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.