Greg wrote: > > "I've been looking on the net for geophones, and I keep seeing a spec I > don't understand. What is the spurious frequency of a geophone?" > A geophone is of course a pendulum with a natural frequency set by the spring constants, etc. In the illustrative drawings, you only see a mass and a spring. In real life, there are other elements of the suspension system that hold the moving coil in position laterally and such. And since the spring is a flat coil, may have other modes of flexure. It turns out that these other elements also create other pendulums with their own resonant frequencies, typically much higher because the spring constants are much stiffer. The other resonant frequencies are called spurious resonances. For example, if you tap the side of a vertical geophone, the mass will shake back and forth (as well as the desired direction) at some high frequency. Back in the old days, nobody cared about these spurious resonances, because seismic reflection crews only recorded data well below 100 Hz. In the last couple of decades, it became stylish to record higher frequencies because it gave you much better resolution of the geologic structure--you could see thinner layers. The curves always showed geophones as flat above the natural frequency, but in reality, there were often large peaks in that band. Geophysicists discovered the existence of these spurious resonances, and modern geophones addressed the problem by improved designs and then by specifying where spurious resonances might be found in the response curve. They can't be eliminated, but they can be pushed up in frequency until we still don't care. For example, for high resolution reflection, you might want to recover data from 28 to 500 Hz, and so you would want the spurious frequency up around 800 or so. None of these will matter for earthquake recording, since those kinds of frequencies only travel a few hundred feet at best, but you might have local noise sources in that frequency range. A reasonable anti-alias filter will eliminate them. -- Doug Crice web site http://www.georadar.com GeoRadar Inc. e-mail dcrice@............ 19623 Via Escuela Drive phone 408-867-3792 Saratoga, CA 95070 USA fax 408-867-4900 _____________________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>