Bob McClure's Horizontal Seismometer Design

Date Mon, 14 Oct 2002 230600 EDT

These are photos of my first horizontal sensor, originally constructed in 1958 with optical pickoff. I recently refitted it with magnetic pickoff. The red thing at one end of the base plate is the 4-pole magnet assembly. It is supported at the right height by a clear plastic platform with three machine bolt legs. The vertical column on the other end of the base plate has jewel cup bearings from a big old D'Arsenval meter (probably once used by Thomas Edison) mounted in it and on top of it. The vertical column is rigidly supported by two stays of aluminum angle stock.


The garden gate pendulum is an 8-inch length of 6-32 threaded brass rod. The coil is threaded onto it at one end, and an antique steel gramophone needle is attached to the pivot end. For added mass, about 10 turns of 1/8" plumber's solder is wrapped around the pendulum rod, next to the coil. The pendulum rod is supported by a length of 1/4 inch wide aluminum strip, secured to the pendulum rod by nuts. Hanging downward at that point on the rod is an unused copper damping vane. The upper end of the aluminum strip is fitted with a pointed hook of stainless steel wire, which rests in the upper pivot jewel. The entire pendulum assembly weighs 66 grams.

In between the solder and the vane, a bracket normally used to support a curtain rod is screwed to the base. It, and a long screw with a wire hanging down from it form stops for limiting the lateral movement of the pendulum when the magnet is not in place.

A one foot metal ruler is lying on the base to show the scale. The base is a 10 by 16 by 7/8 inch slab of salvaged aluminum alloy.

The coil assembly has brass screw terminals. These are connected to the output barrier strip by several foot lengths of #38 magnet wire. The wires run from the coil terminals along the pendulum to a point just short of the pivot, and hang downward from there in the form of a multiturn bifilar coil, and from there go to the barrier strip. The coiled section reduces any torque the wires might otherwise exert on the pendulum.

The sensor coil is rectangular, consisting of 780 turns of #38 magnet wire wound on a 2.75 by 0.375 inch plastic core, 1/8 inch thick. It was one of the separators that kept the magnets separated from each other when they were shipped. The coil is potted with Elmer's wood glue, and mounted in a 1/8 inch thick plastic holder, which also holds the screw terminals and has a threaded hole in it for mounting the holder to the pendulum. The coil has a free travel of 5 mm in the magnet.

The magnet assembly consists of 4 each 3 inches by 0.5 inch by 0.2 inch NdFeB magnets, with poles on the 3 x 0.5 inch faces, two 3 inch by 2 inch by 0.25 inch steel plates, and two 0.172 inch plastic spacers. Each plate holds two magnets of opposing polarity, laid side-by-side, and separated by a plastic spacer. One plate has 3/16" drilled holes in each corner. In these holes are mounted jack screws, which hold the plates and their magnets apart at the desired gap spacing, presently set at 0.205 inches. The measured field in the gap is 6000 gauss. The field reverses direction at the center line of the assembly. It was not necessary to glue the magnets to the steel plates. They are more attracted to their plates than to each other across the gap.

The output sensitivity is about 78 volt-second/meter. Damping is accomplished by shunting the 300 ohm sensor coil with a 78K-ohm resistor, plus the 100K-ohm input impedance of the amplifier. The period is set at about 9 seconds, and the damping is about 0.6. I can extend the effective period to over 50 seconds by digital post-filtering.

I hope this conveys something about what I have been up to lately. Although I have not recorded a verifiable event as yet, I am delighted with the way this sensor is working. Maybe my next project will be a vertical sensor, designed along similar lines, using light construction and very strong magnets. I realize that it is more challenging to build a decently long period vertical sensor.

Bob McClure

Date: Fri, 2 Jan 2004 21:51:05 EST
Subject: Sensor photos

Hi John,

The microseisms are high today, so I photographed my mini-sensor while not operating, using my new Canon A60. The attached photos are reduced from original size, but good enough detail is preserved. The device is not workmanlike in appearance, but performs as well as local site conditions allow. The coil and magnet in this photo are not the ones in the original description, but shorter and wider. The four-pole magnet principle is the same. Each magnet is now 50 mm X 18 mm X 6mm, rather than the original 3 inch X 0.5 inch X 0.2 inch. Output is nearly the same, 0.78 volts per centimeter per second. The pivots work very well, and consist of sapphire cups salvaged from a voltmeter (or watt-hour meter?) which are imbedded in the support mast, and sharp steel points on the pendulum.

The baseplate is somewhat irregularly-shaped aluminum plate, approximately 16" X 12" X 3/4". The overall pendulum length is about 12 inches, with a radius of gyration of about 8.5 inches.  A gasketed plastic cover (not shown) measuring 12.5" long by 8" wide by 7.125 inches high encases the working parts to eliminate drafts. For protection against humidity, the entire sensor, together with a bucket of calcium chloride, is placed in a clear plastic leaf bag. I have operated at 13 seconds period for months now, and re-centering is done about every three weeks.

I post-process the effective period to 24 seconds or more with a digital filter of my own design. Additional post processing consists of low pass filtering (smoothing) accomplished with non-real-time filters of my own design, which filter without introduction of phase shift or time delay in the processed waveform.

Above is a picture of today's reception, with an abundance of 7 second period microseisms. I wrote the program for the analyzer, with some help from Dataq. It is posted as a third party download at


Successful Recording!


Picture of new horizontal.