SUMMARY:
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.
BACKGROUND:
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.
LINKS:
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.
