Electronics > Projects, Designs, and Technical Stuff
Seismic Sensors
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radioactive:
Interesting document comparing various sensor types.

Review of sensors for low frequency seismic vibration measurement
https://cds.cern.ch/record/1322403/files/CERN-ATS-Note-2011-001-TECH.pdf

After reading about the FBA type sensors, I'm wondering if this video from AppliedScience might be relevant.  Any thoughts on if it would be possible to use this approach for a vertical arrangement?  Please watch all the way to the end where it is demonstrated that you can pull the wavelength a few nm either way from center via supply modulation.
GK:
Spent ~13 of my 15 years in government assisting in upgrading and maintaining the (since disbanded) seismic monitoring network for the whole state of South Australia and have been on more field trips to deploy temporary equipment for aftershock surveys than I can remember.

I've never seen a geophone used to monitor natural seismicity. I've only had anything to do with geophones for induced seismicity, such as when letting off charges or for teaching purposes banging a metal plate on the ground with a sledgehammer. I have some geophones which I've collected and have thought about sticking them in the ground, but they are just too insensitive and really don't have the low frequency response required to make a decent seismometer.

We always operated on a shoestring budget and the state seismologist kept quite a large amount of antique/legacy equipment in active use just to keep the monitoring network as large as practically possible. We had a lot of Ranger SS-1 short-period seismometers in use - originally developed for the moon (the lunar Ranger Missions) and only recently (a few years ago) finally discontinued by Kinemetrics. From a hobby perspective with a limited budget, a used SS-1 from ebay or some specialist test/scientific equipment reseller would be a excellent start. They are mechanically simple and easily repaired. An SS-1 suitably installed will pick up those 7 and 8s overseas no problem.

The best seismometers for distant earthquakes though are the long-period units with huge/heavy sprung masses. The farther away the epicenter of the earthquake, the lower the frequency of the shock waves. Think of a stone dropped into a body of water - as the ripples radius outwards they dissipate as the wavelengths grow longer and longer. We had some long-period Benioff seismometers dating back to the 50s/60s that weighed, IIRC, a couple of hundred kg each. They had no high-frequency response to speak of so the noise was really low. These were installed in concrete bunkers on huge concrete plinths. Probably not practical for the average hobbyist, but in response to a big enough overseas wobble (our in-house technical term for earthquakes in general) they could continue to respond for an hour or more.

A couple of our modern sensors for picking up local seismicity, that I recall, were the 3ESP and the CMG-5T from Guralp:

http://www.essearth.com/product/weak-motion-seismometer/
http://www.essearth.com/product/strong-motion-feedback-accelerometer/




EEVblog:

--- Quote from: max_torque on January 06, 2019, 01:50:05 pm ---
--- Quote from: EEVblog on January 05, 2019, 01:10:15 pm ---I used to work at the largest seismic water tank in the southern hemisphere

--- End quote ---

I've googled "seismic water tank" and all i get is lots of links to how to design water storage tanks that are earthquake proof!  I assume the test tank you are talking about Dave is to provide isolation for precision measurements?  Be interesting to hear more!

--- End quote ---

Yes, above ground test tanks vibration isolated on huge rubber mounts and other mechanisms. IIRC the one on the Thomson Marconi Sonar site was maybe 6-8m in diameter and 20m deep. I think the Marconi one was a bit smaller, but also above ground.
I've tried but can't find any photos of them, this was in the days before digital cameras and the photo happy craze we have these days.
Maybe I should try and ask if it's still there and get a tour?
radioactive:

--- Quote ---Maybe I should try and ask if it's still there and get a tour?
--- End quote ---
  I would like to watch that!
diegogmx:
One interesting method i know modern seismometers use is to have a test mass as an lvdt moving element, and using a control loop and a coil to keep the lvdt output at 0, the seismometer output waveform would be the output of the control loop, they use one of these for each axis, buying one of these would most likely be prohibitively expensive yet it would be an interesting (and probably damn hard) project for summer (unless you are in the north hemisphere) :)
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