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Device for Seismic Noise Analysis

A device that monitors the statistics of the magnitude and the 3-D direction of seismic noise might detect earthquakes just before they hit.

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Building collapse is what actually kills people when major earthquakes hit impoverished urban areas. There are 7 earthquakes that have each killed over 100,000 people in the past century. 223,000 people died in Haiti ten years ago. The world has already forgotten. A reliable warning on the order of tens of seconds or more would let people move to safer places, but so far such a system does not yet exist.

The earth is always in motion. This motion is "seismic noise." A new femtoampere amplifier IC now allows the reliable measurement of the vector magnitude and 3 dimensional origin of this noise and determination of the statistics of these values in real time. The project reports a new, unique, ultra-sensitive and easily networked digital seismic device built with off the shelf components. It outputs statistical values and traditional seismic data as vectors. Small local seismic signals that were previously lost in the seismic noise can be readily identified.

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...

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  • 1 × Wooden base 2 inch thick 8 inch diameter hardwood bowl blank from woodworking store.
  • 1 × Center mass - 3 inch chromium steel or fluorite ball Available on Amazon and eBay.
  • 3 × Piezo crystal microphone or buzzer elements Available on Amazon and eBay.
  • 1 × 3 channel charge amplifier circuit board Pad2Pad, per custom specs- schematic & bill of materials free on request
  • 3 × Texas Instruments LMP7721 JFET femtoampere op amplifier And passive surface mount componetns - from Digikey. See Pad2Pad BOM.
  • 3 × Arduino Yun microcontroller Linux processor for networking and ARM processor for Arduino code
  • 3 × Magnetic door stops Available at Lowe's and Home Depot stores
  • 1 × Real time clock module Board and Arduino library per Adafruit.com

  • Data collection results

    michael d.5 days ago 0 comments

    At 2:32 AM EST on April 22, 2017 a typical very small 30-40 second seismic event was recorded at the Knoxvile USGS seismometer. It was also recorded at the Copper Ridge, Tn seismometer, so it is not a highly local event. The following images display the output from their device and our device in response to that event.

    This is the USGS seismometer reading, downloaded from their website. The relevant event downloaded from their website is cropped out and marked with asterisks.

    This is the vector magnitude graph for the 2 - 3 AM time period of our vector seismic device showing its response to the same event. 1 second per time point.

    Here is the vector magnitude data from the event on a shorter time scale - x axis is seconds.

    The next graph shows the distribution of the vector magnitude over that hour. The brief tremor would not affect the overall distribution of the noise over an hour and those timepoints were removed. The important point is that this data does not conform to an ideal gaussian probability (red curve.) This data is skewed to the right, as would be expected if a significant proportion of seismic noise is coming from local sources. It is exactly this hypothetical characteristic of seismic noise that this project was hoping to detect and exploit.

    The next graph is the 2D geospatial distribution of the averaged location of the noise energy timepoints between 2 AM and 3 PM. -Pi radians would represent South), 0 radians would represent an average of North) +Pi radians would be South again. Each one second time point is the average of 50 calculated vectors. For pure noise with a well calibrated device, the peak of the gaussian curve should be at zero. Local noise from random sources would not be expected to shift the gaussian distribution of noise direction. For this time interval the actual measured average was -0.04818 radians.

    The next graph is the combined probability statistic for the 2 - 3 AM time period. The combined probability is basically the overall probability of having readings of that particular average magnitude coming from that particular average 3D radial direction. This graph shows a 5 second running average of the data. Note that this is a logarithmic scale - the variation in the probability statistic is actually quite large.

    Here is the probability data on a shorter time scale - again, x axis is in seconds. The data is from the same time interval shown above for the vector magnitude data. This is the actual data, not a 5 second running average.

    There was no real discrepancy this morning between the onset of the combined probability event and the vibration magnitude event, possibly because this was not a local event. An earlier seismic event some months ago was only 12 miles away form the device and it was predicted approximately 50 seconds in advance by the probability statistic. More experience with truly local events is clearly needed.

    .

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  • 1

    This picture is of my "indoor" seismometer, with it's electronics in an antique wooden box next to it. It is networked and completely functional, but because of its noisy indoor location, it is not useful for serious seismology.

    The device consists of a heavy wooden base, mechanical supports for piezoelectric sensors, a spherical center mass, the sensors themselves, a high quality but very simple 3 channel charge amplifier based on an ultrasensitive Texas Instruments femtoampere op amp, a clock module, the Arduino YUN microcontroller and its program. Because the YUN has built in wireless networking, the data stored on its SD card can be accessed for processing externally, even remotely from the web. The "indoor" seismometer is enclosed in a glass display cloche, to minimize temperature and air current variations. "Working" seismometers are enclosed in weighted styrofoam coolers.

    One seismometer is bolted to a 300 pound concrete base under a fiberglass housing in the back yard and two others are on a massive concrete garage slab.

    I have used several different hardwood bases - walnut, spalted maple and cherry. Different sizes and shapes are possible, but a circular base 2 inches thick and 8 inches diameter seems to work well. Beautiful pieces of wood can be purchased on Amazon - look for "bowl blank". Rubber stoppers (hardware store next to the corks) work well as pedestal bases.

    The sensor elements are basically piezelectric buzzers in plastic cases similar to this

    - http://www.ebay.com/itm/Lot-of-2-Piezo-Buzzer-70dB-2kHz-Supply-1-to-25v-Square-waves-/112066085579?hash=item1a17a8c2cb .

    The center hole is drilled out to allow a plastic bead to contact the piezo element directly.

    The sensor elements are glued directly to adjustable supports. Magnetic door stops (Lowe's, Home Depot) work well, as do inexpensive mini tripod mounts like these.

    The adjustable supports are bolted to the wooden base by 1/4" threaded bolts.

    The center mass ball is supported by the 3 beads in the center holes of the sensors and the sensors are tilted at a 45 degree angle with respect to the horizontal. Minerals and rocks are easily available as 3 inch spheres, as are chromium steel balls. The sensors themselves must be precisely angled and leveled - this is extremely important, as the acceleration of gravity is a very significant part of the program's vector calculations. The gravitational acceleration must affect all three sensors equally. Smart phone apps like iLevel (for the iphone) can provide highly accurate leveling information. A bubble level or smartphone is used to level the wooden base.

    The 3 channel charge amplifier and low-pass filter electronics are based on the following simple design. Shown is one channel. The TI op amp is a dual design, so only 3 relatively inexpensive op amps are needed. The Pad2Pad circuit board designed by me can be ordered by anyone - contact me for permission and instructions. The Digikey.com bill of materials can also be provided on request of the circuit board information. It is a surface mount design. Relatively good soldering skills (or patience and willingness to learn!) are required for assembly.

    The Arduino YUN program is freely available on request to interested individuals, but as with the circuit board design, I simply would like to know who is using it.


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Andrew Bolin wrote 03/31/2017 at 02:14 point

Great write-up, I'm interested to see how your results come out!

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