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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 before they happen.

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Building collapse is what actually kills people when major earthquakes hit impoverished urban areas with substandard housing. 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 - now forgotten. A reliable warning on the order of tens of seconds or more would let people move to safer places, but 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 precise 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 seismic data in vector format and statistical data. Small local seismic signals that were previously lost in the seismic noise can be readily identified.

This completely open source project breaks down into four main parts.

1) Part 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."

The role of the hacker here might be to challenge that bit of conventional wisdom. Let's see where it goes if we put the magnifying glass on seismic noise instead of intentionally ignoring it.

The hypothesis behind this project was that if we really study the seismic noise and mathematically get to know its behavior in seismically active locations, we will be able to tell the difference between this noise and the telltale snaps, crackles and pops that must happen locally right before a geological fault line lets loose.

Basically, I am reporting a new kind of design for a seismic 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 one 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 neutral to all 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 very insensitive and they are best used for strong man-made signals in geological exploration. However, piezoelectric pressure sensors still have the theoretical 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 (mostly) coming from?" in addition to "How loud is it?" in real time. This has not been done before.

The design is fixed to a wooden base on rubber feet. A stainless steel or mineral sphere is supported by 3 hard insulating plastic beads resting directly on 3 inexpensive piezoelectric buzzer elements. These elements are themselves symmetrically arranged 120 degrees apart and precisely tilted at a 45 degree angle around the center ball. The buzzer elements are mounted on adjustable supports, like 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 or with an iPhone. An enclosure provides protection against short term temperature changes and air currents. The device can be placed under a glass bell jar or withinin an airtight styrofoam cooler box, for example. The electronics are either housed in a closed box or within the styrofoam cooler box. To be useful, the devices need a very sturdy foundation. The devices either rest on a thick concrete slab...

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newground.pcb

The Pad2Pad.com design file that contains all needed instructions for manufacture of the PCB. It also contains its own version of the BOM.

pcb - 147.48 kB - 04/28/2017 at 19:30

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BOM.ods

Bill of materials

application/vnd.oasis.opendocument.spreadsheet - 13.39 kB - 04/28/2017 at 03:13

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  • 1 × Wooden base - see instructions 2 inch thick 8 inch diameter hardwood bowl blank from woodworking store.
  • 1 × Center mass - 3 inch chromium steel or mineral ball Available on Amazon and eBay. See instructions.
  • 3 × Piezo crystal microphone or buzzer elements - see instructions Available on Amazon and eBay.
  • 1 × Assembled 3 channel charge amplifier circuit board - see instructions Pad2Pad, per custom specs- schematic & bill of materials free on request
  • 3 × Texas Instruments LMC662 femtoampere op amplifier And passive surface mount componetns - from Digikey. See Pad2Pad BOM.

View all 9 components

  • Much less seismic noise intensity at lower frequencies

    michael d.05/18/2017 at 00:14 0 comments

    The overall vector magnitude of the seismic noise decreased by about 90% when the 1 Hz filter was substituted for the 15 Hz filter. Very interestingly, the magnitude distribution is much more gaussian than the output from the 15 Hz device, as well. Will this be good or bad for what the device is intended to do?

    I have increased the amplification of the second stage of the amplifier by a factor of 2 to partially compensate for the diminished noise intensity. The overall signal response to small local events seems to have decreased, but there is expected to be no change in the probability data output. Too soon to tell.The software variables are still being tweaked to deal with the new arrangement. It will be interesting to see how the data from the 1 Hz and 15 Hz devices compare while running side by side! Hopefully I will have some data to post in the next week or two.

  • Looking at lower frequencies

    michael d.05/13/2017 at 19:51 0 comments

    The charge amplifier on one of the seismometers has been changed to respond to 1.5 Hz and below. It was a simple resistor change, of the rightmost resistor on the handwritten schematic down below. The resistor was 1000 Ohms and now it is 10,000 Ohms.

    Up until now the amplifier has been using a 15 Hz low pass filter. It's going to take a couple of days to collect enough data to get the device's sensors calibrated. The plan is to run the 1.5 Hz device and a 15 Hz device side by side until "something seismic" happens, then compare and contrast the results.

  • Thanks for the prize!

    michael d.05/08/2017 at 22:40 0 comments

    I just found out my project was selected to be one of the prizewinners in this round.

    Wow! I am humbled by the other winners and proud to be included with them. There were many awesome projects that were not selected, as well. I hope that these people will not get discouraged. Hang in there and make your project better and better!

    I hope that this recognition will bring more "lookers" who may be interested in working together with me to create networked seismic noise monitoring systems in seismically active regions.

    This thousand dollars is going to be given to Doctors Without Borders because of their fantastic record of dealing with earthquake emergencies.

    Thanks!

