PSN-L Email List Message

Subject: Re: Forwarded question
From: Doug Crice dcrice@............
Date: Mon, 19 Jul 1999 14:42:34 -0700


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

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Larry Cochrane <cochrane@..............>