What's the difference between all of the different models?
What are spurious frequencies?
How old are geophones? When was the first one made?
Mine says "Pollard" on it or looks like a stethoscope on one end.
What makes me an expert on geophones?
I had a link to email me and the spam spiders got ahold of it. So now there's no way to send corrections, additions, etc... Sorry. The old email address is now filtered into ye ole bit bucket.
What is a geophone?
A geophone is a small, cheap instrument for measuring ground motion.
There are many different varieties for different applications. They are
designed for earthquakes, machine vibrations, oil exploration, mining, etc...
Who makes/sells them?
Here are a few links / phone numbers to companies that either make, lease or sell geophones:
GeoSpace
Mitcham Industries
Geotool
GISCOGEO
R. T. Clark
Sercel (Mark Products)
I/O
How much are they?
It depends on a lot of different factors. Basically the price is between
free to beyond $1000.00 U.S. dollars. I have two used 10 Hz geophone kits
that I bought for about $10.00 U.S. dollars each. The more complicated
/ lower resonant frequency / multiple channel / name brand / new geophones
cost more money. The older / simpler / single channel / used geophones
are cheaper.
What's the difference between all of the different
models?
There are huge differences between the models and options for geophones.
The exterior case is optional on a lot of geophones. Some have coaxial
connectors and some have binding post connectors, but most have two little
pins that you connect your leads to. The resonant frequency is one
of the main factors in the price. Lower resonant frequencies are more difficult
to achieve in a small box with a light weight and a low price. Basically
you want the resonant frequency to be close to what your looking for in
signals. Also your application should be a factor. You probably don't need
a 1 Hz resonant frequency to watch local earthquakes, but you would want
one for distant earthquakes. The frequency response of an instrument is probably centered around the resonant frequency and is very narrow in width.
What do they look like?
They look like little metal cylinders mostly. They vary in size and proportions with different connectors and some go into little cases. There are some pictures on the web sites listed elsewhere on this page.
Who uses them?
Oil companies, security companies, mining companies and seismologists
both amateur and professional.
How do they work?
Most of the models have a coil hanging from a spring in the center of some magnets. When the case is moved up and down the mass tends to stay put and induces small currents into the coil as it moves through the magnetic field. It measures velocity of motion.
The current is used to move a pen over paper or gets recorded into a computer for analysis.
What are spurious frequencies?
This information was given to me and I tried to paraphrase it for simplicity. Basically, it's the noise from high frequency oscillations in the suspension spring. The spring inside the geophone that supports the mass, can pick up higher frequencies and add noise to the signal. Some values I saw while poking around are pretty high and would not effect seismic signals. At least not distant seismic events.
How old are geophones? When was the first one made?
Real seismometers go back to the 1900's. Geophones probably came shortly after.
Mine says "Pollard" on it or looks like a stethoscope on one end.
It's not a typical geophone with a coil and a magnet. It's for listening for underground leaks in pipes.
Here's a good link with pictures.
What makes me an expert on geophones?
Actually, I'm an amateur and I have a passing interest in seismology. Please, do not base your thesis on things I've written.
last updated 06/26/2006