Regarding extending the operating period of a geophone:

See "A Versatile Equalization Circuit for Increasing Seismometer
Velocity response Below the Natural Frequency", by Peter M. Roberts;
BSSA, (Bulletin of the Seismological Society of America), Vol 79,
pp 1607-1697, October 1989.

The paper describes extending the period of a 1-hz seismometer (L4-C)
to 10 seconds. The major problem is that the equalization requires
a total amplifier gain of 488, so amplifier noise is a the limiting
problem. Noise power spectra are not presented.

An instrument company makes a three-component 1-hz replacement
for the L4-C using a variation of the circuit and 4.5hz geophones
installed in a solid cylinder about the size of a single L4-C.
It is a good medium gain instrument for network studies in active
areas, but like the 1-hz seismometer, only senses the 6-second
microseisms if they are abnormally large. 

The major problem with period extension schemes using small geophones
(equalization, VBB feedback, NIC circuits, etc.) is that the force
of the moving mass (20 - 30 grams) on the suspension is quite small 
compared with the constants of the leaf springs, such that mechanical 
self noise contributes a significant amount of noise. This makes them
excellent for being rugged devices, but not particularly sensitive
compared with the noise models. Examples of geophone noise levels are
given in the paper by Riedesel. At 1 hz, there is a factor of about
100 times more nose from a 4.5hz geophone compared with an L4-C with
a 1000 gram mass.


"Limits of Sensitivity of Inertial Seismometers with Velocity 
Transducers and Electronic Amplifiers"; by Mark A Riedesel, R.D.Moore, 
and J.A.Orcutt; Bulletin of the Seismological Society of America, 
Vol. 80, No. 6, December 1990.

Regards,
Sean-Thomas
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