1997_06_geophone_spur_freq - CANADIANJOURNALOF EXPLOR.4T!ON...

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CANADIAN JOURNAL OF EXPLOR.4T!ON OEOPHYSlCS “lx 33. NOS. I & 2 (DECEMBER ,997). Pi 4&54 GEOPHONE SPURIOUS FREQUENCY: WHAT IS IT AND HOW DOES IT AFFECT SEISMIC DATA QUALITY? KEES FABER AND PETER W. MAXWELL* ABSTRaCT The Spurious Frequency of a geophone is normally spccificd by manufacturers but is less frequently understood by users. The Natural Frequency is well understood as the resonant frequency of the spring mass system in the working axis of the gcaphone. All dis- continuities in the transfer function of the geophone above the natu- ral frequency are termed spurious resonances. 24 bit technology and digital filtering have pushed the anti-alias (or high-cut) frequencies of modern recording systems to greater than 415 Nyquist, extending the available recording bandwidth, however also getting ckxer to thr spurious danger zone. We can now no longcr ignore the spurious frequencies of the geophones with which WC record such broadband data. Tbrougb this paper we wilt explain this often overlooked geo- phone parameter by discussing the mechanism within the geophone element that produces the rcso”a”ce, how we measure that ia”- nancr and fhc effect it can have on seismic data ihruugh cnamplcs from some field tests. SPURIOUS FREQUENCY: WHAT IS IT? Geophones are designed to respond to extremely small ground displacements. A particle velocity of 0.1 mm/s, which generates an amplitude of 3 mV in a geophone is caused by a displacement of the ground of only 160 nanome- tres at 100 Hz, the displacement is even smaller at higher fre- quencies. It is therefore understandable why any additional mechanical micro-movement within the geophone will easily result in voltage signals comparable to those produced hy these extremely small ground displacements. Traditionally the springs used in geophones are flat mem- brane discs with spring arms linked to concentric rings. They are designed to move linearly in their working axis but by nature are also free to move perpendicular to this axis (Fig. I). This lateral movement is kept as small as possible but is essential to allow freedom of movement of the geo- phone coil in the main axis. The spring mass resonance of the geophone in its working axis is called the natural fre- quency. The spurious frequency is the resonance of the sys- tem perpendicular to this normal working axis and is a com- bination of multiple modes of movement. Ground motion in this plane, either transverse or rotational, may cause the geo- phone to resonate at the spurious frequency, which is taken as the lowest of these resonance modes. The spring is designed to have high stiffness in the direction perpendicular to its working axis, hence the high frequency nature of the spurious resonance. The coil motion in this plane is virtually undamped by the coil form. The eddy current damping is small because of the limited variation of magnetic flux within the coil when it is moved in the plane of the radial magnetic field. However due to the construction of the
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This note was uploaded on 05/09/2010 for the course EARTH SCIE APPLIED GE taught by Professor Es during the Spring '09 term at IIT Bombay.

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1997_06_geophone_spur_freq - CANADIANJOURNALOF EXPLOR.4T!ON...

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