Sherlock Investigates: Seismic Signals Below 1 Hz
Very Low Frequency Seismic Signals (Below 1 Hz) can be detected from earthquakes around the world. This detector (receiver) is in the corner of the basement to keep it isolated from local seismic noises.
The city noises from trucks, trains and windblown trees cause seismic pollution mainly above 1Hz. The frequency of this detector is 0.25 Hz (which is period of 4 seconds), which is good for filtering out local noise and for detecting earthquakes.
The equipment needed to detect these signals is a detector, a DC amplifier and a recorder.
The detector is a swinging “Garden Gate.” It is literally a swinging gate 3 feet long, hanging on a nail point and swinging at the base on a knife edge to eliminate friction, The gate arm holds a small multi turn RF coil that is positioned in the field of an old Magnetron magnet. That signal is sent to a DC amplifier.
The DC amplifier drives the servo pen motor in a Geotech Helicorder that writes with a hot pen on heat sensitive paper. The recorder is running at 1 RPM and there is a WWVB receiver that adds time ticks to the signal line. See Photographs.
The Helicorder and all the equipment is backed up on a UPS.
This Seismic Observatory started running in 1980 before the USGS got all the data on the web.
Now you can see all the Earthquake Seismographs at: https://earthquake.usgs.gov/monitoring/operations/heliplot.php?virtual_network=ANSS