Is there any advantage to wiring two geophones in series? Is their sensitivity limited by spring and mass so you gain nothing, assuming you have enough gain.
They were on sale at $9.95, just went up to $13.95.
Mikek
If you're making a phase array.
Spring and mass should be linear all the way down.
-- When I tried casting out nines I made a hash of it.
If the vibrations hit both together, but the noise sources are unsynchronized, you gain a bit of s/n advantage. Probably not worth it, given that a truck a few blocks away or people walking in the building will make massive seismic noise. It's surprising how much the world vibrates. I've filled 55 gallon drums with water to get a bit of stability.
An array could be directional!
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Probably similar to these...
They're not really accelerometers, they measure velocity above their resonant frequency.
Note the use of a damping resistor to flatten the resonant peak. These were/are used in seismic surveys where a string of geophones is connected to a multi-channel seismograph and some sort of impulse (explosive, weights, sledge hammer) is induced in the ground.
They're no good for earthquake type seismics being far too high a frequency and far too insensitive. They're a moving coil construction with coils wound in opposite senses near opposite fixed magnet poles so as to cancel external field pickup. Horizontal versions are available but less common.
I was involved in constructing triaxial packs of these which connected to portable 3 channel FM cassette recorders (plus 1 flutter channel) for demolition vibration monitoring. It was a while back.
Cheers
-- Clive
No. These are resonant, and (the one I got) intended for vertical orientation. Good for analyzing a motor vibration, or for hammer seismometry, but not much else a hobbyist would be interested in.
I'm intending to couple one to a table saw with clay, and see about the various pulleys' balance. Someday.
A cloth sack full of sand is remarkably good if you don't have anything better. We operated an atomic force microscope 200m from a major road with nothing more sophisticated and it worked for most of the time. (much to my surprise)
If he wants to do that it is worth having four and combining all pairs.
Classic Golumb ruler spacing for four antenna is 1,3,2
x x x x
Which gets you every baseline phase delay length from 1 to 6 inclusive.
-- Regards, Martin Brown
(...)
That doesn't seem to stop vendors from selling earthquake recorders based on such geophone sensors:
"Do Low-Cost Seismographs Perform Well Enough for Your Network?"
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
No. Such geophones already have two opposing coils wound in opposite directions. The idea is cancel any external magnetic fields.
So, what are you designing? If you have nothing planned, perhaps a ground penetrating 3D sonar to locate gophers?
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Nothing planned, just curious.
Installing a BOG antenna is my front burner project. Replacing one I lost to hurricane clean up. But, I want to try to broadband a BOG. If you want to talk BOG (Beverage On Ground) antennas with lumped variable inductances to receive lower frequencies than the length would provide for. As I understand it, Beverage antennas are not a resonant antenna, but today I'm lead to believe that, BOGs ARE resonant antennas.
Mikek
If you are at all interested, here's a drawing of the proposed Lumped inductance BOG circuit.
is it just possible these vendors are being a little bit economical with the truth?
Seriously. Geophones and seismometers have an overlap of course, but earthquakes are tens or hundreds of millihertz.
Cheers
-- Clive
It's not that simple; earth movement happens at a range of frequencies, it's the damping with distance that makes short-period instruments useless at long distances.
Looking for earthquakes, you'd want to cast a few hundred to a few thousand miles net; prospecting for oil, you're more interested in the LOCAL strata, in range of your hammer or thumper (one-piston earth-mover engine).
A big earthquake, nearby, will show up on a geophone. An event in the Tonga Trench, half the world away, won't have Atlanta observing unless they have that long-period sensor that isn't a 'geophone'.
Two examples from my knowledge.
Australia's National Measurement Institute is in West Lindfield, a bush area with few main roads, none nearby. The railways wanted to dig a tunnel under it, but it would have cost at least $100M extra to relocate the NMI. They re-routed the tunnel instead. Someone messed that up too; the electric supply was not enlarged enough to be able to restart a full peak-hour train on the increased up-slope. D'oh!
Nick Hacko is setting up a watchmaking factory in Sydney... or he was, until they found they could only use their expensive precision CNC mill at night, when there were no trucks on Old Pittwater Rd or especially on the carpark ramps near their facility. He's building the factory an hour or so out in the Southern Highlands instead.
Clifford Heath.
