PSN-L Email List Message

Subject: VBB conversion of Geotech LPs
From: S-T Morrissey sean@...........
Date: Fri, 31 Mar 2000 13:18:56 -0600 (CST)


What a lucky man; its too bad that all your Geotech LP seises
are not verticals. The vertical data is much more interesting, since
the horizontal generally shows very little of the P-wave. Also,
the pier/site tilt problem is much greater for a horizontal, which
will affect where you put them.

The LP seises are good candidates for converting to a VBB instrument
if the coil resistance is low enough. I  don't know the parameters of
the Geotech units, but a coil resistance of more than a few hundred
ohms, when used as the feedback coil, will be unstable. You may have to
wind new coils, or find a way to tap the existing winding.

One of the first LP sensors that I converted to a VBB is the old
Sprengnether 5100 of the WWNSS design: 15 seconds To, 500 ohm coils,
and generator constant Gn of 83 N/A. The high coil Rf works in the 
transfer function because Gn is high as well as the mass is relatively large
at 11.2 kg. I use an integrator at 400 seconds with the other parameters
of RI = 402k, Cf = 12.29uf , Rp = 2.1 megohm, for a Tn of 280 seconds;
Rf is actually 469 ohms, and r is 612900 volts/meter from the LVDT.
This gives a velocity output of 11000 Volts/meter/second. I couple the
output through a 3000 second high-pass filter to the display recorder
in the lobby of our building. It has been operating this way since 1996.
The very long period response readily shows 30 to 60 second barometric
noise, especially when tornado weather is in the area.

If you are going to convert a number of units to a VBB system, we need
to make some headway on getting the circuit boards available. I have
laid out new boards (voltage regulator, oscillator, VRDT amp/demodulator,
and the quad amp board) for the new instrument, but I have not had
any actually produced yet. When that happens, I can make a set available
at cost, and then maybe Larry can assemble them into a working system.



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