PSN-L Email List Message

Subject: Torsion Seismometer in a jar
From: kevin.mckee@.......
Date: Thu, 15 Oct 2009 16:27:19 -0400

Hi -
I just joined this e-mail list, and noticed some of the discussion about the seismometer project I posted at

There seemed to be some concern that the system would perform poorly or be very noisy. There was some discussion over the LED being a cause for noise. I performed a number of tests before I completed the seismometer (several years ago now) to satisfy my own concerns in that regard. I used various incandescent bulbs, LEDs and sensors, and in some cases, locked the pendulum in a stationary position to test whether the sensor was picking up noise from the LED. In fact, I got a flat line even when the amplifier was maxed out--the LED was significantly quieter than other light sources. 

I've actually found the two devices that I've built (facing orthogonally) to be very sensitive.  I've put together a rough distance-magnitude threshold detection plot that I can forward to anyone who is interested. The weakest quake I've detected is a magnitude 2.0, about 0.05 degrees (about 3 miles) away.  The common phases are easily picked out of the signal, depending on the quake origin. 

The larger factor in the devices' sensitivity is that I mounted them on a basement slab under the stairs to my front porch, which means that  they detects the comings and goings of my family too (I sometimes used this fact to check up on my daughters' returns from dates).

I believe that I am detecting microseismic background with the instrument too, rather than air currents. 

Regarding the fact that the sensor is a displacement sensor vice a velocity sensor--I don't see that as a problem. As with a capacitive /radio frequency force-balance sensor, electronic filters and amplifiers can be used to translate the signal into a velocity response, if that is what is desired. In fact using the appropriate filter amplifier combination gets around any concerns like those expressed about the resolution of the optical sensing devices--electronic integration works very well for that.

Kevin McKee, Burke, VA

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