COMPARISON OF THE 4.5 HZ GEOPHONE FREQUENCY SPECTRUM WITH AND WITHOUT USING NEGATIVE FEEDBACK RELATED TO MEASUREMENT MICROTREMOR

• Main Author: Bulan Simarmata, Sondang
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/60966
• Institution: Institut Teknologi Bandung

The earth's surface always moves at seismic frequencies, either because of natural activities such as natural phenomena or because of earth activities. Seismic sources are divided into two types, natural sources and artificial sources. Natural sources are usually used for the use of passive seismic methods while artificial sources are usually used for the use of active methods. This research is motivated by the measurement of microtremor that utilizes a passive method. The frequency range used in these measurements is a low frequency of about 1 Hz. Thus, it takes a sensor or instrumentation that has a response to low frequencies. One of the measurement instruments used is a geophone which record vibration waves which are then converted into electrical signals or voltages. The geophone used in this study is a 4.5 Hz geophone. To increase the range of the geophone at low frequencies, the catch of the geophone frequency is widened by using negative feedback. This negative feedback circuit consists of a converter that converts the input voltage into current and a low pass filter that allows low frequencies to pass. Once measured, the data is in the form of a time domain which will then be converted into a frequency domain using the Fast Fourier Transform. From these data, a spectrogram will be obtained to obtain a visualization of the data in the frequency domain. Data processing is carried out using the Signal Analyzer application in Matlab. Measurements were made during the day and at night. From the measurement frequency spectrum during the day, obtained by using negative feedback, the gain is about 40 dB against a low frequency of 1 Hz. Meanwhile, measurements at night showed a gain of about 36 dB using negative feedback at the low frequency of 1 Hz. This negative feedback circuit can be applied to measurements but the signal gain and measurement duration must in set so that they can be used for microtremor measurements