  • Another minor seismic event

    michael d.05/01/2017 at 20:58 0 comments

    This morning at 1:00 AM another very small distant event showed up on several of the East Tennessee machines - at the Knoxville, Bacon Ridge and Sewanee locations and on the Copper Ridge short period machine,

    Here is that small tremor, cropped out from the Knoxville CERI/USGS record on the web-

    Here is the event as it showed up in the vector magnitude data of one of the garage slab machines -

    Here is the vector magnitude data from the 100 seconds around that event-

    Here is the location data for that same time interval - expressed in radial distance (in radians) from the average compass location of the noise. As with most (but no all) of the distant events that have been recorded so far, the location seems to bounce around - possibly due to echoes and reflection from our regional mountain chain - the Smokey Mountains.

    To give meaning to this graph, the next graph shows the distribution of the radial (compass) position of the noise for the 2 hours surrounding the event. The averaging is done in such a way that the distribution should peak at North, or 0 on this graph. The 0.8 value at the peak of the small tremor (see the graph above) is the highest value recorded in that 2 hour interval. Basically, the values greater than 0.4 radians and less than -0.4 radians are statistical outliers.

    The next graph shows the combined probability statistic of the event (green) along with its 5 second running average (blue). Again, this is a logarithmic scale, so the probability values change by 6 orders of magnitude as the event progresses.

    As with the event from several days ago, there was no major advance warning from the statistics data, possibly because of the distance of the event from our machine. There was an unusual brief 2 order of magnitude drop to around -5.8 in the graph about 10 seconds before the main event, however.

  • Data collection results

    michael d.04/22/2017 at 18:20 4 comments

    At 2:32 AM EST on April 22, 2017 a typical tiny 30-40 second seismic event was detected at the Knoxville CERI/USGS seismometer. It was also recorded at the Copper Ridge, Tn CERI seismometer, so it is not a local event. The following images display the output from their device and our device in response to that remote event. This particular event is chosen to be shown because (1) it happened this morning and (2) it is a typical tiny event that shows off the capabilities of this device well.

    This is the CERI/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. The graph shows 1 second per time points, therefore 3600 readings per hour. The peak at 2:32 AM is approximately 1/10 of the device's full scale.

    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 tiny and brief tremor would not significantly affect the overall distribution of the noise over an hour, but those time points 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 whole purpose of the femtoampere amplifier is to broaden the distribution, so subtle changes in the "right side" shoulder region can be discriminated easily. This is only one hour's worth of data - the incoming data is matched to a probability table that is based on the first 100,000 measurements after the program starts up. Then every new data point is assigned a probability value from that table as well.

    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. Again, each new location measurement is matched up to a probability table derived from 100,000 measurements and assigned a probability from that table.

    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. The two independent probabilities are multiplied in order to get the combined probability result. Note - there is essentially no correlation between the location and the magnitude when large numbers of noise level signals are analysed. It's clearly different for event related signals. 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. Because this is a logarithmic scale, there is a million-fold change in this value within seconds of the appearance of the tremor event. The traditional USGS seismometer barely detected the event (see above).

    There was no real discrepancy this morning between the onset of the combined probability event and the vibration...

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View all 5 project logs

  • 1

    This picture is of my "indoor" seismometer, with its 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 any serious seismology. It is never quiet, even at night, because of vibrations related to air conditioning or heating fans, dog activity, weather, etc. Things DO go bump in the night, a lot, and I have proof... The center mass is an agate sphere, in keeping with the geological aspect of this project.

    The basic device design consists of a heavy and rigid 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 and an enclosure. 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.

    The photograph above shows one of the devices, the Yun board (left), the clock module (middle, with the LED) and the three channel charge amplifier (right).

    The next photograph shows another device with a heavy 4" walnut base, a 4 pound chromium steel ball and rubber feet. The device behind it is the last version of an earlier design that used 3 steel balls suspended from a center post. It is no longer in use. The single mass design makes fewer assumptions about the vertical component of the seismic noise. It would also be much more stable in case of a strong seismic event.

    Two of the noise vector devices rest directly on a massive 2-car garage concrete slab.

    One device is bolted through its styrofoam container to a 300 pound poured concrete base outdoors, under a weatherproof "fake rock" fiberglass housing. As with the others, it is connected to the home network through a wireless router.

    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, usually 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 of the piezo casing is drilled out to allow a hard plastic bead (JoAnn fabric store) 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 on eBay.

    The adjustable supports are bolted to the wooden base by 1/4" threaded bolts (hardware store) that have been cut to size with a Dremmel tool.

    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. Beautiful minerals and rocks are easily available from multiple sources on the web 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 of the center mass must affect all three sensors equally. Smart phone apps like iLevel (for the iphone) can provide highly accurate leveling and tilting information. A bubble level or smartphone is used to level the wooden base.

    Inexpensive large scale manufacture is needed to produce a device for use in the third world. A molded plastic base incorporating all the necessary angles and distances without the need for adjustable elements would be the way to go. Glue the sensors in place, drop a lead or steel ball in and it would be done. Someone with a 3D printer could produce a prototype - anybody interested?

    The Pad2Pad circuit board designed by me can be ordered by any interested party through Pad2Pad - contact me for further instructions if there is difficulty ordering it or Pad2Pad requires further permission from me. The design is based on ideas from a Texas Instruments white paper on piezoelectric sensor instrumentation.

