A three channel geophone amplifier


A three channel geophone amplifier
(This device is covered in my book.)
The three axis geophone

Apart from seismometers like the Lehman seismometer or the simple vertical seismometer there is yet another way to measure seismic waves traveling through the earth.

This can be accomplished, too, with so called geophones - microphones with a very low resonance frequency. Normally this frequency is 4.5Hz for quite cheap geophones or down to 1Hz for very expensive, research grade instruments.

On the left, the outer appearance of a three axis, 4.5Hz geophone can be seen. This device has been taken out of a string of geophones which are used for underground surveys, for example for oil explorations and the like.

The picture on the right shows the interior of the three axis geophone. Each of the three geophones can be seen quite easily - they are mounted so that each of the geophones is aligned with the X, Y and Z axes.

On top of the Z geophone the damping resistor can be seen - this resistor is used to damp the geophone membrane after being excited by a seismic wave reaching the instrument.

Unfortunately (due to the quite high resonance frequency) geophones like this one can be used only to measure local seismic events since high frequency parts of seismic waves as generated by teleseismic events are filtered away by the earth itself.

The interior of the three axis
The interior of the amplifier

Nevertheless, since I got this three axis instrument I had to build a three channel amplifier to explore the instrument and to start some measurements. The first idea I had was to build three amplifiers like the one used for the Lehman seismometer but this would have been quite cumbersome since it would have been necessary to implement a three stage rotary switch to control the amplification.

To avoid this I chose to use a four channel VCA, the SSM2164, which is quite cheap since it has been designed for modern consumer electronics like MP-3 players, etc. This VCA has a gain control range in excess of 100dB and thus is quite usable for a high gain multi channel amplifier.

The amplifier shown on the left has a maximum gain of 100000 on all three channels which can be controlled by a single 10 turn precision potentiometer.