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ASMEWin98Manuscript - IMPROVING THE PERFORMANCE OF A...

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IMPROVING THE PERFORMANCE OF A GEOPHONE THROUGH CAPACITIVE POSITION SENSING AND FEEDBACK Aaron Barzilai Dept. of Mechanical Engineering Stanford University Stanford, CA Tom VanZandt Center for Space Microelectronics Technology Jet Propulsion Laboratory Pasadena, CA Tom Kenny Dept. of Mechanical Engineering Stanford University Stanford, CA ABSTRACT This paper reports on the improved performance of a geo- phone obtained by measuring the displacement of its proof mass capacitively rather than inductively and operating the sensor as a closed loop system. A measurement of the displacement of the proof mass rather than the velocity measurement made with a conventional geophone provides better low frequency perfor- mance. As a result, a geophone can be considered for use in a broader array of applications. A capacitive geophone can operate in either an open loop or closed loop mode. This paper describes a closed loop geophone system with a frequency response similar to the state of the art Streckeisen seismometers. INTRODUCTION Seismologists and geophysicists rely on a wide variety of sen- sors to study the Earth. Historically, a popular sensor in these communities has been the geophone, a highly sensitive ground motion transducer that have been in use for decades(Sheriff & Geldart, 1995). Figure 1 shows both a schematic drawing and a cross-sectional view of a geophone, which uses the motion of a spring supported coil in the field of a permanent magnet to generate an output signal. Studies of local and regional seismic- ity often rely on geophones. However, these sensors are most commonly used as sensors for seismic reflection and refraction surveys, techniques to image the three-dimensional structure of oil and gas deposits beneath the Earth’s surface. For imaging ap- plications, large, two-dimensional arrays of sensors are deployed on the surface to record seismic waves as they propagate be- low the ground. By measuring travel times and amplitudes of various components of the waves, the underground structures encountered by the waves can be determined. The combination of attributes offered by a geophone make it a good choice for these applications and others. Riedesel et al.(1990) showed that a geophone can have a minimum instru- mental noise as low as 0.1 n g / Hz, which is quieter than the minimum seismic noise of the Earth(Peterson, 1993). The nom- inally 4.5 Hz GS-11D geophone by OYO Geospace used in this work has a minimum instrumental noise of 0.6 n g / Hz(Barzilai
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performance than inductive sensing, since velocity is directly proportional to the frequency of the motion. Additionally, the STS-2 and others operate in a closed loop mode, enabling the manufacturer to modify the frequency response appropriately. This improved performance of the STS-2 is accompanied by other drawbacks. High end seismometers can cost as much as $15,000 each. These seismometers are very fragile and much larger than a geophone. An STS-2 has a volume of 11,000 cm 3 and mass of 13 kg, while a GS-11D has a volume of 30 cm 3 and a mass of 140 g.
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