpretation. The joint interpretation aspect has been commonly used in any geophysical survey method. The joint inversion aspect has had several pilot studies and has shown quite positive developments, both in terms of quality and quantity. Meanwhile, research on joint data acquisition instrumentation aspect has not been much published. Until now, the data-acquisition process of joint surveys still uses instrumentation for each method separately. Therefore, it is necessary to research and develop a joint survey data acquisition instrumentation system for the DCR method and the SR method in a single device. This study aims to design and implement an instrumentation system that can carry out the joint data-acquisition process for both the DCR method and the SR method simultaneously in a single device. The design of the main sensor is a combination of an electrode as a DCR sensor and a geophone as an SR sensor. The process of reading signals and data acquisition uses ADC with 16 bit resolution for DCR and 24 bit resolution for SR. One microcontroller is used to control the ADC for detecting and acquiring signals. The microcontroller also controls the process of data communication from the survey point (node) to the control and data collection center (gateway). The process of filtering, monitoring data quality, and storing the measured signal are carried out at the gateway using a PC which as well plays as an interfacing device the system with the surveyor/ user. In the DCR method, the current measurement range is 0~1000mA with 1mA of accuracy and 60dB of a dynamic ratio, while for a voltage is -2500~2500mV with 1mV of accuracy and 74dB of dynamic ratio. In the SR method, the measurement range is -2500~2500mV with 1mV of accuracy and 74dB of dynamic ratio and 2~200Hz of frequency range. This instrumentation system works with a multichannel method where one system handles many nodes and each node works automatically according to the specified configuration. The process of controlling and storing data for each node is carried out centrally at the gateway. Communication between the gateway and all nodes can use two options, namely wire-based using the RS-485 protocol or wirelessly using WiFi. A field test was carried out in the Gedebage area in the southern city of Bandung, which is geologically an ancient lake sediment area. The reliability of the instrumentation was exhibited by the ability in performing data acquisition and data transmission processes using 24 nodes simultaneously. The joint data-acquisition process can be accomplished in less than 30 minutes. The obtained data which after processing can provide information on subsurface layers. The result of the join interpretation found top soil at the top layer up to a depth of 3 m, underneath there is clayey silt up to a depth of 40 m. The geophysical parameters of the measurement results are validated with geological data in the form of core drilling data up to a depth of 40m, indicating that obtained data of this instrumentation is valid. The use of instrumentation can be applied for some underground investigation such as searching for sliding plane of the landslides, geological structure, groundwater level, underground freshwater source, cavity, buried thing, etc. In case of vertical investigation, the instrument is also applicable for survey of downhole seismic, downhole resistivity, crosshole seismic, crosshole resistivity, or joint survey of seismic-resistivity downhole/crosshole. Using scheduled survey, the instrument can perform time-lapse monitoring, mainly for DCR method. By replacing the geophone to the proper harmonic frequency, the instrument also can be used for passive seismic survey, as well by activating automatic gain control the instrument is also usable for shallow reflection survey For further research is suggested to perfect the method and algorithm of wireless communication which has a farther reach of distance and a greater bandwidth hence it can involve more nodes with a farther survey radius. In addition, further research is also suggested to design and implement an integrated data acquisition instrumentation and inversion system for both methods.