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Other methods of bandwidth extension

The force-balance principle permits the construction of high-performance, broadband seismic sensors but is not easily applicable to geophone-type sensors because fitting a displacement transducer to these is difficult. Sometimes it is desirable to broaden the response of an existing geophone without a mechanical redesign.

The simplest solution is to send the output signal of the geophone through a filter that removes its original response (this is called an inverse filtration) and replaces it by some other desired response, preferably that of a geophone with a lower eigenfrequency. The analog, electronic version of this process would only be used in connection with direct visible recording; for all other purposes one would implement the filtration digitally as part of the data processing. Suitable filter algorithms are contained in seismic software packages such as listed in section 9.

Alternatively, the bandwidth of a geophone may be enlarged by strong damping. This does of course not enhance the gain outside the passband but rather reduce it within the passband; nevertheless after appropriate amplification the net effect is an extension of the bandwidth towards longer periods. Strong damping is obtained by connecting the coil to a preamplifier whose input impedance is negative. The total damping resistance, which is otherwise limited by the resistance of the coil (eq.36), can then be made arbitrarily small. The response of the overdamped geophone is flat to acceleration around its free period. It can be made flat to velocity by an approximate (band-limited) integration. This technique is used in the Lennartz Le-1d and Le-3d seismometers whose electronic corner period can be up to 40 times larger than the mechanical one. Although these are not strictly force-balance sensors, they take advantage of the fact that active damping (which is a form of negative feedback) greatly reduces the relative motion of the mass.


next up previous contents
Next: Seismic noise. Site selection. Up: Force-balance accelerometers and seismometers Previous: Velocity broadband seismometers
Erhard Wielandt
2002-11-08