The project breaks down into three main parts.
1) Seismic sensor
From Wikipedia, the free encyclopedia -
"In geology and other related disciplines, seismic noise is a generic name for a relatively persistent vibration of the ground, due to a multitude of causes, that is a non-interpretable or unwanted component of signals recorded by seismometers."
In true hacker fashion, let's challenge that bit of conventional wisdom and see where we can take it.
Basically, I am reporting a new design for a device for the purpose of responding to the low frequency ( <15 Hz ) baseline noise movements of the earth's crust. Seismic noise is present everywhere on earth. Some of it is local and some of it arrives from far away. Ocean waves are a major cause of distant noise. Most useful sensitive seismometers utilize a mechanical moving element with a fixed resonant frequency. Because noise by definition is a composite of a wide range of frequencies, for our purposes this device must not have any significant frequency biases. It must be relatively blind to the frequencies in its band. Mechanical designs therefore can't be used for this purpose. Piezoelectic seismic accelerometers are practically frequency independent for seismic purposes and these "geophones" are commercially available, but they are usually insensitive and best used for strong man-made signals in geological exploration. However, piezoelectric pressure sensors still have the potential for extremely high sensitivity. They have no moving parts or resonant frequencies in the seismic range, they have minimal frequency biases and are widely avaiable in the form of extremely inexpensive ($0.11) but high quality microphone elements. Because of the need for frequency independent noise floor analysis, I needed to design an ultrasensitive inertial piezo instument that pushes its seismic sensitivity to the very limit of what is possible. The other goal is to extract 3D directional information - in other words, to be able to ask "where is the noise coming from?" in addition to "How loud is it?" in real time.
The design is fixed to a wooden base on rubber feet. A stainless steel or fluorite ball is supported by 3 hard plastic beads resting directly on 3 piezoelectric microphone elements, which are themselves symmetrically arranged 120 degrees apart and tilted at a 45 degree angle around the center ball. The microphone elements are either mounted on magnetic doorstops available in hardware stores or inexpensive camera tripod heads. Each piezo element provides equal support to the central ball. Movement of the base in any vertical or horizontal direction accelerates the mass and changes the compression force of the ball against its sensors. One of the sensors is aligned to true north as a direction reference. The precise geometry of the device allows for the mathematical calculation of the vector magnitude and spatial origin of the seismic noise in real time.
The base of the device is leveled with a bubble inclinometer. A glass bell jar provides protection against short term temperature changes and air currents. Alternatively, the device is placed in an airtight styrofoam cooler box. The electronics are either housed in a closed antique box or within the styrofoam cooler box. The device rests on a thick concrete slab of a two car garage or an outdoor poured concrete slab sheltered by a fiberglass utility box. The test site is in a forested suburban location in a very seismically quiet region of Eastern Tennessee.
2) Integrating electronic charge amplifier
As the base of the device moves, the ball is accelerated by the piezo crystals themselves; the sensors then pump a charge (electrons) into or out of the amplifier. An ultra-sensitive Texas Instruments LMP7721 JFET femtoampere op amplifier (sensitive to changes of <10^2 electrons/second !) integrates, time averages and translates this discrete charge to an output voltage which an Arduino Yun microcontroller...Read more »