Homebuilt Seismometers

By Allan Coleman
E-mail address:  adcoleman3   "at "  verizon   "dot"   net
Web page revised: 19 February 2006

This web page deals with the design and construction of several homebuilt seismometers developed by an amateur instrument builder. The intent is to share ideas with other amateurs who may find the information disclosed here interesting.
During the last 20 years just as many seismometers were constructed by the author. In the beginning their output signals were fed into analog mechanical recorders to produce ink on paper tracings of seismic events. More recently the seismic signals were digitally recorded and analyzed using powerful software tools. The first dozen or so designs consisted of a simple conventional electromagnetic transducer outputting a velocity signal but their sensitivity dropped off quickly towards the lower frequencies. A couple of seismometers had their output signal differentiated to drive a feedback transducer mounted on the pendulum, for improving system linearity and broadening the frequency response. Poor low frequency sensitivity to teleseismic events was still a concern. Starting in the late 1990s, seismometers (each identified with a unique Roman numeral) began to be made with a displacement transducer because of their favorable sensitivity to low frequecy signals. These later designs also utilized an electrical triple feedback path to provide an output signal flat to velocity over a broad range. 
Which component seismometer to build, horizontal or vertical? Pendulums on long period horizontal component seismometers are prone to ground tilt noise which causes the displacement transducer to drift out of range, requiring frequent re-leveling to keep the transducer centered. By motorizing one of the leveling screws, tilt compensation can be done manually (remotely) or automatically when needed.
A long period vertical component's suspended mass reacts to barometric pressure fluctuations (a major long period noise source) unless it is enclosed in a rigid airtight container. Suspension springs suffer from long term creep and thermal effects causing the displacement transducer to drift off center. Periodic drift compensation by manipulating the base of the suspension spring with a small electric motor to return the pendulum to equilibrium.    
Brief descriptions of the more successful feedback designs are linked to this page, see below.
One of the first practical feedback designs (the model MkXI) used optoelectronic devices for the displacement transducer. By varying the light levels striking a pair photodiodes proved adequate for teleseismic detection, but their sensitivity (because they were unshielded) to ambient AC electrical noise, required heavy low pass filtering of the front end circuitry. This limited the top end frequency response to just a few Hertz. Click on the link below to view a PDF for a description of the instrument and a selection of sample seismograms.

MkXI Optoelectronic Horizontal BB Seismometer

An electronic circuit for a variable capacitance displacement transducer was developed for use on the model MkXVII horizontal seismometer. This type of transducer was relatively easy to fabricate without the electrical noise experienced by the optoelectronic transducer. Before the transducer drifted too far off center, one of the leveling screws was motorized so re-leveling could be performed automatically. Below is a link to a paper describing its construction.

MkXVII Horizontal BB Seismometer

Follow the link below to see a paper describing the model MkXX vertical component seismometer. Its housing was airtight, with a motorized means of manipulating the spring suspension whenever the displacement transducer wondered too far off center.

MkXX Vertical BB Seismometer

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