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Sherlock Investigates: Seismic Signals Below 1 Hz

Created by Sherlock on 2023-08-30


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:




Sherlock Investigates: Seismic Signals Below 1 Hz
It has been a while (April 2028) since we have enjoyed a citizen scientist instalment from Sherlock, and an engaging one it is! The seismograph reminds me of one that I made from plans in Scientific American (when it was a refereed journal). That one built on a circular graph using a clock as the driver. I never could eliminate local noises such as siblings. HI.
Sherlock Investigates: Seismic Signals Below 1 Hz
Nice work on a citizen science instrument! I designed the low-noise preamp for the long-period seismometers used in the Regional Seismic Test Network (RSTN) deployed by LLNL and Sandia in 1982 to monitor nuclear testing. Such systems are now deployed worldwide for that purpose. Fascinating equipment, installed deep in old mines in the desert. The techs took forever to level it… the requirement was a free-period longer than 100 seconds. The Peterson Low Earth Noise model peaks at 0.25 Hz; I seem to recall that it’s driven by wave action on countless seashores. Managing 1/f noise in the preamp was challenging to say the least. Digital telemetry links were expensive back then, too. I understand that many hundreds of years ago the Chinese used enormous stones somehow suspended in caves as seismic masses for their mechanical recorders. The noise floor of the instrument is limited only by the Brownian motion of the mass…