    The need for soldering might be a barrier to makers interested in building these devices. I am looking into the possibility of selling the charge amplifier PCB board and all of its components on Tindie.com. It will be sold either as a kit with the DigiKey bill of materials or as a pre -assembled board.

    Newground.pcb is the Pad2Pad.com project file and it has been uploaded to the uploaded files section of this project. The Pad2Pad design program itself is free and downloadable from their website - it is needed to work with, view or modify the Newground.pcb file.


    The Pad2Pad.com BOM is included in the uploaded files section of this project and it is also part of the Newground.pcb file. The Findchips.com web page suggested by Hackaday will not accept the BOM in the Pad2Pad format or open source Open Office format, unfortunately.

    The PCB is a surface mount design but uses fairly large (well, large for surface mount, anyway!) SMT 1206 components for the most part. Some soldering skills (or patience and willingness to learn!) are required for assembly.

    The 3 channel charge amplifier and low-pass filter electronics are based on the following simple design. Shown is one channel only, but they are identical. The TI op amp is a dual design, so only 3 relatively inexpensive op amps are needed.

    The Arduino YUN program (906 lines, including spaces and comments) is open source and freely available on request to any interested individual. I am supplying it on a "on request" basis because I would like to know (and collaborate or at least stay in touch with!!! ) the individuals who are using it.


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Tindie wrote 2 days ago point

Congratulations on being one of the Internet of Useful Things Hackaday Prize Finalists!

  Are you sure? yes | no

Peter Walsh wrote 05/07/2017 at 00:10 point

1) That "not a gaussian" curve looks suspiciously like a Levy distribution. It's one of the three known stable distributions (others being Gaussian and Cauchy). It's what you get when the variation is proportional to the offset from the mean, and comes up occasionally IRL for things such as annual flooding and geomagnetic reversals.

http://www.gummy-stuff.org/Levy.htm

2) I don't know what amplification you're actually getting, but a quick back-of-the-envelope estimate guessing the parameters of the piezoelectric sensor indicates that the output of your amplifier is 1,000x the input signal.

I have a project with a charge amplifier circuit that I've been working on for awhile, and I'm getting a factor of 1,000,000x the input signal, which are 3MHz pulses. (Measuring individual alpha particles.) I'm accurately measuring 6uV pulses that are 1/3uS long, which is about 3x the noise floor.

If you think higher amplification would benefit your project and want to compare notes or try a different circuit, send me a PM.

3) Kickass project, looking forward to future posts.

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michael d. wrote 05/09/2017 at 00:48 point

Peter - 

Thanks for the comments.

Your project page looks really interesting. I would really like to look at your charge amplifier circuit out of sheer curiosity about what it looks like. Sounds like you have a great project there too. You are looking at transient pulses whereas I am looking at ultra-slow seismic waves affecting the behavior of piezo crystals, so we are optimizing for different things.

The amplification in this piezo charge amplifier first stage is somewhat difficult to define because of the huge but necessary (500 megaOhm) resistor, the ultra-low femtoampere leakage current of the TI op amp inverting input, the high resistance of the piezo crystal and the very nature of "charge " vs. "voltage". 

This is not at all a perfect analogy, but you can think of a piezo crystal as a sponge - you squeeze it and electrons get squeezed out - when you un-squeeze it they go back in. Unlike a sponge, though, if you stop squeezing the electrons go back in too, even if you haven't un-squeezed it yet. Therefore, it really responds to the derivative of the squeeze, so to speak. The electrons go "out" of the sponge/crystal in a charge amplifier circuit but not "around the block" as in a typical circuit. The charge amplifier first stage is then followed by a classic 10X voltage amplifying second stage. The second stage could be arbitrarily high, but 10X is just fine for our data collection purposes and it keeps the electronic noise down to a minimum.

To summarize, the first stage charge amplifier is very sensitive to the static electric field produced by the piezo element's charge output and the second stage amp gives us tens of millivolts of output in response to the acceleration of the center mass caused by the seismic noise. 

Mike

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David H Haffner Sr wrote 04/29/2017 at 22:17 point

This is another fantastic project here, and I truly wish you luck on this, what a breakthrough it would be!

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Thomas wrote 04/28/2017 at 18:39 point

Did you try using ordinary TL072 for the charge amplifiers? The bias current is a bit higher, but I don't expect the offset to be higher than 0.5V (which can be canceled out with a floating average). 

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michael d. wrote 04/29/2017 at 01:49 point

The original design used TL082 op amps - it  "worked", but was not nearly as sensitive. The standard deviation of the noise increased significantly when we swapped the op amps.  The idea for the change came out of a Hackaday discussion (!). This project depends on resolving the noise into the widest possible noise distribution. The wider it is, the more the program can detect subtle changes in what is happening down there!!

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Thomas wrote 04/29/2017 at 03:01 point

Thanks, now it's clear :-)

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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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