Many years ago, when I worked at IBM, they were trying to bring a FIB tool in our building. Every once in a while they couldn't focus it, no matter what they did. They brought in all sorts of "experts". By accident (people from different groups talking in the hall) it was discovered that there was a shake table on the floor below them. It wasn't used often but when it was, it shook the whole building.
Oops. Been there myself. In about 1969, I was working for a company that make their own thick film hybrid circuits and using either an SS White Airbrasive sand blaster, or a home made CO2 laser for trimming the resistors. The laser tube and optics were about 5ft long and mounted on a concrete block wall. A few feet from the other side of the wall were the railroad tracks. If we attempted to trim resistors while a train went past, the kerf looked very much like a seismograph plot. These resistors had the correct resistance, but would drift with temperature and aging as resistive debris fell into the kerf. Break times were coordinated with the train schedules, and I rigged a vibration detector to the tracks which could warn of an approaching train up to a mile away. We mounted the laser tube on a floating and damped contrivance, not so much to deal with the train vibrations, but rather to deal with the need to refocus the spot every time the train went past. Finally, we built a trimming station where everything was mounted to one work table, which floating on shock absorbers and dampers and NOT connected to the wall.
At my next employer, they were sandblasting radio crystals to specific frequencies. Although there was no evidence of a seismograph shaped kerf, I convinced management to try floating and shock proofing the cutting fixture to see if it might help. It did. I eventually discovered that vibration from the pump motor in the sand blaster was causing the accuracy and drift problems. A geophone might have been useful for finding the source of the vibrations, but a cardboard ear trumpet was good enough.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
I am no way expert on this, but I did run test with a cheap 6 axis accelerometer for inertial navigation, I did read the paper Jeff referred to but found some graphs difficult to read, but it sort of confirms my own measurements (I ran days long looking for tide effects on gravitation for example). I found the 6 axis accelerometers are very temperature sensitive, long time drift. Then some university repeated it in a good oven.. better results. Seems to me for higher frequencies than days it should not be a problem, Some tests with the MPU6050:
As the frequency decreases below the 10Hz resonance, the geophone coil will move less and less, and it's the coil velocity which produces the output.
I worked for a year for a seismometer manufacturer, designing and tweaking the control electronics.
Their 'real' seismometers had a frequency response expressed in seconds.
200s (5mHz) would be a typical low frequency for something portable. They used masses supported on knife edges with zero length springs for support. The mass is servoed to maintain it's balance position using voice coils and capacitive feedback - a force balance accelerometer. In addition there is a motorised system for coarse adjustment/levelling.Usually triaxial - one issue was getting the responses the same between horizontal and vertical given the differences in the mass supporting springs. A triaxial unit was about the size of a bucket, smaller for higher frequency less sensitive devices.
Cheers
-- Clive
Yes OK, I can follow that, but I was referring to 6 axis accelerometers like the MPU6050. lookup that datasheet, quite impressive. I have one running 24/7 in one Raspberry Pi based navigation project. Any motion or vibration or change in attitude is detected.. 16 bits ADCs. The mechanical vibrating sensors themselves run at about 30 kHz. On the low frequency detection side you are only limited by temperature drift. and with low I mean days. And the noise limit... 16 bits is not nothing. For what it is worth... I like the chip, and it is cheap, SPI ebay module with it only a few $.
An LVDT and a small mass has nanometer vibration sensitivity, I discovered.
My first experience with things like this was designing the electronics for a Taleyvel inductive inclination sensor, to be used on the C5A to align things. People walking around, on a concrete slab floor, made vibration and changed inclination.
Laser gadgets now are no doubt far more sensitive.
An array of vibration pickups, and a lot of signal processing, could be a sort of radar to spot people and vehicles moving around. Maybe pick up voices. I'm sure the CIA or someone has done that.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
In 1987 I visited the HP Corvallis facility, where they were making inkjet printers, calculators, and X Windows software. Those HP buildings were all built to the same plan. Earth-quake proofing was done in a way that when you were seated in a cubicle upstairs, you could feel a wave moving in the floor as someone approached down the aisle, and after they'd passed. It was very strange to me to feel a large building move like that in response to individual people moving inside.
They had built foundations for a new IC fab, with a huge rubber suspension system running deep into the ground, because there was a freight train line just under a mile away. Even then they would have had to suspend certain operations based on the railway schedule. The building had been abandoned at the foundation level because it was already out-dated, even though they'd spent $10s of millions on it.
Clifford Heath.
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