Date: Sun, 7 Mar 2004 21:35:12 EST
Hi John and all,
I am now two-thirds of the way to a complete station. I purchased a vertical
sensor from my seismo buddy, Victor Aiello. He made it, mostly from
salvaged parts. The base was from a junked Sprengnether, and other salvaged
material made up most of the rest of the device. The flexure hinges use 10 mil
fishing line leader instead of thin metal. The upper end of the spring is not
free to pivot after adjustment. This is not the best possible suspension, but
it got away from problems with stick-slip with the original loop attachment
design. The coil design is not optimal, since its inner diameter is needlessly
small. The spring is a hardware store item, probably used mostly for holding
doors closed. The magnet was once a part of a Varian Vac-Ion pump, from our
Sperry Tube Division days.
The sensor is set for a natural period of five seconds. Damping is obtained by
shunting the 2200 ohm coil with a 3900 ohm resistor. This reduces the output,
but the response is still about 70 vs/m. I use my digital filter to extend the
response to at least 16 seconds, and the processed event waveforms correspond
rather well with those obtained by the nearby PAL station, except for the very
long period stuff.
The pendulum is somewhat temperature sensitive, and moves up and down slightly
with the house furnace cycle. Sometimes, late on cold nights, long period
disturbances arise from air currents rising from the base. I have not yet done
anything about that, and probably won't have to until next winter. A plastic
cover rests on the base, and a large cardboard box covers everything.
I am still pleased with the results I get, considering the drawbacks of my
site. Microseismic and cultural noise is very different between horizontal and
vertical here. The horizontal noise is a persistent 0.72 Hz ringing, in addition
to the usual 4 to 7 seconds microseisms. There is very little high frequency
noise. In contrast, there is no ringing on the vertical, the micros come in
somewhat stronger, and high frequency noise from nearby trains and heavy
vehicles is evident.
A photo of the sensor is attached. The pendulum is shown higher up than
normally operated.
Regards,
Bob